Dijet Azimuthal Decorrelations Research Articles

QCD resummation of dijet azimuthal decorrelations in pp and pA collisions

We study the azimuthal angular decorrelations of dijet production in both proton-proton (pp) and proton-nucleus (pA) collisions. By utilizing soft-collinear effective theory, we establish the factorization and resummation formalism at the next-to-leading logarithmic accuracy for the azimuthal angular decorrelations in the back-to-back limit in pp collisions. We propose an approach where the nuclear modifications to dijet production in pA collisions are accounted for in the nuclear modified transverse momentum dependent parton distribution functions (nTMDPDFs), which contain both collinear and transverse dynamics. This approach naturally generalizes the well-established formalism related to the nuclear modified collinear parton distribution functions (nPDFs). We demonstrate strong consistency between our methodology and the CMS measurements in both pp and pA collisions, and make predictions for dijet production in the forward rapidity region in pA collisions at LHC kinematics and for mid-rapidity kinematics at sPHENIX. Throughout this paper, we focus on the application of this formalism to a simultaneous fit to both collinear and transverse momentum dependent contributions to the transverse momentum dependent distributions.

Dijet azimuthal decorrelation in e+e− annihilation

We examine non-global and clustering logarithms in the distribution of the azimuthal decorrelation between two jets in e+e−→ dijet events, where the jets are defined with E-scheme recombination in the generalized kt algorithm. We calculate at one loop and to all orders the leading global single logarithms in the distribution of the said observable. We also compute at fixed order up to four loops at finite Nc the non-global and clustering logarithms, and numerically resum them to all orders in the large-Nc approximation. We compare our results at O(αs) and O(αs2) with those of the EVENT2 fixed-order Monte Carlo program and find agreement of the leading singular behavior of the azimuthal decorrelation distribution. We find that the impact of non-global logarithms on the resummed distribution in the anti-kt algorithm is substantial, while it is significantly smaller in the kt algorithm. Furthermore, the combined clustering and non-global logarithms in the kt algorithm have an even smaller effect on the distribution. Finally, we use the program Gnole to calculate the resummed distribution at NLL accuracy, thus achieving state-of-the-art accuracy for the resummation of this quantity.

Sudakov effects in central-forward dijet production in high energy factorization

We discuss central-forward dijet production at LHC energies within the framework of high energy factorization. In our study, we profit from the recent progress on consistent merging of Sudakov resummation with small-x effects, which allows us to compute two different gluon distributions which depend on longitudinal momentum, transverse momentum and the hard scale of the process: one for the quark channel and one for the gluon channel. The small-x resummation is included by means of the BK equation supplemented with a kinematic constraint and subleading corrections. We test the new gluon distributions against existing CMS data for transverse momentum spectra in forward-central dijet production. We obtain results which are largely consistent with our earlier predictions based on model implementation of Sudakov form factors. In addition, we study dijet azimuthal decorrelations for the forward-central jets, which are known to be sensitive to the modeling of soft radiation.

Dijets with Large Rapidity Separation at CMS

Search for asymptotic Balitsky–Fadin–Kuraev–Lipatov (BFKL) effects at CMS in dijet events with large rapidity separation between jets in proton-proton collisions at 7 TeV is presented. CMS data on cross-section ratios and azimuthal decorrelations for dijets with large rapidity separation between jets are compared with predictions by various Monte Carlo (MC) event generators based on Dokshitzer–Gribov–Lipatov–Altarelli–Parisi (DGLAP) and BFKL evolutions. None of DGLAP-based generators can describe all measured observable by CMS at 7 TeV. NLL BFKL predictions are consistent with CMS data on dijet azimuthal decorrelations at 7 TeV. More data at higher LHC energies are needed to reveal possible effects.

Measurement of dijet azimuthal decorrelations in pp collisions at s=8 TeV with the ATLAS detector and determination of the strong coupling

A measurement of the rapidity and transverse momentum dependence of dijet azimuthal decorrelations is presented, using the quantity $R_{\Delta \phi}$. The quantity $R_{\Delta \phi}$ specifies the fraction of the inclusive dijet events in which the azimuthal opening angle of the two jets with the highest transverse momenta is less than a given value of the parameter $\Delta \phi_\mathrm{max}$. The quantity $R_{\Delta \phi}$ is measured in proton--proton collisions at $\sqrt{s}=$8 TeV as a function of the dijet rapidity interval, the event total scalar transverse momentum, and $\Delta \phi_\mathrm{max}$. The measurement uses an event sample corresponding to an integrated luminosity of 20.2 fb$^{-1}$ collected with the ATLAS detector at the CERN Large Hadron Collider. Predictions of a perturbative QCD calculation at next-to-leading order in the strong coupling with corrections for non-perturbative effects are compared to the data. The theoretical predictions describe the data in the whole kinematic region. The data are used to determine the strong coupling $\alpha_{\mathrm{S}}$ and to study its running for momentum transfers from 260 GeV to above 1.6 TeV. An analysis that combines data at all momentum transfers results in $\alpha_{\mathrm{S}}(m_{Z}) = 0.1127^{+0.0063}_{-0.0027}$.

