Direct measurement of photons from the electron-hadron bremsstrahlung at the EIC

TL;DR

The paper addresses the challenge of achieving sub-percent absolute luminosity accuracy at the EIC by pursuing direct measurement of bremsstrahlung photons, leveraging insights from HERA. It compares a PbWO4-based SGC suitable for very low luminosity with a copper-QFi spaghetti calorimeter (MGC) designed for high-rate operation, using Geant4 simulations and Bremge-generated bremsstrahlung spectra. Results show the SGC is limited by radiation damage and viable only for short calibration runs, while the MGC demonstrates high-rate tolerance with fast Cherenkov signals and good linearity, enabling precise energy-scale calibration under realistic EIC conditions. A key takeaway is that the copper-QFi approach offers a viable path to high-precision absolute and relative luminosity measurements, contingent on prototype validation and the development of fast SiPM-based readout electronics (e.g., 200 MHz digitization).

Abstract

Direct detection of bremsstrahlung photons, in principle, offers the most straightforward and most robust method of luminosity determination at the EIC, but requires an extraordinary performance of the photon detector. In this paper, we first discuss the extreme working conditions for such detectors at the EIC and the resulting technology choices. Then, we report the initial results of Monte Carlo simulations, using Geant4, of the proposed sampling calorimeter, which is made of a copper absorber with embedded quartz fibres read out by silicon photomultipliers. Finally, the tentative requirements for appropriate readout electronics are formulated.

Figures (9)

• Figure 1: (left) The bremsstrahlung diagram in the leading order, and (right) a simple sketch of the layout of the bremsstrahlung detectors at the EIC with two movable calorimeters for direct photon detection and two movable synchrotron radiation filters (SR), where the dashed rectangle represents the $e^+e^-$ pair spectrometer, PS, for the detection of converted photons, which includes two dipole magnets D.
• Figure 2: Distribution of the total energy of bremsstrahlung photons per bunch crossing at low (left) and high (right) EIC luminosity, assuming the photon threshold energy of 10 MeV. The nominal $ep$ luminosity of $10^{34}\;\rm cm^{-2}s^{-1}$ corresponds to an average number of such photons $\mu=39$. The distribution in the insert corresponds to the $e$Au luminosity of $5\cdot10^{33}$ cm$^{-2}$s$^{-1}$ per nucleon.
• Figure 3: (left) Distributions of the dose per 1 fb$^{-1}$ due to bremsstrahlung in a PbWO$_4$ calorimeter, assuming the 10 GeV and 275 GeV electron and proton beams, respectively -- the longitudinal dose distribution for $y=0$, and (right) the transverse distribution at the maximum of the longitudinal distribution.
• Figure 4: (left) Distributions of the dose due to bremsstrahlung in central fibres of copper-quartz "spaghetti" calorimeter, assuming a 10 GeV electron beam colliding with 275 GeV protons per 100 fb$^{-1}$, and (right) in fibres 30 mm away horizontally from the central fibre. The calorimeter is tilted vertically by 5 degrees.
• Figure 5: Pattern of fibres.