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HL-LHC counts down to LS3

Oliver Brüning and Markus Zerlauth describe the latest progress and next steps for the validation of key technologies, tests of prototypes and the series production of equipmentince the start of physics operations in 2010, the Large Hadron Collider (LHC) has enabled a global user community of more than 10,000 physicists to explore the high-energy frontier. This unique scientific programme – which has seen the discovery of the Higgs boson, countless measurements of high-energy phenomena, and exhaustive searches for new particles – has already transformed the field. To increase the LHC’s discovery potential further, for example by enabling higher precision and the observation of rare processes, the High-Luminosity LHC (HL-LHC) upgrade aims to boost the amount of data collected by the ATLAS and CMS experiments by a factor of 10 and enable CERN’s flagship collider to operate until the early 2040s.

Following the completion of the second long shutdown (LS2) in 2022, during which the LHC injectors upgrade project was successfully implemented, Run 3 commenced at a record centre-of-mass energy of 13.6 TeV. Only two years of operation remain before the start of LS3 in 2026. This is when the main installation phase of the HL-LHC will commence, starting with the excavation of the vertical cores that will link the LHC tunnel to the new HL-LHC galleries and followed by the installation of new accelerator components. Approved in 2016, the HL-LHC project is driving several innovative technologies, including: niobium-tin (Nb₃Sn) accelerator magnets, a cold powering system made from MgB₂ high-temperature superconducting cables and a flexible cryostat, the integration of compact niobium crab cavities to compensate for the larger beam crossing angle, and new technology for beam collimation and machine protection.

Efforts at CERN and across the HL-LHC collaboration are now focusing on the series production of all project deliverables in view of their installation and validation in the LHC tunnel. A centrepiece of this effort, which involves institutes from around the world and strong collaboration with industry, is the assembly and commissioning of the new insertion-region magnets that will be installed on either side of ATLAS and CMS to enable high-luminosity operations from 2029. In parallel, intense work continues on the corresponding upgrades of the LHC detectors: completely new inner trackers will be installed by ATLAS and CMS during LS3 (CERN Courier January/February 2023 p22 and 33), while LHCb and ALICE are working on proposals for radically new detectors for installation in the 2030s (CERN Courier March/April 2023 p22 and 35).

Civil-engineering complete

The targeted higher performance at the ATLAS and CMS interaction points (IPs) demands increased cooling capacity for the final focusing quadrupole magnets left and right of the experiments to deal with the larger flux of collision debris. Additional space is also needed to accommodate new equipment such as power converters and machine-protection devices, as well as shielding to reduce their exposure to radiation, and to allow easy access for faster interventions and thus improved machine availability. All these requirements have been addressed by the construction of new underground structures at ATLAS and CMS. Both sites feature a new access shaft and cavern that will house a new refrigerator cold box, a roughly 400 m-long gallery for the new power converters and protection equipment, four service tunnels and 12 vertical cores connecting the gallery to the existing LHC tunnel. A new staircase at each side of the experiment also connects the new underground structures to the existing LHC tunnel for personnel.

**Buildings and infrastructure** The new gallery at Point 1 (ATLAS) for power converters and other equipment (left); the surface building for cooling and ventilation at Point 5 (CMS) (middle); and the underground service cavern (right). Credit: I Garcia Obrero

Civil-engineering works started at the end of 2018 to allow the bulk of the interventions requiring heavy machinery to be carried out during LS2, since it was estimated that the vibrations would otherwise have a detrimental impact on the LHC performance. All underground civil-engineering works were completed in 2022 and the construction of the new surface buildings, five at each IP, in spring 2023. The new access lifts encountered a delay of about six months due to some localised concrete spalling inside the shafts, but the installation at both sites was completed in autumn 2023.

The installation of the technical infrastructures is now progressing at full speed in both the underground and surface areas (see “Buildings and infrastructure” image). It is remarkable that, even though the civil-engineering work extended throughout the COVID-19 shutdown period and was exposed to market volatility in the aftermath of Russia’s invasion of Ukraine, it could essentially be completed on schedule and within budget. This represents a huge milestone for the HL-LHC project and for CERN.

A cornerstone of the HL-LHC upgrade are the new triplet quadrupole magnets with increased radiation tolerance

A cornerstone of the HL-LHC upgrade are the new triplet quadrupole magnets with increased radiation tolerance. A total of 24 large-aperture Nb₃Sn focusing quadrupole magnets will be installed around ATLAS and CMS to focus the beams more tightly, representing the first use of Nb₃Sn magnet technology in an accelerator for particle physics. Due to the higher collision rates in the experiments, radiation levels and integrated dose rates will increase accordingly, requiring particular care in the choice of materials used to construct the magnet coils (as well as the integration of additional tungsten shielding into the beam screens). In order to have sufficient space for the shielding, the coil apertures need to be roughly doubled compared to the existing Nb-Ti LHC triplets, thus reducing the β^* parameter (which relates to the beam size at the collision points) by a factor of four compared to the nominal LHC design and fully exploiting the improved beam emittances following the upgrade of the LHC injector chain.

For the HL-LHC, reaching the required integrated magnetic gradient with Nb-Ti technology and twice the magnet aperture would require a much longer triplet. Choosing Nb₃Sn allows fields of 12 T to be reached, and therefore a doubling of the triplet aperture while keeping the magnet relatively compact (the total length is increased from 23 m to 32 m). Intensive R&D and prototyping of Nb₃Sn magnets started 20 years ago under the US-based LHC Accelerator Research Program (LARP), which united LBNL, SLAC, Fermilab and BNL. Officially launched as a design study in 2011, it has since been converted into the Accelerator Upgrade Program (AUP, which involves LBNL, Fermilab and BNL) in the industrialisation and series-production phase of all main components.

