Deep exclusive meson electroproduction focus on vector mesons

Deep exclusive meson electroproduction: focus on vector mesons Discussions with/material borrowed from: L. Favart, S. Kananov, P. Marage, C. Riedl, K. Park, …

Regge theory: Exchange of families of mesons in the t-channel

Regge theory: Exchange of families of mesons in the t-channel M(s, t) ~ sa(t) where a(t) (trajectory) is the relation between the spin and the (squared) mass of a family of particles M->sa(t) stot~1/s x Im(M(s, t=0))->sa(0)-1 [optical theorem] ds/dt~1/s 2 x |M(s, t)|2 ->s 2 a(t)-2 stot ->[ea(t)lns(s)] ds/dt s t

Some signatures of the (asymptotic) « hard » processes: Q 2 dependence: W dependence: Ratio of yields: s. L~1/Q 6 s~|x. G(x)|2 s. T~1/Q 8 (for gluon handbag) r/w/f/(J/Y)~9/1/2/8 Saturation with hard scale of a. P(0), b, … SCHC : checks with SDMEs s. L/s. T~Q 2 (for gluon handbag)

H 1, ZEUS , Q 2>>

HERMES H 1, ZEUS COMPASS CLAS H 1, ZEUS , Q 2>>

HERMES H 1, ZEUS COMPASS CLAS H 1, ZEUS , Q 2>> + « older » data from: E 665, NMC, Cornell, …

HERMES H 1, ZEUS COMPASS CLAS H 1, ZEUS , Q 2>> + « older » data from: E 665, NMC, Cornell, …

W dependence Steepening W slope as a function of Q 2 indicates « hard » regime (reflects gluon distribution in the proton)

W dependence Steepening W slope as a function of Q 2 indicates « hard » regime (reflects gluon distribution in the proton) Two ways to set a « hard » scale: *large Q 2 *mass of produced VM Universality : r, f at large Q 2+M 2 similar to J/y

a. P(0) increases from “soft” (~1. 1) to “hard” (~1. 3) as a function of scale m 2=(Q 2+MV 2)/4. Hardening of W distributions with m 2

Q 2 dependence s. L~1/Q 6 => Fit with s~1/(Q 2+MV 2)n r: Q 2 >0 Ge. V 2 => n=2+/- 0. 01 J/y: 2 2 Q >10 Ge. V => n=2. 5+/- 0. 02 (S. Kananov) Q 2 >0 Ge. V 2 => n=2. 486 +/- 0. 08 +/-0. 068 Q 2 dependence is damped at low Q 2 and steepens at large Q 2 Approaching handbag prediction of n=6 (Q 2 not asymptotic, fixed W vs fixed x. B, stot vs s. L, Q 2 evolution of G(x)…)

t dependence b decreases from “soft” (~10 Ge. V-2) to “hard” (~4 -5 Ge. V-2) as a function of scale m 2=(Q 2+MV 2)/4

Ratios r/w/f/(J/Y) ~ 9/1/2/8

s. L / s. T (almost) compatible with handbag prediction (damping at large Q 2)

SDMEs HERMES H 1 (almost) no SCHC violation

At high energy (W>5 Ge. V), the general features of the kinematics dependences and of the SDMEs are relatively/qualitatively well understood Good indications that the “hard”/p. QCD regime is dominant for m 2=(Q 2+MV 2)/4 ~ 3 -5 Ge. V 2. Data are relatively well described by GPD/handbag approaches

HERMES H 1, ZEUS COMPASS CLAS H 1, ZEUS , Q 2>>

JLab & CLAS in Hall B Duty cycle 100% Emax 6 Ge. V Pmax 80%

Exclusive r 0, w, f & r+ electroproduction on the proton @ CLAS 6 K. Lukashin et al. , Phys. Rev. C 63: 065205, 2001 (f@4. 2 Ge. V) C. Hadjidakis et al. , Phys. Lett. B 605: 256 -264, 2005 (r 0@4. 2 Ge. V) L. Morand et al. , Eur. Phys. J. A 24: 445 -458, 2005 (w@5. 75 Ge. V) J. Santoro et al. , Phys. Rev. C 78: 025210, 2008 (f@5. 75 Ge. V) S. Morrow et al. , Eur. Phys. J. A 39: 5 -31, 2009 (r 0@5. 75 Ge. V) A. Fradi, Orsay Univ. Ph. D thesis (r+@5. 75 Ge. V) } } } e 1 -b (1999) e 1 -6 (2001 -2002) e 1 -dvcs (2005)

