Observation of Ξb0 Baryon
 Figure 1 : Spin 1/2 baryons consisting of u, d, s, b quarks. |
 Figure 2 : Decay mode of Ξb0. |
 Figure 3 : Invariant mass distribution of Ξc+ reconstructed from Ξ−π+π+. |
 Figure 4 : Invariant mass distribution of Ξb0 reconstructed from Ξc+ and π−. |
 Figure 5 : Plot of a gausisan fit of the invariant mass distribution of Ξb0 in Figure 4. |
So far, in the CDF experiment in which the Osaka City University High Energy Physics Laboratory participates, we have reported the discovery of new baryons containing b quarks: Σb±, Σb*± (High Energy Physics Laboratory News, October 26, 2006), and Ξb− (High Energy Physics Laboratory News, June 18, 2007). We are pleased to announce the discovery of a new particle, Ξb0. The Ξb0 is a baryon with a spin of 1/2 and an internal structure composed of the quarks (u, s, b). Unlike the previously discovered Ξb−, it contains a u quark instead of a d quark. Figure 1 classifies spin 1/2 baryons composed of u, d, s, and b quarks based on the number of b quarks. The Ξb0 falls into the category with one b quark. Incidentally, baryons containing two b quarks have not yet been discovered.
The data analysis for this discovery was conducted with a statistical amount corresponding to 4.2 fb−1 of proton-antiproton collisions at a center-of-mass energy of 1.96 TeV. The Ξb0 was identified by reconstructing the decay Ξb0 → Ξc+π−, Ξc+ → Ξ−π+π+. The Ξ− further decays into Λπ−, and the Λ decays into pπ−. Unlike the previous discovery of Ξb−, where the final state included a μ+μ− pair from J/ψ, serving as a marker for the reaction, the final state of the Ξb0 consists entirely of pions or protons, making it difficult to identify without clear markers. However, thanks to the high-performance silicon vertex detector (SVX), we achieved our goal. Figure 2 shows the decay mode of the Ξb0 and the particle detection in the SVX.
Figure 3 shows the invariant mass distribution reconstructed from Ξ− and two π+. A clear peak of Ξc− can be seen at 2.47 GeV/c2. Figure 4 shows the results obtained by reconstructing Ξc+ and π−, with a significant peak observed near 5.8 GeV/c2. Based on the peak width of Ξc− in Figure 3, the mass precision of Ξb0 is estimated to be 20 MeV/c2. Fitting the peak in Figure 4 using this value results in Figure 5, yielding a Ξb0 signal count of 25.3 +5.6−5.4. Statistically processing this data, the probability of the same peak arising from statistical fluctuations in the background events is calculated to be 3.6×10−12, corresponding to 6.8σ. This indicates that the discovery of Ξb0 is almost certain. The mass of Ξb0 has been measured as:
m(Ξb0) = 5787.8±5.0(stat)±1.3(syst) MeV/c2 .
In the CDF experiment, we currently have about twice the amount of data used in this analysis, so there may be further discoveries of new particles in future analyses.
- Press release of Fermilab
- arXiv preprint “Obvervation of the Ξb0 Baryon”