Lighting up the Higgs sector with photons at CDF
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Transcript of Lighting up the Higgs sector with photons at CDF
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Lighting up the Higgs sector with photons at CDF
Fermilab Joint Experimental-Theoretical Seminar
Karen Bland
for the CDF collaboration
May 20, 20111
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Tevatron and CDF Performance
• Thanks to accelerator division for delivering luminosity!
• And CDF for keeping the detector running well!
• Delivered luminosity ~11.0 fb-1 !!
• CDF acquired luminosity: ~9.2 fb-1
• Using 7.0fb-1 for results shown here
2
7.0 fb-1
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Outline
3
• Introduction
• Photon ID and Efficiency
• SM Hγγ Search
• Fermiophobic hγγ Search
• Summary and Conclusions
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Outline
4
• Introduction– Theoretical Overview
– Motivation
• Photon ID and Efficiency
• SM Hγγ Search
• Fermiophobic hγγ Search
• Summary and Conclusions
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The Standard Model• Higgs boson is only SM particle that
hasn’t been observed!• Through the Higgs mechanism:
(1) Electroweak symmetry is broken(2) Other SM particles acquire mass
• But mass of Higgs boson a free parameter…• Has to be determined experimentally if exists• What we know so far:
• Tevatron exclusion regions mostly based on search channels other than Hγγ
• Hγγ plays a role in the low mass region…
• Which is the favored Higgs mass region from electroweak constraints
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Tevatron exclusionalso reaching LEP limit at low mass
Hγγ contributes sensitivity hereHγγ contributes sensitivity here
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Hγγ contributes sensitivity hereHγγ contributes sensitivity here
The Standard Model• Higgs boson is only SM particle that
hasn’t been observed!• Through the Higgs mechanism:
(1) Electroweak symmetry is broken(2) Other SM particles acquire mass
• But mass of Higgs boson a free parameter…• Has to be determined experimentally if exists• What we know so far:
6
Tevatron exclusionalso reaching LEP limit at low mass
• There is a lot of work being done still at the Tevatron experiments to extend this exclusion region, so stay tuned!
• A new combination will come out this summer
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SM Higgs Production at the TevatronGluon Fusion
Associated Production
Vector Boson Fusion
7Introduction: Theoretical Overview
• ggH is largest cross section• Excluded from channels where Higgs decays to quarks due to multijet backgrounds (like Hbb)
• ggH is largest cross section• Excluded from channels where Higgs decays to quarks due to multijet backgrounds (like Hbb)
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SM Higgs Production at the TevatronGluon Fusion
~ 1000 fb @ 120 GeV
Associated Production~ 225 fb @ 120 GeV
Vector Boson Fusion~ 70 fb @ 120 GeV
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Produced only rarely:◦ One out of every 1012 collisions◦ That’s about 2 Higgs bosons
produced each week
• ggH is largest cross section• Excluded from channels where Higgs decays to quarks due to multijet backgrounds (like Hbb)• Hγγ gains by using all three production
methods (~1300 fb)
• ggH is largest cross section• Excluded from channels where Higgs decays to quarks due to multijet backgrounds (like Hbb)• Hγγ gains by using all three production
methods (~1300 fb)
Introduction: Theoretical Overview
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SM Hγγ Decay• Dominant low mass
decay mode is Hbb
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• Hγγ Br < 0.25%
• Signal expectation @ 120 GeV:N = σ × L × Br
= 1300fb × 7.0fb-1 × 0.002
~ 18 Hγγ events produced
(~ 6 reconstructed)Introduction: Theoretical Overview
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• No SM Hγγ observation expected at the Tevatron!
• However, contributes sensitivity to Tevatron search in difficult region ~125 GeV
• Even has sensitivity comparable to ZHllbb ~140 – 150 GeV:– Br(Hbb) falls steeply for
mH > 140 GeV– Br(Hγγ) relatively flat– Translates to relatively flat
sensitivity compared to other channels
Introduction: Motivation 10
• Many beyond SM scenarios include a larger Br(Hγγ)
• New results for one such scenario shown later in the talk
Is a Hγγ search interesting at the Tevatron?
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• Clean signature compared to Hbb– Photons (or electrons from photon conversions) easier
to identify/reconstruct than b-jets– Larger fraction of Hγγ events accepted in
comparison– Total acceptance:
• ~35% accepted for ggH• ~30% accepted for VH and VBF
– Largest efficiency losses from fiducial requirements and ID efficiency
– Also improves reconstructed mass resolution…
11Introduction: Motivation
Is a Hγγ search interesting at the Tevatron?
