In high energy

particle physics Particle physics or high energy physics is the study of fundamental particles and forces that constitute matter and radiation. The fundamental particles in the universe are classified in the Standard Model as fermions (matter particles) an ...

nucleon In physics and chemistry, a nucleon is either a proton or a neutron, considered in its role as a component of an atomic nucleus. The number of nucleons in a nucleus defines the atom's mass number (nucleon number). Until the 1960s, nucleons were ...

lepton In particle physics, a lepton is an elementary particle of half-integer spin ( spin ) that does not undergo strong interactions. Two main classes of leptons exist: charged leptons (also known as the electron-like leptons or muons), and neutr ...

scattering, the semi-inclusive deep inelastic scattering (SIDIS) is a method to obtain information on the nucleon structure.H. Avakian, B. Parsamyan and A. Prokudin, “Spin orbit correlations and the structure of the nucleon,”
Riv. Nuovo Cim. 42, no.1, 1-48 (2019). It expands the traditional method of

deep inelastic scattering Deep inelastic scattering is the name given to a process used to probe the insides of hadrons (particularly the baryons, such as protons and neutrons), using electrons, muons and neutrinos. It provided the first convincing evidence of the reality ...

(DIS). In DIS, only the scattered lepton is detected while the remnants of the shattered nucleon are ignored (inclusive experiment). In SIDIS, a high

momentum In Newtonian mechanics, momentum (more specifically linear momentum or translational momentum) is the product of the mass and velocity of an object. It is a vector quantity, possessing a magnitude and a direction. If is an object's mass an ...

hadron In particle physics, a hadron (; grc, ἁδρός, hadrós; "stout, thick") is a composite subatomic particle made of two or more quarks held together by the strong interaction. They are analogous to molecules that are held together by the ele ...

, a.k.a. as the ''leading hadron'' is detected in addition to the scattered lepton. This allows us to obtain additional details about the scattering process kinematics.

Usefulness

The leading hadron results from the

hadronization Hadronization (or hadronisation) is the process of the formation of hadrons out of quarks and gluons. There are two main branches of hadronization: quark-gluon plasma (QGP) transformation and colour string decay into hadrons. The transformation ...

of the struck

quark A quark () is a type of elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nuclei. All commonly o ...

. This latter retains the information on its motion inside the nucleon, including its transverse momentum which allows to access the

transverse momentum distributions In high energy particle physics, specifically in hadron-beam scattering experiments, transverse momentum distributions (TMDs) are the distributions of the hadron's quark or gluon momenta that are perpendicular to the momentum transfer between the b ...

(TMDs) of

partons In particle physics, the parton model is a model of hadrons, such as protons and neutrons, proposed by Richard Feynman. It is useful for interpreting the cascades of radiation (a parton shower) produced from quantum chromodynamics (QCD) processes a ...

. Likewise, by detecting the leading hadron, one essentially ''tags'' (i.e. identifies) the quark on which the scattering occurred. For example, if the leading hadron is a

kaon KAON (Karlsruhe ontology) is an ontology infrastructure developed by the University of Karlsruhe and the Research Center for Information Technologies in Karlsruhe. Its first incarnation was developed in 2002 and supported an enhanced version of ...

, we know that the scattering occurred on one of the

strange quark The strange quark or s quark (from its symbol, s) is the third lightest of all quarks, a type of elementary particle. Strange quarks are found in subatomic particles called hadrons. Examples of hadrons containing strange quarks include kaons ( ...

s of the nucleon's quark sea. In DIS the struck quark is not identified and the information is an indistinguishable sum over all the quark flavors. SIDIS allows to disentangle this information.

Experiments

SIDIS measurements were pioneered at

DESY The Deutsches Elektronen-Synchrotron (English ''German Electron Synchrotron''), commonly referred to by the abbreviation DESY, is a national research center in Germany. It operates particle accelerators used to investigate the structure of matt ...

by the

HERMES Hermes (; grc-gre, Ἑρμῆς) is an Olympian deity in ancient Greek religion and mythology. Hermes is considered the herald of the gods. He is also considered the protector of human heralds, travellers, thieves, merchants, and orato ...

experiment. They are currently (2021) being carried out at

CERN The European Organization for Nuclear Research, known as CERN (; ; ), is an intergovernmental organization that operates the largest particle physics laboratory in the world. Established in 1954, it is based in a northwestern suburb of Gene ...

by the

COMPASS A compass is a device that shows the cardinal directions used for navigation and geographic orientation. It commonly consists of a magnetized needle or other element, such as a compass card or compass rose, which can pivot to align itself with ...

experiment and several experiments at

Jefferson Lab Thomas Jefferson National Accelerator Facility (TJNAF), commonly called Jefferson Lab or JLab, is a US National Laboratory located in Newport News, Virginia. Its stated mission is "to provide forefront scientific facilities, opportunities and ...

. SIDIS will be an important technique used in the future Electron Ion Collider scientific program.A. Accardi et al., “Electron Ion Collider: The Next QCD Frontier - Understanding the glue that binds us all,”
2012.

References

{{reflist Quantum chromodynamics Nuclear physics Scattering