TAKAHARA, Akihisa for the PHENIX Collaboration (CNS, University of Tokyo and RIKEN)

1. J/y Photoproduction in ultra-peripheral Au+Au collisions at √sNN=200 GeV measured by RHIC-PHENIX TAKAHARA, Akihisa for the PHENIX Collaboration (CNS, University of Tokyo and RIKEN)

2. A Z1e b > 2R A v~c Z2e  Characteristics of Ultra Peripheral Collisions Weizsacker-Williams (EPA): • The electromagnetic field is equivalent to a large flux of quasi-real photons, and can be calculated per (Fermi)-Weizsacker-Williams: b>2R no nuclear overlap. Possibility to study g- induced reactions • Coherence condition: • wavelength > nucleus size . Very low photon virtuality b RHIC, LHC LA/LAA RHIC max. photon energies(EPA):3 GeV~g/R

3. Physics motivation for UPC J/y gluon distribution in Nuclei is not same free proton Theoretical predictions And 2004 result RHIC UPC J/y Q2=2.5GeV2 x~0.01 • Direct Measurement of gluon distributions at low-x • search for Nuclear shadowing

4. coherent and incoherent distribution • Strikman et al calculate that quasi-elastic (incoherent) J/y cross-section comparable to coherent production • Incoherent J/y produced from photo-nucleon interaction • Much larger t distribution expected for incoherent • Both process emit only J/y and neutron. pTdistribution is important to divide them • Strikman’s predictions say at central rapidity, coherent process is dominant, but at forward rapidity, incoherent process is dominant at PHENIX Strikman, Tverskoy, Zhalov, PLB 626 p. 72-79、2005 n incoherent n incoherent coherent coherent

5. RHIC-PHENIX • Luminosity • Au+Au(200GeV) : 2 x 1026 [cm-2s-1] • p+p (500GeV): 2 x 1031 [cm-2s-1] • 2007-RUN • for central • AuAu • 200GeV • ~530/μb • Central arm (|y|<0.35) • electron (using 2007 data) • Forward arm (1.2<|y|<2.2) • muon(using 2010 data) • 2010-RUN • for forward • AuAu • 200GeV • ~800/μb

6. Signal from UPC J/y and its trigger n • Signal from UPC J/y→ll • a lepton pair without any other tracks • 50~60% UPC events are associated with nuclear break up • PHENIX UPC trigger • BBC_VETO(reject nuclear overlap) • EMCAl(for central)/Muon track(for forward) • ZDC detect at least a neutron BBC e+ e- Coherent UPC 50-60% PHENIX MB :4kHz PHENIX ERT2x2(EMcal):8kHz to reduce trigger rate, 3rd condition was required BBC(3<|y|<3.9) BBC is main vertex detector of PHENIX 1stcondition means we can’t use it

7. p e+ g e- UPC J/y+Xnmeasurement at PHENIX Central arm(|y|<0.35,X>1) + • Offline analysis cuts • |collision vertex determined from tracks reconstructed in the PAD chambers|< 30cm • number of tracks==2 • North or south BBC charge==0 • energy deposit of ZDC>30GeV(just confirm there are no noise trigger)

8. UPC J/y+Xn(y>0)Yn(y<0) measurement at PHENIX Forward arm(X>1,Y>1) 2010 run forward UPC mass dist (North 1.2_<y<2.2) 5 interaction length→ to get vertex information, both side ZDC fire was required Clear J/ψ peak • Offline analysis cuts • |vertex determined from ZDC|<30cm • number of tracks==2 (in central and forward) • North or south BBC charge==0 • energy deposit of ZDC>30GeV • Just noise cut • Both RXNP charge <1000a.u.

9. Real data for dielectron(|y|<0.35) Dimass distribution Unlike like ZDC energy • Clear J/y peak • Only unlikesign pair (over 2 GeV) • Clear Coherent(low pT) peak dipT distribution pT(GeV/c)

10. Real data for dimuon(1.2<|y|<2.2) North Unlike like South Unlike like • Clear J/y peak • Only unlike sign pair (2 GeV>mass) • Coherent(low pT) peak is not so clear • But still 0 pT peak dipTdistribtuion 2.7GeV<dimass<3.5 GeV North South

11. Neutron emission for dimuon J/y North ZDC South ZDC • Same side:1300GeV • Opp side :700GeV Dimuon North opp side Multi Photon excitation Dimuon South Same side Multi photon excitation +Nuclear break up by recoiled neutron • If there was nuclear over lap, the asymmetry can’t be explained • Suggest UPC (incoherent) process

