Dilepton Decay Channel Research Articles

Complete NLO corrections to top-quark pair production with isolated photons

We compute for the first time the so-called complete NLO corrections to top-quark pair production with one and two isolated photons in the di-lepton top-quark decay channel. The Narrow Width Approximation is used for the modeling of unstable top quarks and W bosons. Higher-order QCD and EW effects as well as photon bremsstrahlung are consistently included at all stages: in production and top-quark decays. We present results at the integrated and differential fiducial cross-section level for both processes for the LHC Run II center-of-mass energy of s\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\sqrt{s} $$\\end{document} = 13 TeV. In addition, we investigate the scale choice in photonic observables. Finally, the individual size of each subleading contribution is discussed in detail and the origin of the main subleading corrections is scrutinised. For the latter case, alternative calculations are performed in which the subleading NLO corrections are included only in the production of tt¯γ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ t\\overline{t}\\gamma $$\\end{document} and tt¯γγ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ t\\overline{t}\\gamma \\gamma $$\\end{document}.

Tt¯H Interactions and T-odd Correlations at Hadron Colliders

Abstract We explore C P -violation effects of Higgs-top interactions in the associated production of a Higgs boson with top-pair in the dileptonic decay modes of top-quark originating from proton proton collisions pp → t t ¯ H → ( l + ν l b ) ( l − ν l ¯ b ¯ ) H at NLO in QCD matched to parton shower via T-odd observables using momenta of various particles involved in the process. In particular, we predict the constraints on the C P -violating t t ¯ H coupling obtained through the production asymmetries associated with the T-odd observables in the dileptonic decay channel of the t t ¯ pair for the Large hadron collider (LHC) with centre-of-mass energy of 13 TeV and an integrated luminosity of 139 fb−1. We also present the corresponding limits for future hadron colliders, namely the High Luminosity LHC (HL-LHC) and the Future Circular Collider (FCC-hh). Our estimates of the t t ¯ H interaction strength reveal that the upper bound on pseudoscalar coupling c p corresponding to the largest asymmetry would be of about 1.57 × 10−2 at 2σ C.L. for c s = 1 at the LHC with S =13 TeV for an integrated luminosity of 139 fb−1. The respective limits for HL-LHC and FCC-hh with the projected Luminosities of 3 ab−1 and 30 ab−1 are found to be to 2.7 × 10−3 and 1.3 × 10−4 respectively at 2σ C.L.

Energy dependence of coherent photonuclear production of J/ψ mesons in ultra-peripheral Pb-Pb collisions at sqrt{{textrm{s}}_{textrm{NN}}} = 5.02 TeV

The cross section for coherent photonuclear production of J/ψ is presented as a function of the electromagnetic dissociation (EMD) of Pb. The measurement is performed with the ALICE detector in ultra-peripheral Pb-Pb collisions at a centre-of-mass energy per nucleon pair of sqrt{{textrm{s}}_{textrm{NN}}} = 5.02 TeV. Cross sections are presented in five different J/ψ rapidity ranges within |y| < 4, with the J/ψ reconstructed via its dilepton decay channels. In some events the J/ψ is not accompanied by EMD, while other events do produce neutrons from EMD at beam rapidities either in one or the other beam direction, or in both. The cross sections in a given rapidity range and for different configurations of neutrons from EMD allow for the extraction of the energy dependence of this process in the range 17 < Wγ Pb,n < 920 GeV, where Wγ Pb,n is the centre-of-mass energy per nucleon of the γPb system. This range corresponds to a Bjorken-x interval spanning about three orders of magnitude: 1.1 × 10−5 < x < 3.3 × 10−2. In addition to the ultra-peripheral and photonuclear cross sections, the nuclear suppression factor is obtained. These measurements point to a strong depletion of the gluon distribution in Pb nuclei over a broad, previously unexplored, energy range. These results, together with previous ALICE measurements, provide unprecedented information to probe quantum chromodynamics at high energies.

