Positron Beam for Measurements of Generalized Parton Distributions Generalized Parton Distributions e + @ JLab (i) Generalized parton distributions (ii)

Positron Beam for Measurements of

Generalized Parton Distributions

e+ @ JLab

(i) Generalized parton distributions(ii) Polarized positron production(iii) Polarization transfer physics(iv) PEPPo(v) Perspectives(vi) Conclusions

Institut de Physique NucléaireOrsay, France

Eric Voutier

Trento, June 8-12, 2015New Directions in Nuclear Deep Inelastic Scattering

?

Generalized Parton Distributions

• GPD concept• Electro-production of photons

• Experimental observables

Trento, June 8-12, 2015

GPD concept

GPDs are the appropriate framework to deal with the partonic structure of hadrons and offer the unprecedented possibility to access the spatial

distribution of partons.

Parton Imaging

M. Burkardt, PRD 62 (2000) 071503 M.Diehl, EPJC 25 (2002) 223

GPDs can be interpreted as a 1/Q resolution distribution in the

transverse plane of partons with longitudinal momentum x.

GPDs = GPDs(Q2,x,x,t) whose perpendicular component of the momentum transfer to the nucleon is Fourier conjugate to the transverse position of partons.

GPDs encode the correlations between partons and contain information about the dynamics of the system like the angular momentum or the distribution of the strong forces experienced by quarks and gluons inside hadrons.

X. Ji, PRL 78 (1997) 610 M. Polyakov, PL B555 (2003) 57

A new light on hadron structure

Eric Voutier

Trento, June 8-12, 2015 3/31

x

)0,,( xH

GPDs enter the cross section of hard scattering processes via an integral over the intermediate quarks longitudinal momenta

t

GPDs

x x

Deep Exclusive Scattering

GPDs can be accessed via exclusive reactions in the Bjorken kinematic regime

Q2 >> M2 -t << Q2

GPD concept Eric Voutier

),,(),,(),,( 1

1

1

1txi

x

txdx

ix

txdx

GPD

GPDGPDP

Trento, June 8-12, 2015 4/31

Photon Electroproduction

The Bethe-Heitler (BH) process where the real photon is emitted either by the incoming or outgoing electron interferes with DVCS.

DVCS & BH are indistinguishable but the BH amplitude is exactly calculable and known at low t.

The relative importance of each process is beam energy and kinematics dependent.

Electro-production of photons

leptonic plane

e-’ j

p

e- g*

hadronic plane

g

Out-of-plane angle entering the harmonic development of the reaction amplitudeA.V. Belitsky, D. Müller, A. Kirchner, NPB 629 (2002) 323

Polarization observables help to single-out the DVCS amplitude.

P0 = 6 GeV/c

γpepe

Eric Voutier

Trento, June 8-12, 2015 5/31

N(e,e′gN) Differential Cross SectionUnpolarized Target

M. Diehl at the CLAS12 European Workshop, Genova, February 25-28, 2009

INTlINTlDVCSlDVCSBHeP0 σ~Pσeσ~Pσσσ

Even in j Odd in j

γNNγσ INT Ae γNNγσ~INT Am

EHHF )(4

~)()()()( 22211 tF

M

ttFtFtFC I

Polarized electrons and positrons allow to separate the 4 unknown components of the g electro-production cross section.

INTDVCSBH00 σσσσ

INTDVCS00 σ~2σ~2σσ

INT0000 σ2σσ

INT00000000 σ~4σσσσσσσσ

Electron observables

Electron & positron observables

Eric Voutier



6/31

N(e,e′gN) Differential Cross SectionPolarized Target

M. Diehl at the CLAS12 European Workshop, Genova, February 25-28, 2009

INTlINTlDVCSlDVCSBHleP0

ePS σPσ~eσPσ~σPSσσ

EEHHF ~)(224

)(11

)(2)(11

2

)(2)(1~

)()( 1 tFM

ttFtFtFtFtFtFC ILP

EHEHF ~)(2)(1241

2~

)(2)(11

2

)(1

2)(

2

2)(

11

2

)(241

1)(

2

1)( 1222

tFtFM

ttFtFtFtFtF tF

M

ttF

M

tC ITP

Polarized targets allow to access other GPD combinations

INTDVCS00 σ~2σ~2σσ

Additional observables Four new cross section components that may be separated from Rosenbluth-like experiments, or the combination of polarized electrons and positrons measurements at the same kinematics.

