麻豆淫院

Baryons, composite particles made up of three quarks bound together via the so-called strong force, make up the most visible matter and have thus been the focus of numerous physics studies. Studying the rare processes via which unstable baryons decay into other particles could potentially contribute to the discovery of new physics that is not explained by the Standard Model of particle physics.

The LHCb experiment is a large-scale research effort involving scientists working at universities worldwide, aimed at studying the physics of particles containing so-called b-quarks and searching for new physics beyond the Standard Model, relying on a large and specialized detector located at CERN. In a paper recently in 麻豆淫院ical Review Letters, the LHCb collaboration reported the observation of an ultra-rare process via which a type of baryon known as sigma-plus (危鈦�) decays into a proton and two muons with opposite charges.

"The motivation for our study originally came from the measurement of the HyperCP experiment at Fermilab, which had evidence for this decay with 3 decays seen in 2005," Francesco Dettori, Professor at the University of Cagliari and member of the LHCb Collaboration, told 麻豆淫院.

"The interest in their measurement is that looking at the muon pair of the three candidates, it showed they had the same combined mass, instead of a distribution as expected. This would have indicated the presence of an intermediate particle never seen before, and outside of the current understanding of particle physics (known as the Standard Model)."

The results recently collected at Fermilab 20 years ago attracted the attention of many particle physicists worldwide, some of whom proposed possible explanations for the observed decay process (危⁺鈫抪饾渿⁺饾渿^鈭�), which are rooted in theories beyond the Standard Model. Other experiments focusing on dimuon pairs were unable to observe this ultra-rare decay process, but due to its scope and scale, the LHCb experiment had all the tools necessary to search for this ultra-rare decay.

"We analyzed data collected in 2016鈥�2018 in proton-proton collisions at the Large Hadron Collider," said Gabriele Martelli, associate researcher at INFN, Section of Perugia. "The 危⁺ baryon is just slightly heavier than the proton, so in these high energy collisions, almost any collision also produces one of these particles. In particular, we estimated that 10¹⁴ (a hundred trillion) 危 baryons were produced in the experiment during this period."

The very rare decay that the team was searching for has a distinctive signature that differs from other decays, as the 危 baryon is relatively long lived. As part of their study, the LHCb collaboration searched for the decay vertex up to tens of centimeters away from the proton-proton interaction.

"At the same time, the two muons and the proton, being charged, are seen by our detectors and identified through different stages of particle identification," said Martelli. "While the signature is easy to reconstruct, we have to fight with a large quantity of random combinations that could mimic this decay, and we employed machine learning methods to reject them."

Ultimately, the researchers were able to observe the 危⁺鈫抪饾渿⁺饾渿^鈭� decay, which is the rarest of all baryon decays studied to date. This is a remarkable scientific achievement that highlights the sensitivity of the LHCb detector at CERN.

"Having observed hundreds of these decays gave us the possibility to measure precisely its probability and other properties, and compare them to the Standard Model predictions," said Dettori. "Historically rare decays have been the place where particles were discovered through their quantum effects, well before the accelerators had energy to produce them directly. The most notable case being the 'charm quark' discovered to explain a decay which was measured to be rarer than predicted."

The LHCb experiment has already helped to improve the understanding of various particle physics phenomena explained by the Standard Model. As the precision and sensitivity of the detector improve further, however, the researchers involved in the experiment could gain access to decay processes that are increasingly rare, which could in turn lead to the observation of new particles and physical interactions.

"Using data collected from 2023 on, with the LHCb upgraded detector, we should be able to measure thousands of these decays and probe not only their probability but also other characteristics," added Martelli.

"The next property we want to study is the difference between matter and antimatter (known as Charge-Parity symmetry violation) in this decay, by comparing the 危 decays with the anti-危 one. The measurement of this CP symmetry in many decays is fundamental to find clues on the matter/antimatter imbalance of the universe."

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