For the general reader:
Last week I showed you, without any technicalities, how to recognize the elementary forces of nature in the pattern of particle masses and lifetimes. This week we’ll start seeing what we can extract just from the particles’ masses alone… and what we cannot. Today we’ll focus on quarks and the strong nuclear force.
A key factor in nature, which plays an enormous role in everyday life, is the mass of a typical atom. [Note: on this website, “mass” always means “rest mass”, which does not increase with a particle’s speed.] This in turn arises mainly from the masses of protons and neutrons, which are about equal, and tiny: about 0.00000000000000000000000000167 kg (or 0.00000000000000000000000000368 pounds). Since those numbers are crazy-small, physicists use a different measure; we say the mass is about 1 GeV/c2, and more precisely, 0.938 GeV/c2. In any case, it’s tiny on human scales, but it’s some definite quantity, the same for every proton in nature. Where does this mass come from; what natural processes determine it?
You may have heard the simplistic remark that “a proton is made of three quarks” (two up quarks and a down quark), which would suggest these quarks have mass of about 1/3 of a proton, or about 0.313 GeV/c2. But something’s clearly amiss. If you look at websites and other sources about particle physics, they all agree that up and down quark masses are less than 0.01 GeV/c2; these days they usually say the up quark has mass of 0.002 GeV/c2 and the down quark has 0.005 GeV/c2. So if the proton were simply made of three quarks, it would naively have a mass of less than 1% of its actual mass.
What’s going on? A first little clue is that different sources sometimes quote different numbers for the quark masses. There are six types of quarks; from smallest mass to largest, they are up, down, strange (u,d,s, the three light quarks), charm, bottom (c,b, the two somewhat heavy quarks) and top (t, the super-heavy quark.) [Their names, by the way, are historical accidents and don’t mean anything.] But some websites say the up quark mass is 0.003 instead of 0.002 GeV/c2, a 50% discrepancy; the bottom quark’s mass is variously listed as 4.1 GeV/c2, 4.5 GeV/c2, and so forth. This is in contrast to, say, the electron’s mass; you’ll never see websites that disagree about that.
The origin of all these discrepancies is that quarks (and anti-quarks and gluons) are affected by the strong nuclear force, unlike electrons, Higgs bosons, and all the other known elementary particles. The strong forces that quarks undergo make everything about them less clear and certain. Among numerous manifestations, the most dramatic is that quarks (and anti-quarks and gluons) are never observed in isolation. Instead they’re always found in special combinations, called “hadrons“. A proton is an example, but there are many more. And the strong nuclear force can have a big effect on their masses.
The Modern Proton and the Masses of Quarks
A proton, in fact, is not simply made from three quarks, the way a hydrogen atom is simply made from a proton and an electron. As I described in this article, it’s vastly more complex; it’s made from three quarks plus lots of gluons plus lots of pairs of other quarks and anti-quarks. So the simple intuition we get from atoms does not apply to hadrons like the proton.
Since we never find quarks outside of hadrons, and hadrons are generally complicated, this poses real problems for measuring or even uniquely defining what quark masses are. Whereas one can isolate an electron and easily measure its rest mass (perhaps by wiggling it back and forth, or seeing how quickly it accelerates in a known electric field), one can’t do anything similar for a quark, since one can’t isolate it. In the end, this makes the very definition of a quark’s mass a bit ambiguous, and certainly subtle!
What’s a poor physicist to do? Well, since the culprit is the strong nuclear force being so darn strong and complicated, wouldn’t it be nice if we could make it weaker and see what happens? Sure would. And what do you know? The strong nuclear force does this for us, all on its own! For objects with mass well below the proton’s mass, the strong nuclear force is super-strong indeed; but for objects well above the proton’s mass, it becomes weaker (relative to other forces at that mass scale) and everything about it becomes a lot simpler. [Caution for future experts: there is an oversimplification in this statement.] The transition between super-strong and not-so-strong occurs in the region of 0.3 – 1.0 GeV/c2 in mass, or 0.3 – 1.0 GeV in energy… right around the proton mass. In fact, this is why protons and neutrons, and thus atoms, have the masses they do. The masses of atoms are set by the mass range in which the strong nuclear force transitions from kind-of-strong to really-really-strong.
Today I’m going to give you evidence for this incredibly important and surprising phenomenon, called “asymptotic freedom”, discovered in 1973 and awarded Nobel Prizes in 2004. We’ll do this through the patterns seen in some hadrons’ masses. Along the way we’ll see how to estimate the masses of the top, bottom and charm quarks (the three “heavy quarks”), gain some interesting insights into the masses of the up, down and strange quarks (the three “light quarks”), and see how the latter feed into the proton mass.
Our strategy will be to investigate some exotic “atoms” made, at least nominally, from a quark and an anti-quark. Hadrons of this class are called “mesons“, examples go by the names of pion, rho, kaon, omega, but we won’t care about the names. We’ll see that if the quark and anti-quark are both heavy (c or b), the atoms appear relatively simple, and in fact this is still roughly true even if only one of them is heavy. But if both the quark and anti-quark are light (u, d and/or s), then this is not true; the behavior of the corresponding meson is very different from an atom. Such mesons are more like protons, with an extremely complex interior.
