The top quark, others new phenomena observed at the CDF in {ie359-1} collisions at √s = 1.96 TeV (Review)

Upgraded Tevatron luminosity in Run-II (started 2001) has opened a new level of modern heavy-quark studies compared to that one of Run-I. Now top event samples contain hundreds of event statistics for investigation. This review mainly covers the mass measurements of the top quark produced at √s = 1.96 TeV in {ie359-2} collisions at the Collider Detector of Fermilab with the integrated luminosity samples up to 1 fb^?1. As an example of the top quark mass measurements we consider so-called Matrix Element method in “lepton + jets” and “dilepton” channels of the top-quark decay. The CDF top quark mass obtained in lepton +jets mode is the world most precise single measurement of this important physics parameter. The review summarizes the essential results of the CDF top-quark mass measurement achieved and published for the recent 2003–2008 period. We consider also b-quark baryon discoveries like Σ _b , Σ _b ^* , Θ _b as well as first observation of {ie359-3} oscillations. Let us mention here that a CDF/JINR-group created significant contribution to the new CDF complex and physics investigation.     

Review of the top quark mass measurement at the CDF in p \bar p collisions at \sqrt s = 1.96 TeV

Upgraded Tevatron RUN-II luminosity has opened a new chapter in a modern heavy quark studies—a top physics: a dozen level of a Tevatron Run-I top events statistic now is being replaced by hundreds events. This work reviews a mass measurement of the top quark produced at $\sqrt s $ = 1.96 TeV in p $\bar p$ collisions at the Collider Detector at Fermilab (CDF) with the integrated luminosity samples up to 359 pb^?1. The review covers the M(top) measurement using mainly the so-called template methods both in the “lepton + jets” and “dilepton” channels of top quark decay. The CDF top quark mass obtained in the lepton + jets mode is currently the world’s most precise single measurement of this important physical parameter. This work summarizes the essential results of the CDF top quark mass measurement achieved and published for the recent 2003–2005 period.     

Measurement of the top quark mass in dileptonic top quark pair decays with √s = 7 TeV ATLAS data

The top quark mass in dileptonic top quark pair decays was measured using 4.7 fb^–1 of √s = 7 TeV proton-proton (pp) collision data recorded by the ATLAS experiment at the LHC in 2011. The event topology is characterized by the presence of two charged leptons, at least two neutrinos and several jets, two of which originate from bottom quarks. Using the template method and the m _?b observable, defined as the average invariant mass of the two charged lepton plus b-jet pairs in each event, the top quark mass is measured to be 173.09 ± 0.64(stat) ± 1.50(syst) GeV. This proceeding is based on a preliminary result, which has been superseded meanwhile.     

Observability of light charged Higgs decay to muon in top quark pair events at LHC

In this paper the charged Higgs signal through the decay to a pair of muon and neutrino (H ^±→μν) is analyzed. The analysis attempts to estimate the amount of muonic signal of the charged Higgs at LHC at a center of mass energy of 14 TeV. The signal process is the top quark pair production with one of the top quarks decaying to a charged Higgs (non SM anomalous top decay) and the other decaying to a W boson which is assumed to decay hadronically to two light jets. Due to the small branching ratio of charged Higgs decay to muon, results are quoted for data corresponding to an integrated luminosity of 300 fb⁻¹ which is expected to be collected at the LHC high luminosity regime. It is shown that a signal significance close to 5σ down to below 1σ is achievable for a charged Higgs mass in the range 80 GeV<m(H ^±)<150 GeV taking the top quark pair production with both top quarks decaying to W bosons as the main irreducible background.     

Heavy-flavor tagging and the supersymmetry reach of the CERN Large Hadron Collider

