JLab PAC33, January 16, 2008 Polarization transfer in WACS1
p p
Polarization transfer in Wide-AngleCompton ScatteringProposal 08-017
D. Hamilton, R. Gilman, A. Nathan, and
B. Wojtsekhowski, co-spokespersons
Outline
•Mechanism of the reaction is a key question
•Compton e -> e process and polarization observables
•WACS present results and experimental program
•Proposed experiment and expected results
JLab PAC33, January 16, 2008 Polarization transfer in WACS2
p p
Two basic options for mechanism:
Collective response - several partons involved in highmomentum interaction with the photons
Individual response - one quark absorbs incident photon andthe same quark emits scattered photon
Mechanism of the process
JLab PAC33, January 16, 2008 Polarization transfer in WACS3
p p
• Regge poles - VMD - since 1960’s …, Laget
• pQCD - two-gluon- Brodsky, …, Guichon&Vanderhaeghen, Brooks&Dixon, Thomson et al.
• Diquark model - Guichon&Kroll, 1996
• Leading quark- Brodsky et al., 1972
• GPDs (handbag)- Radyushkin, Kroll et al.
• CQM- G.Miller
Main issues:
• Competing mechanisms
• Interplay between hard and soft processes
• Threshold for onset of asymptotic regime
• Role of hadron helicity flip
Studies of the RCS process
JLab PAC33, January 16, 2008 Polarization transfer in WACS4
p p
Exact formalism for CS off electron
JLab PAC33, January 16, 2008 Polarization transfer in WACS5
p p
Compton in 1925
Directed Quanta ofScattered X-Rays
Arthur Compton andAlfred Simon, PR26, 3 (1925)
recoil electron
photo electron
JLab PAC33, January 16, 2008 Polarization transfer in WACS6
p p
Cross section of WACSvs pQCD calculations
normalization problems, calculations are 3-10 times below data
Calculation byThomson et al
Calculation byBrooks & Dixon
Cornell data
JLab data
JLab PAC33, January 16, 2008 Polarization transfer in WACS7
p p
• Single-quark mechanism
“handbag” diagram accounts for
scattering from quark and
introduces the FFs for ++,+-, ..
Form factors: RV, RT, RA
KN-like polarization observables
quark coupling is small compare t
WACS in GPD approach
Form factors allows to fit cross section
JLab PAC33, January 16, 2008 Polarization transfer in WACS8
p p
The plan of proposed experiment
proposed
-u = 3.6
-u = 2.3
p
p
JLab PAC33, January 16, 2008 Polarization transfer in WACS9
p p
Result: test of reaction mechanism
Proposed for E ~ 4.3 GeV for cm angles 90o and 110o
( a 70o cm data point will be obtained in E07-002)
JLab PAC33, January 16, 2008 Polarization transfer in WACS10
p p
Result: test of reaction mechanism
Proposed for E ~ 4.3 GeV for cm angles 90o and 110o
( a 70o cm data point will be obtained in E07-002)
JLab PAC33, January 16, 2008 Polarization transfer in WACS11
p p
Experiment concept: similar to E99-114
Beam 80% polarized, 80 μA
Ee= 4.8 , <Ēγ>~ 4.3 GeV
Photon flux ~ 1013 eqv. /s; in thisexperiment it will be boosted by anadditional factor 5 with larger calorimeter
JLab PAC33, January 16, 2008 Polarization transfer in WACS12
p p
Selection of the WACS events
Magnet separates e'/γ, no veto needed, allows higher luminosity
JLab PAC33, January 16, 2008 Polarization transfer in WACS13
p p
Focal Plane Polarimeter
FPP used in many Hall A experiments
FOM = εAy2 ~ 0.02, so we need ≈1-2 M RCS events
Calibrated with ep elastic scattering
JLab PAC33, January 16, 2008 Polarization transfer in WACS14
p p
Present proposal (KLL,LT) has photon energy 4.3 GeV, large s, -t, -u
E99-114 Results vs new plan
s = 6.9 GeV2
-t = 4.0
-u = 1.13
PRL 98, 152001
PRL 94, 242001
JLab PAC33, January 16, 2008 Polarization transfer in WACS15
p p
WACS research program
CS research has significant chance to discover
the basic reaction mechanism
CS research would be extended for cross sections
up to to s = 20 GeV2 with the 12 GeV upgrade, could
possibly find onset of pQCD in exclusive reactions
JLab PAC33, January 16, 2008 Polarization transfer in WACS16
p p
Proposed Kinematics
Eγ = 4.3 GeV, s = 8.9 GeV2
Large -t,-u
Precision
data for KLL,
and KLT
JLab PAC33, January 16, 2008 Polarization transfer in WACS17
p p
Beam Time Request
21 days beam on target in Hall A
JLab PAC33, January 16, 2008 Polarization transfer in WACS18
p p
Summary
Mechanism of Real Compton Scattering could be foundfrom proposed experiment - need two data points
WACS has largely the same physics as FFs and DVCS
Experimental technique is well understood and tested
This polarization experiment belongs to 6-GeV program
Beam time request: 21 days beam on target in Hall A
JLab PAC33, January 16, 2008 Polarization transfer in WACS19
p p
Backup slides
JLab PAC33, January 16, 2008 Polarization transfer in WACS20
p p
Beam Time Request - by hours
21 days of beam in Hall A
JLab PAC33, January 16, 2008 Polarization transfer in WACS21
p p
Reply to TAC comments
• Installation time ~ < 1 month in Hall A.
• As suggested by RadCon group the photon radiator willbe made from Pb, for which case rad-budget is twice lower
than for the standard Cu radiator.
JLab PAC33, January 16, 2008 Polarization transfer in WACS22
p p
Polarization observables in QED
JLab PAC33, January 16, 2008 Polarization transfer in WACS23
p p
GPD-based prediction for KLS
JLab PAC33, January 16, 2008 Polarization transfer in WACS24
p p
NLO GPD-based calculations
NLO corrections to the cross section are 10-20%
Corrections to polarization observables ~ 1-2% at cm=90o
~ 10-15% at 120o
JLab PAC33, January 16, 2008 Polarization transfer in WACS25
p p
GPDs and form factors of WACS
GA at w
JLab PAC33, January 16, 2008 Polarization transfer in WACS26
p p
Systematic Uncertainties
By far the largest contribution tosystematic uncertainty from dilutionfactor/background asymmetrystability.
In previous analysis, backgroundwas fitted with polynomialdistribution.
In future analysis, a dedicated MonteCarlo (already developed) will beused to better understandbackground.
Expected improvement in systematicuncertainty by factor two or more- dKLLsyst ~ 0.01
