arXiv subjects

S. Amerio

Publications and source records attributed to S. Amerio.

At least 397 records · Page 22Linked to original sources

arXiv2013

Differential branching fraction and angular analysis of the decay $B_s^0\toϕμ^{+}μ^{-}$

The determination of the differential branching fraction and the first angular analysis of the decay $B_s^0\toϕμ^{+}μ^{-}$ are presented using data, corresponding to an integrated luminosity of $1.0\,{\rm fb}^{-1}$, collected by the LHCb experiment at $\sqrt{s}=7\,{\rm TeV}$. The differential branching fraction is determined in bins of $q^{2}$, the invariant dimuon mass squared. Integration over the full $q^{2}$ range yields a total branching fraction of ${\cal B}(B_s^0\toϕμ^{+}μ^{-}) = (7.07\,^{+0.64}_{-0.59}\pm 0.17 \pm 0.71)\times 10^{-7}$, where the first uncertainty is statistical, the second systematic, and the third originates from the branching fraction of the normalisation channel. An angular analysis is performed to determine the angular observables $F_{\rm L}$, $S_3$, $A_6$, and $A_9$. The observables are consistent with Standard Model expectations.

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arXiv2013

First observation of the decay $B_s^0 \rightarrow ϕ\bar{K}^{*0}$

The first observation of the decay $B_s^0 \rightarrow ϕ\bar{K}^{*0}$ is reported. The analysis is based on a data sample corresponding to an integrated luminosity of 1.0 fb$^{-1}$ of $pp$ collisions at $\sqrt{s} = 7 TeV$, collected with the LHCb detector. A yield of $30 \pm 6$ $B_s^0 \rightarrow (K^+K^-)(K^-π^+)$ decays is found in the mass windows $1012.5 < M(K^+K^-) < 1026.5 MeV/c^2$ and $746 < M(K^-π^+)< 1046 MeV/c^2$. The signal yield is found to be dominated by $B_s^0 \rightarrow ϕ\bar{K}^{*0}$ decays, and the corresponding branching fraction is measured to be ${\cal B}(B_s^0 \rightarrow ϕ\bar{K}^{*0}) = (1.10 \pm 0.24 (stat) \pm 0.14 (syst) \pm 0.08 (f_d/f_s)) \times 10^{-6}$, where the uncertainties are statistical, systematic and from the ratio of fragmentation fractions $f_d/f_s$ which accounts for the different production rate of $B^0$ and $B_s^0$ mesons. The significance of $B_s^0 \rightarrow ϕ\bar{K}^{*0}$ signal is 6.1 standard deviations. The fraction of longitudinal polarization in $B_s^0 \rightarrow ϕ\bar{K}^{*0}$ decays is found to be $f_0 = 0.51 \pm 0.15 (stat) \pm 0.07 (syst)$.

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arXiv2013

Indirect measurement of $sin^2θ_W$ (M_W) using e+e- pairs in the Z-boson region with $p\bar{p}$ collisions at a center-of-momentum energy of 1.96 TeV

Drell-Yan lepton pairs are produced in the process $p\bar{p} \rightarrow e^+e^- + X$ through an intermediate $γ^*/Z$ boson. The lepton angular distributions are used to provide information on the electroweak-mixing parameter $sin^2θ_W$ via its observable effective-leptonic $sin^2θ_W$, or $sin^2θ^{lept}_{eff}$. A new method to infer $sin^2θ_W$, or equivalently, the W-boson mass M_W in the on-shell scheme, is developed and tested using a previous CDF Run II measurement of angular distributions from electron pairs in a sample corresponding to 2.1 fb-1 of integrated luminosity from $p\bar{p}$ collisions at a center-of-momentum energy of 1.96 TeV. The value of $sin^2θ^{lept}_{eff}$ is found to be 0.2328 +- 0.0010. Within a specified context of the standard model, this results in $sin^2θ_W$ = 0.2246 +- 0.0009 which corresponds to a W-boson mass of 80.297 +- 0.048 GeV/c^2, in agreement with previous determinations in electron-position collisions and at the Tevatron collider.

