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FCC-ee: beyond a Higgs factory

Long-lived heavy neutral lepton — **Discovery potential** An artistic rendering of the production and decay of a long-lived heavy neutral lepton at FCC-ee. Credit: norest.design/FCC

The proposed 100 km-circumference Future Circular Collider (FCC) at CERN features, as a first stage, an electron–positron Higgs and electroweak factory (FCC-ee) operating at centre-of-mass energies from 91 GeV (the Z mass) to a maximum of 365 GeV (above the tt production threshold). The same tunnel is then planned to host a hadron collider (FCC-hh) operating at the highest possible energies, at least 100 TeV. The complete FCC programme, whose financial and technical feasibility is currently under study, offers unprecedented potential in terms of the reach on phenomena beyond the Standard Model (SM). The proposed Circular Electron Positron Collider project in China adopts the scheme envisioned for the FCC-ee, with a somewhat less ambitious overall physics programme.

While the original goal of a future lepton collider is the precise study of the interactions of the scalar boson discovered in 2012 at the LHC, seeking answers to open questions in particle physics requires many high-precision measurements of the other three heaviest SM particles: the W and Z electroweak bosons and the top quark. Beyond the exploration of the Higgs sector, FCC-ee offers a rich range of opportunities to indirectly and directly discover new phenomena.

Studies of Higgs-boson interactions are prime tests of the dynamics of electroweak symmetry breaking and of the generation of elementary-particle masses. At FCC-ee, the Higgs boson will dominantly be produced by radiation off a Z boson. With around one million such e⁺e^– → ZH events recorded in three years of operation, a per-mil precision is targeted on the cross-section measurement. This corresponds to probing phenomena coupled to the scalar SM sector at energy scales approaching 10 TeV. The Higgsstrahlung process is, however, sensitive to gauge interactions beyond those of the Higgs boson (see “Higgs production” figure), which can themselves be affected by new physics. A robust test of the SM’s consistency will require independent experimental determination of these interactions. The precision available today is insufficient, however, and calls for new electroweak measurements to be performed.

Electroweak and top-quark precision

FCC-ee will provide these missing pieces, and much more. An unprecedented number (5 × 10¹²) of Z bosons will be produced with an exquisite knowledge of the centre-of-mass energy (100 keV or lower, thanks to the availability of transverse polarisation of the beams), thereby surpassing the precision of all previous measurements at LEP and SLC by several orders of magnitude. Uncertainties of the order of 100 keV on the Z-boson’s mass and 25 keV on its width can be achieved, as well as precisions of around 10^–5 on the various charged fermion couplings, and of 3 × 10^–5 on the QED coupling strength α_QED (m_Z). Impressive numbers of pairs of tau leptons (1.7 × 10¹¹) and 10¹² each of c and b quarks will be produced in Z decays, allowing order-of-magnitude improvements on tau and heavy-flavour observables compared to other planned facilities.

Proposed FCC layout — **Going large** The proposed layout of the Future Circular Collider. Credit: CERN

At the WW threshold, with 10⁸ W bosons collected at a centre-of-mass energy of 161 GeV and threshold scans with an energy uncertainty of about 300 keV, a unique W-boson mass precision of 0.5 MeV will be reached. Meticulous measurements of di-boson production will be essential for the Higgs programme, given the gauge-symmetry relations between triple-gauge-boson and Higgs-gauge-boson interactions. Hadronic W and Z decays will also provide measurements of the QCD coupling strength with per-mil uncertainties – a factor of 10 better than the current world average.

Stepping up to a centre-of-mass energy of 350 GeV, e⁺e^– → tt measurements would deliver an impressive determination of the top-quark mass with 10 MeV statistical uncertainty, thanks to energy scans with a 4 MeV precision. At the highest FCC-ee energies, the determination of the top quark’s electroweak couplings, which affect Higgs processes, can be performed to sub-percent precision.

These high-precision FCC-ee measurements in the Higgs, electroweak and top-quark sectors will be sensitive to a large variety of new-physics scenarios. High-mass physics with SM couplings, for example, can be tested up to scales of the order of 50 TeV. Regardless of mass scale, mixing of new particles with known ones at the level of a few tens of ppm will also produce visible effects.

Probing new physics at the Z pole

Given that new light particles are constrained to be feebly coupled to the SM, large e⁺e^– luminosities are needed to search for them. By examining an astounding number of Z-boson decays, FCC-ee will explore uncharted territories in direct searches for feebly coupled light states, such as heavy neutral leptons and axion-like particles. If not directly produced, the former are also probed indirectly through precision electroweak measurements.

Contributions to Higgsstrahlung — **Higgs production** Representative tree- and loop-level contributions to Higgsstrahlung, the main Higgs production process at FCC-ee. Couplings marked with a black dot vanish in the Standard Model, while all others may also receive new-physics contributions. Uncertainties in interactions not involving a Higgs boson could limit the precision to which Higgs interactions can be extracted. Gauge invariance also relates Higgs interactions to others that are better probed in processes involving only gauge bosons and fermions. Credit: CERN

Heavy neutral leptons (N) are sterile particles, such as those invoked in neutrino mass-generation mechanisms. The mixing of these states with neutrinos would induce interactions with electroweak bosons and charged leptons, for example N¯W, NνZ or NνH. Heavy neutral leptons can have a wide range of masses and be searched for at FCC-ee, both directly and indirectly, with unparalleled reach. When heavier than the muon and mixing with either the e or µ ﬂavours, they lower the µ → eν_eν_µ decay rate and affect the extraction of the Fermi constant, leading to deviations from the SM in many precision electroweak observables. When lighter than the Z boson, they could be produced in Z → νN decays. FCC-ee will bring order-of-magnitude improvements over LEP bounds in both regimes (see “Heavy neutral leptons” figure). The direct sensitivity improves even more dramatically than the indirect one: in the parameter space where N have sizeable lifetimes, displaced vertices provide a spectacular, background-free, signature (see “Discovery potential” image). This region of great interest corresponds to weak-scale leptogenesis, in which right-handed neutrinos participate in the generation of the baryon asymmetry of the universe.

Mass and mixing of new heavy neutral leptons — **Heavy neutral leptons** Present and prospective 95% C.L. limits on the mass and mixing of new heavy neutral leptons, N. The dominant bounds in the displayed mass range arise from direct Z → Nν searches and, indirectly, from the measurement of electroweak precision observables (preliminary dashed line for a mixing to the τ flavour). FCC-ee would improve constraints by orders of magnitude in both regimes, extending to masses way beyond those pictured. Credit: CERN

Axion-like particles (ALPs) are pseudoscalar singlets with derivative couplings to the SM, which may be generated in the breaking of global symmetries at high scales. They could contribute to the dark-matter relic abundance and, in a specific range of parameter space, provide a dynamical explanation for the absence of CP violation in the strong interaction. Having symmetry-protected masses, ALPs can be naturally light. For masses smaller than twice that of the electron, they can only visibly decay to photons. Suppressed by a potentially large scale, their couplings to the SM may be tiny. ALPs lifetimes could thus be large. A coupling to either hypercharge or weak isospin would allow them to be produced in Z-boson decays together with a photon and to decay to photon pairs. Searching for this signature, FCC-ee will probe couplings more than an order of magnitude smaller than those accessible at the LHC (see “Axion-like particles” figure). Pairs of ALPs could possibly also be produced in the decay of the Higgs boson, whose small width enhances branching fractions and allows small couplings to be probed. Producing Higgs bosons in larger numbers, hadron colliders are, however, more efficient at probing such interactions.

