Search for the standard model higgs boson decaying to muons via weak gauge boson fusion
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INDIA – CMS, 5-6 April 2007. Search for the Standard Model Higgs Boson Decaying to Muons via Weak Gauge Boson Fusion. Ashok Kumar and Suman Beri Panjab University – Chandigarh Ritva Kinnunen Helsinki Institute of Physics. Cross-sections and Branching Fractions.

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Search for the standard model higgs boson decaying to muons via weak gauge boson fusion

INDIA – CMS, 5-6 April 2007

Search for the Standard Model Higgs Boson Decaying to Muons via Weak Gauge Boson Fusion

Ashok Kumar andSuman Beri

Panjab University – Chandigarh

Ritva Kinnunen

Helsinki Institute of Physics

Ashok Kumar


Cross-sections and Branching Fractions

Though the H → m+m-branching fraction is tiny and the backgrounds from Z→m+m-

and ttbar production are large, but it is an interesting channel because it provides

information on the Hmm coupling


Two search methods possible:

- Inclusive search profiting from the larger cross section, fast simulation

study in CMS IN 2005/013

- Search in the weak gauge boson fusion, exploits an efficient background

reduction with forward jet tagging

Ashok Kumar


Event Generation and Reconstruction

  • Z+jet generation with qq’ -> gZ and gq -> qZ,

  • preselection: 10 < pT < 100 GeV, Z-> mm, ee, tt

  • 2) preselection cuts: W-> mn, pTm1 > 20 GeV, pTm2 > 10 GeV ,|hm|<2.5, Mmm > 10 GeV

  • preselection cuts: pTq > 20 GeV, |hq|<5, |hq1 –hq2| > 4.2, hq1 * hq2 < 0,

  • Mq1,q2 > 900 GeV, Mmm > 70 GeV

Standard ORCA packages for reconstruction:

- Muons:GlobalMuonReconstructor

- Hadronic jets: TransientJetAlgorithm with Jet reconstruction cone = 0.5, Corrections: MCJet with SplittedEcalPlusHcalTowerInput, Ettower > 0.5 GeV, Etower > 0.8 GeV and with ET(raw jet) > 20 GeV

- MET from EcalPlusHcalTowers with Type1 jet corrections

Ashok Kumar


Event Selection

Higgs boson mass reconstruction

Select events with exactly two isolated muons with pTm > 20 GeV,passing the L1 and

HLT di-muon or single muon triggers

Muon isolation:no tracks with pT>1 GeV in a cone ofDR = 0.4around the muon

New values due to the potential bb+jets background, pTm >15 GeV and DR = 0.3 were used in the first version

Reconstructed muons

MC muons

Results from a gaussian fit: s = 1.42 GeV (1.18%), <mmm> = 120.1 GeV

Ashok Kumar


Central variables for event selection

1. No neutrinos in the signal event: an upper bound on ETmisscan be used to

suppress the ttbar background

2. Expect more boosted di-muon system in the signal than in the Z+jet background:

cut in pTmm

Rejection of tt ~4 forETmiss< 50 GeV

Rejection of Z+jets ~12 forpTmm>60 GeV

Ashok Kumar


Number of signal and background events and cut efficiencies

after central event cuts and mass window

No signal visibility possible even with large integrated

luminosity, assume that Higgs boson mass is known from

other processes to select the mass window

Ashok Kumar


Forward jet tagging in the mass window1)

Selection of forward jets: two hardest jets anywhere in rapidity withhj1*hj2 < 0

Select lowest possible value,

ET > 20 GeV,

due to PYTHIA generation of Z+jet


No optimization of this variable

Signal vs qqZ differences due to

generator level cuts on qqZ

Differences may arise also due to the possibility

of the Z originating from a final state quark

  • Due to the small mass window, statistics low for Z+jets:

  • - need to open the mass window and

  • - assume decoupling of pTmm and tagging cuts

Ashok Kumar


Rapidity gap between the tagging jets

Expect large rapidity gap for the q jets in qq -> qqH

|hq1 –hq2| > 4.2 at the generator level

for qqZ

|hj1 - hj2| >4.5

Ashok Kumar


Di-jet invariant mass

With all (previous) event selection cuts, including |hj1 - hj2| >4.5

Optimal cut value:Mtaggingjets > 1 TeV

Ashok Kumar


Central jet veto

  • Selection of central jets:

  • All corrected jets between the two tagging jets within

  • hmin + 0.5 < hjet < hmax – 0.5 and with ET > 20 GeV

Statistics too low for Z+jet

to obtain the efficiency curve

ETjet = 20 GeV taken as the


Ashok Kumar


Number of signal and background events and cut efficiencies

for forward jet tagging and central jet veto cuts

Ashok Kumar


Comparison of PYTHIA and ALPGEN generation

for the Z+jets background

Data samples:

