Lighting up the Higgs sector with photons at CDF Fermilab Joint Experimental-Theoretical Seminar...

Lighting up the Higgs sector with photons at CDF

Fermilab Joint Experimental-Theoretical Seminar

Karen Bland

for the CDF collaboration

May 20, 20111

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

Outline

3

• Introduction

• Photon ID and Efficiency

• SM Hγγ Search

• Fermiophobic hγγ Search

• Summary and Conclusions

Outline

4

• Introduction– Theoretical Overview

– Motivation

• Photon ID and Efficiency

• SM Hγγ Search



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

5

Tevatron exclusionalso reaching LEP limit at low mass

Hγγ contributes sensitivity hereHγγ contributes sensitivity here

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

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)

SM Higgs Production at the TevatronGluon Fusion

~ 1000 fb @ 120 GeV

Associated Production~ 225 fb @ 120 GeV

Vector Boson Fusion~ 70 fb @ 120 GeV

8

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

SM Hγγ Decay• Dominant low mass

decay mode is Hbb

9

• 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

• 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?

• 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


• 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


Outline

13

• Introduction• Photon ID and Efficiency

– Introduction– Central Photons– Forward Photons– Conversion Photons

• SM Hγγ Search• Fermiophobic hγγ

Search• Summary and Conclusions

p

p

Silicon Vertex Detector

Central Tracker

Muon Chambers

ElectromagneticCalorimeterCDF Detector

Solenoid

Hadronic Calorimeter

Photon ID and Efficiency - Introduction

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

15

Central

Plug

Cross sectional view of one detector quadrantPhoton ID and Efficiency - Introduction

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

16

Inside jets

Background

Signal


Photon Identification• ΕΜ calorimeter segmentation:

– Δη×Δϕ ~ 0.1×15° (|η|<1)– Not fine enough to fully reject

π0/η jets

17

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


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

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pTtrk

trk|z0−ztrk |< 5cmΔR< 0.4

∑

Photon ID and Efficiency – Central Photons

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

• 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)

• 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

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

22

• 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

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

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

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

Outline

26

• Introduction• Photon ID and Efficiency• SM Hγγ Search

– Event Selection– Background Modeling– Results– Tevatron Combination



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

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

28

Systematic Uncertaintieson Hγγ Signal

29

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

30

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

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

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

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)

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

Mass Distributions

36

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

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

38

• 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

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

ATLAS SM Hγγ

• First preliminary result uses 38pb-1

• 95% upper C.L. limits @ ~25xSM

41

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

Outline

43

• Introduction• Photon ID and Efficiency• SM Hγγ Search• Fermiophobic hγγ

Search– Theory Motivation– Differences in search

from SM– Results


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

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

Fermiophobic Higgs (hf) Decay

46Introduction

Suppressed by m2b/m2

W

• hbb no longer dominant

• Dominant low mass decay mode is now hγγ


= 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)

Fermiophobic Higgs (hf) Decay

47Introduction

• hbb no longer dominant

• Dominant low mass decay mode is hγγ


= 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

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

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


Low pTγγ Bin


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

50

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

51

Backup

52

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

53

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

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

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

56

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R =N(Z →e + trid)

N(Z →ee)

Photon ID and Efficiency – Conversion Photons

Lighting up the Higgs sector with photons at CDF Fermilab Joint Experimental-Theoretical Seminar...

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