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GPDs measurements with CLAS: Deeply virtual Compton Scattering and p0 electroproduction

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the data at low t. GPD model agrees fairly. with the data at high t. DVCS cross section analysis ... 2Mn n=Ee-Ee' t = (p-p')2. f (angle between leptonic and ... – PowerPoint PPT presentation

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Title: GPDs measurements with CLAS: Deeply virtual Compton Scattering and p0 electroproduction


1
GPDs measurements with CLAS Deeply virtual
Compton Scattering and p0 electroproduction
  • Silvia Niccolai (CLAS
    Collaboration)

2
GPDs measurements with CLAS Deeply virtual
Compton Scattering and p0 electroproduction
  • CLAS at JLab
  • First CLAS results for DVCS
  • The e1-dvcs experiment
  • epg event selection
  • Results BSA
  • Preliminary DVCS cross sections
  • GPDs and DVMP
  • epp0 selection
  • Preliminary cross sections
  • Results BSA
  • Upcoming experiments and CLAS12

3
Jefferson Laboratory Newport News, USA
Continuous Electron Beam Accelerator Facility
Imax 200 mA Duty Factor 100 sE/E 2.5
10-5 Beam Pol 80
Emax 6 GeV
Hall B

4
The CLAS detector
  • Toroidal magnetic field (6 supercond. coils)
  • Drift chambers (argon/CO2 gas, 35000 cells)
  • Time-of-flight scintillators
  • Electromagnetic calorimeters
  • Cherenkov Counters (e/p separation)
  • Performances
  • large acceptance for charged particles
  • 8ltqlt142, ppgt0.3 GeV/c, ppgt0.1GeV/c
  • good momentum and angular resolution
  • Dp/p 0.5- 1.5, Dq, Df 1 mrad

5
DVCS _at_ CLAS first results
NON-DEDICATED experiments
ep?epX
ep?epg
S. Chen et al., PRL 97 (2006)
ltQ2gt 1.82 GeV2 ltxBgt 0.16 lt-tgt 0.31 GeV2

Target-spin (L) asymmetry (H, H)
Beam-spin asymmetry (H)
BSA, BCA and TSA (T L) at HERMES handbag
dominance confirmed
Low statistics Limited kinematics Uncertainties
on determination of epg final state
6
The e1-dvcs experiment at CLAS
  • Experiment March - May 2005
  • Ee 5.77 GeV
  • Polarization 76 - 82
  • Current 20-25 nA
  • Integrated luminosity 3.33? 107 nb-1

424 PbWO4 crystals 16cm x 1.3cm x 1.3cm 18
radiation lengths APD light readout
Superconducting solenoid magnet (shielding for
Moeller electrons)
7
Channel selection
TOF
ep?epg in CLAS
CC
DC
All 3 particles are detected
EC
  • electron ID
  • EC, CC, DC and TOF
  • proton ID
  • DC and TOF
  • photon ID
  • IC or EC

IC
8
BSA selection of the DVCS final state
  • Exclusivity cuts
  • PXT lt 90 MeV/c (150 MeV/c) ep?epgX
  • Cone angle ?(?X) lt1.2 (2.7) ep?epX
  • Coplanarity angle between (?p) et (?p)lt1.5
    (3)
  • EX lt 300 MeV (500 MeV)

Ph.D. Thesis of F.X. Girod
9
Evaluation of p0 background
epp0?epg(g) background calculated with Monte
Carlo simulation and experimental epp0?epgg data
ep ?epg ep?ep?0?ep?(?) Nep?X Nep? Nep?0(1?)
??0 ? N?0(2?)?Acc?0(2?) N?0(1?)?Acc?0(1?)
p0 background 5 in average
Ph.D. Thesis of F.X. Girod
10
BSA coverage and f distributions
Data integrated over t
  • 13 Q2, xB bins
  • 5 t bins
  • 12 f bins

