the discovery potential of the higgs boson at cms in the four lepton final state

David FutyanUC Riverside

1Higgs to Four Leptons at CMS18th May 2006

The Discovery Potential of the Higgs Boson at CMS in the Four Lepton Final

State

The Discovery Potential of the Higgs Boson at CMS in the Four Lepton Final

State

David Futyan

UC Riverside



Overview

Introduction:LHC and CMS

Motivation for Higgs boson searches

Decay channels observable at the LHC

Signal and background processes:Cross-sections and branching ratios

Event generation and simulation

Online selection

Offline reconstruction of electrons and muons

Offline event selection

Evaluation of background from data

Significance with background systematics

Potential for measurement of Higgs boson properties:Mass, width, cross-section

Experimental systematic uncertainties



The LHC (Large Hadron Collider)

CERNCERN

CMSCMS

SwitzerlandSwitzerland

FranceFranceALTASALTAS

LHCbLHCb

ALICEALICE

GenevaGeneva

Proton-proton collider:√s = 14 TeV

Luminosity = 1034 cm-2s-1

17 miles in circumference

Due to begin operation summer 2007



The CMS Detector (Compact Muon Solenoid)

Muon Chambers

4 Tesla SuperconductingSolenoid

Silicon Tracker

Silicon Pixel Detector

HCAL

Crystal ECAL

Magnet Return Yoke

General purpose detector

Over 2000 people from

160 institutes

12500 tonnes

General purpose detector

Over 2000 people from

160 institutes

12500 tonnes



CMS Detector Slice



CMS Under Construction



The Higgs Boson

A key objective of the LHC is to elucidate the origin of mass.

Higgs mechanism:Provides an explanation for electroweak symmetry breaking in the Standard Model:

• Gives rise to the massive Z and W vector bosons and the massless photon.

=>Lies at the core of the Standard Model - without the Higgs mechanism the SM is neither consistent nor complete.

Provides mechanism through which gauge bosons and fermions acquire mass.

Predicts the existence of one physical scalar, neutral Higgs boson.



Higgs Boson Mass Constraints

Higgs boson mass not predicted by the theory - free parameter of the standard Model:

Must be determined experimentally.

Current limits:Combined lower limit from direct searches

at LEP: mH > 114.4 GeV/c2 (95% CL).

Higgs boson contributes to radiative

corrections to electroweak observables.

Consistency fits to electroweak

precision measurements from LEP, SLC,

Tevatron yield an indirect upper limit:

mH < 207 GeV/c2 (95% CL).



The Higgs Boson Production at the LHC

Dominant production mechanisms:

Gluon-gluon fusion contributes around 80% of the total

Decay channels:



Decay channels of SM Higgs boson which yield highest sensitivity for discovery at the LHC:

H W+W- 2l2

H ZZ(*) 4l

H γγ

LHC Search Channels for the Higgs Boson

l = electron or muon

LEP Limit = 114.4 GeV/c2

EX

CL

UD

ED



The HZZ4l Channel

Most sensitive channel for the discovery of the Higgs boson at the LHC for a wide range of masses.

Exceptionally clean signature of 4 isolated high pT leptons, with relatively

small backgrounds.

For mH>2mZ: “Golden channel”, with 2 real Z bosons

Mass of Higgs boson can be directly reconstructed from the invariant mass of the 4 leptons

Direct measurement of mass

Direct measurement of width for large mH (>200 GeV)



General Strategy

Detailed analyses have been developed largely independently for each of the three final states:

4e: LLR (France), Split (Croatia), Rome/INFN

4Florida, FNAL, Cambridge,

2e2: UC Riverside, Bari/INFN (Italy)

Common to all channels (allows coherent combination of results):Event generation, detector simulation.

Signal and background production and decay processes considered and their NLO cross-sections and branching ratios.

Straight forward counting experiment approach - Cut based analyses:

• Look for local event excess over expected background.

Details of event selections developed differently in each of the 3 channels.

Analyses are designed as if real data were being analyzed:Full detailed simulation of CMS detector geometry and response.

Simulation of LHC conditions in first years of running at L = 2×1033cm-2s-1.

Full treatment of systematic errors included in significance evaluation.

Techniques developed to measure the size of the residual background from LHC data.



Production Cross-Section and Branching Ratio

BR(HZZ(*)4l) is the branching ratio to a final state containing only e and including decay products.

