 production in p+p
collisions in STAR
Manuel Calderón de la Barca Sánchez
UC Davis
STAR Collaboration
23d Winter Workshop on Nuclear Dynamics
Big Sky, Montana
2-15-07
Goal: Quarkonia states in A+A
Charmonia: J/y, Y’, cc
Bottomonia: (1S), (2S), (3S)
Key Idea: Melting in the plasma

Color screening of static potential between heavy quarks:
– J/y suppression: Matsui and Satz, Phys. Lett. B 178 (1986) 416
Suppression of states is determined by TC and their binding energy
 Lattice QCD: Evaluation of spectral functions  Tmelting (next talk!)

Sequential disappearance of states:
 Color screening  Deconfinement
 QCD thermometer  Properties of QGP
When do states really melt?
Tdiss(y’)  Tdiss(cc)< Tdiss((3S)) < Tdiss(J/y)  Tdiss((2S)) < Tdiss((1S))
H. Satz, HP2006
 :Pros for theory interpretation
, ’, ’’ sequential suppression



(1S) no melting at RHIC (nor LHC?)  standard candle
(reference)
(2S) likely to melt at RHIC (analog J/y)
(3S) melts at RHIC (analog y’)
Pros


co-mover absorption negligible
recombination negligible at RHIC
 Both of these affect charmonia, but not
bottomonia.
 : Experimental Pros and Cons
Cons

Mass resolution pushed to the limit
 Ratio extraction (2S/1S) and (3S/1S) possible, but difficult

extremely low rate
 BR x ds/dy(1s+2s+3s)=91 pb
 from NLO calculations.
 Luminosity limited (RHIC II will substantially help)
 pp Run 6 ~ 9 pb-1 (split into 2 triggered datasets)
Pros

Efficient trigger
 ~80%
 works in p+p up to central A+A!

Large acceptance at midrapidity
 Run VI = Run IV x 4

Small background at M~10 GeV/c2.
 STAR’s strength are the  states
STAR Detectors Used for  Analysis
• EMC
• Acceptance: || < 1 , 0 <  < 2
• PID : EMC Tower (energy)  p/E
• High-energy tower trigger  enhance high-pT sample
• Essential for quarkonia triggers
• Luminosity limited for 
• TPC
• Tracking and dE/dx PID for electrons & positrons
 Mass Resolution and expected s

STAR detector does not resolve
individual states of the 
 Finite
p resolution (B=0.5 T)
 e-bremsstrahlung


Yield is extracted from combined
++ states
FWHM ≈ 0.7 GeV/c2
W.-M. Yao et al. (PDG), J. Phys. G 33, 1 (2006);
R. Vogt et al., RHIC-II Heavy Flavor White Paper
State
Mass [GeV/c2]
9.46030

10.02326

10.3552

++
Bee [%]
(dσ/dy)y=0
Bee×(dσ/dy)y=0
2.38
2.6 nb
62 pb
1.91
0.87 nb
17 pb
2.18
0.53 nb
12 pb
91 pb
STAR  Trigger
Sample -triggered Event
• e+e- candidate
• mee = 9.5 GeV/c2
• cosθ = -0.67
• E1 = 5.6 GeV
• E2 = 3.4 GeV

Offline:
charged tracks +
EMC tower
Fast L0 Trigger (Hardware)
 Select events with at least one  high
energy tower (E~4 GeV)

L2 trigger (Software)
 Clustering, calculate mee, cos q.


Very clean to trigger up to central Au+Au
Offline: Match TPC tracks to triggered
towers
 Acceptance in STAR

Simulations Run 6 Conditions
 Including detector variations:
Calorimeter crates
removed/recovered
Hot towers masked
 Two Trigger setups:
Acc = 0.272±0.01 for |y|<0.5
(set 1)
Acc = 0.263±0.019 for |y|<0.5
(set 2)
– Set 2 used in results shown
today.
 Trigger Efficiency

