Photoproduction at proton and ion colliders
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Transcript of Photoproduction at proton and ion colliders
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Photoproduction at proton and ion colliders
Photoproduction at hadron colliders
Photonuclear and interactions
Physics from Photoproduction
Unique Possibilities
RHIC & Tevatron results
Future Prospects: the LHC & RHIC
LHC is the highest energy p collider in the world
Conclusions
Spencer Klein, LBNL & UC Berkeley
0 photoproduction in STAR
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Photons from Hadrons Relativistic hadrons carry strong electromagnetic fields
Weizsacker-Wiliams: a field of almost-real photons Virtuality Q2 < (h/RA)2
• Q2 0 significant for e+e- production, and (maybe) with proton beams
Photon Emax ~ h/RA
3 GeV with gold at RHIC 80 GeV with lead at the LHC ~10%+ of proton energy at both machines
Photon flux ~ Z2
Higher intensity with heavy ions Important for considering multiple interactions between a single
ion pair RHIC & LHC have higher luminosities for light ions --> often
better overall rates “Optimum” ion depends on the problem
Au
Au
Coupling ~ form factor
, P, or meson
X
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Photonuclear Interactions Photon coupling Z2 ~ 0.6 for heavy ions
Multi-photon reactions common -gluon/Pomeron/meson interactions -Pomeron/meson can be coherent
Exclusive final states Coupling ~ A 2 (bulk) A 4/3 (surface) Large cross-sections for vector meson production
Require b > 2RA
no hadronic interactions <b> ~ 20-60 fermi at RHIC
Cross sections are huge ~ 5 barns for coherent 0 production with lead at the LHC
Need plots
Au
Coupling ~ form factor
Au
XP/gluons/meson
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interactions Rate ~ ~ Z4
Rates to produce hadrons are much smaller than for photoproduction of vector mesons
Prolific production of e+e- pairs = 200,000 barns with lead at the LHC QED process --> proposed as luminosity
monitor. Coulomb corrections are significant
Wmax ~ 2 kmax
6 GeV with gold at RHIC 160 GeV with lead at the LHC Higher for lighter ions
Au
Au
X
Luminosity
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Unique Possibilities at hadron colliders
Highly charged photon emitters Multi-photon interactions
Impact parameter tagging Symmetric initial state
Quantum interference Ion interactions
Pair production with capture
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Factorization Multiple photons are emitted independently
(Gupta, 1950)
To lowest order, interactions are independent
(neglecting quantum correlations) True for + nuclear breakup with heavy ions Seems to work well for e+e- and 0 at RHIC
Cf. Yuri Gorbunov’s talk
...)()( 212 bPbbPd
Au
Au P
Au*
Au*
0
G. Baur et al, Nucl. Phys. A729, 787 (2003)
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Impact Parameter Tagging Nuclear excitation ‘tag’s small b Multiple photon exchange
Mutual Coulomb excitation Au* decay via neutron emission
Detected in zero degree calorimeters simple, unbiased trigger
Multiple Interactions probable P(0, b=2R) ~ 1% at RHIC P(2EXC, b=2R) ~ 30%
Non-factorizable diagrams are small for AA
)()( 022 bPbbPd EXC
Au
Au
P
Au*
Au*
(1+)0
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Tagged Photon Spectra Single photon exchange (low
energy limit) N() ~ 1/k The presence of additional
tagging photons biases the collision toward small b
For single tagging (2-photon exchange) N() ~ independent of k
G. Baur et al, Nucl. Phys. A729, 787 (2003)
k (GeV)
N(k
)
Photon spectra at RHIC
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Strong Coupling/Multiple Interactions
Z2 ~ 0.6 with gold/lead ‘Extra’ photons are cheap
Higher order diagrams could be important Multi-photon reactions are important
They factorize Mutual Coulomb excitation of colliding
nuclei is a useful tag Simple experimental trigger Selects events with small impact
parameters
Au
Au
P
Au*
Au*
0
)()( 022 bPbbPd EXC
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Zero Degree Calorimeters & Luminosity measurement
Mutual Coulomb dissociation is used as to monitor luminosity Calculable in QED
Accuracy limited by uncertainty in hadronic interaction radius
Require 1 neutron in each ZDC (& no activity in central detector) via Giant Dipole Resonance
PHENIX used dA() --> pnA to monitor *luminosity
-->l+l- proposed at LHC Uncertainty in hadronic interaction radius
limits accuracy
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e+e- production w/ STAR With Coulomb excitation in 200 GeV
gold-on-gold collisions 52 events
Mee > 140 MeV/c2, |Yee|< 1 Equivalent photon method fails for pair
pT spectrum Photon virtuality is important
Cross-section is consistent with all-orders QED calculation ~ Inconsistent with lowest order
calculation ~ 30% higher than all-orders in STAR
acceptance Pair pT
Lepton pT
Points dataSolid line – LO calcDashed – all orders
STAR data, calcs by A. Baltz PRL 100, 062302 (2007)
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e+e- production in CDF 1.8 TeV pp collisions 16 events with Mee > 10 GeV Fits theoretical calculations
LPAIR Proposed e+e- pairs as a
luminosity monitor at the LHC QED is well understood Problem: what is the effective
bmin, at which there are no hadronic interactions?
