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Higgs Searches at the LHC: An Experimenter’s Perspective. Robert Cousins, UCLA 31 st Johns Hopkins Workshop on Current Problems in Particle Theory Heidelberg, 2 August 2007. Four Excellent Talks <2 Weeks Ago at EPS. ... And many more at SUSY07!.

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higgs searches at the lhc an experimenter s perspective

Higgs Searches at the LHC:An Experimenter’s Perspective

Robert Cousins, UCLA

31st Johns Hopkins Workshop on Current Problems in Particle Theory

Heidelberg, 2 August 2007

four excellent talks 2 weeks ago at eps
Four Excellent Talks <2 Weeks Ago at EPS

... And many more at SUSY07!

So, in this talk, I will not attempt to “archive” more than can be absorbed in the time allotted for my talk.
  • While giving a broad overview illustrated by “official” CMS and ATLAS results, I will attempt to emphasize some aspects of Higgs searches where more work might be useful. Focus here on first observation, but many more issues will follow re couplings, etc.
  • Among the numerous general resources available which aided me in preparing this talk, I mention in particular the CMS Physics Technical Design Report, and lecture notes by D. Rainwater, http://arxiv.org/abs/hep-ph/0702124.
  • Much more in: A. Djouadi, arXiv:hep-ph/0503172, 0503173. Also: V. Buescher and K. Jakobs, Int. J. Mod. Physics A, Vol 20, Nr. 12 (2005), 2523-2602. hep/ph-0504099

SM Higgs Production



  • gluon-gluon Fusion
  • W,Z Boson Fusion
  • Associated WH, ZH prod.
  • t tbar H production

BSM can change this in many ways, e.g. ,gg→Hbb.

Note: tot ~ 1011 pb, b ~ 109 pb, jet > 100 GeV ET > 106 pb

Need control regions in data to understand bkgnd.


SM Higgs decay modes

BSM: γγ, , and bb changed in many ways, even within MSSM (M. Carena et al., hep-ph/0202167).

For effective Lagrangian approach to BSM ggHγγ, see Manohar and Wise, hep-ph/0601212: can be “dramatic”.

the experimental challenge
The Experimental Challenge
  • Production cross section times decay branching ratio for H → g g is ~10-13 of the pp inelastic cross section.
  • For H → Z Z(*)→ 4 leptons, it is even smaller.
  • These are inhumanly small numbers, lower even than searched-for rare decays of kaons and muons (10-11 to 10-12 B.R.). Top quark discovery at Tevatron was <10-10 level.
  • Thus the challenge at the LHC is to push the state of the art in both “hadron collider” techniques and “rare decay” techniques.
  • Experience from both is to rely on theory and M.C. as little as possible, tuning both to real data. Measured ratios of similar processes (so that unknown systematics cancel at least partially) are typically the most robust.
  • Be prepared for unexpected backgrounds.
cross section of cms
Cross Section of CMS

4T central B,

-2T in return yoke

the approximate state of the art in m c studies

Figure from VBF studies by Asai, et al., Eur Phys J C 32, s02, s19-s54 (2003), also showing ATLAS TDR results. Since superseded in some modes. ttH re-examined by Cammin and Schumacher.

For γγ, L. Carminati at PhysLHC-06 Cracow), NLO cuts analysis with K factors: S~6 from 120 to 140 GeV.

CMS Physics TDR (2006), and refs therein. Work continues; VBF -> WW re-examined; ttH pessimistic.

The Approximate State of the Art in M.C. Studies

With K-factors

with K factors

LHC: ~1 fb-1 in 2008, increasing to 100 fb-1/year at design luminosity.

h g g
H → g g
  • B.R. ~0.002 at MH~115-140. Classic bump-hunting on smooth background but (!) S/B ~ 1/20.
  • Experimental challenges:
    • g energy and angle resolution
      • Both CMS and ATLAS optimized for this
    • Reduce fake photons, reduce photons from 0’s
      • Preshower, isolation (form of veto).
  • Beyond simple cuts:
    • CMS: ANN, classify events by quality, combine with weights
    • ATLAS: include kinematic variables in likelihood

How safe is this? How to control? How to convince skeptics with more info than a mass peak?

