Post on 20-Dec-2015
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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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MC Acceptance calculation in 7D
Variable Range No. Bins
Bin Width
Q2(GeV2) 1.00-5.50
5
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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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 ]