Electroweak Symmetry Breaking (Historical Perspective)

Home , Robert Mills (physicist), Vitaly Ginzburg, Yoichiro Nambu

40th SLAC Summer Institute · 2012 History is not just a thing of the past!

2 Symmetry

Indistinguishable before and after a transformation

Unobservable quantity would vanish if symmetry held

Disorder order = reduced symmetry

3 Symmetry

Bilateral

Translational, rotational, …

Ornamental

Crystals

4 Symmetry

http://www.pirx.com/iMol/ Fullerene C60 ball and stick created from stick a PDB created and ball C60 Fullerene [ Piotr Rotkiewicz's using Wikipedia, English Source: iMol]. {{gfdl}}

CsI

5 Symmetry (continuous)

6 Symmetry matters.

7 8 Symmetries & conservation laws

Spatial translation Momentum

Time translation Energy

Rotational invariance Angular momentum

QM phase Charge

9 Symmetric laws need not imply symmetric outcomes.

10 symmetries of laws ⇏ symmetries of outcomes by Wilson Bentley, via NOAA Photo Library Studies among the Snow Crystals ... the SnowStudies among Crystals

11 Broken symmetry is interesting.

12 Two-dimensional Ising model of ferromagnet

http://boudin.fnal.gov/applet/IsingPage.html 13 Continuum of degenerate vacua 14 Nambu–Goldstone bosons

V Betsy Devine

Yoichiro Nambu 2

Massless NG boson 1 Massive scalar boson

NGBs as spin waves, phonons, pions, … Jeffrey Goldstone 15 Symmetries imply forces. I: scale symmetry to unify EM, gravity

Hermann Weyl (1918, 1929) 16 NEW

Complex phase in QM ORIGINAL

Global: free particle Local: interactions

17 Maxwell’s equations; QED

massless spin-1 photon coupled to conserved charge no impediment to electron mass

(eL & eR have same charge)

James Clerk Maxwell (1861/2) 18 19 QED Fermion masses allowed Gauge-boson masses forbidden Photon mass term 1 2 µ 2 m A Aµ violates gauge invariance:

AµA (Aµ µ) (A ) = AµA µ ⇥ µ µ ⇤ µ Massless photon predicted 22 observed: m 10 me

20 Symmetries imply forces. II: non-Abelian gauge symmetry

Robert Mills C. N. Yang Ron Shaw (1954)

21 Can one choose independently at each point in spacetime the convention to name proton and neutron?

Local isospin symmetry implies 3 massless gauge bosons ☹ coupled to isospin no impediment to nucleon mass

(NL & NR have same isospin)

22 Might hiding the symmetry help?

Seems to add massless NGBs ☹ to massless gauge bosons ☹

Goldstone theorem proved with ever-increasing rigor

23 Gerrit-Jan Flim (left) and Heike Kamerlingh Onnes at the helium liquefactor, ca. 1920 (Leiden Institute of Physics). Dirk van Delft | MUSEUM BOERHAAVE, LEIDEN UNIVERSITY

