G2p Analysis Minutes

Minutes of the weekly analysis meetings

Agenda

8/19/2015

Present: Chao, Jie, Min, Jixie, Kalyan, Melissa
By Phone: Vince, Ryan, Toby, Karl, Pengjia

Feature Presentations:

Melissa
- Showed a method for determining the correction to asymmetries for pion contamination. The
  measured asymmetry can be written in terms of the electron and pion asymmetries, multiplied
  by the fraction of events of that particle. To differentiate between electrons and pions, a cut is
  made on events that trigger the Cherenkov detector (electrons) and events that do not trigger
  the Cherenkov (pions). Kalyan commented that this method of selecting pions may be too blind,
  and suggested making a 2D cut on E/p vs Cherenkov instead. To determine the fraction of
  electrons/pions, "good event" cuts are applied (loose acceptance cuts, single track events and
  pion rejector cuts) and the number of electrons/pions that survive the cuts are counted. Although
  the pion asymmetry is much larger than the electron asymmetry, the number of residual pions
  is very small, so the overall correction is small. For this example the 2.2 GeV, 5T longitudinal
  setting was used, but the correction may be larger for the 5T transverse settings, which she
  will do next. More details can be seen in her slides here.

Min
- Gave an update on acceptance studies. She used the 1.7 GeV, 2.5T setting (with 3rd septum
  configuration) to test the acceptance cuts using the yield ratio of data to simulation. She
  showed a plot of target θ vs φ for both data and simulation, and applied a graphical cut to get
  rid of events on the edges. Looking at the resulting 1D plots shows reasonable agreement
  between θ and φ. Vince pointed out that there seems to be some additional structure in the
  dp distribution, and suggested expanding the δ cut out to &pm; 4% to see if there is more
  hidden structure. Min said she has seen this in other settings as well and will look into it. She
  also tried adjusting the cuts on θ, φ and δ and looked at how the ratio of yields from data/
  simulation changed; the ratio starts to drop off as the cuts get wider. Next she will work on
  calculating the yields using W bins. More details can be seen in her slides here.

8/12/2015

Present: Jie, Chao, Kalyan, Min, Jixie, Melissa
By Phone: Pengjia, Ryan, Toby, Karl, Ellie, Alex

Feature Presentations:

Chao
- Gave an update on optics studies. Previously, Min has shown comparisons of data/simulation
  where she looked at different columns of sieve holes. Chao continued this study by cutting on
  individual sieve holes. Looking at the center sieve hole, the φ distribution looks reasonable, but
  the θ distribution has some issues. Specifically, the data is wider on the left side (negative θ)
  than the simulation. He showed results for several sieve holes, including one that was in the
  row as the center hole, but in the farthest column on the left side of the sieve slit. In this case
  the θ distribution had similar width for both data and simulation, but the height of the simulation
  distribution was larger. This may suggest that something is wrong with the acceptance in the
  simulation. The problem with θ seems to be systematic; for most of the sieve holes there are
  more events on the left side of the data than in the simulation. Kalyan asked whether the
  thickness of the sieve slit was taken into account, Chao says it was included in the simulation
  and optimizer. Jixie suggested comparing the dp distributions for each sieve hole. More details
  can be seen in Chao's slides here.

7/29/2015

Present: Chao, Min, JP, Melissa
By Phone: Alex, Pengjia

Feature Presentations:

Min
- Gave an update on acceptance studies. Previously, Chao showed a correction for the beam
  position in g2psim. Min checked this correction by looking at optics runs. On the data she
  applied a cut on dp (to select elastic events) and on the focal plane (to get rid of junk
  events). On the simulation, only an aperture cut was applied. She compared the data and
  simulation results for θ and φ both before and after drifting from the sieve slit to the target.
  The effect of the target field on θ is not very large, though there is a shift down in angle.
  However, the shape of the θ distribution is different between data and simulation. The
  effect of the target field on φ is significant; it's not clear why this is the case. To try to
  determine the reason for the discrepancies, she applied a cut along each column of sieve
  slit holes in φ and compared the data and simulation results for θ,φ and dp. Column 2
  seems to have the best agreement (for dp), columns 1 and 2 have a shift while columns
  3-6 have a difference in width. Chao suggested also cutting along each row of sieve slit
  holes to better study the effect of the drift on φ. Min will work on identifying the differences
  between data and simulation, specifically whether the boundaries are different between
  the two. More details can be seen in her slides here.

