Author: Juan Rojo

I recently had the pleasure of taking part in the 8th general meeting of the FASER Collaboration, hosted by the University of Bern. It was a wonderful week full of great science, exciting results, and plenty of ideas for the future.

The programme spanned the full breadth of the collaboration’s efforts: from neutrino cross-sections at TeV energies to forward charm production; from searches for dark photons, axion-like particles, and quirks to muon deep-inelastic scattering at the energy frontier as a probe of proton structure; and from new detector concepts to innovative reconstruction algorithms and out-of-the-box theory insights. There is no shortage of initiatives to further enrich an already blossoming programme.

While reaping the fruits of the recently completed Run 3 dataset and planning some of its legacy analyses, we also discussed at length the prospects for a thriving TeV neutrino and muon programme, alongside long-lived-particle and BSM searches, during Run 4 in the High-Luminosity LHC era, when FASER would be exposed to an unprecedented luminosity. It was clear from the discussions that, cliché but true, the best is yet to come.

It was a pleasure to attend with a strong delegation from the Nikhef Theory group and VU Physics and Astronomy: Jelle Koorn, Paulina Hernandez Sainz, Peter Krack, and Elie Hammou, and to meet in person so many of our friends and close collaborators from FASER, including Felix Kling, Jamie Boyd, Jonathan Feng, and many others, after months of interaction over Mattermost and Zoom.

Many thanks to Akitaka Ariga and his Bern colleagues for the flawless organisation. We return home fully energised and full of ideas for the next steps. The coming months will be intense but also very interesting, so stay tuned for news from FASER!

Thrilled to share that my ERC Advanced Grant project UNICORN “UNlocking the strong Interactions with COllideR Neutrinos” has been awarded. The five-year project will be hosted at the VU Amsterdam and embedded in the Nikhef Theory Group, and will support a team of several PhD candidates postdoctoral researchers starting in late 2026.

For 15+ years, the intense flux of high-energy neutrinos produced in the forward direction at the LHC went undetected. Their observation in 2023 by the FASER and SND@LHC experiments opened a new “collider neutrino” era, giving access to the highest-energy neutrinos ever produced in a laboratory. UNICORN will build a comprehensive theoretical framework needed to model their production and scattering, combining state-of-the-art QCD calculations, Monte Carlo simulations, and machine learning, and apply it to the full FASER and SND@LHC Run 3 datasets, as well as laying the groundwork for a thriving LHC neutrino program at the High-Luminosity LHC. In a nutshell, UNICORN will transform FASER and SND@LHC into precision microscopes to unravel proton structure. Exploiting LHC neutrinos now is a once-in-a-lifetime opportunity, with no comparable experiment expected before the 2070s.

The scientific payoff of UNICORN reaches well beyond the strong interactions. Sharper knowledge of proton structure feeds directly into precision predictions for Higgs and electroweak physics at the High-Luminosity LHC and into searches for New Physics, while data-driven control of the prompt atmospheric flux supports the interpretation of neutrino telescope observations such as the 120 PeV event recorded by KM3NeT, the most energetic particle ever detected.

With UNICORN, I will:

• carry out a data-driven extraction of the LHC forward neutrino fluxes, with full energy and rapidity dependence, to stress-test QCD models of forward particle production and validate explanations of the cosmic-ray muon puzzle;
• turn the neutrino fluxes from charm decays into high-resolution probes of small-x QCD, constraining the gluon down to momentum fractions of x around ten to the minus seven, searching for non-linear and BFKL dynamics, and delivering precise predictions for the prompt atmospheric flux at neutrino telescopes;
• measure neutrino and muon deep-inelastic scattering structure functions at TeV energies and includes them in the NNPDF global analysis of proton structure, yielding improved predictions for Higgs and electroweak observables and PDFs tailored to New Physics searches; and
• develop state-of-the-art event generators for neutrino and muon DIS combining higher perturbative accuracy with data-driven tunes, and integrates it with Neural Simulation-Based Inference techniques to extract optimally sensitive, unbinned observables from FASER data.

Join the team

Vacancies for PhD candidates and postdoctoral researchers supported by UNICORN will be announced soon on the Nikhef jobs portal. Stay tuned!

