A few days ago, scientists at the European Center for Nuclear Research (CERN) announced the discovery of a new particle. Using the newly souped-up Large Hadron Collider (LHC), definitive evidence for a particle made up of 5 quarks, a pentaquark, was observed. Not having seen a diary posted about it here, other than an item in the Overnight News Digest, I thought I could devote this diary to what's been reported about it, with perhaps a little background. I should offer here that while I am a scientist, I'm not a particle physicist, so what I'm giving is my understanding of the popular descriptions of elementary particle theory and nothing much deeper than that.
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Let's start simple: What's a quark?
Back in 1932, James Chadwick discovered the neutron, and at that point, all of the constituents needed to make up all of ordinary matter became definitively known: electron, proton and neutron. Each atom has a nucleus consisting of a number of protons equal to its atomic number, as well as a certain number of neutrons. In a neutral atom, the number of negatively charged electrons orbiting the nucleus is equal to the number of protons. For elementary atomic structure, that's all you need to know. Just three particles are all that's necessary to describe the behavior of most atoms.
At this point, if physicists had been smart, they would have dismantled their particle accelerators and gone fishing. Instead, they continued their experiments and built ever bigger and more powerful particle accelerators. Using these instruments, they found a plethora of new particles that did not seem to have anything to do with ordinary matter. They were all short-lived, decaying in small fractions of a second, but the fact that they existed for any time at all had to be explained. Over the course of the 1950s, the particle zoo had grown huge, with little rhyme or reason. There had to be some way to systematize this zoo.
There were two known varieties of these particles: baryons and mesons. Protons and neutrons are baryons because they are stable particles that respond to the strong nuclear force. They are held together in the tiny atomic nucleus despite the very large electrical repulsive force that exists between the like-charged protons; that's why the force that holds the nucleus together is called the strong force--it has to be stronger than electrical repulsion. Mesons are particles proposed to mediate the strong force; two baryons can be held together by trading a meson, just as two charged particles trade photons (light quanta) to mediate the force between them. (A big difference here though is that mesons are matter while photons are not.)
In 1964, George Zweig and Murray Gell-Mann independently came up with a way to make sense of the particle zoo: they proposed that all of these observed particles were not elementary themselves, but in fact composed of some combination of still more elementary particles. Zweig called these particles "aces," but Gell-Mann called them "quarks," and Gell-Mann's moniker stuck. Each baryon was proposed to consist of three quarks, and each meson to consist of quark-antiquark pair. (An antiquark is the antimatter counterpart to the quark.) Zweig and Gell-Mann were able to account for all particles known at that time using just three distinct types of quarks (and their antiparticles). Further, and more importantly, they were able to predict the existence of other particles that had not yet been discovered, but later were, as other combinations of quarks.
[A full discussion of the different types of quarks and the forces that act between them would require a much longer diary, so I'm not going there. If you're curious, look here.]
Theoretical physicists being what they are, they immediately started playing around with the idea of the quark, and considered what other kinds of combinations (besides combinations of 2 and 3) might form observable particles. It didn't take long before the pentaquark was proposed, actually consisting of four quarks and an antiquark. During the 5 decades following, experimentalists have claimed to have discovered the pentaquark, only later to determine that they observed a glitch: false alarm.
It took until this year, and the ultra high-energy LHC at CERN, to produce definitive evidence for the pentaquark's existence. More experiments must be performed to confirm its properties and to further investigate its nature. One question yet to be answered is whether the pentaquark is itself a distinct particle, with all quarks interacting directly, or if it's just a baryon and a meson stuck together.
Myself, I have been wondering about the utility of the LHC after the discovery of the Higgs boson. Thus far, the only results I've seen announced since the Higgs are this pentaquark and the discovery of the tetraquark last year. While these are interesting discoveries, they are well within the realm of the Standard Model, the theory that ties together the electromagnetic, and the strong and weak nuclear forces. What would really shake things up is a completely unexpected discovery, one that the Standard Model could not predict. There are various theories (e. g. supersymmetry) that predict the existence of other types of particles, but to this point there has been no experimental evidence of such, or even a hint of where to find them. This is the challenge to the LHC: Break physics. The theories we have work too well. We won't make any really new discoveries until we find a case where the known theories don't work. Until the LHC makes a discovery of this sort, it will not have proven its worth.
Enough editorializing. On to the comments!
TOP MOJO
July 15, 2015
(excluding Tip Jars and first comments)
Got mik!
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July 15, 2015
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