Minimum Transverse Momentum Research Articles

Multijet event shape variables for Mueller-Navelet jet topologies

This paper presents a new set of multijet event shape variables introduced to further understand the Mueller-Navelet jet topology. This topology consists of having at least one pair of jets with a very large rapidity separation between them, treating additional jet activity inclusively. This multijet topology is expected to shed light on the radiation pattern that is expected in the high-energy limit of the strong interaction. The paper relies on a Monte Carlo event generator analysis. One set of predictions uses the ex event generator, which is based on Balitsky-Fadin-Kuraev-Lipatov (BFKL) perturbative quantum chromodynamics (pQCD) evolution with a resummation of large logarithms of energy at leading-logarithmic accuracy. The ex predictions are compared with a fixed-order next-to-leading order pQCD calculation using matched to the parton shower of ythia8 (NLO+PS), which is the standard for NLO generator predictions at the LHC. We find that both approaches lead to compatible results at current LHC energies, assuming the current experimental constraints for the reconstruction of low transverse momentum jets in ATLAS and CMS. This shows the reliability of the BFKL approach at describing the behavior of the strong interaction in the preasymptotic limit of high center-of-mass energies. Differences between the NLO+PS and the BFKL-based approaches are found when the jet multiplicity is increased or when the minimum transverse momentum of the jets is decreased. Published by the American Physical Society 2024

Study of intrinsic-kT in the Parton Branching Method

The work presented here focuses on the production of Drell-Yan lepton pairs in hadron-hadron collisions with very low transverse momentum which is sensitive to the contribution from the non-perturbative transverse motion of partons inside hadrons as well as to the multiple soft gluon emissions which have to be resumed. To study the two contributions, we use Parton Branching Method which describes the evolution of transverse momentum dependent parton distributions and provides very precise predictions for inclusive observables such as the Drell–Yan cross section as a function of transverse momentum. In this study, we tune a parameter that describes the internal transverse motion of partons within hadron (intrinsic-kT) depending on the soft gluon contribution which is varied applying a lower limit to the transverse momentum of the emitted parton in branching up to 2 GeV. It will be shown that the parameter presented by the width of the Gaussian distribution increases with the minimal transverse momentum. This means that by reducing the contribution of soft gluons, more intense internal transverse motion is required (larger intrinsic-kT) to describe experimental data. Moreover, it was observed that the value of the intrinsic-kT increases with the collision energy and such a trend is more pronounced when the minimum transverse momentum of the parton at the branching is higher.

Machine learning-based jet and event classification at the Electron-Ion Collider with applications to hadron structure and spin physics

We explore machine learning-based jet and event identification at the future Electron-Ion Collider (EIC). We study the effectiveness of machine learning-based classifiers at relatively low EIC energies, focusing on (i) identifying the flavor of the jet and (ii) identifying the underlying hard process of the event. We propose applications of our machine learning-based jet identification in the key research areas at the future EIC and current Relativistic Heavy Ion Collider program, including enhancing constraints on (transverse momentum dependent) parton distribution functions, improving experimental access to transverse spin asymmetries, studying photon structure, and quantifying the modification of hadrons and jets in the cold nuclear matter environment in electron-nucleus collisions. We establish first benchmarks and contrast the estimated performance of flavor tagging at the EIC with that at the Large Hadron Collider. We perform studies relevant to aspects of detector design including particle identification, charge information, and minimum transverse momentum capabilities. Additionally, we study the impact of using full event information instead of using only information associated with the identified jet. These methods can be deployed either on suitably accurate Monte Carlo event generators, or, for several applications, directly on experimental data. We provide an outlook for ultimately connecting these machine learning-based methods with first principles calculations in quantum chromodynamics.

Minijet quenching in a concurrent jet+hydro evolution and the nonequilibrium quark-gluon plasma

Mini-jets, created by perturbative hard QCD collisions at moderate energies, can represent a significant portion of the total multiplicity of a heavy-ion collision event. Since their transverse momenta are initially larger than the typical saturation scale describing the bulk of the equilibrating QGP, their typical stopping distances are larger than the usual hydrodynamization time, so they do not in general hydrodynamize at the same pace than the bulk of the collision. Therefore, in general mini-jets cannot be described solely by a unique pre-equilibrium stage that bridges the initial, over-occupied glasma state, with the hydrodynamical evolution. In this work we make use of a new concurrent mini-jet+hydrodynamic framework in which the properties of the hydrodynamically evolving QGP are modified due to the injection of energy and momentum from the mini-jets. We study the system for different choices of the minimum transverse momentum associated to mini-jet production. In order to achieve a realistic description of charged particle multiplicity, the amount of entropy associated to the low-$x$ initial state needs to be reduced. Moreover, the fact that the injected momentum from the randomly oriented mini-jets is not correlated with the spatial gradients of the system reduces overall flow, and the value of the QGP transport coefficients needs to be reduced accordingly. They are an important new source of fluctuations, resulting in a spikier, notably modified hydrodynamical evolution. We discuss the impact of the mini-jets on a number of observables, such as $p_T$ spectra and $p_T$-differential flow $v_n$ for a wide range of centrality classes. In contrast to elliptic, triangular or quadrangular flow, we find that directed flow, $v_1$, has the strongest potential to discriminate between different mini-jet production rates.

