search for the sm higgs boson in the h → γγ channel with cms at lhc marco pieri ucsd april 4 th...

Search for the SM Higgs Boson in the H→γγ Channel with CMS

at LHC

Marco Pieri

UCSD April 4th 2006

4-Apr-06 Search for the SM Higgs Boson with CMS Marco Pieri 2

Outline

LHC collider and CMS Detector

Higgs Searches at LHC

H→γγ search with CMS Trigger Ecal calibration Background simulation Photon isolation Vertex estimation Photon conversion identification and π0 rejection Selection and background measurement C.L. for discovery or exclusion Analysis optimization

Summary and Outlook


LHC Collider

First beams 2007 - Pilot Run

2008 start of physics Only ~1.5 years to the

first collisions Now mainly studying

the low luminosity phase

LHC operation (pp s =14 TeV) Low luminosity phase (Pile-up ~4

events/beam crossing) ℒ ~ 2 x 1033 cm-2s-1

Int ℒ ~ 30 fb-1

High luminosity phase (Pile-up ~20 events/beam crossing) ℒ ~ 1 x 1034 cm-2s-1

Int ℒ ~ 300 fb-1

CMSCMS

AtlasAtlas

LHCbLHCb

ALICEALICE


CMS Detector

MUON BARREL

Silicon MicrostripsPixels

ECAL Scintillating PbWO4 crystals

Cathode Strip Chambers Resistive Plate Chambers

Drift Tube Chambers

Resistive Plate Chambers

SUPERCONDUCTINGCOIL

IRON YOKE

TRACKER

MUONENDCAPS

Total weight : 12,500 tOverall diameter : 15 mOverall length : 21.6 mMagnetic field : 4 Tesla

HCAL Plastic scintillator/brasssandwich

CALORIMETERS


CMS Magnet

Superconducting coil: length 13m, diameter 5.9 m

Magnetic field 4 Tesla Energy stored 2.5

GJoule Coil has been cooled

down to 4.5K


DIRECT SEARCHES AT LEP GAVE NEGATIVE RESULTS SM Higgs boson

MH>114.1 GeV @95% CL Some hints of possible Higgs signal were reported the last year of

LEP running MSSM neutral Higgs bosons

Mh, MA>92.9, 93.3 GeV @95% CL

Current Status of Direct Higgs Searches


INDIRECT CONSTRAINTS ON THE SM HIGGS BOSON

SM Electroweak fits to all high Q2 measurements give: MH=89+42

-30 GeV MH<207 GeV @ 95% CL

(taking into account direct LEP limit)

Other Constraints on Higgs Mass

MSSM HIGGS BOSON In the MSSM Mh ≲ 135 GeV In the decoupling limit, for MA≳150 GeV

h behaves like HSM

Standard model searches directly apply H→γγ channel is the most sensitive at LHC


SM Higgs Production at LHC

NLO Cross sections M. Spira et al.

gg fusion

IVB fusion


SM Higgs Decays

When WW channel opens up pronounced dip in the ZZ BR

…


SM Higgs Search Channels

ProductionDECAY

Inclusive VBF WH/ZH ttH

H → γγ YES YES YES YES

H → bb YES

H → ττ YES

H → WW* YES YES YES YES

H → ZZ*, Z ℓ+ℓ-, ℓ=e,μ YES YES YES YES

Low mass MH ≲ 200 GeV

H → γγ and H → ZZ* → 4ℓ are the only channels with a very good mass resolution ~1%


Current status of CMS analysis

Physics TDR (Technical Design Report for the LHCC)

End of last year Volume I Detector performance,

calibration, analysis tools Now Volume II

Physics channels (more or less detailed)

H→γγ search is being finalized these days

End of the year Volume III Startup physics and activities All what must be done to start

CMS data taking and analysis

Continuous effort on software, data management and computing The readiness of this (together

with hardware performance) will determine the early success of the experiment


forward jets

Photons from Higgs decay

qqH → qqγγ MH = 120 GeV

H→ γγ Signal

SIGNAL: two isolated photons with large Et

Gluon-gluon fusion WW and ZZ fusion (Weak Boson Fusion) WH, ZH, ttH (additional leptons) Total σ x BR ~90 fb for MH = 110-130 GeV Very good mass resolution better than 1%

