CMS-BPH-20-004

CMS-BPH-20-004 ; CERN-EP-2021-020
Observation of a new excited beauty strange baryon decaying to $\Xi_{\mathrm{b}}^{-}\pi^{+}\pi^{-}$
CMS Collaboration
8 February 2021
Phys. Rev. Lett. 126 (2021) 252003
Abstract: The $\Xi_{\mathrm{b}}^{-}\pi^{+}\pi^{-}$ invariant mass spectrum is investigated with an event sample of proton-proton collisions at $\sqrt{s} =$ 13 TeV, collected by the CMS experiment at the LHC in 2016-2018 and corresponding to an integrated luminosity of 140 fb $^{-1}$ . The ground state $\Xi_{\mathrm{b}}^{-}$ is reconstructed via its decays to $\mathrm{J}/\psi \Xi^{-}$ and $\mathrm{J}/\psi \Lambda \mathrm{K}^{-}$ . A narrow resonance, labeled $\Xi_{\mathrm{b}}(6100)^{-}$ , is observed at a $\Xi_{\mathrm{b}}^{-}\pi^{+}\pi^{-}$ invariant mass of 6100.3 $\pm$ 0.2 (stat) $\pm$ 0.1 (syst) $\pm$ 0.6 ( $\Xi_{\mathrm{b}}^{-}$ ) MeV, where the last uncertainty reflects the precision of the $\Xi_{\mathrm{b}}^{-}$ baryon mass. The upper limit on the $\Xi_{\mathrm{b}}(6100)^{-}$ natural width is determined to be 1.9 MeV at 95% confidence level. Following analogies with the established excited $\Xi_{\mathrm{c}}$ baryon states, the new $\Xi_{\mathrm{b}}(6100)^{-}$ resonance and its decay sequence are consistent with the orbitally excited $\Xi_{\mathrm{b}}^{-}$ baryon, with spin and parity quantum numbers $J^P=$ 3/2 $^-$ .
Links: e-print arXiv:2102.04524 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ;

Figures	Summary	Additional Figures	References	CMS Publications

Figures
png pdf	Figure 1: The $\Xi_{\mathrm{b}}(6100)^{-} \to \Xi_{\mathrm{b}}^{-}\pi^{+}\pi^{-}$ decay topology, where the $\Xi_{\mathrm{b}}^{-}$ decays to $\mathrm{J}/\psi \Xi^{-}$ (left) or to $\mathrm{J}/\psi \Lambda \mathrm{K^{-}}$ (right). The numbers in blue are average decay lengths.
png pdf	Figure 1-a: The $\Xi_{\mathrm{b}}(6100)^{-} \to \Xi_{\mathrm{b}}^{-}\pi^{+}\pi^{-}$ decay topology, where the $\Xi_{\mathrm{b}}^{-}$ decays to $\mathrm{J}/\psi \Xi^{-}$ . $\mathrm{J}/\psi \Lambda \mathrm{K^{-}}$ . The numbers in blue are average decay lengths.
png pdf	Figure 1-b: The $\Xi_{\mathrm{b}}(6100)^{-} \to \Xi_{\mathrm{b}}^{-}\pi^{+}\pi^{-}$ decay topology, where the $\Xi_{\mathrm{b}}^{-}$ decays to $\mathrm{J}/\psi \Xi^{-}$ . $\mathrm{J}/\psi \Lambda \mathrm{K^{-}}$ . The numbers in blue are average decay lengths.
png pdf	Figure 2: Invariant mass distributions of the selected $\Xi_{\mathrm{b}}^{-}$ candidates in the ${\mathrm{J}/\psi \Xi^{-}}$ (left) and ${\mathrm{J}/\psi \Lambda \mathrm{K^{-}}}$ (right) decay channels with the fit results superimposed. The vertical solid (dashed) lines show the mass windows discussed in the text and used in the reconstruction of the ${\Xi_{\mathrm{b}}^{-} \pi^{+} \pi^{-}}$ candidates in ${\mathrm{J}/\psi \Xi^{-}}$ and ${\mathrm{J}/\psi \Lambda \mathrm{K^{-}}}$ ( $\mathrm{J}/\psi \Sigma^{0} \mathrm{K^{-}}$ ) channels.
png pdf	Figure 2-a: Invariant mass distribution of the selected $\Xi_{\mathrm{b}}^{-}$ candidates in the ${\mathrm{J}/\psi \Xi^{-}}$ ${\mathrm{J}/\psi \Lambda \mathrm{K^{-}}}$ decay channel with the fit results superimposed. The vertical solid line shows the mass window discussed in the text and used in the reconstruction of the ${\Xi_{\mathrm{b}}^{-} \pi^{+} \pi^{-}}$ candidates in the ${\mathrm{J}/\psi \Xi^{-}}$ channel.
png pdf	Figure 2-b: Invariant mass distribution of the selected $\Xi_{\mathrm{b}}^{-}$ candidates in the ${\mathrm{J}/\psi \Lambda \mathrm{K^{-}}}$ decay channel with the fit results superimposed. The vertical solid (dashed) lines show the mass windows discussed in the text and used in the reconstruction of the ${\Xi_{\mathrm{b}}^{-} \pi^{+} \pi^{-}}$ candidates in the ${\mathrm{J}/\psi \Lambda \mathrm{K^{-}}}$ ( $\mathrm{J}/\psi \Sigma^{0} \mathrm{K^{-}}$ ) channel.
png pdf	Figure 3: Distributions of the invariant mass difference ${\Delta M}$ for the selected ${\Xi_{\mathrm{b}}^{-} \pi^{+} \pi^{-}}$ candidates, with the $\Xi_{\mathrm{b}}^{-}$ reconstructed in the ${\mathrm{J}/\psi \Xi^{-}}$ and ${\mathrm{J}/\psi \Lambda \mathrm{K^{-}}}$ channels (left) or partially reconstructed in the $\mathrm{J}/\psi \Sigma^{0} \mathrm{K^{-}}$ channel (right). The result of the simultaneous fit is also shown.
png pdf	Figure 3-a: Distribution of the invariant mass difference ${\Delta M}$ for the selected ${\Xi_{\mathrm{b}}^{-} \pi^{+} \pi^{-}}$ candidates, with the $\Xi_{\mathrm{b}}^{-}$ reconstructed in the ${\mathrm{J}/\psi \Xi^{-}}$ and ${\mathrm{J}/\psi \Lambda \mathrm{K^{-}}}$ channels. The result of the simultaneous fit is also shown.
png pdf	Figure 3-b: Distribution of the invariant mass difference ${\Delta M}$ for the selected ${\Xi_{\mathrm{b}}^{-} \pi^{+} \pi^{-}}$ candidates, with the $\Xi_{\mathrm{b}}^{-}$ partially reconstructed in the $\mathrm{J}/\psi \Sigma^{0} \mathrm{K^{-}}$ channel. The result of the simultaneous fit is also shown.

