Experimental Verification of the Standard Model

No. 5] Proc. Jpn. Acad., Ser. B 97 (2021) 211

Review Experimental veriﬁcation of the standard model of particle physics

† By Tomio KOBAYASHI*1,

(Edited by Takaaki KAJITA, M.J.A.)

Abstract: The history concerning an experimental veriﬁcation of the standard model of particle physics is reviewed with special emphasis on results from experiments using the highest- energy particle colliders, namely, PETRA, LEP and LHC. This article covers physics subjects from discovering the gluon and precise measurements at LEP, to discovering the Higgs boson. It also covers some searches for physics beyond the standard model, particularly supersymmetry, as well as recent developments of some particle detectors that were used in those experiments.

Keywords: standard model, gluon, Higgs boson, supersymmetry

1. Introduction Table 1. Fermions in the standard model as of 1974 1-st 2-nd 3-rd charge All theoretical ingredients for the standard particle model of particle physics were available by 1974, generation generation generation (e) and were ready to be scrutinized by experiments. up-type uct2/3 During 40 years thereafter, a number of discoveries quark and ﬁne measurements were made, and the standard down-type dsb!1/3 model became established. quark

The standard model of particle physics is based neutrino 8e 87 8= 0 on theories that describe the elementary particles and charged e 7=!1 their interactions, namely the electro-magnetic, weak lepton and strong forces. Particle charges are in the unit of the elementary charge (e). The present particle contents of the standard Particles in the higher generation are heavier than the ones in model are given in Table 1 (fermions) and in Table 2 the lower generation, except that neutrinos are assumed to be (bosons). The particles in the hatched areas were not massless. The particles in hatched area were not found until 1974. found before 1974. Fermions (spin 1/2) were supposed to be the basic constituents of all material in Table 2. Bosons in the standard model as of 1974 the universe. Vector bosons (spin 1) are carriers of 2 the basic forces of the standard model, and a scaler particle spin charge (e) mass (GeV/c ) boson (spin 0) is responsible for the mass of all photon . 10 0 particles in the standard model. gluon g 10 0 The fundamental theory of the standard model charged weak boson W 1 ’1 80.385 is the gauge theory. The electro-magnetic interaction neutral weak boson Z 1 0 91.188 is described by the U(1) gauge theory: quantum Higgs boson H 0 0 125.1 electrodynamics (QED). The electro-magnetic force The particles in hatched area were not found until 1974. and the weak force are combined into the electroweak (EW) force in the SU(2) # U(1) gauge theory, giving a massless photon (.) and massive weak bosons (W ’, Z 0). The Higgs mechanism was *1 International Center for Elementary Particle Physics introduced in the EW theory, so as to give masses (ICEPP), The University of Tokyo, Hongo 7-3-1, Bunkyo-ku, to the standard-model particles, and as a conse- Tokyo 113-0033, Japan. † quence a spinless neutral Higgs boson (H) should Correspondence should be addressed: T. Kobayashi, ro- 358, Asahi, Chiba 289-2516, Japan (e-mail: [email protected] exist. The strong force between quarks, and hence tokyo.ac.jp). between hadrons, is described by SU(3) gauge theory doi: 10.2183/pjab.97.013 ©2021 The Japan Academy 212 T. KOBAYASHI [Vol. 97, with 3 color charges. This theory, called quantum DESY then decided to buildpffiffiffi the world highest chromodynamics (QCD), predicts the existence of energy eDe! collider, PETRA ( s ¼ 46:8 GeV max.), gluons as the carrier of the strong force. But none of of which one of the main objectives was to find the the bosons, except for the photon, were found before top quark (t), a counterpart of the b quark in the 3-rd 1974. generation quark doublet. PETRA completed its The fermionic part of the standard model construction and started operation in 1978. The consists of quarks, which have all 3 interactions ICEPP team formed the international group, the (i.e., electro-magnetic, weak and strong forces), and JADE collaboration, together with German and UK leptons, which have 2 interactions other than the teams to participate in experiments at PETRA. strong interaction. Quarks, as well as leptons, appear The author joined the ICEPP team as a research as a pair (like u and d, or 8e and e) to form an SU(2) staff member in 1977, and participated in the JADE doublet of the weak interaction. Fermions in the 1-st experiment. The activities of ICEPP continued from generation are supposed to form almost all of the experiments at DESY to two experiments at the materials in the universe, since stable atoms or nuclei European Organization for Nuclear Research are made of them. (CERN): OPAL experiment using LEP eDe! collider Until 1974, all of the hadrons, including the from 1980, and ATLAS experiment using the LHC strange particles, were explained by the quark model proton-proton collider from 1992. The author was with 3 quarks (u, d and s). While the lepton doublet involved in all 3 experiments (JADE, OPAL and of the 2-nd generation (87, 7) was already found, the ATLAS) until 2015. counterpart of the s-quark was missing. The GIM This article describes the important findings mechanism1) was proposed to cure a strangeness- made by the experiments at PETRA, LEP and LHC, changing neutral-current problem, which predicted as well as those from other experiments, in the light the existence of a 4-th quark (charm, c). of an experimental verification of the standard model In November 1974, the J/A particle was of particle physics. discovered at Brookhaven National Laboratory The standard model was known to have its own (BNL) and at Stanford Linear Accelerator Center limitation, namely: i) it does not have the gravity (SLAC). Soon afterwards the J/A was confirmed to interaction, ii) it has a charge quantization problem, be a ground-state charmonium, which is a meson iii) too many parameters, iv) a hierarchy problem, composed of c and c (anti-particle of c quark). The v) a generation problem, vi) no solutions to dark other charmonium states as well as the open charm matter, dark energy, and baryon asymmetry in the states (both charmed mesons and charmed baryons) universe, and so on. These necessitate a new theory were discovered one after another, so that the which includes the standard model. apparent flaw of the standard model framework had There are a number of theories which can solve been removed. Hence, the discovery of J/A was called some of these problems. Some of the candidates are: the “November Revolution”, and it was the beginning theories with supersymmetry, quark/lepton substruc- of an experimental verification of the standard model tures, technicolor models, quantum gravity theories, of particle physics. etc. During the course of experimentally verifying In 1975 the = lepton was discovered at SLAC, the standard model of particle physics, especially the and in 1977 the particle (the ground-state supersymmetry theories and the technicolor theories, bottomonium bb) was discovered at Fermi National these came up as the main candidate theories just Accelerator Laboratory (Fermilab), which strongly beyond the standard model. In this article some of indicated the existence of 3-rd generation fermions. the searches for new physics beyond the standard The International Center for Elementary Par- model are also presented, with emphasis on these ticle Physics (ICEPP) was established in 1974 at the two types of theories. The details of these theories University of Tokyo in order to participate in the are briefly described in the relevant sections of this DASP experiment using the DORIS accelerator at article. Deutsches Elektronen Synchrotron (DESY). DORIS D ! 2. PETRA period (1978–) is a positron-electron (e e ) collider of whichpffiffiffi the maximum center-of-mass (c.m.) energy ( s) was The PETRA (Positronen-Electronen Tandem 11.2 GeV. The ICEPP team made an essential Ring Anlage) eDe! collider was constructed at contribution to the discovery of the Pc particle, one DESY, and started operation in 1978. By thep endffiffiffi of the charmonium states. of 1979, it provided eDe! collisions between s ¼ No. 5] Experimental verification of the standard model of particle physics 213

Fig. 1. Sectional view of the JADE detector.