The branching fraction for the decays of gluinos to third generation quarks is expected to be enhanced in classes of supersymmetric models where either third generation squarks are lighter than other squarks, or in mixed-higgsino dark matter models constructed so as to be in concordance with the measured density of cold dark matter. In such scenarios, gluino production events at the CERN Large Hadron Collider should be rich in top and bottom quark jets. Requiring b jets in addition to E _T ^miss should, therefore, enhance the supersymmetry signal relative to Standard Model backgrounds from V + jet, VV and QCD backgrounds (V=W,Z). We quantify the increase in the supersymmetry reach of the LHC from b-tagging in a variety of well-motivated models of supersymmetry. We also explore “top tagging” at the LHC. We find that while the efficiency for this turns out to be too low to give an increase in reach beyond that obtained via b-tagging, top tagging can indeed provide a confirmatory signal if gluinos are not too heavy. We also examine c jet tagging but find that it is not useful at the LHC. Finally, we explore the prospects for detecting the direct production of third generation squarks in models with an inverted squark mass hierarchy. This is signaled by b jets + E _T ^miss events being harder than in the Standard Model, but softer than those from the production of gluinos and heavier squarks. We find that while these events can be readily separated from the SM background (for third generation squark masses ∼300–500 GeV), the contamination from the much heavier gluinos and squarks remains formidable if these are also accessible.     

Measurement of the top quark charge at the atlas detector

We have investigated the possibility of reconstructing the top quark charge through measurement of its decay product charges at the ATLAS detector. Verification of the hypothesis about alternative interpretation of top-quark experimental data has been considered as well. The method of “semileptonic B-meson decay” was applied for reconstructing the b-jet charge. A statistical significance of more than 5σ can be achieved using this method after analyzing 1 fb⁻¹ of the t̄t-pairs data. The analysis was carried out with HERWIG and PYTHIA generators and using the GEANT4 detector simulation software package. The text was submitted by the authors in English.     

Identified charged particles in quark and gluon jets

A sample of 2.2 million hadronic Z decays, selected from the data recorded by the Delphi detector at Lep during 1994–1995 was used for an improved measurement of inclusive distributions of and p and their antiparticles in gluon and quark jets. The production spectra of the individual identified particles were found to be softer in gluon jets compared to quark jets, with a higher multiplicity in gluon jets as observed for inclusive charged particles. A significant proton enhancement in gluon jets is observed indicating that baryon production proceeds directly from colour objects. The maxima, , of the -distributions for kaons in gluon and quark jets are observed to be different. Received: 24 January 2000 / Revised version: 15 May 2000 / Published online: 8 September 2000     

Use of \Lambda_b polarimetry in top quark spin-correlation functions

Due to the absence of hadronization effects and the large mass, top quark decay will be uniquely sensitive to fundamental electroweak physics at the Tevatron, at the LHC, and at a future linear collider. A “complete measurement” of the four helicity amplitudes in decay is possible by the combined use of andW polarimetry in stage-two spin-correlation functions (S2SC). In this paper, the most general Lorentz-invariant decay density matrix is obtained for the decay sequence where and [or ], and likewise for . These density matrices are expressed in terms of b-polarimetry helicity parameters which enable a unique determination of the relative phases among the amplitudes. Thereby, S2SC distributions and single-sided b-W-interference distributions are expressed in terms of these parameters. The four b-polarimetry helicity parameters involving the amplitude are considered in detail. polarimetry signatures will not be suppressed in top quark analyses when final angles-and-energy variables are used for . Received: 26 January 2001 / Published online: 15 March 2001     

Precise measurement of the top-quark mass in the lepton+jets topology at CDF II

We present a measurement of the mass of the top quark from proton-antiproton collisions recorded at the CDF experiment in Run II of the Fermilab Tevatron. We analyze events from the single lepton plus jets final state (tt-->W(+)bW(-)b-->lnubqq'b). The top-quark mass is extracted using a direct calculation of the probability density that each event corresponds to the tt final state. The probability is a function of both the mass of the top quark and the energy scale of the calorimeter jets, which is constrained in situ by the hadronic W boson mass. Using 167 events observed in 955 pb(-1) of integrated luminosity, we achieve the single most precise measurement of the top-quark mass, 170.8+/-2.2(stat.)+/-1.4(syst.) GeV/c(2).     

