# Presentation: 859396

## Saturation, Low-x, and QCDat RHIC, Part 3

- T. Hallman
- Villaggio Guglielmo, Italy

##       Ultra-peripheral relativistic heavy ion collisions: a unique testing ground for QCD

## The interest in UPC’s

**What are Ultra-Peripheral Collisions:**
- Electromagnetic interactions that occur at impact parameters b larger than twice the nuclear radius, where no hadronic interactions occur
- Flux of “almost real” virtual photons (a la Enrico Fermi in 1924) scales with Z2 so heavy ions are an intense source of photons and cross sections can be large
- The study both two-photon and photonuclear interactions is possible
- With the Lorentz boosts, two-photon energies up to 6 GeV are achievable at RHIC
- At RHIC,  -Au cms energy of ~30 GeV/nucleon and and -proton cms energy of 300 GeV are achievable

**Some topics of interest:**

## The interest in UPC’s

**The gluon distribution in heavy nuclei:**** **
- Photoproduction of coherent J/ production (virtual photon fluctuates to a quark-anti-quark pair which interacts coherently with the entire nucleus) can provide a window in gluon distributions not measurable with DIS.
- Avoids problems in studies using hadronic interactions are subject to uncertainties due to higher order corrections and the Cronin effect.
- STAR will study the gluon distribution of heavy nuclei using two photoproduction channels: coherent J/ production, and open charm.
- The total cross section isn’t very sensitive to shadowing, but the rapidity distribution is.
- If gluon shadowing is “large”, as predicted in HIJING and the diffractive model of Frankfurt and Strikman,  shadowing may reduce mid-rapidity J/ production by as much a 40%.

## The interest in UPC’s

**Meson Spectroscopy**
- Photonuclear interactions are a prolific source of vector mesons. At RHIC design luminosity, the exclusive 0 production rate with gold beams is 120 Hz!  Although the ‘ground state’ 0, , andmesons are well understood, their excited states are not. For example, the particle data group points out that the (1450) and (1700) may be 1 particle or 2
- Data from UPC’s at RHIC (and elsewhere) can resolve this question. If they are two particles, they will likely have different absorption cross sections, and consequently, the mass spectrum from -A interactions will have a different shape from studies at e+e- colliders. These excited states have production rates ~ 1% of the ‘ground’ states.

## Interest in UPCs

**Interferometry**
- The symmetric geometry in heavy ion collisions allows for interesting tests of quantum mechanics. STAR has already observed the interference between when nucleus 1 emits a photon which scatters from nucleus 2, emerging as a vector meson, and vice versa.
- This process requires a non-local wave function which retains amplitudes for all possible particle decays even after the decay occurs.
**Electrodynamics in Strong Fields**
- The usual picture of two-photon interactions is that each beam particle emits a photon, and the photons collide.
- Since the coupling constant Z~0.6 at RHIC, this picture may fail.
- Extensive theoretical effort to study e+e- pair production in UPCs at RHIC; these studies have found relatively small changes in the total cross section.
- Preliminary results support these calculations, finding cross sections close to the lowest-order prediction. However, the additional photons are likely to make their presence felt in other ways, by changing the angular distributions, and in the production of multiple pairs.

## Photo production of the 0 in UP AuAu Collisions at RHIC

- The pT and p-p invariant mass spectrum of two-track events collected in 200 GeV Au-Au collisions
- with the minimum bias trigger, from coherent photoproduction of ro and direct p-p pairs accompanied
- by mutual Coulomb excitation.  The pT spectrum is peaked below 100 MeV/c, as expected for a
- photon from the electromagnetic field of one nucleus scattering coherently from the other nucleus,
- emerging as a vector meson.

##  photoproduction
Two-source interference
 photoproduction in dA
Coherent production of 4-prong final states
Correlations in mutual Coulomb excitation

- Two-source interference
-  photoproduction in dA
- Coherent production of 4-prong final states
- Correlations in mutual Coulomb excitation

- Ongoing analyses in STAR

## Slide 9

## 200 GeV 0 dataset (Preliminary)

-  mass spectra show 0, interference with direct  production
- Accompanied by mutual Coulomb excitation (MCE) [left] and 1n1n MCE [right]

****** ****Inv. mass**

****** ****Inv. mass**

- 0

- Inteference
- Term

## Main Preliminary Results

- d/dy for  photoproduction
- With and without mutual excitation
- M spectrum:  and direct 
- pT spectrum for 
- Coherent + Incoherent production
-  helicity analysis
- Helicity is conserved in  photoproduction
- Spectra & results for 1n,1n excitation

**d********/dy**:
- Data + 3 theoretical predictions

- pT spectrum

**Blue – coherent**
**Red- incoherent**

-  (AuAu → AuAu) = 509  34  107 mb
- Consistent with theory prediction based on Glauber
- but not parton saturation model

- inc/ coh = 0.29  0.03  0.03

- Ratio of  to direct similar toHERA
- And independent of photon energy

- Preliminary

- Preliminary

## 0 interference paper

## 0 interference paper

- Interference between 0 production  on two target nuclei observed
- Fit spectra to
- R(t) = Int (t)/Noint(t)
- Int(t), Noint(t) are t-spectra with &
-     without interference
- c = 1 – ‘full’ interference
- c = 0 – no interference
- c =96 +/- 5 +/ ?%
- Systematic error still under evaluation

- t ~ t = pT2  (GeV2)

- Data
- Interference
- No interference

- Minbias data, |y|<0.5

**Preliminary**

## dAu -->  analysis (Preliminary data)

- dAu -->  dAu  & pn Au 
- Photon mostly comes from Au
- Breakup channel (pn) cleaner due to use of ZDC in trigger
- Measured d/dy, d/dpT, d/dM

- M

- d/dy

- d/dt

- Preliminary

- Preliminary

- Preliminary

## Au Au --> Au* Au*

- Nuclei may be electromagnetically excited to a Giant Dipole Resonance & decay by single neutron emission
- Vector transition --> 1—
- Neutron pT follows photon (transverse) polarization
- b is the same for both nuclei
- Convolute two cosines
-  =1- 2

- Nucl. Phys. A729, 787 (2003)

- (transverse view)

- n (East)

- n (West)

- 2

- 1

## UPC trigger & rates in 2007 run

- Trigger on low multiplicity + ZDC coincidence
- As of June 6th – 1.74M UPC triggers in 228 hrs
- Mean rate 2 Hz (after prescale)
- Expect ~ 2.2M UPC triggers
- If trigger yield (N/Ntrigger) is what we hope
- based on best 2002 yields
- ~ 50,000 
- 600 4-prong (’)
- Other good stuff

## J/ trigger in 2007 run

- L0 trigger
- Two EMC towers with E>~ 1.2 GeV
- Separated by at least 600 in 
- Low multiplicity in CTB
- L2 not used for UPCs
- Rate: 2,380 triggers as of June 6th
- Expect ~ 3,100 total

## 4-prong analysis Preliminary data)

- Coherent 4-particle (Q=0) production seen in 2004 data
- Use 2007data for publication
- Better understanding of trigger efficiency, statistics

- 4-particle combinations

- pT (GeV/c)

- 4 Mass (GeV)

- ~100 events

- Preliminary

- Preliminary

## Outlook

- UPC’s in heavy ion collisions at RHIC and in the future at LHC provide a fertile field for fundamental QCD studies which avoid problems with other measurements
- In STAR, particularly when the DAQ1000 upgrade becomes operational, this physics should really “come into its own”.
- It is a great time to think about this problem theoretically and publish your work