I tremendously enjoyed Higgs and Effective Field Theories (#HEFT2026) in the wonderful city of Valencia, hosted in the beautiful setting of the Jardí Botànic: the perfect background for a week of intense discussions on effective field theories and the search for physics beyond the Standard Model:

https://indico.cern.ch/event/1609241

EFTs give us a powerful, model-independent way to ask one of the deepest questions in physics: what lies beyond the Standard Model? Rather than betting on any single new-physics scenario, EFTs let us parametrise the possible imprints of heavy new particles in a systematic way, and then confront that framework with data across an enormous range of processes, from the LHC to low-energy precision experiments. It’s a powerful interplay of formal theory, phenomenology, and experiment, and it sits right at the frontier of how we’ll make discoveries in the coming years of particle physics.

A lot of fun to present ongoing work with Nikhef (National Institute for Subatomic Physics) and VU Physics and Astronomy colleagues on “The SMEFT as a New Physics Microscope at the FCC-ee“, where the central message is that discovery at a future Higgs and electroweak factory like the Future Circular Collider Study will look nothing like the bump-hunting of the LHC era. New physics will reveal itself as subtle deviations between ultra-precise measurements and equally precise theory predictions. Resolving those signals demands global analysis tools sharp enough to act as a genuine microscope, and that’s where the modern tools such as the #SMEFiT framework come in.

Many thanks for my collaborators Ilaria Brivio Elie Hammou Kamil Laurent Wopke Telman Jordy de Vries Vaisakh Plakkot Lemonia Gialidi Roan van Brussel for their final push to deliver some cool results in time for HEFT26.

Beyond the talks themselves, what makes HEFT so valuable is the scientific atmosphere. The programme left real room for discussion over coffee, during the sessions, and well into the evenings, bringing together theorists and phenomenologists from very different backgrounds. These are exactly the conversations where ideas get sharpened and, with a bit of luck (fingers crossed), where new collaborative projects are born.

cc the many friends and colleagues EFT aficionados Jaco ter Hoeve Alejo Rossia Luca Mantani Admir Greljo Ella Cole Jorge de Blas Raquel Gomez Ambrosio and many others joining the HEFT26 fun!

Following my talk at the Nikhef (National Institute for Subatomic Physics) Neutrino Platform on Tuesday, today I had the privilege of presenting on the same topic, Physics with TeV Neutrinos and Muons from the LHC, at the PLANCK 2026 & 6th EuCAPT Symposium at CERN

https://indico.cern.ch/event/1552644/

a joint conference bringing together the high energy physics and astroparticle theory communities. A perfect audience for this topic, and a great opportunity to establish new connections.

A few highlights that resonated particularly with this audience:

1) The FASER Experiment was born as a BSM experiment. Its original goal was to search for light dark sector particles: dark photons, feebly-interacting particles (FIPs), long-lived particles (LLPs). Thanks to a new theoretical infrastructure for neutrino production and scattering, we can now extend FASER beyond its original mandate and turn it into a precision QCD microscope

2) the CERN LHC operating at 7 TeV in the centre-of-mass frame is equivalent to a ~100,000 TeV fixed-target experiment: a direct laboratory counterpart of the ultra-high-energy cosmic ray interactions that experiments like Auger, IceCube, and KM3NeT probe. The recent KM3NeT Neutrino detection of KM3-230213A, the most energetic elementary particle ever detected, makes this connection more urgent than ever. We need reliable QCD modelling of neutrino production and scattering all the way up to 100 PeV, and FASER data can calibrate this.

3) TeV muons produced at the LHC and reaching FASER are not just a background: they enable muon deep-inelastic scattering in a kinematic regime that overlaps with the future Electron-Ion Collider. In particular, charm production in muon DIS is the ultimate probe of intrinsic quarks in the proton, and with data already on tape, definitive evidence may be within reach with FASER.

The LHC forward region is a goldmine for particle physics, astroparticle physics, and the search for new phenomena. With the complete Run 3 dataset on tape for FASER, the best is yet to come!

Can we discover New Physics through Precision at Future Colliders such as CERN’s Future Circular Collider?

We can but we need to use a multi-pronged approach, since searching for new physics at the FCC-ee will be very different than at the (HL-)LHC!

