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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
3
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
10th October 2014 5
• 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
10th October 2014 6
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)
10th October 2014 7
• 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
10th October 2014 8
• 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-
10th October 2014 9L. Chanaka De Silva Hawaii 2014, Hawaii, USA
• 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
10th October 2014 10
• 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
€
dσ
dt∝Fg
2 (x, t)
*
*H. DeVries, C.W. De Jager, C. DeVries, ATOMIC DATA AND NUCLEAR DATA TABLES 36,495536 (1987)
10th October 2014 13
• 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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