Jobs – Page 6 of 505 – CERN Courier

Music city tunes in to accelerators

By some counts, there are more than 300 distinct branches of science, a number that continues to grow. In physics alone, which began with astronomy five millennia ago, there are now at least two dozen subdivisions in most taxonomies. Over the past three decades, the science of beams has evolved into a distinct discipline with its own subjects and methods, dedicated peer-reviewed journals – like Physical Review Accelerators and Beams, which turned 25 last year – and nearly two dozen regular regional and international conferences and workshops.

Today, around 5000 accelerator scientists and engineers work in more than 50 countries, collaborating with a pool of technical experts three to four times that size. While most are deeply involved in operations and upgrades, their careers also include designing and constructing new facilities, beam-physics research, developing critical technical components, and project leadership. Their work often involves technology transfer, industrial applications, education and training of future experts, and public and academic outreach.

A global field

The need for regular meetings of the entire field has long been recognised. Historically, regional conferences like the biannual particle-accelerator conferences (PACs) in the US (1965–2009), the biannual EPACs in Europe (1988–2008) and the triannual APACs in Asia (1998–2007) served this purpose. These gatherings covered all types of accelerators, particles and use-cases. As the field became truly global, leaders established the series of international PACs (IPACs), which rotate through the regions in a three-year cycle, convening about 1500 attendees. The 15th IPAC took place from 19 to 24 May in Nashville, Tennessee, with almost 200 registrants from Asia, more than 400 from Europe and nearly 700 from the US.

The “beef” of the conference was in the reports from facilities, but no one person can summarise all the progress, and I must restrict myself to personal highlights in fields that are close to my heart. Fascinating progress was reported on energy-recovery linacs (ERLs) and associated technologies such as superconducting RF and fixed-field-alternating-gradient accelerators, following the recent success of the CBETA accelerator test facility at Cornell. Another hot topic in my eyes was design work and experimental studies towards strong hadron cooling for the Electron–Ion Collider. This year’s progress in industrial and medical accelerators is also impressive, with noteworthy presentations on radioisotope production and radiotherapy (Oliver Kester, TRIUMF and Michael Galonska, GSI), light sources for semiconductor manufacturing (Bruce Dunham, SLAC), accelerator-driven fusion (Richard Magee, TAE Technologies), and 96 exhibitions from companies and institutions worldwide.

CERN’s FCC-ee project was discussed in several sessions. Nuria Catalan-Lasheras (CERN) gave a memorable talk demonstrating impressive progress on high-power klystrons (RF sources). At present, klystrons have about 55% efficiency – RF power divided by wall-plug power – but she noted that they have the potential to go to as high as about 85% efficiency. The path is clear: increase voltage and decrease current, thereby reducing the “microperveance” of the klystrons. This will be crucial at FCC-ee, which must continuously replenish 100 MW of synchrotron radiation losses with 100 MW of RF power. The klystron efficiency improvement alone can save more than 60 MW – fully a third of the current power consumption of the CERN accelerator complex.

The “beef” of the conference was in the reports from facilities, but no one person can summarise all the progress

Muon colliders were presented as a unique opportunity to achieve a substantial energy increase compared to hadrons (Diktys Stratakis, Fermilab). Due to the point-like nature of the muon, the full centre-of-mass energy is available for probing new physics processes in every collision. Therefore, a 10 TeV muon collider can provide comparable high-energy-physics breakthroughs to a 100 TeV proton–proton collider, where colliding partons only carry a fraction of the proton’s energy. Due to its compactness, the cost of a 10 TeV muon collider compares to that of the FCC-ee and is likely to be many times lower than any other alternative concept that can achieve 10 pCM (parton centre-of-mass) energies (T Roser et al. 2023 JINST 18 P05018). The challenge lies in developing technologies for muon production, cooling and acceleration in the next two decades. In the upcoming 19 to 25 years it should be technically feasible for the accelerator community to demonstrate the technologies of a) high-intensity and short proton bunches; b) high-power proton targets; c) muon cooling; d) fast muon acceleration; e) 10 to 12 T superconducting magnets lined with tungsten inserts to protect coils from the muon decay products, and; f) effective spreading of the narrow cones of ultra-high-energy neutrinos by wiggling the beams, to avoid damage caused by the chargeless neutrinos when the muons decay.

