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

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.

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 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.

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.

Direct Subprocesses

Some Other Sub processes

One Fragmentation Sub process

Two Fragmentation Sub process

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

√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

√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

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.

√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

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

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

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

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

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.

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

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.