Hi,
here are my minutes from Wednesday:
-Wire chamber calibrations (DH)
With Doug's wire positions relative to the chamber center of the gas
volume and Aaron's positions relative to the center by the tooling
balls in both cases the 1.2mm shift of one chamber relative to the
other two in the right sector seems to be apparent (from straight
track analysis with near-wire events rejected to improve
resolution). The earlier reported 1.2cm discrepancy in z between Doug
and Aaron refers to the location of the origin of the two coordinate
frames (center of gas volume versus center of tooling balls) and is
compensated by the appropriate choice of wire positions. Both versions
seem to be correct.
We're back to where we were three weeks ago.
Strategy:
1) apply adhoc geometry shift 1.2mm,
2) use cosmics for straight tracks (which provide higher resolution)
1) does not satisfy, 2) is the way to go. CC provides list of cosmics
runs that fulfill requirements (BLAST magnet off, detector closed, wire
chamber on, hall lights off, correct trigger) to Doug.
If there are really geometrical shifts of entire chambers (three in
each sector), these should be revealed by a straight-track analysis
based on cosmics tracks.
It is understood that it be crucial for the analysis completion in all
channels that the "kinematic corrections" are not "ad hoc".
-MC studies of kinematic offsets due to energy loss and internal
radiation (EG)
We could convince ourselves that the momentum loss of the protons in
the BLAST acceptance Q^2 > 0.12 (GeV/c)^2 (p_p >~ 350 MeV/c) is
NO MORE THAN 6 MeV/c for the slowest protons, i.e. <2%. (Note that the
"kinematic offsets" seen in ep elastic are of the order 30-50 MeV,
i.e. >~ 10%).
There are visibly no big differences for reconstructed momenta for the
MC samples with MASCARAD on or off, i.e. of the two effects, energy
loss and internal radiation, it is the energy loss that dominates the
momentum shift, while still small.
For electrons, both energy loss and shifts due to internal radiation
are even smaller.
For deuterons, the energy loss effects are likely larger, however
deuteron momenta are not used for anything but for PID.
Though the energy loss and radiative corrections are smaller than the
observed kinematic offsets, it is nevertheless necessary to account
for the energy loss in the reconstruction if one wants to determine
the net offsets (which then should be applicable to all reactions).
For the (elastic) analyses that are based on MC-data comparisons: Applying
the energy loss correction in the reconstruction may not be necessary when
included in the MC generation. (This is true if Q2 is determined by
the angle in ep elastic or ed elastic).
In most other cases however (i.e. in quasielastic and inelastic
channels), the value of Q^2 at which an observable (such as GEn or
GMn) is evaluated will depend on the magnitude of applied momentum
correction. So far, the "kinematic corrections" have been a
convolution of energy loss effects and "net kinematic offset" into an
"effective kinematic offset". With the consideration above, this has
to be disentangled in order to make the then-determined "net kinematic
offset" applicable to all channels.
Best regards,
Michael
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