Hi,
here are the minutes of the meeting from Wednesday 03/08/2006:
-Spin angle:
Spin angle uncertainty quoted with 0.5 degrees is still the dominant
systematics in most channels. Most sensitive is GEn with 12% per
degree. In GMn, systematic error due to the spin angle (2.5%) exceeds
the statistical error (1-2%).
There is a 1.5 degree discrepancy between yield-averaged spin angle of
the most recent spin angle map of June 2005 and the average spin angle
from the T20 aymmetry analysis by Chi (demonstrated already in the
January collaboration meeting).
Note that yield averaging of the map depends on the location of the z
distribution of the yield and is thus also subject to "kinematic
corrections".
A new measurement of the spin angle map is envisioned based on a
compass principle, where the field angle is measured directly, as
opposed to the previous method of measuring a longitudinal and
transverse field amplitude. Genya reported on the method, the device
from Jlab, the expected difficulties. The compass device is a magnetic
probe (permanent magnet or magnetized iron) that sits on an air pillow
and that can be sled along the z axis. A mirror is attached to the
probe and the angle of the probe orientation is measured by the
direction of the reflected light from a laser that sits on the axis.
Problem sources are predominantly
-Alignment:
+The geometrical axis may not be aligned with the magnetic axis of
the probe.
+The orientation of the mirror may not be correctly aligned with the
geometrical axis of the probe. The probe with the attached mirror
can be rotated around the z-axis. 90 degree rotations allow to
account for misalignments.
-Size of the probe: It is 1 inch long. There may be concerns that with
a "sampling size" of 1in the observed average field angle may be
different from the angle exactly on the axis due to different field
gradients in x/y/z. Aki was asked to provide plots of the x/z
components of the holding field in the vicinity of the z-axis (as
function of x or y) in order to see how narrow each holding field
component is distributed along the z-axis.
Another possibility may be to replace the probe with one smaller in
size, as the field to be measured is on the order of a few hundred
Gauss while the actual probe was designed to measure fields of a few
tens of Gauss.
-Kinematic corrections/geometry offsets
+Doug is working on straight track analysis of cosmic runs. Lists of
cosmics runs from 2005 and during hydrogen running end of 2004 are
now available.
+Adrian showed us plots of the momentum offsets p_e-p_e(theta_e) and
p_p-p_p(theta_p) for both sectors. This again confirms the picture:
Momentum offsets for protons and electrons in the right sector are of
similar size but with opposite signs. To first order these offsets
will likely be corrected for by a geometrical shift on one chamber
plane. In the left sector, the proton momentum is almost as expected
from the angle, while the electron momentum is about 30MeV/c off
(measured value too small)
+Adam showed us comparisons of measured yields in quasielastic
d(e,e'p) from the 2005 dataset in comparison with Monte Carlo, for
reconstructed momenta and angles. The general trend of momentum
offsets seen in ep elastic is also seen in the comparison of
quasielastic yields and MC.
-Two pion production and pion production from deuterium
Aki demonstrated his PID analysis for pi+ and pi- identification and
showed us yields for p(e,e'pi-)pi+p, d(e,e'pi-)pp, d(e,e'pi+)nn.
The two pion production yields are quite low, note that (e,e'pi-)
from the proton target necessarily involves two positive charges in
the undetected state, of which one must be an additional undetected
pi+.
The pi+- production yields from deuterium with two nucleons in the
final state (nn,pp) are sizable. In case of pi+, a "dineutron" plus a
continuum is seen in the missing mass spectrum; in case of the pi- the
missing mass is cut off at 2.07 GeV [Aki: can you plot missing mass of
d(e,e'pi-) just like incase of d(e,e'pi+) without cutting?]
[It would be very interesting to also investigate the d(e,e'd)pi0 yield.]
Best regards,
Michael
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