Beyond the Standard Model physics, also called new physics, is a very active field of research in particle physics. We are talking here of about 40-50 new publications each week in average, to quote some numbers. That is also the field I am working on for almost 15 years, which makes me happy to talk about it.
In previous posts that I wrote last year, I have mentioned that the Standard Model of particle physics was one of the most tested and most successful theory explaining the life of the elementary particles.
Since this was last year, this motivates me to first wish a happy 2017 year to all my Steemian fellows, and hope that all of you will get a full 2017 package that includes health, happiness and of course good STEM.
After a 15 days break in Canada, I am now back online (in Korea this week) and will resume my regular scientific posting activities.
One may therefore wonder why it is necessary to go beyond the Standard Model, and why so many people are working on beyond the Standard Model physics. This by the way also consists of one very large part of the experimental program in high-energy physics (at the Large Hadron Collider, the LHC, at CERN, to mention one set of experiments).
[image credits: The Particle Adventure]
BEYOND THE STANDARD MODEL PHYSICS IN A NUTSHELL
Despite its numerous successes, the Standard Model of particle physics features several conceptual issues and practical limitations.
It is therefore acknowledged as a theory that must originates from a more fundamental theory. Or to say it in layman’s terms, the Standard Model is simply the visible tip of a huge iceberg, and we have no clue about what lies below sea level.
For this reason, beyond the Standard Model physics is at the core of the present high-energy physics program. Particle physicists indeed want to unravel the hidden mysteries of nature and understand how our universe works.
In particular, the ATLAS and CMS collaborations have carried out, at the LHC, an extensive program of searches for new particles and phenomena. Since we do not know what type of new physics could be there and which one is likely to be observed, we must be pragmatic in order to be sure not to miss any of its possible signature. The only way is then to seek every single predicted phenomena by any possible theory.
Of course there may still be holes in the current research program, and our job is to identify them and fill them. This is also part of my job.
STATUS OF THE NEW PHYSICS SEARCHES AT THE LHC
Although there are hundreds of different searches for new physics at the LHC, the results are unambiguous: any possible signal is absent.
There are however some caveats behind this statement, and new physics theories evading all constraints, predicting thus the absence of any signal, also exist.
In parallel, some strange signals are observed, but we are not confident enough in those to make any claim. These deviations from the Standard Models have strengths below 3 sigmas. This means that there is more than one chance in 370 that we have a fluctuation in data instead of a real new phenomenon. More data will however come and help clarifying those situations.
It is now time to discuss the limitations of the Standard Model that motivate considering new physics.
THE HIERARCHY PROBLEM
[image credits: the particle zoo]
One of the most unsatisfactory conceptual limitation of the Standard Model is the so-called hierarchy problem. This issue finds its name in the fact that the electroweak symmetry breaking scale and the Planck scale are separated by many orders of magnitude.
In a few words, the Standard Model is entirely based on symmetries, which makes it elegant. This works very well with respect to data, with the exception of one little issue: particles are all predicted massless. The symmetry picture cannot however be totally wrong, as, as I said, it works well with respect to experimental data. Therefore, we want to embed a mechanism allowing us to both make the particles massive and keep the symmetry-based model building. This leads to the concept of symmetry breaking, which I refer, for more information, to this older post of mine.
The important point is that the symmetry breaking concept introduces an energy scale that will dictate the magnitude of all particle masses, the so-called electroweak scale. The electroweak scale is thus roughly the scale of the W and Z boson masses, and is the one currently probed at the LHC.
The Planck scale is on the other hand the energy at which gravity becomes non negligible, and where the Standard Model must definitely be replaced by a more fundamental theory that we do not know today.
We have thus two scales at stake, and they differ by several orders of magnitude.
Because of this hierarchy, the parameters of the Standard Model must be tuned up to their 30th decimal if we do not want the Higgs boson mass to be equal to the Planck scale once quantum corrections are included. And quantum corrections must always be included for getting correct predictions.
This make physicists feeling very uncomfortable: if one changes the 30th digit of one parameter, the predicted Higgs boson mass is sent far far away from the electroweak scale, in contrast to what data tells us.
As a result, we believe that we need new physics to clarify the situation. Physics beyond the Standard Model effects will in this way help to stabilize the Higgs boson mass with respect to the quantum corrections.
For instance, supersymmetric theories removes the sensitivity to the Planck scale by the virtue of partner particles to each of the Standard Model particles. All the dependence on the high scale induced by each Standard Model particle is exactly cancelled by the one originating from the partner.
[image credits: NASA]
The second most striking feature demanding the existence of new physics is uncontrovertibly dark matter. And there is no candidate particle in the Standard Model that could play the role of dark matter.
This of course holds only if one assumes that dark matter exists. To make a long story short, there are many cosmological and astrophysical observations that suggest the existence of dark matter. Other explanations, like a modification of Newtonian mechanics, are also possible, but the dark matter paradigm is currently the one that fits data the best. The only missing piece is its observation (see my hitchhiking guide to the quest for dark matter for more information with this respect).
Most theories extending the Standard Model therefore exhibit a particle that could play the role of dark matter and whose properties allow the theory to match astrophysical and cosmological observations.
To keep the supersymmetry example of above, one finds supersymmetric neutralinos, gravitinos or sneutrinos that could be appropriate dark matter particles.
UNIFICATION OF THE FUNDAMENTAL INTERACTIONS
[image credits: Pixabay]
Another good motivation for new physics lies in the complicated structure underlying the symmetries behind the fundamental interactions. We actually have three symmetries for the three fundamental interactions, and it would be way more appealing to embed all of them into a single more general symmetry.
Moreover, each of the three Standard Model symmetries is connected to an interaction strength that varies with the energy. In the Standard Model, there is an energy regime, not too far from the Planck scale, where the three strengths are almost equal. This feature motivates the development of new physics theories with a unified interaction pattern and with a unique interaction strength.
For instance, in grand unified theories, the three interaction strengths do unify by construction. Grand unified theories additionally feature the unification of all particles that are then seen as the different components of a very small number of basic quantities.
There are many more conceptual issues in the Standard Model of particle physics that make us think that this theory is incomplete. I however do not want to make this post more lengthy than it already is, and I will consequently restrain myself to the few motivations depicted above.
In this post, I have tried to show some motivations underlying most of my work and the dreams of many particle physicists: discovering new physics, or new phenomena, and trying to understand where they come from.
In the future and if there is an interest (please let me know as a comment), I may write a few more posts on the most popular paradigms that are currently under investigation both theoretically and experimentally, trying to show why they are so appealing.
And once again, all the best for 2017!
[SOURCES]: all of this came from my mind and 15 years of research in the field!