Post on 14-Jan-2016
description
Deeply Virtual ω Production
Michel Garçon & Ludyvine Morand (SPhN-Saclay)
for the CLAS collaboration
MENU 2004, Beijing
e-
e-’
p p’
ω
*
Which picture prevails at high Q2 ?Which picture prevails at high Q2 ?
Meson and Pomeron (or two-gluon) exchange …
… or scattering at the quark level ?
π, f2, Pρ0 σ, f2, P
ω π, f2, P
Φ P
Flavor sensitivity of DVMP on the proton:
ω
ρ0 2u+d, 9g/4
ω 2u-d, 3g/4
Φ s, g
ρ+ u-d
γ*LωL
6
2
4
),(),,)((
1)(1
Q
tfdxdztxbEaH
ixz
z
QQdt
d MSL
(Photoproduction)
(SCHC)
CLAS: high luminosity run at 5.75 GeVCLAS: high luminosity run at 5.75 GeV
First JLab experiment with GPDs in mind
(october 2001 – january 2002)
- polarized electrons, E = 5.75 GeV
- Q2 up to 5.5 GeV2,
-Integrated luminosity: 30 fb-1
- W up to 2.8 GeV
W = 2.8
2
.4
2
1.8 G
eV
e- p e- p ωπ+ π- π0
Identification of channelIdentification of channel e- p e- p ωπ+ π- π0
m2
(2m)2
- 0
n +…
0
Separation of :
+ - 0 (M = 783 MeV, = 8 MeV)
0 + - (M = 770 MeV, = 151 MeV)
through missing mass
Determination of CLAS efficiency in 4DDetermination of CLAS efficiency in 4D
Analysis Code
CLAS GEANT simulation Event Generator
Ngen Nacc
For each 4D bin : eff = Nacc/Ngen
4 kinematical variables=> 4D efficiency table
Q2, xB, t,
eff ~ 5%
Q2 (
GeV
2 )
xB
28 millions events 20 days
through simulation
xB from 0.16 to 0.70
Q2 f
rom
1.6
to 5
.6 G
eV2
+ binning in or t
Background subtractionBackground subtraction
Extraction of σγ*p->pωExtraction of σγ*p->pω
*
0
( , )1 1 1( , )
2
2 2( , ) int
2 QnFp p radBR eff eff effCC EC wQ x L Q xV B B
BQxBx
*p
p
(b
)
Q2 (GeV2)
0.16 < xB < 0.22 0.22 < xB < 0.28 0.28 < xB < 0.34
0.58 < xB < 0.640.52 < xB < 0.580.46 < xB < 0.520.40 < xB < 0.46
0.34 < xB < 0.40
Q2 (GeV2)Q2 (GeV2)Q2 (GeV2)
Bin to bin syst. errors included = 15-18%
Main sources :• background subtraction = 8 or 12%• efficiency calculation = 8%
Comparison with previous dataComparison with previous data
Compatibility with DESY data (1977)
Disagreementwith Cornell data (1981)
Q2 range much extended with our data
(deg)
d/d
(b
/rd)
Differential cross sections in Differential cross sections in
* 1cos 2 2 1 cos
2p pd
T L T TT Ld
=> First indication of helicity non conservation in s-channel
TT e
t T
L (b
)
0.40 < xB < 0.46
0.22 < xB < 0.28 0.28 < xB < 0.34
0.46 < xB < 0.52 0.52 < xB < 0.58 0.58 < xB < 0.64
0.16 < xB < 0.22 0.34 < xB < 0.40
Q2 (GeV2) Q2 (GeV2) Q2 (GeV2) Q2 (GeV2)0, 0TT LT
Differential cross sections in tDifferential cross sections in t
Large t range, up to 2.7 GeV2.
Small t : diffractive regime (d/dt ebt)
Large t : cross sections larger than anticipated
Comparison with JML modelComparison with JML model
Model includes exchange of π0, f2, P
12( )2
12
F QQ
p p’
ω*
π0
Small |t|
* ω
p p’
Large |t|THIS WORK
12( , )2
12 ( )
F Q tQ
t
2
22
)(1
)0(1)(
t
t
(J.-M. Laget, to be published in Phys. Rev. D)
Second part of data analysis: study of ω decaySecond part of data analysis: study of ω decay
• ω channel identification
• Determination of CLAS efficiency in 6D
• Study of ω decay
Angular distribution of ω decay products
(study of e-pπ+π-X final state)
Determination of ω polarization Test of s-channel helicity conservation (SCHC)
6D efficiency table
Q2, xB, t, , θN, φN
eff ~ 0,5%
0
Study of ω decay (2)Study of ω decay (2)
041 1
1( ) 1 2 cos 2
2rNW N
3 1 1 2(cos ) 1 3 1 cos
4 2 204 0400 00N NrW r
3 1 1 2(cos , , ) 1 3 1 cos4 2 2
22 Re sin 2 cos sin cos 2
2 2 2cos 2
04 0400 00
04 0410 1 1
1 1 1 111 00 10 1 1
2 210
sin cos 2 Re sin 2 cos sin sin 2
2sin 2 2 Im sin 2 sin Im sin sin 2
2 (1
1 1
N N N
N N N N
N
r r
r r
r r r rN N N N N
N N N Nr
W
r
2 2 2) cos sin cos 2 Re sin 2 cos sin cos 2
22 (1 )
5 5 5 511 00 10 1 1
6 6sin 2 Im sin 2 sin Im sin sin 210 1 1
N N N N N N
N N N
r r
N
r r
r r
N
cosN
ω decay study : φN dependenceω decay study : φN dependence
41 00
1r => other indication of non conservation of helicity in the s-channel
Determination of 041 1r
Contribution of L from VGG (GPDs) and JML (Regge) models
Contribution of L from VGG (GPDs) and JML (Regge) models
VGG model:M. Vanderhaeghen, P. Guichon, M. Guidal
<W> = 2.1 GeV
<W> = 2.8 GeV
L
T + L
Conclusions and perspectivesConclusions and perspectives
• Precise measurements of the e-p e-p reaction at high Q2
and over a wide range in t• Exclusive reactions are measurable at high Q2 (even with the current « modest » CEBAF energy) • First significant analysis of decay in electroproduction
• Cross sections larger than anticipated at high t• SCHC does not hold for this channel• Evidence for unnatural parity exchange
0 exchange very probable even at high Q2
What do we learn ?What do we learn ?