Towards a new frontier

The physics potential of FCC-ee clearly extends much beyond its original purpose as a Higgs and electroweak factory. Upgrading the facility to FCC-hh will require a new machine based on high-field superconducting magnets, although key parts of FCC-ee infrastructure would be usable at both colliders. Compared to the LHC, FCC-hh will collect about 10 times more integrated luminosity and increase the direct discovery reach for high-mass particles – such as Z′ or W′ gauge bosons, gluinos and squarks, and even WIMP dark matter – by a factor of around 10, up to scales of about 50 TeV. It would also serve as a giga Higgs factory, producing more than 10¹⁰ Higgs bosons during its planned 25 years of data taking, albeit not in the ultraclean collision environment of FCC-ee.

**Axion-like particles** Producing 5 × 10¹² Z bosons, FCC-ee would be particularly sensitive to their decay to a photon and an axion-like particle, when the latter couples to either U(1)_Y hypercharge (blue lines) or SU(2)_L weak isospin (orange lines). Dashed lines delimit the parameter space accessible at the LHC with an integrated luminosity of 300 fb^–1. The reach of FCC-ee on the coupling to photons requires four decays within 1.5 m of the beam axis compared to 100 at the LHC and assumes all axion-like particles decay to photon pairs. Credit: Source: *Eur. Phys. J. C* 79 74 and 474, *JHEP* **1712** 044

Beyond exquisite precision on Higgs-boson couplings to other SM particles, a 100 TeV proton–proton collider comes to the fore in revealing how the Higgs boson couples to itself, which is connected to the electroweak phase transition in the early universe and ultimately to the stability of the vacuum. The rate of Higgs pair-production events, which in some part occur through the Higgs self-interaction, would grow by a factor of 40 at FCC-hh with respect to the LHC and enable this unique property of the Higgs boson to be measured with a statistical accuracy reaching ±2%. Such a measurement would comprehensively explore classes of models that rely on modifying the Higgs potential to drive a strong first-order phase transition at the time of electroweak symmetry breaking, a necessary condition to induce baryogenesis.

Stepping up to an energy of 350 GeV would deliver an impressive determination of the top-quark mass

Following the highly successful model of LEP and its successor, the LHC, the integrated FCC programme offers a far-reaching particle-physics programme at the limits of known technology to significantly push the frontier of our knowledge of the fundamental particles and interactions. A conceptual design report was published in 2019, estimating that operations could begin as soon as 2040 for FCC-ee and 2065 for FCC-hh. Exploring the financial and technical feasibility of this visionary project
is one of the highest priority recommendations of the 2020 update of the European strategy for particle physics, with a decision on whether or not to proceed expected by the next strategy update towards the middle of the decade.

CLIC: beyond a Higgs factory

**CLIC here** Impressions of the CLIC accelerator and its detectors. Credit: CLIC collaboration

The Compact Linear Collider (CLIC) is conceived in its first stage to be an 11 km-long electron–positron collider operating at a centre-of-mass energy of 380 GeV. Unlike other Higgs-factory proposals that start around 240 GeV, CLIC benefits at the initial stage not only from top-quark production, but also from two Higgs-boson production modes – Higgsstrahlung (e⁺e^– → HZ) and WW fusion – giving extra complementary input for global interpretations of the data.

A defining feature of a linear collider is that its collision energy can be raised by extending its length. While the European strategy update recommended a circular hadron collider at the energy frontier as a long-term ambition, CLIC represents a compelling alternative were a circular machine found not to be feasible. CLIC has the potential to be extended in several stages up to 50 km and a maximum energy of 3 TeV, giving access to a wide range of physics processes (see “Multichannel” figure). Some important processes such as Higgsstrahlung production fall with energy, while others such as double-Higgs production require higher energies, and processes occurring through vector-boson fusion grow with energy. In general, the beyond-Standard-Model (BSM) sensitivity of scattering processes such as ZH, WW and two-fermion (including top-pair) production rises strongly with energy, so the higher-energy stages bring further sensitivity to potential new physics both indirectly and directly.

Lepton colliders can in general explore much closer to kinematic limits than hadron colliders

In contrast to the ILC (see ILC: beyond the Higgs), CLIC operates via a novel two-beam scheme, whereby radio-frequency power extracted from a high-current, low-energy drive beam is used to accelerate the colliding beams. Were a decision to be made to upgrade CLIC from 380 GeV to 1.5 TeV, the length of the main linacs would have to be extended to 29 km, as well as moving and adding accelerator modules. Going from an energy of 1.5 to 3 TeV, as well as further lengthening of the main linacs, a second drive-beam complex must be added. CLIC’s combination of Higgs- and top-factory running, and multi-TeV extension potential, makes it illuminating to study the physics prospects of the initial stage in parallel with those of the ultimate energy.

Higgs physics

At 380 GeV, with 1 ab^–1 of integrated luminosity CLIC would produce around 160,000 Higgs bosons. This stage would enable precision determinations well beyond the HL-LHC, for example in the single-Higgs couplings to WW, ZZ, bb, and cc. Due to the known kinematic constraints in the collision environment, it also allows an absolute determination of the Higgs couplings, as opposed to the ratios accessible at the LHC. The corresponding precision on Higgs-coupling measurements is increased considerably by the enhanced statistics at 1.5 TeV, where CLIC could produce 1 million Higgs bosons with an integrated luminosity of 2.5 ab^–1 as well as opening sensitivity to other processes. A linear collider like CLIC provides considerable flexibility, for example: collecting at 380 GeV 1 ab^–1 in 8 years or 4 ab^–1 in 13 years, as studied recently, before a possible jump to 1.5 TeV.

**Physics by extension** The three stages of CLIC’s baseline design. Credit: CERN

The 1.5 TeV energy stage gives access to two double-Higgs production mechanisms: double-Higgsstrahlung (e⁺e^– → ZHH) and vector-boson fusion (e⁺e^– → HHν_eν_e). Such production of Higgs-boson pairs allows the Higgs self-coupling to be probed directly. While the 1.5 TeV stage could reach a precision of –29%/+67% using a rate-only analysis, at 3 TeV an ultimate Higgs self-coupling precision of –8%/+11% is expected, also exploiting differential information. Furthermore, the ability to measure both the ZHH and HHν_eν_e processes allows for an unambiguous determination of the Higgs self-coupling even if it is far from its Standard Model value. Unlike indirect determinations from ZH measurements at lower Higgs-factory energies, the precision of CLIC’s direct Higgs-self-coupling measurement is largely preserved in global fits. CLIC could thus robustly verify that the Higgs self-coupling assumes the value predicted by the Standard Model, or uniquely identify the new-physics effects responsible for potential tensions with the Standard Model in Higgs observables.