Sum of gg fusion, WW fusion, ZZ fusion BR(HZZ(*)4l), including BR(e)



Enhancement for 4e and 4 Final States

For 4e and 4 final states, enhancement of signal cross-section due to constructive final state interference between like-sign electrons or muons originating from different Z(*) bosons:

Calculated usingCompHEP



Signal Monte Carlo Event Generation

Signal samples generated with PYTHIA for 18 mass points between 115 and 600 GeV.

Higgs production mechanisms simulated:gg fusion, WW fusion, ZZ fusion.

Z bosons forced to decay to e,with forced to decay to e10000 events generated per mass point, for each final state (4e, 4, 2e2)Events re-weighted to correspond to:

€

σ pp → H( ) × BR H → ZZ( ) × BR Z → ll( )2

€

BR Z → ll( ) = 0.101where



Background Processes

Reducible backgrounds:

qq/gg tt W+W-bb 4l + X (PYTHIA)

qq/gg (Z(*)/γ*) bb 4l + X (CompHEP interfaced with PYTHIA)

Irreducible non-resonant continuum background:

qq Z(*)/γ*)(Z(*)/γ*) 4l (PYTHIA)

Process σLO(pb)NLO

K-factorσNLO(pb)

ttW+W-bb - - 840

e+e-bb 115 2.40.3 276

bb 116 2.40.3 279

Z(*)/γ*)(Z(*)/γ*) 18.7KNLO(m4l)

+0.228.9

(Z(*)/γ*) bb {



t-channel dominates:90% m4l<2mZ, ~100% m4l>2mZ

s-channel simulated for 4 final state only

MCFM generator used to calculate an K-factor to account for all NLO processes:

Function of 4 lepton invariant mass:

Z(*)/γ*)(Z(*)/γ*) Background

Z(*)/γ*

Z(*)/γ*

l+

l-

l+

l-

q

q

Z(*)/γ*

Z(*)/γ*

q

ql-l-

l+

l+

q = u,d,s,c or b

LO cross-section 18.07pb (from MCFM generator):



Significant NNLO box diagram process

not included in the simulation:

TOPREX generator used to obtain ratio σ(ggZZ4l)/σ(qqZZ4l): ~20%

Total NLO cross-section = σLO (K(m4l) + 0.2) = 29pb (for average K(m4l)=1.35)

All events re-weighted at analysis level using this m4l dependent K-factor.

Z(*)/γ*)(Z(*)/γ*) Background

Z(*)/γ*

Z(*)/γ*g

g

l+

l-

l+

l-

q



Other Potential Backgrounds

Zcc can also give 4 leptons in the final state:Investigated with full detector simulation - found to be negligible

Other potential sources of background investigated at generator level:Wbb

Wcc

Single top

bbbb

bbcc

cccc

All found to be negligible

}All leptons non-isolated

}One or more fake leptons



Detector Simulation

Generated Monte Carlo events for all generated samples are passed through a highly detailed simulation of the CMS detector, including:

Precise simulation of the complete detector geometry: all material in the detector including cables, services etc.

Detailed simulation of the 4T magnetic field.

Full simulation of detector response for all detector components: information used as input to the analysis fully simulates real LHC data.

Generated events are mixed with pile-up events to simulate the LHC conditions at “low luminosity” (2×1033cm-2s-1)

Several inelastic pp collisions per bunch crossing

Corresponds to conditions during the initial phase of data taking.



Cross-Section Times Branching Ratio

Generator level kinematic preselection includes the final state lepton flavor requirement (4e, 2 or 2e2), plus generator level cuts:

Electrons: pT>5GeV, ||<2.5

Muons: pT>3GeV, ||<2.4

For 2e2 case:

tt Zbb ZZ

σfb 840x103 555x103 28.9x103

σ.BR.fb

744 390 37.0



4-lepton Invariant Mass After Generator Pre-selection

Same on linear scale

s-channel ZZ production

mH=140 GeV signal



Online Selection

LHC bunch crossing rate is 40MHz. Multiple events per bunch crossing

CMS Trigger consists of a Level-1 trigger followed by a High Level Trigger. HLT is a software trigger involving full reconstruction of physics objects.

Triggers chosen for HZZ4l channels:

Single triggers were also considered for the 2e2 channel but were found not to benefit the final significance.

