The Drell–Yan process occurs in high energy
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 ...
–hadron scattering. It takes place when a
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 ...
of one hadron and an
antiquark
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 ...
of another hadron annihilate, creating a
virtual photon
A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they always ...
or
Z boson
In particle physics, the W and Z bosons are vector bosons that are together known as the weak bosons or more generally as the intermediate vector bosons. These elementary particles mediate the weak interaction; the respective symbols are , , and ...
which then decays into a pair of oppositely-charged
leptons
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 ...
. Importantly, the energy of the colliding quark-antiquark pair can be almost entirely transformed into the mass of new particles. This process was first suggested by
Sidney Drell
Sidney David Drell (September 13, 1926 – December 21, 2016) was an American theoretical physicist and arms control expert.
At the time of his death, he was professor emeritus at the Stanford Linear Accelerator Center (SLAC) and senior fel ...
and
Tung-Mow Yan Tung-Mow Yan (; born 1937) is a Taiwanese-born American physicist, who has specialized in theoretical particle physics; primarily in the structure of elementary particles, the standard model, and quantum chromodynamics. He is professor emeritus at C ...
in 1970
[
::And ''erratum'' in ] to describe the production of
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 ...
–
antilepton
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 ...
pairs in high-energy hadron collisions. Experimentally, this process was first observed by J.H. Christenson ''et al.''
[
] in proton–uranium collisions at the
Alternating Gradient Synchrotron
The Alternating Gradient Synchrotron (AGS) is a particle accelerator located at the Brookhaven National Laboratory in Long Island, New York, United States.
The Alternating Gradient Synchrotron was built on the innovative concept of the alternati ...
.
Overview
The Drell–Yan process is studied both in fixed-target and collider experiments. It provides valuable information about the
parton distribution functions
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 ...
(PDFs) which describe the way the momentum of an incoming high-energy nucleon is partitioned among its constituent partons. These PDFs are basic ingredients for calculating essentially all processes at hadron colliders. Although PDFs should be derivable in principle, current ignorance of some aspects of the
strong force
The strong interaction or strong force is a fundamental interaction that confines quarks into proton, neutron, and other hadron particles. The strong interaction also binds neutrons and protons to create atomic nuclei, where it is called the n ...
prevents this. Instead, the forms of the PDFs are deduced from experimental data.
Drell–Yan process and deep inelastic scattering
PDFs are determined using the world data from
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 ...
, Drell–Yan process etc.
The Drell–Yan process is closely related to the deep inelastic scattering; the Feynman diagram
of the Drell–Yan process is obtained if the Feynman diagram of deep inelastic scattering is rotated by 90°.
A time-like virtual photon or Z boson is produced in ''s''-channel in the Drell–Yan process
while a space-like virtual photon or Z boson is produced in ''t''-channel in the deep inelastic scattering.
Sensitivity to light sea quark flavor asymmetry in the proton
It had been naively believed that the quark sea in the proton was formed by
quantum chromodynamics
In theoretical physics, quantum chromodynamics (QCD) is the theory of the strong interaction between quarks mediated by gluons. Quarks are fundamental particles that make up composite hadrons such as the proton, neutron and pion. QCD is a type ...
(QCD) processes that did not discriminate between up and down quarks.
However, results of deep inelastic scattering of high energy muons on a proton and a deuteron
targets by CERN-NMC
[
][
]
showed that there are more
's than 's in the proton.
The Gottfried sum measured by NMC was 0.235±0.026, which is significantly smaller than
the expected value of 1/3.
This means that (''x'')-(''x'') integrated over Bjorken ''x'' from 0 to 1.0
is 0.147±0.039, indicating a flavor asymmetry in the proton sea.
Recent measurements using Drell–Yan scattering probed the flavor asymmetry of the proton.
[
][
]
To
leading order in the strong interaction coupling constant, α
s, the Drell-Yan cross section is given by