CP-violation, asymmetries and interferences in toverline{t}phi

In this paper, we use the associated production of top-quark pairs ( toverline{t} ) with a generic scalar boson (ϕ) at the LHC (pp → toverline{t}phi ) to explore the sensitivity of a large set of observables to the sign of the CP mixing angle (α), present in the coupling between the scalar boson and the top quarks. The mass of the scalar boson is set to mϕ = 125 GeV (the Standard Model Higgs boson mass) and its coupling to top-quarks is varied such that α = 0°, 22.5°, 45.0°, 67.5°, 90.0°, 135.0° and 180.0°. Dileptonic final states of the toverline{t}phi system are used (pp → bℓ+νℓ overline{b} ℓ− overline{nu} ℓb overline{b} ), where the scalar boson is expected to decay according to ϕ → boverline{b} . A new method to reconstruct the scalar mass, originally designed for the low mass regime is used, improving the resolution of the Higgs mass by roughly a factor of two. A full phenomenological analysis is performed using Standard Model (SM) background and signal events generated with MadGraph5_aMC@NLO, in turn reconstructed using a kinematical fit. The most sensitive CP-observables are selected to compute Confidence Level (CL) limits as a function of the sign of the top quark Yukawa couplings to the ϕ boson. We also explore the sensitivity to interference terms using differential distributions and angular asymmetries. Given the significant difference between the pure scalar (σ0+) and pure pseudo-scalar (σ0−) production cross section values, it is unlikely the toverline{t}phi channel alone will be sensitive to the sign of the CP-mixing angle or interference terms, even at the end of the LHC. Using the {b}_2^{toverline{t}phi } and {b}_4^{toverline{t}phi } variables, exclusion limits at 95% CL for the CP-even and CP-odd components of the top quark Yukawa couplings are expected to be set to overset{sim }{kappa } ∈ [-0.698,+0.698] and |κ| ∈ [0.878,1.04], respectively, at the end of the High Luminosity phase of the LHC (HL-LHC) by using the dileptonic decay channel alone.

Pinning down the gauge boson couplings in WW\u03b3 production using forward proton tagging

In this paper, we explore the potential of the LHC to measure the rate of pp → p WWγ p process, also to probe the new effective couplings contributing to the WWγ and WWγγ vertices. The analysis is performed at the sqrt{s} = 13 TeV, in the dileptonic decay channel, and assuming 300 fb−1 integrated luminosity (IL). In addition to the presence of two opposite sign leptons, a photon, and missing energy, the distinctive signature of this process is the presence of two intact protons flying few millimeters from the initial beam direction in both sides of interaction points which suppress the background process effectively. To exploit this feature of signal we benefit from forward detectors (FDs) placed about 200 meters from the interaction point to register the kinematics of tagged protons. In order to overcome the major sources of backgrounds, we introduced three categories of selection cuts dealing with objects that strike the central detector, protons hitting the FDs, and correlations of central objects and protons, respectively. We also evaluate the probability of pile-up protons to be tagged in the FDs as a function of the mean number of pile-up. Then the sensitivity of the LHC to observe this process and constraints on multi-boson effective couplings are extracted. The obtained expected limits show very good improvements for dimension-8 quartic couplings and competitive bounds on dimension-6 anomalous triple couplings w.r.t. the current experimental limits. Therefore, we propose this process to the LHC experiments as a sensitive and complementary channel to study the multi-gauge boson couplings.

Charged Higgs bosons in naturally aligned two-Higgs-doublet models at the LHC

Measurements of a Higgs boson at the Large Hadron Collider (LHC) have become increasingly consistent with the predictions of the Standard Model (SM). This fact puts severe constraints on many potential low-energy extensions of the Higgs sector of the SM. In the well-known two-Higgs-doublet model (2HDM), an ``alignment limit'' of parameters readily furnishes one SM-like scalar and can be achieved naturally through an underlying symmetry. Among the other physical states of the 2HDM, a charged scalar ${H}^{\ifmmode\pm\else\textpm\fi{}}$ would provide striking evidence of new physics if observed. We propose a novel technique for the observation of the process $pp\ensuremath{\rightarrow}tb{H}^{\ifmmode\pm\else\textpm\fi{}}\ensuremath{\rightarrow}t\overline{t}b\overline{b}$ in the dileptonic decay channel at the LHC. The reconstruction of events in this channel is complicated by multiple $b$ jets and unobserved neutrinos in the final state. To determine the neutrino momenta, we implement a neutrino weighting procedure to study, for the first time, the $t\overline{t}b\overline{b}$ signature. We further train a pair of boosted decision trees to reconstruct and classify signal events. We determine the resulting reach within the context of naturally aligned 2HDMs, such as the maximally symmetric two-Higgs-doublet model (MS2HDM). By testing at the integrated luminosity of $150\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ achieved in run 2 of the LHC, we find that this channel may restrict the parameter space of a type-II MS2HDM with charged Higgs masses as high as 680 GeV.

