Probing vector-like top partner from same-sign dilepton events at the LHC

The standard model is a successful theory but is lacking a mechanism for neutrino mass generation as well as a solution to the naturalness problem. In the models that are proposed to simultaneously solve the two problems, heavy Majorana neutrinos and top partners are usually predicted to lead to a new decay mode of the top partner mediated by the heavy Majorana neutrinos: $Tto b,{W}^{+}to b,{{ell }}^{+}{{ell }}^{+}qbar{q^{prime} }$. In this paper, we will study the observability of such a new signature via the pair production process of a top partner ${pp}to Tbar{T}to 2b+{{ell }}^{pm }{{ell }}^{pm }+4j$ in a model independent way. By performing Monte Carlo simulations, we present the 2σ exclusion limits of the top partner mass and mixing parameters at the HL-LHC.     

On the possibility of the search for top-antitop resonances at LHC

The possibility of discovering top-antitop resonances with masses of 700, 1000, 1500, 2000, and 3000 GeV/c ² at proton-proton collision energy of 14 TeV in the center of a mass system is considered. The minimum cross sections for the resonance production necessary for their detection in a lepton + jets channel of $ t\mathop t\limits^ - The possibility of discovering top-antitop resonances with masses of 700, 1000, 1500, 2000, and 3000 GeV/c ² at proton-proton collision energy of 14 TeV in the center of a mass system is considered. The minimum cross sections for the resonance production necessary for their detection in a lepton + jets channel of pair decay with a 5σ confidence level is estimated. The analysis is performed both with and without taking into account the possibility of hadronic jets reconstruction from b quarks (b-tagging). Original Russian Text ? E.V. Khramov, A.N. Tonoyan, V.A. Bednyakov, N.A. Rusakovich, 2008, published in Pis’ma v Zhurnal Fizika Elementarnykh Chastits i Atomnogo Yadra, 2008, No. 6 (148), pp. 865–873.     

Majorana neutrinos and same-sign dilepton production at LHC and in rare meson decays

We discuss same-sign dilepton production mediated by Majorana neutrinos in high-energy proton–proton collisions for at the LHC energy TeV, and in the rare decays of the , and B mesons of the type . For the pp reaction, assuming one heavy Majorana neutrino of mass , we present discovery limits in the plane where are the mixing parameters. Taking into account the present limits from low-energy experiments, we show that at LHC one has sensitivity to heavy Majorana neutrinos up to a mass TeV in the dilepton channels , and , but the dilepton states will not be detectable due to the already existing constraints from neutrinoless double beta decay. We work out a large number of rare meson decays, both for the light and heavy Majorana neutrino scenarios, and argue that the present experimental bounds on the branching ratios are too weak to set reasonable limits on the effective Majorana masses. Received: 24 April 2001 / Published online: 29 June 2001     

Top-quark mass measurement from dilepton events at CDF II

We report a measurement of the top-quark mass using events collected by the CDF II detector from pp collisions at square root of s = 1.96 TeV at the Fermilab Tevatron. We calculate a likelihood function for the top-quark mass in events that are consistent with tt --> bl(-)nu(l)bl'+ nu'(l) decays. The likelihood is formed as the convolution of the leading-order matrix element and detector resolution functions. The joint likelihood is the product of likelihoods for each of 33 events collected in 340 pb(-1) of integrated luminosity, yielding a top-quark mass M(t) = 165.2 +/- 6.1(stat) +/- 3.4(syst) GeV/c2. This first application of a matrix-element technique to tt --> bl+ nu(l)bl'- nu(l') decays gives the most precise single measurement of M(t) in dilepton events. Combined with other CDF run II measurements using dilepton events, we measure M(t) = 167.9 +/- 5.2(stat) +/- 3.7(syst) GeV/c2.     

FCNC-induced heavy-quark events at the LHC from supersymmetry

We analyze the production and subsequent decay of the neutral Higgs bosons h≡h⁰,H⁰,A⁰$h\equiv h^0,H^0,A^0$ of the MSSM into electrically neutral quark pairs of different flavors (qq^′≡tc,bs$q{q}^{\prime }\equiv tc,bs$, depending on h  ) at the LHC, i.e. σ(pp→h→qq^′)$\sigma (pp\to h\to q{q}^{\prime })$, and compare with the direct FCNC production mechanisms σ(pp→qq^′)$\sigma (pp\to q{q}^{\prime })$. The cross-sections are computed in the unconstrained MSSM with minimal flavor-mixing sources and taking into account the stringent bounds from b→sγ$b\to s\gamma$. We extend the results previously found for these FCNC processes, which are singularly uncommon in the SM. Specifically, we report here on the SUSY-EW part of σ(pp→h→qq^′)$\sigma (pp\to h\to q{q}^{\prime })$ and the SUSY-QCD and SUSY-EW contributions to σ(pp→bs)$\sigma (pp\to bs)$. In this way, the complete map of MSSM predictions for the qq^′$q{q}^{\prime }$-pairs produced at the LHC becomes available. The upshot is that the most favorable channels are: (1) the Higgs boson FCNC decays into bs, and (2) the direct production of tc   pairs, both of them at the ∼1 pb$\sim 1\text{pb}$ level and mediated by SUSY-QCD effects. If, however, the SUSY-QCD part is suppressed, we find a small SUSY-EW yield for σmax_{(pp→h→tc)}∼¹⁰⁻⁴ pb$\sigma {(pp\to h\to tc)}_{\max }\sim {10}^{\mathrm{-4}}\text{pb}$ but, at the same time, σmax_{(pp→h→bs)}∼0.1–1 pb$\sigma {(pp\to h\to bs)}_{\max }\sim 0.1\text{–}1\text{pb}$, which implies a significant number (∼¹⁰⁴–¹⁰⁵$\sim {10}^4\text{–}{10}^5$) of bs   pairs per 100 fb⁻¹$100{\text{fb}}^{\mathrm{-1}}$ of integrated luminosity.     

