Meinhard Regler, an expert in detector development and software analysis, passed away on 22 September 2024 at the age of 83.
Born and raised in Vienna, Meinhard studied physics at the Technical University Vienna (TUW) and completed his master’s thesis on deuteron acceleration in a linac at CERN. In 1966 he joined the newly founded Institute of High Energy Physics (HEPHY) of the Austrian Academy of Sciences. He settled in Geneva to participate in a counter experiment at the CERN Proton Synchrotron, and in 1970 obtained his PhD with distinction from TUW.
In 1970 Meinhard became staff member in CERN’s data-handling division. He joined the Split Field Magnet experiment at the Intersecting Storage Rings and, together with HEPHY, contributed specially designed multi-wire proportional chambers. Early on, he realised the importance of rigorous statistical methods for track and vertex reconstruction in complex detectors, resulting in several seminal papers.
In 1975 Meinhard returned to Vienna as leader of HEPHY’s experimental division. From 1993 until his retirement at the end of 2006 he was deputy director and responsible for the detector development and software analysis groups. As a faculty member of TUW he created a series of specialised lectures and practical courses, which shaped a generation of particle physicists. In 1978 Meinhard and Georges Charpak founded the Wire Chamber Conference, now known as the Vienna Conference on Instrumentation (VCI).
Meinhard continued his participation in experiments at CERN, including WA6, UA1 and the European Hybrid Spectrometer. After joining the DELPHI experiment at LEP, he realised the emerging potential of semiconductor tracking devices and established this technology at HEPHY. First applied at DELPHI’s Very Forward Tracker, this expertise was successfully continued with important contributions to the CMS tracker at LHC, the Belle vertex detector at KEKB and several others.
Meinhard is author and co-author of several hundred scientific papers. His and his group’s contributions to track and vertex reconstruction are summarised in the standard textbook Data Analysis Techniques for High-Energy Physics, published by Cambridge University Press and translated into Russian and Chinese.
All that would suffice for a lifetime achievement, but not so for Meinhard. Inspired by the fall of the Iron Curtain, he envisaged the creation of an international centre of excellence in the Vienna region. Initially planned as a spallation neutron source, the project eventually transmuted into a facility for cancer therapy by proton and carbon-ion beams, called MedAustron. Financed by the province of Lower Austria and the hosting city of Wiener Neustadt, and with crucial scientific and engineering support from CERN and Austrian institutes, clinical treatment started in 2016.
Meinhard received several prizes and was rewarded with the highest scientific decoration of Austria
Meinhard was invited as a lecturer to many international conferences and post-graduate schools worldwide. He chaired the VCI series, organised several accelerator schools and conferences in Austria, and served on the boards of the European Physical Society’s international group on accelerators. For his tireless scientific efforts and in particular the realisation of MedAustron, Meinhard received several prizes and was rewarded with the highest scientific decoration of Austria – the Honorary Cross for Science and Arts of First Class.
He was also a co-founder and long-term president of a non-profit organisation in support of mentally handicapped people. His character was incorruptible, strictly committed to truth and honesty, and responsive to loyalty, independent thinking and constructive criticism.
In Meinhard Regler we have lost an enthusiastic scientist, visionary innovator, talented organiser, gifted teacher, great humanist and good friend. His legacy will forever stay with us.
Registration is now open for the Open Symposium of the 2026 update to the European Strategy for Particle Physics (ESPP). It will take place from 23 to 27 June at Lido di Venezia in Italy, and see scientists from around the world debate the inputs to the ESPP (see “A call to engage”).
The symposium will begin by surveying the implementation of the last strategy process, whose recommendations were approved by the CERN Council in June 2020. In-depth working-group discussions on all areas of physics and technology will follow.
The rest of the week will see plenary sessions on the different physics and technology areas, starting with various proposals for possible large accelerator projects at CERN, and the status and plans in other regions of the world. Open questions, as well as how they can be addressed by the proposed projects, will be presented in rapporteur talks. This will be followed by longer discussion blocks where the full community can get engaged. On the final day, members of the European Strategy Group will summarise the national inputs and other overarching topics to the ESPP.
Karel Šafařík, one of the founding members of the ALICE collaboration, passed away on 7 October 2024.
Karel graduated in theoretical physics in Bratislava, Slovakia (then Czechoslovakia) in 1976 and worked at JINR Dubna for over 10 years, participating in experiments in Serpukhov and doing theoretical studies on the phenomenology of particle production at high energies. In 1990 he joined Collège de France and the heavy-ion programme at CERN, soon becoming one of the most influential scientists in the Omega series of heavy-ion experiments (WA85, WA94, WA97, NA57) at the CERN Super Proton Synchrotron (SPS). In 2002 Karel was awarded the Slovak Academy of Sciences Prize for his contributions to the observation of the enhancement of the production of multi-strange particles in heavy-ion collisions at the SPS. In 2013 he was awarded the medal of the Czech Physical Society.
As early as 1991, Karel was part of the small group who designed the first heavy-ion detector for the LHC, which later became ALICE. He played a central role in shaping the ALICE experiment, from the definition of physics topics and the detector layout to the design of the data format, tracking, data storage and data analysis. He was pivotal in convincing the collaboration to introduce two layers of pixel detectors to reconstruct decays of charm hadrons only a few tens of microns from the primary vertex in central lead–lead collisions at the LHC – an idea considered by many to be impossible in heavy-ion collisions, but that is now one of the pillars of the ALICE physics programme. He was the ALICE physics coordinator for many years leading up to and including first data taking. Over the years, he also made multiple contributions to ALICE upgrade studies and became known as the “wise man” to be consulted on the trickiest questions.
Karel was a top-class physicist, with a sharp analytical mind, a legendary memory, a seemingly unlimited set of competences ranging from higher mathematics to formal theory, and from detector physics to high-performance computing. At the same time he was a generous, caring and kind colleague who supported, helped, mentored and guided a large number of ALICE collaborators. We miss him dearly.
Günter Wolf, who played a leading role in the planning, construction and data analysis of experiments that were instrumental in establishing the Standard Model, passed away on 29 October 2024 at the age of 86. He significantly shaped and contributed to the research programme of DESY, and knew better than almost anyone how to form international collaborations and lead them to the highest achievements.
