EM Probes in STAR
A Look into the Future
Thomas Ullrich, STAR/BNL
International Workshop on
Electromagnetic Probes of Hot and Dense
Matter
ECT, Trento
June 10, 2005
Current STAR Layout and EM Capabilities
Detectors used for EM Probe Detection:
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TPC: tracking pT > 0.2 GeV/c, PID via dE/dx for pT < 0.7-1 GeV/c (-1.3 < h < 1.3)
BEMC & EMC: e/g PID best for p > 1.5 GeV/c, trigger (0 < h < 2)
ToF: electron PID (Df  p/30 -1 < h < 0)
PMD: g detection, p > 20 MeV/c (2.3 < h < 3.7)
FPD: e,g PID for p > 10 GeV, xF > 0.4, small pT (3.4 < h < 4)
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Current EM Capabilities
EMC+BEMC:
 not optimized for low pT EM probes
 large coverage and efficiency for


high-pT electrons (p > 1.5 GeV/c)  open charm, , Z (s = 500 GeV)
high-E photons  high-pT p0, g-jet, jet-jet
ToF Patch:
 good electron PID for pT < 3 GeV/c in conjunction with TPC


successfully used for non-photonic single electrons (open charm)
acceptance of present “prototype” too small for e+e- physics
PMD:
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photon detection down to 20 MeV/c
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DCC studies
g multiplicity and rapidity distributions in forward region
FPD:
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only for low-pT, high-p, xF > 0.4 physics (only p+p, d+Au. or peripheral
Au+Au)

p0, open charm, J/Y (), at high xF
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Electron PID with MRPC TOF/TPC and EMC
EMC
ToF
1.
2.
3.
4.
5.
1.
2.
use TPC for p and dE/dx
use Tower E  p/E
use SMD shape to reject hadrons
e/h discrimination power ~ 105
works for pT > 1.5 GeV/c
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use TPC and ToF PID
works for pT < 3 GeV/c
g and p0 Studies Using the TPC Only
STAR reconstructs p0, g from
conversions in material inside the TPC
 Material budget crucial

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Sweet spot: ~6% radiation
length from vertex to TPC
eff(p0) ~ eff(e)4
PRC 70 (2004) 044902
 130 GeV Au+Au
 Inclusive g from 0 to 2.5 GeV/c
 DE/E = 2%
 Fraction of p0 gg contribution to
inclusive yield decreases in most
central events
 Large systematic uncertainties


~40% p0 normalization
complex interplay of corelated
und un-correlated uncertainties
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Same Idea: Photonic Single Electron Background Subtraction
Opening Angle
Works well for photonic background
rejection in single electron studies:
1. Combine candidate electron with opposite
sign tracks anywhere in TPC
2. Reject tagged track when
m
< mcut ~ 0.1 – 0.15 MeV/c2
Rejection Efficiency:
• g conversion and p0 Dalitz decay
reconstruction efficiency ~60%
Invariant Mass Square
g conversion and p0 Dalitz decay
reconstruction efficiency :
~60% at pT>1.0 GeV/c
Signal
Rejected
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Studies on EM Probes in STAR
PMD:
62 GeV Au+Au
Centrality dependence
of dNg/dy
(nucl-ex/0502008)
FPD:
Forward p0 production
in 200 GeV p+p
(PRL 92 (2004)
171801)
Excellent (e,g)-h
separation
Other studies:
gg-HBT using TPC and EMC/TPC ( a la WA98)
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The Next Step: Upgrades
Barrel Electromagnetic Calorimeter (EMC)
 Current ¾ barrel will be instrumented to full azimuthal coverage,
-1 < h < 1, for next RHIC run
Barrel Time of Flight (TOF)
 Current prototype patches to be upgraded to full azimuth, -1 < h < 1.
 Project is in President’s budget.
Forward Meson Spectrometer (FMS)
 Full azimuthal EM Calorimetry 2.5 < h < 4.0
 Possibility of charm measurements in this region
 Proposal submitted to NSF
Data acquisition upgrade (DAQ1000)
 Upgrade TPC readout an order of magnitude, ~double effective
Luminosity
Heavy Flavor Tracker (HFT)
 High precision (<10 um) measurements for displaced vertices
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Relevant for EM Probes: ToF and HFT
Heavy Flavor Tracker (HFT)
Time-of-Flight: MRPC
Two layers

1.6 cm radius
4.8 cm radius
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24 ladders
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2 cm by 20 cm
MIMOSA Active Pixel Sensor
(CMOS)
Precise (<10 mm) , thin and low
power
50 mm thick chip - air cooling
0.36% radiation length
2
Power budget 100 mW/cm
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
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p/K separation up to 1.6 GeV/c
p/K separation up to 3 GeV/c
Thus cover wider range of
(p,K,p) pT
Full ToF: -1 < h < 1, 2p
Relevant for EM Probes: ToF and HFT
Heavy Flavor Tracker (HFT)
SVT + HFT
Time-of-Flight: MRPC
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Clean D meson sample (v2 !)
Test statistical models

Pythia
p-p 200 GeV
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Au-Au
Statistical
recombination*
D+/D0
0.33
0.455
Ds+/D0
0.20
0.393
Lc+/D0
0.14
0.173
J/y/D0
0.0003
0.0004 (No
suppr.)
ToF + EMC
ToF + TPC
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A. Andronic et al., PLB 571,36 (2003).
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Disentangle b,c contributions
to non-photonic singel electron
spectra
sbb through B J/Y + X (?)