Probing transverse momentum broadening in heavy ion collisions

We study the dijet azimuthal de-correlation in relativistic heavy ion collisions as an important probe of the transverse momentum broadening effects of a high energy jet traversing the quark–gluon plasma. We take into account both the soft gluon radiation in vacuum associated with the Sudakov logarithms and the jet PT-broadening effects in the QCD medium. We find that the Sudakov effects are dominant at the LHC, while the medium effects can play an important role at RHIC energies. This explains why the LHC experiments have not yet observed sizable PT-broadening effects in the measurement of dijet azimuthal correlations in heavy ion collisions. Future investigations at RHIC will provide a unique opportunity to study the PT-broadening effects and help to pin down the underlying mechanism for jet energy loss in a hot and dense medium.

Measurement of dijet azimuthal decorrelation in ppcollisions at [Formula: see text

A measurement of the decorrelation of azimuthal angles between the two jets with the largest transverse momenta is presented for seven regions of leading jet transverse momentum up to 2.2,mathrm{TeV}. The analysis is based on the proton-proton collision data collected with the CMS experiment at a centre-of-mass energy of 8,mathrm{TeV} corresponding to an integrated luminosity of 19.7,text {fb}^{-1}. The dijet azimuthal decorrelation is caused by the radiation of additional jets and probes the dynamics of multijet production. The results are compared to fixed-order predictions of perturbative quantum chromodynamics (QCD), and to simulations using Monte Carlo event generators that include parton showers, hadronization, and multiparton interactions. Event generators with only two outgoing high transverse momentum partons fail to describe the measurement, even when supplemented with next-to-leading-order QCD corrections and parton showers. Much better agreement is achieved when at least three outgoing partons are complemented through either next-to-leading-order predictions or parton showers. This observation emphasizes the need to improve predictions for multijet production.

Jet production and αs measurement at CMS

The measurements of inclusive jet, dijet and trijet cross sections, of the dijet azimuthal decorrelations, the ratio of the inclusive 3-jet to 2-jet cross sections are presented. The measurements use the samples of proton-proton collisions collected with the CMS detector at the LHC at a center-of-mass energy of 7 and 8 TeV.

Dijet azimuthal decorrelations for Δϕ dijet < 2π/3 in perturbative QCD

We point out an inconsistency in perturbative QCD predictions previously used for dijet azimuthal decorrelations for azimuthal angles of $\Delta\phi_{\rm dijet} < 2\pi/3$ between the two jets. We show how the inconsistency arises and how the calculations can be modified to provide more accurate results that exhibit a smaller scale dependence and give a better description of the data than the inconsistent results. We also explain how the quality of the predictions strongly depends on a perceivedly minor detail in the definition of the dijet phase space and give recommendations for future measurements.

Diffraction &amp; Forward Physics in CMS: results and perspectives

In this plenary talk given at ISMD2014 the interesting CMS analyses on diffractive proton-proton cross section and Mueller-Navelet dijet azimuthal decorrelations performed at √ s = 7 TeV were presented.

Hadronic results from CMS experiment at the LHC

The Compact Muon Solenoid (CMS) is one of the two large multipurpose experiments at the LHC. We present the CMS results related to tests of QCD predictions for jet, dijet, and multijet production in pp collisions at center-of-mass energies of 7 TeV and 8 TeV. The results also include recent hadronic event shape measurements, as extraction, charged hadron measurements, dijet azimuthal decorrelations, searches for BFKL effects in the azimuthal correlations of forward-backward and forward-central jets at 7 TeV, and studies of the transition from the perturbative to the non-perturbative regime using minijets at 8 TeV.

Dijet azimuthal decorrelations at the LHC in the parton Reggeization approach

We study inclusive dijet azimuthal decorrelations in proton-proton collisions at the CERN LHC invoking the hypothesis of parton Reggeization in t-channel exchanges at high energies. In the parton Reggeization approach, the main contribution to the azimuthal angle difference between the two most energetic jets is due to the Reggeon-Reggeon-Particle-Particle scattering, when the fusion of two Reggeized gluons into a pair of Yang-Mills gluons dominates. Using a high-energy factorization scheme with the Kimber-Martin-Ryskin unintegrated parton distribution functions and the Fadin-Lipatov effective vertices we obtain good agreement of our calculations with recent measurements by the ATLAS and CMS Collaborations at the CERN LHC.