In the conference’s closing talk, I reviewed three dozen future-collider proposals, analysed the ultimate energies potentially attainable in all types of colliding beams and accelerators within reasonable cost and power consumption limits, and laid out arguments that energies beyond a PeV (thousands of TeV) can be achieved, concluding that muons are the particles of the future for high-energy physics.

I can attest to IPAC’s success in fostering real-life interactions in the global accelerator landscape

The prize session was a highlight, with acceptance speeches from KEK’s Kaoru Yokoya (APS Wilson Prize) and SLAC’s Gennady Stupakov (IEEE NPSS PAST Award). Yokoya outlined his participation in various electron–positron machines and proposals such as the TRISTAN e⁺e^– collider and the ILC. Stupakov emphasised the importance of beam-dynamics theory in the age of computer modelling and simulations.

Ever since the first edition in Kyoto in 2010, I can attest to IPAC’s success in fostering real-life interactions in the global accelerator landscape. After the conference, I counted more than a hundred encounters of 5 minutes or more – something that would be difficult to achieve at a smaller or more specialised conference. It was pleasing to see many Chinese colleagues attend this US-based conference, but I did not identify any participants from Russia – a concerning development for our science’s international spirit. I hope political barriers will not interfere with next year’s IPAC’25 in Taiwan.

On a personal note, I would like to thank the organisers for putting together great scientific and social programmes, and the dedicated Joint Accelerator Conferences Website team, whose tireless efforts ensured that virtually all conference proceedings – papers, talks and posters – were available online by the final day, setting a standard that other fields of high-energy physics could greatly benefit from.

ALICE does the double slit

In the famous double-slit experiment, an interference pattern consisting of dark and bright bands emerges when a beam of light hits two narrow slits. The same effect has also been seen with particles such as electrons and protons, demonstrating the wave nature of propagating particles in quantum mechanics. Typically, experiments of this type produce interference patterns at the nanometre scale. In a recent study, the ALICE collaboration measured a similar interference pattern at the femtometre scale using ultra-peripheral collisions between lead nuclei at the LHC.

In ultra-peripheral collisions, two nuclei pass close to each other without colliding. With their impact parameter larger than the sum of their radii, one nucleus emits a photon that transforms into a virtual quark–antiquark pair. This pair interacts strongly with the other nucleus, resulting in the emission of a vector meson and the exchange of two gluons. Such vector-meson photoproduction is a well-established tool for probing the internal structure of colliding nuclei.

In vector-meson photoproduction involving symmetric systems, such as two lead nuclei, it is not possible to determine which of the nuclei emitted the photon and which emitted the two gluons. Crucially, however, due to the short range of the strong force between the virtual quark–antiquark pair and the nucleus, the vector mesons must have been produced within or close to one of the two well-separated nuclei. Because of this and their relatively short lifetime, the vector mesons decay quite rapidly into other particles. These decay products form a quantum-mechanically entangled state and generate an interference pattern akin to that of a double-slit interferometer.

In the photoproduction of the electrically neutral ρ⁰ vector meson, the interference pattern takes the form of a cos(2φ) modulation of the ρ⁰ yield, where φ is the angle between the two vectors formed by the sum and difference of the transverse momenta of the two oppositely charged pions into which the ρ⁰ decays. The strength of the modulation is expected to increase as the impact parameter decreases.

Using a dataset of 57,000 ρ⁰ mesons produced in lead–lead collisions at an energy of 5.02 TeV per nucleon pair during Run 2 of the LHC, the ALICE team measured the cos(2φ) modulation of the ρ⁰ yield for different values of the impact parameter. The measurements showed that the strength of the modulation varies strongly with the impact parameter. Theoretical calculations indicate that this behaviour is indeed the result of a quantum interference effect at the femtometre scale.