Screening of heavy quark free energies at finite temperatureand non-zero baryon chemical potential

We analyze the dependence of heavy quark free energies on the baryon chemical potential μ_b in 2-flavor QCD by performing a 6th order Taylor expansion in the chemical potential which circumvents the sign problem. The bare quark mass at corresponds to a pion mass of about 770 MeV and is thus not in the range of physical quark masses but the quark mass dependence is known to be small above T_c. At N_τ = 4 the lattices are coarse, however, we are using improved (p4 staggered) fermions. The Taylor expansion coefficients of color singlet and color averaged free energies are calculated and from this the expansion coefficients for the corresponding screening masses are determined. We find that for small μ_b the free energies of a static quark-antiquark pair decrease in a medium with a net excess of quarks and that screening is well described by a screening mass which increases with increasing μ_b. The μ_b-dependent corrections to the screening masses are well described by perturbation theory for T ≳ 2T_c. In particular, we find for all temperatures above T_c that the expansion coefficients for singlet and color averaged screening masses differ by a factor 2. PACS. 11.15.Ha, 11.10.Wx, 12.38Gc, 12.38.Mh     

Independent measurement of the top quark mass and the light- and bottom-jet energy scales at hadron colliders

A method for the simultaneous determination of the energy scales for bottom-quark jets and light jets, the jet energy resolution, and the top quark mass at hadron colliders is presented. The method exploits the unique kinematics of events with top-antitop pair production, where one of the top quarks involves a leptonic and one a hadronic boson decay. The paper shows a feasibility study of how this simultaneous measurement can be performed at the upcoming LHC experiments ATLAS and CMS.     

Bottomonium spectrum with coupled-channel effects

We study the bottomonium spectrum in the nonrelativistic quark model with coupled-channel effects. The mass shifts and valence b[`(b)]b\bar{b} component are evaluated to be rather large. We find that the hadronic loop effects can be partially absorbed into a reselection of the model parameters. No bottomonium state except ϒ(5 ¹ S ₀) and ϒ(5 ³ S ₁) with mass around 10890 MeV is found in the quark models both with and without coupled-channel effects, so we suggest that Y _b (10890) is an exotic state beyond the quark model, if it is confirmed to be a new resonance. The predictions for the χ _b (3P) masses are consistent with the ATLAS measurements. If some new bottomonium-like states are observed at LHCb or SuperB in the future, we can determine whether they are conventional bottomonium or exotic states by comparing their masses with the mass spectrum predicted in our work.     

Properties of the hadronic jets in π− + p and π− + C interactions at 40 GeV/c

The charged π-meson and proton mixed hadronic jets in the fragmentation regions of π⁻ + p and π⁻ + C interactions at 40 GeV/c are extracted and their properties are investigated. It has been shown that the characteristics of the jets in the π⁻-meson fragmentation region are universal in jet events, do not depend on the type of the target and are defined only from the quark contents of the π⁻ meson. The jet characteristics in the proton fragmentation region are different and depend upon the quark contents of the proton. Mixed hadronic jets with defined values of the electric charge were studied. The resonance structures related to the Δ isobars are discovered in the effective mass distributions of the jets. It has been shown that the small azimuthal angle high-p _T correlations are observed and strong back-to-back correlations exist. The text was submitted by the authors in English.     

A path integral formula for quark condensate states in?a?modified?PQCD

A modified version of PQCD considered in previous works is investigated here in the case of retaining only the quark condensate. The Green function generating functional is expressed in a form in which Dirac’s delta functions are now absent from the free propagators. The new expansion implements the dimensional transmutation effect through a single interaction vertex in addition to the standard ones in massless QCD. The new vertex suggest a way for constructing an alternative to the SM, in which the mass and CKM matrices could be generated by the instability of massless QCD under the production of the top quark and other fermions condensates, in a kind of generalized Nambu–Jona-Lasinio mechanism. The results of a two loop evaluation of the vacuum energy indicate that the quark condensate is dynamically generated. However, the energy as a function of the condensate parameter is again unbounded from below in this approximation. Assuming the existence of a minimum of the vacuum energy at the experimental value of the top quark mass m _q =173 GeV, we evaluate the two particle propagator in the quark–anti-quark channel in zero order in the coupling and a ladder approximation in the condensate vertex. Adopting the notion from the former top quark models in which the Higgs field corresponds to the quark condensate, the results suggest that the Higgs particle could be represented by a meson which might appear at energies around twice the top quark mass.     

