Three-Jet Production in Deep-Inelastic Scattering at HERA

TL;DR

Three-jet production in neutral current DIS is measured for the first time at HERA, spanning $5< Q^2<5000$ GeV$^2$ and $25<M_{\rm 3jet}<140$ GeV with jets defined in the Breit frame. Next-to-leading order pQCD with hadronization corrections describes both the three-jet cross section $d\sigma_{3\text{jet}}/dQ^2$ and the ratio $R_{3/2}$ across the full $Q^2$ range, while leading order underestimates at low $Q^2$. Normalized distributions of jet energy fractions and angular variables show Bremsstrahlung-driven patterns that deviate from uniform phase-space predictions but are well captured by NLO QCD. The results provide robust tests of perturbative QCD in DIS and offer constraints on the strong coupling, PDFs (notably the gluon density), and hadronization effects.

Abstract

Three-jet production is studied for the first time in deep-inelastic positron-proton scattering. The measurement carried out with the H1 detector at HERA covers a large range of four-momentum transfer squared 5 < Q^2 < 5000 GeV^2 and invariant three-jet masses 25 < M_(3jet) < 140 GeV. Jets are defined by the inclusive k_T algorithm in the Breit frame. The size of the three-jet cross section and the ratio of the three-jet to the dijet cross section R_(3/2) are described over the whole phase space by the predictions of perturbative QCD in next-to-leading order. The shapes of angular jet distributions deviate significantly from a uniform population of the available phase space but are well described by the QCD calculation.

Figures (5)

• Figure 1: A sketch of the angles $\theta_3$ and $\psi_3$ which are defined by the momenta $\vec{p}_3$, $\vec{p}_4$, $\vec{p}_5$ of the three jets and the proton beam ($\vec{p}_B$) in the three-jet center of mass frame.
• Figure 2: The inclusive three-jet cross section (a) measured as a function of the four-momentum transfer squared $Q^2$. The predictions of perturbative QCD in leading order (dotted line) and in next-to-leading order with (solid line) and without hadronization corrections (dashed line) are compared to the data. Also shown is the ratio of the measured cross section and the theoretical prediction, including the effects from variations of $\alpha_s(M_Z)$, the renormalization scale $\mu_r$ and the gluon density in the proton. The ratio $R_{3/2}$ of the inclusive three-jet cross section to the inclusive dijet cross section (b) is compared to the leading order (dotted line) and the next-to-leading order calculations (central value of the light band) including hadronization corrections. The sensitivity of the NLO calculation to parameter variations is indicated as in (a). (The data at $55 < Q^2 < 100\,{\rm GeV}^2$, which are affected by the cut on $\theta_{\rm positron}$ in eq. (\ref{['eq:kin']}), are not included in this Figure.)
• Figure 3: The inclusive three-jet cross section measured as a function of (a) the Bjorken scaling variable $x_{\rm Bj}$ and (b) the invariant three-jet mass $M_{\rm 3jet}$. The predictions of perturbative QCD in leading order (dotted line) and next-to-leading order (solid line) with hadronization corrections are compared to the data.
• Figure 4: The distributions of the jet energy fractions $X_3$ (top) and $X_4$ (bottom) in the three-jet center-of-mass frame at low $Q^2$ (left) and high $Q^2$ (right). The data are compared to the predictions of perturbative QCD in next-to-leading order (solid line) and to a three-jet phase space model (dotted line).
• Figure 5: Distributions of $\cos \theta_3$ (top) and the angle $\psi_3$ (bottom) in the three-jet center-of-mass frame at low $Q^2$ (left) and high $Q^2$ (right). The data are compared to the predictions of perturbative QCD in next-to-leading order (solid line) and in leading order (dashed line) and to a three-jet phase space model (dotted line).