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Higgs Searches a t Tevatron. M axim Titov, CEA Saclay , France. On behalf of the. COLLABORATIONS. 18 August 2011, Moscow, Russia. Reaching the Higgs Horizon. Introduction Challenges and Analysis strategies Standard Model Higgs Searches New Tevatron combined result - PowerPoint PPT Presentation
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On behalf of the
Maxim Titov, CEA Saclay, France
COLLABORATIONS
18 August 2011, Moscow, Russia
• Introduction • Challenges and Analysis strategies • Standard Model Higgs Searches • New Tevatron combined result• Beyond Standard Model Higgs
The excellent performance of the Tevatron during last few years has
sparked the realisation that a Higgs might be observed at Tevatron, thus
ensuring a complimentarity of Tevatron and LHC Searches
Exciting interplay of Higgs physics and direct supersymmetry searches
A light Standard Model Higgs hypothesis is in agreement with all indirect tests
MW mt2, logmH
mH< 161 GeV (95% CL)
mH 92 2634 GeV
Indirect constraints• Precision electroweak observables
are sensitive to the Higgs boson mass via quantum corrections.
First the Higgs Boson
has tobe discovered !!!
Is it a Standard Modelor MSSM Higgs...?
Mass: determines SM Higgs profile
Width/partial width/ couplings
Spin and CPquantum numbers
Higgs self-coupling
Tevatron/ LHC
Higgs is a journey,not a destination
LHC/sLHC/
Linear Collider
The absence of a light Higgs implies New Physics beyond SM
7.02.52.5# int./ crossing
3963963500Bunch crossing (ns)
50-6015-203 Ldt (pb-1/week)
4.0 10321x10321.6 1030Typical L (cm-2s-1)
1.961.961.8s (TeV)
36 3636 366 6Bunches in Turn
Run IIbRun IIaRun I
World’s highest energy proton-antiproton collider
Expect > 10 fb-1 analyzable data by end of September 2011
Results today using
up to 8.5 fb-1
Two general purpose detectors:
• Central tracking system embedded in a solenoidal magnetic field:
• Silicon vertex detector • Tracking chamber (CDF)
• Fiber tracker (DØ)
• Preshowers• Electromagnetic and hadronic
calorimeters• Muon system
Rapidity coverage: CDF DzeroTracking 2.0 2.5 Calorimeter 3.6 4.0 Muon 1.0 2.0 B-field 1.4 T 2.0 T
Main production mechanisms (115<mH<180 GeV):
• Gluon fusion (gg H): ~0.8-0.2 pb• Associated production (VH, V=W,Z): ~0.2-0.02 pb• Vector boson fusion (VBF): ~0.1-0.02 pbs
mH < 135 GeV:
VH (V=W,Z) production with H→bb decay
mH > 135 GeV:
gg→H production with H→WW→lnln decay
Analyze all decay channels to achieve the best sensitivity (e.g.):
• Direct: gg H tt, gg• VBF: qqH qqbb, qqH qqWW • ttH l+jets, ttH all jets
Combine all CDF/DO channels
~ 400 -700 Higgs events
produced with 10 fb -1
• QCD Multijets (data driven methods) Jets faking leptons, ET
miss from mismeasured jets
• Z/g + jets (ALPGEN/PYTHIA, NNLO theory cross-section, data-based corrections to model pT(Z)) mismeasured jets or leptons yielding MET • W+jets, W+g(ALPGEN/PYTHIA, NNLO theory cross-section, data-based corrections to model pT(W) and/or data driven methods) jets or gfaking lepton
• Diboson - WW, WZ, ZZ (PYTHIA, normalized to NLO cross-section; NLO correction for pT and di-lepton opening angle)
• ttbar, single top (ALPGEN/PYTHIA, COMPHEP; normalized to NNLO)
• gg H bb final state overhelmed by QCD
• Main channel: associated production WH / ZH with H→bb
Extremely challenging (requires Excellent b-tagging, dijet mass resolution, bkgds understanding)
WH lnbb ZHnnbbZHl l bb
WH→lnbb: lepton+MET+2 b-jets Largest signal rate Larger V+jets background
ZH→llbb: dilepton+2 b-jets Smallest Higgs signal rate Low background Kinematically constrained
ZH→lnbb: MET+2 b-jets Comparable signal rate to WH(+WH→lnbb with missing lepton) Challenging instrumental background
Step 1: Identify events consistent with leptonic W/Z decays and >= 2 jets
Trigger on high pT electrons, muons or ETmiss
• Wln: e or m and high ETmiss
• Zll: ee or mm consistent with Z resonance • Znn: no charged leptons; high Et
miss and 2 acoplanar jets
Step 2: b-tagging (reduces backgrounds by two orders of magnitude) B-tagging exploits information on:
• Lifetime: displaced tracks and/or vetices• Mass: secondary vertex mass• Soft leptons
Use MVA for improved performance: NN for b-to-c discrimination after
secondary vertex tagging; NN for b-to-light: continuous tagger
(multiple operating points)
13131313
S:B ~ 1:4000 S:B ~ 1:75S:B ~ 1:400
Before b-tagging =1 tightb-tag
≥2 looseb-tags
e.g. DO: NIMA620 490-517 (2010)
b-jet eff. ~ 50%mis-tag rate ~ 0.5%
Step 3: Validate background modelling in control regions
Similar control regions for other final states and heavy flavour enhanced samples
Multijet enhanced:
loosening missing ETmiss
(and related variables)
W+jets enhanced:
require isolated lepton
Top enhanced:
require isolated leptonand two 2b-tag jets
• The invariant mass of the bb pair is the most sensitive variable to the Higgs
• An improvement in resolution has a direct impact on the search sensitivity
Exploit information from tracker,preshower, jet shape variables,semileptonic b-decays with the NN
15% resolution improvement
How to choose 2jets from 3jets or more?
