Drell-Yan process

Drell-Yan process. In particle physics, the Drell-Yan process is a fundamental mechanism for the production of lepton-antilepton pairs in high-energy collisions of hadrons. It was first described theoretically by Sidney Drell and Tung-Mow Yan of the Stanford Linear Accelerator Center in 1970. The process provides a crucial testing ground for quantum chromodynamics and the parton model, and serves as a primary background and signal process in experiments at facilities like the Large Hadron Collider.

Overview

The process occurs when a quark from one incoming hadron and an antiquark from another annihilate via a virtual photon or Z boson, producing a lepton pair such as muon-antimuon or electron-positron. This mechanism is conceptually analogous to the time-reversed version of electron-positron annihilation in storage rings like those at DESY or CERN. Its discovery was pivotal in confirming the partonic structure of protons and other hadrons, as initially revealed in deep inelastic scattering experiments at the Stanford Linear Accelerator Center. The clean experimental signature of the resulting dilepton pair makes it a vital tool for probing the internal structure of colliding particles and for precision measurements of fundamental parameters.

Theoretical description

The theoretical framework is built upon the parton model, formalized within the context of quantum chromodynamics. The cross-section is factorized into parton distribution functions, which describe the momentum distributions of quarks and gluons within the hadrons, and a hard scattering matrix element calculable in perturbative QCD. This factorization, similar to that used in deep inelastic scattering, was rigorously established within QCD factorization theorems. The underlying annihilation proceeds through an intermediate virtual photon or Z boson, making it a purely electroweak interaction at leading order. Higher-order corrections involving gluon radiation are critical for precise predictions and are calculated using techniques like those developed for the Dokshitzer–Gribov–Lipatov–Altarelli–Parisi equations.

Experimental observation and significance

The first clear observations were made in the 1970s using proton-uranium collisions at the Alternating Gradient Synchrotron at Brookhaven National Laboratory and later in pion-beam experiments at Fermilab. It became a cornerstone process for testing QCD predictions and measuring parton distribution functions, notably contributing to the understanding of the sea quark content of the proton. At the Super Proton Synchrotron and the Tevatron, measurements provided stringent tests of the Standard Model. At the Large Hadron Collider, it is a critical irreducible background for searches for new physics, such as Higgs boson production via vector boson fusion, and for the discovery of new particles like the Z' boson. Precision measurements of the process also constrain potential contributions from physics beyond the Standard Model.

Cross-section and kinematic distributions

The differential cross-section depends on key kinematic variables: the invariant mass of the lepton pair, its transverse momentum, and the rapidity. The invariant mass distribution exhibits a steep fall-off, with prominent resonances at the masses of the J/ψ meson, the Υ meson, and the Z boson. The transverse momentum spectrum is sensitive to initial-state gluon radiation and is described by resummation techniques developed by theorists like Guido Altarelli. Measurements of the forward-backward asymmetry, particularly near the Z boson pole, provide precise determinations of the weak mixing angle and are sensitive to the chiral couplings of quarks to the Z boson. These distributions are benchmark data for global fits performed by groups like the CTEQ Collaboration and the NNPDF Collaboration.

Related processes and extensions

Closely related processes include the production of W bosons, which proceeds via quark-antiquark annihilation into a W boson that subsequently decays leptonically, a cornerstone measurement at the Tevatron and the Large Hadron Collider. The production of quarkonium states like the J/ψ meson can be described by analogous color-singlet mechanisms within non-relativistic QCD. Extensions to heavier initial states involve the production of top quark pairs or the hypothetical leptoquark. In heavy-ion collisions at the Relativistic Heavy Ion Collider and the Large Hadron Collider, the process is used as a penetrating probe of the created quark-gluon plasma, with modifications to its yield providing evidence of the Color Glass Condensate initial state.

Category:Particle physics Category:Quantum chromodynamics Category:Scattering experiments