H → γγ MH = 115 GeV Jets from qq are at

high rapidity and large Δη


H→ γγ Background

BACKGROUND ‘irreducible’ backgrounds, two real photons

gg→ γγ (box diagram) qq→ γγ (born diagram) pp→ γ+jets (2 prompt γ)

‘reducible’ backgrounds, at least one fake photons pp→ γ+jets (1 prompt γ + 1 fake γ) pp→ jets (2 fake γ) pp→ ee (Drell Yan) when electrons are mis-identified as photons

Process Pthat (GeV) Cross section (pb) Events/1 fb-1

pp→γγ (born) >25 82 82K

pp→γγ (box) >25 82 82K

pp→ γ+jets >30 90x104 90M

pp→jets >25 1x108 1x1011

Drell Yan ee - 4x103 4M


Cross section and K-factors

Signal cross sections and BR (NLO M. Spira)

K-factors for the background

pp→γγ (born) 1.5

pp→γγ (box) 1.2

pp→ γ+jets (2 prompt) 1.72

pp→γ+ jets (1 prompt+ 1 fake) 1

pp→jets 1

M=115 GeV M=120 GeV M=130 GeV M=140 GeV M=150 GeV

σ (gg fusion)(pb) 39.2 36.4 31.6 27.7 24.5

σ (IVB fusion) (pb) 4.7 4.5 4.1 3.8 3.6

σ (HW, HZ, Hqq) (pb) 3.8 3.3 2.6 2.1 1.7

Total (pb) 47.6 44.2 38.3 33.6 29.7

BR (H→ γγ) 2.08x10-3 2.21x10-3 2.24x10-3 1.95x10-3 1.40x10-3

Inclusive σ x BR (fb) 99.3 97.5 86.0 65.5 41.5


Outline of the Analysis

Trigger (Level-1+High Level Trigger) Calibration of the electromagnetic calorimeter

Background Simulation

Photon isolation Vertex estimation Conversion identification π0 rejection

Selection, background estimation C.L. extraction for discovery or exclusion Effect of systematic errors Analysis optimization

Results


CMS Trigger Design – Two stages

Level 1 hardware trigger

High Level Trigger (HLT)

“Offline” code running on PC farm

40 MHz

100 kHz

1 Tbit/s

100 Hz

Beam crossing rate 40 MHzInteraction rate 1 GHzMax Level 1 trigger rate 100 kHzEvent size 1 MByte


DAQ ArchitectureR

ead

ou

t B

uild

ers

12.5 kHz +12.5 kHz +12.5 kHz

Dat

a to

su

rfac

e

Aggregate data flow ~1 Tbit/s In the process of ordering PCs and network elements


Electromagnetic trigger towers are classified in three categories depending on the energy deposition in the calorimeter trigger towers: unidentified, non-isolated, isolated.

Single isolated Et>23 GeV

Double isolated Et>12 GeV

Double non-isolated Et>19 GeV

Total electron+photon Level-1 trigger rate: 4.4 kHz Level-1 trigger efficiency for H→ γγ larger than 99.5%

Level-1 Trigger


H → γγ signal has two isolated photons Dominant background from di-jets and γ+jet has at least one candidate

from jet fragmentation that is not well isolated

We keep early conversions in the double stream

HLT trigger efficiency 88% Trigger is relatively easy for H→ γγ because of high Et photons Total rate for photons after HLT ~6 Hz

High Level Trigger for Photons

HLT photon selection


ECAL Calibration

Use electrons from W→eν decays Match the measured ECAL cluster energy with the electron momentum

measured in the silicon tracker

Investigating alternative methods that use π0 or η

Calibration precision in barrel Effect on Higgs mass


Events must be pre-selected events at the generator level and we only simulate and reconstruct those events that more likely pass the analysis selection

For jets also the generation is a very heavy task Generator level selection:

charged track isolation at generator level allow more than em interacting particle to contribute to the energy of the

photon candidate Results of the selection are verified after simulation and reconstruction

using data samples generated with looser pre-selection

BG Simulation

Background coming from pp→jets and pp→γ+jets contributes more than half the total background for H→γγ search