The HL-LHC inner-triplet magnets are designed and constructed in a collaboration between AUP and CERN. The 10 (eight for installation and two spares) Q1 and Q3 cryo-assemblies, which contain two 4.2 m-long individual quadrupole magnets (MQXFA), will be provided as an in-kind contribution from AUP, while the 10 longer versions for Q2 (containing a single 7.2 m-long quadrupole magnet, MQXFB, and one dipole orbit-corrector assembly) will be produced at CERN. The first of these magnets was tested and fully validated in the US in 2019 and the first cryo-assembly consisting of two individual magnets was assembled, tested and validated at Fermilab in 2023. This cryo-assembly arrived at CERN in November 2023 and is now being prepared for validation and testing. The US cable and coil production reached completion in 2023 and the magnet and cryo-assembly production is picking up pace for series production.

The first three Q2 prototype magnets showed some limitations. This prompted an extensive three-phase improvement plan after the second prototype test to address the different stages of coil production, the coil and stainless-steel shell assembly procedure, and welding for the final cold mass. All three improvement steps were implemented in the third prototype (MQXFBP3), which is the first magnet that no longer shows any limitations, neither at 1.9 K nor 4.5 K operating temperatures, and thus the first from the production that is earmarked for installation in the tunnel (see “Quadrupole magnets” image).

**Dipole magnets** Prototypes of a single-aperture dipole D1 (left) and a twin-aperture dipole D2 (right) on the test bench in SM18. Credits: E Todesco/CERN; CERN-PHOTO-202303-078-10

Beyond the triplets, the HL-LHC insertion regions require several other novel magnets to manipulate the beams. For some magnet types, such as the nonlinear corrector magnets (produced by LASA in Milan as an in-kind contribution from INFN), the full production has been completed and all magnets have been delivered to CERN. The new separation and recombination dipole magnets – which are located on the far side of the insertion regions to guide the two counterrotating beams from the separated apertures in the arc onto a common trajectory that allows collisions at the IPs – are produced as in-kind contributions from Japan and Italy. The single-aperture D1 dipole magnets are produced by KEK with Hitachi as the industrial partner, while the twin-aperture D2 dipole magnets are produced in industry by ASG in Genoa, again as an in-kind contribution from INFN. Even though both dipole types are based on established Nb-Ti superconductor technology (the workhorse of the LHC), they push the conductor into unchartered territory. For example, the D1 dipole features a large aperture of 150 mm and a peak dipole field of 5.6 T, resulting in very large forces in the coils during operation. Hitachi has already produced three of the six series magnets. The prototype D1 dipole magnet was delivered to CERN in 2023 and cryostated in its final configuration, and the D2 prototype magnet has been tested and fully validated at CERN in its final cryostat configuration and the first series D2 magnet has been delivered from ASG to CERN (see “Dipole magnets” image).

A novel cold powering system featuring a flexible cryostat and MgB₂ cables can carry the required currents at temperatures of up to 50 K

Production of the remaining new HL-LHC magnets is also in full swing. The nested canted-cosine-theta magnets – a novel magnet design comprising two solenoids with canted coil layers, needed to correct the orbit next to the D2 dipole – is progressing well in China as an in-kind contribution from IHEP with Bama as the industrial partner. The nested dipole orbit-corrector magnets, required for the orbit correction within the triplet area, are based on Nb-Ti technology (an in-kind contribution from CIEMAT in Spain) and are also advancing well, with the final validation of the long-magnet version demonstrated in 2023 (see “Corrector magnets” image).

Superconducting link

With the new power converters in the HL-LHC underground galleries being located approximately 100 m away from and 8 m above the magnets in the tunnel, a cost- and energy-efficient way to carry currents of up to 18 kA between them was needed. It was foreseen that “simple” water-cooled copper cables and busbars would lead to an undesirable inefficiency in cooling-off the Ohmic losses, and that Nb-Ti links requiring cooling with liquid helium would be too technically challenging and expensive given the height difference between the new galleries and the LHC tunnel. Instead, it was decided to develop a novel cold powering system featuring a flexible cryostat and magnesium-diboride (MgB₂) cables that can carry the required currents at temperatures of up to 50 K.

**Corrector magnets** An HL-LHC corrector package in its cryostat (left) and construction of the corrector-magnet cold mass (right). Credits: OPEN-PHO-ACCEL-2023-035-3; E Todesco, CERN

With this unprecedented system, helium boils off from the magnet cryostats in the tunnel and propagates through the flexible cryostat to the new underground galleries. This process cools both the MgB₂ cable and the high-temperature superconducting current leads (which connect the normal-conducting power converters to the superconducting magnets) to nominal temperatures between 15 K and 35 K. The gaseous helium is then collected in the new galleries, compressed, liquefied and fed back into the cryogenic system. The new cables and cryostats have been developed with companies in Italy (ASG and Tratos) and the Netherlands (Cryoworld), and are now available as commercial materials for other projects (CERN Courier May/June 2023 p37).

Three demonstrator tests conducted in CERN’s SM18 facility have already fully validated the MgB₂ cable and flexible-cryostat concept. The feed boxes that connect the MgB₂ cable to the power converters in the galleries and the magnets in the tunnel have been developed and produced as in-kind contributions with the University of Southampton and Puma as industrial partner in the UK and the University of Uppsala and RFR as industrial partner in Sweden. A complete assembly of the superconducting link with the two feed boxes has been assembled and is being tested in SM18 in preparation for its installation in the inner-triplet string in 2024 (see “Superconducting feed” image).

IT string assembly

The inner-triplet (IT) string – which replicates the full magnet, powering and protection assembly left of CMS from the triplet magnets up to the D1 separation dipole magnet – is the next emerging milestone of the HL-LHC project (see “Inner-triplet string” image). The goal of the IT string is to validate the assembly and connection procedures and tools required for its construction. It also serves to assess the collective behaviour of the superconducting magnet chain in conditions as close as possible to those of their later operation in the HL-LHC, and as a training opportunity for the equipment teams for their later work in the LHC tunnel. The IT string includes all the systems required for operation at nominal conditions, such as the vacuum (albeit without the magnet beam screens), cryogenics, powering and protection systems. The installation is planned to be completed in 2024, and the main operational period will take place in 2025.

