Vector meson photo-production at RHIC using \ /s NN = 200 GeV Au+Au collisions

1. Vector meson photo-production at RHIC using \/sNN = 200 GeV Au+Au collisions L. Chanaka De Silva, Creighton University For the STAR collaboration Fourth Joint Meeting of the Nuclear Physics Divisions of the American Physical Society and The Physical Society of Japan Hawaii, USA, 7 -11 October 2014 Hawaii 2014, Hawaii, USA 10th October 2014 L. Chanaka De Silva

2. Talk outline • Ultra peripheral collisions • STAR UPC trigger • STAR Highlights • Future directions • Offline analyses cuts • Data and comparison to theory • Summary and outlook Hawaii 2014, Hawaii, USA 10th October 2014 L. Chanaka De Silva

3. High Energy Photonuclear Reactions Ultra Peripheral Collision (UPC) • 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 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 Hawaii 2014, Hawaii, USA 10th October 2014 L. Chanaka De Silva

4. The STAR Experiment at RHIC Y. Gorbunov Alex Schmah STAR - Solenoidal Tracker At RHIC • Large acceptance; TPC: -1 < η < 1 and –π < ϕ < π • Excellent PID; ToF and TPC dE/dx STARUPC Trigger • 1≤ beam neutrons ≤ 6 in both ZDCs • veto on small BBC tiles 3 < |η| < 5 • 2 ≤ TOF hits ≤ 6 • Low multiplicity Hawaii 2014, Hawaii, USA 10th October 2014 L. Chanaka De Silva

5. Highlighting results from STAR π+π-π+π- photonuclear production, 2010 Energy dependence of total ρ0 cross section, 2012 STAR publications • 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 ρ0 production differential cross section, 2008 Resonant ρ0 production, 2008 ρ0 production cross section, 2008 First π spectra from two different triggers, 2002 e+e- pair rapidity compared to theory, 2004 Hawaii 2014, Hawaii, USA 10th October 2014 L. Chanaka De Silva

6. Moving forward ρ0 photo-production with higher statistics 2010 AuAu 200 GeV data • 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 J/ψ photo-production using 2010 AuAu 200 GeV data • 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] Hawaii 2014, Hawaii, USA 10th October 2014 L. Chanaka De Silva

7. Analysis cuts for ρ and J/ψ • 37 Million200 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 hastwo 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) Hawaii 2014, Hawaii, USA 10th October 2014 L. Chanaka De Silva

8. Pt Distributions 2010 AuAu 200 GeV data Only acceptance corrected ρ -> π+π- J/ψ -> e+e- • 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 Hawaii 2014, Hawaii, USA 10th October 2014 L. Chanaka De Silva

9. Inv. Mass Distributions 2010 AuAu 200 GeV data ρ J/ψ Interference Polynomial Background Pure ρ0 Direct ππ • 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 Hawaii 2014, Hawaii, USA 10th October 2014 L. Chanaka De Silva

10. Vector Meson Rapidity Distributions 2010 AuAu 200 GeV data ρ -> π+π- J/ψ -> e+e- • 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 Hawaii 2014, Hawaii, USA 10th October 2014 L. Chanaka De Silva

11. Rapidity Distribution of Vector Meson Ratio 2010 AuAu 200 GeV data STAR preliminary • 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 • 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 Hawaii 2014, Hawaii, USA 10th October 2014 L. Chanaka De Silva

12. t-Distribution and diffraction peaks ρ * • 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 *H. DeVries, C.W. De Jager, C. DeVries, ATOMIC DATA AND NUCLEAR DATA TABLES 36,495536 (1987) Hawaii 2014, Hawaii, USA 10th October 2014 L. Chanaka De Silva

13. Summary • 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 Hawaii 2014, Hawaii, USA 10th October 2014 L. Chanaka De Silva