HEIKE KAMERLINGH ONNES AND THE ROAD TO LIQUID HELIUM Superconductivity (1911) In 1908, Heike Kamerlingh Ontricnes effectfirst liquefied in the helium voltage in a cryogenic leads by making more with broken mercury threads! Fig.7 O riginal drawing by Gerrit Jan Flim showing the setup for the persistent-current experiments lead immersed in liquid helium and laboratory whose excellence and scale were unparalleled. Creating, staffing of May 1914. Left: front view showing the lead coil in the helium cryostat and the copper and running the Leiden laboaratory required more than just scientific skill. everything of the same metal. It didn’t The generation of strong magnetic fields compensation coil in the liquid air Dewar (actually, during the experiment, both coils were on carrying a persistent current of 200 A. work, because the transition from solid now became within reach. But the ex- the same height as the compass needle). Right: top view showing also the compass needle in Today, the most compelling demon- On 10 July 1908, Heike Kamerlingh Onnes Heike Kamerlingh Onnes was born in fruitful exchange between theory and the middle pointing north demonstrating good compensation of the fields from the Pb and the (1853-1926) liquified helium for the the cityto of Groningenliquid in 1853mercury [1]. His experiment. turned out to be the periment, even announced in Kamer- stration of persistent currents is the first time, briefly rendering his Dutch father owned a tile factory in a small copper coils (Archive of the Museum Boerhaave, Leiden). laboratory ‘the coldest spot on earth’. village,source a two hour drive of on a horseback. considerable MISSION thermoelectric lingh Onnes’s Nobel lecture and carried levitation of a permanent magnet by a This paper tells the story of Leiden Uni- Heike studied at the University of Gro- When Kamerlingh Onnes started as versity’s famed cryogenics laboratory ningen.voltage At the age of 17 ofhe started 0.5 stud- mV.a professorStill, in the experimental October physics ex- out on 17 January 1914, brought a great superconductor. and the man behind it, whose scientific ying chemistry, his favourite topic at in Leiden in 1882, he immediately de- accomplishments earned him the No- high school.periment After passing his produced propae- cided to transform the the historic building into a plot, deception. The magnetic field generated The excitement about persistent cur- bel Prize in Physics in 1913. The central deutic exam he moved to Heidelberg, research laboratory. Why this consid- question is how Kamerlingh Onnes then famous for its international aca- erable effort? Because of his scientific was able to succeed so brilliantly in demicshown environment, for in a Wanderjahr Fig.6 mission:, of to the test the abruptmolecular laws of reap- with a lead coil turned out to destroy rents on a macro scale spread quickly. developing his cryogenics laboratory (year of travel). Why Heidelberg? Be- Johannes Diderik van der Waals and in – undoubtedly an exceptional feat in cause pearanceof Robert Bunsen, in ofthose thedays doing mercury so to give international resistance prestige at the superconductivity at 4.2 K already Paul Ehrenfest, who witnessed the terms of its scale and its almost in- the most famous chemist in Europe. to Dutch physics. Van der Waals had dustrial approach at the turn of the In his4.20 first semester, K. HeikeThe enjoyed part the publishedof the his thesis plot on the abovecontinuity the at 600 Gauss (60 mT). Gone were the experiment, wrote in a letter to H.A. century. Key factors in his success were chemistry lab very much. But when the of the liquid and gas phase in 1873, a Kamerlingh Onnes’s organisational time cametransition to start some own research, temperature milestone in molecular (TC physics) is – notice of par- dreams of producing magnetic fields as Lorentz: “Unsettling, to see the ring talent, his personality and his inter- Bunsen’s conservatism and aversion that molecules were not yet generally national orientation. The liquefaction to mathematics got Heike to switch to accepted in those days [2]. As a student of helium opened up unexplored ter- the physicsticular department, interest led by Gustav in because Groningen, Kamerlingh the Onnes gradual had high as 10 T (100.000 Gauss): “An un- of electrons goes round and round and ritories of extreme cold and cleared the Kirchhoff. Important for Heike was been attracted to Van der Waals’ results path for the eventual discovery of su- that Kirchhoffincrease was a modern shows physicist, thatand to thethe kinetic electrons theory of Clausius, at 4.2 foreseen difficulty is now found in our round […] virtually without friction.” perconductivity on 8 April 1911. in the sense that he propagated the Maxwell and Boltzmann. way, but this is well counterbalanced His colleague Kuenen proposed an ex- by the discovery of the curious property periment, depicted in Fig.8, in which 04 100 years of supercondictivity 100 years of supercondictivity 05 Heikewhich Kamerlingh is the cause ofOnnes it”. the loop of lead could be interrupted, Cryostat with mercury resistor and mercury Historic plot of resistance (Ω) versus Fig.5 Fig.6 24 leads for the 26 October 1911 experiment (same temperature (K) for mercury from the 26 or even repeatedly opened and closed color scheme as in Fig. 2): seven October 1911 experiment showing the PERSISTENT CURRENTS from outside, thus forming the first U-shaped glass capillaries in series (inner superconducting transition at 4.20 K. The last notes about superconductivity (mechanical) persistent mode switch. diameter 0.07 mm), each with a mercury Within 0.01 K the resistance jumps from reservoir at the top and contact leads also made immeasurably small (less than 10 -5 Ω) in the archives are dealing with the per- Upon Ehrenfest’s suggestion Kamer- of glass capillaries filled with mercury. External to 0.1 Ω. sistent mode experiments carried out lingh Onnes repeated the experiment contacts were made through Pt wires (denoted during the spring and summer of 1914. with a single lead ring, which also by Hgxx) shown in the top right drawing. Kamerlingh Onnes concentrated on worked. Still, Kamerlingh Onnes never the question how small the resistance believed that the “micro-residual re-