We’ll extract these basic features of quarks, atom-like mesons and the strong nuclear force from just one figure! That’s the point of this post.
Four atoms: Hydrogen, anti-hydrogen, positronium and protonium
Since we’re going to focus on quark-antiquark “atoms”, let me first introduce you to four less complicated atoms, one familiar and three exotic. I want to show you how much we can learn from their masses. The technique, easy to understand in this context, can then be applied to mesons and quarks, with mixed results.
In ordinary life, hydrogen is the simplest atom; it consists of one electron (of charge -1) and one proton (of charge +1). But in high-tech experiments, physicists can make the anti-particles of electrons and protons (called positrons and anti-protons) and create three other simple electrically-neutral atoms, giving us four altogether; as shown in Figure 2, they are hydrogen (electron and proton), positronium (electron and positron), protonium (anti-proton and proton), and anti-hydrogen (anti-proton and positron.)
These four atoms have three different masses; that’s because hydrogen and anti-hydrogen have exactly the same mass (because two types of objects that are each other’s anti-particle always have exactly the same mass.) And also, the mass of an atom is roughly the sum of the masses of the objects it contains. Because of this, we can easily estimate the masses of the proton (and anti-proton) and of the electron (and positron) from the masses of these atoms.
- Divide the protonium mass by 2 to get an estimate of the proton mass.
- Divide the positronium mass by 2 to get an estimate of the electron mass.
- As a check: their sum should give hydrogen’s mass.
For these atoms, it works great. We’ll have only partial success with mesons.
Simple Shifts To Atomic Masses
The reason it works so well with atoms, and less well with mesons, has to do with two types of shifts of an atom’s mass — changes that are extremely small for a relatively weak force like electromagnetism, but can be large for the strong nuclear force.
- The mass of the atom is slightly shifted by the energy needed to hold the atom together; that extra energy affects the atom’s mass because E=mc2. It’s a tiny effect for these atoms, largest for protonium where it is a (negative) 1 in 10,000 effect, but for mesons it will matter much more.
- Two-particle atoms like this secretly have two configurations; if you pull the atom off the shelf you may find it in either one. Electrons and protons are intrinsically spinning, in a limited sense; but because quantum physics is weird, you’ll always find [Figure 3] their spin orientations in a hydrogen atom are either (a) anti-aligned (with “total spin 0“), or (b) aligned (with “total spin 1“). Because aligning the spins requires extra energy, these two “spin states” of the atom have very slightly different masses, a few parts per billion for protonium and positronium, and less for hydrogen.
[For electron-positron atoms, these two configurations are referred to as para-positronium and ortho-positronium.]
These mass shifts are so very small because the electromagnetic force is not very powerful, relatively speaking. [For those who read my slightly more technical posts, it is because the strength of the force is small: α = 1/137.04… << 1.] For the strong nuclear force, though — even where it is only kind-of-strong, but especially where it is really-really-strong, these shifts are a big deal!
Many Mesons From A Few Quarks
It’s certainly naive to take a simple strategy that works for atoms and apply it to mesons. But it will partially work, and where it fails, it will teach us something.
First, though, we need to deal with the top quark. The super-heavy quark’s mass is so big, and its lifetime so small, that the strong nuclear force has neither enough time nor enough strength to mess with it very much. Consequently, despite what I said above, experiments do observe the top quark in (extremely brief) isolation, during which time it decays to other particles that leave evidence in experiments (namely jets, which you can read about here.) From this evidence, the top quark can be identified as a sharp peak in data; Figure 4 shows an example from the CMS experiment operating at the Large Hadron Collider [LHC]. The location of the peak measures its mass — close to 172 GeV/c2.
But back now to the other five quarks. Roughly, just as the electromagnetic force can bind an electron and a positron into positronium, the strong nuclear force can bind a quark and anti-quark into atom-like “mesons”. With five types of quarks and anti-quarks, that’s 5×5=25 types of mesons, each with two spin states, for 50 total. However, some of these are each others’ anti-particles and have the same mass, so the number of mesons with independent masses is only 30 (=5*6/2 mesons x 2 spin states.) Of these, 27 have so far been observed in experiments; that’s enough for our purposes.
When I plot the masses of all of these mesons in Figure 6, putting their spin 0 states (green) and their spin 1 states (red) next to each other, patterns become obvious, and indeed I’ve organized the dots horizontally to make some of them clear.
The masses come in clusters: there are 6 red dots around 0.8-1.0 GeV/c2, whose corresponding green dots lie well below that; then there are three green-red pairs around 2 GeV/c2 and three around 5, and isolated dots at 3, 6 and 10 GeV/c2. This clustering is already a sign of the quark-antiquark structures within.