Search for dijet resonances in events with an isolated charged lepton using sqrt{s} = 13 TeV proton-proton collision data collected by the ATLAS detector

A search for dijet resonances in events with at least one isolated charged lepon is performed using 139 fb−1 of sqrt{s} = 13 TeV proton-proton collision data recorded by the ATLAS detector at the LHC. The dijet invariant-mass (mjj) distribution constructed from events with at least one isolated electron or muon is searched in the region 0.22 < mjj< 6.3 TeV for excesses above a smoothly falling background from Standard Model processes. Triggering based on the presence of a lepton in the event reduces limitations imposed by minimum transverse momentum thresholds for triggering on jets. This approach allows smaller dijet invariant masses to be probed than in inclusive dijet searches, targeting a variety of new-physics models, for example ones in which a new state is produced in association with a leptonically decaying W or Z boson. No statistically significant deviation from the Standard Model background hypothesis is found. Limits on contributions from generic Gaussian signals with widths ranging from that determined by the detector resolution up to 15% of the resonance mass are obtained for dijet invariant masses ranging from 0.25 TeV to 6 TeV. Limits are set also in the context of several scenarios beyond the Standard Model, such as the Sequential Standard Model, a technicolor model, a charged Higgs boson model and a simplified Dark Matter model.

The q_T subtraction method: electroweak corrections and power suppressed contributions

Building upon the formulation of transverse-momentum resummation for heavy-quark hadroproduction, we present the first application of the q_T subtraction formalism to the computation of electroweak corrections to massive lepton pairs through the Drell–Yan mechanism. We then study the power suppressed contributions to the q_T subtraction formula in the parameter r_{mathrm{cut}}, defined as the minimum transverse momentum of the lepton pair normalised to its invariant mass. We analytically compute the leading power correction from initial and final-state radiation to the inclusive cross section. In the case of initial-state radiation the power correction is quadratic in r_{mathrm{cut}} and our analytic result is consistent with results previously obtained in the literature. Final-state radiation produces linear contributions in r_{mathrm{cut}} that may challenge the efficiency of the q_T subtraction procedure. We explicitly compute the linear power correction in the case of the inclusive cross section and we discuss the extension of our calculation to differential distributions.

Overview of jet physics results from ALICE

Recent results from jet shower-shape and substructure analyses done by the ALICE collaboration in central Pb–Pb collisions at √sNN = 2.76 TeV and in pp collisions at √s = 7 TeV are reviewed. The presented jet shower- shape observables are angularity g and transverse momentum dispersion pTD, which were studied for a small resolution parameter R = 0.2, track-based jets with a minimum constituent transverse momentum (pT) cut-off of 0.15 GeV/c. Jet substructure is explored for track-based anti-kT jets with R = 0.4 by means of iterative declustering and grooming techniques, which were used to measure the absolutely-normalized leading subjet momentum fraction zg and the number of hard splittings in the reclustered jet shower, nSD. These observables provide complementary information on the jet fragmentation and help to discriminate between different scenarios for medium-induced modifications of the parton shower in heavy-ion collisions due to jet quenching.

New observables for multiple-parton interactions measurements usingZ+jetsprocesses at the LHC

Multiple-parton interactions (MPI) play a vital role in hadron-hadron collisions. This paper presents a study of the MPIs with simulated $Z+\mathrm{jets}$ events in proton-proton collisions at a center-of-mass energy of 13 TeV. The events are simulated with powheg, followed by hadronization and parton showering using pythia8. The events with dimuon invariant mass in the range of $60--120\text{ }\text{ }\mathrm{GeV}/{c}^{2}$ are selected for the analysis. The charged particle jets having a minimum transverse momentum of $5\text{ }\text{ }\mathrm{GeV}/c$ and an absolute pseudorapidity less than 2 are used to construct the observables for measurements of the MPIs. The proposed observables and phase-space region presented in this paper are found to have enhanced sensitivity to MPIs. The increased sensitivity to MPIs will lead to precise constraints on the parameters of the MPI models.

