The case for the upgrade rests on what the High-Luminosity LHC itself will deliver. When the accelerator completes its current upgrade programme, it will increase the integrated luminosity delivered to experiments by roughly an order of magnitude compared with Run 3, flooding the forward region with an unprecedented flux of neutrinos. These particles, born in the decays of heavy-flavour hadrons such as those containing charm quarks, are produced at extreme angles relative to the beamline, at pseudorapidities between about 7 and 8, and stream hundreds of metres along the tunnel. No general-purpose detector can catch them. SND@LHC, sitting off the collision axis, was conceived precisely to exploit this blind spot, and its first observations of both muon neutrinos and electron neutrinos from LHC collisions confirmed that collider neutrino physics has moved from theory to reality.

The upgraded detector, referred to in the paper as SND@HL-LHC while the collaboration keeps its original name, must contend with a fundamental technological problem: the emulsion-based tracking system that served the experiment during Run 3 cannot cope with the particle rates of the high-luminosity era. Emulsion films integrate everything that passes through them and offer no event-by-event electronic readout. The upgrade therefore replaces them with a fully electronic, silicon-based tracking system capable of real-time operation while preserving the spatial resolution needed to reconstruct neutrino interactions in a high-occupancy environment. The design also adds two capabilities the original detector never had: a precision timing system for rejecting background, and a magnetic spectrometer that measures the momentum and charge of outgoing muons, allowing, for the first time, a clean distinction between neutrino and antineutrino interactions.

The detector itself remains remarkably compact, roughly three metres long with a cross-section of about one square metre, a footprint dictated by the tight confines of the TI18 tunnel. Its heart is the neutrino target region, a stack of 58 tungsten plates, each seven millimetres thick, interleaved with silicon microstrip layers salvaged and repurposed from the CMS Microstrip Tracker. The stack weighs about 1.3 tons and corresponds to 116 radiation lengths and 4 nuclear interaction lengths, dense enough to stop and measure the showers produced when neutrinos strike tungsten nuclei. The silicon modules resolve positions to better than 30 micrometres, and consecutive layers are rotated by 90 degrees to provide precision tracking in both spatial coordinates. To keep the irradiated silicon stable, the target operates at approximately minus 20 degrees Celsius inside an insulated enclosure cooled by a closed-loop dry-air system with full redundancy and safety monitoring.

Downstream of the target sits the second major component: a magnetised hadronic calorimeter built from 34 layers of 50-millimetre-thick iron slabs interleaved with more recycled CMS silicon modules. The iron serves double duty, absorbing hadronic showers so their energy can be measured and providing the medium for a magnetic field of approximately 1.7 tesla, generated by a single-piece copper coil. This field bends the trajectories of muons leaving the target, allowing momentum measurements up to about 1 teraelectronvolt with a resolution of roughly 20 percent. Finite-element simulations confirm a homogeneous field within the active volume, with non-uniformities of only a few percent and negligible stray fields outside. Upstream, three planes of plastic scintillator bars read out by silicon photomultipliers form a veto system with a rejection efficiency at the level of one part in a billion against the few-kilohertz flux of beam muons, while a dedicated timing system, based on either fast scintillator tiles or thin-gap resistive plate chambers, aims at sub-50-picosecond resolution to tie every event to the correct LHC bunch crossing.

The most consequential part of the paper is its comparison of two installation scenarios, both using the identical detector. The Baseline configuration, which is the approved version of the upgrade, places the detector directly on the existing tunnel floor with no civil-engineering work, leaving its centre about 57 centimetres above and 39 centimetres to one side of the collision axis. That position yields an average pseudorapidity coverage of 6.9 to 7.7 but an azimuthal acceptance of only about 66 milliradians, squeezed by the clearance between detector and ceiling. The Extended configuration instead lowers the detector by 43 centimetres and shifts it 29 centimetres horizontally, requiring the removal of 4.5 cubic metres of concrete. The reward is dramatic: the transverse distance to the collision axis shrinks to roughly 14 by 10 centimetres, pseudorapidity coverage extends beyond 7.8, and the azimuthal acceptance grows roughly fourfold to about 250 milliradians, which the authors identify as the dominant contribution to the overall acceptance gain.

