# Presentation: 108208

## Recent results in Ultra-Peripheral Collisionsfrom STAR

- What are ultra-peripheral collisions?
- Exclusive0 production
- 0 interferometry
- e+e- pair production (V. Morozov dissertation, 2003)
- Conclusions

- Spencer Klein, LBNL (for the STAR Collaboration)

## Coherent Interactions

**Coherent Interactions**

- b > 2RA;
- no hadronic interactions
- <b> ~ 20-60 fermi at RHIC
- Ions are sources of fields
- photons
- N ~ Z2
- Pomerons or mesons (mostly f0)
- A 2 (bulk) A 4/3 (surface)
- Fields couple coherently to ions
- Photon/Pomeron wavelength  = h/p> RA
- amplitudes add with same phase
- P < 30 MeV/c
- P|| < 3 GeV/c
- Strong couplings --> large cross sections

- Au

- Au

- Coupling ~ nuclear form factor

- , P, or meson

## Unique Features of Ultra-peripheral collisions

- Very strong electromagnetic fields
-  --> e+e- and  --> qq
- Multiple interactions between a single ion pair
- Unique Geometry
- 2-source interferometer
- Nuclear Environment
- Particle Production with capture
- Large  for e-

## Exclusive 0 Production

- A virtual photon from one nucleus 		    fluctuates to a qq pair which scatters 		     elastically from the other nucleus and 		     emerges as a vector meson
- Photon emission follows the Weizsacker-Williams method
- For heavy mesons (J/), the scattering is sensitive to nuclear shadowing
- Coherence --> Rates are high
-  ~ 8 % of (had.) for gold at 200 GeV/nucleon
- 120 /sec at design luminosity
- Other vector mesons are copiously produced

- Au

- Au

- 0

- 

- qq

## Nuclear Excitation

- Nuclear excitation ‘tag’s small b
- Multiple Interactions are independent
- Au* decay via neutron emission
- simple, unbiased trigger
- Higher order diagrams
- smaller <b>
- Harder photon spectrum
- Production at smaller |y|
- Single (1n) and multiple (Xn, X>0) neutron samples

- 0  with gold  @ RHIC

- d/dy

- y

- Exclusive - solid
- X10 for XnXn - dashed
- X100 for 1n1n - dotted

## Slide 6

## Slide 7

## 0 Analysis

- Exclusive Channels
- 0 and nothing else
- 2 charged particles
- net charge 0
- Coherent Coupling
- pT < 2h/RA  ~100 MeV/c
- back to back in transverse plane
- Trigger
- Back to back hits in Central Trigger barrel

## 200 GeVExclusive 0

**200 GeV****Exclusive ********0**

- 1.5 Million topology triggers
- 2 track vertex
- non-coplanar;  < 3 rad to reject cosmic rays
- and  model background shape
- pairs from higher multiplicity events have similar shape
- scaled up by ~2
- Incoherent 0 (w/ pT>150 MeV/c) are defined as background in this analysis
- asymmetric M peak

- M()

- 0 PT

- Signal region: pT<0.15 GeV

- Preliminary

## 200 GeV XnXn data

- 1.7 million minimum bias triggers
- Select events with a 2 track vertex
- and  model background
- single (1n) and multiple (Xn) neutron production
- Coulomb excitation
- Giant Dipole Resonance
- Rapidity distribution matches Soft Pomeron model calculation

- After detector simulation

- After detector simulation

- Soft Pomeron

- pT

## M

- Mspectrum includes 0 + direct +-
- Same 0: +- ratio as is observed in p--> +- p at HERA

- M()

- XnXn sample

- ZEUS p --> (0 + +- )p

- e+e- and hadronic backgrounds

- M

- d/dMb/GeV

- STAR Au --> (0 + +- )Au*

## Cross Section Comparison

- 130 GeV data
- Normalized to 7.2 b hadronic cross section
- Systematic uncertainties: luminosity, overlapping events, vertex & tracking simulations, 1n selection, etc.
- Exclusive 0 bootstrapped from XnXn
- limited by statistics for XnXn in topology trigger
- Good agreement
- factorization works

## Interference

**Interference**

- 2 indistinguishable possibilities
- Interference!!
- Like pp bremsstrahlung
- no dipole moment, so
- no dipole radiation
- 2-source interferometer with separation b
-  is negative parity so
-  ~ |A1 - A2eip·b|2
- At y=0
- =0[1-cos(pb)]
- b is unknown
- Reduction for pT <<1/<b>

