One of the key tasks of any particle collider is measurement. In practice, this is often done by fitting data to a simulation, which depends on many parameters. Sometimes, when the effects of varying different parameters are highly correlated, a large ensemble of data may be needed to resolve parameter-space degeneracies. An important example is measuring the top-quark mass, where other physical and unphysical parameters in the simulation must be marginalized over when fitting the top-quark mass parameter. We compare three different methodologies for top-quark mass measurement: a classical histogram fitting procedure, similar to one commonly used in experiment optionally augmented with soft-drop jet grooming; a machine-learning method called DCTR; and a linear regression approach, either using a least-squares fit or with a dense linearly-activated neural network. Despite the fact that individual events are totally uncorrelated, we find that the linear regression methods work most effectively when we input an ensemble of events sorted by mass, rather than training them on individual events. Although all methods provide robust extraction of the top-quark mass parameter, the linear network does marginally best and is remarkably simple. For the top study, we conclude that the Monte-Carlo-based uncertainty on current extractions of the top-quark mass from LHC data can be reduced significantly (by perhaps a factor of 2) using networks trained on sorted event ensembles. More generally, machine learning from ensembles for parameter estimation has broad potential for collider physics measurements.
By studying quark ensembles with infinite correlation length we formulate the quantum field theory model that, as we show, is exactly integrable and develops an instability of its standard vacuum ensemble (the Dirac sea). We argue such an instability is rooted in high ground state degeneracy (for 'realistic' space-time dimensions) featuring a fairly specific form of energy distribution, and with the cutoff parameter going to infinity this inherent energy distribution becomes infinitely narrow and leads to large (unlimited) fluctuations. Analysing some possible vacuum ensembles such as the Dira…
The free energy of a static quark-antiquark pair is obtained in an interacting dyon ensemble near the deconfinement temperature. Comparing the results with the noninteracting case, we observe that the string tension between the quark-antiquark pair increases for the interacting ensemble. As a result, the confinement temperature decreases.
"Exotic quarks" in the title refers to a modification of quark periodicity condition on the thermal circle by introduction of some phases -- known also as "flavor holonomies" -- different quark flavors. These phases provide a valuable tool, to be used for better understanding of deconfinement and chiral restoration phase transitions: by changing them one can dramatically modify both phase transitions. In the language of instanton constituents -- instanton-dyons or monopoles -- it has a very direct explanation: the interplay of flavor and color holonomies can switch topological zero modes betwe…
This is the second paper of the series aimed at understanding the ensemble of instanton-dyons, now with two flavors of light dynamical quarks. The partition function is appended by the fermionic factor, $(det T)^{N_f}$ and Dirac eigenvalue spectra at small values are derived from the numerical simulation of 64 and 128 dyons. Those spectra show clear chiral symmetry breaking pattern at high dyon density.
The features of hot and dense gas of quarks which are considered as the quasi-particles of the model Hamiltonian with four-fermion interaction are studied. Being adapted to the Nambu-Jona-Lasinio model this approach allows us to accommodate a phase transition similar to the nuclear liquid-gas one at the proper scale and to argue an existence of the mixed (inhomogeneous) phase of vacuum and normal baryonic matter as a plausible scenario of chiral symmetry (partial) restoration. Analyzing the transition layer between two phases we estimate the surface tension coefficient and speculate on the pos…
We study the charged Higgs production at LHC via its associated production with top quark. The kinematic cuts are optimized to suppress the background processes so that the reconstruction of the charged Higgs and top quark is possible. The angular distributions with respect to top quark spin are explored to study the $Htb$ interaction at LHC.