  • Also in VBF. What is interplay between VBF and inclusive?
  • Once established, mass measurement to fraction of 1%.
discovery potential of h gg
Discovery potential of H->gg

CMS optimized: Artificial Neural Net with kinematics and g isolation as input, s/b per event

ATLAS likelihood: pT, angles


Significance for SM Higgs MH=130 GeV for 30 fb-1, NLO:

CMS Physics TDR: 6.0 cut-based, 8.2 optimized

ATLAS: 6.3 cut-based, 30-40% better with likelihood

h z z 4 leptons
H → Z Z(*)→ 4 leptons
  • Studied and discussed for years, since relatively clean and sensitive over large MH range, especially 4μ.
  • Background is so low that bkgnd statistical uncertainty from sidebands may be an issue: profitable to do more work on measuring backgrounds using other sign/flavor combinations, relaxing cuts, etc.?
  • At low MH, continuum ZZ(*) bkgnd peaks above the signal: need to be sure off-shell extrapolation is reliable. (Typically one requires one on-shell Z.)
  • How low in MH can one push this channel?
  • Can other kinematic variables (e.g. pT) be used convincingly?
  • What is best way to optimize cuts (robust yet powerful)?
    • Separate cuts for leptons 1, 2, 3, and 4?
    • How strongly should cuts depend on mass?
    • Multi-variate? (Event generators...)
h z z e e cms ptdr
H → Z Z(*)→ e+ e- μ+ μ- (CMS PTDR)

tt and Zbb bkgnds reduced by isolation, impact parameter cuts: both to be understood from data.


4l bonus: Higgs JCP. Generalization of an old idea...

...with much richer potential information.

... or with θ’s measured in Z frames

See Rainwater (2007) and refs therein, incl. VBF extension...

CERN workshops:[hep-ph/0608079][CERN-2006-009].

Not for the first year!

h w w 2 l 2
H → W W(*)→ 2l 2ν
  • H → W W(*) is dominant decay mode above ~135 GeV, dramatically increases width of H and reduces other modes to “rare” except ZZ(*).
  • A data analyst’s dream (?): since no mass peak, uses about every trick in the book... and chance for early discovery if MH ~ 2MW and bkgnds understood!
  • ATLAS updating old PDTR result. CMS studied 2μ2ν as a benchmark channel for muons, also other 2l 2ν.
  • Backgrounds (several still with 15% uncertainty or greater); higher order effects, spin correlations are important; need full generators.
    • Continuum WW (and WZ and ZZ)
    • tt, tWb (jet veto) and some bb (impact parameter), isolation
    • Drell-Yan dimuons (angle btw muons is large unless jet present)
    • Events with jets faking electrons, in particular W+ jets
  • Sensitivity in a variety of kinematic quantities, incl spin correlations, φμ μ : muons tend to come together when WW from spin 0.
  • Cuts vs multivariate? Discussion of background estimation from data. What is optimal way to combine μμ, eμ, eechannels?
h production by weak vector boson fusion
H Production by (Weak) Vector Boson Fusion

No color string to snap in central region

  • In last few years, widely studied following earlier work (e.g., Rainwater & Zeppenfeld, PRD 60,113004 and dozen refs therein): H decay modes ττ, γ γ, WW.
  • ATLAS (Asai et al.) says VBF ττ mode is more promising at low MH than (non-VBF) γ γ, and VBF WW mode better than non-VBF.
  • MH measurement relies on resolving MET along two axes of (non-back-to-back) ττ. How will this work in real data?
  • Will central region be as “quiet” as predicted? Is some sort of veto (calo, track, combination?) adequate, or better off with multi-variate?
  • How well can backgrounds be understood from data? See discussions in Rainwater (2007) and Asai et al. (2003), and CMS PTDR.

ATLAS fig.


(Weak) Vector Boson Fusion (sim with ATLFAST)

Asai, et al., Eur Phys J C 32, s02, s19-s54 (2003).


(Weak) Vector Boson Fusion, ττ → lepton + tau jet ...

