J/ψ photoproduction on nuclei

Transcription

1 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

2 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

3 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)

4 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> 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: ; Eskola et al, JHEP 1310 (013) 13 Future: Electron-Ion Collider in the US, Accardi et al, ArXiv: ; LHeC@CERN, LHEC Study Group, J. Phys. G39 (01) Option right now: Charmonium photoproduction in Pb-Pb UPCs@LHC 4

5 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: ; Wagner s talk on Wedneday I will stay at LO in my talk. 5

6 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) 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) 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) ; Belitsky, Mueller, Kirchner, NPB 69 (00) 33, Kumericki s talk on Monday 6

7 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

8 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

9 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) 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) Pb-08, Q =4 GeV 0.4 EPS09 LTA g A (x)/ag N (x) 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 x 9

10 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) Pb-08, Q =4 GeV, x= b, fm! 10

11 S Pb Comparison to SPb from ALICE UPC data Guzey, Zhalov JHEP 1310 (013) 07 J/ψ, Q =3 GeV LTA+CTEQ6L1 ALICE 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= Similarly good description using EPS09+CTEQ6L. x ψ(s), Q =4 GeV LTA+CTEQ6L Cannot be described by simple versions of the dipole model, Lappi, Mantysaari 013 S Pb Guzey, Zhalov, arxiv: 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

12 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) 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: / dy [mb] dσ coh Pb+Pb Pb+Pb+J/ψ CMS µb (.76 TeV) CMS data ALICE data Impulse approximation Leading twist approximation y 1

13 Outlook for Collision energy increase snn=.76 TeV 5.0 TeV probes ga(x,q ) at x= 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)] x= High Low F A (t) VG, Strikman, Zhalov, work in progress t, GeV 13

14 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 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

15 Additional Slides

16 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, ) 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

17 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: Q =.5 GeV quark, Fit B quark, Fit A β βf j/ip Q =.5 GeV gluon, Fit B gluon, Fit A Necessary information for numerical prections. Important that gp >> qp. β 17

18 Effective cross sections σ and σsoft = 18

19 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 TOTAL PbPb->PbPb (s) snn =5.0 TeV Suppression due to nuclear shadowing same for J/ψ and ψ : y 0nXn y 0nXn d 0/dy d J/ /dy = at y=0. d /dy, mb d /dy, mb 0.1 Guzey, Kryshen, Zhalov, PRC 93 (016) y y 19

20 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].