Particle Physics at a Crossroads

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Particle Physics at a Crossroads

• Meenakshi Narain
• Brown University

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• 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
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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.

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Astro-Physics

• Now (15 billion years)

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Stars form (1 billion years)
Atoms form (300,000 years)
Nuclei form (180 seconds)
Protons and neutrons form (10-10 seconds)
Particle Physics
Quarks differentiate (10-34 seconds?)
Fermilab 410-12 seconds LHC 10-13 Seconds
??? (Before that)
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What do we know about nature?

• Forces
• The particle spectrum

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Standard Model of Particles and Forces
invariant under U(1)
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Standard Model of Particles and Forces
invariant under SU(2)
U(1)
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Standard Model of Particles and Forces
invariant under SU(2)
U(1)
SU(2) x U(1)
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Standard Model of Particles and Forces
invariant under SU(3)
U(1)
SU(2) x U(1)
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Standard Model of Particles and Forces
couples to all massive particles
SU(3) x SU(2) x U(1) symmetry
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Standard Model of Particles and Forces
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What we would like to know

• Can all the forces be unified?
• The particle spectrum and
• The mystery of mass

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Theory

• ..add

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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
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The standard model

• 18 parameters
• gauge couplings
• photon ?
• W and Z bosons g
• gluon ?s
• Higgs-boson coupling
• mZ or mW
• Higgs-fermion coupling
• me m? m?
• mu md ms mc mb mt
• Higgs mass
• mH
• quark mixing parameters
• ?1 ?2 ?3 ?

• many observables
• ? 1/127.934(27)
• G 1.16637(1) 10-5 GeV-2 / (g/MW)2
• mZ 91.1876(21) GeV
• sin2?eff 0.23148(17)
• couplings of Z to fermions
• ? scattering cross sections
• mW 80.426(34) GeV
• ?W 2.139(69) GeV
• mt 174.3(5.1) GeV
• unknown
• mH

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The standard model

• global electroweak fit
• are all measurements consistent with one set of
  parameters?
• mt 174.0 4.5 GeV
• mH 9158-37 GeV
• sin2 ?eff 0.23142 0.00015
• mW 80.386 0.019 GeV
• ?2/dof 25.5/15 (4.4)

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Global Fit to Existing Data
All interactions and couplings are
defined Measure the 21 Free Parameters of the
Theory Masses, CKM, Couplings, etc.
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Experimental limits on Higgs mass

• Indirect
• Higher order corrections link SM parameters
• e.g. MW Mtree
• Measure MW, mt (or others) ? constrain MH
• LEP,TeV,NuTeV,SLC
• global fit MH lt 211 GeV _at_ 95 CL
• (LEPEWWG
  Winter 2003)
• Direct
• LEP ee-?ZH
• MH gt 114.4 GeV _at_ 95 CL
• (LHWG Note/2002-01)

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The Connection between the Top Quark, W Boson
and the Higgs Boson mass

• Higher order corrections link SM parameters
• e.g. MW Mtree
• Measure MW, mt (or others) ? constrain MH
• Indirect
• LEP,TeV,NuTeV,SLC (LEPEWWG Winter 2005)
• MH9145-32GeV/c2 and MHlt186 GeV/c2 _at_95CL
• Direct
• LEP ee-?ZH
• MH gt 114.4 GeV _at_ 95 CL (LHWG
  Note/2002-01)

W
W
t
W
W
W
Higgs
b
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Indirect constraints on Higgs mass

• Top mass
• systematics ? MC model, jet scale
• W mass
• systematics ? production and decay model

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Theoretical limits on Higgs mass
MH too large Higgs self coupling blows up
at some scale ?
MH too small for scalar field values O(?)
the Higgs potential becomes unstable

• If SM is valid up to ? ¼ Planck Scale
• 130 . MH . 180 GeV

e.g. Riseelmann, hep-ph/9711456
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The hierarchy problem

• SM provides an excellent EQFT.
• Higgs seems light.
• 1-loop correction is quadratically
• sensitive to cutoff scale.

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SM with 10TeV cutoff
We need less than 1 fine-tuning!
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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

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Is there anything beyond the SM?

• Problems of the SM
• Many free parameters
• Hierarchy Planck scale vs ewk scale
• ?large corrections to scalar masses (MH)
• ?fine tuning required to keep MH 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

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New physics candidates?

• SUSY
• Extra Dims.
• Other strong interactions
• And

• -gtstop,gauginos,higgsino cancel the corresponding
  top, gauge and higgs contributions Different
  statistics!
• -gt10 tuning needed from current exp.
• -gtCutoff at 1 TeV
• -gtStrongly coupled gravity at TeV energies
• -gtTechnicolor,topcolor

Stop loops can lift the mass above 114GeV
The Little Higgs
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Supersymmetry

• Symmetry between fermions and bosons
• Natural solution to hierarchy problem
• Additional corrections to MH precisely cancel
  divergences
• More complicated Higgs sector
• 2 Higgs doublets ?5 physical scalar particles
• CP-even h0, H0, CP-odd A0, charged H
• MSSM
• Mh . 135 GeV
• SUSY with gauge coupling unification
• Mh . 205 GeV (QuirosEspinosa hep-ph/9809269)

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Can the Higgs be heavy?

• Global fit to electroweak data ? MHlt211 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 MH. 450 GeV
• Chivukula, Hölbling, hep/ph-0110214

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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 ltlt MPlanck
• 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)

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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 lt 2 TeV
• heavy gauge bosons (to cancel W,Z loops)
• masses lt 5 TeV
• heavy scalar (to cancel Higgs loop)
• mass lt 10 TeV

Arkani-Hamed, Cohen, Georgi