→ γγ → γγ H search and direct photon pair H search and direct photon pair production differential cross section production differential cross section measurement at DØ measurement at DØ Xuebing Bu USTC/FNAL URA thesis award talk June/1/2011, Fermilab 44th Users' meeting Xuebing Bu, URA talk 1
→γγ Why search for H at Tevatron ? ➢ Higgs boson is undiscovered ! ➢ Clean signature. ➢ Current direct experimental search + indirect constraint prefer light Higgs boson. ➢ Contribute to the Tevatron Standard Model Higgs combination, especially in the intermediate mass region around 125 GeV. ➢ Golden channel for the discovery of Standard Model Higgs at LHC. June/1/2011, Fermilab 44th Users' meeting Xuebing Bu, URA talk 2
→ γγ H ● Standard Model Higgs: ● Gluon fusion ● Associated production ● Vector boson fusion ● Examine the inclusive di- γγ photon dataset ( +X) to search for the high mass resonance. June/1/2011, Fermilab 44th Users' meeting Xuebing Bu, URA talk 3
DØ detector γ γ June/1/2011, Fermilab 44th Users' meeting Xuebing Bu, URA talk 4
Event selection ● Select 2 photons – Transverse energy E T > 25 GeV – Isolated in calorimeter – Isolated in tracker – Energy shower shape consistent with photon EM calorimeter – Neural network discriminant Preshower (ONN) > 0.1 – No spatially well-matched tracks Fiber Tracker – No pattern of hits in the tracker in a road be consistent with an electron Silicon Tracker June/1/2011, Fermilab 44th Users' meeting Xuebing Bu, URA talk 5
Backgrounds Using ONN=0.75 as the boundary ● ● Reducible background to separate the events into 4 categories: γ → – Z/ * ee , both electrons are – Npp: both pass the ONN>0.75 misidentified as photons, estimated with Geant Monte – Npf: first passes, second fails Carlo. – Nfp: vice-versa γγ ( +jet,jet+jet), when γ – Non- – Nff: both fail the jet(s) is(are) misidentified as photon(s), estimated from data. ● Irreducible background – direct γγ – estimated from data. June/1/2011, Fermilab 44th Users' meeting Xuebing Bu, URA talk 6
j1 , j2 are jet ONN>0.75 efficiencies, estimated from MC, validated from data. 1 , 2 are photon ONN>0.75 efficiencies, estimated from MC, corrected from data. E = Includes potential signal. The quoted uncertainties are statistical only. June/1/2011, Fermilab 44th Users' meeting Xuebing Bu, URA talk 7
γγ γ Non- ( +jet, jet+jet) background ➔ Shape from reversing the ONN cut (0.1) for one photon candidate. ➔ Normalization to the number of events from 4X4 matrix method. Ratio June/1/2011, Fermilab 44th Users' meeting Xuebing Bu, URA talk 8
γγ Direct background ➔ Estimated from data using side- band fitting after the subtraction of the reducible background (Z → ee, non- γγ ). ➔ The exponential fitting function is validated using NLO From NLO prediction predictions. preliminary From subtracted data June/1/2011, Fermilab 44th Users' meeting Xuebing Bu, URA talk 9
The results input input M H =100 GeV as an example ! June/1/2011, Fermilab 44th Users' meeting Xuebing Bu, URA talk 10
Direct photon pair production cross section Check the validity of the predictions of perturbative quantum ● chromodynamics (pQCD) and soft-gluon resummation methods implemented in theoretical calculations. → γγ search Dominant background for the H ● – The sensitivity could be enhanced if other kinematic information, e.g. the di-photon transverse momentum, included. N data − N bkg d = L ⋅ Acc ⋅ trigger ⋅ sel ⋅ dX , , cos M , p T data vs. resbos, diphox and pythia June/1/2011, Fermilab 44th Users' meeting Xuebing Bu, URA talk 11
Differential cross sections June/1/2011, Fermilab 44th Users' meeting Xuebing Bu, URA talk 12
Differential cross sections 30 GeV M 50 GeV 50 GeV M 80 GeV 80 GeV M 350 GeV June/1/2011, Fermilab 44th Users' meeting Xuebing Bu, URA talk 13
Differential cross sections in 30 GeV M 50 GeV 2 d 2 2 d d dM ⋅ d dM ⋅ dcos dM ⋅ dp T June/1/2011, Fermilab 44th Users' meeting Xuebing Bu, URA talk 14
Differential cross sections in 50 GeV M 80 GeV 2 d 2 2 d d dM ⋅ d dM ⋅ dcos dM ⋅ dp T June/1/2011, Fermilab 44th Users' meeting Xuebing Bu, URA talk 15
Differential cross sections in 80 GeV M 350 GeV 2 d 2 2 d d dM ⋅ d dM ⋅ dcos dM ⋅ dp T June/1/2011, Fermilab 44th Users' meeting Xuebing Bu, URA talk 16
Conclusions ➔ We presented the measurements of single and double differential cross sections for direct photon pair production: Show necessity to include higher order corrections beyond NLO and resummation to all ➔ orders of soft and collinear initial state gluons to describe di-photon production. June/1/2011, Fermilab 44th Users' meeting Xuebing Bu, URA talk 17
Conclusions ➔ We presented the measurements of single and double differential cross sections for direct photon pair production: Show necessity to include higher order corrections beyond NLO and resummation to all ➔ orders of soft and collinear initial state gluons to describe di-photon production. ➔ We performed the first Higgs → γγ search at Tevatron. Setting limits a factor of ~15 above Standard Model prediction at 130 GeV. ➔ Recent Tevatron Higgs → γγ search results: June/1/2011, Fermilab 44th Users' meeting Xuebing Bu, URA talk 18
back-up June/1/2011, Fermilab 44th Users' meeting Xuebing Bu, URA talk 19
SM Higgs at Tevatron 135 GeV decay ➢ Major decay channels H->bb @ MH<135 GeV H->WW @ MH>135 GeV ➢ Current results (limits @95% CL) – SM LEP direct search: M H >114.4 GeV – SM indirect constraint + LEP direct search: M H <185 GeV Tevatron is sensitive over the whole “interesting” mass range. June/1/2011, Fermilab 44th Users' meeting Xuebing Bu, URA talk 20
E = 0.07 < iso < 0.1 June/1/2011, Fermilab 44th Users' meeting Xuebing Bu, URA talk 21
γγ Direct production (DDP) – irreducible background Challenge to model by theoretical ● prediction so far, each generator has its own deficit – pythia: LO + parton shower – diphox: NLO, except the gg box contribution (LO) – resbos: NLO + Resummation of collinear/soft ISR gluons + ~NLO single fragmentation However, in the high mass region ● (M γγ >50GeV), M γγ spectrum agrees reasonably very well between diphox and resbos. June/1/2011, Fermilab 44th Users' meeting Xuebing Bu, URA talk 22
Event yield Good agreement between data and SM background prediction: June/1/2011, Fermilab 44th Users' meeting Xuebing Bu, URA talk 23
gg → H → WW (arXiv: 0909.2305) June/1/2011, Fermilab 44th Users' meeting Xuebing Bu, URA talk 24
CL Limits calculation June/1/2011, Fermilab 44th Users' meeting Xuebing Bu, URA talk 25
June/1/2011, Fermilab 44th Users' meeting Xuebing Bu, URA talk 26
June/1/2011, Fermilab 44th Users' meeting Xuebing Bu, URA talk 27
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