12 GeV and 31.6 GeV, which gave rise to many New flavor quark pair production in eDe! important results. PETRA continued working until collisions should enhance the multihadronic event 1986, and its attained highest collision energy was rate, so that the search was made through measure- 46.8 GeV. ments of the R value: There were 4 experiments at PETRA: JADE, ðeþe ! hadronsÞ Mark J, TASSO and PLUTO (which was replaced by R ¼ : ½1 ðeþe ! þÞ CELLO from 1980). A sectional view of the JADE P 3 2 detector is shown in Fig. 1. The ICEPP team was It is equal to i qi in the simple quark model, involved in the JADE experiment, and contributed where qi is the charge (in the unit of e) of the quark i, to the construction and operation of the electro- and the factor 3 comes from the number of quark magnetic shower calorimeter, which consisted of 2520 colors. The results were consistent with the produc- lead glass counters in the barrel part, and 192 lead tion of the known 5 quarks (u, d, s, c, b), and there glass counters on the two end caps of the detector. was no sign of any excess concerning the R values. 2.1. Top quark search. One of the main All 4 experiments made another more sensitive objectives of the PETRA experiments was to find the search for the top quark, by looking into the event top quark (t). Many theorists predicted the t quark shape of the multihadronic events. Just above the mass to be at around 13.5 GeV, mainly from the fact production threshold of a new heavy quark pair, its that the masses of s, c, b quarks are 90.5 GeV, event shape was supposed to be spherical, while the 91.5 GeV, 94.5 GeV, respectively. The first results lighter quark pair productions would give 2-jet-like concerning the t quark search using the data taken event shapes. until 1979 were given by MARK J,2) PLUTO,3) There were a number of event shape variables JADE,4),5) and TASSO.6) used in the analyses, such as the thrust (T), 214 T. KOBAYASHI [Vol. 97,

pffiffiffi ! / 3 Fig. 2. Distribution of the eigenvalues Q1,pQffiffiffi2, Q3 of the sphericity tensor. Q1 is plotted versus (Q3 Q2) . (a) Data from all energies combined and (b) model prediction at s ¼ 30 GeV including top quark (mt F 14 GeV) production. The dashed lines show the cuts Q1 > 0.075 and sphericity S > 0.55.

sphericity (S), spherocity (SB), etc., which were and the sphericity axis is given by the principal axis defined as follows: of the momentum ellipsoid corresponding to the "#X eigenvalue Q3. jpikj T ¼ max X ; ½2 Figure 2(a) is a Dalitz plot (named the “Q-plot”) 5) pi used by the JADE experiment, where the perpen- "#X dicularpffiffiffi sides of the triangle are Q1 and (Q3 ! Q2)/ p2 ¼ min X i? ½3 3, so that the hypotenuse is the sphericity, Sp.ffiffiffi Each S 2 ; pi event from the various c.m. energies ( s)is "#X represented by a point in the plot. While the 2 4 pi? spherical events expected from the top quark S0 ¼ min X : ½4 production would give a wide distribution in the pi plot, no significant accumulation of the events was Here, pik and pi? are the longitudinal and transverse observed in the upper part of the triangle. momenta of the hadron (i) relative to the jet axis, As a comparison, the results of a model which is determined by varying the direction of this calculation are shown in Fig. 2(b). This model is axis for each event. based on the quark-antiquark pair, qq (q F u, d, s, The sphericity value can also be obtained by c, b, t), productions, assuming that the top quark diagonalizing the sphericity tensor, decays through the chain t ! b ! c ! s, and on the X cascade mechanism from the quark fragmentation pipi into hadrons. The top quark mass (mt) was fixed to ¼ iX ½5 T 2 ; 14 GeV, and the c.m. energy to 30 GeV in the Monte- pi Carlo simulation. The normalization for the number i of events was based on the accumulated luminosity. where pi, is the ,-component (, F x, y, z) of the In order to draw quantitative results, the observed momentum of the i-th particle. From the resulting and expected events in the regions S > 0.55 and eigenvalues, Q1, Q2, Q3 (Q1 < Q2 < Q3, Q1 D Q2 D Q1 > 0.075 were compared. A Monte-Carlo simula- Q3 F 1), the sphericity value is calculated accord- tion without top quark production yields no events ingly to in this region. 3 In conclusion, no evidence was observed con- ¼ ð þ Þ ½6 S 2 Q1 Q2 ; cerning the production of a top quark with mt between 11 and 14 GeV. Similar results were also No. 5] Experimental verification of the standard model of particle physics 215

Fig. 3. Values of R F <(eDe! ! hadrons)/<(eDe! ! 7D7!)asa function of c.m. energy W. The results from JADE and TASSO data are summed. obtained by Mark J,2) PLUTO3) and TASSO6) using various event shape variables. Similar to the existence of J/A and for c and b quarks, respectively, the models of heavy quarkonia predict the ttbound states to lie below the threshold for the tt continuum. The experiments at PETRA made fine energy scans to search for the narrow states at around the 30 GeV c.m. energy region, and at later stages up to the highest energies at PETRA. Figure 3 shows the energy scan results made by JADE and TASSO at around the 30 GeV energy region, where the R values (Eq. [1]) are plotted as a function of the 7) c.m. energies. No significant structure waspffiffiffi seen. PETRA increased the energy up to s ¼ 46:8 GeV, and searched for the tt narrow resonance, but the results were negative.8)–10) The top-quark search was then taken over by AMY, TOPAZ and VENUS experiments using the eDe! collider TRISTAN at the National Laboratory for High Energy Physics pffiffiffi in Japan (KEK),pffiffiffi of which the maximum attained Fig. 4. “Q-plot” (described in the text) for (a) data at s ¼ 27:7 energy was s ¼ 64 GeV. The obtained result was and 30.0 GeV, (b) qq model prediction, (c) qqg model prediction. ! excluding the top quark with a mass below The planarity (Q2 Q1) axis is orthogonal to the sphericity axis. F 930 GeV.11)–13) The dotted line indicates planarity 0.07. 2.2. Discovery of the gluon. The most important result from the PETRA experiments was 14) discovering the gluon, the carrier of the strong force. 3p:0ffiffiffi7:4 GeV) of SLAC, and by PLUTO at DORIS Quark-parton models predicted the two-jet ( s ¼ 3:19:5 GeV) of DESY,15) using the sphericity structure in multihadron production in the eDe! distributions. annihilation reaction at higher energies, which was The QCD theory of strong interactions predicts expected from the process eDe! ! qq, with a sub- a specific type of event structure deviating from the sequent fragmentation of the quarks in to hadrons. two-jet structure due to the gluon bremsstrahlung D ! This phenomenon had been clearly observedpffiffiffi by process, e e ! qqg . This implicates that the data experiments at the SPEAR eDe! storage ring ( s ¼ should contain events with planar or three-jet 216 T. KOBAYASHI [Vol. 97,

16) configurations. JADE looked intop theffiffiffi Q-plot. Figure 4(a) shows the data at s ¼ 27:7 and 30.0 GeV, which can be compared with the qq model prediction, Fig. 4(b), and the qqg model prediction, Fig. 4(c). The transverse momentum (q?) of secon- dary quarks in the hadron-jet cascade is described by 2 exp 2 2 2 < d=d q? q?= q , where q was set to the standard value of 250 MeV. A significant deviation from the two-jet model is apparent. To see this difference more clearly, projecting the Q-plot onto the planarity (Q2 ! Q1) axis, which is orthogonal to the sphericity axis, was taken. In Fig. 4, the dotted line indicates a planarity of 0.07. Figure 5 shows the planarity distribution compared with the model predictions. These data show a substantial excess of events in the high planarity region over the qq model with

Fig. 6. Three-jet event observed by the JADE experiment at PETRA. The view is along the eDe! beam line. The JADE lead-glass shower calorimeter is shown in a perspective, overlaid with charged particle trajectories measured by the inner tracking detector. The shower counters, in which energy had been deposited, are marked in black. No. 5] Experimental veriﬁcation of the standard model of particle physics 217

