Quarkonia photoproduction in
Ultraperipheral A+A collisions
at RHIC & LHC

1st ALICE Physics Week

Erice, 4 – 10 December 2005

David d'Enterria

LLR, Ecole Polytechnique, Palaiseau

Overview

Physics topics: g g , g p collisions at e+e-, ep colliders

g g , g A collisions in UltraPeripheral A+A

Experimental aspects: Ultraperipheral A+A collisions

(signatures, brackgd, triggers, detectors, analysis cuts, ...)

Results: Quarkonia g-production in UPC @ RHIC (PHENIX)

Prospects: Quarkonia g-production in UPC @ LHC

Summary

Overview

Physics topics: g g , g p collisions at e+e-, ep colliders

g g , g A collisions in UltraPeripheral A+A

Experimental aspects: Ultraperipheral A+A collisions

(signatures, brackgd, triggers, detectors, analysis cuts, ...)

Results: Quarkonia g-production in UPC @ RHIC (PHENIX)

Prospects: Quarkonia g-production in UPC @ LHC

Summary

g g , g p collisions

High-energy photon can interact point-like (e.g. Compton scatt.) or quantum fluctuating into fermion-antifermion or vector-meson (J=1--):

In practice: g  g0 , but g  V,qq fluctuations interact strongly and

High-energy gg, gp collisions complementary to more “conventional” e+e-,

give largest contribution to g g and g p cross-sections:

s(e+e-  hadrons) µ 1/s

s(gg  hadrons) ~ ln(s)

s(gp  hadrons) ~ ln(s)

Rise as pp,pp colls.

consistent w/ Regge

phenomenology

_

_

ep (DIS), pp, pp collisions to study QCD/QED (or even beyond-SM).

_

Equivalent g beams at e+e-, ep colliders

EM field of relativistic charged particle = flux of “equivalent” photons.

Weizsacker-Williams (EPA) formula for an e+- beam:

Characteristics of g-induced collisions at e+e- (LEP), ep (HERA) colliders:

Peaked at lower c.m. energies than parent beams: 0.1√s <~ Wgg,gp<~ 0.5√s

Quasi-real (Q2~0) photons: parent e± scattered very close to beam, produced particles have low pT.

Schematic kinematics for gg and gp:

Soft bremsstrahlung g spectrum

g g , g p physics at e+e-, ep colliders

Typical diagrams for gg and gp collisions:

Main interest of g-induced collisions at LEP & HERA:

Precision QCD w/ low bckgd & simpler initial state than hadronic colls.

Measurements: s(tot.had), Ggg of 0-+2++, VV production, g structure function,

hard photoproduction (Quarkonia, jets, heavy-Q), ...

Physics topics: Regge theory, low-energy q spectrosc., BFKL dynamics,

hard scatt. factorization, G(x,Q2) distribution, diffractive interactions, ...

Equivalent g beams in UPC A+A collisions

Heavy-ions (w/ charge Z) produce stronger

electromagnetic fields than e± beams due to

the coherent action of all proton charges.

EPA formula for flux of photons in electromagnetic

(i.e. ultraperipheral, b>bmin~ 2RA ) A+A collisions:

Characteristics of photon beams:

Flux ~ Z2 (~7·103 for Pb) and s(gg) ~ Z4 (i.e. ~4·107) larger than e± beams !

“Coherence condition” : g wavelength > nucleus size since all protons

Egmax ~ 3 (80) GeV at RHIC (LHC)

Wgg ~ 6 (160) GeV at RHIC (LHC)

WgA ~ 30 (900) GeV at RHIC (LHC)

nucleus form factor dependence

interact coherently ⇒ very low photon virtuality:

g g , g A physics in UPC A+A collisions

Typical diagrams for gg and gA collisions:

Main interest of g-induced collisions in UPC A+A collisions

Precision QCD: Low bckgd & simpler initial state than nuclear A+A colls.

Measurements: Dilepton pairs, hard photo-production (Quarkonia, jets, heavy-Q), ...

Physics topics: QED in strong regime (Zaem~1), nuclear GA(x,Q2) function, small-x physics, QQbar dynamics in cold nuclear matter, ccbar (bbar) spectroscopy,. ..

Quarkonia g-production in UPC A+A

HERA: gpVp (V=J/,¡) very sensitive to gluon distrib. at small-x:

Likewise RHIC,LHC: g+A V+A (V=J/,¡ ) in UPC A+A clean measurement of:

Nuclear GA(x,Q2) at small-x: x(J/) ~4·10-4 , x(¡)~10-3 at LHC (y=0)

QQbar propagation in cold nuclear matter.

Discriminates diff.