• JML model (Regge) : Good agreement when introducing a t dependence in the form factor suggests coupling to a « point » object at high t (hep-ph/0406153)
• VGG model (GPDs) :Direct comparison not possible since L not measuredNevertheless handbag diagram contributes only about 1/5 of measured cross sections no incompatibility→ ω most challenging/difficult channel to access GPD
• Explore physical significance of high t behaviour • Systematic study of other processes (e-p e-p 0, , ) in view of an interpretation in terms of GPDs• Measurements feasible up to Q2=8-9 GeV2 with CEBAF@12 GeV
PerspectivesPerspectives
Comparison to modelsComparison to models
linea
r
saturation
Regge theory applied to vector meson productionRegge theory applied to vector meson production
α(t)
t
(Jean-Marc Laget et al., see also talk of Marco Battaglieri)
Regge trajectories exchange in t channel
0
parameters
=
coupling constants g ij
12( )2 21 /
F QQ
Extension to electroproduction case
*
add a parameter = electromagnetic form factor
2( )F Q0 exchange dominance
at W ~ few GeVin ω photoproduction
FACTORIZATION
GPD formalism for vector mesonsGPD formalism for vector mesons
In the Bjorken regime :
Collins et al.
*L LM
t small
1 11 1 1( , , )
0 1M
L H Ez
A dz dx a b x tz Q ix xi
Amplitude:
e-
p
p p+
q q-
x+x-
e-
p
M
*
L
LM
H, E
z
t=2
2 1*6L L L Lp pM
QA
Cross section:
Scaling law
= 0, , (see talk by Michel Guidal)
Kinematical domain explored
Reson
ance
sW
=1,
8
W=M Inelastic
x B = 0,1
x B = 0,8
valen
ce quarks
ν (GeV)0,1 1 10 100
Q2 (
GeV
2 )
0,1
1
10M 1,8
2
2
QxB M
,q
xB
Elastic pQCD
Regge
gluons sea quarks
Elastic
Resonances
Deepinelastic
W (GeV)
2 2 2 2( ) 2W P q M M Q
M 1,8
Electron identification
e-
10 RL
6 RLOUT
IN
Information from DC + CC + EC
e--
Pion rejection
Electron selection
Hadron identification
Information from DC + SC
DC 22
)(Mp
pM
SC DCSC
SC
l
c t
0)( MSC M
+
p
Identification of channel e- p e- p ωπ+ π- π0through missing mass
0
m2
0
Background subtractionBackground subtraction
Event weighting : w=1/eff(Q2,xB,t,)
y(m) = Skewed Gaussian + Pol2
(peak + radiative tail) (background)
Fit of MX[e-pX] with :
Subtraction of y(m).
Integration of event number between 720 and 850 MeV.
Determination of CLAS efficiency in 6D
6 kinematical variables=> 6D efficiency table Q2, xB, t, , θN, φN
118 millions events 63 days
through simulation
For each 6D bin : eff = Nacc/Ngen eff ~ 0,5%
Analysis Code
GPPGSIM Event Generator
Ngen Nacc
Phase space (1)
Phase (2)
Experimental Q2, xB, t, , … distributions
e-p+-X
e-p+X
CLAS system efficiency
4D
6D
Formalism for ω decayFormalism for ω decay
Decay angular distribution :
elements of ω spin density matrix ε parameter of virtual photon polarization R = σL/σT (Rp : = T + L)
αρij
Deduce :
• ω polarization via
• SCHC test : (necessary condition)
• If SCHC, then separation σL, σT
• Test of hypothesis of natural parity exchange in the t-channel (NPE) :(necessary condition)04 04 1 1
00 1 11 2 2 1 12 011r r r r
βrijαρij
04 0400 001R r r
41 00
1r
0-, 1+UNPE0+,1- NPE
p p’
ω*
cos , , ; rijN NW
, ,f Ririj j
i, j meson helicity
virtual photon polarization
Study of ω decay (2)Study of ω decay (2)
Determination of 0400r
Bx
ω decay study (4)ω decay study (4)
2cos
sin 2
5 1
2 25
Re
0400
0 co2
410 s
4
r
r
N
N
…
04 04 1 100 1 11 2 2 1 12 011r r r r
exchange of unnatural parity
particle in the t-channel
Method of moments :2
Determination of 15 rij
parameters which should vanish if SCHC holds
combinaison which should vanish if NPE holds
decay study (cont.)
