Diffractive Vector Meson Photoproduction in ultra-peripheral heavy ion collisions with

STAR

Exclusive0 photoproduction in AuAu and dAu collisions

0 interferometry

4-prongs – the *0?

e+e- pair production

Conclusions

Akio Ogawa(BNL), Spencer Klein(LBL)For STAR Collaboration

A. Ogawa, BNL

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 photon emission and scattering Rates are high ~ 8 % of (had.) for gold at 200 GeV/nucleon

120 /sec at design luminosity Other vector mesons are copiously produced Incoherent scattering can also be studied

Au

Au 0

qq

A. Ogawa, BNL

The

Collaboration

STARSTAR

~ 400 collaborators41 institutions9 countries

Solenoid Tracker At RHIC

A. Ogawa, BNL

A. Ogawa, BNL

0 photo- production 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

Au

Au 0

qq

A. Ogawa, BNL

200 GeVExclusive 0

Enhancement at pT < 2h/RA ~100 MeV/c

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 peakM()

0 PT

Signal region:

pT<0.15 GeV

Preliminary

A. Ogawa, BNL

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 ρ

σ

Au

Au

PAu*

Au*

0

0 with gold @ RHIC

d/

dyy

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

n

n

A. Ogawa, BNL

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

A. Ogawa, BNL

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*

A. Ogawa, BNL

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

STARPRL 89, 027302 (2002)

TheoryPRL 89, 012301 (2002)

0wit h XnXn 36.6±.4±8.9 b 7 b0wit h 1n1n .5±0.4±0.6 b 3.5 bExclusive0 410±190±100 b 350 b

A. Ogawa, BNL

Interference in AuAu 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

A. Ogawa, BNL

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)

-

0

0+

+

A. Ogawa, BNL

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

A. Ogawa, BNL

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

A. Ogawa, BNL

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

A. Ogawa, BNL

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

A. Ogawa, BNL

d 0pn Topology trigger + ZDC for Au breakup

Clear single neutron signal M well fit by 0 + direct

0 mass = 766 ± 1 MeV = 159 ±13 MeV

~ particle data book values 0:direct +- ratio slightly lower than AuAu data

t spectrum is similar to ZEUS slope b ~ 11.5 GeV-2

Dropoff at small t Too little energy to dissociate the deuteron

t (GeV2)

Deu

tero

n do

es n

ot d

isso

ciat

e

M (GeV)M (GeV)

Preliminary

A. Ogawa, BNL

pT

4-prong analysis Very preliminary ‘Model’ reaction

A->0*(1450/1700) --> ++-

Expect ~ 100 events Follows 2-prong analysis

pT < 100 MeV/c Excess seen for ++-

Over ++-

Only at low pT

Analysis on a fraction of data Background subtracted mass

spectrum peaks at ~1.5 GeV

Neutral 4 pion combos

Charged 4 pion combos

En

trie

s

Net Signal

0

mass (GeV)

En

trie

s

Preliminary

A. Ogawa, BNL

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)

A. Ogawa, BNL

Conclusions & Outlook STAR has observed photonuclear 0 production in AuAu

and dAu collisions The 0 cross sections agree with theoretical predictions. 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. We observe coherent 4-prong events, likely the *0. The cross section for e+e- pair production is consistent with

lowest order quantum electrodynamics. In 2004, we have multiplied our data sample, and hope to

observe photoproduction of the J/.

Back up

A. Ogawa, BNL

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

A. Ogawa, BNL

0 production in dAu The photon usually comes from the Au

The coherent (no breakup) reaction has a small contribution due to photons from the deuteron

d --> 0d Coherent, coupling to entire deuterons

d --> 0pn Incoherent, couples to individual nucleons

Both are ‘usually’ two photon processes Factorization does not hold here

The deuteron is small; 0 pT can be large

A. Ogawa, BNL

d 0d? No neutron detected

d 0d Deuteron form factor

d 0pn where the neutron missed the ZDC Simulations in progress

Au 0Au Mostly at pT < h/Rau

Studies are in progress to understand these contributions

0 mass, width close to particle data book values

Ratio of 0: direct similar to d 0pn

t (GeV2)

M (GeV)

Preliminary