Another big draw is proton decay – a prediction so cool, you could have a GUT based entirely on its appeal – which has scientists transfixed. It has remained one of the most fascinating enigmas of particle physics. Experiments have limited the average lifetime of the proton to more than 10^{34} years. Even then this time is only four orders of magnitude beyond the age of the universe. These constraints are therefore key, as they dictate our taxonomy of particles in the universe and how we think they all fit together.
Proton decay is more complex than just what can be tested in the laboratory. Limits from dark matter decay products set a lower bound of 10^17 years on the proton lifetime. Plus, clues from the Cosmic Microwave Background (CMB) favor a lower bound of 2 × 10^17 years. These results are very important! They continue to open up deeper questions about the stability of protons under distorting conditions across time and space.
Proton decay could proceed through two primary channels: either into a neutral pion and a positron or a positive pion and a neutrino. The experimental limit for the first decay channel is about 62 times larger than for the second one. That’s important because it reflects a huge variation in the possible decay modes.
Theoretical Constraints and Implications
In fact, the implications of proton decay are widespread, affecting everything from cosmic scale events to day to day earthly life. Proton decay could provide a second, more interesting source of Carbon-14 (14C). Protons must therefore live at least about 10 19 years. This has deep implications on our understanding of nuclear processes and isotopic compositions.
Moreover, if proton decay were to occur within Earth's core, it would generate heat sufficient to melt the solid inner core. Recent experimental results have placed the proton lifetime above 2 × 10^18 years. This conclusion is further supported by the fact that Earth’s inner core is in a solid state.
The stability of protons is equally important in stellar environments. A typical neutron star with a mass of about 1.4 solar masses hosts on the order of 2 × 10^56 protons. Proton decay might still occur at a significantly higher rate than laboratory experiments suggest. If a dark matter particle lighter than the proton minus pion mass affects this decay, it would have profound astrophysical implications.
Insights from Pulsars and Cosmic Observations
The coolest known pulsar, PSR J2144–3933 with a temperature less than 42,000 K, is roughly 0.3 billion years old. It provides a unique window into proton decay on astrophysical time scales. In this way, by measuring the temperature of a pulsar, scientists can extract valuable insights. That data allows them to figure out how many protons have decayed in the last billion years.
Our own Davoudiasl and Denton have made strides in this area by considering how these unknown forces could affect the rate of proton decay. Their studies conclude that the proton's lifetime must be greater than 1.5 × 10^18 years to align with observed cosmic phenomena and terrestrial conditions.
"One case that hadn't been investigated was the stability of the proton. Earth experiments show that the proton is incredibly stable, but those only apply here, in our part of the galaxy, and now, over the last several decades. What if proton decay depended on time or space?" – Peter Denton
"Could be realized, for example, if a hypothetical long-range force can affect the mass of a new particle into which a proton can decay, making the proton lifetime much shorter when those decays are energetically allowed." – Hooman Davoudiasl
Challenges and Future Research Directions
The search for proton decay continues to be an exciting and important field of study in physics. Though these new limitations have led to groundbreaking understanding, they have exposed the lack of our current knowledge. Proton decay may vary temporally or spatially. This potentiality far exceeds the scale of most local experiments and makes it hard to judge its complexity based on local experimentation.
In the future, we hope to dive deeper into these exciting possibilities, possibly leveraging machine vision and other new technology and methodologies. By studying different proton environments across the universe and taking advantage of new frontier observational methods, scientists believe they can finally uncover how and why proton decay occurs.