August 3, 2005ASI-Praha 2005 Karen Dow Polarized Electron Scattering from Polarized Deuterium at BLAST ASI: SPIN-Praha-2005 Karen Dow MIT Bates Linac

August 3, 2005ASI-Praha 2005 Karen Dow BLAST Collaboration

August 3, 2005ASI-Praha 2005 Karen Dow The BLAST Experiments Experiments use:  Longitudinally polarized electron beam  Isotopically pure target of polarized deuterons or protons from the Atomic Beam Source  Large acceptance detector Deuterium as the simplest N-N system; also useful as neutron target. Physics results discussed in this talk:  Quasielastic scattering from the deuteron  G E n  T 20

August 3, 2005ASI-Praha 2005 Karen Dow The Bates Linac and South Hall Ring 850 MeV 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 mA; 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 Siberian Snake

August 3, 2005ASI-Praha 2005 Karen Dow Beam Performance

August 3, 2005ASI-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 CsI 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, 2005ASI-Praha 2005 Karen Dow The Compton Polarimeter Beam polarization typically 65%, systematic uncertainty < 3% PRELIMINARY

August 3, 2005ASI-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, 2005ASI-Praha 2005 Karen Dow BLAST Spectrometer: Coils – 8 copper coils provide a toroidal field of up to 3.8 kG 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 – 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 BEAM TARGET

August 3, 2005ASI-Praha 2005 Karen Dow BLAST

August 3, 2005ASI-Praha 2005 Karen Dow BLAST σθσθ 0.5° σφσφ σzσz 1 cm σpσp 3% Typical ep elastic event Measured resolutions:

August 3, 2005ASI-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, 2005ASI-Praha 2005 Karen Dow Kinematics Three planes to consider: scattering (electron) orientation (target spin) reaction (reaction products) Electron left Electron right Target spin Target spin oriented in horizontal plane at 32° or 47° beam left. Asymmetry most sensitive to G E n when q-vector is perpendicular to target spin.

August 3, 2005ASI-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, 2005ASI-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, A e =A d V =A ed T =0 In the absence of the L=2 moment for the deuteron, A d T =0 A ed V is also sensitive to L=2, as well as MEC, IC, RC (for perpendicular kinematics) (for parallel kinematics)

August 3, 2005ASI-Praha 2005 Karen Dow Quasielastic (e,e′p) Use a missing mass cut to ensure reaction is quasi- elastic, since only electron and proton are detected Theory from Arenhövel, implemented in BLAST GEANT Monte Carlo. Bonn potential, with MEC + IC + RC.

August 3, 2005ASI-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 A ed V at low Q 2 in the lowest missing momentum bins can be compared with theory to extract hP z.. Insensitive to choice of potential.

August 3, 2005ASI-Praha 2005 Karen Dow Quasielastic A ed V PRELIMINARY

August 3, 2005ASI-Praha 2005 Karen Dow Quasielastic A d T PRELIMINARY

August 3, 2005ASI-Praha 2005 Karen Dow Thesis work of Vitaliy Ziskin (MIT) G E n 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, 2005ASI-Praha 2005 Karen Dow Neutron detectors concentrated in the beam right sector (momentum transfer roughly perpendicular to target spin), maximizes sensitivity to G E n 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 laser- flasher system Coincidence time difference (nsec):

August 3, 2005ASI-Praha 2005 Karen Dow To extract G E n : 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 G E n that gives the best fit to the asymmetry as a function of missing momentum 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 hP z from (e,e′p) quasielastic

August 3, 2005ASI-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 A1 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, 2005ASI-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, 2005ASI-Praha 2005 Karen Dow T 20 Thesis work of Chi Zhang (MIT) Deuterium ground state described by 3 form factors: G C, G M and G Q 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 P zz

August 3, 2005ASI-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 G M G Q );T 22 related to G M 2 P zz obtained by normalizing two lowest Q points to Abbott’s parametrization III

August 3, 2005ASI-Praha 2005 Karen Dow T 20 e - left, d + right e - right, d + left Everything Colpanarity Kinematics Full cuts Kinematics Coplanarity cut σ = 1° Kinematics cut Timing cut Clean e-d elastic sample Relative timing

August 3, 2005ASI-Praha 2005 Karen Dow T 20 PRELIMINARY

August 3, 2005ASI-Praha 2005 Karen Dow T 20 and T 21 PRELIMINARY

August 3, 2005ASI-Praha 2005 Karen Dow Conclusions Extensive high quality data taken on tensor and vector polarized deuterium Preliminary extractions of T 20, G E n and (e,e′p) vector and tensor asymmetries Data also on polarized hydrogen; pion production on deuterium; G M n from inclusive scattering on deuterium.

August 3, 2005ASI-Praha 2005 Karen Dow BLAST Collaboration