Ansätze & evolutions

of G(x,Q2)

Perturbative process:

Note: sgA (LHC) > 3--6·sgA(HERA)

J/

J/

Overview

Physics topics: g g , g p collisions at e+e-,ep colliders

g g , g A collisions in UPC A+A

Experimental aspects: Quarkonia in UPC collisions

(signatures, brackgd, triggers, detectors, analysis cuts, ...)

Results: Quarkonia g-production in UPC @ RHIC (PHENIX)

Prospects: Quarkonia g-production in UPC @ LHC

Summary

Existing g g , g A measurements @ RHIC

Measured processes in A+A UPC collisions:

STAR:

(1) Coherent rproduction: g+A ®A*+ r(®p+p-)

(2) Dielectron continuum at low minv : g +g ®(A*) + e+e–

PHENIX (Preliminary):

(1) Coherent J/production: g+A ®A*+ J/(®e+e–)

(2) Dielectron continuum at high minv : g+g ®(A*) + e+e–

r

PRL 89 272302 (02)

PRC 70 031902 (04)

D.d'E, QM'05 poster

Proceeds. Acta Phys. Slovaka

g g , g A collisions: experimental signatures

Central rapidities:

(1) Low multiplicities: N<~15

(2) Low total transverse momentum (“coherence condition”):

pT < √2 ℏ/RA or pT~minv /g ~ 30 – 50 MeV

(3) Zero net charge: even # of charged tracks of opposite signs.

(4) Narrow dN/dy peaked at y=0 (narrower for larger minv ).

Forward rapidities:

(5) Large probability of multiple e.m. interactions

(3g exch.): Mutual Coulomb excitation (GDR)

leading to A* dissociation via (forward) neutron

(Xn) emission: P~30-50% (J/).

Important for tagging/trigger purposes !

Note: Coulomb-dissoc. probab. factorizes

in UPC cross-section calculations

A + A  A + A + g  A + A + X (X = J/, ...)

[All here also valid for: A + A  A + A+ g+ g  A + A + X ]

A + g  A + X or  A + g+g

g +A  J/Ψ+A : UPC trigger example

L1 UltraPeripheral Trigger:

Veto on coincident BBC (|y| ~3-4):

[avoid periph. nuclear, beam-gas colls.]

Neutron(s) in at least one ZDC (E>30 GeV)

[sensitive to Au* Coulomb dissociation]

Large energy (E > 0.8 GeV) cluster in EMCal:

[e+e- decay from J/]

Definition:

Events collected (~0.4% of MB trigger):

UPC AuAu: 8.5 M

MB AuAu (BBCLL1): 1122 M (equivalent ∫L = 120 mb-1)

PHENIX Run-4 AuAu UPC trigger:

Sensitive to g + Au ®Au* + J/(®e+e-)

UPC: (ZDCN || ZDCS) && (!BBCLL1noVtx) && (ERT2x2)

UPC-BBCSxorN : (ZDCLL1N || ZDCLL1S) && (BBCLL1S ^ BBCLL1N) && (ERT2x2)

g +A  J/Ψ+A : possible background sources

“Non-physical”:

(1) Cosmic rays: no ZDC, no good vtx.

(2) Beam-gas: no good vtx., large multiplicity, asymmetric dN/dy

Physical processes:

(3) Peripheral nuclear A+A: “large” multiplicity, large pT

(4) Hadronic diffractive (Pomeron-Pomeron, rapidity gap evt.):

forward proton emission, larger pT: pT(gg) < pT(PP), like-sign pairs.

Hard-diffractive J/ production.

(5) Incoherent UPC g+nn+J/: pT(gg) < pT(gP), wider & asymm. dN/dy,

≥2 neutrons (induced nuclear break-up) w/ same direction as J/.

(6) Other coherent UPC processes:

gg  e+e- (Important !) , gA  jet(s)+A (lower cross-sections) ?

A + A  A + A + g  A + A + J/

Trigger

level

Final

signal

PHENIX UPC-measurement detectors

DC + PCs: Full central-arm

charged tracking (e± momentum).

RICH + EMCal: e± identification

in central rapidity.

ZDC: Forward neutron detection

(Au* dissociation):

PHENIX UPC analysis cuts

Global cuts:

Std. vtx. cut: | zvtx | < 30 cm

Multiplicity(tracks)<15 [removes non-UPC events]

Loose PID e ± cuts (compared to std. AuAu-nuclear analysis):

RICH: n0>=2 [# of photo-tubes within nominal ring radius]

CNT-EMCal matching (plus no dead tower within 2x2).

