[Reminder: I'll be interviewed today at 5 p.m. Eastern time, at http://www.blogtalkradio.com/virtuallyspeaking/2012/02/15/matt-strassler-tom-levenson-virtually-speaking-science , which you can listen to either live or later. My interviewer, Tom Levenson, is an eminent science journalist who has written fascinating and surprising books on Einstein and on Newton, among others, won awards for his work on television (e.g. NOVA), has a great blog (and also posts here), and is a professor of science writing at MIT. Should be fun!]
Since a number of readers were surprised to learn, from yesterday’s article about the benefits of increasing the energy of the protons at the Large Hadron Collider [LHC], that protons are very complicated and have a lot more in them than just two up quarks and a down quark, I thought I’d put up a plot or two that gives some indication of how particles are distributed inside a proton. Caution: the answers you get, and the physical intuition you obtain, depends in some subtle ways on exactly what you ask, so you should pay some attention to precisely which question I’m answering below. The details matter.
The two plots in the Figure show exactly the same thing, just with a different vertical scale, so that certain things that are hard to see on one plot are clearer on the other. And what they show is this: if a proton is flying toward you in a Large Hadron Collider [LHC] proton beam, and you strike something inside that proton, how likely are you to have hit an up quark, or down quark, or gluon, or up antiquark, or down antiquark, that carries a fraction x of the proton’s energy? From these plots we can learn:
- From the fact that the curves all grow very rapidly at small x (clearest in the lower plot), you learn that most particles in the proton carry much less than 10% (that is, x < 0.1) of the proton’s energy, and you are most likely to hit one of those, with lower energies being much more likely than higher ones. [10% isn't small, by the way; in 2012 the LHC beams will have 4 TeV = 4000 GeV per proton, so 10% means 400 GeV --- whereas to make a 124 GeV Higgs particle from two gluons you would only need 62 GeV per gluon.]
- From the fact that (lower plot) the yellow curve is so much higher than the others, you learn that if you do hit something that carries less than 10% of the proton’s energy, then it is most likely a gluon; and quarks and antiquarks (the other curves) are about equally likely once you get below about 2% of the proton’s energy.
- From the fact that (upper plot) the gluon curve dips below the quark curves at higher x, you learn that if you hit something with much above 20% (that is, x>0.2) of the proton’s energy —- which is very, very rare — it is most likely a quark, and about twice as likely to be an up quark as a down quark. [Here we see a remnant of the idea that "a proton is two up quarks and a down quark."]
- The curves all fall off very quickly as x increases; you are very unlikely indeed to hit anything with more than 50% of the proton’s energy.
These observations are reflected indirectly in Figure 1 of this article on what you gain in going from 7 TeV to 8 TeV at the LHC. Here are a couple of other things that are not obvious from the plots above:
- Most of the energy of the proton is shared (roughly equally, for no special reason) among the small number of higher-energy quarks and the huge number of lower-energy gluons.
- However the number of particles is dominated by the lower-energy gluons, with very low-energy quarks and antiquarks coming in next.
The number of quarks and antiquarks is huge… but: The total number of up quarks minus the total number of up antiquarks is 2, and the total number of down quarks minus the total number of down antiquarks is 1. And as we saw above, these excess quarks carry a significant fraction (but not the vast majority) of the energy of a proton as it is flying toward you. In these senses only, the proton is substantially made from two up quarks and a down quark.
By the way, all of this information was obtained from a fascinating combination of experiments (mostly scattering of electrons or neutrinos off of protons or off of the atomic nuclei of heavy hydrogen [``deuterium'', which contains one proton and one neutron]), assembled together using the detailed equations that describe the electromagnetic, strong nuclear and weak nuclear forces. It’s a long story going back to the very late 1960s and early 1970s. And it works beautifully when used to predict the phenomena observed in proton-proton or proton-antiproton colliders like the Tevatron and LHC.