The Standard Model is a physical theory of a spectacularly successful sort. It is built on beautiful and deep mathematics, covers almost all known physical phenomena, and agrees precisely with the result of every single experiment ever d...
The Standard Model is a physical theory of a spectacularly successful sort. It is built on beautiful and deep mathematics, covers almost all known physical phenomena, and agrees precisely with the result of every single experiment ever done to test it. It leaves open a very small number of questions: why this specific combination of small symmetry groups and their representations? What determines the parameters of the model (18 if you ignore neutrino masses, 7 more if you include them)? What about gravity? Does it need to be extended to account for dark matter?
For several decades now, there has been a very active and heavily advertised field of “Beyond Standard Model” physics, the study of extensions of the standard model that remain consistent with experimental bounds. While BSM models have played a role in guiding experimentalists towards things to look for that are not already ruled out by what is known, they have never come anywhere near fulfilling the hope that they might provide some insight into the SM itself. They provide no explanation of the unexplained aspects of the Standard Model, instead adding a great deal of additional unexplained structure. Perhaps the simplest and most widely studied example is the minimal supersymmetric extension of the SM, which not only explains none of the 25 undetermined SM parameters, but adds more than 100 additional such parameters to the list.
Theorists have traditionally followed what has been described as “Albert Einstein’s dream that the laws of nature are sublimely beautiful, inevitable and self-contained”, and the SM is our closest approach so far to Einstein’s dream. If you shared this dream, the known BSM models would never have much appealed to you, since they just added complexity and extra unexplained parameters. You also would not have been at all surprised by the strong negative results about such models that are one of the two major achievements so far of the LHC (the other is the Higgs discovery). If you’re a follower of Einstein’s dream, the obvious reaction to the LHC results so far would be to rejoice in the vindication of this dream, welcome the triumph of the simplicity of the SM, and hope that further study of the Higgs sector will somehow provide a hint of a better idea about where the SM parameters come from (almost all of them are Higgs couplings).
Remarkably, a very different story is being sold to the public by those who had a great deal invested in now failed BSM models. In this story, the BSM models were the ones of Einstein’s dream: they were “natural”, and their failure leaves us with the “unnatural” Standard Model.
An article entitled Is Nature Unnatural? is the source of the above quote about Einstein, and it tells us that
Decades of confounding experiments have physicists considering a startling possibility: The universe might not make sense…
In peril is the notion of “naturalness,” Albert Einstein’s dream that the laws of nature are sublimely beautiful, inevitable and self-contained. Without it, physicists face the harsh prospect that those laws are just an arbitrary, messy outcome of random fluctuations in the fabric of space and time…
“The universe is impossible,” said Nima Arkani-Hamed, 41, of the Institute for Advanced Study, during a recent talk at Columbia University [more about this talk here].
What is behind this sort of claim that down is up is abuse of the English word “naturalness”, which in this particular case has been adopted by theorists to refer a technical property better described as “not quadratically sensitive to the cut-off scale”. There’s a lot to be said (and a lot that has been said on this blog) about the precise technical issue here. It’s a real one, and likely an important hint about the true nature of the Higgs sector of the SM and where all those undetermined parameters come from. Getting