7/22/2015

Present: JP, Min, Jixie, Melissa
By Phone: Alex, Ryan, Toby, Ellie, Karl, Pengjia

Feature Presentations:

Melissa
- Showed various checks for packing fraction runs that have large discrepancies, including
  beam position, beam energy, spectrometer central momentum setting, septum current, and
  the location of anneals. For both the 2.2 and 1.1 GeV settings, there are significant shifts in
  the x and y beam position (2mm or more). She showed the average and standard deviation
  of epics readings for the LHRS septum current for each run. For several runs, there are small
  number of epics readings that drift from the average current. It's not clear if this is the
  current is actually changing or if it is a readout problem. She will look into the yield vs time
  to try to determine this. JP suggested making a raster cut on the center of the beam to get
  rid of any potential scraping. Pengjia says there is a way to cut on raster size, but there will
  be an additional BPM uncertainty. Melissa will discuss this more offline with Pengjia. More
  details can be seen in her slides here.

Pengjia
- Gave an update on calculating asymmetries/cross sections using the MAID model. This time,
  he used the Bosted model to get the unpolarized cross sections for the proton. Using unradiated
  cross section models, he still saw a factor of 6 difference when compared with data. Ryan also
  calculated asymmetries using unradiated models, and the results were consistent with Pengjia's,
  so it seems the radiative effects contribute significantly to the factor of 6 difference. More
  details can be seen in his slides here.

7/15/2015

Present: JP, Min, Chao, Jixie, Melissa
By Phone: Ryan, Toby, Ellie, Karl, Vince, Pengjia

Feature Presentations:

Chao
- Doing a check of simulation/data results for the 2.2 GeV, longitudinal setting in order
  to check that the optics calibration matrix will work for all momentum settings (not
  just elastic). He showed a comparison for several different momentum settings.
  Vince suggested simulating more events, since the statistics start to drop off for
  lower momentum settings. In general, the data is broader then the result from
  simulation. In the θ distribution, the "tails" seen in the data are not symmetric. JP
  suggested carefully selecting cuts to remove any background from the data to
  ensure a good comparison with simulation. More details can be seen in his
  slides here.

Pengjia
- More discussion on Pengjia's slides from last week. Pengjia has seen a factor of
  six difference between his asymmetry calculated from data compared to the
  asymmetry determined using the MAID model. Ryan did not see this factor of six
  in his study, but in addition to using radiated XS models, he is using a combination
  of the Bosted and MAID models (Pengjia is using just the MAID model). Karl
  commented that the XS from MAID is not very good for our kinematics. Pengjia
  also showed an estimated NH3 XS. JP commented that it was surprising that the
  delta peak wasn't visible. Pengjia will look into this for next time.

7/8/2015

Present: JP, Min, Chao, Melissa
By Phone: Toby, Vince

Feature Presentations:

Pengjia
- Working on comparing asymmetries determined from MAID to those calculated from
  data; last time he showed a factor of 6 difference between the two results. This
  time, he showed two different methods for calculating the differential XS from the
  virtual photon XS. The first method calculates the differential XS directly from
  the virtual photon XS, and the second method calculates the differential XS using
  F₁, F₂, g₁ and g₂. He also showed two different methods for calculating the
  asymmetry from the virtual photon XS; one method uses A₁ and A₂, while the other
  is calculated from the virtual photon XS. There was a discrepancy between the two
  methods, but he found that there was actually a mistake in one of the equations.
  While the results from the two methods agree with each other, they are still a factor
  of 6 larger than the results from data. More details can be seen in his slides here.