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• Great mass resolution:– Mass resolution limited only by electromagnetic (EM)
calorimeter– 1σ width ~3 GeV or less
(Mjj width is ~16 GeV) – Resolution ~5x better than
best jet algorithms for Hbb– Great background
discrimination using Mγγ alone– Search for narrow resonance– Sideband fits can be used to
estimate background
12
€
σMγγ
≈ 2.5%
Introduction: Motivation
Is a Hγγ search interesting at the Tevatron?
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Outline
13
• Introduction• Photon ID and Efficiency
– Introduction– Central Photons– Forward Photons– Conversion Photons
• SM Hγγ Search• Fermiophobic hγγ
Search• Summary and Conclusions
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p
p
Silicon Vertex Detector
Central Tracker
Muon Chambers
ElectromagneticCalorimeterCDF Detector
Solenoid
Hadronic Calorimeter
Photon ID and Efficiency - Introduction
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Photon Identification• “Central”
– |η|<1.1
• “Plug”– 1.2<|η|<2.8– Tracking efficiency
lower than in central region
– Easier to miss a track and reconstruct fake object as a photon
– Higher backgrounds then for plug photons
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Central
Plug
Cross sectional view of one detector quadrantPhoton ID and Efficiency - Introduction
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Photon Identification
• Basic Photon Signature:– Compact EM cluster– Isolated– No high momentum track associated
with cluster– Profile (lateral shower shape)
consistent with that of a prompt photon
• Unlike that of π0/η γγ decays (the largest background for prompt photons)
• Hard to do this with calorimeters alone
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Inside jets
Background
Signal
Photon ID and Efficiency - Introduction
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Photon Identification• ΕΜ calorimeter segmentation:
– Δη×Δϕ ~ 0.1×15° (|η|<1)– Not fine enough to fully reject
π0/η jets
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Hadronic Calorimeter
Electromagnetic Calorimeter
Shower maximum detector
Signal Background
• Shower max detector– ~6 radiation lengths into EM
calorimeter– Finely segmented– Gives resolution to better reject
π0/ηγγ– Αlso refines EM cluster
position measurement to better match associated tracks
Photon ID and Efficiency - Introduction
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Central Photon Identification
• Three level selection• (1) Loose requirements
– Fiducial in shower max detector
– Ratio of hadronic to electromagnetic transverse energy (Had/EM) < 12.5%
– Calorimeter isolation• .• Cut slides with
– Track isolation < 5 GeV
18
€
I = ETTot (ΔR < 0.4) − ET
EM
€
ETEM
€
pTtrk
trk|z0−ztrk |< 5cmΔR< 0.4
∑
Photon ID and Efficiency – Central Photons
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Central Electron Identification
• Three level selection• (1) Loose requirements
– Fiducial in shower max detector
– Ratio of hadronic to electromagnetic transverse energy (Had/EM) < 12.5%
– Calorimeter isolation• .• Cut slides with
– Track isolation
– pTtrk1 < 5
GeV
19
€
I = ETTot (ΔR < 0.4) − ET
EM
€
ETEM
€
pTtrk
trk|z0−ztrk |< 5cmΔR< 0.4
∑
Photon ID and Efficiency – Central Photons
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• 98% signal efficiency (8% better than standard ID cuts)• 87% background rejection (23% better than standard ID cuts)
Central Photon Identification NN discriminant constructed
from seven well understood variables:– Ratio of hadronic to EM
transverse energy– Shape in shower max
compared to expectation– Calorimeter Isolation– Track isolation– Ratio of energy at shower
max to total EM energy– Lateral sharing of energy
between towers compared to expectation
20
s/sqrt(b) for Hγγ vs NN cut gives optimum cut of 0.74
Trained using inclusive photon MC and jet MC (with ISR photons removed and energy reweighting)
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• ID efficiency checked in data and MC from Ze+e– decays
• Z mass constraint applied to get a pure sample of electrons to probe
• Effect of pile-up seen through Nvtx dependence
• Net efficiencies obtained by folding εvtx into Nvtx distribution of diphoton data and signal MC (a weighted average)
Central Photon ID Efficiency
• Net photon ID efficiency: Data: 83.2% MC: 87.8%
• MC scale factor of 94.8% applied• Total systematic uncertainty of 2%
applied from:– Differences between electron vs photon
response (checked in MC)– Data taking period dependence– Fits made to Z mass distribution
• Small uncertainties using this method!21Photon ID and Efficiency – Central Photons
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Plug Photon ID and Efficiency
Standard CDF Cut-Based ID
• Fiducial in shower max detector
• Ratio of hadronic to EM transverse energy* < 5%
• Calorimeter isolation* < 2 GeV
• Track isolation* < 2 GeV
• Shape in shower max compared to expectation
Same Efficiency Technique as for Central Photons
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• Net photon ID efficiency:– Data: 73.2%– MC: 80.6%
• MC scale factor of 90.7% applied• Total systematic uncertainty of 4.5%
* Slides with EM energy or ET Photon ID and Efficiency – Foward Photons
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Photon Conversions
• γe+e–
• Colinear tracks moving in approximately same direction
• Occurs in presence of detector material• More material, higher the probability of
converting
23
COT inner cylinder
port cards, cables
ISL outer screen
L7
L6L00, L0-L4
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PhotonConversions
• Conversion probability at CMS substantially higher*…
• ~70% of Hγγ events have at least one photon that converts!!