ExtrapolatingW-associated jet-production ratios at the LHC

Electroweak vector-boson production, accompanied by multiple jets, is an important background to searches for physics beyond the Standard Model. A precise and quantitative understanding of this process is helpful in constraining deviations from known physics. We study four key ratios in $W + n$-jet production at the LHC. We compute the ratio of cross sections for $W + n$- to $W + (n-1)$-jet production as a function of the minimum jet transverse momentum. We also study the ratio differentially, as a function of the $W$-boson transverse momentum; as a function of the scalar sum of the jet transverse energy, $H_T^{\rm jets}$; and as a function of certain jet transverse momenta. We show how to use such ratios to extrapolate differential cross sections to $W+6$-jet production at next-to-leading order, and we cross-check the method against a direct calculation at leading order. We predict the differential distribution in $H_T^{\rm jets}$ for $W+6$ jets at next-to-leading order using such an extrapolation. We use the BlackHat software library together with SHERPA to perform the computations.

Measurement of the cross section ratio [formula omitted] in [formula omitted] collisions at [formula omitted

The first measurement of the cross section ratio σtt¯bb¯/σtt¯jj is presented using a data sample corresponding to an integrated luminosity of 19.6 fb−1 collected in pp collisions at s=8 TeV with the CMS detector at the LHC. Events with two leptons (e or μ) and four reconstructed jets, including two identified as b quark jets, in the final state are selected. The ratio is determined for a minimum jet transverse momentum pT of both 20 and 40 GeV/c. The measured ratio is 0.022±0.003(stat)±0.005(syst) for pT>20 GeV/c. The absolute cross sections σtt¯bb¯ and σtt¯jj are also measured. The measured ratio for pT>40 GeV/c is compatible with a theoretical quantum chromodynamics calculation at next-to-leading order.

Z0 Boson measurement with the ALICE central barrel in pp collisions at 14TeV

The possibility to detect the Z0 in the ALICE central barrel is studied via the electronic decay channel Z0->e+e-. The signal and the background are simulated with the leading order event generator PYTHIA 6. The total cross-sections are taken from NLO calculations. Based on test beam data, the electron identification performance of the Transition Radiation Detector is extrapolated to high momenta. The expected yields for minimum-bias pp collisions at 14 TeV are presented. An isolation cut on the single electron, together with a minimum transverse momentum cut, allows to obtain a clear signal. The expected background is of the order of 1 % with the main contribution coming from misidentified pions from jets.

Constrained MC for QCD evolution with rapidity ordering and minimum kT

With the imminent start of LHC experiments, development of phenomenological tools, and in particular the Monte Carlo programs and algorithms, becomes urgent. A new algorithm for the generation of a parton shower initiated by the single initial hadron beam is presented. The new algorithm is of the class of the so-called “constrained MC” type algorithm (an alternative to the backward evolution MC algorithm), in which the energy and the type of the parton at the end of the parton shower are constrained (predefined). The complete kinematics configurations with explicitly constructed four momenta are generated and tested. Evolution time is identical with rapidity and minimum transverse momentum is used as an infrared cut-off. All terms of the leading-logarithmic approximation in the DGLAP evolution are properly accounted for. In addition, the essential improvements towards the so-called CCFM/BFKL models are also properly implemented. The resulting parton distributions are cross-checked up to the 10 −3 precision level with the help of a multitude of comparisons with other MC and non-MC programs. We regard these tests as an important asset to be exploited at the time when the presented MC will enter as a building block in a larger MC program for W / Z production process at LHC.

Heavy ion event generator HYDJET++ (HYDrodynamics plus JETs)

Heavy ion event generator HYDJET++ (HYDrodynamics plus JETs)

Missing heavy flavor backgrounds to Higgs boson production

We investigate characteristics of the signal and backgrounds for Higgs boson decay into $WW$ at the Fermilab Tevatron and CERN Large Hadron Collider. In the lepton-pair-plus-missing-energy final state, we show that the background receives an important contribution from semileptonic decays of heavy flavors. Lepton isolation cuts provide too little suppression of these heavy flavor contributions, and an additional 4 to 8 orders-of-magnitude suppression must come from physics cuts. We demonstrate that an increase of the minimum transverse momentum of nonleading leptons in multilepton events is one effective way to achieve the needed suppression, without appreciable loss of the Higgs boson signal. Such a cut would impact the efficiency of searches for supersymmetry as well. We emphasize the importance of direct measurement of the lepton background from heavy flavor production.

Photoproduction of dijets with high transverse momenta at HERA

Differential dijet cross sections are measured in photoproduction in the region of photon virtualities Q^2 < 1 GeV^2 with the H1 detector at the HERA ep collider using an integrated luminosity of 66.6 pb^{-1}. Jets are defined with the inclusive k_T algorithm and a minimum transverse momentum of the leading jet of 25 GeV is required. Dijet cross sections are measured in direct and resolved photon enhanced regions separately. Longitudinal proton momentum fractions up to 0.7 are reached. The data compare well with predictions from Monte Carlo event generators based on leading order QCD and parton showers and with next-to-leading order QCD calculations corrected for hadronisation effects.

