physics of ultraperipheral nuclear collisions janet seger
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Physics of Ultraperipheral Physics of Ultraperipheral Nuclear CollisionsNuclear Collisions
Janet SegerJanet Seger
May 23, 2008May 23, 2008 22
Introduction to UPC physicsIntroduction to UPC physicsExperimental results from RHICExperimental results from RHICLooking toward the LHCLooking toward the LHC
May 23, 2008May 23, 2008 33
Ultraperipheral Nuclear CollisionsUltraperipheral Nuclear Collisions Nuclei miss each other geometricallyNuclei miss each other geometrically
b b > > RR11 + + RR22
Long-range electromagnetic Long-range electromagnetic interactioninteraction Exchange of nearly-real photon(s)Exchange of nearly-real photon(s)
Weizsacker-Williams formalismWeizsacker-Williams formalism Photon flux ~ ZPhoton flux ~ Z22
Exclusive interactionExclusive interaction Coherent emission limits pCoherent emission limits pTT and energy of photon and energy of photon
Z
Z
b > 2R
50 MeV/cTA
cp
R
A
L
R
ck
max
May 23, 2008May 23, 2008 44
Photon interactionsPhoton interactions
Non-pert. QED Non-pert. QED Produces lepton or quark pairsProduces lepton or quark pairs
PhotonuclearPhotonuclearVector Meson DominanceVector Meson Dominance
Photon fluctuates to a Photon fluctuates to a vector meson ( vector meson ())
Vector meson photoproduction -- dominant Vector meson photoproduction -- dominant coherent processcoherent process
Incoherent processes Incoherent processes g, g, qqShadowing, exoticsShadowing, exotics
May 23, 2008May 23, 2008 55
High Photon FluxesHigh Photon Fluxes
Photon fluxes high at ion Photon fluxes high at ion colliderscolliders
High probability of multiple High probability of multiple photon exchangephoton exchange
Vector meson can be Vector meson can be accompanied by nuclear accompanied by nuclear Coulomb excitationCoulomb excitation 3-3- exchange at lowest order exchange at lowest order Coulomb excitation Coulomb excitation neutrons neutrons Useful for tagging UPCsUseful for tagging UPCs
May 23, 2008May 23, 2008 66
Modeling Photonuclear InteractionsModeling Photonuclear Interactions
Klein/Nystrand: Phenomenological model based on scaling data of p to A
Starlight Monte Carlo agrees well with data
Photon spectrum: Weizsäcker-Williams
Input photon-nucleon data: parameterized from results at HERA and fixed target
Scaling p A: Neglecting cross terms - fluctuates into V which scatters
elastically Shadowing through a Glauber model nuclear momentum transfer from form factor
(excellent analytical parameterization)
J. N
ystr
and,
S.
Kle
in n
ucl-e
x/98
1100
7J.
Nys
tran
d, S
. K
lein
PR
C 6
0(19
99)0
1490
3
May 23, 2008May 23, 2008 77
Starlight predictionsStarlight predictions
No Breakup
With Breakup (Xn,Xn)
With Breakup (1n,1n)
A.Baltz, S.Klein, J.Nystrand Phys. Rev. Lett. 89(2002)012301
May 23, 2008May 23, 2008 88
Heavy Vector MesonsHeavy Vector Mesons
J/, (pVp) calculable
from pQCD 2-gluon exchange Sensitive probe of g(x), g2(x)
Low-mass states at high rapidity probe low xLow-mass states at high rapidity probe low xRyskin, Roberts, Martin, Levin, Z. Phys C 76 (1997) 231, Frankfurt LL, McDermott MF, Strikman M, J. High Energy Physics 02:002 (1999) and Martin AD, Ryskin MG, Teubner T Phys.Lett. B454:339 (1999)
May 23, 2008May 23, 2008 99
Kinematic range of UPCsKinematic range of UPCs
at LHC
J/ at LHC
J/ at RHIC
y=0 J/
RHIC Wp = 25 GeV
x ≈ 2 x 10-2
LHC PbPb
Wp = 130 GeV
x ≈ 6 x 10-4
Wp = 230 GeV
x ≈ 2 x 10-3
Wp: photon-proton CM energy
x : Bjorken-x of gluon Q2 = MV
2/4
May 23, 2008May 23, 2008 1010
Gluon shadowing suppresses VM Gluon shadowing suppresses VM photoproductionphotoproduction
FS
Z,
Act
a P
hysi
cs P
olon
ica
B34
Blue = impulse approx.
