star electro-magnetic physics status and future aspect guoji lin (yale) for star collaboration rhic...

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


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.

, c B l X Vector meson ( , , ...), /J l l

Rich Information throughout the evolution.


STAR Detectors

STAR BEMC: primary detector for photon measurement. Trigger on high pt shower.

TPC+SVT: conversion photon reconstruction.

Year 2004

TPC+TOF: provied e, μ pid ability.

New MTD: greatly enhance the middle and high pt μ pid ability

HFT: improve electron id, significantly suppress photonic electron background.

HFT


Part I: Direct Photon HBT


Motivation

WA98, PRL 93 (2004) 022301

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.

Qinv

(GeV/c)


Photon ReconstructionTPC photons via conversion: Select e+ and e- tracks from PID by energy loss in TPC. Quality cuts are applied to each e+/e- pair.

π0 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.

π0 signal after background subtraction from EMC-TPC photon pairs

STAR Preliminary

STAR Preliminary


Correlation Function

Correlation function from EMC-TPC photon pairs

2 21 24 sin

2invQ E E

• 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.

A full GEANT simulation will be done to understand the reason of the peak.

STAR Preliminary


New Approach to Photon HBTProposal 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.


Model of Direct Photon ProductionA model of complete space-time source structu

re 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)


Results Using Direct Photons Only

Rlong, Rout variation with pt reflect the emission size dependence on time.


Results from ‘Complete’ Simulation

80M equivalent central STAR events.

3 bins of pair kt for Qosl.

1-D observables: Qinv, Qosl, Qxyz.

Qout, Qside, Qlong


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.


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.


Part II: Muon Telescope Detector


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• ecorrelation charm, bottom production• Advantage over electron PID: no conversionmuch less Dalitz decayOnline trigger in central Au+Au

Hadron Rejection:Usual Muon Detectors: 100 – 200


Muon ID at Low pt

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 pt.


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)


MTD Simulation

Muon detecting efficiency Pion detecting efficiency

Secondary muon from pion decay. Can be subtracted with DCA cut.

Pt (GeV) Pt (GeV)

eff eff


Hadron Rejection and Trigger at RHIC

pT (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.

Cuts Nhit/event

No cut 70

TOF 1.6

Eloss 7.6

TOF&Eloss 0.72

TOF (-400ps,100ps) 0.23


Test Scintillator Trays

1) Two spare scintillator trays outside the magnet2) p+p trigger rate: 1 per 100K events3) Acceptancexeff: 1/200*0.2 = 1/10004) Enhancement for leading charged hadron: 1005) Run6 p+p 2.5M events

equivalent to 250M p+p TPC eventsMinbias p+p events in STAR so far: ~10M

Pt (GeV)

eff


A Promising Muon Detector

New Detector System: developed for QCDLab from BNL

High pT muon Heavy Flavor leptonic dec

ay Quarkonia IMR dilepton, DY

High-pT hadron trigger

Red: MTD angle

Blue: other angles

STAR Preliminary

STAR Preliminary


Part III: Electron (dielectron) physics in HFT


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) % X0 0.282%

Beam Pipe Thickness 0.5 mm or 0.14% X0

30 μm silicon pixels give 10 μm space point resolution. Great help to single electron and di-electron study.


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!


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.


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 π0 Dalitz decays by measuring both electrons, of a pair, in the TPC.


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 fro

m HFT.


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star electro-magnetic physics status and future aspect guoji lin (yale) for star collaboration rhic...

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