Quark Properties Research Articles

Top Quark Properties Measurements with the ATLAS Experiment

Results on recent measurements of top quark properties with the ATLAS experiment at the European Laboratory, CERN, are shown. The measurements are performed using the full data set recorded during the LHC Run-I. The full data set consists of a collected integrated luminosities ∫Tdt of 4.6 fb-1 recorded at a proton-proton collision energy of √s = 7 TeV and 20.3 fb-1 collected at 8 TeV. The mentioned top quark properties include: spin correlation, charge asymmetry, W-boson polarization, color flow, top mass and top width in events with a top and anti-top quark pair (tt). An introduction to the LHC and the ATLAS detector is included and latest main results from this experiment. The contents include the current world benchmark results for the different properties and plans for future measurements during the ongoing LHC Run-II.

Heavy quark masses from production near threshold

In this paper, we review the precision determination of the bottom and top quark masses from the total pair-production cross-section near threshold. The theory prediction of the cross-section includes QCD corrections up to third-order. We further discuss the combined impact of Higgs corrections, the QED Coulomb potential, non-resonant production, and P-wave production on the extraction of top quark properties.

Nontrivial center dominance in high temperature QCD

We investigate the properties of quarks and gluons above the chiral phase transition temperature [Formula: see text], using the renormalization group (RG) improved gauge action and the Wilson quark action with two degenerate quarks mainly on a [Formula: see text] lattice. In the one-loop perturbation theory, the thermal ensemble is dominated by the gauge configurations with effectively [Formula: see text] center twisted boundary conditions, making the thermal expectation value of the spatial Polyakov loop take a nontrivial [Formula: see text] center. This is in agreement with our lattice simulation of high temperature quantum chromodynamics (QCD). We further observe that the temporal propagator of massless quarks at extremely high temperature [Formula: see text] remarkably agrees with the temporal propagator of free quarks with the [Formula: see text] twisted boundary condition for [Formula: see text], but differs from that with the [Formula: see text] trivial boundary condition. As we increase the mass of quarks [Formula: see text], we find that the thermal ensemble continues to be dominated by the [Formula: see text] twisted gauge field configurations as long as [Formula: see text] and above that the [Formula: see text] trivial configurations come in. The transition is similar to what we found in the departure from the conformal region in the zero-temperature many-flavor conformal QCD on a finite lattice by increasing the mass of quarks.

Measurement of the top quark mass using charged particles inppcollisions ats=8 TeV

A novel technique for measuring the mass of the top quark that uses only the kinematic properties of its charged decay products is presented. Top quark pair events with final states with one or two charged leptons and hadronic jets are selected from the data set of 8 TeV proton-proton collisions, corresponding to an integrated luminosity of 19.7 inverse femtobarns. By reconstructing secondary vertices inside the selected jets and computing the invariant mass of the system formed by the secondary vertex and an isolated lepton, an observable is constructed that is sensitive to the top quark mass that is expected to be robust against the energy scale of hadronic jets. The main theoretical systematic uncertainties, concerning the modeling of the fragmentation and hadronization of b quarks and the reconstruction of secondary vertices from the decays of b hadrons, are studied. A top quark mass of 173.68 +/- 0.20 (stat) +1.58 -0.97 (syst) GeV is measured. The overall systematic uncertainty is dominated by the uncertainty in the b quark fragmentation and the modeling of kinematic properties of the top quark.

Measurements of top quark properties in top pair production and decay at the LHC using the CMS detector

Measurements are presented of the properties of top quarks in pair production and decay from proton-proton collisions at the LHC. The data were collected at centre-of-mass energies of 7 and 8 TeV by the CMS experiment during the years 2011 and 2012. The top quark-antiquark charge asymmetry is measured using the difference of the absolute rapidities of the reconstructed top and anti-top kinematics, as well as from distributions of the top quark decay products. The measurements are performed in the decay channels of the tt‾ pair into both one and two leptons in the final state. The polarization of top quarks and top pair spin correlations are measured from the angular distributions of top quark decay products. The W-boson helicity fractions and angular asymmetries are extracted and limits on anomalous contributions to the Wtb vertex are determined. The flavor content in top-quark pair events is measured using the fraction of top quarks decaying into a W-boson and a b-quark relative to all top quark decays, R=B(t→Wb)/B(t→Wq), and the result is used to determine the CKM matrix element Vtb as well as the width of the top quark resonance. All of the results are found to be in good agreement with standard model predictions.

