STAR Electro-magnetic Physics Status and Future Aspect Guoji Lin (Yale) For STAR Collaboration RHIC & AGS Users’ Meeting, BNL, June 5-9

06/05/2006 RHIC & AGS Users' Meeting 2 Outline Motivation of E&M physics E&M physics in STAR  Direct photon HBT  New Muon Telescope Detector (MTD)  Electron (dielectron) physics in HFT Conclusion

06/05/2006 RHIC & AGS Users' Meeting 3 Why E&M Physics? π, K, p… dilepton Direct γ Drell-Yan Hadrons are created at late freeze-out stage, experience strong interaction Photon and leptons are created at all stages of collisions, only have electro-magnetic interaction---- almost do not interact with the medium. Rich Information throughout the evolution.

06/05/2006 RHIC & AGS Users' Meeting 4 STAR Detectors STAR BEMC: primary detector for photon measurement. Trigger on high p t shower. TPC+SVT: conversion photon reconstruction. Year 2004 TPC+TOF: provied e, μ pid ability. New MTD: greatly enhance the middle and high p t μ pid ability HFT: improve electron id, significantly suppress photonic electron background. HFT

06/05/2006 RHIC & AGS Users' Meeting 5 Part I: Direct Photon HBT

06/05/2006 RHIC & AGS Users' Meeting 6 Motivation WA98, PRL 93 (2004) Due to photon’s electromagnetic nature of interaction, Direct photon HBT correlation can provide information about the space-time distribution of the hot matter prior to freeze-out. It was observed at SPS energies Correlation function is calculated using EMC- TPC photon pairs: The conversion photon reconstruction efficiency in TPC is low. The granularity of BEMC tower hinders the observation of two very close BEMC photons. HBT signal for 80 M SIMULATED 200 GeV Au+Au central events using EMC TPC photon pairs. Q inv (GeV/c)

06/05/2006 RHIC & AGS Users' Meeting 7 Photon Reconstruction TPC photons via conversion:  Select e + and e - tracks from PID by energy loss in TPC.  Quality cuts are applied to each e + /e - pair. π 0 signal after background subtraction from TPC-TPC photon pairs EMC photons via energy deposited:  Energy is measured by BEMC tower.  η and φ positions are measured by SMD.  Charged particles are rejected. π 0 signal after background subtraction from EMC-TPC photon pairs STAR Preliminary

06/05/2006 RHIC & AGS Users' Meeting 8 Correlation Function Correlation function from EMC-TPC photon pairs A big peak is obsearved at small Q inv area. The reason is still unknown. The existence of this peak is insensitive to the geometrical and quality cuts of photons. Removing TPC photons with energy greater than 1 GeV substantially reduces the peak. A full GEANT simulation will be done to understand the reason of the peak. STAR Preliminary

06/05/2006 RHIC & AGS Users' Meeting 9 New Approach to Photon HBT Proposal for R&D towards a measurement of direct photon HBT with STAR A.Chikanian, E. Finch, R. Majka, J. Sandweiss Yale University Two critical changes to the STAR detector: 1.Install a photon converter of about 0.1 radiation length at r≈45 cm inside the inner field cage. The TPC detection efficiency is about 7%. 2.A “shashlyk” calorimeter with improved energy resolution (on the order of 5%/√E) and good efficiency for photons down to around 100 MeV of energy. Use 1 γ in TPC, 1 γ in calorimeter.

06/05/2006 RHIC & AGS Users' Meeting 10 Model of Direct Photon Production A model of complete space-time source structure as well as the momentum spectrum: Three 'eras' of direct photon generation (corresponding, roughly, to (1) initial hard scattering, (2) QGP production, and (3) hadron gas production). Temperature are fitted to give an overall spectrum matching the prediction given in D. d’Enterria and D. Peressounko nucl-th/ Every piece of matter emits photons with Boltzmann p t spectrum corresponding to temperature at its proper time. The longitudinal expansion follows the Bjorken’s model with flat rapidity distribution in lab.frame. There is no transverse expansion (for simplicity). (1) (2) (3)

06/05/2006 RHIC & AGS Users' Meeting 11 Results Using Direct Photons Only R long, R out variation with p t reflect the emission size dependence on time.

06/05/2006 RHIC & AGS Users' Meeting 12 Results from ‘ Complete ’ Simulation 80M equivalent central STAR events. 3 bins of pair k t for Q osl. 1-D observables: Q inv, Q osl, Q xyz. Q out, Q side, Q long

06/05/2006 RHIC & AGS Users' Meeting 13 First Photon Experiment with a Converter and the STAR Calorimeter An interesting experiment can be done with the addition of a converter as in the proposed  HBT experiment, but using the existing calorimeter. Detailed study of the η meson (yield, p t spectrum). Other resonances involving photons are under study and may be possible. π 0 HBT.  HBT for direct photon yields at low p t may well be feasible and is under careful study.

06/05/2006 RHIC & AGS Users' Meeting 14 Direct Photon v 2 Statistical errors only %: decay photon v 2 and inclusive photon v 2 are close together. 5-20% & 5-70%: inclusive photon v 2 looks systematically higher than decay photon v 2. Wednesday’s talk for more detail.

06/05/2006 RHIC & AGS Users' Meeting 15 Part II: Muon Telescope Detector

06/05/2006 RHIC & AGS Users' Meeting 16 Why Muon Identification at Mid-rapidity? Dimuon continuum QGP thermal radiation Quarkonia (J/  etc) QGP Color Screen Drell-Yan virtual photon Initial photon production Vector Meson  Chiral Symmetry Restoration e  correlation charm, bottom production Advantage over electron PID: no  conversion much less Dalitz decay Online trigger in central Au+Au Hadron Rejection: Usual Muon Detectors: 100 – 200

06/05/2006 RHIC & AGS Users' Meeting 17 Muon ID at Low p t 0.15<pT<0.25 GeV/c, DCA<3cm At high pT,  separation is 0.5  e π μ μ is well separated at low p t.