In the ongoing Run 3 of the LHC and in the next run, Run 4, ALICE is expected to collect more than 15 million ρ⁰ mesons from lead–lead collisions. This enhanced dataset will allow a more detailed analysis of the interference effect, further testing the validity of quantum mechanics at femtometre scales.

Building on success, planning for the future

From 29 January to 1 February, the Chamonix Workshop 2024 upheld its long tradition of fostering open and collaborative discussions within CERN’s accelerator and physics communities. This year marked a significant shift with more explicit inclusion of the injector complex, acknowledging its crucial role in shaping future research endeavours. Chamonix discussions focused on three main areas: maximising the remaining years of Run 3; the High-Luminosity LHC (HL-LHC), preparations for Long Shutdown 3 and operations in Run 4; and a look to the further future and the proposed Future Circular Collider (FCC).

Immense effort

Analysing the performance of CERN’s accelerator complex, speakers noted the impressive progress to date, examined limitations in the LHC and injectors and discussed improvements for optimal performance in upcoming runs. It’s difficult to do justice to the immense technical effort made by all systems, operations and technical infrastructure teams that underpins the exploitation of the complex. Machine availability emerged as a crucial theme, recognised as critical for both maximising the potential of existing facilities and ensuring the success of the HL-LHC. Fault tracking, dedicated maintenance efforts and targeted infrastructure improvements across the complex were highlighted as key contributors to achieving and maintaining optimal uptime.

As the HL-LHC project moves into full series production, the technical challenges associated with magnets, cold powering and crab cavities are being addressed (CERN Courier January/February 2024 p37). Looking beyond Long Shutdown 3 (LS3), potential limitations are already being targeted now, with, for example, electron-cloud mitigation measures planned to be deployed in LS3. The transition to the high-luminosity era will involve a huge programme of work that requires meticulous preparation and a well-coordinated effort across the complex during LS3, which will see the deployment of the HL-LHC, a widespread consolidation effort, and other upgrades such as that planned for the ECN3 cavern at CERN’s North Area.

The vision for the next decades of these facilities is diverse, imaginative and well-motivated from a physics perspective

The breadth and depth of the physics being performed at CERN facilities is quite remarkable, and the Chamonix workshop reconfirmed the high demand from experimentalists across the board. The unique capabilities of ISOLDE, n_TOF, AD-ELENA, and the East and North Areas were recognised. The North Area, for example, provides protons, hadrons, electrons and ion beams for detector R&D, experiments, the CERN neutrino platform, irradiation facilities and counts more than 2000 users. The vision for the next decades of these facilities is diverse, imaginative and well-motivated from a physics perspective. The potential for long-term exploitation and leveraging fully the capabilities of the LHC and other facilities is considerable, demanding continued support and development.

In the longer term, CERN is exploring the potential construction of the FCC via a dedicated feasibility study that has just delivered a mid-term report – a summary of which was presented at Chamonix. The initiative is accompanied by R&D on key accelerator technologies. The physics case for FCC-ee was well made for an audience of mostly non-particle physicists, concluding that the FCC is the only proposed collider that covers each key area in the field – electroweak, QCD, flavour, Higgs and searches for phenomena beyond the Standard Model – in paradigm-shifting depth.

Environmental consciousness

Sustainability was another focus of the Chamonix workshop. Building and operating future facilities with environmental consciousness is a top priority, and full life-cycle analyses will be performed for any options to help ensure a low-carbon future.

Interesting times, lots to do. To quote former CERN Director-General Herwig Schopper from 1983: “It is therefore clear that, for some time to come, there will be interesting work to do and I doubt whether accelerator experts will find themselves without a job.”

