Do Protons Decay? The Moon May Help Find the Answer
Does proton decay exist and how to look for it? This is what the recently presented paper tells us study, in which a team of international scientists is exploring the concept of using samples from the Moon to look for evidence of proton decay, which remains a hypothetical type of particle decay that has not yet been observed and continues to elude particle physicists. This research could potentially help solve one of the long-standing mysteries in all of physics, as it could allow new research into the deeper levels and laws of nature in general.
Universe Today discusses this research with Dr. Patrick Stengel, postdoc in the cosmology group at the INFN Ferrara department, about the motivation for the research, the significant results, the significance of the search for proton decay, the implications for confirming the existence of proton decay and turning their concept into reality. So what's behind this research?
Dr. Stengel told Universe Today that the research began around 2018 with lead author Dr. Sebastian Baum and others, with the idea of using “paleodetectors” — a method of studying particles that span vast periods of geological time. This led to a discussion with study co-author Dr. Joshua Spitz, who became interested in paleodetectors after several papers had looked at their potential to find dark matter, and one of Dr. Spitz’s graduate students about how paleodetectors could be used to detect the existence of proton decay. However, the team published a study that found that proton decay was impossible to detect on Earth due to atmospheric neutrinos.
“About a year after completing the paper on atmospheric neutrinos, Spitz proposed looking at mineral samples from the Moon,” Dr. Stengel tells Universe Today. “Due to the absence of an atmosphere, the flux of neutrinos caused by cosmic rays on the Moon is much less saturated than on Earth. Accordingly, fewer neutrino interactions caused by cosmic rays should be observed on the Moon, which allows, at least in principle, to look for proton decay in paleodetectors.”
For the study, scientists proposed a hypothetical concept for the use of paleodetectors, which involves collecting mineral samples from depths of more than 5 kilometers below the surface of the Moon and analyzing them for the presence of proton decay either on the Moon itself or on Earth. The researchers note that these lunar paleodetector samples could yield proton lifetimes of up to 10^34 years. For comparison, the age of the Universe is approximately 13.7 x 10^9 years. So what are the most significant findings from this study?
Dr. Stengel tells Universe Today: “For lunar mineral samples that are sufficiently radiopure to reduce radiogenic background and buried at sufficient depth to be shielded from other cosmic ray backgrounds, we show that the sensitivity of paleodetectors to proton decay could in principle be competitive with next-generation conventional proton decay experiments.”
As noted, proton decay remains a hypothetical type of particle decay and was first proposed in 1967 by Soviet physicist and Nobel Prize winner Dr. Andrei Sakharov. As the name suggests, proton decay is hypothesized to occur when protons decay into particles smaller than an atom, also called subatomic particles. As noted in this recent study and various previous studies, proton decay has not yet been discovered or observed. However, there is a hypothesis that it can help to better understand our Universe and the origin of life, and quantum tunneling is proposed as a proton decay process. In this regard, what is the significance of the search for proton decay, and what consequences might its existence have for science, in particular for particle physics, in general?
Dr. Stengel told Universe Today: “Proton decay is a general prediction of theories of particle physics beyond the Standard Model (SM). In particular, proton decay may be one of the only low-energy predictions of the so-called Grand Unified Theories (GUTs) that attempt to unify all the forces that mediate the interactions of the SM into one force at very high energies. Physicists have been developing and constructing experiments to search for proton decay for more than 50 years.”
Dr. Stengel continues: “The detection of proton decay, whether with a mineral detector or in a more traditional experiment, would have incredible implications for science in general and particle physics in particular. Such a discovery would be the first confirmation of particle physics beyond the SM. Depending on how well the proton decay signal can be characterized, we may learn something about a fundamental theory of nature.”
As noted, the hypothetical concept proposed in this study, using paleodetectors to detect proton decay on the Moon, would require collecting samples at least 5 kilometers below the lunar surface. By comparison, the deepest humans have ever collected samples from beneath the lunar surface was just under 300 centimeters, in the drill core samples obtained by the Apollo 17 astronauts.
On Earth, the deepest man-made borehole is the Kola Superdeep Borehole in northern Russia, with a true vertical depth of approximately 12.3 kilometers and requiring several boreholes and several years to construct. While the study notes that the proposed concept of using paleodetectors on the Moon is “clearly futuristic,” what steps are needed to move the concept from futuristic to realistic?
Dr. Stengel told Universe Today: “As we try not to stray too far from our respective areas of expertise in particle physics, we decided not to speculate on the actual logistics of conducting such an experiment on the Moon. However, we felt that the concept was timely, since various scientific agencies in different countries are considering the possibility of returning to the moon and are planning a wide program of experiments.”
Dr. Stengel continues: “As you said, mineral samples would need to be mined at least 5 km deep into the lunar crust. So a drilling rig capable of reaching such depths would need to be delivered to and operated on the Moon. While this logistical challenge seems daunting, we note that NASA, for example, envisions that fairly large payloads will eventually be delivered to the Moon as part of the Artemis program.”
As noted, this research is being conducted as part of NASA's Artemis program, which plans to return astronauts to the lunar surface for the first time in more than 50 years with the goal of landing the first woman and person of color on the lunar surface. Additionally, as scientific interest in paleodetectors continues to grow, the concept proposed in this study may prove useful not only for detecting proton decay, but also for better understanding our place in the Universe. Finally, it turned out that only a small sample would be required for the proposed concept to work.
Dr. Stengel tells Universe Today: “Because paleodetectors have been exposed to proton decay for billions of years, only one kilogram of the target material is needed to compete with conventional experiments. Combined with scientific motivation and the recent desire to return people to the Moon for scientific research, we believe that paleodetectors may be the final frontier in the search for proton decay.”
