Photoproduction at Hadron Colliders Results from STAR at RHIC Spencer Klein, LBNL (for the STAR Collaboration)

Photoproduction at Ion Colliders n Heavy Ions have strong electromagnetic fields u Equivalent to high intensity photon beam F Weizsacker-Williams F Photons are almost-real Here: focus on  A reactions n At the LHC  W  p ~ 10 TeV ~ 50x HERA  W  A/n ~ 1 TeV ~ 10x fixed target u Measure structure functions in protons and nuclei at low-x n With ions, the high photon flux is allows multiple interactions between a single ion pair Au 00  qq

Photon tagging n Nuclear excitation ‘tag’s small b n Multiple Interactions are independent n Au* decay via neutron emission u simple, unbiased trigger n Multiphoton events have: u smaller u Harder photon spectrum F Production at smaller |y| u Photon polarizations follow E field F Polarizations are collinear Au  P Au*  00 RHIC – Au  -spectra with and w/o nuclear excitation N(   (GeV) G. Baur et al. 2003

Photoproduction at proton colliders n Z=1 --> lower photon flux n Luminosities much higher than in AA u More than compensates for lower flux n Backgrounds may be higher than in AA  (p(  )p --> pJ/  p) ~ 0.1% of  (pp --> J/  X)  An exclusive J/  final state with 2 rapidity gaps should give a clean photoproduction sample  Caveat – background from double- diffractive production e.g.  c -->  J/  u Sensitive to gluon density Klein & Nystrand, 2004

Exclusive Vector meson  Production at RHIC n A virtual photon from one nucleus fluctuates to a qq pair which scatters elastically from the other nucleus and emerges as a vector meson  For heavy mesons (J/  ), the scattering is sensitive to nuclear shadowing n Coherent photon emission and scattering   ~ 8 % of  (had.) for gold at 200 GeV/nucleon F 120 /sec at RHIC design luminosity u Other vector mesons are copiously produced F LHC is a vector meson factory up to 230,000  0 & 780 J/  /sec (with Ca beams) Au 00  qq

Triggering on  0 in STAR Exclusive  0   0 and nothing else in TPC u Trigger on 2 charged particles in central trigger barrel  0 + mutual Coulomb excitation   0 in TPC + signals in forward (zero degree) calorimeters u Trigger on neutron signals in calorimeters

200 GeV Exclusive  0 n 1.5 Million triggers in 2002 n 2 track vertex  non-coplanar;  < 3 rad to reject cosmic rays Backgrounds from     and       scaled up by ~2  Incoherent  0 (w/ p T >150 MeV/c) are defined as background here asymmetric M  peak from interference with direct     production u Ratio comparable to that seen at HERA Signal region: p T <0.15 GeV Prelim i nary  0 P T M(     )

200 GeV XnXn data n 1.7 million ZDC coincidence triggers in 2002 n Require a 2 track vertex     and      model background n single (1n) and multiple (Xn) neutron production u 1n mostly from Giant Dipole Resonance n Cross section and rapidity distribution match soft Pomeron model After detector simulation Soft Pomeron pTpT Preliminary  0 P T  0 Rapidity

 0 production in dAu n Photons usually come from Au u Small contribution due to photons from deuteron  d -->  0 d u Coherent coupling to entire deuteron F A=2, so coherence is modest  d -->  0 pn u Coupling to individual nucleons n R d ~ 2 fm and R p ~ 0.7 fm   0 p T can be large  d -->  0 pn (neutron detected in ZDC) M   fit to  0 + direct   0 mass, width consistent with particle data book  0 :direct  ratio slightly lower than AuAu data Preliminary M  (GeV)

 d --> pn  0 Deuteron dissociates t (GeV 2 ) t  = p T 2 spectra n Slopes are similar u Deuteron coherence not a large effect here HERA finds b=11 GeV -2 for  p -->  p u Not exactly comparable measurements t (GeV 2 ) Preliminary  d --> d  0 Deuteron stays intact dN/dt – a exp(-bt) b =9.2 GeV -2 dN/dt – a exp(-bt) b =8.4 GeV -2  from Au Not enough energy for dissociation

