FarBackward

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EPIC Far-Backward Detector Working Group

Conveners:

  • Nick Zachariou (nick.zachariou@york.ac.uk)
  • Igor Korover (korover@mit.edu)
  • Jaroslav Adam (adamjaro@centrum.cz)
  • Krzysztof Piotrzwowski (piotrzkowski@agh.edu.pl)

Meetings: INDICO on Thursdays @ 10:00am EDT, unless noted otherwise via email.

Far-Backward Sub-systems

The path of the electron beam downstream of the interaction point is denoted as the far-backwards region along which photons from e+p and e+A interactions will travel. These photons come predominantly from the bremsstrahlung process used for luminosity determination using the direct photon detector and a pair spectrometer for complementary measurements. The luminosity measurement provides the required normalization for all physics studies. At the broadest scale it determines absolute cross sections, such as needed for the structure function F2 and derived PDFs. On an intermediate scale, it is also required to combine different running periods, such as runs with different beam energies needed to measure FL, or runs with different beam species to study A dependencies. Asymmetry measurements are conducted using beams with bunches of both spin states. On the finest scale, the relative luminosity of the different bunch crossings is needed to normalize the event rates for the different states; the uncertainty on the relative bunch luminosity is a limiting factor for asymmetry measurements. The bremsstrahlung process was used successfully for the measurement of luminosity by the HERA collider experiments. It has a precisely known QED cross-section which is large, minimizing theoretical uncertainty and providing negligible statistical uncertainty. Thus the scale uncertainty of the luminosity is determined by the systematic uncertainties of the counting of bremsstrahlung events, and complementary measurements allow us to reduce systematic uncertainties. Bremsstrahlung and low-Q2 processes produce electrons with momenta slightly below the beam energy, which can be bent out of the beam by lattice dipoles and measured by taggers.

  • Luminosity Direct Photon Detector

The direct photon detector system is placed on the zero-degree line in the far backward region. This system allows the direct determination of the number of bremsstrahlung photons The calorimeter is also exposed to the direct synchrotron radiation fan and must be shielded, thus degrading the energy resolution. This also imposes a rough low energy cutoff on photons typically ≈ 0.1-1 GeV below which the calorimeter is insensitive. At peak HERA luminosities, the photon calorimeters were sensitive to 1-2 photons per HERA bunch crossing. At an EIC luminosity of 1033 cm−2 s−1, the mean number of such photons per bunch crossing is over 20 for electron-proton scattering and increases with Z2 of the target for nuclear beams. The per bunch energy distributions are broad, with a mean proportional to the number of photons per bunch crossing. The counting of bremsstrahlung photons thus is effectively an energy measurement in the photon calorimeter with all of the related systematic uncertainties (e.g. gain stability) of such a measurement.

  • Luminosity Pair Spectrometer

The pair spectrometer provides a complimentary measurement of luminosity that is outside the primary synchrotron radiation fan and is operated in a reduced rate environment. A small fraction of the bremsstrahlung photons are converted into e+e− pairs in the vacuum chamber exit window. A dipole magnet splits the pairs vertically and each particle hits a separate calorimeter adjacent to the unconverted photon path. The spectrometer rate is directly proportional to the fraction of photons which convert into e+e− pairs, placing stringent requirements on the photon exit window. It must have a precisely known material composition, and a precisely measured and uniform thickness along the photon direction.

Calorimeters are required for both luminosity devices, for triggering and energy measurements. The high rates dictate a radiation hard design, especially for the zero-degree calorimeter, which must also have shielding against synchrotron radiation. The spectrometer must also have precise position detectors to measure the e±. Combined with the calorimeter energy measurement this allows reconstruction of the converted photon positions. The distribution of photon positions is required to correct for the lost photons falling outside the photon aperture and detector acceptances.

  • Low Q^2 Taggers

Downstream of the interaction point the electron beam is accompanied by a flux of electrons at small angles with respect to the beam direction and at slightly lower energy. They are predominantly final state electrons from the bremsstrahlung process. Also, a fraction of the electrons in this region is produced in quasi-real photoproduction with Q2 ≈ 0.

