Classic Edition: Making Quarks Out of Nothing at All

This is the second of a set of old posts, dating back to 2003, discussing the business of experimental particle physics. In this installment, I talk about how you get exotic particles by slamming ordinary ones together at high speed.

In a previous post, I gave a quick outline of the Standard Model of elementary particles, and how it relates to the recent discovery of a new particle. The best illustration of the process is probably the picture on the Ohio University reference page: A deuterium nucleus (one proton and one neutron) is sitting there, minding its own business, when a photon comes along and whacks into it. After being hit by the photon, a bunch of other stuff comes out, including the new "pentaquark" particle.

In terms of the basic quark model set out in the previous post, this seems a little weird. You start out with a collection of six quarks, bound into two nucleons: the neutron is two "down"s and an "up," while the proton is two "up"s and a "down." What you get out is, well strange: you've still got one proton, but now there's also a negative kaon, consisting of a "strange" and an "anti-up" stuck together, and this "pentaquark," containing two "up," two "down," and one "anti-strange." That's a total of four more quarks than you started with. Where did the rest of them come from?

The answer to that question drags in "The World's Most Famous Equation", Einstein's E = m c2 (E is the "Rest energy" of a particle, "m" is the mass, and "c" is the speed of light). This equation tells us that mass and energy are the same thing-- particles have energy just by virtue of having mass, while a sufficiently large quantity of energy can be treated as mass. It's what gives us atomic bombs, and nuclear power, and explains why the stars shine, and why weird things happen in particle physics experiments.

The equivalence of energy and mass is a surprising result, because it runs counter to our everyday experience. You don't think of moving objects as gaining some extra heft by virtue of moving-- if anything, you expect people who move around a lot to lose mass as a result...

The reason for this is that c is a tremendously large number-- 300,000,000 m/s-- so the mass gain due to everyday sorts of motions is negligible. An 80-kg man jogging along at 5 m/s has kinetic energy which would be equivalent to (roughly) 10-14 kg of mass, or 0.00000000001 grams. That's a few thousand times less than the mass of a single cell. It takes a phenomenal amount of energy to add up to a measurable amount of mass.

Of course, when you get down to the subatomic scale, the masses are very small-- A proton has a rest mass of 1.672 10-27 kg (roughly 10-7 joules of energy, or the amount of kinetic energy associated with an ant scurrying along the floor), and its component quarks have even lower masses. It's not exactly easy to generate enough energy to add up to a couple of quark masses, but it's nowhere near as difficult as trying to generate enough to add up to a macroscopic amount of stuff.

The equivalence of mass and energy means that, under the right circumstances, you can switch back and forth from one to the other. Anybody who's read any science fiction is probably aware of one direction of switch: If you bring together a normal particle and it's antimatter complement (a proton and an anti-proton, for example), they'll annihilate one another, and convert their mass entirely into energy (usually in the form of photons). If you could do this on a macroscopic scale, it would be a very efficient way of generating a lot of energy very quickly, which is why SF novels and TV shows tend to power their black-box stardrives with antimatter reactors of some kind.

What's less well known is that the opposite reaction can also occur. If you have a photon with a high enough energy, it can spontaneously create mass, in the form of a matter-antimatter pair. The photon energy required is equal to the mass of two particles of whatever variety you're after, which typically puts these photons well into the gamma ray range. In terms of the quantities people usually think about when dealing with light, these are photons with very short wavelengths-- the photon needed to produce an electron-positron pair has a wavelength that's roughly 1/500,000th of visible light.

(Once you get into this kind of range, the wavelengths are much smaller than the size of a single atom, so it doesn't really make sense to talk about them in wave terms. For that reason, in the gamma-ray region of the electromagnetic spectrum, people stop talking about wave properties, and just refer to the energy content of the photons-- "1 MeV" photons, for example (1 MeV = 1,000,000 eV, where an eV ("electron volt") is the energy a single electron gets from accelerating through a potential of one volt). Due to the equivalence of mass and energy, it's also conventional to give particle masses in terms of the energy content, so an electron is said to have a mass of "0.511 MeV/c2," meaning that the mass is such that multiplying by c2 would give you 511,000 eV of energy.)

