Tuesday, Jun 25, 2013 11:00am EDT
From Hall A Wiki
Agenda
Minutes
- Attendance: David A., Wouter, Seamus, Mark Pitt, Rupesh, KK
- David A:
- Defining what the tracking system needs to do
- Some simulation progress so far..
- KK:
- What is needed for MIE?
- Nilanga agreed to write the MIE tracking section
- No breakdown in MIE proposal for tracking systematics. We just claim that we will get 0.5%.
- Old MIE proposal has some argument for 0.5%, but need more work.
- Want to get something more concrete and construct a systematic error table.
- KK:
- Monte Carlo task: move collimator around, and observe how APV varies -> Use this as geometry argument.
- David A:
- In the simulation, need a correct central theta, beam energy, and distribution.
- KK:
- We need to come up with a nomenclature so that we can put this in MIE, and starting using this moving forward. Perhaps, dig up E158, and see what was used here.
- David A:
- Tracking system is not trying to measure absolute central angle. Tracking system is used to understand acceptance around this: radiation losses, acceptances around this, acceptances in the detector etc.
- Reply on survey to give us a central angle about the collimator, and use the simulation to understand the acceptances/distributions.
- With the thick target (can never get the kinematics at the scattering vertex), can only get some effective kinematics.
- Seamus:
- Need an effective acceptance function (somewhat similar to what we did in PREX)
- First order uncertainty: move collimator around, vary magnetic field, and observe how it affects APV.
- Distribution matching: various distributions
- r,r',phi,phi' describes a track - GEM measurements
- Move GEMs closer to detector - resolution improves
- Direction variables more important for reconstruction
- Mollers: theta_CM vs r: basically all the theta_CM are focused at one point in r <- spectrometers designed to focus Mollers to a point in r.
- Mollers: theta_CM vs r': theta_CM more spread in r',
- eps: theta_ver vs r and theta_ver vs r': different than Mollers, and more features <- perhaps quadratic? theta_ver vs r' has strong correlation, and can be very useful for tracking.
- KK: need to isolate these events
- point target of size Z where elastic will dominate
- very high radiation length tiny calorimeter that can slide up and down, in order to pick up tracks.
- David A:
- radial separation between eps and Mollers will allow us to pick out ep or Mollers cleanly by putting a cut on r.
- perhaps something similar on phi, but less interesting.
- tracking:
- st. line fits to GEMs, and get the correlations at the target variables
- do not need anything as complicated as in QWEAK.
- Seamus:
- need some information upstream of the magnet: either from survey, GEMs or something..
- because we need some absolute calibration
- Seamus:
- Inelastics occupy a large phase-space, but not sure that we can gain anything useful for tracking here. Most of the tracking info is probably going to come from ep.
- KK:
- why it is that we can get away without any tracking information upstream of magnets?
- Eprime: we don't need to measure Eprime event by event
- we can get away without measuring Eprime, but need to justify why we can do this
- David A:
- can't measure Eprime because of radiative tail.
- KK:
- think about thin target, sieve hole, movable collimator: whatever are needed to get the correlations cleanly
- Seamus:
- Carbon foils, sieve holes spaced cleanly: to map out the phase space
- What do the magnetic fields do to the acceptances (especially at the edges)?
- David A:
- acceptance is defined by collimator, spectrometers, detector locations.
- If acceptance has mag field & main detector location to it, which it does with the radiative tail, then we get some distributions with simulation & slightly different distributions in data, how do we quantify our error due to mismatch in distributions?
- we adjust the knobs like theta, Ep etc in simulation and try to quantify this error.
- KK:
- carbon foil, sieve: gives us kinematic factor that multiplies the asymmetry: perhaps can use this.
- Seamus:
- there are always going to be some mismatch between simulation and data, and ..
- Mark Pitt:
- basically measuring an acceptance function:
- QWEAK: theta, Eprime distribution and match data. But for Moller we can't do this.
- David A:
- radiative losses make it very difficult to know theta, Ep at the target.
