Exclusive J/ψ production and gluonic structure

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1 Exclusive J/ψ production and gluonic structure C. Weiss (JLab), Exclusive Meson Production Workshop, JLab, Jan 25 Quarkonium size and structure Parametric: Dynamical scales Numerical: Potential models, Lattice QCD hard x 1 x ? t J /ψ J/ψ photo/electroproduction at W W th FNAL, COMPASS, HERA, EIC Space time picture in rest frame GPD as color dipole moment of nucleon Gluon imaging of nucleon J/ψ photo/electroproduction near threshold Cornell, SLAC, JLab 12 GeV Kinematics of large t min,x Gluonic form factor of nucleon Nuclear targets Connections: Small size configurations, high Q 2 meson production, high t form factors, color transparency...

2 Heavy quarkonium: Scales and size m r Parametric: Non relativistic system Cf. Positronium in QED, v αem m mass mv momentum, inv. size mv 2 binding energy Frankfurt, Koepf, Strikman 98 Effective field theory approach: Non relativistic QCD, mv n expansion Lepage et al 92; Manohar 97; Brambilla 2; Kniehl et al. 22 Numerical: Potential models Eichten et al. 75; Quigg, Rosner 77 Typical c c distances r.2.3 fm 1 fm Transverse size in light cone wave function r 2 T = 2/3 r2 High momentum components with k m account for 3% of R (r = ) Decays J/ψ moderately small, relativistic

3 Heavy quarkonium: Size from lattice QCD γ µ Γ i Γ j Charmonium form factors Separate ground excited states using matrix of correlation functions Dudek et al. 6 Light quarks, hybrid mesons 1.8 temp. p f = () spat. p f = () temp. p f = (1) spat. p f = (1) Artificial J/ψ charge form factor from current with c c coupling G C J/ψ (Q 2 ).6.4 J/ψ charge radius r 2 1/2.26 fm Also η c, radiative transitions.2 J/psi "charge" form factor Dudek, Edwards, Richards Q 2 (GeV 2 )

4 Heavy quarkonium: Probe of color field Q 2 r T Use heavy quarkonium as probe of color fields in light hadrons Fields change with incident energy, size of Q Q configurations Multipole expansion: Dipole +... recoil Exclusive photo/electroproduction Target recoils: Gluonic form factor Q 2 tests/changes mix of Q Q sizes Theoretical challenges! Separate structures of target and probe (factorization), model gluonic structure of target Quarkonium hadron rescattering Theoretically simpler, but difficult to realize at low energies!

5 Photoproduction: Kinematics 11 P + (1 ζ) P, + Τ 11 t Exclusive production γn J/ψ +N Invariant momentum transfer grows near threshold t min,th = 2.2GeV 2 t min [GeV 2 ] Invariant momentum transfer m ψ 2 mn / (m ψ + m N ) Light cone variables ζ Plus momentum transfer, cf. x B large near threshold, but not x 1! T Transverse momentum transfer ζ min Longitudinal momentum transfer m ψ / (m ψ + m N ) t = (ζ 2 m 2 N + 2 T )/(1 ζ) Two regimes W W th t min = 1 2 GeV 2, ζ large cf. nucleon elastic form factors Cornell, SLAC, JLab 12 GeV threshold m ψ 2 /W W [GeV] W W th t min negligible, ζ 1 cf. diffractive processes FNAL, COMPASS, HERA, EIC

6 High W: QCD factorization and dipole picture l coh QCD factorization theorem Collins, Frankfurt, Strikman 96 r T x 1 x 2 N GPD t N Collinear factorization of amplitude GPD Hard scattering Meson dist. amp. GPD as transition matrix element of twist 2 operator: Gluonic form factor of nucleon x 1 = x 2,t = : Usual gluon density Integrand N F +i ()F +i (z ) N z 2 = light like distance Size distribution in integral. McDermott et al. Small sizes ~.1 fm Fixed mass Running mass Space time picture in rest frame Brodsky et al. 94 Coherence length l coh 1fm A = d 2 r T ψ γ (r T ) A Q QN(r T ) }{{} ψ J/ψ (r T ) r 2 T α s GPD(Scale r 2 T ) Distribution of Q Q sizes determined dynamically, changes with energy, electroproduction Q 2 Cf. Color tranparency.2.4 r T (fm) [fm] GPD as transition color dipole moment

7 High W: Data and interpretation Q 2 11 hard GPD T J /ψ φ, ρ b x gluons J/ψ photo/electroproduction at high W well understood HERA data, extensive literature Experimental tests of small size regime Universality of t slopes above Q 2 1GeV 2 GPD/dipole calc s describe cross sections Frankfurt et al. 95; Goloskokov, Kroll 8+;... Gluonic radius of nucleon b 2 g [fm 2 ] J/ψ, H1 5 J/ψ, ZEUS 2 µ + µ - J/ψ, ZEUS 2 e + e - J/ψ, FNAL 82 Fits COMPASS J/ψ JLab12 φ x Frankfurt, Strikman, CW 11, Q 2 3GeV 2 Transverse spatial distribution of gluons Fourier T b impact parameter Distribution changes with x and scale Q 2 : Parton diffusion, DGLAP evolution Fundamental gluonic size of nucleon in QCD: Gluon vs. quark radii, non pert. dynamics Input for small x physics: Evolution equations, saturation Needed for pp@lhc: Underlying event, multiparton processes, diffraction

8 Near threshold: Reaction mechanism local x 1 x ? J /ψ Near threshold kinematics Large t min, up to 2.2 GeV 2 Large longit. momentum transfer x 1 x 2 = ζ σ [nb] Slope B [GeV -2 ] threshold t SLAC 75 CERN 87 Cornell W [GeV] FNAL 82 FNAL 93 Reaction mechanism near threshold Strikman, CW, in progress γggj/ψ vertex local on scale R nucl 1fm Amp CF gg (t) local gluonic form factor. Energy dependence through F gg (t min ) Consistent with existing low energy data. Can be tested with JLab 12 GeV! Theoretical questions Matching collinear short distance expansion? Quantum numbers of gluonic operator? Behavior of two gluon form factor? Correlated configurations in nucleon LCWF? Cf. model of Brodsky, Chudakov, Hoyer, Laget

9 Near threshold: Nuclei and ψn interaction W 11 t Kinematics of ψn scattering t accessible at all W > W th Ideal process for probing color fields in hadrons and nuclei! Physics of low energy ψn interaction 2 Operator expansion: Dipole dipole interaction Fuji, Kharzeev 99 Lab momenta p ψ, p N [GeV] Threshold Total J/ψ N Van der Waals force of QCD Brodsky, Miller 97 Nuclear bound states? Brodsky, de Teramond 9; Luke, Manohar, Savage 92 Near threshold γa J/ψ +X p ψ m ψ m ψ 2m N J/ψ fast, relativistic! E γ [GeV] Produced J/ψ is fast How to study bound states?

10 Summary J/ψ as small size probe of color fields in hadrons moderately small, relativistic High W photo/electroproduction at probes gluon GPD Transverse spatial distribution of gluons at fixed x Near threshold photo/electroproduction probes local gluonic form factor Theory/phenomenology developing New physics accessible with JLab 12 GeV! J/ψ fast in photoproduction How to study low energy ψn interaction, bound states? Connections with other JLab 12 GeV experiments Small size q q configurations in high Q 2 meson production High t form factors gluon vs. quark operators? Color transparency different probes Nuclear short range correlations high momentum components in WF