arXiv subjects

R. Andreassen

Publications and source records attributed to R. Andreassen.

At least 19 recordsLinked to original sources

arXiv2018

Measurement of the relative rate of prompt $χ_{c0}$, $χ_{c1}$ and $χ_{c2}$ production at $\sqrt{s}=7$TeV

Prompt production of charmonium $χ_{c0}$, $χ_{c1}$ and $χ_{c2}$ mesons is studied using proton-proton collisions at the LHC at a centre-of-mass energy of $\sqrt{s}=7$TeV. The $χ_{c}$ mesons are identified through their decay to $J/ψγ$, with $J/ψ\toμ^+μ^-$ using photons that converted in the detector. A data sample, corresponding to an integrated luminosity of $1.0\mathrm{fb}^{-1}$ collected by the LHCb detector, is used to measure the relative prompt production rate of $χ_{c1}$ and $χ_{c2}$ in the rapidity range $2.0<y<4.5$ as a function of the $J/ψ$ transverse momentum from 3 to 20 GeV$/c$. First evidence for $χ_{c0}$ meson production at a high-energy hadron collider is also presented.

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arXiv2015

Observation of the $B^0_s\toη'η'$ decay

The first observation of the $B^0_s\toη'η'$ decay is reported. The study is based on a sample of proton-proton collisions corresponding to $3.0$ ${\rm fb^{-1}}$ of integrated luminosity collected with the LHCb detector. The significance of the signal is $6.4$ standard deviations. The branching fraction is measured to be $[3.31 \pm 0.64\,{\rm (stat)} \pm 0.28\,{\rm (syst)} \pm 0.12\,{\rm (norm)}]\times10^{-5}$, where the third uncertainty comes from the $B^{\pm}\toη' K^{\pm}$ branching fraction that is used as a normalisation. In addition, the charge asymmetries of $B^{\pm}\toη' K^{\pm}$ and $B^{\pm}\toϕK^{\pm}$, which are control channels, are measured to be $(-0.2 \pm1.3)\%$ and $(+1.7\pm1.3)\%$, respectively. All results are consistent with theoretical expectations.

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arXiv2015

Quantum numbers of the $X(3872)$ state and orbital angular momentum in its $ρ^0 Jψ$ decay

Angular correlations in $B^+\to X(3872) K^+$ decays, with $X(3872)\to ρ^0 J/ψ$, $ρ^0\toπ^+π^-$ and $J/ψ\toμ^+μ^-$, are used to measure orbital angular momentum contributions and to determine the $J^{PC}$ value of the $X(3872)$ meson. The data correspond to an integrated luminosity of 3.0 fb$^{-1}$ of proton-proton collisions collected with the LHCb detector. This determination, for the first time performed without assuming a value for the orbital angular momentum, confirms the quantum numbers to be $J^{PC}=1^{++}$. The $X(3872)$ is found to decay predominantly through S wave and an upper limit of $4\%$ at $95\%$ C.L. is set on the fraction of D wave.

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arXiv2015

Dalitz plot analysis of $B^0 \to \overline{D}^0 π^+π^-$ decays

The resonant substructures of $B^0 \to \overline{D}^0 π^+π^-$ decays are studied with the Dalitz plot technique. In this study a data sample corresponding to an integrated luminosity of 3.0 fb$^{-1}$ of $pp$ collisions collected by the LHCb detector is used. The branching fraction of the $B^0 \to \overline{D}^0 π^+π^-$ decay in the region $m(\overline{D}^0π^{\pm})>2.1$ GeV$/c^2$ is measured to be $(8.46 \pm 0.14 \pm 0.29 \pm 0.40) \times 10^{-4}$, where the first uncertainty is statistical, the second is systematic and the last arises from the normalisation channel $B^0 \to D^*(2010)^-π^+$. The $π^+π^-$ S-wave components are modelled with the Isobar and K-matrix formalisms. Results of the Dalitz plot analyses using both models are presented. A resonant structure at $m(\overline{D}^0π^-) \approx 2.8$ GeV$/c^{2}$ is confirmed and its spin-parity is determined for the first time as $J^P = 3^-$. The branching fraction, mass and width of this structure are determined together with those of the $D^*_0(2400)^-$ and $D^*_2(2460)^-$ resonances. The branching fractions of other $B^0 \to \overline{D}^0 h^0$ decay components with $h^0 \to π^+π^-$ are also reported. Many of these branching fraction measurements are the most precise to date. The first observation of the decays $B^0 \to \overline{D}^0 f_0(500)$, $B^0 \to \overline{D}^0 f_0(980)$, $B^0 \to \overline{D}^0 ρ(1450)$, $B^0 \to D_3^*(2760)^- π^+$ and the first evidence of $B^0 \to \overline{D}^0 f_0(2020)$ are presented.