Top-quark physics

CLIC is unique among the proposed electron–positron colliders in producing top-quark pairs at its initial energy stage. Electroweak couplings to third-generation fermions such as the top are particularly relevant in many BSM scenarios. Operating at the top-quark pair-production threshold of around 350 GeV would allow precise measurements of the top-quark mass and width, while cross-section and asymmetry measurements would probe the top-quark interactions. However, comprehensive exploration of top-quark couplings requires several energy stages, and spacing them widely as the CLIC baseline envisages enhances energy-dependent effects.

Electron-beam longitudinal polarisation at ±80% plays an important role in the precision programme at CLIC. Generally the polarisation significantly enhances WW-fusion processes, for example single- and double-Higgs production at higher energies; we make use of this in the baseline scenario by taking more data with left-handed electrons at the later stages. In the interpretation of Standard Model measurements, polarisation also helps to disentangle different contributions. The coupling of the top quark to the Z boson and the photon is one such example.

Indirect searches

Many observables such as cross-sections and differential distributions for WW and two-fermion production, in addition to measurements from the Higgs-boson and top-quark sectors, can be used to constrain potential new physics in the framework of effective field theory. Here, the Standard Model Lagrangian is supplemented by interaction operators of higher dimension that describe the effects of new particles. These particles could be too heavy to be produced at CLIC, but can still be probed through the effects they induce, indirectly, on CLIC observables.

**Multichannel** Important physics processes across CLIC’s full staged energy programme. Credit: CLICdp Collaboration

For many new-physics operators, CLIC is projected to bring an order of magnitude increase in sensitivity over the HL-LHC. The 380 GeV stage already significantly enhances our knowledge of operators relating to modifications of the Higgs couplings, as well as electroweak observables such as triple-gauge couplings. The higher-energy stages are then particularly effective in probing operators that induce corrections to Standard Model predictions which grow with energy. Sensitivity to these operators allows a wide range of new-physics scenarios to be probed without reference to particular models. Comparisons performed for the 2020 update of the European strategy for particle physics show, for example, that sensitivities derived in this way to four-fermion, or two-fermion two-boson contact interactions rise very steeply with the centre-of-mass energy of a lepton collider, allowing CLIC to probe scales up to 100 TeV and beyond.

Precision measurements of Standard Model processes can also be interpreted in the context of particular BSM models, such as the broad classes of composite Higgs and top, or extra-dimension models. At CLIC this represents strong new-physics reach. For example, a 3 TeV CLIC has sensitivity to Higgs compositeness up to a scale of around 18 TeV for all values of the compositeness sector coupling strength (see “Sensitivity” figure, left), and can reach beyond 40 TeV in particularly favourable scenarios; in all cases well beyond what the HL-LHC can exclude. At high masses, a multi-TeV lepton collider such as CLIC also provides the best possible sensitivity to search for new vector bosons such as the Y-universal Z′, which has couplings to quarks and leptons that are comparable (see figure, right).

As a further example, the very high energy of CLIC, and therefore the high propagator virtuality in two-fermion production, means that high-precision differential cross-sections could reveal deviations from Standard Model predictions owing to the presence of new particles in loops. This would allow discovery or exclusion of new states, for example dark-matter candidates, with a variety of possible quantum numbers and masses in the range of several TeV.

Direct searches

Direct searches for new physics at CLIC benefit from the relatively clean collision environment and from triggerless detector readout, both of which allow searches for elusive signatures that are difficult at a hadron collider. Mono-photon final states are an example of such a signature. In simplified dark-matter models containing a dark-matter particle and a mediator, dark-matter particles can be pair-produced in association with a photon, which is observed in the detector. In the case of a scalar mediator, lepton colliders are particularly sensitive and CLIC’s reach for the mediator can exceed its centre-of-mass energy significantly. In the case where the couplings to electrons and quarks are different, e⁺e^– and proton colliders provide complementary sensitivities.

Comparison to Higgs bosons — **Sensitivity** CLIC’s 2σ sensitivity in coupling and scale/mass parameters to composite Higgs bosons, combining different measurements (left) and to a new vector boson, comparing different energy stages (right). Credit: A. Wulzer

Lepton colliders can in general explore much closer to kinematic limits than hadron colliders, and this was recently verified in several examples of pair production, including simplified supersymmetric models and doubly charged Higgs production. Supersymmetric models where the higgsino multiplet is decoupled from all other supersymmetric states can lead to charginos decaying to slightly lighter neutralinos and leaving a “disappearing track stub” signature in the detector. CLIC at 3 TeV would be sensitive to such a higgsino to masses beyond 1.1 TeV, which is what would be required for the higgsino to account for the dark-matter relic mass density.

All the above approaches can be combined to illuminate the electroweak phase transition in the early universe. Models of electroweak baryogenesis can contain new scalar particles to facilitate a strong first-order phase transition, during which the electroweak symmetry is broken. Such scalar singlet extensions of the Higgs sector can be searched for directly; and indirectly from a universal scaling of all Higgs couplings.

Having both the precision capacity of a lepton collider and also the high-energy reach of multi-TeV collisions, CLIC has strong potential beyond a Higgs factory as a discovery machine. Over the next five years CERN will maintain a level of R&D in key CLIC technologies, which are also being adapted for medical applications, such that the project could be realised in a timely way after the HL-LHC if the international community decides to take this route.

Implementing a vision for CERN’s future

**Great shape** *Wandering the immeasurable*, a 15 tonne sculpture tracing 4000 years of scientific knowledge, welcomes visitors to CERN. Credit: CERN-PHOTO-201410-213

The European strategy for particle physics (ESPP), updated by the CERN Council in June 2020, lays the foundations for a bright future for accelerator-based particle physics. Its 20 recommendations – covering the components of a compelling scientific programme for the short, medium and long terms, as well as the societal and environmental impact of the field, public engagement and support for early-career scientists – set out an ambitious but prudent approach to realise the post-LHC future in Europe within the worldwide context.

Full exploitation of the LHC and its high-luminosity upgrade is a major priority, both in terms of its physics potential and its role as a springboard to a future energy-frontier machine. The ESPP identified an electron–positron Higgs factory as the highest priority next collider. It also recommended that Europe, together with its international partners, investigate the technical and financial feasibility of a future hadron collider at CERN with a centre-of-mass energy of at least 100 TeV, with an electron–positron Higgs and electroweak factory as a possible first stage. Reinforced R&D on a range of accelerator technologies is another ESPP priority, as is continued support for a diverse scientific programme.

Implementation starts now

It is CERN’s role, in strong collaboration with other laboratories and institutions in Europe and beyond, to help translate the visionary scientific objectives of the ESPP update into reality. CERN’s recently approved medium-term plan (MTP), which covers the period 2021–2025, provides a first implementation of the ESPP vision.