Top quark pair production and top quark properties at CDF

We present the most recent measurements of top quark pairs production and top quark properties in proton-antiproton collisions with center-of-mass energy of 1.96 TeV using CDF II detector at the Tevatron. The combination of top pair production cross section measurements and the direct measurement of top quark width are reported. The test of Standard Model predictions for top quark decaying into b-quarks, performed by measuring the ratio R between the top quark branching fraction to b-quark and the branching fraction to any type of down quark is shown. The extraction of the CKM matrix element |Vtb| from the ratio R is discussed. We also present the latest measurements on the forward-backward asymmetry (AFB) in top anti-top quark production. With the full CDF Run II data set, the measurements are performed in top anti-top decaying to final states that contain one or two charged leptons (electrons or muons). In addition, we combine the results of the leptonic forward-backward asymmetry in tt‾ system between the two final states. All the results show deviations from the next-to-leading order (NLO) standard model (SM) calculation.

Measurement of Top Quark Properties in Single Top-Quark Production at CMS

Single top-quark t-channel production is exploited for studies of top quark properties. The analyses include the measurement of the CKM matrix element, |Vtb|, search for anomalous couplings of the top quark using a Bayesian neural network analysis, measurement of single top-quark polarization which directly confirms the V-A nature of the tWb production vertex, and the measurement of W-helicity fractions in the phase space sampled by a selection optimized for t-channel single top-quark production, orthogonal to the tt‾ final states used in traditional measurements of these properties. All measurements are found to be consistent with the standard model predictions.

750 GeV composite axion as the LHC diphoton resonance

We propose that the 750 GeV resonance, presumably observed in the early LHC Run 2 data, could be a heavy composite axion that results from condensation of a hypothetical quark in a high-colour representation of conventional QCD. The model, motivated by a recently proposed solution to the strong CP problem, is very economical and is essentially defined by the properties of the additional quark - its colour charge, hypercharge and mass. The axion mass and its coupling to two photons (via axial anomaly) can be computed in terms of these parameters. The axion is predominantly produced via photon fusion ($\gamma\gamma \to {\cal A}$) which is followed by $ Z $ vector boson fusion and associated production at the LHC. We find that the total diphoton cross section of the axion can be fitted with the observed excess. Combining the requirement on the cross-section, such that it reproduces the diphoton excess events, with the bounds on the total width ($\Gamma_{tot} \leqslant 45$ GeV), we obtain the effective coupling in the range $1.6\times 10^{-4}$ GeV$^{-1}\gtrsim C_{{\cal A}} \gtrsim 6.5\times 10^{-5}$ GeV$^{-1}$. Within this window of allowed couplings the model favours a narrow width resonance and $ y_{Q}^2 \sim \mathcal{O}(10)$. In addition, we observe that the associated production $q\bar{q} \to {\cal A}\gamma\to \gamma\gamma\gamma$ can potentially produce a sizeable number of three photon events at future LHC and $ e^{+} e^{-} $ colliders. However, the rare decay $Z\to\mathcal{A}^*\gamma \to \gamma\gamma\gamma$ is found to be too small to be probed at the LHC.

Top quark pair properties using the ATLAS detector at the LHC

The latest measurements of the properties of the top quark using the ATLAS experiment are presented. The top quark mass is one of the fundamental parameters of the Standard Model. A new measurement using the template method in dilepton events in which the event selection was optimised to achieve the smallest total uncertainty is presented. It is combined with a measurement on a multi-dimensional template fit that can constrain the uncertainties on the energy measurements of jets. In addition, measurements aiming to measure the mass in a well-defined scheme are presented. The top quark pair charge asymmetry is an asymmetry predicted to occur beyond leading-order QCD in the Standard Model, and may be significantly enhanced by the presence of new physics. The tt production charge asymmetry is measured inclusively and differentially using the 8 TeV ATLAS datasets in the dilepton channel and with lepton+jets events including boosted topology. Making use of the large number of top quark pairs collected, we also present measurements of the spin correlation between top and anti-top quarks and discuss their sensitivities to new physics.

