Monday, April 5, 2010
In thinking about how to start this story I figured that an itty bitty lesson in particle physics was in order. Now, a word of warning: I'm not, and have never been, a particle physicist. At least not in this dimension. So I'm going to try and keep it simple for your sake, my sake, and the sake of the word count. Here's hoping it comes out intelligible:
In modern theory we have the Standard Model, a name given to the theory of fundamental particles and their reactions (we're talking all the stuff that makes up protons, neutrons electrons, etc.). This theory has been extremely useful the field of particle physics as well as other areas of study that use its principles. You've no doubt heard all the news hype about the Higgs boson, particularly in relation to the Large Hadron Collider (LHC). The Higgs boson is an additional, as yet theorized but undiscovered, particle that lends mass to these other, fundamental particles.
What other particles? I'm glad you asked. They are the fundamental building blocks from which all else is made. In the Standard Model there are 12 fundamental matter particle types (and, of course, their corresponding antiparticles). These 12 types are divided into 2 classes, of 6 particles each, called quarks and leptons (the 6 being 12 if you count antiparticles). Since this story talks about quarks in particular I just define those. Quarks are the constituents of hadrons such as protons and neutrons. Remember I said there were 6 types? They are up (u), down (d), charm (c), strange (s), top (t), and bottom (b). They experience all four of the fundamental forces (electromagnetism, gravitation, strong interaction, and weak interaction), although gravity is negligible at these scales, and are grouped into 3 generations. The first (and only to be found in nature) being up and down quarks, the second charm and strange quarks, and the third top and bottom quarks.
Hopefully, that made something resembling sense. My brain hurts and we are just getting to the story.
A team of physicists, using more computing power and simulations than we can probably conceive of, have calculated the masses of up quarks and down quarks. This new calculation is 20 times more precise than previous ones. It could be a big help to scientists working at facilities such as the LHC, not to mention other theorists who could incorporate it into the Standard Model (which treats quark masses as arbitrary). It is known that quarks are bound together by the strong interacting force (strong force), but this hold is so tight that you can't isolate one quark from another. So you can't measure the mass of a single quark. Particles that are made up of quarks are, as you would expect, even more complex. A proton has two up quarks and one down quark while a neutron has two down quarks and one up quark -- all held together by gluons (quark interactions or vector bosons). Oh yeah, and don't forget to add the quark-antiquark pairs that come into and out of existence. When you start adding things up, the valence quarks (those original 3) make up under 2% of a proton's mass. This is where the theorists step in.
Physicists have used an approach called lattice quantum chromodynamics (lattice QCD) that simulates these particles within a hadron by modeling it as a grid of points (a lattice). When you place the quarks and gluons on the grid and simulate their interactions you can calculate the masses of hadrons. In the past, this procedure lended a sizable uncertainty to the masses of the quarks. Here, the scientists were able to calculate the ratios of the charm-quark mass to the strange-quark mass and then combine this with another group of ratios of strange-quark mass to up-quark mass and down-quark mass. The resulting ratios were of a much smaller uncertainty (from 30% to 1.5%) than in the past. "The team finds that an up quark weighs 2.01 +/- 0.14 megaelectron-volts, whereas a down quark weighs 4.79 +/- 0.16 MeV. That’s 0.214% and 0.510% of the mass of the proton, respectively." More testing needs to be done to see if these results hold up, but this is undoubtedly an extraordinary achievement.
Whew...I need a drink...
Get more of the story here: http://news.sciencemag.org/sciencenow/2010/04/mass-of-the-common-quark-finally.html
(image from startswithabang.com)