Polarized Electron Scattering from Polarized Deuterium at BLAST
Polarized Electron Scattering from Polarized Deuterium at BLAST ASI: SPIN-Praha-2005 Karen Dow MIT Bates Linac August 3, 2005 ASI-Praha 2005 Karen Dow
BLAST Collaboration August 3, 2005 ASI-Praha 2005 Karen Dow
The BLAST Experiments use: Ø Longitudinally polarized electron beam Ø Isotopically pure target of polarized deuterons or protons from the Atomic Beam Source Ø Large acceptance detector Deuterium as the simplest N-N system; also useful as neutron target. Physics results discussed in this talk: Ø Quasielastic scattering from the deuteron Ø GE n Ø T 20 August 3, 2005 ASI-Praha 2005 Karen Dow
The Bates Linac and South Hall Ring • 850 Me. V cw polarized electron beam • Beam polarization longitudinal at target (Siberian snake rotates spin so precession in second half of ring cancels precession from first half) • Storage ring filled to 225 m. A; helicity flipped each fill • Lifetime 25 to 30 minutes • Automated synchronization of ring fill, detector HV ramp, data taking, ring dump using the EPICS control system August 3, 2005 ASI-Praha 2005 Karen Dow Siberian Snake
Beam Performance August 3, 2005 ASI-Praha 2005 Karen Dow
The Compton Polarimeter Online monitor of stored beam polarization: • 5 W, 532 nm cw laser normally incident on stored electron beam • Laser helicity flipped with a Pockels cell; chopper wheel for background measurements • Backscattered photons detected with a Cs. I crystal, to measure count rate as a function of photon energy • Cross section for backscattered photons depends on photon energy, also beam and laser helicities. • Form asymmetry in the photon count rate (as a function of laser helicity), binned by photon energy. Fit for beam polarization. August 3, 2005 ASI-Praha 2005 Karen Dow
The Compton Polarimeter Beam polarization typically 65%, systematic uncertainty < 3% PRELIMINARY August 3, 2005 ASI-Praha 2005 Karen Dow
The Internal Target • Isotopically pure polarized targets of hydrogen and deuterium • Magnets and RF transitions set to populate desired spin state(s) and deliver the atomic beam to a 60 cm long, 15 mm diameter open-ended target cell • 5 minutes per spin state • Atomic Beam Source from NIKHEF, extensively reworked to operate in a 0. 2 Tesla magnetic field • Target thickness: • Target polarizations: August 3, 2005 ASI-Praha 2005 Karen Dow
BLAST TARGET BEAM August 3, 2005 Spectrometer: • Coils – 8 copper coils provide a toroidal field of up to 3. 8 k. G • Drift Chambers: • 3 chambers per sector • 2 superlayers per chamber, ± 5º stereo angle • 3 layers per superlayer • 18 wire hits per track • Time of Flight scintillators – 16 + 4 per sector, 2. 5 cm thick • Čerenkov detectors – 4 per sector, 1 cm Aerogel, n=1. 02 or 1. 03 • Neutron detectors – 10 cm thick in left sector, 30 cm thick in right sector • 2 -level, 8 -channel programmable trigger with buffered digitizers, read out with JLab CODA system ASI-Praha 2005 Karen Dow
BLAST August 3, 2005 ASI-Praha 2005 Karen Dow
BLAST Measured resolutions: σθ 0. 5° σφ 0. 5° σz 1 cm σp 3% Typical ep elastic event August 3, 2005 ASI-Praha 2005 Karen Dow
BLAST Data Collection 2004: • 94 pb-1 on polarized hydrogen • 170 pb-1 on polarized deuterium 2005: • 250 pb-1 on polarized deuterium, still being analyzed. August 3, 2005 ASI-Praha 2005 Karen Dow
Kinematics Electron left Three planes to consider: • scattering (electron) • orientation (target spin) • reaction (reaction products) August 3, 2005 Target spin Electron right Target spin oriented in horizontal plane at 32° or 47° beam left. Asymmetry most sensitive to GEn when q-vector is perpendicular to target spin. ASI-Praha 2005 Karen Dow
Kinematics For the quasielastic reactions, we only detect the scattered electron and one nucleon. Define some useful quantities, where “x” represents the detected nucleon: August 3, 2005 ASI-Praha 2005 Karen Dow
Quasielastic (e, e′p) • • • Thesis work of Aaron Maschinot (MIT) Deuterium is loosely bound, easily distintegrates electromagnetically to two nucleons The cross section can be written as: • In the Born approximation, Ae=Ad. V=Aed. T=0 • In the absence of the L=2 moment for the deuteron, Ad. T=0 • Aed. V is also sensitive to L=2, as well as MEC, IC, RC (for perpendicular kinematics) (for parallel kinematics) August 3, 2005 ASI-Praha 2005 Karen Dow