We analyze, within several parameter scenarios of type-II two-Higgs doublet extensions of the standard model, the impact of heavy neutral Higgs-boson resonances on top-quark pair production and their subsequent decay to dileptonic final states at the LHC (13 TeV). In particular, we investigate the effects of heavy Higgs bosons on top-spin observables, that is, the longitudinal top-quark polarization and top-quark spin correlations. We take into account NLO QCD as well as weak interaction corrections and show that top-spin observables, if evaluated in judiciously chosen top-quark pair invariant…
We present a calculation of the up, down, strange and charm quark masses performed within the lattice QCD framework. We use the twisted mass fermion action and carry out simulations that include in the sea two light mass-degenerate quarks, as well as the strange and charm quarks. In the analysis we use gauge ensembles simulated at three values of the lattice spacing and with light quarks that correspond to pion masses in the range from 350 MeV to the physical value, while the strange and charm quark masses are tuned approximately to their physical values. We use several quantities to set the s…
The presence of the anomalous top quark flavour changing neutral current (FCNC) interactions leads to the production of same-sign top quarks in proton-proton collisions. The results of a search for events with same-sign dileptons and b-jets conducted by CMS collaboration with 10.5 $fb^{-1}$ of data collected in pp colisions at $\sqrt{s}=8$ TeV are used to obtain the constraints on the strength of top quark FCNC interactions. The 95$%$ confidence level upper limits on the branching ratios of top quark decays to a light quark $q=u,c$ and a gauge or a Higgs boson are set to be ${\cal B}R(t\righta…
The production of a single top quark in association with an isolated photon probes the electromagnetic coupling structure of the top quark. We investigate the sensitivity of kinematical distributions at the LHC in single-top-plus-photon production in view of a detection of anomalous electric and magnetic dipole moments of the top quark.
Vector-like quarks (VLQs) are attractive extensions to the Standard Model. They mix with the SM quarks and can lead to rich phenomenology. Determination of VLQ's interaction structure with the SM is then an important issue, which can be inferred from the decay products of VLQs, such as top quark. We calculate the spin-analyzing powers for charged leptons from top quark in VLQ decay for various VLQ scenarios. We find that the top polarization effect will be helpful to distinguish different natures of the VLQ couplings with the SM.
At the LHC, top quark pairs are dominantly produced from gluons, making it difficult to measure the top quark forward-backward asymmetry. To improve the asymmetry measurement, we study variables that can distinguish between top quarks produced from quarks and those from gluons: the invariant mass of the top pair, the rapidity of the top-antitop system in the lab frame, the rapidity of the top quark in the top-antitop rest frame, the top quark polarization and the top-antitop spin correlation. We combine all the variables in a likelihood discriminant method to separate quark-initiated events fr…
The single top quark production has an electroweak nature and provides an additional to the top pair production source of the top quarks. The processes involving single top have unique properties, they are very interesting from both theoretical and experimental view points. Short review of the single top quark production processes is given in the paper.
We present the first chiral-continuum extrapolated up, down and strange quark spin contribution to the proton spin using lattice QCD. For the connected contributions, we use eleven ensembles of 2+1+1-flavor of Highly Improved Staggered Quarks (HISQ) generated by the MILC Collaboration. They cover four lattice spacings $a \approx \{0.15,0.12,0.09,0.06\}$ fm and three pion masses, $M_π\approx \{315,220,135\}$ MeV, of which two are at the physical pion mass. The disconnected strange calculations are done on seven of these ensembles, covering the four lattice spacings but only one with the physica…
We present results for the kaon mixing parameter $B_K$ obtained using improved staggered fermions on a much enlarged set of MILC asqtad lattices. Compared to our previous publication, which was based largely on a single ensemble at each of the three lattice spacings $a\approx 0.09\;$fm, $0.06\;$fm and $0.045\;$fm, we have added seven new fine and four new superfine ensembles, with a range of values of the light and strange sea-quark masses. We have also increased the number of measurements on one of the original ensembles. This allows us to do controlled extrapolations in the light and strange…
In this work we present the results from numerical simulations of an interacting ensemble of instanton-dyons in the $SU(3)$ gauge group with $N_f=2$ flavors of massless quarks. Dynamical quarks are included via the effective interactions induced by the fermionic determinant evaluated in the subspace of topological zero modes. The eigenvalue spectrum of the Dirac operator is studied at different volumes to extract the chiral condensate and eigenvalue gap, with both observables providing consistent values of the chiral transition temperature $T_c$. We find that a sufficient density of dyons is r…
We update our results for $B_K$ obtained using HYP-smeared staggered valence quarks on the MILC asqtad lattices. In the last year, we have added 5 new measurments on the fine ($a\approx 0.09\;$fm) ensembles, and 2 new measurements on the superfine ($a\approx 0.06\;$fm) ensembles. These allow a simultaneous extrapolation in $a^2$ and sea quark masses, reducing the corresponding systematic error significantly. Our updated result is $\hat{B}_K = 0.738 \pm 0.005 (\text{stat}) \pm 0.034 (\text{sys})$.
