Photo-production of J/  and High-Mass e + e - in ultra-peripheral Au+Au collisions at 200 GeV/A by PHENIX Photo-production of J/  and High-Mass e + e.

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Photo-production of J/  and High-Mass e + e - in ultra-peripheral Au+Au collisions at 200 GeV/A by PHENIX Photo-production of J/  and High-Mass e + e - in ultra-peripheral Au+Au collisions at 200 GeV/A by PHENIX [PHENIX, arXiv: , submitted to PLB] Zaida Conesa del Valle Laboratoire Leprince-Ringuet (École Polytechnique, CNRS/IN2P3, France) Quark Matter 2009

2 / 14Quark Matter 2009Z. Conesa del Valle The ultra-peripheral collisions  Weizsacker-Williams (EPA):  Electromagnetic field of an ultra-relativistic particle  photon flux with continuous energy  Characteristics of ultra-peripheral collisions (UPC)  b > 2 R  Nuclei do not collide, possibility to study  - induced reactions   - flux is  (  X)  Z 2 ~ 6·10 3 &  (  )  Z 4 ~ 4·10 7 ⇒ larger than e-beams  Coherence condition:  wavelength > nucleus size ⇒ very low photon virtuality  Maximum center of mass energies: W max,  n  34 GeV & W max,   6 GeV b > 2R A A v~c Z2eZ2e Z1eZ1e [J.Nystrand, NPA752 (2005) 470] [Balz et al P.R.L (2002)] [S.R.Klein, J.Nystrand; PRC60 (1999) ] [Baur et al, N.P.A (2003)] Cf. Plenary talk by T.Lappi

3 / 14Quark Matter 2009Z. Conesa del Valle Physics processes of interest  Dilepton:  test QED on a strongly interacting regime (Z  em ~ 1)  Vector meson:  Low-x (10 -2 ) gluon PDFs, x=(m VM /W  A    QQbar propagation in Cold Nuclear Matter (shadowing, absorption) [J.Nystrand, NPA752 (2005) 470][Baur et al, N.P.A (2003)] [Armesto, J.Phys.G32:R367,2006][M. G. Ryskin, Z. Phys. C 57 (1993) 89]

4 / 14Quark Matter 2009Z. Conesa del Valle HowTo Trigger on UPCs  Experimental challenge  In UPCs nuclei do not collide  Events characterized by a rapidity gap  Veto on the MB interaction trigger (BBC veto)  The way out  Large probability to exchange additional soft photons  Nuclei excitation, most probably via Giant Dipole Resonance mechanism (GDR), decays by (forward) neutron emission  Coincidence probability for J/  is 55  6%  Emitted neutrons serve as triggering tool 1 or 2 ZDC trigger (E~30GeV)  Enrich the electron sample  ERT: EmCal Rich Trigger 2x2 tile threshold at E~0.8 GeV  Used trigger configuration: [Balz et al P.R.L (2002) + private comm.] [Baur et al, N.P.A (2003)] BBC veto  ZDC trg  ERT trg

5 / 14Quark Matter 2009Z. Conesa del Valle The experimental signatures, the analysis  Signatures:  Low particle multiplicities  Low transverse momentum : coherence condition p T < 2ħ/R or p~m ee /  ~ MeV  Zero net charge (N e+ = N e- )  Narrow dN/dy  Analysis:  Tracking DC, PC  Vertex reconstructed from EmCal & PC information. |vertex| < 30 cm (Select events centered on the detector fiducial area)  N. charged tracks == 2 (Selective diffractive criteria)  Electron identification RICH signal, n 0 ≥2 Track-EmCal matching with no dead/noisy tower E 1 > 1 GeV || E 2 > 1GeV select electrons above the ERT trigger turn on curve  Back-to-back electrons