2.3. Discovery of weak bosons. Another big candidate group of GUT, which includes the gauge epoch during this period was the discovery of the group of SM; SU(3) # SU(2) # U(1). Among those weak bosons (W ’, Z 0), which came from CERN in candidates, the minimum simple group is SU(5), and 1983. the next larger group is SO(10). The validity of the theory of EW interactions The minimal SU(5) GUT model covers many had already been proven by various experiments by shortcomings of SM, and gives new predictions, such that time: the discovery of the neutral current by as the gauge coupling unification, proton decay, etc. Gargamalle experiment in 1973,20)–22) a number of However, it soon became apparent that this simple neutrino scattering experiments, atomic parity vio- model has a difficulty to explain the long lifetime of lation experiments, and the inelastic electron scatter- the proton from the measurements of Kamiokande, ing on deuterium at SLAC in 1978.23) These brought Super-Kamiokande,32) and other nucleon decay Nobel Prizes in Physics 1979 to Sheldon Lee experiments. Glashow, Abdus Salam and Steven Weinberg for One possible way out is to go to the larger establishing the theory of the unified weak and symmetry group, or to introduce new ideas beyond electromagnetic interaction.24)–26) the SM. One of such ideas is supersymmetry (SUSY), The EW interaction at the lowest order is which is a symmetry between bosons and fermions. described by a single parameter, 3W, the weak mixing SUSY is an extension of the space-time symmetries 2 angle, in addition to , and GF. The value of sin 3W of quantum field theory, which was introduced in had roughly been determined by previous experi- particle physics as a possible solution to the gauge ments, which predicted the masses of the weak hierarchy problem, i.e., to the naturalness or fine- 27) bosons to be MW 9 80 GeV and MZ 9 90 GeV. tuning problem of the electroweak scale and the The CERN SPS proton-antiproton collider Planck scale against large radiative quantum correc- 33),34) (SppS)p wasffiffiffi designed to provide sufficient collision tions. The quadratic divergence appearing in the energy ( s ¼ 540 GeV) to produce weak bosons. The Higgs boson mass corrections could be eliminated by key element of the SppS was the technique for the cancellating the boson and fermion loop contribu- stochastic cooling of antiprotons, invented by Simon tions. By incorporating SUSY with SU(5) GUT, the van der Meer,28) which made it possible to accumu- proton decay problem could be ameliorated.35),36) late enough antiprotons and feed them into the The idea of a low-energy SUSY,33),34) being SppS. SUSY unbroken above TeV energies, had led to a Carlo Rubbia made decisive contributions in phenomenological model, Minimal Supersymmetric initiating the project, and in forming the experiment Standard Model (MSSM). MSSM can also be UA1 to discover the weak bosons. In 1983 the UA1 regarded as being a supersymmetric extension of and UA2 experiments observed clear signals of W’ SM with the minimal particle content. This requires productions with the isolated high transverse mo- the existence of supersymmetric partner particles mentum leptons with the associated missing trans- corresponding to SM particles. Some early trials to verse energy,29),30) followed by a clean peak of Z 0 in search for new SUSY particles were made based on the invariant mass distribution of lepton pairs by theoretical predictions,37)–39) and a few experimental UA1.31) searches based on those were made in JADE40)–44) Carlo Rubbia and Simon van der Meer were and in other experiments at PETRA. There was no jointly awarded Nobel Prizes in Physics 1984. sign of new particles at the PETRA energies. 2.4. Candidates of theories beyond the Other than SUSY, there was another solution standard model (SM). It was apparent that the to the gauge hierarchy problem. Since the problem SM should not be the final theory, since it does not seemed to be related to the large quantum corrections include gravity, because it does not explain charge to the Higgs boson mass, the elementary Higgs boson quantization (why the charge of the electron is could be replaced by a composite particle, by exactly the same as that of the proton), since it does introducing a new QCD-like theory (“technicolor”) not explain the particle generation, because it has too and additional massless fermions (“technifermions”). many parameters (at least 18), and so on. The technicolor models generate masses for W and Aside from gravity, it is desirable that some kind Z bosons via dynamical electroweak symmetry of theory exists that unifies all of the particles and breaking mechanism. Those models predict new the interactions in SM; Grand Unified Theory (GUT) heavy states, e.g., vector mesons analogous to the to name it. There are a number of choices for the ; and B mesons in QCD, while there are various 218 T. KOBAYASHI [Vol. 97,

Table 3. Fermions in the standard model as of 1988 3. LEP period (1989–) 1-st 2-nd 3-rd charge particle LEP (Large Electron-Positron Collider) was generation generation generation (e) constructed at CERN, and started its operation in up-type uc t2/3 1989. Until 1995, LEP ran at energies of around the quark pffiffiffi Z 0 peak, i.e., s 91 GeV (LEP1). From the end of down-type D ! dsb!1/3 1995 the e e collision energy was increased, reach- quark ing 161 GeV in 1996, surpassing the W DW ! thresh- neutrino 8 87 8= 0 e old. LEP continued working until 2000 above the charged D ! e 7=!1 W W threshold (LEP2). Its attained highest lepton collision energy was 209 GeV. The particles in hatched area were not found until 1988. There were 4 experiments at LEP: OPAL, ALEPH, DELPHI and L3. The general layout of the OPAL detector is shown in Fig. 7.45) The Table 4. Bosons in the standard model as of 1988 ICEPP team was involved in the OPAL experiment particle spin charge (e) mass (GeV/c2) and contributed to the construction and operation photon . 10 0of barrel part of the electro-magnetic shower calo- gluon g 10 0rimeter, which consisted of 9440 lead glass coun- 45)–47) charged weak boson W 1 ’1 80.385 ters. neutral weak boson Z 1 0 91.188 This calorimeter played an essential role in Higgs boson H 0 0 125.1 detecting and identifying the first event at LEP, as The particle in hatched area was not found until 1988. shown in Fig. 8, where the histogram represents the energy deposit in the lead glass counters. All of those lead glass counters worked well (with no dead possibilities for a composite Higgs boson. Experimen- counters) during the entire running period of LEP, tal searches for those new states were carried out at and made indispensable contributions to the high- higher energy machines. quality data analysis of OPAL due to the hermetic 2.5. Summary of this period. After the counter arrangement and the stability of the counter “November Revolution”, i.e., the discovery of the c operation. quark, the discoveries of the = lepton and b quark 3.1. Particle generations. The first result from followed until 1978. During the PETRA period the LEP experiments was determining the number (between 1978 and 1988) new vector bosons (gluon, of particle generations, i.e., the number of light W and Z) were discovered, which were the first neutrino species, N8. As the SM theory assumes that concrete proofs of the validity of the QCD and EW the neutrinos are massless, N8 can be determined theories. from line-shape measurements of the Z 0 resonance. Tables 3 and 4 show the particle content of The OPAL experiment measured the multi- SM, fermions and bosons, respectively, which were hadron cross sections at various c.m. energies at discovered before 1989, where the particles in around the Z 0 peak (Fig. 9). In order to determine 0 hatched area were yet to be found. the mass (mZ) and width (!Z)ofZ , a Breit-Wigner In parallel with the experimental verification of line shape with s-dependent width was used: SM, new theoretical models were proposed in order to 2 pole s Z cure the gauge hierarchy problem of SM and to aim ~ðsÞ¼had : ½7 ð 2 Þ2 þð 2 2 Þ2 for a GUT. One idea was to use SUSY, with which s mZ s =mZ Z the Higgs boson remains to be an elementary particle, The overall normalization was and its mass problem could be cured by canceling the pole 2 12 had boson and fermion loop contributions. Another idea ¼ ~ðs ¼ m Þ¼ e : ½8 had Z m2 2 was to regard the Higgs boson as being a composite Z Z particle, and to introduce a dynamical symmetry- The line-shape formula contains 3 free parame- pole breaking mechanism to generate the masses of W ters: mZ, !Z and had . The solid curve in Fig. 9 shows and Z. There were no experimental signs during this the fit result by the OPAL experiment, where these 3 period for any predictions from those new theories parameters were varied independently.48) The dashed beyond the SM. curve in Fig. 9 represents the SM fit with 3 particle No. 5] Experimental verification of the standard model of particle physics 219

Fig. 7. (Color online) General layout of the OPAL detector.

generations, where the only free parameter is mZ, pole since the values of !Z and had can be obtained mainly from mZ, with minor corrections from other parameters of SM. The data are in good agreement with the expectation of SM. In order to extract the number of light neutrino 0 species (N8) from the Z line-shape measurement, the following analysis was made. It was assumed that the neutrinos had the SM coupling and that no other new physics was reflected in the line shape. The Z 0 total width is written as ¼ SM þð 3ÞSM ½9 Z Z N ; SM where Z is the SM value of the total decay width, SM and is the SM value of the partial decay width into a single neutrino species. The quantities mZ pole and N8 were varied, while had was fixed by the 8) relation given in. The obtained fit result was N8 F Fig. 8. First event at LEP detected by OPAL experiment. The ’ histogram represents the energy deposit in the electro-magnetic 3.12 0.42. shower calorimeter. The inner tracking detector of OPAL was Similar analyses were made by the other LEP – not switched on at that moment. experiments, and similar results were obtained.49) 51) 220 T. KOBAYASHI [Vol. 97,

events in the forward and backward polar angular regions, NF and NB, and using the deﬁnition

NF NB AFB ¼ : ½10 NF þ NB In the SM at the tree level, the following relations between the weak and electromagnetic couplings are given: 1 G ¼ pffiffiffi ; ½11 F 2 2 sin2 mW W 2 sin2 ¼ 1 mW ½12 W 2 ; mZ sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi! 2 4 2 ¼ mZ 1 þ 1 A ½13 mW 2 2 ; mZ

with

2 Fig. 9. Multihadron cross section versus the center of mass A ¼ pffiffiffi ð37:28 GeVÞ ; ½14 energy measured by the OPAL experiment. The solid curve is a 2GF 3-parameter model independent fit. The dashed curve is the best where GF is the Fermi constant, , is the electro- fit from SM, where only mZ is varied. magnetic fine-structure constant, mW is the W boson mass, and 3W is the weak mixing angle. These tree level relations are modified by These measurements made a drastic improvement on including the radiative corrections, depending on a the previous determinations of N8 from SppS experi- chosen renormalization scheme. In the on-shell ments, from PETRA, based on cosmological or renormalization scheme Eq. [12] remains valid in astrophysical arguments. The number of particle higher orders, whereas Eq. [11] is modified as generations was conclusively determined to be 3. 2 2 A 3.2. Precision EW measurements. Since the m sin W ¼ : ½15 W 1 r LEP could produce tens of millions of Z 0 bosons, it became a very good test bed for the EW theory of The radiative correction, "r, depends on all of the SM. During the LEP1 period 4 LEP experiments parameters of SM, particularly the masses of the top 0 collected 17 million Z decays in total. The SM quark, mt, and the Higgs boson, mH. 0 expects the Z boson to decay into all species of The weak mixing angle, 3W, derived from Eq. [15] fermion-pairs, which are kinematically allowed, with is different from the effective mixing angle, 3eff, in the similar probability. neutral-current couplings at the Z 0 peak. The vector Like other experiments at LEP, OPAL made a and axial-vector couplings of Z 0 to fermion pairs (ff), 0 variety of analyses on the Z resonance. Following gVf and gAf respectively, are defined as D ! pffiffiffiffiffi the first line-shape result (e e ! hadrons), several f 2 f gVf f ðT3 2Qf sin eff Þ; ½16 analyses on the leptonic decays of Z 0 and combined pffiffiffiffiffi f ½17 analyses of the hadronic and letonic decay channels gAf f T3 ; 0 f were performed, obtaining precise Z line shape where T3 is the third component of the weak-isospin, parameters and EW couplings of charged lep- and Qf is the fermion charge. The relation between tons.52)–55) these two mixing angles is expressed as Figures 10 and 11 show the cross sections and sin2 f sin2 ; ½18 forward-backward charge asymmetries, respectively, eff f W pffiffiffi D ! as functions of s for e e ! lepton pairs and where 5f is a factor representing the difference in the eDe! ! hadrons, based on the data taken in 1989 two renormalization schemes. and 1990.55) The forward-backward charge asymme- In order to analyze the cross sections and tries were evaluated by counting the numbers of forward-backward asymmetries (Figs. 10 and 11), No. 5] Experimental verification of the standard model of particle physics 221

D ! D ! D ! D ! D ! D ! Fig. 10. Cross sections as functions of the center-of-mass energy for: a e e ! e e with j cos e j < 0:7; b e e ! 7 7 ; c e e ! = = ; d eDe! ! hadrons. The solid lines are the combined ﬁt results. The open and solid points show the 1989 and 1990 data respectively.

OPAL used the following form of the leptonic ^2 4 2 ¼ ½22 al gAl l: differential cross section in the improved Born Approximation: The main results of the combined fits were the 55) 2 following : 2s d 1 ðeþe ! lþlÞ¼ ð1 þ cos2 Þ N ¼ 3:05 0:09; 2 d cos 1 l ¼ 0:998 0:009; 2ReðsÞ 2 2 2 þ ½v^ ð1 þ cos Þþ2a^ cos sin2 l ¼ 0 238þ0:030 1 l l eff : 0:006: þj ð Þj2½ð^2 þ ^2Þ2ð1 þ cos2 Þþ8^2 ^2 cos ½19 Since the top quark mass dependence on the radiative s al vl al vl ; 2 corrections is dominated by the mt term, while the with Higgs mass dependence is logarithmic, a constraint 2 on mt could be obtained: ¼ GF mpZffiffiffi s ½20 2 þ ; 218 GeV at 95% confidence level 8 2 s mZ is Z=mZ mt < : where ", is the QED vacuum polarization correc- The results of all four experiments at LEP, 0 tion, and v^l and a^l are the effective vector and axial- corresponding to 9650 thousand Z decays into vector couplings: hadrons and charged leptons collected in 1989 and 0 56) v^2 4g2 ¼ ð1 4 sin2 l Þ; ½21 1990, were combined to obtain the Z parameters. l Vl l eff The combined results were: 222 T. KOBAYASHI [Vol. 97,

D ! D ! Fig. 11. Forward-backward charge asymmetries as functions of the center-of-mass energy for: a e e ! e e with j cos e j < 0:7; b eDe! ! 7D7! with |cos 3| < 0.95; c eDe! ! =D=! with |cos 3| < 0.9. The solid lines are the combined ﬁt results. The solid points show the 1990 data.

a polarized beam at SLC. The LEP experiments N ¼ 3:00 0:05; measured the cross sections, forward-backward 2 l D ! sin eff ¼ 0:2337 0:0014; asymmetries (l l , qq, bb, cc) and = polarization. þ40þ21 The SLD measured the left-right asymmetry: mt ¼ 1245621 GeV; ½23 1 ¼ NL NR ½24 where the second errors on m were systematic errors ALR ; t NL þ NR hPei coming from the variation of mH in the range of 50– 1000 GeV. where NL and NR are the numbers of Z bosons In 1994 the CDF experiment at Fermilab produced by left and right longitudinally polarized hP i announced discoveringpffiffiffi the top quark produced in electron beams, respectively. e is the luminosity- pp collisions at s ¼ 1:8 TeV using the Tevatron.57) weighted e! beam polarization magnitude, while the By assuming that the top quark decays into W boson eD beam at SLC was not polarized. The combination and a b jet, the top quark mass was estimated to be of all these results yielded precise determinations of þ13 0 58) mt ¼ 174 1012 GeV, which was in good agreement the Z parameters : with the indirect measurement23) by LEP experi- m ¼ 91:1875 0:0021 GeV; ments. This was the first clear evidence for the Z ¼ 2 4952 0 0023 GeV validity of the EW theory in the higher orders. Z : : ; The four LEP experiments gathered 17 million Z BðZ ! hadÞ¼69:911 0:057%; decays in the LEP1 period, while at SLAC the SLD BðZ ! lþlÞ¼3:3658 0:0023%; experiment collected 600 thousand Z decays using No. 5] Experimental verification of the standard model of particle physics 223