- existing ALPGEN production samples for exclusive 2-5 jet final states,

generated with Z-> ee, mm,tt, mll > 40 GeV

- generation of PYTHIA events with the data cards used for the ORCA

production samples but with mll > 40 GeV

Comparison method:

Jet reconstruction within |h | < 5 with the PYTHIA jet reconstruction

package in CMKIN

Main event selection cuts:

- two leptons (electrons or muons) with pT > 15 GeV/c

- cut in the di-lepton pT, pTll > 60 GeV/c

- two tagging jets with the method used for the ORCA analysis

- central jet veto

- No MET cut, no mass window cut

Ashok Kumar


ALPGEN generation

Ashok Kumar


Conclusion from ALPGEN/PYTHIA comparison

Differencies due to harder jets in ALPGEN

- Higher efficiency from forward jet tagging cuts from ALPGEN,

- higher efficiency for central jet veto from PYTHIA

+ normalization in PYTHIA (pT > 10 GeV/c)

-> level of Z+jets background in ORCA simulation not underestimated

Ashok Kumar


Estimate of WW+jets, W->mn background

  • Exploit the ALPGEN samples:

  • s(WW+2jets) = 1.22 pb

  • PYTHIA jet reconstruction

  • Main event relection (as for Z+2jet events)

  • Result: s(selected) = 6.5x10-5 pb, 0.8% of Z+2jet cross section

Estimate of bb+jets background

  • Estimate based on the study of bb background on bbH/A->em+XPTDR2, page 346, Table 11.1:

  • Leptonic triggers, lepton pT cuts (pTm>19 GeV, pTe>29 GeV) and isolation (pT > 1 GeV, DR = 0.4): 37.4 pb, rejection factor ~106

  • Assume that for the remaining selection the efficiency is similar to that of Z+jets

  • Result: ~ 4x10-3 events in the mass window

Ashok Kumar


Signal superimposed on the total background

With all selection cuts, for 60 fb-1

Ashok Kumar


JES uncertainty in the background determination

Signal visibility not obtained with 60 fb-1 , background could be estimated

around the already known Higgs mass (from H->gg)

Main uncertainty expected from JES uncertainty

Uncertainty on the backgrounds from JES uncertainty,

10% MET uncertainty included

tt most sensitive to the

increase of JES uncertainty

due to large jet multiplicity

  • 30% uncertainty in this

    estimate due to MC statistics

Ashok Kumar


Estimate of statistical significance

Total background in the mass bin 3.74 events

Systematic uncertainties:

JES uncertainty dominates for this channel

tt 20%

Z+jets 16%

qqZ 8%

Assume 5% uncertainty for all production cross sections

Total background with systematics 3.74 ± 0.73 events

Significance (Poisson stat., ScPf program):

0.75 with systematic uncertainty

0.80 without systematic uncertainty

Ashok Kumar



Full simulation study on qq → qqH, H → mm with forward jet tagging and central jet veto yields S/B ~ 0.5 in a mass window assuming mH to be known from H → gg for 60 fb-1

Statistical significance = 0.75 and upper bound for sxBR = 12.7 fb at 95% CL, including systematic uncertainty

  • Main backgrounds from Z+jets, ttbar and qqZ were simulated

  • For better optimization of jet tagging and veto cuts, generation of the Z+jets background with a NLO generator needed

  • For high luminosity, Lt > 100 fb-1, inclusive H->mm channel more profitable:

  • background can be extracted with high precision around the known mass value

  • (0.5% from fast simulation for 60 fb-1)

Ashok Kumar


The analysis note was presented in the Physics meeting on

December 8, 2006 by Suman Beri

Minutes of the Physics Meeting of 08/12/06:

AN2006/105 Search for the Standard Model Higgs Boson Decaying into Muons

-  Comments from room: approach of the analysis is not the right one.presently this is approached like a discovery analysis, while there is no

chance in this analysis. Instead one should take into account thealready measured mass of the Higgs, select on a mass window

in the di-muon system  and then optimize the cuts against background.

In any case it will remain a difficult channel for the LHC.

-  Z+jets ALPGEN files are now available so these files can be used tomake the background analysis of that channel, or at least cross checkexplicitely against PYTHIA

-  Concerns from the referees are to be taken to be taken into account

STATUS: Not approved. Sent back to the Higgs group to re-discuss the analysis strategy

Ashok Kumar