Simulations based on the DPMJET, FLUKA, GENIE and POWHEG generators, assuming the full 3000 inverse femtobarns of integrated luminosity expected over roughly a decade of HL-LHC operation, quantify what that geometric gain buys. In the Extended configuration the total neutrino interaction rate rises by approximately a factor of five, and the energy spectra shift toward higher values because the detector intercepts a more energetic component of the forward flux. The consequences ripple through the entire physics programme. For muon neutrino and antineutrino deep-inelastic scattering cross-sections above 350 gigaelectronvolts, a region never previously explored that bridges fixed-target experiments and IceCube, the statistical uncertainty falls from about 3 percent in the Baseline layout to below 1 percent in the Extended one, while the dominant flux systematic can be constrained to about 5 percent thanks to overlapping measurements by the LHCf experiment during Run 3.

The upgrade also promises precision tests of fundamental symmetries. Because electron neutrinos in the detector’s acceptance originate predominantly from charm decays, their measurement probes forward charm production and the gluon distribution of the proton at Bjorken-x values below ten to the minus five, a regime relevant both for future colliders such as the FCC and for predictions of ultra-high-energy atmospheric neutrinos. Per-bin statistical uncertainties on these charm-related measurements improve from about 10 percent in the Baseline to 5 percent in the Extended configuration. Tests of lepton flavour universality sharpen considerably: the ratio of electron-neutrino to tau-neutrino interactions, sensitive to the cross-sections of the two species, would be measured with a statistical uncertainty of about 6 percent in the Baseline and 2 percent in the Extended layout, compared with 30 percent in Run 3. Perhaps most tantalising, the Extended configuration could deliver the first direct observation of tau antineutrinos, which have never been seen experimentally: simulations predict roughly 120 tau antineutrino charged-current interactions in the target, about 9 observable signal events against a background of 2, yielding an observation significance of about 5 sigma, where the Baseline manages only around 3 sigma.

Beyond the Standard Model, the detector doubles as a hunting ground for feebly interacting particles such as dark photons, heavy neutral leptons and axion-like particles produced in the far-forward region, and the paper evaluates sensitivity to light dark matter scattering off protons via a leptophobic baryonic mediator. Here too the Extended geometry wins, since the larger acceptance and closer approach to the collision axis increase the dark matter flux intersecting the detector and extend the reach in mediator coupling into territory not excluded by other experiments. The collaboration also foresees roughly 600 to 1300 neutrino interactions coincident with events recorded by ATLAS, enabled by sub-50-picosecond timing synchronisation, which would directly link a neutrino interaction to its production mechanism. Whatever installation scenario is ultimately realised, the experience gained will feed directly into the SND detector at the proposed SHiP facility, for which the upgraded instrument serves as a full-scale prototype, making this modest tunnel experiment a testing ground for the next generation of collider neutrino physics.

Subject of Research: Upgrade of the SND@LHC forward neutrino detector for the High-Luminosity LHC, comparing installation scenarios and physics reach

Article Title: SND@LHC upgrade for the high-luminosity LHC: physics reach and installation scenarios

Article References: SND@LHC Collaboration, Abbaneo, D., Ahmad, S., Albanese, R., Alexandrov, A., Alicante, F., Aloschi, F., Androsov, K., Anokhina, A., Arellano, L. G., Asawatangtrakuldee, C., Torres, M. A. A., Bangaru, N., Battilana, C., Bay, A., Bertocco, A., Betancourt, C., Bick, D., Biswas, R., … Saá, J. Z. (2026). SND@LHC upgrade for the high-luminosity LHC: physics reach and installation scenarios. The European Physical Journal C, 86(9), Article 1030. https://doi.org/10.1140/epjc/s10052-026-15924-5

Image Credits: AI Generated

DOI: 10.1140/epjc/s10052-026-15924-5

Keywords: SND@LHC, High-Luminosity LHC, neutrinos, CERN, particle physics, forward physics, heavy-flavour decays, dark matter, lepton flavour universality, tau antineutrino, silicon trackers, magnetic spectrometer

The measurement matters because helium is not merely the second-most abundant element in the cosmos; it is a fossil record of the universe’s infancy. Roughly ninety percent of all helium in existence today was forged during Big Bang nucleosynthesis, the brief epoch when the expanding universe was cool enough for protons and neutrons to bind together but still hot and dense enough to drive nuclear reactions. The remaining ten percent was manufactured inside stars through nuclear fusion over the past 13.5 billion years. By measuring how much helium exists in gas clouds that have never been enriched by stellar processes, astronomers can effectively read a number that was written into the cosmos before the first stars ever ignited.

