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arXiv2012

A search for flavour changing neutral currents in top-quark decays in pp collision data collected with the ATLAS detector at sqrt(s) = 7 TeV

A search for flavour changing neutral current (FCNC) processes in top-quark decays by the ATLAS Collaboration is presented. Data collected from pp collisions at the LHC at a centre-of-mass energy of sqrt(s) = 7 TeV during 2011, corresponding to an integrated luminosity of 2.1 fb^-1, were used. A search was performed for top-quark pair-production events, with one top quark decaying through the t->Zq FCNC (q=u,c) channel, and the other through the Standard Model dominant mode t->Wb. Only the decays of the Z boson to charged leptons and leptonic W-boson decays were considered as signal. Consequently, the final-state topology is characterised by the presence of three isolated charged leptons, at least two jets and missing transverse momentum from the undetected neutrino. No evidence for an FCNC signal was found. An upper limit on the t->Zq branching ratio of BR(t->Zq)<0.73% is set at the 95% confidence level.

hep-ex↗
arXiv2013

Neutralino dark matter confronted by the LHC constraints on Electroweak SUSY signals

The supersymmetric particles (sparticles) belonging exclusively to the electroweak sector of the minimal supersymmetric standard model (MSSM) may hold the key to the observed dark matter relic density in the universe even if all strongly interacting sparticles are very heavy. The importance of the light EW sparticles in DM physics and in producing spectacular collider signals is emphasized. It is shown that even the preliminary data on the direct searches of these sparticles at the LHC, significantly constrain the parameter space of the MSSM compatible with the observed relic density and provide useful hints about the future search prospects. If in addition to the electroweak sparticles the gluinos are also within the reach of the LHC experiments, then the gluino mass limits in the light slepton scenario obtained via the canonical jets + $\met$ channel may be relaxed by as much as 25 $%$ compared to the existing limits. But the corresponding same sign dilepton (SSD) + jets + $\met$ signal will yield enhanced limits competitive with the strongest limits currently available. This is illustrated with the help of benchmark scenarios at the generator level using PYTHIA. If the gluinos are just beyond the current reach of the LHC, then the generic n-lepton + m-jets + missing energy signal may discriminate between different DM producing mechanisms by comparing the signals corresponding to different values of n. This is illustrated by simulating the signals for n = 0 and n = 2 (the SSD signal).

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arXiv2013

Combining Higher-Order Resummation with Multiple NLO Calculations and Parton Showers in GENEVA

We extend the lowest-order matching of tree-level matrix elements with parton showers to give a complete description at the next higher perturbative accuracy in alpha_s at both small and large jet resolutions, which has not been achieved so far. This requires the combination of the higher-order resummation of large Sudakov logarithms at small values of the jet resolution variable with the full next-to-leading order (NLO) matrix-element corrections at large values. As a by-product this combination naturally leads to a smooth connection of the NLO calculations for different jet multiplicities. In this paper, we focus on the general construction of our method and discuss its application to e+e and pp collisions. We present first results of the implementation in the GENEVA Monte Carlo framework, where we employ N-jettiness as the jet resolution variable, combining its next-to-next-to-leading logarithmic resummation with fully exclusive NLO matrix elements, and PYTHIA8 as the backend for further parton showering and hadronization. For hadronic collisions, we take Drell-Yan production as an example to apply our construction. For e+e- -> jets, taking alpha_s(mZ) = 0.1135 from fits to LEP thrust data, together with the PYTHIA8 hadronization model, we obtain good agreement with LEP data for a variety of 2-jet observables.