Fig. 12. Feynman diagrams for the process eDe! ! W DW ! at the tree level.

l ¼ 1:0050 0:0010; 2 l sin eff ¼ 0:23153 0:00016;

N ¼ 2:9840 0:0082; where B(Z ! had) is the Z branching fraction into hadrons, and B(Z ! l Dl !) is the Z branching fraction into chaged lepton pairs of single species assuming the lepton universality. The four LEP experiments performed measurements at LEP2, collecting 3 fb!1 of the integrated luminosity in total, so that the precise EW studies Fig. 13. (Color online) LEP measurements of the W-pair pro- were made on W boson pair production. By combin- duction cross section, compared to the theoretical predictions. ing the results of the four experiments, the fundamental properties of the W boson were obtained.59) In particular, the mass and width of W, mW and !W, and the branching fraction of W decays to hadrons, ’t Hooft and Martinus J.G. Veltman, who proved the B(W ! had), were determined to be: renormalizability of the theory.

mW ¼ 80:376 0:033 GeV; To go a step further, like the top-quark mass was predicted from the indirect precision measurements, ¼ 2:195 0:083 GeV; W the Higgs boson mass can also be obtained in a ð ! hadÞ¼67 41 0 27% B W : : : similar way. By combining the indirect EW measure- According to the SM, W bosons are pair- ments and the direct measurements of mt and mW,a produced via the processes at the tree level, as shown fit was performed to predict the Higgs-boson mass, 58) in Fig. 12, involving t-channel 8e exchange and s- mH. The obtained result was: channel . and Z exchange. The s-channel diagrams, þ74 m ¼ 129 GeV; i.e., the triple gauge couplings, manifest the non- H 49 Abelian nature of the SU(2) # U(1) gauge theory. Its or mH < 285 GeV at the 95% confidence level. direct study was made possible at LEP2, while at 3.3. Direct search for Higgs boson. The SM LEP1 and at SLC the couplings of Z 0 to fermion Higgs boson was expected to be directly produced at pairs were studied precisely. LEP1 mainly in association with a virtual Z 0 boson Figure 13 shows the combined LEP W-pair (Z*). The search channels at an early stage of LEP1 production cross section as a function of the center- were therefore eDe! ! Z*H 0, with Z* ! (eDe! or of-mass energy. The experimental data are compared 7D7! or ) and H 0 ! (qq or =D=!).60) By searching with the theoretical predictions: 1) with all the H 0 for the whole LEP1 period, OPAL did not find diagrams in Fig. 12 (YFSWW/RacoonWW), 2) with any positive signals, and obtained a mass lower limit 61) no ZWW vertex, and 3) with only the 8e exchange of 60 GeV. The other LEP experiments obtained diagram. The need for a diagram with a ZWW vertex similar results. is apparent, and it is a remarkable confirmation of AT LEP2 the Higgs boson could be produced in the non-Abelian nature of the EW theory. association with a real Z 0 boson, where all of the All of these precision measurements proved the decay channels of Z 0 could be used in the analysis validity of the EW theory, not only in the tree level, (Z 0 ! eDe!, 7D7!, =D=!, and qq). For the Higgs but also in the quantum correction level. This boson above the LEP1 limit, the main decay channels brought Nobel Prizes in Physics 1999 to Gerardus were H 0 ! bb and H 0 ! =D=!. None of the LEP 224 T. KOBAYASHI [Vol. 97, experiments obtained significant signals for H 0 production. The combined results of the four LEP experiments were mH > 114.4 GeV at the 95% confidence level.62) These results of the direct H 0 search are consistent with the indirect search mentioned in the previous subsection. Within the framework of the SM, the mass of the Higgs boson was constrained by LEP experiments to be in the narrow region: 114.4 GeV < mH < 285 GeV. 3.4. Verification of QCD. Following analyses of jet productions in eDe! collisions, mostly based on data from PETRA experiments, the abundant LEP collider data on the Z 0 resonance as well as at LEP2 were used for detailed QCD studies. One of the essential tests of perturbative QCD is “running” of the QCD coupling constant, ,s, which is the only free parameter in the theory when the quarks are treated as being massless. This manifests Fig. 14. (Color online) Summary of LEP measurements of , 2 F the asymptotic freedom of QCD, a steady reduction s(Q), where Q s. Results from PETRA and TRISTAN experiments are also included. The curves represent the QCD of the strong coupling as the energy scale of the predictions of ,s(Q) for the world average of ,s(mZ). interaction increases. , The LEP measurementspffiffiffi of s covered a wide range of energies, from s ¼ 1:78 GeV (the mass of = lepton) to 206 GeV. The hadronic partial decay In 2004 Nobel Prizes in Physics were awarded to 0 ! width of the Z p, ffiffiffihad, can be used to extract a precise David J. Gross, H. David Politzer and Frank Wilczek value of ,s at s ¼ mZ, since it is one of the most for discovering asymptotic freedom in the theory of precisely measured quantities, and its QCD correc- strong interactions.66)–70) tions are known up to the next-next-to-leading order The experimental investigations of the gauge ð 3Þ (NNLO), i.e.,toO s . The combined result of LEP structure of QCD were further pursued at LEP. The experiments was: key element giving rise to the asymptotic freedom is þ0 0058 the gluon self-coupling, i.e., that gluons can interact ðm Þ¼0:1226 : ; s Z 0:0038 with themselves. This was studied in angular 63) obtained from the ratio RZ ¼ had=lþl . Similarly, correlations and energy distributions of four-jet 71),72) a significant determination of ,s was made at a small events. The results were combined to determine energy scale, obtained from the normalized hadronic the color factors of QCD; CA associated with gluon branching fraction of = leptons, R= F !(= ! hadrons emission from a gluon, and CF associated with gluon 73) 8=)/!(= ! e8e8=): emission from a quark : ¼ 2 89 0 21 sðm Þ¼0:322 0:005ðexp:Þ0:030ðtheo:Þ: CA : : ; CF ¼ 1:30 0:09; Determinations of ,s from hadronic event shape observables and from jet productionpffiffiffi rates were which are in excellent agreement with the gauge made from LEP1 to LEP2 energies, s ¼ 91:2 GeV– structure constants of QCD (CA F 3, CF F 4/3). 206.0 GeV. It should be fair to mention here that similar The LEP measurements of ,s are summarized in results had been obtained by the TRISTAN experi- Fig. 14,63) together with PETRA64) and TRISTAN65) ments by that time, though with a bit less precision. measurements. The data are compared with the QCD The angular correlations among the jets in the four- predictions of ,s for the world average of ,s(mZ) F jet events were analyzed, and the non-Abelian nature 0.1183 ’ 0.0027. This clearly exhibits that the of the QCD was studied. The first result was 74) specific energy dependence of ,s, and hence the obtained by AMY, excluding the Abelian model concept of asymptotic freedom, are verified by the of QCD at the 90% C.L., followed by TOPAZ and experimental data. VENUS,75) raising the exclusion level to 95%. No. 5] Experimental verification of the standard model of particle physics 225