e 1 -6 experiment (Ee =5. 75 Ge. V) (October 2001 – January 2002)

ep ep p+(p-) Mm(epp+ X) p+ e Mm(ep. X) (p-) p

sr (g*p pr 0) vs W

r+ r 0 w f C. Hadjidakis et al. , Phys. Lett. B 605: 256 -264, 2005 (r 0@4. 2 Ge. V) S. Morrow et al. , Eur. Phys. J. A 39: 5 -31, 2009 (r 0@5. 75 Ge. V) L. Morand et al. , Eur. Phys. J. A 24: 445 -458, 2005 (w@5. 75 Ge. V) J. Santoro et al. , Phys. Rev. C 78: 025210, 2008 (f@5. 75 Ge. V) K. Lukashin, Phys. Rev. C 63: 065205, 2001 (f@4. 2 Ge. V) A. Fradi, Orsay Univ. Ph. D thesis, 2009 (r+@5. 75 Ge. V)

ep->epf ( K+[K-]) GK s. L f. L

+ VGG GPD model

VGG GPD model GK GPD model

t γ, π, ρ, ω… -2ξ x+ξ x-ξ ~ ~ H, H, E, E (x, ξ, t) “ERBL” region -1 Antiquark distribution -ξ 0 “DGLAP” region ξ q q Distribution amplitude ERBL +1 Quark distribution DGLAP W~1/x x

DDs + “meson exchange” DDs w/o “meson exchange” (VGG) “meson exchange”

Comparison with r 0, w, f

VMs (r 0, w, f) the only exclusive process [with DVCS] measured over a W range of 2 orders of magnitude (s. L, T, ds/dt, SDMEs, …) At high energy (W>5 Ge. V), transition from “soft” to “hard” (m 2 scale) physics relatively well understood (further work needed for precision understanding/extractions) At low energy (W<5 Ge. V), large failure of “hard” approach. This is not understood. Is the GPD/handbag approach setting at much larger Q 2 or is the widely used GPD parametrisation in the valence region completely wrong ? A lot of new data expected soon from JLab@11 Ge. V, COMPASS (transv. target), HERA new analysis, …

GPDs/handbag ? ? ? GPDs/handbag W~1/x

g. N->Np

sr (g*p pr 0) vs W

Angular distribution analysis, cos qcm Relying on SCHC (exp. check to the ~25% level)

Longitudinal cross section s. L (g*Lp pr. L 0)

Interpretation “a la Regge” : Laget model g*p pr 0 g*p pw g*p pf Free parameters: *Hadronic coupling constants: g. MNN *Mass scales of EM FFs: (1+Q 2/L 2)-2

s. L (g*Lp pr. L 0) s, f 2 Pomeron Regge/Laget

Exclusive r + electroproduction

Channel selection e p e’ [n] r+ e’ [n] p+p 0 e’ [n] p+ g g p+ EC e gg (p-) (p) One event in CLAS IC

Invariant mass IM(p+p 0) Total cross section s (Q 2, x. B) r+ Y R A IM L E R P IN

Regge “hadronic” approach r+ r+ + p+ r+ n n GPD “partonic” approach H, E r+ L ρ0 eu Hu - ed Hd eu. Eu - ed. Ed w eu Hu + e d Hd eu. Eu + e d. Ed ρ+ Hu - Hd Eu - Ed

“Hadronic approach”: Laget model r+ r 0

“Partonic approach”: GPDs (*) r+ r 0 VGG GPD model GK GPD model

Exclusive f electroproduction

ep->epf ( K+[K-])

