recent results in ultra-peripheral collisions from star what are ultra-peripheral collisions?...
TRANSCRIPT
![Page 1: Recent results in Ultra-Peripheral Collisions from STAR What are ultra-peripheral collisions? Exclusive 0 production 0 interferometry e + e - pair](https://reader030.vdocuments.net/reader030/viewer/2022033105/5697bfcf1a28abf838ca9be3/html5/thumbnails/1.jpg)
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)
![Page 2: Recent results in Ultra-Peripheral Collisions from STAR What are ultra-peripheral collisions? Exclusive 0 production 0 interferometry e + e - pair](https://reader030.vdocuments.net/reader030/viewer/2022033105/5697bfcf1a28abf838ca9be3/html5/thumbnails/2.jpg)
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
![Page 3: Recent results in Ultra-Peripheral Collisions from STAR What are ultra-peripheral collisions? Exclusive 0 production 0 interferometry e + e - pair](https://reader030.vdocuments.net/reader030/viewer/2022033105/5697bfcf1a28abf838ca9be3/html5/thumbnails/3.jpg)
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-
![Page 4: Recent results in Ultra-Peripheral Collisions from STAR What are ultra-peripheral collisions? Exclusive 0 production 0 interferometry e + e - pair](https://reader030.vdocuments.net/reader030/viewer/2022033105/5697bfcf1a28abf838ca9be3/html5/thumbnails/4.jpg)
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
![Page 5: Recent results in Ultra-Peripheral Collisions from STAR What are ultra-peripheral collisions? Exclusive 0 production 0 interferometry e + e - pair](https://reader030.vdocuments.net/reader030/viewer/2022033105/5697bfcf1a28abf838ca9be3/html5/thumbnails/5.jpg)
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
![Page 6: Recent results in Ultra-Peripheral Collisions from STAR What are ultra-peripheral collisions? Exclusive 0 production 0 interferometry e + e - pair](https://reader030.vdocuments.net/reader030/viewer/2022033105/5697bfcf1a28abf838ca9be3/html5/thumbnails/6.jpg)
S. Klein, LBNL
![Page 7: Recent results in Ultra-Peripheral Collisions from STAR What are ultra-peripheral collisions? Exclusive 0 production 0 interferometry e + e - pair](https://reader030.vdocuments.net/reader030/viewer/2022033105/5697bfcf1a28abf838ca9be3/html5/thumbnails/7.jpg)
S. Klein, LBNL
![Page 8: Recent results in Ultra-Peripheral Collisions from STAR What are ultra-peripheral collisions? Exclusive 0 production 0 interferometry e + e - pair](https://reader030.vdocuments.net/reader030/viewer/2022033105/5697bfcf1a28abf838ca9be3/html5/thumbnails/8.jpg)
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
![Page 9: Recent results in Ultra-Peripheral Collisions from STAR What are ultra-peripheral collisions? Exclusive 0 production 0 interferometry e + e - pair](https://reader030.vdocuments.net/reader030/viewer/2022033105/5697bfcf1a28abf838ca9be3/html5/thumbnails/9.jpg)
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
![Page 10: Recent results in Ultra-Peripheral Collisions from STAR What are ultra-peripheral collisions? Exclusive 0 production 0 interferometry e + e - pair](https://reader030.vdocuments.net/reader030/viewer/2022033105/5697bfcf1a28abf838ca9be3/html5/thumbnails/10.jpg)
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
![Page 11: Recent results in Ultra-Peripheral Collisions from STAR What are ultra-peripheral collisions? Exclusive 0 production 0 interferometry e + e - pair](https://reader030.vdocuments.net/reader030/viewer/2022033105/5697bfcf1a28abf838ca9be3/html5/thumbnails/11.jpg)
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*
![Page 12: Recent results in Ultra-Peripheral Collisions from STAR What are ultra-peripheral collisions? Exclusive 0 production 0 interferometry e + e - pair](https://reader030.vdocuments.net/reader030/viewer/2022033105/5697bfcf1a28abf838ca9be3/html5/thumbnails/12.jpg)
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
![Page 13: Recent results in Ultra-Peripheral Collisions from STAR What are ultra-peripheral collisions? Exclusive 0 production 0 interferometry e + e - pair](https://reader030.vdocuments.net/reader030/viewer/2022033105/5697bfcf1a28abf838ca9be3/html5/thumbnails/13.jpg)
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
![Page 14: Recent results in Ultra-Peripheral Collisions from STAR What are ultra-peripheral collisions? Exclusive 0 production 0 interferometry e + e - pair](https://reader030.vdocuments.net/reader030/viewer/2022033105/5697bfcf1a28abf838ca9be3/html5/thumbnails/14.jpg)
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)
-
+
+
![Page 15: Recent results in Ultra-Peripheral Collisions from STAR What are ultra-peripheral collisions? Exclusive 0 production 0 interferometry e + e - pair](https://reader030.vdocuments.net/reader030/viewer/2022033105/5697bfcf1a28abf838ca9be3/html5/thumbnails/15.jpg)
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
![Page 16: Recent results in Ultra-Peripheral Collisions from STAR What are ultra-peripheral collisions? Exclusive 0 production 0 interferometry e + e - pair](https://reader030.vdocuments.net/reader030/viewer/2022033105/5697bfcf1a28abf838ca9be3/html5/thumbnails/16.jpg)
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
![Page 17: Recent results in Ultra-Peripheral Collisions from STAR What are ultra-peripheral collisions? Exclusive 0 production 0 interferometry e + e - pair](https://reader030.vdocuments.net/reader030/viewer/2022033105/5697bfcf1a28abf838ca9be3/html5/thumbnails/17.jpg)
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
![Page 18: Recent results in Ultra-Peripheral Collisions from STAR What are ultra-peripheral collisions? Exclusive 0 production 0 interferometry e + e - pair](https://reader030.vdocuments.net/reader030/viewer/2022033105/5697bfcf1a28abf838ca9be3/html5/thumbnails/18.jpg)
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
![Page 19: Recent results in Ultra-Peripheral Collisions from STAR What are ultra-peripheral collisions? Exclusive 0 production 0 interferometry e + e - pair](https://reader030.vdocuments.net/reader030/viewer/2022033105/5697bfcf1a28abf838ca9be3/html5/thumbnails/19.jpg)
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
![Page 20: Recent results in Ultra-Peripheral Collisions from STAR What are ultra-peripheral collisions? Exclusive 0 production 0 interferometry e + e - pair](https://reader030.vdocuments.net/reader030/viewer/2022033105/5697bfcf1a28abf838ca9be3/html5/thumbnails/20.jpg)
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
![Page 21: Recent results in Ultra-Peripheral Collisions from STAR What are ultra-peripheral collisions? Exclusive 0 production 0 interferometry e + e - pair](https://reader030.vdocuments.net/reader030/viewer/2022033105/5697bfcf1a28abf838ca9be3/html5/thumbnails/21.jpg)
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.