CERN: The European Strategy for Particle Physics

CERN: The European Strategy for Particle Physics

CERN’s Director for Research and Computing, Professor Eckhard Elsen, spoke to SciTech Europa about the new update of the European Strategy for Particle Physics.

Planning the European Strategy for Particle Physics is an open, inclusive and evidence-driven process and takes into account the worldwide particle physics landscape and developments in related fields.

In October, during its 190th session, the CERN Council formally launched the update of the European Strategy for Particle Physics, a two-year process involving the whole community and aiming at developing a common vision for the future of particle physics in Europe. The process is expected to be concluded in May 2020, with the approval of the updated European Strategy for Particle Physics by CERN’s Council.

To inform this vital process, the particle-physics community across universities, laboratories and national institutes have been invited to submit written input by 18 December 2018. This exercise will be followed by a Scientific Open Symposium to be held in Granada, Spain, from 13 to 16 May 2019, where the community is invited to debate the future orientation of European particle physics. This event and the written community input received will lead to the writing of a ‘briefing book’ which will inform the Strategy Drafting Session that will take place in Bad Honnef, Germany, from 20-24 January 2020.

SciTech Europa spoke to CERN’s Director for Research and Computing, Professor Eckhard Elsen, about what this review means for the particle physics landscape both in Europe and around the world, and about some of the new and exciting proposals being put forward for both the tools and the experiments needed to produce new physics in the European Strategy for Particle Physics.

Could you begin by outlining what the European Strategy for Particle Physics is and why the new revision is important?

The European Strategy for Particle Physics is revised every five to seven years, with this being the third edition of that series (the last one being in 2013). This emerged from the realisation that the future projects for particle physics would need to be of a size that requires long term planning. It also became clear that the extent of the international cooperation would also need to be global – indeed, while this is spoken of as a European strategy, it is also important to note that this happens in concert with both our American and Asian colleagues.

The new European Strategy for Particle Physics revision is expected to appear in May 2020, and it will enable us to take stock of the field, to see what we have learned, and to identify our vision for the future. Then, it will guide us to establish the level of co-operation that allows us to address the tasks which have been identified to tackle the most relevant physics questions. In many ways, revising the European Strategy for Particle Physics is a sobering exercise for the field.

Regarding the revision procedure: it has recently been launched through the CERN council and is essentially a call to the European particle physics community to provide their input on what they would like to see moving forwards. There is a target date of 18 December, and many groups will submit a ten page document detailing their ideas. Those documents will be compiled in a ‘briefing book’, and in May 2019 we will hold what is known as a ‘Town Meeting’, in which the community will come together to discuss the breadth of the field and absorb the ideas that have emerged so far.

Following that, the new European Strategy for Particle Physics will be developed via a process which is in place to ensure that we do not have simply a shopping list, but rather something that is realistic and strategic for the future and, of course, something that is also supported by our funders. Thus, after one year from the Town Meeting, we hope to arrive at a strategy that indicates the directions to be taken.

Again, it is important to emphasise that while this is a European Strategy European Strategy for Particle Physics, two representatives from both the Americas and Asia will contribute to the discussion, meaning that the end result is very well intertwined with efforts in the respective regions of the world where strategies updates are also scheduled to emerge in the coming years.

Do views and opinions on what the future of particle physics should look like differ between regions – USA, China, Europe etc.?

The Large Hadron Collider (LHC) provides the highest energy currently available anywhere in the world, which means that we hold a leadership position with regard to particle physics priorities at the energy frontier.

In the 2013 review of the strategy, three big projects were identified for the future of particle physics: the upgrade to the LHC, a coordinated activity on neutrino research (which is something that the USA and, potentially, Japan, are picking up now), and a new tool, the International Linear Collider, which may be hosted in Japan. Two of these are on track – the High-Luminosity LHC in Europe and the DUNE/LBNF neutrino facility in the USA – while we are still waiting for a decision on the International Linear Collider from Japan. That climate of sharing and co-operation was thus firmly established.

Regarding the International Linear Collider, we are expecting a statement before the end of the year from Japan. This would be a political indication of whether they would like to host it. This is important as it will inform us about what we can concentrate on, and invest in, in Europe and the USA. This decision has been pending since the last strategy update in 2013, and we are curious to see whether Japan will be the host.

Now, as we look towards the future, because of the long lead times involved we have to look very far ahead. To take the LHC as an example, this was first conceived in the 1980s (in parallel to similar efforts in the USA) and took over 20 years before the first experiments began. It is thus clear that we need to start thinking about future projects now, so that they can be developed over the next 20 (plus) years. Indeed, we need to start thinking about the projects that will happen after the High Luminosity LHC (HL-LHC), which we anticipate will run to the mid-2030s.

While neutrinos were emphasised in the 2013 European Strategy for Particle Physics review, the decision was made to discontinue CERN’s experimental activities in this area. As such, how is CERN working to ensure that it – and the European community – remains involved in this field?

Neutrino physics is now a beautiful example of international collaboration. The previous management made the bold decision to discontinue the neutrino beam, which had seen neutrinos being sent from CERN to the Gran Sasso experiments in Italy, some 700km away. This had resulted in compelling physics results, but it had become clear that with parallel endeavours taking place in the USA and Japan meant that it no longer made sense to further entertain these beamlines here in Europe. As such, the beams for neutrino production at CERN were turned off.

However, as we did not want to leave the European neutrino physics community alone, we launched the Neutrino Platform for the development of detector technology to be placed in accelerator-driven neutrino beams in the USA or Japan, and this has been an astounding success.

