XIII th International Conference on Meson-Nucleon Physics and the Structure of the Nucleon, Rome, September 30 – October 4, 2013

XIIIth International Conference on Meson-Nucleon Physics and the Structure of the Nucleon, Rome, September 30 – October 4, 2013

Physics Perspectives at JLab witha Polarized Positron Beam

PEPPo @ JLab

(i) Physics motivations(ii) Polarized positrons

production (iii) Perspectives(iv) Conclusions

Eric Voutier Laboratoire de Physique Subatomique et de Cosmologie

Grenoble, France

Rome, September 30 – October 4, 2013

Physics Motivations

• Electromagnetic form factors (U,P)• Generalized parton distributions (U,P)

• Inclusive structure functions (U,P)• Search for the U-boson of dark matter (U)

• Charge conjugation violation to access C3q (P)

• International Linear Collider• Positron-ion Collider (LHeC, e-RHIC, EIC, MEIC, ENC)

• Thermal positron facility

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

3/21

Electromagnetic form factors Eric Voutier


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 VoutierElectromagnetic 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

Rome, September 30 – October 4, 2013 4/21

Generalized parton distributions Eric Voutier

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


N(e,e′gN) Differential Cross Section

Eric Voutier

M. 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

6/21

Experimental Observables

Eric VoutierGeneralized parton distributions

V. Burkert, V. Guzey, JPos09, AIP Conf. Proc. 1160 (2009)

Calculations of experimental observables within a dual parametrization of the nucleon GPDs are predicting very significant effects comparing electrons and

positrons.

A beam current larger than 10 nA would be ideal for a polarized positron DVCS program at CLAS12.

CALU

2π

0sinφLU Asinφdφ

π1A


Polarized Positrons Production

• PEPPo concept• Proof-of-principle experiment

• Preliminary experimental results


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

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

e- → g → e+

PEPPo concept Eric Voutier


Polarized Electrons for Polarized Positrons Polarized positrons can be created by materialization of

polarized photons issued from the bremsstrahlung of polarized electrons.

H. Olsen, L. Maximon, PR 114 (1959) 887 E.A. Kuraev, Y.M. Bystritskiy, M. Shatnev, E. Tomasi-Gustafsson, PRC 81 (2010) 055208

Eric Voutier

Bremsstrahlung and Pair Creation

BREMSSTRAHLUNG PAIR CREATION

Finite lepton mass calculations of the bremsstrahlung and pair creation processes predict very efficient polarization transfers.

PEPPo concept


March 2012

May 2011

Eric VoutierProof-of-principle experiment

E11-105 @ CEBAF InjectorJ. Grames, E. Voutier et al.

The proof-of-principle PEPPo experiment, installed at the CEBAF injector, intented to demonstrate the feasibility of using

bremsstrahlung of polarized electrons to produce polarized positrons.

The positron yield and polarization distibutions have been measured.


PEPPoT1

T2

S1

S2

D

DPT

Pe-

Calorimeter

e+

e-

Principle of Operation

PEPPo did measure the longitudinal polarization transfer from electrons to positrons in the 3.2-6.3 MeV/c momentum range.

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

proportional to Pe-.

o Pg transfers to e+ into longitudinal (Pe+) and

transverse (Pt) polarization components. On the

average Pt=0.

Ie = 1 µAT1 = 1 mm

Eric VoutierProof-of-principle experiment

G. Alexander et al, PRL 100 (2008) 210801, NIMA 610 (2009) 451


Pe- = 83.7% ± 2.8%Stat. ± 0.6%Syst.

PT = 7.53% ± 0.04%Stat. ± 0.06%Syst.

12/21

Eric Voutier

Data Taking

Data were taken using two major sequences:

I. Use the polarized electron beam directly from the CEBAF injector to calibrate the analyzing power of the Compton transmission polarimeter;

II. Use the Compton transmission polarimeter to measure the polarization transfer from electrons to positrons.

Preliminary experimental results

Vacuumwindow

TriggerScintillator

ReconversionTarget C

ALORIMETER

e+

photon

The DAQ trigger for positron measurements is a coincidence between a thin scintillator placed prior the reconversion target, and the central crystal (PMT5).

Drastic reduction of the neutral background.


Eric Voutier

Electron Analyzing Power

PEPPo

On-line

Analys

is A high quality measurement of the electron analyzing power has been achieved and is currently in the final analysis stage.