Quasielastic (e, e′p) • Use a missing mass cut to ensure reaction is quasielastic, since only electron and proton are detected • Theory from Arenhövel, implemented in BLAST GEANT Monte Carlo. Bonn potential, with MEC + IC + RC. August 3, 2005 ASI-Praha 2005 Karen Dow
Quasielastic (e, e′p) • At the quasielastic peak, d(e, e′p)n can be thought of as elastic scattering from hydrogen with a spectator neutron. Thus, the measured asymmetry Aed. V at low Q 2 in the lowest missing momentum bins can be compared with theory to extract h. Pz. . Insensitive to choice of potential. August 3, 2005 ASI-Praha 2005 Karen Dow
Quasielastic Aed PRELIMINARY August 3, 2005 ASI-Praha 2005 Karen Dow V
Quasielastic PRELIMINARY August 3, 2005 T Ad ASI-Praha 2005 Karen Dow
• Thesis work of Vitaliy Ziskin (MIT) • GEn is a fundamental quantity, difficult to measure as there are no free neutron targets, and the electromagnetic response is dominated by the neutron magnetic form factor • Higher precision measurements needed at low Q 2, for understanding the neutron charge distribution and for interpreting parity violation experiments; data will provide a sensitive test of QCD at low energies • Access using spin degrees of freedom, with vector-polarized deuterium as a neutron target • Cross section can be written as: (for perpendicular kinematics) (for parallel kinematics) August 3, 2005 ASI-Praha 2005 Karen Dow
• • Neutron detectors concentrated in the beam right sector (momentum transfer roughly perpendicular to target spin), maximizes sensitivity to GEn Neutron detection efficiency about 30% (right sector), 10% (left sector) TOF and wire chambers used as a charged particle veto Neutron momentum from time-of-flight; timing calibrations from cosmics and a laserflasher system August 3, 2005 Coincidence time difference (nsec): ASI-Praha 2005 Karen Dow
• Cut on invariant mass (quasi-elastic scattering) and missing mass (mass of the proton) • Use hydrogen and empty cell measurements to check for background from cell walls • h. Pz from (e, e′p) quasielastic To extract GEn: • • At each Q 2 compare the measured asymmetry to a Monte Carlo calculation using Arenhövel’s theory (Bonn potential, MEC+IC+FSI) Select the value of GEn that gives the best fit to the asymmetry as a function of missing momentum August 3, 2005 ASI-Praha 2005 Karen Dow
• Data from polarization experiments; random and systematic uncertainties added in quadrature. BLAST preliminary data in red; more to be analyzed • Blue curve is the Platchkov fit using the Galster parameterization • Black dashed curve is the A 1 collaboration fit using the Friedrich and Walcher parametrization • Red dashed curve is the BLAST fit (F&W form), constrained to have the correct slope at Q 2=0. • August 3, 2005 ASI-Praha 2005 Karen Dow
• Fourier transform (not relativistically correct) of BLAST fit shows a positive core due primarily to the dipole term; constituent quarks • Bump term gives a negative shell, due to diffuse pion cloud • August 3, 2005 ASI-Praha 2005 Karen Dow
T 20 • • • Thesis work of Chi Zhang (MIT) Deuterium ground state described by 3 form factors: GC, GM and GQ Rosenbluth separation can only provide two pieces of information; unpolarized cross section is: • Need a third measurement to extract all three form factors; use a tensor-polarized target, so the cross section has a term that depends on Pzz August 3, 2005 ASI-Praha 2005 Karen Dow
T 20 • Measure asymmetry in the cross section for elastic electron scattering from tensor-polarized deuterium: • Extract elastic T 20 from measured asymmetry; T 21 from parametrization or measurement (proportional to GMGQ); T 22 related to GM 2 • Pzz obtained by normalizing two lowest Q points to Abbott’s parametrization III August 3, 2005 ASI-Praha 2005 Karen Dow
T 20 Everything Colpanarity Kinematics Full cuts Kinematics e- left, d+ right e- right, d+ left Relative timing • Coplanarity cut σ = 1° • Kinematics cut • Timing cut Clean e-d elastic sample August 3, 2005 ASI-Praha 2005 Karen Dow
T 20 PRELIMINARY August 3, 2005 ASI-Praha 2005 Karen Dow
T 20 and T 21 PRELIMINARY August 3, 2005 ASI-Praha 2005 Karen Dow
Conclusions • Extensive high quality data taken on tensor and vector polarized deuterium • Preliminary extractions of T 20, GEn and (e, e′p) vector and tensor asymmetries • Data also on polarized hydrogen; pion production on deuterium; GMn from inclusive scattering on deuterium. August 3, 2005 ASI-Praha 2005 Karen Dow
BLAST Collaboration August 3, 2005 ASI-Praha 2005 Karen Dow
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