Erice, October 2001 Pablo Yepes, Rice U. 1

Ultra-Peripheral Collisions at STAR

Pablo Yepes, Rice University

for the CollaborationS T A R

Workshop on Electromagnetic Probes of Fundamental

Physics Erice (Sicily) Oct 16-21, 2001

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Ultra-Peripheral Collisions (UPC)

• Nuclei “miss” each other (b > 2RA) No hadronic interactions

• Ions are sources of fields Photons: ~ Z2 very high fluxes Pomerons: A 2 (bulk)- A 4/3 (surface)

• Fields couple coherently to ions P < h/RA, ~30 MeV/c for heavy ions

P|| < h/RA ~ 3 GeV/c at RHIC

Au

Au

Coupling ~ nuclear form factor

, P, or meson

(Au Au, 200GeV/nucleon,

University of Frankfurt)

time

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•It is thought to factorize as function of impact parameter

•Decay yields neutrons at zero degrees wrt beam direction (ZDC)

In addition to UPC production, nuclei can exchange one or more separate photons and become mutually excited.

Nuclear Excitation

Au

Au

PAu*+n

Au*+n

Process

Au+Au -> Au*+Au* +UPC

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• Vector mesons: A , , , J/,… Very large cross sections. Can we address physics

of meson factories? Production cross sections --> (VN) Vector meson spectroscopy (, , ,…) Wave function collapse

• EM particle production: e+e-,,mesons Strong Field QED: Z ~ 0.6 meson spectroscopy

~ charge content of scalar/tensor mesons particles without charge (glueballs) won’t be

seen• Mutual Coulomb excitation (GDR & higher)

Luminosity measurement, impact parameter tag

Some Physics Topics

Production occurs in/near one ion

VM

e+e-, qq,...s

Z ~ 0.6; is N > 1?

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A typical STAR events=200 GeV/A

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Year 2000 Samples

• Data: 400 K Minimum Bias:

Ntracks<10 (Signal in both ZDC’s)

7 h dedicated run with special trigger

• Monte Carlo: 100 K: 100 K di-electrons

Level 0 Trigger with Central Trigger Barrel:

•Top-Bottom Veto•1-2 hit North and 1-2 South•Output 20-40 Hz

Level 3 Trigger:•Vertex and Multiplicity•Output: 1-2 Hz

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Analysis Selection

Minbias • 2 +/- Primary Tracks• rvert< 15 cm, |zvert|<200 cm• Transverse Angle > 2.7 rad• Total pT<0.2 GeV

TriggeredAs Minbias plus anti-cosmic cut:

Opening Angle < 3 rad.Backgrounds:

incoherent photonuclear interactions grazing nuclear collisions beam gas

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pT and y Acceptance and Efficiency

From MC sample

|y|<1

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Opening Angle in Transverse Plane

Minbias Triggered

Cut Cut

Trigger simulationapplied to MC!

MC normalizedto data

MC normalizedto data

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Theta* with Pion Mass Assumption

Minbias Triggered

Theta*: decay angle relative to beam axis in center of mass mother

Trigger simulationapplied to MC!

MC normalizedto data

MC normalizedto data

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Triggered Sample• Track dE/dx consistent with • Sample divided:

No signal in ZDC: No nuclear breakup

Signal in ZDC: Nuclear breakup• peak for pT < 2h/ ~ 100 MeV/c

0 PT

M()

Preliminary

Preliminary

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‘Minimum Bias’ PT

0 PT

Preliminary

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Direct + - production

• Direct + - is independent of energy• The two processes interfere: 1800 phase change at

M(0) changes + - lineshape

• good data for p --> p (HERA + fixed target)• poor data for A

+ - fraction should decrease as A rises

-

+

-

A -- > 0A -- > + - A

+

A -- > + - A

0

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0 lineshape

ZEUS p --> (0 + +- )p

Set =0 for STAR

STAR Au --> (0 + +- )Au

Preliminary

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A Look at e+e-

• ‘Minimum bias data set 2 track Q=0 vertex

• Find electrons by dE/dx p< 140 MeV/c

• Select identified pairs

• pT peaked at 1/<b>

e

Blue - all particlesred - e+ e- pairs

Preliminary

Pt (GeVc)

P (GeV/c)

dE/d

x (k

eV/c

m)

Eve

nts

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Present and Future: 2001 Run

• Around 10 times more minbias events

• Dedicated trigger for UPC running in parallel for the last part of the run

TriggeredMinbias Very Preliminary

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Conclusions

• For the first time, we have observed three peripheral collisions processes Au + Au -- > Au + Au + 0

Au + Au -- > Au* + Au* + 0

Au + Au -- > Au* + Au* + e+e-

• We see interference between 0 and direct

• In 2001: more data, more triggers, more luminosity,more energy, more channels, more acceptance.