Proton Taggers

Complete: 3

What are the proton taggers?

Proton taggers is a short name for so called near-beam detectors, special tracker devices installed on the beam pipe hundreds metres away from the interaction point and aimed to detect protons which lost a small fraction of their energy or were scattered at small angles (forward protons). This package provides a method to simulate the acceptance of the proton taggers to be installed 420 and 220 m from IP5 (in both beam directions).

Implementation in Fast Simulation

Currently the only module defined in ProtonTaggers is ProtonTaggerFilter. The module is implemented in a form of EDFilter. Basically it can be viewed as a trigger code, which only passes through those events which have some forward protons in the taggers. As an input it needs MC generated events (in the form of HepMCProduct). The only output is a boolean value showing whether the event has been accepted. Conditions upon which an event is accepted are steered from .cfg file. You can choose between detector locations (220 and 420 metres or both) and minimum number of protons (1 or 2). Apart from a trigger bit the module does not add anything to the event.

This package is based on the acceptance simulation procedure from ORCA-based FAMOS (module FastTotem). It does a simple table-based calculation of the probability of detecting a forward proton (given its momentum vector) with a Roman Pot type detector installed in one of the four pre-defined locations (220 or 420m, two beam directions). Note that old FAMOS was able to simulate the measurement of proton fractional momentum loss. However the detector resolution simulation in FAMOS does not reflect the recent improvements in the near-beam detector design, and therefore has not been ported to CMSSW. In principle, proton taggers also allow for measurement of the t kinematic variable, connected to proton scattering angle, however this measurement has never been implemented in FAMOS.

How to use

The ProtonTaggers package is included in CMSSW starting from 1_7_0_pre6. It is also possible to use it with older releases. For that go to your CMSSW_X_Y_Z/src directory, do cvs co FastSimulation/ProtonTaggers; cd FastSimulation/ProtonTaggers; scarmv1 b.

To use the package you need to include in your .cfg file a piece of text like the following.

   module forwardProtonFilter = ProtonTaggerFilter
      # ... choose where you want a proton to be detected for beam 1 (clockwise)
      #     0 -> ignore this beam
      #     1 -> only 420 (FP420)
      #     2 -> only 220 (TOTEM)
      #     3 -> 220 and 420 (region of overlay)
      #     4 -> 220 or 420 (combined acceptance)
      uint32 beam1mode = 1

      # ... and for beam 2 (anti-clockwise)
      uint32 beam2mode = 1

      # ... choose how the information for the two beam directions should be combined
      #     1 -> any of the two protons (clockwise or anti-clockwise) is enough
      #     2 -> both protons should be tagged
      #     3 -> two protons should be tagged as 220+220 or 420+420 (makes sence with beamXmode=4)
      #     4 -> two protons should be tagged as 220+420 or 420+220 (makes sence with beamXmode=4)
      uint32 beamCombiningMode = 2

... and add forwardProtonFilter in your path. In this example only events with two protons tagged by the two arms of FP420 will survive after the filter. The results of the filter (true or false) can then be found in the TriggerResults container in the resulting Event stream. SelectEvents parameter of PoolOutputModule can be used to select the "triggered" events to be written on disk. See example here: FastSimulation/ProtonTaggers/test/testProtonTaggers.cfg (you might need to get the newest version with cvs co)

Known limitations

Like other FastSim packages, this package is aimed to be a fast and simple tool for quick physics studies which cannot wait for a heavy full simulation. However it has limitations:

1. The acceptance tables currently in use are quite old and may not provide a very high precision simulation.

2. The module reads generated data only in the form of HepMCProduct. It does NOT read pileup simulated as SimTracks/!SimVertex's.

3. Only one proton, with highest Pz, is simulated in each beam direction. This may not be appropriate in the presence of high pileup.

Review Status

PatrickJanot - 12 Sep 2007 Template page
DmitryZaborov - 12 Sep 2007 First documentation for ProtonTaggers
DmitryZaborov - 05 Oct 2007 Add usage example
DmitryZaborov - 28 Oct 2007 Add "Known issues" section, update example
DmitryZaborov - 29 Oct 2007 Revise instructions for use

Responsible: PatrickJanot
Last reviewed by: DmitryZaborov

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Topic revision: r6 - 2007-10-29 - DmitryZaborov

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