“…this consensus has been brought about, not by shifts in philosophical preference or by the influence of astrophysical mandarins, but by the pressure of empirical data.” -Steven Weinberg
One of the most fundamental questions we could ever ask about all of existence is “What makes up the Universe?”
I don’t mean “stars and galaxies,” like you see above. That might make up the Universe on the largest scales, but that’s taking a look at the question of what the fundamental constituents of the Universe compose themselves into.
The other side of the coin is to look at ever smaller and smaller scales at matter, and try to figure out what the smallest, indivisible things are. To take anything, whether it’s a supercluster, star, planet, human or amoeba, and break it down into the smallest things possible.
Beyond the cellular level, beneath the molecules and atoms and protons of existence, we can finally reach the fundamental, indivisible particles that cannot be split into anything smaller. These particles — the quarks and gluons, the charged leptons and neutrinos, the electroweak gauge bosons and the Higgs — make up the Standard Model of elementary particles. Combined with gravity, as far as we can tell, these are the the most fundamental constituents of all the normal* matter and energy in the Universe.
We learn about these particles — their existence, charges, masses, and (for the unstable ones) their lifetimes and decay properties — not from building ever-more-powerful microscopes. Instead, we build ever-more-powerful particle accelerators and colliders, and measure what comes out with gigantic detectors.
By accelerating particles as close as possible to the speed of light, and colliding them with either particles-or-antiparticles moving with an equal and opposite amount of momentum, we can use a giant fraction of that energy (up to 100%) to simply create new particles; anything allowable by the laws of physics.
You build your giant detectors around the collision point, and measure what comes out; this is how your modern particle accelerators work. Now, here’s the thing: you don’t make the vast majority of these fundamental particles — especially quarks and gluons — on their own.
Because of how long they give, they combine with other fundamental particles and form bound states, such as mesons or baryons, before they decay.
One of the variations of mesons that exist consist of a bottom quark (or antiquark) and a strange antiquark (or quark) pair. Both the bottom quark and the strange quark are unstable, and there are a variety of decay products that can arise from this particle.
One of the decay channels — and it’s a rare one — is that this particle could decay into a muon and an anti-muon.
Now, here’s where it gets interesting: all fundamental particles that exist in the Universe couple to one another. The question of exactly how they couple and by exactly how much is what keeps many theorists in particle physics employed, because the underlying physics is known, but the calculations of things like event rates, branching fractions/ratios and decay parameters needs to be calculated. Moreover, if there is any interesting physics beyond the Standard Model, it will show up as a departure in the decay rates/ratios of particles such as this.
In other words, you can see what the Standard Model prediction is: about 3.5 × 10-9 of the bottom-strange mesons will decay into muon-antimuon pairs, and about 1.1 × 10-10 of the bottom-down mesons will decay into muon-antimuon pairs.
Any departure from this becomes very strong evidence for physics — and fundamental particles — that are outside of the Standard Model. However, a measurement of these branching ratios that lines up with the Standard Model will very severely constrain alternatives to the Standard Model, especially Supersymmetry (SUSY).
Basically, the lower-in-mass the (theoretical) superparticles are, the more they affect that rate. So if we measure the rate to be what the Standard Model tells us to be, we can constrain the mass of any (possible) SUSY particles to be very large, and the more we constrain that rate to be in agreement with what the Standard Model tells us, the worse it looks for supersymmetry as having any relevance for our Universe.
The first actual measurement, rather than a constraint, of that decay!
And… do you want to know what they found? Whether we have evidence that the Standard Model is perfect, or whether there’s something new out there, on the brink of discovery?
I’ve got to say, the results are in, and what they basically state is that there is no need for any physics beyond the Standard Model, as they measure rates that about 3.2 × 10-9 of the bottom-strange mesons (with error bars of about +1.5 and -1.2) will decay into muon-antimuon pairs, and less than 9.4 × 10-10 of the bottom-down mesons decay into muon-antimuon pairs, both the best constraints ever. (Full PDF here.)
To paraphrase a particle theorist friend of mine:
This constrains the masses of the supersymmetric particles to be anywhere from about 1 TeV up to infinity. I’m betting on infinity.
In other words, there is currently not a shred of experimental evidence in support of the need for — or existence of — SUSY.
For those of you keeping score, no SUSY at all energies means no on the question of string theory, so don’t hold your breath on that front, either. We may have reached the end of what we can learn about the fundamentals of the Universe from particle physics, and the more the data from the LHC continues to agree with the Standard Model, the less attractive “new physics” theories like SUSY, extra dimensions and technicolor become.
The Standard Model still has problems, and there must be physics — at some point — beyond it, but this could be it for particle physics. Is there any particle physics beyond the standard model? So far, the evidence says no.
* – Dark matter not included.