Transcript Document

STAR di-lepton measurements
Lijuan Ruan for the STAR Collaboration
(Brookhaven National Laboratory)
Outline:
• Motivation and Introduction
• Results from 200 GeV p+p collisions
• Projections from 200 GeV Au+Au collisions
• Conclusions and outlook
4/1/2010
MRPC workshop at USTC, Lijuan Ruan (BNL)
1
What have we learnt so far at RHIC
A hot, dense medium with partonic degrees of freedom
created at RHIC:
Jet quenching
Baryon enhancement, number of constituent quark
scaling in elliptic flow
…
Next:
Is the system thermalized and how does the system thermalize?
What are the properties of the strongly-coupled system?
What is the phase structure of QCD matter?
What exotic particles are produced at RHIC?
What is the mechanism for partonic energy loss?
Does QCD matter demonstrate novel symmetry properties?
What is the nature of the initial state in nuclear collisions
…
4/1/2010
MRPC workshop at USTC, Lijuan Ruan (BNL)
2
STAR’s di-lepton program
The initial temperature of sQGP; the mass origin of hadrons;
color screening features of heavy quarkonia …
Measurements
low mass di-leptons
Physics
thermal radiation of QGP;
in-medium modifications of vector
meson (  ), chiral symmetry
restoration
intermediate mass di-leptons
thermal radiation of QGP;
heavy flavor modification;
resonances in sQGP
large mass: heavy quarkonia
T of QGP, color screening,
quarkonium production mechanism
4/1/2010
MRPC workshop at USTC, Lijuan Ruan (BNL)
3
Di-electron continuum at NA60
NA60, PRL96,162302(2006)
NA60, PRL100,022302(2008)
1. In-medium modifications of rho0 spectra in In-In collisions at NA60
2. Intermediate mass di-electrons show less radial flow.
4/1/2010
MRPC workshop at USTC, Lijuan Ruan (BNL)
4
Di-lepton Spectrum at PHENIX
PHENIX, PRC81,034911(2010)
1. In-medium modifications of rho0 can not explain the enhancement at 0.15-0.7 GeV/c2
2. Charm contribution to di-lepton spectra is significant at low mass at RHIC.
3. Charm contribution to di-lepton spectra might be dominant in the intermediate mass
region. Its correlation makes a big difference to access the thermal radiation contribution
from
QGP.
4/1/2010
MRPC workshop at USTC, Lijuan Ruan (BNL)
5
STAR detector: TPC, EMC & MRPC-TOF
Time Projection Chamber
1.
2.
3.
Tracking
Ionization energy loss (dE/dx):
(, K) < 0.7 or > 3 GeV/c,
proton < 1 or > 3 GeV/c
Coverage -1.8<<1.8
Electro-magnetic Calorimeter
1.
A new technology (TOF) ---Multi-gap Resistive Plate Chamber
adopted from CERN-Alice.
1.
2.
3.
Good timing resolution,
(, K) ~ 1.6 GeV/c, proton ~ 3 GeV/c
Coverage: -0.9<<0.9
Coverage
(4% y2008, 72% y2009, 100% y2010)
electrons and photons
4/1/2010
MRPC workshop at USTC, Lijuan Ruan (BNL)
6
STAR Time of Flight detector performance
200 GeV d+Au collisions
200 GeV d+Au collisions
STAR Collaboration, nucl-ex/0309012
|1/-1|<0.03
TOF PID: (, K) ~ 1.6, proton ~ 3 GeV/c
STAR Collaboration, PLB616(2005)8
TOF enables clean electron PID up to
PT < 3 GeV/c.
STAR Collaboration, PRL94(2005)062301
STAR Collaboration, nucl-ex/0407006
M. Shao et al., NIMA 558(2006)419
4/1/2010
MRPC workshop at USTC, Lijuan Ruan (BNL)
7
Electron ID in p+p: high efficiency and purity
STAR Preliminary
STAR Preliminary
The dE/dx cut efficiency on e
f(p )
T
high electron purity: 99%
Efficiency: greater than 60% on the dE/dx cut
Acceptance for |eta|<1 with TOF
in y2009: 56%
In y2010: 78%
4/1/2010
MRPC workshop at USTC, Lijuan Ruan (BNL)
8
Di-lepton background
Mixed-event normalized to like-sign within 0.4 - 1.5 GeV/c2
Cross-pair bg. can only be described by like-sign method.
For minimum bias events, we subtracted like-sign bg at Mee<0.7
GeV/c2, mixed-event bg subtraction was applied at Mee>0.7 GeV/c2.
4/1/2010
MRPC workshop at USTC, Lijuan Ruan (BNL)
9
Di-lepton background
Mixed-event normalized to like-sign within 1 - 3 GeV/c2
Cross-pair bg. can only be described by like-sign method.
For minimum bias events, we subtracted like-sign bg at Mee<0.7
GeV/c2, mixed-event bg subtraction was applied at Mee>0.7 GeV/c2.
4/1/2010
MRPC workshop at USTC, Lijuan Ruan (BNL)
10
Di-lepton invariant mass spectrum in p+p
collisions --- low mass range
di-lepton at y2009 p+p collisions: signal excess above background
ω: S/B~0.7, significance: 5 σ
: S/B~1, significance: 4 σ
4/1/2010
MRPC workshop at USTC, Lijuan Ruan (BNL)
11
Di-lepton invariant mass spectrum in p+p
collisions --- full range
S/B ranges from 0.1 to 100 versus mass in p+p collisions
4/1/2010
MRPC workshop at USTC, Lijuan Ruan (BNL)
12
Cocktail Simulation in 200 GeV p+p at STAR
STAR: π, KS, K*, , ρ
PHENIX: η, ω, J/
Z. Tang et al., PRC 79,051901(R)(2009); M. Shao et al.,
J. Phys. G: Nucl. Part. Phys. 37 (2010) 085104
ccbar: star measured cross section as input;
STAR Collaboration, PRL94(2005)062301
bbar/ccbar ratio from PYTHIA
Particle spectra from data were fit with Tsallis function, and
were used as an input to full GSTAR simulation
to generate e+e- invariant mass cocktail.
