STAR Electro-magnetic Physics Status and Future Aspect

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

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 RHIC & AGS Users' Meeting

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

4. STAR Detectors Year 2004 STAR BEMC: primary detector for photon measurement. Trigger on high pt shower. New MTD: greatly enhance the middle and high ptμ pid ability HFT TPC+TOF: provied e, μ pid ability. TPC+SVT: conversion photon reconstruction. HFT: improve electron id, significantly suppress photonic electron background. RHIC & AGS Users' Meeting

5. Part I: Direct Photon HBT RHIC & AGS Users' Meeting

6. Motivation • 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. 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 Qinv (GeV/c) HBT signal for 80 M SIMULATED 200 GeV Au+Au centralevents using EMC TPC photon pairs. WA98, PRL 93 (2004) 022301 RHIC & AGS Users' Meeting

7. Photon Reconstruction EMC photons via energy deposited: • Energy is measured by BEMC tower. • η and φ positions are measured by SMD. • Charged particles are rejected. 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. STAR Preliminary STAR Preliminary π0 signal after background subtraction from TPC-TPC photon pairs π0 signal after background subtraction from EMC-TPC photon pairs RHIC & AGS Users' Meeting

8. Correlation Function • A big peak is obsearved at small Qinv 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. STAR Preliminary Correlation function from EMC-TPC photon pairs A full GEANT simulation will be done to understand the reason of the peak. 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: • 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%. • 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. 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/0503054 • Every piece of matter emits photons with Boltzmann pt 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) RHIC & AGS Users' Meeting

11. Results Using Direct Photons Only Rlong, Rout variation with pt reflect the emission size dependence on time. RHIC & AGS Users' Meeting

12. Results from ‘Complete’ Simulation Qout, Qside, Qlong 1-D observables: Qinv, Qosl, Qxyz. 3 bins of pair kt for Qosl. 80M equivalent central STAR events. 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, pt spectrum). • Other resonances involving photons are under study and may be possible. • π0 HBT. •  HBT for direct photon yields at low pt may well be feasible and is under careful study. RHIC & AGS Users' Meeting

14. Direct Photon v2 Statistical errors only. 20-60%: decay photon v2 and inclusive photon v2 are close together. 5-20% & 5-70%: inclusive photon v2 looks systematically higher than decay photon v2. Wednesday’s talk for more detail. RHIC & AGS Users' Meeting

15. Part II: Muon Telescope Detector 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 (f,r) Chiral Symmetry Restoration • e-m correlation charm, bottom production • Advantage over electron PID:no g conversionmuch less Dalitz decayOnline trigger in central Au+Au Hadron Rejection: Usual Muon Detectors: 100 – 200 RHIC & AGS Users' Meeting

17. Muon ID at Low pt π e μ 0.15<pT<0.25 GeV/c, DCA<3cm μ is well separated at low pt. At high pT, m/p separation is 0.5s RHIC & AGS Users' Meeting

18. A Three-layer Prototype MTD 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) π RHIC & AGS Users' Meeting

19. MTD Simulation Secondary muon from pion decay. Can be subtracted with DCA cut. eff eff Pt (GeV) Pt (GeV) Muon detecting efficiency Pion detecting efficiency RHIC & AGS Users' Meeting

20. pT (GeV/c) Hadron Rejection and Trigger at RHIC 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. RHIC & AGS Users' Meeting

21. Test Scintillator Trays eff Pt (GeV) • Two spare scintillator trays outside the magnet • p+p trigger rate: 1 per 100K events • Acceptancexeff: 1/200*0.2 = 1/1000 • Enhancement for leading charged hadron: 100 • Run6 p+p 2.5M eventsequivalent to 250M p+p TPC eventsMinbias p+p events in STAR so far: ~10M RHIC & AGS Users' Meeting

22. A Promising Muon Detector • New Detector System:developed for QCDLab from BNL • High pT muon • Heavy Flavor leptonic decay • Quarkonia • IMR dilepton, DY • High-pT hadron trigger STAR Preliminary Red: MTD angle Blue: other angles STAR Preliminary RHIC & AGS Users' Meeting

23. Part III: Electron (dielectron) physics in HFT RHIC & AGS Users' Meeting

24. The Heavy Flavor Tracker • 30 μm silicon pixels give 10 μm space point resolution. • Great help to single electron and di-electron study. 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! 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 pt electron/positron spectra and substantially increase the statistical significance of the measurements. RHIC & AGS Users' Meeting

27. Vector Mesons with Dileptons • 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 π0 Dalitz decays by measuring both electrons, of a pair, in the TPC. Number of central Au+Au events required to observe a 3-signal for  and  in their leptonic decay channels under different detector configurations. 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. RHIC & AGS Users' Meeting

29. Back up RHIC & AGS Users' Meeting

30. RHIC & AGS Users' Meeting