H→γγsearch and direct photon pair production differential cross section measurement at Dφ.pdf

1、At a hadron collider，diphoton (γγ）production allows detailedstudies of the Standard Model (SM)，as well as as searchesfor new phenomena，such as new heavy resonances，extra spatialdimensions or cascade decays of heavy new p

2、articles. Within theSM，continuum γγ+X production is characterized by a steeplyfallingγγ mass spectrum，on top of which a heavy resonancedecaying into γγ can potentially be observed. In particular，thisis considered one of

3、the most promising discovery channels for aSM Higgs boson at the LHC，despite the small branching ratio ofBR (H →γγ)～0.2％ for 110

4、ion，followed by associated production with a W orZ boson，and vector boson fusion. While the SM Higgs productionrate at the Tevatron is not sufficient to observe it in the γγmode，the Hgg and Hγγ couplings，being loop-media

5、ted，are particularlysensitive to new physics effects. Furthermore，in somemodels beyond the SM，for instance，fermiophobic Higgs，withno couplings to fermions，the BR (H →γγ）can be enhancedsignificantly relative to the SM pre

6、diction，while has the SM-likeproduction cross sections except the gluon fusion is absent.
　　 In this thesis，we present a search for a light Higgs boson in the diphoton final state using 4.2±0.3 fb?1 of the DφRun Iida

7、ta，collected at the Fermilab Tevatron collider from April 2002to December 2008. Good agreement between the data and theSM background prediction is observed. Since there is no evidencefor new physics，we set 95％ C.L. Limit

8、s on the productioncross section times the branching ratio (σ×BR(H →γγ)）relativeto the SM-like Higgs prediction for different assumed Higgsmasses. The observed limits (σ(limit)/σ(SM)）range from 11.9 to 35.2for Higgs mass

9、es from 100 to 150 GeV，while the expected limitsrange from 17.5 to 32.0. This search is also interpreted in thecontext of the particular fermiophobic Higgs model. The correspondingresults have reached the same sensitivit

10、y as a single LEPexperiement，setting a lower limit on the fermiophobic Higgs ofMhf>102.5 GeV (Mhf>107.5 GeV expected). We are slightlybelow the combined LEP limit (Mhf>109.7 GeV).We also provideaccess to theMhf>125 GeV r

11、egion which was inaccessibleat LEP.
　　 During the study，we found the major and irreducible backgrounddirect γγ (DPP）production is not well modelled by thecurrent theoretical predictions：RESBOS，DIPHOX or PYTHIA. There

12、is ～20％ theoretical uncertainty for the predicted values. Thus,for our Higgs search，we use the side-band fitting method to estimateDPP contribution directly from the data events. Furthermore,DPP production is also a sign

13、ificant background in searchesfor new phenomena，such as new heavy resonances，extra spatialdimensions，or cascade decays of heavy new particles. Thus,precise measurements of the DPP cross sections for various kinematicvari

14、ables and their theoretical understanding are extremelyimportant for future Higgs and new phenomena searches.
　　 In this thesis，we also present a precise measurement of theDPP single differential cross sections as a

15、function of the diphotonmass，the transverse momentum of the diphoton system，theazimuthal angle between the photons，and the polar scattering angleof the photons，as well as the double differential cross sectionsconsidering

16、 the last three kinematic variables in three diphotonmass bins，using 4.2 fb?1 data. These results are the first oftheir kind at D? Run II，and in fact the double differential measurementsare the first of their kind at Tev