Ultra-Peripheral Collisions at RHIC

What are ultra-peripheral collisions?

Physics Interest

STAR Results at 130 GeV/nucleon:

Au + Au --> Au + Au + 0

0 production with nuclear excitation

e+e- pair production

A peek at 200 GeV/nucleon & beyond

Conclusions

Spencer Klein, LBNL (for the STAR Collaboration)

Coherent Interactions b > 2RA;

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

Ions are sources of fields photons

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 < h/RA, ~30 MeV/c for heavy ions

P|| < h/RA ~ 3 GeV/c at RHIC

Strong couplings --> large cross sections

Au

Au

Coupling ~ nuclear form factor

, P, or meson

Unique Features of Ultra-peripheral collisions

Very strong electromagnetic fields --> e+e- and --> qq Multiple production Coherent Decays

Unique Geometry 2-source interferometer

Nuclear Environment Particle Production with capture

Large for e-

Specific Channels Vector meson production

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

Electromagnetic particle production

leptons,mesons Strong Field (nonperturbative?) QED

Z ~ 0.6 meson spectroscopy

~ charge content of scalar/tensor mesons is small for glueballs

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 Weizsacker-Williams flux

The photon fluctuates to a qq pair The pair scatters elastically from the other nucleus qq pair emerges as a vector meson ~ 590 mb; 8 % of AuAu(had.) at 200 GeV/nucleon

120 Hz production rate at RHIC design luminosity RHIC, , , * rates all > 5 Hz J/’, *, *, copiously produced, a challenge

The LHC is a vector meson factory ~ 5.2 b; 65% of PbPb (had.) 230 kHz 0 production rate with calcium

Au

Au

0

qq

Elastic Scattering with Soft Pomerons

Glauber Calculation parameterized HERA data

Pomeron + meson exchange all nucleons are the same

~ A2 (weak scatter limit) All nucleons participate J/

~ A 4/3 (strong scatter limit) Surface nucleons participate Interior cancels (interferes) out

~ A 5/3 (0) depends on (Vp)

sensitive to shadowing?

Y = 1/2 ln(2k/MV)

RHIC - Au HE

RA

data + G

lauberK

lein & N

ystrand, 1999

HERA param.

Shadowing & Hard Pomerons If pomerons are 2-gluon ladders

P shadowing ~ (gluon shadowing)2

Valid for cc or bb d/dy & depend on gluon

distributions reduces mid-rapidity d/dy

Suppression grows with energy reduced ~ 50% at the LHC

colored glass condensates may have even bigger effect high density phase of gluonic

matter Y = 1/2 ln(2k/MV)

RHIC - Au

Leading Tw

ist Calculation

Frankfurt, S

trikman &

Zhalov, 2001

No shadowing

Shadowed

HERA param.

d/d

y

Nuclear Excitation

Nuclear excitation ‘tag’s small b Multiple photon exchange

Mutual excitation Au* decay via neutron emission

simple, unbiased trigger Multiple Interactions probable

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

Non-factorizable diagrams are small for AA )()( 02

2 bPbbPd EXC

Au

Au

P

Au*

Au*

(1+)0

Interaction Probabilities & d/dy Excitation + 0 changes b distribution

alters photon spectrum low <b> --> high <k>

Baltz, Klein & Nystrand (2002)

b [fm]

0 with Gold @ RHIC

P(b

)

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

0 with gold @ RHIC

d/d

y

y

Photoproduction of Open Quarks A --> ccX, bbX sensitive to gluon structure function. Higher order corrections problematic Ratio (A)/(p) --> shadowing

removes most QCD uncertainties Experimentally feasible (?)

high rates known isolation techniques

Physics backgrounds are gg--> cc, --> cc cross section is small gg background appears controllable by requiring a rapidity

gap

Production occurs in one ion

QQ-->open charm

g

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)

