Higgs Searches at CDF
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Transcript of Higgs Searches at CDF
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Alexei Safonov (Texas A&M University)For the CDF Collaboration
*Higgs Searches at CDF
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“Higgs Searches at CDF”, A. Safonov, Aspen, CO, February 14, 2012 2
*Outline*Brief introduction to Higgs search strategies at the Tevatron*Summer 2011 combination and ingredients*New CDF analyses since Summer 2011*Ongoing work and plans*Summary
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*Higgs Production at Tevatron*Dominated by gluon fusion
*But large decay modes often come with large backgrounds
*Associated Production*While smaller cross section, offers cleaner final states
*Vector Boson Fusion (VBF) *Even smaller cross-section, but can help increase the overall sensitivity
*Have to use all accessible modes and many decay channels*Improving analysis techniques
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*Higgs Search Strategies*Sensitivity strongly depends on
backgrounds for a specific channel:*“High mass” m >~135 GeV:*WW (and ZZ) decay modes *Clean final states with leptons, any
production mode will do*“Low mass” m < ~135 GeV:*Associated production WH/ZH: *Rely on leptons/neutrinos to reduce
background, go after all decay modes (but bb dominates)
*Gluon fusion: *H→bb overwhelmed by backgrounds*H→tt:ok, but small BR
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*Summer 2011 Combination
*Of course LHC results since then excluded masses above ~135 GeV
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*Ingredients of the Combination*High mass region
dominated by H→WW*Low mass region:*WH+ZH→MET+b
b*WH→lnbb*ZH→llbb*H→WW makes
key impact above 120 GeV
H→WW
ZH→llbb
VH→MET+bb
WH→lnbb
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*Selected Recent Analyses
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*Search for H→ with 10 fb-1*One of very recently updated results
*Follows the same technique as the 7 fb-1 analysis*At least one central photon ET>25
GeV*Many improvements including NN ID
for central photons, converted photon recovery in central region
*Fit for a bump plus a smooth curve (from the rest of the distribution after excluding 12 GeV window around searched Higgs mass)*Each of 4 categories separately
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*Search for H→ with 10 fb-1*Combine four channels at the
level of likelihoods*Bayesian limits for mH)<130
GeV at ~12xs(SM) except m~120 GeV*Previous limit used in the
combination was ~14-19xs(SM)
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*Search for H→tt w/ 1+ Jets*Result from Fall’11, L=8.3 fb-1
*Allow multiple production modes *Sensitivity dominated by VH (H→tt,
V→jets)*Channels: e/m+t+1jet, e/m+t+≥2jets, e+m+1jet,
e+m+≥2jets,
*Deploy Support Vector Machine (SVM) to discriminate against main backgrounds using event kinematics properties:*Z→tt+jets
*tt-bar (only for em+jets channel)
*QCD muti-jets
*Fit the distribution for the SVM outcome to extract limits (or claim discovery if there is significant excess)
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*SVM-Based H→tt Search Limits*Final exclusion limit is obtained by combining the four
channels at the level of likelihoods *Observed limit about 12xs(SM)*Combined limit is dominated by e/m+t+≥2jets channel
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*Search for H→tt Using MMC *Also a recent analysis using L=7.8
fb-1
*Traditional cut-based search, but attempts to improve di-tau mass reconstruction using MMC technique* Elagin, Murat, Pranko, and A.S., NIM A654, 481
(2011)
*MMC uses likelihood to solve an under-constrained system of equations*Uses Bayesian-like priors to pick
solutions consistent with tau decay kinematics and experimental uncertainties in measuring MET*A large improvement over collinear
approximation as MMC can handle back-to-back events (and no tail)
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*Search for H→tt Using MMC *Attempt to minimize systematic
uncertainties due to backgrounds by measuring normalizations from data*Require one of two taus to decay leptonically, another hadronically *No e+m events in this search
*Define 8 channels: *(1 or 3 prong) x (e/m) x (0/1+jets)*Count jets with ET>30 GeV
*Fit ditau mass reconstructed with MMC for signal + background
e+th channelNpr(t)=1
e+th channelNpr(t)=3
Njet=0
Njet=0
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*Search for H→tt Using MMC
e+th channelNpr(t)=1
m+th channelNpr(t)=3
m+th channelNpr(t)=1
e+th channelNpr(t)=3
Njet>0
*Use of MMC equivalent to ~x1.6 more data vs transverse mass*Sub-channel with Njet>0 is sensitive to VH production, reach can
be further improved by utilizing kinematics selections (will be in 10 fb-1 analysis)
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*Search for H→tt Using MMC *Combined limit of 8 channels:
~14xs(SM)@m=115 GeV/c2
*0-jet and “1+”-jet channels complementary*Updated analysis will use improved event
kinematics discrimination
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*Near Future Plans
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*Are We Done Yet?*Luminosity used in key
channels:*H→WW: 8.2 fb-1
*WH+ZH→MET+bb: 7.8 fb-1
*WH→lnbb: 7.5 fb-1
*ZH→llbb: 7.5-7.9 fb-1
*Less in secondary channels: weighted luminosity for low mass searches is ~7.5 fb-1
*CDF full certified dataset is now 10 fb-1
*Expect updated analyses showing up at La Thuile, Tevatron combination is more likely to be shown at Moriond
*Expect sensitivity scaling better than 1/sqrt(L) due to ongoing improvements to analyses
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*Tevatron and LHC Expectations *Why bother if the LHC
expects to settle the question by the end of 2012?*Will we know if what we
found is indeed the SM Higgs?*E.g. branching for H→bb:* Current CMS expected limit is
3-4xs(SM) for the bb channel*Complementary sensitivity in
the very low mass range*E.g. plot on the right: region
below 120 seems already excluded by CMS+Tevatron*Tevatron data will allow important cross-checks and help
answer questions that may not be answered by the LHC alone until after the long shutdown in 2012
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*Summary*CDF is busy updating its results to include the full 10 fb-1 dataset*Numerous improvements to analysis techniques are expected to improve sensitivity beyond just √L*Expect to see individual analyses at La Thuile, combination is
more likely to be first presented at Moriond*Apart from complementarity to global LHC+Tevatron sensitivity, Tevatron results will have an impact on our understanding the nature of Higgs should it be discovered at the LHC in 2012*One example is sensitivity to the H→bb mode, which is an important check-mark in distinguishing between SM non-SM Higgs cases