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!
Tom
(published on 10/22/2007)
