21 June 2010

The Case For Supersymmetry

LuboŇ° Motl is a character in the science blogosphere. He is strident, utterly self-confident and not one to suffer fools kindly. His political stances and Internet manner can make a stir even when he is not talking about science. But, today, he is (mostly) talking about science.

He is an ardent supporter of string theory and supersymmetry and makes a particularly bold show of support in the wake of the possibility that new physics including a scenario with five Higgs bosons could have experimental support as he explains in a post today.

A key point regarding his support of supersymmetry is that it goes hand in hand with string theory. If string theory is right, supersymmetry is the most plausible version of string theory that could fit the real world data.

He has acknowledged that "Fermilab's D0 Collaboration claimed the evidence for a new source of CP-violation in their comparison of muon-muon and antimuon-antimuon final states . . . [is] a huge claim." And, he acknowledges that it may not end up being true. Indeed, he's proud that physicists have such high standards that a finding that is statistically 95% likely to be true doesn't mean much, unlike other disciplines with lower standards. Likewise he notes that supersymmetry "may or may not be seen by the LHC." This is all extremely ordinary and boring. No reputable physicist on the planet is saying anything different.

But, the frame into which Motl puts the finding is remarkable, not so much for its content (many physicists have invested immense amounts of time on the same theories, essentially betting their careers on them) as for its confident style.

He first hits the basics. He explains how a single Higgs boson would fill out the Standard Model by giving particles masses, and how a supersymmetric model would require five Higgs bosons.

Essentially, the mathematical structure of the Standard Model would need four Higgs bosons except for the fact that the three particles that give rise to the nuclear weak force serve the same role leaving the Standard Model just one particle short of what it needs for all of its mathematics to be work. The Higgs is also needed in the Standard Model to give quarks mass.

He then explains that a five Higgs boson model is necessary for a supersymmetric theory in order to make the more elaborate mathematics behind a theory with far more particles work, you need two doublets (i.e. 8) Higgs bosons less three weak force particles that serve the same role in the model.

Thus, the Standard Model with one Higgs boson, and a supersymmetric extension of it with five, both make the math work and produce a theoretical structure that could explain what we observe.

Supersymmetry is a highly constrained property of the Universe that . . . predicts some new phenomena but the tightly organized system controlling all the new particles and interactions actually makes supersymmetric extensions the most conservative models of new physics that you may add to the Standard Model.

That's not enough to feel confident that the LHC should observe the supersymmetry.

But if you're competent, if you understand how supersymmetry (at some scale) follows from (i.e. is predicted by) all realistic vacua of string theory, if you appreciate how it helps to produce the dark matter candidate particles and to preserve the gauge coupling unification, and how it helps to solve the hierarchy problem (the puzzle why the Higgs doesn't want to become as heavy as the Planck scale), you may start to understand why it seems more likely than not that the LHC should actually observe SUSY.

In other words, supersymmetry flows from string theory, could explain dark matter, and could motivate the properties of the basic laws of nature that the Standard Model doesn't even try to explain.

This is particularly bold because, if the LHC doesn't discover anything but one Higgs boson and one graviton, and there is so far only the faintest hint outside that mathematics that it might. Further, the make or break moment when some of the predicted particles will either be discovered or ruled out is a just a few years away.

If the LHC doesn't find anything new (and even if it finds only one Higgs particle) then supersymmetry, and by implication string theory, becomes much less plausible as a theory. The showdown is approaching and Motl has accurately shown that we may soon have some strong pointers towards whether the main thrust of theoretical physics for the last few decades is right or is likely to be wrong.

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