HL-LHC insertion regions — **Inner-triplet string** The key elements constituting the new insertion regions for the high-luminosity LHC experiment, showing the quadrupole (Q) and dipole (D) assemblies, superconducting-link feedboxes (DFHX and DFH), cryogenic distribution line (SQXL) and auxiliary equipment. Credit: A Kosmicki/CERN

The entire IT string – measuring about 90 m long – just fits at the back of the SM18 test hall, where the necessary liquid-helium infrastructure is available. The new underground galleries are mimicked by a metallic structure situated above the magnets. The structure houses the power converters and quench-protection system, the electrical disconnector box, and the feed box that connects the superconducting link to normal-conducting powering systems. The superconducting link extends from the metallic structure above the magnet assembly to the D1 end of the IT string where (after a vertical descent mimicking the passage through the underground vertical cores) it is connected to a prototype of the feed box that connects to the magnets.

The inner-triplet string is the next emerging milestone of the HL-LHC project

The installation of the normal-conducting powering and machine-protection systems of the IT string is nearing completion. Together with the already completed infrastructures of the facility, the complete normal-conducting powering system of the string entered its first commissioning phase in December 2023 with the execution of short-circuit tests. The cryogenic distribution line for the IT string has been successfully tested at cold temperatures and will soon undergo a second cooldown to nominal temperature, ahead of the installation of the magnets and cold-powering system this year.

Collimation

Controlling beam losses caused by high-energy particles deviating from their ideal trajectory is essential to ensure the protection and efficient operation of accelerator components, and in particular superconducting elements such as magnets and cavities. The existing LHC collimation system, which already comprises more than 100 individual collimators installed around the ring, needs to be upgraded to address the unprecedented challenges brought about by the brighter HL-LHC beams. Following a first upgrade of the LHC collimation and shielding systems deployed during LS2, the production of new insertion-region collimators and the second batch of low-impedance collimators is now being launched in industry.

String and installation — **Inner-triplet string** The string in SM18 (left) and the installation of electrical infrastructure and signal cables in December 2023 (right). Credit: I Garcia Obrero/CERN; CERN-PHOTO-202312-289-5

LS2 and the subsequent year-end technical stop also saw the completion of the novel crystal-collimation scheme (CERN Courier November/December 2022 p35). Located in “IR7” between CMS and LHCb, this scheme comprises four goniometers with bent crystals – one per beam and plane – to channel halo particles onto a downstream absorber (see “Crystal collimators” image). After extensive studies with beam during the past few years, crystal collimation was used operationally in a nominal physics run for the first time during the 2023 heavy-ion run, where it was shown to increase the cleaning efficiency by a factor of up to five compared to the standard collimation scheme. Following this successful deployment and comprehensive machine-development tests, the HL-LHC performance goals have been conclusively confirmed for both proton and ion operations. This has enabled the baseline solution using a standard collimator inserted in IR7 (which would have required replacing a standard 8.3 T LHC dipole with two short 11 T Nb₃Sn dipoles to create the necessary space) to be descoped from the HL-LHC project.

Crab cavities

A second cornerstone of the HL-LHC project after the triplet magnets are the superconducting radiofrequency “crab” cavities. Positioned next to the D2 dipole and the Q4 matching-section quadrupole magnet in the insertion regions, these are necessary to compensate for the detrimental effect of the crossing angle on luminosity by applying a transverse momentum kick to each bunch entering the interaction regions of ATLAS and CMS. Two different types of cavities will be installed: the radio-frequency dipole (RFD) and the double quarter wave (DQW), deflecting bunches in the horizontal and vertical crossing planes, respectively (see “Crab cavities” image). Series production of the RFD cavities is about to begin at Zanon, Italy under the lead of AUP, while the DQW cavity series production is well underway at RI in Germany under the lead of CERN following the successful validation of two pre-series bare cavities.

**Crystal collimators** Installation of new crystal collimators in the LHC tunnel at Point 7 in 2023. Credit: CERN-PHOTO-202302-036-4

A fully assembled DQW cryomodule has been undergoing highly successful beam tests in the Super Proton Synchrotron (SPS) since 2018, demonstrating the crabbing of proton beams and allowing for the development and validation of the necessary low-level RF and machine-protection systems (CERN Courier March/April 2022 p45). For the RFD, two dressed cavities were delivered at the end of 2021 to the UK collaboration after their successful qualification at CERN. These were assembled into a first complete RFD cryomodule that was returned to CERN in autumn 2023 and is currently undergoing validation tests at 1.9 K, revealing some non-conformities to be resolved before it is ready for installation in the SPS in 2025 for tests with beams. Series production of the necessary ancillaries and higher-order-mode couplers has also started for both cavity types at CERN and AUP after the successful validation of prototypes. Prior to fabrication, the crab-cavity concept underwent a long period of R&D with the support of LARP, JLAB, UK-STFC and KEK.

On schedule

2023 and 2024 are the last two years of major spending and allocation of industrial contracts for the HL-LHC project. With the completion of the civil-engineering contracts and the placement of contracts for the new cryogenic compressors and distribution systems, the project has now committed more than 75% of its budget at completion. An HL-LHC cost-and-schedule review held at CERN in November 2023, conducted by an international panel of accelerator experts from other laboratories, congratulated the project on the overall good progress and agreed with the projection to be ready for installation of the major equipment during LS3 starting in 2026.

**Crab cavities** The insertion of a beam screen in a DQW crab cavity (left) and the arrival and unpacking of the RFD cryomodule (right), both in the SM18 hall in November 2023. Credit: OPEN-PHO-ACCEL-2023-030-2; CERN-PHOTO-202311-255-11

The major milestones for the HL-LHC project over the next two years will be the completion and operation of the IT-string installation in 2024 and 2025, and the completion of the installation of the technical infrastructures in the new underground galleries. All new magnet components should be delivered to CERN by the end of 2026, while the drilling of the vertical cores connecting the new and old underground areas should complete the major construction activities and mark the start of the installation of the new equipment in the LHC tunnel.