We can see a sign of the strong nuclear force’s asymptotic freedom and its transition around 1 GeV/c2 from super-strong to not-so-strong. For all mesons above 1 GeV/c2 where both spin-states have been observed, the separation between them is small; this gives us some reason to hope that these cases are not so different from ordinary atoms. By contrast, for the mesons at or below 1 GeV/c2, the separations are very significant in all but one case, so these mesons are clearly subject to very strong internal forces. They are likely much more complicated than atoms — and more like protons. (By the way, the proton has a higher spin state too, called the Delta, with a mass 30% higher than the proton’s.)
Extracting Quark Masses
Despite this, let’s apply the naive procedure we used to extract the masses of protons and electrons. In Figure 7, I’ve done this graphically, with arrows representing the mass of the quarks that we would extract from the procedure; red, yellow, green, blue and purple arrows indicate the masses of the u, d, s, c and b quarks. Two arrows connected together, one for the quark and one for the anti-quark, then give our naive guess for the mass of a corresponding meson, whose quark/anti-quark content I’ve indicated. Three minor details:
- Each meson has the same mass as its anti-particle, which has quark and anti-quark types interchange.
- Mesons may contain many gluons and many quark/anti-quark pairs in addition to the quark and anti-quark shown in the label.
- The quark/anti-quark content of three low-mass mesons is more complicated (they represent a “superposition” of quark/antiquark combinations.)
In many cases the arrows do point to where the dots are; in particular, this
- works well for all spin-1 mesons
- works well for both spin-1 and spin-0 mesons that have at least one b or c quark (or anti-quark)
- completely fails for most spin-0 mesons with u, d and s quarks!
Despite some partial success, there’s something very funny about the quark masses that we get this way. They’re always too high. Approximately,
(Different ways of applying today’s method give somewhat different results, but always qualitatively similar.) This is striking for two reasons:
- For each quark, our mass estimate comes out about 0.3 to 0.5 GeV/c2 higher than modern methods obtain.
- The sum of the masses for two u quarks and one d quark (or two d quarks and one u quark) comes within 15% of the proton’s or neutron’s mass, much closer than one obtains from the modern methods.
The Central Role of the Strong Nuclear Force
What is this telling us? One way to think about it — but this is only a rule of thumb, not an exact or rigorous statement backed by clear theoretical argument or calculation — is that the strong nuclear force, through its addition to a hadron of all those gluons, quark/anti-quark pairs, and binding energy, essentially shifts the quarks’ masses, adding three to five tenths of a GeV/c2 to the elementary mass of a quark. This shift is a small effect for the heavy quarks c and b, but is a dramatic effect for the light quarks u, d and s. Our method picks out the quark masses after this shift. Scientists, however, have highly sophisticated (though still imperfect) methods for undoing this shift, and picking out the more fundamental values of the quark masses — and this is what you see on most websites.
The importance of this shift is enormous! As we’ve seen, without it a proton or neutron would have a mass of less than 0.01 GeV/c2. And so the strong nuclear force’s mass shift contributes 99% of a proton’s and neutron’s mass — which means it contributes 99% of your mass, too.
These issues reflect the fact that most mesons, in particular those with light quarks or antiquarks in them, are not at all like hydrogen atoms, and aren’t made from just two particles. The mass shift somehow captures the presence of all those other particles, and the energy required to keep them both trapped and in motion.
Why is the shift roughly 0.3-0.4 GeV/c2 per quark? Qualitatively, its size makes sense: it is right around the mass at which the strong nuclear force transitions from super-strong to not-so-strong. A shift of 10 GeV/c2 would be way too big, and a shift of only 0.001 GeV/c2, while possible, might be surprisingly small. But why is the shift 0.3 rather than 0.15 or 0.5 GeV/c2? and why is it basically the same in all spin-1 mesons and in “baryons” (the class of hadrons to which protons and neutrons belong)? No one has fully understood the answers, either conceptually or mathematically. One clue as to how this shift works is seen in the fact that the spin-0 meson masses are so small — that the mass shifts are smaller there. This is well-understood, but it is a long story for a future post.
These discrepancies between what one means by quark masses are not new; they confused physicists for a long time. In fact, with all these mysterious shifts and ambiguities, you’d be reasonable to wonder if physicists even now actually understand quarks and the strong nuclear force. To see that we do, I’ve shown in Figure 7 the results of computer simulations of the light quarks interacting with the strong nuclear force. With just two inputs (the masses of the lightest two types of spin-0 mesons), a whole host of things can be successfully calculated:
- the masses of the full set of spin-1 mesons
- the masses of the remainder of the spin-0 mesons (the most difficult of all the calculations, for technical reasons)
- the masses of spin 1/2 and spin 3/2 “baryons” (including those of protons and neutrons.)
So the equations for the strong nuclear force do show that light quarks with masses as low as 0.002 GeV/c2 can turn into mesons with masses ranging from 0.14 to 1.02 GeV/c2, as well as baryons from the proton’s 0.938 GeV/c2 on up. Our understanding isn’t complete, but our equations are right, and our ability to extract information from them is likely to improve as computers and math develop further.