Standard Model Higgs - PowerPoint PPT Presentation

1 / 41
About This Presentation
Title:

Standard Model Higgs

Description:

GRAVITATION (couples to energy) ELECTROMAGNETISM (couples to ... real experimental issues: tagging, calibrations, underlying event, ... Peter Skands ... – PowerPoint PPT presentation

Number of Views:40
Avg rating:3.0/5.0
Slides: 42
Provided by: peters53
Category:

less

Transcript and Presenter's Notes

Title: Standard Model Higgs


1
Standard Model Higgs
  • at the LHC

Peter Z. Skands Fermilab Theoretical Physics
Department (Significant parts taken from TASI
PASI lectures by M. Carena (Fermilab), S. Dawson
(BNL), D. Rainwater (U Rochester) )
2
The Role of Particle Physics
  • To discover what the Universe is made of and how
    it works.
  • Particle Accelerators reproduce in a controlled
    lab environment forms of matter and energy last
    seen in the early universe.
  • Elementary particles are the ultimate
    constituents of matter
  • Five basic forces act between these elementary
    matter particles
  • GRAVITATION (couples to energy)
  • ELECTROMAGNETISM (couples to electromagnetic
    charge)
  • THE STRONG FORCE (couples to colour charge)
  • THE WEAK FORCE (couples to weak isospin)
  • MASS rest energy (where does it come from?)

Relativity
Gauge Forces
?
3
What we Know
  • The photon and gluon appear to be massless
  • The W and Z bosons are very heavy
  • MW 80.404 GeV 0.030 GeV
  • MZ 91.1875 GeV 0.0021 GeV
  • There are 6 quarks (actually 36)
  • Mt 171.4 2.1 GeV (CDFDØ July 2006)
  • Expected ? 1.5 GeV at end of Run II
  • Mt gtgt other fermion masses
  • There appear to be 3 neutrinos
    with small but non-zero masses
  • The pattern of fermions appears to replicate
    itself 3 times

Dec 8 2006 DØ Reported 3.4s Evidence for
single top
4
The Higgs Mechanism
  • Spontaneous Symmetry Breaking

5
Why is the photon massless?
  • U(1) gauge theory with spin-1 photon Aµ
  • Local gauge invariance
  • Could we put in a mass term for A?
  • Not gauge invariant!
  • So charge conservation ? ? massless

?
6
Adding a charged scalar
  • U(1) gauge theory with spin-1 Aµ and charged
    scalar f
  • V(f) Most general renormalizable potential
    allowed by U(1) invariance
  • ? L is still invariant under U(1) rotations

7
Two solutions
  • U(1) gauge theory with spin-1 Aµ and charged
    scalar f
  • µ2 gt 0
  • QED with m?0 and mfµ
  • Unique minimum at f0
  • µ2 lt 0
  • Minimum energy state at
  • Vacuum breaks U(1) symm.

8
A closer look at µ2 lt 0
  • µ2 lt 0 Minimum energy state at
  • Write complex f in (Mod,Arg) notation
  • v constant offset ? minimum-energy (vacuum)
    state
  • h modulus d.o.f. ? Higgs field, with mass mh
    -2µ2 gt 0
  • ? argument d.o.f. ? massless Goldstone boson

m?ev
9
What happens to the Goldstone boson?
  • What happens to the ? field?
  • After gauge transformation (unitary gauge)
  • L becomes
  • ? disappears, enslaved to longitudinal part of A
    (eaten)
  • Physical degrees of freedom
  • before SSB massless A (2 d.o.f) complex scalar
    f (2 d.o.f.)
  • after SSB massive A (3 d.o.f.) real Higgs
    scalar (1 d.o.f.)

10
Abelian Higgs Mechanism
  • Spontaneous breaking of a gauge symmetry by the
    non-zero VEV of a charged scalar field
  • ?The Goldstone degree of freedom of the scalar is
    enslaved (eaten) to serve as the longitudinal
    component of the gauge field
  • ?On this background, the gauge field propagator
    appears to be massive
  • In QED, this is what happens in a superconductor
  • For the weak force it happens in free space.

11
SM Higgs Mechanism
  • SU(2)WxU(1)Y gauge symmetry.
  • Add a scalar doublet with Y1
  • For µ2lt0 there is a minimum at
  • (direction not fixed)
  • ?One massive Higgs scalar
  • And 3 Goldstone bosons
  • (same for charged components)

12
SM Mass Spectrum
  • When replacing H by its vev HVV ? masses
  • LHiggs
  • This mass matrix has one zero eigenvalue and 3
    others g2, g2, and g2g2
  • Fermions acquire mass due to Yukawa (Hff)
    couplings

13
Current Status
  • Brief Summary

14
Unitarity
  • WLWL scattering
  • Pertubative scattering P gt 1for s 1 TeV2
  • Need something (e.g. Higgs) to unitarize theory
  • If SM Higgs, then mH lt 800 GeV

15
Perturbativity and Stability
  • The Higgs quartic coupling determines the Higgs
    mass
  • Running due to self-coupling and top quark loops

from M. Carena, TASI 2006
16
? Theoretical Constraints
  • Self-coupling can become non-perturbative ? 8 ?
    upper bound
  • Radiative corrections ? deeper minima at large
    values of f ? lower bound

from M. Carena, TASI 2006
17
Precision Tests of the Standard Model
  • The Standard Model has been tested to very high
    precision (one part in a thousand) at experiments
    around the world
  • Although the Higgs has not been seen and its mass
    is unknown, via quantum corrections it enters
    electroweak observables (masses, decay rates, )
  • All electroweak parameters have at most
    logarithmic dependence on mh, however, a
    preferred value of mh can still be determined

