Transcript Advanced Statistical Techniques in Particle Physics

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
R. Cousins, Johns Hopkins/Heidelberg, 2 Aug 2007
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Four Excellent Talks <2 Weeks Ago at EPS
... And many more at SUSY07!
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• 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/ph0504099
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SM Higgs Production
pb
NLO
a)
b)
c)
d)
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.
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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”.
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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.
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ATLAS
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CMS
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Cross Section of CMS
Particle
type
Tracking
EM
Calorimeter
Hadronic
Calorimeter
Muon
g
e
4T central B,
-2T in return yoke

Jet
neutrinos
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ATLAS
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The Approximate State of the Art in M.C. Studies
With K-factors
with K factors
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 reexamined 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.
LHC: ~1 fb-1 in 2008, increasing to 100 fb-1/year at design luminosity.
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H→gg
• 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%.
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Discovery potential of H->gg
SM
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
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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...)
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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.
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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!
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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 2μ 2ν 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, ee channels?
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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 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.
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(Weak) Vector Boson Fusion (sim with ATLFAST)
Asai, et al., Eur Phys J C 32, s02, s19-s54 (2003).
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(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.
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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
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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)
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A Couple Slices in MSSM Parameter Space
A. Djouadi, arXiv:hep-ph/0503173
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MSSM Charged Higgs H+, HDominant 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
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Scenarios with Increased Hb Coupling
(MSSM large tan)
Subsequent decay modes studied: μμ, ττ
Re-emphasizes importance of early SM studies of b quarks
(in copious tt production) and tau’s (in Z), and modes
such as Zbb.
Status in CMS
Physics TDR:
ATLAS update for μμ: S. Gentile, et al., arXiv:0705.2801v1
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Includes parity-violating sign
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Karl Jakobs
at SUSY07
Invisible Higgs decays ?
Possible searches:
tt H  ℓnb qqb + PTmiss
Z H  ℓℓ
+ PTmiss
qq H  qq
+ PTmiss
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)
Sensitivity:
Problems / ongoing work:
• ttH and ZH channels have low rates
95% CL
• 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
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.
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Discussion
• 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.
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NNLO calculation is not always
needed for initial discovery of
di-object resonance.
Nor do you initially need absolute rate to 5%.
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Vetoes
• 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.
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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.
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Single-Top 5+ Years into Tevatron Run II
D0: 3.4  “first evidence”
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.html
How much does one want to rely on multi-variate techniques for
early discovery physics at LHC?
How to do the controls?
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Higgs and SUSY
searches share
many issues...
Note multi-b
production.
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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 wellcablibrated. 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!
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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.]
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Conclusion
• 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.
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Thanks
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.
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