Strange correlations benchmark hadronisation

ALICE figure 1 — **Fig. 1.** Pearson correlation coefficient between the net-Ξ number and net-kaon number, as a function of the charged-particle multiplicity of the collision. Model calculations are shown as coloured bands. Credit: arxiv.2405.19890

In high-energy hadronic and heavy-ion collisions, strange quarks are dominantly produced from gluon fusion. In contrast to u and d quarks, they are not present in the colliding particles. Since strangeness is a conserved quantity in QCD, the net number of strange and anti-strange particles must equal zero, making them prime observable to study the dynamics of these collisions. Various experimental results from high-multiplicity pp collisions at the LHC demonstrate striking similarities to Pb–Pb collision results. Notably, the fraction of hadrons carrying one or more strange quarks smoothly increases as a function of particle multiplicity in pp and p–Pb collisions to values consistent with those measured in peripheral Pb–Pb collisions. Multi-particle correlations in pp collisions also closely resemble those in Pb–Pb collisions.

Explaining such observations requires understanding the hadronisation mechanism, which governs how quarks and gluons rearrange into bound states (hadrons). Since there are no first-principle calculations of the hadronisation process available, phenomenological models are used, based on either the Lund string fragmentation (Pythia 8, HIJING) or a statistical approach assuming a system of hadrons and their resonances (HRG) at thermal and chemical equilibrium. Despite having vastly different approaches, both models successfully describe the enhanced production of strange hadrons. This similarity calls for new observables to decisively discriminate between these two approaches.

The data indicate a weaker opposite-sign strangeness correlation than that predicted by string fragmentation

In a recently published study, the ALICE collaboration measured correlations between particles arising from the conservation of quantum numbers to further distinguish the two models. In the string fragmentation model, the quantum numbers are conserved locally through the creation of quark–antiquark pairs from the breaking of colour strings. This leads to a short-range rapidity correlation between strange and anti-strange hadrons. On the other hand, in the statistical hadronisation approach, quantum numbers are conserved globally over a finite volume, leading to long-range correlations between both strange–strange and strange–anti-strange hadron pairs. Quantum-number conservation leads to correlated particle production that is probed by measuring the yields of charged kaons (with one strange quark) and multistrange baryons (Ξ^– and Ξ⁺) on an event-by-event basis. In ALICE, charged kaons are directly tracked in the detectors, while Ξ baryons are reconstructed via their weak decay to a charged pion and a Λ-baryon, which is itself identified via its weak decay into a proton and a charged pion.

Figure 1 shows the first measurement of the correlation between the “net number” of Ξ baryons and kaons, as a function of the charged-particle multiplicity at midrapidity in pp, p–Pb and Pb–Pb collisions, where the net number is the difference between particle and antiparticle multiplicities. The experimental results deviate from the uncorrelated baseline (dashed line), and string fragmentation models that mainly correlate strange hadrons with opposite strange quark content over a small rapidity range fail to describe both observables. At the same time, the measurements agree with the statistical hadronisation model description that includes opposite-sign and same-sign strangeness correlations over large rapidity intervals. The data indicate a weaker opposite-sign strangeness correlation than that predicted by string fragmentation, suggesting that the correlation volume for strangeness conservation extends to about three units of rapidity.

The present study will be extended using the recently collected data during LHC Run 3. The larger data samples will enable similar measurements for the triply strange Ω baryon, as well as the study of higher cumulants.

German community discusses future collider at CERN

German particle-physics community in Bonn — **Bonn workshop** The German particle-physics community gathered at Bonn to discuss the opportunities of a future collider at CERN. Credit: M Schott

More than 150 German particle physicists gathered at Bonn University for a community event on a future collider at CERN. More precisely, the focus set for this meeting was to discuss the opportunities that the FCC-ee would offer should this collider be built at CERN. The event was organised by the German committee for particle physics, KET, and took place from 22 to 24 May. Representatives from almost all German institutes and groups active in particle physics were present, an attendance that shows the large interest in the collider to be built at CERN after the successful completion of the HL-LHC programme.

The main workshop was preceded by a dedicated session with more than 80 early-career scientists, organised by the Young High Energy Physicists Association, yHEP, to bring the generation that will benefit most from a future collider at CERN up to speed on the workshop topics. It included a presentation by former ECFA chair Karl Jakobs (Freiburg University) “From Strategy Discussions to Decision-Taking for Large Projects”, explaining the mechanisms and bodies involved in setting a project like the FCC-ee on track.