INTDVCSBH σ4σ4σ4σσσσ



7/31

Eric Voutier

Model Calculations

Experimental observables


Eric Voutier

8/31

Evaluation of cross section and asymmetries is performed considering GPD H and E only, within a dual parametrization.

The experimental configuration assume the detection of the full epg final state in CLAS12 with an 11 GeV electron/positron beam.

Statistics of projected data involve 1000 h data taking time at 1035 cm-2.s-1 for electrons, and 2×1034 cm-2.s-1 for positrons (8 nA, 10 cm LH2).

H. Avakian, V. Burkert, V. Guzey, JPos09 (2009)

sx×sy < 2×2 mm2 sE/E < 10-3 Polarized positrons

Cross Sections



Eric Voutier

9/31


Significant differences between polarized and unpolarized lepton beams supports again importance of a polarized positron beam.

Moments



Eric Voutier

10/31


The sin(f) moments linked to the imaginary part of the interference amplitude express the important benefit of a polarized positron beam for GPD physics at JLab.

Projected Data



Eric Voutier

11/31


High quality data with modest polarized positron beam current can be achieved, allowing to separate

the pure interference contribution to the photon electro-production process.

Polarized Positron Production

• Sokolov-Ternov self-polarization• Photon circular polarization transfer


Storage Ring Scheme

Polarized positrons have been obtained in high energy storage ring taking advantage of the Sokolov-Ternov effect which

leads to positrons polarized parallel to the magnetic field.

Sokolov-Ternov self-polarization

5

3

2

22e

γ

ρ

e

cm

35

8τ

Polarization builds up exponentially with a time constant characteristic of the energy and the curvature of the positrons

t ~ 20 mn @ HERA

Not compatible with CW beam delivery at JLab.

Eric Voutier

Trento, June 8-12, 2015 13/31

Fixed Target Schemes

The principle of polarization transfer from circular photons to longitudinal positrons has been demonstrated in the context of the ILC project.

T. Omori et al, PRL 96 (2006) 114801

G. Alexander et al, PRL 100 (2008) 210801

P(e+) = 73 ± 15 ± 19 %

Compton Backscattering Undulator

1.3 GeV

Require independent ~GeV to multi-GeV electron beam.

Photon circular polarization transfer Eric Voutier

Trento, June 8-12, 2015 14/31

E.G. Bessonov, A.A. Mikhailichenko, EPAC (1996) A.P. Potylitsin, NIM A398 (1997) 395

e- → g → e+

J. Grames, E. Voutier et al., JLab Experiment E12-11-105, 2011

Polarized Bremsstrahlung

Eric Voutier


Photon circular polarization transfer

15/31

Sustainable polarized electron intensities up to 4 mA have been demonstrated from a superlattice photocathode.

R. Suleiman et al., PAC’11, New York (NJ, USA), March 28 – April 1, 2011

The purpose of the PEPPo (Polarized Electrons for Polarized Positrons) experiment at the CEBAF injector was to demonstrate

feasibility of using bremsstrahlung radiation of polarized electrons

for the production of polarized positrons.

Polarization Transfer Physics

• Ultra-relativistic approximation • Finite lepton mass calculations


Ultra-relativistic approximation

H. Olsen, L. Maximon, PR114 (1959) 887 The most currently used framework to evaluate polarization transfer for polarized

bremsstrahlung and pair creation processes is the O&M work developped in the Born approximation for relativistic particles and small scattering angles.

