Jet Transverse Momentum Research Articles

Azimuthal separation in nearly back-to-back jet topologies in inclusive 2- and 3-jet events in {text {p}} {text {p}} collisions at sqrt{s}=13,text {Te}text {V}

A measurement for inclusive 2- and 3-jet events of the azimuthal correlation between the two jets with the largest transverse momenta, varDelta phi _{12}, is presented. The measurement considers events where the two leading jets are nearly collinear (“back-to-back”) in the transverse plane and is performed for several ranges of the leading jet transverse momentum. Proton-proton collision data collected with the CMS experiment at a center-of-mass energy of 13,text {Te}text {V} and corresponding to an integrated luminosity of 35.9{,text {fb}^{-1}} are used. Predictions based on calculations using matrix elements at leading-order and next-to-leading-order accuracy in perturbative quantum chromodynamics supplemented with leading-log parton showers and hadronization are generally in agreement with the measurements. Discrepancies between the measurement and theoretical predictions are as large as 15%, mainly in the region 177^circ< varDelta phi _{12} < 180^circ . The 2- and 3-jet measurements are not simultaneously described by any of models.

IR-improved DGLAP parton shower effects in W+jets in pp collisions at s=7 TeV

We use HERWIRI1.031, a new Monte Carlo event generator for hadron-hadron scattering at high energies, to study the phenomenological effects of our approach of exact amplitude-based resummation in precision QCD calculations. W + jet(s) events with exact NLO QCD corrections are generated in the MG5_aMC@NLO framework and showered by both HERWIRI1.031 and HERWIG6.5 with PTRMS = 0 and PTRMS = 2.2 GeV/c, respectively. Here, PTRMS is the rms value of the intrinsic Gaussian transverse momentum distribution for the partons inside the proton. The differential cross sections for many observables are presented such as the jet rapidities and the jet transverse momenta as well as other event observables such as the scalar sums of transverse momenta of the jets, the missing transverse energy of the jets and the dijets' observables. Finally, we compare our results with the ATLAS and CMS measurements of the W production cross sections in association with jets.

Study of jet-medium interactions using jet shape observables in heavy ion collisions at LHC energies with JEWEL

Based on a perturbative quantum chromodynamics inspired dynamical model of jet-medium interactions, Jewel, we have studied possible modifications to inclusive jet yields and a set of jet shape observables, namely, the fragmentation functions and radial momentum distributions when jets propagate through a deconfined partonic medium created in collisions of heavy nuclei at Large Hadron Collider (LHC) energies. Jets are reconstructed with anti-kT algorithm in the pseudorapidity range for resolution parameter R = 0.2, 0.3 and 0.4. For background subtraction, a Jewel-compatible 4-Momenta subtraction technique (4MomSub) has been used. The modification of inclusive jet-yields in Pb–Pb collisions relative to proton–proton interactions, quantified by , are seen to be in reasonable agreement with ALICE, ATLAS and CMS data over a broad transverse momentum range. Jewel is able to capture the qualitative features of the modifications to the fragmentation functions and radial momentum distributions in data but not always quantitatively. This quantitative discrepancy may be related to the simplified treatment of recoil partons in the background model and partly due to background subtraction procedure itself. Nevertheless, observed modifications to jet shape observables in Jewel corroborates the fact that in-medium fragmentation is harder and more collimated than the fragmentation in vacuum. We further observe that these modifications depend on the transverse momentum of jets and it seems that medium resolves the core structure of low momentum jets below 100 GeV c–1 at LHC energies.