The HL-LHC will push the largest scientific instrument ever built to unprecedented levels of performance and extend the flagship collider of the European and US high-energy physics programme by another 15 years. It is the culmination of more than 25 years of R&D, with close cooperation with industry in CERN’s member states and the establishment of new accelerator technologies for the use of future projects. All hands are now on deck to ensure the brightest future possible for the LHC.

CERN welcomes host-state presidents

Wrapping up a two-day state visit to Switzerland, president of the French Republic, Emmanuel Macron (right) came to CERN on 16 November accompanied by the president of the Swiss Confederation Alain Berset (left). CERN Director-General Fabiola Gianotti took the host-state leaders on a tour of the ATLAS cavern and to the recently inaugurated Science Gateway. Speaking to journalists during the visit, Macron said: “If I came here today, it is to reiterate my confidence in the scientific community and our ambition to maintain our leadership in this domain.” (translated)

Webb sheds light on oldest black holes

JWST image of distant galaxies — **Back in time** Overlays of the JWST image of the distant galaxies with the Chandra X-ray image. The left version shows all the sources as seen using the JWST, while the right indicates the position of the distant galaxy, which clearly correlates Webb’s image with a high-intensity X-ray source seen using the Chandra data. Source: *Nat. Astron.* 2023 doi:10.1038/s41550-023-02111-9

While it is believed that each galaxy, including our own, contains a supermassive black hole (SMBH) at its centre, much remains unknown about the origin of these extreme objects. The seeds for SMBHs are thought to have existed as early as 200 million years after the Big Bang, after which they accreted mass for 13 billion years to turn into black holes with sizes of up to tens of billions of solar masses. But what were the seeds of these massive black holes? Some theories state that they were formed after the collapse of the first generation of stars, thereby making them tens to hundreds of solar masses, while other theories attribute their origin to the collapse of massive gas clouds that could produce seeds with masses of 10⁴–10⁵ solar masses.

The recent joint detection of a SMBH dating from 500 million years after the Big Bang by the James Webb Space Telescope (JWST) and the Chandra X-ray Observatory provides new insights into this debate. The JWST, sensitive to highly redshifted emission from the early universe, observed a gravitationally lensed area to provide images of some of the oldest galaxies. One such galaxy, called UHZ1, has a redshift corresponding to 13.2 billion years ago, or 500 million years after the Big Bang. Apart from its age, the observations allow an estimate of its stellar mass, while the SMBH expected to be at its centre remains hidden in these wavelengths. This is where Chandra, which is sensitive in the 0.2 to 10 keV energy range, came in.

Observations by Chandra of the area of the cluster lens, Abell 2744, which magnifies UHZ1, shows an excess at energies of 2–7 keV. The measured emission spectrum and luminosity correspond to that from an accreting black hole with a mass of 10⁷ to 10⁸ solar masses, which is about half of the total mass of the galaxy. This can be compared to our own galaxy where the SMBH is estimated to make up only 0.1% of the total mass.

Such a mass can be explained by a seed black-hole of 10⁴ to 10⁵ solar masses accreting matter for 300 million years. A small seed is more difficult to explain, however, because such sources would have to continuously accrete matter at twice their Eddington limit (the point at which the gravitational pull of the object is cancelled by the radiation pressure it applies through the accretion to the surrounding matter). Although super-Eddington accretion is possible, as this limit assumes for example spherical emission of the radiation, which is not necessarily correct, the accretion rates required for light seeds are difficult to explain.

The measurements of a single early galaxy already provide strong hints regarding the source of SMBHs. As JWST continues to observe the early universe, more such sources will likely be revealed. This will allow us to better understand the masses of the seeds, as well as how they grew over a period of 13 billion years.

Third environment report demonstrates progress

CERN’s third environment report, published on 4 December, chronicles progress made in various high-priority environmental domains during the years 2021 and 2022, and reflects a proactive approach to environmental protection across the laboratory.

CERN’s strategy with respect to the environment is based on three pillars: minimise the lab’s impact on the environment, reduce energy consumption and increase energy reuse, and develop technologies that can help society to preserve the planet. For a large part of the latest reporting period, CERN’s accelerator complex was undergoing a long shutdown that ended in July 2022 with the start of Run 3 (scheduled to end in 2025). The report charts progress made in domains such as waste, noise, ionising radiation and biodiversity, land use and landscape change. It specifically covers measures taken to reach objectives set out in the first report published in 2020: limiting the rise in electricity and water consumption and reducing direct emissions (“Scope 1”) of fluorinated gases from large experiments.

CERN is committed to limiting the rise in electricity consumption to 5% up to the end of Run 3 compared to the 2018 baseline year (which corresponds to a maximum target of 1314 GWh), while delivering significantly increased performance of its facilities. It is also committed to increasing energy reuse. A total of 1215 GWh was consumed in 2022, and the accelerator complex is now more efficient, delivering more data per unit of energy consumed (CERN Courier May/June 2022 p55). In light of the energy crisis, CERN implemented additional energy-saving measures as a mark of social responsibility, and further explored diversification of energy sources and heat-recovery projects. The process to obtain the internationally recognised ISO 50001 energy-management certification was also undertaken in the reporting period, and has since been awarded.

CERN’s objective is to reduce direct greenhouse-gas emissions by 28% by the end of Run 3 compared to 2018, which corresponds to a maximum target of 138,300 tCO₂e. In 2022, 184,300 tCO₂e direct emissions were generated, with a comprehensive programme to ensure progress towards the objective. For example, the experiments have increased efforts to repair leaks in gas systems and worked towards replacing current gases with more environmentally friendly ones. With respect to indirect greenhouse-gas emissions (“Scope 3”), CERN first reported these in the second environment report (2019–2020) spanning catering, commuting and duty travel. This third report now includes scope 3 emissions arising from procurement, which represent 92% of this total, and details the main sources of related emissions.