Toby
- Showed an update of his scattering angle study for model reconstruction. He is
  using BPM and optics data to determine the scattering angle. He showed an example
  of the calculated scattering angle; JP questioned whether the range was really from
  0-16 deg. Toby pointed out that there really aren't many events at the boundaries,
  the distribution is peaked between 4.5-9 deg. He then uses this scattering angle as
  input for the Bosted model. At low ν the simulation results match reasonably well
  with the data, but the discrepancy starts to become larger as ν increases. He took
  a closer look at the reconstructed scattering angle for each central momentum, and
  found that a second peak is visible for smaller values of p0. If this second peak was
  real, the simulation should recreate the data, so this could suggest a problem with
  the reconstruction. However, Chao pointed out that, for this energy setting, the optics
  calibration is not complete (currently using the longitudinal optics matrix), so the
  reconstructed θ and φ may not be correct yet. For next time, Toby will test this
  analysis using the 1.7 GeV setting, which has calibrated optics. More details can
  be seen in his slides here.

Melissa
- Gave a summary of packing fraction analysis. For some settings, the variation in the
  yields (and P_f) is due to a fluctuation in the beam position. This will hopefully be
  resolved by Jie's beam position/acceptance study. For the 2.2 GeV, 2.5T transverse setting
  there is also an issue of a shift in ν between runs. For the 1.1 GeV, 2.5T transverse
  setting, the yields vary depending on the beam current used for each run. JP suggested
  a few things to check as the possible cause for these discrepancies including drift in the
  BCM calibration, anneals of the target material, drift in field (HRS or septum), etc. She
  has posted a technote draft here, feedback would be appreciated. More details can be seen in her slides here.

Min
- Gave an update on her acceptance study. Last time, she showed that the simulation results
  are more narrow than the data. For this time, she tried cutting on just the center hole,
  and found that the dp distribution from data better matched the simulation. JP suggested
  cutting on each of the different holes individually to determine which hole causes the dp
  distribution to be wider. More details can be seen in her slides here.

Chao
- Gave an update on optics analysis, currently checking whether simulation results match
  with data, starting with optics data. First he checked whether the geometry was correct,
  some deviation was found in the recent optics meeting. The position of the BPMs is hard-
  coded into the simulation, and the drifting algorithm is used to drift the electrons
  backward to the BPMs to simulate the readout of BPM A and B. He compared the θ
  distribution before drifting in the target field using a fixed and non-fixed BPM location.
  The results using a fixed BPM match well with the data. He is currently working on
  checking this for all momentum settings, and will follow up with Pengjia about the BPM
  problem. More details can be seen in his slides here.

7/1/2015

Present: Chao, Vince, Jie, JP, Min, Melissa
By Phone: Pengjia

Feature Presentations:

Min
- Gave an update on acceptance studies. She showed a summary table of each energy
  setting and septum configuration. There hasn't been any problems in the 2.2 GeV, 5T
  longitudinal and 1.7 GeV, 2.5T transverse settings, but some settings (2.2 GeV, 2.5T
  transverse, 1.2 GeV, 2.5T transverse and 2.2 GeV, 5T transverse) have a discrepancy
  in the focal plane between data and simulation. She compared the data divided by the
  Mott XS to the simulation results without XS, and found that the simulation results
  of θ and φ are narrower then the data. JP commented that, while including the Mott
  XS might change the shape of the distribution, it shouldn't change the boundary. She
  work on figuring out the cause for this discrepancy before calculating the acceptance.
  More details can be seen in her slides here.

Pengjia
- Comparing asymmetries determined from the MAID model to those calculated from data.
  Last time he showed a factor of 6 difference between the two results. He determined the
  scattering angle and Q² by fitting the data and compared them to the quantities calculated
  from MAID. He also showed the results of calculating the quantities g₁, g₂, F₁, F₂, A_L, A_T,
  dXS_L, dXS_T, XS_tot andXS_mott using MAID. He hasn't included radiative corrections in his
  calculations, but this probably won't account for the factor of 6. More details can be seen
  in his slides here.