Many particle physicists currently investigate what should be the next large high-energy physics facility to be built, as a successor to the Large Hadron Collider at CERN. Last December, I contributed to a publication in which we tried to assess the best choice from the European standpoint, after accounting for the various (European) options on the market.
[image credits: open photos @ CERN]
The machine that is presently exploited in Europe, the LHC at CERN, has already done a lot. After two successful runs of operation, it indeed discovered the Higgs boson in 2012 and constrained quite strongly many options for new phenomena. Whilst we have deep indications we should discover something new at some point, the form of this new physics is completely unknown.
Moreover, the moment at which we should see something is also unknown. This could happen at the LHC, or not. There is no no-lose theorem here… The next 35 years of data taking at the LHC are therefore really exciting as the future results may shed some light on physics beyond the Standard Model.
Particle physics has entered an era of exploration and we do not know what we may find, the deep details about how to find it and when this will happen. This is after all what exploration is about: trying to discover new phenomena without any certainty about finding them soon.
The crucial point is that along this exploration, one should consider all options and make sure not to miss anything.
Why worrying about the next steps today?
The simple answer is that we are already late!
We need time to understand what are the options, how to build each of them, select the best one and effectively build it. We can take the LHC as an example. Whilst its exploitation started in 2008, the first studies date back from the mid 1980s. Moreover, the machine will run until roughly 2040. This is the kind of timeframe we are talking about here!
[image credits: open photos @ CERN]
In the paper of last December, I collaborated a group of researchers and we investigated the various European options. In other words, we assessed all options that could be built at CERN and lead to a machine that would start at the end of the LHC operations.
The project that we considered as the best for European particle physics is the so-called FCC project where the FCC acronym simply stands for a Future Circular Collider. This consist in a circular collider that is long of 100 kilometres and in which various types of particles can collide.
This machine has in particular the possibility to operate in an electron-positron collision mode and in a proton-proton mode. And there is no need to choose! We could have both, which would guarantee 100 years of physics at CERN! Moreover, having the two options operating one after the other could be achieved at a decent price (of several tens of billions, but spread over many years and many countries).
A future electron-positron collider at CERN
In its electron-positron collider mode, the machine is proposed to produce 1,000,000 Higgs bosons, 1,000,000 top quarks, 5,000,000,000 Z bosons and 100,000,000 W bosons. Those are the heaviest particles of the Standard Model, and having so many of them will all us to probe their properties with an unprecedented precision.
From the current results of the LHC, we know that new phenomena are elusive: there is indeed no trace of them so far. Theoretically, we however also know they should be there. One potential avenue consists in scrutinising very deeply all known particles and hunt for deviations in their properties.
Let’s now quantify a bit the improvement in precision we are after with the new collider in the picture below.
[image credits: arXiv]
Each entry corresponds to the accuracy at which a given property of the Higgs boson can be measured at different colliders. The smaller is the bar, the better is the measurement. The expectation from the entire LHC run is given in green, whilst the improvements arising from a future electron-positron collider (FCC-ee) is shown in purple.
This figure shows that where we expect a precision of 1–10% at the LHC, the FCC would allow for a sub-percent precision for many quantities. This consists in the main goal of a future electron-positron collider programme at CERN: provide handles on new phenomena that could hide as tiny deviations in the properties of known particles.
A high-energy proton-proton collider at CERN
A big advantage of having an electron-positron machine in a 100 km tunnel is that it can be easily upgraded into a very energetic proton-proton collider at a decent cost (of again a couple of tens of billions USD, which is cheap after accounting for the timescale and the spread over the participating countries).
This corresponds to another avenue to new phenomena that could be explored. With a 100 km tunnel at hand, we could indeed get proton-proton collisions occurring at an energy larger than 7 times the LHC energy, and at a much larger rate.
In this way, we will get handles on more energetic and rarer phenomena. The key Standard Model measurements would be those related to the self-interactions of the Higgs boson, that are barely reachable at the LHC (so that there is a lot of room for the unexpected in those). In addition, dark matter, supersymmetry or compositeness will be pushed to more extreme corners, to name a few of the best candidates for physics beyond the Standard Model.
[image credits: arXiv]
This is illustrated with the above figure, that describes the reach in mass for various supersymmetric processes. In supersymmetric theories, each known particle possesses a partner that is heavily searched for. The expectation of those searches, if run at a potential future FCC collider at CERN, are shown as blue bars on the figure. The mass scale corresponds to how heavy could these new particles be to be discovered. For comparison, the brown coloured bars show the capabilities of the LHC.
Take your favourite model, similar figures can be made! In a few words, the mass reach for any potential new particle will be extended by a lot thanks to the FCC!
Summary - a vibrant high-energy physics community
This programme around the two discussed future colliders has clear benefits for the particle physics community. We will learn more about the Standard Model and the physics that lies beyond it. In other words, with 80-100 years of physics experiments we will get a better and deeper understanding of how the universe works.
It is true that the machine costs are not negligible at all (several tens of billions of USD). We however must keep in mind that the budget is spread across many countries and over many decades.
In exchange, a highly-qualified work force will be trained within the course of the project, both for the design of the machine, its operation and data analysis. This is where the benefits for the society comes in: high qualification by training!
Moreover, building the project requires non-existing (yet) high-field magnetics, complex cryogeny, novel civil engineering techniques, the development of new computing methods, data handling, and so on. All developments will come back to the society, for free, without any patent.
Will governments pay for this? That is a question I haven’t the answer for now… But I hope so, honestly. I personally think that the cost-benefit analysis is in favour of the society.
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