Higgs physics at lhc
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A very biased look at. Higgs Physics at LHC. Jeff Forshaw University of Manchester. WHEPP8, Mumbai, January 2004. Standard Model Higgs D iscovery potential at LHC Heavy Higgs i.e. lightest Higgs much heavier than Standard Model upper limit Real triplet model

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Higgs Physics at LHC

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A very biased look at

Higgs Physics at LHC

Jeff Forshaw

University of Manchester

WHEPP8, Mumbai, January 2004

  • Standard Model HiggsDiscovery potential at LHC

  • Heavy Higgsi.e. lightest Higgs much heavier than Standard Model upper limitReal triplet model

  • Light Higgs i.e. lightest Higgs much lighter than Standard Model lower limitMSSM with explicit CP violation

  • Invisible Higgs i.e. lightest Higgs has significant branching into invisible particles

  • No Higgs i.e. strong dynamicsWW scattering

In collaboration with Douglas Ross (Southampton), Agustin Sabio-Vera (Cambridge), Ben White (Manchester), Jon Butterworth (UCL/ZEUS/ATLAS), Brian Cox (Manchester/H1/ATLAS), Jae Sik Lee (Manchester), James Monk (Manchester), Apostolos Pilaftsis (Manchester).

Precise data (0.1%) from LEP, SLC and Tevatron imply a light Higgs boson when interpreted within the Standard Model

See lepewwg.web.cern.ch

Similar conclusions in

minimal SUSY extensions,

i.e. < 135 GeV

Higgs production at LHC

from Keith Ellis

Discovery potential for a SM Higgs

Note new role ofVBF for “low” massHiggs

e.g. see LHC Higgs working Group meetings (Cranmer, Mellado, Quayle and Wu: 5σ in each of dilepton and l+jet channels and room for improvement!)

Is it possible to improve even more in the <140 GeV region?

  • De Roeck, Khoze, Martin and Ryskin propose to sidestep pileup even at high luminosity by using tracking information. Could then allow to use H->bb decay channel. Identify primary vertex and cut on tracks emanating from there, e.g. no tracks above some threshold ~1 GeV between tag jets and b-jets.

  • Needs experimental study.



Another possible contribution in the VBF channel?

  • “Two loop” QCD contribution not so far considered..Leading order (one loop) jet+Higgs+jet is ~10% of VBF for typical cuts [del Duca et al]

  • But can be estimated by computing imaginary part

Same colour topologyas VBF – so will surviveminijet veto

Heavy Higgs

Introduce a real triplet (Y=0):

[Lynn & Nardi, Blank & Hollik]

Tree level violation ofcustodial symmetry

Oblique Quantum Corrections

i.e. not vertex or box

[Lynn, Stuart; Peskin, Takeuchi]

In the triplet model we work to lowest order inQuantum corrections are expected to be small:

S, T and U are gauge invariant and finite:

General observable

New physics enters only via S, T and U

To lowest order in

Hence quantum corrections are small

quantum loops give (for no mixing in neutral sector)

Expected since Y=0

Expected since approximately custodial

is the mass difference between the neutral and

charged (mainly) triplet higgses.

  • Direct correction to W mass since

  • Indirect correction to all observables since

tree level

But tree level corrections are interesting:

For SM contribution


13 observables

Tree Level Effects

95% CL


Standard Model

Quantum Level Effects

95% CL


Some general remarks [Peskin & Wells]

  • There are other ways to accommodate a heavier higgs boson:

  • S < 0 extra SU(2)xSU(2) multiplets [Dugan & Randall] new singlet majorana fermions [Gates & Terning]

  • T > 0 4th generation [e.g. Dobrescu & Hill] 2 higgs doublets [Chankowski et al]

  • New vector bosons[e.g. Casalbuoni et al, extra dimensions]would be seen at LHC

If NO new particles at LHC/FLC then the crucial informationcould come from even more precise electroweak measurements:

GigaZ (~1 month of linear collider) and/or


[Figure from Peskin & Wells]

Conclusion: Triplet Model

  • The data do not force us into a light Higgs

  • Real triplet model: tree level mechanism “naturally”produces*

  • May be very helpful to improve precision (GigaZ)

“Quasi-decoupling”: lightest Higgs can have mass up to strong dynamics scale (500 GeV) without anyother consequences.