The research team, working with data from the Large Binocular Telescope in Arizona, targeted metal-poor nebulas, which are clouds of gas and dust that contain extremely low concentrations of elements heavier than hydrogen and helium. Because carbon, nitrogen, oxygen, and other heavier elements are produced and dispersed by successive generations of stars, a nebula with very few of them is considered a close analogue to the pristine material that filled the early universe. The astronomers analyzed helium signals in both optical and infrared light, which allowed them to determine the temperature and density of the gas in these faraway systems. Those physical parameters are essential, because the apparent strength of helium emission lines depends on them, and any error in temperature or density translates directly into an error in the inferred helium abundance.

After collecting 48 high-quality galactic samples, the team assembled a dataset designed to significantly expand researchers’ ability to infer the universe’s primordial helium abundance. Previous efforts had constrained that abundance only to a precision of about two percent. The new analysis reduces the uncertainty to nearly half a percent, which Richard Pogge, a professor of astronomy at Ohio State and a founding member of the project, described as a huge leap for computational astrophysics. That improvement did not come from a single dramatic discovery but from the painstaking accumulation of well-characterized observations, careful modeling of the emission physics, and rigorous control of systematic errors that can creep in when light from distant galaxies passes through Earth’s atmosphere before reaching a telescope.

The precision matters because the primordial helium fraction is theoretically tied to conditions in the universe when it was only about one second old. At that moment, the cosmos was filled with a bath of neutrinos, tiny and extraordinarily abundant subatomic particles that interact only weakly with matter. The predicted yield of helium from Big Bang nucleosynthesis depends on several parameters, including the number of neutrino species, the density of ordinary matter, and the expansion rate of the universe during those first minutes. If the helium abundance measured in metal-poor galaxies differed substantially from current predictions, it could signal that the early universe contained additional particle species, or that some other aspect of early-universe physics departs from established theory, potentially opening the door to new and undiscovered physics.

Until this result, it had remained unclear whether the many subatomic particles that underpin modern particle physics all appeared simultaneously in the first moments after the Big Bang, or whether they arrived later in some alternative, sequential fashion. By comparing their helium measurements with archival observations of the cosmic microwave background, the relic radiation released roughly 400,000 years after the Big Bang when the universe first became transparent, the team confirmed that the number of neutrino species present at the Big Bang is consistent with the standard model of particle physics. The comparison works because both the helium abundance and the properties of the cosmic microwave background encode information about the same early epoch, so agreement between them provides a powerful consistency check on cosmological models.

Reaching that conclusion required overcoming formidable observational challenges. Extremely metal-poor galaxies are vanishingly rare in the nearby universe, and the team had to locate suitable examples, including the tiny dwarf galaxy Leo P, whose gas retains a chemical composition close to primordial. The researchers also had to account for how Earth’s atmosphere could distort their data, since absorption and scattering by air alter the brightness of the spectral lines used to measure helium. Infrared observations were particularly important in this respect, because key helium emission features fall at wavelengths where atmospheric effects and detector calibration demand especially careful treatment. Weller, who leads the infrared data reduction for the project, and her colleagues developed techniques to correct for these effects and to quantify the remaining uncertainties.

The scientific payoff extends beyond cosmology into the story of our own origins. As Weller noted, everything needed for life on Earth was once fused inside a star, and understanding precisely where those elements come from informs how the universe evolved and how it will continue to evolve. The first generation of stars formed from gas containing only the hydrogen and helium left over from the Big Bang, along with traces of lithium. Those stars then produced carbon, nitrogen, oxygen, and the other heavier elements that later seeded subsequent generations of stars, planets, and ultimately living organisms. Measuring the primordial helium abundance therefore anchors the entire chemical narrative of cosmic history, providing the starting composition from which all later enrichment proceeded.

