direct soft photon production in au+au collisions at √s nn = 200 gev: first measurements at star

D.Arkhipkin, R.Zulkarneev, Y. Zoulkarneeva, Workshop of European Research Group on Ultrarelativistic Heavy Ion Physics., March 9th 2006

Direct Soft Photon Production in Au+Au Collisions at √s

NN = 200 GeV: First Measurements at STAR


Outline

● Object of Study

● Analysis➔ Physics Goals & Setup ➔ MC studies➔ Raw data➔ Corrections

● Current Results

● Outlook and Future Plans


Object of Study: What is meant by “direct photon”?

Initial hard scattering and pre-equilibrium photons

Hadron Decays

Direct

Thermal Radiation from QGP and Hadron Gas

Photons are produced in all stages of relativistic heavy-ion collision: from initial hard scattering to final hadronic state

For simplicity we shall distinguish two sources only: (1) an early one in which photons are produced in the initial parton cascade leading to thermalization of a quark-gluon plasma and (2) a later one where photons are radiated from thermalized matter the quark-gluon plasma phase (QGP) and the hadronic phase (HG).

Direct photons: sum of the prompt +pre-equilibrium and thermal (QGP+HRG) components.


Object of Study: What is meant by “soft”?

Nuclear Theory, nucl-th/0503054

WA 98 :Yield of Direct PhotonsWA 98 :Yield of Direct Photons

RHICRHIC

Direct photons in ultra-relativistic collisions of heavy nuclei were observed by WA98 experiment at CERN SPS and PHENIX experiment at RHIC. There is no experiment covering the soft photon region! No experiment covers P

T 40 ... 1000 MeV !


Physics goals and setup

➢ Direct photon cross-section

➢ Inclusive photon cross-section : AuAu, dAu

➢ Direct photon ratio (to better distingwish direct photon signal excess) : AuAu/dAu

Primary physics goals:

Current experimental setup:

➢ pT range : 40 - 160 MeV/c photons, || < 1.0

➢ Collision type : AuAu 200, dAu 200

➢ All measurements on the same detector setup, systematics effect is minimized


MC studies : low pT

primary photons

• Decay photons from primary vertex (collision)• p

T < 0.5 GeV

• || < 1.0

Photon's Yield in Simulated Collisions• Au+Au 200GeV Hijing 1.382• d+Au 200GeV Hijing 1.382 • p+p 200GeV Pythia 6.203

Inclusive Photons = Direct + Decayed Inclusive Photons = Direct + Decayed

signal background

Background needs to be evaluated using Simulation Models:


MC studies: Au+Au, d+Au, p+p collisions

Photons resulting from 0, and other hadronic decays (, , etc) are considered p

t< 0.2 GeV : most of background comes from 0

~ 90% --> 0

less 10% --> less 2% --> cocktail

• Decay photons from primary vertex• p

t < 0.5 GeV

• || < 0.5• M = 0.5 T

✗ AuAu200, dAu200 : simulated with Hijing 1.382 + GSTAR (propagation throughout the STAR detector)

✗ pp200 : simulated with Pythia 6.203 + GSTAR


MC studies (continuation): background photon yield at various centralities (Au+Au 200 GeV)

Nuclear Geometry and Centrality Selection

Centrality 20-60 %

(Nch)

Combined bins often used:

Centrality

Nch_range

0-10% 520 and up

20-60% 57...319

0-80% (MinBias) 14 and up


MC studies: d+Au, p+p background photon yield

d+Au p+p


Au+Au / d+Au background photon yield ratio

AuAu 0-10% central AuAu 20-60% central


STAR Experiment: Photon Reconstruction Setup

Method: photon conversion on TPC gas

Excellent TPC PID capabilities : 99.99% pure electron sample up to 0.08 GeV using dE/dx over momentum cuts.

Cuts used in analysis:●dAu z-vertex < 50.0 cm●AuAu z-vertex < 30.0 cm●|Nsigma (electron)| < 3.0●DCA between tracks < 1.5 cm●Opening angle < 0.2 radian●Fit points > 10

Photons are reconstructed via e+e- conversion in STAR Time Projection Chamber (TPC). Photon Momentum reconstruction was done by the measured momenta of e+e- pairs produced from conversions.


STAR Experiment: Can we see soft photons?

Extensive QA done: d+Au, Au+Au nice opening angle, invariant mass, DCA distributions showing great photon reconstruction!

AuAu statistics:

●~5M AuAu 200 used●0.5M events passed through cuts (z-vertex, MinBias trigger)●~1K photons extracted

dAu statistics:

●~25M dAu 200 used●10.5M events passed through cuts (z-vertex, MinBias trigger)●~1K photons extracted

Corrected for pT-dependent reco efficiency using AuAu and dAu

embedding data.

Parameters:

●Pseudorapidity : [-1, 1]●Electron p

T : [0.02MeV, 0.08Mev]We see enough already, 50% of AuAu dataset is

still to be used in analysis!


Experiment: difficulties encountered

MC photon (reconstructed e+e- pair) conversion radius

Real photon (reconstructed e+e- pair) conversion radius

TPC conversion radius: ●MC is different from real dataConversion efficiency:●Embedding shows higher conversion rate than real data


Experiment: photon reconstruction efficiency evaluation using embedding

An embedded MC photon is considered found, if it is converted into e+e- pair (at a particular distance from the collision center limited within the TPC

volume) and both e+ and e- tracks were reconstructed.

•eff_pT = [ (Reco number of MC photons) / (Total number of MC photons) ]*100% at particular p

t bin

eff_ = [(Reco number of MC photons) / (Total number of MC photons) ]*100% at particulat bin

•The momentum resolution reflects the quality of the track reconstruction. The resolution was also determined by embedding MC simulated particle (photon)

into real events. It shows momentum divergence between MC track and associated reconstructed track. A MC photon reconstruction assumption is the same

as for the efficiency.

Pt Efficiency

Eta efficiency

AuAu200

dAu200dAu200


Experiment: momentum resolution


While work is still in progress,...

STAR preliminary

AuAu200 inclusive photons yield over dAu200 inclusive photons yield ratio


Raw inclusive dN/dP yields: d+Au, Au+Au

Only the basic corrections applied!


Outlook

● While work is still in progress, first results are quite promising!

● AuAu/dAu ratio is ready, more work on simulation needs to be done to understand the phenomena:

➔ Hijing tuning

➔ Other models needs to be considered

● High hopes on the rest of dataset.

● STAR upgrades may provide new ways to explore low-p

T photons.


Future: STAR upgrades

Crystal Detector R&D Proposal for Soft Photon Measurement in STAR

The proposed soft photon program covers photon transverse momentum range of 20 MeV to 1000 MeV at mid-rapidity. Physics program will focus on three aspects: 1) thermal photon radiation from the Quark Gluon Plasma (QGP)

phase in nucleus-nucleus collisions at RHIC; 2) measurement of radiative decays of particles such as 0 and

c

states and explore spectroscopy of particles with soft photon and 0 in the final state;

3) soft photon radiations from bulk quark matter through processes such as jets traversing dense partonic matter, radiation from collective high angular momentum state and possible synchrotron radiation from high longitudinal E/B field from non-equilibrium stage during the transition from the Color Glass Condensate to Quark-Gluon Plasma.

direct soft photon production in au+au collisions at √s nn = 200 gev: first measurements at star

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