• Similarly for ATLAS• Much more important at LHC
experiments! * J. Nysten, Nuclear Instruments and Methods in Physics
Research A 534 (2004) 194-198
Photon ID and Efficiency – Conversion Photons 24
• Use central only• Then for two photons, %
of events lost from a single central photon converting is:– 26% for CC channel– 15% for CP channel
• CDF had only one Run I measurement using converted photons: γ cross section PRD,70, 074008 (2004)
• Hγγ is the first Run II CDF photon analysis using conversions
p ≈ 15% for central γ
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Conversion ID
• Base selection:– |η|<1.1– Oppositely signed high quality tracks– Proximity: r- sep and Δcotθ– e + (γ e+e–) “trident” veto
photon radiated via bremmstrahlung• Other tighter selection on calorimeter
and tracking variables applied to further reduce backgrounds
• 7% uncertainty in conversion ID taken as systematic from Ze+trident studies
• Base selection:– |η|<1.1– Oppositely signed high quality tracks– Proximity: r- sep and Δcotθ– e + (γ e+e–) “trident” veto
photon radiated via bremmstrahlung• Other tighter selection on calorimeter
and tracking variables applied to further reduce backgrounds
• 7% uncertainty in conversion ID taken as systematic from Ze+trident studies
25
r-ϕ separation (cm)
cotθ = pz/pT
cut ~94% efficient
cut ~95% efficient
ΔcotθPhoton ID and Efficiency – Conversion Photons
Example trident
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Outline
26
• Introduction• Photon ID and Efficiency• SM Hγγ Search
– Event Selection– Background Modeling– Results– Tevatron Combination
• Fermiophobic hγγ Search
• Summary and Conclusions
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Event Selection• Inclusive photon trigger
– Single photon ET > 25 GeV– Trigger efficiency after offline selection obtained from trigger
simulation assuming zvtx = 0 and trigger tower clustering• Use photon ID as previously described• Photon pT > 15 GeV• Four orthogonal diphoton categories:
– Central-central photons (CC)– Central-plug photons (CP)– Central-central conversion photons (CC conv) where one
converts– Central-plug conversion photons (CP conv) where central
convertsSM Hγγ Search 27
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Signal Shapes• Widths ~3
GeV (or less) for each channel
• Use 2σ width to determine signal window 12 GeV
• Shapes used to fit for signal in the data when setting limits
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Systematic Uncertaintieson Hγγ Signal
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Primary Background Composition
• Regular Photon Backgrounds– Real SM photons via QCD
interactions– Jets faking a photon
(mostly from π0/ηγγ)– Misidentified electrons such
as in Drell-Yan Z/γ* e+e–
• Conversion backgrounds– Real SM photons converting– Photons from π0/η jets converting– Combinatorics– Prompt conversions from Dalitz
decays π0e+e–γ at small radius
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Data-Driven Background Model• Assume a null hypothesis• Fit made to sideband regions of Mγγ distribution• We use a 6 parameter polynomial times exponential to model
smooth portion of the data• Fit is then interpolated into the 12 GeV signal region• Example shown here for a test mass at 115 GeV for CC channel
31
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Data-Driven Background Model• CP and CP conversion channels also contaminated by Z
background• Breit-Wigner function added to smooth distribution to
model this, where mean and width are bounded in fit• Example shown here for a test mass at 115 GeV for CP
channel
32
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Background Model• Windowed fit shown to indicate Higgs mass region being tested• Interpolated fit used to obtain data-fit residuals• Used to inspect for signs of a resonance for each mass and channel• No significant resonance observed so will set limits on σ×Br(Hγγ)
33CC ChannelCC Channel CC Conversion ChannelCC Conversion Channel
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34CP ChannelCP Channel CP Conversion ChannelCP Conversion Channel
Background Model• Windowed fit shown to indicate Higgs mass region being tested• Interpolated fit used to obtain data-fit residuals• Used to inspect for signs of a resonance for each mass and channel• No significant resonance observed so will set limits on σ×Br(Hγγ)
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Background Rate Uncertainty
• Parameters of fit function varied within uncertainties to obtain a new test fit
• Integral in 12 GeV signal region calculated for test fit
• Repeated millions of times
• Largest upper and lower differences from standard fit stored