Transverse energy from minijets in ultrarelativistic nuclear collisions: A next-to-leading order analysis

We compute in next-to-leading order (NLO) perturbative QCD the amount of transverse energy produced into a rapidity region $\Delta Y$ of a nuclear collision from partons created in the few-GeV subcollisions. The NLO formulation assumes collinear factorization and is based on the subtraction method. We first study the results as a function of the minimum transverse momentum scale and define and determine the associated $K$-factors. The dependence of the NLO results on the scale choice and on the size of $\Delta Y$ is also studied. The calculations are performed for GRV94 and CTEQ5 sets of parton distributions. Also the effect of nuclear shadowing to the NLO results is investigated. The main results are that the NLO results are stable relative to the leading-order (LO) ones even in the few-GeV domain and that there exists a new kinematical region not present in the LO, which is responsible for the magnitude of the $K$-factors. The dependence on the size of $\Delta Y$ is found to be practically linear within two central units of rapidity, and the scale dependence present at RHIC energies is seen to vanish at the LHC energies. Finally, the effect of NLO corrections to the determination of transverse energies and multiplicities in nuclear collisions is discussed within the framework of parton saturation.

Production of transverse energy from minijets in next-to-leading order perturbative QCD

We compute in next-to-leading order (NLO) perturbative QCD the transverse energy carried into the central rapidity unit of hadron or nuclear collisions by the partons freed in the few-GeV subcollisions. The formulation is based on a rapidity window and a measurement function of a new type. The behaviour of the NLO results as a function of the minimum transverse momentum and as a function of the scale choice is studied. The NLO results are found to be stable relative to the leading-order ones even in the few-GeV domain. Consequences of the NLO computation for the estimates of the final state charged particle multiplicity and transverse energy per unit rapidity in ultrarelativistic heavy ion collisions at RHIC and LHC are also studied.

Ratios of W+/- gamma and Z gamma cross sections: New tools in probing the weak boson sector at the Fermilab Tevatron.

The ratios ${\cal R}_{\gamma ,\ell}=B(Z\to\ell^+\ell^-)\cdot\sigma(Z\gamma) /\allowbreak B(W\to\ell\nu)\cdot\sigma(W^\pm\gamma)$, ${\cal R}_{\gamma , \nu}= B(Z\to\bar\nu\nu)\cdot\sigma(Z\gamma)/\allowbreak B(W\to\ell\nu)\cdot\sigma(W^ \pm\gamma)$, ${\cal R}_{W\gamma}=\sigma(W^\pm\gamma)/\allowbreak\sigma(W^\pm)$, and ${\cal R}_{Z\gamma}=\sigma(Z\gamma)/\allowbreak\sigma(Z)$ are studied as tools to probe the electroweak boson self-interactions. As a function of the minimum photon transverse momentum, ${\cal R}_{\gamma ,\ell}$ and ${\cal R}_{ \gamma ,\nu}$ are found to directly reflect the radiation zero present in $W^ \pm\gamma$ production in the Standard Model. All four ratios are sensitive to anomalous $WW\gamma$ and/or $ZZ\gamma/Z\gamma\gamma$ couplings. The sensitivity of the cross section ratios to the cuts imposed on the final state particles, as well as the systematic uncertainties resulting from different parametrizations of parton distribution functions, the choice of the factorization scale $Q^2$, and from higher order QCD corrections are explored. Taking into account these uncertainties, sensitivity limits for anomalous three gauge boson couplings, based on a measurement of the cross section ratios with an integrated luminosity of 25~pb$^{-1}$ at the Tevatron, are estimated.

QCD minijet cross sections.

We calculate the lowest-order QCD jet cross section for a variety of different sets of structure functions, QCD scales, and parton minimum transverse momentum {ital p}{sub {ital T}}{sup min}. We compare our theoretical results with the experimental data for the jet differential and total cross sections including the minijet regime in order to study the correlations between the various parameters. We find that for a constant {ital K} factor the choice of scale {ital Q}{sup 2}=p{sub T}{sup 2} provides the best representation of the experimental data. With a {ital K} factor of order 2, a theoretical cutoff of {ital p}{sub {ital T}}{sup min}{similar to}3 GeV seems to describe the observed total minijet cross section with {ital E}{sub {ital T}}{sup jet} ({ital E}{sub {ital T}}{sup raw}){ge}5 GeV. We extrapolate our results to yield predictions for the expected minijet cross section at the Fermilab Tevatron Collider which should have some resolving power to distinguish the different sets of structure functions and choices of scale.

The lund monte carlo for e +e − jet physics

The lund monte carlo for e +e − jet physics