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Exclusive electroproduction of the on the proton at CLAS

Outline:Physics motivations:GPDs

CLAS experiment: e1-dvcsData analysis: cross section

n

Ahmed FRADI, IPN Orsay

Bosen Workshop 2007

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A  ’’hard’’ part exactly calculable in pQCD, which describes the interaction between the virtual photon and a quark of the nucleon and the exchange of a gluon.

A “soft’’ part which represents the non-perturbative structure of the nucleon and describes this structure in terms of 4 GPDs.

A second ’’soft’’ part which describes the structure of the meson with the distribution amplitude z).

For the electroproduction of mesons, the reaction amplitude can be factorized in 3 parts:

Large Q2, small t

Mesons : L

-1<x<1 t=

the dominant process is the handbag diagram  

Physics motivations:

GPDs (Generalized Parton Distributions) (Ji, Radyushkin, Collins, Strikman, Frankfurt,…)

~~

pn(=p+)

H,E,H,E(x,,t)

x-

t

x+

z)

Meson

~~

3

H, H, E, E (x,ξ,t)~ ~

“Ordinary” parton distributions

H(x,0,0) = q(x), H(x,0,0) = Δq(x) ~

x

Elastic form factors

H(x,ξ,t)dx = F1(t)(Dirac FF) ( ξ)

x

Ji’s sum rule

2Jq = x(H+E)(x,ξ,0)dx

gq LGL 21

21

(nucleon spin)

x+ξ x-ξ

tγ, π, ρ, ω…

GPDs are not completely unknownGPDs are not completely unknown

-2ξ

E(x,ξ,t)dx = F2(t) (Pauli FF) (

ξ)

X.Ji,Phys.Rev.Lett.78,610(1997);

Phys.Rev.D55,7114(1997)

Elastic scattering

Exclusive scattering

Deep inelastic scattering

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Physics motivations : interest of +

Separate flavors Hq,Eq :

Vector meson H,E

Hu+1/3Hd

Hu - Hd

CLAS analysis S. Morrow/M. Guidal(almost finalized)

The GPDs H(x,,t) and E(x,,t) must satisfy a polynomiality rule: the nth x moment of GPDs must be a polynomial in of order n+1.In GPDs models based on Double Distributions,for n odd the n+1 order is missing.The so-called D-term has been introduced1 to take into account this missing power n+1 .

D-term: must be an odd function of x.

D-term can be interpreted as an isoscalar scalar(0+) meson contribution

(e p e p 0)

D-term

*0

p p

D-term

+

p n

*

NO D-term

(1) M.Polyakov and C.Weiss,Phys.Rev. D60,114017(1999)

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Prediction for the cross section:GPDs (VGG)

M.Vanderhaeghen,P.A.M. Guichon and M.Guidal,Phys.rev.D 60 094017 (1999)

≈ 0 /5

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Electromagnetic CalorimeterElectron ID, detection of neutral particles

Time-of-Flight Counters

Measure speed → mass (particle identification)

Gas Cherenkov Counters

Separation e/

Hydrogen target

Drift Chambers:

to determine the trajectories and momenta of charged particles

Torus Coil :

to bend the trajectory of

charged particles

beam

Hall B / JLab (VA,USA)

CLAS : CEBAF Large Acceptance Spectrometer

Inner Calorimeter: detection of photons in the forward direction

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e1 - dvcs

Beam energy = 5.75 GeV

.1 < xB < .8

Q2 up to 5 GeV2

Integrated Luminosity ≈ 40fb-1

e p e n e’ n +0 n e

(February-June 2005)

Detected in CLAS Missing mass

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Cross section (*p n + 0)

V (Q2, xB) : the virtual photon flux .

Lint : integrated luminosity ≈ 40fb-1.

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Lint Q2 xB

N+0(Q2, xB)(Q2, xB)

p n

V (Q2, xB) Acc(Q2, xB)

Acc(Q2, xB): Acceptance of the CLAS detector.

Q2 xB :bin width .

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Channel selectione p e n e’ n +0 n e

Neutron missing mass

invariant mass

N +0

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Counts/20 MeV

100% e1-dvcs statistics

+0 invariant mass =√( p p

N +0

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1

Lint Q2 xB

N+0(Q2, xB)(Q2, xB)

p n

V (Q2, xB) Acc(Q2, xB)

Cross section (*p n + 0)

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MC Acceptance calculation in 7D

Variable Range No. Bins

Bin Width

Q2(GeV2) 1.00-5.50

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0.90

xB 0.10-0.80 4 0.18

-t(GeV2) 0.00-5.00

4

1.25

0-360 3 120

cosHS+ -1.0 - +1.0 3 0.66

HS+ 0-360 3 120

IM[+0] (GeV)

0.25-1.80 5 0.31

W(GeV) 1.71-3.05 4 0.34 120 million events generated with a realistic generator and simulated with a GEANT CLAS simulator.

Acc

Acc(Q2,xB,t,…) = rec(Q2,xB,t,…) / gen(Q2,xB,t,…)

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Q2

xb

-t W

+0 invariant mass

Cos+

Kinematical variables( Data+ Simulation: phase space) Acc

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Cross section (*p n + )

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Lint Q2 xB

N+0(Q2, xB)(Q2, xB)p n

V (Q2, xB) Acc(Q2, xB)

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Reduced cross section : pn

Preliminary

Arbitrary unitsStatistical errors only

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Cross section (*p n + )

Cross section (*p n + )

Background subtraction

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Background subtraction: for each (Q2,xB) bin

Fit :5 parameters

Skewed BreitWigner : 4 parameters

+ normalisation

+ mass

+ width

+ skew parameter

Phase space:simulation

1 parameter (background)

IM [+0 ] (GeV)

total fit result

p →n (Q2,xB)

d /d IM [+0 ]

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Fit to invariant mass for each (Q2,xB,t) bin

Good fits

unexpected peaks !

ddt(Q2,xB,t)

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To-do list

Improve the fits to the +0 invariant mass.

Extract * p →n + and d/ dt

Separate the longitudinal( L) from the tranverse(T) cross section by fitting the cosHS+ and relying on SCHC(« s-channel helicity conservation » ).

Comparison with theory: GPDs(VGG),Regge approach(JML),…