Proton Decay
- nicolelyu812
- Feb 20, 2024
- 2 min read
Updated: Aug 20, 2024
From high school (or whatever), we have learnt that atoms compose of a nucleus and electrons, and the nucleus contains protons and often neutrons. Protons, just like a nucleus, compose of something else too - quarks. When we hear about “proton decay”, one might imagine the proton disintegrating itself into 3 isolated quarks. Unfortunately, due to the constraints of quantum chromodynamics, quarks can’t exist by themselves. So what does a proton decay into? We’re not sure. In fact it’s still hypothetical.
Proton decay is not predicted in the standard model, unfortunately. The standard model requires all decay pathways to follow multiple conservations, including baryon number conservation. As the least massive particle consisting of three quarks, the proton is already the lightest baryon (with a baryon number 1). The standard model says it can’t decay into lighter baryon to maintain the baryon number.
So how can we search for it? Aimlessly? Well…. all atoms compose of the same nucleons so yeah… We just gather a bunch of protons together and wait for one of them to decay. The more protons we get, the more likely that one will decay within a feasible timeframe (like 5 years). Currently, most experiments gather a large pool of water with detectors all around it to detect—
How to detect a decay signal?
From theoretical prediction, one decay channel of the proton is
One proton decays into one neutral pion (which soon decays into two gamma photons) and one positron.
When the positron is emitted from the nucleus, it travels near the speed of light in a vacuum (c). Very very fast. On the other hand, light is “slowed down” in water as it is optically denser than air or vacuum. So the positron is actually faster than light when they both travel in water! This creates a light equivalent of a “sonic boom”, creating a cone of photons around the positron as it travels through the water. The photons, making a faint blue light, is called Cherenkov radiation. When the cone hit the array of detectors, a ring is shown on the detector output. After we check that the ring is indeed from a proton decay event and not something else like atmospheric neutrino shenanigans, then it’s (maybe) success! That’s how we know a proton has decayed.
History and efforts
Several large experiments have contributed to the search of protond decay signals. Fun-but-no-so-fun fact - although experiments have been going on for many years, there is no confirmed proton decay signal yet. We wish it means that the lifetime of the proton is simply very, very long.
One of the earliest experiments is the Irvine-Michigan-Brookhaven (or IMB) experiment. Monitoring protons in ultrapure water, the detector revealed the long lifetime of a proton, much longer than what some Grand Unified Theories predicted.
The trophy of the most-famous would no doubt go to the Super-Kamiokande (Super-K) experiment and its predecessor, Kamiokande experiment. Through these experiments, the lower bound of the proton half life goes to around 10^34 years!! Shocking. But the quest continues with even bigger detectors like Hyper-Kamiokande under construction.
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