quark matter 2002falk meissner, lbnl falk meissner lawrence berkeley national laboratory for the...

Quark Matter 2002 Falk Meissner, LBNL

Falk Meissner Lawrence Berkeley National Laboratory

For the STAR Collaboration

Quark Matter 2002 19.07.02

Coherent Vector Meson Production in Coherent Vector Meson Production in Ultra-Peripheral Heavy Ion CollisionsUltra-Peripheral Heavy Ion Collisions

•Ultra Peripheral Collisions•Exclusive Meson Production•2000/2001 Data Sets and Analysis•Cross Sections•Photon-Photon Interactions•Summary


Ultra-Peripheral CollisionsUltra-Peripheral Collisions

Au

Au

XPP

Nuclei ‘miss’ geometrically and interact via long range fields

Coupling strengthCoupling strengthlarge cross sections• Photon- Z2 Equivalent Photon Approximation (Weizsaecker-Williams, Fermi)

• Pomeron-P A4/3 (surface) to A 2 (volume)

Coherent coupling to extended charge of both nuclei• P> R_A Small transverse momentum: pT < h/2RA~ 90

MeV• Longitudinal component PL < h/2RA ~ 6 GeV• Quasi Real Photons


Exclusive Vector Meson Production Exclusive Vector Meson Production AA’’VAVA

• Extrapolate p’Vp to A’VA with Glauber calculation

Large cross sections:350 mb at sNN

1/2=130 GeV 590 mb at sNN

1/2=200 GeVS.Klein, J.Nystrand, Phys. Rev C60 014903 (1999), A. Baltz, S. Klein,J. Nystrand PRL 89, 012301 (2002)

• Factorize as function of impact parameter

• Nuclear breakup by single (1n1n) and multiple (xnxn )neutron emission

• single/multiple neutron emission selects different impact parameters

Exclusive production AuAu AuAu0

)()( 022 bPbbPd GDR

… with nuclear excitation AuAu Au*Au*0


Experimental Signature of UPCExperimental Signature of UPC• Two oppositely charged

tracks with vertex • Low total pT

• Back-to-back in transverse plane

ZDC ZDC

CTB-Topology

Challenge: Trigger !

Topology requirements in central trigger barrel (CTB)

Coincident signals from nucl. Breakupin zero degree calorimeters (ZDC)


Transverse Momentum & Transverse Momentum & Invariant Mass SpectraInvariant Mass Spectra

• 2000: 130 GeV; 9hrs dedicated, 30k triggers

• 2001: 200 GeV; trigger mix 1.5M topology triggers• Peak at low pT coherent interaction• Background model from like-sign pairs

normalized to dataNo neutron signal in ZDC gold nuclei

remain in ground state

Signal region: pT<0.15 GeV

Topology Trigger AuAu AuAu0

200GeV

Preliminary


Meson Production with Nuclear Meson Production with Nuclear Break-upBreak-up

Minimum Bias (ZDC) Trigger AuAu Au*Au*0

ZDC Response

Signal region: pT<0.15 GeV

•2000: ~400k triggers L~60/mb•2001: ~1.7M triggers L~250/mb

200GeV

Preliminary


Mass FitMass Fit

• Acceptance corrected• Breit-Wigner+direct pion

pair production+ BG • Background contribution at

low mass from e+e- pairs • Amplitude ratio similar

to lepton nucleon scattering

130 GeV-

+

A+-A


Cross Section Cross Section (AuAu (AuAuAuAu(*)(*)AuAu(*)(*)))130 GeV - nucl-ex/0206004, 200 GeV Preliminary Luminosity normalization from 7.2b hadronic AuAu cross sectionSystematic uncertainties ~20%

Theory: A. Baltz, S. Klein J. NystrandPreliminaryTotalNo Breakup

xn,xn

1n,1n


• Compare to lepton-nucleon scattering

• A dependence of photo-nuclear cross section

• VM Production mechanism depends on scale - c.m.s. energy, pT, quark masses

• J/sensitive to gluon distribution in the nucleus

A-Scaling and Energy DependenceA-Scaling and Energy Dependence


Two-Photon Interaction Two-Photon Interaction AuAuAu*Au*e+e-

Identified e+e - Pairs via dE/dx only at low momentum p<0.13 GeVNon-Identified coherent e+e- pairs - background for production; extrapolate identified e+e- to all momenta with MC

Coherent e+e- Pairs pT < h/b (<b>~40fm)

Physics Topics:Strong field QED Z ~ 0.6Large cross section Z44

Au

Au

e-

e+

200GeV, 0.25T Magnetic Field


SummarySummaryFirst measurements for coherent meson production in ultra-First measurements for coherent meson production in ultra-

peripheral heavy ion collisions with and without nuclear peripheral heavy ion collisions with and without nuclear excitation excitation

Au + Au -> Au + Au + and Au + Au -> Au* + Au* + .• low pT= coherent coupling• Cross Sections agree with predictions =>Approximate factorization of rho-production and nuclear excitation =>Weizsaecker-Williams photon flux from large relativistic charges ok. => Glauber extrapolation of N to Au ok• This is just the beginning; Future analysis topics:

– Vector meson spectroscopy ,excited states (*,…)– Multiple VM production– Hard diffraction - higher mass states J/– Strong field QED - e+e- pairs– Interference of decaying particles

RHIC is a good place to study diffractive and electromagnetic processes in heavy ion collisions. Lots of data and physics topics.


RapidityRapidityExtrapolationExtrapolation

• Y-distribution differs for prod. with (xnxn,1n1n) and without

(0n0n) breakup

Nucl.Breakup

STAR Data•Acceptance is flat in pT and Mass•Analysis |y|<1•Extrapolate to 4pi with MC


Compilation of VM cross Sections Compilation of VM cross Sections


Entangled Nonlocal WavefunctionsEntangled Nonlocal Wavefunctions• VM are short lived (~10-23 s) decay before traveling distance b• Decay products interfere

• J/s many decays: e+ e-, +- +- , hadrons• At the decay time(s) neither amplitude can know

about the other (no overlap)Either independent decays --> little/no interferenceOR The wave function retains amplitudes for all possible

decays, long after the decay occurs The wave function only collapses when it interacts

with the detector Quantum mechanics predicts choice 2 Example of the Einstein-Podolsky-Rosen paradox

+

-

+

-

b

-

e+

e-

b

J/

J/

+0

???


2-slit interferometer with separation of impact paramter b!

<b> ~ 40 fermi >>RA ~ 7 fmJ have negative parity JPC = 1- -

Amplitudes have opposite signs ~ |A1 - A2eip·b|2

destructive interference at pT=0 (maximal at y=0 when A1=A2)

Can’t differentiate between projectile and target

Expected Signal

No Interference

Interference

InterferenceInterference

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

quark matter 2002falk meissner, lbnl falk meissner lawrence berkeley national laboratory for the...

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