I believe that Nature is such that an observer can study it, and then based on the results of his studies one can construct a physics theory that correctly predicts the outcome of all currently existing and future experiments. The reason for doing my research is to contribute to the quest of finding the ultimate theory of everything.

Unfortunately, when studying the smallest scales of Nature, we do not have the luxury of having a much smaller probe than the studied particle itself! This means, that the above "Swiss watch" study-strategy would not work with particles.

Fermilab accelerators (Click on image to enlarge.)

CDF detector (Click on image to enlarge.)

Luckily, there is another way to study the structure of the Swiss watch. Take two watches, smash them together very hard. Carefully find and study the collision products (where they landed, what they are, ... ). Then it is possible from the information gathered about the collision products to reconstruct the structure of the watch. This is precisely the way we study particles and their interactions. We accelerate particles, smash them together at very high energies. Then we employ very sophisticated detectors to gather as much information as possible about the collision products. Based on this information, we are then able to extract tremendous amount of knowledge about the structure of particles and about the physics that governed the collision.

Particle content of the Standard Model (Click on image to enlarge.)

On the other side, the Standard Model has quite a few input parameters whose values are not determined within the theory. They must come from experimental measurements.
The Standard Model is also not able to describe the behavior of matter at exotic, extremely high-energy conditions, frequently found in the universe now and throughout its history.

Therefore, we must conclude that the Standard Model is not the ultimate theory. We believe that there must be a theory beyond the Standard Model. One such candidate theory is Supersymmetry.

Superpartners (Click on image to enlarge.)

In what sense does SUSY double the world? All elementary particles belong to one of two categories, bosons or fermions, as far as the "spin" attribute is concerned. Bosonic and fermionic particles behave very differently, acting almost as if being from two different worlds. The sharp difference between these two categories led physicists to propose that a new symmetry, called the supersymmetry, exists that would unify the bosons and fermions. Supersymmetry predicts that each boson has a fermionic superpartner and each fermion has a bosonic superpartner. These SUSY partners will have all of the same attributes as their ordinary counterparts, except the spin. The spin will differ by 1/2 unit. For example, the superpartner of the Standard Model's top quark (top quark is a fermion, spin 1/2 particle) will be the stop quark (a boson, spin 0 particle).

In order to explain how to use colliders to search for new, yet unseen particles, I need to discuss:

• probabilistic behavior of Nature
• E=mc²
• decays of particles
• particle detectors
• un-interesting debris that mimics new particles
• filtering the signal from debris

With the introduction of quantum physics, when physicists zoomed into small scales they realized that the equations are such that they can predict the outcome of experiments only with a certain probability. In other words, every experiment has multiple possible outcomes. Some of these are more likely, some are less likely. The only things that we can calculate and predict are the probabilities that a given result will occur.

This has a very deep implication for particle physics. When we collide particles in colliders, we cannot be sure what we will get. Luckily, we can at least calculate the probability that the experiment will result in a given process of interest.

For example, when we collide protons with antiprotons at the center of mass energy 1.8 trillion electron volts, SUSY predicts that the probability to create a superpartner of the top quark is approximately one part in a billion! This means that we had better have at least 100 billion collisions in order to be able to feel confident that if the stop quark exists in Nature at least some will be produced. Yes, it is a tough game of randomness. You never know ahead of time what you will get!

When energy materialize into TOP quarks. (Click on image to enlarge.)

In order to answer these questions, we need to get Einstein's equation E=mc² involved. It simply tells us, that any energy "E" is basically equivalent to mass "m". In other words, when we are smashing particles at very high-energies, the incoming energy to the collision can materialize in the form of a supersymmetric top quark with a certain mass "m". Of course, this will not always happen. Sometimes the energy will materialize in the form of electrons or neutrons or ... . Only with the probability of one part in a billion one expects to get a stop quark. Pretty small chance, right?

Not quite!

Our supersymmetric extension of the Standard Model predicts that stop quarks exist only for a fraction of a second. This is because Nature is made such a way that it likes to exist in a "as-low-as possible" energy stage. In other words, if particles are heavy they carry a lot of energy (remember, E=mc²) and they prefer to decay to lighter particles. Stops decay very fast. Your store would run out of stops in a billionth of a billionth of a billionth of a second.

Therefore, we have only one choice. We need to create stop quarks with collisions and study their decay products.

The answer is that we recognize the stop quark via traces it leaves in the particle detector. The collisions are set up to happen in the middle of a giant particle-sensitive electronic device, called the detector. If stop quarks are produced, the SUSY theory predicts, that they would decay into:

1. A narrowly collimated jet (spray) of particles originating from a bottom quark. (Bottom quark is one of the six predicted quarks in the Standard Model. It has been observed at Fermilab.) These jets can be traced in the detector.
2. An electron like object (lepton) which can also be traced and recognized in the detector.
3. Two neutral very weakly interacting particles (neutrino and a supersymmetric neutralino), not leaving traces in the detector. Even though these particles do not leave traces in our detectors, we can still deduce their presence in the collision events. How? Since they escape undetected, the total energy measured in the detector must be unbalanced. Therefore, we add all the energy in the detector, and if we see that, for example, substantially more energy was detected in the upper hemisphere of the detector, it is a good indication that through its lower part some neutral particles leaked out.

To illustrate the difficulty of this "find a needle in a hay stack" game, I mention that for each produced SUSY top signal event we expect several million Standard Model events having approximately the same look in the detector as the stop quark. In order to deal with the enormous background, we use powerful computers to simulate both signal and background events.

We then impose sophisticated statistical and topological techniques to get as high background reduction as possible, while keeping as much signal as possible. When we are confident that we can reliably distinguish signal from background, we apply the developed techniques on real collision data and we hope some will be caught in the filters.

Stay tuned, maybe today is this lucky day.