K were still predominantly scattered by below TC actually was and designed an sistance” was really zero, revealing that phonons (Planck vibrators). From the elegant experiment using the lead coil he was not aware of the new (quantum) sudden jump it was clear that a totally in a closed-loop configuration. When state of matter he had discovered. That

new and unexpected phenomenon had this device is cooled through TC in an Fig.8 Design for the June 1914 experiment with (left) the cryostat with insert, (center) the mechani- insight came in 1933 with the experi- been discovered, which Kamerlingh applied magnetic field, any change cal persistent mode switch (superconducting “key”) with p and q lead rings, and a, b, and ments of Meissner and Ochsenfeld in c current leads and voltage leads, and (right) the cutting machine (Archive of the Museum Onnes from thereon called ‘supracon- in field will generate a current in the Boerhaave, Leiden). Berlin and the explanation by the ductivity’. closed loop which, if the resistance is young Gorter in Haarlem telling us really zero, will circulate for ever. The that a superconductor actually is a per- MATERIALS AND MAGNETS magnetic field produced by that circu- fect diamagnetic rather then a perfect An interesting entry form 20 June 1912, lating current was probed by a compass conductor, and finally in 1957 with the “Discussed with Holst …. to alloy mer- needle and the current value followed microscopic explanation by Bardeen, cury with gold and Cd”, indicates the from the compensating effect of the Cooper and Schrieffer. first step to exploring other materials magnetic field from an almost identi- then pure mercury. Surprisingly, the cal copper coil positioned on the other resistance disappeared as before and side of the needle. In Fig.7 two sketches Kamerlingh Onnes concluded that of the set up made by Gerrit Jan Flim they could have saved a lot of time are reproduced. The experiment worked previously spent on the preparation of well and Kamerlingh Onnes would have pure mercury. ”Even with the amal- loved to demonstrate it to his colleagues gam used for the backing of mirrors, at the monthly meeting of the KNAW, the resistance was found to be zero.” In but he didn’t have the means to do December 1912 also lead and tin were that. Although not possible in 1914, in found to become superconducting at 1932 Flim flew to London for the tra- about 7 K and 3.8 K, respectively. Since ditional Friday evening lecture of the then, the experiments were continued Royal Institution bringing with him with these materials: no disasters any- a portable dewar containing a ring of

14 100 years of supercondictivity 100 years of supercondictivity 15 Meissner effect (1933)

Thanks to Felicia Svoboda 25 Magnetic ﬁelds excluded

Walther Meißner Robert Ochsenfeld Pb: 40 nm penetration

26 Ginzburg–Landau model (1950)

T > Tc TT < < TTcc

ψ ψ00 Free Energy Free Energy Free Energy Free Energy

Order Parameter ψ OrderOrder ParameterParameter ψψ

Photon acquires mass in superconductor

27 Vitaly Ginzburg Lev Landau

28 BCS theory (1957)

John Bardeen Leon Cooper Robert Schrieffer

29 Some hints

(1962) Photon can acquire mass in 1+1-dimensional QED

Julian Schwinger

(1963) Superconductor: massive photon, hidden gauge symmetry. Model for strong interactions?

Phil Anderson 30 Spontaneous symmetry breaking

Higgs Kibble Guralnik Hagen Englert Brout† 1964– : Goldstone theorem doesn’t apply to gauge theories! Each would-be massless NGB joins with a would-be massless gauge boson to form a massive gauge boson, leaving an incomplete multiplet of massive scalar bosons. 31 Simplest example: Abelian Higgs model = Ginzburg–Landau in relativistic notation Yields massive photon + a massive scalar particle “Higgs boson”

No mention of weak interactions. No question of fermion masses (not an issue for Yang–Mills theory).

32 Spontaneously broken gauge theory

Longitudinal component 1 Higgs boson

1981: massive collective mode (Raman scattering in NbSe2)

33 Physics World 2 July 1989 29

Why do things have mass? Why is the universe bigger than a football? Why do people want to spend millions on tunnels under Geneva and Texas? The unbearable

