Ultra-peripheral Collisions at RHIC Spencer Klein, LBNL for the STAR collaboration

Ultra-peripheral Collisions at RHICSpencer Klein, LBNL

for the STAR collaborationUltra-peripheral Collisions: What and Why

Interference in Vector Meson Production

Au + Au --> Au + Au + 0

0 production with nuclear excitation

Direct +- production & interference

e+e- pair production

Conclusions

Resu l ts

Coherent Interactions

b > 2RA

no hadronic interactions Ions are sources of fields

photons ~ Z2 --> very high fluxes

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

Au

Au

Coupling ~ nuclear form factor

, P, or meson

Specific Topics Vector meson production

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

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 in STAR

One nucleus emits a photon The photon fluctuates to a qq pair

vector meson dominance --> treat as vector meson The pair scatters elastically from the other nucleus

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

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

Au

Au

0

qq

Interference 2 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

For pT << 1/b

destructive interference

No Interference

Interference

pT (GeV/c)

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)

-

0

A typical STAR event200 GeVnucleon cm)

Analysis Approach 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

Exclusive 0

Trigger on low-multiplicity events back-to-back topology

~ 7 hours of data prototype trigger

2 track vertex in interaction diamond non-coplanar, < 3 rad

reject cosmic rays track dE/dx consistent with No neutrons in ZDC peak for pT < 2h/ ~ 100 MeV/c and model background

0 PT

M()

Preliminary

Nuclear Excitation

Multiple Interactions are possible P(0, b=2R) ~ 0.5% P(2GDR, b=2R) ~ 30% Factorization should hold

Diagram on rt. should dominate

Au* decay mostly by neutron emission

Au

Au

P

Au*

Au*

0

)()( 022 bPbbPd EXC

‘Minimum Bias’ Dataset Trigger on >1 neutron signal

in both zero degree calorimeters

~800,000 triggers Event selection same as

peripheral 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 change at M(0) changes + - lineshape

good data for p --> p (HERA + fixed target) poor data for A

+ - 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

A peek at --> e+e-

‘Minimum bias dataset 2 track Q=0 vertex

Find electrons by dE/dx p< 140 MeV/c

Select identified pairs pT peaked at 1/<b>

e

Blue - all particlesred - e+ e- pairs

Preliminary

Pt (GeVc)

P (GeV/c)

dE/d

x (k

eV/c

m)

Eve

nts

Conclusions For the first time, we have observed three peripheral

collisions processes Au + Au -- > Au + Au + 0

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

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

We see interference between 0 and direct

Peripheral collisions is in it’s infancy next year: more data, more triggers, more luminosity,more

energy, more channels, more acceptance, more... The 0 pT spectrum is sensitive to whether particle decay

triggers wave function collapse