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Particle Physics at a Crossroads

Meenakshi NarainBrown 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)

Fermilab4×10-12 seconds

LHC10-13 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

…Theory…

• ..add…

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

complexscalar field

Higgs boson

mix

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

– sin2eff = 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

The standard model

• global electroweak fit– are all measurements consistent with one set of

parameters?

• mt = 174.0§ 4.5 GeV

• mH = 91+58-37 GeV

• sin2 eff = 0.23142§ 0.00015

• mW = 80.386§ 0.019 GeV

2/dof = 25.5/15 (4.4%)

Global Fit to Existing Data

All interactions and couplings are definedMeasure 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. MW = Mtree + +

–Measure MW, mt (or others) constrain MH

–LEP,TeV,NuTeV,SLC global fit: MH < 211 GeV @ 95% CL

(LEPEWWG Winter 2003)

•Direct–LEP: e+e-ZH

MH > 114.4 GeV @ 95% CL (LHWG Note/2002-01)

WW t

b

2

2

W

t

M

m

W WW

Higgs

W

H

M

Mln

FEMZ GM ,,

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)– MH=91+45

-32GeV/c2 and MH<186 GeV/c2 @95%CL• Direct

– LEP: e+e-ZH • MH > 114.4 GeV @ 95% CL (LHWG Note/2002-01)

FEMZ GM ,,

WW t

b

2

2

W

t

M

m

W WW

Higgs

W

H

M

Mln

Indirect constraints on Higgs mass

• Top mass

– systematics MC model, jet scale

• W mass

– systematics production and decay model

now 2 fb-1 15 fb-1

l+jets5.1

2.7 1.3 GeV

dilepton 2.8 1.3 GeV

now 2 fb-1 15 fb-1

Wl 34 27 17 MeV

Theoretical limits on Higgs mass

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

updated EW precision

updated direct limit

M

Planck,gravity

MH too large: Higgs self coupling blows up at some

scale

MH too small: for scalar field

values O() the Higgs potential

becomes unstable

e.g. Riseelmann, hep-ph/9711456

The hierarchy problem

• SM provides an excellent EQFT.

• Higgs seems light. • 1-loop correction is

quadratically

sensitive to cutoff scale.

SM with 10TeV cutoff

-100 -50 0 50 100

Higgs

Tree

Top

Gauge

Scalar

1-loop

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 scalelarge 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

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 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 (Quiros&Espinosa hep-ph/9809269)

Can the Higgs be heavy?

• Global fit to electroweak data MH<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 MH. 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 << 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)]– …

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