IN A few weeks Higgs field are cal- time, the Large led Higgs bosons. Electron-Positron heaviness of being These are what the colliding ring acce- particle people are lerator (LEP) at IAN AITCHISON after. CERN in Geneva It should be will start operating. One of its prime tasks will be to join the clear, then, that the discovery of Higgs bosons, and the hunt for the particle physicists' currentlManyy most wanted ﬁngersdetermination of theiinr interactionthes witpieh other …known character - the Higgs boson. This search will intensify as elementary particles, is expected to have direct bearing on other, even more powerful, accelerators come into commis- three deep questions: the role of symmetry, and of sion in the last decades of this century. What has this symmetry breaking, in the fundamental forces; the origin of particle done to deserve such ruthless (and costly) tracking mass; and the nature of the vacuum. I shall briefly discuss down? In short: it has broken the symmetry. each of these three points and then, on the experimental This belief that the laws of nature should embody a high side, take a quick look at the prospects for discovering the degree of symmetry is ancient, profound, and empirically Higgs boson at existing and future accelerators. Finally I well justified, but the discernment of such symmetry has shall mention a most embarrassingly wrong prediction that often been a subtle and imaginative art. After many decades the Higgs concept leads to, when viewed within the of intensive experimental and theoretical work particle framework of Einstein's theory of gravitation. physicists now believe that they have identified an intricate and beautiful symmetry shared by the electromagnetic force Electromagnetic and weak forces: (which binds electrons to nuclei to form atoms) and the missing symmetry weak force (which is responsible for certain kinds of It would take me too far afield to explain the nature of the radioactivity, including the kind which enters cruciallyPeter in Higgs,mathematica 50 Yearsl symmetr of Weaky whic Interactions,h appears to be Wingspread shared by the (1984) the combustion cycle of the stars). The curious thing, electromagnetic and weak forces. Suffice it to say that it is a however, is that the symmetry seems, at first sight, to be possible only if all the particles are entirely massless. This restriction applies equally to the particles which are currently regarded as the constituents of matter (quarks and The Schwinger-Anderson- leptons), and to those (such as photons) which are the Englert-Brout-Higgs- quanta of the force fields acting between the matter mechanism constituents. Though some of these particles (the photon, Guralnik-Hagen-Kibble boson and perhaps the neutrinos) are indeed massless, most are quite certainly not: for instance, the electron's massIan ,Aitchison, though “TheUnti l unbearable1962 it was widel heavinessy believed thaof t being,”the gauge Physics symmetry Worldof (July 1989) of course very tiny on the scale of everday things, is electromagnetism required photons to be massless. In that year, experimentally extremely well determined and finite. The Julian Schwinger wrote a paper exposing a loophole in the mere existence of particles with mass, then, apparently accepted argument. The next year, Phil Anderson showed how means that the 'electroweak symmetry', as it is called, is a superconductor provides a working model of a situation in broken. which the laws of electromagnetism still hold, but photons 34 behave as if they have a mass. He suggested extending this idea Is there anything more to this than an aesthetic dis- to theories of the strong (nuclear) force then in vogue. The first appointment? The answer is a quite definite yes: without explicit demonstration, within the framework of relativistic the symmetry, the theory fails to make sense mathematic- quantum field theory, of the way in which a gauge field ally. Thus a way had to be found to preserve the symmetry - quantum could acquire mass came in 1964, with the paper by so as to ensure mathematical consistency - while at the same F Englert and Robert Brout, received by Physical Review time breaking it - so as to give the particles mass! In the Letters on 26 June. Englert and Brout performed calculations in present context, a way in which this trick can be pulled was a kind of lowest order perturbation theory, and conjectured first pointed out by F Englert and Robert H Brout of the that their result would be generally true. On 31 August the University of Brussels, and, independently, by Peter Higgs same journal received Peter Higgs' article, in which the now of the University of Edinburgh (see box). This particular classic 'Higgs model' was introduced, and solved as a classical kind of symmetry breaking phenomenon is now known as field theory problem. Further aspects of the theory were treated the 'Higgs mechanism'. Its central ingredient is the in a Physical Review Letter (received 12 October 1964) by Dick Hagen, G Guralnik, and Tom Kibble. In 1966 there followed a postulated existence of a field - called the 'Higgs field' - comprehensive and authoritative article by Higgs in Physical which does not vanish in the supposedly empty vacuum, Review, and in 1967 the series closed with a detailed paper (also and whose non-zero average value in the vacuum is related in Physical Review) by Kibble, in which the 'non-Abelian' case to the particles' masses. As with any field, however, the was fully treated. None of these papers, however, suggested Higgs field can oscillate about its average value, and application of the ideas to the weak interactions. That crucial according to quantum theory these oscillations would step was taken by Steven Weinberg and Abdus Salam in 1967. generally be manifested as quanta: for example, those of the So, whose boson is it? electromagnetic field are photons. The quanta of the What of Yang–Mills (isospin) theory?

After SSB, still not the theory of nuclear forces Right idea, wrong symmetry, wrong constituents

Precursor of Quantum Chromodynamics based on SU(3) color gauge symmetry for interactions among quarks

35 In contrast to biological evolution, unsuccessful lines in theoretical physics do not become extinguished, never to rise again. We are free to borrow potent ideas from the past and to apply them in new settings, to powerful effect.

