Download - Diffractive Vector Meson Photoproduction in ultra-peripheral heavy ion collisions with STAR
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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
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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
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A. Ogawa, BNL
The
Collaboration
STARSTAR
~ 400 collaborators41 institutions9 countries
Solenoid Tracker At RHIC
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A. Ogawa, BNL
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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
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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
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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
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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
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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*
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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
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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
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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+
+
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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
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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
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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
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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
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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
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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
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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)
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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/.
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Back up
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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
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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
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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