The final state bremsstrahlung electrons provide a powerful tool for calibrating and verifying the luminosity measurement with photons. Tagging bremsstrahlung electrons and counting corresponding photons in the photon detectors provides a direct measure of the luminosity detector acceptance in the tagged energy range. This is of paramount importance to precisely determine the pair conversion probability for the luminosity spectrometer, which depends on the exit window composition and thickness. Such measurements will require special runs with low bunch currents to ensure that there is only one bremsstrahlung electron/photon pair in the system per bunch crossing.

Tagging of low-Q2 processes provides an extension of the kinematic range of DIS processes measured with electrons in the central detector. It crosses the transition from DIS to hadronic reactions with quasi-real photons. Taggers provide useful acceptance in the range Q2 < 10−2 GeV2. Application of the electron taggers for low-Q2 physics will face a challenge from the high rate of bremsstrahlung electrons, which can be addressed by tagger design and correlation with information from the central detector.

ePIC Far-Backward Tasks Table

EPIC Geometry Implementation

  • Description: Initial inclusion and testing of Far-Backward systems in EPIC simulation framework.
  • Work Start: October 2022
  • Expected Duration: 2-4 months
  • Required Expertise: Medium/High
  • Task assigned to: Simon Gardner (Taggers), Dhevan Gangadharan, Aranya Giri (Luminosity)
  • Notes: Ongoing
  • Status: Revised Tagger geometries have been merged into the main epic repository; Lumi systems are under revision in a separate linked branch of epic.
  • Contact: Jaroslav Adam, Simon Gardner, Dhevan Gangadharan
  • Links: EPIC DD4hep geometry, Far-Backward EPIC Inclusion Breakdown

EPIC Reconstruction Implementation

  • Description: Initial inclusion and testing of Far-Backward systems in EPIC simulation framework.
  • Work Start: November 2022
  • Expected Duration: 2-4 months
  • Required Expertise: Medium/High
  • Task assigned to: Simon Gardner (Taggers)
  • Notes: Ongoing
  • Status: Preliminary plugins and algorithms for the taggers are being implemented in a branch.
  • Contact: Jaroslav Adam, Simon Gardner
  • Links: Far-Backward EPIC Inclusion Breakdown

Radiation dose in Sensitive elements

  • Description: Quantify dose in each of the 3 subsystems and each of the two technology
  • Work Start: Anticipated
  • Expected Duration: 2-4 months
  • Required Expertise: Medium
  • Task assigned to:
  • Notes: Will likely need input from global integration group.
  • Contact:
  • Links:

Event Pile up

  • Description: Quantify event pile up in each of the 3 subsystems and each of the two technology
  • Work Start: October 2022
  • Expected Duration: 2-4 months
  • Required Expertise: Medium
  • Task assigned to:
  • Notes:
  • Contact:
  • Links:

Vacuum system at Tagger stations

  • Description: Quantify impedance/synchrotron radiation
  • Work Start: October 2022
  • Expected Duration: 2-4 months
  • Required Expertise: Medium/High
  • Task assigned to:
  • Notes:
  • Contact:
  • Links:

Luminosity Exit Window

  • Description: Quantify exit window for luminosity monitors
  • Work Start: October 2022
  • Expected Duration: 2-4 months
  • Required Expertise: Medium/High
  • Task assigned to:
  • Notes:
  • Contact:
  • Links:

Pair Spectrometer

  • Description: Study size of PS calorimeters, tracking, magnet
  • Work Start: October 2022
  • Expected Duration: 2-4 months
  • Required Expertise: Medium/High
  • Task assigned to:
  • Notes:
  • Contact:
  • Links:

Getting Started and Resources

EIC tutorial 1: Setting up your environment: Indico

Geometry by DD4Hep: https://dd4hep.web.cern.ch/dd4hep/

Data models

Reconstruction by JANA2: https://jeffersonlab.github.io/JANA2/

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