Observation of the decay $B_c \rightarrow J/\psi K^+ K^- \pi^+ $

The decay $B_c\rightarrow J/\psi K^+ K^- \pi^+$ 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 $\B_c \rightarrow J/\psi K^+ K^- \pi^+$ decays to that of $B_c \rightarrow J/\psi \pi^+$ decays is measured to be $0.53\pm 0.10\pm0.05$, where the first uncertainty is statistical and the second is systematic.


Introduction
The B + c meson is of special interest, as it is the only meson consisting of two heavy quarks 2 of different flavours. It is the heaviest meson that decays through weak interactions, with 3 either the c orb quark decaying or through their weak annihilation [1,2]. Although 4 the B + c meson was discovered in 1998 by the CDF collaboration [3], relatively few decay 5 channels were observed [4] prior to LHCb measurements [5][6][7][8][9]. 6 In the factorisation approximation [10], the B + c → J/ψ K + K − π + decay 1 is characterised 7 by the form factors of the B + c → J/ψ W + transition and the spectral functions for the sub- The LHCb detector [15] is a single-arm forward spectrometer covering the pseudorapidity 20 range 2 < η < 5, designed for the study of particles containing b or c quarks. The signal B + c → J/ψ K + K − π + and normalisation B + c → J/ψ π + decays are reconstructed 54 using the J/ψ → µ + µ − channel. Common selection criteria are used in both channels with 55 additional requirements to identify kaon candidates in the signal channel. of the kaon and pion hypotheses [16] is required to satisfy ∆ ln L K/π > 2 (< 0).

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To ensure that they do not originate from a pp interaction vertex (PV), hadrons must 61 have χ 2 IP > 4, where χ 2 IP is defined as the difference in χ 2 of a given PV reconstructed with 62 and without the considered hadron. When more than one PV is reconstructed, that with 63 the smallest value of χ 2 IP is chosen.

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Oppositely-charged muons that have a transverse momentum greater than 0.55 GeV/c and that originate from a common vertex are paired to form J/ψ candidates. The quality 66 of the vertex is ensured by requiring that the χ 2 of the vertex fit (χ 2 vtx ) is less than 20. The 67 vertex is required to be well-separated from the reconstructed PV by selecting candidates 68 with decay length significance greater than 3. The invariant mass of the J/ψ candidate is 69 required to be between 3.020 and 3.135 GeV/c 2 .

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The selected J/ψ candidates are then combined with a π + meson candidate or 71 a K + K − π + combination to form B + c candidates. The quality of the common vertex 72 is ensured by requiring χ 2 vtx < 35 (16) for the signal (normalisation) channel, and that 73 the χ 2 values for the distance of closest approach for the K + K − , K − π + and K + π + combi- to point to the PV. When more than one PV is reconstructed, that with the smallest 80 value of χ 2 IP is chosen. The χ 2 per degree of freedom for this fit is required to be less 81 than 5. This requirement also reduces the potential contamination from decay chains with The invariant mass distribution of the selected B + c → J/ψ K + K − π + candidates is shown in For B + c → J/ψ K + K − π + candidates, the resonant structures in the K − π + , K + K − ,

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K + K − π + , J/ψ K + K − , J/ψ K − π + and J/ψ K + systems are studied and the possible contri-107 butions from the decays B + c → B 0 K + and B + c → B + K − π + , followed by subsequent decays   subtract the estimated background contribution from the corresponding mass distributions.

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The results are shown in Fig. 2.

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The binned K − π + invariant mass distribution, presented in Fig. 2(a), is fitted with As the ratio of branching fractions is measured, many potential sources of systematic 124 uncertainty cancel in the ratio of efficiencies for the normalisation and signal decays.

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The overall efficiency for both decays is the product of the geometrical acceptance of 126 the detector, reconstruction, selection and trigger efficiencies. These are estimated using 127 simulation and the ratio of the efficiencies is found to be where the uncertaintty is statistical only. Systematic uncertainties that do not cancel in 129 this ratio are discussed below and summarised in Table 2.  uncertainty. The total uncertainty assigned to track reconstruction and selection is 4.2 %.

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The systematic uncertainty associated with kaon identification is studied using a kine- and includes effects related to pion identification criteria.

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The dependence of the B + c → J/ψ K + K − π + decay reconstruction and selection efficiency 154 on the decay model implemented in the simulation is estimated from a comparison of 155 the K + K − π + invariant mass distributions in data and simulation, which has the greatest 156 dependence on the decay model. This combined efficiency is recomputed after reweighting 157 the K + K − π + mass distribution to that observed in data. The relative difference of 2.5 % 158 observed is taken as the systematic uncertainty due to the decay model.

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Other systematic uncertainties are related to the widths of the K + K − π + and  Potential uncertainties related to the stability of the data taking conditions are tested 176 by studying the ratio of the yields of B + → J/ψ K + π + π − and B + → J/ψ K + decays for 177 different data taking periods. According to this study an additional systematic uncertainty The decay B + c → J/ψ K + K − π + is observed for the first time, and a signal yield of 78 ± 14 185 is reported. This analysis uses a data sample corresponding to an integrated luminosity 186 of 1 fb −1 at a centre-of-mass energy of 7 TeV and 2 fb −1 at 8 TeV. The significance, taking 187 into account the systematic uncertainties due to the fit function, peak position and mass 188 resolution in the default fit, is estimated to be 6.2 standard deviations.

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Using the B + c → J/ψ π + mode as a normalisation channel, the ratio of branching 190 fractions is calculated as where N is the number of reconstructed decays obtained from the fit described in Sect. 4.
The ratio of branching fractions is measured to be = 0.53 ± 0.10 ± 0.05, where the first uncertainty is statistical and the second systematic. The largest contri-194 bution to the B + c → J/ψ K + K − π + decay is found to be from B + c → J/ψ K * 0 K + decays.