<Emphasis Type="Italic">Z</Emphasis><Superscript>0</Superscript>-tagged quark jets at the large hadron collider

The Large Hadron Collider will allow studies of hard probes in nucleus-nucleus collisions which were not accessible at the Relativistic Heavy Ion Collider—even the study of small cross-section Z ⁰-tagged jets becomes possible. Going beyond the measurement of back-to-back correlations of two strongly interacting particles to measure plasma properties, we replace one side by an electromagnetic probe which propagates through the plasma undisturbed and therefore provides a measurement of the energy of the initial hard scattering. We show that at sufficiently high transverse momentum the Z ⁰-tagged jets originate predominately from the fragmentation of quarks and anti-quarks while gluon jets are suppressed. We propose to use lepton-pair tagged jets to study medium-induced partonic energy loss and to measure in-medium parton fragmentation functions to determine the opacity of the quark gluon plasma.     

Measurement of the tt production cross section in pp collisions at square root s = 1.96 TeV using missing E(T) + jets events with secondary vertex b tagging

We present a measurement of the tt production cross section in pp collisions at square root s = 1.96 TeV which uses events with an inclusive signature of significant missing transverse energy and jets. This is the first measurement which makes no explicit lepton identification requirements, so that sensitivity to W --> tau nu decays is maintained. Heavy flavor jets from top quark decay are identified with a secondary vertex tagging algorithm. From 311 pb(-1) of data collected by the Collider Detector at Fermilab, we measure a production cross section of 5.8 +/- 1.2(stat)(-0.7)(+0.9)(syst) pb for a top quark mass of 178 GeV/c2, in agreement with previous determinations and standard model predictions.     

On pair production of scalar top quarks in <Emphasis Type="Italic">e</Emphasis><Superscript>+</Superscript><Emphasis Type="Italic">e</Emphasis><Superscript>−</Superscript> collisions at ILC and a possibility of their mass reconstruction

We study pair production of scalar top quarks (stop, ) in e ⁺ e ⁻ collisions with the subsequent decay of the top squarks into b quarks and charginos . We simulate this process by using PYTHIA6.4 for the beam energy 2E _b = = 350, 400, 500, 800, 1000 GeV. A set of criteria for physical variables is proposed, which provides good separation of stop signal events from top quark pair production being the main background. These criteria allow us to reconstruct the mass of the top squark with an integrated luminosity of 1000 fb⁻¹ provided that the neutralino mass is known. The article is published in the original.     

Determination of the b quark mass at the Z mass scale

In hadronic decays of Z bosons recorded with the OPAL detector at LEP, events containing b quarks were selected using the long lifetime of b flavoured hadrons. Comparing the 3-jet rate in b events with that in d,u,s and c quark events, a significant difference was observed. Using calculations for massive quarks, this difference was used to determine the b quark mass in the renormalisation scheme at the scale of the Z boson mass. By combining the results from seven different jet finders the running b quark mass was determined to be . Evolving this value to the b quark mass scale itself yields , consistent with results obtained at the b quark production threshold. This determination confirms the QCD expectation of a scale dependent quark mass. A constant mass is ruled out by 3.9 standard deviations. Received: 14 May 2001 / Published online: 17 August 2001     

Measurement of the tt production cross section in pp collisions at square root of s = 1.96 TeV

We present a measurement of the top quark pair production cross section in pp collisions at square root of s = 1.96 TeV using 318 pb(-1) of data collected with the Collider Detector at Fermilab. We select tt[over ] decays into the final states enu+jets and mu nu+ jets, in which at least one b quark from the t-quark decays is identified using a secondary vertex-finding algorithm. Assuming a top quark mass of 178 GeV/c2, we measure a cross section of 8.7 +/- 0.9(stat)(-0.9)+1.1(syst) pb. We also report the first observation of tt[over ] with significance greater than 5sigma in the subsample in which both b quarks are identified, corresponding to a cross section of 10.1(-1.4)+1.6(stat)(-1.3)+2.0(syst) pb.     

Precise measurement of the top-quark mass from lepton + jets events

We measure the mass of the top quark using top-quark pair candidate events in the lepton+jets channel from data corresponding to 1 fb;{-1} of integrated luminosity collected by the D0 experiment at the Fermilab Tevatron collider. We use a likelihood technique that reduces the jet energy scale uncertainty by combining an in situ jet energy calibration with the independent constraint on the jet energy scale (JES) from the calibration derived using photon+jets and dijet samples. We find the mass of the top quark to be 171.5+/-1.8(stat.+JES)+/-1.1(syst.) GeV.