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arXiv2015

Measurement of $CP$ asymmetries and polarisation fractions in $B_s^0 \rightarrow K^{0}\bar{K}{}^{0}$ decays

An angular analysis of the decay $B_s^0 \rightarrow K^{*0}\bar{K}{}^{*0}$ is performed using $pp$ collisions corresponding to an integrated luminosity of $1.0$ ${fb}^{-1}$ collected by the LHCb experiment at a centre-of-mass energy $\sqrt{s} = 7$ TeV. A combined angular and mass analysis separates six helicity amplitudes and allows the measurement of the longitudinal polarisation fraction $f_L = 0.201 \pm 0.057 {(stat.)} \pm 0.040{(syst.)}$ for the $B_s^0 \rightarrow K^*(892)^0 \bar{K}{}^*(892)^0$ decay. A large scalar contribution from the $K^{*}_{0}(1430)$ and $K^{*}_{0}(800)$ resonances is found, allowing the determination of additional $CP$ asymmetries. Triple product and direct $CP$ asymmetries are determined to be compatible with the Standard Model expectations. The branching fraction $\mathcal{B}(B_s^0 \rightarrow K^*(892)^0 \bar{K}{}^*(892)^0)$ is measured to be $(10.8 \pm 2.1 {\ \rm (stat.)} \pm 1.4 {\ \rm (syst.)} \pm 0.6 \ (f_d/f_s) ) \times 10^{-6}$.

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arXiv2015

First Observation of CP Violation in B0->D(*)CP h0 Decays by a Combined Time-Dependent Analysis of BaBar and Belle Data

We report a measurement of the time-dependent CP asymmetry of B0->D(*)CP h0 decays, where the light neutral hadron h0 is a pi0, eta or omega meson, and the neutral D meson is reconstructed in the CP eigenstates K+ K-, K0S pi0 or K0S omega. The measurement is performed combining the final data samples collected at the Y(4S) resonance by the BaBar and Belle experiments at the asymmetric-energy B factories PEP-II at SLAC and KEKB at KEK, respectively. The data samples contain ( 471 +/- 3 ) x 10^6 BB pairs recorded by the BaBar detector and ( 772 +/- 11 ) x 10^6, BB pairs recorded by the Belle detector. We measure the CP asymmetry parameters -eta_f S = +0.66 +/- 0.10 (stat.) +/- 0.06 (syst.) and C = -0.02 +/- 0.07 (stat.) +/- 0.03 (syst.). These results correspond to the first observation of CP violation in B0->D(*)CP h0 decays. The hypothesis of no mixing-induced CP violation is excluded in these decays at the level of 5.4 standard deviations.

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arXiv2015

Measurement of the $η_c (1S)$ production cross-section in proton-proton collisions via the decay $η_c (1S) \rightarrow p \bar{p}$

The production of the $η_c (1S)$ state in proton-proton collisions is probed via its decay to the $p \bar{p}$ final state with the LHCb detector, in the rapidity range $2.0 < y < 4.5$ and in the meson transverse-momentum range $p_T > 6.5$ GeV/c. The cross-section for prompt production of $η_c (1S)$ mesons relative to the prompt $J/ψ$ cross-section is measured, for the first time, to be $σ_{η_c (1S)}/σ_{J/ψ} = 1.74 \pm 0.29 \pm 0.28 \pm 0.18 _{B}$ at a centre-of-mass energy $\sqrt{s} = 7$ TeV using data corresponding to an integrated luminosity of 0.7 fb$^{-1}$, and $σ_{η_c (1S)}/σ_{J/ψ} = 1.60 \pm 0.29 \pm 0.25 \pm 0.17 _{B}$ at $\sqrt{s} = 8$ TeV using 2.0 fb$^{-1}$. The uncertainties quoted are, in order, statistical, systematic, and that on the ratio of branching fractions of the $η_c (1S)$ and $J/ψ$ decays to the $p \bar{p}$ final state. In addition, the inclusive branching fraction of $b$-hadron decays into $η_c (1S)$ mesons is measured, for the first time, to be $B ( b \rightarrow η_c X ) = (4.88 \pm 0.64 \pm 0.29 \pm 0.67 _{B}) \times 10^{-3}$, where the third uncertainty includes also the uncertainty on the $J/ψ$ inclusive branching fraction from $b$-hadron decays. The difference between the $J/ψ$ and $η_c (1S)$ meson masses is determined to be $114.7 \pm 1.5 \pm 0.1$ MeV/c$^2$.