- thin targets, wire targets with rastered/unrastered beam
- Acceptance function:
- fit theta vs r horizontal slices, and compare this distribution to data.
- Dave:
- link dr, r to theta_ver with a functional fit, but how are we going to get the uncertainty?
- Seamus:
- big question: GEM resolution
- David A:
- GEMs are probably overkill
- no point in getting more resolution than the resolution of the spectrometers
- real reason that we need resolution is to identify real/good tracks
- Seamus:
- 5mrad resolution will probably suffice. much better than what GEMs can do.
- KK: naive model:
- combination of foil target, sieve hole to validate field and acceptance function
- use H2 gas run to demonstrate that we can calculate cross-section
- run with LH2 production target and look at radiative effects.
- KK:
- for MIE, if we stick with 3 GEMs upstream of the detectors, as long as we do not need a larger lever arm, we can do away with the Roman pot.
- David A:
- prefer 4 GEM planes better: redundancy, efficiency headroom.
- KK:
- can always reduce the GEM surface area, and rotate more.
- rotation system:
- most of our sensitivity are going to be in radial direction
- if we do not know phi GEM position much, then not a big deal
- radius on a rotator is much easier
- radial survey much easier than phi - and more precise?
- KK:
- not so sure about this.
- phi defocusing completely dominates in some regions.
- KK:
- tie the GEMs together for rotation? over 5 m?
- David A:
- 4 GEM planes: self calibration easy.
- if the main detectors are surveyed very well, then we can use the main detectors to
- KK:
- need some crude trigger scintillator, that can come off or turned off to trigger for charged particles.
- Mark Pitt/David A:
- need to move scintillator out of the way, to prevent radiation damage.
- David A:
- use thin detectors as trigger?
- KK:
- useful to have thin detectors by itself.
- David A:
- have trigger scintillator housed on the GEM housing, so it rotates with the GEMs?
- KK:
- 3 or 4 GEM planes.
- David A:
- all GEMs locked together locked tougher much easier to deal with.
- KK:
- 2 GEMs couple together, 1 m apart?
- 4 GEMs
- Mark Pitt:
- pairs of VDCs, HDCs in QWEAK: relatively stable during rotation
- GEMs in vacuum: worried about exit window thickness?
- KK:
- can make the exit window thin enough
- David A:
- sieve collimator?
- KK:
- roll in/out in front of the acceptance collimator
- prefer to do it for all 7 collimator
- David A:
- needs to be surveyed as well, as well as the primary collimator.
- KK:
- during E158, did a similar thing, and repeatedly used this sieve collimator
- Dave:
- need to completely block out primary collimator?
- very useful in QWEAK for background studies.
- KK:
- need to think this through.
- Seamus:
- What is the dependence on the magnetic field?
- position uncertainties,
- David A:
- how does the radial distribution of eps change when we move the coil?
- what does r' do when the coil is moved?
- Seamus:
- need slopes for this
- David A:
- slopes for the absolute magnetic field
- slopes for beam position on the target - first order cancellation around the sextants, but won't have perfect symmetry.
- Seamus:
- need to consider raster as well, the plots in the document discussed in this meeting was generated with the raster turned on.
- KK:
- can do this only with optics target?
- David A/Seamus:
- need eps because the phase space is different for different tragets/processes.
- Seamus:
- elastic C might even be better for this?? need to think this through..
- KK:
- Work on Near term, longer term plan.
- Near term: what goes into MIE.
- David A:
- Do we have Al or carbon elastic generator?
- Does a nuclear target give us any advantage??
- Can we trust anything without any radiative effects included?
- David A:
- need to see the affects in asymmetry, to r' distribution as we move collimator.
_________________________________________ _________________________________________
- David A:
- would probably want main detectors in event mode, because we would want to look at pulse height distributions.
- Mark Pitt:
- QWEAK used two different PMT bases, because of 10^6 difference in rates between current and event mode.
- KK:
- PREX used the same base, modified PMT voltages between current and event mode running.
- PREX production voltage 600 V, tracking voltage 2000V; lost only 1 of 7-8 PMTs: could do something similar.