Channel Trigger

single electron || double electron

Single muon || double muon

double electron || double muon



HLT Selection Efficiencies

tt: 0.399 ± 0.001Zbb: 0.661 ± 0.001ZZ: 0.896 ± 0.004

tt: 0.399 ± 0.001Zbb: 0.661 ± 0.001ZZ: 0.896 ± 0.004

For 4 channel, HLT efficiency is close to 100% for all samples

4e 2e2



Muon Reconstruction and Selection

Muons are reconstructed with high efficiency with CMS:

Require + and - reconstructed with pT>7 in the barrel and pT>13 in the

endcaps.

Require M(μ+μ-)>12GeV for all permutations (excludes low mass resonances).

These cuts have little effect on signal efficiency.



Electron Reconstruction

Lowest pT electron in HZZ4e events around 10GeV:

Electrons radiate on average half their energy before reaching the ECAL due to:Strong magnetic field (4 Tesla)

1 X0 of material in the inner tracker

Energy is radiated as photons which may in turn convert to e+e- before reaching the ECAL - significant spread of energy in .



Electron Reconstruction

Sophisticated algorithms developed, motivated by the HZZ4e analysis, in order to achieve good reconstruction efficiency for low pT electrons:

Use of Gaussian Sum Filter tracking - electron track is reconstructed right out to ECAL surface. Measure bremsstrahlung energy loss:

Categorization of electrons according to amount of radiated energy, ECAL cluster shape, cluster-track matching.

Combine ECAL energy and tracker

momentum measurements based on

measurement uncertainties:

( ) inoutinbrem pppf −=



Electron Selection

Electron reconstruction has a significant background from fakes (e.g. +/0 overlap from underlying event).

Selection important to exclude potential backgrounds which can fake one or more electrons.

4e analysis: Cut based selection:Ecalo/pTrack < 3.Track cluster matching: || <0.02 and ||<0.1

EHCAL/EECAL <0.2

pT> 5 GeVLoose isolation: pT/pT < 0.5 (cone R=0.2)

2e2 analysis:Likelihood developed based on similar variables.Require likelihood>0.2.Select electron and positron with highest likelihoods.



Electron Reconstruction Efficiency (4e channel)



For the signal, and for the irreducible ZZ background, all 4 leptons are isolated and originate from the primary vertex.

For the reducible tt and Zbb backgrounds, 2 of the leptons are associated with b-jets → non-isolated and with displaced vertices.

For all three channels, offline selection consists of two set do cuts:Vertex/Impact parameter and Isolation cuts - reduce Zbb and tt only.

Kinematic cuts :lepton pT and lepton invariant mass cuts - reduce all backgrounds.

The offline selection for the 2e2 channel is described on the following slides.

Offline Event Selection



Vertex and Impact Parameter Cuts (2e2)

3 variables chosen: High background rejection for 95% signal efficiency. Largely uncorrelated:

(1) Transverse distance from +- vertex to beam line < 0.011 cm

(2) 3D Distance between +- and e+e- vertices < 0.06 cm

(3) Transverse impact parameter significance of lepton with highest IP significance < 7

Combined Efficiency (%)

Signal 89-91

tt 14.5 ± 0.2

Zbb 13.0 ± 0.1



Tracker Isolation (2e2)

Cut on pT of all reconstructed tracks in the event which satisfy:

pT>0.9 GeVAt least 5 hitsWithin region defined as the sum of cones of size R<0.25 around each lepton, excluding veto cones of size R>0.015 around each lepton.Consistent with originating from the reconstructed primary vertex to within z<0.2cm

Cut



Kinematic Distributions for Reconstructed Leptons

Shown for events passing HLT and with e+e-+- reconstructed



Kinematic Cuts (2e2)

Lepton pT cuts:pT

1 > thr1

pT2 > thr2

pT3 > thr3

pT4 > thr4

+- and e+e- invariant mass cuts:mZ1

< thr5

mZ2 > thr6

Four lepton invariant mass cuts:thr7 < mH

< thr8

leptons sorted in decreasing order of pT

mZ1 = max(m+-,me+e- ), mZ2 = min(m+-,me+e-

)



Kinematic cuts are optimized simultaneously together with the isolation pT

threshold.

Cut optimization performed using MINUIT by maximizing significance, defined

by the Log-Likelihood ratio:

Cuts optimized independently for each Higgs mass.

To exclude effects of limited MC statistics: For each cut obtained from the

automatic optimization:

Plot ScL vs cut value with all other cuts fixed.

Assign final cut value by inspection, such that ScL is as close as possible to the

maximum whilst retaining smooth variation of cut value as a function of mH.