CDF, PRL 98,112001 (2007)
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STAR - 0 Photoproduction Exclusive 0 triggered w/ ‘Topology’
trigger 0-with mutual Coulomb excitation
triggered with ZDC coincidence Select events with 2 primary tracks
+ some ‘global’ tracks
Au
Au 0
STAR, Phys. Rev. C77, 34910 (2008)
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0 Photoproduction Mass Spectra Coherent production events
pT < ~ 150 MeV/c
M fit to 0 + direct +- spectrum w/ interference term Relativistic Breit-Wigner w/ phase space
0:direct +- ratio same as p @ HERA
Exclusive 0 0 + Mutual Coulomb Excitation
0
0: interferenceGray -background
STAR, Phys. Rev. C77, 34910 (2008)
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0 rapidity distribution 0 w/ mutual Coulomb excitation Photon energy
k = Mv/2 exp(y) 2-fold directional ambiguity leads to
symmetric distributions Cross-section depends on qq-N, which
depends on the hadronic model Saturation models of Goncalves & Machado
is ruled out A->A(k) can be determined by combining
exclusive and w/ mutual Coulomb excitation Errors are large
S T A R
STAR, Phys. Rev. C77, 34910 (2008)
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0 pT spectra Coherent + Incoherent form factors
Fit to dual exponential Incoherent production
bN = 8.8 ±1.0 GeV-2
nucleon form factor
Coherent production bAu = 388.4 ±24.8 GeV-2
bAu ~ R
A2
Data sensitive to RA
Measure hadronic radius of gold • 3% statistical uncertainty• Significant theoretical corrections/uncertainty
(incoh)/(coh) ~ 0.29 ±0.03 Extrapolated form factor to t=0
S T A R
pT2 ~ t
STAR, Phys. Rev. C77, 34910 (2008)
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Polarized Photons Electric fields parallel to b
photon polarization follows b Use 1 reaction to determine the polarization, to study
a 2nd reaction Correlated Decay angles Use -- > +- as ‘analyzer
+- decay plane often parallels photon pol.
reasonable analyzing power Study VM production angles, single spin A
asymmetries, etc. 2
2||
2
2||
2
dd
ddA
b,E
(transverse view)
+
-
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Interference 2 indistinguishable 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
b is unknown For pT << 1/<b>
destructive interference
No Interference
Interference
pT (GeV/c)
y=0
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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)
+
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Einstein-Podolsky-Rosen paradox For each +from decay, can choose to measure x
or p Both x - localize production point (sometimes) Both p - measure p of VM; see interference; delocalize production
The +- system is an example of the Einstein-Podolsky-Rosen paradox
(Gedanken) test for superluminal collapse
b
-+
+
X/p m
eas.X/p
mea
s.
Klein &
Nystran
d, 2003
0
0
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Interference and Nuclear Excitation
Smaller <b> --> interference at higher <pT> Suppression for pT < h/<b>
0 prod at RHIC: 0.1 < |y| < 0.6
Noint ~ exp(-bt)Int.
Noint ~ exp(-bt)Int.
tXnXnNo breakup criteria
t
dN/d
t
dN/d
t
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Interference Analysis ‘topology’ (multiplicity+) trigger for exclusive 0, Neutrons in both ZDCs trigger for mutual Coulomb excitation. Tight cuts -- > clean 0, but lower efficiency
Exactly 2 tracks 1 vertex with Q=0 0.55 & GeV<M<0.92 GeV
Fit dN/dt spectrum Require accurate dN/dt w/o interference Efficiency is ~ independent of t
Momentum smearing affects two low-t bins• Included in Monte Carlo
Interference is maximal at y=0, decreasing as |y| rises 2 rapidity bins 0/0.05 < |y| < 0.5 & 0.5<|y|<1.0
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0 pT spectrum in UPCs Scattering (Pomeron) pT
Modified nucleon form factor Glauber calculation for absorption position-dependent photon flux
Photon pT
Weizsacker-Williams + form factor Add components in quadrature w/o interference dN/dt ~ exp(-bt)
b = x * RA2
STAR simulations Woods-Saxon + Glauber calculation
• (H. Alvensleben et al., 1970)
Modifications change x Interference
Only cause for dN/dt -->0 at small t
2min
2TT ptpt
Woods-Saxon Form Factor
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2 Monte Carlo samples: Interference No interference w/ detector simulation
Detector Effects Small Data matches Int Inconsistent with Noint Interference clearly observed 973 events Fit to dN/dt = A exp(-kt)* [1+c(R(t)-1)]
Exponential for nuclear form factor R = Int(t)/NoInt(t) Separates nuclear form factor (exponential) & interference
Analysis Technique
dN/d
t (a
.u.)
