Direct observation of dijets in central Au+Au collisions at RHIC
Ultra-peripheral Collisions at RHIC Spencer Klein, LBNL for the STAR collaboration
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Transcript of Ultra-peripheral Collisions at RHIC Spencer Klein, LBNL for the STAR collaboration
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Ultra-peripheral Collisions at RHICSpencer Klein, LBNL
for the STAR collaborationUltra-peripheral Collisions: What and Why
Interference in Vector Meson Production
Au + Au --> Au + Au + 0
0 production with nuclear excitation
Direct +- production & interference
e+e- pair production
Conclusions
Resu l ts
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Coherent Interactions
b > 2RA
no hadronic interactions Ions are sources of fields
photons ~ Z2 --> very high fluxes
Pomerons or mesons (mostly f0) A 2 (bulk)- A 4/3 (surface)
Fields couple coherently to ions P < h/RA, ~30 MeV/c for heavy ions
P|| < h/RA ~ 3 GeV/c at RHIC
Au
Au
Coupling ~ nuclear form factor
, P, or meson
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Specific Topics Vector meson production
A -- > ’, , , J/,… A Production cross sections --> (VN) Vector meson spectroscopy (, , ,…) Wave function collapse Vector Meson superradiance
Electromagnetic particle production
leptons,mesons Strong Field QED
Z ~ 0.6 meson spectroscopy
~ charge content of scalar/tensor mesons particles without charge (glueballs) won’t be seen
Mutual Coulomb excitation (GDR & higher) Luminosity measurement, impact parameter tag
Production occurs in/near one ion
VM
e+e-, qq,...s
Z ~ 0.6; is N > 1?
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Exclusive 0 Production in STAR
One nucleus emits a photon The photon fluctuates to a qq pair
vector meson dominance --> treat as vector meson The pair scatters elastically from the other nucleus
also Photon- meson contribution qq pair emerges as a vector meson is large: 380 mb for Au at 130 GeV/nucleon
5% of hadronic cross section 120 Hz production rate at RHIC full energy/luminosity
Au
Au
0
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Interference 2 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
For pT << 1/b
destructive interference
No Interference
Interference
pT (GeV/c)
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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)
-
0
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A typical STAR event200 GeVnucleon cm)
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Analysis Approach 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
Nuclear breakup possible Backgrounds:
incoherent photonuclear interactions
grazing nuclear collisions beam gas
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Exclusive 0
Trigger on low-multiplicity events back-to-back topology
~ 7 hours of data prototype trigger
2 track vertex in interaction diamond non-coplanar, < 3 rad
reject cosmic rays track dE/dx consistent with No neutrons in ZDC peak for pT < 2h/ ~ 100 MeV/c and model background
0 PT
M()
Preliminary
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Nuclear Excitation
Multiple Interactions are possible P(0, b=2R) ~ 0.5% P(2GDR, b=2R) ~ 30% Factorization should hold
Diagram on rt. should dominate
Au* decay mostly by neutron emission
Au
Au
P
Au*
Au*
0
)()( 022 bPbbPd EXC
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‘Minimum Bias’ Dataset Trigger on >1 neutron signal
in both zero degree calorimeters
~800,000 triggers Event selection same as
peripheral no ZDC cuts
and model background phase space in pT
small
0 PT
M()
Preliminary
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Direct + - production
Direct + - is independent of energy The two processes interfere
1800 phase change at M(0) changes + - lineshape
good data for p --> p (HERA + fixed target) poor data for A
+ - fraction should decrease as A rises
-
+
-
A -- > 0A -- > + - A
+
A -- > + - A
0
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0 lineshape
Data
Fit
0
+-
Fit all data to 0 + +-
interference is significant+- fraction is high (background?)
ZEUS p --> (0 + +- )p
Set =0 for STAR
STAR Au --> (0 + +- )Au
Preliminary
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A peek at --> e+e-
‘Minimum bias dataset 2 track Q=0 vertex
Find electrons by dE/dx p< 140 MeV/c
Select identified pairs pT peaked at 1/<b>
e
Blue - all particlesred - e+ e- pairs
Preliminary
Pt (GeVc)
P (GeV/c)
dE/d
x (k
eV/c
m)
Eve
nts
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Conclusions For the first time, we have observed three peripheral
collisions processes Au + Au -- > Au + Au + 0
Au + Au -- > Au* + Au* + 0
Au + Au -- > Au* + Au* + e+e-
We see interference between 0 and direct
Peripheral collisions is in it’s infancy next year: more data, more triggers, more luminosity,more
energy, more channels, more acceptance, more... The 0 pT spectrum is sensitive to whether particle decay
triggers wave function collapse