Analysis of d(e,e’p)n in BLAST
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Transcript of Analysis of d(e,e’p)n in BLAST
Analysis of d(e,e’p)n in BLAST
Aaron MaschinotMassachusetts Institute of Technology
Spin 2004 Conference
Trieste, Italy
SPIN 2004 Trieste, Italy
• Loosely-bound deuterium readily breaks up electromagnetically into two nucleons e + d e’ + p + n
• Most generally, the d(e,e’N)N cross section can be written as:
• In the Born approximation,
• Additionally, vanishes in the L = 0 model for the deuteron (i.e. no L = 2 admixture) is a good measure of L = 2 component
• Also, is also a good measure of L = 2 as well as subnuclear degrees of freedom (e.g. MEC, IC, RC)
Deuteron Electro-disintegration
S h,Pz ,Pzz S0 1PzAdV PzzAd
T h Ae PzAedV PzzAed
T
Ae AdV Aed
T 0
AdT
AdT
AedV
L=0+
L=2
+
SPIN 2004 Trieste, Italy
The BLAST Program
• Bates Large Acceptance Spectrometer Toroid
• Located at the MIT-Bates Linear Accelerator Facility in Massachusetts, USA
• Utilizes polarized beam and polarized targets 0.850GeV longitudinally polarized electron beam polarized internal atomic beam source (ABS) target
• Large acceptance, left-right symmetric spectrometer detector simultaneous parallel/perpendicular, in-plane/out-of-plane asymmetry
measurements Toroidal magnetic field
• Ideally suited for a comprehensive analysis of the spin-dependent electromagnetic response of few-body nuclei at momentum transfers up to 1GeV2
SPIN 2004 Trieste, Italy
Polarized Beam at Bates
• 1GeV longitudinally polarized electron beam 0.5GeV linear accelerator with recirculator
• Polarized beam fills South Hall storage ring location of BLAST experiment
• Longitudinal polarization maintained by Siberian snakes
• 25 minute lifetime @ 175mA ring current
SPIN 2004 Trieste, Italy
Beam Polarization Measurements
• Beam polarization measured via a Compton polarimeter polarization ~ amount of back-scattered photons nondestructive measurement of polarization
• Long-term beam polarization stability average beam polarization = 65% ± 4%
PRELIMINARY
SPIN 2004 Trieste, Italy
The BLAST Targets
• Internal Atomic Beam Source (ABS) target
• Hydrogen and Deuterium gas targets
• Can quickly switch between polarization states
• Hydrogen polarization in two-state mode Vector : +Pz -Pz
• Deuterium polarization in tri-state mode (Vector, Tensor) :
(-Pz, +Pzz) ( +Pz, +Pzz)
(0, -2Pzz)
• Flow = 2.2 1016 atoms/s, Density = 6.0 1013 atoms/cm2, Luminosity = 4.0 1031 /cm2/s @ 140mA
• Actual polarization magnitudes from data analysis
• 3He target ready for future running
SPIN 2004 Trieste, Italy
The BLAST Spectrometer
• Left-right symmetric detector simultaneous parallel and
perpendicular asymmetry determination
• Large acceptance covers 0.1GeV2 ≤ Q2 ≤ 1GeV2
out-of-plane measurements
• DRIFT CHAMBERS momentum determination,
particle identification• CERENKOV COUNTERS
electron/pion discrimination• SCINTILLATORS
TOF, particle identification• NEUTRON COUNTERS
neutron determination• MAGNETIC COILS
4.5kG toroidal field
DRIFT CHAMBERS
CERENKOVCOUNTERS
SCINTILLATORS
NEUTRON COUNTERS
TARGET
BEAM
BEAM
SPIN 2004 Trieste, Italy
Drift Chambers
• Three wire chambers on either side
• Two superlayers of cells per chamber
• Three sense wires per cell
• 3 2 3 = 18 hit wires for ionizing particle
• 954 total sense wires, 9888 total wires
• Large acceptance 20° ≤ ≤ 80° , -17° ≤ ≤ 17° 1sr total solid angle
• Each wire 98% efficient
SPIN 2004 Trieste, Italy
Event Reconstruction
• C++ OOP reconstruction library using ROOT• Resolutions are a “work in progress”
much progress has been made in the last six months
current goal
p 3% 2%
0.5° 0.3°
0.5° 0.5º
z 1cm 1cm
.
SPIN 2004 Trieste, Italy
Missing Mass and Momentum
• Only the e- and p+ are measured actually measure d(e,e’p)X need cuts to ensure that X = n
• Define “missing” energy, momentum, and mass:
• Demanding that mM = mn helps ensure that X = n
2M
2MM
pM
pdM
pEm
pqp
EmE
SPIN 2004 Trieste, Italy
Monte Carlo d(e,e’p)n Asymmetries
• Using theoretical model from H. Arenhövel• Data take into account detector acceptance• Target polarization vector, ,set at 32º on
the left side can access different asymmetry
components
q
p d
p d
q
p d //
q
electron side
sideasymmetry component
left right perpendicular
right left parallel
SPIN 2004 Trieste, Italy
Background Contributions
• Empty target runs provide a measure of background
• Negligible contribution at small pM
• Larger contribution at high pM due to scattering off of Aluminum target
AREAL AMEAS fREAL 1
fREAL rREAL
rREAL rBGR
Perpendicular Parallel
SPIN 2004 Trieste, Italy
Beam-Vector Asymmetry Results
• 200kC of data analyzed so far• 450kC projected total data
• Vector polarization determined from fitting asymmetry below pM = 0.15GeV• Visible correlation with full subnuclear-effects model
SPIN 2004 Trieste, Italy
Tensor Asymmetry Results
• Tensor polarization from independent T20 fit
• L=2 “dips” reproducible in the data
• Still working on systematic checks; results are preliminary
SPIN 2004 Trieste, Italy
Determining the Vector Polarization
• In the quasi-elastic (QE) limit, d(e,e’p)n is well understood: reduces to p(e,e’p) with spectator n <1% model uncertainty in
• Large asymmetry, high detector efficiency small statistical uncertainty
• QE d(e,e’p)n pM pN = 0
small uncertainty up to pM = 0.15GeV
AedV
perp para
h•Pz 0.467±0.006 0.460±0.006
h 0.65±0.04
Pz 0.72±0.04 0.71±0.04
SPIN 2004 Trieste, Italy
Conclusions
• Both the d(e,e’p)n beam-vector and tensor asymmetries are good measures of the L = 2 deuterium component.
• The d(e,e’p)n beam-vector asymmetry is a good measure of subnuclear effects (and relativistic corrections).
• Both asymmetries are being measured in BLAST
• Final asymmetry results with 450kC expected within six months
• Results will offer much discerning power between models