36 Asymptotic freedom in QCD

1 27 2m ln c s(2mc) 6⇥ ⇥

37 REPORTS mud,correspondingtoMp ≅ 135 MeV,are difficult. vector meson (r, K*) octets that do not require (25, 26)andforoursetup(27)], simulating di- They need computationally intensive calculations, the calculation of disconnected propagators. rectly at physical mud in large enough volumes, with Mp reaching down to 200 MeV or less. Typical effective masses are shown in Fig. 1. which would be an obvious choice, is still ex- 5) Controlled extrapolations to the contin- 3) Shifts in hadron masses due to the finite tremely challenging numerically. Thus, the stan- uum limit, requiring that the calculations be size of the lattice are systematic effects. There dard strategy consists of performing calculations performed at no less than three values of the are two different effects, and we took both of at a number of larger mud and extrapolating the lattice spacing, in order to guarantee that the them into account. The first type of volume de- results to the physical point. To that end, we use scaling region is reached. pendence is related to virtual pion exchange be- chiral perturbation theory and/or a Taylor expan- Our analysis includes all five ingredients tween the different copies of our periodic system, sion around any of our mass points (19). listed above, thus providing a calculation of the and it decreases exponentially with Mp L.Using 5) Our three-flavor scaling study (27)showed light hadron spectrum with fully controlled sys- MpL > 4 results in masses which coincide, for that hadron masses deviate from their continuum tematics as follows. all practical purposes, with the infinite volume values by less than approximately 1% for lattice 1) Owing to the key statement from renor- results [see results, for example, for pions (22) spacings up to a ≈ 0.125 fm. Because the sta- malization group theory that higher-dimension, and fore baryons (23, 24)]. Nevertheless, for one tistical errors of the hadron masses calculated in local operators in the action are irrelevant in the of our simulation points, we used several vol- the present paper are similar in size, we do not continuum limit, there is, in principle, an un- umes and determined the volume dependence, expect significant scaling violations here. This is limited freedom in choosing a lattice action. which was included as a (negligible) correction at confirmed by Fig. 2. Nevertheless, we quantified There is no consensus regarding which action all points (19). The second type of volume de- and removed possible discretization errors by a would offer the most cost-effective approach to pendence exists only for resonances. The cou- combined analysis using results obtained at three the continuum limit and to physical mud.Weuse pling between the resonance state and its decay lattice spacings (19). an action that improves both the gauge and products leads to a nontrivial-level structure in We performed two separate analyses, setting fermionic sectors and heavily suppresses non- finite volume. Based on (20, 21), we calculated the scale with MX and MW.Theresultsofthese physical, ultraviolet modes (19). We perform a the corrections necessary to reconstruct the reso- two sets are summarized in Table 1. The X set is series of 2 + 1 flavor calculations; that is, we nance masses from the finite volume ground- shown in Fig. 3. With both scale-setting proce- include degenerate u and d sea quarks and an state energy and included them in the analysis dures, we find that the masses agree with the additional s sea quark. We fix ms to its approxi- (19). hadron spectrum observed in nature (28). mate physical value. To interpolate to the phys- 4) Though important algorithmic develop- Thus, our study strongly suggests that QCD ical value, four of our simulations were repeated ments have taken place recently [for example is the theory of the strong interaction, at low on November 29, 2008 with a slightly different ms.Wevarymud in a range that extends down to Mp ≈ 190 MeV. Table 1. Spectrum results in giga–electron volts. The statistical (SEM) and systematic uncertainties 2) QCD does not predict hadron masses in on the last digits are given in the first and second set of parentheses, respectively. Experimental physical units: Only dimensionless combinations masses are isospin-averaged (19). For each of the isospin multiplets considered, this average is (such as mass ratios) can be calculated. To set the within at most 3.5 MeV of the masses of all of its members. As expected, the octet masses are more overall physical scale, any dimensionful observ- accurate than the decuplet masses, and the larger the strange content, the more precise is the able can be used. However, practical issues in- result. As a consequence, the D mass determination is the least precise. fluence this choice. First of all, it should be a quantity that can be calculated precisely and X Experimental (28) MX (X set) MX (W set) www.sciencemag.org whose experimental value is well known. Sec- r 0.775 0.775 (29) (13) 0.778 (30) (33) ond, it should have a weak dependence on mud, K* 0.894 0.906 (14) (4) 0.907 (15) (8) so that its chiral behavior does not interfere with N 0.939 0.936 (25) (22) 0.953 (29) (19) that of other observables. Because we are con- L 1.116 1.114 (15) (5) 1.103 (23) (10) sidering spectral quantities here, these two con- S 1.191 1.169 (18) (15) 1.157 (25) (15) ditions should guide our choice of the particle X 1.318 1.318 1.317 (16) (13) whose mass will set the scale. Furthermore, the D 1.232 1.248 (97) (61) 1.234 (82) (81) particle should not decay under the strong in- S* 1.385 1.427 (46) (35) 1.404 (38) (27) Downloaded from teraction. On the one hand, the larger the strange X* QCD1.533 explains1.565 most (26) (15) of visible1.561 mass (15) (15) content of the particle, the more precise the mass W 1.672 1.676 (20) (15) 1.672 determination and the weaker the dependence on Light hadrons (dynamical fermions) mud.ThesefactssupporttheuseoftheW baryon, the particle with the highest strange content. On Fig. 3. The light hadron the other hand, the determination of baryon dec- spectrum of QCD. Hori- 2 uplet masses is usually less precise than those of zontal lines and bands are m = E0/c the octet. This observation would suggest that the experimental values the X baryon is appropriate. Because both the with their decay widths. W and X baryon are reasonable choices, we Our results are shown by carry out two analyses, one with MW (the W set) solid circles. Vertical error and one with MX (the X set). We find that for all bars represent our com- three gauge couplings, 6/g2 =3.3,3.57,and3.7, bined statistical (SEM) and both quantities give consistent results, namely systematic error estimates. a ≈ 0.125, 0.085, and 0.065 fm, respectively. To p, K,andX have no error fix the bare quark masses, we use the mass ratio bars, because they are pairs M /M ,M /M or M /M ,M /M .We used to set the light quark p W K W p X K X mass, the strange quark determine the masses of the baryon octet (N, S, mass and the overall L, X)anddecuplet(D, S*, X*, W)andthose scale, respectively.

