Post on 18-Jan-2016
transcript
Recent results in Ultra-Peripheral Collisionsfrom STAR
What are ultra-peripheral collisions?
Exclusive0 production
0 interferometry
e+e- pair production (V. Morozov dissertation, 2003)
Conclusions
Spencer Klein, LBNL (for the STAR Collaboration)
S. Klein, LBNL
Coherent Interactions b > 2RA;
no hadronic interactions <b> ~ 20-60 fermi at RHIC
Ions are sources of fields photons
N ~ 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 < 30 MeV/c
P|| < 3 GeV/c Strong couplings --> large cross sections
Au
Au
Coupling ~ nuclear form factor
, P, or meson
S. Klein, LBNL
Unique Features of Ultra-peripheral collisions
Very strong electromagnetic fields --> e+e- and --> qq Multiple interactions between a
single ion pair Unique Geometry
2-source interferometer Nuclear Environment
Particle Production with capture Large for e-
S. Klein, LBNL
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 --> Rates are high
~ 8 % of (had.) for gold at 200 GeV/nucleon 120 /sec at design luminosity
Other vector mesons are copiously produced
Au
Au 0
S. Klein, LBNL
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
Au
Au
PAu*
Au*
0
0 with gold @ RHIC
d/
dyy
Exclusive - solidX10 for XnXn - dashedX100 for 1n1n - dotted
S. Klein, LBNL
S. Klein, LBNL
S. Klein, LBNL
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
Trigger Back to back hits in Central
Trigger barrel
S. Klein, LBNL
200 GeVExclusive 0
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 peak
M()
0 PT
Signal region:
pT<0.15 GeV
Preliminary
S. Klein, LBNL
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
S. Klein, LBNL
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*
S. Klein, LBNL
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
STAR PRL 89, 027302 (2002)
Theory PRL 89, 012301 (2002)
0 with XnXn 36.6 2.4 8.9 mb 27 mb 0 with 1n1n 2.50.40.6 mb 3.25 mb Exclusive 0 410190100 mb 350 mb
S. Klein, LBNL
Interference 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
S. Klein, LBNL
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)
-
+
+
S. Klein, LBNL
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
S. Klein, LBNL
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
S. Klein, LBNL
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
S. Klein, LBNL
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
S. Klein, LBNL
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
S. Klein, LBNL
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)
STAR Preliminary
STAR Preliminary
S. Klein, LBNL
Conclusions & Outlook STAR has observed photonuclear 0 production.
The 0 cross sections agree with theoretical models. 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. The cross section for e+e- pair production is consistent with lowest order
quantum electrodynamics. The equivalent photon approximation does not describe the pT spectrum.
2003 run: Good dA --> 0 sample. Meson production via double Pomerons from pp pending.