Precise predictions for toverline{t}gamma /toverline{t} cross section ratios at the LHC

With the goal of increasing the precision of NLO QCD predictions for the ppto toverline{t}gamma process in the di-lepton top quark decay channel we present theoretical predictions for the mathrm{mathcal{R}}={sigma}_{toverline{t}gamma }/{sigma}_{toverline{t}} cross section ratio. Results for the latter together with various differential cross section ratios are given for the LHC with the Run II energy of sqrt{s}=13 TeV. Fully realistic NLO computations for toverline{t} and toverline{t}gamma production are employed. They are based on matrix elements for {e}^{+}{nu}_e{mu}^{-}{overline{nu}}_{mu }boverline{b} and {e}^{+}{nu}_e{mu}^{-}{overline{nu}}_{mu }boverline{b}gamma processes and include all resonant and non-resonant diagrams, interferences, and off-shell effects of the top quarks and the W gauge bosons. Various renormalisation and factorisation scale choices and parton density functions are examined to assess their impact on the cross section ratio. Depending on the transverse momentum cut on the hard photon a judicious choice of a dynamical scale allows us to obtain 1%–3% percent precision on mathrm{mathcal{R}} . Moreover, for differential cross section ratios theoretical uncertainties in the range of 1%-6% have been estimated. Until now such high precision predictions have only been reserved for the top quark pair production at NNLO QCD. Thus, mathrm{mathcal{R}} at NLO in QCD represents a very precise observable to be measured at the LHC for example to study the top quark charge asymmetry or to probe the strength and the structure of the t- overline{t} -γ vertex. The latter can shed some light on possible new physics that can reveal itself only once sufficiently precise theoretical predictions are available.

Proton PDFs constraints from measurements using the ATLAS experiment

High-precision measurements of Standard Model (SM) processes provide information on different aspects contributing to the process, such as parton distribution functions (PDFs), and comparisons with the current precision reached theoretically on the calculations of the cross sections of such processes. This document describes ATLAS measurements, performed at different centre-of-mass energies, of vector boson (W and Z) cross sections and cross-section ratios. It also discusses measurements of ratios of Z-boson and top-quark pair production cross sections, which provide important information on the proton PDFs. Finally, a measurement involving the di-lepton decay channel of top-quark pairs at 8 TeV is presented, because of its direct sensitivity to the gluon PDF.

Measurement of normalized differential tt¯ cross sections in the dilepton channel from pp collisions at s√=13 TeV

Normalized differential cross sections for top quark pair production are measured in the dilepton (e^+e^−, μ^+μ^−, and μ^∓e^±) decay channels in proton-proton collisions at a center-of-mass energy of 13 TeV. The measurements are performed with data corresponding to an integrated luminosity of 2.1 fb^(−1) using the CMS detector at the LHC. The cross sections are measured differentially as a function of the kinematic properties of the leptons, jets from bottom quark hadronization, top quarks, and top quark pairs at the particle and parton levels. The results are compared to several Monte Carlo generators that implement calculations up to next-to-leading order in perturbative quantum chromodynamics interfaced with parton showering, and also to fixed-order theoretical calculations of top quark pair production up to next-to-next-to-leading order.

Measurement of normalized differential mathrm{t}overline{mathrm{t}} cross sections in the dilepton channel from pp collisions at sqrt{s}=13 TeV

Normalized differential cross sections for top quark pair production are measured in the dilepton (e+e−, μ+μ−, and μ∓e±) decay channels in proton-proton collisions at a center-of-mass energy of 13 TeV. The measurements are performed with data corresponding to an integrated luminosity of 2.1 fb−1 using the CMS detector at the LHC. The cross sections are measured differentially as a function of the kinematic properties of the leptons, jets from bottom quark hadronization, top quarks, and top quark pairs at the particle and parton levels. The results are compared to several Monte Carlo generators that implement calculations up to next-to-leading order in perturbative quantum chromodynamics interfaced with parton showering, and also to fixed-order theoretical calculations of top quark pair production up to next-to-next-to-leading order.

Top quark mass studies with toverline{t}j at the LHC

A precise measurement of the top quark mass, a fundamental parameter of the Standard Model, is among the most important goals of top quark studies at the Large Hadron Collider. Apart from the standard methods, numerous new observables and reconstruction techniques are employed to improve the overall precision and to provide different sensitivities to various systematic uncertainties. Recently, the normalised inverse invariant mass distribution of the toverline{t} system and the leading extra jet not coming from the top quark decays has been proposed for the ppto toverline{t}j production process, denoted as ℛ(mtpole, ρs). In this paper, a thorough study of different theoretical predictions for this observable, however, with top quark decays included, is carried out. We focus on fixed order NLO QCD calculations for the di-lepton top quark decay channel at the LHC with sqrt{s}=13 TeV. First, the impact on the extraction of mt is investigated and afterwards the associated uncertainties are quantified. In one approach we include all interferences, off-shell effects and non-resonant backgrounds. This is contrasted with a different approach with top quark decays in the narrow width approximation. In the latter case, two cases are employed: NLO QCD corrections to the ppto toverline{t}j production process with leading order decays and the more sophisticated case with QCD corrections and jet radiation present also in top quark decays. The top quark mass sensitivity of ℛ(mtpole, ρs) is investigated and compared to other observables: the invariant mass of the top anti-top pair, the minimal invariant mass of the b-jet and a charged lepton as well as the total transverse momentum of the toverline{t}j system. Once top quark decays are included the invariant mass of the toverline{t} system shows better sensitivity to the top quark mass extraction and smaller dependence on the off-shell effects and non-resonant contributions of the top quark and the W gauge boson than the ℛ(mtpole, ρs) observable.