CMS Physics TDR, full sim and reconstruction

Asai, et al. (2003). ATLFAST.

... VBF needs further study in all modes.

tth h bb
ttH, H → bb

Proving to be a very tough channel.

J.Cammin and M.Schumacher ATL-PHYS-2003-024:

S/sqrt(B) = 2.8, MH = 120 GeV, 30 fb-1 , being revisited.

CMS NOTE 2006/119

higgs beyond the standard model
Higgs Beyond the Standard Model
  • Vast literature by now, detailing many possibilities: benchmarks in MSSM; extensions beyond MSSM; substitutes for fundamental scalar. (EPS and SUSY07.)
  • I will not attempt to discuss all the plots in various parameter spaces, but rather focus on a couple novel experimental signatures with respect to SM Higgs.
    • Now at least 5 states, including charged Higgs bosons, CP-odd state, (even doubly-charged state in 3-doublet model).
    • Enhanced coupling to b quarks, tau in some scenarios; other scenarios such as decays dominant to invisible particles. Re-emphasizes need to understand b, tau, missing ET.
    • Possibility of H decaying to SUSY particles (e.g., for ATLAS, Hansen et al., hep-ph/0504216)
  • Emphasizes need to measure quantum numbers and couplings (in both production and decay)
a couple slices in mssm parameter space
A Couple Slices in MSSM Parameter Space

A. Djouadi, arXiv:hep-ph/0503173

mssm charged higgs h h
MSSM Charged Higgs H+, H-

Dominant production is at a tbH vertex. For heavy H:

For lighter H, on-shell tt production following by tHb.

Decays mostly to  for mass < 180 GeV; tb mode opens above but seems hopeless, so  remains the focus.

Tau polarization opposite to tau’s from W decay: useful handle!

Events are complex, with complex backgrounds (tt, tW, W+jets); b jets must be understood; some current search strategies are dominated by systematic errors.

Current effort is on how to reduce systematic errors with subsidiary measurements, ratios. (SM top, Z, etc.)

Refs: CMS Physics TDR; Mohn et al., ATL-PHYS-PUB-2007-006

scenarios with increased hb coupling mssm large tan
Scenarios with Increased Hb Coupling (MSSM large tan)

Re-emphasizes importance of early SM studies of b quarks (in copious tt production) and tau’s (in Z), and modes such as Zbb.

Subsequent decay modes studied: μμ, ττ

Status in CMS Physics TDR:

ATLAS update for μμ: S. Gentile, et al., arXiv:0705.2801v1


Karl Jakobs at SUSY07

Invisible Higgs decays ?

Possible searches: tt H  ℓnb qqb + PTmiss

Z H  ℓℓ + PTmiss

qq H  qq + PTmiss


- J.F. Gunion, Phys. Rev. Lett. 72 (1994)

- D. Choudhury and D.P. Roy, Phys. Lett. B322 (1994)

- O. Eboli and D. Zeppenfeld, Phys. Lett. B495 (2000)

All three channels have been studied:

key signature: excess of events above SM backgrounds with large PTmiss ( > 100 GeV/c)


  • Problems / ongoing work:
  • ttH and ZH channels have low rates
  • More difficult trigger situation for qqH
  • backgrounds need to be precisely known
  • (partially normalization using ref. channels
  • possible)
  • non SM scenarios are being
  • studied at present
  • first example: SUSY scenario

95% CL

ATLAS preliminary

higgs bosons in non minimal models
Higgs Bosons in Non-Minimal Models
  • Little Higgs
  • Doubly charged Higgs: Spectacular resonance in same-sign dimuons
  • Extra dimensions
  • Radions, Higgs in radion decays

Experimental issues similar to the rest in this talk: resolution, tag jets, photon ID and isolation, b-tagging, background measurement.

  • In the last 25 years, an enormous amount of effort has gone into developing Higgs search strategies and predicting how well they will perform. A lot of this effort involved reducing uncertainties in predicting background.
  • As the exciting time of real LHC data approaches, uncertainties in predicting how well search strategies will perform are relevant only in deciding where to concentrate the search effort... Soon we will measure background rates, and refine the search strategies!
  • So let’s remind ourselves of some principles of experimental HEP. Techniques developed at the Tevatron, LEP, and B factories will help us a lot, but we still have work to do while anticipating first beam.

