The end of the Run 2 of the Large Hadron Collider - the dawn of a new class of interactions

작년

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?

YES!

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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Indeed, this is very interesting. I have couple of very silly questions, if I may ask. From my little and naive understanding about particle physics, what I understood in 2012 is that higgs boson creates a higgs field, and how a particle interacts with this field is what gives it its mass. Is Yukawa interaction the name of mechanism describing interaction between higgs field and a particle. Or is it something else with its own boson?

My second question is how is such an interaction measured. I understand that if you have to measure gravitational or EM interaction you can figure it out by figuring how something moves in EM or gravitational field. Strong and weak force though bit complicated, I get the essence of it. But I I found it hard to imagine that what kind of experimental setup would measure this interaction?

PS: You may find my questions silly, I am just a biologist trying to understand this. I hope you dont mind these naive questions.

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Indeed, this is very interesting. I have couple of very silly questions, if I may ask

Always! :)

From my little and naive understanding about particle physics, what I understood in 2012 is that higgs boson creates a higgs field, and how a particle interacts with this field is what gives it its mass. Is Yukawa interaction the name of mechanism describing interaction between higgs field and a particle. Or is it something else with its own boson?

There are several questions in there.

First, the fields are the basic objects in particle physics, and the particles can be seen as excitations of the fields. For more information, feel fee to have a quick look to this. So we have the Higgs field, and actually, we also have fields associated with all other particles. In short, a particle does not "create" a field.

Then, the Yukawa interactions in the Standard Model corresponds to interactions of a Higgs field and two fermionic fields (for instance, an electron-positron pair). Those Yukawa interactions then give rise to two effects:

  • the fermion masses;
  • an interaction between the Higgs boson and the fermions.

Since we know the masses of the particles (they are measured), the strength of the initial Yukawa interaction is determined (as everything is linked). We have two effects but one single parameter controling them.

My second question is how is such an interaction measured. I understand that if you have to measure gravitational or EM interaction you can figure it out by figuring how something moves in EM or gravitational field. Strong and weak force though bit complicated, I get the essence of it. But I I found it hard to imagine that what kind of experimental setup would measure this interaction?

Let's take an example to answer this.

  • Step1: We can select among all LHC collisions those featuring what would be expected when a Higgs boson is produced in association with a pair of top quarks. Of course, background mimicking this will be selected too. After further constraining what we select, one obtains a bunch of events satisfying all constraints.
  • Step2: We can compute theoretically the expectation for the background. The difference between step1 and step2 consists in the Higgs signal.
  • Step 3: we compute theoretically predictions for the signal with different values for the Yukawa coupling (with the mass fixed). We are expecting a given value connected to the top-quark mass, but uncertainties imply that other values are possible too.
  • Step 4: we do the same for varied process and obtain the last figure of the post.

Does it clarify?

PS: You may find my questions silly, I am just a biologist trying to understand this. I hope you dont mind these naive questions.

I love them! Do not hesitate to ask for more!

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the fermion masses;
an interaction between the Higgs boson and the fermions.

wait a minute! what gives mass to bosons then?

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In the Standard Model, the bosons are connected to the fundamental interactions (electromagnetism, weak interactions and strong interactions). The Higgs field is sensitive to the weak interactions (it is slightly more complex... but here I simplify a lot), and it thus interacts with the weak bosons. These interactions are then connected to the boson masses.

As for the fermion, a coupling to the Higgs always give interactions on the one hand, and masses on the other hand.

the machine will soon enter a two-year-long upgrade phase

No more discoveries for 2 years?


I was watching a numberphile vid yesterday about Zeno's paradox, and at a certain point in the vid (10:53 min) he said that it would be interesting to ask a physicist if you can divide time and space infinitely, and a caption indicated that they will soon ask a physicist, but as far as I can tell they didn't.

And so I'm asking you! :D

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No more discoveries for 2 years?

Not really as all data from the run 2 that just finished has not been analyzed yet. There will be no more new recorded data, but a lot of work is planned to exploit as best as possible what we have so far (LHC analyses take easily months, or sometimes even years, before being finalized).

I was watching a numberphile vid yesterday about Zeno's paradox, and at a certain point in the vid (10:53 min) he said that it would be interesting to ask a physicist if you can divide time and space infinitely, and a caption indicated that they will soon ask a physicist, but as far as I can tell they didn't.