The opening session of the main workshop featured a fresh view on “The physics case for an e⁺e^– collider at CERN” by Margarete Mühlleitner (KIT Karlsruhe), who spread excitement about the strong and comprehensive physics case from super-precise measurements of the properties of the Z boson, the W boson and the top quark to what most people associate with a future e⁺e^– collider: precision measurements of the Higgs boson and insights about its connection to many of the still open questions of particle physics like dark matter or the matter–antimatter asymmetry. Markus Klute (KIT Karlsruhe) gave an in-depth review of the FCC-ee project. The midterm results of the FCC feasibility study indicate that no showstoppers were found in all the aspects studied so far and that the integrated FCC programme offers unparalleled exploration potential through precision measurements and direct searches. The picture was rounded off by a presentation from Jenny List (DESY, Hamburg) who talked about alternative options to realise an e⁺e^– Higgs factory at CERN, and the perspective of the early-career researchers was highlighted by Michael Lupberger (Bonn University). While all these presentations concentrated on the science and technology of the FCC-ee or alternatives, Eckart Lilienthal, representing the German Ministry of Education and Research, BMBF, reminded the audience that a future collider project at CERN needs an affordable financial plan and that – given the large uncertainties at present – this requires the community to prepare for different scenarios including one without the FCC-ee. Lilienthal confirmed that the future of CERN remains of the highest priority to BMBF.

The event was an important step in building consensus in the German community for a future collider project at CERN

The workshop went on to review many aspects of the FCC-ee and possible alternatives in more detail: accelerator R&D, detector concepts and technologies, computing and software, theory challenges as well as sustainability. The workshop witnessed the first meetings of the newly established German detector R&D consortia on silicon detectors, gaseous detectors and calorimetry. They will receive BMBF funding for the next three years and will allow German groups to strongly participate in the recently formed international DRD consortia in the context of the ECFA detector roadmap.

The path ahead

The workshop concluded with discussion sessions on the future collider scenarios for CERN, the engagement of the German community and a path to prepare the German input to the update of the European Strategy for Particle Physics. A series of three additional community workshops will be held in Germany before this input is due in March 2025.

The Bonn event was an important step in building consensus in the German community for a future collider project at CERN. The FCC-ee project generated a lot of interest and many groups plan to embark more strongly on this project. Contributions concerning the physics case, theory challenges, detector design and development, software, computing, and accelerator development were discussed. Alternative options for a future collider project at CERN need to be kept open to address the unanswered fundamental questions of particle physics in case the FCC-ee is not built at CERN. This event was clear evidence that a bright future for CERN remains of highest priority for the German particle-physics community and funding agency.

Exploring the Higgs potential at ATLAS

ATLAS figure 1 — **Fig. 1.** 95% confidence upper limits on the signal strength for inclusive gluon–gluon and vector–boson fusion HH production from each of the five analyses and their statistical combination. Credit: ATLAS Collab. 2024 arXiv:2406.09971

Immediately after the Big Bang, all the particles we know about today were massless and moving at the speed of light. About 10^–12 seconds later, the scalar Higgs field spontaneously broke the symmetry of the electroweak force, separating it into the electromagnetic and weak forces, and giving mass to fundamental particles. Without this process, the universe as we know it would not exist.

Since its discovery in 2012, measurements of the Higgs boson – the particle associated with the new field – have refined our understanding of its properties, but it remains unknown how closely the field’s energy potential resembles the predicted Mexican hat shape. Studying the Higgs potential can provide insights into the dynamics of the early universe, and the stability of the vacuum with respect to potential future changes.

The Higgs boson’s self-coupling strength λ governs the cubic and quartic terms in the equation describing the potential. It can be probed using the pair production of Higgs bosons (HH), though this is experimentally challenging as this process is more than 1000 times less likely than the production of a single Higgs boson. This is partly due to destructive interference between the two leading order diagrams in the dominant gluon–gluon fusion production mode.

