Physics of Ultraperipheral Nuclear Collisions Janet Seger.

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

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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

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Rev

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tt.

100,

06

2302

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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