On February 7th, a research paper entitledInclusive approach to hunt for the beauty-charmed baryonsΞbcwas published inPhysical Review D (Letter)by Prof. Qin Qin from School of Physics, Huazhong University of Science and Technology (HUST), and collaborators, including his PhD student Yang Guohe, Dr. Shi Yuji from Bonn University, Prof. Wang Wei from Shanghai Jiao Tong University, Prof. Yu Fusheng from Lanzhou University and Prof. Zhu Ruilin from Nanjing Normal University.
While the Large Hadron Collider, the giant facilitylocated on the Swiss-French border,is best known for discovering the God particle-the Higgs boson at two of its detectors ATLAS and CMS, another detector LHCb has been a prominent hunter of new hadrons, e.g., the first pentaquarkPc and the firstdoubly charmed baryonXcc. These new hadrons are gradually filling in the period table of hadrons as shown in Fig. 1, a more fundamental version of the period table of chemical elements. Especially,Pc opened a new period andXcc opened a new main group, respectively.Thefirst charming beautyXbc is one of the next primary targets of LHCb, which will again open a new main group of theperiod table of hadrons.
TheXbc baryon contains two different heavy quarks, beauty and charm, and a light quark. It resembles a ‘double-star’ core surrounded by a light ‘planet’, forming a distinctive internal structure from all established hadrons. Such a unique structure will provide us with new points of view to decipher the strong interaction. In the past few years, experimentalists have made abundant efforts to search for the beauty-charmed baryons but not succeeded yet. Traditional exclusive searches forXbc are faced with a major difficulty: TheXbc baryon contains a beauty quark, which decays with an extremely low probability into any particular exclusive final state, so all exclusive searches have very low reconstruction efficiencies. To overcome this difficulty, Qin and collaborators proposed to huntXbc via an inclusive decay channel,Xbc→Xcc + X with X representing all possible particles, as shown in Fig. 2. The most advantageous feature of this process is thatXbc will fly a certain distance before decaying intoXcc, thus forming aXcc displaced from the primary collision point. BecauseXbc is almost the only source of this phenomenon, observation of displacedXcc would indicate the discovery ofXbc. This approach is similar to the discovery of Neptune via Uranus perturbation in astronomical observation.
Based on effective theories of quantum chromodynamics and the heavy quark symmetry, Qin and collaborators further developed the heavy diquark effective theory, and hereby calculate theXbc→Xcc + X decay probability theoretically. It turns out that the decay branching ratio is approximately 10%, which is larger than any of the exclusive decay processes by orders of magnitude. Taking into account the production rate ofXbc and the detection efficiency ofXcc, it is evaluated that the LHC will be able to collect approximately 300Xbc baryons through this inclusive decay process when it completed its 3rdrunning phase. Therefore, it is highly hopeful thatXbc will be discovered via this inclusive approach, which will then be another milestone in the field of new hadron spectrum.
Fig. 1.The period table of hadrons.
Fig. 2.Sketch ofXbcproduction and decay at the LHC.