The DUNE experiment will be located in a mine a mile underground in South Dakota, USA, to record neutrinos sent from Fermilab. The technology for DUNE eventually consists of four modules by far exceeding the size of Olympic swimming pools filled with liquid argon and is being developed in Europe. A recent prototype representing a genuine building block of DUNE has nicely confirmed the viability of the technology and the principle at the Neutrino platform at CERN. The point being that the expertise for these detectors largely rests in Europe, with CERN being a focal point for neutrino physicists worldwide to develop detectors.

Europe has thus economised in the investment but has nevertheless kept the edge in detector development. The application is not only limited to accelerator-based neutrino physics, as these detectors can also be used to detect proton decay, for example.

What do you feel will be the biggest changes to the particle physics landscape when we look towards the post-LHC era?

The HL-LHC has been clearly identified as the key endeavour for the next 10-15 years, but beyond that time scale things are less certain.

The particle physics landscape has changed dramatically since the 2013 strategy update due to the discovery of the Higgs particle. The Higgs boson has become a great tool to use for further exploration: it is the only scalar particle in the Standard Model; it is the particle that couples to mass and thus opens new paths of exploration.

We very well understand its couplings within the Standard Model, and yet we know that there is new physics out there – such as that concerning dark matter, for example. If dark matter exists and it is attached to a particle, then this particle will have mass and most likely will also couple to the Higgs particle. We can therefore view the Higgs Particle as a portal to this ‘new world’, and what we understand so far from the very successful running of the LHC is that the coupling to this new world must be of a strength that requires very high sensitivity in experimental observations.

So far, explorations of the Higgs boson have been able to establish what is known as the ‘third generation’, with limited precision and so only discerns extreme deviations. That is, the heaviest of the three families of quarks and leptons was established; we understand the coupling to the top and bottom quarks, and we have seen the coupling to the heaviest Tau Lepton, the sibling of the electron, but we have not yet been able to measure the individual couplings to the second generation, not to speak of the first, and so there is ample room for new physics to be hiding.

This is the clear mandate for why we want to go to a higher luminosity, and everything is prepared for that. Indeed, we are extremely lucky because the detector technology is following suite with this desire to measure with higher precision; the progress being made there should enable us to achieve success by 2035. Of course, that doesn’t mean that we will have discovered new physics, but at least we will have very severely constrained the energies at which the new physics might exist, and so we will know where to look next.

So in that sense, the discovery of the Higgs is really informing the European Strategy for Particle Physics moving forwards?

Yes, absolutely. There is no doubt about what we have to do until 2035 or so. The question is what we do afterwards. And that, of course, will be informed by the measurements that result over the next 10-15 years. If new physics does appear directly then we certainly will reconsider our current priorities. But in the absence of that, because of the long lead times, we need to think about the best tools for the future now.

There are two big contenders here. The first is essentially a much bigger LHC, which will see the construction of a 100km ring reaching energies of around 100 TeV employing new high-field superconducting magnets for proton collisions. The other option is a linear lepton collider that could accelerate electrons up to 3TeV with a complementary physics programme. Both tunnel versions would support an initial lepton collider programme for precision physics to fill the gap that would be left if the ILC were not built in Japan. Lepton colliders complement the HL-LHC precision programme by e.g. being able to measuring the recoil of the Higgs particle decaying into invisible particles.

So, there is a clear mandate for a lepton collider to complement the LHC. A very high energy proton collider means that much higher energies can be achieved, and if the new physics hides beyond the TeV scale then that would be the right choice.

Both options have their merits, and I am sure that discussions around which one should be chosen will occupy a lot of the discussions time over the next year or so. And it is important to begin to take these decisions because both collider options require significant investment in technology development.

What do you hope the Physics Beyond Colliders programme will achieve?

Looking at physics that can be done without a collider is something we have been doing for some time. Now, we want to take stock of the ideas and select the most promising because we cannot afford all of them (although, of course, they are in no way comparable to the two large scale projects proposed for the post-2035 landscape).

To take dark matter as an example once again, dark matter may perhaps be realised through Weakly Interacting Massive Particles (WIMPS) which are being searched for at the LHC. There are models which suggest light axions could be contenders for dark matter particles. If so, the LHC would not necessarily be the right tool to look for that physics. If we were to investigate whether these particles couple to photons or leptons, which will probably be in a rather low mass region, then it will be necessary to conduct other experiments, and there have been several interesting proposals put forward for this.
We have little guidance from theory for light dark matter. So we must make sure that from the experimental side we cover all ground including physics without colliders. Physics Beyond Colliders is one such activity.

To make the point again: at high energy colliders you may directly produce new particles. In lower energy experiments you compare precise measurements with clear predictions from theory to see whether there is room for new physics entering through some amplitude in the cross section.

With sufficient precision it is possible to make gains of a few orders of magnitude over earlier measurements. The electric dipole moment is one such example. This moment should be zero for the proton – or be so small that we will be unable to measure it for a long time to come. So if we made a precision measurement and observed a finite electric dipole moment for the proton, that is a deviation of the charge distribution of a proton away from a perfectly spherical symmetric distribution we knew there were contributions from new physics occurring inside the proton itself, and that would be a revelation. This experiment is one of the proposals that has been made within the Physics Beyond Colliders study, and so it is clear that there are some very exciting areas here – however, it should be noted that this particular proposal would require a small ring for circulating protons, and so essentially would still make use of an accelerator.

The Town Meeting in May, 2019 in Spain will thus have a lot to discuss. We hope that it will be a place where the community can voice their thoughts and opinions, and by 2020 we will turn this into a coherent strategy that guides us into the future.

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