members of the light pseudoscalar (p, K)and BMW 38

1226 21 NOVEMBER 2008 VOL 322 SCIENCE www.sciencemag.org LETTE RS TO THE ED I TOR

one unit and no change of parity, it can be given only The sign of the asymmetry coeAicient, o., is negative, by the Gamow-Teller interaction. This is almost im- that is, the emission of beta particles is more favored in perative for this experiment. The thickness of the the direction opposit. e to that of the nuclear spin. This radioactive layer used was about 0.002 inch and con- naturally implies that the sign for Cr and Cr' (parity tained a few microcuries of activity. Upon demagnetiza- conserved and pa.rity not conserved) must be opposite. tion, the magnet is opened and a vertical solenoid is The exact evaluation of o. is difficult because of the raised around the lower part of the cryostat. The many eA'ects involved. The lower limit of n can be whole process takes about 20 sec. The beta and gamma estimated roughly, however, from the observed value counting is then started. The beta pulses are analyzed of asymmetry corrected for backscattering. AL velocity on a 10-channel pulse-height analyzer with a counting v(c=0.6, the value of n is about 0.4. The value of interval of 1 minute, and a recording interval of about (I,)/I can be calculated from the observed anisotropy 40 seconds. The two gamma counters are biased to of the gamma radiation to be about 0.6. These two accept only the pulses from the photopeaks in order to quantities give the lower limit of the asymmetry discriminate against pulses from Compton scattering. parameter P(n P(=I,)/I) approximately equal to 0.7. A large beta asymmetry was observed. In Fig. 2 we In order to evaluate o, accurately, many supplementary have plotted the gamma anisotropy and beta asym- experiments must be carried out to determine the metry vs time for polarizing field pointing up and various correction factors. It is estimated here only to pointing down. The time for disappearance of the beta show the large asymmetry effect. According to I-ee and asymmetry coincides well with that of gamma ani- Yang' the present experiment indicates not only that sotropy. The warm-up time is generally about 6 minutes, conservation of parity is violated but also that invari- and the warm counting rates are independent of the ance under charge conjugation is violated. 4 Further- field direction. The observed beta asymmetry does not more, the invariance under time reversal can also be change sign with reversal of the direction of the de- decided from the momentum dependence of the asym- magnetization field, indicating that it is not caused by metry parameter P. This effect will be studied later. remanent magnetization in the sample. The double nitrate cooling salt has a highly aniso- tropic g value. If the symmetry axis of a crysial is not l.3 set parallel to the polarizing field, a small magnetic GAMMA-AN I SOTROPY field vill be produced perpendicular to the latter. To 0) EQUATORIAL COUNTER check whether the beta asymmetry could be caused by b) POLAR COUNTER I.I such a magnetic field distortion, we allowed a drop of ~ w A x g CoC12 solution to dry on a thin plastic disk and cemented ld x , X 4 ' „ I.O the disk to the bottom of the same housing. In this way ~ x x K the cobalt nuclei should not be cooled su%ciently to + Z 0'9 Z I— produce an appreciable nuclear polarization, whereas P Z the will behave as beta asym- o O.8 housing before. The large V mef. ry was not observed. Furthermore, to investigate 0.7— possible internal magnetic effects on the paths of the I I I I I electrons as they find their way to the surface of the OPY CALCULATED FROM (a) 8(b) crystal, we prepared another source by rubbing CoC1& 0.3 — ~i~ ) W(0) solution on the surface of the cooling salt until a W(~up) reasonable amount of the crystal was dissolved. AVe then 0TH POLARIZING F I ELD DOWN allowed the solution to dry. No beta asymmetry was β-decay: parityI not conserved! O. observed with this specimen. 3lore rigorous experimental checks are being initi- but in view of the implications of these 60 I I ated, important Polarized Co I I I I.20 METRY (AT PULSE observations, we report them now in the hope that they X HEIGHT IOV) EXCHANGE Diay stimulate and encourage further experimental u n, GAS IN investigations on the parity question in either beta or cf I.OO hyperon and meson decays. Z Q The inspiring discussions held with Professor T. D. &z O3O ~0 Lee and Professor C. N. Yang by one of us (C. S. Ku) Oo 0.80 are gratefully acknowledged. * YVork partially supported by the U. S. Atomic Energy I I I I I I I I Commission. 0 2 4 6 8 IO l2 I4 16 I8 ' T I ME I N M I NU T ES T. D. Lee and C. N. Yang, Phys. Rev. 104, 254 (1956). ~ Ambler, Grace, Halban, Kurti, Durand, and Johnson, Phil. FIG. 2. Gamma anisotropy and beta asymmetry for Mag. 44, 216 (1953). Unpolarizingobservablefield pointing upobservedand pointing down. ' Lee, Oehme, and Yang, Phys. Rev. (to be published' ).