**Fabiola Gianotti** is Director-General of CERN. Her second term of office began in January 2021. Credit: CERN-PHOTO-201512-261-6

Starting this year, CERN will deploy efforts on the feasibility study for a Future Circular Collider (FCC) as recommended by the ESPP update. One of the first goals is to verify that there are no showstoppers to building a 100 km tunnel in the Geneva region, and to gather pledges for the necessary funds to build it. The estimated FCC cost cannot be met only from CERN’s budget, and special contributions from non-Member States as well as new funding mechanisms will be required. Concerning the enabling technologies, the first priority is to demonstrate that the superconducting high-field magnets needed for 100 TeV (or more) proton–proton collisions in a 100 km tunnel can be made available on the mid-century time scale. To this end CERN is implementing a reinforced magnet R&D programme in partnership with industry and other institutions in Europe and beyond. Fresh resources will be used to explore low- and high-temperature superconducting materials, to develop magnet models towards industrialisation and cost reduction, and to build the needed test infrastructure. These studies will also have vast applications outside the field. Minimising the environmental impact of the tunnel, the colliders and detectors will be another major focus, as well as maximising the benefits to society from the transfer of FCC-related technologies.

The 2020 MTP includes resources to continue R&D on key technologies for the Compact Linear Collider and for the establishment of an international design study for a muon collider. Further advanced accelerator technologies will be pursued, as well as detector R&D and a new initiative on quantum technologies.

Continued progress requires a courageous, global experimental venture involving all the tools at our disposal

Scientific diversity is an important pillar of CERN’s programme and will continue to be supported. Resources for the CERN-hosted Physics Beyond Colliders study have been increased in the 2020 MTP and developments for long-baseline neutrino experiments in the US and Japan will continue at an intense pace via the CERN Neutrino Platform.

Immense impact

The discovery of the Higgs boson, a particle with unprecedented characteristics, has contributed to turning the focus of particle physics towards deep structural questions. Furthermore, many of the open questions in the microscopic world are increasingly intertwined with the universe at large. Continued progress on this rich and ambitious path of fundamental exploration requires a courageous, global experimental venture involving all the tools at our disposal: high-energy colliders, low-energy precision tests, observational cosmology, cosmic rays, dark-matter searches, gravitational waves, neutrinos, and many more. High-energy colliders, in particular, will continue to be an indispensable and irreplaceable tool to scrutinise nature at the smallest scales. If the FCC can be realised, its impact will be immense, not only on CERN’s future, but also on humanity’s knowledge.

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Seeking consensus

**Moving south** Joachim Mnich, former DESY director of particle physics, joined the CERN directorate on 1 January. Credit: DESY

You started out studying electrical engineering. Why the switch to physics, and what have been your main research interests?

Actually, I studied them both in parallel, having started out in electrical engineering and then attending physics courses after I found myself getting a bit bored. I graduated with a Masters in electrical engineering, and then pursued a PhD in particle physics, working on the MARK-J experiment at DESY studying muon pairs, which allowed us to make estimates of the Z mass and sin²θ. To some level at MARK-J we could already test electroweak theory. Afterwards, I did a postdoc at CERN for two years on the L3 experiment, and ended up staying on L3 for 12 years. My background in engineering has helped several times during my career. For example, I acted as an interface between the physicists at CERN and the engineers in Aachen who designed and built the complicated L3 readout electronics, as they couldn’t always speak the same language.

How do you remember your LEP days?

It was a marvellous time, certainly some of the best years of my life. For the firsts few years at L3 I didn’t do any physics analysis – I was down in the tunnel dealing with the readout electronics. After a few years I was able to pick up physics again, going back to electroweak physics, and becoming the coordinator of the line-shape group that was in charge of measurements of Z parameters. I later became L3 analysis coordinator. I was there for essentially the whole duration of LEP, leaving CERN at the end of 1999 and joining the CMS group at Aachen University.

What are your key achievements since becoming DESY’s director for particle and astroparticle physics in 2009?

I came to DESY shortly before the experiments at HERA stopped and became director as the analyses were ramping down and LHC activities were ramping up. Certainly, one of the biggest achievements during this time was helping DESY transition from having local experiments onsite to a laboratory that now plays a key role in the CMS and ATLAS experiments. DESY became one of the largest Tier-2 data centres of the worldwide LHC computing grid, plus it had a lot of experts on proton structure and in detector operation who were highly welcomed by the LHC experiments. DESY joined the LHC relatively late, in 2008, but now has a very strong involvement in the ATLAS and CMS trackers, for example, and has set up a large infrastructure to build one end-cap tracker for ATLAS and one for CMS. DESY also joined the Belle experiment at KEK, and continues to be one of the leading labs in the development of detector R&D for future colliders. Smaller scale experiments at DESY also picked up speed, in particular axion searches. Recently the 24th dipole for the ALPS-II experiment was installed, which is really impressive. The motivation for astroparticle physics was always more concentrated at DESY’s Zeuthen site, and two years ago it was decided to create an independent division for astroparticle physics to give it more visibility.

How has the transition from collider physics to X-ray science changed life at DESY?

Well, there is no longer the burden at DESY to operate large accelerators and other facilities for particle physics, so those resources are now all directed towards photon science, such as the operation of the PETRA light source, the FLASH facility and the European XFEL. On the other hand, the laboratory has also grown over the last decade, to the benefit of photon science. However, if you count the number of DESY authors in ATLAS and CMS, it is still the second or third largest laboratory, so DESY is still very significant in particle physics.

How would you sum-up the state of high-energy physics today?

I’m optimistic, otherwise I wouldn’t be here! Often when I talk to students, I tell them that the best is yet to come in particle physics. Yes it’s true, we do not have at the moment a scenario like we had for the LHC, or for the SppS, which had clear targets to discover new particles, but if you look back in history, this hasn’t been the case very often. We would not have built several machines, including LEP, if that was the case. Discovery doesn’t have to necessarily mean new particles. So that’s why I am optimistic for the future of the field, because we have the Higgs boson now, which is a very special particle. It’s the first of its kind – not another quark or lepton. Studying the Higgs in detail might be the key to new insights into fundamental physics. This is also the central theme of the recent European strategy update.

I don’t think the question of linear vs circular is a technology one

What do you see as your main opportunities and challenges during the next five years?

CERN is a very complicated thing. I have been away for 20 years now, so I am still in a learning phase. It is very clear what our challenges are though. We have to make the next LHC run a success, and we also need to prepare for the HL-LHC. The world is looking on us for that. The second most important thing is the implementation of the European strategy update, and in particular, the preparation for the longer-term future of CERN. We have to prepare a convincing plan for the post-LHC collider, to be ready for decision at the next strategy update at the latest.

What is in store for computing?

Computing will remain a major challenge. LHC Run 3 will start soon and we have to prepare for it now, including securing the necessary funds. On the horizon there is the high-luminosity LHC, with an enormous increase in data volumes that would by far exceed the available capacities in a flat-budget scenario. We will have to work in close collaboration with the experiments and our international partners to address this challenge and be open to new ideas and emerging technologies. I believe that the new Prévessin Computing Centre will be instrumental and enhance collaboration among the experiments and the IT department.

What involvement did you have in the European strategy update?