NNLO calculation is not always needed for initial discovery of di-object resonance.

Nor do you initially need absolute rate to 5%.

  • Veto: requiring the absence of some particle, signature, etc. Notoriously difficult to predict effect, going back to the days of NIM electronics.
  • Example vetoes:
    • Jet activity in central region, for VBF signature.
    • Too many b quarks, when background is enhanced in b’s (e.g. when background is tt).
    • Typical isolation criteria.
  • Note: Optimal criteria for defining object (e.g., b quark) for veto are not necessarily the same as for positive ID.
  • Especially with pile-up rates of 20 events per beam crossing, will require great care and creative ways to calibrate.
likelihoods multivariate techniques
Likelihoods, Multivariate Techniques
  • Neyman-Pearson Lemma: Best discriminating variable for distinguishing two simple hypotheses (no fitted parameters) is the ratio of the likelihoods under the two hypotheses. If possible to write down correctly with all the correlations, etc., then that’s it.
    • “Poor person’s version”: multiply 1D or 2D likelihoods as if no correlations. At least one can see the plots entering the calculation.
  • Machine-learning techniques (ANN, BDT, etc.) can sometimes do better when it is hard to write down likelihood ratio with full correlations. (Essentially that is what they are attempting to do; see H. Prosper in http://www.ippp.dur.ac.uk/Workshops/02/statistics/proceedings.shtml). More and more experience in HEP.
  • Very powerful, but can be very hard to track down puzzling behavior.
single top 5 years into tevatron run ii
Single-Top 5+ Years into Tevatron Run II

How much does one want to rely on multi-variate techniques for early discovery physics at LHC?

How to do the controls?

CDF: “The question arises to which extent the results of the Matrix Element (ME), the Likelihood Function (LF), and the Neural Networks (NN) techniques are compatible... our compatibility measure ...is 0.65%.” [same data!]


D0: 3.4  “first evidence”

beyond first observation what is it what else is there
Beyond First Observation: What is it? What else is there?

More precise measurements and more precise theoretical calculations move into spotlight.

  • Challenge to compare theory and expt for production cross section, with effect of cuts on kinematic distributions, etc. Event generators to highest possible order (and with flexibility for model tuning) are welcome!
  • Can we discern new physics interfering (+ or -) with the top loop in ggH ?
  • Mass: O(0.1%) over wide range once detectors well-cablibrated. Width: see discussion by Rainwater 2007.
  • Spin: angles, e.g., leptons from (spin 0) HWW tend to be in same direction.
  • Multiple production and decay modes: if M~130 GeV, several to compare!
statistics for lhc
Statistics for LHC
  • Will build on the considerable experience of Tevatron, LEP, B factories, et al.
  • ATLAS and CMS already discussing common (multiple) methods for comparing and combining channels and experiments.
  • Aim is to have supported tools in ROOT for various frequentist and Bayesian methods.
  • Incorporating systematic uncertainties still a challenge! [Talks at PhyStat 2005 at Oxford.]
  • Over many years, Higgs-hunting strategies have evolved from concepts, to generator-level studies, to full simulation with reconstruction, with data-driven background techniques.
  • Now the focus is shifting even more from projections of “how well will we be able to do” to “how precisely will we do it”. Understanding and controlling systematic errors, in particular as the analyses become more complicated, is at the forefront. Real data will come soon!
  • A general area where theorists can help is in guidance on what kinematic distributions are reliable discriminants, especially if fed into a multivariate soup. Similarly, which parts of phase space make reliable control regions for predicting background in signal regions.
  • Work is underway to have coherence in (various) statistical techniques, combining channels, etc.

To many for discussions and references, including:

  • CMS Higgs physics analysis group conveners Alexandre Nikitenko and Yves Sirois, and CMS Physics Coordinator Paris Sphicas; and Claudio Campagnari.
  • ATLAS Higgs working group conveners Louis Fayard and Markus Schumacher; and Karl Jakobs.