Born in Ulm, Germany in 1937, Wolf studied physics in Tübingen. At the urging of his supervisor Helmut Faissner, he went to Hamburg in 1961 where the DESY synchrotron was being built under DESY founder Willibald Jentschke. Together with Erich Lohrmann and Martin Teucher, he was involved in the preparation of the bubble-chamber experiments there and at the same time took part in experiments at CERN.
The first phase of experiments with high-energy photons at the DESY synchrotron, in which he was involved, had produced widely recognised results on the electromagnetic interactions of elementary particles. In 1967 Wolf seized the opportunity to continue this research at the higher energies of the recently completed linear accelerator at Stanford University (SLAC). He became the spokesperson for an experiment with a polarised gamma beam, which provided new insights into the nature of vector mesons.
In 1971, Jentschke succeeded in bringing Wolf back to Hamburg as senior scientist. He remained associated with DESY for the rest of his life and became a leader in the planning, construction and analysis of key DESY experiments.
Together with Bjørn Wiik, as part of an international collaboration, Wolf designed and realised the DASP detector for DORIS, the first electron–positron storage ring at DESY. This led to the discovery of the excited states of charmonium in 1975 and thus to the ultimate confirmation that quarks are particles. For the next, larger electron–positron storage ring, PETRA, he designed the TASSO detector, again together with Wiik. In 1979, the TASSO collaboration was able to announce the discovery of the gluon through its spokesperson Wolf, for which he, together with colleagues from TASSO, was awarded the High Energy Particle Physics Prize of the European Physical Society.
Wolf’s negotiating skills and deep understanding of physics and technology served particle physics worldwide
In 1982 Wolf became the chair of the experiment selection committee for the planned LEP collider at CERN. His deep understanding of physics and technology, and his negotiating skills, were an essential foundation for the successful LEP programme, just one example of how Wolf has served particle physics worldwide as a member of international scientific committees.
At the same time, Wolf was involved in the planning of the physics programme for the electron–proton collider HERA. The ZEUS general-purpose detector for experiments at HERA was the work of an international collaboration of more than 400 scientists, that Wolf brought together and led as its spokesperson for many years. The experiments at HERA ran from 1992 to 2007, producing outstanding results that include the direct demonstration of the unification of the weak and electromagnetic force at high momentum transfers, the precise measurement of the structure of the proton, which is determined by quarks and gluons, and the surprising finding that there are collisions in which the proton remains intact even at the highest momentum transfers. In 2011 Wolf was awarded the Stern–Gerlach Medal of the German Physical Society, its highest award for achievements in experimental physics.
When dealing with colleagues and staff, Günter Wolf was always friendly, helpful, encouraging and inspiring, but at the same time demanding and insistent on precision and scientific excellence. He took the opinions of others seriously, but only a thorough and competent analysis could convince him. As a result, he enjoyed the greatest respect from everyone and became a role model and friend to many. DESY owes its reputation in the international physics community not least to people like him.
Should we start with your father’s involvement in the founding of CERN?
I began hearing my father talk about a new European laboratory while I was still in high school in Rome. Our lunch table was always alive with discussions about science, physics and the vision of this new laboratory. Later, I learned that between 1948 and 1949, my father was deeply engaged in these conversations with two of his friends: Gilberto Bernardini, a well-known cosmic-ray expert, and Bruno Ferretti, a professor of theoretical physics at Rome University. I was 15 years old and those table discussions remain vivid in my memory.
So, the idea of a European laboratory was already being discussed before the 1950 UNESCO meeting?
Yes, indeed. Several eminent European physicists, including my father, Pierre Auger, Lew Kowarski and Francis Perrin, recognised that Europe could only be competitive in nuclear physics through collaborative efforts. All the actors wanted to create a research centre that would stop the post-war exodus of physics talent to North America and help rebuild European science. I now know that my father’s involvement began in 1946 when he travelled to Cambridge, Massachusetts, for a conference. There, he met Nobel Prize winner John Cockcroft, and their conversations planted in his mind the first seeds for a European laboratory.
Parallel to scientific discussions, there was an important political initiative led by Swiss philosopher and writer Denis de Rougemont. After spending the war years at Princeton University, he returned to Europe with a vision of fostering unity and peace. He established the Institute of European Culture in Lausanne, Switzerland, where politicians from France, Britain and Germany would meet. In December 1949, during the European Cultural Conference in Lausanne, French Nobel Prize winner Louis de Broglie sent a letter advocating for a European laboratory where scientists from across the continent could work together peacefully.
My father strongly believed in the importance of accelerators to advance the new field that, at the time, was at the crossroads between nuclear physics and cosmic-ray physics. Before the war, in 1936, he had travelled to Berkeley to learn about cyclotrons from Ernest Lawrence. He even attempted to build a cyclotron in Italy in 1942, profiting from the World’s Fair that had to be held in Rome. Moreover, he was deeply affected by the exodus of talented Italian physicists after the war, including Bruno Rossi, Gian Carlo Wick and Giuseppe Cocconi. He saw CERN as a way to bring these scientists back and rebuild European physics.
How did Isidor Rabi’s involvement come into play?
In 1950 my father was corresponding with Gilberto Bernardini, who was spending a year at Columbia University. There Bernardini mentioned the idea of a European laboratory to Isidor Rabi, who, at the same time, was in contact with other prominent figures in this decentralised and multi-centered initiative. Together with Norman Ramsay, Rabi had previously succeeded, in 1947, in persuading nine northeastern US universities to collaborate under the banner of Associated Universities, Inc, which led to the establishment of Brookhaven National Laboratory.
What is not generally known is that before Rabi gave his famous speech at the fifth assembly of UNESCO in Florence in June 1950, he came to Rome and met with my father. They discussed how to bring this idea to fruition. A few days later, Rabi’s resolution at the UNESCO meeting calling for regional research facilities was a crucial step in launching the project. Rabi considered CERN a peaceful compensation for the fact that physicists had built the nuclear bomb.
How did your father and his colleagues proceed after the UNESCO resolution?
Following the UNESCO meeting, Pierre Auger, at that time director of exact and natural sciences at UNESCO, and my father took on the task of advancing the project. In September 1950 Auger spoke of it at a nuclear physics conference in Oxford, and at a meeting of the International Union of Pure and Applied Physics (IUPAP), my father– one of the vice presidents – urged the executive committee to consider how best to implement the Florence resolution. In May 1951, Auger and my father organised a meeting of experts at UNESCO headquarters in Paris, where a compelling justification for the European project was drafted.