Measurement of the production of charm jets tagged with D0 mesons in pp collisions at sqrt{mathrm{s}}=7 TeV

The production of charm jets in proton-proton collisions at a center-of-mass energy of sqrt{s}=7 TeV was measured with the ALICE detector at the CERN Large Hadron Collider. The measurement is based on a data sample corresponding to a total integrated luminosity of 6.23 nb−1, collected using a minimum-bias trigger. Charm jets are identified by the presence of a D0 meson among their constituents. The D0 mesons are reconstructed from their hadronic decay D0 →K−π+. The D0-meson tagged jets are reconstructed using tracks of charged particles (track-based jets) with the anti-kT algorithm in the jet transverse momentum range 5<{p}_{{mathrm{T}}_{,mathrm{jet}}}^{mathrm{ch}} < 30 GeV/c and pseudorapidity |ηjet| < 0.5. The fraction of charged jets containing a D0-meson increases with {p}_{{mathrm{T}}_{,mathrm{jet}}}^{mathrm{ch}} from 0.042 ± 0.004 (stat) ± 0.006 (syst) to 0.080 ± 0.009 (stat) ± 0.008 (syst). The distribution of D0-meson tagged jets as a function of the jet momentum fraction carried by the D0 meson in the direction of the jet axis left({z}_{parallel}^{mathrm{ch}}right) is reported for two ranges of jet transverse momenta, 5<{p}_{{mathrm{T}}_{,mathrm{jet}}}^{mathrm{ch}} < 15 GeV/c and 15<{p}_{{mathrm{T}}_{,mathrm{jet}}}^{mathrm{ch}} < 30 GeV/c in the intervals 0.2<{z}_{BigVert}^{mathrm{ch}}<1.0 and 0.4<{z}_{BigVert}^{mathrm{ch}}<1.0 , respectively. The data are compared with results from Monte Carlo event generators (PYTHIA 6, PYTHIA 8 and Herwig 7) and with a Next-to-Leading-Order perturbative Quantum Chromodynamics calculation, obtained with the POWHEG method and interfaced with PYTHIA 6 for the generation of the parton shower, fragmentation, hadronisation and underlying event.

BSM W W production with a jet veto

We consider the impact on W W production of the unique dimension-six operator coupling gluons to the Higgs field. In order to study this process, we have to appropriately model the effect of a veto on additional jets. This requires the resummation of large logarithms of the ratio of the maximum jet transverse momentum and the invariant mass of the W boson pair. We have performed such resummation at the appropriate accuracy for the Standard Model (SM) background and for a signal beyond the SM (BSM), and devised a simple method to interface jet-veto resummations with fixed-order event generators. This resulted in the fast numerical code MCFM-RE, the Resummation Edition of the fixed-order code MCFM. We compared our resummed predictions with parton-shower event generators and assessed the size of effects, such as limited detector acceptances, hadronisation and the underlying event, that were not included in our resummation. We have then used the code to compare the sensitivity of W W and Z Z production at the HL-LHC to the considered higher-dimension operator. We have found that W W can provide complementary sensitivity with respect to Z Z, provided one is able to control theory uncertainties at the percent-level. Our method is general and can be applied to the production of any colour singlet, both within and beyond the SM.

Probing the nonstandard top-gluon couplings through tt¯γγ production at the LHC

In this paper, we investigate the anomalous chromoelectric and chromomagnetic dipole moments of the top quark through top pair production in association with two photons at the LHC. We first present the strategy to reconstruct this process assuming a different source for background processes. Then, we focus on the existing constraints from inclusive top-pair production from the Tevatron and LHC, adding the new LHC measurement. Afterwards, we introduce the new cross section ratio $R_{2\gamma/\gamma}=\sigma_{t\bar{t}\gamma\gamma}/\sigma_{t\bar{t}\gamma}$ and show the usefulness of this ratio in canceling most of the systematic uncertainties and its special functionality to constrain dipole moments. Finally, we use the scalar sum of the transverse momentum of jets, $H_{T}$, in order to define a signal-dominated region and obtain limits on these anomalous top couplings using different amounts of expected data from the LHC.

