J/ψ photoproduction on nuclei Vadim Guzey Petersburg Nuclear Physics Institute (PNPI), National Research Center Kurchatov Institute, Gatchina, Russia Outline: - Ultraperipheral collisions (UPCs), coherent photoproduction of vector mesons and nuclear shadowing - Gluon nuclear shadowing from J/ψ photoproduction at the LHC - Outlook and Conclusions Workshop Nucleon and nuclear structure through di-lepton production ECT*, Trento, Italy, Oct 4-8, 016 1
Ultraperipheral collisions (UPCs) Ions can interact at large impact parameters b >> RA+RB =10-0 fm ultraperipheral collisions (UPCs) strong interaction suppressed interaction via quasi-real photons, Fermi (194), von Weizsäcker; Williams (1934) B B J/ψ, - UPCs correspond to empty detector with only two lepton tracks - Nuclear coherence by veto on neutron production by Zero Degree Calorimeters and selection of small pt A A Coherent photoproduction of vector mesons in UPCs: d AA!AAJ/ (y) dy = N /A (y) A!AJ/ (y)+n /A ( y) A!AJ/ ( y) Photon flux: Photon flux from QED: - high intensity ~ Z - large photon energies Photoproduction cross section N /Z (k) = Z em [ K 0 ( )K 1 ( ) = k(r A / L ) y =ln[w /( L m N M V )] = J/ψ rapidity (K 1( ) K0( ))] UPCs = γp and γa interactions at unprecedentedly large energies, Baltz et al., The Physics of Ultraperipheral Collisions at the LHC, Phys. Rept. 480 (008) 1
Nuclear shadowing Nuclear shadowing (NS) = suppression of cross section on a nucleus compared to sum of cross sections on individual nucleons: σa < A σn. Observed for beams of nucleons, pions, real and virtual photons, neutrinos, other hard probes of large energies (> 1 GeV) Explained by multiple rescattering of the projectile on target nucleons destructive interference among amplitudes for interaction with 1,, nucleons nucleons in rear of the nucleus see smaller (shadowed) flux: σa~a /3. NS in photoproduction of ρ, ɸ on nuclei: - tests dynamics of soft high-energy γp and γa scattering: color fluctuations, inelastic (Gribov) shadowing, Frankfurt, VG, Strikman, Zhalov, PLB 73 (016) 51 NS in photoproduction of J/ψ, ψ(s), Υ on nuclei: - new constraints on nuclear gluon distribution ga(x,µ ) at small x, VG Kryshen, Strikman, Zhalov, PLB 76 (013) 90; VG, Zhalov, JHEP 10 (013) 07; JHEP 0 (014) 046; VG, Kryshen, Zhalov, PRC 93 (016) 05506 3
Gluon nuclear shadowing Gluon nuclear shadowing = ga(x,q ) < A gn(x,q ) for small x H. Pauukunen, NPA 96 (014) 4 Piller, Weise (000) Global QCD fits shadowing For heavy nuclei, shadowing as large as 0% for Q >1 GeV and x>0.005. The nuclear gluon distribution ga(x,µ ) is known with large uncertainties. pa@lhc data help little and mostly in antishadowing region, Armesto et al, arxiv: 151.0158; Eskola et al, JHEP 1310 (013) 13 Future: Electron-Ion Collider in the US, Accardi et al, ArXiv:11.1701; LHeC@CERN, LHEC Study Group, J. Phys. G39 (01) 075001 Option right now: Charmonium photoproduction in Pb-Pb UPCs@LHC 4