Regarding water consumption, CERN is committed to keeping the increase in its water consumption below 5% up to the end of Run 3 compared to 2018 (which corresponds to a maximum target of 3651 ML) despite a growing demand for water cooling at the upgraded facilities. Since 2000, CERN has radically decreased its water consumption by about 80%. The report also explores how waste is managed. CERN’s aim over the reporting period has been to increase its recycling rate for non-hazardous waste, which represents over 70% of the total waste generated. In 2022 this recycling rate was 69% compared to 56% in 2018.

Biodiversity, land use and landscape change are another important focus of the report, as is the latest on how CERN’s technology and knowledge benefit society, notably with the new CERN Innovation Programme on Environmental Applications launched in March 2022.

Benoît Delille, head of the CERN Occupational Health and Safety and Environmental Protection unit, concludes: “Over the years since we embarked on our first environment report, we have learned a great deal about our footprint, implemented mechanisms to better understand and control it, and increased our efforts to identify and develop technologies stemming from our core research that have the potential to benefit the environment.”

US unveils 10-year strategy for particle physics

On 8 December, the high-energy physics advisory panel to the US Department of Energy and National Science Foundation released a 10-year strategic plan for US particle physics. The Particle Physics Project Prioritization Panel (P5) report recommends projects across high-energy physics for different budget scenarios. Extensive input from the 2021 Snowmass exercise and other community efforts was distilled into three overarching themes: decipher the quantum realm; explore new paradigms in physics; and illuminate the hidden universe, each of which has been linked to science drivers that represent the most promising avenues of investigation for the next 10 years and beyond.

“The Higgs boson had just been discovered before the previous P5 process, and now our continued study of the particle has greatly informed what we think may lie beyond the standard model of particle physics,” said panel chair Hitoshi Murayama (UCB). “Our thinking about what dark matter might be has also changed, forcing the community to look elsewhere – to the cosmos. And in 2015, the discovery of gravitational waves was reported. Accelerator technology is changing too, which has shifted the discussion to the technology R&D needed to build the next-generation particle collider.”

Independent of the budget scenario, realising the full scientific potential of existing projects is the highest P5 priority, including the High-Luminosity LHC, DUNE and PIP-II, and the Vera C Rubin Observatory. In addition, the panel recommends continued support for the medium-scale experiments NOvA, SBN, T2K and IceCube; DarkSide-20k, LZ, SuperCDMS and XENONnT; DESI; Belle II and LHCb; and Mu2e.

On the hot topic of future colliders, the P5 report endorses an off-shore Higgs factory, naming FCC-ee and ILC, to advance studies of the Higgs boson following the HL-LHC. The US should actively engage in design studies to establish the technical feasibility and cost of Higgs factories and convene a targeted panel to make decisions in US accelerator physics at the time when major decisions concerning an off-shore Higgs factory are expected, at which point the US should commit funds commensurate with its involvement in the LHC and HL-LHC. Looking further into the future “and ultimately aim to bring an unparalleled global facility to US soil”, the P5 report supports vigorous R&D toward a 10 TeV parton-centre-of-momentum collider, including a targeted programme to establish the feasibility of a 10 TeV muon collider at Fermilab – dubbed “our Muon Shot”.

Astro-matters

Looking outward, the panel identified several critical areas in cosmic evolution, neutrinos and dark matter where next-generation facilities could make a dramatic impact. Topping the list are: CMB-S4, which will use telescopes in Chile and Antarctica to study the cosmic microwave background (CERN Courier March/April 2022 p34); early implementation of a planned accelerator upgrade at Fermilab to advance the timeline of DUNE (in addition to a re-envisioned second phase of DUNE and R&D towards an advanced fourth detector); and a comprehensive Generation-3 dark-matter experiment to be coordinated with international partners and preferably sited in the US. Here, states the report, the impact of the more constrained budget scenario is severe, and could force the US to cede leadership in Generation-3 and to descope or delay elements of DUNE: “Limiting of DUNE’s physics reach would negatively impact the reputation of the US as an international host, and more limited contributions to an off-shore Higgs factory would tarnish our standing as a partner for future global facilities.”

Multi-messenger observatories with dark-matter sensitivity, including IceCube Gen-2 for the study of neutrino properties, and small-scale dark-matter experiments employing innovative technologies, are singled out for support. In addition, the panel recommends that the Department of Energy create a new competitive programme to support a portfolio of smaller, more agile experiments in high-energy physics.

The P5 report supports vigorous R&D toward a 10 TeV parton-centre-of-momentum collider

Investing in the scientific workforce and enhancing computational and technological infrastructure are described as “crucial”, with increased support for theory, general accelerator R&D, instrumentation and computing needed to bolster areas where US leadership has begun to erode. The report also urges broader engagement with and support for the workforce, suggesting that all projects, workshops, conferences and collaborations incorporate ethics agreements that detail expectations for professional conduct and establish mechanisms for transparent reporting, response and training.

“In the P5 exercise, it’s really important that we take this broad look at where the field of particle physics is headed, to deliver a report that amounts to a strategic plan for the US community with a 10-year budgetary timeline and a 20-year context,” said P5 panel deputy chair Karsten Heeger (Yale). “The panel thought about where the next big discoveries might lie and how we could maximise impact within budget, to support future discoveries and the next generation of researchers and technical workers who will be needed to achieve them.”

Polish companies and institutes – quality partners for collaboration

Polish industry has been collaborating with CERN for years and has been very successful in various technical domains, such as cryogenics, mechanical engineering, electrical engineering and IT. The contracts and orders entrusted to Polish companies have been carried out with good quality. Polish companies proves that they are solid business partners and demonstrate a high level of specialisation.

Industrial supplies for CERN were provided by KrioSystem in Wrocław and Turbotech in Płock, CHEMAR in Kielce and RAFAKO in Racibórz. CERN also operates devices manufactured by the ZPAS company in Wolibórz, while Polish company ZEC Service has been awarded CMS Gold awards for the delivery and assembly of cooling installations. Creotech Instruments – a company established by a physicist and two engineers who met at CERN – is a regular manufacturer of electronics for CERN and enjoys a strong collaboration with CERN’s engineering teams. Polish companies also transfer technology from CERN to industry, such as TECHTRA in Wrocław, which obtained a license from CERN for the production and commercialisation of GEM (Gas Electron Multiplier) foil. Deliveries to CERN are also carried out, inter alia, by FORMAT, Softcom or Zakład Produkcji Doświadczalnej CEBEA from Bochnia. FIBRAIN, a manufacturer in the photonics and fiber optics sector, supplied CERN with launchers, which are used during measurement work in the CERN Tunnel.