+

Interference and Nuclear Excitation

Smaller <b> --> interference at higher <pT>

0 prod at RHIC J/ prod at the LHC

Solid - no breakup criteriaDashed lines 1n1n and XnXn breakup

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

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

Enhancement (ala HBT) for production from same ion (away from y=0) Vector meson superradiance

toward a vector meson laser

p < h/RA

Like production coherence Large fraction of pairs

Stimulated decays?P

(b)

b (fermi)

Production with gold at RHIC

Electron Production w/ Capture e+e-

Electron is bound to nucleus Probe of atomic physics non-perturbative uncertain, ~ 100-200 barns

Focused +78Au beam RHIC Rate ~ 10,000 particles/sec

beam ~ 40-80 mW LHC rate ~ 1M particles/sec

beam ~ 10-40 W can quench superconducting magnets

limits LHC luminosity w/ Pb Could extract as external beam

Z ~ 0.6; is N > 1?

s

e-

e+

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

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

Backgrounds: incoherent photonuclear

interactions grazing nuclear collisions beam gas interactions

Triggering Strategies Ultra-peripheral final states

1-6 charged particles low pT

rapidity gaps small b events

‘tagged’ by nuclear breakup

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

9 hours of data ~ 30,000 events Backgrounds

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

Exclusive 0

2 tracks in interaction region vertex in diamond reject cosmic rays

non-coplanar; < 3 rad peak for pT < 150 MeV/c and give background shape

pairs from higher multiplicity events have similar shape

scaled up by 2.1 high pT 0 ?

asymmetric M peak

M()

pT<0.15 GeV

0 PT

Signal region:

pT<0.15 GeV

‘Minimum Bias’ Dataset

Trigger on neutron signals in both ZDCs

~800,000 triggers Event selection same as peripheral and model background single (1n) and multiple (Xn)

neutron production Coulomb excitation

Giant Dipole Resonance Xn may include hadronic interactions? Measure (1n1n) & (XnXn)

0 PT

ZDC Energy (arbitrary units)

Signal region:

pT<0.15 GeV

Direct + - production

The two processes interfere 1800 phase shift at M(0)

changes + - lineshape good data with p (HERA + fixed target) +- : 0 ratio should depend on ():(A)

decrease as A rises?

-

+

-

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

+

A -- > + - A

0

0 lineshape

Fit to 0 Breit-Wigner + +-

Interference is significant+- fraction is comparable to ZEUS

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

e+e- and hadronic backgrounds

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

Preliminary

Md/

dM

m

b/G

eV

d/

dM

b

/GeV

M

dN/dy for 0(XnXn)

d/dy are different with and without breakup

XnXn data matches simulation

Extrapolate to insensitive region

Nucl.Breakup

Soft Pomeron, no-shadowing, XnXn

After detector simulation

Cross Section Comparison

Normalized to 7.2 b hadronic cross section Systematic uncertainties: luminosity, overlapping events, vertex & tracking simulations, single neutron

selection, etc. Exclusive 0 bootstrapped from XnXn

limited by statistics for XnXn in topology trigger Good agreement

factorization works

STAR Theory0 with XnXn 36.6 2.4 8.9 mb 27 mb0 with 1n1n 2.50.40.6 mb 3.25 mbExclusive 0 410190100 mb 350 mb

Baltz, K

lein & N

ystrand (2002)

P (GeV/c)

A look 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

A peek at the 2001 data 200 GeV/nucleon

higher s higher luminosity

‘Production’ triggers Minimum Bias data:

10X statistics Topology Data

~20X statistics Physics

precision 0 and pT spectra

--> f2(1270) (e+e-) and theory comparison 4-prong events (*(1450/1700)?)

0 spectra - 25% of the min-bias data

Physics Recap RHIC is a high luminosity and A collider The strong fields and 2-source geometry allow many unique

studies 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 Tests of strong field QED 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-

The cross sections for 0 production are in agreement with theoretical models

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

subsystems, ... Ultra-peripheral collisions is in it’s infancy

STAR0 paper out in ~ 1 week It works!!! come back next year!