The ATLAS collaboration recently compiled a series of results targeting HH decays to bbγγ, bbττ, bbbb, bbll plus missing transverse energy (E_T^miss), and multilepton final states. Each analysis uses the full LHC Run 2 data set. A key parameter is the HH signal strength, μ_HH, which divides the measured HH production rate by the Standard Model (SM) prediction. This combination yields the strongest expected constraints to date on μ_HH, and an observed upper limit of 2.9 times the SM prediction (figure 1). The combination also sets the most stringent constraints to date on the strength of the Higgs boson’s self-coupling of –1.2 < κ_λ < 7.2, where κ_λ = λ/λ_SM, its value relative to the SM prediction.

Each analysis contributes in a complementary way to the global picture of HH interactions and faces its own set of unique challenges.

Despite its tiny branching fraction of just 0.26% of all HH decays, HH → bbγγ provides very good sensitivity to μ_HHthanks to the ATLAS detector’s excellent di-photon mass resolution. It also sets the best constraints on λ due to its sensitivity to HH events with low invariant mass.

The HH → bbττ analysis (7.3% of HH decays) exploits state-of-the-art hadronic–tau identification to control the complex mix of electroweak, multijet and top-quark backgrounds. It yields the strongest limits on μ_HH and the second tightest constraints on λ.

HH → bbbb (34%) has good sensitivity to μ_HH thanks to ATLAS’s excellent b-jet identification, but controlling the multijet background presents a formidable challenge, which is tackled in a fully data-driven fashion.

Studying the Higgs potential can provide insights into the dynamics of the early universe

The decays HH → bbWW and HH → bbττ in fully leptonic final states have very similar characteristics and are thus targeted in a single HH → bbll+E_T^miss analysis. Contributions from the bbZZ decay mode, where one Z decays to charged light leptons and the other to neutrinos, are also considered.

Finally, the HH → multilepton analysis is designed to catch decay modes where the HH system cannot be fully reconstructed due to ambiguity in how the decay products should be assigned to the two Higgs bosons. The analysis uses nine signal regions with different multiplicities of light charged leptons, hadronic taus and photons. It is complementary to all the exclusive channels discussed above.

For the ongoing LHC Run 3, ATLAS designed new triggers to enhance sensitivity to the hadronic HH → bbττ and HH → bbbb channels. Improved b-jet identification algorithms will increase the efficiency in selecting HH signals and distinguishing them from background processes. With these and other improvements, our prospects have never looked brighter for homing in on the Higgs self-coupling.

Estonia becomes 24th Member State

On 30 August CERN welcomed Estonia as its 24th Member State, marking the end of a formal application process that started in 2018 and crowning a period of cooperation that stretches back three decades.

“Estonia is delighted to join CERN as a full member because CERN accelerates more than tiny particles, it also accelerates international scientific collaboration and our economies,” said Estonia president Alar Karis. “We have seen this potential during our time as Associate Member State and are keen to begin our full contribution.”

The bilateral relationship formally began in 1996, when Estonia and CERN signed a first cooperation agreement. Estonia has been part of the CMS collaboration since 1997, participating in data analysis and the Worldwide LHC Computing Grid, for which Estonia operates a Tier 2 centre in Tallinn. Researchers from Estonia also contribute to other experiments including CLOUD, COMPASS, NA66 and TOTEM, and to studies for future colliders, while Estonian theorists are highly involved in collaborations with CERN.

“Estonia and CERN have been collaborating closely for some 30 years, and I am very pleased to welcome Estonia to the ever-growing group of CERN Member States,” said Director-General Fabiola Gianotti. “I am sure the country and its scientific community will benefit from increased opportunities in fundamental research, technology development, and education and training.”

Estonia has held Associate Member State status in the pre-stage to membership of CERN since February 2021. As a full Member State, Estonia will now have voting rights in the CERN Council, enhanced opportunities for Estonian nationals to be recruited by CERN and for Estonian industry to bid for CERN contracts.

“On behalf of the CERN Council, I warmly welcome Estonia as the newest Member State of CERN,” said Council president Eliezer Rabinovici. “I am happy to see the community of CERN Member States enlarging, and I am looking forward to the enhanced participation of Estonia in the CERN Council and to its additional scientific contributions to CERN.”

