Lighting up the Higgs sector with photons at CDF

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Lighting up the Higgs sector with photons at CDF Baylor HEP Seminar 1 Karen Bland November 12, 2011

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Lighting up the Higgs sector with photons at CDF. Baylor HEP Seminar. Karen Bland November 12, 2011. Outline. Introduction Tevatron and CDF Detector Photon ID and Efficiency SM H  γγ Search Fermiophobic h  γγ Search Summary and Conclusions. Outline. Introduction - PowerPoint PPT Presentation

Transcript of Lighting up the Higgs sector with photons at CDF

Page 1: Lighting up the Higgs sector  with photons at CDF

Lighting up the Higgs sector with photons at CDF

Baylor HEP Seminar

1

Karen Bland

November 12, 2011

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Outline

• Introduction

• Tevatron and CDF Detector

• Photon ID and Efficiency

• SM Hγγ Search

• Fermiophobic hγγ Search

• Summary and Conclusions

2

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Outline

• Introduction– Theoretical Overview

– Motivation

• Tevatron and CDF Detector

• 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:

• Lower mass SM Higgs boson mass preferred by electroweak constraints

• Search focus: mH =114 – 145 GeV

• SM Higgs boson not to be discovered at Tevatron

• However through spring 2012 Tevatron, more sensitive in much of search region than LHC experiments4

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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:

• Lower mass SM Higgs boson mass preferred by electroweak constraints

• Search focus: mH =114 – 145 GeV

• SM Higgs boson not to be discovered at Tevatron

• However through spring 2012 Tevatron, more sensitive in much of search region than LHC experiments5

Tevatron experiments are still working very hard to

improve analysis techniques and add final datasets to fill in as much of the remaining gaps as

possible.

The Hγγ analysis contributes to this effort

Tevatron experiments are still working very hard to

improve analysis techniques and add final datasets to fill in as much of the remaining gaps as

possible.

The Hγγ analysis contributes to this effort

Hγγ contributes sensitivity hereHγγ contributes sensitivity here

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SM Higgs Production at the Tevatron

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Gluon Fusion

Associated Production

Vector Boson Fusion

• 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 TevatronProduced only rarely:

◦ One out of every 1012 collisions◦ That’s about 2 Higgs bosons

produced each week

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Gluon Fusion~ 1000 fb @ 120 GeV

Associated Production~ 225 fb @ 120 GeV

Vector Boson Fusion~ 70 fb @ 120 GeV

• 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)

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SM Hγγ Decay• Dominant low mass

decay mode is Hbb

• Hγγ Br < 0.25%

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• Signal expectation @ 120 GeV:N = σ × L × Br

= 1300fb × 7.0fb-1 × 0.002

~ 18 Hγγ events produced

(~ 6 reconstructed)

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• Small Br, however contributes sensitivity to Tevatron search in difficult region ~125 GeV:

• Many beyond SM scenarios include a larger Br(Hγγ)

• New results for one such scenario shown later in the talk

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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…

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Is a Hγγ search interesting at the Tevatron?

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• Great mass resolution:– Mass resolution limited only by electromagnetic (EM)

calorimeter and ability to select correct vertex of event (natural width negligible)

– 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 onsmoothly falling background

– Fits to non-signal region of mass spectrum can be used to estimate background

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σMγγ

≈ 2.5%

Is a Hγγ search interesting at the Tevatron?

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Outline

• Introduction• Tevatron and CDF

Detector• Photon ID and

Efficiency• SM Hγγ Search• Fermiophobic hγγ

Search• Summary and

Conclusions

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Tevatron

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• pp collisions at √s = 1.96 TeV• Peak luminosity 414×1032 cm-2s-1

• Shut down on Sept. 30th, 2011• 11.9 fb-1 delivered• 9.9 fb-1 stored on tape at CDF

• pp collisions at √s = 1.96 TeV• Peak luminosity 414×1032 cm-2s-1

• Shut down on Sept. 30th, 2011• 11.9 fb-1 delivered• 9.9 fb-1 stored on tape at CDF

Results shown here use 7.0 fb-1

Results shown here use 7.0 fb-1

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CDF Detector and Particle Identification

14Want a detector that can differentiate between

different types of final state particles

“Jets” come from quarks

or gluonsfragmenting

Muons long-lived and won’t interact in

calorimetry;leave trackin tracking

detector andmuon

chambers

Hadrons interact in calorimetry via cascades of nuclear interactions

(much more complex than EM cascades)

e’s and γ’s interact in calorimetry via electromagnetic cascades (i.e. ionization and

bremmstrahlung for e’s and photoelectric effect, Compton scattering, and pair production for γ’s)

Charged particles leave

a “track”in the tracking

chambers

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p

p

Silicon Vertex Detector

Central Tracker

Muon Chambers

ElectromagneticCalorimeterCDF Detector

Solenoid

Hadronic Calorimeter

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Outline

• Introduction

• Tevatron and CDF Detector

• Photon ID and Efficiency– Introduction

– Central Photons

– Forward Photons

– Conversion Photons

• SM Hγγ Search

• Fermiophobic hγγ Search

• Summary and Conclusions

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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 quadrant