€

ScL = 2lnQ s

bs

N

NN

b

s eN

NQ −

+

⎟⎟⎠

⎞⎜⎜⎝

⎛+= 1where

Optimization of Selection Cuts (2e2)



Optimised Kinematic Cuts (2e2)



σ.BR. After Each Cut

x-axis categories: Preselection, L1, HLT, 4 leptons, Vertex, Isolation, Lepton pT, Z mass, Higgs mass

2e22e2



4 Lepton Invariant Mass Before/After Offline Selection

Beforeoffline selection

Afteroffline selection

mH=130 GeV mH=200 GeV

2e22e2



Final Selected Events per fb-1 and NS/NB

mH (GeV) 120 140 160 180 200 250 300 400 500

N signal for 10fb-1 1.9 11.7 7.8 8.7 36.4 29.1 19.4 18.0 9.6

N background for 10fb-1 1.5 2.0 2.0 4.0 16.2 13.6 4.1 3.7 2.6

2e22e2



Summary of Offline Selection for 4e Channel

Longitudinal impact parameter significance for all electrons < 13

Transverse impact parameter significance of reconstructed Z(*) bosons:< 30 for highest me+e-

< 15 for lowest me+e-

Isolation, required separately for each electron, cone size R<0.2:Tracker isolation: (pT

tracks)/pTe < 0.1

Hadronic isolation: (ETHCAL)/pT

e < 0.2

Electron quality requirementsFurther cuts on track-cluster matching, cluster shape, HCAL/ECAL

Kinematic cuts on lepton pT, mZ1, mZ2, m4e



Summary of Offline Selection for 4 Channel

Find that only the following cuts are critical:Isolation: Tracker and calorimeter: threshold applied to the least isolated muon

Single pT threshold for each mass applied to all but the lowest pT muon• Lowest pT muon already required to have pT>7(13) in the barrel (endcaps)

Four lepton invariant mass cuts

Additional cuts (impact parameter, +- inv. mass) do not significantly improve results.

Cut optimization procedure similar to 2e2 analysis, but uses a minimization program named GARCON recently developed by the HZZ4 group.

CalorimeterIsolation for least isolatedmuon



Evaluation of the Z(*)/γ*)(Z(*)/γ*) Background

Systematic error on the no. on background events in the signal region enters into the significance calculation.

Direct simulation of Z(*)/γ*)(Z(*)/γ*)4l subject to the following uncertainties:Theoretical uncertainties:

• PDFs and QCD scale variations• NLO and NNLO production cross-section uncertainties• Relies entirely on existing SM constraints and theoretical knowledge

Experimental uncertainties:• LHC luminosity• MC modeling of detector response, material budget etc• Energy scales (ECAL calibration) and resolution• electron and muon reconstruction and kinematic selection efficiencies• Electron and muon islolation efficiencies

Such uncertainties are difficult to evaluate from first principles.

More robust approach is to evaluate the size of the background directly using the LHC data.



Evaluation of the Z(*)/γ*)(Z(*)/γ*) Background from Data

2 Approaches:

1) Use single Z boson production:Single Z bosons will be produced with a high rate at the LHC.Production cross-section will rapidly be measured directly to a high precisionCan use ratio of production cross-sections for Z(*)/γ*)(Z(*)/γ*) and single Z production to evaluate the Z(*)/γ*)(Z(*)/γ*) background.Cancellation of luminosity uncertainties.

Reduction of PDF and QCD scale uncertainties for low mH.Partial cancellation of experimental uncertainties.

2) Direct measurement through counting the number of events in the sidebands (i.e. excluding the signal peak) of the 4-lepton invariant mass distribution:

Full cancellation of all uncertainties except PDF and QCD scale uncertainties (not fully cancelled because may affect the shape of the m(4l) distribution).Disadvantage: Limited by statistics of the background rate in the sidebands.

Approach 2 is used here as the most robust solution.



Evaluation of Z(*)/γ*)(Z(*)/γ*) Background from Sidebands

Points represent a simulation of LHC data for the relevant integrated luminosities:Total no. of events generated randomly from a Poisson distribution with mean = total expected events from all processes (signal and background).

For each event, 4 lepton invariant mass generated randomly according to the histogram formed from the sum of the MC distributions for signal and background.

