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

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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.

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•  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.

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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.

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•  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

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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

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•  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

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•  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

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•  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

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•  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

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•  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

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•  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

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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

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•  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

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•  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

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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

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•  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.

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Summary

Theoretical inputs are very welcome.

Backup

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pT dependence at the LHC

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•  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

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•  Both PYTHIA and EPOS underestimate the yield

•  The percolation model agrees better with data.