* Quantum mechanism also viable

Light Higgs

MSSM with CP Violation

  • Higgs sector CP violation naturalSince the soft SUSY breaking trilinear couplings and gaugino masses can be complex

  • Easy to arrange for lightest Higgs to have weak coupling to Z.Hence it may not have been seen at CERN

  • Light Higgs scenario is more general*, e.g. if Higgs mixes with anything with a reduced coupling to the Z (as occurs with the radion in Randall-Sundrum).

[Pilaftsis; Carena, Ellis, Pilaftsis, Wagner]

*See also 2HDM

CPX Scenario

  • Mixing of h, H and A via top and bottom squark loopsat one-loop (and gluino loops at two-loop).

  • EDM constraints can be avoided without fine-tuning.

  • Benchmark scenario (CPX):

Carena, Ellis, Pilaftsis, Wagner

  • Difficulties in detecting such a light Higgs at Tevatron and LHC via conventional search channels.

  • Dedicated LEP analysis underway in an attempt toexclude the low mass regions.

  • Possibility to look in diffraction?


GeV (tagged protons needed)

S/B > 1 anticipated at LHC (de Roeck et al)

KMR predict 3 fb for a 115 GeV higgsincludes gap survival factor 1/50

[Khoze, Martin & Ryskin]

Decay to b quarks viable since QCD background is heavily suppressed.

Crucial to measure “exclusive” dijets at Run II to ascertain the reliability of the theoretical calculations which do at present sufferfrom large uncertainties.

R = ratio of CPX ggH coupling to that in SM

Diffractive Higgs production cross-section at LHC (solid) and Tevatron (dotted). MSSM parameters chosen to lie in the region not currently excluded by LEP, i.e.

Conclusions: Light Higgs

  • Rate may well be high enough to be explored at the LHC – especially in tau decay channel (but a detailed study of S/B needed).

  • Central production with tagged protons may wellbe a useful tool which is able to complement more traditional search strategies.

  • Need for suitable forward detectors.

Invisible Higgs

  • Not hard to imagine models where the Higgs decaysinvisibly, e.g. to a pair of neutralinos.

  • Zeppenfeld & Eboli propose that such a Higgs could be discovered in VBF: “gaps between jets with missing ET” signature.

  • Would need to work in low lumi phase.

  • ATLAS & CMS have supported the original idea with detailed studies: 5 sigma discovery with 30/fb for a wide range of parameter space.

Possible “fly in the ointment”:

  • Cut on missing ET>100 GeV helps kill QCD background.

  • Signal has tag jets close in azimuth and this is also effective in reducing QCD background

  • Absence of QCD radiation in interjet region suppresses LO QCD jj but not QCD singlet exchange.

Large momentum transfer colour singlet

May need to fall back on ZH->dilepton+missing ET…

Godbole, Guchait, Mazumbar, Moretti, Roy

No Higgs

What if there is no new “light” physics?

  • Suppose new physics is at a scale Q >> E (E = energy of experiment) [e.g. could be a heavy Standard Model Higgs or some new strong dynamics]

  • Can use effective field theory approach to parametrizeignorance of the new physics

  • Q cannot be much beyond 1 TeV since we know thatthe Standard Model without a Higgs breaks down ataround 1 TeV (WW -> WW violates unitarity)


The only dimension twooperator allowed by gaugeinvariance and custodial symmetry

Goldstone (“pion”) bosons of thebroken chiral symmetry.

