Study of Direct Photon Pair Production in Hadronic Collisions at

√s=14 TeV(Preliminary Results)

Sushil Singh ChauhanDepartment 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 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.

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 EhadT in R

– Photon is isolated if ET < ETCUT in R

R2 min= [ y(γ1) – y(γ2) ]2 + Φ2 γγ

22 ISOR

ISORR

hadTT EE

√s=1.8 TeV, Pt1≥14.90 GeV, Pt2≥13.85 GeV, η<|1.0|, CTEQ6MR=0.4, Et=2 GeV, Rmin=0.3

√s=1.8 TeV, Pt1≥14.90 GeV, Pt2≥13.85 GeV, η<|1.0|, CTEQ6MR=0.4 GeV, Et=2 GeV, GeV, Rmin=0.3

Discrepancy at low QT

• The differential cross section for small QT is

• QCD prediction is reliable when QT≈ Q (hard scale), and less reliable when QT <<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|, CTEQ6MR=0.4, Et=2 GeV, GeV, Rmin=0.3

Fragmentation Contribution For Diphoton at D0

0.0000001

0.000001

0.00001

0.0001

0.001

0.01

0.1

1

10

0 10 20 30 40 50 60 70 80

Qt=|Pt1+Pt2| GeV

d(s

igm

a)/

dQ

t (

pb

/GeV

/c)

Direct

One Fragmentation

Two Fragmentation

√s=1.8 TeV, Pt1≥14.90 GeV, Pt2≥ 13.85 GeV, GeV, η<|1.0|, CTEQ6MR=0.4, Et=2 GeV, GeV, Rmin=0.3

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

√s=14 TeV, Pt1≥40 GeV, Pt2≥25 GeV,η<|2.5|, CTEQ6MR=0.4, Et=5 GeV, GeV,Rmin=0.3

Diphoton at LHC (CTEQ6M),M=M(gamma,gamma)/2

0.00001

0.0001

0.001

0.01

0.1

1

10

0 20 40 60 80 100

Qt=|Pt1+Pt2| GeV

d(s

igm

a)/

dQ

t (p

b/G

eV

/c)

Direct

One Fragm.

Two Fragm

√s=14 TeV, Pt1≥40 GeV, Pt2≥25 GeV, η<|2.5|, CTEQ6MR=0.4, Et=5 GeV, GeV, Rmin=0.3

√s=14 TeV, Pt1≥40 GeV, Pt2≥25 GeV, η<|2.5|, CTEQ6MR=0.4, Et=5 GeV, GeV,Rmin=0.3

√s=14 TeV, Pt1≥40 GeV, Pt2≥25 GeV, η<|2.5|, CTEQ6MEt=5 GeV, GeV,Rmin=0.3

Comparision with different Radius of Isolation Cone at LHC

0.001

0.01

0.1

1

10

0 20 40 60 80 100 120

Qt=|Pt1+Pt2| (GeV)

d(s

igm

a)/

dQ

t (

pb

/GeV

/c)

Direct Contribution, R=0.3

Direct Contribution, R=0.7

√s=14 TeV, Pt1≥40 GeV, Pt2≥25 GeV, η<|2.5|, CTEQ6MEt=5 GeV, GeV, Rmin=0.3

Comparision of Mass Spectrum of Diphoton at LHC with different R values

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

70 80 90 100 110 120 130 140 150

Invariant Mass(M) (GeV)

d(s

igm

a)/

dM

(p

b/G

eV

/c*c

)

at R=0.7,Et=5 GeV

at R=0.3, Et=5 GeV

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 fa1/h1(x1,Q2) fa2/h2 (x2,Q2) σ(a1a2→γγ) To introduce the transverse kinematics of the initial-state partons ,we extend

each integral over the PDF to the kt-space.

dx fa/h (x,Q2) → dx d2kt g(kt) fa/h (x,Q2)

we assume a Gaussian type of Kt distribution, where

g(kt)=( exp(-k2t /<k2

t>) /(π<k2t>))

<k2t>=4*<kt>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 )

Effect of Kt Smearing

0.01

0.1

1

0 5 10 15 20 25 30

Pt=|Pt1+Pt2| (GeV)

d(s

igm

a)/

dP

t (

pb

/GeV

/c)

D0 data

Kt=4.0 GeV, CTEQ5L

With ISR gluonsuppressed, CTEQ5L

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.