Quark jet versus gluon jet: fully-connected neural networks with high-level features

Jet identification is one of the fields in high energy physics that machine learning has begun to make an impact. More often than not, convolutional neural networks are used to classify jet images with the benefit that essentially no physics input is required. Inspired by a recent work by Datta and Larkoski, we study the classification of quark/gluon-initiated jets based on fully-connected neural networks (FNNs), where expert-designed physical variables are taken as input. FNNs are applied in two ways: trained separately on various narrow jet transverse momentum $p_{TJ}$ bins; trained on a wide region of $p_{TJ} \in [200,~1000]$ GeV. We find their performances are almost the same. The performance is better when the $p_{TJ}$ is larger. Jet discrimination with FNN is studied on both particle and detector level data. The results based on particle level data are comparable with those from deep convolutional neural networks, while the significance improvement characteristic (SIC) from detector level data would at most decrease by $15\%$. We also test the performance of FNNs with full set or subsets of jet observables as input features. The FNN with one subset consisting of fourteen observables shows nearly no degradation of performance. This indicates that these fourteen expert-designed observables could have captured the most necessary information for separating quark and gluon jets.

Machine-learning-based jet momentum reconstruction in heavy-ion collisions

The precise reconstruction of jet transverse momenta in heavy-ion collisions is a challenging task. A major obstacle is the large number of (mainly) low-${p}_{\mathrm{T}}$ particles overlaying the jets. Strong region-to-region fluctuations of this background complicate the jet measurement and lead to significant uncertainties. In this paper, a novel approach to correct jet momenta (or energies) for the underlying background in heavy-ion collisions is introduced. The proposed method makes use of common machine learning techniques to estimate the jet transverse momentum based on several parameters, including properties of the jet constituents. Using a toy model and HIJING simulations, the performance of the new method is shown to be superior to the established standard area-based background estimator. The application of the new method to data promises the measurement of jets down to extremely low transverse momenta, unprecedented thus far in data on heavy-ion collisions.

Sorting Out Quenched Jets.

We introduce a new "quantile" analysis strategy to study the modification of jets as they traverse through a droplet of quark-gluon plasma. To date, most jet modification studies have been based on comparing the jet properties measured in heavy-ion collisions to a proton-proton baseline at the same reconstructed jet transverse momentum (p_{T}). It is well known, however, that the quenching of jets from their interaction with the medium leads to a migration of jets from higher to lower p_{T}, making it challenging to directly infer the degree and mechanism of jet energy loss. Our proposed quantile matching procedure is inspired by (but not reliant on) the approximate monotonicity of energy loss in the jet p_{T}. In this strategy, jets in heavy-ion collisions ordered by p_{T} are viewed as modified versions of the same number of highest-energy jets in proton-proton collisions, and the fractional energy loss as a function of jet p_{T} is a natural observable (Q_{AA}). Furthermore, despite nonmonotonic fluctuations in the energy loss, we use an event generator to validate the strong correlation between the p_{T} of the parton that initiates a heavy-ion jet and the p_{T} of the vacuum jet which corresponds to it via the quantile procedure (p_{T}^{quant}). We demonstrate that this strategy both provides a complementary way to study jet modification and mitigates the effect of p_{T} migration in heavy-ion collisions.

Full NLO corrections to 3-jet production and mathbf {R_{32}} at the LHC

We present the evaluation of the complete set of NLO corrections to three-jet production at the LHC. To this end we consider all contributions of mathcal{{O}}(alpha _s^nalpha ^m) with n+m=3 and n+m=4. This includes in particular also subleading Born contributions of electroweak origin, as well as electroweak virtual and QED real-radiative corrections. As an application we present results for the three- over two-jet ratio R_{32}. While the impact of non-QCD corrections on the total cross section is rather small, they can exceed -10% for high jet transverse momenta. The R_{32} observable turns out to be very stable against electroweak corrections, receiving absolute corrections below 5% even in the high-p_T region.