3 1 1 2(cos , , ) 1 3 1 cos4 2 2
22 Re sin 2 cos sin cos 2
2 2 2cos 2
04 0400 00
04 0410 1 1
1 1 1 111 00 10 1 1
2 210
sin cos 2 Re sin 2 cos sin sin 2
2sin 2 2 Im sin 2 sin Im sin sin 2
2 (1
1 1
N N N
N N N N
N
r r
r r
r r r rN N N N N
N N N Nr
W
r
2 2 2) cos sin cos 2 Re sin 2 cos sin cos 2
22 (1 )
5 5 5 511 00 10 1 1
6 6sin 2 Im sin 2 sin Im sin sin 210 1 1
N N N N N N
N N N
r r
N
r r
r r
cosN N
TT TL
1 1 5 511 00
3( ) cos 2 c 114 0os 0r r r rW
But :
' '
' '
1 * *1 1 11 11 1 111
, ,
1 * *0 1 01 01 0 100
, ,
111
100
N N N N
N N N N
r
r
T T T T
T T T T
' '
' '
5 * * * *10 11 11 10 10 1 1 1 1 1011
, ,
5 * * * *00 01 01 00
51
00 0 1 0 1 00
1
500 00
, ,
N N N N
N N N N
r
r
T T T T T T T T
T T T T T T T T
and
If SCHC then : 1 1 5 511 00 00 011r r r r and 0TTT L
R extraction
'
2 2001 0 100
,N N
T T
0 if SCHC
'
240000
,N N
T
1 if SCHC
0040
400 00
10rR
R
10400 R
rR
SCHC
Theory :
Observation :
'
1 *01 0 100
,
10 00
N N
T Tr
' '
5 * *01 00 0 1 0000
, ,
50 00
N N N N
T T Tr T
So one can’t extract R.
Same helicity amplitudesthan the ones that come into play in 0
00
04
1
1
000400
Rr
r
R extraction (cont.)
041
1
000400
Rr
r
Suppose SCHC holds, then :
One has found : 0400 0,44r
Then :1 0,44
1,460,54 1 0,44
L
TR
But : T L
So : 0,561
0,821
T
L
RR
R
0 meson production and JML model
Comparison with JML model (cont.)<W> = 2.1 GeV
<W> = 2.8 GeV
Within this model, Data still favor a t-dependent πγω form factor. π0 exchange dominant for the whole Q2 range. σT dominant for the whole Q2 range.
Form Factors => transverse position
Q2 = -t
Exclusive elastic reactionse-
e-
p
2Q
p
MotivationsMotivationsHigh Q2 reactions give access to internal dynamics of the nucleon
Deep inclusive reactions
Parton Distributions
longitudinal momentum fraction quarks contribution to nucleon spin
x = xB
e-
e-
px
2Q
X
Deep exclusive reactions
Generalized Parton Distributions
correlations ! quarkangular momentum
e-
e-
p p
ou M
x
2Q
x
t
x, t, ξ
e-
e-
*,q
Link with FF and PD
0 access to correlations ,
x, dependence quark longitudinal impulsion
t dependence quark transverse impulsion
Ji sum rule :
GPD formalism (cont.)
g
gq
J
LG
J
L
q
21
21
qqqq
VGG (GPDs) model
2 211
2 11 1
(1 )( , ) ( ) ( ) ( , )
(1 )
b
bqDD x d d x C q D x
2, / 2, , a Q tH E DD x e
x,t, dependence parametrization:
2 parameters : bvalence et bsea
Corrections to leading order:
In fact Q2 non infinite
1 12
2
x i k perpx i
Q
In pratice :non perturbative effects at small Q2 averagedby « frozing » the strong coupling constantS to 0.56
Contribution of L from VGG (GPDs) model (cont.)
* p
p
b
)
Q2 (GeV2) Q2 (GeV2)
Q2 (GeV2) Q2 (GeV2)
xxBB=0.31=0.31 xxBB=0.38=0.38
xxBB=0.45=0.45 xxBB=0.52=0.52
0 meson results at 4.2 GeV (Regge)
C. Hadjidakis et al.
0 meson results at 4.2 GeV (GPDs)xxBB=0.31=0.31 xxBB=0.38=0.38
xxBB=0.45=0.45 xxBB=0.52=0.52Q2 (GeV2) Q2 (GeV2)
Q2 (GeV2) Q2 (GeV2)
* Lp
p L
b)
C. Hadjidakis et al.
G parity definition
yi IG Ce