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Elementary Particle Physics from Theory to Experiment

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Elementary Particle Physics From Theory to Experiment Carlos Wagner Physics Department, EFI and KICP, Univ. of Chicago HEP Division, Argonne National Laboratory Society of Physics Students, Univ. of Chicago, Nov. 10, 2014 Particle Physics studies the smallest pieces of matter, 1 1/10.000 1/100.000 1/100.000.000 and their interactions. Friday, November 2, 2012 Forces and Particles in Nature Gravitational Force Electromagnetic Force Attractive force between 2 massive objects: Attracts particles of opposite charge k e e F = 1 2 d2 1 G = 2 MPl Forces within atoms and between atoms Proportional to product of masses + and - charges bind together Strong Force and screen each other Assumes interaction over a distance d ==> comes from properties of spaceAtoms and timeare made fromElectrons protons, interact with protons via quantum neutrons and electrons. Strong nuclear forceof binds e.m. energy together : the photons protons and neutrons to form atoms nuclei Strong nuclear force binds! togethers! =1 m! = 0 protons and neutrons to form nuclei protonD. I. S. of uuelectronsd formedwith Modeled protons by threeor by neutrons a theoryquarks, based bound on together Is very weak unless one ofneutron theat masses high energies is huge,udd shows by thatthe U(1)gluons gauge of thesymmetry strong interactions like the earth protons and neutrons SU (3) c are not fundamental Friday,p November! u u d 2, 2012 formed by three quarks, bound together by n ! u d d the gluons of the strong interactions Modeled by a theory based on SU ( 3 ) C gauge symmetry we see no free quarks Very strong at large distances confinement " in nature ! Weak Force Force Mediating particle transformations Observation of Beta decay demands a novel interaction Weak Force Short range forces exist only inside the protons 2 and neutrons, with massive force carriers: gauge bosons ! W and Z " MW d FW ! e / d Similar transformations explain the non-observation of heavier elementary particles in our everyday experience. Modeled by SU (2)L gauge symmetry assignsFriday, November 2 2, 2012isospin & u # &) L # charges ± 1 ''( $ ! $ ! 2 $d ! , $ e ! % "L % "L Explains nuclear fusion in the Sun! and ultimately, Sunlight and ultimately, Sunlight Standard Model Particles There are 12 fundamental gauge fields: But very different masses! 8 gluons, 3 Wµ’s and Bµ m3/m2 and m2/m1 a few tens or hundreds and 3 gauge couplings g1, g2, g3 3 mµ mτ me = 0.5 10− GeV, m 200, m 20 The matter fields: e µ 3 families of quarks and leptons with same Largest hierarchies 5 quantum numbers under gauge groups mt 175 GeV mt/me 10 ⇥ 9 neutrino masses smaller than as 10− GeV! Only left handed fermions transform under the weak SM gauge group Lectures onSSupUe(3)rsymmetrSy U(2)L U(1)Y Carlos E.M. Wagner, Argonne and EFI Fermion× and gauge× boson masses forbidden by symmetry Friday, November 2, 2012 Antiparticles Dirac equation leads to negative energy states Solution to this problem : States filled (the Dirac sea) Holes in the Dirac sea : Antiparticles Positron discovered a few years later Particle Physics Property : When two particles collide, they may be converted into other forms of energy (particles energetically accessible) Friday, November 2, 2012 Particle Physics Property : When two particles collide, they may be converted into other forms of energy (particles energetically accessible) Colliding Colliding Shaquille Anti-Shaquille Rate of production of a given particle depends on its strength of interactions with colliding particles. Hulk Hogan (and anti-Hogan) produced more frequently than Hawking Friday, November 2, 2012 In the ElectroweakStandardElectro wModel,eak SymmetrySymmetr explicit massesy Breaking:Br eaking:are forbidden by symmetry TheThe HiggsHiggs MechanismMechanism andand the Origin of MassMass AA scalarscalar (Higgs)(Higgs) fieldfield isis introduced.introduced. The Higgs field acquires aa nonzerononzero valuevalue toto minimizeminimize itsits energyenergy SpontaneousSpontaneous Breakdown ofof thethe symmetrysymmetry : SU(2)L U(1)(1)Y UU(1)(1)em L × Y ⇥⇥ em VacuumVacuum becomes a source ofof energyenergy = a source of mass 0 <Hφ" >==v v AA physicalphysical statestate (Higgs(Higgs boson)boson) appearappear associated ⇥ to fluctuations inin thethe radialradial directiondirection .. GoldstoneGoldstone modes:modes: LongitudinalLongitudinal component ofof massivemassive GaugeGauge fields.fields. MassesMasses ofof fermionsfermions andand gaugegauge bosonsbosons proportional to their couplingscouplings toto thethe HiggsHiggs field:field: m2 vv22 MMWW ,Z==ggWW,,ZZ vv mmtoptop == hhtoptop v H = !λ ii µµ ii ii dd j i u j == iiΨ¯Ψ¯ γγµΨΨ ΨΨ¯¯ hh HHd + Ψ¯ h (i⇥2H∗)u ++ h.c.h.c. LL L,RL,RDD µ L,RL,R −− LL ijij R L ij 2 R ii i,ji,j ⇤⇤ ⇤⇤ ⇥⇥ Friday, November 2, 2012 TheThe Discovery Discovery of the of Higgs the Higgs would puts put thethe lastlast piece piece of the of theStandard Standard Model Model in placein place How did we search for the Higgs ? Colliding particles at the Tevatron and the LHC Tevatron Energy = 2,000 proton masses LHC Energy = 14,0008 proton masses Chicago Mountains Swiss Alps Geneva ATLAS LHC CMS Friday, November 2, 2012 The complete set of Standard Model particles Progress in developing the SM has been the result of collaboration of theory and experiment Reasons for Proposal and Later Solutions to 4 Puzzles (1932) ! 