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