(i) Physics motivations (ii) The PEPPo experiment (iii) PEPPo measurements (iv) Outlook Eric Voutier Institut de Physique Nucléaire d’Orsay Orsay, France

PEPPoWorld Wide Access to Polarized

Positrons

PEPPo @ CEBAF-Injector.JLab

(i) Physics motivations(ii) The PEPPo experiment(iii) PEPPo measurements(iv) Outlook

Eric Voutier Institut de Physique Nucléaire d’Orsay

Orsay, France

Paphos, November 2rd-7th, 2015XIth Conference on Electromagnetic Interactions with Nucleons and Nuclei

Physics Motivations

• Electromagnetic form factors (U,P)• Inclusive structure functions (U,P)

• Generalized parton distributions (U,P)• Search for the U-boson coupling to dark matter (U,P)

• Charge conjugation violation to access C3q (P)

• International Linear Collider• Positron-ion and circular Colliders (LHeC, EIC, MEIC)

• Thermal positron facility

Paphos, November 2nd-7th, 2015

Two Photons Physics

Following the very first measurements of polarization transfer observables in electron elastic scattering, the validity of the 1g exchange approximation of the electromagnetic interaction has

been questioned.

P.A.M. Guichon, M. Vanderhaeghen, PRL 91 (2003) 142303

3/17

Electromagnetic form factors

ε),Q(F~δeF~ε),Q(G~δe)Q(GG~ε),Q(G~δe)Q(GG~

23l3

2El

2EE

2Ml

2MM

Within the 2g exchange hypothesis, the electromagnetic structure of the nucleon may be parametrized by 3 generalized form factors, corresponding to 8 unknow quantities.

Eric Voutier


Electromagnetic form factors

Experimental ObservablesM.P. Rekalo, E. Tomasi Gustafsson, NPA 742 (2004) 322 C. Carlson, M. Vanderhaeghen, ARNPS 57 (2007)

171

313022 ~,~2~,~2 FGfGFGfGGG EEMMEMR

31~,~~12 FGfGGGGGP EMMEMEtR

3222 ~,~21 FGfGGP MMMlR

Combining polarized electrons and positrons allows a model independent separation of the electromagnetic form factors of the nucleon.

Unpolarized e± elastic scattering and polarization transfert observables off the nucleon involve up to 5 unknown quantities.

5 unknown contributions for 6 independent observables

Cross Section

PolarizationTransfert

4/17

Eric Voutier


N(e,e′gN) Differential Cross SectionM. Diehl at the CLAS12 European Workshop, Genova, February 25-28, 2009

INTlINTlDVCSlDVCSBHeP0 σ~Pσeσ~Pσσσ

Polarized electrons and positrons allow to separate the unknown amplitudes of the cross section for electro-production of

photons.

INTDVCSBH00 σσσσ

INTDVCS00 σ~2σ~2σσ

INT0000 σ2σσ

INT00000000 σ~4σσσσσσσσ

Electron observables

Electron & positron observables

Generalized parton distributions

INTlINTlDVCSlDVCSBHleP0

ePS σPσ~eσPσ~σPSσσ

INTDVCS00 σ~2σ~2σσ

Additional observables

INTDVCSBH σ4σ4σ4σσσσ

Eric Voutier

5/17Paphos, November 2nd-7th, 2015

PEPPo Experiment

• Polarized positron production • Proof-of-principle experiment

• Electron & positron polarimetry


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 VoutierPolarized positron production

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. 7/17Paphos, November 2nd-7th, 2015

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 from 8.25 MeV/c e-

to e+ 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


Compton Transmission Polarimeter

FADC 250

Hamamatsu R6236-100

CsI crystals a (6×6×28 cm3) are coupled to PMTs equiped with custom amplified basis to extend the PMT life-time in the high rate environment of PEPPo.

The ~2 µs long and 2 V optimized signal is fed into the JLab custom FADC250 that sampled the signal at 250 MHz.

The flexibility of the FADC250 allows for 3 data taking modes :- Sample (500 samples /detector event);- Semi-integrated (1 integral / detector

event);- Integrated (1 integral /helicity gate

event).

Electron and positron polarimetry Eric Voutier


Analyzing Magnet Response

Experimental asymmetries are measured with respect to beam helicity ; they are linearly proportional to the target polarization, itself proportional to the target magnetization.

IHWP inserted

A. Adeyemi (Hampton University)

Eric Voutier

10/17

Electron and positron polarimetry

G. Alexander et al., NIMA 610 (2009) 451

Paphos, November 2nd-7th, 2015PT = 7.06% ± 0.05%Stat. ± 0.07%Syst.