Exclusive J/ψ photoproduction In leading logarithmic approximation of perturbative QCD and non-relativistic ~7 approximation for charmonium wave function (J/ψ, ψ(s)): qj k d T!J/ T (W, t = 0) dt = C(µ ) xg T (x, µ ) M. Ryskin (1993) l+ x = M J/ W, µ = M J/ /4=.4 GeV C(µ )=MJ/ 3 ee 3 s (µ )/(48 em µ 8 ) a In collinear factorization for exclusive processes, at LO in αs and NR expansion for J/ψ wave function: d T!J/ T (W, t = 0) dt = 16 3 ee 3 e.m. M 5 V S (µ )H g (,,t=0,µ ) NLO corrections are very large, Ivanov, Schafer, Szymanowski, Krasninov, EPJ C 75 (015), 75; Jones, Martin, Ryskin, Teubner, J. Phys. G43 (015), no.3, 03500, but can be tamed by choice of factorization scale µ=mc and other tricks, Jones et al, arxiv:1610.07; Wagner s talk on Wedneday I will stay at LO in my talk. 5
Exclusive J/ψ photoproduction () At high energies (small ξ) and LO in αs, GPDs can be connected to PDFs in a weakly model-dependent way. At low µ0, x1, ξ skewness can be neglected All skewness at µ > µ0 due to evolution, Frankfurt, Freund, VG, Strikman, PLB 418 (1998) 345; Shuvaev et al., RPD 60 (1999) 014015 DGLAP for GPDs solved using either brute-force method, Frankfurt, Freund, VG, Strikman, PLB 418 (1998) 345, or conformal symmetry, Shuvaev et al., RPD 60 (1999) 014015 H g (,,t=0,µ )=R g xg(x B,µ ) R g = +3 p ( +5/) (4 + ) 1., for xg 1/x with 0. At LO, somewhat overestimates HERA DVCS data, Freund, McDermott, Strikman, PRD67 (003) 036001; Belitsky, Mueller, Kirchner, NPB 69 (00) 33, Kumericki s talk on Monday 6
Coherent J/ψ photoproduction on nuclei Application to nuclear targets: A!J/ A(W p )= (1 + A )R g,a (1 + )R g d p!j/ p (W p,t= 0) dt apple GA (x, µ ) AG N (x, µ ) A(t min ) Small correction ka/n 0.95 From HERA and LHCb Gluon shadow. Rg From nuclear form factor: approximation Nuclear suppression factor S direct access to Rg S(W p )= " Pb!J/ Pb IA Pb!J/ Pb# 1/ = apple A/N G A (x, µ ) AG N (x, µ ) = apple A/N R g A(t min )= Z tmin 1 dt F A (t) experiment Impulse Approximation theory: leading twist NS model Side remark: we choose µ =3 GeV to correctly reproduce W-dependence of HERA data on γp J/ψp. 7
Leading twist nuclear shadowing model Combination of Gribov-Glauber NS model with QCD factorization theorems for inclusive and diffractive DIS shadowing for individual partons j, Frankfurt, Strikman (1999) + Interaction with nucleons: model-indep via diffractive PDFs: j (x) = 16 xf j/n (x, µ ) Z 0.1 x dx P f D(4) j/n (x, µ,x P,t= 0) Interaction with 3 nucleons: via soft hadronic fluctuations of γ* soft(x) = R d P ( ) 3 R d P ( ) P(σ) probability to interact with cs σ In quasi-eikonal approximation in low-x limit, Frankfurt, Guzey, Strikman 01: xf j/a (x, µ )=Af j/n (x, µ ) j f j/n(x, µ ) [ j soft (x)] Z d b e 1 j soft (x)t A(b) 1+ j soft (x) T A (b)! 8
Leading twist nuclear shadowing model () Model gives nuclear PDFs at µ =3-4 GeV for subsequent DGLAP evolution. Name leading twist since diffractive structure functions/pdfs measured at HERA scale with Q. Gluon diffractive PDFs are large, ZEUS, H1 006 predict large shadowing for ga(x,µ ), Frankfurt, Guzey, Strikman, Phys. Rept. 51 (01) 55 1.4 Input: Leading twist (LTA) vs. EPS09 Results of DGLAP evolution: from Q =4 GeV to Q =10 and 10,000 GeV 1.4 g A (x)/ag N (x) 1. 1 0.8 0.6 Pb-08, Q =4 GeV 0.4 EPS09 LTA 0. 10-5 10-4 10-3 10-10 -1 g A (x)/ag N (x) 1. 1 0.8 0.6 Pb-08 x For quarks, the agreement between LTA and EPS09 is much better. Q =4 GeV 0.4 Q =10 GeV Q =10,000 GeV 0. 10-5 10-4 10-3 10-10 -1 x 9