Data

outbckgdMCMeasured

inbckgd NN α= Data

outbckgdMCMeasured

inbckgd NN α=

MC

outbckgd

inbckgd

MC N

N=α

MC

outbckgd

inbckgd

MC N

N=α

∫L = 9.2 fb-1 ∫L = 5.8 fb-1

2e22e22e22e2



Background Systematic Errors

Theoretical uncertainty

on the ratio α

Statistical error on background measurement

from data:

€

B2 = ΔBstat2 + ΔBtheory

2

€


2

€

Bstat /B =1

Nbckgdout

Data

High statistical error at high mH due to low statistics in sidebands due to hard lepton pT cuts and large signal width.2e22e2



Background Systematic Errors: Theory

Systematic uncertainty from PDFs and QCD scale estimated using the MCFM event generator.

20 eigenvectors of the CTEQ6M PDFs varied by 1σ.

QCD normalization and factorization scales varied independently up and down by factor 2 from nominal values R = F = 2mZ.

€


2

€


2

€

Btheory



Significance Calculation

Counting experiment significance, ScP:Defined as no. of sigmas of a Gaussian distribution equivalent to Poisson probability of observing equal to or greater than NObs events, given B expected events:

An extended form of the ScP estimator is used which takes into account the systematic uncertainty on B.€

1

2πe

−x 2

2ScP

∞

∫ dx =(μB )ie−μ B

i!i= Nobs

∞

∑

€

1

2πe

−x 2

2ScP

∞

∫ dx =(μB )ie−μ B

i!i= Nobs

∞

∑



Significance for 2e2 Channel

mH (GeV) 120 140 160 180 200 250 300 400 500

N signal at ∫L for 5σ 28.0 10.7 13.4 19.6 21.2 21.7 13.1 14.6 17.8

N back at ∫L for 5σ 21.4 1.8 3.5 9.1 9.4 10.1 2.8 3.0 5.3



Combined Significance for 30 fb-1

Without systematicUncertainties:

€

ScL = 2lnQ

€

Q =

p ni | bi + si( )i=1,3

∏

p ni | bi( )i=1,3

∏where



Combined Significance for 30 fb-1

Systematic uncertainties included:



Higgs Mass Measurement from Gaussian Fit

Statistical error on measurement of mH:Measured Higgs mass from Gaussian fit for high statistics

mH=140 GeV mH=200 GeV mH=500 GeV

€

stat =σ GaussFit

NS

Shown as fraction of true mass

2e22e2 2e22e2



Higgs Width Measurement from Gaussian Fit

Direct measurement of width possible with stat<30% for mH200 GeV

€

σGaussFit = ΓH2 + σ meas

2

€

stat =σ GaussFit

2NS

Shown as fraction of true width

2e22e2



Higgs Cross Section Measurement Uncertainty

€

σ 2 = Δstat 2 + Δsyst 2 + ΔL2 + ΔB2

€

σ 2 = Δstat 2 + Δsyst 2 + ΔL2 + ΔB2

€

syst 2 = 2Δεe2 + 2Δεμ

2 + Δε iso2

€

syst 2 = 2Δεe2 + 2Δεμ

2 + Δε iso2

1% 1% 2%3%

3%

€

σ =Nobs

Lε

€

σ =Nobs

Lε

€

stat = Nexp

Shown as fraction of expected no. of signal events

2e22e2



Summary

Standard Model Higgs boson with mass in range 130≤mH≤500 GeV

observable in the channel HZZ(*)4l with > 5σ significance with 10fb-1 of integrated luminosity, excluding a 15 GeV gap close to mH=170 GeV (40fb-1).

If mass lies in the range 190≤mH≤400 GeV, 5σ significance can be attained

with 4fb-1.

Size of ZZ*/γ* background determined from data in sidebands with systematic uncertainty included in ScP significance calculation.



Aknowledgements

HZZ4e:S. Baffioni, C. Charlot, F. Ferri, R. Salerno, Y. Sirois (LLR, France)

N. Godinovic, I. Puljak (Split, Croatia)

P. Meridiani (Rome and INFN, Italy)

HZZ4:S. Abdullin (FNAL)

D. Acosta, P. Bartalini, R. Cavanaugh, A. Drozdetskiy, A. Korytov, G. Mitselmakher, Y. Pakhotin, B. Scurlock (Florida)

A. Sherstnev (Cambridge)

HZZ2e2:D. Futyan, D.Fortin (UC Riverside)

D. Giordano (Bari and INFN, Italy)



Backup Slides



Electron Experimental Systematic Uncertainties

Material budget: Change in amount of material traversed by electron before reaching the ECAL affects:

electron identification and selection efficiencies

energy scale and resolution

Material budget can be measured using single electron events, using the observed fraction of energy lost through bremsstrahlung, since energy radiated is proportional to material thickness traversed:

where pin and pout are the measured momenta at the innermost and outermost point on the GSF electron track.

