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1 Falk Meissner Lawrence Berkeley National Laboratory For the STAR Collaboration Schleching, March 2001 Ultra-Peripheral Collisions at RHIC Introduction.

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Presentation on theme: "1 Falk Meissner Lawrence Berkeley National Laboratory For the STAR Collaboration Schleching, March 2001 Ultra-Peripheral Collisions at RHIC Introduction."— Presentation transcript:

1 1 Falk Meissner Lawrence Berkeley National Laboratory For the STAR Collaboration Schleching, March 2001 Ultra-Peripheral Collisions at RHIC Introduction and Physics Trigger and Data Set Analysis and First Results Nuclear Excitation in UPC Summary

2 Schleching, March 2001Falk Meissner, LBNL 2 Coherent Coupling Coherent Coupling to both nuclei: photon  Z 2, Pomeron  A 4/3 to A 2 ( A 4/3 in the limit     ; A 2 in the weak limit) Large cross section: 380 mb for Au @ 130Gev/nuclon Small transverse momentum: p T ~2h  /R A ~ 60 MeV Nuclei may be mutually excited Au 00  P Ultra-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 > 2R A

3 Schleching, March 2001Falk Meissner, LBNL 3 First Goal - Proof of Principle UPC-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

4 Schleching, March 2001Falk Meissner, LBNL 4 2-slit interferometer !  J  have negative parity: destructive interference at p T =  Fundamental Quantum Mechanics at Work Can’t differentiate between projectile and target Expected Signal No Interference Interference An Example : Interference S. Klein and J. Nystrand, Phys. Rev. Lett. 84(2000)2330 ]

5 Schleching, March 2001Falk Meissner, LBNL 5 STAR Detector Coils Silicon Vertex Tracker E-M Calorimeter Trigger Barrel Time Projection Chamber Forward Time Projection Chamber Electronics Platforms Magnet

6 Schleching, March 2001Falk Meissner, LBNL 6 Heavy 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)

7 Schleching, March 2001Falk Meissner, LBNL 7 Experimental Signature of UPC 0nly two oppositely charged tracks Low total p T Back-to-back in transverse plane Trigger Backgrounds: Cosmic rays Beam-gas Events Debris from upstream events Typical Event :

8 Schleching, March 2001Falk Meissner, LBNL 8 Ultra-Peripheral Collisions Trigger 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

9 Schleching, March 2001Falk Meissner, LBNL 9 Event Selection Criteria Vertex within interaction region |z vertex | < 200 cm and |x,y vertex | < 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 )

10 Schleching, March 2001Falk Meissner, LBNL 10 First Results: Invariant Mass & Transverse Momentum Spectra Observe about 100  0 events in the data for the peripheral trigger Peripheral Trigger: Au+Au -> Au +Au +   Preliminary Signal region: p T <0.1 GeV Select Signal: Low p T region p T <0.1MeV

11 Schleching, March 2001Falk Meissner, LBNL 11 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 Excitation Au  P Au*+n  00 Process Au+Au -> Au * +Au * +   

12 Schleching, March 2001Falk Meissner, LBNL 12 Select Signal: Low p T region p T <0.1 MeV Observe about 200  0 events in ~500,000 events for the minbias trigger, a rate of 0.5*10^-3 is consistent with expectations Minimum Bias Trigger: Au+Au -> Au * +Au * +   Invariant Mass & Transverse Momentum Spectra Preliminary Signal region: p T <0.1 GeV Select Signal: Low p T region p T <0.1MeV

13 Schleching, March 2001Falk Meissner, LBNL 13 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 Signals Au+Au -> Au+Au +    and Au+Au --> Au * +Au * +   Observe two different processes ! (for two track events)

14 Schleching, March 2001Falk Meissner, LBNL 14 Preliminary Data: Minimum bias trigger 0.62 { "@context": "http://schema.org", "@type": "ImageObject", "contentUrl": "http://images.slideplayer.com/11/3220792/slides/slide_14.jpg", "name": "Schleching, March 2001Falk Meissner, LBNL 14 Preliminary Data: Minimum bias trigger 0.62

15 Schleching, March 2001Falk Meissner, LBNL 15 Summary 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 Ultra-Peripheral Collisions in heavy ion interactions


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