Red = leading twist shadowing
May 23, 2008May 23, 2008 1111
Gluon shadowing alters rapidity dist.Gluon shadowing alters rapidity dist.
FS
Z,
Phy
s Le
tt B
540
Black Impulse Approx.
Red Alvero et al. gluon density
Blue H1 Gluon density
May 23, 2008May 23, 2008 1212
Experimental Characteristics of UPCsExperimental Characteristics of UPCs Low central multiplicities
“cleaner” than hadronic collisions
Zero net charge Low total transverse momentum Low virtualities Narrow dN/dy peaked at mid-
rapidity Large probability of multiple
electromagnetic interactions Coulomb excitations Emission of neutrons
Require: good tracking, particle ID, selective triggering
May 23, 2008May 23, 2008 1313
Triggering on UPCsTriggering on UPCs
Typically requireTypically requireLow multiplicityLow multiplicityDissociation of excited nucleus (neutrons in Dissociation of excited nucleus (neutrons in
ZDC)ZDC)Reduces statistics but increases triggering Reduces statistics but increases triggering
efficiencyefficiency
Sometimes includeSometimes includeEM Calorimeter towers for J/psiEM Calorimeter towers for J/psiBack-to-back event topologyBack-to-back event topology
May 23, 2008May 23, 2008 1414
UPCs at RHICUPCs at RHIC
200 GeV Au-Au collisions200 GeV Au-Au collisionskkmaxmax ~ 3 GeV, W ~ 3 GeV, WNN ~ 35 GeV ~ 35 GeVElectron pairs, vector meson photoproduction Electron pairs, vector meson photoproduction
studied so farstudied so farProof of principle for UPC studiesProof of principle for UPC studies
Develop trigger algorithmsDevelop trigger algorithmsTest UPC modelsTest UPC modelsConsistent with HERA measurementsConsistent with HERA measurements
May 23, 2008May 23, 2008 1515
Electron pairsElectron pairs2-photon interaction2-photon interactionZZ ~ 0.6 ~ 0.6Expect non-perturbative QED effectsExpect non-perturbative QED effects
Pair pT Minv
Lowest order
Higher order
A.
J. B
altz
, P
hys.
Rev
. Le
tt.
100,
06
2302
466
(20
08).
S T A R
May 23, 2008May 23, 2008 1616
Coherent Coherent photoproduction at photoproduction at RHICRHIC
Select coherent events with pT < 0.15 GeV/c
Mass distribution fit with Breit-Wigner signal Söding interference term for
direct +- production Second order polynomial to
describe background
A: amplitude for ρ0
B: amplitude for direct +-
S T A R
May 23, 2008May 23, 2008 1717
Many properties consistent with ZEUSMany properties consistent with ZEUS
Ratio of non-resonant to resonant pion productionRatio of non-resonant to resonant pion production 200 GeV: |B/A| = 0.84 ± 0.11 GeV 200 GeV: |B/A| = 0.84 ± 0.11 GeV -1/2 -1/2
130 GeV: |B/A| = 0.81 ± 0.28 GeV 130 GeV: |B/A| = 0.81 ± 0.28 GeV -1/2 -1/2
No angular dependence or rapidity dependenceNo angular dependence or rapidity dependence
s-channel helicity conservation
ParameterParameter STARSTAR ZEUSZEUS
-0.03 ± 0.03 ± 0.06 -0.03 ± 0.03 ± 0.06 0.01 ± 0.030.01 ± 0.03
-------- 0.01 ± 0.02 0.01 ± 0.02
-0.01 ± 0.03 ± 0.05-0.01 ± 0.03 ± 0.05 -0.01 ± 0.02-0.01 ± 0.02
0400r
][ 0410re
0411r
S T A R
May 23, 2008May 23, 2008 1818
Extend pExtend pTT range for measurement range for measurement of of ρρ00 productionproduction
Fit function:Fit function:
Incoherent productionIncoherent production d = 8.8 ±1.0 GeVd = 8.8 ±1.0 GeV-2-2– access to the nucleon form factor – access to the nucleon form factor
Coherent productionCoherent production b = 388.4 ±24.8 GeVb = 388.4 ±24.8 GeV-2 -2 – access to nuclear form factor – access to nuclear form factor
(incoh)/(coh) ~ 0.29 ±0.03
Incoherent ProductionIncoherent Production
)*exp(*)*exp(* tdctbadt
d
To the pT2 range: (0.002,0.3) GeV2
Coherent
Incoherent
S T A R
May 23, 2008May 23, 2008 1919
Model predictions for Model predictions for cross cross sectionsection