Measurement of the combined rapidity and [formula omitted] dependence of dijet azimuthal decorrelations in [formula omitted] collisions at [formula omitted

We present the first combined measurement of the rapidity and transverse momentum dependence of dijet azimuthal decorrelations, using the recently proposed quantity RΔϕ. The variable RΔϕ measures the fraction of the inclusive dijet events in which the azimuthal separation of the two jets with the highest transverse momenta is less than a specified value of the parameter Δϕmax. The quantity RΔϕ is measured in pp¯ collisions at s=1.96 TeV, as a function of the dijet rapidity interval, the total scalar transverse momentum, and Δϕmax. The measurement uses an event sample corresponding to an integrated luminosity of 0.7 fb−1 collected with the D0 detector at the Fermilab Tevatron Collider. The results are compared to predictions of a perturbative QCD calculation at next-to-leading order in the strong coupling with corrections for non-perturbative effects. The theory predictions describe the data well, except in the kinematic region of large dijet rapidity intervals and small Δϕmax.

A new quantity for studies of dijet azimuthal decorrelations

We introduce a new measurable quantity, R Δϕ , for studies of the rapidity and transverse momentum dependence of dijet azimuthal decorrelations in hadron-hadron collisions. In pQCD, R Δϕ is computed as a ratio of three-jet and dijet cross sections in which the parton distribution functions cancel to a large extent. At the leading order, R Δϕ is proportional to αs, and the transverse momentum dependence of R Δϕ can therefore be exploited to determine αs. We compute the NLO pQCD theory predictions and non-perturbative corrections for R Δϕ at the LHC and the Tevatron and investigate the corresponding uncertainties. From this, we estimate the theory uncertainties for αs determinations based on R Δϕ at both colliders. The potential of R Δϕ measurements for tuning Monte Carlo event generators is also demonstrated.

Dijet physics with CMS detector at the Large Hadron Collider

The results from various dijet distributions in proton–proton collisions at a centre-of-mass energy of 7 TeV, with 2010 and 2011 data from the CMS experiment, are presented. The measurements of the dijet mass spectra, centrality ratio, azimuthal decorrelation and angular distribution are shown. Sensitivity of the phenomenological parameters used to model different event generators is also investigated. Prospects for observing evidence for new physics in these distributions are presented.

Jet and Multijet Results from CMS

QCD measurements with jets are presented, from proton-proton collisions at a centre-of-mass energy of 7 TeV at the CERN LHC. The data were collected with the CMS detector during the 2010 data-taking period, and correspond up to an integrated luminos of 36 pb−1. The measured inclusive-jet and dijet production cross sections are compared to perturbative QCD predictions at next-to-leading-order, and are found to be in good agreeme Observables sensitive to multijet production, such as the hadronic event shapes, the dijet azimuthal decorrelations, and the ratio of the 3-jet to 2-jet production cross section, are compared to the predictions of various QCD Monte-Carlo generators.

Dijet Azimuthal Decorrelations inppCollisions ats=7 TeV

Measurements of dijet azimuthal decorrelations in pp collisions at sqrt(s) = 7 TeV using the CMS detector at the CERN LHC are presented. The analysis is based on an inclusive dijet event sample corresponding to an integrated luminosity of 2.9 inverse picobarns. The results are compared to predictions from perturbative QCD calculations and various Monte Carlo event generators. The dijet azimuthal distributions are found to be sensitive to initial-state gluon radiation.

Measurement of Dijet Azimuthal Decorrelations in $p{\bar p}$ Collisions

Correlations in the azimuthal angle between the two largest transverse momentum jets have been measured using the DØ detector in [Formula: see text] collisions at [Formula: see text]. The analysis is based on an inclusive dijet event sample in the central rapidity region corresponding to an integrated luminosity of 150 pb-1. Azimuthal correlations are stronger at larger transverse momenta. These are well-described in NLO pQCD, except at large azimuthal differences where soft effects are significant.

QCD and diffraction results from DØ Run II

This paper presents recent results in QCD and diffraction physics obtained by the DØ experiment at the Fermilab Tevatron pp¯ collider at s=1.96 TeV. The data used correspond to an integrated luminosity up to 150 pb−1. The different measurements are jet inclusive and dijet cross sections, dijet azimuthal decorrelation, diffractively produced Z bosons and elastic scattering study.