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arXiv2013

Measurements of branching fractions of leptonic and hadronic Ds+ meson decays and extraction of the Ds+ meson decay constant

We present measurements of absolute branching fractions of hadronic and leptonic Ds+ decays to K- K+ pi+, anti-K0 K+, eta pi+, mu+ nu_mu and tau+ nu_tau and report a search for the leptonic Ds+ to e+ nu_e decays. The results are obtained from a data sample of 913 fb^-1 collected at or near the Y(4S) and Y(5S) resonances with the Belle detector at the KEKB asymmetric-energy e+e- collider. The branching fractions of hadronic decays are measured to be Br(Ds+ -> K- K+ pi+) = (5.06 +- 0.15 +- 0.21)%, Br(Ds+ -> anti-K0 K+) = (2.95 +- 0.11 +- 0.09)%, and Br(Ds+ -> eta pi+) = (1.82 +- 0.14 +- 0.07)%, where the first and second uncertainties are statistical and systematic, respectively. The branching fractions of leptonic decays are measured to be Br(Ds+ -> mu+ nu_mu) = (0.531 +- 0.028 +- 0.020)%, and Br(Ds+ -> tau+ nu_tau) = (5.70 +- 0.21 +0.31-0.30)%, which are combined to determine the Ds+ meson decay constant f_Ds = (255.5 +- 4.2 +- 5.1) MeV. We find no significant signal for Ds+ -> e+ nu_e decays and set an upper limit of Br(Ds+ -> e+ nu_e) < 1.0 (0.83) x 10^-4 at 95% (90%) confidence level.

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arXiv2012

Gaugino production in proton-proton collisions at a center-of-mass energy of 8 TeV

Motivated by hints for a light Standard Model-like Higgs boson and a shift in experimental attention towards electroweak supersymmetry particle production at the CERN LHC, we update in this paper our precision predictions at next-to-leading order of perturbative QCD matched to resummation at the next-to-leading logarithmic accuracy for direct gaugino pair production in proton-proton collisions with a center-of-mass energy of 8 TeV. Tables of total cross sections are presented together with the corresponding scale and parton density uncertainties for benchmark points adopted recently by the experimental collaborations, and figures are presented for up-to-date model lines attached to them. Since the experimental analyses are currently obtained with parton showers matched to multi-parton matrix elements, we also analyze the precision of this procedure by comparing invariant-mass and transverse-momentum distributions obtained in this way to those obtained with threshold and transverse-momentum resummation.

hep-ph↗
arXiv2013

$τ^-\to K^-η^{(\prime)}ν_τ$ decays in Chiral Perturbation Theory with Resonances

We have studied the $τ^-\to K^-η^{(\prime)}ν_τ$ decays within Chiral Perturbation Theory including resonances as explicit degrees of freedom. We have considered three different form factors according to treatment of final-state interactions. In increasing degree of soundness: Breit-Wigner, exponential resummation and dispersive representation. We find that although the first one fails in accounting for the data on the $Kη$ mode, the other two approaches provide good fits to them which are sensitive to the $K^\star(1410)$ pole parameters, that are determined to be $M_{K^{\star\prime}}\,=\,(1330^{+27}_{-41})$ MeV and $Γ_{K^{\star\prime}}\,=\,(217^{+68}_{-122})$ MeV. These values are competitive with the standard determination from $τ^-\to(Kπ)^-ν_τ$ decays. The corresponding predictions for the $τ^-\to K^-η^{\prime}ν_τ$ channel respect the current upper bound and hint to the discovery of this decay mode in the near future.

hep-ph↗
arXiv2013

Measurement of the differential cross-section of B+ meson production in pp collisions at sqrt(s) = 7 TeV at ATLAS

The production cross-section of B+ mesons is measured as a function of transverse momentum pT and rapidity y in proton--proton collisions at center-of-mass energy sqrt(s) = 7 TeV, using 2.4 fb-1 of data recorded with the ATLAS detector at the Large Hadron Collider. The differential production cross-sections, determined in the range 9<pT<120 GeV and y<2.25, are compared to next-to-leading-order theoretical predictions.