The bigger the mass of the particle, the bigger the energy needed to produce a particle-antiparticle pair. Electrons are the easiest particles to generate, and the energies go up from there. Quarks require more energy to create than electrons do, and different types of quarks require different amounts of energy, as they have different masses. As noted in the comments to the previous post, particle mass is what distinguishes the different "generations" of particles. "Generation I" particles like "up" and "down" quarks have lighter masses than "strange" and "charm" quarks ("generation II"), which are lighter than "top" and "bottom" quarks ("generation III"). This is also what gives you the prohibition against moving up in generations-- you can split a heavy particle into lighter ones, but you can't go from a light particle to a heavy one without dumping in a lot of energy.

But once you get enough energy together, you can convert that energy into particle-antiparticle pairs, essentially creating new stuff out of thin air. This is the whole idea behind particle accelerators-- if you crank a beam of protons up to very close to the speed of light, you give those particles a fantastic amount of energy. When they slam into something, that kinetic energy is released, and can be converted into new kinds of particles. This is how all the particles other than protons, neutrons, and electrons were detected.

The pair creation effect is also why single quarks are never seen-- if you start with two quarks that are stuck together, you need to put energy into the system in order to try to pull them apart. The farther apart they get, the more energy they acquire (think of it as sort of like stretching a spring-- the more you stretch it, the harder you need to pull), until there's enough energy in the system to create a new quark-anti-quark pair. When that happens, two new particles pop into existence, and you end up with two new pairs of quarks. The harder you try to pull them apart, the heavier the new particles that pop into existence, but you can never pull a single quark out of a pair.

So, bringing this all the way back around to answer the question that started this post off, the "pentaquark" is formed by creating particles out of thin air (improbable as that may seem). A deuterium nucleus is sitting there, mining its own business, when a hugely energetic photon slams into it. The photon energy (and some of the energy that was holding the deuterium nucleus and its components together) goes into creating four new quarks out of nothing: an "up" and an "anti-up", and a "strange" and an "anti-strange." The "strange" and the "anti-up" pair off, and leave as a kaon, while the "anti-strange" and the "up" join up with the three quarks that used to be the neutron, and form the "pentaquark" (which later decays into a neutron and another kaon).

Put that way, it almost sounds easy... Of course, the tricky part is figuring out just what happened in the incredibly short period of time it takes those particles to pop into existence, fly apart, and decay into other things. But that will wait for another post.

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I'm not really up on these things, but the impression I have recently is that the whole pentaquark thing has fallen apart both theoretically and experimentally.

By Aaron Bergman (not verified) on 27 Sep 2006 #permalink

To be honest, I had forgotten that that was the focus of the whole thing-- I just cut-and-pasted the source from the old articles. It's the ultimate in lazy blogging, but Wednesdays are bad days for me.

I haven't followed the "pentaquark" story since then, so I couldn't really say. I'll see what I can turn up later.

Inside the proton are three quark types and these quark types are resting against each other: this produces a small region between the quarks which is called the (singularity) because each quark has a mass no matter how small it is.
Such a relationship within the proton or neutron creates a sense of purpose for the quarks as they do not have any force propelling them apart or pulling them together: Their object is to control the singularity ( to sit at the centre point if emptied of their strings or loops / this would reduce the quarks oscillation in time and space ) The result of reducing a quarks oscillation in time and space would create a gravity that could distort and destroy the proton shell and suck in remaining valance electrons: who could not fire enough emission photons to restore the proton system: To ping and never receive the return ping. Time stops / space becomeâs important as energy is quickly converted to mass.
The conditions within the proton are opposite to the conditions outside the proton shell: Therefore energy within the proton acts as waves and produces a resonance billions of times per one billionth of every second: an effect which in itself produces physical matter.
When energy is to escape the proton inner region it collapses as particles and produces the zoo of animals we have come to know: Therefore when differing anounts of energy exit the proton either by decay or collision, what we are seeing is the collapse of pure enrgy relative to its charge or mass to produce a particle type. This could go on forever especially if you introduce infinite fractions decimal or otherwise into the argument, as nothing is never really nothing, and even a string has a charge.

By carpenetr (not verified) on 22 Oct 2010 #permalink