Higgs Boson at CMS: gearing up for discovery
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Transcript of Higgs Boson at CMS: gearing up for discovery
Andrey Korytov, UF 19 February 2008, Cambridge 1
Higgs Boson at CMS: gearing up for discovery
Andrey Korytov
Andrey Korytov, UF 19 February 2008, Cambridge 2
Outline
• Introductory remarks: what we already know
• LHC, CMS
• SM Higgs frontrunners: o H WW2l2o HZZ4lo H 2
• A few other SM Higgs production/decay channels
• A few words on MSSM Higgs
• Concluding remarks
Andrey Korytov, UF 19 February 2008, Cambridge 3
SM Higgs Trivia: intelligent design
• Start from scalar fieldo doublet pseudo-scalar in SM
• Require local gauge invarianceo need massless gauge fields Ao lagrangian acquires terms
• Mexican hat potentialo min V() is not at =0 o non-zero vacuum expectation value v0—ether of 21 centuryo expand around minimumo effective mass terms for gauge bosonso effective mass for h-field itself
• Free lunch: o force interact with fermions with ad hoc couplings f o effective fermion masses (within the P-violation framework!)
Two important points: • Higgs boson mass is the only free parameter• (Higgs-particle coupling) ~ (mass of particle)
o Production mechanisms: first one needs to produce heavy particleso Decay channels: higgs likes to decay to heaviest particles it can
decay to
( )x
( ) ( ) ( )x U x x
g AA2 42( )
2V
2 20~ vhm
2 20~ vAm g
2 20~ vf fm
0v h
Andrey Korytov, UF 19 February 2008, Cambridge 4
What we know: theory
After renormalization• (Q)
• If mH were small at 1 TeV, runs down with Q, flips sign at some scale Q, and vacuum breaks loose
• If mH were large at 1 TeV, runs up with Q, explodes at some scale, theory becomes non-perturbative, and theorists can retire
SM Higgs has a very narrow window of opportunity to be self-sufficient due to a fine-tuned accidental cancellation of large correction factors
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Higgs mass MH (GeV)
Andrey Korytov, UF 19 February 2008, Cambridge 5
What we know: direct search at LEP
}MJJ=MH =?
}MJJ=MZ=91 GeVZ0
H bb
jet (b-tagged)
jet (b-tagged)
q
q jet
jet
e+
e-
Z0
LEP Energy209 GeV
Andrey Korytov, UF 19 February 2008, Cambridge 6
What we know: direct search at LEP
Tight Cuts The final word:No discovery...Consistency with background: ~1.7
MH > 114.4 GeV @95% CL
Phys. Lett. B565 (2003) 61
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What we know: direct search at Tevatron
At MH= 115 Expected: 3.8xSM Observed: 4.2xSM
At MH= 160 Expected: 1.9xSM Observed: 1.1xSM
Z/W+H, Hbb
HWW
Andrey Korytov, UF 19 February 2008, Cambridge 8
What we know: circumstantial evidence
Presence of too light or two heavy Higgs in loops would make various SM precision measurements less self-consistent: mH<144 GeV at 95% CL (unconstrained by the LEP 114-GeV limit)mH<182 GeV at 95% CL (if combined with the LEP 114-GeV limit)
LEP EWK Working Group
W
H
Andrey Korytov, UF 19 February 2008, Cambridge 9
Large Hadron Collider
2008: first collisions (Lint ~ 10+ pb-1 ?)
