1. Vector meson photo-production at RHIC using \ /s NN = 200 GeV Au+Au collisions L. Chanaka De Silva Hawaii 2014, Hawaii, USA10 th 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 Japan Hawaii, USA, 7 -11 October 2014

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

3. 10 th 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 Z 2 – Photon flux is well described in Fermi-Weizsäcker-Williams approximation – Hadronic processes are strongly suppressed – Maximum CM photon energy at RHIC ~ γh/R A ~ 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

4. 10 th 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 multiplicity L. Chanaka De Silva Hawaii 2014, Hawaii, USA Alex Schmah Y. Gorbunov

5. 10 th October 2014 5 ρ 0 photo-production in AuAu collisions at \ /s NN = 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 \ /s NN = 200 GeV at the STAR detector -Phys. Rev. C 81 (2010) 44901 Observation of two-Source Interference in the photo-production reaction AuAu -> AuAuρ 0  Phys. Rev. Lett. 102 (2009) 112301 ρ 0 photo-production in ultra-peripheral relativistic heavy ion collisions at \ /s NN = 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 ρ 0 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 ρ 0 production cross section, 2008 Resonant ρ 0 production, 2008 ρ 0 production differential cross section, 2008 π + π - π + π - photonuclear production, 2010 Energy dependence of total ρ 0 cross section, 2012

6. 10 th October 2014 6 ρ 0 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 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,Q 2 ) 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 d 2 σ/dtdy Diffraction pattern detail helps constraining dipole cross section models

7. 10 th 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<M inv <1.1 GeV/c 2 J/ψ: 2.5<M inv <3.5 GeV/c 2 Background modeled with like-sign pairs, scaled to match data for p T > 0.2 GeV/c yDistribution –coherent cut; p T < 0.15 GeV/c –0.05 < |y| < 1.0 (Lower limit for cosmic rays/upper limit for STAR acceptance) L. Chanaka De Silva Hawaii 2014, Hawaii, USA

8. 10 th 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 p T < 0.15 GeV/c are used Only statistical error bars are presented L. Chanaka De Silva Hawaii 2014, Hawaii, USA 2010 AuAu 200 GeV data Only acceptance corrected ρ -> π + π - J/ψ -> e + e -

9. 10 th October 2014 9 L. 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 ρ 0 Direct ππ Interference Polynomial Background

10. 10 th 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 data ρ -> π + π - J/ψ -> e + e -

11. 10 th October 2014 11 L. 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 preliminary

12. Study of Au partonic form factor via Fourier transform of d 2 σ/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 10 th October 2014 12 L. Chanaka De Silva Hawaii 2014, Hawaii, USA ρ * *H. DeVries, C.W. De Jager, C. DeVries, ATOMIC DATA AND NUCLEAR DATA TABLES 36,495536 (1987)

13. 10 th 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 model s 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