The cost of such an endeavour was beyond the means of any single nation. This led to an intergovernmental conference under the auspices of UNESCO in December 1951, where the foundations for CERN were laid. Funding, totalling $10,000 for the initial meetings of the board of experts, came from Italy, France and Belgium. This was thanks to the financial support of men like Gustavo Colonnetti, president of the Italian Research Council, who had already – a year before – donated the first funds to UNESCO.
Were there any significant challenges during this period?
Not everyone readily accepted the idea of a European laboratory. Eminent physicists like Niels Bohr, James Chadwick and Hendrik Kramers questioned the practicality of starting a new laboratory from scratch. They were concerned about the feasibility and allocation of resources, and preferred the coordination of many national laboratories and institutions. Through skilful negotiation and compromise, Auger and my father incorporated some of the concerns raised by the sceptics into a modified version of the project, ensuring broader support. In February 1952 the first agreement setting up a provisional council for CERN was written and signed, and my father was nominated secretary general of the provisional CERN.
He worked tirelessly, travelling through Europe to unite the member states and start the laboratory’s construction. In particular, the UK was reluctant to participate fully. They had their own advanced facilities, like the 40 MeV cyclotron at the University of Liverpool. In December 1952 my father visited John Cockcroft, at the time director of the Harwell Atomic Energy Research Establishment, to discuss this. There’s an interesting episode where my father, with Cockcroft, met Frederick Lindemann and Baron Cherwell, who was a long-time scientific advisor to Winston Churchill. Cherwell dismissed CERN as another “European paper mill.” My father, usually composed, lost his temper and passionately defended the project. During the following visit to Harwell, Cockcroft reassured him that his reaction was appropriate. From that point on, the UK contributed to CERN, albeit initially as a series of donations rather than as the result of a formal commitment. It may be interesting to add that, during the same visit to London and Harwell, my father met the young John Adams and was so impressed that he immediately offered him a position at CERN.
What were the steps following the ratification of CERN’s convention?
Robert Valeur, chairman of the council during the interim period, and Ben Lockspeiser, chairman of the interim finance committee, used their authority to stir up early initiatives and create an atmosphere of confidence that attracted scientists from all over Europe. As Lew Kowarski noted, there was a sense of “moral commitment” to leave secure positions at home and embark on this new scientific endeavour.
During the interim period from May 1952 to September 1954, the council convened three sessions in Geneva whose primary focus was financial management. The organisation began with an initial endowment of approximately 1 million Swiss Francs, which – as I said – included a contribution from the UK known as the “observer’s gift”. At each subsequent session, the council increased its funding, reaching around 3.7 million Swiss Francs by the end of this period. When the permanent organisation was established, an initial sum of 4.1 million Swiss Francs was made available.
In 1954, my father was worried that if the parliaments didn’t approve the convention before winter, then construction would be delayed because of the wintertime. So he took a bold step and, with the approval of the council president, authorised the start of construction on the main site before the convention was fully ratified.
This led to Lockspeiser jokingly remarking later that council “has now to keep Amaldi out of jail”. The provisional council, set up in 1952, was dissolved when the European Organization for Nuclear Research officially came into being in 1954, though the acronym CERN (Conseil Européen pour la Recherche Nucléaire) was retained. By the conclusion of the interim period, CERN had grown significantly. A critical moment occurred on 29 September1954, when a specific point in the ratification procedure was reached, rendering all assets temporarily ownerless. During this eight-day period, my father, serving as secretary general, was the sole owner on behalf of the newly forming permanent organisation. The interim phase concluded with the first meeting of the permanent council, marking the end of CERN’s formative years.
Did your father ever consider becoming CERN’s Director-General?
People asked him to be Director-General, but he declined for two reasons. First, he wanted to return to his students and his cosmic-ray research in Rome. Second, he didn’t want people to think he had done all this to secure a prominent position. He believed in the project for its own sake.
When the convention was finally ratified in 1954, the council offered the position of Director-General to Felix Bloch, a Swiss–American physicist and Nobel Prize winner for his work on nuclear magnetic resonance. Bloch accepted but insisted that my father serve as his deputy. My father, dedicated to CERN’s success, agreed to this despite his desire to return to Rome full time.
How did that arrangement work out?
My father agreed but Bloch wasn’t at that time rooted in Europe. He insisted on bringing all his instruments from Stanford so he could continue his research on nuclear magnetic resonance at CERN. He found it difficult to adapt to the demands of leading CERN and soon resigned. The council then elected Cornelis Jan Bakker, a Dutch physicist who had led the synchrocyclotron group, as the new Director-General. From the beginning, he was the person my father thought would have been the ideal director for the initial phase of CERN. Tragically though, Bakker died in a plane crash a year and a half later. I well remember how hard my father was hit by this loss.
How did the development of accelerators at CERN progress?
The decision to adopt the strong focusing principle for the Proton Synchrotron (PS) was a pivotal moment. In August 1952 Otto Dahl, leader of the Proton Synchrotron study group, Frank Goward and Rolf Widerøe visited Brookhaven just as Ernest Courant, Stanley Livingston and Hartland Snyder were developing this new principle. They were so excited by this development that they returned to CERN determined to incorporate it into the PS design. In 1953 Mervyn Hine, a long-time friend of John Adams with whom he had moved to CERN, studied potential issues with misalignment in strong focusing magnets, which led to further refinements in the design. Ultimately, the PS became operational before the comparable accelerator at Brookhaven, marking a significant achievement for European science.
It’s important here to recognise the crucial contributions of the engineers, who often don’t receive the same level of recognition as physicists. They are the ones who make the work of experimental physicists and theorists possible. “Viki” Weisskopf, Director-General of CERN from 1961 to 1965, compared the situation to the discovery of America. The machine builders are the captains and shipbuilders. The experimentalists are those fellows on the ships who sailed to the other side of the world and wrote down what they saw. The theoretical physicists are those who stayed behind in Madrid and told Columbus that he was going to land in India.
Your father also had a profound impact on the development of other Big Science organisations in Europe
Yes, in 1958 my father was instrumental, together with Pierre Auger, in the founding of the European Space Agency. In a letter written in 1958 to his friend Luigi Crocco, who was professor of jet propulsion in Princeton, he wrote that “it is now very much evident that this problem is not at the level of the single states like Italy, but mainly at the continental level. Therefore, if such an endeavour is to be pursued, it must be done on a European scale, as already done for the building of the large accelerators for which CERN was created… I think it is absolutely imperative for the future organisation to be neither military nor linked to any military organisation. It must be a purely scientific organisation, open – like CERN – to all forms of cooperation and outside the participating countries.” This document reflects my father’s vision of peaceful and non-military European science.
