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
Rome, September 30 – October 4, 2013
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
Rome, September 30 – October 4, 2013 5/21
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
Rome, September 30 – October 4, 2013
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
Rome, September 30 – October 4, 2013 7/21
Polarized Positrons Production
• PEPPo concept• Proof-of-principle experiment
• Preliminary experimental results
Rome, September 30 – October 4, 2013
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
Rome, September 30 – October 4, 2013 9/21
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
Rome, September 30 – October 4, 2013 10/21
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.
Rome, September 30 – October 4, 2013 11/21
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
Rome, September 30 – October 4, 2013
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.
Rome, September 30 – October 4, 2013 13/21
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.
Preliminary experimental results
Rome, September 30 – October 4, 2013 14/21
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
Preliminary experimental results
Rome, September 30 – October 4, 2013
Positron Polarization
15/21
Perspectives
• Positron beam at CEBAF• Technological challenges
Rome, September 30 – October 4, 2013
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
Eric VoutierPositron beam at CEBAF
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
Rome, September 30 – October 4, 2013 18/21
Eric VoutierPositron beam at CEBAF
Rome, September 30 – October 4, 2013 19/21
e+ Production Scenarii
CEBAF INJ (10-100 MeV) FEL (100 MeV) CEBAF (12 GeV)
Eric VoutierTechnological challenges
Rome, September 30 – October 4, 2013 20/21
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.
Rome, September 30 – October 4, 2013 21/21
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
Rome, September 30 – October 4, 2013
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
Proof-of-principle experiment
(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.
Rome, September 30 – October 4, 2013
Proof-of-principle experiment
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
Proof-of-principle experiment
Rome, September 30 – October 4, 2013
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
Preliminary experimental results
Rome, September 30 – October 4, 2013
Eric VoutierPositron beam at CEBAF
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.
Rome, September 30 – October 4, 2013