high-pT Jets Research Articles

Searches for New Physics in Multijet Final States

A variety of new physics models predict heavy resonances that decay to multiple hadronic jets. These models include axigluons, colorons, diquarks, excited quarks, Randall-Sundrum gravitons, string resonances, and Z' models, among others. Other models make the prediction that high-pT jets will be suppressed, resulting in jet extinction. Using the data collected in 2012 at a center-of-mass energy of 8 TeV, the CMS Collaboration has made a baseline inclusive jet cross section measurement for comparison with new-physics searches, and then performed searches for jet extinction and resonances that decay to two hadronic jets. The results of these searches will be presented. No evidence of new physics has been observed, and these results set new limits on the parameters of these models.

Jet production in (un)polarizedppcollisions: Dependence on jet algorithm

We investigate single-inclusive high-pT jet production in longitudinally polarized pp collisions at RHIC, with particular focus on the algorithm adopted to define the jets. Following and extending earlier work in the literature, we treat the jets in the approximation that they are rather narrow, in which case analytical results for the corresponding next-to-leading order partonic cross sections can be obtained. This approximation is demonstrated to be very accurate for practically all relevant situations, even at Tevatron and LHC energies. We confront results for cross sections and spin-asymmetries based on using cone- and kt-type jet algorithms. We find that jet cross sections at RHIC can differ significantly depending on the algorithm chosen, but that the spin asymmetries are rather robust. Our results are also useful for matching threshold-resummed calculations of jet cross sections to fixed-order ones.

Extracting the wavefunction of the LSP at the LHC

We consider associated production of squarks and gluinos with the lightest supersymmetric particle (LSP), or states nearly degenerate in mass with it. Though sub-dominant to pair production of color SU(3)-charged superpartners, these processes are directly sensitive to the wavefunction composition of the lightest neutralinos. Exploiting event-shape variables – including some introduced here for the first time – we are able to identify the composition of the LSP by selecting events involving a single high-pT jet recoiling against missing transverse energy. We illustrate the proposed technique on a set of benchmark cases and propose methods for applying these results in more realistic experimental environments.

Studying the underlying event in Drell-Yan and high transverse momentum jet production at the Tevatron

We study the underlying event in proton-antiproton collisions by examining the behavior of charged particles (transverse momentum pT > 0.5 GeV/c, pseudorapidity |\eta| < 1) produced in association with large transverse momentum jets (~2.2 fb-1) or with Drell-Yan lepton-pairs (~2.7 fb-1) in the Z-boson mass region (70 < M(pair) < 110 GeV/c2) as measured by CDF at 1.96 TeV center-of-mass energy. We use the direction of the lepton-pair (in Drell-Yan production) or the leading jet (in high-pT jet production) in each event to define three regions of \eta-\phi space; toward, away, and transverse, where \phi is the azimuthal scattering angle. For Drell-Yan production (excluding the leptons) both the toward and transverse regions are very sensitive to the underlying event. In high-pT jet production the transverse region is very sensitive to the underlying event and is separated into a MAX and MIN transverse region, which helps separate the hard component (initial and final-state radiation) from the beam-beam remnant and multiple parton interaction components of the scattering. The data are corrected to the particle level to remove detector effects and are then compared with several QCD Monte-Carlo models. The goal of this analysis is to provide data that can be used to test and improve the QCD Monte-Carlo models of the underlying event that are used to simulate hadron-hadron collisions.

Non-global logarithms and jet algorithms in high-p T jet shapes

We consider jet-shape observables of the type proposed recently, where the shapes of one or more high-pT jets, produced in a multi-jet event with definite jet multiplicity, may be measured leaving other jets in the event unmeasured. We point out the structure of the full next-to-leading logarithmic resummation specifically including resummation of non-global logarithms in the leading-Nc limit and emphasising their properties. We also point out differences between jet algorithms in the context of soft gluon resummation for such observables.

PHENIX and the reaction plane: Recent results

During the past several years, experiments at RHIC have established that a dense partonic medium is produced in Au+Au collisions at = 200 GeV. Subsequently, a primary goal of analysis has been to understand and characterize the dynamics underlying this new form of matter. Among the many probes available, the measurements with respect to the reaction plane have proven to be crucial to our understanding of a wide range of topics, from the hydrodynamics of the initial expansion of the collision region to high-pT jet quenching phenomena. Few tools have the ability to shed light on such a wide variety of observables as the reacion plane. In this talk, we discuss recent PHENIX measurements with respect to the reaction plane, and the implications for understanding the underlying physics of RHIC collisions.