Electroweak chiral lagrangian

Can relax the assumption of custodial symmetrywithout any problem – just more parameters.Actually we need to do this if we are to get away with no light higgs boson (i.e. in light of precision data). Bagger, Falk & Swarz

Longhitano; Appelquist & Bernard

One-loop amplitude for WW->WW. Calculated using the equivalencetheorem. Physical WW->WW amplitudes obtained by crossing and isospin symmetry

Focus on quartic couplings of vector bosons:

Pade approximation.Matches one-loop perturbationtheory result

Elastic unitarity


  • Effective theory only valid for E<<Q

  • Can try to extrapolate to higher E by insisting thatpartial waves are unitary [considerable ambiguity]

  • Pade (and N/D) schemes have been implemented intoPYTHIA

Resonance Map in Pade scheme

A =900 GeV scalarB =1300 GeV vectorC = 1800 GeV vectorD = 800 GeV scalar and 1300 GeV vectorE = no resonancesSM = 1 TeV HiggsTC = Re-scaled QCD

What can LHC do in 1 year in scenarios A-E?

See Dobado et al

Probing the new physics at LHC

Focus on semi-leptonicdecay channel



Missing ET + e or m

Background is manyorders of magnitudebigger than signal…


Analysis: Leptonic W

  • Combine all charged leptons 1 by 1 with neutrino (missing pT) to give W 4-vector up to 2- fold ambiguity (mass fixed to central value)

  • Cut at 320 GeV on W transverse momentum (plot (d))

[Distributions are area normalised]

Analysis: Hadronic W

(a) and (b) : pT and hof highest pT jet

(c) mass of highest pT jet in region | h| < 4

(hadronic W reconstructed as 1 jet 98% of time)

  • Cut at pT > 320 GeV and 70 GeV < MJ < 90 GeV

Subjet method for identifying energetic W bosons

Cut: 1.6 < log(pTy1/2) < 2.0

Use kt algorithm tofind scale at whichW-jet candidate resolves into two subjets.

For a genuine W, we expect the scale at which the subjets are resolved (i.e. ypT2) to be of order MW2

The Hadronic Environment I

(a) Top quark veto :

Rejection Cut:130 GeV < MWJ < 240 GeV

(b) Tag jets : Must have forward and backward tag jets satisfying

Cut: pT > 20 GeV, E > 300 GeV,

4.5 > |h| > 2

Divide event into 3 regions :

“forward” of most forward W

“backward” of most backward W

“central” (contains both W candidates)

Forward (backward) tag jet is highest pT jet in forward (backward) region

The Hadronic Environment II

(d) Minijet veto: no colour exchanged between protons

Minijets: all jets (apart from W candidate) with |h| < 2

Cut: Nminijets (pT > 15 GeV) < 2

(c) Hard pT: pT of 2 tag jets + 2 W’s = 0 for signal

Cut: pT < 50 GeV

WW mass and cos q*distributions

A =900 GeV scalarB =1300 GeV vectorC = 1800 GeV vectorD = 800 GeV scalar and 1300 GeV vectorE = no resonances

Simulated measurement 100 fb-1

Underlying Event

The minijet cut is sensitive to underlying event (and pileup) although the approach developed here is less sensitive than in previous analyses

(a) Hadronic W width affected greatly by underlying event model

(b) Subjet analysis insensitive to models

(c) and (d) : The minijet distributions are sensitive, particularly below 20 GeV. The 15 GeV threshold we use is marginal …

but measurement of the underlying event in data should allow tuning of models and cut

Conclusions (WW Scattering)

  • It may be that there is no new physics until we enterthe TeV region (LHC). In which case we may well want a ~2 TeV linear collider.

  • WW scattering is an excellent place to look for evidence of the new physics: With 1 year high luminosity should be able to measureWW cross-section differential in WW mass and hencethe mass of any new resonances (up to about 1.5 TeV). In some scenarios, it may be possible to measure the spin of the resonance(s) too.

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