Instead of using two largest pT jets, use two most b-like jets from bID information.
S/B remains small, need advanced (multivariate) analysis techniques
Step 4: Optimize separation via multivariate technique• Exploit information from several discriminant variables and their correlations
• Improves sensitivity compared to cut-based analysis by ~15-20%
• However, must be very careful with the choice of training sample
• Many checks performed in different kinematic regions to validate the modeling of the inputs to the MVA method and its output;
Same optimization/techniques in similarfinal states as for Higgs searches:
• Single top
• Diboson hadronic decays
VS
WZ+ZZnnbb,nncc:
17171717
WZ ZZmeas 6.9 1.3(stat.) 1.8(syst.) pb
WZ ZZtheo 4.6 0.3 pb
D0 NOTE-6223 (2011)For mH=115 GeV• WH→lnbb: σ = 26 fb• ZH→nnbb: σ = 15 fb• ZH→llbb: σ = 5 fb Total VH: σ = 46 fb
Replace Z with H• WZ→lnbb: σ = 105 fb• ZZ→nnbb: σ = 81 fb• ZZ→llbb: σ = 27 fb Total VZ: σ = 213 fb
• Use the final discriminant distribution (e.g. NN output) to perform hypothesis testing (S+B vs B-only)
In the absence of excess, set limits using:
A Bayesian method (flat prior signal, credibility intervals) The CLS method (log-likelihood test statistic CLS = CLS +B /CLB)
Upper cross section limit for Higgs production relative to SM prediction
Observed limit
(solid line)from data
Expected limit (dot dashed line) and predicted 1σ/2σ (green/yellow bands)variations from background only pseudo-experiments
ZHllbb
WHlvbb
(7.9 fb-1) (7.5 fb-1 ) (7.8 fb-1)
VHvvbb
Exp (obs) - 3.9 (4.8) x SM @ MH=115 GeV
CDF NOTE-10583 (2009)CDF NOTE-10596 (2011)
CDF NOTE-10593 (2011)
CDF NOTE-10572 (2011)
Exp (obs) – 2.7 (2.6) x SM @ MH=115 GeV
Exp (obs) – 2.9 (2.3) x SM @ MH=115 GeV
ZHllbb
WHlvbb
(8.6 fb-1) (8.5 fb-1 ) (8.4 fb-1)
VHvvbb
D0 NOTE-6166 (2011)
Exp (obs) – 4.8 (4.9) x SM @ MH=115 GeV
D0 NOTE-6220 (2011)
Exp (obs) – 3.5 (4.6) x SM @ MH=115 GeV
D0 NOTE-6223 (2011)
Exp (obs) – 4.0 (3.2) x SM @ MH=115 GeV
95% CL Limits at mH = 115 GeV:
Channel Exp/obs Limit (/SM)
WHlnbb (7.5 fb-1) 2.7/2.6
ZHnnbb (7.8 fb-1) 2.9/2.3
ZHl+l-bb (7.9 fb-1) 3.9/4.8
WHlnbb (8.5 fb-1) 3.5/4.6
ZHnnbb (8.4 fb-1) 4.0/3.2
ZHl+l-bb (8.6 fb-1) 4.8/4.9
VH/VBFjjbb (4.0fb-1) 17.8/9.1
ttHl+jets (7.5 fb-1) 11.722.9
ttHjets (5.7 fb-1) 20.2/28.1
Tevatron Combination H gg:
D0: arXiv: 1107.4960
Channel Exp/obs Limit (/SM)
tt+1j,2j (6.0 fb-1) 15.2/14.7
lltt/lntt(6.2 fb-1) 17.3/18.5
tt+2j (5.4 fb-1) 12.8/32.8
Dominant decay mode for mH> 135 GeV: H WW
Clean environment can takeadvantage of gg → H production:
• 2 opposite charge high pT leptons
• Missing ET (E T miss)
Signal contribution also from associated production (W/Z+H) and VBF (qqH):
~ 35 % more signal
Consider all final states with 2 high-pT leptons and ETmiss
Step 1: Preselect events with two isolated high-pT leptons
Split analysis according to:
• D0: Lepton flavor: ee, em, mm • CDF: Signal purity based on lepton
quality• CDF: Low (<16 GeV) di-lepton mass
Different instrumental/fake backgrounds Different background compositions
Suppress the dominant Z/g background:
Use kinematics, in particular ETmiss
based variables that ensure ETmiss is
significant and not due to mis-measured object.
