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Physicists must find the Higg's Boson's counterparts to prove supersymmetry

The Large Hadron Collider is being used to look for more particles after the Higgs Boson
October 29, 2014, 12:00pm
LHC detector Image: µµ/Flickr

The Large Hadron Collider is hardly done smashing. As you read this, physicists are firing test beams around the Super Proton Synchotron, the accelerator ring just inside the LHC that supplies the main attraction with proton beams. Next year, the LHC itself will be up and running, colliding particles at energies twice that of its first run.

The LHC is just getting started, in a sense. The Higgs boson? After so much anticipation and fanfare, you'd almost think that discovering the Higgs boson was the conclusion of something.

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Hardly. The LHC might have revealed the Higgs, but what it didn't find has caused a full-on crisis within particle physics, a disquieting rift between experimental results and theoretical predictions. The Higgs boson should have a family, a whole village of particles linking the two distinct categories comprising the Standard Model of Physics, fermions (which usually make up matter) and bosons (which usually handle forces).

The LHC's first round of experiments found nothing of the sort, however: just the Higgs. This calls into question one of the most hopeful, and seemingly necessary proposals for New Physics: supersymmetry. A 2013 paper summarizes the anxiety well enough:

The first three years of the LHC experiments at CERN have ended with 'the nightmare scenario': all tests, confirm the Standard Model of Particles so well that theorists must search for new physics without any experimental guidance.

The problem with the Standard Model is that it predicts all particles in the universe to be massless. Basically, existence should consist of energy, nothing more—no dust and gas to gather into galaxies and planets and humans, just radiation.

This is obviously not the case, and so the Standard Model is not the case. It's more a framework of convenience until we find the next thing. And the next thing was thought to be supersymmetry. Supersymmetry is an enormous principle related to particles and particle interactions that attempts to explain the differences between the four fundamental forces of nature: the electroweak force (particle decay), the strong force (the attraction that holds atomic nuclei together), electromagnetism (electricity, light, chemistry, most everything conjured by the word "force," save for gravity), and gravity.

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The Standard Model:

The strengths of each of these forces are so different from each other that it's almost impossible to fathom. These differences are manifested only at the very low energies/temperatures that particles experience today; in the superheated mess of the very early universe, in which no particles had mass, there would have just been a single unified force. A simple realm, but hardly fit for life.

The universe cooled, and this superheated mess condensed, just as water droplets condense on the outside of a chilled glass. Everything, all of these particles and forces, became differentiated, acquired definition from their prior plainness. This includes the Higgs field, a substrate that extends throughout the universe. When a particle interacts with this field it's able to acquire mass. The catch is that once something has mass via the Higgs boson, it keeps interacting with the Higgs field.

As Cornell physicist Csaba Csaki explains, the Higgs field acts as a "very special kind of damping force," e.g. a force that slows down an object moving through a particular medium. This force is proportional to the acceleration of the object/particle.

This whole Higgs field interaction doesn't quite work with the Standard Model or, rather, everything comes out wrong. The problem is that the Higgs field itself acquires mass from the Standard Model particles it interacts with, and if there's just one Higgs particle, that turns out to be a very big mass—too big.

"Imagine you have a very hot bath of particles," Csaki says. "This means that the particles inside the bath have very high energies, and are flying around with large velocities. Now, imagine you shoot into this bath a particle with very small energy, that interacts with the other energetic particles. What you expect is that the particle you shot in will eventually after a few collisions have roughly the same order of magnitude of energy as the other particles in the bath."

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"Now, if you fish out the same particle and find that it did not pick up a correction to its energy, you will be very surprised," she continues, "and say that something special must have happened: Perhaps the particle did not interact at all, or there is some reason why the particle can not gain energy beyond a certain size."

"This is exactly what is happening with the Higgs," Csaki says.

The hot bath the Higgs is actually cruising around in is the vacuum of empty space. Except, empty space is hardly a vacuum at all and is instead a sea of short-lived "virtual" particle pairs blinking in and out of existence as the result of quantum uncertainty. Simply, a true void is too certain for the quantum world, which is indeterminate by definition. Instead of emptiness, we have a very odd zero-point foam, full of energy that, while highly transient, adds up to a whole lot. It's thought that this vacuum (or dark) energy is the force behind the universe's accelerating expansion.

"This Higgs boson would actually feel and interact with those very energetic short-lived pairs of particles," Csaki explains, "which will result in its mass getting pushed up. What supersymmetry would do is to make sure that there are several different types of particles in the vacuum that can pop in and out of the vacuum, and their contributions to the Higgs mass would mainly cancel."

Within the supersymmetry principle, every particle in the Standard Model has a sort of shadow. This shadow is a bit like antimatter, but with less fireworks. The shadow particle's role is in shouldering enough of this extra mass to make the Higgs boson's mass a lot more reasonable and more like what's so-far been observed.

So, with this other shadow particle taking care of its partner's mass, the world makes sense again and supersymmetry has saved the day. Not only that, but many supersymmetric theories (supersymmetry is a principle, not a theory in itself) offer up a super-light variety of particle that would seem to fit the bill for dark matter. Nice!

Well, it would be nice if there was any evidence for it. Supersymmetry predicts a lot of bonus particles, and yet none of them have ever been observed. The doubling isn't so crazy when you consider that antimatter theories did the exact same thing and those particles turned out to exist just as predicted, but antimatter came easy, in comparison.

There's still some outside hope for the supersymmetric world. Most theories involving supersymmetric principles were excluded by the first round of LHC results. The next LHC run will involve much larger energies and much heavier masses. These missing superpartners aren't very likely to be this heavy, sadly, but it's not impossible. The catch is that heavier superpartners begin to look contrived, less a feature of naturalness, something considered to be a necessary part of supersymmetry.

If supersymmetry doesn't exist or is invalid, we're still stuck a big mess. At the very least, this means that particle physics isn't about to start being boring any time soon. Only time and super-high energy particle collisions will tell.