Production of the neutral top-pion π0t in association with a high pT jet at the LHC

In the framework of the topcolor-assisted technicolor (TC2) model, we study the production of the neutral top-pion π0t in association with a high-pT jet at the LHC, which proceeds via the partonic processes gg → π0tg, gq → π0tq, qq̄ → π0tg, gb(b̄) → π0tb(b̄), and bb̄ → π0tg. We find that it is very challenging to detect the neutral top-pion π0t via the process pp → π0t + jet + X → tt̄ + jet + X, while the possible signatures of π0t might be detected via the process pp → π0t + jet + X → (t̄c + tc̄)+jet + X at the LHC.

Substructure of high-pTjets at the LHC

We study high-pt jets from QCD and from highly-boosted massive particles such as tops, W, Z and Higgs, and argue that infrared-safe observables can help reduce QCD backgrounds. Jets from QCD are characterized by different patterns of energy flow compared to the products of highly-boosted heavy particle decays, and we employ a variety of jet shapes, observables restricted to energy flow within a jet, to explore this difference. Results from Monte Carlo generators and arguments based on perturbation theory support the discriminating power of the shapes we refer to as planar flow and angularities. We emphasize that for massive jets, these and other observables can be analyzed perturbatively.

Generation of complete events containing very high-p T jets

The study of very high transverse-momentum jets will be an important issue at the LHC, in particular since the corresponding cross sections will be considerably larger than at RHIC energies. Jets are expected to provide information on QGP formation, due to the energy loss of fast partons in the medium. Jet cross sections can in principle be compared to simple pQCD calculations, based on the hypothesis of factorization. But often it is useful or even necessary to not only compute the production rate of the very high-p T jets, but in addition the “rest of the event”. The proposed talk is based on recent work, where we try to construct an event generator—fully compatible with pQCD—which allows one to compute complete events, consisting of high-p T jets plus all the other low p T particles produced at the same time. Whereas in “generators of inclusive spectra” like Pythia one may easily trigger on high-p T phenomena, this is not so obvious for “generators of physical events”, where in principle one has to generate a very large number of events in order to finally obtain rare events (like those with a very high-p T jet). We shall discuss how we overcome these difficulties in the framework of the EPOS model.

High-pT jet fragmentation in Au+Au collisions

The large single-particle high-pT suppression (RAA) observed in heavy-ion collisions at RHIC energies suggests a considerable loss of energy by hard-scattered partons. Additional experimental information is sought to improve the understanding of medium-induced energy loss beyond that given by RAA. Angular di-jet correlations are an important experimental probe currently available to investigate the mechanisms giving rise to jet quenching in heavy-ion collisions. In a comparison between Au+Au and p + p, it is observed that the quantities representing jet widths and shapes for the two species become comparable at high transverse momentum (pT > 7 GeV/c). This apparent similarity is interpreted as being due to some combination of the following: (a) an inability of di-jet correlations to serve as a penetrating probe of the medium, or (b) minimal medium-induced modification of the observed high-pT jets due to large fluctuations in energy loss probabilities. Additional experimental probes are discussed which offer the possibility to discriminate between such explanations.

Elliptic flow arising from ridges due to semi-hard scattering

Azimuthal anisotropy in heavy-ion collisions is studied by taking into account the ridges generated by semi-hard scattering of intermediate-momentum partons, which can be sensitive to the initial spatial configuration of the medium in non-central collisions. In a simple treatment of the problem where the recombination of only thermal partons is considered, analytical formulas can be derived that yield results in accord with the data on v2 for pT<1–2 GeV/c. Centrality dependence is described by a geometrical factor. Ridge phenomenology is used to determine the initial slopes of v2 at low pT for both pion and proton. For higher pT, shower partons from high-pT jets must be included, but they are not considered here.

Hard physics with heavy ions in CMS

The LHC will collide protons at √s = 14 TeV and lead beams at √sNN = 5.5 TeV as well as possibly other AA and pA combinations. The physics program of the Compact Muon Solenoid (CMS) includes the study of heavy-ion collisions, where the collision energy, higher than at RHIC, will allow the study of the dense partonic system with hard probes. A broad palette of these probes is within the reach of CMS. Some are likely to be modified, relative to p-p collisions, by the presence of the hot medium, like quarkonia (J/ψ and γ) or high-pT jets. Others, like heavy quarks, Z0 bosons and photons, will act as reference candles. This article overviews the hard physics reach and unique abilities of CMS in studying the medium produced in heavy-ion collisions at the LHC.

Heavy-ion physics with CMS

The Large Hadron Collider (LHC) at CERN will collide protons at and lead ions at . The research program of the Compact Muon Solenoid (CMS) experiment includes a strong heavy-ion physics agenda, especially in the measurement of hard processes. The high centre-of-mass energies available at the LHC will allow high statistics studies of high-density QCD with hard probes: heavy quarks and quarkonia with an emphasis on the J/ψ and ϒ, high-pT jets, direct photons as well as Z0 bosons. In addition to the detailed studies of hard probes, CMS will measure charged particle multiplicity and energy flow with respect to the reaction plane on an event-by-event basis. Studies of the CMS capabilities and physics reach for different high-density QCD observables, using the full detector simulation and reconstruction, are presented.