Probing color reconnection with underlying event observables at the LHC energies

In this work, we study the underlying-event (UE) activity as a function of the highest jet or charged-particle transverse momentum (${p}_{\mathrm{T}}^{\text{jet}}$ or ${p}_{\mathrm{T}}^{\text{leading}}$) in terms of number and summed-${p}_{\mathrm{T}}$ densities of charged particles in the azimuthal region transverse to the leading jet or particle direction. Contrary to inelastic $pp$ LHC data, the UE observables normalized to the charged-particle density obey an approximate Koba-Nielsen-Olesen (KNO) scaling. Based on PYTHIA 8.2 simulations of $pp$ collisions at LHC energies, we show that the small breaking of the KNO scaling is due to the increasing importance of multiple partonic interactions (MPI) at higher $\sqrt{s}$. This in turn makes that with increasing energy, the ${p}_{\mathrm{T}}$ spectra in the UE get harder than in inelastic $pp$ collisions. Color reconnection (CR) models the interactions among outgoing partons just before the hadronization; therefore, it modifies the ${p}_{\mathrm{T}}$-spectral shape. Motivated by this, we studied the UE activity considering charged particles within different ${p}_{\mathrm{T}}$ intervals in $pp$ collisions at $\sqrt{s}=7\text{ }\text{ }\mathrm{TeV}$. Although MPI saturate for ${p}_{\mathrm{T}}^{\text{jet}}&gt;10\text{ }\text{ }\mathrm{GeV}/c$, the UE still increases with increasing ${p}_{\mathrm{T}}^{\text{jet}}$. We demonstrate that the saturation of both number and summed-${p}_{\mathrm{T}}$ densities, which are commonly claimed, are only observed for low-${p}_{\mathrm{T}}$ charged particles ($0.5&lt;{p}_{\mathrm{T}}&lt;2\text{ }\text{ }\mathrm{GeV}/c$). Moreover, for the ${p}_{\mathrm{T}}$-integrated case (${p}_{\mathrm{T}}&gt;0.5\text{ }\text{ }\mathrm{GeV}/c$), the summed-${p}_{\mathrm{T}}$ density is not sensitive to the variation of CR; however, at low-${p}_{\mathrm{T}}$, it is reduced with increasing CR, whereas an opposite behavior is found at intermediate ${p}_{\mathrm{T}}$ ($2&lt;{p}_{\mathrm{T}}&lt;10\text{ }\text{ }\mathrm{GeV}/c$). Finally, we show that CR produces flowlike behavior only in the UE region, and the effects are reduced with increasing ${p}_{\mathrm{T}}^{\text{jet}}$ due to the hardening of UE. The outcomes encourage the measurement of inclusive and identified charged-particle ${p}_{\mathrm{T}}$ spectra (over a wide range of ${p}_{\mathrm{T}}$) associated to UE aimed at better understanding the similarities between $pp$ and heavy-ion data discovered at the LHC.

Charged jet cross section and fragmentation in proton-proton collisions at s=7 TeV

We report the differential charged jet cross section and jet fragmentation distributions measured with the ALICE detector in proton-proton collisions at a centre-of-mass energy $\sqrt{s}=$ 7 TeV. Jets with pseudo-rapidity $\left| \eta \right| < {\rm 0.5}$ are reconstructed from charged particles using the anti-$k_{\rm T}$ jet finding algorithm with a resolution parameter $R$ = 0.4. The jet cross section is measured in the transverse momentum interval 5 $\leq p_{\rm T}^{\rm ch \; jet} <$ 100 GeV/$c$. Jet fragmentation is studied measuring the scaled transverse momentum spectra of the charged constituents of jets in four intervals of jet transverse momentum between 5 GeV/$c$ and 30 GeV/$c$. The measurements are compared to calculations from the PYTHIA model as well as next-to-leading order perturbative QCD calculations with POWHEG + PYTHIA8. The charged jet cross section is described by POWHEG for the entire measured range of $p_{\rm T}^{\rm ch \; jet}$. For $p_{\rm T}^{\rm ch \; jet}$ $>$ 40 GeV/$c$, the PYTHIA calculations also agree with the measured charged jet cross section. PYTHIA6 simulations describe the fragmentation distributions to 15%. Larger discrepancies are observed for PYTHIA8.

