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Particle today

Giulia Zanderighi (CERN & University of Oxford) Physics Today’s high Today’s high energy colliders ParticleCol lPhysicsider is fundamentalProcess research,status Toda as opposedy’s high toenergy many appliedcolliders current and upcoming ex- sciencesColl id(medicine,er Pr biology,ocess ,status nano-science, material science ...) HERA (A & B) e±p running cpuerrimenetntsancdolluidpecopmroitongnsex- THeERvaCtrAoolln(idA(eI&r&B)II) Preopcp¯pess rsutanntuinsg The ultimate aim is to± understand the fundamentalcpuerrimenetan llawstsal nincdov loofluidlpv enature.ceopmQroiCDtongn Inse xa- nutshellTHeERvatrALoH n(theAC(I&& mainB)II) objectivesep±p¯p todaystarurtsnn are2in0g0 to7 understandpe⇒riments collide all involve QCD TevatrLoHnC(I & II) pp¯ starurtsnn2in0g07 ⇒ theHER origin,A: ma structure,inly measu compositionrements of p aofrto then d euniversensitieaslla nindvdoilffrveacQtioCDn LHC pp starts 2007 ⇒ theTHeERv constituentsatrA:onm:aminalyinmly eofdai ssmattercuorevemreynotsf thofeptoarptoannderneslaititeeds manedasduiffrreamcetionnts theLTHeHERv CfundamentalatrA:doenms:igamninaelyidnmltoy eforcesdaisscuorev embetweenreynotsf thofe pthemtoarptoann derneslaititeeds manedasduiffrreamcetionnts LTeHvCadtridsoecnso:ivgmenraethidneltoyHdiigsgcosvaenrdy omfethaesutorepit’asndprroeplaetertidesmeasurements LHCduindsrecasovivegelnrpethodsetosiHbilgegpshaysnidcsmbeeaysounrde thit’sepSMroperties unravel possible physics beyond the SM Oudriscaobvileityr thtoe Hdiisgcgosvaenr dnmeweapsaurrtiecilt’essparonpdertotiems easure their unravel possible physics beyond the SM pOurorpearbtiielitys litomitedidscboyvtherenqeuwaliptyaortifcoleusr uannddertostamndeiansguoref QCtheDir pOurorpearbtiielitys litomitedidscboyvtherenqeuwaliptyaortifcoleusr uannddertostamndeiansguoref QCtheDir The one-loop amplitude for six - April 2006 – p.2/20 properties limited by the quality of our understanding of QCD The one-loop amplitude for six gluon scattering - April 2006 – p.2/20

The one-loop amplitude for six gluon scattering - April 2006 – p.2/20

Since Particle Physics aims at pushing the boundaries of our knowledge, it faces big technological challenges, e.g. • the development of accelerators with highest , operating at high magnetic field, low temperatures • treatment of huge amount of data (fast processing of information, storage) • detectors with highest tracking/calorimetry/timing information • .... Often this leads to great -offs, e.g. • world wide wed (www) • radiotherapy, diagnose • new computational methods and tools ... Particle Physics

Hence, particle physics lead, and will continue to lead to important technological advances. Still, the pleasure and challenge of exploring the unknown maintains a key role in this field

Furthermore, even if it is not an applied science the by far most relevant scientific and technological progress resulted from fundamental breakthroughs in particle physics (e.g. electromagnetisms, relativity, theory...)

Do encourage young talented students to work on fundamental sciences! Particle Physics

Particle physics went through an incredible journey in the 20’s century

Since these developments are covered in many books, I will instead focus only on those discoveries that have a close connection to today’s activities

Some references

• L. Lederman (1993) • M. Veltman Facts and Mysteries in Elementary Particle Physics (2003) • F. Close Particle Physics: A Very Short Introduction (2004) • F. Close The New Cosmic Onion: and the of the (2006) • G.Giudice A Zeptospace Odyssey, A Journal into the physics of the LHC (2010) All that remains to do in physics is to fill in the sixth decimal place Albert Michelson 1894 All that remains to do in physics is to fill in the sixth decimal place Albert Michelson 1894

CERN and Particle Physics

At CERN’s Large (LHC), the approach is to accelerate to highest possible energies, make them collide and produce new particles

High energy is important as heavy particles need high energy: E = m c2

This is even more so at a collider since the elementary particle entering the hard scattering is a or gluon in the proton that carries only a fraction of the proton’s energy TheLarge HHadr aLHCdTheroonn Coll i dmachineLargeer Hadron Collider