These are just a few examples of companies from Poland that are successfully supplying products to CERN and other Big Science centers.

Among the scientific institutes, the National Center for Nuclear Research in Otwock near Warsaw and the Institute of Nuclear Physics of the Polish Academy of Sciences in Krakow are particularly active.

Looking for partners? Come to us!

Find new business contacts

The Świerk Science and Technology Park, at the National Center for Nuclear Research is the point of contact with Big Science centers for Polish industry and houses an ILO office for CERN, ITER and XFEL.

If you are looking for partners from Poland, in the area of R&D, come to us. We will help you establish valuable business contacts.

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Lawrence W Jones 1925–2023

**Lawrence Jones** also had a strong interest in entomology. Credit: University of Michigan

Experimental particle physicist Lawrence W Jones, a well-respected mentor and educator who contributed to important developments in accelerators and detectors, passed away on 30 June 2023.

Born in Evanston, Illinois on 16 November 1925, he enrolled at Northwestern University in autumn 1943 but was drafted into the US army a few months later. He served in Europe during World War II in 1944 and 1945, returning to Northwestern to complete a BSc in zoology and physics in 1948, followed by an MSc in 1949. After completing a PhD from the University of California, Berkeley in 1952, Jones went to the University of Michigan to begin a lifetime career in the physics faculty. In 1962 he acted as dissertation adviser to future Nobel laureate Samuel Ting and was promoted to full professor in 1963. He served as the physics department chair from 1982 to 1987 and was named professor emeritus in 1998.

Jones collaborated in the 1950s in the Midwestern Universities Research Association, a collaboration of US universities that developed key concepts for colliding beams, and built the first fixed-focus alternating gradient accelerator. Over the course of his career, Jones also contributed to the development of scintillation counters, optical spark chambers and hadron calorimeters. He participated in experiments designed to measure inelastic and elastic scattering, particle production, dimuon events, neutrino physics and charm production.

Jones came to CERN as a Ford Foundation Fellow (1961–1962) and as a Guggenheim Fellow (1964–1965), and then contributed to cosmic-ray experiments on Mount Evans, Colorado and nearby Echo Lake. In 1983 he joined the L3 experiment at LEP, which was led by his former student Ting. The Michigan team, led by Byron Roe, helped to design, construct and install the experiment’s hadron calorimeter – a key component used to determine the number of elementary neutrino families. Jones also contributed to the construction of L3 cosmics, a programme to trigger on and measure cosmic rays using the detector’s precision muon detector and surrounding solenoidal magnet.

Jones’ interest in entomology led to a species of beetle (Cryptorhinula jonsi) being named after him. On the first Earth Day, in 1970, Jones introduced the term “liquid hydrogen fuel economy” and, in 1976, he joined the advisory board of the International Association for Hydrogen Energy. He had a long involvement with the Ann Arbor Ecology Center, which he led in 1974–1975, and became co-chair of the Michigan Environmental Council’s Science Advisory Committee in 2000.

Electroweak milestones at CERN

Celebrating the 1973 discovery of weak neutral currents by the Gargamelle experiment and the 1983 discoveries of the W and Z bosons by the UA1 and UA2 experiments at the SppS, a highly memorable scientific symposium in the new CERN Science Gateway on 31 October brought the past, present and future of electroweak exploration into vivid focus. “Weak neutral currents were the foundation, the W and Z bosons the pillars, and the Higgs boson the crown of the 50 year-long journey that paved the electroweak way,” said former Gargamelle member Dieter Haidt (DESY) in his opening presentation.

History could have turned out differently, said Haidt, since both CERN and Brookhaven National Laboratory (BNL) were competing in the new era of high-energy neutrino physics: “The CERN beam was a flop initially, allowing BNL to snatch the muon-neutrino discovery in 1962, but a second attempt at CERN was better.” This led André Lagarrigue to dream of a giant bubble chamber, Gargamelle, financed and built by French institutes and operated by CERN with beams from the Proton Synchrotron (PS) from 1970 to 1976. Picking out the neutral-current signal from the neutron-cascade background was a major challenge, and a solution seemed hopeless until Haidt and his collaborators made a breakthrough regarding the meson component of the cascade.

The ten years between the discovery of neutral currents and the W and Z bosons are what took CERN from competent mediocrity to world leader
Lyn Evans

By early July 1973, it was realised that Gargamelle had seen a new effect. Paul Musset presented the results in the CERN auditorium on 19 July, yet by that autumn Gargamelle was “treated with derision” due to conflicting results from a competitor experiment in the US. ‘The Gargamelle claim is the worst thing to happen to CERN,’ Director-General John Adams was said to have remarked. Jack Steinberger even wagered his cellar that it was wrong. Following further cross checks by bombarding the detector with protons, the Gargamelle result stood firm. At the end of Haidt’s presentation, collaboration members who were present in the audience were recognised with a warm round of applause.

On behalf of the Gargamelle collaboration, former member Dieter Haidt recounted the story of CERN’s first big discovery, which confirmed the electroweak theory. Credit: CERN-PHOTO-202310-256-37

From the PS to the SPS
The neutral-current discovery and the subsequent Gargamelle measurement of the weak mixing angle made it clear not only that the electroweak theory was right but that the W and Z were within reach of the technology of the day. Moving from the PS to the SPS, Jean-Pierre Revol (Yonsei University) took the audience to the UA1 and UA2 experiments ten years later. Again, history could have taken a different turn. While CERN was working towards a e⁺e^– collider to find the W and Z, said Revol, Carlo Rubbia proposed the radically different concept of a hadron collider — first to Fermilab, which, luckily for CERN, declined. All the ingredients were presented by Rubbia, Peter McIntyre and David Cline in 1976; the UA1 detector was proposed in 1978 and a second detector, UA2, was proposed by CERN six months later. UA1 was huge by the standards of the day, said Revol. “I was advised not to join, as there were too many people! It was a truly innovative project: the largest wire chamber ever built, with 4π coverage. The central tracker, which allowed online event displays, made UA1 the crucial stepping stone from bubble chambers to modern electronic ones. The DAQ was also revolutionary. It was the beginning of computer clusters, with same power as IBM mainframes.”

