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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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By AnnihilatorX on 11/13/2012 7:50:13 AM , Rating: 1
What is wrong with trying to fix theory when it doesn't explain observation? What else are they going to do? Find some obscure Mathematics plucked out of the blue, make predictions and hope one would fit all reality and hence theory of everything?

So you do not like Newtonian physics, general relativity either because Newton's law was based on observation and general relativity came about because Eistein saw Newton's law did not fit reality?


By bug77 on 11/13/2012 9:34:23 AM , Rating: 2
The guy said the attempt to reconcile theory with observation is what might have gotten out of hand.

I'm not convinced that's he case, but it happened before. In the 19th century, while trying to explain the propagation of light (thought to be just a wave) through the vacuum. A new medium was proposed to explain this: aether. Throughout its existence, this medium earn all sort of crazy properties, in an attempt to match theories and observation. We all know how that ended: there was no aether, but light wasn't just a wave either. Right now, we think dark matter must exist because our observations seem to point to the fact that galaxies should really be much heavier. Just as in the case of light, the correct explanation could be entirely different - but we don't know that yet.

For a reading on aether: http://en.wikipedia.org/wiki/Luminiferous_aether


By drycrust3 on 11/13/2012 10:05:50 AM , Rating: 2
quote:
What is wrong with trying to fix theory when it doesn't explain observation?

What is wrong isn't the "trying to fix" bit, but that the theory "doesn't explain [the] observation" bit. A good theory explains the observation, and it enables predictions that turn out to be true. In fact, one could argue that if you can't make fairly accurate predictions with a theory then it isn't much use.


By inko1nsiderate on 11/13/2012 5:23:36 PM , Rating: 2
Full stop. SUSY was not proposed to solve the Dark Matter problem. SUSY was introduced to solve the Hierarchy problem, or essentially cancel out the quadratic divergences in the Higgs mass to protect the mass of the Higgs from higher energy physics. The Dark Matter candidates in SUSY come from trying to solve the Proton decay problem SUSY brings up. SUSY, without R parity, allows the decay of the Proton to be very large (~15 minutes) and so by introducing R parity this problem was solved. By introducing R parity it also happens to allow a natural description for Dark Matter because odd R parity particles cannot decay into even R parity particles, leading to a potentially stable neutral particle. The breaking of SUSY then allows these odd R parity neutral particles to become WIMPs, and thus potentially explain cosmological Dark Matter.

Now, if we are going to say there is no theoretical understanding of Dark Matter, then we are also going to say something that is flat out wrong. It is very easy to make a model with Dark Matter. Exceedingly easy. So much so that pretty much any given model of physics beyond that Standard Model can have additional Dark Matter particles inserted. For instance, you can make Dark Matter that also leads to Neutrino mass. You can also have Dark Matter from SUSY (coming from R parity), and then add on extra Dark Matter particles (as in other species of Dark Matter or just potential Dark Matter particles with the lightest mass particle of all candidates being the cosmological Dark Matter).


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