Thursday, July 05, 2012

The Standard Model - summarized

In a most excellent overview of the Higgs(es?) news, The Economist manages the best concise summary of the Standard Model that I've read anywhere (emphases mine) ...

The Higgs boson: Gotcha! | The Economist:

... the Standard Model, the best explanation to date for how the universe works—except in the domain of gravity, which is governed by the general theory of relativity. The model comprises 17 particles. Of these, 12 are fermions such as quarks (which coalesce into neutrons and protons in atomic nuclei) and electrons (which whizz around those nuclei). They make up matter. A further four particles, known as gauge bosons, transmit forces and so allow fermions to interact: photons convey electromagnetism, which holds electrons in orbit around atoms; gluons link quarks into protons and neutrons via the strong nuclear force; W and Z bosons carry the weak nuclear force, which is responsible for certain types of radioactive decay. And then there is the Higgs.

The Higgs, though a boson (meaning it has a particular sort of value of a quantum-mechanical property known as spin), is not a gauge boson. Physicists need it not to transmit a force but to give mass to other particles. Two of the 16 others, the photon and the gluon, are massless. But without the Higgs, or something like it, there is no explanation of where the mass of the other particles comes from.

For fermions this is no big deal. The Standard Model’s rules would let mass be ascribed to them without further explanation. But the same trick does not work with bosons. In the absence of a Higgs, the rules of the Standard Model demand that bosons be massless. The W and Z are not. They are very heavy indeed, weighing almost as much as 100 protons. This makes the Higgs the keystone of the Standard Model...

I've read elsewhere that in the absence of the Higgs particles would zip around at the speed of light. Evidently, not so! The problem is rather with the W and Z bosons. That's quite different, but there's something about this summary that feels more authoritative.

I've pasted that text into Notational Velocity/SimpleNote so I have it in my extended memory.

There's more in the article ...

...  the model requires its 20 or so constants to be exactly what they are to an uncomfortable 32 decimal places. Insert different values and the upshot is nonsensical predictions, like phenomena occurring with a likelihood of more than 100%.

... One way to look beyond the Standard Model is to question the Higgs’s status as an elementary particle. According to an idea called technicolour, if it were instead made up of all-new kinds of quark held together by a new interaction, akin to but distinct from the strong force, the need for fine-tuning disappears.

Alternatively, the Higgs can maintain its elementary status, but gain siblings. This is a consequence of an idea called supersymmetry, or susy for short. Just as all the known particles of matter have antimatter versions in the Standard Model, in the world of susy every known boson, including the Higgs, has one or more fermion partners, and every known fermion has one or more associated bosons....

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