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

INDIA – CMS, PU, 5-6 April 2007Ashok KumarPU & GNDU

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

Ashok Kumar Ashok Kumar andand Suman BeriSuman BeriPanjab University – ChandigarhPanjab University – Chandigarh

Ritva KinnunenRitva Kinnunen Helsinki Institute of Physics Helsinki Institute of Physics

INDIA – CMS, 5-6 April 2007INDIA – CMS, 5-6 April 2007


Though the H → branching fraction is tiny and the backgrounds from Z→- and ttbar production are large, but it is an interesting channel because it provides information on the H coupling

Cross-sections and Branching FractionsCross-sections and Branching Fractions

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

[pb

]


Event Generation and ReconstructionEvent Generation and Reconstruction

generator reconstructed

events used x BR x BR x

preselection

Signal, mH = 120 GeV PYTHIA 8000 1.1 fb 1.1 fb

Z+jet, 10 < pT < 100 GeV1) PYTHIA 288413 5.1 nb 5.1 nb

tt, preselection cuts on muons2) PYTHIA 268244 840 pb 25.1 pb

qqZ, preselection cuts on jets3) MadGraph 10000 76 fb

1) Z+jet generation with qq’ -> gZ and gq -> qZ, preselection: 10 < pT < 100 GeV, Z-> , ee, 2) preselection cuts: W-> , pT

> 20 GeV, pT > 10 GeV ,||<2.5, M > 10 GeV

3) preselection cuts: pTq > 20 GeV, |q|<5, |q1 –q2| > 4.2, q1 * q2 < 0,

Mq1,q2 > 900 GeV, M > 70 GeV

Standard ORCA packages for reconstruction: - Muons: GlobalMuonReconstructor- Hadronic jets: TransientJetAlgorithm with Jet reconstruction cone = 0.5, Corrections: MCJet with SplittedEcalPlusHcalTowerInput, Et

tower > 0.5 GeV, Etower > 0.8 GeV and with ET(raw jet) > 20 GeV- MET from EcalPlusHcalTowers with Type1 jet corrections


Higgs boson mass reconstructionHiggs boson mass reconstruction

Select events with exactly two isolated muons with pT> 20 GeV, passing

the L1 andHLT di-muon or single muon triggers Muon isolation: no tracks with pT>1 GeV in a cone of R = 0.4 around the muon

MC muons

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

Event Selection

Reconstructed muons

New values due to the potential bb+jets background, pT>15 GeV

and R = 0.3 were used in the first version


Central variables for event selectionCentral variables for event selection

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

suppress the ttbar background2. Expect more boosted di-muon system in the signal than in the Z+jet background: cut in pT

Rejection of tt ~4 for ETmiss < 50 GeV Rejection of Z+jets ~12 for pT

>60 GeV


Signal qqZ Z+jets ttbar

Events for 60 fb-1 66 5160 3.05 x 108 1.25 x 106

L1 61.8 (93.5%) 5142 (99.7%) 1.18 x 108 (38.7%) 1.25 x 106(100%)

HLT 57.5 (93.0%) 4930 (95.9%) 4.84 x 107 (41.0%) 1.00 x 106(80.2%)

PV 56.6 (98.5%) 4863 (98.6%) 4.74 x 107 (97.8%) 9.97 x 105(99.4%)

2 ,pT>20 GeV 41.9 (74.4%) 3668 (75.4%) 2.09 x 107 (44.1%) 3.79 x 105(38.0%)

muon isolation 28.8 (68.9%) 2546 (69.4%) 1.44 x 107 (68.9%) 1.60 x 105(42.2%)

MET < 50 GeV 25.7 (89.2%) 2179 (85.6%) 1.37 x 107 (95.1%) 4.26 x 104(26.6%)

pT > 60 GeV 16.6 (64.4%) 1396 (64.1%) 1.10 x 106 (8.0%) 2.27 x 104(53.3%)

118 < m<122 GeV 11.1 (67.0%) 2.06 (0.15%) 2968 (0.27%) 383.6 (1.69%)

Number of signal and background events and cut Number of signal and background events and cut efficienciesefficiencies

after central event cuts and mass windowafter 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


Forward jet tagging Forward jet tagging in the mass windowin the mass window1)1)

Selection of forward jets: two hardest jets anywhere in rapidity with j1*j2 < 0

Signal vs qqZ differences due to generator level cuts on qqZDifferences may arise also due to the possibility of the Z originating from a final state quark

Select lowest possible value, ET > 20 GeV,due to PYTHIA generation of Z+jetbackgroundNo optimization of this variable