SPIN STRUCTURE OF NUCLEON AND SPIN TRANSFER IN HIGH ENERGY FRAGMENTATION PROCESS

Spin transfer in high energy fragmentation process is determined by the hadronization mechanism and spin structure of hadrons. It can be studied by measuring the polarizations of hyperons and/or vector mesons in e+e- annihilation, in the current fragmentation region of polarized deeply inelastic lN-scatterings, and high pT-jets in polarized pp-collisions. Theoretical calculations have been made using different models. In this talk, I will briefly summarize the main features of the models, the results obtained and the comparison with available data. They can be used for future tests by experiments.

Prospects for the observation of a Higgs boson with [formula omitted] associated with one high-[formula omitted] jet at the LHC

We investigate the sensitivity of the LHC experiments to the Standard Model Higgs using H→ττ→l+l−p̸t associated with one high-PT jet in the mass range 110<MH<150 GeV/c2. Cut and Neural Network based event selections are chosen to optimize the expected signal significance with this decay mode. A signal significance of about 6.2σ can be achieved for MH=120 GeV/c2 with 30 fb−1 of integrated luminosity for one experiment only. Additionally, experimental issues related to tagging forward jets, reconstructing missing transverse momentum and the application of a central jet veto are simplified considerably.

The equivalence of fluctuation scale dependence and autocorrelations

We define optimal per-particle fluctuation and correlation measures, relate fluctuations and correlations through an integral equation and show how to invert that equation to obtain precise autocorrelations from fluctuation scale dependence. We test the precision of the inversion with Monte Carlo data and compare autocorrelations to conditional distributions conventionally used to study high-pt jet structure.

Calorimetric transverse energy-energy correlations as a probe of jet quenching

The sensitivity of calorimetric energy-energy correlation function to the medium-induced energy loss of fast partons in high multiplicity heavy ion interactions is demonstrated at the appropriate selection of events for the analysis, namely, the availability of one high-pt jet in an event at least and using the procedure of ``thermal'' background subtraction. Without jet trigger this correlation function manifests the global structure of transverse energy flux: the correlator is isotropic for central collisions, and for non-central collisions it is sensitive to the azimuthal anisotropy of energy flow reproducing its Fourier harmonics but with the coefficients squared.

Heavy ion physics with the CMS experiment at the large hadron collider

The LHC will collide protons at and lead beams at as well as other AA and pA combinations. The physics program of the compact muon solenoid (CMS) includes the study of heavy ion collisions. The collision energy, much higher than at RHIC, will study the dense partonic system with very hard probes: heavy quarks and quarkonia with emphasis on b and ϒ, W± and Z0 bosons, high-pT jets and photons. The production of these particles is likely to be modified, relative to pp collisions, by the presence of the hot medium.

The new physics at RHIC. From transparency to high p suppression

Heavy ion collisions at RHIC energies (Au + Au collisions at√SNN = 200GeV) exhibit significant new features as compared to earlier experiments at lower energies. The reaction is characterized by a high degree of transparency of the collision partners leading to the formation of a baryon-poor central region. In this zone, particle production occurs mainly from the stretching of the color field. The initial energy density is well above the one considered necessary for the formation of the Quark Gluon Plasma, QGP. The production of charged particles of various masses is consistent with chemical and thermal equilibrium. Recently, a suppression of the high transverse momentum component of hadron spectra has been observed in central Au + Au collisions. This can be explained by the energy loss experienced by leading partons in a medium with a high density of unscreened color charges. In contrast, such highpt jets are not suppressed in d + Au collisions suggesting that the highpt suppression is not due to initial state effects in the ultrarelativistic colliding nuclei.

Recent QCD results from DØ

We present some of the most recent QCD results from DO. The Run I measurement of the inclusive k T jet cross section is one of the first analyses on hadron-hadron colliders that uses the k T -algorithm instead of the traditional cone one. We observe a good agreement with NLO QCD predictions and also with the previous Run I measurement which utilised the cone algorithm. The agreement is marginal at low p T end. The difference is attributed to the different behaviour of jet algorithms after hadronization. We also present preliminary Run II results on the inclusive jet cross section and the inclusive dijet mass spectrum. The analysis is performed on the sample of $34\,\mbox{pb}^{-1}$ . Due to a 9% increase in the center of mass energy and a higher instantaneous luminosity, in the near future, the Run II data will provide greater statistics for high p T jets than in Run I.