LakLake of Geneva

CMS

LHCb

ALICE ATLAS

TheLar ge HadronCollider is a 27 km longcollider ring housed in a tunnel about 100 m SUSY2009,!Northeastern!!!!!!!!!!!!!!!!!!! LHC!Entering!Operation 7 5"June"09,!!P!Jenni!!(CERN) underground near Geneva The LHC in a nutshell

The machine: 2 beams of protons, circulating in two magnetized rings of 27 km, steered by 1200 , 9 Tesla, operating at 1.5 K

Run 1: pp or HI collisions at ECM = 7/8 TeV, ongoing Run2: 13TeV 25 fb-1 of data collected in Run 1, i.e. 25000 trillions of pp collisions

Four main experiments: ATLAS & CMS: Higgs studies, New Physics searches LHCb: flavour physics, CP violation, heavy-quark physics ALICE: heavy ion collisions, study of quark-gluon plasma Collision recorded by ATLAS Collision recorded by ATLAS Aim

By analyzing carefully the collisions the hope is to • discover the direct production of new particles • find that some particles are produced at a different rate than what we expect

Collisions look very complicated. Is it very difficult? yes!

But such deviations, if confirmed, would be the sign of New Physics (new particles, new forces), i.e. of Physics Beyond our of particles physics

Let’s start discussing the Standard Model then content in the Standard Model Force carriers in the Standard Model

Electromagnetic Weak force • Strong (QCD) • electric • • colour charge • massless • massive W±/Z • 8 massless • coupling e • coupling gw • coupling gs • U(1) • SU(2) • SU(3)

0 eV/c2 80.4 GeV/c2 91.1 GeV/c2 0 eV/c2 0 � ±1 W+- 0 Z 0 g 1 1 1 1 The Higgs

Higgs boson • responsible for generation • only scalar (spin=0) elementary particle so far

125.5 GeV/c2 0 H 0

More on the later Feynman diagrams

Interaction through exchange of gauge . Feynman diagrams still used a lot today

Example: e+e- → Z → e+e-

space

time Feynman diagrams

Interaction through exchange of gauge bosons. Feynman diagrams still used a lot today

Example: e+e- → Z → e+e-

space gauge boson

time Precision calculations through higher order corrections in the coupling • loop corrections (virtual particles) • real emissions The Standard Model (SM) Lagrangian: encodes all possible ways in which SM particles interact with each other The Standard Model (SM) Lagrangian: encodes all possible ways in which SM particles interact with each other Running couplings

Abelian case (quantum electrodynamics):

Higher energies ↔ smaller distances, less screening from vacuum polarization ↔ coupling increases with energy (intuitive) Running couplings

Non-abelian case (electroweak and strong force):

Include extra contributions with field-strength self-interaction

Overall effect: coupling decreases with energy, i.e. the closer the more “free” particles are. The more you try to pull particles apart, the stronger the interaction Running couplings stronger weaker

At collider energies are “perturbative”, i.e. the coupling constant is smaller than one and a description though a power expansion is possible Coupling unification? stronger weaker

Improved “unification” of gauge couplings is one of the strong argument used in support of (we will come back to this later) QCD: confinement

Strong interaction becomes stronger at low energies (larger distances). What does it mean?

When you try to pull apart two quarks, you pull out of the vacuum a pair of quark + anti-quark, so that you build two “colour singlet” states that can move apart This simple picture is used today to model the hadronization of quarks into colourless (quark-antiquark) and (3quarks) If quarks are confined, how do we know that they exist? QCD matter sector

e e u 2/3 up + q d s q

down strange − 1/3 p X 1st 2nd describing DIS of an quark generation on a proton

• The light quark's existence was validated by the MIT/SLAC's deep inelastic scattering (DIS) experiments in 1968: strange was a necessary component of Gell-Mann and Zweig's three-, it also provided an explanation for the and discovered in cosmic rays in 1947 QCD matter sector

Feynman diagram describing the mixing of a kaon into its anti-particle. The black boxes d s indicate weak effective four- u c interactions 2/3 up charm + K c c K¯

d s s¯ d¯ electric charge

down strange − 1/3

2 2 2 mc st nd G sin 1 2 F c M 2 quark generation W

• In 1970 Glashow, Iliopoulos, and Maiani (GIM mechanism) presented strong theoretical arguments for the existence of the as-yet undiscovered , based on the absence of flavor-changing neutral currents

[S. L. Glashow, J. Iliopoulos and L. Maiani, Phys. Rev. D 2 (1970) 2] QCD matter sector

u c 2/3 up charm + d s electric charge

down strange − 1/3

Computer reconstruction of st nd 1 2 a ψ′ decay in the Mark I detector at SLAC, making a quark generation near-perfect image of the Greek letter ψ • Charm quarks were observed almost simultaneously in November 1974 at SLAC and at BNL as charm anti-charm bound states (charmonium). The two groups had assigned the discovered meson two different symbols, J and ψ. Thus, it became formally known as the J/ψ meson (Nobel Prize 1976) QCD matter sector