12. Comparison with pp for dielctron Dimass distribution Unlike like • Even pp(ncoll=1 limit), • J/y events are associated with additional tracks • At pT ~2.5 GeV/c, unlike/like~2 • UPC and PP J/ypT distribution is different dipT distribution Unlike Like ~1GeV/c peak Number of tracks in central arm UPC(without central track cut) PP Normalized at 2

13. Comparison with pp for dimuon Dimass distribution Unlike like • Even pp(ncoll=1 limit), • J/y events are associated with additional tracks • At pT ~2.5 GeV/c, unlike/like~2 • UPC and PP J/ypT distribution is different • Central multiplicity distribution suggest little most peripheral contamination • About 3 J/y per arm. dipT distribution Unlike like Number of tracks in central arm(not muon arms) UPC(without central track cut) PP(0track/non 0tracks~30%) Normalized at 0

14. Contamination form diffractive process • Diffractive J/y should have just 2 tracks • UPC like events ! • Typically, diffractive collisions /all pp~30% • 10k J/y was generated by PYTHIA (pp 200GeV,msel2(minimum byas)) • 0/10k J/y was generated by diffractive process • Can be neglected

15. Background sources for dielectron(|y|<0.35) Rapidity distribution STARLIGHT simulation for gg->dilepton • In central(|y|<0.35) region, γγ->dielectron is main background source • “Simulated continuum curve +Gaussian” fit • J/ygaus+trig,detecterAccxeff(mass)x exp • Exp slope was fixed by simulation

16. Background sources for dimuon(|y|<0.35) • HERA:gp->J/y measurement • y(2s)/J/y=7% • Eur. Phys. J. C 24, 345–360 (2002) • gg->dimuon can be neglected this rapidity region • Expected most peripheral contamination • doesn’t have enough statistics to explain all background North dipT <0.5GeV/c J/y y(2s) Total background • Background by UPC process are suggested • UPC ccbar • gAu->dipion->dimuon

17. Integrated cross sectionJ/y+XnCentral2004&2007 sys errors Acc(over all) 5% simulation 12% Lumi 4% ERT 0.2% Njpsi1.4% BBC1.6% 2004+2007 2004 PHENIX 76  31 (stat) 15 (syst) b J. Nystrand, Nucl. Phys. A 752(2005)470c; A.J. Baltz, S.R. Klein, J. Nystrand, PRL 89(2002)012301; S.R. Klein, J. Nystrand, Phys. Rev. C 60(1999)014903 M. Strikman, M. Tverskoy and M. Zhalov, Phys. Lett. B 626 72 (2005) V. P. Goncalves and M. V. T. Machado, arXiv:0706.2810 (2007). Yu. P. Ivanov, B. Z. Kopeliovich and I. Schmidt, arXiv:0706.1532 (2007). 2007 PHENIX

18. J/y+Xn Invariant Yield@(-0.35<y<0.35) • theoretical • calculations for Coherent(＠y=0) • noshadowing 113μb • DS10 μb • EKS 83μb • Kopeliovich GBW 61μb • Kopeliovich KST 54μb • EPS 53μb • Strikmanimpulse 40 μb • Strikmanglauber 30μb • We can see both coherent and incoherent distribution • 46.7 ±13μb for pT < 0.4GeV(upper limit of coherent) • compatible with calculations including strong suppression of gluons at low x

19. J/y+Xn(y>0)Yn(y<0) • The pTdistributions at forward rapidity shows that incoherent process is very visible at forward(can’t see coherent peak) • There are no theoretical predictions with XnYn condition

20. Summary and Outlook Summary • PHENIX measured J/ ψphoto-production yield and its pTdependence in a broad rapidity region. • characteristics of UPC signals is obviously different from pp • - J/ψ +Xn result at mid-rapidity is consistent with calculations suggesting strong gluon shadowing • - important contribution from incoherent processes in J/ψ+Xn(y<0)Yn(y>0) at forward rapidity, looking forward for calculations for this exclusive process Outlook • New vertex detectors will help in the further study of UPC events at RHIC.

25. gg->dimuon distribution • gg->dimuon distribution isvery sharp in this region • Because of edge of detector, Acceptance x efficiency is very low at Y=1.2

26. Detail of fitting • J/ygaus+trig,detecterAccxeff(mass)x exp • Exps lope was fixed by simulation

27. Detail of fitting North dipT <0.5GeV/c J/y y(2s) Total background • Divide into pT bins • Due to hadron suppresser • Fitting function • Acceff(dimass)x • Gaus1(J/y) • Gaus2(J/y tail) • Gaus3(y(2s)) • +exp(background) • Shape of gaus 1&2 was fixed to pp data • Gaus3/Gaus(1+2) is fixed to 7%