Measurement of Multijet Production in ep Collisions at High Q^2 and Determination of the Strong Coupling alpha_s

TL;DR

The paper reports high-precision measurements of inclusive jet, dijet, and trijet cross sections in neutral-current deep-inelastic e p scattering at HERA, using an improved hadronic final-state reconstruction and a regularised unfolding to correct detector effects. By comparing to NLO pQCD with hadronisation and electroweak corrections, it extracts the strong coupling constant alpha_s(M_Z) and its running, achieving the most precise NLO determination from jet data in a single experiment with alpha_s(M_Z) ≈ 0.1165 (exp 0.0008, pdf/theo 0.0038). Normalised multijet cross sections yield the best experimental precision and are consistent with the world average, while the dijet/trijet results reveal sensitivities to higher-order corrections. Overall, the results demonstrate strong agreement with pQCD within current theoretical uncertainties and underscore the need for NNLO computations to further sharpen alpha_s determinations from DIS jet production.

Abstract

Inclusive jet, dijet and trijet differential cross sections are measured in neutral current deep-inelastic scattering for exchanged boson virtualities 150 < Q^2 < 15000 GeV^2 using the H1 detector at HERA. The data were taken in the years 2003 to 2007 and correspond to an integrated luminosity of 351 pb^{-1}. Double differential Jet cross sections are obtained using a regularised unfolding procedure. They are presented as a function of Q^2 and the transverse momentum of the jet, P_T^jet, and as a function of Q^2 and the proton's longitudinal momentum fraction, Xi, carried by the parton participating in the hard interaction. In addition normalised double differential jet cross sections are measured as the ratio of the jet cross sections to the inclusive neutral current cross sections in the respective Q^2 bins of the jet measurements. Compared to earlier work, the measurements benefit from an improved reconstruction and calibration of the hadronic final state. The cross sections are compared to perturbative QCD calculations in next-to-leading order and are used to determine the running coupling and the value of the strong coupling constant as alpha_s(M_Z) = 0.1165 (8)_exp (38)_{pdf,theo}.

Figures (21)

• Figure 1: Deep-inelastic $ep$ scattering at different orders in $\alpha_s$: (a) Born contribution $\mathcal{O}(\alpha_{\mathrm{em}}\xspace^2)$, (b) example of boson-gluon fusion $\mathcal{O}(\alpha_{\mathrm{em}}\xspace^2 \alpha_s\xspace)$, (c) example of QCD Compton scattering $\mathcal{O}(\alpha_{\mathrm{em}}\xspace^2 \alpha_s\xspace)$ and (d) example of a trijet process $\mathcal{O}(\alpha_{\mathrm{em}}\xspace^2 \alpha_s\xspace^2)$.
• Figure 2: Mean values of the $P_{\mathrm{T,bal}}$-distributions and the double-ratio of data to MC simulations as function of $P_{\mathrm{T}}^{\mathrm{da}}$, as measured in the one-jet calibration sample and in an independent dijet sample. Results for data are compared to RAPGAP and DJANGO. The open boxes and the shaded areas illustrate the statistical uncertainties of the MC simulations. The dashed lines in the double-ratio figure indicate a $\pm\unit[1]{\%}$ deviation.
• Figure 3: Distributions of $Q^2$ and $y$ for the selected NC DIS data on detector level in the extended analysis phase space. The data are corrected for the estimated background contributions, shown as gray area. The predictions from DJANGO and RAPGAP are weighted to achieve good agreement with the data. The ratio of data to prediction is shown at the bottom of each figure.
• Figure 4: Distributions of $P_{\rm T}^{\rm jet}$ and $\eta^{\mathrm{jet}}_{\mathrm{lab}}\xspace$ for the selected inclusive jet data on detector level in the extended analysis phase space. The are been corrected for the estimated background contributions, shown as gray area. The predictions from DJANGO and RAPGAP are weighted to achieve good agreement with the data. The ratio of data to prediction is shown at the bottom of each figure.
• Figure 5: Distributions of $\langle P_{\mathrm{T}} \rangle_{2}$ and $\xi_2$ for the selected dijet data on detector level in the extended analysis phase space. The data are corrected for the estimated background contributions, shown as gray area. The predictions from DJANGO and RAPGAP are weighted to achieve good agreement with the data. The ratio of data to prediction is shown at the bottom of each figure.