New phenomena in particle physics - searching for our dear SUSY (aka supersymmetry)

avatar
(Edited)

In my previous post on the physics beyond the Standard Model of particle physics, I presented few motivations for this very active field of research. At the end of the post, I offered to describe the most popular paradigms that are considered for new physics studies, and here we are.



[image credits: particle central]

Supersymmetry is the class of models which I focused on when I entered the field of particle physics. Very popular at that time, supersymmetry, also abbreviated as SUSY, is still in the pipeline. It is even at the basis of a very important fraction of the experimental searches for new phenomena.

Although supersymmetric signals are expected to show up at the energies probed at the Large Hadron Collider at CERN, nothing has been found so far. Nevertheless, supersymmetric theories have a very rich phenomenology and are thus widely accepted as at least playgrounds to test new physics.


FROM THE FUNDAMENTAL INTERACTIONS TO SYMMETRIES

Before discussing anything related to supersymmetry, it is important to emphasize that we have two classes of particles in nature, that differ according to their spin (an intrinsic property of each particle).

We have particles whose spin is integer, known as bosons (like the Higgs boson, the photon or the gluon) and particles with a half-integer spin, known as fermions (like all elementary matter constituents such as the electron, the quark or the muon). Both have very different properties.



[image credits: homemade]

The word supersymmetry contains the word symmetry, and this is not a random choice. Supersymmetry is in fact one possible symmetry of nature. The particularity of supersymmetry is that it is a symmetry that connects bosons and fermions.

This type of symmetry changes the bosonic/fermionic nature of the particles and thus generalizes the usual way symmetries are seen. A supersymmetry transforms a boson into a fermion, and vice versa. Moreover, particles mapped by a supersymmetry should have similar properties (mass, charge, etc). This has profound consequences.


THE SIMPLEST SUPERSYMMETRIC MODEL AND THE SUPERPARTNERS



[image credits: CERN (but cannot find where anymore)]

The simplest theory we could try to build is a supersymmetric version of the Standard Model. As said above, supersymmetry transforms a boson into a fermion and vice versa. We however do not have enough particles to allow supersymmetry to work within the Standard Model particles.

In particular as they are some requirements on the common properties of the particles connected by a supersymmetry. The solution is simple: let us add as many new particles as required for supersymmetry to be a symmetry of nature.

Each particle of the Standard Model has now a partner, or a superpartner, connected by a supersymmetry operation. We have hence electrons and selectrons, quarks and squarks, photons and photinos, Higgs bosons and higgsinos, etc. A lot of cool names, and a lot of potential particles to discover.

But can nature be supersymmetric? By symmetry reasons, a Standard Model particle and its superpartner should have the same mass so that the superpartners should have already be observed.

As this is not the case, we must construct a model by enforcing supersymmetry in a first step, but then we break it spontaneously to introduce mass splittings between the Standard Model particles and their partners. I refer to this post of mine for more information on the symmetry breaking mechanism.

Symmetry breaking is standard, so no big deal here :)


CURING THE ISSUES OF THE STANDARD MODEL



[image credits: pixabay]

Nature does not necessarily need to be supersymmetric, but it could be. And if it is, we have a natural solution to many of the issues that I have mentioned in my previous post. This makes supersymmetry attractive as an option for describing what we do not know about nature and get below the Standard Model iceberg.

First of all, by construction, supersymmetry unifies the concept of bosons and fermions with the fundamental interactions. We have symmetries explaining the strong, weak and electromagnetic interactions, as well as symmetries mapping the Standard Model bosons to their superpartners.

In my last post, I mentioned the hierarchy problem implying that the Higgs boson mass was very sensitive to the quantum corrections except if the model parameters were tuned to their 30th decimal. In supersymmetry, this is not needed anymore. The effects of the superpartners exactly cancel those of the Standard Model, and the Higgs mass thus behaves nicely, provided the supersymmetric guys are not too heavy. This is why we expect them to show up at the LHC.

On top of that, the lightest superpartner has in general an interesting property. It can be made neutral, weakly interacting and stable. These are exactly the properties of a particle that could play the role of the dark matter. Bingo! Supersymmetry can explain dark matter.

