The future of particle physics - from the Large Hadron Collider to future circular colliders

I like to dream about the future. Especially when I have the chance to participate to the discussions that will drive this future. Particle physicists are currently preparing the future of their research field, and important decisions will be taken in the next few years.

_{[image credits: CERN]}

In this article, I will discuss one possible option for the next collider machine to be built, a very large circular collider that will have an energy 7 times larger than today’s best existing machine (the Large Hadron Collider at CERN), that is expected to record 10 times more data, and that may be 100km long.

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THE LARGE HADRON COLLIDER - A RECAP

The Large Hadron Collider (or the LHC in short) is the largest particle collider ever built by humans and is currently operating at CERN, the largest physics laboratory in the world.

The high-energy physics community has today a wonderful machine allowing us to push forward the frontier of knowledge. The LHC is so far famous for its discovery of the Higgs boson (please have a look to my theory and experiment articles on this topic), the missing link of the Standard Model that evaded detection during many years until 2012.

And as a first lovely consequence of the Higgs discovery, a fresh Nobel Prize!

But the story was only starting in 2012 (the year in which the Higgs boson was discovered). The LHC machine is expected to run until 2035 and collect way more data compared with what has already been recorded.

In short, many options for new discoveries are open. Although Nature seems to be nasty with us and hide any new phenomenon from detection, there are many motivations to expect new phenomena (see the couple of posts on this topic that I have recently written, here and there).

We now need to wait (at least) until new results to be announced at the next Moriond conference in March 2017, which is the most important winter conference for particle physics.

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INTERMEZZO - CHASING AMBULANCES

In the meantime, one of the funny thing that I can do as a theorist is to chase ambulances, or in other words not too significant excesses and deficits in data with respect to the expectation of the Standard Model.

I have currently found a good one recently that seems to have been overlooked by everyone so far. More about it when the related (scientific, and maybe steemit) article will be out… No spoilers! ^^

Whether I believe in this ambulance or not is not the question. What makes it interesting is not the fact that the ambulance might be something real or not (this, we can learn in being patient enough).

The interesting question to ask is the following. If an excess or a deficit (a deviation in short) with respect to the Standard Model is real, are we well prepared to perform all measurements that may be necessary to understand what is going on?

And to do that, there is only one way.

• First, we need to build one or more particle physics models that could explain the deviation. This can range from complicated to simple models.

• We then need to check the predictions of such models and confront them to existing data. All data. This will imply constraints on the model parameters.

• Finally, when we have found some surviving benchmark setups that accommodate both the deviation that we are chasing and all other existing (null) results, we can check how to further falsify the theory.

In this last step, it may turn out that some key predictions of the model are untested. We consequently add an item in the LHC search program. This is why chasing ambulances is in my opinion useful.

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THE NOT TOO FAR FUTURE IS TODAY

It is now time to go back to the topic. Approximating all dates, the LHC first studies date from 1985, the machine has started in 2005 and will operate until 2035.

To recap this timeframe, we needed 20 years to understand what could be done in terms of physics, decide that this was useful for high-energy physics, design and finally build the machine and the associated experiments. And of course, to finally start the whole thing.

This 20-years period is then followed by 30 years of physics and data taking.

The LHC will stop in 2035 and it is important to be ready to start a new series of experiments right after this.

Based on history, we may need about 20 years to investigate the potential of such a next machine and also, what this machine could be.

Today is thus the right period to think about the future!!!

This is why physicists are thinking about ideas of future experiments that could be built, in particular at CERN but potentially elsewhere a well. And for each idea, we need to think about the physics potential, and the costs for the society (note that there will be in principle a benefit given back to the society, as for the LHC](https://steemit.com/science/@lemouth/the-cern-large-hadron-collider-and-its-economical-impact-on-the-society)).

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A FUTURE CIRCULAR COLLIDER?

There are many options for a future machine on the menu, and one I particularly like consists of a future circular hadron collider. To describe it shortly, this is nothing but a VLHC, a Very Large Hadron Collider. We can take the LHC and multiply the entire setup by a not so small scaling factor (check out the map).

Such a concept is actually abbreviated as an FCC, or a Future Circular Collider. More information can be found on this CERN website or this IHEP website, the former website being the CERN option and the latter website being the corresponding Chinese option.

But why is such a machine useful? We have three case stories depending on what will be found or what will not be found at the LHC.

• First, the LHC may observe something new. We will try to characterize what this thing is via several physics theories. Most new physics theories however predict a full spectra of new particles to observe with many of them possibly out of the LHC reach. For this reason, a more powerful machine will be needed.

• Second, one may imagine that the LHC will observe deviations from the Standard Model, but not any new particle itself. We have actually here an indirect effect of a new particle. By studying it, we can derive the mass scale at which the new particles that generated the effect lie. And to get there, there will be only one way: a new more powerful machine.

• Finally, we may be unlucky and just find nothing at the LHC. Therefore, the only way to find a new phenomenon will be to measure all the Standard Model observables as precisely as possible. In particular (taking one example among many), all Higgs-boson self interactions should be measured: The trilinear one (an interaction between three Higgs bosons) being roughly extractable from LHC data and the quartic one (an interaction between four Higgs bosons) being out of reach. We therefore need a more powerful machine to do that.

In short, we will need a more powerful machine to investigate further all possible findings, or non-findings, of the LHC.

CONCLUSIONS - GETTING PREPARED

Physicists have started a couple of years ago to evaluate the physics potential of such a very large circular collider.

We are talking about a 100km long collider, to be built either at CERN or in China (both have plans with this respect, although it is clear that only one collider will be built). The energy of such a machine will be 7 times the one of the LHC and the experiments to be built along this machine will be expected to record a very large amount of data (10 times more than the LHC).

This implies many challenges!

I am working on the capacities to precisely measure the quartic Higgs interaction strength at this future collider, and it turns out that this will be extremely difficult (as collaborators and I have shown last year), but actually feasible (as the same collaborators and I will show next month).

On the other hand, experimental colleagues have started R&D developments concerning the detectors that should be built, their features, etc, as well as the IT people have started to think about managing data.

We move on on all aspects, in order to be ready for 2018, the year where the European strategy for particle physics will be updated.

In the meantime, please stay tuned!