Fragmentation of J/ψ in jets in pp collisions at [formula omitted

The fragmentation of jets containing a J/ψ meson is studied in s=5.02TeV pp data using an integrated luminosity of L=27.39pb−1. The fraction of the jet transverse momentum pT,jet carried by the J/ψ is measured for prompt J/ψ and J/ψ coming from b hadron decays, named nonprompt. Whereas the fragmentation function of nonprompt J/ψ is well-modeled by simulations using a Monte Carlo generator, the prompt J/ψ are found to be accompanied by a larger level of jet activity. The fraction of J/ψ mesons that are produced inside a jet is also reported, and found to be larger in data than in simulation, for both prompt and nonprompt J/ψ.

Measurement of the nuclear modification factor for inclusive jets in Pb+Pb collisions at [formula omitted] with the ATLAS detector

Measurements of the yield and nuclear modification factor, RAA, for inclusive jet production are performed using 0.49 nb−1 of Pb+Pb data at sNN=5.02TeV and 25 pb−1 of pp data at s=5.02TeV with the ATLAS detector at the LHC. Jets are reconstructed with the anti-kt algorithm with radius parameter R=0.4 and are measured over the transverse momentum range of 40–1000 GeV in six rapidity intervals covering |y|<2.8. The magnitude of RAA increases with increasing jet transverse momentum, reaching a value of approximately 0.6 at 1 TeV in the most central collisions. The magnitude of RAA also increases towards peripheral collisions. The value of RAA is independent of rapidity at low jet transverse momenta, but it is observed to decrease with increasing rapidity at high transverse momenta.

Determination of αs in NNLO QCD using H1 jet cross section measurements

Measurements of jet cross sections in neutral current deep-inelastic scattering (NC DIS) using data taken with the H1 detector at HERA are accomplished by the precision measurement of double-differential inclusive jet, dijet and trijet cross sections at low photon virtualities 5.5 &lt; Q2 &lt; 80 GeV2, and by extending previous inclusive jet measurements in the range 150 &lt; Q2 &lt; 15000 GeV2 to low transverse jet momenta 5 &lt; PT &lt; 7 GeV. The strong coupling constant at the Z-boson mass, αs(mZ), is determined in next-to-next-to-leading order (NNLO) QCD using H1 inclusive jet and dijet cross section measurements. Complementary, αs(mZ) is determined together with parton distribution functions of the proton (PDFs) from jet and inclusive DIS data measured by the H1 experiment. The running of the strong coupling is tested at different values of the renormalisation scale and the results are found to be in agreement with the QCD expectations.

Measurement of photon–jet transverse momentum correlations in 5.02 TeV Pb + Pb and pp collisions with ATLAS

Jets created in association with a photon can be used as a calibrated probe to study energy loss in the medium created in nuclear collisions. Measurements of the transverse momentum balance between isolated photons and inclusive jets are presented using integrated luminosities of 0.49 nb−1 of Pb + Pb collision data at sNN=5.02 TeV and 25 pb−1 of pp collision data at s=5.02 TeV recorded with the ATLAS detector at the LHC. Photons with transverse momentum 63.1<pTγ<200 GeV and |ηγ|<2.37 are paired with all jets in the event that have pTjet>31.6 GeV and pseudorapidity |ηjet|<2.8. The transverse momentum balance given by the jet-to-photon pT ratio, xJγ, is measured for pairs with azimuthal opening angle Δϕ>7π/8. Distributions of the per-photon jet yield as a function of xJγ, (1/Nγ)(dN/dxJγ), are corrected for detector effects via a two-dimensional unfolding procedure and reported at the particle level. In pp collisions, the distributions are well described by Monte Carlo event generators. In Pb + Pb collisions, the xJγ distribution is modified from that observed in pp collisions with increasing centrality, consistent with the picture of parton energy loss in the hot nuclear medium. The data are compared with a suite of energy-loss models and calculations.