Fit ? sin?/(1?cos??
Ph.D. Thesis of F.X. Girod
11
BSA a vs. t
GPD model agrees fairly with the data at high t
GPD model misses the data at low t
() Guidal, Polyakov, Radyushkin, Vanderhaegen,
PRD 72 (2005)
() Cano and Laget, PL B551 (2003)
Ph.D. Thesis of F.X. Girod
12
DVCS cross section analysis
Comparison DATA-MC
-t
f
Q2
xB
Ph.D. Thesis of H.S. Jo
13
DVCS cross section analysis
0.2lt-tlt0.4 GeV2
Acceptance calculated for each 4-dimensional bin
Ph.D. Thesis of H.S. Jo
14
Unpolarized Cross Sections
0.09lt-tlt0.2 GeV2
0.2lt-tlt0.4 GeV2
0.4lt-tlt0.6 GeV2
PRELIMINARY (arbitrary units)
0.6lt-tlt1 GeV2
1lt-tlt1.5 GeV2
1.5lt-tlt2 GeV2
Ph.D. Thesis of H.S. Jo
15
Difference of Polarized Cross Sections
0.09lt-tlt0.2 GeV2
0.2lt-tlt0.4 GeV2
0.4lt-tlt0.6 GeV2
PRELIMINARY (arbitrary units)
0.6lt-tlt1 GeV2
1lt-tlt1.5 GeV2
1.5lt-tlt2 GeV2
Largest kinematical coverage ever obtained for
DVCS 0.1ltxBlt0.58, 1ltQ2lt4.6, 0.09lt-tlt2
Ph.D. Thesis of H.S. Jo
16
Hard exclusive meson production and GPDs
4 Generalized Parton Distributions (GPDs) H
H conserve nucleon helicity E
E flip nucleon helicity
e
p, ?, ?
t

e
gL

(Q2)
x?
x-?
Vector mesons (r, w, f)
Pseudoscalar mesons (p, h)
Factorization proven only for longitudinally
polarized virtual photons and valid at high Q2
and small t
p0 2DuDd
h 2Du-Dd
?0 2ud
? 2u-d
? u-d
quark flavor decomposition accessible via meson
production
17
Deeply virtual p0 electroproduction
  • Q2 - (e-e)2
  • xB Q2/2Mn nEe-Ee
  • t (p-p)2
  • f (angle between leptonic and
  • hadronic plane)

reduced cross section for gp?pp0
18
Channel selection
TOF
ep?epp0 (p0?gg) in CLAS
CC
DC
p0
All 4 particles are detected
EC
  • electron ID
  • EC, CC, DC and TOF
  • proton ID
  • DC and TOF
  • photon ID
  • IC or EC

IC
19
Exclusivity cuts
MM2(epp0)
IM(gg)
MM2(ep)
MM(ep0)
p0
w
p0
h
p
M2(GeV2/c4)
M(GeV/c2)
M(GeV/c2)
M2(GeV2/c4)
  • 3s cuts (for each 4-dim bin) on
  • gg invariant mass
  • ep missing mass squared
  • ep0 missing mass
  • epp0 missing mass squared

20
Exclusivity cuts
MM2(epp0)
IM(gg)
MM2(ep)
MM(ep0)
p0
w
p0
h
p
M2(GeV2/c4)
M(GeV/c2)
M(GeV/c2)
M2(GeV2/c4)
p0
p
p0
21
Background subtraction
Q2 1.75 GeV2/c2 xB 0.175 -t 0.175 GeV2/c2
Yield vs f
f ()
Black events within 3s from the mean Red
events from side bands (each 3s wide)
22
Kinematical coverage and binning
  • W gt 2 GeV/c2
  • 7 Q2 bins (1 - 4.6 GeV2/c2)
  • 7 xB bins (0.1 0.55)
  • 8 -t bins (0.09 2 GeV2/c2)
  • 20 f bins

