Measurement of Dijet Production at Low Q^2 at HERA

TL;DR

This paper measures dijet production at low photon virtualities at HERA, comparing experimental data with QCD predictions, revealing limitations of NLO calculations and exploring parton shower models for better understanding.

Contribution

It provides detailed measurements of dijet cross sections at low Q^2 and evaluates the performance of NLO QCD and Monte Carlo models in this regime.

Findings

NLO QCD fails at low Q^2 and low E_T regions.

LO Monte Carlo with resolved photons describes data better.

Parton showers help include higher order QCD effects.

Abstract

Triple differential dijet cross sections in e^\pm p interactions are presented in the region of photon virtualities 2<Q^2<80GeV^2, inelasticities 0.1<y<0.85, jet transverse energies E_T1>7GeV, E_T2>5GeV, and pseudorapidities -2.5 < eta_1^*, eta_2^* <0. The measurements are made in the gamma^* p centre-of-mass frame, using an integrated luminosity of 57pb^-1. The data are compared with NLO QCD calculations and LO Monte Carlo programs with and without a resolved virtual photon contribution. NLO QCD calculations fail to describe the region of low Q^2 and low jet transverse energies, in contrast to a LO Monte Carlo generator which includes direct and resolved photon interactions with both transversely and longitudinally polarised photons. Initial and final state parton showers are tested as a mechanism for including higher order QCD effects in low E_T jet production.