The cross section is huge for the process pp→jets

Process Pthat cut (GeV) σ (Pythia LO) Events per 1 fb-1

pp→γ+jets >25 9.0x104 pb 9.0x107

pp→jets >35 1.0x108 pb 1.0x1011


BG Simulation II

Pthat cut (GeV)

σ gen(pb)

σ sim(pb)

Red. factor

Ineff.parton cuts (%)

Ineff.particle cuts

Ineff. total

pp→γ+jet 25 9.0x104 2.6x102 350 0.1 3.4 3.5

pp→jets 30 1.8x108 4.4x103 41000 0.0 14.1 14.1

pp→jets 40 6.0x107 4.3x103 14000 1.3 14.1 15.4

pp→jets 45 3.7x107 4.3x103 8700 3.4 14.1 17.5

pp→jets 50 2.5x107 4.1x103 6000 6.5 14.1 20.6

With this selection it was possible to simulate 5 million jet events corresponding to 1 fb-1 integrated luminosity

Total pre-selection inefficiency ~20% Events lost are mainly with low Et photons, not very important


Primary Vertex Determination

LHC beams have a longitudinal spread of 7.5 cm Longitudinal interaction spread ~5 cm Vertex estimated from the underlying event and recoiling jet We use a combination of the two methods:

Maximal scalar sum of tracks pt

Maximal vector sum of tracks pt

The efficiency of determining the right vertex is ~83% Higgs events after selection

Efficiency for the different types of background is similar Can also use identified converted photons – not done yet


Primary Vertex Determination II

Process Eff (%)

H→γγ (gg fusion) 82

H→γγ (IVB fusion) 89

pp→γγ (born) 71

pp→γγ (box) 72

pp→γ+jet (2 prompt) 78

pp→γ+jet (1 prompt + 1fake) 86

pp→jets 90

Efficiency of determining the primary vertex within 5 mm from the true one


Photon Conversions

CMS tracker is rather thick ~30% of the photons convert before reaching the ECAL Energy resolution somewhat deteriorated Conversions can be identified in the tracker Identified conversions may also help to distinguish γ’s from π0’s

Total2 tracks1 track


R9: Sum of 9/cluster energy

Unconverted photons have large R9

Converted photons, photons in jets and electrons have small R9

π0’s also have small R9

Energy resolution is better for large R9

Shower Shape Variables


0.88<R9< 0.95

Photon Energy Resolution

σfit=0.64%

σfit=1.35%

σfit=1.91%

σfit=0.92%

σfit=1.02%

σfit=0.78%

Barrel Endcaps

R9>0.95

R9<0.88

Photons with Et > 40 GeV


Photon Isolation

Reducible backrounds (π0’s and mis-identified jets) have other particles near at least one photon candidate

Most of discriminating variables are built by summing up the Et or Pt of calorimeter deposits or tracks within a cone

ΔR = (Δη2+ Δφ2)

To study the performance of isolation variables we use individual photon candidates

Signal is: H→γγ gg-fusion 2nd highest Et cluster with Et>40 GeV matched with a generated photon within ΔR<0.2, background is: γ+jet 2nd highest Et super-cluster with Et>40 GeV NOT matched with a generated photon

Plot the distribution of the variables for signal and background, move the cut and compute Eff. Sig and Eff. Background

ΔR


Combined Isolation Performance Plot

EndcapsBarrel

The sum of ECAL, HCAL and Track Et variables can be considered as a global isolation variable

Cone sizes and Et thresholds have been optimized for the different detectors


All isolation variables are more or less correlated We used a Neural Network with 2, 3 or 5 of following inputs:

ΔR of the 1st track with Pt>1.5 GeV/c Sum ECAL Et within ΔR<0.3 The shower shape variable R9

Sum HCAL Et within ΔR<0.35 Sum tracks Et within ΔR<0.2

Did not use kinematical information, easy to combine these variables with reconstructed mass and photons Et in an optimized H→γγ analysis

Isolated Photon Identification with NN


Selection for Inclusive Analysis

Photon selection: photon candidates are reconstructed using the hybrid clustering algorithm in the barrel and the island clustering algorithm in the endcaps ET1, ET2 > 40, 25 GeV |η|<2.5 Both photon candidates should match L1 isolated triggers

with ET > 12 GeV within ΔR < 0.5 Track isolation

No tracks with pt>1.5 GeV present within ΔR<0.3 around the direction of the photon candidate