LT shadowing: Impact parameter dependence Shadowing arises from rescattering on target nucleons at given impact parameter b. Removing integral over b impact-parameter-dependent nuclear PDFs: xf j/a (x, b, µ )=T A (b)xf j/n (x) ga(x,b)/[ta(b)gn(x)] j f j/n(x, µ ) e 1 j [ soft (x)] dependence of nuclear shadowing for 40 Ca (upper green surfaces) and 08 Pb (lower red surfaces). The graphs show the ratio a function of x and the impact parameter E b at Q = 4 GeV. The top panel corresponds to ū-quarks; the bottom panel he evaluation of nuclear shadowing, model FGS10_H was used (see the text). (For interpretation of the references to color in r is referred to the web version of this article.) j soft (x)t A(b) 1+ j soft (x) T A (b) Can be only indirectly determined using global QCD fits, EPS09s npdfs, Helenius et al (01) Can be probed and tested in: - centrality dependence of hard pa/aa processes, Helenius et al (01) - t dependence of exclusive γ*a and γa processes, e.g., γ*a γa, Frankfurt, VG, Strikman 01, γa J/ψA, VG, Strikman, Zhalov, work in progress g A (x,b)/t A (b)g N (x) 1.4 1. 1 0.8 0.6 0.4 0. Pb-08, Q =4 GeV, x=10-4 0 4 6 8 10 b, fm! 10
S Pb Comparison to SPb from ALICE UPC data 1.1 0.9 1 0.8 0.7 0.6 0.5 0.4 0.3 0. 0.1 0 Guzey, Zhalov JHEP 1310 (013) 07 J/ψ, Q =3 GeV LTA+CTEQ6L1 ALICE 10-4 10-3 10-10 -1 Abelev et al. [ALICE], PLB718 (013) 173; Abbas et al. [ALICE], EPJ C 73 (013) 617 Good agreement with ALICE data on coherent J/ψ photoproduction in Pb-Pb UPCs@.76 TeV first direct evidence of large gluon nuclear shadowing at x=0.001. Similarly good description using EPS09+CTEQ6L. x 1.1 0.9 1 0.8 0.7 0.6 0.5 0.4 0.3 ψ(s), Q =4 GeV 0. 0.1 LTA+CTEQ6L1 0 10-4 10-3 10-10 -1 Cannot be described by simple versions of the dipole model, Lappi, Mantysaari 013 S Pb Guzey, Zhalov, arxiv:1404.6101 Adam et al. [ALICE], PLB751 (015) 358 We predict similar suppression for J/ψ and ψ(s) tension with ALICE data on ψ(s) photoproduction in Pb-Pb UPCs at y=0 wait for better precision Run data. x 11
Coherent J/ψ photoproduction in Pb-Pb UPCs with forward neutron emission UPCs can be accompanied by e.m. excitation of colliding ions followed by forward neutron emission, Baltz, Klein, Nystrand, PRL 89 (00) 01301 CMS data in 0nXn-channel converted to the total coherent cross section agrees very well with our predictions of large gluon shadowing, CMS Collab., arxiv:1605.06966 / dy [mb] dσ coh 9 8 7 6 5 4 Pb+Pb Pb+Pb+J/ψ CMS -1 159 µb (.76 TeV) CMS data ALICE data Impulse approximation Leading twist approximation 3 1 0 0.5 0 0.5 1 1.5.5 3 3.5 y 1
Outlook for Run @LHC Collision energy increase snn=.76 TeV 5.0 TeV probes ga(x,q ) at x=6 10-4 for y=0. Photoproduction of Υ mesons to study Q dependence of ga(x,q ). UPC with forward neutron emission possibility to disentangle low-w and high-w contributions in dσ/dy UPC cross section access to much lower x, VG, Strikman, Zhalov, EPJ C (014) 74: 94 Measurement of t-dependence of γa J/ψA: complementary constraint on ga(x,q ) and determination of 3D gluon distribution in nuclei ga(x,b,q ) d A!J/ A (W ) dt / applez d be i~q T ~b g A (x, b, µ ) (d /dt)/[d /dt(t=0)] 10 0 10-1 10-10 -3 x=0.0006 High Low F A (t) VG, Strikman, Zhalov, work in progress 10-4 0 0.005 0.01 0.015 0.0 0.05 0.03 0.035 0.04 t, GeV 13
