Hall B Collaboration Meeting
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Hall B Collaboration Meeting Newport News, February 22-25, 2012
DDeeply eeply VVirtual irtual CCompton ompton SScatteringcattering
off off 44HeHe
eg6 @ CLAS.JLabeg6 @ CLAS.JLab
(i) Physics motivations(ii) Experimental setup(iii) 4He(e,e’p)3H cross check (iv) 4He(e,e’4He) analysis(v) Physics background channels(vi) Conclusion
Laboratoire de Physique Subatomique et de CosmologieGrenoble, France
Yohann Perrin, Eric Voutier
?eg6 group (E07-009, E08-024)
Physics Motivations
Newport News, February 22-25, 2012
Coherent Nuclear DVCS
Nuclear DVCS probes the partonic structure of nuclei and offers the opportunity to investigate the role of the transverse degrees of freedom in the modifications of the nuclear parton distributions, as compared to free nucleons.
S. Scopetta, PRC 70 (2004) 015204 ; 79 (2009) 025207 S. Liuti, K.Taneja, PRC 72 (2005) 032201 ; 034902
.J. Gomez et al., PRD 49 (1994) 4348
RA
x
Nuclear modifications(t=-0.1 GeV2)
Binding (t=-0.1 GeV2)
RA(t=0)
txH
tF
tF
txH
txA
txAtxR
N
N
A
ApLU
ALU
A ,,
,,
,,
,,,,
Generalized EMC Ratio
The ratio of beam-spin asymmetries on the nucleus and on the nucleon is predicted to be more sensitive to
peculiar features of the EMC effect modeling.
S. Liuti, K.Taneja, PRC 72 (2005) 032201 ; 034902V. Guzey, PRC 78 (2008) 025211 V. Guzey, A.W. Thomas, K. Tsushima, PLB 673 (2009) 9 ; PRC 79 (2009) 055205
Physics Motivations
Newport News, February 22-25, 2012
Incoherent Nuclear DVCS
-t = 0.000 GeV2
-t = 0.100 GeV2
-t = 0.207 GeV2
4He(e,e’He)
-t = 0.000 GeV2
-t = 0.095 GeV2
-t = 0.329 GeV2
4He(e,e’p)3H
The role of the transverse degrees of freedom in the EMC effect are pedicted to be more prominent in incoherent DVCS than
in coherent DVCS.
Similarly to conventional (e,e’p) physics, the concept of bound nucleon GPDs can be adressed via incoherent DVCS.
Physics Motivations
Newport News, February 22-25, 2012
Spinless Nucleus
A. Belitsky, D. Müller, A. Kirchner, A. Schäfer, PR D64 (2001) 116002 V. Guzey, M. Strikman, PRC 68 (2003) 015204
At leading order of the coupling constant, the partonic structure of spin 0 nuclei is characterized by one twist-2 (HA) and two twist-3 quark GPDs (+ gluon ones).
23
232
21
0HeLU
)()()()()()(
)()()(A
4
AAAAA
AA
tFtF
tF
HHH
H
mee
m
Because of the simple GPD structure of spin 0 nuclei, the twist-2 beam spin asymmetry (BSA) allows for a model-independent simultaneous extraction of the real and the imaginary parts of the twist-2 Compton form factor.
tdtMQtxHxdx fA
AffA
2/2
1
1
2
5
4,,,
Momentum fraction of the target carried by the quark
Nuclear D-term accessible via the real part of the DVCS
amplitude.M. Polyakov, PLB 555 (2003) 57
Spatial Distribution of Strong Forces
Experimental Setup
Newport News, February 22-25, 2012
E07-009 & E08-024
The standard CLAS detector package is supplemented with a radial time projection chamber (RTPC), a forward electromagnetic calorimeter (IC), and a solenoidal magnet for Møller shielding.
Solenoidal Magnet
Inner Calorimeter
Radial Time Projection Chamber
Experimental Setup
Newport News, February 22-25, 2012
CLAS (e,p)
S. Dhamija, L. El Fassi, R. Paremuzyan, C. Salgado, C. Smith, B. Toryaev, M. Ungaro
The CLAS is used in a conventional way for electron
and proton detection and identification.
Experimental Setup
Newport News, February 22-25, 2012
IC ()
F.-X. Girod
The invariant mass of 2-events in IC is used for the calorimeter calibration, taking into account the interaction vertex as given by CLAS.