QCD predicts that quarks and gluons behave particle was to introduce SUSY in SM. As it was differently regarding the fragmentations due to their described in subsection 2.4, MSSM appeared to be different color charges. At LEP1 a study was made very promissing, but no sign of SUSY had been using hadronic Z 0 decays, and identifying the gluon observed. jets in bbg events using the b-tagging method. Soon after the LEP experiments started, the Various distributions, such as rapidity, energy, collaborators of the DELPHI experiment published transverse momentum with respect to the jet axes, an interesting analysis concerning the gauge coupling etc. were compared for charged particles in light unification using DELPHI data.80) Their result quark and gluon jets. It was observed that the showed that the GUT prediction of gauge coupling charged-particle multiplicity ratio of gluon to quark unification, i.e., the tree coupling constants would jets to be 2.29 ’ 0.09 (stat.) ’ 0.15 (syst.), in good become equal at a single unification point, did not agreement with the QCD expectation of CA/CF F work in SM. In contrast, the MSSM led to good 2.25.76) agreement with a single unification scale of 1016 GeV, 3.5. Other discoveries in SM. There were two which was consistent with the limit on the proton more major discoveries in SM from experiments other lifetime. In addition, their fit result indicated the than the high-energy collider experiments. SUSY scale to be at around 1000 GeV, which gave Following the discovery of the sixth quark (top) a strong hope for the SUSY particles to have masses in 1994 at Fermilab, the sixth lepton (tau neutrino, in the TeV region. Figure 15 shows the latest analysis 8=) was discovered in 2000 also at Fermilab by the result taken from Ref. 81 (2018). DONUT experiment.77) They recorded and analyzed Direct searches for SUSY particles (scalar the high-energy neutrino interactions in nuclear leptons, scalar quarks, charginos and neutralinos) emulsion targets. After = decay searches, they were carried out by LEP experiments. OPAL, like 0 observed four 8= events with an estimated back- other LEP experiments, made searches in the Z ground of 0.34 events, which was consistent with the decay channels,82),83) and at each energy point of SM expectation. LEP2 up to its highest energies.84)–86) None of them This completed the list of all fermions in SM showed any sign of direct productions of SUSY with 3 generations (Table 1). Regarding the bosons particles. (Table 2), all of the vector bosons were made present Another direction for new physics beyond the by this time, while only the scaler boson (the Higgs) SM was dynamical EW symmetry breaking. In such was yet to be found. theories, massive weak bosons acquire masses Another discovery was made at KEK and at through the mechanism that their longitudinal SLAC, which observed the CP violation in B-meson components are identified as composite Nambu- decays. The CP violation had first been discovered in Goldstone bosons arising from dynamical symmetry K-meson decay. However, it could not been explained breaking in a strongly-coupled extension of the SM. until a theory formulated by M. Kobayashi and T. The first and simplest models of dynamical EW Maskawa in 1973, by introducing 6 quarks within the symmetry breaking were technicolor theories,87),88) as framework of the EW theory, while there were only 3 described in subsection 2.4. quarks known then to exist.78) The Belle experiment M.E. Peskin and T. Takeuchi introduced new at KEKB and the Babar experiment at PEP-II parameters, S and T, representing the EW radiative observed clear signals of CP violation in B-meson corrections, and calculated the contributions from decays, so that the validity of the Kobayashi- various models of new physics.89) Early precision Maskawa theory was proven. Details of the discovery measurements at LEP and SLC, especially on the were described by F. Takasaki in Proc. Jpn. Acad., value of S, soon excluded a large parameter space of Ser. B published in 2012.79) the simple technicolor models,90),91) giving a necessity 3.6. Search for new physics beyond SM. At for theories to incorporate new ideas. the beginning of the LEP period, there were two main The first window of new physics beyond the SM directions for new physics beyond the SM. The key was opened in the neutrino sector. In the framework issue was the mechanism for EW symmetry breaking of the SM, neutrinos are assumed to be precisely in the SM, which resulted in the question as to massless. The accumulation of experimental evidence whether the Higgs boson is elementary or composite. from Kamiokande, Super-Kamiokande, SNO and The most typical and seemingly quite natural other solar neutrino experiments brought the discov- way to keep the Higgs boson as an elementary ery of neutrino oscillations, i.e., both atmospheric 226 T. KOBAYASHI [Vol. 97,

Fig. 15. (Color online) Running couplings in SM and MSSM using two-loop renormalization group revolution. This ﬁgure was taken from Ref. 81 (2018).

neutrinos (87) and solar neutrinos (8e) oscillate with energy SUSY, which gave a strong motivation for different species of neutrinos. This meant that SUSY searches in the next period. neutrinos have masses, though they are much lighter – than that of other fermions. The search for the 4. LHC period (2009 ) origins of the neutrino masses, neutrino mixing The LHC (Large Hadron Collider), proton-pffiffiffi parameters and the CP violation in the lepton sector proton collider, producing a collision energy ( s)of would open up a new field of physics beyond the 14 TeV, was constructed at CERN in the LEP tunnel SM. Details of the discovery of neutrino oscillations after termination of LEP operation. LHC started were described in Proc. Jpn. Acad., Ser. B by T. operating in September 2008 with the injection of a Kajita on atmospheric neutrinos (in 2010)92) and by single beam, but just after 9 days it experienced an M. Nakahata on solar neutrinos (in 2011).93) incident that damaged a large part of the collider 3.7. Summary of this period. From the view- machine. It took more than a year for recovery and point of verifying the SM, this period was very restarting its operation in November 2009 with productive and fruitful. Starting from determining successful collisions at an injection energy of the particle generations, high-energy collider experi- 450 GeV D 450 GeV. ments produced results that essentially confirmed the As the restoration couldpffiffiffi not be fully completed validity of the EW theory and QCD at the higher for the design energy of s ¼ 14 TeV within a short order quantum level with high-precision measure- ptimeffiffiffi of 91 year, it was decided to start running at ments of various observables. s ¼ 7 TeV.p Inffiffiffi March 2010 LHC succeeded beam There were also important discoveries, such as collisions at s ¼ 7 TeV, and started physics runs. the top quark, tau neutrino, and CP violation in the There are 4 experiments at LHC: ATLAS, CMS, quark sector. As a result, all of the ingredients of SM LHCb and ALICE. ATLAS94) and CMS95) are were discovered and confirmed, except for the Higgs general-purpose detectors aimed at studies of physics boson. Whether the Higgs boson exists or not, and at the highest possible energies. LHCb is a specialized whether its properties conform to the SM expecta- detector for physics related to b quark, whereas tions or not, remained to be the biggest and most ALICE is dedicated to heavy-ion (Pb) collision mode urgent questions regarding the next-generation ex- of LHC. periments. The ICEPP team was involved in the ATLAS In addition to verifying the SM, new observa- experiment from its formation of the collaboration tions toward physics beyond the SM appeared during in 1992. The ATLAS-Japan group was formed in this period. One is the discovery of neutrino 1994 by 12 Japanese institutes, including ICEPP, oscillations, and therefore non-zero neutrino masses. KEK, Kobe University and others, to participate Another observation was the possibility of low- in the ATLAS collaboration. The contributions of No. 5] Experimental verification of the standard model of particle physics 227 the ATLAS-Japan group covered a large area in the integrated luminosity of 1.5 pb!1. All of the relevant ATLAS experiment: a silicon inner tracker, a SM particles can be clearly seen, which shows the solenoidal magnet, a forward muon trigger detector, power of lepton identification in the high-energy a trigger/data acquisition system as well as software hadron collider environment, and that the experi- with Geant4, and Tier-2 data analysis center for the ment is ready for physics studies. worldwide LHC computing GRID system. Muon analyses, as well as electron analyses, were 4.1. Confirmation of SM.pffiffiffi From the start of elaborated using whole data in 2010, corresponding !1 97) LHC beam collisions at s ¼ 900pGeVffiffiffi in 2009, to an integrated luminosity of 40 pb , and applied followed by high-energy runs at s ¼ 7 TeV in to studied of the Drell-Yan process, i.e., dilepton 2010 and in 2011, LHC experiments successfully (l Dl !) production via the s-channel exchange of a accumulated sufficient data to confirm the perform- virtual photon or Z boson in pp collisions, based on ance of the detectors, and to tune the simulation the data of 2010 and 2011. The Drell-Yan process software. has a small experimental uncertainty and low back- Since the proton is not a point-like particle, it is grounds, allowing for a precision test of SM, as well as necessary to know its structure functions in order to providing important information on the partonic analyze pp collision processes. In high-energy pp structure of the proton. ATLAS allowed for analyses collisions they are expressed in the parton distribu- in the low invariant mass region (between 26 GeV tion functions (PDFs), which had been precisely and 66 GeV), including a part of data collected in determined by the previous fixed-target experiments 2011 with an integrated luminosity of 1.6 fb!1,98) and and collider experiments (Tevatron and HERA), as in the high invariant mass region (between 116 GeV well as by the LHC experiments themselves.96) and 1500 GeV) based on the full 2011 data set with One of the plots showing how quickly the an integrated luminosity of 4.9 fb!1.99) Both results ATLAS (and the CMS too) could calibrate the data were consistent with the SM expectations. and tune the necessary software for analyses is given By extending the lepton analyses, ATLAS (and in Fig. 16. It shows the reconstructed invariant mass also CMS) conducted measurements concerning distribution of opposite-sign muon pairs, in which inclusive W boson production,100) as well as diboson D ! 101)–103) thepffiffiffi events were selected in the first data sample at (W W , ZZ and WZ) productions. Further, s ¼ 7 TeV, taken in 2010, corresponding to an by including hadronic jet informations, top-quark pair production could be studied, in which the top quark decays as t ! bW.104) All of these processes were in good agreement with predictions of the SM. 4.2. Discovery of Higgs boson.pffiffiffi In 2012 LHC increased the collision energy to s ¼ 8 TeV, and started physics runs in April. By June, LHC had delivered to each experiment more than 5 fb!1, i.e., paffiffiffi similar amount as in the previous year’s run at s ¼ 7 TeV. Both ATLAS and CMS analyzed those data, and in July they announced the discovery of a new particle with its mass at around 125 GeV, which is consistent with the SM Higgs boson.105),106) The search for the SM Higgs boson through the decay channel H ! ZZ (*) ! 4l, where l F e, 7, provides good sensitivity over a wide mass range (110–600 GeV), due to the excellent momentum resolution of the ATLAS (and also CMS) detector. Figure 17 shows the distribution of the 4-lepton invariant mass measured by the ATLAS experiment, compared to the SM background expectation. The signal expectation for an SM Higgs boson with mH F Fig. 16. (Color online) Reconstructed invariant mass distribu- 125 GeV is also shown. tion of opposite-signp muonffiffiffi pairs. The events were selected in the first data sample at s ¼ 7 TeV taken in 2010 corresponding to The major backgrounds come from continuum the integrated luminosity of 1.5 pb!1. productions of ZZ (*) (including Z (*).* and .*.*), 228 T. KOBAYASHI [Vol. 97,