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arXiv2013

Cornering light Neutralino Dark Matter at the LHC

We investigate the current status of the light neutralino dark matter scenario within the minimal supersymmetric standard model (MSSM) taking into account latest results from the LHC. A discussion of the relevant constraints, in particular from the dark matter relic abundance, leads us to a manageable simplified model defined by a subset of MSSM parameters. Within this simplified model we reinterpret a recent search for electroweak supersymmetric particle production based on a signature including multi-taus plus missing transverse momentum performed by the ATLAS collaboration. In this way we derive stringent constraints on the light neutralino parameter space. In combination with further experimental information from the LHC, such as dark matter searches in the monojet channel and constraints on invisible Higgs decays, we obtain a lower bound on the lightest neutralino mass of about 24 GeV. This limit is stronger than any current limit set by underground direct dark matter searches or indirect detection experiments. With a mild improvement of the sensitivity of the multi-tau search, light neutralino dark matter can be fully tested up to about 30 GeV.

hep-ph↗
arXiv2013

Study of $B_{(s)}^0 \to K_{\rm S}^0 h^{+} h^{\prime -}$ decays with first observation of $B_{s}^0 \to K_{\rm S}^0 K^{\pm} π^{\mp}$ and $B_{s}^0 \to K_{\rm S}^0 π^{+} π^{-}$

A search for charmless three-body decays of $B^0$ and $B_{s}^0$ mesons with a $K_{\rm S}^0$ meson in the final state is performed using the $pp$ collision data, corresponding to an integrated luminosity of $1.0\,\mbox{fb}^{-1}$, collected at a centre-of-mass energy of $7\mathrm{\,Te\kern -0.1em V}$ recorded by the LHCb experiment. Branching fractions of the $B_{(s)}^0 \to K_{\rm S}^0 h^{+} h^{\prime -}$ decay modes ($h^{(\prime)} = π, K$), relative to the well measured $B^0 \to K_{\rm S}^0 π^{+} π^{-}$ decay, are obtained. First observation of the decay modes $B_s^0 \to K_{\rm S}^0 K^{\pm} π^{\mp}$ and $B_s^0 \to K_{\rm S}^0 π^{+} π^{-}$ and confirmation of the decay $B^0 \to K_{\rm S}^0 K^{\pm} π^{\mp}$ are reported. The following relative branching fraction measurements or limits are obtained \begin{eqnarray*} \nonumber \frac{{\cal B}(B^0 \to K_{\rm S}^0 K^{\pm} π^{\mp})}{{\cal B}(B^0 \to K_{\rm S}^0 π^{+} π^{-})} &=& 0.128 \pm 0.017 \, ({\rm stat.}) \pm 0.009 \, ({\rm syst.}) \,, \\ \nonumber \frac{{\cal B}(B^0 \to K_{\rm S}^0 K^{+} K^{-} )}{{\cal B}(B^0 \to K_{\rm S}^0 π^{+} π^{-})} &=& 0.385 \pm 0.031 \, ({\rm stat.}) \pm 0.023 \, ({\rm syst.}) \,, \\ \nonumber \frac{{\cal B}(B_s^0 \to K_{\rm S}^0 π^{+} π^{-} )}{{\cal B}(B^0 \to K_{\rm S}^0 π^{+} π^{-})} &=& 0.29\phantom{0} \pm 0.06\phantom{0} \, ({\rm stat.}) \pm 0.03\phantom{0} \, ({\rm syst.}) \pm 0.02 \, (f_s/f_d) \,, \\ \nonumber \frac{{\cal B}(B_s^0 \to K_{\rm S}^0 K^{\pm} π^{\mp})}{{\cal B}(B^0 \to K_{\rm S}^0 π^{+} π^{-})} &=& 1.48\phantom{0} \pm 0.12\phantom{0} \, ({\rm stat.}) \pm 0.08\phantom{0} \, ({\rm syst.}) \pm 0.12 \, (f_s/f_d) \,, \\ \nonumber \frac{{\cal B}(B_s^0 \to K_{\rm S}^0 K^{+} K^{-} )}{{\cal B}(B^0 \to K_{\rm S}^0 π^{+} π^{-})} &\in& [0.004;0.068] \; {\rm at \;\; 90\% \; CL} \,. \end{eqnarray*}