Laget s. T+es. L W=2. 9 Ge. V W=2. 45 Ge. V W=2. 1 Ge. V GK s. L f. L

Exclusive w electroproduction

ep->epw ( p+p-[p 0])

cos(qcm) distribution fcm distribution

Cross section s(g*p pw) Laget Regge model for g*p pw Laget s. T+es. L Laget es. L VGG es. L (H&E) Issue with GPD approach if p 0 exchange dominant : ~ p 0 ->E while ~ E subleading in handbag for VM production

Cross section s(g*p pw) –Comparison with GPD calculation (VGG)-

r+ r 0 w f C. Hadjidakis et al. , Phys. Lett. B 605: 256 -264, 2005 (r 0@4. 2 Ge. V) S. Morrow et al. , Eur. Phys. J. A 39: 5 -31, 2009 (r 0@5. 75 Ge. V) L. Morand et al. , Eur. Phys. J. A 24: 445 -458, 2005 (w@5. 75 Ge. V) J. Santoro et al. , Phys. Rev. C 78: 025210, 2008 (f@5. 75 Ge. V) K. Lukashin, Phys. Rev. C 63: 065205, 2001 (f@4. 2 Ge. V) A. Fradi, Orsay Univ. Ph. D thesis, 2009 (r+@5. 75 Ge. V)

GPDs/handbag ? ? ? GPDs/handbag W~1/x

Motivation to go to higher Q 2 (but stay in valence region): Approach asymptotic regime and test validity of power corrections If (power corrected) handbag diagram in valence region: same Q 2 dependence at low W than at large W: r 0 and f should be different from w and r+, these latter having higher-twist t-channel exchanges If higher twist contribution in valence region: cross section will drop faster as a function of Q 2 at low W than at large W:

Overview of existing data (valence region) r 0, w, f & r+ electroproduction on the proton @ CLAS 6 GPDs or not GPDs ? Perspectives with CLAS 12 & EIC

100% acceptance & integrated over all variables but (x. B, Q 2) 6 Ge. V e fixed p target Counting ra for 1000 hou at 1034 cm-2 s Limitation co from phase s

100% acceptance & integrated over all variables but (x. B, Q 2) 6 Ge. V e fixed p target Counting ra for 100 hour at 1034 cm-2 s Limitation co from phase s

100% acceptance & integrated over all variables but (x. B, Q 2) 11 Ge. V e fixed p target Counting ra for 1000 hou at 1035 cm-2 s Limitation co from phase s

100% acceptance & integrated over all variables but (x. B, Q 2) 11 Ge. V e fixed p target Counting ra for 1000 hou at 1035 cm-2 s Limitation co from phase s

100% acceptance & integrated over all variables but (x. B, Q 2) 11 Ge. V e 60 Ge. V p Counting ra for 1000 hou at 1034 cm-2 s Limitation co from luminos

100% acceptance & integrated over all variables but (x. B, Q 2) 11 Ge. V e 60 Ge. V p Counting ra for 1000 hou at 1034 cm-2 s Limitation co from luminos

6 Ge. V e fixed p target 11 Ge. V e 60 Ge. V p

CLAS@6 Ge. V S. Morrow et al. , Eur. Phys. J. A 39: 5 -31, 2009 (r 0@5. 75 Ge. V)

Back-up slides

IM(pp+)

IM(pp-)

Interpretation in terms of GPDs ? LO (w/o kperp effect) LO (with kperp effect) Soft overlap (partial) Handbag diagram calculation has kperp effects to account for preasymptotic effects

Background Subtraction (normalized spectra) 1) Ross-Stodolsky B-W for r 0(770), f 0(980) and f 2(1270) with variable skewedness parameter, 2) D++(1232) p+p- inv. mass spectrum and p+p- phase space.

ds/dt (g*p p r 0) Large tmin ! (1. 6 Ge. V 2) Fit by ebt

Longitudinal cross sections I Y R NA IM L E R P s. L(r+)[mb] ds. L/dt (r+)[mb]

Comparison with r 0, w, f

Increasing Q 2: e - 2 Q ~ Me. V 2 ? Q 2>> e - 2 Q >>Ge. V 2

GPDs parametrization based on DDs (VGG/GK model) Strong power corrections… but seems to work at large W…

ds/dt (g*p p r 0) Large tmin ! (1. 6 Ge. V 2) Fit by ebt