With the European Strategy for Particle Physics 2026 now concluded and the FCC-ee emerging as the preferred candidate for the next major collider, quantifying its physics potential in a rigorous and reproducible way has never been more important. That is exactly what our new paper sets to do:

https://inspirehep.net/literature/3147037

Together with my amazing #SMEFiT collaborators Tommaso Armadillo, Eugenia Celada, Jaco ter Hoeve Fabio Maltoni Luca Mantani Alejo Rossia Simone Tentori Marion Thomas and Eleni Vryonido, we present a state-of-the-art global analysis of the new physics reach of FCC-ee, benchmarked against other future colliders options such as LEP3 and the Linear Collider Facility. Results in the global SMEFT fit, coupling modifiers, and effective couplings are presented, as well as exclusion bounds for representative UV complete models matched to the SMEFT.

Along with the physics results, we release an updated version of the open source SMEFiT framework (https://smefit.science/) that allows every result in this paper to be fully reproduced, extended, and customised. Users can add new collider options, observables, or UV models with minimal effort. If you work on future collider phenomenology, this framework may be interesting for you for you!

The headline physics message: FCC-ee is a uniquely powerful machine for new physics searches, driven by the extraordinary precision of its Tera-Z run and the combination with the 240 and 365 GeV runs through quantum EFT effects.

A big shout out also to Jorge de Blas for many illuminating discussions about the corresponding HEPfit studies, to CERN Theory Division for their support while part of this work was being carried out, to Nikhef (National Institute for Subatomic Physics) for many exciting discussions on future collider, and to many other colleagues for suggestions and encouragement.

A milestone for the FCC-ee physics program – laying the groundwork for a new generation of future collider studies.

NSBI for Proton Structure: We present a new approach to determine the proton structure at the LHC using Neural Simulation Based Inference (NSBI), moving beyond traditional analyses based on binned observables.

Instead of compressing measurements into histograms (and inevitably losing information), we directly exploit the full, high-dimensional structure of collider events. By leveraging AI-driven inference, this allows us to extract PDFs with improved precision and reduced information loss.

As a proof of concept, we apply this framework to top-quark pair production and demonstrate clear gains over standard (binned, low dimensional) methodologies. In turn, this analysis benefits other gluon-initiated channels, with direct impact on LHC flagship measurements such as Higgs production via gluon fusion, where PDF uncertainties remain a key limiting factor.

More broadly, our work contributes toward an ongoing shift in collider physics: from binned dimensional observables to fully unbinned, high-dimensional, ML-assisted measurements, unlocking more of the information already present in the data.

Many thanks for my wonderful collaborators, a real Dream Team of theorists, experimentalists, and ML experts which has worked really hard in the last months to realise a idea which arose during the discussions with Robert Schoefbeck at the last NNPDF Collaboration meeting in Morimondo 2025.

https://inspirehep.net/literature/3145457

Effective Field Theories such as the SMEFT and HEFT have become an almost unavoidable component of the modern particle physicist’s toolbox. They provide access to New Physics at high scales, beyond the direct reach of particle colliders, by exploiting the subtle quantum effects that connect processes taking place at very different energies. At the LHC, EFT-based analyses are already becoming competitive with direct (resonance) searches for new physics, and at future facilities — such as the Future Circular Collider (FCC-ee) operating in its electron–positron mode — they will provide a unique microscope (or better, attoscope) in the quest to find what lies beyond the Standard Model.

This week the SMEFiT Collaboration held a long-overdue general meeting at CERN to review recent progress, assess the status of ongoing projects, and plan the next steps. SMEFiT is a global analysis framework, with strong contributions from Nikhef and VU Amsterdam, that interprets measurements from LEP and the LHC within the SMEFT and in terms of UV completions of the Standard Model matched onto it. Much of our recent work has focused on comparative studies of future colliders — quantifying their reach for key targets such as the self-interactions of the Higgs boson — and on deploying state-of-the-art theory calculations, including renormalisation-group-evolution effects that produce an even richer, and often counter-intuitive, pattern of correlations between processes and the parameters of the EFT.

Over two days of energetic discussion we addressed which datasets should take highest priority; how to go beyond dimension-six operators; how best to use the SMEFT to fingerprint possible BSM anomalies; how to connect with low-energy data; how to interface AI-assisted optimal observables; and how to extend SMEFiT to further targets such as the global electroweak fit — alongside the technical but essential matters of code development, improvements, and speed-ups, since exploring a parameter space of more than 60 dimensions is no walk in the park.