PEPPo Measurements

• Polarized positron production • Proof-of-principle experiment

• Electron & positron polarimetry


Proof-of-principle experiment Eric Voutier


The location of the 511 keV peak is used for in-situ monitoring of the effective gain of the read-out chain, and link to radioactive source

calibration data.

Compton physics asymmetries are obtained from the polarization sensitive energy deposit spectra in the central crystal.

The coincidence time spectra allows for accidental subtraction.

Polarized positron characterization Eric Voutier

511 keV from e+ annihilation at rest

Bremsstrahlung end-point

Energy (FADC units)

Energy Deposit Spectra


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 VoutierPolarized positron characterization


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

Positron Polarization

Eric VoutierPolarized positron characterization

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).15/17

D. Abbott1,P. Adderley1, A. Adeyemi3, P. Aguilera1, M. Ali1, H. Areti1, M. Baylac2, J. Benesch1, G. Bosson2, B. Cade1, A. Camsonne1, L. Cardman1, J. Clark1, P.

Cole4, S. Covert1, C. Cuevas1, O. Dadoun5, D. Dale4, H. Dong1, J. Dumas1,2, E.

Fanchini2, T. Forest4, E. Forman1, A. Freyberger1, E. Froidefond2, S. Golge6, J. Grames1,

P. Guèye3, J. Hansknecht1, P. Harrell1, J. Hoskins14, C. Hyde7, B. Josey13, R. Kazimi1,

Y. Kim1,4, D. Machie1, K. Mahoney1, R. Mammei1, M. Marton2, J. McCarter11, M. McCaughan1, M. McHugh10, D. McNulty4, T. Michaelides1, R. Michaels1, B.

Moffit1, C. Muñoz Camacho8, J.-F. Muraz2, K. Myers9, A. Opper14, M. Poelker1,

J.-S. Réal2, L. Richardson1, S. Setiniyazi5, M. Stutzman1, R. Suleiman1, C. Tennant1, C.-Y. Tsai12, D. Turner1, A. Variola5, E. Voutier2,8, Y. Wang1, Y. Zhang9

1 Jefferson Lab, Newport News, VA, US 2 LPSC, Grenoble, France 4 Hampton University, Hampton, VA, USA 5 Idaho State University & IAC, Pocatello, ID, USA 3 LAL, Orsay, France

6 North Carolina University, Durham, NC, USA 7 Old Dominion University, Norfolk, VA, US 11 IPN, Orsay, France 12 Rutgers University, Piscataway, NJ, USA

8 The College of William & Mary, Williamsburg, VA, USA 9 University of Virginia, Charlottesville, VA, USA 13 Virginia Tech, Blacksburg, VA, USA 14University of New Mexico

10 George Washington University, Washington, DC, USA

PEPPo Collaboration

Many thanks for advice, equipment loan, GEANT4 modeling support, and funding toSLAC E-166 Collaboration

International Linear Collider ProjectJefferson Science Association Initiatives Award


Outlook

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.2g physics, GPDs…

Eric Voutier


The PEPPo experiment @ the CEBAF injector has been a first step

in an R&D effort for a polarized positron beam at JLab.

17/17

PEPPo II is building-up with the aim of a polarized positron beam for CLAS12

Ee+ > 4 GeV Ie+ > 10 nA Pe+ > 50%

Please join !!

Outlook

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.2g physics, GPDs…

PEPPo II is building-up with the aim of a polarized positron beam for CLAS12

Ee+ > 4 GeV Ie+ > 10 nA Pe+ > 50%

Please join !!

Eric Voutier


The PEPPo experiment @ the CEBAF injector has been a first step

in an R&D effort for a polarized positron beam at JLab.

Magnetic Flux Measurement

Eric Voutier

PT = 7.06% ± 0.05%Stat. ± 0.07%Syst.

Commissionning

The iron core target is equipped with 3 pick-up coils measuring the magnetic flux generated by the magnet current variation (ramping-up, polarity reversal).

Specific cycling procedures are used during the experiment to monitor the target polarization.

E. Froidefond, E. Voutier, PEPPo TN-14-02 (2014)

22/36Paphos, November 3rd-7th, 2015

Documents

(i) Physics motivations (ii) The PEPPo experiment (iii) PEPPo measurements (iv) Outlook Eric Voutier Institut de Physique Nucléaire d’Orsay Orsay, France