positron beam for measurements of generalized parton distributions generalized parton distributions...
TRANSCRIPT
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
Trento, June 8-12, 2015
Electro-production of photons
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
Trento, June 8-12, 2015
Electro-production of photons
7/31
Eric Voutier
Model Calculations
Experimental observables
Trento, June 8-12, 2015
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
Experimental observables
Trento, June 8-12, 2015
Eric Voutier
9/31
H. Avakian, V. Burkert, V. Guzey, JPos09 (2009)
Significant differences between polarized and unpolarized lepton beams supports again importance of a polarized positron beam.
Moments
Experimental observables
Trento, June 8-12, 2015
Eric Voutier
10/31
H. Avakian, V. Burkert, V. Guzey, JPos09 (2009)
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
Experimental observables
Trento, June 8-12, 2015
Eric Voutier
11/31
H. Avakian, V. Burkert, V. Guzey, JPos09 (2009)
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
Trento, June 8-12, 2015
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
Trento, June 8-12, 2015
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
Trento, June 8-12, 2015
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
Trento, June 8-12, 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
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
Trento, June 8-12, 2015
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
Trento, June 8-12, 2015
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
Trento, June 8-12, 2015
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
Positron 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+ > 50%dp/p = 10-2
ex = 1.6 mm.mradey = 1.7 mm.mrad
Eric Voutier
Trento, June 8-12, 2015 28/31
Positron beam at CEBAF
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
Trento, June 8-12, 2015