The presence of the superpartners also modifies the dependence of the fundamental interaction strengths with the energy. And as a result, the three fundamental interaction couplings unify at a scale roughly lying three orders of magnitude below the Planck mass scale. We have now an energy regime where three of the fundamental forces can potentially unify.

There are many other advantages for a supersymmetric theory but I will try to prevent this post from exploding in length. So that I stop here.


OBSERVING SUPERSYMMETRY?



[image credits: CERN]

I have so far described the gains associated with imposing a theory to be supersymmetric. But the next step would be to detect the consequences of such.

Since the superpartners are sensitive to the fundamental interactions in the same way as their Standard Model counterparts (supersymmetry is a symmetry after all), they can be produced at colliders such as the LHC and thus searched for.

These superparticles are however unstable and will decay. A superpartner always (OK not always but let us keep the scenario dark-matter motivated) decays into another superpartner and a Standard Model particle. But the lightest superpartner cannot decay and is stable. A particle can indeed only decay into a lighter particle and there is none available for the lightest superpartner.

Freshly produced superpartners will then decay into each other, forming a nice cascade, until the lightest one, stable and weakly interacting, will leave the detectors invisibly. These cascade decays can be observed, including the missing energy carried by the escaping lightest superpartners, and they are thus actively searched for. However, unfortunately, there is no signal so far.


SUMMARY

With this first post trying to give elements about the numerous theories of particle physics beyond the Standard Model, I have introduced the concept of supersymmetry, a symmetry connecting bosons with fermions. As a result of this connection, each Standard Model particle gets a superpartner, a particle with the same properties but a different bosonic/fermionic nature.

Whilst doubling the number of particles may sound complicated, this allows one to address many of the conceptual issues of the Standard Model, like the hierarchy problem, the dark matter issue or the unification of the fundamental interactions.

In the next episode, I will discuss a former competitor to supersymmetry in terms of popularity, extra-dimensional models. They are still considered today, but not as much as 10 years ago.



0
0
0.000
10 comments
avatar

One question, when you say “And as a result, the three fundamental interaction couplings unify at a scale roughly lying three orders of magnitude below the Planck scale. We have now an energy regime where three of the fundamental forces can potentially unify.” is it then that that Gravity is the only fundamental force left over, and if so what happened to the other 3 forces. Did they rap space into a black hole. I did not know that there are magnitudes below the Planck scale. I thought that was the smallest unit, or is that just for the Standard Model.

0
0
0.000
avatar

It actually depends which Planck scale one is talking about.

What I call the Planck scale is the Planck energy scale which is huge, roughly equal to 10^19 GeV. The unification scale is thus roughly 10^16 GeV. And for the sake for the comparison, the scale probed at the LHC is roughly of 10^3-10^4 GeV. We still have a long way to go.

You can always convert the Planck mass into a Planck length by adjusting a prefactor depending on the Planck constant and the speed of light. And this one is small, as a length is the inverse of an energy in the unit system commonly used in particle physics.

I have edited my post to make this clearer. Thanks for reading it, and pointing it out!

0
0
0.000
avatar

After the Higgs boson was confirmed, I was so thrilled I took an open university course with Dr. Nanopoulos on cosmology and particle physics. After 10 courses I decided that I must be the dumbest person on earth. You just made it interesting again... thanks!

0
0
0.000
avatar

Thanks a lot for your nice comment. Particle physics and cosmology is not easy and can even be really hard depending on one's background in physics and math.

I do not know much about the level of Nanopoulos' lectures (I have never attended them, never read the notes, etc.), so that it is hard to me to comment on this. But I am pretty sure it is not a matter of being dumb or clever, but instead of picking up the right course at the right level.

0
0
0.000
avatar
(Edited)

Great post! Upvoted and thank you so very much for sharing. The graphics chosen were illustrating the matter very well.

