International Journal of Modern Physics A

I give a brief introduction to jet measurements at the Large Hadron Collider, the subject of this special issue of the International Journal of Modern Physics A.

We review the history of jets in high energy physics, and describe in more detail the developments of the past ten years, discussing new algorithms for jet finding and their main characteristics, and summarising the status of perturbative calculations for jet cross sections in hadroproduction. We also describe the emergence of jet grooming and tagging techniques and their application to boosted jets analyses.

Jets are one of the most prominent physics signatures of high energy proton–proton (p–p) collisions at the Large Hadron Collider (LHC). They are key physics objects for precision measurements and searches for new phenomena. This review provides an overview of the reconstruction and calibration of jets at the LHC during its first Run. ATLAS and CMS developed different approaches for the reconstruction of jets, but use similar methods for the energy calibration. ATLAS reconstructs jets utilizing input signals from their calorimeters and use charged particle tracks to refine their energy measurement and suppress the effects of multiple p–p interactions (pileup). CMS, instead, combines calorimeter and tracking information to build jets from particle flow objects. Jets are calibrated using Monte Carlo (MC) simulations and a residual in situ calibration derived from collision data is applied to correct for the differences in jet response between data and Monte Carlo. Large samples of dijet, $Z$+jets, and $\gamma$+events at the LHC allowed the calibration of jets with high precision, leading to very small systematic uncertainties. Both ATLAS and CMS achieved a jet energy calibration uncertainty of about 1% in the central detector region and for jets with transverse momentum $p_T>100\mathrm{\text{GeV}}$. At low jet $p_T$, the jet energy calibration uncertainty is less than 4%, with dominant contributions from pileup, differences in energy scale between quark and gluon jets, and jet flavor composition.

This review discusses the measurements of the inclusive jet and dijet cross section performed by the experimental collaborations at the LHC during what is now being called LHC Run 1 (2009–2013). It summarises some of the experimental challenges and the techniques used in the measurements of jets cross sections during the LHC Run 1.

During the first years of the LHC operation a large amount of jet data was recorded by the ATLAS and CMS experiments. In this review several measurements of jet-related observables are presented, such as multi-jet rates and cross sections, ratios of jet cross sections, jet shapes and event shape observables. All results presented here are based on jet data collected at a centre-of-mass energy of 7 TeV. Data are compared to various Monte Carlo generators, as well as to theoretical next-to-leading-order calculations allowing a test of perturbative Quantum Chromodynamics in a previously unexplored energy region.

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 ${\alpha }_s(Q)$ 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 ${\alpha }_s(Q)$ running at the TeV scale. I conclude with a brief outlook on possible future developments.

Hadronic jets are extremely abundant at the LHC, and testing QCD in various corners of phase-space is important to understand backgrounds and some specific signatures of new physics. In this article, various measurements aiming at probing QCD in configurations where the theory modelling becomes challenging are presented. Azimuthal angle de-correlations are sensitive to hard as well as soft QCD emission, and in most of the events jets are produced in a back-to-back configuration. Events where jets have a large rapidity separation are also rare, and those without additional radiation between the jets are exponentially suppressed. The modelling of radiation between very forward and backward jets is complicated, and may require theoretical tools different with respect to those normally used for central, high-$p_T$ events. Observables can be constructed that are sensitive to all these effects, such as the study of azimuthal angle de-correlations in events where the two leading jets have large rapidity separations. The two general-purpose detectors of the LHC have measured these observables, and for some of them interesting deviations with respect to the most commonly used theoretical models are observed.

The LHC experiments ATLAS and CMS have measured $V$ + jets and $t\bar{t}$ + jets final states over a large energy range in data collected between 2010 and 2012 at $\sqrt{s}=7\mathrm{\text{TeV}}$ and $\sqrt{s}=8\mathrm{\text{TeV}}$. The results have been compared to pQCD calculations at NLO and have been used to validate novel Monte Carlo techniques.

We summarize measurements of $\mathrm{\text{b}}$ and $\mathrm{\text{c}}$ jet production at the LHC, which are an important signature and background for decays of massive particles such as $\mathrm{\text{H}}\to \mathrm{\text{b}}\overline{\mathrm{\text{b}}}$. These include measurements of the inclusive and dijet production of heavy quark jets, $\mathrm{\text{b}}$ and $\mathrm{\text{c}}$ jets produced in association with vector bosons $\mathrm{\text{Z}}$ and $\mathrm{\text{W}}$, and decays of boosted $\mathrm{\text{Z}}$ bosons into pairs of $\mathrm{\text{b}}\bar{\text{b}}$. The current status of $\mathrm{\text{b}}$ tagging and $\mathrm{\text{b}}$ jet energy scale is also reviewed. These measurements test perturbative QCD in the four and five-flavor number schemes, and provide insight into the relative importance of heavy flavor production through flavor creation, flavor excitation and gluon splitting channels. The $\mathrm{\text{W+c}}$ measurement provides additionally a powerful way to probe the strange quark and antiquark sea in the proton. The recent studies looking separately at production of one and two $\mathrm{\text{b}}$ jets find generally good agreement with theory predictions for two $\mathrm{\text{b}}$-jet production, while some discrepancies are observed for singly produced $\mathrm{\text{b}}$ jets, particularly at large $\mathrm{\text{b}}$-jet $p_T$, where gluon splitting becomes dominant.

Jet observables have been exploited extensively during the LHC Run 1 to search for physics beyond the Standard Model. In this article, the most recent results from the ATLAS and CMS collaborations are summarized. Data from proton–proton collisions at 7 and 8 TeV center-of-mass energy have been analyzed to study monojet, dijet, and multijet final states, searching for a variety of new physics signals that include colored resonances, contact interactions, extra dimensions, and supersymmetric particles. The exhaustive searches with jets in Run 1 did not reveal any signal, and the results were used to put stringent exclusion limits on the new physics models.

In this document I present a brief review of the concepts of jet physics employed in heavy ion physics. Fast partons originating from scatterings with large momentum transfer are produced at very short time-scales and subsequently propagate through the strongly interacting medium produced in the collisions of heavy nuclei. They feature the only experimental handle available to directly study the interaction of a well defined probe and the medium. Consequently they are ideally suited to investigate the nature of the medium produced in these collisions and the mechanism of interaction between the medium and the partons. The experimental methods necessary to reconstruct jets originating from fragmenting partons in the environment of high particle multiplicity heavy ion collisions will be discussed. Physics observables suited to investigate the parton medium interaction will be introduced and a summary of recent results on jet physics in heavy ion collisions is presented.