How is it possible for one person to contribute so profoundly to science and global collaboration?
My father’s ability to accept defeats and keep pushing forward was key to his success. He was an exceptional person with a clear vision and unwavering dedication. I hope that by sharing these stories, others might be inspired to pursue their goals with the same persistence and passion.
Could we argue that he was not only a visionary but also a relentless advocate?
He travelled extensively, talked to countless people, and was always cheerful and energetic. He accepted setbacks but kept moving forwards. In this connection, I want to mention Eliane Bertrand, later de Modzelewska, his secretary in Rome who later became secretary of the CERN Council for about 20 years, serving under several Director-Generals. She left a memoir about those early days, highlighting how my father was always travelling, talking and never stopping. It’s a valuable piece of history that, I think, should be published.
International collaboration has been a recurring theme in your own career. How do you view its importance today?
International collaboration is more critical than ever in today’s world. Science has always been a bridge between cultures and nations, and CERN’s history is a testimony of what this brings to humanity. It transcends political differences and fosters mutual understanding. I hope CERN and the broader scientific community will find ways to maintain these vital connections with all countries. I’ve always believed that fostering a collaborative and inclusive environment is one of the main goals of us scientists. It’s not just about achieving results but also about how we work together and support each other along the way.
Looking ahead, what are your thoughts on the future of CERN and particle physics?
I firmly believe that pursuing higher collision energies is essential. While the Large Hadron Collider has achieved remarkable successes, there’s still much we haven’t uncovered – especially regarding supersymmetry. Even though minimal supersymmetry does not apply, I remain convinced that supersymmetry might manifest in ways we haven’t yet understood. Exploring higher energies could reveal supersymmetric particles or other new phenomena.
Like most European physicists, I support the initiative of the Future Circular Collider and starting with an electron–positron collider phase so to explore new frontiers at two very different energy levels. However, if geopolitical shifts delay or complicate these plans, we should consider pushing hard on alternative strategies like developing the technologies for muon colliders.
Ugo Amaldi first arrived at CERN as a fellow in September 1961. Then, for 10 years at the ISS in Rome, he opened two new lines of research: quasi-free electron scattering on nuclei and atoms. Back at CERN, he developed the Roman pots experimental technique, was a co-discoverer of the rise of the proton–proton cross-section with energy, measured the polarisation of muons produced by neutrinos, proposed the concept of a superconducting electron–positron linear collider, and led LEP’s DELPHI Collaboration. Today, he advances the use of accelerators in cancer treatment as the founder of the TERA Foundation for hadron therapy and as president emeritus of the National Centre for Oncological Hadrontherapy (CNAO) in Pavia. He continues his mother and father’s legacy of authoring high-school physics textbooks used by millions of Italian pupils. His motto is: “Physics is beautiful and useful.”
This interview first appeared in the newsletter of CERN’s experimental physics department. It has been edited for concision.
What motivates you to be CERN’s next Director-General?
CERN is an incredibly important organisation. I believe my deep passion for particle physics, coupled with the experience I have accumulated in recent years, including leading the Deep Underground Neutrino Experiment, DUNE, through a formative phase, and running the Science and Technology Facilities Council in the UK, has equipped me with the right skill set to lead CERN though a particularly important period.
How would you describe your management style?
That’s a good question. My overarching approach is built around delegating and trusting my team. This has two advantages. First, it builds an empowering culture, which in my experience provides the right environment for people to thrive. Second, it frees me up to focus on strategic planning and engagement with numerous key stakeholders. I like to focus on transparency and openness, to build trust both internally and externally.
How will you spend your familiarisation year before you take over in 2026?
First, by getting a deep understanding of CERN “from within”, to plan how I want to approach my mandate. Second, by lending my voice to the scientific discussion that will underpin the third update to the European strategy for particle physics. The European strategy process is a key opportunity for the particle-physics community to provide genuine bottom-up input and shape the future. This is going to be a really varied and exciting year.
What open question in fundamental physics would you most like to see answered in your lifetime?
I am going to have to pick two. I would really like to understand the nature of dark matter. There are a wide range of possibilities, and we are addressing this question from multiple angles; the search for dark matter is an area where the collider and non-collider experiments can both contribute enormously. The second question is the nature of the Higgs field. The Higgs boson is just so different from anything else we’ve ever seen. It’s not just unique – it’s unique and very strange. There are just so many deep questions, such as whether it is fundamental or composite. I am confident that we will make progress in the coming years. I believe the High-Luminosity LHC will be able to make meaningful measurements of the self-coupling at the heart of the Higgs potential. If you’d asked me five years ago whether this was possible, I would have been doubtful. But today I am very optimistic because of the rapid progress with advanced analysis techniques being developed by the brilliant scientists on the LHC experiments.
What areas of R&D are most in need of innovation to meet our science goals?
Artificial intelligence is changing how we look at data in all areas of science. Particle physics is the ideal testing ground for artificial intelligence, because our data is complex there are none of the issues around the sensitive nature of the data that exist in other fields. Complex multidimensional datasets are where you’ll benefit the most from artificial intelligence. I’m also excited by the emergence of new quantum technologies, which will open up fresh opportunities for our detector systems and also new ways of doing experiments in fundamental physics. We’ve only scratched the surface of what can be achieved with entangled quantum systems.
How about in accelerator R&D?
There are two areas that I would like to highlight: making our current technologies more sustainable, and the development of high-field magnets based on high-temperature superconductivity. This connects to the question of innovation more broadly. To quote one example among many, high-temperature superconducting magnets are likely to be an important component of fusion reactors just as much as particle accelerators, making this a very exciting area where CERN can deploy its engineering expertise and really push that programme forward. That’s not just a benefit for particle physics, but a benefit for wider society.
How has CERN changed since you were a fellow back in 1994?
The biggest change is that the collider experiments are larger and more complex, and the scientific and technical skills required have become more specialised. When I first came to CERN, I worked on the OPAL experiment at LEP – a collaboration of less than 400 people. Everybody knew everybody, and it was relatively easy to understand the science of the whole experiment.