Klein, NystrandKlein, Nystrand: vector : vector dominance model (VDM) & dominance model (VDM) & classical mechanical approach classical mechanical approach for scattering, based on for scattering, based on γγpp→→ρρp p experiments resultsexperiments results PRC 60 (1999) 014903PRC 60 (1999) 014903
Frankfurt, Strikman, ZhalovFrankfurt, Strikman, Zhalov: : generalized vector dominance generalized vector dominance model + Gribov-Glauber model + Gribov-Glauber approachapproach PRC 67 (2003) 034901 PRC 67 (2003) 034901
Goncalves, MachadoGoncalves, Machado: QCD : QCD dipole approach (nuclear effects dipole approach (nuclear effects and parton saturation and parton saturation phenomenon) phenomenon) Eur.Phys.J. C29 (2003) 271-275Eur.Phys.J. C29 (2003) 271-275
May 23, 2008May 23, 2008 2020
Energy and A-dependence of Energy and A-dependence of cross sectioncross section
STAR Preliminary
62 GeV Au-Au
62 GeV
STAR Preliminary
200 GeV d-Au
STAR
Preliminary
May 23, 2008May 23, 2008 2121
Excited Excited state(s) state(s) γγAu Au ρρ ππ+ + ππ – – ππ+ + ππ ––
STAR observes broad peak STAR observes broad peak around 1510 MeV/caround 1510 MeV/c22
May be production of excited May be production of excited states states (1450) and/or (1450) and/or (1700)(1700)
STAR preliminary
May 23, 2008May 23, 2008 2222
J/Psi at RHIC (PHENIX)J/Psi at RHIC (PHENIX)
D’E
nter
ria,
nucl
-ex/
0601
001
dN/dmee (background subtracted) w/ fit to (MC) expected dielectron continuum and J/Ψ signals
May 23, 2008May 23, 2008 2323
Comparison with TheoryComparison with Theory
Large error bars!Large error bars! Need Need
more/better datamore/better data
D’Enterria, nucl-ex/0601001
Strikman, et al., Phys. Lett B626
May 23, 2008May 23, 2008 2424
UPCs at the LHCUPCs at the LHC
2.75 TeV Pb beams2.75 TeV Pb beamskkmaxmax = 81 GeV, W = 81 GeV, Wpp ~ 950 GeV ~ 950 GeVCompared to RHIC:Compared to RHIC:
Greater energyGreater energyGreater photon fluxGreater photon flux Increased cross Increased cross
sectionssectionsLower xLower x
May 23, 2008May 23, 2008 2525
New UPC physics at the LHCNew UPC physics at the LHC Elastic Vector Meson production
+A J +A expected prod rate ~ 1x107/ year
+A +A expected prod rate ~ 1x105/ year
sensitive probe of g(x,Q2) Photonuclear production of
heavy quarks +gcc
Photonuclear jet production; photon+partonjet+jet; e.g. +g q+q R. Vogt hep-ph/0407298, M. Strikman, R. Vogt,
S. White PRL 96(2006)082001.
May 23, 2008May 23, 2008 2626
LHC detectorsLHC detectors
CMS
ALICE
ATLAS
Very good tracking, PID
Extends to pT =0.05 GeV/c, but || < 1
No ZDC trigger
Tracking to || < 2.4, but pT > 0.2 GeV/c
Good rapidity coverage– can measure rapidity gaps
Tracking to || < 2.4, but pT > 0.5 GeV/c
Good rapidity coverage– can measure rapidity gaps
May 23, 2008May 23, 2008 2727
ConclusionsConclusions UPCs allow study of photon-induced interactionsUPCs allow study of photon-induced interactions
Low-multiplicity environmentLow-multiplicity environment Can be separated from hadronic backgroundCan be separated from hadronic background
RHIC and LHC are high-luminosity RHIC and LHC are high-luminosity A collidersA colliders RHIC energies comparable to HERARHIC energies comparable to HERA LHC energies will extend beyondLHC energies will extend beyond
Experience at RHICExperience at RHIC demonstrated feasibility of UPC studiesdemonstrated feasibility of UPC studies Developed trigger algorithmsDeveloped trigger algorithms and J/and J/ cross sections cross sections Agreement with HERA resultsAgreement with HERA results
LHC will probe interesting new physicsLHC will probe interesting new physics Higher energy, lower xHigher energy, lower x Shadowing effects, jetsShadowing effects, jets