Hi again! So, this time I’m going to tell you about two ideas which are fairly new to me but nonetheless interesting (and, I think, closely related): General searches for new phenomena at the Large Hadron Collider (LHC) and Black Swan events.
The usual framework
Since the LHC started operations a few years ago, a significant amount of effort has been put into two endeavours: the search -and its (now almost certain) discovery in 2012- of the Higgs boson and the search for supersymmetry (SUSY) and other extensions of the Standard Model (SM), that is of our current theory for how particles interact with each other.
The Higgs boson was known as “the missing piece” of the SM, a boson that allows itself and other particles to acquire mass. SUSY and other models aim to solve several of the major problems of the SM, such as the so-called naturalness or hierarchy problem and the lack of a Dark Matter candidate. They also predict the existence of new phenomena that can be confirmed or excluded to a certain extent by the experiments at LHC. Long story short: there are good reasons to believe that one of these models actually represents reality, and so we have to build our fancy experiments to check, don’t we?
Typically, a search for new physics at the LHC consists in choosing a concrete scenario of an extension of the SM and studying the experimental signatures (i.e. combinations of measured particles) where the new phenomena may manifest themselves. If they don’t, as has been the case so far, we quantify the extent to which our theory is excluded. There are also searches based on specific experimental signatures without being guided by any model, where the theoretical interpretation could happen a posteriori.
A counting exercise
Hundreds of scientists at the LHC experiments devote their time to search for new physics as described above. The total number of signatures explored by such studies is around 100 or 200
, which encompass predictions from the most promising theories beyond the SM. (The numbers and reasoning in this and the following paragraph are taken from here.)
But what if we count all possible combinations of measured particles that could be produced in a collision at the LHC? If we take into account up to eight measured particles of five kinds (electrons, photons, muons, jets, b-jets, and neutrinos), the number is already over 1200! The goal of general searches for new phenomena is to close this gap by analyzing the hundreds of signatures that are not taken into account in other studies.
There are two subtleties in the statement above: Firstly, most general search analyses don’t distinguish electrical charge, meaning that for example the electron and the positron are considered as the same object. Secondly, jets and b-jets are in reality several particles produced in a narrow cone and usually reconstructed as a single object for physics analyses.
Intermezzo: Where are the Black Swans?
I just started reading Nassim Nicholas Taleb‘s acclaimed book “The Black Swan”, where he describes the impact of highly improbable (a.k.a. Black Swan) events in various areas of knowledge and society. Taleb defines Black Swan events as having three main features:
- Rarity, in the sense of being outliers to our expectations,
- Carrying an extreme impact, and
- Being retrospectively predictable (i.e. only after the event has happened).
We do not know when or where could Black Swan events happen, by definition. At best, we can only try to increase our chances to discover Black Swans by being exposed to them.
The new physics that may appear in the “gap” signatures is pretty much of the Black Swan kind. Such discovery would be outside of our expectations, since it would correspond to unknown (and un-hypothesized) phenomena. If the discovery of a particle predicted by SUSY would be a game changer, a Black Swan would leave every theorist in the field scratching their heads for a while. Finally, as it has happened in the past with unexpected discoveries, a new theory would be devised to come to the rescue.
Looking for whatever, everywhere
General searches face the monumental challenge of looking for Black Swan phenomena to make our quest for new physics more robust (or less fragile) to the unexpected. The single assumption made is that new physics will appear in signatures containing particles with high transverse momentum. The idea is to systematically study all possible signatures by looking for a deviation of data from the SM prediction in few variables that are sensitive to new physics. For example, the creation and further decay of a new particle would appear as a “bump” in the spectrum of invariant mass of the decay products.
The result of a general search reports all found deviations. The quantity used to assess whether a particular deviation is interesting is the p-value, the probability of a deviation of the SM prediction from the data values. Moreover, the prediction is based on (imperfect) Monte Carlo simulations of the detectors and the SM processes in the collisions; the appearance of a significant deviation would require a more detailed study of the corresponding signature. As a final step, some artificial signal is injected to the data to check if the analysis is sensitive to the presence of new physics.
I will explain in more detail how general searches are performed and what has been done in the past, since it is the main subject of my thesis. For now, I leave you with this picture of a real black swan: