hall b collaboration meeting

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


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


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


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


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


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


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


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.



3 level of physics cuts :

reconstructed q’ out-of-plane angle

missing mass



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


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.



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.



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.



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).



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



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


Coherent DVCS Assesment

Pion Contamination

Helium 3 Contamination

Tritium Contamination

Conclusion & Outlook


Summary

hall b collaboration meeting

Documents

incoherent dvcs

incoherent nuclear dvcs

dvcs amplitude

ep physics

clas e

partonic structure of

partonic structure of

spin asymmetries