1. Study of Direct Photon Pair Production in Hadronic Collisions at √s=14 TeV (Preliminary Results) Sushil Singh Chauhan Department of Physics & Astrophysics University of Delhi, Delhi

2. Outline  Importance of direct photon pair production study.  Comparison of the result for DØ experiment.  Discussion on low Qt discrepancy.  Prediction at LHC energy.  Effect of isolation cone cut.  Kt smearing model for LO using Pythia.  Work to do.

3. Importance of This Analysis  The direct di-photon is one of the background subprocess for SM Higgs at LHC energy.  It is an irreducible background in the mass range 90-140 GeV at LHC energy.  Study of isolation cone cut effect at LHC energy.  Effect of the fragmentation contribution on the results.  Study of infrared sensitivity of diphoton Pt spectrum.

4. Code Used * The partonic level code called DIPHOX is used for this process. * This code does a full NLO calculation for this process. * It takes the fragmentation contribution into account. * Collinear singularity are removed using phase space slicing technique. * It suffers from infrared divergence.

5. Direct Subprocesses

6. Some Other Sub processes

7. One Fragmentation Sub process

8. Two Fragmentation Sub process

9. Isolation Cut Parameters Definition To isolate a photon, – Define a cone of size R in η- Φ space –Sum up the hadronic E had T in R – Photon is isolated if E T < E T CUT in R  R 2 min = [ y( γ1) – y(γ2) ] 2 + Φ 2 γγ

10. √s=1.8 TeV, Pt1≥14.90 GeV, Pt2≥13.85 GeV, η<|1.0|, CTEQ6M R=0.4, Et=2 GeV, R min =0.3

11. √s=1.8 TeV, Pt1≥14.90 GeV, Pt2≥13.85 GeV, η<|1.0|, CTEQ6M R=0.4 GeV, Et=2 GeV, GeV, R min =0.3

12. Discrepancy at low Q T The differential cross section for small Q T is QCD prediction is reliable when Q T ≈ Q (hard scale), and less reliable when Q T <<Q. In this region photon pair is accompanied by multiple soft gluon radiation. To calculate reliably, multiple soft gluon emission must be taken into account. Fragmentation part is free of such divergence.

13. √s=1.8 TeV,Pt1≥14.90 GeV, Pt2≥13.85 GeV, η<|1.0|, CTEQ6M R=0.4, Et=2 GeV, GeV, R min =0.3

14. √s=1.8 TeV, Pt1≥14.90 GeV, Pt2≥ 13.85 GeV, GeV, η<|1.0|, CTEQ6M R=0.4, Et=2 GeV, GeV, R min =0.3

15. √s=14 TeV, Pt1≥40 GeV, Pt2≥25 GeV, η<|2.5|, CTEQ6M, R=0.4, Et=5 GeV, GeV, R min =0.3

16. √s=14 TeV, Pt1≥40 GeV, Pt2≥25 GeV,η<|2.5|, CTEQ6M R=0.4, Et=5 GeV, GeV,R min =0.3

19. √s=14 TeV, Pt1≥40 GeV, Pt2≥25 GeV, η<|2.5|, CTEQ6M Et=5 GeV, GeV,R min =0.3

21. Kt –Smearing Model We parameterized the ISR gluon in terms of Kt smearing. This provides an additional transverse impulse to the outgoing partons. The expression for LO cross section is σ(h1h2→γγ)=∫ dx1 dx2 f a1/h1 (x1,Q 2 ) f a2/h2 (x2,Q 2 ) σ(a1a2→γγ) To introduce the transverse kinematics of the initial-state partons,we extend each integral over the PDF to the k t -space. dx f a/h (x,Q 2 ) → dx d 2 k t g(k t ) f a/h (x,Q 2 ) we assume a Gaussian type of K t distribution, where g(k t )=( exp(-k 2 t / ) /(π )) =4* 2 /π Pythia adds Kt to each colliding parton with a Gaussian variance.

22. Effect of Kt- smearing Model to LO calculation of Di-Photon K-factor = dσ(LO + Kt smearing) dσ( LO )

23. Work to do Correction to NLO Qt spectrum for DØ & LHC using Kt smearing model. To get the Pt spectrum for ISR gluon at LHC energy. Study the effect of different PDFs on the present results. Prediction for different η regions at LHC energy. Study of stringent isolation cut. Detailed study of fragmentation at LHC energy Study of scale uncertainty.