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Ultraperipheral heavy ion collisions

Timoshenko S., Emelyanov V.

Moscow Engineering Physics Institute (State University)

(for the STAR Collaboration)

XXXIII International Conference on High Energy Physics (ICHEP'06), Moscow 2006.

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The STAR Collaboration~400 collaborators, 49 institutions, 8 countries

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Overwiev

Introduction in UPC

Vector meson production

STAR

AuAu

dAu

4 prong analysis

interference

Conclusion

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Introduction in ultraperipheral collisions

The two nuclei geometrically “miss” each other b > 2RA

Ions are source of fields photons s ~ Z4

pomerons sp ~ Z2A2 – for ‘heavy’ states

sp ~ Z2A5/3 - for lighter mesons

Photon and pomeron can couple coherently to the nuclei if its have:

• Small transverse momentum:

pT < h/RA~ 30 MeV

• Maximum longitudinal componentpL < h/RA ~ 3 GeV

A

nuclear form factor

, P

b

A

A

Pomeron carry the strong interaction but is colorless and it has the quantum number of the vacuum JP = 0++

E ~ 3 (80) GeV at RHIC (LHC)W ~ 6 (160) GeV at RHIC (LHC)

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Vector meson production Vector mesons production (photon-pomeron interaction)

Exclusive vector meson production can be described in terms of elastic scattering. Photon emitted by one nucleus fluctuates to virtual ‘qq’ pairs, this state elastically scatters from the other nucleus (absorb some of the photon wave function) , then ‘qq’ pairs can be emerge as a vector meson. The “pomeron” represents the absorptive part of nuclear cross section.

, J/Y ,Y

Klein S. and Nystrand J.,hep-ph/0311164

00 V q u,d,s,c,b q l e, , lV , , ,J / ,| c | c | V c | qq c | l l

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

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STAR

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Types of trigger Topology trigger (no break up d)

CTB (central trigger barrel) 240 scintillator slats CTB divided into 4 azimuthal

quadrants Hits in the North and South

sectors Top and bottom reject cosmic

ray Low multiplicity (2 or 4 events) Vertex position

typical event

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Types of trigger Topology trigger + ZDCs (0 in TPC + signals in forward (zero

degree calorimeters) Topology trigger + West ZDC: Au+d->Au+pn

required break up d Topology trigger + both ZDC: Au+Au->AuAu+Xn

Backgrounds peripheral hadronic events cosmic rays, beam gas interactions, pile-up

TPCAu d(Au)

ZDC-West ZDC-East

CTB-topology

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AuAu -> 0Au*Au* 200 GeV

Signal region:

pT<0.15 GeV

0 Rapidity

After detector simulation

1.7 million ZDC coincidence triggers in 2002 Require a 2 track vertex and - model background

scaled up to 2 single (1n) and multiple (Xn) neutron production

1n mostly from Giant Dipole Resonance Cross section and rapidity distribution match soft Pomeron model

STAR Preliminary

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d -> 0pn Mass Spectra

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2 2

M MdA B

dM M M iM

Topology trigger + ZDC for d breakup

Data shows clear 1n signal Large, clean 0 sample

~13400 events Well fit by 0 + direct

0 mass, width consistent with particle data book

0:direct pp ratio slightly lower than AuAu data

= 2.63±0.32±0.73 mb

preliminary

no cut pT

STAR Preliminary

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d --> 0pn t spectra Fit to t ~ pT

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Sensitive to nucleon form factor

ZEUS used F(t)=e-bt+ct*t

for proton form factor slope b ~ 9 GeV-2

Same as ZEUS R=√4b=(1.2±0.3) fm

Good fit to ZEUS at large t Drop off at small t,

Too little energy to dissociate the deuteron

Seen by fixed target experiments

Deuteron does not dissociate

t (GeV/c)2

ds/d

t, m

b/G

eV2

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4 prong analysis Very preliminary ‘Model’ reaction

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

Expect ~ 100 events Follows 2-prong analysis

pT < 100 MeV/c Excess for ++-

• Over ++-

• Only at low pT

Analysis on a fraction of data Background subtracted mass

spectrum peaks at ~1.5 GeV

mass (GeV)

En

trie

s

Neutral 4 pion combos

Charged 4 pion combos

En

trie

s

Net Signal

STAR Preliminary

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Interference 2 indistinguishable possibilities

Interference!!

2-source interferometerwith separation b

is negative parity For pp, AA parity transform ->

~ |A1 - A2eip·b|2

At y=0=0[1 - cos(pb)] For pbar p: CP transform ->

~ |A1 + A2eip·b|2

b is unknown Reduction for pT <<1/<b>

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

t (GeV/c)2

dN/d

t

int

noint

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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

syst. uncertainties: ±8(syst)±15%(theory) c=0 -- > no interference c=1 -- > “full” interference

Data and interference model match c = 1.01 +/- 0.08

0.78 +/- 0.13

dN/d

tdN

/dt

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

AuAu -> r0Au*Au* 200 GeV

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Conclusion

STAR has observed incoherent photonuclear 0 meson production in dAu collisions. The 0 cross sections and kinematic distributions

agree with theoretical models. dN/dt reflects d/Au size/form factor

STAR has observed (likely the 0*) production in AuAu collisions

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.