Calorimeter isolation Sum of Et of the ECAL basic clusters within 0.06<ΔR<0.35

around the direction of the photon candidate <4 GeV Sum of Et of the HCAL towers within ΔR<0.3 around the

direction of the photon candidate<8 GeV(6 GeV) in barrel (endcaps)

Sum of the 2 less than 10 GeV(8 GeV) in barrel (endcaps)

L1 + HLT inefficiency negligible after selection


Mass Spectrum of Selected Events

All plots are normalized to an integrated luminosity of 1 fb-1 and the signal is scaled by a factor 10

Fraction of signal is very small (signal/background ~0.1) Use of background MC can be avoided when we will have data Data + signal MC can be used for optimizing cuts, training NN and

precise BG estimation


Results

Higgs efficiency for MH=120 GeV

Background expectation at MH=120 GeV

Before isolation BG expectation ~ 300 times larger

Box(fb/GeV)

Born(fb/GeV)

γ+ jets 2 prompt (fb/GeV)

γ+ jets 1 prompt+ 1 fake(fb/GeV)

Jets(fb/GeV)

Total(fb/GeV)

H→γγ (MH=120 GeV) eff. in window of 2.5 GeV (%)

32 46 62 51 64 255 24.4

MH After photon selection Final

120 GeV 52.7% 34.4%


Discovery Significance/Exclusion C.L.

Use Log Likelihood Ratio frequentistic approach Log likelihood ratio between the signal+background hypothesis

and the BG only hypothesis

Mean/Median BG only Mean/Median Sig + BG


Effect of Systematic Errors

Input for CL calculation is: Background expectation from fit to the data (sidebands) Signal expectation from MCOrigin of systematic errors Error on the BG estimation (statistical from fit of sidebands +

uncertainty of the form of the fitted function) Error on the signal (theoretical σxBR, integrated luminosity,

detector + selection efficiency)Effect of systematic errors Systematic errors on the signal does not change the expected

discovery CL Systematic error on the signal makes exclusion more difficult Systematic error on the BG makes exclusion and discovery

more difficult


Main Systematic Errors

SIGNAL Theoretical error on cross

section times BR (~15%) Integrated luminosity (5%) Higgs Qt distribution – effect

to be evaluated Selection efficiency (≲5%)

For now assume a total of 20% (anyway not important in case of discovery)

BACKGROUND Statistical error on the fit of

the sidebands (~0.3% for ~20 fb-1)

Systematic error on the shape of the fitted function (~0.3%)

No other errors when data available


Large effect on the 5σ discovery of the systematic error on the background (due to the small s/b in this channel)

A signal/background cut is applied at 0.02

Results for Discovery/Exclusion

MH=120 GeV 3σ evidence

Int L (fb-1)

5σ discovery

Int L (fb-1)

95% exclusionInt L (fb-1)

Counting 11.3 31.5 3.6

Mass distribution 10.7 30 3.4

Counting WITH 0.5% error on BG, 20% Error on Signal

13.2 53.5 4.2

Mass distribution WITH 0.5% error on BG , 20% Error on Signal

12.7 47 4.0


How to Improve the Sensitivity

We consider the following: ‘Reducible’ background tends to have smaller R9

‘Reducible’ background contribution is larger in the endcaps Split the sample into 4 by using the 4 combinations of the

requirements: Both photons in the barrel or not min(R91, R92) larger or smaller than 0.93

this is equivalent to having different channels

Can also split more the sample by dividing the ECAL into 4 pseudo-rapidity regions and for each region have progressively tightening cuts

When increasing the number of categories the uncertainty on the BG fit should approximately increase by (number of categories)

The uncertainty is basically uncorrelated between the different categories


Four Categories

Barrel, min(R91, R92)>0.93

Endcap, min(R91, R92)>0.93 Endcap, min(R91, R92)<0.93

Barrel, min(R91, R92)<0.93


Discovery/Exclusion with More Categories

MH=120 GeV 3σ discovery

Int L (fb-1)

5σ discovery

Int L (fb-1)

95% exclusionInt L (fb-1)

