1 Falk Meissner Lawrence Berkeley National Laboratory For the STAR Collaboration Schleching, March...

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Falk Meissner Lawrence Berkeley National Laboratory

For the STAR Collaboration

Schleching, March 2001

Ultra-Peripheral Collisions Ultra-Peripheral Collisions at RHICat RHIC

•Introduction and Physics•Trigger and Data Set•Analysis and First Results•Nuclear Excitation in UPC•Summary

Schleching, March 2001 Falk Meissner, LBNL

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Coherent CouplingCoherent Coupling to both nuclei: photon Z2, Pomeron

A4/3 to A 2 ( A4/3 in the limit ; A2 in the weak limit)

• Large cross section: 380 mb for Au@130Gev/nuclon

• Small transverse momentum: pT ~2h/RA~ 60 MeV

• Nuclei may be mutually excited

Au

Au

0

P

Ultra-Peripheral CollisionsUltra-Peripheral Collisions

Exclusive production of mesons Au+Au --> Au+Au +

Two nuclei ‘miss’ each other and interact via their photon fields at impact parameters b > 2RA


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First Goal - Proof of Principle First Goal - Proof of Principle

UPC-Physics TopicsUPC-Physics Topics

• Coherent vector meson production

A ’VA • Non-local interference of unstable particles • Vector meson spectroscopy • Production cross sections

• Electromagnetic particle production

’leptons,mesons

• Strong Field QED Z ~ 0.6

Observe exclusive production Au Au Collisions


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2-slit interferometer !

J have negative parity:

destructive interference at pT=

Fundamental Quantum Mechanics at Work

Can’t differentiate between projectile and target Expected Signal

No Interference

Interference

An Example : InterferenceAn Example : Interference

S. Klein and J. Nystrand, Phys. Rev. Lett. 84(2000)2330

]


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STAR DetectorSTAR Detector

Coils

Silicon Vertex Tracker

E-M Calorimeter

Trigger Barrel

Time Projection Chamber

Forward Time Projection Chamber

Electronics Platforms

Magnet


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Heavy Ion CollisionHeavy Ion Collision

RHIC 2000 run: 130 GeV/nucleon

Up to ~2000 tracks per event

Trigger: •Multiplicity in Central Trigger Barrel •Neutron Deposit in Zero Degree Calorimeter

(Au Au, 200GeV/nucleon, University Frankfurt)


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Experimental SignatureExperimental Signature of UPC of UPC

0nly two oppositely charged tracks

•Low total pT

•Back-to-back in transverse plane

Trigger Backgrounds:

•Cosmic rays

•Beam-gas Events

•Debris from upstream events

Typical Event :


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Ultra-Peripheral Collisions Ultra-Peripheral Collisions TriggerTrigger

Level 0 • Back to back hits in

Central Trigger Barrel• Coincidence 1 North + 1 South hit• Veto on top + bottom (reject cosmic rays)• Rate 20-40 Hz

Level 3-online reconstruction

• Vertex position • Charged multiplicity• Accepted 1-2 Hz

Data Set • ~ 7 hours of dedicated data collection• 30,000 trigger


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Event Selection CriteriaEvent Selection Criteria

• Vertex within interaction region

|zvertex| < 200 cm and |x,yvertex| < 2 cm

• 2 tracks with net charge zero

• Opening angle < 3.0 rad

(reject cosmic background)

• Pion identification for both tracks via dE/dx

• ADC counts of ZDC <8

(reject signals above pedestal, e.g. hadronic peripheral collisions and beam-gas events)


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First Results: Invariant Mass & First Results: Invariant Mass & Transverse Momentum SpectraTransverse Momentum Spectra

Peripheral Trigger:

Au+Au -> Au +Au +

Preliminary

Signal region: pT<0.1 GeV

Select Signal: Low pT region pT<0.1MeV


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•Believed to factorize as function of impact parameter

•Decay yields neutrons in Zero Degree Calorimeter (ZDC) -> minimum bias trigger

•Data set: ~400,000 events with minimum bias trigger

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

Nuclear ExcitationNuclear Excitation

Au

Au

PAu*+n

Au*+n

0

Process

Au+Au -> Au*+Au* +


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Select Signal: Low pT region pT<0.1 MeV

Minimum Bias Trigger: Au+Au -

> Au*+Au* +

Invariant Mass & Transverse Invariant Mass & Transverse Momentum SpectraMomentum Spectra

Preliminary

Signal region: pT<0.1 GeV

Select Signal: Low pT region pT<0.1MeV


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Ultra Peripheral Trigger Minimum Bias Trigger

•Pedestal peak at ADC sum = 4•Higher ADC values usually in east or west only (beam gas events)

•Single neutron peak around ADC =9 coincident in east and west•Higher ADC values from hadronic peripheral events

Pedestal

Reject events ADC>7 Reject events ADC>30

Single neutron peak

Compare ZDC SignalsCompare ZDC Signals

Au+Au -> Au+Au + and Au+Au --> Au*+Au* + Observe two different processes !

(for two track events)


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Preliminary

Data: Minimum bias trigger 0.62<M<0.92 GeV

Reasonable agreement between data and Monte Carlo

Not yet addressed by the Monte Carlo:

• Background

•Full spectrometer simulation

•Interference

Monte Carlo Simulation vs. DataMonte Carlo Simulation vs. DataSSTARlight TARlight - Monte Carlo Generator for UPC • Simulates photon-photon, photon-Pomeron, photon-meson interactions•Simulates rapidity, transverse momentum and angular distributions

Transverse Momentum Transverse Momentum SpectrumSpectrum

S. Klein and J. Nystrand, STAR Note 347


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SummarySummary

The observation of exclusive production in both peripherally triggered and minimum bias data sets demonstrates existence of

both interactions

Au + Au -> Au + Au +

Au + Au -> Au* + Au* + .

Clear signals at the mass and at low transverse momentum for both processes.

First observation of First observation of Ultra-Peripheral Collisions Ultra-Peripheral Collisions in in

heavy ion interactionsheavy ion interactions

1 Falk Meissner Lawrence Berkeley National Laboratory For the STAR Collaboration Schleching, March...

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