Vector meson photo-production at RHIC

using \/sNN = 200 GeV Au+Au collisions

L. Chanaka De Silva Hawaii 2014, Hawaii, USA 10th October 2014 1

For the STAR collaboration

L. Chanaka De Silva, Creighton University

Fourth Joint Meeting of the Nuclear Physics Divisions of the

American Physical Society and The Physical Society of JapanHawaii, USA, 7 -11 October 2014

10th October 2014

• Ultra peripheral collisions

• STAR UPC trigger

• STAR Highlights

• Future directions

• Offline analyses cuts

• Data and comparison to theory

• Summary and outlook

2L. Chanaka De Silva Hawaii 2014, Hawaii, USA

10th October 2014

• High energy photonuclear reaction

• Impact parameter (b) is larger than the sum of radii of participating nuclei

• Nuclei interact via strong electromagnetic fields– EM field can be viewed as a flux of quasi-real photons

– Photon flux is proportional to Z2

– Photon flux is well described in Fermi-Weizsäcker-Williams approximation

– Hadronic processes are strongly suppressed

– Maximum CM photon energy at RHIC ~ γh/RA ~ 3GeV

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Ultra Peripheral Collision (UPC)

Physics motivations• To study non linear effects at low-x in the gluon

distribution of the target

• To measure nuclear gluon shadowing and saturation (via vector meson photo-production)

• To study partonic form factors

L. Chanaka De Silva Hawaii 2014, Hawaii, USA

10th October 2014 4

STAR - Solenoidal Tracker At RHIC• Large acceptance; TPC: -1 < η < 1 and –π < ϕ < π

• Excellent PID; ToF and TPC dE/dx

STAR UPC Trigger• 1≤ beam neutrons ≤ 6 in both ZDCs

• veto on small BBC tiles 3 < |η| < 5

• 2 ≤ TOF hits ≤ 6

• Low multiplicityL. Chanaka De Silva Hawaii 2014, Hawaii, USA

Alex Schmah Y. Gorbunov

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• ρ0 photo-production in AuAu collisions at \/sNN = 64 GeV with the STAR detector

- Phys. Rev. C 85 (2012) 14910

• Observation of π+π-π+π- photo-production in ultra peripheral heavy-ion collisions at \/sNN = 200 GeV at the STAR detector

- Phys. Rev. C 81 (2010) 44901

• Observation of two-Source Interference in the photo-production reaction AuAu -> AuAuρ0

- Phys. Rev. Lett. 102 (2009) 112301

• ρ0 photo-production in ultra-peripheral

relativistic heavy ion collisions at \/sNN = 200 GeV with STAR

- Phys. Rev. C 77 (2008) 34910

• Production of e+e− pairs accompanied by nuclear dissociation in ultra peripheral heavy-ion collisions

- Phys. Rev. C 70 (2004) 031902(R)

• Coherent ρ0 production in ultra peripheral heavy-Ion collisions

- Phys. Rev. Lett. 89 (2002) 272302

STAR publications

L. Chanaka De Silva Hawaii 2014, Hawaii, USA

First π spectra from two different triggers, 2002 e+e- pair rapidity compared to theory, 2004

ρ0 production cross section, 2008 Resonant ρ0 production, 2008

ρ0 production differential cross section, 2008

π+π-π+π- photonuclear production, 2010 Energy dependence of total ρ0 cross section, 2012

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ρ0 photo-production with higher statistics 2010 AuAu 200 GeV data

L. Chanaka De Silva Hawaii 2014, Hawaii, USA

J/ψ photo-production using 2010 AuAu 200 GeVdata• To study non linear effects at low-x in the gluon

distribution of the target

• To measure nuclear gluon shadowing and saturation (via vector meson photo-production)

• Cross section for mid rapidity J/ψ production can provide insight in to nuclear gluon distribution dσ/dy ~ [g(x,Q2)2]

• We focus on observing the diffractive pattern with higher statistics, and also repeat measurements we had in earlier publications• Study of Au partonic form factor via Fourier transform of d2σ/dtdy• Diffraction pattern detail helps constraining dipole cross section models

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GA (x,Q2)∝g(x,Q2)