My overarching approach is built around delegating and trusting my team
But I don’t think the scientific culture of CERN and the particle-physics community has changed much. When I visit CERN and meet with the younger scientists, I see the same levels of excitement and enthusiasm. People are driven by the wonderful mission of discovery. When planning the future, we need to ensure that early-career researchers can see a clear way forward with opportunities in all periods of their career. This is essential for the long-term health of particle physics. Today we have an amazing machine that’s running beautifully: the LHC. I also don’t think it is possible to overstate the excitement of the High-Luminosity LHC. So there’s a clear and exciting future out to the early 2040s for today’s early-career researchers. The question is what happens beyond that? This is one reason to ensure that there is not a large gap between the end of the High-Luminosity LHC and the start of whatever comes next.
Should the world be aligning on a single project?
Given the increasing scale of investment, we do have to focus as a global community, but that doesn’t necessarily mean a single project. We saw something similar about 10 years ago when the global neutrino community decided to focus its efforts on two complementary long-baseline projects, DUNE and Hyper-Kamiokande. From the perspective of today’s European strategy, the Future Circular Collider (FCC) is an extremely appealing project that would map out an exciting future for CERN for many decades. I think we’ll see this come through strongly in an open and science-driven European strategy process.
How do you see the scientific case for the FCC?
For me, there are two key points. First, gaining a deep understanding of the Higgs boson is the natural next step in our field. We have discovered something truly unique, and we should now explore its properties to gain deeper insights into fundamental physics. Scientifically, the FCC provides everything you want from a Higgs factory, both in terms of luminosity and the opportunity to support multiple experiments.
Second, investment in the FCC tunnel will provide a route to hadron–hadron collisions at the 100 TeV scale. I find it difficult to foresee a future where we will not want this capability.
These two aspects make the FCC a very attractive proposition.
How successful do you believe particle physics is in communicating science and societal impacts to the public and to policymakers?
I think we communicate science well. After all, we’ve got a great story. People get the idea that we work to understand the universe at its most basic level. It’s a simple and profound message.
Going beyond the science, the way we communicate the wider industrial and societal impact is probably equally important. Here we also have a good story. In our experiments we are always pushing beyond the limits of current technology, doing things that have not been done before. The technologies we develop to do this almost always find their way back into something that will have wider applications. Of course, when we start, we don’t know what the impact will be. That’s the strength and beauty of pushing the boundaries of technology for science.
Would the FCC give a strong return on investment to the member states?
Absolutely. Part of the return is the science, part is the investment in technology, and we should not underestimate the importance of the training opportunities for young people across Europe. CERN provides such an amazing and inspiring environment for young people. The scale of the FCC will provide a huge number of opportunities for young scientists and engineers.
We need to ensure that early-career researchers can see a clear way forward with opportunities in all periods of their career. This is essential for the long-term health of particle physics
In terms of technology development, the detectors for the electron–positron collider will provide an opportunity for pushing forward and deploying new, advanced technologies to deliver the precision required for the science programme. In parallel, the development of the magnet technologies for the future hadron collider will be really exciting, particularly the potential use of high-temperature superconductors, as I said before.
It is always difficult to predict the specific “return on investment” on the technologies for big scientific research infrastructure. Part of this challenge is that some of that benefits might be 20, 30, 40 years down the line. Nevertheless, every retrospective that has tried, has demonstrated that you get a huge downstream benefit.
Do we reward technical innovation well enough in high-energy physics?
There needs to be a bit of a culture shift within our community. Engineering and technology innovation are critical to the future of science and critical to the prosperity of Europe. We should be striving to reward individuals working in these areas.
Should the field make it more flexible for physicists and engineers to work in industry and return to the field having worked there?
This is an important question. I actually think things are changing. The fluidity between academia and industry is increasing in both directions. For example, an early-career researcher in particle physics with a background in deep artificial-intelligence techniques is valued incredibly highly by industry. It also works the other way around, and I experienced this myself in my career when one of my post-doctoral researchers joined from an industry background after a PhD in particle physics. The software skills they picked up from industry were incredibly impactful.
I don’t think there is much we need to do to directly increase flexibility – it’s more about culture change, to recognise that fluidity between industry and academia is important and beneficial. Career trajectories are evolving across many sectors. People move around much more than they did in the past.
Does CERN have a future as a global laboratory?
CERN already is a global laboratory. The amazing range of nationalities working here is both inspiring and a huge benefit to CERN.
How can we open up opportunities in low- and middle-income countries?
I am really passionate about the importance of diversity in all its forms and this includes national and regional inclusivity. It is an agenda that I pursued in my last two positions. At the Deep Underground Neutrino Experiment, I was really keen to engage the scientific community from Latin America, and I believe this has been mutually beneficial. At STFC, we used physics as a way to provide opportunities for people across Africa to gain high-tech skills. Going beyond the training, one of the challenges is to ensure that people use these skills in their home nations. Otherwise, you’re not really helping low- and middle-income countries to develop.
What message would you like to leave with readers?
That we have really only just started the LHC programme. With more than a factor of 10 increase in data to come, coupled with new data tools and upgraded detectors, the High-Luminosity LHC represents a major opportunity for a new discovery. Its nature could be a complete surprise. That’s the whole point of exploring the unknown: you don’t know what’s out there. This alone is incredibly exciting, and it is just a part of CERN’s amazing future.
In 1955, after a year of graduate study at Harvard, I joined a group of a dozen or so students committed to studying elementary particle theory. We approached Julian Schwinger, one of the founders of quantum electrodynamics, hoping to become his thesis students – and we all did.
Schwinger lined us up in his office, and spent several hours assigning thesis subjects. It was a remarkable performance. I was the last in line. Having run out of well-defined thesis problems, he explained to me that weak and electromagnetic interactions share two remarkable features: both are vectorial and both display aspects of universality. Schwinger suggested that I create a unified theory of the two interactions – an electroweak synthesis. How I was to do this he did not say, aside from slyly hinting at the Yang–Mills gauge theory.
By the summer of 1958, I had convinced myself that weak and electromagnetic interactions might be described by a badly broken gauge theory, and Schwinger that I deserved a PhD. I had hoped to partly spend a postdoctoral fellowship in Moscow at the invitation of the recent Russian Nobel laureate Igor Tamm, and sought to visit Niels Bohr’s institute in Copenhagen while awaiting my Soviet visa. With Bohr’s enthusiastic consent, I boarded the SS Île de France with my friend Jack Schnepps. Following a memorable and luxurious crossing – one of the great ship’s last – Jack drove south to Padova to work with Milla Baldo-Ceolin’s emulsion group in Padova, and I took the slow train north to Copenhagen. Thankfully, my Soviet visa never arrived. I found the SU(2) × U(1) structure of the electroweak model in the spring of 1960 at Bohr’s famous institute at Blegsdamvej 19, and wrote the paper that would earn my share of the 1979 Nobel Prize.