LHCb measures the weak mixing angle

LHCb figure 1 — **Fig. 1.** The LHCb measurement of the forward–backward asymmetry as a function of the absolute difference between the pseudorapidities of the two muons produced in the Z boson decay, |Δη|. Credit: LHCb Collab.

At the International Conference on High-Energy Physics in Prague in July, the LHCb collaboration presented an updated measurement of the weak mixing angle using the data collected at the experiment between 2016 and 2018. The measurement benefits from the unique forward coverage of the LHCb detector.

The success of electroweak theory in describing a wide range of measurements at different experiments is one of the crowning achievements of the Standard Model (SM) of particle physics. It explains electroweak phenomena using a small number of free parameters, allowing precise measurements of different quantities to be compared to each other. This facilitates powerful indirect searches for beyond-the-SM physics. Discrepancies between measurements might imply that new physics influences one process but not another, and global analyses of high-precision electroweak measurements are sensitive to the presence of new particles at multi-TeV scales. In 2022 the entire field was excited by a measurement of the W-boson mass that is significantly larger than the value predicted within these global analyses by the CDF collaboration, heightening interest in electroweak measurements.

The weak mixing angle is at the centre of electroweak physics. It describes the mixing of the U(1) and SU(2) fields, determines couplings of the Z boson, and can also be directly related to the ratio of the W and Z boson masses. Excitingly, the two most precise measurements to date, from LEP and SLD, are in significant tension. This raises the prospect of non-SM particles potentially influencing one of these measurements, since the weak mixing angle, as a fundamental parameter of nature, should otherwise be the same no matter how it is measured. There is therefore a major programme measuring the weak mixing angle at hadron colliders, with important contributions from CDF, D0, ATLAS, CMS and LHCb.

Since the weak mixing angle controls Z-boson couplings, it can be determined from measurements of the angular distributions of Z-boson decays. The LHCb collaboration measured around 860,000 Z-boson decays to two oppositely charged muons, determining the relative rate at which negatively charged muons are produced closer to the LHC beamline than positively charged muons as a function of the angular separation of the two muons. Corrections are then applied for detector effects. Comparison to theoretical predictions based on different values of the weak mixing angle allows the value best describing the data to be determined (figure 1).

The unique angular coverage of the LHCb detector is well-suited for this measurement for two key reasons. First, the statistical sensitivity to the weak mixing angle is largest in the forward region close to the beamline that the LHCb detector covers. Second, the leading systematic uncertainties in measurements of the weak mixing angle at hadron colliders typically arise from existing knowledge of the proton’s internal structure. These uncertainties are also smallest in the forward region.

The value of the weak mixing angle measured by LHCb is consistent with previous measurements and with SM expectations (see “Weak mixing angle” figure). Notably, the precision of the LHCb measurement remains limited by the size of the data sample collected, such that further improvements are expected with the data currently being collected using the upgraded LHCb detector. In addition, while other experiments profile effects associated with the proton’s internal structure to reduce uncertainties, the unique forward acceptance means that this is not yet necessary at LHCb. This advantage will also be important for future measurements: the small theoretical uncertainty means that the forthcoming Upgrade 2 of the LHCb experiment is expected to achieve a precision more than a factor of two better than the most precise measurements to date.

Become a Particle Physicist in Eight Simple Moves

Simone Ragoni is passionate about outreach. His Instagram page, quarktastic, has more than 10 thousand followers, and is one of the very few that successfully makes particle physics and academia relatable. He wrote Become a Particle Physicist in Eight Simple Moves while completing his PhD on the ALICE experiment. The first move is to sip his favourite beverage: coffee.

As a social-media manager and communicator, I’ve been following Ragoni for years. His main tool is humour. And I’m proof it works. I will always remember the basic structure of a proton, because life is indeed full of “ups” and “downs”.

Did I say gentle humour? Nah. Ragoni goes all the way. But he confesses that his humour can only be understood by a handful of people. Particle physics is esoteric – and readers will want to join the club. His book invites you into the world of a young particle physicist. Being a nerd is the new cool.