I was a member of the European strategy group in my capacity as research director for particle physics at DESY. The strategy group contained the scientific delegates to council, plus about a dozen people from the national laboratories. I was in Bad Honnef in January 2020 for the final drafting session – it was an interesting time. If you had asked me on the Monday of that week what the result at the end would be, I would have said there was no way that we could reach consensus on a strategy. But we did, even if deciding on the specific facility to be built was beyond the ESPP mandate.

Should a post-LHC electron–positron Higgs factory be linear or circular?

Its shape is not my principal concern – I want one to be built, preferably at CERN. However, if we can get additional resources from outside the field to have one built in Japan or China, then we should grab the opportunity and try a global collaboration. I think even for the next project at CERN, we also need support from outside Europe. I don’t think the question of linear vs circular is a technology one – I think we have already mastered both technologies. We have pros and cons for both types of machine, but for me it is important that we get support for one of them, and the feasibility study that has been requested for a large circular tunnel in the Geneva area is an important step.

Infrared imaging of an ATLAS tracker end-cap petal — **LHC at DESY** Infrared imaging at DESY of an ATLAS tracker end-cap petal to check a new cooling system for HL-LHC operations. Similar work is also taking place at DESY for the CMS upgrade. Credit: DESY

Young people ask me which horse will win the race – I don’t know. I consider it as my task as CERN’s director for research and computing to unite the community behind the next collider because that will be vital for our success. The next collider will be a Higgs factory and there are so many things in common between the various proposals if you consider the detectors or the physics. People should come together and try to push the idea of a Higgs factory in whatever topology. Look, I am a scientist. At DESY I have been working on linear colliders. And in the European XFEL we essentially already have a prototype for the International Linear Collider. But if CERN or China build a circular collider, I will be the first one who signs up for an experiment! I think many others think like me.

What are the main challenges in getting the next collider off the ground?

We have competition now – very severe competition. I see that in Germany everybody is now speaking about life science and biology because of the pandemic, plus there are other key societal challenges such as climate and energy. These are topics that also have an interesting story to tell, and one which might be easier to understand. If someone asks me what the applications of the Higgs boson are, I reply that I don’t know. However, I am convinced that in 50 or 100 years from now, people will know. As particle physicists we have to continue to point out our role in society to motivate the investments and resources for our future plans, not just in science, but in technology and impact on society. If you look at the first accelerators, they were not built with other applications in mind – they were built to understand what the core of matter is. But look at the applications of accelerators, detectors and computing that have spun-off from this. X-ray science is one very strong, unforeseen example.

Would a lack of consensus for the next collider risk making physicists appear unsure about their ambitions?

Of course, there will be people who think that. However, there are also politicians, who I know in the US for instance, who are very supportive of the field. If you compare us to the synchrotron field for instance, there are dozens of light-source facilities around the world. This discipline has the benefit of not having to converge on only one – each country can essentially build its own facility. We have the challenge that we have to get a global consensus. I think many politicians understand this. While it is true that particle physics is not a decisive topic in elections, we have a duty to share our scientific adventure and results with the public. We are very fortunate in Germany that we have had a scientist as chancellor for the past 15 years, which I think this is one of the main reasons Germany is flourishing.

I consider it as my task as a CERN director for research to unite the community

What would be the implication for European particle physics were Japan or China to proceed with a Higgs factory?

I do not have a “gold-plated” answer for this. It really depends on things that are beyond our direct control as physicists. It could be an opportunity for CERN. One of the things that the strategy update confirms is that Europe is the leader of the field scientifically and also technologically, thanks mainly to the LHC. One of the arguments that CERN could profit from is the fact that Europe should want to remain the leader, or at least “a leader” in the field. That might be very helpful for CERN to also get a future project on track. Being the leader in the field is something that CERN, and Europe, can build upon.

What is your philosophy for successful scientific management?

I believe in flat hierarchies. Science is about competition for the best ideas, and the capital of research laboratories like CERN are the people, their motivation and their creativity. Therefore, I intend to foster the CERN spirit of fruitful collaboration in our laboratory but also with all our partners in Europe and the rest of the world.

Together towards new facilities

Testing of silicon sensors — **Well-oiled machine** Collaborative R&D already underpins the instrumentation challenges of the LHC detectors, such as the testing of silicon sensors for the CMS high-granularity calorimeter under development for the high-luminosity LHC. Credit: CERN-PHOTO-201904-099-14

The recently completed European strategy for particle physics (ESPP) outlines a coherent and fascinating vision for an effective and efficient exploration of the most fundamental laws of physics. Scientific recommendations for the field provide concrete guidance and priorities on future research facilities and efforts to expand our current knowledge. The depth with which we can address open mysteries about the universe depends heavily on our ability to innovate instrumentation and research infrastructures.

The ESPP calls upon the European Committee for Future Accelerators (ECFA) to develop a global detector R&D roadmap to support proposals at European and national levels. That roadmap will define the backbone of the detector R&D needed to implement the community’s vision for both the short and long term. At its plenary meeting in November, ECFA initiated a roadmap panel to develop and organise the process to realise the ESPP goals in a timely fashion. In addition to listing the targeted R&D projects required, the roadmap will also consider transformational, blue-sky R&D relevant to the ESPP.

Six technology-oriented task forces will capture each of the major components in detector instrumentation: gaseous and liquid detectors; solid-state detectors; photon detection and particle-identification; calorimetry; and quantum and emerging technologies. Along with three cross-cutting task forces devoted to electronics, integration and training, these efforts will proceed via in-depth consultation with the research community. An open symposium for each task force, due to be held in March or April 2021, will inform discussions that will eventually culminate in a roadmap document in the summer. To identify synergies and opportunities with adjacent research fields, an advisory panel – comprising representatives from the nuclear and astrophysics fields, the photon- and neutron-physics communities, as well as those working in fusion and space research – will also be established.

The roadmap will also consider transformational, blue-sky R&D relevant to the ESPP

In parallel, with a view to stepping up accelerator R&D, the European Laboratory Directors Group is developing an accelerator R&D roadmap as a work-plan for this decade. Technologies under consideration include high-field magnets, high-temperature superconductors, plasma-wakefield acceleration and other high-gradient accelerating structures, bright muon beams, and energy-recovery linacs. The roadmap, to be completed on a similar timeline as that for detectors, will set the course for R&D and technology demonstrators to enable future facilities that support the scientific objectives of the ESPP.

Gathering for a Higgs factory

The global ambition for the next-generation accelerator beyond the HL-LHC is an electron–positron Higgs factory, which can include an electroweak and top-quark factory in its programme. Pending the outcome of the technical and financial feasibility study for a future FCC-like hadron collider at CERN, the community has at this stage not concluded on the type of Higgs factory that is to emerge with priority. The International Linear Collider (ILC) in Japan and the Future Circular Collider (FCC-ee) at CERN are listed, with the Compact Linear Collider (CLIC) as a possible backup.

**Jorgen D’Hondt**, Vrije Universiteit Brussel, completed his three-year term as ECFA chair in December 2020. Credit: Vrije Universiteit Brussel

It goes without saying, and for ECFA within its mandate to explore, that the duplication of similar accelerators should be avoided and international cooperation for creating these facilities should be encouraged if it is essential and efficient for achieving the ESPP goal. At this point, coordination of R&D activities is crucial to maximise scientific results and to make the most efficient use of resources.