Data (w/ fit)NointIntLS Background
STAR Preliminary
t
R =
Int
(t)/
Noi
nt(t
)
t (GeV2) = pT2
dN/dt for XnXn, 0.1<|y|<0.6
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Results
Efficiency corrected spectra Two rapidity bins, 2 triggers
Cut |y|<0.05 for topology to remove cosmic rays
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Results
2/DOF > 1 for 2 samples Much study, no answers… Scale errors up by 2/DOF (PDG prescription)
Systematic errors due to trigger (10% for topology), other detector effects (4%), background (1%), fitting & nuclear radius(4%), theoretical issues (5%)
Final, combined result: c=0.87 ± 0.05 (stat.) ± 8 (syst.)% Limit on decoherence, due to decay or other factors < 23% at a
90% confidence level
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k values photon flux ~ 1/b2
P(b) ~ N (b)(b)
For b ~ few RA, production is asymmetric Smaller apparent size Small change in (tot)
<b> ~ 3 RA for minimum bias data F(t) is Fourier transform of production
density Parameter b ~ (diameter)2
Affects the interference pattern Impact parameter beff < bgeom
Neglected here
N(b) ~ 1/(b-RA)2
N(b) ~ 1/b2
N(b) ~ 1/(b+RA)2
Au
Au
Woods-Saxon (b)WS weighted by 1/b2
b (fm)
P(b
) =
(b
)N(
b)
A linear (toy) model
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Photon is almost always emitted by gold No two-fold ambiguity--> easy extraction
d --> d, pn0 both seen Deuteron dissociation tagged by neutron
in Zero Degree calorimeter Allows cleaner trigger
Coherent reaction has larger t-slope Incoherent process reduced at small t
Not enough momentum transfer for dissociation
0 photoproduction in deuteron-gold interactions
Neutron-tagged data
t, GeV2
S T A R
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Photoproduction of Expected to be largely through a radially excited
and/or Peak at low pT from coherent enhancement Studies of resonant substructure are in progress
STAR preliminary
M(GeV)pT(GeV/c)
S T A R
B. Grube, Wkshp. on HE Photon Collisions at the LHC
S T A R
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J/ photoproduction at RHIC
e+e- pair + 1 nucleus breakup Nuclear breakup needed for
trigger J/ + continuum --> e+e-
~ 12 events in peak Cross sections for both J/ and
continuum e+e- ~ as expected
D’Enterria, PHENIX, nucl-ex/0601001
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J/ photoproduction at the Tevatron CDF selects exclusive J/
photoproduction is sensitive to gluon structure of nuclei ~ |g(x,MV
2/4)|2
334 exclusive +- signal events. “Background” from double
Pomeron production c --> J/
Some ’ Cross section determination in
progress
M() (GeV/c2)
J. Pinfold, Wkshp. on HE Photon Collisions at the LHC
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UPCs at the LHC CMS, ALICE and ATLAS plan programs “Yellow Book” gives physics case
K. Hencken et al., Phys. Rept. 458, 1 (2008). Gluon structure Functions at low-x
Including saturation tests The ‘black disc’ regime of QCD Search for exotica/new physics
--> Higgs, Magnetic monoples, etc. Some techniques, and some final states overlap with pp
diffraction (double-Pomeron) studies Roman pots can be used for full kinematic reconstruction in pp
t_perp can help separate , P and PP interactions
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Structure Functions at the LHC Many photoproduction reactions probe structure
functions --> qq; the quarks interact with target gluons Q2 ~ (Mfinal state/2)2
x ~ 10-4 at midrapidity x ~ 10-6 in forward regions
J/, ’,states Open charm/bottom/top? Dijets The twofold ambiguity can be avoided using
pA collisions or by comparing d/dy with and w/o mutual Coulomb excitation
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J/ photoproduction ~ g(x,Q2)2
x ~ few 10-4 for J/@ the LHC x ~ few 10-2 for J/@ RHIC Q2 ~ Mv
2/4
Coherent production pT < h/RA
High rates 3.2 Hz production with Pb
Detection is easy Shadowing is ~ factor of several
Theoretical uncertainties cancel in (pp)/(AA) or (pA)
M. Strikman, F. Strikman and M. Zhalov, PL B540, 220 (2002)y
y
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Dijets
With calorimetery to |y|<3, probe down to x~10-4 in 1 month
Use standard jet triggers
Ra
te/b
in/m
on
th @
L=
4*1
026/c
m2/s
M. Strikman, R. Vogt and S. White, PRL86, 082001 (2006)
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“Black Disk” Regime At high enough energies/low enough x, nuclei
look like black (absorptive) disks Studies of HERA data suggest that the Black Disk
Regime (BDR) should be visible at the LHC for Q2 < 4 GeV2
Photons fluctuate to qq dipoles, with separation d dipole-target ~ d2
Smaller and smaller dipoles interact Photons contain many small dipoles --> increasing
number of interactions --> tot (p) rises rapidly
Increase in high pT interactions Fraction of diffractive events increases
M. Strikman
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e+e- pairs at the LHC ~ 200,00 barns with Pb beams
Significant background for small-radius detectors (also @RHIC)
20 pairs/beam crossing (at 1027/cm2/s & 100 nsec crossing) e+, e- have small pT (typically me)
Hard to trigger ALICE strategy – random events
Expect ~ MHz rates in inner detectors Measure cross-section, Mee, pT spectra, etc.
