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H search and direct photon pair H search and direct photon pair production differential cross section production differential cross section measurement at D measurement at D Xuebing Bu USTC/FNAL URA thesis award

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SLIDE 1

June/1/2011, Fermilab 44th Users' meeting Xuebing Bu, URA talk 1

H H search and direct photon pair → γγ search and direct photon pair → γγ production differential cross section production differential cross section measurement at DØ measurement at DØ

Xuebing Bu USTC/FNAL URA thesis award talk

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SLIDE 2

June/1/2011, Fermilab 44th Users' meeting Xuebing Bu, URA talk 2

Why search for H at Tevatron →γγ ?

➢ 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.

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SLIDE 3

June/1/2011, Fermilab 44th Users' meeting Xuebing Bu, URA talk 3

H → γγ

  • Standard Model Higgs:
  • Gluon fusion
  • Associated production
  • Vector boson fusion
  • Examine the inclusive di-

photon dataset ( +X) γγ to search for the high mass resonance.

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SLIDE 4

June/1/2011, Fermilab 44th Users' meeting Xuebing Bu, URA talk 4

DØ detector

γ γ

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SLIDE 5

June/1/2011, Fermilab 44th Users' meeting Xuebing Bu, URA talk 5

Event selection

  • Select 2 photons

–

Transverse energy ET > 25 GeV

–

Isolated in calorimeter

–

Isolated in tracker

–

Energy shower shape consistent with photon

–

Neural network discriminant (ONN) > 0.1

–

No spatially well-matched tracks

–

No pattern of hits in the tracker in a road be consistent with an electron Silicon Tracker Fiber Tracker Preshower EM calorimeter

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SLIDE 6

June/1/2011, Fermilab 44th Users' meeting Xuebing Bu, URA talk 6

Backgrounds

  • Reducible background

– Z/ *

ee γ → , both electrons are misidentified as photons, estimated with Geant Monte Carlo.

– Non-

γγ ( +jet,jet+jet), when γ the jet(s) is(are) misidentified as photon(s), estimated from data.

  • Irreducible background –

direct γγ

– estimated from data.

  • Using ONN=0.75 as the boundary

to separate the events into 4 categories:

–

Npp: both pass the ONN>0.75

–

Npf: first passes, second fails

–

Nfp: vice-versa

–

Nff: both fail

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SLIDE 7

June/1/2011, Fermilab 44th Users' meeting Xuebing Bu, URA talk 7

are jet ONN>0.75 efficiencies,

estimated from MC, validated from data. are photon ONN>0.75 efficiencies, estimated from MC, corrected from data.

j1 , j2 1,2

=

E

The quoted uncertainties are statistical only. Includes potential signal.

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SLIDE 8

June/1/2011, Fermilab 44th Users' meeting Xuebing Bu, URA talk 8

➔ Shape from reversing the ONN cut (0.1) for one photon candidate. ➔ Normalization to the number of events from 4X4 matrix method.

Ratio

Non- ( +jet, jet+jet) background γγ γ

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SLIDE 9

June/1/2011, Fermilab 44th Users' meeting Xuebing Bu, URA talk 9

Direct background γγ

➔ Estimated from data using side-

band fitting after the subtraction

  • f the reducible background

(Z→ee, non-γγ).

➔ The exponential fitting function

is validated using NLO predictions.

preliminary From NLO prediction From subtracted data

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SLIDE 10

June/1/2011, Fermilab 44th Users' meeting Xuebing Bu, URA talk 10

The results

input input MH=100 GeV as an example !

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SLIDE 11

June/1/2011, Fermilab 44th Users' meeting Xuebing Bu, URA talk 11

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.

  • Dominant background for the H

→ γγ search

– The sensitivity could be enhanced if other kinematic information,

e.g. the di-photon transverse momentum, included.

d  dX = N data−N bkg L⋅Acc⋅trigger⋅sel⋅

M  , pT

 , ,cos 

data vs. resbos, diphox and pythia

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SLIDE 12

June/1/2011, Fermilab 44th Users' meeting Xuebing Bu, URA talk 12

Differential cross sections

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SLIDE 13

June/1/2011, Fermilab 44th Users' meeting Xuebing Bu, URA talk 13

30 GeV  M    50GeV 50 GeV  M    80GeV 80 GeV  M    350GeV

Differential cross sections

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SLIDE 14

June/1/2011, Fermilab 44th Users' meeting Xuebing Bu, URA talk 14

Differential cross sections in

30GeV M 50GeV d

2

dM ⋅dpT

 

d

2

dM ⋅d  d

2

dM ⋅dcos

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SLIDE 15

June/1/2011, Fermilab 44th Users' meeting Xuebing Bu, URA talk 15

Differential cross sections in

d

2

dM ⋅dpT

 

d

2

dM ⋅d  d

2

dM ⋅dcos 50GeV M 80GeV

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SLIDE 16

June/1/2011, Fermilab 44th Users' meeting Xuebing Bu, URA talk 16

Differential cross sections in

80GeV M 350GeV d

2

dM ⋅dpT

 

d

2

dM ⋅d  d

2

dM ⋅dcos

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SLIDE 17

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

  • rders of soft and collinear initial state gluons to describe di-photon production.
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SLIDE 18

June/1/2011, Fermilab 44th Users' meeting Xuebing Bu, URA talk 18

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

  • rders of soft and collinear initial state gluons to describe di-photon production.

➔ We performed the first Higgs→

search at Tevatron. Setting limits a factor of ~15 γγ above Standard Model prediction at 130 GeV.

➔ Recent Tevatron Higgs→

search results: γγ

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SLIDE 19

June/1/2011, Fermilab 44th Users' meeting Xuebing Bu, URA talk 19

back-up

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SLIDE 20

June/1/2011, Fermilab 44th Users' meeting Xuebing Bu, URA talk 20

SM Higgs at Tevatron

decay ➢ Major decay channels H->bb @ MH<135 GeV H->WW @ MH>135 GeV

135 GeV

➢ Current results (limits

@95% CL)

– SM LEP direct search:

MH>114.4 GeV

– SM indirect constraint + LEP

direct search: MH<185 GeV Tevatron is sensitive over the whole “interesting” mass range.

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SLIDE 21

June/1/2011, Fermilab 44th Users' meeting Xuebing Bu, URA talk 21

=

E

0.07 < iso < 0.1

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SLIDE 22

June/1/2011, Fermilab 44th Users' meeting Xuebing Bu, URA talk 22

Direct production (DDP) – γγ 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

(Mγγ>50GeV), Mγγ spectrum agrees reasonably very well between diphox and resbos.

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SLIDE 23

June/1/2011, Fermilab 44th Users' meeting Xuebing Bu, URA talk 23

Event yield

Good agreement between data and SM background prediction:

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SLIDE 24

June/1/2011, Fermilab 44th Users' meeting Xuebing Bu, URA talk 24

gg→H→WW (arXiv: 0909.2305)

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SLIDE 25

June/1/2011, Fermilab 44th Users' meeting Xuebing Bu, URA talk 25

CL Limits calculation

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SLIDE 26

June/1/2011, Fermilab 44th Users' meeting Xuebing Bu, URA talk 26

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SLIDE 27

June/1/2011, Fermilab 44th Users' meeting Xuebing Bu, URA talk 27