Fermilab, SLAC, CERN,
18
The Blue Band Plot
19
LEP and Tevatron
  • Precise measurements of W and top masses can rule
    out SM above TeV
  • Tevatron shooting for top mass at or below 1.5
    GeV uncertainty

20
Before LHC
  • Run II search ultimately depends on combination
    of many weak channels, impaired by low lumi, but
    still interesting window remains

21
Higgs at the LHC
  • Discovery and Measurement Strategies

22
Higgs at Hadron Colliders
Much progress recently in computing NLO (and even
NNLO) QCD corrections, see http//maltoni.home.ce
rn.ch/maltoni/TeV4LHC/SM.html
23
SM Higgs Decay Modes
  • Uncertainties due to uncertainties in as, mt, mb,
    and mc
  • At low mh, mostly bb, tt, but also cc and gg can
    be important.
  • At large mh, H?VV
  • h?gg, h???, h?Z? generated at one loop, but due
    to heavy particles in loop ? relevant
    contributions

24
Choosing a Channel
  • What channel works best at a hadron collider
    isnt so obvious
  • Tevatron uses multiple channels
  • If H?bb dominant then (light Higgs)
  • If H?WW/ZZ dominant then (heavy Higgs)
  • Requiring leptons in the final state help reject
    QCD but not ttbar
  • Jet-jet mass resolution 15 GeV seriously
    impairs search for a narrow resonance

25
Signal Cross Sections
  • While gg?H rises QCD-like, the VH channels
    become relatively quite small

Tevatron
LHC
26
Backgrounds
27
Higgs at the LHC
  • The story of ttH
  • H? ?? and H? tt
  • Weak Boson Fusion
  • Heavy Higgs, H? VV

28
Light Higgs ttH at LHC
  • Idea ttH coupling is large and ttH final state
    should be easy to distinguish ? Little
    Background?
  • However, original studies did not treat QCD
    carefully, especially ttbb and ttjj were
    underestimated

29
Light Higgs ttH current outlook
  • Now ttH looks marginal at best
  • S/B now about 1/6 for mh 120 GeV
  • Shape only differs very slightly ? any shape
    uncertainty will make 5s impossible, even if L?8

30
Heavy Higgs ttH
  • ttH, H?WW is viable!
  • Complicated final states WWWWbb ? multileptons
  • Best channels same-sign dilepton, trilepton
  • LOTS of nasty never-before-calculated bkgs
  • tt Z /? (jj) , ttWjj, ttWW, tttt
  • lots of diagrams, large QCD uncertainties
  • If HWW coupling known ? only direct Yt measurement

31
Heavy Higgs ttH _at_ ATLAS
  • Works over large mh range

32
Light Higgs gg?H?photons
  • Idea rare decay might win because bkg is also
    EW, not QCD.
  • BR(H?photons) 0.2 for light 110ltmhlt140 GeV
  • Might not be discovery but gets mass to 1
  • Requires very good jet (fake photon) rejection
    j? , jj bkgs non-trivial detector sim estimates
    still range over factor 2

33
Light Higgs What about H?tt?
  • Problem taus not observed directly
  • Problem lots of missing energy, how to
    reconstruct mass?
  • Important taus must not be back-to-back
  • ?Limited usefulness for gg?H?tt

34
Weak Boson Fusion
  • Use the LHC as a W/Z collider
  • Quarks get scattered forward in detector, and no
    QCD current over central rapidity region

35
WBF H?tt
  • WBF ? H ? tt ? one or two leptons

Backgrounds
36
WBF H?tt _at_ LHC
  • ATLAS and CMS say it works very well!
  • For 110 lt mh lt 150 GeV (100 fb-1)

37
WBF H?WW
  • For WW recall angular correlation
  • At threshold, Ws at rest ? mll m??, construct
    transverse mass
  • Works well even away from threshold
  • Detector effects smear things out

38
WBF H?WW _at_ LHC
  • ATLAS and CMS say Jacobian peak still there!
  • May even work down to mh 120 GeV (but serious
    backgrounds ? more study needed, e.g. ttj at NLO)

mh 140 GeV
mh 160 GeV
39
WBF Remaining Issues
  • So WBF turns out to be great, but how well do we
    understand it?
  • Minijet Veto (QCD radiation, underlying event)
    currently at primitive stage, but can be tested
    on data, e.g. WBF Zjj
  • tt jets (off-shell effects, normalization and
    shape changes at NLO)
  • Contamination from gluon fusion jets.
    Partially understood
  • real experimental issues tagging,
    calibrations, underlying event,

40
The Combination Machine
  • mh lt 120 GeV
  • 200 700 GeV
  • Inclusive H?ZZ?4l
  • gt 800 GeV
  • WBF H?WW? l?jj
  • WBF H?ZZ?ll??

NO ESCAPE ROUTE FOR SM HIGGS AT LHC!
41
Putting it together
42
The LHC Potential
  • ATLASCMS total sensitivity combining all
    channels ? 5s discovery possible for all masses
    with 5fb-1 (?)
  • For mh 120 GeV, combination of many different
    channels necessary, hence requires a good
    understanding of the detectors
  • Note mh 120 GeV is simultaneously most
    sensitive region for Tevatron
  • Mass resolution for 300 fb-1
  • 0.1 to 1 from H?ZZ?4l or H???
Write a Comment
User Comments (0)
About PowerShow.com