Multiple pairs likely from 1 collision
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LHC – Plans & Issues J/,’, --> leptons is relatively easy
CMS, ALICE, ATLAS are pursuing Di-jets
ATLAS is pursuing Triggering is problematic
Beam gas, neutrons, grazing hadronic collisions... even cosmic ray muons
ZDC coincidences might help with backgrounds Not always available at Level 0
FP420, TOTEM & other forward detectors Use tagging to select --> Higgs, sleptons…
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Bound Free Pair Production
A + A --> A + Ae- + e+
1e- atom has lower Z/A Less bending in dipoles
Momentum ~ unchanged --> beam ~ 280 barns w/ lead at the LHC
280,000 ions/s at LL = 1027/cm2/s 28 watts!
Hits beampipe ~ 136 m from the IP Enough energy to quench superconducting
magnets? Limits LHC luminosity w/ heavy ions
Limit is ~ planned luminosity Quench calculations are touchy
IPPb 82+
Pb 81+
SK, NIM A459, 51 (2001); R. Bruce et al. PRL 99, 144801 (2007)
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Observation of BFPP at RHIC Copper beams at RHIC
theory ~ 200 mb magnetic rigidity) ~ 1/29
1eAu defocuses before striking beampipe 1eCu beam hits beampipe ~ 144
m from interaction point Look for showers using pre-existing PIN
diodes Small enough that acceptance simulations are
problematic Observe correlations between luminosity
(measured at IP) and PIN diode rates
R. Bruce et al. PRL 99, 144801 (2007)
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BFPP results
Colors are PIN photodiode signals at different locationsBlue is optimal location
BFPP has been observed, with cross section ~ expected
RHIC (ZDC) luminosity monitor
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Future Directions at RHIC STAR “DAQ1000” increases data rate by a factor of 10 &
TOF system improves particle identification Substructure in 4-pion events 00 pairs
Roman pots for STAR Mostly for pp diffraction, but might be able to measure scattered
deuteron in dA collisions, improving UPC reconstruction Phase I optimized for elastic scattering
• Requires special low- beam optics Phase II has larger acceptance
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00 production4 diagrams interfere
Depends on pT sum and difference between two Away from y=0, the top and bottom diagrams
dominate
Stimulated emission at low pT
Possibility to look for stimulated decays Needs more luminosity, and background rejection
from ’ decays, but looks possible
])[()(
)()(~)()(
)(2)(2
12212211
22211211
xpxpixpxpi
xpxpixpxpi
TTTT
TTTT
eeyAyA
eyAeyAAmp
bpT /
)]cos([~ 21 bppA TT
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Angular Correlations in 00 & mutual nuclear excitation to
Giant Dipole Resonances ~ any states decaying via a vector decay In linearly polarized 0 decays, the angle
between the 0 polarization and the +/- pT (wrt the direction of motion) follows cos( +/- direction acts as an analyzer
The 0 polarization follows the polarization The angle between the +/- pT in 00 decays is
the convolution of the two cos() distributions C() = 1 + 1/2cos(2)
Possible way to study linear polarization
Angle between +/- pT in double VM decays
G. Baur et al.,Nucl. Phys. A729, 787 (2003)
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Conclusions Many photoproduction reactions can be studied at hadron
colliders. STAR & PHENIX at RHIC, and CDF at the Tevatron have studied
a variety of topics, including vector meson production and e+e- pair production.
Many unique reactions can be studied with heavy ion collisions. Multiple interactions among a single ion pair.
Bound-free e+e- production will limit the LHC luminosity with lead.
The LHC and RHIC have promising future programs. Until the ILC is complete, the LHC will be the highest energy
and p collider in the world.