It was especially encouraging to see the PhD candidates and postdocs in the collaboration take the lead in many of the discussions, communicating a clear and ambitious vision for the EFT program at the LHC and beyond. I learned a great deal, and look forward to seeing which projects come to light in the coming months.

Many thanks to Fabio Maltoni and the CERN-TH secretariat for their help with the local organisation.

I had the pleasure of visiting two great scientific institutions in Abu Dhabi, on a trip that brought together particle physics, astroparticle physics, and quantum science.

At Khalifa University I was warmly hosted by the (astro-)particle physics groups of Arjen van Vliet, Rachik Soualah, and colleagues. Although a relative newcomer to high-energy physics, Khalifa University already has active groups in major particle and astroparticle physics experiments, from ATLAS to KM3NeT, and is involved in CERN’s efforts towards future colliders such as the Future Circular Collider. I gave the Physics department colloquium on recent progress in particle physics, with a focus on collider neutrinos at FASER and its planned upgrades, which open new windows onto the neutrino sector, hadron structure, and quantum chromodynamics, and which inform theoretical predictions and simulations for astroparticle physics experiments.

I also visited, one day before, the Center for Quantum Research at the Technology Innovation Institute, a centre set up just a few years ago, in part from scratch, by my PhD advisor José Ignacio Latorre, and which now hosts outstanding groups across quantum software, computing, communication, and sensing. In my colloquium there, “HEP meets Quantum science and technology”, I presented recent developments at the interface between particle physics and quantum science: quantum-information ideas as new guiding principles in particle physics, such as the maximal entanglement principle proposed a few years ago together with José Ignacio Latorre, Alba Cervera-Lierta, and Luca Rottoli; tests of quantum mechanics at the energy frontier through entanglement in top-quark pair production at ATLAS and CMS; and the intrinsic charm content of the proton, where machine learning lets us probe the quantum wave function of the proton.

These visits were full of stimulating discussions, and they left me optimistic about the future of quantum science and high-energy and astroparticle physics in the United Arab Emirates: with continued investment in fundamental science and technology, the local institutes are well placed to play an important role in global efforts in the coming years. I am grateful to my hosts at Khalifa University and the Technology Innovation Institute for their warm welcome, and I look forward to returning to visit these groups again.

This week I had the pleasure of participating in the new edition of the SEARCH 2025 conference at CERN, a stimulating event exploring the discovery potential and breakthroughs of high-energy physics experiments such as the Large Hadron Collider and its High-Luminosity upgrade, with ample time for joint brainstorming and discussion among the participants.

As part of the “Putting precision and searches together” session, I shared some thoughts on how theory, understood in the broad sense, is shaping the future of particle physics searches. I discussed how the boundary between measurements and searches is increasingly blurred, and how we should exploit this to our advantage. Advances in precision QCD, parton distributions, SMEFT interpretations, and AI-driven observables all play a key role in pushing the LHC’s discovery reach, in most cases well beyond the wildest projections that were on the table when the LHC was first conceived. These are exciting times for high-energy physics, as theory and experiment work more closely than ever to bridge precision and discovery in both directions.

Many thanks to the SEARCH 2025 organizers for an inspiring meeting, and in particular to Monica Dunford and Maurizio Pierini for their kind invitation!

I had a wonderful time at the yearly NNPDF Collaboration meeting, held in the beautiful abbey of Morimondo. It was a stimulating few days discussing recent progress and future plans within the NNPDF framework, ranging from the implementation of new datasets from ATLAS, CMS, and LHCb, to higher-order theoretical calculations and the development of novel machine learning techniques to keep unraveling the fascinating inner world of proton structure.

Special thanks to our invited speakers for their great and inspiring talks: Tilman Plehn (on accurate and uncertainty-aware ML for LHC physics), Robert Schöfbeck (on unbinned measurements for optimal parameter inference), and Eram Rizvi (on precision PDFs at ATLAS for electroweak and EFT analyses). Their lively contributions brought many new ideas to the discussion, and as usual the challenge will be to decide which ones to pursue.

We also welcomed many new colleagues to the NNPDF community, in particular our two new PhD candidates at Nikhef and VU Amsterdam, Jelle Koorn and Kamil Laurent. Welcome to the team, and looking forward to lots of great QCD and ML science together. The study of QCD and proton structure remains utterly fascinating, and the best may be yet to come.