CMS-BPH-19-003

CMS-BPH-19-003 ; CERN-EP-2019-288
Study of excited $\Lambda_{\mathrm{b}}^{0}$ states decaying to $\Lambda_{\mathrm{b}}^{0}\pi^{+}\pi^{-}$ in proton-proton collisions at $\sqrt{s} = $ 13 TeV
CMS Collaboration
18 January 2020
Phys. Lett. B 803 (2020) 135345
Abstract: A study of excited $\Lambda_{\mathrm{b}}^{0}$ baryons is reported, based on a data sample collected in 2016-2018 with the CMS detector at the LHC in proton-proton collisions at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of up to 140 fb$^{-1}$. The existence of four excited $\Lambda_{\mathrm{b}}^{0}$ states: $\Lambda_{\mathrm{b}}(5612)^{0}$, $\Lambda_{\mathrm{b}}(5620)^{0}$, $\Lambda_{\mathrm{b}}(6146)^{0}$, and $\Lambda_{\mathrm{b}}(6152)^{0}$ in the $\Lambda_{\mathrm{b}}^{0}\pi^{+}\pi^{-}$ mass spectrum is confirmed, and their masses are measured. The $\Lambda_{\mathrm{b}}^{0}\pi^{+}\pi^{-}$ mass distribution exhibits a broad excess of events in the region of 6040-6100 MeV, whose origin cannot be discerned with the present data.
Links: e-print arXiv:2001.06533 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Invariant mass distribution of the selected ${\Lambda_{\mathrm{b}}^{0} {\pi^{+} \pi^{-}}}$ candidates near threshold. The vertical bars on the data points display the statistical uncertainties in the data. The overall fit result is shown by the thick solid line, with the thin and dashed lines representing the signal and combinatorial background components, respectively.
png pdf	Figure 2: Invariant mass distribution of the selected ${\Lambda_{\mathrm{b}}^{0} {\pi^{+} \pi^{-}}}$ candidates in the high-mass region. The vertical bars on the data points represent the statistical uncertainties in the data. The overall fit result is shown by the thick solid line. The thin lines present the contributions from the two signal peaks and the broad enhancement. The dashed line displays the combinatorial background.
png pdf	Figure 3: Comparison of the invariant mass distribution of the selected ${\Lambda_{\mathrm{b}}^{0} {\pi^{+} \pi^{-}}} $ candidates in the SS background (band) and OS signal (points) channels. The vertical bars on the OS data points and the width of the bands for the SS distribution indicate the statistical uncertainties only.
png pdf	Figure 4: Two-dimensional distribution of the ${\Lambda_{\mathrm{b}}^{0} {\pi^{+} \pi^{-}}} $ mass versus the ${\Lambda_{\mathrm{b}}^{0} {\pi^{+} }} $ mass (left) and ${\Lambda_{\mathrm{b}}^{0} {\pi^{-} }} $ mass (right), in the range $ {m_{\Lambda_{\mathrm{b}}^{0} {\pi^{+} \pi^{-}}}} < $ 6100 MeV. The scale on the right of each plot gives the number of candidates per 8 MeV $\times$ 12.5 MeV bin.
png pdf	Figure 4-a: Two-dimensional distribution of the ${\Lambda_{\mathrm{b}}^{0} {\pi^{+} \pi^{-}}} $ mass versus the ${\Lambda_{\mathrm{b}}^{0} {\pi^{+} }} $ mass. in the range $ {m_{\Lambda_{\mathrm{b}}^{0} {\pi^{+} \pi^{-}}}} < $ 6100 MeV. The scale on the right of the plot gives the number of candidates per 8 MeV $\times$ 12.5 MeV bin.
png pdf	Figure 4-b: Two-dimensional distribution of the ${\Lambda_{\mathrm{b}}^{0} {\pi^{+} \pi^{-}}} $ mass versus the ${\Lambda_{\mathrm{b}}^{0} {\pi^{-} }} $ mass. in the range $ {m_{\Lambda_{\mathrm{b}}^{0} {\pi^{+} \pi^{-}}}} < $ 6100 MeV. The scale on the right of the plot gives the number of candidates per 8 MeV $\times$ 12.5 MeV bin.
png pdf	Figure 5: The distribution of ${m_{{\Lambda_{\mathrm{b}}^{0} {\pi^{+} \pi^{-}}}}}$ for OS (points) and SS (band) candidates after the possible $\Sigma_{\mathrm{b}}^{\pm}$ and $\Sigma_{\mathrm{b}}^{*\pm}$ baryon contributions in the $\Lambda_{\mathrm{b}}^{0} {\pi^{\pm} } $ mass spectrum are vetoed. The vertical bars on the OS data points and the width of the band for the SS distribution indicate the statistical uncertainties only.

