Discovery of a New Baryon Containing a b quark
 Figure 1 : Generation and species of quarks. |
 Figure 2 : Invariant mass distribution of Λb0 and π．The left peak corresponds to Σb± , and the right peak corresponds to Σb*±. |
 Figure 3 : Series of baryon states with J=3/2 consisting of light quarks (u, d, s) and a b quark. |
In the CDF experiment, in which the High Energy Physics Laboratory of Osaka City University participates, a new baryon containing a b quark, Σb± (J=1/2), and its spin-excited state Σb*± (J=3/2) were discovered. A “baryon” is a particle consisting of three quarks, and examples include protons and neutrons, which make up the nuclei at the center of atoms around us. Figure 1 shows the types of quarks currently known, arranged by generation, with mass increasing with generation. While protons consist of (u, u, d) and neutrons of (u, d, d), the newly discovered Σb+ is composed of (u, u, b) and Σb− of (d, d, b). Due to the heavy mass of the b quark, the production rate is low, requiring high energy and high statistics for production, which is why it had not been discovered in other experiments until now at CDF. Research on QCD, which describes the interactions between quarks, predicted that Σb(*)± would decay into Λc0 and π±. Therefore, the data analysis involved reconstructing the sequential decay Λb0→ Λc+π−, Λc+→ pK−π+, and combining these with the tracks of separate π± to calculate the invariant mass. Figure 2 shows the reconstructed invariant mass distribution. Two peaks indicating the presence of particles are indeed confirmed and identified as Σb± (left) and Σb*± (right) based on comparison with theoretical mass values. The measured masses are as follows:
| m(Σb−) | = | 5816 | +1.0 | (stat) | ± | 1.7(syst) | MeV/c2 |
| −1.0 |
| m(Σb+) | = | 5808 | +2.0 | (stat) | ± | 1.7(syst) | MeV/c2 |
| −2.3 |
| m(Σb*−) | = | 5837 | +2.1 | (stat) | ± | 1.7(syst) | MeV/c2 |
| −1.9 |
| m(Σb*+) | = | 5829 | +1.6 | (stat) | ± | 1.7(syst) | MeV/c2 . |
| −1.8 |
Figure 3 shows the theoretically possible series of baryon states (three-quark states) with spin J=3/2 made of four types of quarks: u, d, s, and b. From past experiments, all baryons composed of the three lighter quarks (u, d, s) have been discovered. The recent discovery by CDF is the first confirmation of a J=3/2 baryon containing one b quark. Theoretically, four more similar particles are predicted, and although not shown in Figure 3, baryons containing c quarks are also expected to exist. However, the top (t) quark, discovered at Fermilab in 1995, is considered unable to be a constituent of baryons due to its extremely short lifetime before decay.