1 Ultraperipheral heavy ion collisions Timoshenko S., Emelyanov V. Moscow Engineering Physics Institute (State University) (for the STAR Collaboration) XXXIII International Conference on High Energy Physics (ICHEP'06), Moscow 2006.

2 The STAR Collaboration ~400 collaborators, 49 institutions, 8 countries

3 Overwiev  Introduction in UPC  Vector meson production  STAR AuAu dAu 4 prong analysis interference  Conclusion

4 Introduction in ultraperipheral collisions  The two nuclei geometrically “miss” each other b > 2R A  Ions are source of fields photons s  ~ Z 4 pomerons s  p ~ Z 2 A 2 – for ‘heavy’ states s  p ~ Z 2 A 5/3 - for lighter mesons  Photon and pomeron can couple coherently to the nuclei if its have: Small transverse momentum: p T < h/R A ~ 30 MeV Maximum longitudinal component p L <  h/R A ~ 3 GeV A nuclear form factor , P b A A Pomeron carry the strong interaction but is colorless and it has the quantum number of the vacuum J P = 0 ++ E  ~ 3 (80) GeV at RHIC (LHC) W  ~ 6 (160) GeV at RHIC (LHC)

5 Vector meson production  Vector mesons production (photon-pomeron interaction) Exclusive vector meson production can be described in terms of elastic scattering. Photon emitted by one nucleus fluctuates to virtual ‘qq’ pairs, this state elastically scatters from the other nucleus (absorb some of the photon wave function), then ‘qq’ pairs can be emerge as a vector meson. The “pomeron” represents the absorptive part of nuclear cross section. , J/ Y,Y Klein S. and Nystrand J., hep-ph/  ~ 8 % of (had.) for gold at 200 GeV/nucleon 120 events/sec at RHIC design luminosity

6 STAR

7 Types of trigger  Topology trigger (no break up d) CTB (central trigger barrel)  240 scintillator slats  CTB divided into 4 azimuthal quadrants  Hits in the North and South sectors  Top and bottom reject cosmic ray Low multiplicity (2 or 4 events) Vertex position typical event

8 Types of trigger  Topology trigger + ZDCs (  0 in TPC + signals in forward (zero degree calorimeters) Topology trigger + West ZDC: Au+d->  Au+pn  required break up d Topology trigger + both ZDC: Au+Au->  AuAu+Xn  Backgrounds  peripheral hadronic events  cosmic rays, beam gas interactions, pile-up TPC Au d(Au) ZDC-WestZDC-East CTB-topology

9 AuAu ->  0 Au * Au * 200 GeV Signal region: p T <0.15 GeV  0 Rapidity After detector simulation  1.7 million ZDC coincidence triggers in 2002  Require a 2 track vertex      and    - model background  scaled up to 2  single (1n) and multiple (Xn) neutron production  1n mostly from Giant Dipole Resonance  Cross section and rapidity distribution match soft Pomeron model STAR Preliminary

10 d ->  0 pn Mass Spectra  Topology trigger + ZDC for d breakup Data shows clear 1n signal  Large, clean  0 sample ~13400 events  Well fit by  0 + direct   0 mass, width consistent with particle data book  0 :direct pp ratio slightly lower than AuAu data  = 2.63±0.32±0.73 mb preliminary no cut p T STAR Preliminary

11 d -->  0 pn t spectra  Fit to t ~ p T 2  Sensitive to nucleon form factor ZEUS used F(t)=e -bt+ct*t for proton form factor slope b ~ 9 GeV -2  Same as ZEUS R=√4b=(1.2±0.3) fm  Good fit to ZEUS at large t  Drop off at small t, Too little energy to dissociate the deuteron Seen by fixed target experiments Deuteron does not dissociate t (GeV/c) 2 d s /dt, m b/GeV 2

12 4 prong analysis  Very preliminary  ‘Model’ reaction A->  *(1450/1700) - > +    +  - Expect ~ 100 events  Follows 2-prong analysis p T < 100 MeV/c Excess for  +    +  - Over  +    +  - Only at low p T  Analysis on a fraction of data  Background subtracted mass spectrum peaks at ~1.5 GeV   mass (GeV) Entries Neutral 4 pion combos Charged 4 pion combos Entries Net Signal STAR Preliminary

13 Interference  2 indistinguishable possibilities Interference!!  2-source interferometer with separation b   is negative parity  For pp, AA parity transform ->  ~ |A 1 - A 2 e ip·b | 2 At y=0= 0 [1 - cos(pb)]  For pbar p: CP transform ->  ~ |A 1 + A 2 e ip·b | 2  b is unknown Reduction for p T   0 w/ mutual Coulomb dissoc.  0.1< |y| < 0.6 t (GeV/c) 2 dN/dt int noint

14 Interference  Efficiency corrected t  1764 events total  R(t) = Int(t)/Noint(t) Fit with polynomial  dN/dt =A*exp(-bt)[1+c(R(t)-1)] A is overall normalization b is slope of nuclear form factor  b = 301 +/- 14 GeV /- 15 GeV -2 syst. uncertainties: ±8(syst)±15%(theory) c=0 -- > no interference c=1 -- > “full” interference  Data and interference model match  c = / / 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 AuAu -> r 0 Au*Au* 200 GeV

15 Conclusion  STAR has observed incoherent photonuclear  0 meson production in dAu collisions. The  0 cross sections and kinematic distributions agree with theoretical models. dN/dt reflects d/Au size/form factor  STAR has observed         (likely the  0* ) production in AuAu collisions  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.