Tables
png pdf	Table 1: Systematic uncertainties (in MeV) in the measured masses. A dash means that the corresponding uncertainty is negligible, and "N/A" means that it does not apply.

Summary

In summary, using the pp collision data recorded with the CMS detector at $\sqrt{s} = $ 13 TeV, corresponding to an integrated luminosity of up to 140 fb$^{-1}$, the existence of the $\Lambda_{\mathrm{b}}(5612)^{0}$ and $\Lambda_{\mathrm{b}}(5620)^{0}$ baryons is confirmed. Their masses, with respect to the $\Lambda_{\mathrm{b}}^{0}$ mass, are measured to be 292.72 $\pm$ 0.12 $\pm$ 0.01 MeV and 300.56 $\pm$ 0.07 $\pm$ 0.01 MeV, respectively, where the first uncertainty is statistical and the second is systematic. By adding the known $\Lambda_{\mathrm{b}}^{0}$ mass of 5619.60 $\pm$ 0.17 MeV [19], we report the mass measurements
M($\Lambda_{\mathrm{b}}(5912)^{0}$) = 5912.32 $\pm$ 0.12 $\pm$ 0.01 $\pm$ 0.17 MeV,M($\Lambda_{\mathrm{b}}(5920)^{0}$) = 5920.16 $\pm$ 0.07 $\pm$ 0.01 $\pm$ 0.17 MeV,where the third uncertainty is the uncertainty in the world-average $\Lambda_{\mathrm{b}}^{0}$ mass. The obtained values are consistent with the world-average values and have similar precision. In addition, the $\Lambda_{\mathrm{b}}^{0}\pi^{+}\pi^{-}$ invariant mass spectrum is investigated in the mass range up to 6400 MeV. A narrow peak is observed with a mass close to 6150 MeV, with a significance over 5 standard deviations, consistent with the superposition of the $\Lambda_{\mathrm{b}}(6146)^{0}$ and $\Lambda_{\mathrm{b}}(6152)^{0}$ baryons recently observed by the LHCb Collaboration [22]. Masses of these states are measured to be
M($\Lambda_{\mathrm{b}}(6146)^{0}$) = 6146.5 $\pm$ 1.9 $\pm$ 0.8 $\pm$ 0.2 MeV,M($\Lambda_{\mathrm{b}}(6152)^{0}$) = 6152.7 $\pm$ 1.1 $\pm$ 0.4 $\pm$ 0.2 MeV,where the first uncertainty is statistical, the second is systematic, and the third is the uncertainty in the world-average $\Lambda_{\mathrm{b}}^{0}$ mass value. The corresponding mass differences with respect to the $\Lambda_{\mathrm{b}}^{0}$ mass are
M($\Lambda_{\mathrm{b}}(6146)^{0}$) $-$ M($\Lambda_{\mathrm{b}}^{0}$) = 526.9 $\pm$ 1.9 $\pm$ 0.8 MeV,M($\Lambda_{\mathrm{b}}(6152)^{0}$) $-$ M($\Lambda_{\mathrm{b}}^{0}$) = 533.1 $\pm$ 1.1 $\pm$ 0.4 MeV.These measurements are not as precise as, but are in good agreement with the LHCb results [22].
In addition, a broad excess of events is observed in the region 6040-6100 MeV, not present in the same-sign $\Lambda_{\mathrm{b}}^{0}\pi^{+}\pi^{-}$ distribution. If it is fit with a single Breit-Wigner function, the returned mass and width are 6073 $\pm$ 5 (stat) MeV and 55 $\pm$ 11 (stat) MeV. However, it is not excluded that this enhancement is an overlap of more than one state with close masses or is created by the partially reconstructed decays of higher-mass states. More data are needed to elucidate the nature of this excess.

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