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arXiv2015

Measurement of $B_c^+$ production in proton-proton collisions at $\sqrt{s}=8$ TeV

Production of $B_c^+$ mesons in proton-proton collisions at a center-of-mass energy of 8 TeV is studied with data corresponding to an integrated luminosity of $2.0~{\rm fb}^{-1}$ recorded by the LHCb experiment. The ratio of production cross-sections times branching fractions between the $B_c^+\to J/ψπ^+$ and $B^+\to J/ψK^+$ decays is measured as a function of transverse momentum and rapidity in the regions $0 < p_{\rm T} < 20~{\rm GeV}/c$ and $2.0 < y < 4.5$. The ratio in this kinematic range is measured to be $(0.683\pm0.018\pm0.009)\%$, where the first uncertainty is statistical and the second systematic.

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arXiv2015

Search for long-lived particles decaying to jet pairs

A search is presented for long-lived particles with a mass between 25 and 50 GeV$/c^2$ and a lifetime between 1 and 200 ps in a sample of proton-proton collisions at a centre-of-mass energy of $\sqrt{s}=7$ TeV, corresponding to an integrated luminosity of 0.62 fb$^{-1}$, collected by the LHCb detector. The particles are assumed to be pair-produced by the decay of a Standard Model-like Higgs boson. The experimental signature of the long-lived particle is a displaced vertex with two associated jets. No excess above the background is observed and limits are set on the production cross-section as a function of the long-lived particle mass and lifetime.

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arXiv2015

Precise measurements of the properties of the B_1(5721)^(0,+) and B_2*(5747)^(0,+) states and observation of B^(+,0) pi^(-,+) mass structures

Invariant mass distributions of B+pi- and B0pi+ combinations are investigated in order to study excited B mesons. The analysis is based on a data sample corresponding to 3.0 fb-1 of pp collision data, recorded by the LHCb detector at centre-of-mass energies of 7 and 8 TeV. Precise measurements of the masses and widths of the B_1(5721)^(0,+) and B_2*(5747)^(0,+) states are reported. Clear enhancements, particularly prominent at high pion transverse momentum, are seen over background in the mass range 5850--6000 MeV in both B+pi- and B0pi+ combinations. The structures are consistent with the presence of four excited B mesons, labelled B_J(5840)^(0,+) and B_J(5960)^(0,+), whose masses and widths are obtained under different hypotheses for their quantum numbers.

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arXiv2015

Angular analysis of the $B^0 \rightarrow K^{*0} e^+ e^-$ decay in the low-$q^2$ region

An angular analysis of the $B^0 \rightarrow K^{*0} e^+ e^-$ decay is performed using a data sample, corresponding to an integrated luminosity of 3.0 ${\mbox{fb}^{-1}}$, collected by the LHCb experiment in $pp$ collisions at centre-of-mass energies of 7 and 8 TeV during 2011 and 2012. For the first time several observables are measured in the dielectron mass squared ($q^2$) interval between 0.002 and 1.120${\mathrm{\,Ge\kern -0.1em V^2\!/}c^4}$. The angular observables $F_{\mathrm{L}}$ and $A_{\mathrm{T}}^{\mathrm{Re}}$ which are related to the $K^{*0}$ polarisation and to the lepton forward-backward asymmetry, are measured to be $F_{\mathrm{L}}= 0.16 \pm 0.06 \pm0.03$ and $A_{\mathrm{T}}^{\mathrm{Re}} = 0.10 \pm 0.18 \pm 0.05$, where the first uncertainty is statistical and the second systematic. The angular observables $A_{\mathrm{T}}^{(2)}$ and $A_{\mathrm{T}}^{\mathrm{Im}}$ which are sensitive to the photon polarisation in this $q^2$ range, are found to be $A_{\mathrm{T}}^{(2)} = -0.23 \pm 0.23 \pm 0.05$ and $A_{\mathrm{T}}^{\mathrm{Im}} =0.14 \pm 0.22 \pm 0.05$. The results are consistent with Standard Model predictions.

