measurement of j/ψ production in p+p collisions at s = 500 ...• stronger-than-linear rise of open...
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Measurement of J/ψ production in p+p collisions at √s = 500 GeV
at STAR experiment Rongrong Ma (BNL)
Hard Probes 2015 McGill University, Montreal, Canada
June 29 – July 3, 2015
• Motivation • STAR experiment • J/ψ measurements – Cross section – Yield ratio of ψ(2s) to J/ψ – J/ψ yield vs. event activity
• Summary
06/30/2015 Rongrong Ma (BNL), HP 2015 2
Outline
J/ψ measurements in heavy-ion collisions • Quarkonia are predicted to be sequentially
melted in Quark Gluon Plasma due to color-screening of the parton constituents àa thermometer of the medium.
• Suppression of J/ψ was proposed as a direct probe of deconfinement.
06/30/2015 Rongrong Ma (BNL), HP 2015 3
• The pp results serve as a reference for the same measurements in heavy-ion collisions.
• Need to fully understand the production mechanism in pp collisions.
Do we understand J/ψ in pp? • NRQCD long-distance matrix
at Next-to-Leading Order from world-data fitting.
06/30/2015 Rongrong Ma (BNL), HP 2015 4
Phys. Rev. D84 (2011) 051501
Do we understand J/ψ in pp? • NRQCD long-distance matrix
at Next-to-Leading Order from world-data fitting.
06/30/2015 Rongrong Ma (BNL), HP 2015 5
• Good data-theory agreement over 0 < pT < 30 GeV/c
• Color Glass Condensate effective theory to calculate cross section at low pT
Low pT
Phys. Rev. D84 (2011) 051501 Phys.Rev.Le4. 113 (2014) 192301
A closer look: event multiplicity dependence
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• Collective effects in high-multiplicity pp collisions? • Do we see similar or different behavior at RHIC?
• Stronger-than-linear rise of open charm production vs event activity.
• Similar behavior seen for inclusive J/ψ at both mid- and forward-rapidity.
• Several ideas on the market: – PYTHIA 8: c and b quarks produced
in Multi-Parton-Interaction -> underestimate yield at large multiplicity
– Percolation model: string screening -> quadratic rise at high multiplicity
– Hard process is associated with larger gluon radiation
arXiv:1505.00664
D meson
The Solenoid Tracker At RHIC (STAR)
Ø TPC: precise momentum and energy loss
Ø TOF: fast detector Ø BEMC: trigger on and
identify electrons Ø MTD: trigger on and
identify muons Ø Cover 45%
geometrical acceptance within |η|<0.5
06/30/2015 Rongrong Ma (BNL), HP 2015 7
Muon Telescope Detector
Barrel ElectroMagne3c Calorimeter
Time Projec3on Chamber
• Mid-rapidity detector: |η| < 1, 0 < φ < 2π
Time-‐Of-‐Flight
J/ψ measurements at high pT
06/30/2015 Rongrong Ma (BNL), HP 2015 8
• Electron identification – One decay electron fire trigger – 0.3 < ptrack/Ecluster < 1.5 – |nσe|<2
• Decay channel: J/ψ à e+ + e- • Data set: p+p collisions at 500 GeV taken in 2011 • High Tower (HT) trigger with a threshold of 3.5 GeV/c
using BEMC – Sampled integrated luminosity ~ 22 pb-1
nσ e =1Rlog (dE / dx)measured
(dE / dx)electron
Extract J/ψ yield
06/30/2015 Rongrong Ma (BNL), HP 2015 9
• Clear signals for ψ(2s) and J/ψ • Combinatorial background:
estimated by like-sign pairs and subtracted.
• Correlated background: estimated by fitting Crystal ball function (signal) + exponential function
Ø Signal extraction: bin counting within [2.7,3.2] GeV/c2
J/ψ cross section above 4 GeV/c
06/30/2015 Rongrong Ma (BNL), HP 2015 10
• Cross section measured within 4 < pT < 20 GeV/c • NRQCD prediction agrees well with data
STAR preliminary NRQCD PRL 106 (2011) 042002 PRD 84 (2011) 114001 JHEP05 (2015) 103
J/ψ cross section above 4 GeV/c
06/30/2015 Rongrong Ma (BNL), HP 2015 11
• Cross section measured within 4 < pT < 20 GeV/c • NRQCD prediction agrees well with data • Follows xT scaling with n ~ 5.6, which reflects number of active
partons in J/ψ production
s/T
= 2pTX-210 -110
]2dy
)[nb/
(GeV
/c)
Tdp Tpπ
/(2σ2B
d5.
6s
810
910
1010
1110
1210
1310
1410
1510
1610
1710STAR p+p 500GeV, this analysisSTAR p+p 200GeV, PLB722, 56
630GeV, PLB256, 112pUA1 p+ 1.96TeV, PRD71, 032001pCDF p+
STAR preliminary
STAR preliminary NRQCD PRL 106 (2011) 042002 PRD 84 (2011) 114001 JHEP05 (2015) 103
Yield ratio of ψ(2s) to J/ψ
06/30/2015 Rongrong Ma (BNL), HP 2015 12
• Follows world data trend with pT, and no obvious collision energy dependence
• Help to pin down the feed-down contribution from ψ(2s) to J/ψ
(GeV/c)T
p0 1 2 3 4 5 6 7 8 9 10
)ψ
(J/
σdψ
J/(2
s))/B
ψ('σd
(2s)
ψB
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08 HERA-B (e channel ), 42GeV, EPJC49, 545 channel), 42GeV, EPJC49, 545µHERA-B (
PHENIX, 200GeV, PRD85, 092004CDF, 1.8TeV, PRL79, 572STAR 500GeV, this analysis
STAR preliminary
~18% of systematic uncertainty
J/ψ measurements at low pT
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• Multi-gap Resistive Plate Chamber (MRPC) – gas detector, avalanche mode
• Installed behind the return iron bars of the magnet – 5 interaction length
• Muon identification utilizing precise timing measurement.
