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