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Recent Results on the Possibility of Observing a Standard Model Higgs Boson Decaying to WW (*) Majid Hashemi University of Antwerp, Belgium.

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Presentation on theme: "Recent Results on the Possibility of Observing a Standard Model Higgs Boson Decaying to WW (*) Majid Hashemi University of Antwerp, Belgium."— Presentation transcript:

1 Recent Results on the Possibility of Observing a Standard Model Higgs Boson Decaying to WW (*) Majid Hashemi University of Antwerp, Belgium

2 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

3  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

4  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

5  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

6  The signal is H ->WW->l 1 l 2 which 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

7 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),p T l1, p T l2  The neural net analysis is also performed using samples of signal and background after the preselection. Analysis steps

8  Kinematic distributions of signal and background events:  A low lepton pair Δ φ 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

9 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

10  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

11  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

12 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

13 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

14 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!

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