6 / 14Quark Matter 2009Z. Conesa del Valle Possible signal and background sources  Non-physical sources:  Cosmic rays:  no vertex,  no ZDC.  Beam gas interactions:  no vertex,  large multiplicities.  Trigger criterion gets rid of those  Physical sources:  Peripheral nuclear A+A collisions:  large multiplicities,  large p T.  Hadronic diffractive (Pomeron-Pomeron, rapidity gap):  forward proton emission,  larger p T : p T (  ) < p T (PP),  expect like-sign pairs too.  Analysis cuts gets rid of them Incoherent UPC:  +n  n+J/   wider p T : p T (  ) < p T (  P),  asymmetry dN/dy,  >2 neutrons (induced nuclear break-up) w/ same direction as J/ . Coherent UPC:   e + e -,   X+J/  A  jet(s)+A We are sensitive to coherent and incoherent UPC ! [D. D’Enterria et al., nucl-ex/ (2005)]

7 / 14Quark Matter 2009Z. Conesa del Valle The measured invariant mass  28 unlike-sign pairs and no like-sign pairs of m ee > 2 GeV/c 2  Clean sample, with zero net charge !  Invariant mass fit input:  Coherent continuum shape derived from theoretical STARLIGHT-MC input + full detector simulation and reconstruction dN/dm e+e- = A · exp( c m e+e- ); c = -1.9  0.1 GeV/c 2  J/  Gaussian fit shape  width 155 MeV/c 2 consistent with MC  Systematic uncertainties derived varying the continuum slope by 3  and by using a power-law form N(J/  ) = 9.9  4.1 (stat)  1.0 (syst) N (e + e - ) = 13.7  3.7 (stat)  1.0 (syst) in m ee  [2.0,2.8] GeV/c 2

8 / 14Quark Matter 2009Z. Conesa del Valle (    e + e - ) transverse momentum distribution  N(e + e - ) = 13.7  3.7  1.0 m ee  [2.0,2.8] GeV/c 2  Slicing in mass  N(e + e - ) = 7.4  2.7  1.0 m ee  [2.0,2.3] GeV/c 2  N(e + e - ) = 6.2  2.5  1.0 m ee  [2.3,2.8] GeV/c 2     e + e - spectra is peaked at very low pt ( p T  100 MeV/c 2 ) Evidence of the    e + e - coherent nature !

9 / 14Quark Matter 2009Z. Conesa del Valle Coherent di-electron (    e + e - ) cross section  Cross section Results agree with QED theoretical (STARLIGHT) calculations even though we are in a strongly interacting regime !  Caveats / leftovers:  Lacking of other model comparisons on this kinematical region… input from theorists is most welcome !  Recent calculations seem to suggest that higher order corrections would suppress the e + e - cross-section Cf. Baltz: 29% reduction on 140 < m ee < 165 MeV/c 2 even if this holds true for higher masses our measurement would still be in agreement with the theoretical calculations [Baltz, Phys.Rev.Lett. 100 (2008) ] STARLIGHT: WW approx. in impact parameter space at LO

10 / 14Quark Matter 2009Z. Conesa del Valle J/  transverse momentum distribution  Coherent (  A) produced J/  should lead to p T  200 MeV/c 2  The low p T J/  consistent with the Au nuclear form factor F dN ee / dp T = A ·p T ·|F(p T )| 2  coherent (  A) J/  production  But there seems to be also an incoherent (  n) J/  component The bulk has low p T ~ 90 MeV, and is consistent with coherent prod.

