direct photon production in p+p and d+au collisions at s=200gev from star
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Direct photon production in p+p and d+Au collisions at s=200GeV from STAR. outline: motivation experiment and dataset analysis technique results in d+Au 2003 systematic errors first results / outlook p+p 2005 conclusions. M.J. Russcher , Utrecht University / NIKHEF. - PowerPoint PPT PresentationTRANSCRIPT
Direct photon production in p+p and d+Au collisions at s=200GeV from STAR
M.J. Russcher, Utrecht University / NIKHEF
outline:• motivation• experiment and dataset• analysis technique• results in d+Au 2003• systematic errors• first results / outlook p+p 2005• conclusions
M.J. Russcher for the STAR collaboration 2
motivation
• Au+Au
– low pT: thermal component
– high pT: tagged dir - jet events, a well-calibrated energy loss study
• d+Au – initial state effects
– nuclear kT , CGC
– with p+p HI baseline result
• in general– no interaction with the medium– directly produced and for prompt no fragmentation uncertainty
• p+p
– testing pQCD
– gluon density
direct photons are produced in processes like quark-anti-quark annihilation and quark-gluon Compton scattering
q + q + g
q + g + q
P3.2: S. Chattopadhyay, - charged in p+p and d+Au
M.J. Russcher for the STAR collaboration 3
• TPC– charged particle veto
– -1.6 < < 1.6
experimental setup
• BEMC– 4800 PbSc towers
– -1<<1, 0<<2, = 0.05 x 0.05
• BBC– minbias trigger p+p
– -3.4 < < 5.0 • ZDC– minbias d+Au
from ZDC-Au
BEMC
BEMC
TPC
-1 < < 1
BBC BBC
ZDC ZDC
M.J. Russcher for the STAR collaboration 4
experimental setup (2)
• BSMD– wire-proportional counter = 0.007 x 0.007
0 reconstruction at high pT
• high ET trigger
– d+Au 2003: 1720 b-1 (4.5 GeV)
– p+p 2005: 2830 nb-1 (3.5 GeV)
– both datasets have 50% of BEMC installed and triggering
M.J. Russcher for the STAR collaboration 5
dir subject to a large background of decay photons,
such as: 0 (~80%),
1. measure all photon candidates in BEMC
2. subtract neutral hadron contamination incl
3. 0 spectrum in same dataset, mT scaling for other mesons
4. simulate decay photons decay
5. dir = incl – decay
double ratio:
– R = (incl/0)data (/0)decay = 1 + dir /decay
– advantage: cancellation of systematics
– dir (1 – R-1) incl
analysis method
M.J. Russcher for the STAR collaboration 6
d+Au results
M.J. Russcher for the STAR collaboration 7
0 in d+Au
– 0 – ~1000 b-1 after event cuts
– reasonable agreement with pQCD calculation folded with nuclear density functions
– 20-30% uncertainty due to 5% error on absolute energy scale
calculations by W. Vogelsang
EPJ C43, 311
0 < y < 1
– error on yield extraction is 5-10% from uncertainty on background in peak region
– no cancellation in double ratio
pT (GeV/c)
dn/dM
M.J. Russcher for the STAR collaboration 8
double ratio in d+Au
– incl in same dataset as 0
– signal consistent with pQCD NLO calculation
– further reduction of error bars needed for dir
both data and calculation include contribution from fragmentation photons
R = 1 + dir / decay
calculations by W. Vogelsang
sizeable fragmentation contribution
M.J. Russcher for the STAR collaboration 9
systematic errors
– beam background events contaminate high ET
datasample– characterized by high
energy in BEMC and no associated tracks reconstructed in TPC
– cut on EBEMC/pT,TPC to reject events
– cut has been varied to estimate the systematic error
work in progress: pattern recognition tool to better identify beam background events
M.J. Russcher for the STAR collaboration 10
systematic errors
R = 1 + dir/decay 3 GeV/c 10 GeV/c
yield extraction 5% 10%
energy scale 3% 3%
beam background <1% 4%
eta/pion 2% 2%
fit to pions 3% 3%
Efficiency (stat.) 10% 4.5%
bsmd gain 5% 5%
stat. error 4% 7%
dominant errors on R:– yield extraction– beam background – efficiency correction factors– BSMD gain
energy scale almost cancels
reduction needed for spectrum– more stats for efficiency correction– in situ calibration of BSMD– study effect of jet-structure
errors on double ratio d+Au:
M.J. Russcher for the STAR collaboration 11
first results and outlook for p+p
M.J. Russcher for the STAR collaboration 12
0 in p+p
– preliminary, subset of data
– Lsampled = 0.4 pb-1
– consistent with pQCD+KKP
– disfavors Kretzer FFs
– agrees well with published PHENIX Run-2 result
– PRL91, 241803
0 < y < 1
M.J. Russcher for the STAR collaboration 13
meson in p+p
– clear signal in subset of data shown for 4 < pT < 12 GeV/c
– reconstructed from – more challenging than 0
because of BR ~= 40%, less abundant and acceptance loss
– spectrum in progress
– /0 = 0.45 0.05 assumed for d+Au
minv [GeV/c2]
minv [GeV/c2] minv [GeV/c2]
minv [GeV/c2]
M.J. Russcher for the STAR collaboration 14
outlook to direct photons in p+p
– total statistical error on double ratio shown with pQCD + scale uncertainty
– large improvement compared to d+Au
– p+p result dominated by systematics
– better calorimeter performance expected to lead to smaller systematic errors
calculations by W. Vogelsang
not data, statistical errors only
M.J. Russcher for the STAR collaboration 15
summary and conclusions
• first BEMC direct photon result by STAR
– d+Au double ratio R = 1 + dir/decay consistent with pQCD
– investigating possible reduction of systematic errors for dir spectrum
• analysis in p+p well on its way 0 shows agreement with pQCD + KKP and PHENIX
Run-2 data– clear signal of meson, spectrum in the pipeline – systematic error evaluation for double ratio
in progress
• access to nuclear effects for pions and photons• provides baseline for analysis in Au+Au.
M.J. Russcher for the STAR collaboration 16
backup slides
M.J. Russcher for the STAR collaboration 17
p+p and d+Au dataset
• BEMC trigger– high ET deposition in a single
calorimeter cell
– d+Au:
– 1720 b-1 (ET > 4.5 GeV)
– p+p:– 2830 nb-1 (ET > 3.5 GeV)
• acceptance– 50% of BEMC (0 < < 1)
– 5-10% dead towers
– 20% dead strips
0 trigger efficiency
100%
M.J. Russcher for the STAR collaboration 18
kT / world data
M.J. Russcher for the STAR collaboration 19
eta to pion ratio
• PHENIX PRL96 202301 / 15%, so 0.45 +/-
0.05 means < +/-2% for simulated decay background