E1 > 1 GeV || E2 > 1 GeV [offline high-pT trigger threshold]

Pair cuts:

arm1 != arm2 [back-to-back di-electrons from J/y ~at rest]

Residual background subtraction:

minv[unlike-sign ee pairs] – minv[like-sign ee pairs]

Overview

Physics topics: g g , g p collisions at e+e-,ep colliders

g g , g A collisions in UPC A+A

Experimental aspects: Ultraperipheral A+A collisions

(signatures, brackgd, triggers, detectors, analysis cuts, ...)

Results: Quarkonia g-production in UPC @ RHIC (PHENIX)

Prospects: Quarkonia g-production in UPC @ LHC

Summary

AuAu UPC results (I): dN/dminv, dN/dpT ee pairs

dN/dminv ,dN/dpT distributions after QA, global-, single- & pair- cuts

for unlike-sign (red) and like-sign (yellow) pairs:

Very small wrong-sign background (located in “non-coherent”

high pT region) well reproduced by MC.

ERT

threshold

effects

important

Clear coherent peak at

low pT for unlike pairs

~60 “surviving”

pairs

AuAu UPC results (II): dN/dminv e+e- pairs

dN/dminv after e+e- continuum subtraction

J/Ψ peak & width in good agreement

w/ theoretical input + full MC resp.+reco

Shape of e+e- continuum in good

agreement w/ theoretical input

+ full-MC resp.+ reco

mJ/Ψ ~ 3.10 GeV ± 130 MeV

NJ/Ψ= 10 ± 3 (stat)

± 3 (syst)

dN/dminv (backgd subtracted) & with

2 fits of expected e+e- continuum shape

(normalized at mee = 1.8 – 2.2 GeV/c2)

Monte Carlo: dN/dminv J/Ψ & e+e- continuum

Accepted/

geometry

Good agreement with expected signals from “Starlight” MC

AuAu UPC results (III): dN/dpT e+e- pairs

dNee/dpT peaked at low

pT~90 MeV/c as expected

for coherent production.

Good detailed agreement

w/ theoretical expectations [*]

[*] Nuclear form factor fit from

J.Nystrand, nucl-th/0112055

AuAu UPC results (IV): J/Ψ cross-section

Measured J/Ψ yield at y=0

consistent w/ theoret. calcs. [1,2]

Main syst. uncertainty: coherent e+e- continuum under J/Ψ

(work in progress).

Reduction of stat. errors need larger luminosity.

Current uncertainties preclude

yet detailed study of crucial model ingredients:

GA(x,Q2), s(J/Ψ absorption)

PHENIX PRELIM INARY

dσJ/Ψ/dy|y=0 = 1/BR × 1/(Acc| y=0.·ε) × 1/εtrig × 1/Lint × NJ/Ψ /Δy =

= 1/(5.9%)×1/(5.7%·56.4%)×1/(90%)×1/120 μb-1×(10 ± 3± 3) =

= 48. ± 16. (stat) ± 18. (syst) μb

[1] Starlight: J.Nystrand, S. Klein NPA 752(2005)470

[2] Strikman et al., hep-ph/0505023

Overview

Physics topics: g g , g p collisions at e+e-,ep colliders

g g , g A collisions in UPC A+A

Experimental aspects: Ultraperipheral A+A collisions

(signatures, brackgd, triggers, detectors, analysis cuts, ...)

Results: Quarkonia g-production in UPC @ RHIC (PHENIX)

Prospects: Quarkonia g-production in UPC @ LHC

Summary

Quarkonia g-production in UPC A+A @ LHC (I)

Some key figures:

Max. sgA(LHC) ≈ 900 GeV > 3 – 5×sgp(HERA)

Max. sgg (LHC) ≈ 160 GeV ≈ sgg (LEP)

LHC (y=0): x(J/) ~4·10-4 , x(¡)~10-3

LHC (y=3): x(J/) ~ 2·10-5 , x(¡)~10-4

g+A J/+A

in UPC A+A versus √sNN:

LHC QQbar cross-sections:

(first time ¡ measurable in UPC)

AuAu @ RHIC

PbPb @ LHC

g+A J/,¡ )+A

Most results from UPC Chapter ALICE PPR II [J.Nystrand et al.]

sQQbar (LHC) ~ 40 – 100×sQQbar(RHIC)

Quarkonia g-production in UPC A+A @ LHC (II)

Cross-sections, rates:

High sensitivity to different G(x,Q2) [~30% reduction of G(x,Q2) Þ 0.5·sJ/,¡)

Rapidity distrib. g+A J/,¡ +A in UPC A+A:

g+A J/,¡ )+A

Most results from UPC Chapter ALICE PPR II [J.Nystrand et al.]

mm decay (ALICE m-spectrom.)

~1/2 – 1/5 less rates than ee

mode at y=0

Summary

UPC A+A collisions generate high-energy g beams for “non-QGP” studies: g+g, g+A physics at sgg,gA larger than LEP & HERA.