39 Parity violated in weak interactions

Chien-Shiung Wu (1956) Eric Ambler 40 Chiral quarks and leptons

R 3 R 2 eR 1 bR tR sR cR 3 dR 2 uR 1 L L eL

tL cL uL bL sL dL

41 An electroweak theory

Weak isospin (left-handed) + weak hypercharge phase symmetry

3 massless gauge bosons coupled to weak isospin ☹ 1 massless hyperphoton coupled to weak hypercharge ☹ massless quarks & leptons ☹ Sheldon Glashow 42 An electroweak theory (1967)

Contrive a vacuum to hide EW symmetry (need 4 new ﬁelds)

Massive W+, W–, Z0 Massless photon Massive Higgs boson

Steven Weinberg Abdus Salam 43 Hide EW symmetry to give masses to quarks, leptons, and gauge bosons

“Higgs mechanism” breaks SU(2)L ⊗ U(1)Y → U(1)em g g’

MW = gv/2

MZ = MW/cosθW

tanθW = g’/g v = 246 GeV

44 Gauge symmetry (group-theory structure) tested in + + e e W W – + W W W– W+

e– e+ e– e+

(a) (b)

W– W+

e– e+ e– e+

45 QuiggFig02.pdf 6/16/09 1:29:27 PM

Electroweak symmetry is real

No ZWW vertex

Only υe exchange

20 (pb)

WW σ

LEP data Standard model

02/17/2005 0 160 180 200 √s (GeV) 46 Higgs bosons: incomplete multiplets

(w1, w2, z, h) form O(4) multiplet

+ – 0 w1, w2, z become longitudinal W , W , Z h becomes H, remembers its roots

High-energy behavior, unitarity bound, …

See end of §III, Phys. Rev. D16, 1519 (1977)

47 Fermion mass after SSB

By decree, Weinberg & Salam add interactions between fermions and scalars that give rise to quark and lepton masses.

e (eL)eR + eR(†eL) me = ev/2 ⇥ ςe : picked to give right mass, not predicted fermion mass implies physics beyond standard model

Highly economical, but is it true?