Summary

In summary, we report the observation of a new excited beauty strange baryon, decaying to

$\Xi_{\mathrm{b}}^{-}\pi^{+}\pi^{-}$ . The analysis uses proton-proton collision data collected by the CMS experiment at

$\sqrt{s} =$ 13 TeV, corresponding to an integrated luminosity of 140 fb

$^{-1}$ . The measured mass difference of this state is

$M(\Xi_{\mathrm{b}}(6100)^{-}) - M(\Xi_{\mathrm{b}}^{-}) - 2\,m^{\text{PDG}}_{\pi^{\pm}} =$ 24.14

$\pm$ 0.22 (stat)

$\pm$ 0.05 (syst) MeV. The known

$\Xi_{\mathrm{b}}^{-}$ mass of 5797.0

$\pm$ 0.6 MeV [20] is used to obtain

$M(\Xi_{\mathrm{b}}(6100)^{-}) =$ 6100.3

$\pm$ 0.2 (stat)

$\pm$ 0.1 (syst)

$\pm$ 0.6 (

$\Xi_{\mathrm{b}}^{-}$ ) MeV. It is particularly remarkable that if the

$\Xi_{\mathrm{b}}(6100)^{-}$ baryon were only 13 MeV heavier, it would be above the

$\Lambda_{\mathrm{b}}^{0}\mathrm{K}^{-}$ mass threshold and could decay to this final state. The natural width of this resonance is compatible with zero and a 95% confidence level upper limit of 1.9 MeV has been determined.

Following analogies with the established excited

$\Xi_{\mathrm{c}}$ baryon states [20], and considering several theoretical predictions [12, 13, 21], the new

$\Xi_{\mathrm{b}}(6100)^{-}$ resonance and its decay sequence are consistent with the orbitally excited

$\Xi_{\mathrm{b}}^{-}$ baryon, with the light diquark spin

$j_{\mathrm{qs}}=$ 1 and

$J^P=$ 3/2

$^-$ . This suggests that it is the beauty analogue of the

$\Xi_{\mathrm{c}}(2815)$ baryon [41]. The observation of this baryon and the measurement of its properties provide information that should help to distinguish between different theoretical models used to calculate the properties of the excited

$\Xi_{\mathrm{b}}$ states.

