study of direct photon pair production in hadronic collisions at √s=14 tev (preliminary results)...
TRANSCRIPT
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.