arXiv subjects

H. Carranza-Mejia

Publications and source records attributed to H. Carranza-Mejia.

116 records · Page 7Linked to original sources

arXiv2013

Measurement of the Λ_b^0, Ξ_b^- and Ω_b^- baryon masses

Bottom baryons decaying to a J/ψ meson and a hyperon are reconstructed using 1.0 fb^{-1} of data collected in 2011 with the LHCb detector. Significant Λ_b^0 \rightarrow J/ψΛ, Ξ_b^-\rightarrow J/ψΞ^- and Ω_b^- \rightarrow J/ψΩ^- signals are observed and the corresponding masses are measured to be M(Λ_b^0) = 5619.53 \pm 0.13 (stat) \pm 0.45 (syst) MeV/c^2, M(Ξ_b^-) = 5795.8 \pm 0.9 (stat) \pm 0.4 (syst) MeV/c^2, M(Ω_b^-) = 6046.0 \pm 2.2 (stat) \pm 0.5 (syst) MeV/c^2, while the differences with respect to the Λ_b^0 mass are M(Ξ_b^-)-M(Λ_b^0) = 176.2 \pm 0.9 (stat) \pm 0.1 (syst) MeV/c^2, M(Ω_b^-)-M(Λ_b^0) = 426.4 \pm 2.2 (stat) \pm 0.4 (syst) MeV/c^2. These are the most precise mass measurements of the Λ_b^0, Ξ_b^- and Ω_b^- baryons to date. Averaging the above Λ_b^0 mass measurement with that published by LHCb using 35 pb^{-1} of data collected in 2010 yields M(Λ_b^0) = 5619.44 \pm 0.13 (stat) \pm 0.38 (syst) MeV/c^2.

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arXiv2013

Exclusive $J/ψ$ and $ψ(2S)$ production in $pp$ collisions at $\sqrt{s}=7$ TeV

Exclusive $J/ψ$ and $ψ(2S)$ vector meson production has been observed in the dimuon channel using the LHCb detector. The cross-section times branching fractions to two muons with pseudorapidities between 2.0 and 4.5 are measured to be $ σ_{pp\rightarrow J/ψ(\rightarrow μ^{+} μ^{-})} (2.0 <η_{μ^{\pm}}< 4.5) = 307 \pm 21 \pm 36 \text{pb}, σ_{pp\rightarrow ψ(2S) (\rightarrow μ^{+} μ^{-})} (2.0 <η_{μ^{\pm}}< 4.5) = 7.8 \pm 1.3 \pm 1.0 \text{pb}, $ where the first uncertainties are statistical and the second are systematic. The measurements are found to be in good agreement with results from previous experiments and theoretical predictions. The $J/ψ$ photoproduction cross-section has been measured as a function of the photon-proton centre-of-mass energy. The results are consistent with measurements obtained at HERA and confirm a similar power law behaviour for the photoproduction cross-section.

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arXiv2013

Measurement of the cross-section for Z->e+e- production in pp collisions at sqrt{s}=7TeV

A measurement of the cross-section for pp->Z->e+e- is presented using data at sqrt{s}=7TeV corresponding to an integrated luminosity of 0.94 fb^{-1}. The process is measured within the kinematic acceptance pT>20GeV/c and 2<η<4.5 for the daughter electrons and dielectron invariant mass in the range 60--120GeV/c^2. The cross-section is determined to be σ(pp->Z->e+e-)=76.0+-0.8+-2.0+-2.6pb where the first uncertainty is statistical, the second is systematic and the third is the uncertainty in the luminosity. The measurement is performed as a function of Z rapidity and as a function of an angular variable which is closely related to the Z transverse momentum. The results are compared with previous LHCb measurements and with theoretical predictions from QCD.

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arXiv2013

Measurement of the fragmentation fraction ratio $f_{s}/f_{d}$ and its dependence on $B$ meson kinematics

The relative production rate of $B^{0}_{s}$ and $B^{0}$ mesons is determined with the hadronic decays $B^{0}_{s} \rightarrow D^{-}_{s}π^{+}$ and $B^0 \rightarrow D^{-}K^{+}$. The measurement uses data corresponding to 1.0 fb$^{-1}$ of $pp$ collisions at a centre-of-mass energy of $\sqrt{s}=7$ TeV recorded in the forward region with the LHCb experiment. The ratio of production rates, $f_{s}/f_{d}$, is measured to be $0.238 \pm 0.004 \pm 0.015 \pm 0.021 $, where the first uncertainty is statistical, the second systematic, and the third theoretical. This is combined with a previous LHCb measurement to obtain $f_{s}/f_{d} = 0.256 \pm 0.020$. The dependence of $f_{s}/f_{d}$ on the transverse momentum and pseudorapidity of the $B$ meson is determined using the decays $B^{0}_{s} \rightarrow D^{-}_{s}π^{+}$ and $B^{0} \rightarrow D^{-}π^{+}$. There is evidence for a decrease with increasing transverse momentum, whereas the ratio remains constant as a function of pseudorapidity. In addition, the ratio of branching fractions of the decays $B^{0} \rightarrow D^{-}K^{+}$ and $B^{0} \rightarrow D^{-}π^{+}$ is measured to be $0.0822 \pm 0.0011 (\textrm{stat}) \pm 0.0025 (\textrm{syst})$.