First SppS collisions took place on 10 July 1981, and by mid-January 1983 ten candidate W events had been spotted by the two experiments. The W discovery was officially announced at CERN on 25 January 1983. The search for the Z then started to ramp up, with the UA1 team monitoring the “express line” event display around the clock. On 30 April, Marie Noelle Minard called Revol to say she had seen the first Z. Rubbia announced the result at a seminar on 27 May, and UA2 confirmed the discovery on 7 June. “The SppS was a most unlikely project but was a game changer,” said Haidt. “It gave CERN tremendous recognition and paved the way for future collaborations, at LEP then LHC.”

Although they were in close competition to find the W and Z, Pierre Darriulat (left, UA2), Carlo Rubbia (middle, UA1) and Jean-Pierre Revol (right, UA1) are connected by the achievements and friendship. Credit: CERN-PHOTO-202310-256-122

Former UA2 member Pierre Darriulat (Vietnam National Space Centre) concurred: “It was not clear at all at that time if the collider would work, but the machine worked better than expected and the detectors better than we could dream of.” He also spoke powerfully about the competition between UA1 and UA2: “We were happy, but it was spoiled in a way because there was all this talk of who would be ‘first’ to discover. It was so childish, so ridiculous, so unscientific. Our competition with UA1 was intense, but friendly and somewhat fun. We were deeply conscious of our debt toward Carlo and Simon [van der Meer], so we shared their joy when they were awarded the Nobel prize two years later.” Darriulat emphasised the major role of the Intersecting Storage Rings and the input of theorists such as John Ellis and Mary K Gaillard, reserving particular praise for Rubbia. “Carlo did the hard work. We joined at the last moment. We regarded him as the King, even if we were not all in his court, and we enjoyed the rare times when we saw the King naked!”

Our competition with UA1 was intense, but friendly and somewhat fun
Pierre Darriulat

The ten years between the discovery of neutral currents and the W and Z bosons are what took CERN “from competent mediocrity to world leader”, said Lyn Evans in his account of the SppS feat. Simon van der Meer deserved special recognition, not just for his 1972 paper on stochastic cooling, but also his earlier invention of the magnetic horn, which was pivotal in increasing the neutrino flux in Gargamelle. Evans explained the crucial roles of the Initial Cooling Experiment and the Antiproton Accumulator, and the many modifications needed to turn the SPS into a proton-antiproton collider. “All of this knowledge was put into the LHC, which worked from the beginning extremely well and continues to do so. One example was intrabeam scattering. Understanding this is what gives us the very long beam lifetimes at the LHC.”

Long journey
The electroweak adventure began long before CERN existed, pointed out Wolfgang Hollik, with 2023 also marking the 90^th anniversary of Fermi’s four-fermion model. The incorporation of parity violation came in 1957 and the theory itself was constructed in the 1960s by Glashow, Salam, Weinberg and others. But it wasn’t until ‘t Hooft and Veltman showed that the theory is renormalizable in the early 1970s that it became a fully-fledged quantum field theory. This opened the door to precision electroweak physics and the ability to search for new particles, in particular the top quark and Higgs boson, that were not directly accessible to experiments. Electroweak theory also drove a new approach in theoretical particle physics based around working groups and common codes, noted Hollik.

The afternoon session of the symposium took participants deep into the myriad of electroweak measurements at LEP and SLD (Guy Wilkinson, University of Oxford), Tevatron and HERA (Bo Jayatilaka, Fermilab), and finally the LHC (Maarten Boonekamp, Université Paris-Saclay and Elisabetta Manca, UCLA). The challenges of such measurements at a hadron collider, especially of the W-boson mass, were emphasised, as were their synergies with QCD in measurements in improving the precision of parton distribution functions.

The electroweak journey is far from over, however, with the Higgs boson offering the newest exploration tool. Rounding off a day of excellent presentations and personal reflections, Rebeca Gonzalez Suarez (Uppsala University) imagined a symposium 40 years from now when the proposed collider FCC-ee at CERN has been operating for 16 years and physicists have reconstructed nearly 10¹³ W and Z bosons. Such a machine would take the precision of electroweak physics into the keV realm and translate to a factor of seven increase in energy scale. “All of this brings exciting challenges: accelerator R&D, machine-detector interface, detector design, software development, theory calculations,” she said. “If we want to make it happen, now is the time to join and contribute!”

Kaon physics at a turning point

Only two experiments worldwide are dedicated to the study of rare kaon decays: NA62 at CERN and KOTO at J-PARC in Japan. NA62 plans to conclude its efforts in 2025, and both experiments are aiming to reach important milestones on this timescale. The future experimental landscape for kaon physics beyond this date is by no means clear, however. With proposals for next-generation facilities such as HIKE at CERN and KOTO-II at J-PARC currently under scrutiny, more than 100 kaon experts met at CERN from 11 to 14 September for a hybrid workshop to take stock of the experimental and theoretical opportunities in kaon physics in the coming decades.

Kaons, which contain one strange and either a lighter up or down quark, have played a central role in the development of the Standard Model (SM). Augusto Ceccucci (CERN) pointed out that many of the SM’s salient features – including flavour mixing, parity violation, the charm quark and CP violation – were discovered through the study of kaons, leading to the Cabibbo-Kobayashi-Maskawa (CKM) quark mixing matrix. The full particle content of the SM was finally experimentally established at CERN with the Higgs-boson discovery in 2012, but many open questions remain.

