Physicists may need new theories to explain how dark matter works

Supersymmetry, or SUSY for short, has been a popular physics theory used to explain away quirks in the Standard Model.  But recent findings from CERN's Large Hadron Collider cast serious doubts on traditional SUSY theory, sending physicists back to the drawing board.

I. Dark Matter -- Does SUSY Offer an Explanation?

When it comes to SUSY, the theory began with a fundamental question -- why were galaxies spinning so fast?

Physicists in the 1900s began to predict the mass of galaxies based on the light of stars within.  What they found was surprising -- the galaxies were spinning faster than they would be if merely adhering to a vanilla version of the Standard Model.

So physicists theorized that the galaxies contained large amounts of so-called "dark matter".  This type of matter is thought to behave in fundamentally different ways from standard matter.  The question facing physicists was how does dark matter behave; physicists sought to solve that question with the theory of super-symmetry, a theory which grew increasingly popular in the particle physics world over the years, spawning several variants.

Dark matter
SUSY is a leading theory to explain the existence of dark matter. [Image Source: NASA]

Under one version of the theory -- the Minimal Supersymmetric Standard Model or MSSM for short -- physicists Howard Georgi (Harvard University) and Savas Dimopoulos (Stanford University) proposed that dark matter consisted of super-particles of masses between 100 GeV and 1 TeV.

The question was how to observe the presence or lack of these high-energy super-particles.  At the time (the 1980s), no particle collider was powerful and sensitive enough to create and detect such pairs.  Then the Large Hadron Collider (LHC) came along.

II. Signs Point to Many SUSY Models Being Flat-Out Wrong

While the LHC is best known for the Higgs boson hunt (scientists currently think they may have observed signs of this much-sought-after particle), the LHC is powerful enough to probe other major unconfirmed physics theories.

SUSY is a perfect example.

The LHC has seven built in particle detectors.  These include flashy detectors like ATLAS and CMS, which have been used in the Higgs boson hunt.

Many popular version of SUSY predict that the "strange" B-meson -- a short-lived 0.5 TeV (in mass) particle that oscillates between a matter and antimater state -- will decay to muons at a far greater rate than the extremely low rate predicted by the vanilla Standard Model.  The source of this shift stems from decay loops such as the chargino and Charged Higgs boson, which SUSY predicts [source] will enhance muon decay rates, by about an order of magnitude.

But it turns out the decay was not as frequent as SUSY expected.  

Bs decay
Bs mu-mu decays occur less frequently than SUSY generally predicts. [Image Source: CERN]

Most detectors failed to observe that kind of decay at all.  And when the LHCb detector finally did spot it, it estimated that only three out of every billion decay results in muon production.

III. Door Opens to New Theories

This at first blush seems an intuitive conclusion -- it would indeed seem odd that the mid-size meson would produce the relatively massive muons on a frequent basis.  But the result does raise major questions -- if SUSY is wrong, what is dark matter made of?

An important thing to note is that while CERN physicists say the new data "squeezes" super-symmetry models, it does not say it invalidates all of them.  For example the so-called AKM model -- theorized by professors Ambrosanio, Kane, Kribs, Martin and Mrenna -- appears to encompass the results in its fringe reaches.

As Prof. Chris Parkes describes to the BBC News, "Supersymmetry may not be dead but these latest results have certainly put it into hospital."
Susy v. SM
SUSY v. SM 2

The observation pushes SUSY to its fringes, raising questions of its validity.
[Image Source: CERN]

Even if the AKM model can accomodate the new results, the fact that they blow up many alternate SUSY models (most of which have over 100 fittable parameters) opens the door to fundamentally different solutions than SUSY to try to explain away symmetry violations.

In other words, the possible fall of SUSY sets the stage for a renaissance of new theory, the kind that equally delights physicists and gives the average member of the public at large a painful headache.

Sources: CERN, BBC News

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