Additional Figures
png pdf	Additional Figure 1: Quark spin configurations for the lightest $\Xi_{\mathrm{b}}$ isodoublets, q corresponds to up and down quarks for $\Xi_{\mathrm{b}}^{0}$ and $\Xi_{\mathrm{b}}^{-}$ , respectively. Here $j_{\mathrm{q} \mathrm{s}}$ denotes the light diquark spin, $J^P$ denotes the total spin-parity of $\Xi_{\mathrm{b}}$ baryon and $L$ denotes the orbital angular momentum between the light diquark and the b quark.
png pdf	Additional Figure 2: $\Xi_{\mathrm{b}} (6100)^-\to \Xi_{\mathrm{b}}^{-} \pi^{+} \pi^{-}$ decay topology, where the $\Xi_{\mathrm{b}}^{-}$ decays to $\mathrm{J}/\psi \Xi^{-}$ (top) or to $\mathrm{J}/\psi \Lambda \mathrm{K^{-}}$ (bottom). The numbers in blue are average decay lengths, calculated as $c\tau$ .
png pdf	Additional Figure 2-a: $\Xi_{\mathrm{b}} (6100)^-\to \Xi_{\mathrm{b}}^{-} \pi^{+} \pi^{-}$ decay topology, where the $\Xi_{\mathrm{b}}^{-}$ decays to $\mathrm{J}/\psi \Xi^{-}$ . The numbers in blue are average decay lengths, calculated as $c\tau$ .
png pdf	Additional Figure 2-b: $\Xi_{\mathrm{b}} (6100)^-\to \Xi_{\mathrm{b}}^{-} \pi^{+} \pi^{-}$ decay topology, where the $\Xi_{\mathrm{b}}^{-}$ decays to $\mathrm{J}/\psi \Lambda \mathrm{K^{-}}$ . The numbers in blue are average decay lengths, calculated as $c\tau$ .
png pdf	Additional Figure 3: Invariant mass distribution of the selected $\Xi_{\mathrm{b}}^{-} \pi \pi$ candidates with no requirements on the $\Xi_{\mathrm{b}}^{-} \pi^{+}$ mass, for the OS (circles) and SS (band) events. The $\Xi_{\mathrm{b}}^{-}$ ground state is fully reconstructed in the $\mathrm{J}/\psi \Xi^{-}$ and $\mathrm{J}/\psi \Lambda \mathrm{K^{-}}$ channels (left) or partially reconstructed in the $\mathrm{J}/\psi \Sigma^{0} \mathrm{K^{-}}$ channel (right). The vertical lines show the masses of the new $\Xi_{\mathrm{b}} (6100)^-$ baryon and of the $\Xi_{\mathrm{b}} (6227)^-$ state, observed by the LHCb Collaboration in the $\Lambda_{\mathrm{b}}^{0} \mathrm{K^{-}}$ and $\Xi_{\mathrm{b}}^{0} \pi^{-}$ decay channels [22].
png pdf	Additional Figure 3-a: Invariant mass distribution of the selected $\Xi_{\mathrm{b}}^{-} \pi \pi$ candidates with no requirements on the $\Xi_{\mathrm{b}}^{-} \pi^{+}$ mass, for the OS (circles) and SS (band) events. The $\Xi_{\mathrm{b}}^{-}$ ground state is fully reconstructed in the $\mathrm{J}/\psi \Xi^{-}$ and $\mathrm{J}/\psi \Lambda \mathrm{K^{-}}$ channels. The vertical lines show the masses of the new $\Xi_{\mathrm{b}} (6100)^-$ baryon and of the $\Xi_{\mathrm{b}} (6227)^-$ state, observed by the LHCb Collaboration in the $\Lambda_{\mathrm{b}}^{0} \mathrm{K^{-}}$ and $\Xi_{\mathrm{b}}^{0} \pi^{-}$ decay channels [22].
png pdf	Additional Figure 3-b: Invariant mass distribution of the selected $\Xi_{\mathrm{b}}^{-} \pi \pi$ candidates with no requirements on the $\Xi_{\mathrm{b}}^{-} \pi^{+}$ mass, for the OS (circles) and SS (band) events. The $\Xi_{\mathrm{b}}^{-}$ ground state is partially reconstructed in the $\mathrm{J}/\psi \Sigma^{0} \mathrm{K^{-}}$ channel. The vertical lines show the masses of the new $\Xi_{\mathrm{b}} (6100)^-$ baryon and of the $\Xi_{\mathrm{b}} (6227)^-$ state, observed by the LHCb Collaboration in the $\Lambda_{\mathrm{b}}^{0} \mathrm{K^{-}}$ and $\Xi_{\mathrm{b}}^{0} \pi^{-}$ decay channels [22].

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