Conclusions l Coherent photoproduction of J/ψ on nuclei in UPCs@LHC gives direct access to nuclear gluon distribution ga(x,µ ) down to x 10-3 at µ 3 GeV. l ALICE and CMS data give first evidence of large nuclear gluon shadowing, which is consistent with predictions of LT NS model and EPS09 npdfs l UPC measurements in pp, pa и AA collisions will continue in Run @LHC. Notably, measurement of t-dependence of γa J/ψA will access 3D nuclear gluon distribution ga(x,b,q ) encoding spacial image of NS. l UPCs@LHC = forerunner of measurements of nuclear gluon distributions at an Electron-Ion Collider. 14
Additional Slides
Diffraction in ep DIS at HERA One of main HERA results is the discovery of large fraction of diffractive events (~10%) [( ) diffraction is a leading twist phenomenon (H1 and ZEUS, 1994-006) e + p! e 0 + X + p 0 k p q k' γ (Q ) p' X (M ) X d 4 t =(p p), x IP = q (p p ) q p Q β = q (p p ) = x! + + M X + Q W + Q, Collinear factorization (Collins 97) diffractive parton distributions F D(4) (x, Q, x P, t) = X = D ep dx P dt dx dq = xq 4 x IP Q Q + M X Measurement of the t-dependence of diffractive cross section: Bdiff = 6 GeV - ± 15% h ere Q is the virtuality of the exchanged photon; x = Q j=q, q,g Z 1 dy y C j D 4 i 1 + (1 y) F D(4) (x, Q, x P, t) y F D(4) L (x, Q, x P, t) y, Q f D(4) j (y, Q, x P, t), p q is the Bjorken variabl F D(4) (x, Q, x P, t) = e B diff(t t min ) F D(4) (x, Q, x P, t min ), re B 6 GeV [6]. Note that this value is sof D(3) (x, Q, x P ) = Z tmin 1 GeV dtf D(4) (x, Q, x P, t), 16
Diffraction in ep DIS at HERA () It is convenient to use (supported by data): f D(3) j (β,q,x IP )=f IP / p (x IP )f j/ip (β,q )+n IR f IR / p (x IP )f j/ir (β,q ) γ γ x x IP β Pomeron flux Pomeron PDFs p p' p p' βf j/ip H1 and ZEUS determined Pomeron PDFs: 0.05 0.04 0.03 0.0 0.01 Q =.5 GeV quark, Fit B quark, Fit A 0 10-4 10-3 10-10 -1 10 0 β βf j/ip 0.8 0.7 0.6 0.5 0.4 0.3 0. 0.1 0 Q =.5 GeV gluon, Fit B gluon, Fit A 10-4 10-3 10-10 -1 10 0 Necessary information for numerical prections. Important that gp >> qp. β 17
Effective cross sections σ and σsoft = 18
Predictions for Run : J/ψ and ψ mesons Combination of LO pqcd and leading twist nuclear shadowing model: Measurement in two channels separation of contributions of small and large Wγp ga(x,µ ) at smaller x. d /dy, mb 4 TOTAL PbPb->PbPbJ/ snn =5.0 TeV LTA d /dy, mb 1.0 0.8 0.6 0.4 0. TOTAL PbPb->PbPb (s) snn =5.0 TeV Suppression due to nuclear shadowing same for J/ψ and ψ : 0.8-4 - 0 4 y 0nXn 0. -4-0 4 y 0nXn d 0/dy d J/ /dy =0.17 0.0 at y=0. d /dy, mb 0.6 0.4 0. d /dy, mb 0.1 Guzey, Kryshen, Zhalov, PRC 93 (016) 05506-4 - 0 4 y -4-0 4 y 19
FIG. 1: The coherent di ractive J/ photoproduction (Q = 0GeV ) cross section in lead-lead collisions at p s NN =.76 TeV computed using fipsat and IIM parametrizations and Boosted Gaussian (thin blue lines) and Gaus-LC (thick black lines) wavefunctions compared with the ALICE data [8, 4].