• Then symmetrized to obtain rate uncertainty for each test mass and channel
35
• Model dependence checked by testing alternate fit functions
• Variation in normalization as compared to standard found to be within uncertainties already obtained
Approximate Systematic Errors on Background (%)
CC 4
CP 1
CC Conv 8
CP Conv 4
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Mass Distributions
36
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12 GeV/c2 signal region for each test mass used to set upper limits set on σ×Br relative to SM prediction
Expected limit of 13.0xSM @ 120 GeV An improvement of ~33% on last result! Observed limit outside 2σ band @ 120 GeV,
but reduced to < 2σ after trial factor taken into account
37
Will be added to SM Higgs
Tevatron combination this summer
New Limits New Limits on on HHγγγγ at at CDF using CDF using
7.0/fb7.0/fb
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DØ’s SM H→γγ Search• Uses a boosted decision tree as
final Hγγ discriminant• Βased on five kinematic
inputs: Mγγ, pT
γγ, ET1, ET
2, Δφγγ
• Example output shown for mass of 120 GeV
• From March 2011• Using 8.2fb-1
• Observed @ 120: 12.4xSM• Expected @ 120: 11.3xSM
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• Observed @ 120: 16.9xSM
• Expected @ 120: 9.1xSM
• Combination significantly extends the sensitivity of the separate CDF/DØ results
• This is deepest existing investigation into this channel – a channel that’s very different from Hbb
39
• Reaching within one order of magnitude of SM prediction… for an analysis that wasn’t expected to happen at the Tevatron!
Tevatron SM Hγγ Combination
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Tevatron vs LHC
• Due to higher jet backgrounds, the LHC is betting on the Hγγ channel rather than Hbb for a low mass Higgs discovery…
• But how are LHC experiments currently doing compared to the Tevatron?
• CMS has no public results yet, so we can look at ATLAS
40
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ATLAS SM Hγγ
• First preliminary result uses 38pb-1
• 95% upper C.L. limits @ ~25xSM
41
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ATLAS SM Hγγ
• Here with 94pb-1 from 2011 only
• Here with 131pb-1 from both 2010 and 2011
42
Atlas expecting to be near ~4 times SM prediction with 1 fb-1
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Outline
43
• Introduction• Photon ID and Efficiency• SM Hγγ Search• Fermiophobic hγγ
Search– Theory Motivation– Differences in search
from SM– Results
• Summary and Conclusions
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Fermiophobic Higgs (hf)
• It’s likely nature doesn’t follow the SM Higgs mechanism…
• We also consider a “benchmark” fermiophobic model
• A two-Higgs doublet model extension to the SM
• Spontaneous symmetry breaking mechanism different for fermions and bosons 5 Higges
• We search for one in which:– No Higgs coupling to fermions– SM Higgs coupling to bosons– SM production cross sections assumed
Introduction 44
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Gluon Fusion~ 1000 fb @ 120 GeV
Fermiophobic Higgs (hf) Production
Introduction 45
Associated Production~ 225 fb @ 120 GeV
Vector Boson Fusion~ 70 fb @ 120 GeV
No gghf
σ ~ 300 fb @ 120 GeV
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Fermiophobic Higgs (hf) Decay
46Introduction
Suppressed by m2b/m2
W
• hbb no longer dominant
• Dominant low mass decay mode is now hγγ
• Signal expectation @ 120 GeV:N = σ × L × Br
= 300fb × 7.0fb-1 × 0.03
~63 (22) hfγγ events produced (reconstructed)
~4x higher than SM expectationBr ~ 13x (120x) higher than SM @ 120 GeV (100 GeV)
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Fermiophobic Higgs (hf) Decay
47Introduction
• hbb no longer dominant
• Dominant low mass decay mode is hγγ
• Signal expectation @ 100 GeV:N = σ × L × Br
= 560fb × 7.0fb-1 × 0.18
~700 (245) hfγγ events produced (reconstructed)
~30x higher than SM expectationBr ~ 13x (120x) higher than SM @ 120 GeV (100 GeV)
Suppressed by m2b/m2
W
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Event Selection
• Inclusive photon trigger– Single photon ET > 25 GeV– Trigger efficiency after offline
selection obtained from trigger simulation assuming zvtx = 0 and trigger tower clustering
• Use photon ID as previously described
• Photon pT > 15 GeV• Four orthogonal diphoton
categories:– Central-central photons (CC)– Central-plug photons (CP)– Central-central conversion photons
(CC conv) where one converts– Central-plug conversion photons
(CP conv) where central converts
• gghf suppressed• Optimize for VH/VBF• Split into three diphoton pt bins:
– High: pT> 75 GeV– Medium: 35 < pT < 75 GeV– Low: pT < 35 GeV
• 4 diphoton categories x 3 Pt bins = 12 total channels
48
Greatest sensitivity!
