The most important experience was actually simply to see how huge the Large Hadron Collider is. We totally didn’t expect the site of every experiment on the 27km ring to resemble an industrial town in its own right, scattered miles across a desert-like terrain with the Mont Blanc and the Jura mountains as the scenic back drop. It was a challenge to walk between the activities we had carefully planned in advance only to find out that some of the were full or required an hour of waiting in line. But the kids have withstood these challenges heroically and were rewarded with a few unforgettable impressions.
This is the text of the mini-lecture on Supersymmetry that CERN Research Physicist, CMS supersymmetry group convener and Deputy LHC Programme Coordinator Filip Moortgat kindly gave us during our visit to CERN and the Large Hadron Collider last week.
Filip Moortgat: Supersymmetry stands out among all the other Beyond Standard Model theories (like extra dimensions and so on). It’s particularly interesting because it answers multiple things at the same time. I would say that most other extensions of the Standard Model solve one problem but not five like supersymmetry.
The first problem: because it connects the internal property of a particle to spacetime, it actually opens a way of gravity entering the Standard Model. As you know, the main problem with the Standard Model is that gravity is not in there. So one of the major forces that we know exists is not in there. Nobody has succeeded to make gravity part of it in a way that is consistent. People hope that supersymmetry can do it, although we’re not there yet.
The second problem is called the hierarchy problem. What that means is that you have a base mass for a particle and then you have corrections to it from all the other particles. What happens is that if you don’t have any other particles beyond the Standard Model particles you get corrections that become gigantic. What you need to do is tune the base mass and these corrections so that you get the mass that we measured for the Higgs Boson or for the w and z bosons. It’s like 10^31 minus 10^31 is a 100 type of tuning, and we find it unnatural. It’s ugly mathematics. In supersymmetry, you get automatic cancelation of these big corrections: You get a big one and then you get minus the big one (the same correction but with a minus in front of it), it cancels out and it’s pretty, it’s beautiful.
The third thing is dark matter, a big problem. 85 procent of the matter in the universe is dark matter (if you also include the energy in the universe, you get different numbers). And the lightest stable supersymmetry particle is actually a perfect candidate for dark matter, in the sense that it has all the properties and if you compute how much you expect it’s exactly what you observe in the universe. It works great. It doesn’t mean that it’s true, it would work great if you could find it.
And then there’re more technical arguments that make things connect together in nicer ways than before. Normally, the electric symmetry is broken in the way that everything becomes zero. All the masses would be zero, the universe would just be floating particles that wouldn’t connect to each other, it would be very boring. But that’s not what happened. To show what actually happened you need to drive one mass squared term negative, which is kind of weird but that is what supersymmetry does automatically! Because the top quark mass is so heavy. Heavier than all the other quarks. For me it’s the most beautiful extension of the Standard Model that gives you a lot of solutions to problems in one go.
The problem is that we haven’t seen anything, yet! We have been looking for it for a long time and we have absolutely zero evidence. We now have reasons to believe that it’s not as light as we have originally thought, that it’s a little bit heavier. Which is not a problem. The LHC has a certain mass range, for supersymmetry it’s typically up to a couple of TeV. But it could be 10 TeV and then we couldn’t get there, we can only get up to 2 or 3 TeV. It could be factor 10 heavier than we think!
This why we are starting to discuss the planning of the Future Collider that will be able to go up the spectre of 10 TeV in mass, for supersymmetry and other theories. There’re several proposals, some of them are linear colliders, but my favourite one is a 100 km circular collider which will connect to the LHC, so that we have one more ring. That ring will actually go under the lake and that would be quite challenging, but in my opinion – although we don’t have any guarantee – we will then have a very good shot, at least in terms of supersymmetry. At the LHC we also have a good shot but don’t have enough reach that we need to really explore the supersymmetry.
When we use conservation of energy and momentum at the collision point, what we do is we measure everybody, we sum it all up and what we need is we need to get the initial state. If something is lacking, then we know there’s something invisible going on. It could be neutrinos, or neutralinos, or it could be something else. So we have to look at the properties and the distributions to figure out exactly what we’re seeing. It’s not a direct detection but it’s a direct derivation if you want, from not seeing something, from lacking something, that we can still say it is consistent with neutralinos.
How do you know if it’s neutrinos or neutralinos?
Neutrinos we know well by now so we know what to expect with neutrinos. Otherwise it could be neutralios but it could be something else. And then to actually prove that it’s neutralinos we have a long program of work.
And is that mainly math?