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arXiv2015

Measurement of indirect $CP$ asymmetries in $D^0\rightarrow K^-K^+$ and $D^0\rightarrow π^-π^+$ decays using semileptonic $B$ decays

Time-dependent $CP$ asymmetries in the decay rates of the singly Cabibbo-suppressed decays $D^0\rightarrow K^-K^+$ and $D^0\rightarrow π^-π^+$ are measured in $pp$ collision data corresponding to an integrated luminosity of 3.0 fb$^{-1}$ collected by the LHCb experiment. The $D^0$ mesons are produced in semileptonic $b$-hadron decays, where the charge of the accompanying muon is used to determine the initial state as $D^0$ or $\bar{D}^0$. The asymmetries in effective lifetimes between $D^0$ and $\bar{D}^0$ decays, which are sensitive to indirect $CP$ violation, are determined to be \begin{align*} A_Γ(K^-K^+) = (-0.134 \pm 0.077 \; {}^{+0.026}_{-0.034})\% \, A_Γ(π^-π^+) = (-0.092\pm 0.145 \; {}^{+0.025}_{-0.033})\% \, \end{align*} where the first uncertainties are statistical and the second systematic. This result is in agreement with previous measurements and with the hypothesis of no indirect $CP$ violation in $D^0$ decays.

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arXiv2015

Search for the lepton flavour violating decay $τ^-\to μ^-μ^+μ^-$

A search for the lepton flavour violating decay $τ^-\to μ^-μ^+μ^-$ is performed with the LHCb experiment. The data sample corresponds to an integrated luminosity of $1.0\mathrm{\,fb}^{-1}$ of proton-proton collisions at a centre-of-mass energy of $7\mathrm{\,Te\kern -0.1em V}$ and $2.0\mathrm{\,fb}^{-1}$ at $8\mathrm{\,Te\kern -0.1em V}$. No evidence is found for a signal, and a limit is set at $90\%$ confidence level on the branching fraction, $\mathcal{B}(τ^-\to μ^-μ^+μ^-) < 4.6 \times 10^{-8}$.

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arXiv2015

Study of the rare $B_s^0$ and $B^0$ decays into the $π^+π^-μ^+μ^-$ final state

A search for the rare decays $B_s^0 \to π^+π^-μ^+μ^-$ and $B^0 \to π^+π^-μ^+μ^-$ is performed in a data set corresponding to an integrated luminosity of 3.0 fb$^{-1}$ collected by the LHCb detector in proton-proton collisions at centre-of-mass energies of 7 and 8 TeV. Decay candidates with pion pairs that have invariant mass in the range 0.5-1.3 GeV/$c^2$ and with muon pairs that do not originate from a resonance are considered. The first observation of the decay $B_s^0 \to π^+π^-μ^+μ^-$ and the first evidence of the decay $B^0 \to π^+π^-μ^+μ^-$ are obtained and the branching fractions, restricted to the dipion-mass range considered, are measured to be $\mathcal{B}(B_s^0 \to π^+π^-μ^+μ^-)=(8.6\pm 1.5\,({\rm stat}) \pm 0.7\,({\rm syst})\pm 0.7\,({\rm norm}))\times 10^{-8}$ and $\mathcal{B}(B^0 \to π^+π^-μ^+μ^-)=(2.11\pm 0.51\,({\rm stat}) \pm 0.15\,({\rm syst})\pm 0.16\,({\rm norm}) )\times 10^{-8}$, where the third uncertainty is due to the branching fraction of the decay $B^0\to J/ψ(\to μ^+μ^-)K^*(890)^0(\to K^+π^-)$, used as a normalisation.

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arXiv2015

LHCb Detector Performance

The LHCb detector is a forward spectrometer at the Large Hadron Collider (LHC) at CERN. The experiment is designed for precision measurements of CP violation and rare decays of beauty and charm hadrons. In this paper the performance of the various LHCb sub-detectors and the trigger system are described, using data taken from 2010 to 2012. It is shown that the design criteria of the experiment have been met. The excellent performance of the detector has allowed the LHCb collaboration to publish a wide range of physics results, demonstrating LHCb's unique role, both as a heavy flavour experiment and as a general purpose detector in the forward region.

