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