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



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).

Eric Voutier

Set 1 (Initial Run)Set 2 (Improved Shielding)

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

Set 1 (Initial Run)Set 2 (Improved Shielding)

ElectronBeam

Polarization

Pe+ = AT / k Ae- Pe- PTPEPPo Preliminary



Positron Polarization

15/21

Perspectives

• Positron beam at CEBAF• Technological challenges


Eric VoutierPositron beam at CEBAF

Positron Collection Concept

W target

Quarter Wave Transformer (QWT) Solenoid

Combined Function Magnet(QD)

Collimatorse-

e+

g

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

24 MeV

126 MeVe-

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. Rome, September 30 – October 4, 2013 17/21


e+ Source Concept

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

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

1mA

I = 300 nAPe+ > 60% Pe-p/p = 10-2

x = 1.6 mm.mrady = 1.7 mm.mrad




e+ Production Scenarii

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

Eric VoutierTechnological 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 Collection

Properties 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.

Conclusions Eric Voutier

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… NP @ low energy, Material Science @ very low energy…

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

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.


Production TargetT1 (0.1-1 mm W)

Beam position monitors

Viewers

Positron Selection Device

Dipoles

Solenoid

Viewer

Collimator

Corrector magnets

Solenoid

Faraday Cup

Annihilation detector

Viewers

Reconversion TargetT2 (2 mm W)

Polarized Analyzing Target

(7.5 cm Fe)

Compton Transmission Polarimeter

Corrector magnets

Quadrupoles

ELEGANT beam optics

PEPPo branch jonction

Eric Voutier

G4PEPPo experiment model


Proof-of-principle experiment

Eric Voutier

DAQ Components

FADC 250

Hamamatsu R6236-100

CsI crystals are coupled to PMT equiped with LPSC custom amplified basis to comfort the PMT life-time (high rate environment).

The ~2 µs long and 2 V optimized signal is fed into the JLab custom FADC 250 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). Rome, September 30 – October 4, 2013


(0, 0, P) (P, 0, 0) (0, P, 0)

Reverse magnet polarity Re

vers

e be

am h

elici

ty

4p spin rotator determines

e- polarization orientation

Rx

Rz

Ry

• Fast reversal of electron beam polarization at 30 Hz through experiment.

• Slow systematic reversal of electron polarization with optical wave-plate (source) and target polarization (Compton).

• Orient polarization in x,y,z direction using 4p spin rotator.

Eric Voutier

P = 83.7% ± 2.8%Stat. ± 0.6%Syst.



Eric Voutier

The iron core target is equipped with 3 pick-up coils measuring the magnetic flux generated upon magnet current variation.

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

Magnetic Flux Measurement

< PT > = 7.53% ± 0.06%Opera ± 0.04%Coil



Eric Voutier

Energy Dependent Analysis

200n

s

Eg

Under progress…

Accidental subtraction is done per helicity state for each energy bin.

Positron polarization will be deduced for each energy bin using simulated analyzing power constrained by electron calibration data.

Set 1Set 2Set 1+2




e+ Figure of Merit

The Figure of Merit is the quantity of interest for the accuracy of a measurement which combines the incident flux of particles and its polarisation.

(GEANT4 simulations based on the full screening case of O&M)

Optimum energy

OptimumFoM

2ee PIFoM

MeV0.25ΔE10Δθ ee

μm 100t% 85 PmA 1I Wee -

J. Dumas, Doctorate Thesis, 2011 (LPSC Grenoble/JLab) Rome, September 30 – October 4, 2013

Eric Voutier

Optimized Polarized Positron Source

Positron beam at CEBAF

J. Dumas, Doctorate Thesis, 2011 (LPSC Grenoble/JLab)

Polarization transfer up to 75% may be expected over the full energy range of the CEBAF injector.

Production efficiencies up to 10-4-10-3 may be expected, depending on the electron beam energy.

These figures are linearly sensitive to the acceptance of the positron collection system.


XIII th International Conference on Meson-Nucleon Physics and the Structure of the Nucleon, Rome, September 30 – October 4, 2013

Documents

electromagnetic structure

mesonnucleon physics

physics perspectives

photons physics

generalized form factors

jlab physics motivations

international conference

electromagnetic interaction