Scholars Gain New Data on Heavy Exotic Hadrons
As part of the Belle experiment, researchers were able to measure the energy dependence of e+e- -> B-anti-B, B-anti-B* and B*-anti-B* reactions in the 10.63 GeV to 11.02 GeV energy range for the first time. The new data will help clarify the nature of the group of exotic Upsilon mesons that have mass in this range. The results of the study were published in the Journal of High Energy Physics. Over 400 researchers have participated in the Belle experiment, including staff members of the HSE International Laboratory of Elementary Particle Physics.
The first Upsilon meson, consisting of b- and anti-b-quarks, was discovered in 1977. Its discovery also coincided with that of the heaviest b-quark, for which the authors of the study—American researchers Leon M. Lederman, Melvin Schwartz and Jack Steinberger—received a Nobel Prize in 1988.
In 2008, researchers discovered that high-energy Upsilon meson states have anomalous properties. The reason for these unexpected properties remains an unanswered question in hadron physics. Theoretical studies agree that there are additional degrees of freedom within these states: a pair comprising a lightweight quark and an anti-quark, or a valent gluon. Such multi-particle bound states are called ‘exotic hadrons’, and until recently, they had not been found. Additional experimental data was needed to distinguish various models of highly excited Upsilon meson structures. This data was collected during the Belle experiment in 2010 and serves as the foundation for the study published in the Journal of High Energy Physics.
The Belle detector was located at the collision point of KEK-B, an electron-positron collider in Japan, with the total beam energy close to 10 GeV. The detector collected data from 1999 to 2010. The main goal of the experiment was to study the properties of B+ and B0 mesons, consisting of a heavy anti-b-quark and one light u- or d-quark. A large number of rare decays of these particles was detected, and differences in the properties of particles and anti-particles (B+ and B-, B0 and anti-B0) were studied in order to uncover the mechanism behind the matter/anti-matter asymmetry of the contemporary universe.
Quarks are elementary particles that form composite particles: baryons and mesons. Quarks have fractional electric charge values and are categorized in pairs in three generations in order of increasing mass: up and down, charm and strange, top and bottom.
A collision between an electron and a positron creates many particles which, in turn, interact or decay. In order to understand the reactions that occur after a particle collision, modern experiments consist of several layers of various detectors. Certain particles are registered and studied in these layers.
The Belle experiment used a silicon detector to determine the interaction point, a drift chamber to track the charged particles, and a caesium iodide-based counter for the photons.
Based on the scan data, the cross-section energy dependence for many reactions was measured. For example, a new heavy Upsilon(10750) meson was observed in the cross-sections of weakly excited Upsilon mesons produced together with a pair of pi+ and pi- mesons. Until recently, only the reactions that make a relatively small contribution to a total cross section were studied. Finally, the cross sections e+e- -> B-anti-B, B-anti-B* and B*-anti-B*, which make the largest contribution to a total cross section, were measured for the first time. This gave the researchers the first full set of data on exotic Upsilon mesons, which allowed several theoretical teams to start working to explain the measurements.
Roman Mizuk, Leading Research Fellow at the International Laboratory of Elementary Particle Physics
The data obtained includes some unexpected discoveries. The cross sections e+e- -> B-anti-B, B-anti-B* and B*-anti-B* demonstrate a complicated dependence on energy, which might provide some important information on the wave functions of exotic meson states. In the future, joint analysis of data on the energy dependence of the cross sections may help to shed light on the question of the structure of Upsilon mesons’ highly excited states.
Roman Mizuk
Leading Research Fellow, International Laboratory of Elementary Particle Physics
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