48 H couplings to W, Z tested

49 World without SSB

Electron and quarks have no mass QCD conﬁnes quarks into protons, etc. Nucleon mass little changed Surprise: QCD hides EW symmetry, gives tiny masses to W, Z Massless electron: atoms lose integrity No atoms means no chemistry, no stable composite structures like liquids, solids, … arXiv:0901.3958 50 Four tasks for the SM Higgs boson

Hide electroweak symmetry (distinguish EM, weak interactions) Give masses to W±, Z0 Give masses and mixings to fermions Keep EW theory from misbehaving

51 High-energy behavior of EW theory

W+W –, ZZ, HH, HZ s-waves approach constants, thanks to gauge cancellations

… satisfy s-wave unitarity _ 1/2 provided MH ≤ (8π√2/3GF) ≈ 1 TeV • If bound is respected, perturbation theory is “everywhere” reliable • If not, weak interactions among W±, Z, H become strong on 1-TeV scale New phenomena are to be found around 1 TeV 52 Does MH < 1 TeV make sense? The peril of quantum corrections

[A PUZZLE RAISED BY THE HIGGS]

s le sse sca ma ak rino we eut n tro N roto ec LHC 9 P tom El t of – p wn ron ot imi 10 U Do eut B L –6 n N s ctro gg 10 Ele au Hi n nge T Z H –3 Muo Stra 10 rm W Cha 0 op 10 T 3 Un Scientiﬁc American exp 10 laine d ga En p k erg 6 ea y S w cal 0 o ? e (G 1 r e eV ct al ) ele sc 9 g- ion 10 on at ? tr !c ity S ni e av 12 u al gr 0 sc m 1 ck tu n n Pla ua 15 Q 0 ? 1 gs rin 18 St 10 53 Gauge symmetry (group-theory structure) tested in + + e e W W – + W W W– W+

e– e+ e– e+

(a) (b)

W– W+

e– e+ e– e+

54 Vacuum energy problem

2 2 Higgs potential V (†)=µ (†)+ (†) | | µ2v2 v4 At the minimum, V ( † )= = | | < 0. 4 4 Identify M 2 = 2µ2 H V =0contributes position-independent vacuum energy density 2 2 MH v 8 4 24 3 10 GeV 10 gcm H 8 46 4 Observed vacuum energy density 10 GeV vac Mismatch by 54 orders of magnitude

55 EWSB and other questions

Origin of fermion masses and mixings Meaning of CP violation Lessons for cosmic inﬂation? Connection to dark matter? Insights for dark energy problem? Link with extra spacetime dimensions? Connection to gravity through supersymmetry?

56 From “Unanswered Questions in EW Theory” 1. What is the agent that hides the electroweak symmetry? Specifically, is there a Higgs boson? Might there be several? 2. Is the Higgs boson elementary or composite? How does the Higgs boson interact with itself? What triggers electroweak symmetry breaking? 3. Does the Higgs boson give mass to fermions, or only to the weak bosons? What sets the masses and mixings of the quarks and leptons? 4. What stabilizes the Higgs boson mass below 1 TeV? 5. Do the different behaviors of left-handed and right-handed fermions with respect to charged-current weak interactions reflect a fundamental asymmetry in the laws of nature? 6. What will be the next symmetry recognized in nature? Is nature supersymmetric? Is the electroweak theory part of some larger edifice? 7. Are there additional generations of quarks and leptons? 8. What resolves the vacuum energy problem? 9. Is electroweak symmetry breaking an emergent phenomenon connected with strong dynamics? Is electroweak symmetry breaking related to gravity through extra spacetime dimensions? 10. What lessons does electroweak symmetry breaking hold for unified theories of the strong, weak, and electromagnetic interactions?

57 The new world is here. Time to explore!

58 ? 59 The 2012 SSI Challenge: Name that boson! If the particle discovered by ATLAS & CMS is the avatar of electroweak symmetry breaking, what would you call it?

60 Submit your proposal plus ≤3 sentences explaining etymology, making the case, etc.

All entries are welcome, but only students may compete for prizes including a bottle of California’s ﬁnest sparkling wine signed by a gaggle of SLAC luminaries

61 Inspirational Reading The case for change: http://j.mp/OibSpI The case for brand loyalty: http://j.mp/MUAse7

62 Supplementary slides

63 Uniﬁed theory: Smaller top mass means smaller αs U

α ❊ 1/ 2mt’

21/6π

2mb 23/6

” π s 2m

α t 2mc “1/ 25/6π

2/27 27/6π Mproton ∝ mt

M “log(E)” U hep-ph/9704332 64 Fermion Masses

100 t charged leptons -1 up quarks 10 down quarks b -2 10 c τ eak Scale 10-3 μ s 10-4 Mass / W d 10-5 u e 10-6

Running mass m(m) … m(U) 65 Might we live in a metastable vacuum?

180

Unstable Metastable 175 eV ) eV mt = 173.2 ± 0.9 GeV G ( t m > 114.4 G H 170 ATLAS+CMS M Stable

165 110 120 130 140 150

MH (GeV)