23
Acceptance calculation comparison data-MC
DATA
MC
24
Acceptance calculation example
25
Reduced cross sections vs f
0.09lt-tlt0.15 (GeV/c)2
Preliminary
gp?pp0 d2s/dtdf
Arbitrary units Statistical errors only
26
Reduced cross sections vs f
0.15lt-tlt0.2 (GeV/c)2
Preliminary
gp?pp0 d2s/dtdf
Arbitrary units Statistical errors only
27
Reduced cross sections vs f
0.2lt-tlt0.3 (GeV/c)2
Preliminary
gp?pp0 d2s/dtdf
Arbitrary units Statistical errors only
28
Reduced cross sections vs f
0.3lt-tlt0.4 (GeV/c)2
Preliminary
gp?pp0 d2s/dtdf
Arbitrary units Statistical errors only
29
Reduced cross sections vs f
0.4lt-tlt0.6 (GeV/c)2
Preliminary
gp?pp0 d2s/dtdf
Arbitrary units Statistical errors only
30
Reduced cross sections vs f
0.6lt-tlt1.0 (GeV/c)2
Preliminary
gp?pp0 d2s/dtdf
Arbitrary units Statistical errors only
31
Reduced cross sections vs f
1.0lt-tlt1.5 (GeV/c)2
Preliminary
gp?pp0 d2s/dtdf
Arbitrary units Statistical errors only
32
Reduced cross sections vs f
1.5lt-tlt2.0 (GeV/c)2
Preliminary
gp?pp0 d2s/dtdf
Arbitrary units Statistical errors only
33
Reduced cross sections fits
NO sT/sL separation
34
c2/NDF
Preliminary
35
sTesL vs. t (GeV2/c2)
Preliminary
Arbitrary units Statistical errors only
36
sLT vs. t (GeV2/c2)
Preliminary
Arbitrary units Statistical errors only
37
sTT vs. t (GeV2/c2)
Preliminary
Arbitrary units Statistical errors only
38
Arbitrary units Statistical errors only
sTesL sLT sTT
Preliminary
Interference terms different from
zero transverse and longitudinal contributions
Q2 2.25 (GeV/c)2 , xB 0.34
39
sTesL vs. t (GeV2/c2)
Preliminary
40
t-slopes
b constant as a function of Q2
b constant as a function of Q2
Preliminary
b decreasing as a function of xb
41
t-slopes
Guidal, Polyakov, Radyushkin, Vanderhaeghen
(2005)
b decreasing as a function of xb
Valence (high x) quarks at the center (small b)
Sea (small x) quarks at the perifery (high b) IF
GPD formalism applies!!
42
Model predictions
J. M. Laget, Regge-inspired model
p(g,p0)p
sTesL
sLT
ds/dt (mb GeV2/c2)
  • Dashed line w, r, b1 exchange (a) and
  • elastic rescattering (b)
  • Full line (a), (b), and p, D, N intermediate
    states

Q22.3 (GeV/c)2 W2.269 GeV/c2 xb0.36
sTT
43
Model predictions
J. M. Laget, Regge-inspired model
p(g,p0)p
sTesL sLT sTT
sTesL
sLT
ds/dt (mb GeV2/c2)
Q22.3 (GeV/c)2 W2.269 GeV/c2 xb0.36
sTT
Fairly good description of the data
44
Beam-spin asymmetry
BSA fit asinf
Rita De Masis Bo Zhaos analysis
45
Beam-spin asymmetry
a vs. t
a 0.08
Rita De Masis Bo Zhaos analysis
46
Where we stand now
  • DVCS
  • The e1-dvcs experiment at CLAS allowed for the
    first time to extract BSA and polarized and
    unpolarized cross sections (still preliminary)
    for DVCS over the widest phase space ever covered
    ? strong constraint for GPD models
  • ep ?epp0
  • measured for the first time at Wgt2 and over a
    wide kinematical range with CLAS, preliminary
    gp?p0p differential cross sections extracted
  • sL/sT separation not possible (accessible via
    Rosenbluth method with 12 GeV upgrade of CEBAF)
  • Transverse contribution not negligible (from
    cross sections and BSA)
  • Laget model predictions give good qualitative
    description of the data