Fig. 17. (Color online) Distribution of the 4-lepton invariant mass measured by the ATLAS experiment, compared to the SM background expectation. The signal expectation for an SM Higgs boson with mH F 125 GeV is also shown.

Z D jets and tt. The excess of events observed near m4l F 125 GeV has a local maximum value of the significance, reaching 3.6<. A search for the SM Higgs boson was also performed through the decay channel H ! .. in the mass range between 110 GeV and 150 GeV. Figure 18 Fig. 18. (Color online) Distributions of the diphoton invariant shows the distributions of the diphoton invariant mass measured by the ATLAS experiment. The inclusive sample mass measured by the ATLAS experiment. is shown in (a) and a weighted version of the same sample in (c). The results of a fit to the data with a signal component fixed to The dominant background is SM diphoton mH F 126.5 GeV and a background component described by a production (..). The contributions also come from fourth-order polynomial is superimposed. The residual of the . D jet and jet D jet production with one or two jets data and weighted data with respect to the respective fitted mis-identified as photons, and from the Drell-Yan background component are displayed in (b) and (d). process. An inclusive sample is shown in Fig. 18(a). The results of a fit to the data with a signal component fixed to mH F 126.5 GeV and a back- category-dependent factors reflecting the signal-to- ground component described by a fourth-order background ratios; Figure 18(d) shows the residual polynomial is superimposed. The residual of the data of data with respect to the fitted background with respect to the respective fitted background component. component is displayed in Fig. 18(b). The excess of events observed near to m4l F In order to increase the sensitivity to a Higgs 126.5 GeV has a local maximum value of the boson signal, the events are separated into ten significance reaching 4.5<. ( ) exclusive categories of different pT and rapidity The decay channel H ! WW * ! e878,with regions, etc., each having different mass resolutions two opposite-charge leptons, has also been used for and signal-to-background ratios. A statistical analy- the SM Higgs boson search, though it has a wider sis of the data employs an unbinned likelihood mass distribution. The significance value at around function constructed from those of the ten categories mH F 125 GeV was found to be 2.8<. of the data sample. Figure 18(c) shows the mass The combined results (H ! ZZ (*) ! 4l, H ! .. spectrum obtained after weighting events with and H ! WW (*) ! e878) show clear evidence for a No. 5] Experimental verification of the standard model of particle physics 229 neutral boson with a measured mass of 126.0 ’ 0.4 tions 2.4 and 3.6, there should be a new physics (stat.) ’ 0.4 (sys.) GeV, which has a significance beyond the SM. of 5.9<, corresponding to a background fluctuation SUSY was the prime candidate among such new probability of 1.7 # 10!9. It is also compatible with physics. However, there are still no signs of SUSY the production and decay of the SM Higgs boson. particles, after LHC Run 2, in the mass region of CMS made a similar analysis, and also obtained a up to 1–2 TeV.112) Although the results depend on clear excess of events with a local significance of assuming various SUSY models, and the limits can be 5.0 < at a mass of 125.3 ’ 0.4 (stat.) ’ 0.5 (sys.) much weaker, models of the low-energy SUSY would GeV, LHC supplied 920 fb!1 to each experiment in need some modifications and/or new ideas. 2012. ATLAS and CMS made further studies of By avoiding difficulties of the technicolor models the spin and parity quantum numbers of the Higgs with the experimental results, many theories of boson using the whole dataset taken so far.107)–109) dynamical EW symmetry breaking were developed. The SM Higgs boson is assumed to have a Most of such theories predict the existence of new spin-parity (JP)0D. This hypothesis was compared resonances in the TeV region. No significant signs to an alternative hypotheses with JP F 0!,1D,1!,2D, have been observed so far by LHC Run 2.112) based on the kinematic properties of the decay If quarks and leptons are composite particles and channels H ! ZZ (*) ! 4l, H ! .. and H ! the composite scale is TeV, or slightly above, the WW (*) ! e878. These studies provided strong effect of the new interactions should be seen at LHC. evidence for the scalar (0D) nature of the Higgs One example is the scattering cross section for qq ! boson. qq, which might differ from the SM predictions. In 2013 Nobel Prizes in Physics were awarded to Another example is the appearance of excited quarks François Englert110) and Peter Higgs111) “for the and leptons, of which the signals are the narrow theoretical discovery of a mechanism that contrib- resonance peaks. After LHC Run 2, no significant utes to our understanding of the origin of mass of deviations from the SM have been observed.112) subatomic particles, and which was recently con- Very interesting new theoretical models firmed through the discovery of the predicted emerged, several years after LHC was decided to be fundamental particle, by the ATLAS and CMS built, to solve the gauge hierarchy problem, which experiments at CERN’s Large Hadron Collider”. does not use SUSY nor technicolor. The theoretical After Run 1, frompffiffiffi 2009 to 2012, LHC raised the framework uses the idea of extra spacial dimensions, collision energy to s ¼ 13 TeV in Run 2, from 2015 which is required to describe a consistent theory of to 2018, and delivered to each of the ATLAS and quantum gravity, i.e., superstring theory. CMS experiments an integrated luminosity of over The first model assumes that the extra dimen- 150 fb!1. sions can be as large as a millimeter scale, while in In Run 1, studies on the Higgs boson were made the original superstring theories the extra dimensions mainly through channels related to couplings of the are supposed be compactified at a scale close to Higgs boson to the vector gauge bosons (W ’, Z and the Planck scale.113) Another model uses a warped .). The excellent performance of LHC Run 2 and geometry that postulates the compactification scale experiments made it possible to study the Yakawa of the extra dimension to be on the order of couplings of the Higgs boson, i.e., the couplings to 1/TeV.114) the charged fermions of the 3-rd generation (t, b and Both models, as well as many varieties of extra- =).112) dimension models appeared later, predict that the So far, the experimental measurements of all the gravitational force would give sizable effects at the observed production and decay channels of the Higgs LHC energy region. In some particular cases LHC boson are consistent with the SM predictions. can even produce mini-black holes. So far, no However, to study the couplings of the Higgs boson signatures for mini-black hole productions have to lighter fermions or the Higgs self-couplings, for been seen at LHC, nor any significant deviations in example, more data are needed. the available distributions from predictions of the 4.3. Directions toward new physics beyond SM.112) the SM. The long exploration to verify the standard 4.4. Limit of SM. If there are no new physics model of particle physics came to an end with the beyond the SM at around the TeV region, or above, discovery of the Higgs boson and its various then the question arises as to where the SM remains confirmations. However, as described in subsec- valid. 230 T. KOBAYASHI [Vol. 97,