We called the new quark flavour “charm”, completing two weak doublets of quarks to match two weak doublets of leptons, and establishing lepton–quark symmetry, which holds to this day
A year earlier, in 1959, Augusto Gamba, Bob Marshak and Susumo Okubo had proposed lepton–hadron symmetry, which regarded protons, neutrons and lambda hyperons as the building blocks of all hadrons, to match the three known leptons at the time: neutrinos, electrons and muons. The idea was falsified by the discovery of a second neutrino in 1962, and superseded in 1964 by the invention of fractionally charged hadron constituents, first by George Zweig and André Petermann, and then decisively by Murray Gell-Mann with his three flavours of quarks. Later in 1964, while on sabbatical in Copenhagen, James Bjorken and I realised that lepton–hadron symmetry could be revived simply by adding a fourth quark flavour to Gell-Mann’s three. We called the new quark flavour “charm”, completing two weak doublets of quarks to match two weak doublets of leptons, and establishing lepton–quark symmetry, which holds to this day.
Annus mirabilis
1964 was a remarkable year. In addition to the invention of quarks, Nick Samios spotted the triply strange Ω– baryon, and Oscar Greenberg devised what became the critical notion of colour. Arno Penzias and Robert Wilson stumbled on the cosmic microwave background radiation. James Cronin, Val Fitch and others discovered CP violation. Robert Brout, François Englert, Peter Higgs and others invented spontaneously broken non-Abelian gauge theories. And to top off the year, Abdus Salam rediscovered and published my SU(2) × U(1) model, after I had more-or-less abandoned electroweak thoughts due to four seemingly intractable problems.
Four intractable problems of early 1964
How could the W and Z bosons acquire masses while leaving the photon massless?
Steven Weinberg, my friend from both high-school and college, brilliantly solved this problem in 1967 by subjecting the electroweak gauge group to spontaneous symmetry breaking, initiating the half-century-long search for the Higgs boson. Salam published the same solution in 1968.
How could an electroweak model of leptons be extended to describe the weak interactions of hadrons?
John Iliopoulos, Luciano Maiani and I solved this problem in 1970 by introducing charm and quark-lepton symmetry to avoid unobserved strangeness-changing neutral currents.
Was the spontaneously broken electroweak gauge model mathematically consistent?
Gerard ’t Hooft announced in 1971 that he had proven Steven Weinberg’s electroweak model to be renormalisable. In 1972, Claude Bouchiat, John Iliopoulos and Philippe Meyer demonstrated the electroweak model to be free of Adler anomalies provided that lepton–quark symmetry is maintained.
Could the electroweak model describe CP violation without invoking additional spinless fields?
In 1973, Makoto Kobayashi and Toshihide Maskawa showed that the electroweak model could easily and naturally violate CP if there are more than four quark flavours.
Much to my surprise and delight, all of them would be solved within just a few years, with the last theoretical obstacle removed by Makoto Kobayashi and Toshihide Maskawa in 1973 (see “Four intractable problems” panel). A few months later, Paul Musset announced that CERN’s Gargamelle detector had won the race to detect weak neutral-current interactions, giving the electroweak model the status of a predictive theory. Remarkably, the year had begun with Gell-Mann, Harald Fritzsch and Heinrich Leutwyler proposing QCD, and David Gross, Frank Wilczek and David Politzer showing it to be asymptotically free. The Standard Model of particle physics was born.
Charmed findings
But where were the charmed quarks? Early on Monday morning on 11 November, 1974, I was awakened by a phone call from Sam Ting, who asked me to come to his MIT office as soon as possible. He and Ulrich Becker were waiting for me impatiently. They showed me an amazingly sharp resonance. Could it be a vector meson like the ρ or ω and be so narrow, or was it something quite different? I hopped in my car and drove to Harvard, where my colleagues Alvaro de Rújula and Howard Georgi excitedly regaled me about the Californian side of the story. A few days later, experimenters in Frascati confirmed the BNL–SLAC discovery, and de Rújula and I submitted our paper “Is Bound Charm Found?” – one of two papers on the J/ψ discovery printed in Physical Review Letters on 5 July 1965 that would prove to be correct. Among five false papers was one written by my beloved mentor, Julian Schwinger.
The second correct paper was by Tom Appelquist and David Politzer. Well before that November, they had realised (without publishing) that bound states of a charmed quark and its antiquark lying below the charm threshold would be exceptionally narrow due the asymptotic freedom of QCD. De Rújula suggested to them that such a system be called charmonium in an analogy with positronium. His term made it into the dictionary. Shortly afterward, the 1976 Nobel Prize in Physics was jointly awarded to Burton Richter and Sam Ting for “their pioneering work in the discovery of a heavy elementary particle of a new kind” – evidence that charm was not yet a universally accepted explanation. Over the next few years, experimenters worked hard to confirm the predictions of theorists at Harvard and Cornell by detecting and measuring the masses, spins and transitions among the eight sub-threshold charmonium states. Later on, they would do the same for 14 relatively narrow states of bottomonium.
Other experimenters were searching for particles containing just one charmed quark or antiquark. In our 1975 paper “Hadron Masses in a Gauge Theory”, de Rújula, Georgi and I included predictions of the masses of several not-yet-discovered charmed mesons and baryons. The first claim to have detected charmed particles was made in 1975 by Robert Palmer and Nick Samios at Brookhaven, again with a bubble-chamber event. It seemed to show a cascade decay process in which one charmed baryon decays into another charmed baryon, which itself decays. The measured masses of both of the charmed baryons were in excellent agreement with our predictions. Though the claim was not widely accepted, I believe to this day that Samios and Palmer were the first to detect charmed particles.
The SLAC electron–positron collider, operating well above charm threshold, was certainly producing charmed particles copiously. Why were they not being detected? I recall attending a conference in Wisconsin that was largely dedicated to this question. On the flight home, I met my old friend Gerson Goldhaber, who had been struggling unsuccessfully to find them. I think I convinced him to try a bit harder. A couple of weeks later in 1976, Goldhaber and François Pierre succeeded. My role in charm physics had come to a happy ending.