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arXiv2015

Observation of two new $Ξ_b^-$ baryon resonances

Two structures are observed close to the kinematic threshold in the $Ξ_b^0 π^-$ mass spectrum in a sample of proton-proton collision data, corresponding to an integrated luminosity of 3.0 fb$^{-1}$ recorded by the LHCb experiment. In the quark model, two baryonic resonances with quark content $bds$ are expected in this mass region: the spin-parity $J^P = \frac{1}{2}^+$ and $J^P=\frac{3}{2}^+$ states, denoted $Ξ_b^{\prime -}$ and $Ξ_b^{*-}$. Interpreting the structures as these resonances, we measure the mass differences and the width of the heavier state to be $m(Ξ_b^{\prime -}) - m(Ξ_b^0) - m(π^{-}) = 3.653 \pm 0.018 \pm 0.006$ MeV$/c^2$, $m(Ξ_b^{*-}) - m(Ξ_b^0) - m(π^{-}) = 23.96 \pm 0.12 \pm 0.06$ MeV$/c^2$, $Γ(Ξ_b^{*-}) = 1.65 \pm 0.31 \pm 0.10$ MeV, where the first and second uncertainties are statistical and systematic, respectively. The width of the lighter state is consistent with zero, and we place an upper limit of $Γ(Ξ_b^{\prime -}) < 0.08$ MeV at 95% confidence level. Relative production rates of these states are also reported.

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arXiv2015

Measurement of the Z+b-jet cross-section in pp collisions at $\sqrt{s}=7{\mathrm{\,Te\kern -0.1em V}}$ in the forward region

The associated production of a Z boson or an off-shell photon $γ^*$ with a bottom quark in the forward region is studied using proton-proton collisions at a centre-of-mass energy of $7{\mathrm{\,Te\kern -0.1em V}}$. The Z bosons are reconstructed in the ${\text{Z}/γ^*}\!\rightarrow{μ^{+}μ^{-}}$ final state from muons with a transverse momentum larger than $20{\mathrm{\,Ge\kern -0.1em V}}$, while two transverse momentum thresholds are considered for jets ($10{\mathrm{\,Ge\kern -0.1em V}}$ and $20{\mathrm{\,Ge\kern -0.1em V}}$). Both muons and jets are reconstructed in the pseudorapidity range $2.0 < η< 4.5$. The results are based on data corresponding to $1.0{\,\mbox{fb}^{-1}}$ recorded in 2011 with the LHCb detector. The measurement of the Z+b-jet cross-section is normalized to the Z+jet cross-section. The measured cross-sections are \begin{equation*} σ(\text{$\text{Z}/γ^*(μ^{+}μ^{-})$+b-jet}) = 295 \pm 60~(\text{stat}) \pm 51~(\text{syst}) \pm 10~(\text{lumi}) {\,\mbox{fb}} \end{equation*} for ${p_{\rm T}}$(jet)$>10{\mathrm{\,Ge\kern -0.1em V}}$, and \begin{equation*} σ(\text{$\text{Z}/γ^*(μ^{+}μ^{-})$+b-jet}) = 128 \pm 36~(\text{stat}) \pm 22~(\text{syst}) \pm 5~(\text{lumi}) {\,\mbox{fb}} \end{equation*} for ${p_{\rm T}}$(jet)$>20{\mathrm{\,Ge\kern -0.1em V}}$.

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arXiv2015

Measurement of the track reconstruction efficiency at LHCb

The determination of track reconstruction efficiencies at LHCb using $J/ψ\rightarrowμ^{+}μ^{-}$ decays is presented. Efficiencies above $95\%$ are found for the data taking periods in 2010, 2011, and 2012. The ratio of the track reconstruction efficiency of muons in data and simulation is compatible with unity and measured with an uncertainty of $0.8\,\%$ for data taking in 2010, and at a precision of $0.4\,\%$ for data taking in 2011 and 2012. For hadrons an additional $1.4\,\%$ uncertainty due to material interactions is assumed. This result is crucial for accurate cross section and branching fraction measurements in LHCb.

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