" u c ! ' 2/3 up charm + d s b electric charge

down strange bottom − 1/3

st nd rd 1 2 3 Unitarity triangle measuring the amount quark generation of CP violation in the standard model • The was theorized in 1973 by Kobayashi and Maskawa in order to accommodate the phenomenon of CP violation, which requires the existence of at least three generations of quarks in Nature (Nobel Prize 2008) [M. Kobayashi and T. Maskawa, Prog. Theor. Phys. 49 (1973) 652] QCD matter sector

The “bump” at 9.5 GeV that lead to the discovery of the bottom quark at u c FNAL in 1977 2/3 up charm + d s b electric charge

down strange bottom − 1/3

1st 2nd 3rd quark generation

• In 1977, physicists working at the fixed target experiment E288 at FNAL discovered the Υ (Upsilon) meson. This discovery was eventually understood as being the of the bottom and its anti-quark (bottomonium) QCD matter sector

Diagram involving the virtual exchange of top quarks that induces a mass difference in the system u c t d W b 2/3 up charm top + B t t B¯ d s b electric charge

down strange bottom − 1/3 ¯b W + d¯

1st 2nd 3rd M G2 m f 2 V 2 m2 quark generation B F B B | td| t

• The measurement of the oscillations of B mesons into its own anti- particles in 1987 by ARGUS led to the conclusion that the top-quark mass has to be larger than 50 GeV. This was a big surprise at that time, because in 1987 the was generally believed to be much lighter QCD matter sector

t, b u c t 2/3 up charm top Z, W Z, W + t d s b t b electric charge

down strange bottom − 1/3 W Z t ¯ 1st 2nd 3rd b quark generation Diagrams that feature a quadratic dependence on the top-quark mass • It was also realized that certain precision measurements of the electroweak vector-boson and couplings are very sensitive to the value of the top-quark mass. By 1994 the precision of these indirect measurements led to a prediction of the top-quark mass between 145 GeV and 185 GeV QCD matter sector

µ+

Top anti-top production in proton anti-proton collision at the µ u c t b 2/3 + up charm top W + t p p¯ d s b ¯ electric charge t down strange bottom − 1/3 ¯ t¯ b jet W 1st 2nd 3rd b quark generation jet

jet

• The top quark was finally discovered in 1995 by CDF and D0 at FNAL. The mass of the top quark is today quite well known, mt = (173.0 ± 0.6 ± 0.8) GeV QCD matter sector

100 u c t up-type quarks t 1 down-type quarks 2/3 up charm top 10 b + 2 10 c d s b top /m 3 s q

electric charge 10

down strange bottom − 1/3 m

4 10 d 1st 2nd 3rd u 5 10 quark generation proton 6 10 charge 2/3 up charm top mass= few MeV ~1.6 GeV ~172 GeV charge -1/3 down strange bottom mass = few MeV ~100 MeV ~5 GeV QCD matter sector

100 u c t up-type quarks t 1 down-type quarks 2/3 up charm top 10 b + 2 10 c d s b top /m 3 s q

electric charge 10

down strange bottom − 1/3 m

4 10 d 1st 2nd 3rd u 5 quark generation 10

6 10 • The masses of the six different quark flavors range from around 2 MeV for the to around 173 GeV for the top. Why these masses are split by almost six orders of magnitude is one of the big mysteries of particle physics

Z decay width

Let’s consider an example: the production and decay of a Z boson, e.g. at an e+e- collider (LEP) one has

From the position of the resonance in the distribution of the decay products one can extract the Z boson mass

What about the width of the distribution? Z decay width

The width of the distribution is sensitive to the total number of possible decay products. For the Z boson we find

• production and decay processes obey a number of conservation rules • [why is there no top quark?] Z decay width

The width of the distribution is sensitive to the total number of possible decay products. For the Z boson we find

• the measurement of the Z-boson width is compatible with 3 families for quarks and and with 3 colors for each quark Extrapolation to today

• similarly with what happened in the past, we might “see” the presence of new particles only indirectly first through the way they affect measurable quantities when they are exchanged as virtual states (this happens if the collider energy is not enough to produce “real” New Physics particles) • when a “real” new particle is produced, it decays similarly to the Z- boson. The particle might manifests itself as a bump in the invariant mass combinations of particles in the final state • from properties of the decay products one can infer properties of the decaying particle (because of conservation rules) • by studying the width of the mass distributions one can learn also about “invisible decay modes” This is what is done today at the LHC