2009: Lint ~ 1+ fb-1
2010: Lint ~ 10+ fb-1
Beyond: Lint ~ 100 fb-1 / yr
Switzerland
France
Geneva airport
6 miles
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Compact Muon Solenoid
Andrey Korytov, UF 19 February 2008, Cambridge 11
CMS: Conceptual Design (cf. ATLAS)
Large Si Tracker in a Large Magnetic Field:• high momentum measurement precision: ~1% up to 100 GeV
Precision PbWO4 EM Calorimeter:• very good energy resolution for photons/electrons: <1% above 30 GeV
Hadron calorimeter has moderate energy resolution: ~10% above 100 GeV Muon system: muon trigger and identification (also important for measuring TeV muons)
Andrey Korytov, UF 19 February 2008, Cambridge 12
CMS: all major components are underground
Endcap Muon Disks go down Tracker Insertion (Dec’07)
Andrey Korytov, UF 19 February 2008, Cambridge 13
CMS commissioning
Underground Muons, Fall 2007
Andrey Korytov, UF 19 February 2008, Cambridge 14
CMS commissioning
Underground Muons, Fall 2007
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CMS Physics Technical Design Report
Physics TDR published in 2006
• Comprehensive/up-to-date overview of CMS physics reach650 pages308 figures 207 tables1.50 kg
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SM Higgs: discovery signatures at L=30 fb-1
Colored cells = { detailed studies available }YES = { sure discovery in the appropriate range of masses at L=30 fb-1 }
Hbb H H HWW HZZ
inclusive YES YES YES
qqH YES YES YES
W/Z+H
ttH
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CMS: SM Higgs search frontrunners
NLO cross sectionsSystematic errors included
NLO cross sectionsSystematic errors included
Benchmark luminosities:• 0.1 fb-1: exclusion limits will start carving into SM Higgs x-section
• 1 fb-1: discoveries become possible around MH~165 GeV
• 10 fb-1:SM Higgs is discovered (or excluded) in full range
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So-called golden channel
I will use this channel to illustrate the level of scrutiny all major search channels now undergo• strategy for data driven background control• strategy for measuring all efficiencies directly from data
HZZ4l
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tt 4 + X
Zbb 4 + X
Higgs signal H 4
ZZ 4 with spectacular peak at m4=mZ
(the s-channel contribution was overlooked in all previous studies)
HZZ4dominant 4 backgrounds
tt Wb + Wb BX + BX X + X 4X
Zbb + BB + X + 2(X) + X 4X
ZZ (dominant after cuts)
t-channel s-channel
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HZZ4analysis strategy
Peak in mdistribution
Cut optimization• mH-dependent (read m4-dependent)• identify most important and not-correlated cuts
m4 mass windowo isolation cut on the least isolated muon (i.e., the same cut for all
muons)o muon pT cut for the 3rd softest muono after applying these cuts, others do not help anymore
• produce smooth cut(m4) functionsThis strategy makes the search automatically optimized for any mass at which Higgs boson may chose to show up
Peak search:• Include statistics and systematics into significance evaluation
Final probabilistic interpretation (significance must be properly de-rated due to a search being conducted in a wide range of masses)
Higgs signal H 4
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HZZ4understanding ZZ bkgd
Knlo(m4)
Box-diagram
Control samples
QCD scale uncertainties
PDF scale uncertainties
Isolation cut uncertainties
Muon efficiency uncertainty
2+ + * + …
2
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~20% over LO
Zecher, Matsuura, van der Bijhep-ph/9404295
HZZ4understanding ZZ bkgd
Formally (by counting vertices), NNLOHowever, - it is the LO for ggZZ and - contribution is large due to large gg “luminosity”
Knlo(m4)
Box-diagram
Control samples
QCD scale uncertainties
PDF scale uncertainties
Isolation cut uncertainties
Muon efficiency uncertainty
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HZZ4ZZ background
Knlo(m4)
Box-diagram
Control samples:• qq Z 2
o very similar origin to ZZ bkgdo huge statistics
• ZZ 4 sidebandso would be perfect, if not for rather complicated shape o and very limited statistics
QCD scale uncertainties
PDF scale uncertainties
Isolation cut uncertainties
Muon efficiency uncertainty
L=10 fb-1
Total 8 eventsExp bkgd 0.8 evtsScL = 4.7
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HZZ4ZZ background
Knlo(m4)
Box-diagram
Control samples
QCD scale uncertainties• estimate of higher-order
contributions
PDF scale uncertainties
Isolation cut uncertainties
Muon efficiency uncertainty
Normalization to Z2Normalization to Z2
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HZZ4ZZ background
Knlo(m4)
Box-diagram
Control samples
QCD scale uncertainties
PDF scale uncertainties
Isolation cut uncertainties • Underlying Event is the main source for energy flow in vicinity of
muons in the irreducible ZZ-bkgd; but UE activity is poorly predicted…• Use data to calibrate UE activity:
o UE activity in Z (qq Z) must be very similar to that in ZZ (qq ZZ)o MC studies (random cone and tag-and-probe techniques) confirm this
statement
Muon efficiency uncertainty: use data
three colors: different UE models— ZZ events- - Z events (random cones)
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HZZ4ZZ background
Knlo(m4)
Box-diagram
Control samples
QCD scale uncertainties
PDF scale uncertainties
Isolation cut uncertainties: use data
Muon efficiency uncertainty: use data
• single muon trigger; well reconstructed muon 0
• take advantage of muon being measured twice: in Tracker and Stand Alone Muon system
• find Z-peak three times…• (efficiency) ~ 1%
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ZZ is a dominant bkgd
HZZ4: signal and background after cuts
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HZZ4Higgs signal over ZZ bkgd