I would say that at some point, when one approaches the Planck time/length, funny effects could occur. But this is just some naive guess :)

These results establish the existence of a new class of fundamental force

1. Gravitational Force
2. Weak Nuclear Force
3. Electromagnetic Force
4. Strong Nuclear Force

5. Yukawa Force (interaction)

??!! Am I right? Will it be something like that if all the Yukawa interactions will be measured in the future?

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The four usual fundamental interactions are those you listed from 1 to 4.

In the Standard Model of particle physics, weak, electromagnetic and strong interactions are embedded in the framework of what we call gauge theories (I recommend some of the links of my post for more information on this). We don't know how to embed gravity in this framework (yet?) and there are many researches on this. In any case, at the fundamental level and currently probed energies, gravity is negligible so that this is not a real problem at the moment (and we have more time to investigate that).

Yukawa interactions are different in the sense that there are not gauge interactions and thus of a different nature. They correspond to the coupling of a scalar particle (the Higgs boson) to a pair of fermions (matter particles). Until the Higgs discovery, while those interactions were present in the Standard Model, there was no evidence for their realization in nature.

I hope this clarifies a little bit.

Will it be something like that if all the Yukawa interactions will be measured in the future?

Not in a close future. Since the strength of the interaction is proportional to the mass of the involved particles, some of them are incredibly weak (which means very rarely occurring in collision processes). But physicists are trying to design strategies to get there (and also consider the capabilities future machines should reach).

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Ok, it is more clear now. It is another thing than gauge theories. It is a coupling for mesons which contributes to their Lagrangians for the interactions with scalars. I think you particle physicists use Dirac Lagrangians for 1/2 spin particles in order to put the mesons interaction into the system. Here the Yukawa interaction represents the interaction between Dirac and Scalar fields.

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It is a coupling for mesons which contributes to their Lagrangians for the interactions with scalars

Those are not exactly the Yukawa interactions I am talking about, as the context here is the one of the fundamental particles (mesons are composite).

Not sure how I missed this. Fascinating as always. What are the hopes for the lhc 3rd run? Detecting some particles predicted by supersymmetry?

Posted using Partiko Android

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Not much hope for run 3 with regards to detecting supersymmetry. The improvements are somehow logarithmic, so that the bounds on supersymmetry that could be extracted from the full run 3 data with respect to those extracted from the full run 2 data are not much different.

Two exceptions:

  • Supersymmetry exactly lies in a region where one is still limited by statistics. More collisions will make a difference.
  • Supersymmetry is slightly too heavy for the run 2 collision energy. Therefore, run 3 with an slightly increased energy can make a difference.

In my opinion, we should instead investigate deviations from the Standard Model for observables where one is currently statistically limited. This is our best handles on new phenomena.

For instance, asking a lot of missing energy and verifying that the shape of the missing energy spectrum is the Standard Model one. The rate for producing a lot of missing energy on the Standard is small.

2 years maintenance time? Wow, that's a number.
Yeah, another meeting at CERN would be nice.
And, as it comes to my mind, one might visit the Gran Sasso guys one day too. :)

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In fact, I didn't say maintenance. I said upgrades. The detectors and the machine will be improved so that one will be able to cope with a larger collision rate.

Gran Sasso is a nice idea too. I have old contacts there :)

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Are there going to be experiments to test the gravitational mass of antimatter? When?

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This will not happen t the LHC. There are however other experiments at CERN that are dedicated to this kind of question. you can try to check what do the ALPHA and GBAR experiments. Note that I also wrote a post on gbar long long long ago, that you may want to check here.

I think I once stumbled on the term "Yukawa interactions" sometimes ago, but it sounded funny to me. Here I am; meeting it again.

I want to ask, do fermions undergo Yukawa interactions?

Welcome back sir

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Thanks @samminator for the nice words!

I want to ask, do fermions undergo Yukawa interactions?

Yes, since here, we are discussing Yukawa interactions that are the interactions between a scalar Higgs boson and a pair of fermions.

Here's hoping to the finding of SUSY partners next time you startup that bad boy again (maybe boring to some wanting to find exotic new physics but I would be so excited with SUSY alone!) And visiting CERN sounds amazing .

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I guess everyone will just be excited by any finding of any kind. SUSY is nice and elegant, but there is however very little chance to find it on day 1 of Run 3. If there is a new particle somewhere, in most of the case we will need to record a lot of collisions to find it.

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