Same cuts were applied in simulation as in data.
4/1/2010
MRPC workshop at USTC, Lijuan Ruan (BNL)
13
ω invariant yields comparison in p+p collisions
PHENIX results:
arXiv:1005.3674
Z. Tang et al., PRC 79,051901(R)(2009)
Tsallis fit is only the fit
function of the previous
published high pT ω
yields reconstructed by
hadron channel!
4/1/2010
MRPC workshop at USTC, Lijuan Ruan (BNL)
14
Di-lepton projection with y2010 Au+Au data
STAR: low material budget, full Time of Flight coverage
beam energies
(Au+Au)
minbias events
(million)
ωee projection
(S/B, significance)
φee projection
(S/B, significance)
200 GeV
355
1/180, 27 σ
1/120, 17 σ
62 GeV
143
15 σ
10 σ
39 GeV
250
22 σ
14 σ
What can STAR do in the near future with data on disk:
the low mass continuum, 0.15-0.7 GeV
vector mesons: ρ, ω, ee
virtual photons
intermediate mass continuum (thermal radiation)
ccbar modification
J/ ee at low pT
…
4/1/2010
MRPC workshop at USTC, Lijuan Ruan (BNL)
15
Charm Contribution to Di-lepton Spectrum at
RHIC
PHENIX, PRC81,034911(2010)
1. Charm contribution to di-lepton spectra is significant at low mass at RHIC.
2. Charm contribution to di-lepton spectra might be dominant in the intermediate
mass region. Its correlation makes a big difference to access the thermal
radiation
4/1/2010 contribution from
MRPCQGP.
workshop at USTC, Lijuan Ruan (BNL)
16
Distinguish heavy flavor and initial lepton pair
production: e-muon correlation
Z. Xu, BNL LDRD 07-007; L. Ruan et al., Journal of Physics G: Nucl.
Part. Phys. 36 (2009) 095001
NA60, PRL100,022302(2008)
e correlation simulation with Muon Telescope
Detector at STAR from ccbar:
S/B=2 (Meu>3 GeV/c2 and pT(e)<2 GeV/c)
S/B=8 with electron pairing and tof association
R. Rapp, hep-ph/0010101
4/1/2010
MTD: construction starts in FY2011;
project completion in FY2014
MRPC workshop at USTC, Lijuan Ruan (BNL)
17
Di-lepton measurements at STAR
FY09/10/11
FY12/13
From FY14 on
Detector
components
TPC+TOF+EMC
TPC+TOF+EMC+MTD(p)
TPC+TOF+EMC+MTD+
HFT
Measurements
J/psiee, Upsilonee
J/psi RAA, v2 vs. pT &Npart,
Upsilon RAA vs. Npart
FY12:
first e-µ measurement
BJ/psiXµµX
di-electron continuum
di-electron spectra vs. pT
FY13:
J/psiµµ, Upsilonµµ
J/psi RAA,v2, vs. pT & Npart,
first look at different
Upsilon states
di-muon continuum
Physics
4/1/2010
Color screening features,
quarkonia production
mechanisms,
vector meson in-medium
modifications,
low mass enhancement,
Intermediate mass
Better understanding on
thermal radiation from
QGP at intermediate mass
and vector meson inmedium modifications
MRPC workshop at USTC, Lijuan Ruan (BNL)
Precise measurements
on different Upsilon
states RAA versus Npart,
J/psiµµ RAA, v2 versus
pT & Npart,
di-muon continuum,
e-µ,
D v2 and RAA
Measure thermal
radiation from QGP at
intermediate mass,
vector meson inmedium modifications,
color screening
features
18
Summary
•
STAR has a bright future on the di-lepton program.
•
Di-lepton continuum measured in 200 GeV p+p collisions, cocktail
simulation is consistent with data.
•
Near term in Au+Au with the TOF:
low mass continuum
vector meson in-medium modifications
virtual photons
intermediate mass thermal radiation
ccbar modification
quarkonia
rare decay and exotics …
•
Long term with the MTD:
e-muon correlation to distinguish heavy flavor production from
initial lepton pair production
di-muon continuum …
4/1/2010
MRPC workshop at USTC, Lijuan Ruan (BNL)
19
Search for rare decay e+e-: an example
e+e- PDG BR <2.7x10-5
SM: ~10-9
New physics: ~10-8, 10-7
RHIC: ideal for such search;
>1011  annual yield at |y|<1;
Clean low-pt electron identification
CLEO Collaboration, Phys.Rev. D 56, 5359 (1997);
CELSIUS/WASA Collaboration, arXiv:0711.3531,
Phys.Rev. D 77, 032004 (2008);
A.E. Dorokhov, arXiv:0905.4577;
J.D. Bjorken et al., Phys.Rev. D 80, 075018 (2009);
M.J. Savage, M.E. Luke and M.B. Wise, Phys.Lett.
B 291, 481 (1992);
L. Bergstrom, Z. Phys. C 14, 129 (1982).
STAR Simulation
~107 
4/1/2010
MRPC workshop at USTC, Lijuan Ruan (BNL)
20