DO (ee, mm) employs Decision Trees trained against Z/g
Step 2: Validation of background modelling and search techniques that share characteristics of the signal
Define control regions to testmodelling for different backgrounds:
W+jets :
same-signdileptons
t-tbar :
opposite-sign
dileptons, >= 2 jets,
b-tag
Diboson cross section measurements:
CDF NOTE-9753 (2009)
CDF NOTE-10358 (2010)
Split analysis according to jet multiplicity:better sensitivity to H+jets final states: qqH, WH, ZH (important for low mass)
Each multiplicity bin correspond toa different dominant background:
• 0 jet: WW• 1 jet: WW; Z/g• ≥2 jets: ttbar MVA optimized for
each channel & mass hypothesis.
Input MVA variables (e.g. D0):
0-jet
1-jet
2-jet
mH = 165 GeV
D0 NOTE-6219 (2011)
CDF NOTE-10599 (2011)
Step 3: Multivariate analysis
Additional sensitivity ( ~ 10%) from same charge dilepton selection
W/ZW/Z Main backgrounds are instrumental:
• Lepton charge mis-ID (Z/g*l+l-)• Jets faking leptons (multijet, W+jet/g)
Final multivariate (BDT, NN) discriminants to analyze data
Exploit: Event topology, lepton kinematics, jet content, relation betweenlepton and ET
miss …
CDF NOTE-10599 (2011)D0: arXiv 1107.1268 (2011)
• Data consistent with the background-only hypothesis within the systematic uncertainties.
• Significant sensitivity at high mass!
CDF Combination: D0 Combination:
Tevatron (CDF + D0) Combination:
S+B versus B-only Hypotheses (LLR = -2lnQ, where Q = Ls+b/Lb)
Most “signal-like’ excess consistent with Higgs of 130 GeV but alsoconsistent with background-onlyhypothesis
D0 NOTE-6229 (2011)
Tevatron Combination:
mH Limit/ SM(GeV) OBS. EXP.
115 1.22 1.17130 2.02 1.37165 0.48 0.58 180 1.17 1.98
July 2011 Tevatron Combination: arXiv:1107.5518
SM Higgs excluded @ 95 % CL:
Observed Exclusion: 100 < mH < 108 and 156 < mH <177 GeVExpected Exclusion: 100 < mH < 109 and 148 < mH < 180 GeV
Observedlimit (data)
• Best current limit for mH<130 GeV
• Unique window into H bb
• H WW analysis sensitive to different signals and backgrounds than LHC around 130-140 GeV
1-3 * SM
Including ongoing analysis improvements and morechannels:
• Exclusion potential for mH < 190 GeV
• 2-3 sensitivity for mH ~ 115-130 GeV
MSSM requires exactly 2 Higgs doublets:• one couples to up-type quarks (vev vu)• another couples to down-type quarks (vev vd)
Important parameter: tan b = vu/vd
tan b ~ 35 = mt/mb is appealing (large tan b)
After EW breaking: 5 physical states ‣ 3 neutral Higgs bosons: h/H (CP-even)
and A (CP-odd) (convention: mh < mH, h/H/A generically denoted j)
‣ 2 charged Higgs bosons: H±
• At tree level: EW breaking controlled by MA and tanβ. Radiative corrections make it more model dependent.
• There must be a light Higgs (h): mh ≤ 135 GeV
H/A/H+ nearly equal masswhen mA large
Higgs coupling to b-quarks enhanced by tan β PROD ~ tan2 b
OverwhelmingQCD background Relatively clean signature
low BR ~10%
High BR ~90%Large multijet background
Reduced backgroundAdditional sensitivity at low mA
Three complimentary channels:
• Both CDF and DØ see ~2 excesses around mA~120-150 GeV
CDF: arXiv: 1106.4782 (2011)
D0: PLB698, 97 (2011)
35
bfbtt
• Tevatron searches does not observe any significant excess
ftt
arXiv: 1106.4555 (2011)
D0 Note 6227 (2011)
• CDF and D0 have paved the way and brought sophistication and maturity into Higgs boson searches at hadron colliders.
• Tevatron is on track to deliver Higgs search results in spring 2012 based on the full 10 fb-1 datasets with promised sensitivity goals
NATURE W
ILL, IN
ALL
LIKEL
IHOOD, S
URPRISE
US !
• Consider main low mass analyses (WHlnbb, ZHnnbb, ZHllbb) at 6 fb-1 and evaluate expected LLR after injecting a SM-like signal at mH=115 GeV
observed limit consistent with a what would be expected from signal+background (but also consistent with background-only)
Tevatron Observed Limit: Signal Injection Test (6 fb-1):
A. Juste, 2011 DPF Meeting, August 2011
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