Recent Results on the Possibility of Observing a Standard Model Higgs Boson

Decaying to WW(*)

Majid Hashemi

University of Antwerp, Belgium

CMS:Size: 21 m long, 15 m wide and 15 m high. Weight: 12 500 tonnes Location: Cessy, France.

ATLAS:Size: 46 m long, 25 m high and 25 m wide. Weight: 7000 tonnes Location: Meyrin, Switzerland.

ALICE:Size: 26 m long, 16 m high, 16 m wide Weight: 10 000 tonnes Location: St Genis-Pouilly, France

LHCb:Size: 21m long, 10m high and 13m wide Weight: 5600 tonnes Location: Ferney-Voltaire, France.

LHC has 4 main detectors:

LHC and its detectors

Results already obtained from LEP (in the same tunnel before upgrading to LHC) shows a Higgs boson mass lower limit of 114.4GeV at 95% C.L. (CERN-EP 2003-011)

Indirect searches including fits to data from electroweak measurements set an upper limit of 193GeV. (CERN-EP 2002-091)

114.4 GeV < m(H) < 193 GeV

LEP and Tevatron results

Tevatron already excluded the mass range between 160 and 170 GeV a month ago!

arXiv: 0903 4001 hep-ex

In CMS there are several production processes for the Higgs boson:

The gg fusion, VBF, associated production WH,ZH, ttH,

However, gg fusion (gg->H) is the dominant process,

The main decay channels are :

H->tautau, H->ɣɣ, for m(H)<135GeV,

H->WW, H->ZZ, for 135<m(H)<700 GeV

Higgs Boson Production processes at LHC

On-going analyses in CMS are already in a good shape and ready for real data to

come in this year,

They include H->tautau, H-> ɣɣ, H->ZZ, H->WW,

All these analyses are already approved by CMS collaboration and are public,

Below are examples of H->ZZ and H->tautau:

Ongoing analyses in CMS

The signal is H ->WW->l1l2which is produced via gg fusion with a small contribution from VBF qqH.

The analysis of qqH has been done separately.

x BR (m(H)=160GeV) = 2.34 pb 234 signal events at 100pb-1 for 14 TeV run

Background processes are : WW, tt, W+jets, single top, Z+jets, W+ɣ, …

12pb, 836pb, 58nb, …

So a large suppression is needed for the discovery of signal.

W+jets : is suppressed by lepton isolation requirements,

2 lepton requirements, …

ttbar : is suppressed by requiring a central jet veto,

WW is reduced by kinematic cuts on the

lepton pair invariant mass, delta phi, …

H->WW signal, and background events

H->WW(*) search strategy and analysis flow

The analysis is performed with the following logic:

Online selection:

Trigger events : online selection of events with 1 muon or 1 electron,

Preselection:

Lepton pair selection : two leptons with pt>10GeV, one with pt>20GeV both in barrel with opposite signs,

Kinematic pre-selection : Met > 30 GeV, lepton pair invariant mass > 12GeV,

Final state selection:

mass dependent cuts on Met,dphi(l1,l2),m(l1,l2),pTl1, pT

l2

The neural net analysis is also performed using samples of signal and background after the preselection.

Analysis steps

Kinematic distributions of signal and background events:

A low lepton pair Δφ and invariant mass is expected for the signal events compared to background.

After all selection cuts there are 31 events of emu signal and 31 background at 1fb-1 ,

Masses lower or higher than m(H)=160GeV leave less signal and more background.

Kinematic distributions and number of selected events

Performing the neural net analysis gives better results than cut based analysis:

Multivariate analysis

At m(H)=160GeV,

Cut based analysis : S/B=70/70,

NN analysis : S/B=67/37

The main background samples are controlled using different strategies:

ttbar events, Control region definition:

For ttbar events a control region is introduced and number of such events is estimated in the signal region using the standard formula:

Control region is close to the signal region

Selection cuts are basically the same dropping central jet veto

The error of the estimation of the background is then calculated using:

=> many of the systematics cancel.

Vary jet Et by +/-7% (jet energy scale uncertainty)

Statistical uncertainty of observed events

Fluctuation of the background in normalization region

R=eff(CJV in signal region)/eff(2jets in normalization region)

≈ 18%

Background studies

WW background, Control region definition:

WW events are enhanced with the same selections as in ttbar but keeping CJV,

Optimization is also applied to increase the WW sample size in the normalization region,

The total error including statistical uncertainty and backgroud fluctuation is ~22%.

W+jet background, fake lepton study:

Fake muons estimate : define the probability of a loosely isolated track to pass muon id,

Fake electron estimate: define the probability of a jet to pass electron id,

Run this analysis on QCD events ( plenty of events) and obtain “fake rates”,

Re-weight signal search which is looking for loosely isolated tracks and jets,

The final error estimate is better because a large sample of QCD is used for fake rate measurement

Background studies

Including all systematic uncertainties the signal significance is calculated for the cut-based and neural net analysis

Certainly the multivariate analysis is performing better and for the central region a 5sigma discovery is possible.

Systematic uncertainties and signal significance

A Higgs boson in the range of 140<m(H)<200 GeV can be excluded at 95%C.L. with the data collected at 1fb-1.

Result

Conclusions:

The CMS results of the search for H->WW were shown,

For a 14 TeV run there is a possibility of exclusing a Higgs boson signal at 1fb-1

for a mass range of 140<m(H)<200 GeV,

There is also a possibility of observing a Higgs boson with a mass around 160 GeV at 5sigma after 1fb-1 data is collected.

For other masses more data and time is needed but this is an early data analysis and current studies show that even at 100pb-1 we may have some news!