I can't help myself but think that the energies released, under the form of superpartners, and observed through supersymmetry analysis will help elucidate the transfers of energies from the illusiveness of the bosonic/fermionic nature, having the mirror-like effect/equivalent in the symmetric phases of energy transfer, which to myself makes me wonder as there seem to have a link between the transfer of energy toward the so-called matter coming from the dark-matter side of things?!? It feels almost as if a corpuscular solid form was moved to its core from a source of energy that is seemingly in a liquid phase or maybe the better term would be plasma phase??? I'm still not clear on the terminology or how to express the the process & nature of what is in my mind. I'm trying to fill in the gap between my experience and the science expressed in your magnificent post.

Namaste :)

0
0
0.000
avatar

Thanks Eric!

I must apologize, but I am not sure to understand your comment... Do you mind clarifying a little bit? Sorry about that :(

0
0
0.000
avatar

Sorry for the jargon... It is a first and rather pale attempt at discussing the matter. No apologies from your end necessary really. LOL! Hey, I just did a few more touch-ups to it and it might point to what is actually in the mind of my experience. I cross my fingers and wish we'll be able to bridge the gap and find a common language to share on this matter.

I thank you for your patience and understanding. Namaste :)

0
0
0.000
avatar

Thanks Eric, for the clarification. Now I can try to formulate an answer ^^

The observation is a bit more complicated. Typically, what will happen is that the collision energy will be high enough to allow one to produce supersymmetric particles, that will then each cascade decay into a dark matter candidate (so that we have two of them in total) plus a bunch of Standard Model particles (and the bunch can be large). The exact definition of the bunch defines the many signals that are searched for.

But then, we also know that the Standard Model can mimic this signature. And we know the Standard Model rate. So the only think we have to do as wait and look for deviation when comparing what we expect (within the Standard Model) with what we observe.

But this is not the end of the story, as different supersymmetric setups can yield the same signature, and different new physics models (that are not supersymmetric) and also yield to such as signature. In short, saying that we have discovered supersymmetry is very hard. We will have to identify the superpartners, measure their properties and check that they match what we would expect in a supersymmetric context.

A lot of work on the table :)


Now, to come back to your question, at the end, we produce two dark matter candidates plus a bunch of other stuff. SO yes, we can see that as an energy transfer between the visible (colliding protons) and the hidden (the two dark matter guys). But there is no plasma here. All of this occurs very quickly and the energy density is too low to get anything. This contrast with heavy ion collisions that also occur at the LHC, once in a while, where a plasma of Standard Model particle can be formed.

I hope all of this helps! This may be the longest comment I have ever written on steemit ^^

0
0
0.000
avatar

Thanks a bunch for the time and efforts in meeting me in the middle sort a speak. ;) You deserve a full 100% of my upvote on this one and, it makes me glad to know that it might have been the longest ply you have ever made on this site, it is very appreciated!

The work on the table is cut for you, I can only agree! Couldn't it be possible to find a vector of energy transfer moving from the dark energy toward the proton? From there, the perspective would be shifted over to the reflection coming towards us, instead of the opposite created by us to research the depths of the dark matter. From here, the understanding would be that matter as we know it would be supported by the dark-matter and matter itself would be a result of dark-matter's synergies, possibly with gravity, to bring about matter?

In relation to the plasma as it only occurs with ion collisions, couldn't there be a plasmic solution of supersymmetric particles? Pardon the play on word but, as we say, I'm "poking in the dark" trying to connect the experience to the science. LOL!!!

Again, a giant thank you for your great answer and patience in relation to my inquiries. Namaste :)

0
0
0.000
avatar

Dark energy or dark matter? For the former, we are very much in the dark theoretically. However, for the latter, we have ideas on how to get information on it. From the rest of your comment, I assume you want to discuss dark matter, which is good as this is something within my expertise, unlike dark energy where I know little.

It is a bit tricky but we can do it. Dark matter is invisible, and thus does not leave any track in a detector, except that we will observe a energy-momentum imbalance. We can actually get information on the invisible by studying the visible (you can find more information here. But no gravity here, as the gravitational strength is way way way too weak at the LHC energy level.

I don't know anyone having done any supersymmetric calculation in the context of heavy ion solution. I believe the expected rates may be too low. (but here, I have no quantitative statement to approve my words). Maybe I should do it if I have time ^^

0
0
0.000