• Double-end readout -> measure hit position along the beam direction.
• Enabled by the new Muon Telescope Detector
• In 2013, 63% of MTD was installed. MTD trigger commissioned in May.
• Installation completed in early 2014 – 122 trays, 1439 readout strips and 2878 readout channels
Extract J/ψ yield
06/30/2015 Rongrong Ma (BNL), HP 2015 14
• Muon identification – Match TPC tracks to MTD – Require z residual below 20 cm
• Decay channel: J/ψ à µ+ + µ- • Data set: p+p collisions at 500 GeV taken in 2013 • MTD dimuon trigger: two hits in MTD • Sampled integrated luminosity ~ 7.7 pb-1
)2 (GeV/cµµM2.5 2.6 2.7 2.8 2.9 3 3.1 3.2 3.3 3.4 3.5
Cou
nts
50
100
150
200
250
300 Run13 p+p @ 500 GeV
> 0 GeV/cψT,J/
p-µ+µ→ψJ/
Unlike-sign pairs
Fit signal+bkg
STAR preliminary
• Background: fitting Gaussian (signal) & expo+pol0
Ø Signal extraction: bin counting within [2.8,3.3] GeV/c2
Characterize event activity
06/30/2015 Rongrong Ma (BNL), HP 2015 15
Multiplicity of TOF matched tracks
|η| < 0.9
Beam-Beam Counter coincidence rate600 800 1000 1200 1400 1600 1800 2000
310×
TofM
ult
0
10
20
30
40
50
60
1
10
210
310
410
510
610Run11 p+p @ 500 GeV
Mean of each BBCrate sliceSTAR preliminary
• Insensitive to pile-up effects
TofMult< TofMult >
J/ψ yield vs. event activity
06/30/2015 Rongrong Ma (BNL), HP 2015 16
• Stronger-than-linear growth in high multiplicity events
<TofMult>TofMult
0 0.5 1 1.5 2 2.5 3 3.5 4
>ψ
J/<N
ψJ/N
0
1
2
3
4
5
6
7
8
9
10
> 0 GeV/cψT,J/
, p-µ+µ→ψJ/
< 8 GeV/cψT,J/
, 4 < p-e+e→ψJ/ > 8 GeV/c
ψT,J/, p-e+e→ψJ/
STAR p+p @ 500 GeV
STAR preliminary
+15% one-sided error along both x- and y- direction
• +15% one-sided global errors along both x- and y- directions – Work in progress to reduce it
• Clear correlation between soft and hard processes – Different trends for J/ψ yield
vs. event activity at low and high pT
Compare to LHC
06/30/2015 Rongrong Ma (BNL), HP 2015 17
• The rising trend is similar at RHIC compared to LHC – Universal dependence of
relative J/ψ yield on event activity at different energy?
ALICE J/ψ: Phys.Le4. B712 (2012) 165-‐175 ALICE D-‐meson: arXiv:1505.00664
Data vs PYTHIA
06/30/2015 Rongrong Ma (BNL), HP 2015 18
Default tune of PYTHIA 8.183 • Both the rising trend and pT dependence observed in data can be reasonably reproduced by PYTHIA8 – It seems to do a better job at
RHIC than LHC
Data vs Percolation model
06/30/2015 Rongrong Ma (BNL), HP 2015 19
• The trend is also qualitatively reproduced by percolation model. – Stronger rise at large
multiplicity than PYTHIA8
• Both the rising trend and pT dependence observed in data can be reasonably reproduced by PYTHIA8 – It seems to do a better job at
RHIC than LHC
• Test with larger multiplicity bins is important
Percolation model: PRC 86 (2012) 034903 private communication
• Inclusive J/ψ cross section are measured above 4 GeV/c via the di-electron channel. – Agrees with NRQCD calculation.
• For the first time, J/ψ is reconstructed via the di-muon channel at STAR using the new MTD.
• The relative J/ψ yield grows rapidly as the event multiplicity increases, and the high pT J/ψ grows even faster than the low pT J/ψ. – Work in progress to reduce the systematic uncertainties.
• PYTHIA8 and percolation model can reproduce the rising trend of the J/ψ. PYTHIA8 can also describe the high pT range. – Test with even higher multiplicity bins is important.
06/30/2015 Rongrong Ma (BNL), HP 2015 20
Summary
Theoretical inputs are very welcome.
Backup
06/30/2015 Rongrong Ma (BNL), HP 2015 21
pT dependence at the LHC
06/30/2015 Rongrong Ma (BNL), HP 2015 22
• Clear pT dependence of the trend.
• Almost a factor 2 of difference between low pT and high pT J/ψ at lowest and highest multiplicity bins
Comparison with models at LHC
06/30/2015 Rongrong Ma (BNL), HP 2015 23
• Both PYTHIA and EPOS underestimate the yield
• The percolation model agrees better with data.
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