Recent results in Ultra-Peripheral Collisionsfrom STAR

What are ultra-peripheral collisions?

Exclusive0 production

0 interferometry

e+e- pair production (V. Morozov dissertation, 2003)

Conclusions

Spencer Klein, LBNL (for the STAR Collaboration)

S. Klein, LBNL

Coherent Interactions b > 2RA;

no hadronic interactions <b> ~ 20-60 fermi at RHIC

Ions are sources of fields photons

N ~ Z2

Pomerons or mesons (mostly f0) A 2 (bulk) A 4/3 (surface)

Fields couple coherently to ions Photon/Pomeron wavelength = h/p> RA amplitudes add with same phase P < 30 MeV/c

P|| < 3 GeV/c Strong couplings --> large cross sections

Au

Au

Coupling ~ nuclear form factor

, P, or meson

S. Klein, LBNL

Unique Features of Ultra-peripheral collisions

Very strong electromagnetic fields --> e+e- and --> qq Multiple interactions between a

single ion pair Unique Geometry

2-source interferometer Nuclear Environment

Particle Production with capture Large for e-

S. Klein, LBNL

Exclusive 0 Production A virtual photon from one nucleus fluctuates

to a qq pair which scatters elastically from the other nucleus and emerges as a vector meson Photon emission follows the Weizsacker-Williams method For heavy mesons (J/), the scattering is sensitive to nuclear

shadowing Coherence --> Rates are high

~ 8 % of (had.) for gold at 200 GeV/nucleon 120 /sec at design luminosity

Other vector mesons are copiously produced

Au

Au 0

qq

S. Klein, LBNL

Nuclear Excitation Nuclear excitation ‘tag’s small b Multiple Interactions are independent

Au* decay via neutron emission simple, unbiased trigger

Higher order diagrams smaller <b> Harder photon spectrum Production at smaller |y|

Single (1n) and multiple (Xn, X>0) neutron samples

)()( 022 bPbbPd EXC

Au

Au

PAu*

Au*

0

0 with gold @ RHIC

d/

dyy

Exclusive - solidX10 for XnXn - dashedX100 for 1n1n - dotted

S. Klein, LBNL

S. Klein, LBNL

S. Klein, LBNL

0 Analysis 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

Trigger Back to back hits in Central

Trigger barrel

S. Klein, LBNL

200 GeVExclusive 0

1.5 Million topology triggers 2 track vertex

non-coplanar; < 3 rad to reject cosmic rays

and model background shape pairs from higher multiplicity

events have similar shape scaled up by ~2 Incoherent 0 (w/ pT>150 MeV/c) are

defined as background in this analysis asymmetric M peak

M()

0 PT

Signal region:

pT<0.15 GeV

Preliminary

S. Klein, LBNL

200 GeV XnXn data

1.7 million minimum bias triggers Select events with a 2 track vertex and model background single (1n) and multiple (Xn)

neutron production Coulomb excitation

Giant Dipole Resonance

Rapidity distribution matches Soft Pomeron model calculation

After detector simulation

Soft PomeronpT

S. Klein, LBNL

M Mspectrum includes 0 +

direct +-

Same 0: +- ratio as is observed in p--> +- p at HERA

-

+

-

+

0

M()

XnXn sample

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

e+e- and hadronic backgrounds

M

d/

dM

b

/GeV

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

S. Klein, LBNL

Cross Section Comparison

130 GeV data Normalized to 7.2 b hadronic cross section Systematic uncertainties: luminosity, overlapping events, vertex & tracking simulations, 1n selection, etc. Exclusive 0 bootstrapped from XnXn

limited by statistics for XnXn in topology trigger Good agreement

factorization works

STAR PRL 89, 027302 (2002)

Theory PRL 89, 012301 (2002)

0 with XnXn 36.6 2.4 8.9 mb 27 mb 0 with 1n1n 2.50.40.6 mb 3.25 mb Exclusive 0 410190100 mb 350 mb

S. Klein, LBNL

Interference 2 indistinguishable

possibilities Interference!!