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arXiv2013

Observation of $B^0_s$-$\bar{B}^0_s$ mixing and measurement of mixing frequencies using semileptonic B decays

The $B^0_s$ and $B^0$ mixing frequencies, $Δm_s$ and $Δm_d$, are measured using a data sample corresponding to an integrated luminosity of 1.0 fb^{-1} collected by the LHCb experiment in $pp$ collisions at a centre of mass energy of 7 TeV during 2011. Around 1.8x10^6 candidate events are selected of the type $B^0_{(s)} \to D^-_{(s)} μ^+$ (+ anything), where about half are from peaking and combinatorial backgrounds. To determine the B decay times, a correction is required for the momentum carried by missing particles, which is performed using a simulation-based statistical method. Associated production of muons or mesons allows us to tag the initial-state flavour and so to resolve oscillations due to mixing. We obtain Δm_s = (17.93 \pm 0.22 (stat) \pm 0.15 (syst)) ps^{-1}, Δm_d = (0.503 \pm 0.011 (stat) \pm 0.013 (syst)) ps^{-1}. The hypothesis of no oscillations is rejected by the equivalent of 5.8 standard deviations for $B^0_s$ and 13.0 standard deviations for $B^0$. This is the first observation of $B^0_s$ mixing to be made using only semileptonic decays.

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arXiv2013

A Direct Measurement of the Total Decay Width of the Top Quark

We present a measurement of the total decay width of the top quark using events with top-antitop-quark pair candidates reconstructed in the final state with one charged lepton and four or more hadronic jets. We use the full Tevatron Run II data set of $\sqrt{s} = 1.96$ TeV proton-antiproton collisions recorded by the CDF II detector. The top-quark mass and the mass of the hadronically-decaying $W$ boson are reconstructed for each event and compared with distributions derived from simulated signal and background samples to extract the top-quark width (\gmt) and the energy scale of the calorimeter jets with {\it in-situ} calibration. For a top-quark mass $\mtop = \gevcc{172.5}$, we find $1.10<\gmt<\gev{4.05}$ at 68% confidence level, which is in agreement with the standard-model expectation of \gev{1.3} and is the most precise direct measurement of the top-quark width to date.

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arXiv2013

Measurement of CP violation in the phase space of $B^{\pm} \rightarrow K^{+} K^{-} π^{\pm}$ and $B^{\pm} \rightarrow π^{+} π^{-} π^{\pm}$ decays

The charmless decays $B^{\pm} \rightarrow K^{+}K^{-}π^{\pm}$ and $B^{\pm} \rightarrow π^{+}π^{-}π^{\pm}$ are reconstructed in a data set, corresponding to an integrated luminosity of 1.0 fb$^{-1}$ of pp collisions at a center-of-mass energy of 7 TeV, collected by LHCb in 2011. The inclusive charge asymmetries of these modes are measured to be $A_{CP}(B^{\pm} \rightarrow K^{+}K^{-}π^{\pm}) =-0.141 \pm 0.040 (stat) \pm 0.018 (syst) \pm 0.007 (J/ψK^{\pm})$ and $A_{CP}(B^{\pm} \rightarrow π^{+}π^{-}π^{\pm}) = 0.117 \pm 0.021 (stat) \pm 0.009 (syst) \pm 0.007 (J/ψK^{\pm})$, where the third uncertainty is due to the CP asymmetry of the $B^{\pm} \rightarrow J/ψK^{\pm}$ reference mode. In addition to the inclusive CP asymmetries, larger asymmetries are observed in localized regions of phase space.