4 Categories 8 22 2.616 Categories 6.6 18.5 2.2

4 Categories WITH 1% error on BG, 20% Error on Signal

8.8 30 3.1

16 Categories WITH 1.5% error on BG, 20% Error on Signal

7 21.5 2.6

The overall effect of splitting into more categories is a decrease of the effect of the systematic error on the CL (mainly because of the increase of s/b for a fraction of events)


Discovery/Exclusion with 16 Categories

With ~25 fb-1 we can discover the Higgs boson with mass between 115 and 140 GeV


Optimized Analysis

Can do better using the optimized signal/background ratio ordering method technique

I introduced this method in L3 for Higgs searches and has been later used for all Higgs searches at LEP

First of all split sample into 6 categories by using the 6 combinations of the requirements: Both photons in the barrel or not min(R91, R92) larger than 0.95, between 0.90 and 0.95 and

smaller than 0.90 Then, for barrel and endcap events use a NN with the following

mass independent input variables: Isolation NN γ1

Isolation NN γ2

Et1/M Et2/M PLHiggs

|Δη| Train the Neural Network using data as background outside the

mass region under study


NN Output and Mass

mass distribution

category 1category 1

Background

Signal

category 1category 1

NN output without mass information

Compute log(s/b) for each event Add the NN output and Mass log(s/b) Than combine all 6 categories into one single plot Use LLR frequentistic method to evaluate the sensitivity


Results of Optimized Analysis

Now get 5σ significance at MH= 120 GeV with an integrated luminosity of 7 fb-1

Important source of improvement is the exploitation of high s/b Weak Boson Fusion events

Expect some degradation from error on the background but less than in the conventional analysis

4% s/b cut


Summary and Outlook

SM Higgs boson (or MSSM h) can be discovered with CMS in the H→γγ channel with 25 fb-1 at low luminosity, with a conventional cut based analysis in the mass range 115-140 GeV

Optimized likelihood analysis needs ~10 fb-1 luminosity for a 5σ signal in the same mass range

Further optimization Include additional tools for:

Photon conversion identification π0 rejection

Largest improvement expected from: Separation of other γγ channels, WBF, WH, ZH, ttH, exploiting their additional

signatures Then obviously from combination with H→ZZ*→4 lepton channels

Only one and a half years from the beginning of LHC operation, must continue to prepare the real analysis: based on data with minimal use of MC information using all possible control samples to verify the performances of the detector

We will hopefully discover soon a low mass Higgs boson at LHC


End of the talk

End of the talk


EXTRA


ECAL Calibration

5 fb-1


Test beam ECAL resolution


Shower Shape Variable R9

rr99=S9/E=S9/ESCSC

non-converted

Note that s/b also improves as we select photons that

didn’t convert.

Note that s/b also improves as we select photons that

didn’t convert.

Jet backgroundJet background

Signal

categories

For highest EtFor highest EtFor highest EtFor highest Et


Analysis flow for each category

Isolation NN photon 1

Isolation NN photon 2(same)

ET1/M

ET2/M

PLhiggs

Neural Net

Mass

SmoothBkgd

SmoothBkgd

Bin Signal andBkgd In LLR(same way)

Define a function of the mass and the NN output and plot events


NN training

Use the background outside the mass region + signal MC When data available use data for the BG

Train

Validate Validate

Train


Combined Signal/Background Variable

ln(s/b) from gg mass

From binned histogram.

ln(s/b) from gg mass

From binned histogram.

ln(s/b) from Neural Net

From fits to s and b.

ln(s/b) from Neural Net

From fits to s and b.

XXXX

====Log Likelihood per

eventLog Likelihood per

event

Rapidly falling Rapidly falling background background

distribution in region distribution in region with significant with significant

signal. signal.

Rapidly falling Rapidly falling background background

distribution in region distribution in region with significant with significant

signal. signal.


