Particle Physics at a Crossroads Meenakshi Narain Brown University
What is the world made of? What holds the world together? Where did we come from?
1. Are there undiscovered principles of nature: New symmetries, new physical laws? 2. How can we solve the mystery of dark energy? 3. Are there extra dimensions of space? 4. Do all the forces become one? 5. Why are there so many kinds of particles? 6. What is dark matter? How can we make it in the laboratory? 7. What are neutrinos telling us? 8. How did the universe come to be? 9. What happened to the antimatter? From “Quantum Universe”
The smallest pieces of matter… Particle physics is the study of smallest known building blocks of the physical universe -- and the interactions between them. The focus is on single particles or small groups of particles, not the billions of atoms or molecules making up an entire planet or star.
Now (15 billion years) Stars form (1 billion years) Atoms form (300,000 years) Nuclei form (180 seconds) Protons and neutrons form (10 -10 seconds) Quarks differentiate (10 -34 seconds?) ??? (Before that) Fermilab 4×10 -12 seconds LHC 10 -13 Seconds Astro-Physics 6 Particle Physics
What do we know about nature? Forces The particle spectrum
“Standard Model” of Particles and Forces invariant under U(1) couples to all charged particles e e
“Standard Model” of Particles and Forces invariant under SU(2) U(1) couples to all weak isospin doublets e W
“Standard Model” of Particles and Forces invariant under SU(2) U(1)SU(2) x U(1) couples to all quarks and leptons Z
“Standard Model” of Particles and Forces U(1)SU(2) x U(1) invariant under SU(3) couples to all quarks u g u
“Standard Model” of Particles and Forces couples to all massive particles SU(3) x SU(2) x U(1) symmetry t H t
“Standard Model” of Particles and Forces >10 orders of magnitude!
What we would like to know Can all the forces be unified? The particle spectrum and The mystery of mass
The standard model The Higgs mechanism –massless spin-1 particles 2 polarization states –Higgs field coupling to fermions quark and lepton masses –massive spin-1 particles 3 polarization states massless field massive field complex scalar field Higgs boson mix
The standard model 18 parameters –gauge couplings: photon: W and Z bosons: g gluon: s –Higgs-boson coupling: m Z or m W –Higgs-fermion coupling: m e m m m u m d m s m c m b m t –Higgs mass: m H –quark mixing parameters: 1 2 3 many observables – = 1/127.934(27) –G = 1.16637(1) 10 -5 GeV -2 / (g/M W ) 2 –m Z = 91.1876(21) GeV –sin 2 eff = 0.23148(17) couplings of Z to fermions scattering cross sections –m W = 80.426(34) GeV – W = 2.139(69) GeV –m t = 174.3(5.1) GeV unknown –mH–mH
The standard model global electroweak fit –are all measurements consistent with one set of parameters? m t = 174.0§ 4.5 GeV m H = 91 +58 -37 GeV sin 2 eff = 0.23142§ 0.00015 m W = 80.386§ 0.019 GeV 2 /dof = 25.5/15 (4.4%)
Global Fit to Existing Data All interactions and couplings are defined Measure the 21 Free Parameters of the Theory Masses, CKM, Couplings, etc.
Experimental limits on Higgs mass Indirect –Higher order corrections link SM parameters –e.g. M W = M tree + + –Measure M W, m t (or others) constrain M H –LEP,TeV,NuTeV,SLC global fit: M H < 211 GeV @ 95% CL (LEPEWWG Winter 2003) Direct –LEP: e + e - ZH M H > 114.4 GeV @ 95% CL (LHWG Note/2002-01) WW t b WW W Higgs
The Connection between the Top Quark, W Boson and the Higgs Boson mass Higher order corrections link SM parameters –e.g. M W = M tree + + –Measure M W, m t (or others) constrain M H Indirect: –LEP,TeV,NuTeV,SLC (LEPEWWG Winter 2005) –M H =91 +45 -32 GeV/c 2 and M H <186 GeV/c 2 @95%CL Direct –LEP: e + e - ZH M H > 114.4 GeV @ 95% CL (LHWG Note/2002-01) WW t b WW W Higgs
Indirect constraints on Higgs mass Top mass –systematics MC model, jet scale W mass –systematics production and decay model now2 fb -1 15 fb -1 l+jets 5.1 2.71.3GeV dilepton2.81.3GeV now2 fb -1 15 fb -1 W l 342717MeV
Theoretical limits on Higgs mass If SM is valid up to ¼ Planck Scale 130. M H. 180 GeV updated EW precision updated direct limit M Planck,gravity M H too large: Higgs self coupling blows up at some scale M H too small: for scalar field values O( ) the Higgs potential becomes unstable e.g. Riseelmann, hep-ph/9711456
Need for new physics Natural cutoff scale of SM is 1 TeV. New physics needs new `quark’,`heavy gauge’ and `higgs’ to cancel each of the quadratic divergences. Maximum scale for new physics if we allow 10% fine tuning:
Is there anything beyond the SM? Problems of the SM –Many free parameters –Hierarchy: Planck scale vs ewk scale large corrections to scalar masses (M H ) fine tuning required to keep M H light –Triviality: self couplings of scalars blow up at high energies –Gravity not included SM can only be the low energy limit of a more comprehensive theory
New physics candidates? SUSY Extra Dims. Other strong interactions And ->stop,gauginos,higgsino cancel the corresponding top, gauge and higgs contributions :Different statistics! ->10% tuning needed from current exp. ->Cutoff at 1 TeV ->Strongly coupled gravity at TeV energies ->Technicolor,topcolor… Stop loops can lift the mass above 114GeV
Supersymmetry Symmetry between fermions and bosons –Natural solution to hierarchy problem Additional corrections to M H precisely cancel divergences More complicated Higgs sector –¸ 2 Higgs doublets 5 physical scalar particles: CP-even: h 0, H 0, CP-odd: A 0, charged: H § –MSSM: M h. 135 GeV –SUSY with gauge coupling unification: M h. 205 GeV (Quiros&Espinosa hep-ph/9809269)
Can the Higgs be heavy? Global fit to electroweak data M H <211 GeV –Assumes no physics beyond SM –If Higgs heavier, there must be new physics at some scale Peskin, Wells PRD 64, 093003 (2001) –e.g. topcolor-seesaw model positive contributions to T allows M H. 450 GeV Chivukula, Hölbling, hep/ph-0110214
Extra Dimensions Large extra dimensions (Arkani-Hamed, Dimopoulos, Dvali) –SM particles localized in 3 dimensions –gravity propagates in extra dimensions falls off faster than r -2 at short distances gravity not tested below m scale –new fundamental scale M << M Planck could be ¼ TeV for mm-size extra dimension or several smaller extra dimensions –no hierarchy problem Other models /phenomenology for extra dimensions –Randall-Sundrum (RS) [PRL 83, 3370 (1999); PRL 83, 4690 (1999)] –[Han, Lykken, Zhang, PRD 59, 105006 (1999)] –[Giudice, Rattazzi, Wells, NP B544, 3 (1999)] –[Cheung, Landsberg, PRD 62 076003 (2000)] –…
Little Higgs Higgs is a pseudo-Nambu-Goldstone boson of a spontaneously broken global symmetry –broken symmetry must contain SM: SU(2)£U(1) –Higgs can be light divergent loops are cancelled by new particles –al least one heavy fermion (to cancel top loop) mass < 2 TeV –heavy gauge bosons (to cancel W,Z loops) masses < 5 TeV –heavy scalar (to cancel Higgs loop) mass < 10 TeV Arkani-Hamed, Cohen, Georgi