A chiral extrapolation of the light vector meson masses in the up, down and strange quark masses of QCD is presented. We apply an effective chiral Lagrangian based on the hadrogenesis conjecture to QCD lattice ensembles of PACS-CS, QCDSF-UKQCD and HSC in the strict isospin limit. The leading orders low-energy constants are determined upon a global fit to the lattice data set. We use the pion and kaon masses as well as the size of the finite volume as lattice ensemble parameters only. The quark mass ratio on the various ensembles are then predicted in terms of our set of low-energy constants. A…
Using 10-16 units of Nvidia DGX-1, we have generated the first gauge ensemble for $N_f=2+1+1$ lattice QCD with physical $(u/d, s, c)$ domain-wall quarks, on the $ 64^4 $ lattice with lattice spacing $a \sim 0.064 $ fm ($ L > 4 $ fm, and $ M_πL > 3 $). The salient feature of this gauge ensemble is that the chiral symmetry is preserved to a high precision and all topological sectors are sampled ergodically. In this paper, we present the first results of the topological susceptibility and the ground-state hadron mass spectra.
The amount of data collected by the CMS experiment at the CERN LHC in the second data taking period provides the possibility to study absolute and differential cross sections of top quark interaction processes with high precision. Utilizing these results, fundamental standard model parameters such as the top quark mass are extracted. Moreover, given standard model measurements are interpreted in the context of Beyond the standard model theories. Recent results on top quark properties and rare or anomalous top quark interactions by CMS are presented in these proceedings. Furthermore, the implic…
We present a calculation of the Mellin moments of the nucleon transverse quark spin densities extracted from the unpolarized and transversity generalized form factors. We use three $N_F=2+1+1$ ensembles of twisted mass fermions with quark masses tuned to their physical values and lattice spacings $a\sim 0.08$~fm, $a\sim 0.07$~fm and $a\sim 0.06$~fm and extrapolate the form factors to the continuum limit. Besides isovector densities we also include results for the tensor charge for each quark flavor using the ensemble with $a\sim 0.08$~fm for which we include the disconnected contributions.
The dependence of the top-quark mass measurement in top-quark production on the parton distribution functions is explored through the differential distribution of the invariant mass of the top-antitop system in top-quark pair production at hadron colliders. Several different proton-proton collider options are considered: 8 TeV, 13 TeV, 13.6 TeV, 14 TeV, and 100 TeV. The top-quark mass is obtained from a chi-squared fit to the invariant mass of the top-antitop-quark pair. The parton distribution function uncertainties are used in the chi-square evaluation. The uncertainties are reduced through …
The polarization of a top quark can be sensitive to new physics beyond the standard model. Since the charged lepton from top quark decay is maximally correlated with the top quark spin, it is common to measure the polarization from the distribution in the angle between the charged lepton and the top quark directions. We propose a novel method based on the charged lepton energy fraction and illustrate the method with a detailed simulation of top quark pairs produced in supersymmetric top squark pair production. We show that the lepton energy ratio distribution that we define is very sensitive t…
Different from other multiple top-quark productions, triple top-quark production requires the presence of both flavor violating neutral interaction and flavor conserving neutral interaction. We describe the interaction of triple top-quarks and up-quark in terms of two dimension-6 operators; one can be induced by a new heavy vector resonance, the other by a scalar resonance. Combining same-sign top-quark pair production and four top-quark production, we explore the potential of the 13 TeV LHC on searching for the triple top-quark production.
