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Q & A: Proton and Neutron decay

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Most recent answer: 06/27/2012
I heard about this proton and neutron decay thing, if this is true would it allow a singularity to decay since it must be composed of infinintly compacted neutrons or protons? and my second question, is proton and neutron decay just some product of some stupid GUT or is it an accepted scientific theory? I say theory becuase it would not be possible to observe the decay over a sensible time scale, or am i mistaken?
- jake (age 14)
central memorial high school, canada
Hi Jake,

Well, first the data. Protons have not been observed to decay, but neutrons decay all the time. The lifetime of a neutron all by itself is about 886 seconds. Neutrons decay into a proton, an electron, and an electron-type antineutrino. This decay proceeds by the (mostly) understood process of the weak interaction, by exchange of a virtual W- boson between a down-type quark in the neutron (changing it into an up-type quark), and the electron and antineutrino. There are still some mysteries about the weak interaction (Why is it there? Why is it weak? Which can be formulated as -- why is the W boson heavy? It has a mass of about 80 proton masses.). Neutrons weigh just a little more than protons so this process proceeds. If protons were heavier, it would be protons that decayed via the weak interaction and not neutrons.

Neutrons decay when by themselves but do not do so when bound inside of atomic nuclei (well, many kinds of nuclei. Some nuclei in fact decay in exactly this way -- one of the neutrons decays). The energy levels inside nuclei are such that if a neutron were to decay into a proton, it would have to find a place in a higher-energy level (because of Pauli's exclusion principle keeping it out of lower-lying energy levels), and the total energy doesn't add up to enough to allow the neutron to decay. But in some nuclei, neutron decay is possible and favored.

A proton cannot decay into a lighter baryon (particle made up of three valence quarks, like a neutron). It must decay into something else, such as maybe a pion and a positron and an electron-type neutrino; this is one of the things people look for when they seek proton decay. The lower limit on the proton lifetime is 1.6 times 10 to the 25th power years, allowing any combination of decay possibilities, and typically around ten to the 31st power to 10 to the 33rd power years for any decay mode by itself. The reason for this discrepancy is that if protons decayed by a variety of different mechanisms, and we do lots of experiments looking for each one separately, we will be less sensitive than if protons all decayed the same way. Each kind of decay can "sneak under our limit" and the sum of all of them can be larger than if they all decayed the same way and some experiment turning up lots of obvious proton decay signals.

You don't necessarily need to wait 10^33 years (or more, who knows?) for protons to decay. All you need is 10^33 protons and watch 'em for a year (all the while, being sensitive to single decay events!). The more the better! The big experiments are gigantic tanks of water underground, with sensitive light-amplifying detectors all around the tank. They are underground because cosmic rays can penetrate inside and create flashes of light which can confuse the results.

GUTs are not accepted scientific theory, because there is no experimental evidence for them. That doesn't mean we shouldn't continue trying to find some!


(published on 10/22/2007)

Follow-Up #1: Can a neutron decay into a hydrogen atom?

If a neutron decays into a proton and an electron does it form a hydrogen atom as an end product?
Yes, it can happen but very, very infrequently.   The main problem is that there is a lot of kinetic energy left over after the decay and the electron and proton don't stay around long enough to bind into a stable hydrogen atom.  Matters are further complicated by the fact that there is also an electron anti-neutrino involved in the reaction. n -> p + e- + vbar.  Since the binding energy of an electron in a hydrogen atom is only 13 eV (electron Volts) and the kinetic energy released in the decay is over 700 million eV the electron and proton tend to just fly apart.  The anti-neutrino, being electrical neutral and weakly interacting, isn't involved in the final state. It just carries off some of the kinetic energy.  From time to time the neutrino will carry off enough energy to leave the electron and proton relatively at rest.  In that case they can form a hydrogen atom.  Experiments have been performed looking for this effect.  It has been observed but seems to occur on average 4 times out of one million neutron decays.  


(published on 06/27/2012)

Follow-up on this answer.