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Higgs Searches at the LHC: An Experimenter


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Title: Higgs Searches at the LHC: An Experimenter

Higgs Searches at the LHC An Experimenters
  • Robert Cousins, UCLA
  • 31st Johns Hopkins Workshop on Current Problems
    in Particle Theory
  • Heidelberg, 2 August 2007

Four Excellent Talks lt2 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,
  • Much more in A. Djouadi, arXivhep-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
  1. gluon-gluon Fusion
  2. W,Z Boson Fusion
  3. Associated WH, ZH prod.
  4. t tbar H production

BSM can change this in many ways, e.g. ,gg?Hbb.
Note ?tot 1011 pb, ?b 109 pb, ?jet gt 100 GeV
ET gt 106 pb Need control regions in data to
understand bkgnd.
SM Higgs decay modes
BSM ??, ??, and bb changed in many ways, even
within MSSM (M. Carena et al., hep-ph/0202167).
For effective Lagrangian approach to BSM
gg?H???, see Manohar and Wise, hep-ph/0601212
can be dramatic.
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 lt10-10 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
4T central B, -2T in return yoke
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
  • B.R. 0.002 at MH115-140. 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 ?0s
  • 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
  • Also in VBF. What is interplay between VBF and
  • Once established, mass measurement to fraction of

Discovery potential of H-gtgg
CMS optimized Artificial Neural Net with
kinematics and g isolation as input, s/b per
event ATLAS likelihood pT, angles
Significance for SM Higgs MH130 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
  • 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
  • How low in MH can one push this channel?
  • Can other kinematic variables (e.g. pT) be used
  • What is best way to optimize cuts (robust yet
  • 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- 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
...with much richer potential information.
... or with ?s measured in Z frames
See Rainwater (2007) and refs therein, incl. VBF
extension... CERN workshopshep-ph/0608079CERN-
2006-009. Not for the first year!
H ? W W() ? 2l 2?
  • 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 analysts dream (?) since no mass peak,
    uses about every trick in the book... and chance
    for early discovery if MH 2MW and bkgnds
  • ATLAS updating old PDTR result. CMS studied 2µ 2?
    as a benchmark channel for muons, also other 2l
  • 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, ?fµ 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, ee channels?

H Production by (Weak) Vector Boson Fusion
No color string to snap in central region
ATLAS fig.
  • In last few years, widely studied following
    earlier work (e.g., Rainwater Zeppenfeld, PRD
    60,113004 and dozen refs therein) H decay modes
    tt, ? ?, WW.
  • ATLAS (Asai et al.) says VBF tt mode is more
    promising at low MH than (non-VBF) ? ?, and VBF
    WW mode better than non-VBF.
  • MH measurement relies on resolving MET along two
    axes of (non-back-to-back) tt. 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.

(Weak) Vector Boson Fusion (sim with ATLFAST)
Asai, et al., Eur Phys J C 32, s02, s19-s54
(Weak) Vector Boson Fusion, tt ? 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
  • Proving to be a very tough channel.

J.Cammin and M.Schumacher ATL-PHYS-2003-024 S/sqr
t(B) 2.8, MH 120 GeV, 30 fb-1 , being
CMS NOTE 2006/119
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
  • 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. Djouadi, arXivhep-ph/0503173
MSSM Charged Higgs H, H-
  • Dominant production is at a tbH vertex. For heavy

For lighter H, on-shell tt production following
by t?Hb. Decays mostly to ??? for mass lt 180 GeV
tb mode opens above but seems hopeless, so ???
remains the focus. Tau polarization opposite to
taus from W decay useful handle! Events are
complex, with complex backgrounds (tt, tW,
Wjets) 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?)
Subsequent decay modes studied µµ, tt
  • Re-emphasizes importance of early SM studies of b
    quarks (in copious tt production) and taus (in
    Z???), and modes such as Zbb.

Status in CMS Physics TDR
ATLAS update for µµ S. Gentile, et al.,
Includes parity-violating sign
Karl Jakobs at SUSY07
Invisible Higgs decays ?
Possible searches tt H ? lnb qqb PTmiss
Z H ? ll
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
All three channels have been studied key
signature excess of events above SM
backgrounds with large PTmiss ( gt 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
  • 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 lets 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
  • Example vetoes
  • Jet activity in central region, for VBF
  • Too many b quarks, when background is enhanced in
    bs (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
  • 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 thats it.
  • Poor persons 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/Wo
    rkshops/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
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! http//www-cdf.fnal.gov/physics/new/top/top
D0 3.4 ? first evidence
  • How much does one want to rely on multi-variate
    techniques for early discovery physics at LHC?
  • How to do the controls?

Higgs and SUSY searches share many issues...
Note multi-b production.
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 M130
    GeV, several to compare!

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