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Measurement of $b$-hadron masses

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Abstract

Measurements of b-hadron masses are performed with the exclusive decay modes B+ -> J/psi K+, B0 -> J/psi K*0, B0 -> J/psi K0S, B0_s -> J/psi phi and Lambda_b -> J/psi Lambda using an integrated luminosity of 35 pb-1 collected in pp collisions at a centre-of-mass energy of 7 TeV by the LHCb experiment. The momentum scale is calibrated with J/psi -> mu+mu- decays and verified to be known to a relative precision of 2 x 10^-4 using other two-body decays. The results are more precise than previous measurements, particularly in the case of the B0_s and Lambda_b masses.

Figures and captions

Reconstructed $J/\psi\rightarrow \mu^+\mu^-$ fitted mass as a function of run number after the momentum calibration procedure discussed in the text. The vertical dashed lines indicate the boundaries of the seven calibration periods. A fit of a constant function (horizontal line) has a $\chi^2$ probability of $6\%$. The shaded area corresponds to the assigned uncertainty on the momentum scale of $0.02\%$.

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Momentum scale bias $\alpha$, extracted from the reconstructed mass of various two-body decays after the momentum calibration procedure described in the text. By construction one expects $\alpha=0$ for the $J/\psi\rightarrow \mu^+\mu^-$ calibration mode. The black error bars represent the statistical uncertainty whilst the (yellow) shaded areas include contributions to the systematic error from the fitting procedure, the effect of QED radiative corrections and the uncertainty quoted by the PDG [3] on the mass of the decaying meson. The (red) dashed lines correspond to the assigned uncertainty on the momentum scale of $0.02\%$.

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Invariant mass distributions for (a) $B^+\rightarrow J/\psi K^+$, (b) $B^0\rightarrow J/\psi K^{*0}$, (c) $B^0\rightarrow J/\psi K^0_{\rm S}$, (d) $ \Lambda^0_b\rightarrow J/\psi\Lambda$, and (e) $B_s^0\rightarrow J/\psi\phi$ candidates. In each case the result of the fit described in the text is superimposed (solid line) together with the background component (dotted line).

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Animated gif made out of all figures.

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Tables and captions

Signal yields, mass values and mass resolutions obtained from the fits shown in Fig. 3 together with the values corrected for the effect of QED radiative corrections as described in the text. The quoted uncertainties are statistical.

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Systematic uncertainties (in $\rm\,MeV\!/\!{\it c}^2$ ) on the mass measurements.

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LHCb measurements, compared to both the best previous measurements and the results of a global fit to available $b$-hadron mass data [3]. The quoted errors include statistical and systematic uncertainties. All values are in $ {\mathrm{\,MeV\!/}c^2}$ .

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Created on 09 December 2018.Citation count from INSPIRE on 09 December 2018.