- No Interference

- 0  w/ mutual Coulomb dissoc.
-   0.1< |y| < 0.6

- t (GeV/c)2

- dN/dt

## Entangled Waveforms

- 0 are short lived, with c ~ 1 fm << b
- Decay points are separated in space-time
- Independent decays to different final states
- no interference
**OR**
- the wave functions retain amplitudes for all possible decays, long after the decay occurs
- Non-local wave function
**non-factorizable**: +-  + -

- -

- b

- (transverse view)

- -

- 

- 

- +

- +

## Interference Analysis

- Select clean 0 with tight cuts
- Lower efficiency
- Larger interference when 0 is accompanied by mutual Coulomb dissociation
- Interference maximal at y=0
- Decreases as |y| rises
- 2 rapidity bins 0.1 < |y| < 0.5  &  0.5<|y|<1.0
- |y|<0.1 is contaminated with cosmic rays

## t for 0.1 < |y| < 0.5 (XnXn)

**t for 0.1 < |y| < 0.5 (XnXn)**

- 2 Monte Carlo samples:
- Interference
- No interference
- w/ detector simulation
- Detector Effects Small
- Data matches Int
- Inconsistent with Noint
- Interference clearly observed
- 973 events

- dN/dt

- 	Data (w/ fit)
- 	Noint
- 	Int
- 	Background

- STAR
- Preliminary

- t (GeV2) = pT2

## XnXn Fitting the 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-2                     	304 +/- 15 GeV-2
- c=0 -- > no interference
- c=1 -- > “full” interference
- c = 	1.01 +/- 0.08       	         	0.78 +/- 0.13
- Data and interference model match

- dN/dt

- dN/dt

- STAR
- Preliminary

- STAR
- Preliminary

-      Data (w/ fit)
-      Noint
-      Int

-      Data (w/ fit)
-      Noint
-      Int

- t (GeV2)

- t (GeV2)

**0.1 < |y| < 0.5**

**0.5 < |y| < 1.0**

## Exclusive 0

- <b> ~ 46 fm
- 5770 events total
- dN/dt = A*exp(-bt)[1+c(R(t)-1)]
- A - overall normalization
- b = 361 +/- 9 GeV-2/         	    368 +/- 12 GeV-2
- Different from minimum bias data
- c = 0.71 +/- 0.16          	   	    1.22 +/- 0.21
- Interference is present

- t

- dN/dt

- dN/dt

-      Data (w/ fit)
-      Noint
-      Int

-      Data (w/ fit)
-      Noint
-      Int

- STAR
- Preliminary

- t

- STAR
- Preliminary

**0.1 < |y| < 0.5**

**0.5 < |y| < 1.0**

## Combining the Data

- The c values are consistent -- > take weighted mean
- c= 0.93 +/- 0.06  (statistical only)
- Data matches predictions
- The b’s for the exclusive 0 and breakup data differ by 20%
- Exclusive 0 :  364 +/- 7 GeV-2
- Coulomb breakup: 303 +/- 10 GeV-2
- Photon flux ~ 1/b2
- More 0 production on ‘near’ side of target
- Smaller apparent size
- Systematic Errors (in progress)
- Change simulation input form factor slope b by 20%
- 3% (2%) change in c(b)
- No Detector simulation
- 18% (1.4%) change in c(b)
- If simulation is 75% ‘right--> 5% systematic error

## Au Au  --> e+e- Au* Au*

- e+e- pairs accompanied by nuclear breakup
- ZEM ~ 0.6
- Higher order corrections?
- Cross section  matches lowest order quantum electrodynamics calculation
- No large higher order corrections
- pT peaked at  ~ 25 MeV
- Matches QED calculation
- By Kai Hencken et al.
- 4 disagreement with  equivalent photon (massless photon) calculation
- V. Morozov PhD dissertation

- Preliminary

- Pair Pt (GeVc)

- Pair Mass (GeV)

- STAR
- Preliminary

- STAR
- Preliminary

## Conclusions & Outlook

- STAR has observed  photonuclear 0 production.
- The 0 cross sections agree with theoretical models.
- Interference between 0 and direct  is seen.
- 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.
- The cross section for  e+e- pair production is consistent with lowest order quantum electrodynamics.
- The equivalent photon approximation does not describe the pT spectrum.
- 2003 run:
- Good dA --> 0  sample.
- Meson production via double Pomerons from pp pending.