Peak search results: • Significance:
o Counting Experimento LLR for m spectrum
• Luminosity needed• Including systematics
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HZZ4Higgs signal over ZZ bkgd
Peak search results: • Significance• Luminosity needed• Including systematics
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HZZ4Higgs signal over ZZ bkgd
Peak search results: • Significance• Luminosity needed• Including systematics
o significance must be de-rated
o effect depends on how we define the control sample: Z2 peak vs ZZ4 sidebands
Calculations done for luminosities, at which the expected significance would be 5, if there were not systematic errors
systematic errors are under control
L=10 fb-1
Total 8 eventsExp bkgd 0.8 evtsScL = 4.7
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HZZ4lcombining four channels
new
electrons are difficult systematic errors are under control
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HZZ4lsignificance de-rating
Search in a broad range of parameter phase space
mH=115-600 GeV
Probability of finding a local excess somewhere is much higher than naïve statistical significance might imply: e.g. S=3 is almost meaningless
A priori assumptions must be clearly defined
Background-onlypseudo experiment
Search for Higgs peak
— actual probability
- - probability implied by local statistical significance
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Standard Model Higgs: H
Andrey Korytov, UF 19 February 2008, Cambridge 34
Standard Model Higgs: H
Backgrounds:• prompt • prompt + jet(brem , )• dijet
Analysis:• Cut-based
o PT, , isolation, M
o events sorted by photon quality (EM shower profile quality)
• Optimized (NN)o loose cuts and sortingo event-by-event kinematical Likelihood Ratioo bkgd pdf from sidebands, signal pdf from MC
• Systematic errors folded in
L=1 fb-1
Signal x 10
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Standard Model Higgs: H
L=7.7 fb-1, Signal x 10(best category of events)
Di-photon mass alone does not provide the full power
CMS
Events sorted by their individual LLR’s
L=7.7 fb-1, Signal not scaled(all categories of events)
CMS
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Standard Model Higgs: H
CMS
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Standard Model Higgs: HWW2l2
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Standard Model Higgs: HWW2l2
Backgrounds:• WW, tt, Wt(b), WZ, ZZ • ggWW (box)
Analysis:• KNLO(pT
WW)
• cuts: o e/ kinematics, isolation, jet veto,
MET
• counting experiment, no peak• background from a control sample:
o signal: 12<mll<40 GeV
o control sample: me>60 GeV
• reduce syst. errors; pay stat. penalty
• systematic errors are folded in
Signal Region Control Sample
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Standard Model Higgs: HWW2l2
CMS
NOT YET PUBLIC
Discovery Exclusion
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VBF channels qqH
qqH, H -- is it better than inclusive H?qqH, HWW -- is it better than inclusive
HWWl?CMS and ATLAS do not seem to agree…
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Difficult (impossible) channel: ttH, Hbb
30 fb-1
ATLAS
SM Higgs: ttH, Hbb
CMS: • careful study of systematic errors in the Physics TDR• syst error control at sub-percent level is needed: not
feasible...
If higgs boson is light, can we use Hbb?
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mtop=174.3 GeV
MSSM Higgs bosons: h, H, A, H±
• SUSY stabilizes Higgs mass• 2 Higgs field doublets needed• Physical scalar particles: h, H, A, H±
• Properties at tree levelo fully defined by 2 free parameters: MA, tano CP-even h and H are almost SM-like in vicinity
of their mass limits vs MA: hmax and Hmin
o large tan ~ suppresses coupling to Z and W ~ enhances coupling to “down”
fermions: b and are very important!o CP-odd A never couples to Z and W:
~ decays: bb, (and tt for small tan)o H± strongly couples to tb and o all Higgs bosons are narrow (<10 GeV)
• Loop corrections o gives sensitivity to other SUSY parameterso mh
max scenario = { most conservative LEP limits }
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ATLASL=300 fb-1
MSSM Higgs or SM Higgs?
SM-like h only:• considerable area…• even at L=300 fb-1
Any handles?• decays to SUSY
particles?• SUSY particle decays?• measure branching
ratios?
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Summary
Standard Model Higgs:• expect to start excluding SM Higgs at L~0.1 fb-1
• discoveries may be expected already at L~1 fb-1
• SM Higgs, if that’s all we have, is expected to be discovered by the time we reach L~10 fb-1
MSSM Higgs:• nearly full (M, tan) plane is expected to be covered at
L~30 fb-1
• there is a serious chance to see only a SM-like Higgs…
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P.S. Word of caution from Tevatron
MH=110 GeV (the best expectations in 2003)• SM Higgs exclusion at 95% CL was expected at L=1.2 fb-1
• Now at L~2 fb-1, the excluded limit is a factor of 6 behind the early expectations
MH=160 GeV (the best current limit)• It took enormous effort (well beyond of what was contemplated in 2003) to bring
the current limit where it is now and approx in agreement with the earlier expectations
The reality may be not as rosy as projections---something to remember as we gear up for the Higgs search at LHC…
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Backup slides
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Standard Model Higgs: qqH, HWW2l2
Backgrounds:• tt, WWjj, Wt
Analysis:• 2 high pT leptons + MET• 2 forward jets (b-jet veto)• central jet veto• counting experiment, no peak:• background from data:
o Signal: all cutso Control sample: no lepton cuts
Result• better than inclusive WW (!!!)
