Discovery of Ξb− Baryon
 Figure 1 : Schematic illustration of the internal structure of Ξb−. |
 Figure 2 : Decay mode of Ξb−. |
 Figure 3 : Invariant mass distribution of (a)Ξ− → Λ π− and (b)J/ψ → μ+μ−. |
 Figure 4 : Invariant mass distribution of Ξb− → J/ψ + Ξ−. |
In a previous issue of OCU HEP Lab. news, we reported the discovery of a new baryon Σb± containing a b quark. This time, another new baryon called Ξb− has been discovered in the CDF experiment, which Osaka City University also participates in. As shown in Figure 1, this baryon (Ξb−) is composed of three quarks (d, s, b) bound together by strong interactions. The Greek letter “Ξ” is used to name baryons with isospin 1/2 that contain an s quark, hence the name “Ξb−” for this particle in the sense of d + s + b. For reference, (u, s, s) is Ξ0, (d, s, s) is Ξ−, (u, s, c) is Ξc+, and (d, s, c) is Ξc0 .
This time, the discovery of Ξb− was made by reconstructing a series of decay tracks: Ξb− → J/ψ + Ξ−, J/ψ → μ+ μ−, Ξ− → Λ π−, and Λ → pπ−. This decay process is illustrated in Figure 2. A magnetic field is applied in the detector, causing charged particles to bend due to the Lorentz force. The direction of the curvature determines the charge of the particles. By kinematically combining Λ reconstructed from p and π− with another π−, Figure 3(a) shows a clear signal of Ξ− with a mass of 1321 MeV/c2. Additionally, Figure 3(b) shows the J/ψ meson, reconstructed from two oppositely charged μ particles, with a mass of 3096 MeV/c2. Collecting event samples containing both Ξ− and J/ψ, and kinematically reconstructing them yields Figure 4, which shows a sharp peak around 5.8 GeV/c2 indicating the presence of Ξb− baryon. There are 18 events between 5.75 GeV/c2 and 5.85 GeV/c2. Fitting these with a Gaussian function using statistical methods provides a mass of
m(Ξb−) = 5792.9 ± 2.4(stat.) ± 1.7(syst.) MeV/c2 .
This value is consistent with theoretical predictions within the error. To further confirm this discovery, the CDF experiment team calculated the probability that the peak in Figure 4 could occur by chance due to statistical fluctuations in an otherwise flat distribution. The probability was found to be an extremely small 4.1×10−15, solidifying the existence of the Ξb− baryon. The discovery of a new baryon is expected to provide crucial insights into the binding mechanisms between quarks.