Drell-Yan process
Encyclopedia
The Drell–Yan process occurs in high energy hadron–hadron scattering. It takes place when a quark
of one hadron
and an antiquark of another hadron annihilate, creating a virtual
photon
or Z boson which then decays into a pair of oppositely-charged leptons. This process was first suggested by Sidney Drell
and Tung-Mow Yan in 1970 to describe the production of lepton
–antilepton pairs in high-energy hadron collisions. Experimentally, this process was first observed by J.H. Christenson et al.
in proton–uranium collisions at the Brookhaven National Laboratory
, Alternating Gradient Synchrotron
.
(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 prevents this. Instead, the forms of the PDFs are deduced from experimental data.
(QCD
) processes that did not discriminate between up and down quarks. To leading order in the strong interaction coupling constant, αs, the ratio of the Drell-Yan cross section from a proton beam on a deuterium target to a proton beam on a proton target is given by
s of the Z boson to quarks. The main observable is the forward–backward asymmetry in the angular distribution of the two leptons in their center-of-mass
frame
.
If heavier neutral gauge bosons exist (see Z' boson), they might be discovered as a peak in the dilepton invariant mass
spectrum in much the same way that the standard Z boson appears by virtue of the Drell–Yan process.
Quark
A quark is an 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. Due to a phenomenon known as color confinement, quarks are never directly...
of one hadron
Hadron
In particle physics, a hadron is a composite particle made of quarks held together by the strong force...
and an antiquark of another hadron annihilate, creating a virtual
Virtual particle
In physics, a virtual particle is a particle that exists for a limited time and space. The energy and momentum of a virtual particle are uncertain according to the uncertainty principle...
photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
or Z boson which then decays into a pair of oppositely-charged leptons. This process was first suggested by Sidney Drell
Sidney Drell
Sidney David Drell is an American theoretical physicist and arms control expert. He is a professor emeritus at the Stanford Linear Accelerator Center and a senior fellow at Stanford University's Hoover Institution. Drell is a noted contributor in the field of quantum electrodynamics and particle...
and Tung-Mow Yan in 1970 to describe the production of lepton
Lepton
A lepton is an elementary particle and a fundamental constituent of matter. The best known of all leptons is the electron which governs nearly all of chemistry as it is found in atoms and is directly tied to all chemical properties. Two main classes of leptons exist: charged leptons , and neutral...
–antilepton pairs in high-energy hadron collisions. Experimentally, this process was first observed by J.H. Christenson et al.
in proton–uranium collisions at the Brookhaven National Laboratory
Brookhaven National Laboratory
Brookhaven National Laboratory , is a United States national laboratory located in Upton, New York on Long Island, and was formally established in 1947 at the site of Camp Upton, a former U.S. Army base...
, Alternating Gradient Synchrotron
Alternating Gradient Synchrotron
The Alternating Gradient Synchrotron is a particle accelerator located at the Brookhaven National Laboratory in Long Island, New York, USA....
.
Overview
The Drell–Yan process is studied both in fixed-target and collider experiments. It provides valuable information about the parton distribution functionsParton (particle physics)
In particle physics, the parton model was proposed by Richard Feynman in 1969 as a way to analyze high-energy hadron collisions. It was later recognized that partons describe the same objects now more commonly referred to as quarks and gluons...
(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 prevents this. Instead, the forms of the PDFs are deduced from experimental data.
Sensitivity to Light Quark Flavor Asymmetry in the Proton
One recent measurement using Drell-Yan scattering probed the flavor asymmetry of the proton. It had been naively believed that the quark sea in the proton was formed by quantum chromodynamicsQuantum chromodynamics
In theoretical physics, quantum chromodynamics is a theory of the strong interaction , a fundamental force describing the interactions of the quarks and gluons making up hadrons . It is the study of the SU Yang–Mills theory of color-charged fermions...
(QCD
QCD
The initialism QCD may refer to:* Quantum chromodynamics, the theory describing the Strong Interaction* Quantum circuit diagram* Quality, Cost, Delivery, a three-letter acronym used in lean manufacturing...
) processes that did not discriminate between up and down quarks. To leading order in the strong interaction coupling constant, αs, the ratio of the Drell-Yan cross section from a proton beam on a deuterium target to a proton beam on a proton target is given by
Z boson production
The production of Z bosons through the Drell–Yan process affords the opportunity to study the couplingCoupling
A coupling is a device used to connect two shafts together at their ends for the purpose of transmitting power. Couplings do not normally allow disconnection of shafts during operation, however there are torque limiting couplings which can slip or disconnect when some torque limit is exceeded.The...
s of the Z boson to quarks. The main observable is the forward–backward asymmetry in the angular distribution of the two leptons in their center-of-mass
Center of mass
In physics, the center of mass or barycenter of a system is the average location of all of its mass. In the case of a rigid body, the position of the center of mass is fixed in relation to the body...
frame
Reference frame
Reference frame may refer to:*Frame of reference, in physics*Reference frame , frames of a compressed video that are used to define future frames...
.
If heavier neutral gauge bosons exist (see Z' boson), they might be discovered as a peak in the dilepton invariant mass
Invariant mass
The invariant mass, rest mass, intrinsic mass, proper mass or just mass is a characteristic of the total energy and momentum of an object or a system of objects that is the same in all frames of reference related by Lorentz transformations...
spectrum in much the same way that the standard Z boson appears by virtue of the Drell–Yan process.