The kaon’s special role in this context was the central topic of the workshop. The study of rare kaon decays provides a unique sensitivity to new physics, up to scales higher than those at collider experiments. In the SM, the rare decay of a charged or neutral kaon into a pion plus a pair of charged or neutral leptons is strongly suppressed, even more so than the similar rare B-meson decays. This is due to the absence at tree-level of flavour-changing neutral current interactions (e.g. s → d) in the SM. Such a transition can only proceed at loop level involving the creation of at least one very heavy (virtual) electroweak gauge boson (figure “Decayed”, left). While experimentally this suppression constitutes a formidable challenge in identifying the decay products amongst a variety of background signals, new-physics contributions could leave a significantly measurable imprint through tree-level or virtual contributions. In contrast to rare B decays, the “gold-plated” rare kaon decay channels K⁺→π⁺νν and K_L→π⁰νν do not suffer from large hadronic uncertainties and are experimentally clean due to the limited number of possible decay channels.

kaons_at_cern_diagram — **Decayed** Diagrams illustrating flavour-changing neutral transitions in rare kaon decays proceed in the SM.

The charged-kaon decay is currently being studied at NA62, and a measurement of its branching ratio with a precision of 15% is expected by 2025. However, as highlighted by NA62 physics coordinator Karim Massri (Lancaster University), to improve this measurement and thus significantly increase the likelihood of a discovery, the experimental precision must be reduced to the level of the theoretical prediction, i.e. 5%. This can only be achieved with a next-generation experiment. The HIKE experiment, a proposed high-intensity kaon factory at CERN currently under approval, would reach the 5% precision goal on the measurement of K⁺→π⁺νν during its first phase of operation. experiment, a future high-intensity kaon factory at CERN currently under approval, will reach the 5% precision goal on the measurement of K⁺→π⁺νν during its first phase of operation. Afterwards, a second phase with a neutral K_L beam aiming at the first observation of the very rare decays K_L→π⁰ℓ⁺ℓ^– is foreseen. With a setup and detectors optimised for the measurement of the most challenging processes, the HIKE programme would be able to achieve unprecedented precision on most K⁺ and K_L decays.

For KOTO, Koji Shimi and Hashime Nanjo reported on the experimental progress on K_L→π⁰ℓ⁺ℓ^– and presented a new bound on its branching ratio. A planned phase two of KOTO, if funded, aims to measure the branching ratio with a precision of 20%. Although principally designed for the study of (rare) bottom-quark decays, LHCb can also provide information about the rare decay of the shorter-lived K_S.Radoslav Marchevski (EPFL Lausanne) presented the status and the prospects for a proposed LHCb-Phase II upgrade.

From the theory perspective, underpinned by impressive new perturbative, lattice QCD and effective-field-theory calculations presented at the workshop, the planned measurement of K⁺→π⁺νν at HIKE clearly has discovery potential, remarked Gino Isidori (University of Zurich). Together with other rare decay channels such as K_L→μ⁺μ^–, K_L→π⁰ℓ⁺ℓ^– and K⁺→π⁺ℓ⁺ℓ^–that would be measured by HIKE, added Giancarlo D’Ambrosio (INFN), the combined global theory analyses of experimental data will allow for discovering new physics if it exists within the reach of the experiment, and for providing solid constraints for new physics.

A decision on HIKE and other proposed experiments in CERN’s North Area will take place in early December.

Going underground in Vienna

From 28 August to 1 September, the 18th International Conference on Topics in Astroparticle and Underground Physics took place at the University of Vienna, organised by HEPHY/Austrian Academy of Sciences (ÖAW), and attracting about 450 participants. An extensive offer of parallel sessions each afternoon spanned direct dark-matter detection, advances in gravitational-wave (GW) searches, neutrino physics, astrophysics and cosmology, cosmic rays and astroparticle physics, as well as intertrack sessions on two or more subjects. A broad stage was also given to outreach and education, featuring science-communication projects from around the world, open science and masterclasses.

The conference provided an excellent review of the status of scientific questions being addressed by experiments in underground labs, including the latest constraints on dark matter from PandaX, LUX-ZEPLIN, SuperCDMS, CRESST and XENONnT. The various techniques for studying dark matter indirectly, for example via cosmic radiation, were reviewed, as well as direct searches at accelerator facilities. The many and diverse efforts ongoing worldwide to understand the nature of neutrinos were covered comprehensively, including the parametrisation of their mixing properties, their absolute mass, whether neutrinos are their own antiparticle, and their role in the early and late universe and in supernova explosions. Two plenary presentations focused on recent highlights in the field: IceCube’s confirmation of neutrinos from the galactic plane, and evidence of a GW background at nanohertz frequencies measured with pulsar timing arrays (CERN Courier September/October 2023 p7). Others summarised the status of cosmology in theory and experiment, cosmic-ray physics and the detection of GWs.

Among participants was Arthur McDonald, co-recipient of the 2015 Nobel Prize in Physics for the discovery of neutrino oscillations, who gave a talk “Using messengers from outer space to understand our universe and its evolution” to a packed audience of all ages in the Festsaal ÖAW. He also celebrated his 80th birthday during the conference, earning a big round of applause.

A total of 110 posters were presented, more than half from early-career scientists. The five winners were: Korbinian Urban (TUM) for “TRISTAN: A novel detector for searching keV-sterile neutrinos at the KATRIN experiment”; Christoph Wiesinger (TUM) for “TAXO – Towards an ultra-low background semiconductor detector for IAXO”; Steffen Turkat (TU Dresden) for “Low-background radioactivity counting at the most sensitive HPGE detector in Germany”; Angelina Kinast (TUM) for “First results on 170 enrichment of CaWO₄ crystals for spin-dependent DM search with CRESST”; and Krystal Alfonso (Virginia Tech) for “Analysis techniques for the search of neutrinoless double-beta decay of Te-130 with CUORE”.

The next edition of TAUP will take place in 2025 in Chengdu, China.

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