IR Design Requirements

This page discusses the requirements imposed by the EIC physics on the IR design.
The following requirements will be discussed in more detail below

1. the detection of neutrons of nuclear break up in the outgoing hadron beam direction
2. the detection of the scattered protons from exclusive and diffractive reaction in the outgoing proton beam direction
3. the beam element free region around the IR and the requirements on the magnetic field of the detector
4. space for low Q² scattered lepton detection
5. space for the luminosity monitor in the outgoing lepton beam direction
6. space for lepton polarimetry

Breakup Neutrons

For exclusive and diffractive reactions in e-A scattering it is essential to detect the neutron of the nuclear break up in the direction of the outgoing beam. The figure below shows the scattering angle distribution for breakup neutrons from a gold nucleus for different excitation energies of the nucleus.

These distributions lead to the requirement of a angular acceptance of +/- 3mrad to allow to detect these neutrons in the ZDC

Scattered protons

To reconstruct the Mandelstam variable t, which represents the momentum transfer to the proton in exclusive reactions, it is critical to detect the forward going scattered proton. t is essential in exclusive reactions as it can be fourier transformed to the impact parameter b, which gives the transverse spatial distribution of partons in the proton. The figure below shows the scattering angle θ for three different center-of-mass energies as a function of the scattered proton momentum.

This poses the following requirement that for different hadron beam energies 100 GeV and 250 GeV protons with a momentum 20\% lower then the beam energy and a scattering angles up to 12 mrad need to be transported through the different magnets.
There is only a very weak correlation between the momentum and the scattering angle. These protons cannot be detected in the main detector. The standard detectors used to detect the scattered proton are roman pots placed at different distances from the IR. Using this detector technology poses an other requirement on the machine performance. To reach as small scattering angles as possible a small emittance of the beam is crucial as there is also an additional requirement of 10 σ clearance from the core of the beam. To have good acceptance at low scattering angle the beam needs to be cooled in transverse direction to achieve a beam angular divergence of ~100μrad