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arXiv2013

Measurement of the $C¶$ asymmetry in $B^+ \rightarrow K^+ μ^+ μ^-$ decays

A measurement of the $C¶$ asymmetry in $B^+ \rightarrow K^+ μ^+ μ^-$ decays is presented using $pp$ collision data, corresponding to an integrated luminosity of 1.0${\,fb}^{-1}$, recorded by the LHCb experiment during 2011 at a centre-of-mass energy of 7 TeV. The measurement is performed in seven bins of $μ^+ μ^-$ invariant mass squared in the range ${0.05<q^{2}<22.00{\mathrm{GeV^2/}c^4}}$, excluding the ${J/ψ}$ and $ψ{(2S)}$ resonance regions. Production and detection asymmetries are corrected for using the $B^+ \rightarrow J/ψK^+$ decay as a control mode. Averaged over all the bins, the $C¶$ asymmetry is found to be ${{\cal A}_{C¶} = 0.000\pm 0.033{(stat.)} \pm0.005 {(syst.)} \pm 0.007{}(J/ψK^+)}$, where the third uncertainty is due to the $C¶$ asymmetry of the control mode. This is consistent with the Standard Model prediction.

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arXiv2013

Measurement of B meson production cross-sections in proton-proton collisions at sqrt(s)= 7 TeV

The production cross-sections of B mesons are measured in pp collisions at a centre-of-mass energy of 7 TeV, using data collected with the LHCb detector corresponding to a integrated luminosity of 0.36 fb-1. The B+, B0 and Bs0 mesons are reconstructed in the exclusive decays B+ -> J/psi K+, B0 -> J/psi K*0 and Bs0 -> J/psi phi, with J/psi -> mu+ mu-, K*0 -> K+ pi- and phi -> K+ K-. The differential cross-sections are measured as functions of B meson transverse momentum pT and rapidity y, in the range 0 < pT < 40 GeV/c and 2.0 < y < 4.5. The integrated cross-sections in the same pT and y ranges, including charge-conjugate states, are measured to be sigma(pp -> B+ + X) = 38.9 +- 0.3 (stat.) +- 2.5 (syst.) +- 1.3 (norm.) mub, sigma(pp -> B0 + X) = 38.1 +- 0.6 (stat.) +- 3.7 (syst.) +- 4.7 (norm.) mub, sigma(pp -> Bs0 + X) = 10.5 +- 0.2 (stat.) +- 0.8 (syst.) +- 1.0 (norm.) mub, where the third uncertainty arises from the pre-existing branching fraction measurements.

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arXiv2013

Search for the production of ZW and ZZ boson pairs decaying into charged leptons and jets in proton-antiproton collisions at sqrt(s)=1.96 TeV

We present a measurement of the production cross section for ZW and ZZ boson pairs in final states with a pair of charged leptons, from the decay of a Z boson, and at least two jets, from the decay of a W or Z boson, using the full sample of proton-antiproton collisions recorded with the CDF II detector at the Tevatron, corresponding to 8.9 fb^(-1) of integrated luminosity. We increase the sensitivity to vector boson decays into pairs of quarks using a neural network discriminant that exploits the differences between the spatial spread of energy depositions and charged-particle momenta contained within the jet of particles originating from quarks and gluons. Additionally, we employ new jet energy corrections to Monte Carlo simulations that account for differences in the observed energy scales for quark and gluon jets. The number of signal events is extracted through a simultaneous fit to the dijet mass spectrum in three classes of events: events likely to contain jets with a heavy-quark decay, events likely to contain jets originating from light quarks, and events that fail these identification criteria. We determine the production cross section to be 2.5 +2.0 -1.0 pb (< 6.1 pb at the 95% confidence level), consistent with the standard model prediction of 5.1 pb.

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arXiv2013

Search for new physics in trilepton events and limits on the associated chargino-neutralino production at CDF

We perform a search for new physics using final states consisting of three leptons and a large imbalance in transverse momentum resulting from proton-antiproton collisions at 1.96 TeV center-of-mass energy. We use data corresponding to 5.8 fb-1 of integrated luminosity recorded by the CDF II detector at the Tevatron collider. Our main objective is to investigate possible new low-momentum (down to 5 GeV/c) multi-leptonic final states not investigated by LHC experiments. Relative to previous CDF analyses, we expand the geometric and kinematic coverage of electrons and muons and utilize tau leptons that decay hadronically. Inclusion of tau leptons is particularly important for supersymmetry (SUSY) searches. The results are consistent with standard-model predictions. By optimizing our event selection to increase sensitivity to the minimal supergravity (mSUGRA) SUSY model, we set limits on the associated production of chargino and neutralino, the SUSY partners of the electroweak gauge bosons. We exclude cross sections up to 0.1 pb and chargino masses up to 168 GeV/c^2 at 95% CL, for a suited set of mSUGRA parameters. We also exclude a region of the two-dimensional space of the masses of the neutralino and the supersymmetric partner of the tau lepton, not previously excluded at the Tevatron.