€

X / X0 ≈ −ln 1− fbrem( )

€

fbrem = pin − pout( ) pinwhere




2% uncertainty shown to have almost no effect on electron reconstruction efficiency:

With electron statistics from single Z production corresponding to ~10fb-1, can measure tracker material thickness to a precision better than 2%:




Electron reconstruction efficiency and energy scale can be controlled using tagged electrons from Zee events:

Select Zee events for which at least one leg is a “golden” electron (no bremsstrahlung), plus kinematic constraint on Z boson mass for second leg.

Use second leg to estimate uncertainties on reconstruction efficiencies and on the energy scale.

Systematic uncertainty on electron reconstruction efficiency and energy scale taken to be <1%



Muon Experimental Systematic Uncertainties

Measure muon reconstruction efficiency from data to better than 1% precision:

Use sample of muon HLT triggers with pT>19 GeV.

Count no. of Z2 events in the resonance of the inv. mass distributions built from:

• HLT muon + reconstructed muons

• HLT muon + all tracks

Ratio gives the efficiency.

Can measure to better than 1%.

Efficiency of isolation cut measured by evaluating energy flow in isolation cones around random directions in Z2 events.

Can measure to better than 2%.

Uncertainty on pT resolution and pT scale evaluated using resonance peaks

from Z2 and J/2 events to high precision.



Monte Carlo Event Generation Details

In all samples, Z and W bosons forced to decay to e,with forced to decay to e No forcing of b decays in Zbb and tt background events.

In Z(*)/γ*)(Z(*)/γ*) and Z(*)/γ*)bb backgrounds, require mZ(*)/γ*) > 5 GeV

Non-perturbative PDFs in the proton taken from CTEQ6 distributionsGlobal QCD analysis combining all existing relevant deep inelastic and jet cross-section measurement results.

QED final state radiation (“internal bremsstrahlung”) simulated by interfacing the event generators with dedicated software package PHOTOS.



True Significance of Local Event Excess

Search for new phenomena in a wide range of parameter space - in this case narrow resonance in very broad range of invariant masses:

Problem of overestimating significance of a “local discovery”

Need to reduce the significance according to the number of chances of getting it:



Reconstructed Invariant Masses of e+e- and +-

Electron pair Muon pair

mH = 130 GeVmH = 130 GeV



CP Nature of Higgs: Shape of MZ* Distribution

Shape of MZ* distribution depends on CP nature of Higgs

Compare theoretical MZ* distributions with result of

convolution of reconstructed MZ* distribution with

efficiency of selection for MZ*.

Similar approach possible using cos distribution of the angle between the planes containing the lepton pairs - important for MH>2MZ



Choice of Trigger

Natural choice from physics viewpoint is to trigger on Z:

(i) Take OR of 2e and 2triggers

Another possible choice is:(ii) Take OR of 1e, 2e, 1 and 2

Consider fraction of events passing (ii) which fail (i). i.e. pass single triggers only:

For background, corresponds to close to half of events using 1e||2e||1||2 trigger results in almost twice as much background as 2e||2trigger

For signal, using 1e||2e||1||2rather than 2e||2increases final no. of events after all offline cuts by <~1% for mH>160 and <5% for mH<160.

But this gain is offset by the fact that the no. of ZZ background events after offline cuts increases by a similar fraction.

Conclusion: use OR of double electron and double muon triggers

Before offline cuts

After offline cuts



Recovery of QCD Internal Bremsstrahlung

At least one IB photon present in 40-45% of HZZ(*)2e2 events

At least one IB photon with pT>5 GeV present for 10-30% of events (increasing with mH)

2/3 emitted by electrons, 1/3 by muons

Distinguish from other photons from the underlying

event using tendency to be collinear with parent lepton.

Z

e

e

γ

R

If >=1 reconstructed photons found within cone of size R<0.3 around any of the 4 reconstructed leptons, photon with smallest R is considered as an IB photon

4-momentum added to Z boson invariant mass prior to Z mass window cuts.

the discovery potential of the higgs boson at cms in the four lepton final state

Documents