Recognising the need for the experimental and theoretical communities involved in physics studies, experiment designs and detector technologies at future Higgs factories to gather, ECFA supports a series of workshops from 2021 to share challenges and expertise, and to respond coherently to this ESPP priority. An international advisory committee will soon be formed to further identify synergies both in detector R&D and physics-analysis methods to make efforts applicable or transferable across Higgs factories. Concrete collaborative research programmes are to emerge to pursue these synergies. With the strategy discussion behind us, we now need to focus on getting things done together.

HEP-based ventilator to be adapted for clinical use

**Breathe easy** The CERN-based High Energy physics Ventilator is to be reengineered for clinical use by a UK-led team. Credit: CERN-PHOTO-202008-107-1

A versatile ventilator to help combat COVID-19 developed by members of the LHCb collaboration is to be re-engineered for manufacture and clinical use. The High Performance Low-cost Ventilator (HPLV) is designed to assist patients in low- and middle-income countries suffering from severe respiratory problems as a result of COVID-19. Following the award of £760,000 by UK Research and Innovation, announced in December, Ian Lazarus of the Science and Technology Facilities Council’s Daresbury Laboratory and co-workers aim to produce and test plans for the creation of an affordable, reliable and easy to operate ventilator that does not rely so heavily on compressed gases and mains electricity supply.

“I am proud to be leading the HPLV team in which we have brought together experts from medicine, science, engineering and knowledge transfer with a shared goal to make resilient high-quality ventilators available in areas of the world that currently don’t have enough of them,” said Lazarus in a press release.

While the majority of people who contract COVID-19 suffer mild symptoms, in some cases the disease can cause severe breathing difficulties and pneumonia. For such patients, the availability of ventilators that deliver oxygen to the lungs while removing carbon dioxide is critical. Commercially available ventilators are typically costly, require considerable experience to use, and often rely on the provision of high-flow oxygen and medically pure compressed air, which are not readily available in many countries.

The HPLV takes as its starting point the High Energy physics Ventilator (HEV), which was inspired by an initiative at the University of Liverpool and developed at CERN in March 2019 during the first COVID-19 lockdown. The idea emerged when physicists and engineers in LHCb’s vertex locator (VELO) group realised that the systems which are routinely used to supply and control gas at desired temperatures and pressures in particle-physics detectors are well matched to the techniques required to build and operate a ventilator (CERN Courier May/June 2020 p8). HPLV will see the hardware and software of HEV adapted to make it ready for regulatory approval and manufacture. Project partners at the Federal Institute of Rio de Janeiro in Brazil – in collaboration with CERN, the University of Birmingham, the University of Liverpool and the UK’s Medical Devices Testing and Evaluation Centre – will now identify difficulties encountered when ventilating patients and pass that information to the design team to ensure that the HPLV is fit for purpose.

“We warmly welcome the HPLV initiative, and look forward to working together with the outstanding HPLV team for our common humanitarian goal,” says Paula Collins, who co-leads the HEV project with CERN and LHCb colleague Jan Buytaert. The HPLV is one of several HEV offshoots involving 25 academic partners, she explains. “In December we also saw the first HEV prototypes to be constructed outside CERN, at the Swiss company Jean Gallay SA, which specialises in engineering for aerospace and energy. We have continued our outreach worldwide, and in particular wish to highlight an agreement being built up with a company in India that plans to modify the HEV design for local needs. None of this would have been possible without the incredible support and advice received from the medical community.”

Cosmic plasma-wakefield acceleration

**No lasers here** Radio waves produced by electrons that have been propelled to relativistic speeds through a long, thin jet at the core of the galaxy Cygnus A. Credit: NRAO/AUI

The ability to accelerate charged particles using the “wakefields” of plasma density waves offers the promise of high-energy particle accelerators that are more compact than those based on radio-frequency cavities. Proposed in 1979, the idea is to create a wave inside a plasma upon which electrons can “surf” and gain energy over short distances. Although highly complex, wakefield acceleration (WFA) driven by laser pulses or electron beams has been successfully used to accelerate electron beams to tens of GeV within distances of less than a metre, and the AWAKE experiment at CERN is attempting to achieve higher energy gains by using protons as drive beams. Recent studies suggest that WFA may also occur naturally, potentially offering an explanation for some of the highest energy cosmic rays ever observed.

So-called Fermi acceleration, first conceived by the eponymous Italian in 1949, is considered to be the main mechanism responsible for high-energy cosmic rays. In this process, charged particles are accelerated due to relativistic shockwaves occurring within jets emitted by black-hole binaries, active galactic nuclei or gamma-ray bursts, to name just a few sources. As a charged particle travels within the jet it gets accelerated each time it passes through the shock wave, allowing it to gain energy until the magnetic field in the environment can no longer contain it. This process predicts the observed power-law spectrum of cosmic rays quite well, at least up to energies of around 10¹⁹ eV. Beyond this energy, however, Fermi acceleration becomes less efficient as the particles start to lose energy due to collisions and/or synchrotron radiation. The existence of ultra-high-energy cosmic rays (UHECRs), which have been observed up to energies of 10²¹ eV, indicates that a different acceleration mechanism could be at play in that energy domain. Thanks to its very high efficiency, WFA could provide such a mechanism.

Although there are clearly no laser beams in astrophysical objects, plasma fields that can support waves can be found in many astrophysical settings. For example, in theories developed by Toshiki Tajima of the University of California at Irvine (UCI), one of the inventors of WFA technology, waves could be produced by instabilities in the accretion disks around compact objects such as black holes. These accretion disks can periodically transition from a highly magnetised to a little magnetised state, emitting electromagnetic waves that can propagate into the disk’s jets in the form of Alfven waves. As these waves continue to propagate along the jets they transform back into electromagnetic waves that can accelerate electrons on the front of the plasma’s “bow wake” and protons on the back of it.

Clear predictions

The energies that are theoretically achievable in cosmic-ray WFA depend on the mass of the compact object, as do the periodicities with which such waves can be produced. This allows clear predictions to be made for a range of different objects, which can be tested against observational data.

Groups based at UCI and at RIKEN in Japan recently tested these predictions on a range of astrophysical objects, spanning from 1 to 10⁹ solar masses. Although not conclusive, these first comparisons between theory and observations indicate several interesting features that require further investigation. For example, WFA models predict periodic emission of both UHECRs – the protons on the back of the bow wake – in coincidence with electromagnetic radiation produced by the electrons from the front of the bow wake. Due to interactions with the intergalactic medium, UHECRs are also expected to produce secondary particles, including neutrinos. WFA could thereby also explain periodic outbursts of neutrinos in coincidence with gamma-rays from, for example, blazars, for which evidence was recently found by the IceCube experiment in collaboration with a range of electromagnetic instruments. Additionally, WFA could explain the non-uniformity of the UHECR sky such as that recently reported by the Pierre Auger Observatory (see CERN Courier December 2017 p15), as it allows for cosmic rays with energies up to 10²⁴ eV to be produced within objects that lie within the location of the observed hot-spot.