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arXiv2013

Measurement of the $B^0_s \to μ^+ μ^-$ branching fraction and search for $B^0 \to μ^+ μ^-$ decays at the LHCb experiment

A search for the rare decays $B^0_s \toμ^+μ^-$ and $B^0 \toμ^+μ^-$ is performed at the LHCb experiment. The data analysed correspond to an integrated luminosity of 1 fb$^{-1}$ of $pp$ collisions at a centre-of-mass energy of 7 TeV and 2 fb$^{-1}$ at 8 TeV. An excess of $B^0_s \toμ^+μ^-$ signal candidates with respect to the background expectation is seen with a significance of 4.0 standard deviations. A time-integrated branching fraction of ${\cal B}(B^0_s \toμ^+μ^-) = (2.9^{+1.1}_{-1.0})\times 10^{-9}$ is obtained and an upper limit of ${\cal B}(B^0 \toμ^+μ^-) < 7.4\times 10^{-10}$ at 95% confidence level is set. These results are consistent with the Standard Model expectations.

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arXiv2013

Measurement of the polarisation amplitudes in B0 -> J/psi K*(892)0 decays

An analysis of the decay B0 -> J/psi K*(892)0 is presented using data, corresponding to an integrated luminosity of 1.0fb^-1, collected in pp collisions at a centre-of-mass energy of 7TeV with the LHCb detector. The polarisation amplitudes and the corresponding phases are measured to be |A_{\parallel}|^2 = 0.227 +- 0.004 (stat.) +- 0.011 (syst.), |A_{\perp}|^2 = 0.201 +- 0.004 (stat.) +- 0.008 (syst.), δ_{\parallel} [rad] = -2.94 +- 0.02 (stat.) +- 0.03 (syst.), δ_{\perp} [rad] = 2.94 +- 0.02 (stat.) +- 0.02 (syst.). Comparing B0 -> J/psi K*(892)0 and \bar{B}0 -> J/psi \bar{K}*(892)0 decays, no evidence for direct CP violation is found.

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arXiv2013

Evidence for a bottom baryon resonance Lambda_b* in CDF data

Using data from proton-antiproton collisions at Ecms=1.96 TeV recorded by the CDF II detector at the Fermilab Tevatron, evidence for the excited resonance state Lambda_b* is presented in its Lambda_b0 pi+ pi- decay, followed by the Lambda_b0 -->Lambda_c+ (-->proton K- pi+) pi- decays. The analysis is based on a data sample corresponding to an integrated luminosity of 9.6/fb collected by an online event selection based on charged-particle tracks displaced from the proton-antiproton interaction point. The significance of the observed signal is 3.5 Gaussian sigmas. The mass of the observed state is found to be 5919.22 +- 0.76 MeV in agreement with similar findings in proton-proton collision experiments.

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arXiv2013

Observation of a resonance in $B^+ \to K^+ μ^+μ^-$ decays at low recoil

A broad peaking structure is observed in the dimuon spectrum of $B^+ \to K^+ μ^+μ^-$ decays in the kinematic region where the kaon has a low recoil against the dimuon system. The structure is consistent with interference between the $B^+ \to K^+ μ^+μ^-$ decay and a resonance and has a statistical significance exceeding six standard deviations. The mean and width of the resonance are measured to be $4191^{+9}_{-8}\mathrm{\,Me\kern -0.1em V/}c^2$ and $65^{+22}_{-16}\mathrm{\,Me\kern -0.1em V/}c^2$, respectively, where the uncertainties include statistical and systematic contributions. These measurements are compatible with the properties of the $ψ(4160)$ meson. First observations of both the decay $B^+ \to ψ(4160) K^+$ and the subsequent decay $ψ(4160) \to μ^+μ^-$ are reported. The resonant decay and the interference contribution make up 20\,% of the yield in the low recoil region, which is larger than theoretical estimates.

