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
Schleching, March 2001 Falk Meissner, LBNL
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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
Schleching, March 2001 Falk Meissner, LBNL
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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
]
Schleching, March 2001 Falk Meissner, LBNL
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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
Schleching, March 2001 Falk Meissner, LBNL
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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)
Schleching, March 2001 Falk Meissner, LBNL
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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 :
Schleching, March 2001 Falk Meissner, LBNL
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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
Schleching, March 2001 Falk Meissner, LBNL
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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)
Schleching, March 2001 Falk Meissner, LBNL
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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
Schleching, March 2001 Falk Meissner, LBNL
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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* +
Schleching, March 2001 Falk Meissner, LBNL
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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
Schleching, March 2001 Falk Meissner, LBNL
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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)
Schleching, March 2001 Falk Meissner, LBNL
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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
Schleching, March 2001 Falk Meissner, LBNL
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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