11 / 14Quark Matter 2009Z. Conesa del Valle J/  cross section vs theoretical calculations I  J/  cross section d  / dy | y=0 = 76  31 (stat)  15 (syst)  b  Model predictions drawn  Starlight: coherent only, parameterization of HERA data  Strikman et al: coherent & incoherent color-dipole +  J/  N = 3mb  Gonçalves-Machado: coherent only color-dipole + Glauber-Gribov shad.  Kopeliovich et al: coherent & incoherent color-dipole + gluon saturation  Looks compatible with coherent predictions, but... measured p t spectra suggests both coherent (  ) and incoherent (  N) J/  production [ 1) P.R.L (2002)…] [ 2) P.L.B626 (2005) 72 ] [ 3) arXiv [hep-ph] ] [ 4) arXiv: [hep-ph] ] [ 1) ] [ 2) ] [ 3) ] [ 4) ]

12 / 14Quark Matter 2009Z. Conesa del Valle J/  cross section vs theoretical calculations II [ZEUS, Eur.Phys.J. C24 (2002) 345] [H1, Eur.Phys.J. C46 (2006) 585] [STAR, Phys.Rev.C77 (2008) ]  Rough comparison with HERA e-p data,   p =     A If coh. incoh. ratio is 50% - 50%   coh = 1.01  0.07   incoh = 0.92  0.08   ~ 1, good agreement with HERA data hard probes scaling!  Similar comparison with STAR  measurement gives  coh = 0.75  0.02, closer to A 2/3 soft scaling Cross-section is consistent with different model predictions  … though current precision precludes yet any detailed conclusion on the basic ingredients: shadowing and nuclear absorption [ 1) P.R.L (2002)…] [ 2) P.L.B626 (2005) 72 ] [ 3) arXiv [hep-ph] ] [ 4) arXiv: [hep-ph] ] [ 1) ] [ 2) ] [ 3) ] [ 4) ]

13 / 14Quark Matter 2009Z. Conesa del Valle Summary  First measurement of J/  e + e - photo-production and of two-photon production of high-mass e + e - in nucleus-nucleus interactions !  Efficient forward neutron tagging trigger,  Clean sample of 28 e + e - pairs and no like-sign pairs for m ee  2.0 GeV/c 2, from which ~ 10 are from J/ .  Their p T spectrum is peaked at low p T ~90MeV as expected for coherent photo-production.   e + e - cross-section at mid-rapidity is 86  23(stat)  16(syst)  b/(GeV/c 2 ) for m ee  [2.0,2.8] GeV/c 2,  …and it is in good agreement with QED theoretical calculations.  J/  photo-production cross-section at mid-rapidity is 76  33(stat)  11(syst)  b  Their measured p T distribution suggests both coherent (  A) and incoherent (  n) J/  photo-production in accordance with predictions,  The J/  cross-section is consistent with different model predictions (pQCD) and with HERA data but precludes yet any detailed conclusion on the gluon- shadowing and J/  nuclear absorption.

14 / 14Quark Matter 2009Z. Conesa del Valle What is next ? Looking forward…  Collected data on 2007 ~ 3 x statistics on 2004  Increased statistics: Improve the statistical uncertainties May allow to separate coherent & incoherent J/  components  Forward rapidity measurements become possible. Models predict distinct rapidity dependences depending on the nuclear shadowing scheme  Further future plans may include the eRHIC program ?  The LHC, new insights  Unexplored kinematic regime  J/  at x~5·10 -4 at y~0   UPC studies will be possible [D. D’Enterria, Eur.Phys,J.A31, 816 (2007)]

Backup slides

16 / 14Quark Matter 2009Z. Conesa del Valle  Rough comparison with HERA e-p data, if coherent incoherent ratio is 50% - 50%  HERA (H1 & ZEUS) input  Result:   coh = 1.01  0.07   incoh = 0.92  0.08   ~ 1, good agreement with HERA data hard probes scaling Comparison with HERA data

17 / 14Quark Matter 2009Z. Conesa del Valle J/  prediction vs shadowing model Filho, Gonçalves, Griep; Phys.Rev.D78: (2008); arXiv: (2008);

18 / 14Quark Matter 2009Z. Conesa del Valle The PHENIX Central Arm e+e+ ee  

19 / 14Quark Matter 2009Z. Conesa del Valle J/  photo-production at CDF [CDF, arXiv: , 7 Feb 2009]