hep-ex↗
arXiv2013

Z' at the LHC: Interference and Finite Width Effects in Drell-Yan

The interference effects between an extra neutral spin-1 Z'-boson and the Standard Model background in the Drell-Yan channel at the LHC are studied in detail. The final state with two oppositely charged leptons is considered. The interference contribution to the new physics signal, currently not fully taken into account by experimental collaborations in Z'-searches and in the interpretation of the results, can be substantial. It may affect limits or discovery prospects of Z' at the LHC. As the Z'-boson interference is model-dependent, a proper treatment would a priori require a dedicated experimental analysis for each particular model. Doing so could potentially improve the sensitivity to new physics, but would require significantly more experimental effort. At the same time, it is shown that one can define an invariant mass window, valid for a wide range of models, for which the contribution of the model-dependent interference to the Beyond the Standard Model signal is reduced to O(10%), comparable to the level of the combined uncertainty from parton densities and higher order corrections. This quasi-model-independent "magic cut" does not scale with the mass of the Z'-boson and is approximately constant over a large range of masses. Such control of the interference effects relies on not-too-small branching ratios of Z' to leptons (typically of at least a few percent) which can be suppressed, however, by additional new decay channels of the Z'; a small width-to-mass ratio alone does not guarantee the interference to be small over an arbitrary kinematic range. Under the general assumption that these new decay channels of Z' are not dominant, one can perform quasi-model-independent analyses, preserving the current scheme used by the experimental collaborations for the Z'-boson search using the suggested invariant mass window cut.

hep-ph↗
arXiv2013

First measurement of time-dependent CP violation in Bs -> K+K- decays

Direct and mixing-induced CP-violating asymmetries in Bs -> K+K- decays are measured for the first time using a data sample of pp collisions, corresponding to an integrated luminosity of 1.0 fb^{-1}, collected with the LHCb detector at a centre-of-mass energy of 7 TeV. The results are C_{KK} = 0.14 \pm 0.11 \pm 0.03 and S_{KK} = 0.30 \pm 0.12 \pm 0.04, where the first uncertainties are statistical and the second systematic. The corresponding quantities are also determined for B0 -> pi+pi- decays to be C_{pipi} = -0.38 \pm 0.15 \pm 0.02 and S_{pipi} = -0.71 \pm 0.13 \pm 0.02, in good agreement with existing measurements.

hep-ex↗
arXiv2013

Analysis of the decay constants of the heavy pseudoscalar mesons with QCD sum rules

In this article, we recalculate the contributions of all vacuum condensates up to dimension-6, in particular the one-loop corrections to the quark condensates $α_s<\bar{q}q>$ and partial one-loop corrections to the four-quark condensates $α_s^2<\bar{q}q>^2$, in the operator product expansion. Then we study the masses and decay constants of the heavy pseudoscalar mesons $D$, $D_s$, $B$ and $B_s$ using the QCD sum rules with two choices: {\bf I} we choose the $\bar{MS}$ masses by setting $m=m(μ)$ and take perturbative corrections up to the order $\mathcal{O}(α_s)$; {\bf II} we choose the pole masses $m$, take perturbative corrections up to the order $\mathcal{O}(α_s^2)$ and set the energy-scale to be the heavy quark pole mass $μ=m_Q$. In the case of {\bf I}, the predictions $f_D=(208\pm11)\,\rm{MeV}$ and $f_B=(189\pm15)\,\rm{MeV}$ are consistent with the experimental data within uncertainties, while the prediction $f_{D_s}=(241\pm12)\,\rm{MeV}$ is below the lower bound of the experimental data $f_{D_s}=(260.0\pm5.4)\,\rm{MeV}$. In the case of {\bf II}, the predictions $f_D=(211\pm14)\,\rm{MeV}$, $f_B=(190\pm17)\,\rm{MeV}$, $f_{D_s}=(258\pm13)\,\rm{MeV}$ and $f_{D_s}/f_D=1.22\pm0.08$ are all in excellent agreements with the experimental data within uncertainties.

hep-ph↗
arXiv2013

Probing Leptonic Interactions of a Family-Nonuniversal Z' Boson

We explore a Z' boson with family-nonuniversal couplings to charged leptons. The general effect of Z-Z' mixing, of both kinetic and mass types, is included in the analysis. Adopting a model-independent approach, we perform a comprehensive study of constraints on the leptonic Z' couplings from currently available experimental data on a number of flavor-conserving and flavor-changing transitions. Detailed comparisons are made to extract the most stringent bounds on the leptonic couplings. Such information is fed into predictions of various processes that may be experimentally probed in the near future.

hep-ph↗
arXiv2013

Measurement of the top quark charge in pp collisions at sqrt(s) = 7 TeV with the ATLAS detector

A measurement of the top quark electric charge is carried out in the ATLAS experiment at the Large Hadron Collider using 2.05 fb-1 of data at a centre-of-mass energy of 7 TeV. In units of the elementary electric charge, the top quark charge is determined to be 0.64 +- 0.02 (stat.) +- 0.08 (syst.) from the charges of the top quark decay products in single lepton ttbar candidate events. This excludes models that propose a heavy quark of electric charge --4/3, instead of the Standard Model top quark, with a significance of more than 8 sigma.

hep-ex↗
arXiv2013

Eviction of a 125 GeV "heavy"-Higgs from the MSSM

We prove that the present experimental constraints are already enough to rule out the possibility of the ~125 GeV Higgs found at LHC being the second lightest Higgs in a general MSSM context, even with explicit CP violation in the Higgs potential. Contrary to previous studies, we are able to eliminate this possibility analytically, using simple expressions for a relatively small number of observables. We show that the present LHC constraints on the diphoton signal strength, tau-tau production through Higgs and BR(B -> X_s gamma) are enough to preclude the possibility of H_2 being the observed Higgs with m_H~125 GeV within an MSSM context, without leaving room for finely tuned cancellations. As a by-product, we also comment on the difficulties of an MSSM interpretation of the excess in the gamma-gamma production cross section recently found at CMS that could correspond to a second Higgs resonance at m_H~136 GeV.

hep-ph↗
arXiv2012

Search for pair production of massive particles decaying into three quarks with the ATLAS detector in sqrt(s) = 7 TeV pp collisions at the LHC

A search is conducted for hadronic three-body decays of a new massive coloured particle in sqrt(s) = 7 TeV pp collisions at the LHC using an integrated luminosity of 4.6/fb collected by the ATLAS detector. Supersymmetric gluino pair production in the context of a model with R-parity violation is used as a benchmark scenario. The analysis is divided into two search channels, each optimised separately for their sensitivity to high-mass and low-mass gluino production. The first search channel uses a stringent selection on the transverse momentum of the six leading jets and is performed as a counting experiment. The second search channel focuses on low-mass gluinos produced with a large boost. Large-radius jets are selected and the invariant mass of each of the two leading jets is used as a discriminant between the signal and the background. The results are found to be consistent with Standard Model expectations and limits are set on the allowed gluino mass.

hep-ex↗
arXiv2013

Search for R-parity-violating supersymmetry in events with four or more leptons in sqrt(s) = 7 TeV pp collisions with the ATLAS detector

A search for new phenomena in final states with four or more leptons (electrons or muons) is presented. The analysis is based on 4.7 fb^-1 of sqrt(s) = 7 TeV proton-proton collisions delivered by the Large Hadron Collider and recorded with the ATLAS detector. Observations are consistent with Standard Model expectations in two signal regions: one that requires moderate values of missing transverse momentum and another that requires large effective mass. The results are interpreted in a simplified model of R-parity-violating supersymmetry in which a 95% CL exclusion region is set for charged wino masses up to 540 GeV. In an R-parity-violating MSUGRA/CMSSM model, values of m_1/2 up to 820 GeV are excluded for 10<tan(beta)<40.

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