Any post-LHC collider, starting operations around late 2030s, would be an inter-generational project and thus it is vital to include the voices of young researchers in the decision making process. Studies planning for a next-generation collider strive to create a global vision for a "sustainable, innovative and diverse" research infrastructure, which also includes a strong desire to engage with young people.

The European Committee for Future Accelerators (ECFA) invited a group of Early-Career Researchers (ECRs) to debate the topics of the current European Strategy Update (ESU) for Particle Physics. Following a one-day meeting at CERN, with the participation of more than 180 ECRs and a survey, a report was published earlier this February. Covering a wide range of topics, from the physics opportunities of post-LHC colliders to sociological aspects, the document serves as input to the ongoing update of the European Strategy for Particle Physics. Two common themes that emerged from these discussions are related to issues of work/life balance as well as the environmental impact and the sustainability of research in high-energy physics.

The survey carried out last year revealed a number of intriguing results reflecting the vividness of the field as well as some concerns for the future of the field and particularly CERN’s role as an international laboratory. A total of 117 responses were collected with respondents representing different nationalities, gender and career level.

The results have shown that more than 85% of the respondents believe that Europe should prepare to build a large collider for the post-LHC era with another 84% concerned about the negative impact that a lack of decision for a post-LHC collider could have for the field. Importantly, there is now a growing consensus (70%) that the next machine should be an electron-positron collider offering high-precision measurements of Standard Model particles and processes.

Figure .1 Results on "Whether the next collider should be a lepton collider" for the European Strategy Update section of the ECR survey, where (1) indicates strongly disagree, (2) disagree, (3) neutral, (4) agree and (5) strongly agree.

Perhaps not surprisingly, about 84% of the respondents agreed that not preparing to build a collider at CERN soon after the LHC could have a negative impact on European research in particle physics. Given Japan’s interest to build the ILC and China’s plans for a circular lepton machine (CEPC), participants replied that if the next lepton collider is built in Asia then CERN should embark on the full FCC programme.

Figure 2. Results on "Whether not building a collider at CERN after the HL-LHC will have a negative impact for the European particle physics research" where (1) indicates strongly disagree, (2) disagree, (3) neutral, (4) agree and (5) strongly agree.

Figure 3. Left: Preferred next-generation collider. Right: Preferred future collider scenario at CERN.

Discussions about the next-generation colliders are linked to the physics opportunities it offers and the type of physics questions we could ask. For the ECRs one of the hottest topics is the study of the Higgs and the electroweak symmetry breaking. Lepton colliders offer a clear advantage in measuring most of the Higgs couplings, decay width and electroweak precision measurements. Lepton colliders as the next step, give us the chance to perform a variety of precision measurements and searches in the electroweak sector, while allowing the total width of the Higgs boson to be measured. It would also benefit from a shorter timescale. However, it was noted that this is balanced by the greater precision that energy-frontier proton-proton colliders have for rare Higgs couplings, in particular the Higgs self-coupling while also allowing to execute most of the major goals in the fields of electroweak and strong interactions,

This trade-off and the pros and cons of the different machines is one of the topics where it was hard to define a trend/consensus. However, there are two points on which everyone agrees. First of all, priority should be put on precision measurements and global fits rather than model-driven searches. Secondly, tighter collaboration between theory and experiment is needed to enhance the precision of measurements and realize the full exploratory potential of future experiments.

A second hot topic is the exploration of possible BSM physics as well as the quest for understanding the nature of Dark Matter. Regarding BSM searches, it was made clear that higher energies are required for a variety of searches for BSM resonances, strong SUSY and many other scenarios lying at a few TeV. However, the very precise measurements even at lower energies of lepton colliders may also help us probe heavier particles. Certainly, the next years and specifically the inputs from the HL-LHC will be critical for defining the goals of future projects in terms of BSM searches. Furthermore, the report stresses the need for diversification of experiments considering both accelerator and non-collider experiments that can be hosted in future upgrades of CERN’s accelerator infrastructure. The “Physics Beyond Colliders” is an important platform where such proposals should be discussed in parallel to the preparation for a new collider at CERN.

