Ultra-peripheral Collisions at RHICSpencer Klein, LBNL

for the STAR Collaboration

Ultra-peripheral Collisions: What and Why

Vector meson interferometry

STAR Results:

Au + Au --> Au + Au + 0

0 production with nuclear excitation

Direct +- production & interference

e+e- pairs

2001 Plans - STAR, PHENIX, PHOBOS

Conclusions

Coherent Interactions b > 2RA;

no hadronic interations <b> ~ 40 fermi

Ions are sources of fields photons

Z2

Pomerons or mesons (mostly f0) 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

large cross sections Enormous variety of reactions

Au

Au

Coupling ~ nuclear form factor

, P, or meson

Specific Channels Vector meson production

A -- > ’, , , J/,… A Production cross sections --> (VN) Vector meson spectroscopy (, , ,…) Wave function collapse Multiple vector meson production

Electromagnetic particle production

leptons,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

Production occurs in/near one ion

VM

e+e-, qq,...s

Z ~ 0.6; is N > 1?

Exclusive 0 Production

One nucleus emits a photon The photon fluctuates to a qq pair The pair scatters elastically from the other nucleus

also Photon- meson contribution qq pair emerges as a vector meson is large: 380 mb for with Au at 130 GeV/nucleon

5% of hadronic cross section 120 Hz production rate at full RHIC energy/luminosity

10% of hadronic cross section , , , * rates > 5 Hz J/’, *, *, all copiously produced, a challenge

Au

Au

0

qq

Interference 2 indistinguishable possibilities

Interference!! Similar to pp bremsstrahlung

no dipole moment, so no dipole radiation

2-source interferometer separation b

,, , J/ are JPC = 1- -

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

b is unknown For pT << 1/<b>

destructive interference

No Interference

Interference

pT (GeV/c)

y=0

Entangled Waveforms VM are short lived

decay before traveling distance b Decay points are separated in space-time

no interference OR

the wave functions retain amplitudes for all possible decays, long after the decay occurs

Non-local wave function non-factorizable: +- + -

Example of the Einstein-Podolsky-Rosen paradox

e+

e-

J/

+

-

J/

b

(transverse view)

+

Multiple meson production P(0) ~ 1% at b=2RA

w/ Poisson distribution P(00) ~ (1%)2/2 at b=2RA ~ 106 00 /year

Identical state enhancement (ala HBT) for production from same ion (away from y=0) p < h/RA

Like production coherence reqt. Large fraction of pairs

Stimulated decays?

b (fermi)

P(b

)

Looking further ahead…. A --> ccX, bbX, ttX (at LHC)

sensitive to gluon structure function high rates leptons/mass peak/separated vertex. backgrounds from grazing hadronic ints.

Double-Pomeron interactions in AA prolific glueball source narrow range of b

small to moderate cross sections harder pT spectrum

experimental signature? Double-Pomeron interactions in pp

pT is different for qq mesons vs. exotica? Tag pomeron momenta with Roman pots

Production occurs in one ion

QQ-->open charm

Transverse view: anarrow range of impact parameters

g

glueballs

Triggering Strategies Ultra-peripheral final states

1-6 charged particles low pT

rapidity gaps small b events

‘tagged’ by nuclear breakup

Nuclear Excitation

Nuclear excitation ‘tag’s small b Multiple Interactions probable

P(0, b=2R) ~ 1% P(2EXC, b=2R) ~ 30%

Factorization appears valid EXC

useful for triggering select small b for interference studies

Au* decay via neutron emission

)()( 022 bPbbPd EXC

Au

Au

P

Au*

Au*

(1+)0

Preliminary

Experimental Studies Exclusive Channels

0 and nothing else 2 charged particles net charge 0

Coherent Coupling pT < 2h/RA ~100 MeV/c

back to back in transverse plane

Nuclear breakup possible Backgrounds:

incoherent photonuclear interactions

grazing nuclear collisions beam gas

Peripheral Trigger & Data Collection Level 0 - prototype, circa 2000

hits in opposing CTB quadrants Rate 20-40 /sec

Level 3 (online reconstruction) vertex position, multiplicity Rate 1-2 /sec

7 hours of data ~ 30,000 events Backgrounds

cosmic rays beam gas upstream interactions out-of-time events

Exclusive 0 2 track vertex

vertex in diamond non-coplanar, < 3 rad

reject cosmic rays track dE/dx consistent with No neutrons in ZDC peak for pT < 100 MeV/c

asymmetric M peak and model background

matches pairs from higher multiplicity events scaled up by 2.1 flat/phase space in pT

mostly at low M()

0 PT

M()

Preliminary

‘Minimum Bias’ Dataset Trigger on >1 neutron signal

in both zero degree calorimeters

~800,000 triggers Event selection same as

peripheral but no ZDC cuts

and model background phase space in pT

small

0 PT

M()

Preliminary

Direct + - production

Direct + - is independent of energy The two processes interfere

1800 phase shift at M(0) changes + - lineshape

good data with p (HERA + fixed target) + - fraction should decrease as A rises

-

+

-

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

+

A -- > + - A

0

0 lineshape

Data

Fit

0

+-

Fit all data to 0 + +-

interference is significant+- fraction is high (background?)

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

Set =0 for STAR

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

Preliminary

P (GeV/c)

A peek at --> e+e-

‘Minimum bias dataset Select electrons by dE/dx

in region p< 140 MeV/c Select identified pairs pT peaked at 1/<b>

Eve

nts

Pt (GeVc)

dE/d

x (k

eV/c

m)

Blue - all particlesred - e+ e- pairs

e

Preliminary

2001 RHIC - higher luminosity, energy, longer run

larger cross sections STAR

improved triggering trigger in parallel with central collisions program 10-100X ‘topological’ data

expanded solid angle (FTPC, MWC,…) 10X minimum bias data

PHOBOS “interest in pursuing it, but will require major trigger reconfiguration

and enhancements before we can get serious data”

Coherent Peripheral Collisions in PHENIX

Study coherent two-photon and photonuclear interactions in coincidence with mutual Coulomb excitation of the nuclei. • Two central tracking arms ||0.35, = 290°• Global detectors (used for triggering): Zero-Degree Calorimeters, Beam-Beam Counters Trigger: Level 1 – ZDC coincidence, BBC veto

Level 2 – 1-5 tracks (hits) in each tracking arm

Goals for 2001: Observe 0 production in the reaction Au+Au Au*+Au*+0; 0 +- ,

A total integrated Au+Au luminosity of 300 b-1

should give a sample of about 20,000 0 ’s, Study the factorization of 0 production and Coulomb excitation, Try to see the 2-source interference in 0 production.

Physics Recap RHIC is a high luminosity and A collider Coherent events have distinctive kinematics Ultra-peripheral collisions allow for many studies

Measurement of (VA) Vector meson spectroscopy A measurement of the 0 pT spectrum can test if particle

decay triggers wave function collapse multiple vector meson production Strong field QED tests Studies of charge content of glueball candidates

Conclusions STAR has observed three peripheral collisions processes

Au + Au -- > Au + Au + 0

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

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

Interference between 0 and direct is seen Next year: higher energy, more luminosity, more data, more

experiments, more detector subsystems, ... More channels, cross sections, pT spectra,...

Ultra-peripheral collisions is in it’s infancy It works!!! come back next year!