Constraints on parton distributions and the strong coupling from LHC jet data

TL;DR

This review assesses how Run I LHC jet data constrain proton parton distributions, especially the gluon at large $x$, and how they enable determinations of the strong coupling constant $\alpha_s(M_Z)$ and its running to TeV scales. It discusses the theoretical framework, including NLO and recent NNLO jet calculations, and the fast-grid tools that make jet data practical for PDF fits. The paper summarizes the impact of ATLAS and CMS jet measurements on gluon PDFs, the consistency of $\alpha_s(M_Z)$ determinations with the PDG average, and the emergence of direct $\alpha_s(Q)$ tests at high scales. It also outlines future prospects, including NNLO dijet completion, electroweak corrections for Run II, and the potential to constrain new colored sectors with high-energy jet data.

Abstract

Jet production at hadron colliders provides powerful constraints on the parton distribution functions (PDFs) of the proton, in particular on the gluon PDF. Jet production can also be used to extract the QCD coupling constant and to test its running with the momentum transfer up to the TeV region. In this review, I summarize the information on PDFs and the strong coupling that has been provided by Run I LHC jet data. First of all, I discuss why jet production is directly sensitive to the gluon and quark PDFs at large-x, and then review the state-of-the-art perturbative calculations for jet production at hadron colliders and the corresponding fast calculations required for PDF fitting. Then I present the results of various recent studies on the impact on PDFs, in particular the gluon, that have been performed using as input jet measurements from ATLAS and CMS. I also review the available determinations of the strong coupling constant based on ATLAS and CMS jet data, with emphasis on the fact that LHC jet data provides, for the first time, a direct test of the $α_s(Q)$ running at the TeV scale. I conclude with a brief outlook on possible future developments.

Figures (7)

• Figure 1: Subprocess decomposition of inclusive jet production at the LHC 8 TeV (left plot) and 14 TeV (right plot), computed with ALPGEN at LO using MSTW08 as input PDFs, as a function of the $p_T$ of the leading jet.
• Figure 2: Left plot: preliminary CMS dataCMS-PAS-SMP-14-002 for dijet production at 8 TeV. The measured cross-sections are compared to NLO QCD theory, using NNPDF2.1 as input PDFs. Right plot: results for the ATLAS 2011 dijet measurementAad:2013tea, now using CT10 as input PDF. In both cases the theoretical calculations include non-perturbative and electroweak corrections.
• Figure 3: Minimum value of Bjorken-$x$ and the scale $m_{34}$ probed in the PDFs for dijet production at the LHC 7 TeV, using the kinematics of the ATLAS 2011 dijet measurementAad:2013tea.
• Figure 4: Correlation coefficients between the gluon PDF (left plot) and the up quark PDF (right plot) for different values of $\left( x,Q^2\right)$ for the kinematics of the CMS 7 TeV inclusive jet cross-section. The computation of these coefficients, performed with NNPDF2.1 NLO, has been performed in all the jet $p_T$ bins of the central rapidity region, $|y|\le 0.5$. Results taken from Ref.Khachatryan:2014waa, additional figures are available from https://twiki.cern.ch/twiki/bin/view/CMSPublic/PhysicsResultsSMP12028.
• Figure 5: Representative results of PDF analyses using jet data obtained by the ATLAS and CMS collaborations. Left plot: impact on the gluon PDF of the ATLAS data on the 2.76/7 TeV inclusive jet cross-section ratioChatrchyan:2013txa. Right plot: constraints on the gluon from the CMS inclusive jet 2011 dataKhachatryan:2014waa. In both cases the results of the PDF fit are compared to a HERA-only baseline fit.