Measurements of the differential jet cross section as a function of the jet mass in dijet events from proton-proton collisions at sqrt{s}=13 TeV

Measurements of the differential jet cross section are presented as a function of the jet mass in dijet events, in bins of jet transverse momentum, with and without a jet grooming algorithm. The data have been recorded by the CMS Collaboration in proton-proton collisions at the LHC at a center-of-mass energy of 13 TeV and correspond to an integrated luminosity of 2.3 fb−1. The absolute cross sections show slightly different jet transverse momentum spectra in data and Monte Carlo event generators for the settings used. Removing this transverse momentum dependence, the normalized cross section for ungroomed jets is consistent with the prediction from Monte Carlo event generators for masses below 30% of the transverse momentum. The normalized cross section for groomed jets is measured with higher precision than the ungroomed cross section. Semi-analytical calculations of the jet mass beyond leading logarithmic accuracy are compared to data, as well as predictions at leading order and next-to-leading order, which include parton showering and hadronization. Overall, in the normalized cross section, the theoretical predictions agree with the measured cross sections within the uncertainties for masses from 10 to 30% of the jet transverse momentum.

Factorization of the Jet Mass Distribution in the Small R Limit

We derive a factorization theorem for the jet mass distribution with a given $$p_T^J$$ for the inclusive production, where $$p_T^J$$ is a large jet transverse momentum. Considering the small jet radius limit (R ≪ 1), we factorize the scattering cross section into a partonic cross section, the fragmentation function to a jet, and the jet mass distribution function. The decoupled jet mass distributions for quark and gluon jets are well-normalized and scale invariant, and they can be extracted from the ratio of two scattering cross sections such as $$d\sigma/(dp_T^JdM_J^2)$$ and $$d\sigma/dp_T^J$$ . When $$M_J \sim p_T^JR$$ , the perturbative series expansion for the jet mass distributions works well. As the jet mass becomes small, large logarithms of $$M_J/(p_T^JR)$$ appear, and they can be systematically resummed through a more refined factorization theorem for the jet mass distribution.

Infrared sensitivity of single jet inclusive production at hadron colliders

Jet production at hadron colliders is a benchmark process to probe the dynamics of the strong interaction and the structure of the colliding hadrons. One of the most basic jet production observables is the single jet inclusive cross section, which is obtained by summing all jets that are observed in an event. Our recent computation of next-to-next-to-leading order (NNLO) QCD contributions to single jet inclusive observables uncovered large corrections in certain kinematical regions, which also resulted in a sizeable ambiguity on the appropriate choice of renormalization and factorization scales. We now perform a detailed investigation of the infrared sensitivity of the different ingredients to the single jet inclusive cross section. We show that the contribution from the second jet, ordered in transverse momentum pT, in the event is particularly sensitive to higher order effects due to implicit restrictions on its kinematics. By investigating the second-jet transverse momentum distribution, we identify large-scale cancellations between different kinematical event configurations, which are aggravated by certain types of scale choice. Taking perturbative convergence and stability as selection criteria enables us to single out the total partonic transverse energy ĤT and twice the individual jet transverse momentum 2 pT (with which ĤT coincides in Born kinematics) as the most appropriate scales in the perturbative description of single jet inclusive production.

Measurement of the Soft-Drop Jet Mass in pp Collisions at sqrt[s]=13 TeV with the ATLAS Detector.

Jet substructure observables have significantly extended the search program for physics beyond the standard model at the Large Hadron Collider. The state-of-the-art tools have been motivated by theoretical calculations, but there has never been a direct comparison between data and calculations of jet substructure observables that are accurate beyond leading-logarithm approximation. Such observables are significant not only for probing the collinear regime of QCD that is largely unexplored at a hadron collider, but also for improving the understanding of jet substructure properties that are used in many studies at the Large Hadron Collider. This Letter documents a measurement of the first jet substructure quantity at a hadron collider to be calculated at next-to-next-to-leading-logarithm accuracy. The normalized, differential cross section is measured as a function of log_{10}ρ^{2}, where ρ is the ratio of the soft-drop mass to the ungroomed jet transverse momentum. This quantity is measured in dijet events from 32.9 fb^{-1} of sqrt[s]=13 TeV proton-proton collisions recorded by the ATLAS detector. The data are unfolded to correct for detector effects and compared to precise QCD calculations and leading-logarithm particle-level MonteCarlo simulations.