No, it’s everything. It needs all the communities to work together, because we need to measure certain properties, distributions with the detector and we will need the theoretical ideas on how to connect these measurements to the properties of the particle. So we will need both the mathematical part and the experimental part. Translating the mathematics into the particle predictions, we will need all of that.
Simon, looking at the dust particles in the sun: “Is brownian motion random? If we look small enough, we might see something deterministic… but it might also be stochastic. What you’re doing, you might get something very little wrong, in which case you get a completely different answer! And how wrong you are in this area is being controlled by the little coins inside your head, or somewhere, which are smaller than an atom! But still, coins are deterministic. So even throwing of a coin is deterministic. It’s pseudo randomness. Looks and feels random but it’s not. If you really closely look how the coin moves then you can predict how the coin is gonna land. Technically, you can have some kind of robot to do that.
So actually, is the Universe random? It’s a very tricky puzzle”.
Me: At the quantum level, a particle can be in two places at once, but once the observer sees it, it seems to choose a specific position.
Simon: “Maybe it even depends on who is looking! Which means that we sometimes see everything wrong!.. Brownian motion is deterministic, we think”.
This is the part that Simon successfully translated into Java:
Around minute 30 of the Attraction and Repulsion Forces Coding Challenge video Simon got stuck with his Java code. Although everything seemed right, his particle didn’t trace nice patterns but went off the screen. Simon was very upset about this, it took me a while to console him. This is where Simon got stuck:
The little particles are attracted to one another but don’t overlap. This is a simpler kind of attraction where every particle has two perception radii: a minimum and a maximum perception radius. Two particles within each other’s maximum radius are attracted to each other but repel each other once one of them enters the other’s minimum radius:
Simon placed this project in a GitHub chat and got some interesting advice (make the mouse a repulser). In the video below he changes the code, creating a repulsing element:
Simon returned to his “old” code (something he wrote about a month ago) and fixed the collision detection in it. He called this “geometry free”collision detection because he doesn’t use any geometry for this project, but only attraction behaviour. The particles repel each other when the distance between them is 20 pixels.
Here the particles are attracted to the black blobs:
The next exciting step in writing his own code about spring force: Simon actually created an interface to allow anyone to build his own shape made of springs and particles! Simon put this project on GitHub and hosted it to make it accessible online.
The online interface to play with: https://simon-tiger.github.io/spring-animation-tool/
He also wrote the instructions himself and placed them in the GitHub Wiki: https://github.com/simon-tiger/spring-animation-tool/wiki/Intro
Videos of the project step by step:
Simon doesn’t consider this project finished. He wants to come up with a way to apply spring force to all the springs simultaneously to make sure the shape’s sides are equal in the final stage.
Simon used Chapter 3 (Oscillation) of Daniel Shiffman’s book The Nature of Code as the theoretical basis for creating his own code. First, he played around with what he calls “soft springs” – multiple spring arrays connecting multiple particles (some of them locked but most of them moving) – allowing for most interesting designs thanks to spring force.
Simon called the video below “a mess” that “doesn’t look promising”, but to me it’s my favourite pattern. To me it resembles a constructivist poster turned alive, something like an El Lissitzky animation:
Other soft springs step by step (Simon explains what soft springs are in the first video):
Simon eventually stepped over t trying to create sets of springs and particles that unfold into certain geometrical shapes, like a trapezoid here:
And finally, a hexagon:
Simon has come up with a new code of his own! It’s about gravitational attraction (particles attracted to targets or moons attracted to planets) and partially based upon the Box2D library but he wrote most of the code himself. Simon used the gravitational constant and Newton’s law of universal gravitation to build this project. The law states that every point mass (m1) attracts every single other point mass (m2) by a force (F) pointing along the line intersecting both points. The force is proportional to the product of the two masses and inversely proportional to the square of the distance between them (r):
Simon had come across this formula before in Daniel Shiffman’s tutorials and just like Daniel he changed r to d (for distance):
float strength = (G * 1 * p.mass) / (distance * distance);
Simon pushed his full code to GitHub at:
Here Simon added some new features, like pressing keys for adding new targets and particles:
Here the force is becoming stronger with more targets, Simon explains:
We’ve also got a video of Simon talking about this project in Dutch (showing it to his math teacher):
Simon loved Daniel Shiffman’s simulation of the Plinko game and added some extra features to Daniel’s code, like a button to make new chips appear. He also saw a television version of Plinko where people were trying to win money, so he added the figures below to indicate your score.
Simon put this project on GitHub, too!
Direct link to the game: https://simon-tiger.github.io/plinko/
You can also both the code and play with the animation via the p5.js web editor (hit Download to get the animation on your computer): http://alpha.editor.p5js.org/simontiger/sketches/Sk23OkW6x