A highlight is when Ragoni describes how to keep those distributions fit. If you know, you know. There is a pun here and the author explains it very well. He next turns to the tedious work that goes into publishing a paper. “Monte Carlo simulations are our real playground,” he writes, “where we unleash all our fantasy, the perfect world where everything is nice.” But particle physicists are cautious. Five sigma is needed to claim a discovery – a one in 3.5 million chance of being wrong. The author concludes with encouragement to make your own measurements using CERN’s open data.

Ragoni’s book is a delightful gift for anyone whom you want to inspire to become a particle physicist of tomorrow or simply to convey the excitement of what you do, with a quirky bonus of being presented bilingually in English and Italian, should you be keen on improving your physics vocab in one of those languages. It is a gateway to the captivating world of particle physics, skilfully blending humour with profound insights, and inspiring readers to explore further and consider joining the ranks of future particle physicists.

I built a physics museum in my classroom

Teaching modern physics to high-school students presents many challenges: overpacked curricula focusing on classical physics; the depth of knowledge needed by students (and teachers) to understand these topics; and students being over-focused on grades and university admissions. By exposing my students to the work being done at major research laboratories around the world, I have managed to find a way to overcome many of those obstacles.

Some time ago, British Columbia removed provincial examinations, giving teachers a bit more freedom to make additions to their curricula. I chose to insert small one- or two-day units throughout the year, which give my students multiple exposure to modern physics topics. These short introductions over a two-year period mean that physics students don’t need to know all the fine details, which decreases their stress and concerns.

Knowledge sharing

Physics teachers are lucky to have access to high-quality professional development via workshops run by CERN, LIGO, the Perimeter Institute (which produces excellent resources for use in physics classes) and others. These often-week-long events give teachers an overview of how a given research facility works, in the hope that they will bring that knowledge back to their students. Along the way, the teachers attend lectures from leading researchers and see first-hand careers in the field that they can bring back to share with their class.

I have been fortunate enough to attend workshops at these facilities. I have also taken part in a research experience at SNOLAB, brought students on tours of TRIUMF and mentored my students as they conducted research at the Canadian Light Source. All these experiences have given me the knowledge and confidence to introduce the facilities and the work done at them to my students in a way that hopefully piques their curiosity.

The pieces provide a starting point for conversations around what these decommissioned parts were used for and the kind of science they supported

While at CERN for the 2019 international teacher programme, I had the opportunity to visit both the CMS and ALICE detectors and to attend lectures from renowned particle physicists. We spent time in S’Cool LAB and visited many of the behind-the-scenes parts of CERN. While all of these experiences left an imprint on my teaching, it was during quiet visits to what was then called the Microcosm garden – which hosts decommissioned pieces of accelerators and detectors as a form of art – that helped transform the physical space in my classroom.

In 2022 my school in British Columbia renovated a large, old classroom to become our new physics lab. Knowing that I had more space to work with than before, I was inspired to start building my own version of the Microcosm garden on my classroom walls. I soon connected with the outreach team at TRIUMF who were excited to help get my project started with a photomultiplier tube, a control panel from a xenon-gas handling system, a paddle scintillator and a light guide. Since then, I have added a Lucas cell from SNOLAB, a piece of the electron gun from the Canadian Light Source and, most recently, a small-strip half-gap prototype from the New Small Wheel upgrade of the ATLAS detector. The pieces provide a starting point for conversations with students around what these decommissioned parts were used for, and the kind of science they supported.

Equipped with some knowledge of what modern research in the field looks like, I have successfully built a system where I am able to inspire students to want to study physics. Since attending my first major workshop in 2018, I have seen an increase in the number of students entering physics majors. Some of them have already gone on to internships at CERN and TRIUMF, after getting their first exposure to these organisations in my classes. My hope is that by having pieces of the facilities I talk about displayed on my classroom walls, this will further inspire more of my students to want to learn about them, possibly setting them on paths to careers in physics.

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