List the following particles in
order from greatest mass to least mass: top quark, strange quark, photon, tau
neutrino, electron, proton, up quark.
Okay, so, of this list of seven
particles, we want to put them in order from greatest to least mass. And as we look over these
particles, we see we have three quarks, a top quark, a strange quark, and an up
quark; a proton and an electron, those are particles that may be more familiar to
us; and then a photon and something called a tau neutrino. When we think about a photon, we
recall that this is a particle that has zero mass. So, whatever the order of the rest
of our list, we know that photon will come last because nothing can have less mass
than zero mass.
Now, this particle right below the
photon, the tau neutrino, this is known as a particle that does have mass, but it
has the least mass of all particles that do. In other words, its mass is not
zero but just barely so. So, working backwards then, we know
the last item on our list will be the photon, and the next to last will be the tau
neutrino. At this point, our remaining
choices are the three quarks and the proton and the electron. Protons we can recall are made up
of quarks. Specifically, one proton is made
from two up quarks and one down quark. And from this, we can reasonably
guess that a proton is more massive than any quark, and indeed this is true. And going further than that, we
know a proton is more massive than an electron, about 2000 times more massive. All this tells us that the particle
at the beginning of our list, the most massive one, is the proton.
That leaves us with our three
quarks and the electron. We want to find out how these
particles rank in terms of mass. One way to help us with this is to
recall, as we did earlier, that a proton is made of two up quarks and one down
quark. It turns out that only about 10
percent of a proton’s mass actually comes from the mass of the quarks that make it
up. But nonetheless, this suggests that
the mass of a quark is greater than the mass of an electron, which, as we recalled,
is about one 2000th that of a proton. And it turns out that our intuition
is correct. An electron is indeed less massive
than any type of quark, which means we can write it as the third to last item on our
ranked list. So now, the only question that
remains is of the top, strange, and up quarks, what is their mass ranking?
As we said, the up quark is part of
what makes up a proton, and it’s part of the lightest set of the quark pair
types. That is if we consider up, down;
charm, strange; and top, bottom, then the mass of each one of these types decreases
as we go from right to left. This shows us that an up or down
quark is less massive than a charm or strange quark, which is less massive than a
top or bottom quark.
And that shows us how we can write
in the last three options on our list, the top, strange, and up quarks. Of the three, the top quark has the
most mass, followed by the strange quark and then, lastly, the up quark. So now, we have our completed
list. In order from greatest to least
mass, we have the proton, the top quark, the strange quark, the up quark, the
electron, the tau neutrino, and the photon.