Mass shift of charmonium states in [formula omitted] collision

The masses of the low lying charmonium states, namely, the J/Ψ, Ψ(3686), and Ψ(3770) are shifted downwards due to the second order Stark effect. In p¯+Au collisions at 6–10 GeV we study their in-medium propagation. The time evolution of the spectral functions of these charmonium states is studied with a Boltzmann–Uehling–Uhlenbeck (BUU) type transport model. We show that their in-medium mass shift can be observed in the dilepton spectrum. Therefore, by observing the dileptonic decay channel of these low lying charmonium states, especially for Ψ(3686), we can gain information about the magnitude of the gluon condensate in nuclear matter. This measurement could be performed at the upcoming PANDA experiment at FAIR.

Demystifying the compressed top squark region with kinematic variables

The ongoing perplexing scenario with no hints of new physics at the Large Hadron Collider can be elucidated amicably if the exotic particle spectrum in many of the well-motivated theoretical models possesses degenerate mass. We investigate the usefulness of different kinematic variables sensitive to the compressed mass region, and propose a search strategy considering a phenomenological supersymmetric scenario where the top squark undergoes a four-body decay due to its extremely narrow mass difference with the lightest supersymmetric particle. Considering a challenging but relatively clean dileptonic decay channel, we demonstrate that one can effectively restrain the significant background from the top quark, which provides a complementary approach to the present CMS analysis. With the new strategic approach the current limit can be extended to a phase-space region that was not explored before.

Z Boson Production in 5.02 TeV pp, p + Pb and Pb + Pb Collisions with ATLAS

Z bosons are short lived and do not participate in the strong interaction. Thus their production yields, observed via dilepton decay channels in proton-lead and lead-lead collisions, provide direct tests of both binary collision scaling and the nuclear modification of parton distribution functions (nPDF). Proton-lead collisions further provide a relatively clean environment for benchmarking nuclear parton distribution functions (PDF). The ATLAS detector has a broad acceptance in the muon and electron channels, with excellent performance even in the high occupancy environment of central heavy-ion collisions. In this talk, Z yields, are presented differentially in rapidity and as a function of centrality in lead-lead and proton-lead collisions.

Δ(1232) Dalitz decay in proton-proton collisions at T=1.25 GeV measured with HADES at GSI

We report on the investigation of $\Delta$(1232) production and decay in proton-proton collisions at a kinetic energy of 1.25 GeV measured with HADES. Exclusive dilepton decay channels $ppe^{+}e^{-}$ and $ppe^{+}e^{-}\gamma$ have been studied and compared with the partial wave analysis of the hadronic $pp\pi^{0}$ channel. They allow to access both $\Delta^+ \to p\pi^0(e^+e^-\gamma)$ and $\Delta^+ \to pe^+e^-$ Dalitz decay channels. The perfect reconstruction of the well known $\pi^0$ Dalitz decay serves as a proof of the consistency of the analysis. The $\Delta$ Dalitz decay is identified for the first time and the sensitivity to N-$\Delta$ transition form factors is tested. The $\Delta$(1232) Dalitz decay branching ratio is also determined for the first time; our result is (4.19 $\pm$ 0.62 syst. $\pm$ 0.34 stat.) $\times $ 10$^{-5}$, albeit with some model dependence.

Measurement of Normalized Differential Cross Section for thett̄Production in the Dilepton Channel in pp Collisions at a center-of-mass energy of 13 TeV

Differential cross sections of top-quark pair production as function of the kinematic variables of leptons, b-jets and top-quarks at particle level are measured in the dilepton decay channel with proton-proton collisions at a center-of-mass energy of 13 TeV. The measurements are performed with Run II data using the CMS detector at the Large Hadron Collider.