This article is adapted from a presentation given at the Institute of High-Energy Physics in Beijing on 20 October 2024 to celebrate the 50th anniversary of the discovery of the J/ψ.
In a paper published in the journal Nature, the CLOUD collaboration at CERN has revealed a new source of atmospheric aerosol particles that could help scientists to refine climate models.
Aerosols are microscopic particles suspended in the atmosphere that arise from both natural sources and human activities. They play an important role in Earth’s climate system because they seed clouds and influence their reflectivity and coverage. Most aerosols arise from the spontaneous condensation of molecules that are present in the atmosphere only in minute concentrations. However, the vapours responsible for their formation are not well understood, particularly in the remote upper troposphere.
The CLOUD (Cosmics Leaving Outdoor Droplets) experiment at CERN is designed to investigate the formation and growth of atmospheric aerosol particles in a controlled laboratory environment. CLOUD comprises a 26 m3 ultra-clean chamber and a suite of advanced instruments that continuously analyse its contents. The chamber contains a precisely selected mixture of gases under atmospheric conditions, into which beams of charged pions are fired from CERN’s Proton Synchrotron to mimic the influence of galactic cosmic rays.
“Large concentrations of aerosol particles have been observed high over the Amazon rainforest for the past 20 years, but their source has remained a puzzle until now,” says CLOUD spokesperson Jasper Kirkby. “Our latest study shows that the source is isoprene emitted by the rainforest and lofted in deep convective clouds to high altitudes, where it is oxidised to form highly condensable vapours. Isoprene represents a vast source of biogenic particles in both the present-day and pre-industrial atmospheres that is currently missing in atmospheric chemistry and climate models.”
Isoprene is a hydrocarbon containing five carbon atoms and eight hydrogen atoms. It is emitted by broad-leaved trees and other vegetation and is the most abundant non-methane hydrocarbon released into the atmosphere. Until now, isoprene’s ability to form new particles has been considered negligible.
The CLOUD results change this picture. By studying the reaction of hydroxyl radicals with isoprene at upper tropospheric temperatures of –30 °C and –50 °C, the collaboration discovered that isoprene oxidation products form copious particles at ambient isoprene concentrations. This new source of aerosol particles does not require any additional vapours. However, when minute concentrations of sulphuric acid or iodine oxoacids were introduced into the CLOUD chamber, a 100-fold increase in aerosol formation rate was observed. Although sulphuric acid derives mainly from anthropogenic sulphur dioxide emissions, the acid concentrations used in CLOUD can also arise from natural sources.
In addition, the team found that isoprene oxidation products drive rapid growth of particles to sizes at which they can seed clouds and influence the climate – a behaviour that persists in the presence of nitrogen oxides produced by lightning at upper-tropospheric concentrations. After continued growth and descent to lower altitudes, these particles may provide a globally important source for seeding shallow continental and marine clouds, which influence Earth’s radiative balance – the amount of incoming solar radiation compared to outgoing longwave radiation (see “Seeding clouds” figure).
“This new source of biogenic particles in the upper troposphere may impact estimates of Earth’s climate sensitivity, since it implies that more aerosol particles were produced in the pristine pre-industrial atmosphere than previously thought,” adds Kirkby. “However, until our findings have been evaluated in global climate models, it’s not possible to quantify the effect.”
The CLOUD findings are consistent with aircraft observations over the Amazon, as reported in an accompanying paper in the same issue of Nature. Together, the two papers provide a compelling picture of the importance of isoprene-driven aerosol formation and its relevance for the atmosphere.
Since it began operation in 2009, the CLOUD experiment has unearthed several mechanisms by which aerosol particles form and grow in different regions of Earth’s atmosphere. “In addition to helping climate researchers understand the critical role of aerosols in Earth’s climate, the new CLOUD result demonstrates the rich diversity of CERN’s scientific programme and the power of accelerator-based science to address societal challenges,” says CERN Director for Research and Computing, Joachim Mnich.
Physics is a universal language that unites scientists worldwide. No event illustrates this more vividly than the general assembly of the International Union of Pure and Applied Physics (IUPAP). The 33rd assembly convened 100 delegates representing territories around the world in Haikou, China, from 10 to 14 October 2024. Amid today’s polarised global landscape, one clear commitment emerged: to uphold the universality of science and ensure the free movement of scientists.
IUPAP was established in 1922 in the aftermath of World War I to coordinate international efforts in physics. Its logo is recognisable from conferences and proceedings, but its mission is less widely understood. IUPAP is the only worldwide organisation dedicated to the advancement of all fields of physics. Its goals include promoting global development and cooperation in physics by sponsoring international meetings; strengthening physics education, especially in developing countries; increasing diversity and inclusion in physics; advancing the participation and recognition of women and of people from under-represented groups; enhancing the visibility of early-career talents; and promoting international agreements on symbols, units, nomenclature and standards. At the 33rd assembly, 300 physicists were elected to the executive council and specialised commissions for a period of three years.
Global scientific initiatives were highlighted, including the International Year of Quantum Science and Technology (IYQ2025) and the International Decade on Science for Sustainable Development (IDSSD) from 2024 to 2033, which was adopted by the United Nations General Assembly in August 2023. A key session addressed the importance of industry partnerships, with delegates exploring strategies to engage companies in IYQ2025 and IDSSD to further IUPAP’s mission of using physics to drive societal progress. Nobel laureate Giorgio Parisi discussed the role of physics in promoting a sustainable future, and public lectures by fellow laureates Barry Barish, Takaaki Kajita and Samuel Ting filled the 1820-seat Oriental Universal Theater with enthusiastic students.
A key focus of the meeting was visa-related issues affecting international conferences. Delegates reaffirmed the union’s commitment to scientists’ freedom of movement. IUPAP stands against any discrimination in physics and will continue to sponsor events only in locations that uphold this value – a stance that is orthogonal to the policy of countries imposing sanctions on scientists affiliated with specific institutions.
A joint session with the fall meeting of the Chinese Physical Society celebrated the 25th anniversary of the IUPAP working group “Women in Physics” and emphasised diversity, equity and inclusion in the field. Since 2002, IUPAP has established precise guidelines for the sponsorship of conferences to ensure that women are fairly represented among participants, speakers and committee members, and has actively monitored the data ever since. This has contributed to a significant change in the participation of women in IUPAP-sponsored conferences. IUPAP is now building on this still-necessary work on gender by focusing on discrimination on the grounds of disability and ethnicity.