Bremsstrahlung and Pair Creation

BREMSSTRAHLUNG PAIR CREATION

Eric Voutier

Trento, June 8-12, 2015 17/31

E.A. Kuraev, Y.M. Bystritskiy, M. Shatnev, E. Tomasi-Gustafsson, PRC 81 (2010) 055208

Bremsstrahlung and Pair Creation

Finite lepton mass calculations

BREMSSTRAHLUNG PAIR CREATION

These reference processes have been revisited considering the finite mass of the leptons, generating additional terms to the leptonic tensor.

Eric Voutier

Trento, June 8-12, 2015 18/31

PEPPo

• Proof-of-principle experiment• Commissionning

• Polarized positrons characterization


Pe-

e-

T1

Polarized Electrons(< 10 MeV/c) strike production target

BREMSSTRAHLUNG

Longitudinal e- (Pe-) produce elliptical g whose circular (Pg) component is proportional to

Pe-

S1

D

D

S2 Pe+

Positron Transverse and Momentum Phase Space Selection

e+

PAIR PRODUCTION

g produce e+e- pairs and transfer Pg into longitudinal (Pe+) and

transverse polarization averages to zero

PEPPo measured the longitudinal polarization transfer

in the 3.07-6.25 MeV/c momentum range.

COMPTON TRANSMISSION

Polarized e+ convert into polarized g (Pg) whose transmission

through a polarized iron target (PT) depends on Pg.PT

Principle of Operation

Ee = 6.3 MeVIe = 1 µAT1 = 1 mm W

Geant4

PEPPo

J. Dumas, PhD Thesis (2011)

T2

PT

Calorimeter

Compton Transmission Polarimeter

Proof-of-principle experiment Eric Voutier

Trento, June 8-12, 2015 20/31

Eric Voutier


Commissionning

21/31

A. AdeyemiTrento, June 8-12, 2015

Eric VoutierCommissionning

Electron Analyzing Power

A high quality measurement of the electron analyzing power has been achieved.

Experimental data are as expected selective with respect to simulations, allowing for the calibration of the polarimeter model.

22/31

Positron Analyzing Power

GEANT4 simulations allow to link the measured electron analyzing power to the expected positron analyzing power of the PEPPo Compton transmission polarimeter.

Eric Voutier

Trento, June 8-12, 2015 23/31

Polarized positron characterization

Significant non-zero experimental asymmetries increasing with positron momentum sign efficient polarization transfer from electrons to positrons.

Positron Polarization

Eric Voutier


Polarized positron characterization

24/31

PEPPo Preliminary

Positron polarization is deduced using measured electron beam and target polarizations, electron analyzing power, experimental

asymmetry, and estimated e+/e- analyzing power ratio (1.1-1.4).

P. Aderley1, A. Adeyemi4, P. Aguilera1, M. Ali1, H. Areti1, M. Baylac2, J. Benesch1, G. Bosson2, B. Cade1, A. Camsonne1, L. Cardman1, J. Clark1, P. Cole5, S.

Covert1, C. Cuevas1, O. Dadoun3, D. Dale5, J. Dumas1,2, E. Fanchini2, T. Forest5, E.

Forman1, A. Freyberger1, E. Froidefond2, S. Golge6, J. Grames1, P. Guèye4, J.

Hansknecht1, P. Harrell1, J. Hoskins8, C. Hyde7, R. Kazimi1, Y. Kim1,5, D. Machie1, K. Mahoney1,

R. Mammei1, M. Marton2, J. McCarter9, M. McCaughan1, M. McHugh10, D. McNulty5, T. Michaelides1, R. Michaels1, C. Muñoz Camacho11, J.-F. Muraz2, K.

Myers12, B. Opper10, M. Poelker1, J.-S. Réal2, L. Richardson1, S. Setiniyazi5, M.

