As you may have noticed, I was quite absent during the last couple of weeks. But there is nothing to worry about that. I did not disappear, and actually only took the opportunity to take a short pre-Xmas vacation break, with the family in South Africa. Hopefully, I will find some time to share a few pictures in the next few days (@mobbs I will try not to forget this time).
[image credits: Harp (CC BY-SA 3.0)]
In the meantime, the Run 2 of the Large Hadron Collider (the LHC at CERN) ended, and the machine will soon enter a two-year-long upgrade phase.
It is therefore potentially the good moment to organize a new SteemSTEM meetup there, with a private visit of the collider and its detectors. In short, reiterating the success story of one year ago.
Would anyone be interested in this?
But more concretely and to come back to the realm of physics, it is also the good moment to think about the main achievements of the LHC so far.
While all these achievements can be stated in short as the fact that Standard Model of particle physics is now complete, there is much more than that.
This proceedings that I have recently read makes the point about recent successes that are not emphasized very much in the discussions with non-particle-physicists. The article being right, I will try to fill this gap with this blog post and discuss the recent discovery of a new type of fundamental interaction at the LHC.
THE SUCCESSES OF THE STANDARD MODEL
The Standard Model if particle physics consists in the ensemble of laws governing the dynamics of the elementary particles. It is in other words the theory that describes the properties of the tiniest building blocks of our universe and how they interact with each other.
From a small number of parameters to be fixed, predictions for thousands and thousands of quantities can be made, and it turns out that they all (mostly) agree with data taken during the current and previous centuries.
[image credits: homemade]
The equation behind it is very compact and can be printed on a cup, that can by the way be bought at CERN. The tablecloth however can’t… ;)
Half of the equation (the first two lines) consists in physics deeply studied and understood in the 20th century including in particular Maxwell equations. It was by the way confirmed an extra time very quickly by the LHC.
The other half (the last two lines) concerns the Higgs boson, that was hunted during 50 years and discovered in 2012. This is clearly where the greatest successes of the LHC come into the game.
Of course, the Higgs boson discovery is the key jewel of all those successes, but this discovery is not the one that I discuss today.
I will instead focus on something else: the interactions of the Higgs boson with the elementary building blocks of matter, that are known as the Yukawa interactions of the Higgs.
THE HIGGS COUPLINGS TO MATTER
The Yukawa interactions consist in the third line of the equation written on the above cup. They are crucial as they allow to describe the masses of all matter particles in the Standard Model. They allow to explain, within the sole theoretical framework of the Standard Model, why the building blocks of all matter in the universe are massive bits and not massless.
As a consequence of this connection to the particle masses, the strength of these Yukawa interactions is predicted to be proportional to the particle masses, which is a prediction that can be tested at the LHC by confronting mass measurements to Yukawa interaction measurements.
[image credits: ATLAS @ CERN]
This test consists of another one of the great recent successes of the Standard Model, as illustrated (among others) on the figure on the left (please ignore the Z and W entries).
The measurements of the Yukawas (reported on the y-axis) are confronted to the particle masses (reported on the x-axis), and their proportionality is apparent.
This thus confirms the theory, at least for some of the particles.
But does it matter?
These results establish the existence of a new class of fundamental force, the Yukawa interaction. Those Yukawa interactions were hypothetic so far (even if included in the Standard Model) and they have now been discovered in data, at least for some of the Standard Model particles (the top quark, the bottom quark, the tau lepton and the muon).
As some of their consequences, it is cool to mention the stability of hydrogen, the size of all atoms and the energy scale of all chemical reactions! Nothing less, nothing more.
But this figure also emphasizes the importance of the exciting next steps. We need to verify that the other 5 Yukawa couplings lies on the dashed blue line above. This is indeed needed to be allowed to state that the Standard Model is complete and fully understood…
TAKE HOME MESSAGE
My return to Steem after a short vacation break coincides with the end of the Run 2 of the Large Hadron Collider at CERN. I wanted to take this coincidence as an opportunity to discuss some of the recent discoveries of the LHC.
While the Higgs boson is what usually pops up automatically in most minds, I decided to focus on some of its properties: its interactions with matter particles. Those interactions are not only connected to the particle masses (this is what the Higgs boson is about after all), but they also represent a new type of fundamental interactions that has never been observed before.
Whilst the equation on the Standard Model cup (see the second image above) seems more and more realized in nature, some of these Yukawa interactions must still be measured. It is therefore too optimistic to claim today that the Standard Model of particle physics is complete (even if this is done very often).
Many more years of research are still needed and both the LHC and future colliders! The fun in fact only starts…
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