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arXiv2013

Observation of the decay $B_c \rightarrow J/ψK^+ K^- π^+ $

The decay $B_c\rightarrow J/ψK^+ K^- π^+$ is observed for the first time, using proton-proton collisions collected with the LHCb detector corresponding to an integrated luminosity of 3fb$^{-1}$. A signal yield of $78\pm14$ decays is reported with a significance of 6.2 standard deviations. The ratio of the branching fraction of $\B_c \rightarrow J/ψK^+ K^- π^+$ decays to that of $B_c \rightarrow J/ψπ^+$ decays is measured to be $0.53\pm 0.10\pm0.05$, where the first uncertainty is statistical and the second is systematic.

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arXiv2013

Model-independent search for CP violation in D0 to K-K+pi-pi+ and D0 to pi-pi+pi+pi- decays

A search for CP violation in the phase-space structures of D0 and D0bar decays to the final states K-K+pi-pi+ and pi-pi+pi+pi- is presented. The search is carried out with a data set corresponding to an integrated luminosity of 1.0fb^-1 collected in 2011 by the LHCb experiment in $pp$ collisions at a centre-of-mass energy of 7TeV. For the K-K+pi-pi+ final state, the four-body phase space is divided into 32 bins, each bin with approximately 1800 decays. The p-value under the hypothesis of no CP violation is 9.1%, and in no bin is a CP asymmetry greater than 6.5% observed. The phase space of the pi-pi+pi+pi- final state is partitioned into 128 bins, each bin with approximately 2500 decays. The $p$-value under the hypothesis of no CP violation is 41%, and in no bin is a CP asymmetry greater than 5.5% observed. All results are consistent with the hypothesis of no CP violation at the current sensitivity.

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arXiv2013

Measurement of form-factor independent observables in the decay $B^{0} \to K^{*0} μ^+ μ^-$

We present a measurement of form-factor independent angular observables in the decay \mbox{$B^0\to K^{*}(892)^{0}μ^+ μ^-$}. The analysis is based on a data sample corresponding to an integrated luminosity of 1.0fb$^{-1}$, collected by the LHCb experiment in $pp$ collisions at a center-of-mass energy of 7TeV. Four observables are measured in six bins of the dimuon invariant mass squared, $q^2$, in the range $0.1<q^2<19.0$GeV$^{2}$/c$^{4}$. Agreement with Standard Model predictions is found for 23 of the 24 measurements. A local discrepancy, corresponding to $3.7$ Gaussian standard deviations, is observed in one $q^2$ bin for one of the observables. Considering the 24 measurements as independent, the probability to observe such a discrepancy, or larger, in one is $0.5\%$.

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arXiv2013

First evidence for the two-body charmless baryonic decay $B^0 \to p \bar{p}$

The results of a search for the rare two-body charmless baryonic decays $B^0 \to p \bar{p}$ and $B_s^0 \to p \bar{p}$ are reported. The analysis uses a data sample, corresponding to an integrated luminosity of 0.9 fb$^{-1}$, of $pp$ collision data collected by the LHCb experiment at a centre-of-mass energy of 7 TeV. An excess of $B^0 \to p \bar{p}$ candidates with respect to background expectations is seen with a statistical significance of 3.3 standard deviations. This is the first evidence for a two-body charmless baryonic $B^0$ decay. No significant $B_s^0 \to p \bar{p}$ signal is observed, leading to an improvement of three orders of magnitude over previous bounds. If the excess events are interpreted as signal, the 68.3% confidence level intervals on the branching fractions are {eqnarray} \cal{B}(B^0 \to p \bar{p}) & = & (1.47 \,^{+0.62}_{-0.51} \,^{+0.35}_{-0.14}) \times 10^{-8} \,, *{0.3cm} \cal{B}(B_s^0 \to p \bar{p}) & = & (2.84 \,^{+2.03}_{-1.68} \,^{+0.85}_{-0.18}) \times 10^{-8} \,, {eqnarray} where the first uncertainty is statistical and the second is systematic.

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