News

Wednesday July 20th, 2011

Observation of Ξb0 Baryon

BaryonsIllstr

Figure 1 : Spin 1/2 baryons consisting of u, d, s, b quarks.

Xi-sub-b-decay-chain

Figure 2 : Decay mode of Ξb0.

Xic+

Figure 3 : Invariant mass distribution of Ξc+ reconstructed from Ξπ+π+.

Xib_0_hist

Figure 4 : Invariant mass distribution of  Ξb0 reconstructed from Ξc+ and π.

Xib_0_fit

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+ → Ξπ+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.