The polarization of a top-quark can be sensitive to new physics beyond the standard model. We propose a novel method to measure top-quark polarization, based on the charged lepton energy fraction in top-quark decay, and illustrate the method with a detailed simulation of top-quark pairs produced in supersymmetric top squark pair production. We show that the lepton energy ratio distribution that we define is very sensitive to the top-quark polarization but insensitive to the precise measurement of the top-quark energy.
Using the method of QCD Sum Rules we estimate the energy of lowest energy top quark meson state with a hybrid admixture. This new estimate uses the previous estimates of hybrid charmonium and upsilon states, but with the mass of the top quark mass being much greater than the charm and bottom quark masses. We discuss production of mixed hybrid top quark mesons and possible detection of the creation of the Quark-Gluon Plasma via Relativistic Heavy Ion Collisions.
Kinematic reconstruction of top quarks allows to define a set of kinematic observables relevant to various physics processes that involve top quarks and provides an additional handle for the suppression of background events. Radiation of photons in association with the top quarks alters the kinematics and the topology of the event, leading to visible systematic effects in measurable observables. The present study introduces an improved reconstruction of the top quark kinematics in the presence of photon radiation. The results are presented for processes with top quark pair production, as well …
In this paper, we examine the sensitivity of the three-top quark production at the LHC to the top quark flavor-changing neutral currents (FCNC) as well as the sensitivity of the four-top production to the strong and weak dipole moments of the top quark. Upper limits at $95\%$ CL on the branching fractions of $\mathcal{B}(t\rightarrow qX)$, where $X = g, Z, γ, H$ and $q=u,c$, are set by performing an analysis on three-top events in the same-sign dilepton channel. We consider the main sources of the background processes and a realistic detector simulation is performed at the center-of-mass energ…
We consider top quark pair production in association with a hard jet through next-to-leading order in perturbative QCD. Top quark decays are treated in the narrow width approximation and spin correlations are retained throughout the computation. We include hard jet radiation by top quark decay products and explore their importance for basic kinematic distributions at the Tevatron and the LHC. Our results suggest that QCD corrections and jet radiation in decays can lead to significant changes in shapes of basic distributions and, therefore, need to be included for the description of ttbar+jet p…
We update our determinations of $f_{D^+}$, $f_{D_s}$, $f_K$, and quark mass ratios from simulations with four flavors of HISQ dynamical quarks. The availability of ensembles with light quarks near their physical mass means that we can extract physical results with only small corrections for valence- and sea-quark mass mistunings instead of a chiral extrapolation. The adjusted valence-quark masses and lattice spacings may be determined from an ensemble-by-ensemble analysis, and the results for the quark mass ratios then extrapolated to the continuum limit. Our central values of the charmed meso…
The High-Luminosity LHC is expected to provide $3 ab^{-1}$ of integrated luminosity. As a result billions of events containing top quarks will be detected at the CMS and ATLAS experiments, allowing for precise measurements of the top quark properties. The experimental challenges that will be faced in a high luminosity environment, with a special focus on top quark related observables are examined. We discuss prospects for measuring top quark anomalous couplings at the HL-LHC. Projections for detecting flavor changing neutral currents involving top quarks are also reviewed.
The discovery of the top quark in 1995 opened a whole new sector of investigation of the Standard Model; today top quark physics remains a key priority of the Tevatron program. Some of the measurements of top quark properties, for example its mass, will be a long-standing legacy. The recent evidence of an anomalously large charge asymmetry in top quark events suggests that new physics could couple preferably with top quarks. I will summarize this long chapter of particle physics history and discuss the road the top quark is highlighting for the LHC program.