Simulation of Trigger response
 Level-0: Fast, Hardware
Trigger, Cut on Single Tower Et
L0 triggered/accepted =
0.928±0.049
 Level-2: Software Trigger, Cut
on invariant mass of tower
clusters
L2 triggered/L0 triggered=
0.855±0.048

Acceptance x Trig Efficiency
~19-21%
 Analysis: Electron Id with TPC and EMC
K
p
d

electrons
e
π
preliminary





 trigger enhances electrons
Use TPC for charged tracks
selection
Use EMC for hadron rejection
Electrons identified by dE/dx
ionization energy loss in TPC
Select tracks with TPC, match to
EMC towers consistent with trigger
preliminary
preliminary
Electron PID Efficiency and Purity
dE/dx cut
dE/dx cut

dE/dx cut
dE/dx cut
Electron Pair PID+Tracking efficiency= 0.47±0.07
STAR  Invariant Mass
preliminary
preliminary
 Signal + Background  unlike-sign electron pairs
 Background  like-sign electron pairs
 (1S+2S+3S) total yield: integrated from 7 to 11 GeV from
background-subtracted mee distribution (0.96 of total)
 Peak width consistent with expected mass resolution
 Cross Section and Uncertainties
 ds 
N
BRee  
 
 dy  y 0 dy      Ldt
=geo×L0×L2×2(e)×mass
geo
L0
L2
2(e)
mass

preliminary
0.263±0.019
0.928±0.049
0.855±0.048
0.47±0.07
0.96±0.04
0.094±0.018
•geo : geometrical acceptance
•L0 : efficiency of L0
•L2 : efficiency of L2
•(e) : efficiency of e reco
•mass: efficiency of mass cut
STAR  Cross Section at Midrapidity
N

48±15(stat.)
0.094±0.018
dy
preliminary
Ldt (5.6±0.8) pb-1
1.0
 '  ''
 ds 
BRee  

 dy  y 0
 91  28 (stat.)  22 (syst.) pb
STAR  vs. theory and world data
prelimina
ry
 '  ''
 ds 
BRee  

 dy  y 0
 91  28(stat.)  22(syst.) pb
STAR 2006 √s=200 GeV p+p
++→e+e- cross section consistent
with pQCD and world data trend
Only RHIC peeks at √s=200 GeV range
Outlook for Run VII Au+Au

Run IV Au+Au
Events sampled per day
Yield estimate:
 17 Week run ~ 100 days
 Run 4 Performance
107
 4-20 M events/day
 For Run VII:
106
 Assume:
 400 – 2000 M events

-1 - 0.3 nb-1
60
mb
Ldt


  cross section in Au+Au
a
 Using
s AB  s pp ( AB)
 with a=0.9, (AB)a ~ 13,500
 dsAuAu/dy|y=0=91 pb x 13500 = 1.2 mb-1
  produced at y=0 in dy=1 ~ 73 – 368
  after acc. & eff. ~ 7 – 37
 Yes, its tough!!!
Summary
Full EMC + trigger  quarkonium program
in STAR
 Run 6: first midrapidity measurement of
++→e+e- cross section at RHIC in p+p
collisions at √s=200 GeV

 BRee×(dσ/dy)y=0=91±28(stat.)±22(syst.) pb
 STAR  measurement is consistent with
pQCD and world data trend
Next run:
Towards a STAR  cross section in
Au+Au collisions at √s=200 GeV

Extra Slides
18
STAR J/y Trigger

L0 (hardware)
 J/y topology trigger: two
towers above ET≈1.2 GeV
 Separated by 60° in φ

L2 (software)
 Match EMC high tower to




High background
contamination ~1.5 GeV/c
Rejection~100  not sampling
full luminosity
Challenging analysis!!!
Efficiency × acceptance ≈ 12%
preliminary
CTB slat  photon
rejection
 Tower clustering
 Cut on mee=√2E1E2(1cosθ)
 Cut on cosθ
STAR J/y Signal
preliminary
• Signal in 200 GeV p+p from 2006
• Tested and working trigger in p+p
• No trigger for Au+Au until full ToF in 2009
• Integrated luminosity in 2006: 377 nb-1
• Analysis in progress
preliminary