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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

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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

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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

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I = ETTot (ΔR < 0.4) − ET

EM

ETEM

pTtrk

trk|z0−ztrk |< 5cmΔR< 0.4

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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

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I = ETTot (ΔR < 0.4) − ET

EM

ETEM

pTtrk

trk|z0−ztrk |< 5cmΔR< 0.4

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Central Photon Identification

• Relative to standard photon selection, increases signal efficiency by 5% and background rejection by 12%

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

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Trained using inclusive photon MC and jet MC (with ISR photons removed and energy reweighting)

s/sqrt(b) for Hγγ vs NN cut gives optimum cut of 0.74

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Central Photon ID Efficiency

• 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)

• 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!23

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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

• Net photon ID efficiency:– Data: 73.2%– MC: 80.6%

• MC scale factor of 90.7% applied• Total systematic uncertainty of 4.5%

24* Slides with EM energy or ET

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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

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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

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• 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 second analysis to use it for Run II

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

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r-ϕ separation (cm)

cotθ = pz/pT

cut ~94% efficient

cut ~95% efficient

ΔcotθExample trident

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Outline

• 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

converts29

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Primary Background Composition• Real SM photons

– Irreducible background

– Via QCD processes from hard interaction

• Fake backgrounds– Reducible backgrounds

– Electrons from Z/γ*e+e–

– Jets fragmenting to neutral mesons (π0/ηγγ) which then decay to pairs of colinear photons and are reconstructed as a single photons

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Data-Driven Background Model• Fit to non-signal (“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 to estimate

background expectation for Higgs mass hypothesis• Example shown here for a test mass at 115 GeV for CC channel

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Data-Driven Background Model• Channels with a plug photon have a non-neglible

contaminated from 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

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Background Model• Vertical red lines show window excluded from fit for Higgs mass

hypothesis 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

33CC ChannelCC Channel CC Conversion ChannelCC Conversion Channel

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Background Model

34CP ChannelCP Channel CP Conversion ChannelCP Conversion Channel

• Vertical red lines show window excluded from fit for Higgs mass hypothesis 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

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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 many times

• Largest upper and lower differences from standard fit stored

• Then symmetrized to obtain rate uncertainty for each test mass and channel

• Model dependence checked by testing alternate fit functions

• Variation in normalization as compared to standard found to be within uncertainties already obtained

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Approximate Systematic Errors on Background (%)

CC 4

CP 1

CC Conv 8

CP Conv 4

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CC Channel Discriminant and some Math Checks

• We show these distributions so you can check our results with some simple math…

• Use 12 GeV signal region, so as an example, here it would be 120 +- 6 GeV

• Real limits are much more complicated, but this is a rough check that results are in the right ball park

• N Signal (S) = 2.2, N Bkg (B) = 271 +- 16.5• 16.5 1σ statistical error on the background

expectation 68% of the time• For S = 2.2, obviously we’re not sensitive to

a SM Higgs observation• How many signal events would we be

sensitive to at say a 95% C.L?• 33 2σ about 95% data fluctuates

between 271 +- 33• The number 33 is simple 95% upper C.L.

limit on the amount of reconstructed signal we’re sensitive to based on bkg expectation alone

• We can excludes models with predict > 33 γγ decays at this mass @ 95% C.L.

• How much is this relative to the SM prediction?: 33/2.2 = 15 This is a simple 95% C.L. upper limit relative to the SM prediction

• Including systematic uncertainties degrades limits by about 10-15%

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Final Discriminants

37Limits add as ~ 1/Li2 similar to a || resister, where Li is limit for an individual channel

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SM Limits• Shown are 95% upper C.L. limits

on σ×Br relative to SM prediction using a Bayesian method

• Most sensitive expected limit is for 120 GeV where limit is ~13.0×SM

• An improvement of ~33% on last result presented!

• Improvements from better central photon ID, including forward photons, and reconstructing photon conversions

• Observed limit at ~28×SM above 2σ but we didn’t consider “look-elsewhere effect”

• With this affect considered, has less than 2σ significance

• This result included in Tevatron Higgs combination

• First SM Higgs result from CDF Run II to be published

• Has also been combined with D0 results to give a Tevatron SM Hγγ result…

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arXiv:1109.4427To be published in PRL

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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, pTγγ, ET

1, ET2, Δφγγ

• Example output shown for mass of 115 GeV

• From March 2011• Using 8.2fb-1

• Observed @ 115: 12.5xSM• Expected @ 115: 15.8xSM• PRL 107, 151801 (2011)

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Tevatron 95% C.L. Limits on Hγγ

For MH of 115 GeV Luminosity Expected/SM Observed/SM

Tevatron Hγγ Combo ≤ 8.2 fb-1 8.5 10.5 40

arXiv:1107.4960

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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…

• Also gain from higher cross sections, calorimeter resolution, and mass resolution

• Limited by Br (as at Tevatron) and multiple interactions (pileup)

• As of Sept, 2011 both CMS (CMS-PAS-HIG-11-021) and ATLAS (arXiv:1108.5895) have results in Hγγ of about 3-4xSM expectation:

• Tevatron is clearly not competitive with LHC in this channel… how about in BSM models?