Stutzman1, R. Suleiman1, C. Tennant1, C.-Y. Tsai13, D. Turner1, A. Variola3, E. Voutier2,11,

Y. Wang1, Y. Zhang12

1 Jefferson Lab, Newport News, VA, US 2 LPSC, Grenoble, France 3 LAL, Orsay, France4 Hampton University, Hampton, VA, USA 5 Idaho State University & IAC, Pocatello, ID, USA 6 North Carolina University, Durham, NC, USA 7 Old Dominion University, Norfolk, VA, US

8 The College of William & Mary, Williamsburg, VA, USA 9 University of Virginia, Charlottesville, VA, USA 10 George Washington University, Washington, DC, USA 11 IPN, Orsay, France

12 Rutgers University, Piscataway, NJ, USA 13 Virginia Tech, Blacksburg, VA, USA

PEPPo Collaboration

Many thanks for advice, equipment loan, GEANT4 modeling support, and funding to

SLAC E-166 CollaborationInternational Linear Collider Project

Jefferson Science Association Initiatives Award Trento, June 8-12, 2015 25/31

Perspectives

• Positron beam at CEBAF• Technological challenges


Positron beam at CEBAF

Positron Collection Concept

W target

Quarter Wave Transformer (QWT) Solenoid

Combined Function Magnet(QD)

Collimators

e-

e+

g

S. Golge, PhD Thesis, 2010 (ODU/JLab)

24 MeV

126 MeV

e-

A collection efficiency of 3х10-4 is predicted at the maximum positron production yield, corresponding to a positron energy of

24 MeV.

The resulting beam can then be accelerated without significant loss, and injected into the CEBAF main accelerator

section.

Eric Voutier

Trento, June 8-12, 2015 27/31


e+ Source Concept

S. Golge, PhD Thesis, 2010 (ODU/JLab)

A. Freyberger at the Town Hall Meeting, JLab, 2011

1mA

I = 300 nAPe+ > 50%dp/p = 10-2

ex = 1.6 mm.mradey = 1.7 mm.mrad

Eric Voutier

Trento, June 8-12, 2015 28/31


e+ Production Scenarii

CEBAF INJ (10-100 MeV) FEL (100 MeV) CEBAF (12 GeV)

Eric Voutier

Trento, June 8-12, 2015 29/31

Technological challenges

Towards a Polarized Positron Beam

High Intensity Polarized Electron SourcePolarized electron gun at JLab did demonstrate high intensity capabilities, though

the polarization at high current was not measured.

The path from a sketched concept to a full design is a correlated multi-parameter problem requiring to adress and solve

several technical/technological challenges.

High Power Production TargetHigh power absorbers (10-100 kW) are challenging for heat dissipation,

radiation management, and accelerator integration.

Optimized Positron CollectionProperties of usable beam for Physics (intensity, polarization, momentum

spread, emittance) are strongly related to the positron production and collection schemes and strategies (1 or 2 targets).

Positron Beam «Shaping»Pre-acceleration (Hadronic Physics) or deccelaration (Material Science) are

required to produce a beam suitable for Phyiscs.

Eric Voutier

Trento, June 8-12, 2015 30/31

Conclusions

Summary

The merits of polarized and/or unpolarized positron beams for the Physics program at JLab is comparable to the benefits of

polarized with respect to unpolarized electrons.

2 g physics, GPDs… NP @ low energy, Material Science @ very low energy…

The PEPPo experiment @ the CEBAF injector has been a first step in this process,

opening a world-wide access to polarized positrons.

An R&D effort is necessary toresolve the several technical and technological challenges

raised by the development of a (un)polarized positron beam for Physics.

31/31

Eric Voutier


Documents

Positron Beam for Measurements of Generalized Parton Distributions Generalized Parton Distributions e + @ JLab (i) Generalized parton distributions (ii)