Measurement of the inclusivett¯production cross section inpp¯collisions ats=1.96 TeVand determination of the top quark pole mass

The inclusive cross section of top quark-antiquark pairs produced in $p\bar{p}$ collisions at $\sqrt{s}=1.96$ TeV is measured in the lepton$+$jets and dilepton decay channels. The data sample corresponds to 9.7 fb${}^{-1}$ of integrated luminosity recorded with the D0 detector during Run II of the Fermilab Tevatron Collider. Employing multivariate analysis techniques we measure the cross section in the two decay channels and we perform a combined cross section measurement. For a top quark mass of 172.5 GeV, we measure a combined inclusive top quark-antiquark pair production cross section of $\sigma_{t\bar{t}} = 7.26 \pm 0.13\,(\mathrm{stat.})\,^{+0.57}_{-0.50}\,(\mathrm{syst.})$ pb which is consistent with standard model predictions. We also perform a likelihood fit to the measured and predicted top quark mass dependence of the inclusive cross section, which yields a measurement of the pole mass of the top quark. The extracted value is $m_t = 172.8 \pm 1.1\,(\mathrm{theo.})\,^{+3.3}_{-3.1}\,(\mathrm{exp.})$ GeV.

Off-shell top quarks with one jet at the LHC: a comprehensive analysis at NLO QCD

We present a comprehensive study of the production of top quark pairs in association with one hard jet in the di-lepton decay channel at the LHC. Our predictions, accurate at NLO in QCD, focus on the LHC Run II with a center-of-mass energy of 13 TeV. All resonant and non-resonant contributions at the perturbative order ${\cal O}(\alpha_s^4 \alpha^4)$ are taken into account, including irreducible backgrounds to $t\bar{t}j$ production, interferences and off-shell effects of the top quark and the $W$ gauge boson. We extensively investigate the dependence of our results upon variation of renormalisation and factorisation scales and parton distribution functions in the quest for an accurate estimate of the theoretical uncertainties. Additionally, we explore a few possibilities for a dynamical scale choice with the goal of stabilizing the perturbative convergence of the differential cross sections far away from the $t\bar{t}$ threshold. Results presented here are particularly relevant for searches of new physics as well as for precise measurements of the top-quark fiducial cross sections and top-quark properties at the LHC.

Semihadronic and hadronic decays ofρ0andωmesons coherently photoproduced in isoscalar nuclei

The interference of the $\rho^0$ and $\omega$ mesons has been studied in the hadronic and leptonic decay channels, i.e., dipion and dilepton decay channels respectively, but this interference is not studied yet in the semihadronic or semileptonic decay channel, e.g., $V \to \pi^0\gamma$. $V$ denotes either $\rho^0$ or $\omega$ meson. To look for the quoted interference in the $\pi^0\gamma$ decay channel as well as the contribution of $\rho$ meson to the cross section, the correlated $\pi^0\gamma$ invariant mass distribution spectra are calculated in the photonuclear reaction in the multi-GeV region. It is assumed that these bosons arise in the final state due to the decay of $\rho^0$ and $\omega$ mesons which are photoproduced coherently in the isoscalar nucleus. The elementary reaction in the nucleus is considered to proceed as $\gamma N \to VN$; $V \to \pi^0\gamma$. The forward propagation of the $\rho$ and $\omega$ mesons and the near forward emission of pion are considered, so that they can be described by the eikonal form. The meson nucleus interactions are evaluated using $t\varrho$ approximation. Replacing the decay vertex $V \to \pi^0\gamma$ by $V \to \pi^+\pi^-$ in the above formalism, it is also used to study the $\rho-\omega$ interference in the $\pi^+\pi^-$ decay channel.

Lightest Kaluza–Klein graviton mode in a back-reacted Randall–Sundrum scenario

In search of extra dimensions in the ongoing LHC experiments, signatures of Randall-Sundrum (RS) lightest KK graviton have been one of the main focus in recent years. The recent data from the dilepton decay channel at the LHC has determined the experimental lower bound on the mass of the RS lightest Kaluza-Klein (KK) graviton for different choices of underlying parameters of the theory. In this work we explore the effects of the backreaction of the bulk scalar field, which is employed to stabilise the RS model, in modifying the couplings of the lightest KK graviton with the standard model (SM) matter fields located on the visible brane. In such a modified background geometry we show that the coupling of the lightest KK graviton with the SM matter fields gets a significant suppression due to the inclusion of the backreaction of the bulk stabilising scalar field. This implies that the backreaction parameter weakens the signals from RS scenario in collider experiments which in turn explains the non-visibility of KK graviton in colliders. Thus we show that the modulus stabilisation plays a crucial role in the search of warped extra dimensions in collider experiments.