The final resolution is 7.1% at the pion mass.
Experimental Setup
Newport News, February 22-25, 2012
RTPC (4He)
N. Baltzell, R. Dupré, R. Paremuzyan, Y. Perrin
Ad hoc cuts for the identification of particles are defined for 24 (,) sectors with respect to the (adc,pRTPC) topology determined with elastic scattering data.
(Mev/c)pTPC
dE/d
x
dcx
baf
Elastic Data
4He(e,’e’p)3H Cross Check
Newport News, February 22-25, 2012
Basic Event Selection
Electron : 1 and only 1 negative track in CLAS + (EC+CC ) pion rejection cut + target windows
rejection
Proton : 1 and only 1 positive track in CLAS
Photon : 1 and only 1 cluster in IC + no neutral particle in EC
Physics cuts are developped with the guidance of the PWIA picture, assuming that the interaction takes place on a quasi-free on-shell
nucleon, leading to kinematical and angular correlations between the reaction products.
A preliminar analysis of the incoherent channel was attempted to check detector calibration, data quality, and analysis algorithm.
4He(e,’e’p)3H Cross Check
Newport News, February 22-25, 2012
3 level of physics cuts :
reconstructed q’ out-of-plane angle
missing mass
4He(e,’e’p)3H Cross Check
Newport News, February 22-25, 2012
The physics analysis will be conducted by N. Baltzell, C. Moody, O. Nusair (ANL)
Helicity Signal
A strong helicity signal is observed for incoherent DVCS candidate events which phase covers a large kinematical domain, similar to the CLAS p-DVCS experiment.
4He(e,’e’ 4He) Analysis
Newport News, February 22-25, 2012
Basic Event Selection
Electron : 1 and only 1 negative track in CLAS + (EC+CC ) pion rejection cut + target windows rejection
Photon : 1 and only 1 cluster in IC + no neutral particle in EC
Helium : 1 and only 1 good positive track in the RTPC + good stoping power
The correlation between the z-vertex locations as reconstructed by CLAS and the RTPC provides an additionnal basic mask to experimental data.
4He(e,’e’ 4He) Analysis
Newport News, February 22-25, 2012
Physics Event Selection
Kinematics of the coherent DVCS reaction allows to reconstruct the expected photon energy from the electron kinematics and the angle of the detected photon.
)cos(θqMω
qMω
2
1q'
γ
22
)cos(θqMω
)cos(θq2ωqωMt
γ
γ22
The out-of-plane angles between the leptonic and hadronic planes as reconstructed from the real photon (CLAS+IC) and the recoil helium (CLAS+RTPC) should be identical.
The momentum of the missing particle of the final state (e4HeX) should be nul.
4He(e,’e’ 4He) Analysis
Newport News, February 22-25, 2012
4He(e,’e’ 4He) Analysis
Newport News, February 22-25, 2012
The missing mass spectra of final state (e4He) is consistent with a coherent DVCS reaction.
The final t-distribution of selected events behaves as may be expected from the fast decrease of helium elastic form factor.
4He(e,’e’ 4He) Analysis
Newport News, February 22-25, 2012
Helicity Signal
A clear helicity signal is observed for coherent DVCS candidate events, and confirmed by the out-of-plane angle distribution of events integrated over (t, Q2, xB).
4He(e,’e’ 4He) Analysis
Newport News, February 22-25, 2012
DVCS Phase Space
Q2 ≥ 1 GeV2
xB ≤ 0.3
-t ≤ 1 GeV2
The candidate experimental phase space for coherent DVCS is limited and statistics does not allow for a multi-dimensional analysis as was performed in the CLAS p-DVCS experiment.
222B
2 GeV2.4QGeV10.3x0.1GeV0.2t
4He(e,’e’ 4He) Analysis
Newport News, February 22-25, 2012
0 < -t < 0.15
0.3 < -t < 0.5 0.5 < -t < 1.0
0.15 < -t < 0.3
DVCS Asymmetries
Large t data are
irrelevant
Physics Background Channels
Newport News, February 22-25, 2012
Coherent DVCS Assesment
Pion Contamination
Helium 3 Contamination
Tritium Contamination
Conclusion & Outlook
Newport News, February 22-25, 2012
Summary
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