Electron PID p < 3 GeV/c
Exactly where needed for J/Y
Low mass dileptons spectra
vector mesons
allows us to trigger on J/Y
 ToF used as fine granular
g veto
ToF

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complement one another
PID of K, p, p allows D meson
measurements up to higher pT
Low Mass Dileptons: What STAR Can Do
Upgraded detectors:
Full TOF+TPC
Electrons PID
SVT+HFT (m-Vertex detector)
Reject electrons not from primary vertex
(g conversion + Dalitz)
NIM Article in preparation:
Studies on Particle Identification with TPC and ToF


γ conversion and π0 , η Dalitz decay background
How can μVertex detector deal with γ conversion subtraction?
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Low-pT Electron PID with ToF
STAR not hadron blind  a low level of hadron contamination crucial
Study in 62.4 GeV Au+Au
Evaluated through dE/dx fits

Hadron contamination increases for pT > 1.5 GeV/c (eff = const.)

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need to accept slightly lower efficiency at intermediate pT
This is the pT range where EMC because effective
Hadron Rejection Power ~ 10-5 for pT < 1 GeV/c

Def: (hadron contamination)  (e/h) / (electron efficiency)
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Low-Mass Dileptons: Background Rejection
Conversion Electrons only
Background inv. mass spectrum
1 M PYTHIA
Events
Require
TPC+SVT+μVertex (HFT):
~98% electrons from gamma
conversion rejected
Dalitz decays become dominant
sources!!!
Dalitz decay background/event:
~5∙10-6/25MeV (ω)
~5∙10-7/25MeV (Φ)
Total background/event :
~10-4/25MeV (ω)
~2∙10-5/25MeV (Φ)
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What one wants …
R. Rapp, hep-ph/0010101
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Vector Mesons Rate Estimate
Assume:
dn/dy|p 1 (pp)
dn/dy|p 300 (Au  Au)
 p  0.15
f p  0.02
s  15 MeV
s f 8MeV
TOF match+PID eff ≈ 80%
TPC+SVT+μVertex eff ≈ 60% (?)
From PDG: BR (e e- )710-5
BR (f e e- )310-4
Preliminary estimates:
Au+Au
#events for ω
with 3σ signal
#events for Φ
with 3σ signal
TOF+TPC
7M
2M
TOF+TPC+SVT+
μVertex (HFT)
800K
(350K)
150K
(50K)
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b Quark Measurements with HFT
B mesons accessible using semileptonic decay electrons
Issue: nonphotonic electrons will be measured, but what is the real fraction of
these from B? Highly model dependent
Using displaced vertex tag is the most promising method
pT ~ 15 GeV/c:
s (Au+Au) ~ 20mb/Gev 10 nb-1
 yields 200k bb pairs
Non-photonic electrons in d+Au
Tagging in Au+Au (w/ HFT)
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DAQ Upgrades (1000 Hz)
Current limit from TPC front-end electronics is 100 Hz
 Limits size of datasets
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~100M events/nominal RHIC run
Affects available luminosity
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Deadtime scales linearly with rate
50 Hz = 50% dead, i.e. 50% drop in luminosity available to rare
triggers: usual compromise
Proposal to replace TPC electronics with ALICE chips to increase
maximum rate by order of magnitude
 Rate of events to disk increased
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though timely processing of events on disk is an issue
Removes deadtime: effective doubling of RHIC luminosity
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Summary
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STAR has proven capabilities for EM probes and heavy flavor measurements
at RHIC
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PMD: Photon multiplicity
FPD: forward g and electron detection - high xF physics
Electron identification using three detector systems (TPC, TOF, EMC) from 1 to >10 GeV/c
Direct reconstruction of charmed mesons
Shortcoming in PID, vertexing, and acceptance
STAR has a clear path for improving its capabilities in the near future
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Completion and extension of calorimetric coverage
Extension of TOF coverage to full azimuth for electrons and combinatoric background
rejection in direct reconstruction
Upgrade of Data Acquisition to increase effective luminosity and untriggered data
samples
Installation of the heavy flavor tracker for displaced vertices for heavy flavor physics
and photonic electron rejection
Low Mass Vector Mesons and Thermal Dileptons Will Become Part
of STAR’s Program
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STAR Collaboration
545 Collaborators from 51 Institutions
in 12 countries
Argonne National Laboratory
Institute of High Energy Physics - Beijing
University of Bern
University of Birmingham
Brookhaven National Laboratory
California Institute of Technology
University of California, Berkeley
University of California - Davis
University of California - Los Angeles
Carnegie Mellon University
Creighton University
Nuclear Physics Inst., Academy of Sciences
Laboratory of High Energy Physics - Dubna
Particle Physics Laboratory - Dubna
University of Frankfurt
Institute of Physics. Bhubaneswar
Indian Institute of Technology. Mumbai
Indiana University Cyclotron Facility
Institut de Recherches Subatomiques de Strasbourg
University of Jammu
Kent State University
Institute of Modern Physics. Lanzhou
Lawrence Berkeley National Laboratory
Massachusetts Institute of Technology
Max-Planck-Institut fuer Physics
Michigan State University
Moscow Engineering Physics Institute
City College of New York NIKHEF
Ohio State University
Panjab University
Pennsylvania State University
Institute of High Energy Physics - Protvino
Purdue University
Pusan University
University of Rajasthan
Rice University
Instituto de Fisica da
Universidade de Sao Paulo
University of Science and Technology of China USTC
Shanghai Institue of Applied Physics - SINAP
SUBATECH
Texas A&M University
University of Texas - Austin
Tsinghua University
Valparaiso University
Variable Energy Cyclotron Centre. Kolkata
Warsaw University of Technology
University of Washington
Wayne State University
Institute of Particle Physics
Yale University
University of Zagreb
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