saturation, low-x, and qcd at rhic, part 3

TJH: Saturation, Low-x, QCD Workshop Villaggio Guglielmo, July 2-18, 20071

Saturation, Low-x, and QCDat 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’sThe 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, , andmesons 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 UPCsInterferometry• 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

Ongoing analyses in STAR


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 – coherentRed- 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 toHERAAnd 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

)]1)((1)[exp( tRcbtAdt

dN

t ~ t = pT2 (GeV2)

DataInterferenceNo 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

PreliminaryPreliminary


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)

)(~)( bpCospP TT

2/)2(1)( CosP

Eb,

(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

Real Data, p+p Run V


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 combinationspT (GeV/c) 4 Mass (GeV)

~100 events

PreliminaryPreliminary


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

saturation, low-x, and qcd at rhic, part 3

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