06/05/2006 RHIC & AGS Users' Meeting 18 A Three-layer Prototype MTD π A prototype MTD with three layers, proposed by Zhangbu Xu: MRPC TOF + Wire Chamber + CTB trays outside the STAR magnet. The detector will have these three layers on top of each other to evaluate the performance of each other in run7 and/or run8. a) R&D on MRPC with Large module, long strip and two-end readout This will have timing of <100ps resolution and spatial resolution of <2cm. ALICE uses 8cmx1.2m MRPC with 3x6 pads (60ps) There is proposal for FAIR/GSI with long strip readout. L0 trigger from MRPC readout b) wire chambers for tracking (Yale has two and the readout from E864) We have two MWPC in a "working condition". Size (active area): 12x12" Read-out: anode wires; connected in 12 strips / Chamber. Cathode strips; 0.2" width, 56 / Chamber. Gas: Ar+CO2(30%), the simplest gas system. HV: "+" polarity, ~2 kV. FEE: readout for E864 straw chamber stations c) Scintillator for dE/dx measurement (we have two spare CTBs and will have more next run)

06/05/2006 RHIC & AGS Users' Meeting 19 MTD Simulation Muon detecting efficiencyPion detecting efficiency Secondary muon from pion decay. Can be subtracted with DCA cut. P t (GeV) eff

06/05/2006 RHIC & AGS Users' Meeting 20 Hadron Rejection and Trigger at RHIC p T (GeV/c) 100 HIJING central events: 18 Events with >=2 hits (dt<20ns) 2 Events with >=2 hits (-400ps<dt<100ps) 3 out of 840 tracks pass our cuts (pT>2 GeV/c). Hadron rejection is about 200. Additional rejection can be obtained using dE/dx and TOF inside the magnet. CutsNhit/event No cut70 TOF1.6 Eloss7.6 TOF&Eloss0.72 TOF (-400ps,100ps)0.23

06/05/2006 RHIC & AGS Users' Meeting 21 Test Scintillator Trays 1)Two spare scintillator trays outside the magnet 2)p+p trigger rate: 1 per 100K events 3)Acceptancexeff: 1/200*0.2 = 1/1000 4)Enhancement for leading charged hadron: 100 5)Run6 p+p 2.5M events equivalent to 250M p+p TPC events Minbias p+p events in STAR so far: ~10M P t (GeV) eff

06/05/2006 RHIC & AGS Users' Meeting 22 A Promising Muon Detector New Detector System: developed for QCDLab from BNL High p T muon  Heavy Flavor leptonic decay  Quarkonia  IMR dilepton, DY High-p T hadron trigger Red: MTD angle Blue: other angles STAR Preliminary

06/05/2006 RHIC & AGS Users' Meeting 23 Part III: Electron (dielectron) physics in HFT

06/05/2006 RHIC & AGS Users' Meeting 24 The Heavy Flavor Tracker Number of pixels 98,304,000 Pixel dimension 30  m  30  m Detector Chip active area 19.2 mm  19.2 mm Detector Chip pixel array 640  640 Number of ladders 24 Ladder active area 192 mm  19.2 mm Number of barrels 2 Inner barrel (6 ladders) r = 1.5 cm Outer barrel (18 ladders) r = 5.0 cm Frame read time 4 ms Pixel read rate, after zero suppression 63 MHz Ladder (w/Al cable) % X % Beam Pipe Thickness 0.5 mm or 0.14% X 0  30 μm silicon pixels give 10 μ m space point resolution.  Great help to single electron and di- electron study.

06/05/2006 RHIC & AGS Users' Meeting 25 Electron ID  Combining TPC dE/dx and TOF gives clear separation of electron and hadrons.  Electro-magnetic shower in EMC matched with a track in TPC. Still a large amount of background from photon conversion!

06/05/2006 RHIC & AGS Users' Meeting 26 Single Electron  A significant fraction of B (~10%) and D (6-17%) meson decays include an electron or positron in the final state: the only source of high transverse momentum electrons and positrons.  The conversion background is reduced by requiring hits in the HFT.  Electrons and positrons from heavy meson decays come from the decay vertex typically displaced from the primary vertex by a few hundred  m. Adding the decay vertex information from the HFT will significantly reduce the background in the high p t electron/positron spectra and substantially increase the statistical significance of the measurements.

06/05/2006 RHIC & AGS Users' Meeting 27 Vector Mesons with Dileptons Detectors   TPC+TOF 8M 2M PC+TOF+SVT+HFT 200K 100K Number of central Au+Au events required to observe a 3-  signal for  and  in their leptonic decay channels under different detector configurations. The large reduction in photonic background will enable us to observed short lived vector meson decays  The conversion background is reduced by requiring hits in the HFT.  Charm semi-leptonic decay background filtered by DCA.  Reject η and π 0 Dalitz decays by measuring both electrons, of a pair, in the TPC.

06/05/2006 RHIC & AGS Users' Meeting 28 Conclusion Rich E&M physics in STAR. Direct photon HBT is under study. A large-area muon detector at mid-rapidity at RHIC is under investigation. Promising electron and dielectron physics from HFT.

06/05/2006 RHIC & AGS Users' Meeting 29 Back up

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