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x=MV

SNNe±y

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dσ

dt∝Fg

2 (x, t)

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• 37 Million 200 GeV Au+Au UPC trigger events ( )• XnXn; Multiple neutron emissions from either nuclei • Z-vertex position; [-100, 100]cm• Fewer than 10 total tracks per event• An event vertex has two tracks associated to it (exclusive

production)• Mass cuts

• ρ: 0.4<Minv<1.1 GeV/c2

• J/ψ: 2.5<Minv<3.5 GeV/c2

• Background modeled with like-sign pairs, scaled to match data for pT > 0.2 GeV/c

• yDistribution– coherent cut; pT < 0.15 GeV/c

– 0.05 < |y| < 1.0 (Lower limit for cosmic rays/upper limit for STAR acceptance)

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L. Chanaka De Silva Hawaii 2014, Hawaii, USA

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• Blue: unlike-sign pairs

• Red: background estimated with like-sign pairs

• The peaks at low transverse momenta is consistent with coherent production

• For analysis, only events with pT < 0.15 GeV/c are used

• Only statistical error bars are presented

L. Chanaka De Silva Hawaii 2014, Hawaii, USA

2010 AuAu 200 GeV dataOnly acceptance corrected

ρ -> π+π- J/ψ -> e+e-

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• ρ mass is fit with a cocktail that includes a Söding term for interference with direct π pair production

• J/Ψ is fit with exponential for e+e- continuum, Gaussian for J/ψ signal and polynomial for background

• Approximately 0.65M ρ mesons are reconstructed

• Approximately 125 J/ψ mesons are reconstructed

• Only statistical error bars are presented

ρ J/ψ 2010 AuAu 200 GeV data

Pure ρ0

Direct ππ

Interference

Polynomial Background

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• After preliminary acceptance corrections

• Dip at y ~ 0 is from the cut to remove cosmic rays

• Trigger/Vertex and tracking efficiency studies are still in-progress

L. Chanaka De Silva Hawaii 2014, Hawaii, USA

2010 AuAu 200 GeV dataρ -> π+π- J/ψ -> e+e-

10th October 2014 11L. Chanaka De Silva Hawaii 2014, Hawaii, USA

• Blue: acceptance-corrected data

• Red: Klein-Nystrand model with mutual Coulomb dissociation

- S.R. Klein, J. Nystrand, Phys. Rev. C 60 (1999) 014903

• Black: Color dipole model, where the dipole nucleon cross section is from the IIM saturation model

- Gonçalves, Machado, Phys. Rev. C 84 (2011) 011902

2010 AuAu 200 GeV data

• Vector meson ratio gives an additional constraint on different photo-production mechanisms. E.g.: LO pQCD models, color dipole models and Glauber models

Color dipole model

STAR pre

limin

ary

• Study of Au partonic form factor via Fourier transform of d2σ/dtdy- t-Distribution is predominantly due to the transverse spread of gluons in the nucleon /

nuclear density profile

• By applying a Fourier transformation on preliminary data, our result for Au radius is consistent with 6.38 fm

• Diffraction pattern detail helps constraining dipole cross section models

10th October 2014 12L. Chanaka De Silva Hawaii 2014, Hawaii, USA

ρ

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dσ

dt∝Fg

2 (x, t)

*

*H. DeVries, C.W. De Jager, C. DeVries, ATOMIC DATA AND NUCLEAR DATA TABLES 36,495536 (1987)

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• STAR has pioneered in UPCs by carrying out broad range of measurements at different center of mass energies

• We have analyzed 37 million UPC trigger events from 2010 Au+Au 200 GeV data set ( )

• After applying offline analyses cuts, STAR observes ~0.65M ρ and 125 J/ψ vector mesons

• We measured and compared the Jψ/ρ vector meson ratio to theory predictions

• Preliminary ratio (~0.0028) is somewhat higher than predicted by both Klein-Nystrand and Gonçalves-Machado models

• STAR result on radius of Au nuclei is consistent with 6.38fm

• Final trigger/vertex efficiency and systematic studies are in progress

• Future: Preparing for the p+A run in 2015 with the inclusion of Roman pots

L. Chanaka De Silva Hawaii 2014, Hawaii, USA

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