Transcript Correlations between photons and charged and dAu collisions at √s

Correlations between photons and charged
particles at large transverse momenta in pp
and dAu collisions at √sNN =200 GeV
Subhasis Chattopadhyay,
Variable Energy Cyclotron Centre, Kolkata 700 064
For the STAR Collaboration
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Motivation of measurement of photons in pp/dAu collisions
Photon-jet measurement important for AA collisions:
• High-pT (> 6 GeV/c) photons are dominantly produced
in hard scattering (photon-jet)
 Photon energy controls initial parton energy scale
 Can be used to quantify parton energy loss
 Spectra of away-side associated particles sensitive to
medium modification (PRC 74, 034906 (2006))
 Surface Bias??
Importance in dAu and pp:
 Provide baseline for AA.
 Understand Initial state properties..
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Identification of direct photons for correlation analysis
Standard methods used for extraction of photons:
Statistical method by
• Reconstruction of inclusive photons
• Subtract photons from decay of p0 , h etc.
Information we do not obtain by statistical method:
Extraction of events rich in gdirect , possibly tag individual
photons
• Study away side jet for photon-tagged events.
•
Method:
• Discriminate photons and p0s using “shower-shape” method.
• For photon-rich events, near-side yield of correlation
function for g-tagged events gets reduced.
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Attempt at g-jet correlation in Au+Au @200GeV
DATA
Correlation between em shower
and charged hadrons
SIMULATION (pp)
p0 simulation
photon simulation
Need g/p0 discrimination
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• Look for discrimination between
direct photon and pion triggers
• Start with favourable environment: pp/dAu
• New discriminatory parameter:
Shower Shape
This talk
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STAR Detector
Detectors used in this analysis
TPC, BEMC,BSMD, Trigger Detectors
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Barrel Electro-Magnetic Calorimeter
Barrel Electromagnetic
Calorimeter
• lead-scintillator sampling
• 2003 RHIC run EMC patch
0 < h< 1 and DF = 2p .
• Designed energy resolution dE/E~16%/√E (GeV).
• Used for trggering high Pt
particles
Shower Maximum Detector(SMD)
Multi-wire gaseous detector
Located at approx. 5 X0
One Calorimeter Module
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Tower and Strips of STAR Calorimeter
EMC Module
Tower
h=0
h=+1
Pre Shower
Shower Max
detector
 wires
The Barrel
Electromagnetic
Calorimeter
tower size –
(Dh, DF) = (0.05, 0.05)
(close to opening angle
of p0 decay > 6.5 GeV/c)
The Shower Maximum
Detector
Good spatial resolution
(Dh,DF)~(0.007,0.007)
h wires
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Enrichment of events using
Shower-shape Analysis
•Shower Maximum
Detector (SMD) provides
detailed shower profile
•High pT p0s decay into
photons with small
opening angle
•Overlapping showers
•Large shower width
• Direct photon shower
profiles are narrow.
•2D shower profile is the
tool
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Selection of photon-rich samples:
2D shower shape analysis (Illustration)
2D Selection regions varied to get optimum results
p0 Bin
Mixed
g Bin
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Photon-Charge correlation (Method)
• High Et ClustersTrigger
particles
ETtrig >ETthreshold1
Measured correlation
Function corrected for
eff of associated particles
(6.5-8.5GeV/c, 8.5-10.5 GeV/c …)
trigger g
• Associated tracks:
ETtrig> pT > pTthreshold2
p0
(> 1GeV/c)
• 3x3 tower Chargedtrack-Veto for pure
photon candidates
associated h
g
incoming partons
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Hijing + GSTAR Simulation (dAu)
Di-jet events
g-jet events
Correlation functions similar for 3 SMD bins in p0 Simulation
Relative reduction in near side yield gives Direct
Photon fraction in the selected sample
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Raw Correlation function (Photon Bin)
• Correlation
Functions fitted with
2Gaussians + const
STAR
STAR PRELIMINARY
PRELIMINARY
•Const term gives
background
•Yield Obtained
(bin counting)
from background
subtracted correlation
functions for 3
different Bins.
Near Angle Peak height reduces relative to away side peak
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Background-subtracted Correlation functions
on selected samples in d+Au collisions
STAR PRELIMINARY
Hard-Probes’06
Proc
Near-side yields reduced towards photon bin
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Correlation Functions: p+p
Reduction in
near angle peak
towards
Photon Bin
Effect more
Prominent for
larger
Ettrigger
Away-side
Yield reduced
Near-side yields reduced towards photon bin
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Reduction in Yields (p+p collisions)
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Extraction of away-side charged particle pt spectra
for “photon-tagged” events for pp collisions
(Et,trig : 8.5 – 10.5 GeV/c)
Method:
• Evaluate p0 fraction (f) from reduction in near side
yield in “photon bin” compared to “ p0 bin”
• Take away-side pT spectra for p0 bin and scale down
by f
• Subtract away-side pt spectra for “scaled p0 bin”
from “photon-bin” and make a (1-f) scaling.
• Compare with Hijing for extracted spectra from p0
and photon bins.
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Extraction of photon spectra
STAR PRELIMINARY
NearSide_Yield (photon Bin)
f= -----------------------------------NearSide_Yield (p0 Bin)
Extracted away-side spectra
(8.5-10.5 GeV) for p0-tagged events
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Spectra for g-tagged Events
Ettrigger from 8.5GeV/c to 10.5 GeV
Extracted pt-spectra
for photon-tagged
events
Matches hijing-pp
Away-side charged
particles spectra
Sys Error (Preliminary):
• Higher p0 Et of
photon bins
• Effect of h, w etc.
• Energy scale
uncertainty
• Yield uncertainty
• SMD Calibration
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SUMMARY and OUTLOOK
• Shower shape method used to obtain photon-rich
sample of events
• Demonstrated by decreasing near-side yield
 Photon fraction increases with pT
(Larger g/p0 ??)
• Spectra of “away-side” associated particles extracted
for “photon-tagged” events
• Slopes agree with expectations from Hijing
• Next steps:
• analyze p+p 2006 (larger stats)
• apply to Au+Au (with large statistics triggered
events)
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Thanks
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Backup
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Removal of trigger-related Background (dAu)
Will put pt/et spectra here
after cut
Background from upstream
Interactions:
energy deposit in calorimeter
but few tracks in TPC
Reject by simultaneous cuts on:
Pt: Total pt of TPC tracks
Et: Total Et from BEMC
FTPC: multiplicity in forward
region
Removes events away from
Pt-et band
Apply cut on et/FTPC
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Systematic checks on Data:
• Extraction of correlation background
using different methods
• Extraction of yield with different D bin width
2D Selection regions varied to get optimum results
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