The closing ceremony brought together the themes of continuity and change. Incoming president Silvina Ponce Dawson (University of Buenos Aires) and president-designate Sunil Gupta (Tata Institute) outlined their joint commitment to maintaining an open dialogue among all physicists in an increasingly fragmented world, and to promoting physics as an essential tool for development and sustainability. Outgoing leaders Michel Spiro (CNRS) and Bruce McKellar (University of Melbourne) were honoured for their contributions, and the ceremonial handover symbolised a smooth transition of leadership.
As the general assembly concluded, there was a palpable sense of momentum. From strategic modernisation to deeper engagement with global issues, IUPAP is well-positioned to make physics more relevant and accessible. The resounding message was one of unity and purpose: the physics community is dedicated to leveraging science for a brighter, more sustainable future.
The World Wide Web, AI and quantum computing – what do these technologies have in common? They all started out as “hacks”, says Jiannan Zhang, founder of the open-source community platform DoraHacks. “When the Web was invented at CERN, it demonstrated that in order to fundamentally change how people live and work, you have to think of new ways to use existing technology,” says Zhang. “Progress cannot be made if you always start from scratch. That’s what hackathons are for.”
Ten years ago, Zhang helped organise the first CERN Webfest, a hackathon that explores creative uses of technology for science and society. Webfest helped Zhang develop his coding skills and knowledge of physics by applying it to something beyond his own discipline. He also made long-lasting connections with teammates, who were from different academic backgrounds and all over the world. After participating in more hackathons, Zhang’s growing “hacker spirit” inspired him to start his own company. In 2024 Zhang returned to Webfest not as a participant, but as the CEO of DoraHacks.
Hackathons are social coding events often spanning multiple days. They are inclusive and open – no academic institution or corporate backing is required – making them accessible to a diverse range of talented individuals. Participants work in teams, pooling their skills to tackle technical problems through software, hardware or a business plan for a new product. Physicists, computer scientists, engineers and entrepreneurs all bring their strengths to the table. Young scientists can pursue work that may not fit within typical research structures, develop their skills, and build portfolios and professional networks.
“If you’re really passionate about something, you should be able to jump on a project and work on it,” says Zhang. “You shouldn’t need to be associated with a university or have a PhD to pursue it.”
For early-career researchers, hackathons offer more than just technical challenges. They provide an alternative entry point into research and industry, bridging the gap between academia and real-world applications. University-run hackathons often attract corporate sponsors, giving them the budget to rent out stadiums with hundreds, sometimes thousands, of attendees.
“These large-scale hackathons really capture the attention of headhunters and mentors from industry,” explains Zhang. “They see the events as a recruitment pool. It can be a really effective way to advance careers and speak to representatives of big companies, as well as enhancing your coding skills.”
In the 2010s, weekend hackathons served as Zhang’s stepping stone into entrepreneurship. “I used to sit in the computer-science common room and work on my hacks. That’s how I met most of my friends,” recalled Zhang. “But later I realised that to build something great, I had to effectively organise people and capital. So I started to skip my computer-science classes and sneak into the business classrooms.” Zhang would hide in the back row of the business lectures, plotting his plan towards entrepreneurship. He networked with peers to evaluate different business models each day. “It was fun to combine our knowledge of engineering and business theory,” he added. “It made the journey a lot less stressful.”
But the transition from science to entrepreneurship was hard. “At the start you must learn and do everything yourself. The good thing is you’re exposed to lots of new skills and new people, but you also have to force yourself to do things you’re not usually good at.”
This is a dilemma many entrepreneurs face: whether to learn new skills from scratch, or to find business partners and delegate tasks. But finding trustworthy business partners is not always easy, and making the wrong decision can hinder the start up’s progress. That’s why planning the company’s vision and mission from the start is so important.
“The solution is actually pretty straight forward,” says Zhang. “You need to spend more time completing the important milestones yourself, to ensure you have a feasible product. Once you make the business plan and vision clear, you get support from everywhere.”
Decentralised community governance
Rather than hackathon participants competing for a week before abandoning their code, Zhang started DoraHacks to give teams from all over the world a chance to turn their ideas into fully developed products. “I want hackathons to be more than a recruitment tool,” he explains. “They should foster open-source development and decentralised community governance. Today, a hacker from Tanzania can collaborate virtually with a team in the US, and teams gain support to develop real products. This helps make tech fields much more diverse and accessible.”
Zhang’s company enables this by reducing logistical costs for organisers and providing funding mechanisms for participants, making hackathons accessible to aspiring researchers beyond academic institutions. As the community expands, new doors open for young scientists at the start of their careers.
“The business model is changing,” says Zhang. Hackathons are becoming fundamental to emerging technologies, particularly in areas like quantum computing, blockchain and AI, which often start out open source. “There will be a major shift in the process of product creation. Instead of building products in isolation, new technologies rely on platforms and infrastructure where hackers can contribute.”
Today, hackathons aren’t just about coding or networking – they’re about pushing the boundaries of what’s possible, creating meaningful solutions and launching new career paths. They act as incubators for ideas with lasting impact. Zhang wants to help these ideas become reality. “The future of innovation is collaborative and open source,” he says. “The old world relies on corporations building moats around closed-source technology, which is inefficient and inaccessible. The new world is centred around open platform technology, where people can build on top of old projects. This collaborative spirit is what makes the hacker movement so important.”
To provide the best experiences, we use technologies like cookies to store and/or access device information. Consenting to these technologies will allow us to process data such as browsing behavior or unique IDs on this site. Not consenting or withdrawing consent, may adversely affect certain features and functions.
Functional
Always active
The technical storage or access is strictly necessary for the legitimate purpose of enabling the use of a specific service explicitly requested by the subscriber or user, or for the sole purpose of carrying out the transmission of a communication over an electronic communications network.
Preferences
The technical storage or access is necessary for the legitimate purpose of storing preferences that are not requested by the subscriber or user.
Statistics
The technical storage or access that is used exclusively for statistical purposes.The technical storage or access that is used exclusively for anonymous statistical purposes. Without a subpoena, voluntary compliance on the part of your Internet Service Provider, or additional records from a third party, information stored or retrieved for this purpose alone cannot usually be used to identify you.
Marketing
The technical storage or access is required to create user profiles to send advertising, or to track the user on a website or across several websites for similar marketing purposes.