Cross-section-constrained top-quark mass measurement from dilepton events at the Tevatron

We report the first top-quark mass measurement that uses a cross-section constraint to improve the mass determination. This measurement is made with a dilepton tt event candidate sample collected with the Collider Detector II at Fermilab. From a data sample corresponding to an integrated luminosity of 1.2 fb(-1), we measure a top-quark mass of 170.7(-3.9)(+4.2)(stat)+/-2.6(syst)+/-2.4(theory) GeV/c(2). The measurement without the cross-section constraint is 169.7(-4.9)(+5.2)(stat)+/-3.1(syst) GeV/c(2).     

Search for anomalous kinematics in tt dilepton events at CDF II

We report on a search for anomalous kinematics of tt dilepton events in pp collisions at square root of s=1.96 TeV using 193 pb(-1) of data collected with the CDF II detector. We developed a new a priori technique designed to isolate the subset in a data sample revealing the largest deviation from standard model (SM) expectations and to quantify the significance of this departure. In the four-variable space considered, no particular subset shows a significant discrepancy, and we find that the probability of obtaining a data sample less consistent with the SM than what is observed is 1.0%-4.5%.     

Inclusive search for new physics with like-sign dilepton events in pp collisions at square root of s =1.96 TeV

We describe a search for anomalous production of events with two leptons (e or mu) of the same electric charge in pp[over ] collisions at a center-of-mass energy of 1.96 TeV. Many extensions to the standard model predict the production of two leptons of the same electric charge. This search has a significant increase in sensitivity compared to earlier searches. Using a data sample corresponding to 1 fb(-1) of integrated luminosity recorded by the CDF II detector, we observe no significant excess in an inclusive selection (expect 33.2+/-4.7 events, observe 44) or in a supersymmetry-optimized selection (expect 7.8+/-1.1 events, observe 13.).     

On minimum-bias effects at the LHC

In general, minimum-bias triggers planned for experiments at the LHC miss a considerable fraction of the total number of events. We exemplify the rejection rate using the Durham model of soft high-energy interactions to obtain quantitative estimates of the signals arising from the ATLAS scintillation counters positioned in the rapidity intervals 2<|η|<4$2<|\eta |<4$, and also from TOTEM detectors covering the intervals 3.1<|η|<6.5$3.1<|\eta |<6.5$. Typically we find that the expected signal is about half of the total cross section, σtot${\sigma }_{\mathrm{tot}}$. We also calculate the cross section for the so-called zero-bias measurement planned by CMS in the extended rapidity interval −5<η<7$-5<\eta <7$. We emphasize that only models which give satisfactory predictions for the measured minimum-bias or zero-bias cross sections can be used to obtain the value of the total inelastic cross section.     

Fluctuations and asymmetric jet events in PbPb collisions at the LHC

Recent LHC results concerning full jet-quenching in Pb Pb collisions have been presented in terms of a jet asymmetry parameter, measuring the imbalance between the transverse momenta of leading and subleading jets. We examine the potential sensitivity of this distribution to fluctuations from the heavy-ion background. Our results suggest that a detailed estimate of the true fluctuations would be of benefit in extracting quantitative information about jet quenching. We also find that the apparent impact of fluctuations on the jet asymmetry distribution can depend significantly on the choice of low-p _t threshold used for the simulation of the hard pp events.     

Next-to-leading QCD effect on the quark compositeness search at the LHC

We present the exact next-to-leading order (NLO) QCD corrections to the dijet production induced by the quark contact interactions at the CERN Large Hadron Collider. We show that, as compared to the exact calculation, the scaled NLO QCD prediction adopted by the ATLAS Collaboration has overestimated the new physics effect on some direct observables by more than 30% and renders a higher limit on the quark compositeness scale. The destructive contribution from the exact NLO correction will also lower the compositeness scale limit set by the CMS Collaboration.     

T-odd top partner pair production in the dilepton final states at the LHC in the littlest Higgs Model with T-parity

In the littlest Higgs model with T-parity, we discuss the pair production of the T-odd top partner(T_) that decays almost 100% into the top quark and the lightest T-odd particle(A_H). Considering the current constraints,we investigate the observability of the T-odd top partner pair production through the process pp→T_T_→tt A_HA_H in the final states with two leptons at 14 TeV LHC. We analyze the signal significance and found that the lower limits on the T-odd top partner mass are approximately 1.2 TeV, 1.3 TeV, and 1.4 TeV at the 2σ confidence level at 14 TeV LHC with an integrated luminosities of 30 fb~(-1), 100 fb~(-1), and 300 fb~(-1), respectively. This lower limit can be increased to approximately 1.5(1.6) TeV if we used 1000(3000) fb~(-1) of the integrated luminosity.