Neutrinos and flavour physics represent two other important strands for future experiments. While the heavy flavour domain benefits from any future collider, numerous specialised smaller experiments in the light sector are needed outside these large-scale scenarios to complete the picture. Similarly, exploring the neutrino sector calls for a combined approach involving both collider experiments as well as fixed-targets, long-baseline and astrophysics experiments. Combining searches at different experimental fronts could help us get a better understanding and shape a more-complete picture about the physics of neutrinos that give us today one of the most pressing experimental facts for physics beyond the Standard Model.

Finally, a topic of high-interest for HEP remains the study of strong interactions. LHC experiments offer a rich QCD programme that shouldn’t be dismissed. For example the apparent collectivity in proton-proton and proton-nucleus collisions is one of the discoveries made at the LHC that needs to be further explored. Moreover, getting a better resolution of parton distribution functions is a prerequisite to improve searches for new physics. Detailed studies of the nuclear structure, hot and dense QCD and heavy-ion physics can only be performed in hadron colliders like the proposed LHeC, FCC-hh and FCC-eh.

Beyond the strict physics challenges for the post-LHC experimental programme of particle physics, the report includes considerations on a number of sociological issues many of which are linked to the work-life balance in today’s HEP. About 60% of the participants admit concern about balancing their personal life with working hours. Perhaps alarmingly, about 70% of the respondents feel that having children could have a negative impact in their academic careers. Rather high is also the percentage of those feeling anxiety about the long-term prospects of their career with only 35% indicating good prospects. A second major challenge related to the difficulty of stabilizing their family life with work is the fact that they often have to move around to different places; a fact that 80% of the ECRs recognise that “has positive impact on the future career and enriches the scientific background but also represents a major challenge for personal aspects”.

Figure 4. Results on whether participants "Feel at ease to express concerns about reconciliation of work and personal life in the workplace": (1) indicates strongly disagree, (2) disagree, (3) neutral, (4) agree and (5) strongly agree.

Given the above mentioned results, ECRs strongly recommend the inclusion of social impact assessment in future project evaluations. Ideally, a significant fraction of project-oriented, short-term funding has to be converted into base-funding, enabling the institutions to realize a healthier and more family friendly environment with a larger fraction of permanent positions.

In the report, the ECR community also expresses their strong belief that as scientists, it is our responsibility to play an exemplary role in tackling humanity’s challenge of climate change. Almost all of the respondents (97%) felt that environmental impacts should be taken into account when taking decisions on future projects. This is interwoven to the nature of particle physics, always being at the forefront of science and technology, exploring uncharted territories and devising the tools and methods for this journey. Technological challenges such as analysing previously unheard-of amounts of data, or detecting extremely rare events in a highly radioactive environment demonstrate that particle physics research is a driver for technological innovation with an enormous impact for society and economy. Development of new accelerator technologies, advancing more efficient computing techniques and implementing new tools for conference/meeting participation to reduce the amount of travelling are goals that should be set forward by the community in light of a post-LHC project. Moreover, 73% of the respondents believe that the environmental impact of future projects should be thoroughly evaluated when making decisions on future projects.

In conclusion, the ECRs who participated in this panel support a new project for a European collider after HL-LHC. However, it is clear that in addition to the physics reach of any new accelerator, the project should also present a reliable assessment of involved timescales and risk mitigation measures as well as a solid environmental impact plan for its implementation. CERN should carefully plan its strategy to maintain its leading role in the European particle physics landscape and strive to serve as a hub for international collaboration on R&D activities needed for building the next generation of experiments in particle physics and contributing to similar activities around the world.

Note: The full report can be found HERE: https://arxiv.org/abs/2002.02837