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

1. Crystal Detector R&D proposalfor Soft Photon Measurement in STAR Presented by Gang Wang (UCLA) University of CaliforniaatLosAngeles Joint Institute for Nuclear Research Institute of Modern Physics at Lanzhou

2. Soft photons: 20 -1000 MeV • Physics motivations : • New physics domain exploration • Radiative decays of particles • EM radiation from bulk medium • Experimental opportunities : • TPC +TOF +HFT +Crystal Detector = unique combination, new particle measurement capability • Larger photon cross-section allows smaller acceptance See next page for details…

3. Physics motivations • Pre-equilibrium and thermal photons • Pre-equilibrium stage of the heavy ion collision and its evolution decidedly unexplored. • Thermal photon radiation from the QGP and HRG phases • Radioactive decays of particles • S0L + g • ccJ/y + g • ~40% J/yfrom feed-down decays • EM radiation from bulk medium • quark jets traversing the QCD color medium • high pT charged hadron – soft photon back-to-back correlation -> properties of the dense partonic matter • high angular momentum of bulk quark matter • dipole radiation • Synchrontron radiationfrompossibleformationoflongitudeinal E/B fields

4. Experiment: direct photons at WA98 Direct PhotonProduction in 158 A GeV 208Pb+208Pb Collisions WA98, Phys. Rev. Lett. 93 (022301), 2004 Range: 0.5 < pT < 3.5 GeV/c Yield of the direct photons at low pT extracted from the strength of the two-photon correlation. (See efforts by the Yale Group) Low-pT range [0.02, 1.0] GeV/c is not accessible

5. Experiment: direct photons at PHENIX Range: 1.0< pT < 10.0 GeV/cDirect photon spectra obtainedwiththestatisticalsubtraction method Probingthe QCD equationofstatewiththermalphotonsinnucleus-nucleus collisionsat RHICDavid d’Enterriaand Dimitri Peressounko, nucl-th/0503054 Low-pT range [0.02, 1.0] GeV/c is not accessible

6. Soft photon measurement pTcoverage : from 20 to 1000 MeV • Can not be measured by PHENIX, ALICE and STAR with present detectors • Predicted yield is large and even with a small crystal patch measurements seem to be possible at STAR in near future • How well (for how much) we can do that ? Needed R&D in FY07 to find out

7. Crystals Inorganic crystals is the only practical option for STAR, CsI(Tl) from IMP is the one we propose to investigate.

8. Monte-Carlo simulations Crystal Detector is implemented within STARSIM framework. It is located in place of BEMC module 88 and TOF tray 88. STAR EAST VIEW Super-cube : 9x20 crystals Crystal Patch Crystal calorimeter: Three projective “super-cubes” 9x20 Total 540 crystals inDhxDf = 0.66x6o Crystals: 2.54 cm x 2.54 cm x 25 cm Interaction point Crystal Patch

9. Crystal Detector: sample MC events Hijing central Au +Au event: clean, two photons photons Hijing central Au +Au event: hadronic background example

10. Occupancy and Energy Deposition Background sources which contribute to detector occupancy : charged tracks and albedo from BEMC modules. Occupancy data indicates that even in central Au +Au events it is possible to identify soft photons cleanly!

11. Photon rates for low-pTregion Preliminary photon count rates for 0.02 < pT < 0.20 GeV are calculated using fully reconstructed Hijing events (starsim + BFC). Cluster finder algorithm efficiency is not included yet, charged hadron background removal is considered to be 100% efficient at this moment. Preliminary estimations show that we will need ~1M central events for ~3% error in yield measurements (low-pT region for all centrality bins) Expected energy resolution 5-10%, depends on cluster size: 3x3, 4x4 crystals and photo detector cutoffs. Small acceptance is compensated by large photon yield!

12. FY07, R&D Goals: MC simulations • Complete understanding of the efficiency and purity of reconstructed soft photons in the Crystal Detector : • Various cluster finder algorithms investigation • Robust procedure for removal of non-photonic backgrounds • Possible hardware veto layer for charged tracks? • Shower shape analysis possibilities and isolation cuts • Extended BEMC albedo studies • Shielding, isolation cuts? • Required dataset for detailed analysis : • 50k fully reconstructed AuAu200 central events (Hijing) with most probable geometry settings (help from S&C group) • Disk space is the most important issue here • Disk space required for studies is ~0.5 Tb • Various performance issues : • Pileup, leakages, optimizations etc…

13. FY07, R&D Goals: Hardware • Define crystal detector element. • crystal, wrapping, coupling with Photo Detector, Photo-detector, readout • lots of things known from literature – need first hand experience. • Optimize light yield/uniformity. • Investigate possibility of using TPC DAQ1000 • PSA/Altro chips for readout of PD. (Counting on help from Tonko) • Make preliminary design of a mechanical support structure and investigate integration into the STAR detector. (help from STSG) • Understand radiation conditions, required rad. hardness tests and monitoring scheme for crystals. • By the end of FY07 we want to define cost/schedule. Requests: • Acquire crystals from IMP. (in progress) • Acquire PD from Hamamatsu. • Acquire/Borrow radioactive sources • Set up a testing lab at BNL

14. Summary • Very first and preliminary results look optimistic. • More detailed R&D work is needed in FY07 and we hope to reach a firm decision on feasibility of the soft photon physics with crystals in STAR • We need more collaborators! Thanks to STAR Software team for their help!

15. Backup slides • Energy resolution: require a lot of explanation • graph 3x3 20-200 MeV band (10% to as low as 3%?) • Shower shape in crystals 3x3 – central fraction, leak from sides put here light yield per MeV, realistic cutoff How crystal patch can be compared with conversion on gas of TPC?