Main motivation: Precision QCD studies w/ low bckgd environment & simpler initial state than pp,A+A collisions.

Physics topics in UPC quarkonia photo-production:

Nuclear GA(x,Q2) at small-x [Gluon saturation, CGC, ...], QQbar propagation in cold nuclear matter, QQbar spectroscopy: g+g  0+-2++ states, ...

Lessons from RHIC:

Efficient trigger w/ forward neutron tagging (A* dissoc.) + high-pT at y=0

Physics signal accessible w/ relative “simple” cuts & analysis

Good theoretical description of J/Ψ (pQCD) & high-mass e+e- (QED)

Main source of syst. uncertainty: coherent g+g  e+e- physics background

Prospects for LHC:

Unexplored kinematic regime (max. energies ever, small-x, g+A ¡ , ...)

Expected rates ~1000 higher than at RHIC.

High sensitivity to nuclear GA(x,Q2) at small-x

Good sensitivity to

different Ansatze &

evolutions of G(x,Q2)

g+A J/,¡ )+A

Backup slides

PHENIX UPC analysis

UPC- Events collected & integrated luminosity

Total data set: 1352 PRDFFs * 0.8 GB/file ~ 1.04 TB

Total equivalent sampled luminosity:

Lint = σAuAu /BBCLL1 x εBBC x εloss = 1122·106/(6.9 b) x 0.92 x 0.79 = 120 mb-1

εloss accounts for # runs excluded (6.7 Mevts instead of 8.5 Mevts)

105 J/Ψ generated (EXODUS)

Realistic input dN/dy and dN/dpT distributions (”StarLight” model)

Theoretical input distribution: J/Ψ

Input: 105 J/Ψ with realistic d2s/dydpT from “Starlight” [J.Nystrand, S.Klein]

PISA production & reco. w/ the same analysis code as for real data

Full MC simulation: J/Ψ

Losses:

(Branching ratio: 5.93%)

Acceptance & vtx & multiplicity cut: ~5.0%

Efficiency loss single & pair cuts: ~ 56.4%

Unlike-sign background: 0

dN/dmee

Peaked at

low pT

(coherence

condition)

Accep. x Effic x BR ~ 2·10-3

dNee/dpT

mJ/Ψ ~ 3.10 GeV +/- 100 MeV

Full MC simulation: e+e- continuum

Input: 2·105 pairs with realistic d2s/dydpTfrom “Starlight” [J.Nystrand, S.Klein]

Full PISA production & reco. w/ the same analysis code as for real data

Accepted/

geometry

Losses:

Acceptance & vtx & multiplicity cut: ~2.1%

Efficiency loss single & pair cuts: ~ 5.5%

Unlike-sign background: 0

dN/dmee

Peaked at very low pT

(coherence condition)

Accep. x Effic: ~10-3

(~3 times lower than for J/Psi)

Fit to power-law

function

dNee/dpT

Summary

Analysis of all statistics of Run-4 AuAu UPC triggered data.

Previous results shown as “work in progress” last DNP'04.

New PRDFF → nDST repass of UPC data at ORNL (pro.66.upc)

New reco analysis (pro.70) w/ official Run-4 (re)calibrators

and electron-based afterburners. New set of analysis cuts.

Full PISA MonteCarlo production & reconstruction with realistic

input distributions for UPC J/ψ and di-electron continuum.

Clear indications of J/ψ & high mass di-lepton continuum from

UPC AuAu collisions (Run-4).

Dielectron continuum: dN/dmee and dNee/dpT in god agreement

with coherent production (very low pair pT, mee spectral shape)

J/ψ signal: dN/dmee peak at 3.1 GeV/c2 w/ 130 MeV width

(good agreement with full PISA MC with realistic input).

J/ψ dN/dpT peaked at very low pT as expected for UPC events.

N= 10 ± 3 counts: dNJ/ψ|y=0= 44. ± 16. (stat) ± 18. (syst) μb

4 plots requested PHENIX-PRELIMINARY for QM'05 (poster)

———————————

 [mb] (req. Xn)

———————————

	590 (170)

	59 ( 17)

	39 ( 13)

J/	0.29 (0.16)

———————————

Large cross-sections expected at RHIC due to:

(i) Large Au charge (AA = Z12Z22 NN ~ 4·105NN)

(ii) High √s (mass: w~2 CM(hc/R) ~ 6 GeV for CM= 100)

Some predictions:

Motivation (cont'd)

Baltz, Klein, Nystrand:

PRC 60(1999)014903,

PRL 89(2002)012301

STAR

measurement

(Coulomb) Excited Au* emits X neutrons

Highest mass vector meson accessible in UPC colls. at RHIC

Slide 1

d7o

2003-08-06T23:02:41

David d'Enterria