In concert with future space-based UHECR detectors such as JEM-EUSO and POEMMA, further analysis of existing data should definitively answer the question of whether WFA does indeed occur in space. The clear predictions relating to periodicity, and the coincident emission of neutrinos, gamma-rays and other electromagnetic radiation, make it an ideal subject to study within the multi-messenger frameworks that are currently being set up.

European projects boost CERN’s medical applications

The MedAustron proton/carbon-ion synchrotron in Austria, constructed in collaboration with CERN, the TERA Foundation, INFN and the CNAO Foundation, with help from PSI. Credit: MedAustron

A CERN-based effort to bring about the next generation of hadron-therapy facilities has obtained new funding from the European Commission (EC) to pursue technology R&D. CERN’s Next Ion Medical Machine Study (NIMMS) aims to drive a new European effort for ion-beam therapy based on smaller, cheaper accelerators that allow faster treatments, operation with multiple ions, and patient irradiation from different angles using a compact gantry system. Its predecessor the Proton-Ion Medical Machine Study (PIMMS), which was undertaken at CERN during the late 1990s, underpinned the CNAO (Italy) and MedAustron (Austria) treatment centres that helped propel Europe to the forefront of hadron therapy.

Covering the period 2021–2024, two recently approved EC Horizon 2020 Research Infrastructure projects will support NIMMS while also connecting its activities to collaborating institutes throughout Europe. The multidisciplinary HITRIplus project (Heavy Ion Therapy Research Integration) includes work packages dedicated to accelerator, gantry and superconducting magnet design. The IFAST project (Innovation Fostering in Accelerator Science and Technology) will include activities on prototyping superconducting magnets for ion therapy with industry, together with many other actions related to advanced accelerator R&D.

“Over the past three years we have collected about €4 million of EC contributions, directed to a collaboration of more than 15 partners, representing about a factor of eight leverage on the original CERN funding,” says NIMMS project leader Maurizio Vretenar. “A key achievement was the simultaneous approval of HITRIplus and IFAST because they contain three strong work packages built around the NIMMS work-plan and associate our work with a wide collaboration of institutes.”

A major NIMMS partner is the new South East European International Institute for Sustainable Technologies (SEEIIST), an initiative started by former CERN Director-General Herwig Schopper and former minister of science for Montenegro Sanja Damjanovic, which aims to build a pan-European facility for cancer research and therapy with ions in South East Europe. CNAO and MedAustron are closely involved in the superconducting gantry design, CIEMAT in Spain will build a high-frequency linac section, and INFN is developing new superconducting magnets, with the TERA Foundation continuing to underpin medical-accelerator R&D.

MEDICIS success

Also successful in securing new Horizon 2020 funding is a project built around CERN’s MEDICIS facility, which is devoted to the production of novel radioisotopes for medical research together with institutes in life and medical sciences. The PRISMAP project (the European medical isotope programme) will bring together key facilities in the provision of high-purity-grade new radionuclides to advance early-phase research into radiopharmaceuticals, targeted drugs for cancer, “theranostics” and personalised medicine in Europe.

MEDICIS is now concluding its programme with the separation of ²²⁵Ac, a fast-emerging radionuclide for the rising field of targeted alpha therapy.

A successful programme towards this goal was developed by MEDICIS during the past two years, with partner institutes providing sources that were purified on a MEDICIS beamline using mass separation, explains Thierry Stora of CERN. “Our programme was particularly impressive this year, with record separation efficiencies of more than 50% met for ¹⁶⁷Tm, the first medical isotope produced at CERN 40 years ago with somewhat lower efficiencies,” he says. “It also allowed the translation of ¹⁵³Sm, already used in low specific activity grades for palliative treatments, to R&D for new therapeutic applications.” MEDICIS is now concluding its programme with the separation of ²²⁵Ac, a fast-emerging radionuclide for the rising field of targeted alpha therapy. “Isotope mass separation at MEDICIS acted as a catalyst for the creation of the European medical isotope programme,” says Stora, who leads the MEDICIS facility.

Together with other project consortia, the MEDICIS and HITRIplus teams are also working to identify the relevance of their research for the EC’s future cancer mission, which is part of its next framework programme, Horizon Europe, beginning this year.

Two further EC Horizon 2020 projects launched by CERN – AIDAinnova, which will enable collaboration on common detector projects, and RADNEXT, which will provide a network of irradiation facilities to test state-of-the-art microelectronics – were approved in November. “These results demonstrate CERN’s outstanding success rate in research-infrastructure projects,” says Svet Stavrev, head of CERN’s EU projects management and operational support section. “Since the beginning of the programme, Horizon 2020 has provided valuable support to major projects, studies and initiatives for accelerator and detector R&D in the particle-physics community.”

A wake-up call from the next generation

Early-career researchers survey — **Fresh perspectives** Selected results from the early-career researchers survey. Credit: Source: arXiv.org:2002.02837

The recent update of the European strategy for particle physics (ESPP) offered a unique opportunity for early-career researchers (ECRs) to shape the future of our field. Mandated by the European Committee for Future Accelerators (ECFA) to provide input to the ESPP process, a diverse group of about 180 ECRs were nominated to debate topics including the physics prospects at future colliders and the associated implications for their careers. A steering board comprising around 25 ECRs organised working groups devoted to topics including detector and accelerator physics, and key areas of high-energy physics research. Furthermore, working groups were dedicated to the environment and sustainability, and to human and social factors – aspects that have been overlooked in previous ESPP exercises. A debate took place in November 2019 and a survey was launched to obtain a quantitative understanding of the views raised.

The feedback from these activities was combined into a report reflecting the opinions of almost 120 signed authors. The survey suggests that more than half of the respondents are postdocs, around two-fifths PhD students and approximately a tenth staff members. Moreover, roughly one-third were female and two-thirds male. Several areas, such as which collider should follow the LHC and environmental and sustainability considerations, were highlighted by the participating ECRs. Among the many topics discussed, we highlight here a handful of aspects that we feel are key to the future of our field.

Building a sustainable future

A widespread concern is that the attractiveness of our field is at risk, and that dedicated actions need to be taken to safeguard its future. Certain areas of work are vital to the field, but are undervalued, resulting in shortages of key skills. Due to significant job insecurity many ECRs struggle to maintain a healthy work–life balance. Moreover, the lack of attractive career paths in science, compared to the flexible working hours and family-friendly policies offered by many companies these days, potentially compromises the ability of our field to attract and retain the brightest minds in the short- and long-term future. With the funding for the proposed Future Circular Collider (a key pillar of the ESPP recommendations) not yet clear, and despite it receiving the largest support among future-collider scenarios in CERN’s latest medium-term financial plan, an additional risk arises for ECRs to back the wrong horse.

The future of the field will depend on the success of reaching a diverse community

It is imperative to holistically include social and human factors when planning for a sustainable future of our field. Therefore, we strongly recommend that long-term project evaluations and strategy updates assess and include the impact of their implementation on the situation of young academics. Specifically, equal recognition and career paths for domains such as computing and detector development have to be established to maintain expertise in the field.