WBF analysis

May tag forward jets and apply the additional following cuts pT

jet > 20 GeV

|jet| < 4.5

Rjet > 0.5

|jet1 –jet2| > 4.0

jet1 *jet2 < 0

pTHiggs> 50 GeV and

Mj1j2> 500 GeV

CompHEP background samples + 3 jets and + 2 jets were produced

PYTHIA underestimates QCD and prompt photon backgrounds when two forward jets are detected

We need more statistics to improve our results with CompHEP


WBF Results

Integrated luminosity needed to reach 5 significance for three different scenarios


WBF PtHiggs


WBF Highest Pt Jet


Higgs boson width


H → ZZ* → 4ℓ

H → ZZ* → ℓ+ℓ-ℓ+ℓ- ℓ=e,μ Irreducible background:

ZZ production Reducible backgrounds

tt and Zbb Very good mass

resolution ~1%

In this channel (and in the H→ γγ) background can be easily estimated from data by fitting the sidebands

Above MH ~ 2MZ the two Z bosons are real and σxBR is larger

Golden channel for Higgs discovery at LHC

Branching ratio dip due to opening of WW channel


Low mass discovery

With 30 fb-1 more than 5 sigma significance for MH>100 GeV Higgs boson can be discovered in more than one channel, possible to

measure its couplings


Whole mass range discovery

All mass range accessible at 5σ significance with 10 fb-1

With a few fb-1 possible to discover the Higgs boson with mass between ~150 and ~500 GeV in the WW and ZZ channels

For mass larger than ~200 GeV use ZZ and WW leptonic decays For mass larger than ~700 GeV use qqH, H → ZZ → ℓℓνν and H → WW → ℓνqq


After discovering the Higgs boson we should measure its parameters Studies for high luminosity (Int L = 300 fb-1)

SM Higgs boson mass direct reconstruction: 4ℓ, γγ, bb likelyhood fit WW

Measurement of Higgs bosons parameters

INT L = 300 fb-1


MSSM Higgs Searches

Two Higgs doublets model 5 Higgs bosons: 2 Neutral scalars h,H 1 Neutral pseudo-scalar A 2 Charged scalars H±

In the Higgs sector all masses and couplings are determined by two independent parameters

Most common choice: tanβ – ratio of vacuum

expectation values of the two doublets

MA – mass of pseudo-scalar Higgs boson

In the MSSM: Mh ≲ 135 GeV


Neutral MSSM Higgs bosons

Decoupling limit (MA≳150 GeV) h behaves like HSM

Standard model searches directly apply MH~MA~MH

±

MA=O(MZ) and large tanβ H behaves similarly to SM Higgs (SM searches apply)

In other cases for large tanβ h(H) → WW,ZZ highly suppressed (A → WW,ZZ never allowed at tree

level) h(H),A almost exclusively decay into bb and ττ and are produced in

association with bb pair Large MA small tanβ

H,A decays almost 100% into tt for lower masses (200-300 GeV) also H → hh and A → Zh

If SUSY particles are light the Higgs bosons may decay into s-particles


h,H production and decay

Decoupling region

Large tanβ mainly

bb, ττ decays Large tanβ hbb, Hbb (and

Abb) production dominates

h,H decays h,H productiontanβ = 30


Results from SM Higgs Searches

In a large part of the MSSM parameter space SM Higgs searches are effective to find the MSSM h boson

In the decoupling region if h observed hard to distinguish SM from MSSM

CMS 5σ discovery contours


MSSM h,H decays

Decoupling region

Large tanβ

bb, ττ decays

Small tanβ H decays into tt when allowed


MSSM Production processes

Large tanβ hbb, Hbb and

Abb production dominates


Higgs Bosons visibility in the MSSM

All the plane is covered but there is a large area where only h can be seen

4 Higgs observable

3 Higgs observable

2 Higgs observable

1 Higgs observable

5σ discovery regions in the MSSM tanβ – MA plane for MH

MAX scenario


Beam Spread

LHC beams have a longitudinal spread of 7.5 cm Longitudinal interaction spread ~5 cm Vertex estimated from the underlying event and recoiling jet

Can also use identified converted photons – not done yet

Higgs signal (MH=120GeV)


Vertex Determination

We use a combination of the two methods: Maximal scalar sum of tracks pt

Maximal vector sum of tracks pt

The efficiency of determining the right vertex is ~83% Higgs events after selection

Efficiency for the different types of background is similar

Process Eff (%)

H→γγ (gg fusion) 82

H→γγ (IVB fusion) 89

pp→γγ (born) 71

pp→γγ (box) 72

pp→γ+jet (2 prompt) 78

pp→γ+jet (1 prompt + 1fake) 86

pp→jets 90

Efficiency of determining the primary vertex within 5 mm from the true one

search for the sm higgs boson in the h → γγ channel with cms at lhc marco pieri ucsd april 4 th...

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