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

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

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

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

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

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)

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

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

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

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

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)

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)

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

qq

STAR, Phys. Rev. C77, 34910 (2008)

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)

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)

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)

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)

+

-

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

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)

+

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

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

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

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

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

Results

Efficiency corrected spectra Two rapidity bins, 2 triggers

Cut |y|<0.05 for topology to remove cosmic rays

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

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

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

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

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

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

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

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

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

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)

“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

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

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…

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)

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)

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

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

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

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)

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.