In the SM a complex scaler Higgs doublet, experiments have also made important contributions þ to constrain new physics beyond the SM. , is added to give masses to the weak vector 0 Luminosity and energy upgrades of the hadron bosons and to fermions through spontaneous sym- collider would certainly play important roles in metry breaking with the potential given by looking for direct evidence of the new physics beyond 2 the SM. V ðÞ¼2y þ ðyÞ : ½25 In parallel, studies of the detailed nature of the Here, 7, which is directly related to the Higgs boson Higgs boson and exploring the mechanism of EW mass, and 6, the quartic coupling, are free parameters symmetry breaking are the important next steps for in the theory. the future high-energy physics experiments, as they In calculating higher order radiative corrections, are the least studied part in the SM. Just as LEP the vector boson part and the fermion part are played very important roles between the discoveries protected from the divergence problem by the gauge from the hadron colliders, the future higher-energy symmetry and the chiral symmetry, respectively. eDe! collider would be an essential tool to open a However, the scalar (i.e., Higgs) part has no window towards the Planck scale. symmetry in the SM to protect from the divergence, although SUSY is a nice candidate for this problem. Acknowledgements Hence the quartic coupling could become non- The author thanks M. Koshiba and S. Orito for perturbative, or negative, which implies that the their support in ICEPP to participate in JADE, Higgs vacuum would become unstable. If either of OPAL and ATLAS experiments. The collaborations these cases occurs below the Planck scale, it means with P. Jenni and G. Mikenberg in the ATLAS that there should exist a new physics beyond the SM experiment, as well as with T. Kondo, K. Tokushuku below that energy scale. and S. Asai in the ATLAS-Japan team are gratefully Radiative corrections of the quartic coupling, 6, acknowledged. depend strongly on the values of the Higgs boson mass (mH) and the top quark mass (mt). The References experimentally measured values of mH and mt indicate that the EW vacuum of the Higgs potential 1) Glashow, S.L., Iliopoulos, J. and Maiani, L. (1970) Weak interactions with lepton-hadron symmetry. is most likely metastable, i.e., the high-energy – 6 Phys. Rev. D 2, 1285 1292. evolution of shows that it becomes negative at 2) Barber, D.P., Becker, U., Benda, H., Boehm, A., 10 12 112) energies of around ¼ O(10 –10 ) GeV. How- Branson, J.B., Bron, J. et al. (MARK J Collab- ever, if mt differs from the currently measured value oration) (1979) Study of electron-positron colli- by 3<, 6 could remain positive all the way to the sions at the highest PETRA energy. Phys. Lett. B – Planck scale, which might open very interesting 85, 463 466. fl 3) Berger, Ch., Genzel, H., Grigull, R., Lackas, W., possibilities, such as Higgs in ation in which the Raupach, F., Krovning, A. et al. (PLUTO SM Higgs boson plays the role of “inflation”,andso Collaboration) (1979) Search for a “top” threshold on. in hadronic eDe! annihilation at energies between – Thus the higher precision measurements of mH 22 and 31.6 GeV. Phys. Lett. B 86, 413 417. and m , as well as exploring the detailed nature of 4) Bartel, W., Canzler, T., Cords, D., Dittmann, P., t Eichler, R., Felst, R. et al. (JADE Collaboration) the Higgs boson and the mechanism of EW symmetry (1979) Total cross section for hadron production breaking, are very important, since these would open by eDe! annihilation at PETRA energies. Phys. a window towards the Planck scale. Lett. B 88, 171–176. 5) Bartel, W., Canzler, T., Cords, D., Dittmann, P., 5. Summary and outlook for future Eichler, R., Felst, R. et al. (JADE Collaboration) (1979) Search for new flavour production at For about 40 years after 1974, all ingredients of PETRA. Phys. Lett. B 89, 136–138. the standard model for particle physics have been 6) Brandelik, R., Braunschweig, W., Gather, K., discovered and verified with high precision. Collider Kadansky, V., Lübelsmeyer, K., Mättig, P. et al. D ! experiments for eDe!,pp, ep, pp colliders have played (TASSO Collaboration) (1980) e e annihilation the major role for the searches and measurements of at high energies and search for the t-quark continuum contribution. Z. Physik C 4,87–93. SM related phenomena. At the same time, verifying 7) Bartel, W., Cords, D., Dittmann, P., Eichler, R., the SM also means searching for new physics beyond Felst, R., Haidt, D. et al. (JADE Collaboration) the SM or looking for the limits of the SM. These (1980) Search for narrow resonances in eDe! No. 5] Experimental verification of the standard model of particle physics 231

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Proﬁle

Tomio Kobayashi was born in Chiba Prefecture in 1950 and graduated from Tokyo Institute of Technology in 1972. He majored in experimental particle physics in the Graduate School of Science, the University of Tokyo, under the supervision of Professor Masatoshi Koshiba. He received his Ph.D for work on the discovery of the gluon in the JADE experiment in 1980. He held a position at ICEPP, the University of Tokyo, as a research associate in 1977, became an associate professor in 1986 and a professor in 1993 in ICEPP. He then worked on international cooperation experiments concerning particle physics until his retirement in 2015. He participated in 3 major international experiments: JADE at DESY, OPAL and ATLAS at CERN. In OPAL he was responsible for constructing the lead-glass electro-magnetic shower calorimeter, and contributed to verifying the standard model of particle physics through precision analyses of electron-positron collision data at LEP. In ATLAS he served as a co-representative of the ATLAS-Japan group and contributed to discovering the Higgs particle. From 2015 to 2020 he worked at KEK, ﬁrst as head of the international cooperation promotion oﬃce and then as director in the research administration department. He received the 1995 European Physical Society Special Prize (as a member of the JADE experiment), the 2012 NISTEP Award, the 2013 Orito Shuji Prize, the 2013 EPS High Energy and Particle Physics Prize (as a member of the ATLAS experiment) and the 2013 Nishina Memorial Prize.