Interference & t-spectra in AuAu n 2 indistinguishable possibilities u Interference!! n 2-source interferometer with separation b  is negative parity n For pp, AA parity transform -->   ~ |A 1 - A 2 e ip·b | 2  At y=0  =  0 [1 - cos(p  b)] n For pbar p: CP transform ->   ~ |A 1 + A 2 e ip·b | 2 n b is unknown u Reduction for p T Interference No Interference  0 w/ mutual Coulomb dissoc.  0.1< |y| < 0.6 t (GeV/c) 2 dN/dt

t for 0.1 < |y| < 0.5 (XnXn) Use tight cuts to select a clean  0 sample n 2 Monte Carlo samples: u Interference u No interference u w/ detector simulation F Detector Effects Small n Drop at low t matches interference calculation n 973 events dN/dt Data (w/ fit) Noint Int Background STAR Preliminary t (GeV 2 ) = p T 2

Fitting the Interference n Efficiency corrected t n 1764 events total n R(t) = Int(t)/Noint(t) u Fit with polynomial n dN/dt =A*exp(-bt)[1+c(R(t)-1)] u A is overall normalization u b is slope of nuclear form factor F b = 301 +/- 14 GeV /- 15 GeV -2 u c=0 -- > no interference u c=1 -- > “full” interference F c = / / n Data and interference model match dN/dt STAR Preliminary STAR Preliminary Data (w/ fit) Noint Int Data (w/ fit) Noint Int t (GeV 2 ) 0.1 < |y| < < |y| < 1.0

Exclusive  0 & results Similar results for exclusive  0 production u Larger so interference only visible for smaller p T u Somewhat less statistical significance n The results are consistent -- > take weighted mean u c= / (statistical) The b’s for the exclusive  0 and breakup data differ by 20%  Exclusive  0 : 364 +/- 7 GeV -2 u Coulomb breakup: 303 +/- 10 GeV -2 u Photon flux ~ 1/b 2 (here b ~ impact parameter)  More  0 production on ‘near’ side of target Smaller apparent size n Preliminary systematic errors u Experimental 8% (detector simulation…) u Theoretical 15% (functional form of interference)

pTpT 4-prong analysis n Very preliminary n ‘Model’ reaction   A->   *(1450/1700) -->  +    +  - u Expect ~ 100 events n Follows 2-prong analysis u p T < 100 MeV/c  Excess for  +    +  -  Over  +    +  - F Only at low p T n Analysis on a fraction of data n Background subtracted mass spectrum peaks at ~1.5 GeV Neutral 4 pion combos Charged 4 pion combos Entries Net Signal   mass (GeV) Entries Preliminary

Au Au  --> e + e - Au* Au* n e + e - pairs accompanied by nuclear breakup Z  EM ~ 0.6 u Higher order corrections? n Cross section matches lowest order quantum electrodynamics calculation u No large higher order corrections n p T peaked at ~ 25 MeV u Matches QED calculation  4  disagreement with equivalent photon (massless photon) calculation n V. Morozov PhD dissertation Preliminary Pair P t (GeVc) Pair Mass (GeV)

Conclusions & Outlook n Photoproduction can be profitably studied at hadron colliders.  The LHC will reach  p energies 10 times higher than HERA. STAR has observed coherent photonuclear  0 and         (likely the  * 0 ) production.  The  0 cross sections and kinematic distributions agree with theoretical models. We observe 2-source interference in  0 production.  The interference occurs even though the  0 decay before the wave functions of the two sources can overlap. n The cross section for e + e - pair production is consistent with lowest order quantum electrodynamics. In 2004, we multiplied our data sample, and hope to observe photoproduction of the J/ .