August 13, 2014 Calibration

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GlueX Calibration Meeting
Wednesday, August 13, 2014
11:00 am, EDT
JLab: CEBAF Center, F326

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  1. Announcements
  2. Commissioning Planning
  3. Simulations
  4. Online Monitoring
  5. AOT


Talks can be deposited in the directory /group/halld/www/halldweb/html/talks/2014-3Q on the JLab CUE. This directory is accessible from the web at .


Attending: Sean (NU); Simon, Lubomir, Mark I., Beni, Manuel, Mike S., Eugene, Elton, Mark D. (JLab); Curtis (CMU); Justin (MIT); Volker, Paul, Sasha, Aristeidis (FSU)

We started by discussing the requirements for the Fall running for each subdetector. Note that the Fall 2014 Commissioning Plan contains the official list of items such as radiators and targets.


  • Curtis & Naomi were out of town so there is no new information beyond what already exists at the CDC commissioning page. For initial calibrations, tracks from any events are sufficient.


  • The first step should be to align the beam with the FDC HV off, so that we are sure that the beam is well inside the "low mass" region of the FDC. One difficult step will be to check the FDC/Start Counter relative alignment. We have a limited ability to move the collimator around.
  • Next, take data with the magnetic field off for FDC alignment. At least 3M events are needed. These are expected to come from the commissioning target and the plastic target attached to the end of the vacuum beam pipe upstream from the magnet.
    • Running with an additional cross-shaped target would be beneficial. Eugene said that this target would have to be placed inside the commissioning target assembly, as there is currently no other place to mount another target upstream. The beam profile monitor could be used as an additional target.
    • Eugene pointed out that we may have to run the solenoid at a very low current instead of zero current. We will need to know how this affects the alignment procedure.
    • Running with a non-standard gas mixture (90/10 Ar/CO2) could be done if the schedule allows.
  • At least 3M tracks are needed with the magnet on to determine resolutions and Lorentz deflection effects.

There was some additional discussion of the properties of the commissioning target

  • The hydrogen target is 2% radiation lengths (R.L.) thick, while a 2 mm plastic target is 0.5^ RL thick
  • More relevant to photoproduction is the difference in the densities (0.1 g/cm^2 versus 1 g/cm^2?)
  • Also, with the plastic target, only ~1/6 of the nuclear recoils will be "clean" protons.
  • Overall, the 2 mm plastic target has very roughly 1/10 of the rate of the hydrogen target?


  • The conditions for FCAL are loose - anything that produces pi0's that can be used for calibrations is fine.
  • The total number of events needed should be determined, but from previous experiences it should be a few million.
  • The effect of EM backgrounds and trigger thresholds needs to be studied.
  • Currently, folks at IU are studying exclusive omega photoproduction events as a source of pi0's. Curtis pointed out that the overall gamma p -> pi+ pi- pi0 cross section is an order of magnitude larger. This shouldn't be a problem since the charged tracks are (presumably) being used to reduce the background and we don't need to fix to a narrow resonance. Inclusive pi0's might be useful after an initial calibration as well.


  • pi0 and eta decays into two photons will be used. Their angular distributions need to be studied in detail, but the pi0's tend to illuminate the downstream end of the detector, while the eta's have a more uniform distribution. Decays that have one shower in the FCAL and one in the BCAL will be especially useful.
  • Eugene pointed out that once the CDC is calibrated, charged tracks that enter the BCAL could be used as well.


  • With the field off, the following samples will be needed:
    • 500k events at low rate to perform geometrical alignment and initial timing calibration
    • 500k events at high rate to study TOF performance with multiple tracks per event
    • 5-6 short runs to perform PMT gain balancing. Note that gain balancing can't be done with vertically travelling cosmic rays since they illuminate the detector in very differently than tracks that travel horizontally. These runs could potentially be taken in parallel with other low intensity running.
    • Since the tracks from photoproduction events illuminate the detector non-uniformly, it would be useful to check the estimates of the number of events needed.
  • With the magnetic field on:
    • Several short runs are needed to study the effect of the solenoid fringe field on the PMT gains.
    • 10 M events under normal running conditions are needed to refine the calibrations. Selecting tracks belonging to particles of known type, e.g., kaons from gamma p -> Lambda K, will enable particle identification studies.
  • Other suggestions included using electrons and photons that convert in the TOF to refine the timing.

Start Counter

  • We reviewed the preliminary plans for the start counter. The data can be taken either with or without the magnetic field on, and it covers a large solid angle, so obtaining enough data will probably not be a problem. However, it should be verified that all of the different regions of the start counter will get enough data. Initial calibrations should be done with the lowest possible beam intensity.
  • Generally, it will be an iterative process, first checking out the electronics, then basic alignment and time calibrations, then time propagation and timewalk corrections.
  • The difference between the time measured in the detector and the RF time should also be determined.
  • It would be very useful if the scalar/discriminator information could be read out for online monitoring. Someone in Hall B supposedly has a setup to read this into EPICS.
  • The start counter should also be aligned with respect to the target.


  • Probably a version of the "standard GlueX trigger" will be used (i.e. primarily FCAL and BCAL energy sums).
  • At least one day will be needed for trigger commissioning.
  • The BCAL and FCAL energy trigger thresholds will need to be studied.
  • Beni suggested that it might be useful to take some data where the full waveforms are written out to disk. This data would be very disk intensive, but could be useful for other detector and trigger studies.


  • To obtain accurate estimates, new simulations including EM background are needed. Sean is organizing the generation of these simulations, along with Simon and Mark I.
  • A new tagged version of sim-recon is needed, so that anyone analyzing the data can work from a common point.
  • An incoherent radiator can be simulated in bggen by, e.g., varying the location of the coherent peak. The incoherent photon simulation should be reviewed.
  • The geometry in the simulation should be updated to reflect the Fall running, including the commissioning targets, the air around the detector and in the missing beam pipe, etc. Simon will be in charge of this.
  • The first order of business should be to determine inclusive charged particle and pi0/eta -> gamma gamma fluxes.
  • Sean mentioned that at NU they had been trying to inclusively reconstruct narrow resonances, and asked if we should more accurately model the recoil nucleons/nuclei from the commissioning targets. The consensus was that this could be done at a later point in time.
    • Beni asked about trying to simulate the expected neutron flux. Eugene said that it is not expected to be large enough to worry about, and that we will have neutron monitors in the Hall anyway.
  • Depending on the EM background, we might not be able to run at zero magnetic field. The lowest magnetic field at which we can run should be determined (300A? maybe lower).
  • Eugene reminded us that the final magnet geometry is slightly different than what is contained in HDDS, and should be updated at some point, though not necessarily for these exercises.

Online Monitoring

  • Initial commissiong/calibration tasks will rely heavily on online monitoring histograms. These histograms will be generated in a standard JANA plugin and read over the network with RootSpy. These plugins need to be written and tested before the Fall running. The BCAL plugin is in an advanced state, the other subdetectors less so.

Future Action Items

  • Update desired event counts (all)
  • Update expected photoproduction event rate calculations
  • Tag new sim-recon version (Mark I.)
  • Update geometry for commissioning running (Simon)
  • Obtain new constants for EM background simulation & magnetic field (Sean)
  • Review incoherent beam simulation
  • Decide number of events & conditions to simulate
  • Run simulations
  • What is the lowest solenoid current that we can run at?
    • How will a low solenoid current affect FDC alignment?
  • Study FCAL & BCAL trigger thresholds