1) Due to the small mass window, statistics low for Z+jets: - need to open the mass window and - assume decoupling of pT

and tagging cuts


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

|q1 –q2| > 4.2 at the generator levelfor qqZ

|j1 - j2| > 4.5

Rapidity gap between the tagging jets


Di-jet invariant mass

Optimal cut value: Mtagging jets > 1 TeV

With all (previous) event selection cuts, including |j1 - j2| > 4.5


Selection of central jets:

- All corrected jets between the two tagging jets within min + 0.5 < jet < max – 0.5 and with ET > 20 GeV

Central jet veto

Statistics too low for Z+jet to obtain the efficiency curve

ETjet = 20 GeV taken as the

threshold


Signal qqZ Z+jets ttbar

Events for 60 fb-1 66 5160 3.05 x 108 1.25 x 106

Central event cuts, 118 < m<122 GeV

11.1 (67.0%) 2.06 (0.15%) 2968 (0.27%) 383.6 (1.69%)

2 tagging jets 8.5 (76.7%) 1.88 (91.3%) 1033 (34.8%) 162.8 (42.4%)

ETj1,ET

j2>20 GeV 7.7 (90.0%) 1.81 (96.5%) 373 (36.1%) 149.9 (92.1%)

|j1-j2|> 4.5 4.0 (51.9%) 1.52 (83.8%) 52.6 (14.1%) 7.7 (5.1%)

mjj > 1 TeV 2.4 (59.0%) 1.36 (89.2%) 4.1 (7.7%) 1.8 (24.0%)

central jet veto 1.7 (69.0%) 0.95 (69.9%) 2.3 (56.3%) 0.5 (27.8%)

Number of signal and background events and cut Number of signal and background events and cut efficienciesefficiencies

for forward jet tagging and central jet veto cutsfor forward jet tagging and central jet veto cuts


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, ,, mll > 40 GeV

- generation of PYTHIA events with the data cards used for the ORCAproduction samples but with mll > 40 GeV

Comparison method:Jet reconstruction within | | < 5 with the PYTHIA jet reconstruction package in CMKINMain event selection cuts: - two leptons (electrons or muons) with pT > 15 GeV/c - cut in the di-lepton pT, pT

ll > 60 GeV/c - two tagging jets with the method used for the ORCA analysis - central jet veto - No MET cut, no mass window cut


2j 3j 4j 5j 2j-5j PYTHIA

x BR (pb) 230 66 17 4.0 317 4500

2l, pT > 15GeV/c 105 30 7.8 1.8 145 831

pTll > 60 GeV/c 35 14 4.1 1.1 54 60

2 jetsj1*j2<0

10 5.0 1.6 0.5 17 19

|j1-j2|> 4.5 0.16 0.14 3.4x10-2 1.5x10-2 0.35 0.12

mjj > 1 TeV 4.7x10-2 5.1x10-2 1.4x10-2 1.2x10-2 0.12 6.7x10-2

ET(veto)< 20 GeV

8.4x10-3 5.3x10-3 3.6x10-4 2.6x10-4 1.4x10-2 2.2x10-2

ALPGEN generation


ALPGEN PYTHIA

efficiency of jet tagging cuts 2.2 x 10-3 1.1 x 10-3

efficiency of central jet veto 12% 33%

Conclusion from ALPGEN/PYTHIA comparison

- 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

Differencies due to harder jets in ALPGEN


Estimate of WW+jets, W-> background

Exploit the ALPGEN samples: (WW+2jets) = 1.22 pb- PYTHIA jet reconstruction- Main event relection (as for Z+2jet events)- Result: (selected) = 6.5x10-5 pb, 0.8% of Z+2jet cross section

Estimate based on the study of bb background on bbH/A->e+X PTDR2, page 346, Table 11.1:- Leptonic triggers, lepton pT cuts (pT

>19 GeV, pTe>29 GeV) and

isolation (pT > 1 GeV, R = 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

Estimate of bb+jets background


Signal superimposed on the total background

With all selection cuts, for 60 fb-1


JES uncertainty in the background determinationJES uncertainty in the background determination

Signal visibility not obtained with 60 fb-1 , background could be estimatedaround the already known Higgs mass (from H->Main uncertainty expected from JES uncertainty

Uncertainty on the backgrounds from JES uncertainty, 10% MET uncertainty included

tt most sensitive to theincrease of JES uncertaintydue to large jet multiplicity

30% uncertainty in this estimate due to MC statistics


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


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

Statistical significance = 0.75 and upper bound for xBR = 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-> channel more profitable:background can be extracted with high precision around the known mass value(0.5% from fast simulation for 60 fb-1)


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

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:

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

Documents

gev met

pt1 gev

z jet background

fbz jet

etraw jet

jet reconstruction cone

gev standard orca packages

signal event