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arXiv2012

First observation of the decays $\bar{B}^0_{(s)}\to D_s^+K^-π^+π^-$ and $\bar{B}^0_s\to D_{s1}(2536)^+π^-$

The first observation of the decays $\bar{B}^0_{s}\to D_s^+K^-π^+π^-$ and $\bar{B}^0\to D_s^+K^-π^+π^-$ are reported using an integrated luminosity of 1.0 fb$^{-1}$ recorded by the LHCb experiment. The branching fractions, normalized with respect to $\bar{B}^0_{s}\to D_s^+π^-π^+π^-$ and $\bar{B}^0_{s}\to D_s^+K^-π^+π^-$, respectively, are measured to be {\br(\bar{B}^0_{s}\to D_s^+K^-π^+π^-)\over\br(\bar{B}^0_{s}\to D_s^+π^-π^+π^-)} &= (5.2\pm0.5\pm0.3)\times10^{-2}, {\br(\bar{B}^0\to D_s^+K^-π^+π^-)\over\br(\bar{B}^0_{s}\to D_s^+K^-^π^+π^-)} &= 0.54\pm0.07\pm0.07, where the first uncertainty is statistical and the second is systematic. The $\bar{B}^0_{s}\to D_s^+K^-π^+π^-$ decay is of particular interest as it can be used to measure the weak phase $γ$. First observation of the $\bar{B}^0_s\to D_{s1}(2536)^+π^-, D_{s1}^+\to D_s^+π^-π^+$ decay is also presented, and its branching fraction relative to $\bar{B}^0_{s}\to D_s^+π^-π^+π^-$ is found to be {\br(\bar{B}^0_s\to D_{s1}(2536)^+π^-, D_{s1}^+\to D_s^+π^-π^+)\over\br(\bar{B}^0_{s}\to D_s^+π^-π^+π^-)} &= (4.0\pm1.0\pm0.4)\times10^{-3}.

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arXiv2012

Measurement of the D+/- production asymmetry in 7 TeV pp collisions

The asymmetry in the production cross-section σof D+/- mesons, A_P = (σ(D+) - σ(D-))/(σ(D+) + σ(D-)), is measured in bins of pseudorapidity ηand transverse momentum p_T within the acceptance of the LHCb detector. The result is obtained with a sample of D+ -> K_S pi+ decays corresponding to an integrated luminosity of 1.0 fb^-1, collected in pp collisions at a centre of mass energy of 7 TeV at the Large Hadron Collider. When integrated over the kinematic range 2.0 < p_T < 18.0 GeV/c and 2.20 < η< 4.75, the production asymmetry is A_P = (-0.96+/-0.26+/-0.18)%. The uncertainties quoted are statistical and systematic, respectively. The result assumes that any direct CP violation in the D+ -> K_S pi+ decay is negligible. No significant dependence on ηor p_T is observed.

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arXiv2012

First evidence for the decay Bs -> mu+ mu-

A search for the rare decays Bs->mu+mu- and B0->mu+mu- is performed using data collected in 2011 and 2012 with the LHCb experiment at the Large Hadron Collider. The data samples comprise 1.1 fb^-1 of proton-proton collisions at sqrt{s} = 8 TeV and 1.0 fb^-1 at sqrt{s}=7 TeV. We observe an excess of Bs -> mu+ mu- candidates with respect to the background expectation. The probability that the background could produce such an excess or larger is 5.3 x 10^-4 corresponding to a signal significance of 3.5 standard deviations. A maximum-likelihood fit gives a branching fraction of BR(Bs -> mu+ mu-) = (3.2^{+1.5}_{-1.2}) x 10^-9, where the statistical uncertainty is 95% of the total uncertainty. This result is in agreement with the Standard Model expectation. The observed number of B0 -> mu+ mu- candidates is consistent with the background expectation, giving an upper limit of BR(B0 -> mu+ mu-) < 9.4 x 10^-10 at 95% confidence level.

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arXiv2012

A study of the $Z$ production cross-section in $pp$ collisions at $\sqrt{s} = 7$ TeV using tau final states

A measurement of the inclusive $Z \to ττ$ cross-section in $pp$ collisions at $\sqrt{s} = 7$ TeV is presented based on a dataset of 1.0 fb$^{-1}$ collected by the LHCb detector. Candidates for $Z \to ττ$ decays are identified through reconstructed final states with two muons, a muon and an electron, a muon and a hadron, or an electron and a hadron. The production cross-section for $Z$ bosons, with invariant mass between 60 and 120 GeV/$c^2$, which decay to $τ$ leptons with transverse momenta greater than 20 GeV/$c$ and pseudorapidities between 2.0 and 4.5, is measured to be $σ_{pp \to Z \to ττ} = 71.4 \pm 3.5 \pm 2.8 \pm 2.5$ pb; the first uncertainty is statistical, the second uncertainty is systematic, and the third is due to the uncertainty on the integrated luminosity. The ratio of the cross-sections for $Z \to ττ$ to $Z \to μμ$ is determined to be $0.93 \pm 0.09$, where the uncertainty is the combination of statistical, systematic, and luminosity uncertainties of the two measurements.

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