47
DVCS asymmetries and GPDs
x xB/(2-xB) k-t/4M2
Polarized beam, unpolarized target


DsLU sinf ImF1H x(F1F2)H kF2Edf
H, H, E
Kinematically suppressed at low t
Unpolarized beam, longitudinal target


H, H
DsUL sinfImF1Hx(F1F2)(H df
Unpolarized beam, transverse target
H, E
DsUT sinfImk(F2H F1E) .. df
Beam charge asymmetry


H, H, E
DsC cosfReF1Hx(F1F2)H-kF2Edf
48
DVCS with longitudinally polarized target
GPDs separation a measurement of
DVCS with polarized target is also necessary
Access to different combinations of GPDs
49
DVCS with longitudinally polarized target
GPDs separation a measurement of
DVCS with polarized target is also necessary
Data taking scheduled for the second half of 2008
  • Dynamically polarized NH3
  • 5 Tesla magnetic field
  • dB/B 10-4
  • NH3 polarization75

Photon detection in IC and EC (view
from target)
Target magnet acts as Moeller shield
50
Moreover
  • More DVMP cross sections (and asymmetries)
    analyses are underway using e1-dvcs data, aiming
    to flavor decomposition of GPDs eph, epr (see
    Michels talk)
  • DVCS asymmetry under study, to study N?D
    transition GPDs
  • Next year at CLAS, second part of e1-dvcs, with
    2 times more data
  • CLAS_at_12 GeV will explore DVCS and DVMP for Q2 up
    to 9 GeV2 and xB up to 0.7

Neutron detector in the central part of
CLAS12 a new project (just starting!!) aimed to
measure nDVCS exclusively European
collaboration France (IPNLPSC) Italy
(INFNGEINFNFR) Korea
51
DVCS cross section binning
0.09lt-tlt0.2 GeV2
0.2lt-tlt0.4 GeV2
0.4lt-tlt0.6 GeV2
0.6lt-tlt1 GeV2
1lt-tlt1.5 GeV2
1.5lt-tlt2 GeV2
Ph.D. Thesis of H.S. Jo
52
Sum Rules and GPD limits
Forward limit (t ?0, x?0)
Hq(x,x0,t0) q(x)
Hg(x,x0,t0) xG(x)
Sum rules
dx Eq(x,x,t) Fq2 (t)
dx Hq(x,x,t) Fq1 (t)


dx Hq(x,x,t) gqA (t)
dx Eq(x,x,t) gqP (t)
Ji sum rule
30(DIS)
1
( H(x,x,t0) E(x,x,t0) ) x dx Jquark
1/2 DS D Lz
-1
53
How to access GPDs
  • Exclusive meson electroproduction
  • Vector mesons (?0)
  • Pseudoscalar mesons (?)
  • Deeply Virtual Compton Scattering
  • Beam charge asymmetry
  • Beam spin asymmetry
  • Longitudinal target spin asymmetry

54
Deeply Virtual Compton Scattering and GPDs
e
  • Q2 - (e-e)2
  • xB Q2/2Mn nEe-Ee
  • x?, x-? longitudinal momentum fractions
  • t (p-p)2
  • x ? xB/(2-xB)

t
g
p
p
GPDs Correlation between quark distributions
both in momentum and coordinate space
 Handbag  factorization valid in the Bjorken
regime high Q2 , ? (fixed xB)
Quark angular momentum (Jis sum rule)
X. Ji, Phy.Rev.Lett.78,610(1997)
55
Accessing GPDs through DVCS
?s s - s - ? I(DVCSBH)
56
DVCS observables and GPDs
Vanderhaegen, Guidal, Guichon
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