jet
jet
ATLAS
ATLASMH=160 GeVHWWe
Signal Region Control Sample
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Standard Model Higgs: qqH, H
Backgrounds:• Zjj, tt
Analysis:• two forward jets, central jet
veto• two leptons (e, , -jet)+MET
l+ l l+ -jet
• mass(l; l or -jet; pTmis)
o despite 3 or 4 ’s present, works quite well in collinear approximation
He
ATLAS 30 fb-1
ATLAS
H
pT
mis
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MSSM Higgs boson: h, H, A production
• x-sections are large, often much larger than SM (dotted line)
• bb(h/H/A) production is very important
h H A
h H A
tan=3
tan=30
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MSSM Higgs: SM-like signatures
CMS:• better detector
simulation• systematics included• contours recessed…
ATLAS:• no systematics
included
CMS 2003 CMS 2006
ATLAS
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MSSM Higgs: heavy neutral H, A• production in association with bb (especially good at large
tan)• bb-decay mode (~80%) is overwhelmed with QCD background• -decay mode (~20%) is the next best• -decays (~0.1%) allow for direct measurement of • better detector simulation (i.e. more realistic) • systematics included
• contours recessed (low MA band, qqH, moved to SM-like Higgs plot)
ATLAS
CMS 2003 CMS 2006
new
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MSSM Higgs: H±
Heavy H± (M>mt):• production via gg tbH± bjj+b and gb tH±
bjj+o H± (H± tb overwhelmed by
bkgd)o tWbjjb
• backgrounds: tt, Wt, W+jets
Light H± (M<mt):• production via gg/qq tt
b+blo t H±b, H± o tWblb
• backgrounds: tt, Wt, Wjjj
new
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Difficult (impossible) channels…
MSSM Higgs: bb(H/A), (H/A)bb MSSM Higgs: H±tb
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MSSM Higgs bosons: h, H, A, H±
Loop corrections give sensitivity to the rest of SUSY sector, more specifically to:• stop quark mixing Xt
• squark masses MSUSY
• gluino mass Mg
• SU(2) gaugino mass M2
• higgsino mass parameter
*Suggested by Carena et al. , Eur.Phys.J.C26,601(2003)
Special benchmark points*:• max stop mixing (mhmax):
o mh < 133 GeVo MSUSY~1 TeVo most conservative LEP limits
• no mixing: o mh < 119 GeVo MSUSY~1 TeV
• gluophobic h o ggh is suppressed
(top+stop loop cancellation)o mh < 119 GeVo MSUSY~350 GeV
• small eff (mix h/H):o and bb-decays
suppressed even for large tan
o mh < 123 GeVo MSUSY~800 GeV
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MSSM Higgs: other benchmark points?
ATLAS studies:• preliminary (no
syst)
• vector boson fusion:o qq(h/H)o h/H, WW,
• caveat for small eff: decoupling from is compensated by WW enhancement
• all four special points are well covered at L=30 fb-1
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ATLASL=300 fb-1
MSSM Higgs or SM Higgs?
SM-like h only:• considerable area…• even at L=300 fb-1
Any handles?• decays to SUSY
particles?• SUSY particle decays?• measure branching
ratios?
Andrey Korytov, UF 19 February 2008, Cambridge 57
MSSM Higgs or SM Higgs?
Decays to SUSY:
• h22
(2l1)+(2l1
)
• Signature:o Four leptonso Large MET
Msleptons=250 GeV ATLAS300 fb-1
BR for different channels: • R = BR(hWW) /
BR(h)• =|RMSSM-RSM|/expimental
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MSSM Higgs: yet another twist
CP-violation in Higgs sector
• complex couplings:o mass eigenstates H1, H2, H3
are mixtures of h, H, Ao production/decay modes
change
• new benchmark point CPX (maximum effect) suggested by Carena et al., Phys.Lett B495 (2000) 155
• new parameterization: MH± ; tan
• uncovered holes remain• more studies needed
ATLAS preliminary
o qqH, HWW, o bbH, Ho tbH± and tH±, H±o …
ATLASL=30 fb-1
not excludedat LEP
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Backup: ATLAS/CMS Summary Plots
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SM Higgs
CMS 2003 CMS 2006
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MSSM SM-like Higgs
ATLAS
CMS 2003 CMS 2006
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MSSM H and A
ATLAS
CMS 2003 CMS 2006
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ATLAS
MSSM H± (old/new plots)
CMS 2003 CMS 2006
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ATLAS