According to modern physics, our universe is believed to have originated from a 'big bang', during which an exactly equal amount of matter and anti-matter was produced. However, various of experimental evidences indicate that the current universe is dominated by matter rather than anti-matter. In order to explain the asymmetry, Sakharov proposed the famous Three Rules in 1967, pointing out that the baryon number (B) violation plays a key role in resolving this puzzle. To this day, extensive research has been conducted on proton decay and the decay of various hadrons that violate baryon number, yet no positive signals have been found. Nevertheless, the commonality of these studies is that the difference between the baryon number and lepton number (L) in the initial and final states remains unchanged. Therefore, exploring new types of baryon number violating phenomena has become increasingly imperative.

Professor Zhao Minggang from Nankai University, in collaboration with Dr. Hao Xiqing from Henan Normal University, present the first search of a process, in which the baryon number in the initial and final states changes, i.e., in  decay with the  possibly oscillates to Λ, based on 1.3 billion  events collected at the BESIII detector. Such oscillations, invoking a quark of second generation, offer a crucial testing ground to explore the theories related to B, L, and B-L conservations, which extend the studies in the neutron-anti-neutron oscillation experiments. In the analysis, the  final state before oscillating is defined as `right sign` (RS), and the  final state after oscillating is defined as `wrong sign` (WS). As is shown in the following figure, the background of the RS distribution is extremely low and the signal is very clean, while in the WS distribution, no events fall in the signal region, indicating no oscillation signal. Taking into account information such as background estimation, detection efficiency, and system uncertainty, the authors estimate the first upper limit of  oscillations as , leading to the upper limit of the oscillation parameter  GeV. This result expands the field of baryon-anti-baryon oscillation experiments and will inspire further theoretical and experimental researches. 

Figure: Distribution of  for  (a) WS events in the signal region and (inset) over the full span, where the filled circle with error bar is from data, the pink filled histogram, normalized arbitrarily, stems from simulated WS signal events, and the arrows in the inset figure show the edges of the signal region; (b) RS events from data, where the filled circles with error bars are from data, the blue solid line represents the result of the fit and the dashed line shows the background contribution.

Recently, the paper summarizing the relevant research results entitled with Search for  Baryon-Number-Violating Oscillations in the Decay +c.c. was published online in Physical Review Letters.

Paper link:

URL: https://link.aps.org/doi/10.1103/PhysRevLett.131.121801

DOI: 10.1103/PhysRevLett.131.121801

（Edited and translated by Nankai News Team.)