Sigmajj

Sigmajj (often written as sigma_jj) is a term used in high-energy physics to denote the dijet production cross section, i.e., the probability for two jets to be produced in a hadronic collision. In proton-proton or proton-antiproton collisions, sigma_jj depends on the kinematic properties of the jets and is commonly expressed as differential cross sections such as d sigma_jj / d m_jj, where m_jj is the invariant mass of the two leading jets, or d sigma_jj / d p_T1 d y1 d p_T2 d y2 for the transverse momenta and rapidities of the jets.

Experimental measurement involves selecting events with at least two jets using a jet algorithm (for example anti-kT with a chosen radius parameter R), reconstructing jet four-momenta, correcting for detector effects, and unfolding to particle- or parton-level cross sections. Theoretical predictions are computed in perturbative quantum chromodynamics (QCD), often at next-to-leading order and sometimes including next-to-next-to-leading order corrections, and rely on parton distribution functions. Monte Carlo event generators simulate full final states including hadronization and detector effects.

Sigmajj is used to test QCD at high energies, constrain parton distribution functions, and search for new physics such as contact interactions, dijet resonances, or extra gauge bosons. Analyses face challenges including jet energy scale uncertainties, pile-up, and backgrounds from QCD multijet processes. The concept plays a central role in collider experiments, including measurements at the Large Hadron Collider and comparisons with earlier accelerator results.