Like pp bremsstrahlung no dipole moment, so no dipole radiation

2-source interferometer with separation b

is negative parity so ~ |A1 - A2eip·b|2

At y=0

=0[1-cos(pb)] b is unknown

Reduction for pT <<1/<b>

InterferenceNo Interference

0 w/ mutual Coulomb dissoc. 0.1< |y| < 0.6

t (GeV/c)2

dN/d

t

S. Klein, LBNL

Entangled Waveforms

0 are short lived, with c ~ 1 fm << b Decay points are separated in space-time

Independent decays to different final states

no interference OR

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

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

-

b

(transverse view)

-

+

+

S. Klein, LBNL

Interference Analysis Select clean 0 with tight cuts

Lower efficiency Larger interference when 0 is accompanied

by mutual Coulomb dissociation Interference maximal at y=0

Decreases as |y| rises 2 rapidity bins 0.1 < |y| < 0.5 & 0.5<|y|<1.0

|y|<0.1 is contaminated with cosmic rays

S. Klein, LBNL

t for 0.1 < |y| < 0.5 (XnXn) 2 Monte Carlo samples:

Interference No interference w/ detector simulation

Detector Effects Small Data matches Int Inconsistent with Noint Interference clearly

observed 973 events

dN/d

t Data (w/ fit)NointIntBackground

STAR Preliminary

t (GeV2) = pT2

S. Klein, LBNL

XnXn Fitting the Interference

Efficiency corrected t 1764 events total R(t) = Int(t)/Noint(t)

Fit with polynomial dN/dt =A*exp(-bt)[1+c(R(t)-1)]

A is overall normalization b is slope of nuclear form factor

b = 301 +/- 14 GeV-2 304 +/- 15 GeV-2

c=0 -- > no interference c=1 -- > “full” interference

c = 1.01 +/- 0.08 0.78 +/- 0.13

Data and interference model matchdN

/dt

dN/d

t

STAR Preliminary

STAR Preliminary

Data (w/ fit) Noint Int

Data (w/ fit) Noint Int

t (GeV2)

t (GeV2)

0.1 < |y| < 0.5

0.5 < |y| < 1.0

S. Klein, LBNL

Exclusive 0

<b> ~ 46 fm 5770 events total dN/dt = A*exp(-bt)[1+c(R(t)-1)]

A - overall normalization b = 361 +/- 9 GeV-2/

368 +/- 12 GeV-2

Different from minimum bias data

c = 0.71 +/- 0.16 1.22 +/- 0.21

Interference is present

t

dN/d

tdN

/dt

Data (w/ fit) Noint Int

Data (w/ fit) Noint Int

STAR Preliminary

t

STAR Preliminary

0.1 < |y| < 0.5

0.5 < |y| < 1.0

S. Klein, LBNL

Combining the Data The c values are consistent -- > take weighted mean

c= 0.93 +/- 0.06 (statistical only) Data matches predictions

The b’s for the exclusive 0 and breakup data differ by 20% Exclusive 0 : 364 +/- 7 GeV-2

Coulomb breakup: 303 +/- 10 GeV-2

Photon flux ~ 1/b2

More 0 production on ‘near’ side of target• Smaller apparent size

Systematic Errors (in progress) Change simulation input form factor slope b by 20%

3% (2%) change in c(b) No Detector simulation

18% (1.4%) change in c(b) If simulation is 75% ‘right--> 5% systematic error

S. Klein, LBNL

Au Au --> e+e- Au* Au* e+e- pairs accompanied by nuclear

breakup ZEM ~ 0.6

Higher order corrections? Cross section matches lowest order

quantum electrodynamics calculation No large higher order corrections

pT peaked at ~ 25 MeV Matches QED calculation

By Kai Hencken et al. 4 disagreement with equivalent photon

(massless photon) calculation V. Morozov PhD dissertation

Preliminary

Pair Pt (GeVc)

Pair Mass (GeV)

STAR Preliminary

STAR Preliminary

S. Klein, LBNL

Conclusions & Outlook STAR has observed photonuclear 0 production.

The 0 cross sections agree with theoretical models. Interference between 0 and direct is seen.

We observe 2-source interference in 0 production. The interference occurs even though the 0 decay before the wave functions

of the two sources can overlap. The cross section for e+e- pair production is consistent with lowest order

quantum electrodynamics. The equivalent photon approximation does not describe the pT spectrum.

2003 run: Good dA --> 0 sample. Meson production via double Pomerons from pp pending.