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Outline

• Introduction

• Tevatron and CDF Detector

• Photon ID and Efficiency

• SM Hγγ Search

• Fermiophobic hγγ Search– Theory Motivation

– Differences in search from SM

– Results

– Tevatron Combination

• Summary and Conclusions

42

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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 Higgs

• We search for one in which:– No Higgs coupling to fermions– SM Higgs coupling to bosons– SM production cross sections assumed

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Gluon Fusion~ 1000 fb @ 120 GeV

Fermiophobic Higgs (hf) Production

No gghf

σ ~ 300 fb @ 120 GeV

44

Associated Production~ 225 fb @ 120 GeV

Vector Boson Fusion~ 70 fb @ 120 GeV

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Fermiophobic Higgs (hf) Decay

• hbb no longer dominant

45

Suppressed by m2b/m2

W

• 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 higher than SM @ 120 GeV

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Fermiophobic Higgs (hf) Decay

• hbb no longer dominant

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• Signal expectation @ 100 GeV:N = σ × L × Br

= 560fb × 7.0fb-1 × 0.18

~700 (245) hfγγ events produced (reconstructed)

~30x higher than SM expectation

Suppressed by m2b/m2

W

• Dominant low mass decay mode is now hγγ

Br ~ 120x higher than SM @ 100 GeV

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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

47

Greatest sensitivity!

Same as SM Search Different for hf search

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Background ModelExample fits for CC for each pT

γγ bin

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Same approach for background model as done for SM

High pTγγ Bin

At 120 GeV:N signal = 2.9s/sqrt(b) = 0.66

Medium pTγγ Bin

At 120 GeV:N signal = 2.5s/sqrt(b) = 0.37

Low pTγγ Bin

At 120 GeV:N signal = 1.3s/sqrt(b) = 0.09

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Results

• At 95% C.L., observed (expected) on 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 from a single experiment

• To be published in PRL with SM result

• Has also been combined with D0 results to give a Tevatron hfγγ result…

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arXiv:1109.4427To be published in PRL

Experiment mhf limit (GeV)

LEP 109.7

Previous CDF PRL (3.0 fb-1) 106

D0’s PRL result (8.2 fb-1) 112.9

CMS Prelim result (1.7 fb-1) 112

CDF new result (7.0 fb-1) 114

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DØ’s fermiophobic hf→γγ Search• Same as SM, but no ggH and

higher Br• Uses a boosted decision tree as final

hfγγ discriminant• Βased on five kinematic inputs: Mγγ,

pTγγ, ET

1, ET2, Δφγγ

• Example output shown for mass of 115 GeV

• From March 2011• Using 8.2fb-1

• Observed @ 115: 12.5xSM• Expected @ 115: 15.8xSM• PRL 107, 151801 (2011)

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Tevatron 95% C.L. Limits on hf

• This is what exclusion looks like • Tevatron results exclude fermiophobic Higgs

bosons with masses below 119 GeV at 95% C.L.• This is the world’s best limit (arXiv:1109.0576) 51

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Summary and Conclusions• CDF SM H and hf γγ soon to be published (we just got the bill)• Improvements at CDF upon previous analyses:

– Inclusion of forward and conversion photons– Better central ID from a NN– Including lower pT regions to hf search

• SM Hγγ search:– Not competitive with LHC, but contributes to final Higgs search at Tevatron– Tevatron Combination ~ 13xSM (observed) 9xSM (expected)

• Fermiophobic hfγγ:– Tevatron setting world’s best limit on this model @ mhf > 119 GeV– CDF limit of 114 GeV most stringent limit by a single experiment

• Both CDF analyses to be updated with full dataset and a few improvements, so stay tuned…

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Backup

53

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Indirect constraints• Precision electroweak observables are

sensitive to the Higgs boson mass via quantum corrections.

mH< 161 GeV (95% CL)

Stalking the Higgs BosonStalking the Higgs Boson

mH = 92−26+34 GeV

Direct searches at LEP• Tantalizing hints (~1.7σ) of a SM-like

Higgs boson with mH~115 GeV:

mH> 114.4 GeV (95% CL)

114.4 < mH< 161 GeV (95% CL)

Combining indirect and direct constraints

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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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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

Added to SM Higgs Tevatron

combination this past summer

57

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 58

φ1 =φ1

+

φ10

⎝ ⎜

⎠ ⎟

⟨φ1⟩=0

v1

⎝ ⎜

⎠ ⎟

φ2 =φ2

+

φ20

⎝ ⎜

⎠ ⎟

⟨φ2⟩=0

v2

⎝ ⎜

⎠ ⎟

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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

Photon ID and Efficiency – Conversion Photons 59

~24% of events

~72% of events

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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

Photon ID and Efficiency – Conversion Photons 60

R =N(Z →e + trid)

N(Z →ee)