Jet quenching in high-energy heavy-ion collisions

Jet quenching in high-energy heavy-ion collisions can be used to probe properties of hot and dense quark–gluon plasma. We provide a brief introduction to the concept and framework for the study of jet quenching. Different approaches and implementation of multiple scattering and parton energy loss are discussed. Recent progresses in the theoretical and phenomenological studies of jet quenching in heavy-ion collisions at RHIC and LHC are reviewed.

Spontaneous violation of mirror symmetry

A symmetry violation model is considered for a system that can spontaneously choose between identical states which differ from each other only in weak properties (R-L). Such mirror symmetry allows reproduction of observed qualitative properties of quark and lepton mixing matrices. The lepton mixing matrix evidences in this case in favor of the inverse mass spectrum and the Dirac nature of SM neutrino. Notwithstanding the Dirac properties of neutrino, an exchange of lepton numbers such as $e^{-}+\mu^{+}\rightarrow e^{+}+\mu^{-}$ is possible but with only leptons participating in the process.

Compact stars on the brane: What could they reveal about extra dimensions?

AbstractIn this work we investigate the properties of compact stars in the Randall‐Sundrum type II brane‐world model. We analyze the properties of quark and hadronic stars using the MIT bag model and a relativistic mean‐field model for the equation of state. We show that the causality limit in brane‐world models coincides with the Schwarzschild limit. Hence, there is a region in the mass‐radius plane that is forbidden in General Relativity for causal equations of state but can be accessed within brane‐world models. We also show that stable compact stars of arbitrarily large mass are possible because they can be supported against collapse by the non‐local bulk effects on the brane. This features provide a potential test for the existence of extra dimensions through the observation of neutron star's masses and radii. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Heavy quark scattering and quenching in a QCD medium at finite temperature and chemical potential

The heavy quark collisional scattering on partons of the quark gluon plasma (QGP) is studied in a quantum chromodynamics medium at finite temperature and chemical potential. We evaluate the effects of finite parton masses and widths, finite temperature $T$, and quark chemical potential ${\ensuremath{\mu}}_{q}$ on the different elastic cross sections for dynamical quasiparticles (on- and off-shell particles in the QGP medium as described by the dynamical quasiparticle model ``DQPM'') using the leading order Born diagrams. Our results show clearly the decrease of the $qQ$ and $gQ$ total elastic cross sections when the temperature and the quark chemical potential increase. These effects are amplified for finite ${\ensuremath{\mu}}_{q}$ at temperatures lower than the corresponding critical temperature ${T}_{c}({\ensuremath{\mu}}_{q})$. Using these cross sections we, furthermore, estimate the energy loss and longitudinal and transverse momentum transfers of a heavy quark propagating in a finite temperature and chemical potential medium. Accordingly, we have shown that the transport properties of heavy quarks are sensitive to the temperature and chemical potential variations. Our results provide some basic ingredients for the study of charm physics in heavy-ion collisions at Beam Energy Scan at RHIC and CBM experiment at FAIR.

Can spin-charge-family theory explain baryon number nonconservation?

