The design study of the Future Circular Colliders (FCC) was initiated at the beginning of 2014, to identify post-LHC accelerator projects at CERN. The study has an emphasis on proton-proton (FCC-hh) and electron-positron (FCC-ee) colliders, and includes the design and optimization of detectors that would comply with the physics programme of both machines.

In this context, two workshops took place at CERN on 2-3 February and 4-5 February 2016 to discuss the progress and the roadmap of the FCC-ee physics and experiment studies. The first event was a FCC-ee mini-workshop entitled Physics behind precision. “Making precision measurements solely for their beauty is not the goal of the FCC-ee”, comments Patrick Janot, who coordinates the physics studies at FCC-ee, along with Alain Blondel. “It is more about discovering new physics through these precise measurements, and understand the quantum structure of the underlying theory in detail.” The second event was the 10^th FCC-ee physics workshop, which covered the progress in defining the FCC-ee physics potential, as well as the route forward. A number of interesting observations on the physics of FCC-ee, based on experimental observations as well as theoretical calculations, were made and will soon appear as publications.

Of course, the first question that may come to mind is “Why do we need to discuss again a new generation of precision machines after LEP and the Super-B factories?” The answer is included in the question: if the LEP performance can be multiplied by a huge factor – profiting from concepts developed for the latest designs of Super B factories – the new machine would be able to accumulate 10¹² to 10¹³ Z decays, i.e. five to six orders of magnitude more than at LEP! The centre-of-mass energy increase offered by the fourfold increase in circumference will also allow millions of Higgs particles or top quarks to be produced, to mention the heaviest and most puzzling two particles in the Standard Model. “The statistical precision of measurements that are sensitive to new physics would be improved by a factor of up to 500 – this is vertiginous, if you think of the challenge to match this with similar systematic precision and theoretical accuracy” says Alain Blondel.

A lepton-lepton collider operating at the intensities and luminosities foreseen for the FCC-ee would offer the high precision and sensitivity needed to better understand the Standard Model shortcomings and to see what might lie beyond, as well as to answer the open physics questions about the recently discovered Higgs particle. “Precision measurements are important as they could allow particles to be searched for and detected, even if they are far too heavy to be directly produced. In that sense, the FCC-ee is complementary to the FCC-hh and the  two together offer an extremely broad, rich and diverse programme of scientific exploration over at least half a century”, adds Alain Blondel.

One of the main goals of these workshops is to understand how this promise can be achieved. Insights were gained in the search for new sensitive observables and in the study of the limitations due to systematic uncertainties arising both from theory and experiments. A broad number of presentations covered these topics, showcasing the progress made on this frontier; in particular on constraining the uncertainties by ancillary measurements achievable at the FCC-ee itself. The workshops also aimed at setting concrete targets and goals for the next months along with discussing a number of new interesting ideas that could be explored with the FCC-ee.

The first idea for the FCC-ee was born in 2011, on the eve of the first evidence from ATLAS and CMS for a new (125 GeV) particle. It was a low-mass Higgs candidate, just above of what could have been seen at LEP. It was thus very tempting to think what could happen if it were possible to build a more powerful electron-positron collider in the LHC tunnel, taking stock of the enormous design progress made for the B-factories. When discussions started of building a new ring for a 100 TeV hadron collider, this led to the observations that a lepton-lepton collider in the new tunnel would be quite an extraordinary machine, and that the combination of the two colliders (not to forget an e-p option) would make a programme reasonably smooth in its funding profile and unmatched in its exploratory potential.

Following last year’s FCC-ee workshop in Pisa and the FCC Week 2015 in Washington DC, significant progress has been made in terms of understanding the underlying physics and designing the machine to accommodate the experiments. Alain Blondel says: “People from the accelerator team worked very hard to come up with design options suitable for the experimental programme of the FCC-ee”. To facilitate the design convergence, a new working group on the “machine-detector interface” was recently established to bring together the detector and accelerator experts. Its key goal is to define the interplay between the detector and accelerator elements so that both operate optimally.

At the workshop new students presented their results on the physics of the FCC-ee. Patrick Janot notes enthusiastically: “The collider will be operated and its data will be analysed by the next and perhaps the next-to-next generation of physicists! We feel that it is part of our mission to share our knowledge of electron-positron physics and to inspire the future generations”. The organizers of the workshop acknowledged that there is room for more people to work on the FCC study. “While it is obviously the time to work on the LHC data and look for signs of new physics or where we should search next, it is important that the LHC physicists and the PhD students spend some of their time on studies for the future”, Patrick Janot adds, “This transition poses some challenges, but I think that the situation is gradually improving as the tantalizing potential of the FCC appears more clearly“. Following an invitation from the organizers, CERN’s DG, Fabiola Gianotti, attended the workshop. In her talk she strongly encouraged the participants to keep working hard on the future circular lepton and hadron collider studies, to ensure that CERN is prepared to make the best choice to face future challenges.

A special ability of the FCC-ee machine is to explore the decays of the Higgs and Z bosons to new (quasi-) invisible decay modes that, if observed, would provide direct evidence for dark matter. The very high luminosities at the Z peak also opens possibilities of discovering new and extremely rare phenomena in flavour or neutrino physics, in a way that is unique among the future lepton-lepton colliders that are currently discussed. Finally, the possible consequences that new observations might have for the FCC-ee were discussed. Alain Blondel comments: “We are all eager to see if the LHC finds something new, though it is hard to predict what this might be, as current data do not seem to point clearly to a specific direction. On the other hand, one should keep in mind that the LHC experiments are measuring Higgs properties right now and it is interesting to understand how well they can do. We are following this aspect carefully in the framework of the FCC-ee study, especially given that a further step will be taken with the High-Luminosity LHC. So whatever is found in the next runs of the LHC should reinforce the interest in the FCC-ee. Having an improved knowledge of the mass and the cross sections of any new particle will help us even better grasp the great potential of the precision measurements possible at the FCC-ee and make more precise predictions how this can be exploited in specific scenarios. Remember how the measurements of the mass and width of the Z boson at LEP predicted the top quark mass, and how, as soon as the top was discovered at that very mass at the Tevatron, the LEP measurements started to be used to constrain the Higgs boson mass.”

All in all, these well-attended workshops offered the chance to monitor the progress of the FCC-ee study, to understand further its complementarity with the FCC-hh machine, and emphasized the challenges to be addressed in the coming years. The workshops concluded by  defining a roadmap for the next steps of the FCC-ee physics and detector study as part of the work for the FCC conceptual design report that will be submitted in 2018, in time for the next European Strategy Update.