Next-generation colliders beyond the LHC will need to overcome major technical challenges in detector physics, software and computing to meet their ambitious physics goals. Our survey and debate showed that young researchers are concerned about a shortage of experts in these domains, where very few staff positions and even less professorships are open for particle physicists specialised in detector development and software and computing. In particular in the light of ever increasing project time scales, a sizable fraction of funding for non-permanent positions must be converted to funding for permanent positions in order to establish a sustainable ratio between fixed-term postdocs and staff scientists.

The possibility for a healthy work–life balance and the reconciliation of family and a scientific career is a must: currently, most of the ECRs consulted think that having children could damage their future and that moving between countries is generally a requirement to pursue a career in particle physics. These might constitute two reasons why only 20% of the polled ECRs have children. Put in a broader perspective, the future of the field will depend on the success of reaching a diverse community, with viable career paths for a wide spectrum of schemes of life. In order to reach this diverse community, it is not enough to simply offer more day-care places to parents. Similarly, the #BlackInTheIvory movement in 2020 shone a spotlight on the significant barriers faced by the Black community in academia – an issue also shared by many other minority groups. Discrimination in academia has to be counteracted systematically, including the filling of positions or grant-approval processes, where societal and diversity aspects must be taken into account with high priority.

The environmental sustainability of future projects is a clear concern for young researchers, and particle-physics institutes should use their prominent position in the public eye to set an example to other fields and society at large. The energy efficiency of equipment and the power consumption of future collider scenarios are considered only partially in the ESPP update, and we support the idea of preparing a more comprehensive analysis that includes the environmental impact of the construction as well as the disposal of large infrastructures. There should be further discussion of nuclear versus renewable energy usage and a concrete plan on how to achieve a higher renewable energy fraction. The ECRs were also of the view that much travel within our field is unnecessary, and that ways to reduce this should be brought to the fore. Since the survey was conducted, due to the ongoing COVID-19 pandemic, various conferences have already moved online, proving that progress can be made on this front.

Collider preference

In the context of the still-open questions in particle physics and potential challenges of future research programmes, the ECRs find dark matter, electroweak symmetry breaking and neutrino physics to be the three most important topics of our field. They also underline the importance of a European collider project soon after the completion of the HL-LHC. Postponing the choice of the next collider project at CERN to the 2030s, for example, would potentially negatively impact the future of the field: there could be fewer permanent jobs in detector physics, computing and software if preparations for future experiments cannot begin after the current upgrades. Additionally, it could be difficult to attract new, young bright minds into the field if there is a gap in data-taking after the LHC. While physics topics were already discussed in great detail during the broader ESPP process, many ECRs stated their discomfort about the way the next-generation scenarios were compared, especially by how the different states of maturity of the projects were not sufficiently taken into account.

About 90% of ECRs believe that the next collider should be an electron–positron machine

About 90% of ECRs believe that the next collider should be an electron–positron machine, concurring with the ESPP recommendations, although there is not a strong preference if this machine is linear or circular. While there was equal preference for CLIC and FCC-ee as the next-generation collider, a clear preference was expressed for the full FCC programme over the full CLIC programme. Given the diverse interest in future collider scenarios, and keeping in mind the unclear situation of the ILC, we strongly believe that a robust and diverse R&D programme on both accelerators and detectors must be a high priority for the future of our field.

In conclusion, both the debate and the report were widely viewed as a success, with extremely positive feedback from ECFA and the ECRs. Young researchers were able to share their views and concerns for the future of the field, while familiarising themselves with and influencing the outcome of the ESPP. ECFA has now established a permanent panel of ECRs, which is a major milestone to make such discussions among early-career researchers more regular and effective in the future.

Heavy flavours probe QGP geometry

**Fig. 1.** Left: the p_T-differential R_AA of muons from heavy-flavour hadron decays at forward rapidity in Xe–Xe (centrality class 0–10%) and Pb–Pb (centrality class 10–20%) collisions. Right: the p_T-differential R_AA of muons and electrons from heavy-flavour hadron decays in semi-central (20–40%) Xe–Xe collisions. Comparisons with transport-model calculations are displayed. Credit: CERN

Charm and beauty quarks are excellent probes of the hot and dense state of deconfined quarks and gluons (quark–gluon plasma, QGP) which is created in high-energy heavy-ion collisions. These heavy quarks are produced in hard-scattering processes at the early stages of the collisions, and interact with the constituents of the newly created QGP through both elastic and inelastic processes. These quarks, which can be studied through their decays into leptons, lose energy while propagating through the QGP medium. Consequently, different production yields are observed at large momenta in nucleus–nucleus collisions compared to proton–proton collisions. This effect can be quantified using the nuclear modification factor, R_AA, which is the ratio of nucleus–nucleus and proton–proton particle yields, scaled by the average number of binary nucleon–nucleon collisions. Comparing measurements in different collision systems sheds light on heavy-quark energy-loss mechanisms, and provides high-precision tomography of the QGP.

The results show that collision geometry plays an important role in heavy-quark energy loss

A new analysis by the ALICE collaboration compares the production of leptons from heavy-flavour hadron decays in Pb–Pb and Xe–Xe collisions at √s_NN = 5.02 and 5.44 TeV, respectively. The measurements use the muon and electron decay channels at forward rapidity and mid-rapidity. The results show that collision geometry plays an important role in heavy-quark energy loss.

Remarkable agreement

A remarkable agreement is observed between the muon yields in head-on Xe–Xe collisions and slightly offset Pb–Pb collisions (figure 1, left). Given the larger size of the lead nucleus, these collision centrality classes – 0–10% and 10–20%, respectively – give rise to similar charged-particle multiplicities, and thus suggest the creation of similar QGP densities and sizes in the colliding systems.

In both cases, the production of muons from heavy-flavour hadron decays is suppressed up to a factor of about 2.5 for 5 GeV < p_T < 6 GeV. This suppression is successfully reproduced by the MC@sHQ+EPOS2 model, which considers both elastic and inelastic energy-loss processes of the heavy quarks in the QGP, but is underestimated by the PHSD model, which only includes elastic processes. The analysis also saw ALICE’s first sensitivity down to p_T = 0.2 GeV using a lower magnetic field (0.2 T) in the solenoid magnet (figure 1, right). The suppression pattern for muons and electrons from heavy-flavour hadron decays is similar at both forward and mid-rapidity, indicating that heavy quarks strongly interact with the medium over a wide rapidity interval. The suppression is smaller in these “glancing” semi-central collisions than in the previously discussed head-on collisions. This is compatible with the hypothesis that the in-medium energy loss depends on the energy density and on the size of the system created in the collision.

The precision of the measurements brings new insights into the nature of parton energy loss and new constraints to the modelling of its dependence on the size of the QGP medium in transport-model calculations. Further constraints will be set by future higher precision measurements during Run 3, when ALICE will measure leptons from charm and beauty decays separately, at both central and forward rapidity. A short run with the much smaller oxygen–oxygen system may also be scheduled and contribute to a deeper understanding of the dependence of system size on in-medium energy loss for heavy quarks.

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