The spin-charge-family theory, in which spinors carry besides the Dirac spin also the second kind of the Clifford object, no charges, is a kind of the Kaluza-Klein theories. The Dirac spinors of one Weyl representation in $d=(13+1)$ manifest in $d=(3+1)$ at low energies all the properties of quarks and leptons assumed by the standard model. The second kind of spins explains the origin of families. Spinors interact with the vielbeins and the two kinds of the spin connection fields, the gauge fields of the two kinds of the Clifford objects, which manifest in $d=(3+1)$ besides the gravity and the known gauge vector fields also several scalar gauge fields. Scalars with the space index $s\in (7,8)$ carry the weak charge and the hyper charge ($\mp \frac{1}{2}, \pm \frac{1}{2}$, respectively), explaining the origin of the Higgs and the Yukawa couplings. It is demonstrated in this paper that the scalar fields with the space index $t\in (9,10,\dots,14)$ carry the triplet colour charges, causing transitions of antileptons and antiquarks into quarks and back, enabling the appearance and the decay of baryons. These scalar fields are offering in the presence of the right handed neutrino condensate, which breaks the ${\cal C}{\cal P}$ symmetry, the answer to the question about the matter-antimatter asymmetry.

Detector development for Jefferson Lab’s 12 GeV Upgrade

Jefferson Lab will soon finish its highly anticipated 12GeV Upgrade. With doubled maximum energy, Jefferson Lab’s Continuous Electron Beam Accelerator Facility (CEBAF) will enable a new experimental program with substantial discovery potential, addressing important topics in nuclear, hadronic and electroweak physics. In order to take full advantage of the high energy, high luminosity beam, new detectors are being developed, designed and constructed to fit the needs of different physics topics. The paper will give an overview of various new detector technologies to be used for 12GeV experiments. It will then focus on the development of two solenoid-based spectrometers, the GlueX and SoLID spectrometers. The GlueX experiment in Hall D will study the complex properties of gluons through exotic hybrid meson spectroscopy. The GlueX spectrometer, a hermetic detector package designed for spectroscopy and the associated partial wave analysis, is currently in the final stage of construction. Hall A, on the other hand, is developing the SoLID spectrometer to capture the 3D image of the nucleon from semi-inclusive processes and to study the intrinsic properties of quarks through mirror symmetry breaking. Such a spectrometer will have the capability to handle very high event rates while still maintaining a large acceptance in the forward region.

Quark Properties from the Hadron Resonance Gas

We show how the quark free energy can be determined from a string and the Hadron Resonance Gas model with one heavy quark below the de-confinement phase transition. We discuss the interesting problem of identification of degrees of freedom at increasing temperatures, as well as the relevance of string breaking and avoided crossings.

Top Quark Mass from the Tevatron and LHC Colliders

The discovery of the top quark in 1995 has been one of the great successes of the CDF and D0 experiments at the Fermilab Tevatron collider. Since then, many measurements of the top quark properties have been performed in different channels and using many methods. The importance of measuring its mass lies in the possibility of verifying the predictions and the consistency of the Standard Model as well as in setting constraints on possible, still unobserved, physics. In 2010, the new CERN experiments, ATLAS and CMS, started to measure the top quark properties exploiting the large amount of data collected at the Large Hadron Collider. In March 2014, the very first combination of measurements from all the four experiments has been performed yielding Mtop = 173.34 ± 0.76 GeV, with a precision below 0.5%. In these proceedings a selected review of the most recent or relevant results obtained by the Tevatron and LHC Collaborations is presented.

Measurements of top quark properties with the ATLAS detector

Measurements of top quark properties with the ATLAS detector

Measurements of top quark properties with the ATLAS detector

A selection of recent top quark measurements performed with the ATLAS detector is presented here, with data taken in 2011 and 2012 at centre-of-mass energies of 7 TeV and 8 TeV, respectively. The analyses cover the determination of the top pole mass by a cross-section measurement, the top/anti-top quark mass difference, the spin correlation and the charge asymmetry. No hints for physics beyond the Standard Model have been found but a great reduction of uncertainties compared to former results was achieved.

Effective approach to top-quark decay and FCNC processes at NLO accuracy

The top quark is expected to be a probe to new physics beyond the standard model. Thanks to the large number of top quarks produced at the Tevatron and the LHC, various properties of the top quark can now be measured accurately. An effective field theory allows us to study the new physics effects in a model-independent way, and to this end accurate theoretical predictions are required. In this talk we will discuss some recent results on top-quark decay processes as well as flavor-changing processes, based on the effective field theory approach.