Tuesday, February 21, 2012

I Come To Praise SUSY, Not To Bury It.

Another summary of the latest non-detection and expanded exclusion range for stops (supersymmetric partners of top quarks) at Resonaances spells out the situation:

To a casual observer, the seminar presenting the results may have resembled a reenactment of the Saint Valentine's day massacre. Nevertheless I will argue here -- more out of contrariness than conviction -- that SUSY may be battered, bruised, covered in slime, and badly bleeding, but formally she is not yet dead. For most particle theorists, the death will be pronounced scalar partners of the top and bottom quarks are excluded up to ~500 GeV, the precise threshold depending on taste, education, and diet. That's because excluding light stops and sbottoms is going to squash any remaining hopes that supersymmetry can address the naturalness problem of electroweak symmetry breaking, for which purpose it was originally invented. However, the LHC collaborations have yet not presented any robust limits on the stop masses. Instead, here's what the LHC have taught us about SUSY so far:

* Since summer 2011 we know that in a generic case when all colored superpartners including stops have comparable masses and decay to the lightest supersymmetric particle (LSP) producing a considerable amount of missing energy then stops have to be heavier than about 1 TeV.

* Since last Tuesday we know if the only squarks below TeV are those of the 3rd generation then gluinos have to be heavier than about 600-900 GeV, depending on the details of the supersymmetric spectrum. . . .

To summarize, the year 2012 will surely go down in history as the Higgs year. But among more studious future historians of science it may also be remembered as the year when SUSY, at least in its natural form, was finally laid to grave...

Another major report on experimental support for SUSY is due in a couple of weeks.

An experimental exclusion of SUSY entirely is quite a bit more profound than it seems at first glance. Nobody but particle physics geeks knows much about SUSY. But, string theory necessarily implies SUSY, so an experimental exclusion of SUSY is also an experimental exclusion of string theory. And, string theory is a beyond the Standard Model theory with a much better P.R. agent than SUSY.

Let me repeat this simply so that those of you who aren't paying attention can get it:

The Large Hadron Collider is on the verge of experimentally ruling out all of the most plausible versions of string theory.

Why is this huge?

Probably about half of the theoretical physicists in academic positions in the United States right now are string theorists. Most of the beyond the Standard Model theories being tested at the LHC are predictions of SUSY or String Theory. String theory, despite its waning prominence, has far more published work and far more adherents among professional physicists than any other beyond the standard model theory that could unify the three Standard Model forces into a "Grand Unified Theory" (i.e. GUT) or unify these with gravity as well into a "Theory of Everything" (i.e. TOE).

At one point people thought that there were a lot more alternatives to String Theory, but then, somebody figured out that all of the different versions are just different ways of describing what is called "M theory".

Sometimes people talk about Loop Quantum Gravity (LQG) as a competitor to String Theory, and as a quantum gravity theory, it is a competitor. But, LQG doesn't even aspire to be a GUT or a TOE.

Various versions of a beyond the Standard Model theory called "Technicolor" are on life support too, because fundamentally, Technicolor is a theory designed to solve theoretical problems that arise if it turns out that there is Higgs boson at a mass on the same order of magnitude as the W and Z bosons. But, the likely discovery of a 125 GeV +/- Higgs boson pretty much makes Technicolor obsolete. The failure of experiments to reveal magnetic monopoles or proton decay has also culled a lot of naively attractive GUT models other than string theory.

Thus, disproving SUSY, together with other recent developments, basically brings theoretical physicists back to square one in the errand of trying to unify all of the fundamental forces found in physics and explain why we have the particles that we do from a more fundamental basis.


Mitchell said...

Supersymmetry is nowhere near being ruled out. What is being tested is the idea that supersymmetry plays a role at the weak scale.

Andrew Oh-Willeke said...

I carefully limted my statements to "plausible" verions. Other SUSY scenarios cease to be very well motivated even though it is possible to fit versions of them to the data since they have so many parameters.