1) Klein Paradox --apparent violation of unitarity (solution:positron existence- pair production possible) ! 2) Wrong Statistics in Nuclei--N-14 nucleus appeared to be bosonic--(solution: neutron not a proton-electron bound state) ! 3) Beta Ray Emission-apparent Energy non conservation (solution:neutrino) ! 4) Energy Generation in Stars (solution: nuclear forces, pep chain, carbon cycle etc.----pion) from G. Segre’10 Friday, November 2, 2012 Higgs Discovery Historical Perspective In order to reflect on the future, it is useful to look at the lessons of the past. I will give a few interesting examples, and then speculate on the future. Higgs mechanism was proposed back in 1964, by several authors, including Higgs, and implemented in the SM by Weinberg, in 1967. What were the prospects of finding the Higgs Boson ten years after its proposal ? Friday, November 2, 2012 A Phenomenological Profile of the Higgs Boson 1976 Eilam Gross, WIS, Freiburg Jan 2012 11 Friday, November 2, 2012 A Phenomenological Profile of the Higgs Boson 1976 Eilam Gross, WIS, Freiburg Jan 2012 12 Friday, November 2, 2012 But the Higgs is not weakly coupled to all fundamental particles ! It is relatively strongly coupled to those particles which had not been discovered at that time Indeed, the W mass, the Z mass and the top quark masses are all of the order of 100 times the proton mass Some of the authors soon realized that these could be used to produce Higgs bosons It is in processes mediated by these particles that we have searched for, and eventually found the Higgs boson ! Friday, November 2, 2012 SearchThe for search the forStandard the Standard Model Model Higgs Higgs at Proton at the LHC Colliders m < 200 GeV • Low mass range HSM H ! "" ,##,bb,WW ,ZZ m > 200 GeV • High mass range HSM H ! WW , ZZ 14 TeV 1 bf-1 in 2009 Friday, November 2, 2012 SM Higgs LHC Cross Sections and Decay Branching Ratios 24 2 1 barn : 10− cm 12 1 pb = 10− barn 15 1 fb = 10− barn L σ = Number of events ⇥ Higgs tends to decay into heavier SM particle kinematically available Friday, November 2, 2012 The low-mass region 2011 data m4l <160 GeV: 2012 data Observed: 39 Expected: 34± 3 Both Experiments look for a Higgs decaying into two Z’s through four lepton channels 2011+2012 data 34 Both see an excess of ZZ events in the 125 GeV mass range. The production rate is consistent with the one expected for a 125 GeV mass Higgs. Friday, November 2, 2012 Properties of the new state Mass and signal strength Observed width consistent with • From a 2-D fit to ZZ* and γγ channels experimental resolution of ~ 2 GeV • expected width of SM Higgs ~ 4 MeV Mass = 126.0 ± 0.4 ± 0.4 GeV Mass = 125.3 ± 0.4 ± 0.5 GeV σ/σSM = 1.4 ± 0.3 σ/σSM = 0.87 ± 0.23 CMS SM SM Strengths in good agreement with prediction for SM Higgs at current precision. November 1, 2012 29 J. Pilcher Friday, November 2, 2012 Combining all channels the LHC experiments found a best fit to the Higgs production 98 rate consistent with that one of a SM Higgs of massCompatibility close to 125 GeV with SM Higgs boson Signal strength ! Overall best-fit signal strength in the combination: Friday, November 2, 2012 !/! = 0.80±0.22 2012 The Status of the Higgs Search J. Incandela for the CMS COLLABORATION 2012theHiggsIncandela TheStatus of SearchJ. theCMSCOLLABORATION for SM th July 4 ! Signal strength in 7 and 8 TeV data are self-consistent 98 Now What? Friday, November 2, 2012 Assume Resonance behaves like a SM Higgs: What are the implications for the future of High Energy Physics? Many questions remain unanswered. Just to list some important ones : Why is gravity so weak or, equivalently, why is the Planck scale so high compared to the weak scale ? (hierarchy problem) What is the origin of the matter-antimatter asymmetry What is the origin of Dark Matter ? Are neutrinos their own antiparticle ? Why are there three generations of fermions ? What is the origin of the hierarchy of fermion masses ? Do forces unify ? Is the proton (ordinary matter) stable ? What about Dark Energy ? Friday, November 2, 2012 Dark Matter and Electroweak Symmetry Breaking Friday, November 2, 2012 Astrophysical Evidence of the Existence of Dark Matter More mass in Galaxies Collisionless form than inferred from of Matter in Galaxies, Stars and Dust carrying most of the Galaxies mass Results from WMAP Ωi : Fraction of critical density Universe density !0 = 1.02 ± 0.02 Dark energy density !" = 0.73 ± 0.04 Total matter density !M = 0.27 ± 0.05 Dark matter is non-baryonic Baryon matter densit y !b = 0.044 ± 0.004 Our Universe: us If Dark Matter is a neutral particle proceeding from the thermal bath, its density fraction is inversely proportional to its annihilation rate.
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