Same as SM Search Different for hf search
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Background ModelExample fits for CC for each pT
γγ bin
49
Same approach for background model as done for SM
High pTγγ Bin
N signal = 2.9s/sqrt(b) = 0.66
Medium pTγγ Bin
N signal = 2.5s/sqrt(b) = 0.37
Low pTγγ Bin
N signal = 1.3s/sqrt(b) = 0.09
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Results
• For comparison:• LEP Limit: 109.7 GeV• Previous CDF PRL result
(3.0/fb): 106 GeV• DØ’s recent (8.2/fb): 112 GeV
• Observed (expected) 95% C.L. limits on σ×B(hfγγ) exclude a Fermiophobic Higgs boson with a mass < 114 GeV (111 GeV)
• A limit of 114 GeV is currently the world’s best limit on a hf Higgs
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Summary and Conclusions
• Improvements for current results:– Inclusion of forward and conversion photons– Better central ID from a NN
• SM Hγγ search:– Observed (expected) 95% C.L. upper limits on σ×Br are
28.3xSM (13.0xSM) @ a mass of 120 GeV– Approximate 33% improvement from the 5.4fb-1 result
• Tevatron SM combination:– Shows significant gains when combined with DØ– Observed (expected) limits of 13.3 (8.5) at a mass of 120 GeV
• Fermiophobic hfγγ:– Excludes hf mass below 114 GeV (111 GeV) from observed (expected)
limits– A limit of 114 GeV is currently the best limits on hf mass
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Backup
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Used two central photons from cut-based ID 12 GeV/c2 signal region for each test mass used
to set upper limits set on σ Br relative to SM prediction
Expected and observed limits in good agreement Expected limits of 19.4xSM @ 120 GeV Most sensitive for range 110 – 130 GeV/c2
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Added to SM Higgs Tevatron
combination this past summer
Previous Previous Limits on Limits on
HHγγγγ at CDF at CDF using 5.4/fbusing 5.4/fb
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Fermiophobic Higgs (hf)
• It’s likely nature doesn’t follow the SM Higgs mechanism…
• We also consider a “benchmark” fermiophobic model
• A Two Higgs Doublet Model (2HDM) extension to SM:
• With vacuum expectation values:
• 5 Physical Higgs Particles: h0, H0, A0, H+, and H–
• 2HDM type-I– Scalar field mixing angle α
can lead to different couplings to fermions for h0 and H0
– sin(α) for H0 and cos(α) for h0
– Limit of απ/2 yields a Higgs with enhanced coupling to bosons: h0hf
• Standard model cross sections assumed
• Not present in MSSM1
1 Akeroyd, hep-ph/9511347, 1995 Introduction 54
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Tight Conversion ID
• Primary electron:– Fiducial– Had/EM < ~5.5%
• Secondary electron:– Fiducial– pT > 1.0 GeV
• Conversion photon– pT > 15 GeV– E/P ratio
(optimized for Ηγγ)– Calorimeter isolation
(optimized for Hγγ)– Rconv > 2.0 cm
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~24% of events
~72% of events
Photon ID and Efficiency – Conversion Photons
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Conversion ID Efficiency
• Search for “tridents” where oneelectron leg brems a photon whichthen converts
• Probed conversions of lower momentum range than those from Hγγ
• Obtain an uncertainty on conversion ID rather than MC scale factor
• In data and MC calculate ratio:
• Rdata/RMC 7% uncertainty
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Photon ID and Efficiency – Conversion Photons