# DIS: What is important

This page describes the important considerations to experimentally realize the physics program described in the EIC White Paper.The measurements that comprise the EIC physics program divide into four classes of scattering reaction:


The following sections discuss the requirements placed on a DIS detector in order to measure these reactions, and how each reaction relates to the key physics measurements at an EIC.

## Inclusive scattering

Inclusive DIS
Inclusive DIS refers to measurements that involve only the scattered lepton.

The requirements for detecting the scattered lepton are critical, and remain the same for semi-inclusive, exclusive and diffractive reactions, as the scattered lepton defines the parton kinematics (through x and Q2) for all reaction types.

#### Key Physics Measurements

Below is a list of measurements from the EIC WP are that rely only on the measurement of the scattered lepton. These measurements are based on the polarised spin structure function g1 and the unpolarised structure functions F2 and FL.

ep:

• The Longitudinal Spin of the Nucleon EIC WP chapter 2.1 (pages 14-32)

eA:

• Gluon saturation through nuclear structure functions EIC WP chapter 3.2.1 (pages 71+), chapter 3.2.2 (page 78) and chapter 3.3 / 3.3.1 (pages 87+)

#### Kinematics describing inclusive DIS

Details of the scattered lepton kinematics can be found here

#### Detector Design Requirements following from inclusive DIS

Details of the requirements imposed on detector performance can be found here

#### Performance Studies for inclusive DIS

Details can be found here

## Semi-inclusive scattering

Semi-inclusive DIS
Semi-inclusive DIS (SIDIS) involves measuring one or more (in the case of correlation measurements) final-state particles in addition to the detection of the scattered lepton. A schematic view of a SIDIS event is shown in the figure.

All the requirements for detection of the scattered lepton apply in SIDIS as for inclusive reactions. However further requirements are posed by the need to detect the final state hadron(s). A measured hadron is typically characterised by its transverse momentum, pT, with respect to the virtual photon (not the incident beams) and its energy fraction, z; see the section on kinematics for further details and definitions.

#### Key Physics Measurements

The following is a list of measurements that rely on the measurement of a hadron or hadrons in addition to the scattered lepton. References are given to the relevant sections of the EIC white paper (WP) for further details.

ep:

• Flavour-separated polarized (anti-)quark parton distributions. EIC WP chapters 2.1 (pages 14-21), 2.2 (pages 22-33).
• Transverse-momentum-dependent parton distributions. EIC WP chapters 2.1 (pages 14-21), 2.3 (pages 34-43).
• Investigate SU(3) flavour symmetry breaking, via (anti-)strange distributions. EIC WP chapter 2.2.2 (pages 25-26).

eA:

• Transverse-momentum-dependent gluon distributions and gluon saturation in nuclei, via di-hadron correlations. EIC WP chapter 3.2.2 (pages 76-86).
• Hadronisation and energy loss during the propagation of a fast-moving color charge in QCD matter. EIC WP chapter 3.3.2 (pages 90-93).

#### Kinematics describing semi-inclusive DIS

Details of the kinematics of the hadronic final state in coincidence with the scattered lepton can be found here

#### Detector Design Requirements following from semi-inclusive DIS

Details of the requirements imposed on detector performance can be found here

#### Performance Studies for semi-inclusive DIS

Details can be found here

## Exclusive scattering

Exclusive DIS
Exclusive DIS describes measurements that involve the measurement of the scattered lepton, the scattered proton (either intact or dissociated into a hadronic final state) and the reconstruction of the final particle produced in the interaction.

For low-x events the proton scatters at such a small angle that very often it escapes the main detector through the beam pipe hole, and its measurement requires forward detectors ("Roman Pots") installed along the beam pipe. The reconstruction of the produced particle, such as a photon or a vector meson decaying into leptons, imposes extra detector requirements, mainly on tracking and calorimetry. The accelerator and detector requirements are discussed on the EIC WP chapter 2.4.4

A note on terminology:

• coherent: proton stays intact
• incoherent: proton breaks up

#### Key Physics Measurements

Below is a list of measurements from the EIC WP are that rely only on the reconstruction of the entire final state.

ep:

• Spatial Imaging of Quarks and Gluons EIC WP chapter 2.4 (pages 44-58)
• Deeply Virtual Compton Scattering (DVCS): e + p → e' + p' + γ. EIC WP chapter 2.4.2
• Production of j/ψ decaying into leptons: e + p → e' + p' + j/ψ, j/ψ → e+ + e-. EIC WP chapter 2.4.2

eA:

#### Kinematics describing exclusive DIS

Details of the kinematics of the final state and the forward scattered proton and nuclei in coincidence with the scattered lepton can be found here

#### Detector Design Requirements following from exclusive DIS

Details of the requirements imposed on detector performance can be found here

#### Performance Studies for exclusive DIS

Details can be found here

## Diffractive scattering

Diffraction
Hard diffractive events, e(k) + N(p) → e′(k′) + N(p′) + X, are events where the proton/nucleus remains intact and the highly virtual photon fragments into a final state, X, that is separated from the scattered proton/nucleus by a large rapidity gap without any particles.

These events are indicative of a color neutral exchange in the t-channel between the virtual photon and the proton over several units in rapidity.

A note on terminology:

• coherent: nucleus stays intact
• incoherent: nucleus breaks up, but nucleons stay intact

#### Key Measurements

Below is a list of measurements from the EIC WP are that use diffractive events.

ep:

• The important measurements are described under exclusive reactions

eA:

• Gluon Saturation: a New Regime of QCD. EIC WP chapter 3.2.1 (pages 64 -86)
• Diffractive Physics. EIC WP chapter 3.2.1 (pages 74+)
• Measurements of Diffractive Events. EIC WP chapter 3.2.2 (pages 82+)

#### Kinematics describing diffractive physics

Details of the kinematics of the final state and the forward scattered nucleus/proton in coincidence with the scattered lepton can be found here

#### Detector Design Requirements following from diffractive events

Details of the requirements imposed on detector performance can be found here

#### Performance Studies for diffractive events

Details can be found here