The LBT Yp project, a collaboration involving researchers at Ohio State, the University of Minnesota, Gonzaga University, Northwestern University, The University of Texas at Austin, and Indiana University, plans to continue this work by constraining less-explored parameters using additional metal-poor galaxies, likely with the aid of vast astronomical archives generated by large survey collaborations such as DESI, the Dark Energy Spectroscopic Instrument. Pogge emphasized that applying the techniques developed by the team to new galaxies will take many years, and expressed satisfaction at passing decades-long findings to the next generation of researchers who will carry the field forward. The co-authors of the study include Jayde Spiegel of Ohio State, Evan Skillman and John H. Miller Jr. of the University of Minnesota, Erik Aver of Gonzaga University, Noah Rogers of Northwestern University, Danielle Berg of The University of Texas at Austin, and John Salzar of Indiana University, with support from the National Science Foundation and Ohio State’s Center for Cosmology and AstroParticle Physics.

For now, the result stands as a striking confirmation that the physics humans have worked out in laboratories on Earth accurately describes conditions in a universe barely a second old. As Pogge put it, finally having atomic data precise enough to show how the universe worked seconds after it began gives scientists the ability to make meaningful constraints on the nature of physics itself. In an era when cosmology is often tested through billion-dollar space missions and enormous survey telescopes, it is a reminder that careful measurements of faint helium lines in a few dozen nearby dwarf galaxies can still deliver some of the most fundamental answers about where everything, including the atoms in our own bodies, came from.

Subject of Research: Precision measurement of the primordial helium abundance in metal-poor galaxies as a test of Big Bang nucleosynthesis and early-universe particle physics

Article Title: Using helium, astronomers blow certainty into early Big Bang conditions

Article References: Using helium, astronomers blow certainty into early Big Bang conditions. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: primordial helium, Big Bang nucleosynthesis, metal-poor galaxies, neutrinos, cosmic microwave background, Large Binocular Telescope, particle physics, cosmology, nebulae, early universe, spectroscopy, The Astrophysical Journal

Neutrinos are elementary particles that are nearly massless, electrically neutral, and famously reluctant to interact with anything. Trillions of them pass through the human body every second without leaving a trace. Yet during the death of a massive star, neutrinos are the dominant actors: roughly 99 percent of the gravitational energy released in a core-collapse event is carried away by an enormous burst of these particles. Because they barely interact with matter, neutrinos are the only messengers capable of escaping directly from the innermost regions of a collapsing stellar core, making them both the engine of the explosion and the only probe of what happens deep inside.

Neutrinos come in three flavors — the electron neutrino, the muon neutrino, and the tau neutrino — and quantum mechanics allows them to oscillate between these states as they travel. This flavor conversion is not merely a curiosity of particle physics. The flavor of a neutrino determines how it interacts with the dense matter in a dying star’s core, and therefore how much energy is deposited where it matters most: in the shock wave that must be revived if the star is to explode. Scientists have long known that flavor conversion occurs, but the prevailing assumption was that it had little bearing on whether the explosion succeeds or fails.

That assumption has now been challenged. Postdoctoral researcher Mariam Gogilashvili and Professor Irene Tamborra, both of the Niels Bohr Institute at the University of Copenhagen, developed a simplified model that allowed them, for the first time, to systematically test the impact of neutrino flavor conversion across a large population of collapsing stars. The challenge was formidable. Simulating the death of a massive star, Tamborra notes, sits at the very frontier of modern computational physics, because the problem couples hydrodynamics, gravity, nuclear physics, and radiation transport, and it is extraordinarily expensive to compute. Incorporating the full quantum kinetics of flavor evolution into such simulations has until now been beyond reach.

The Copenhagen team sidestepped this bottleneck by building a streamlined framework in which flavor conversion could be switched on and off and triggered at different densities within the collapsing star. They then ran simulations of 195 stars with masses ranging from 9 to 120 times that of the Sun, comparing otherwise identical models with and without neutrino flavor conversion. For each star, they tracked whether the shock wave was energized enough to produce a supernova or whether the collapse continued unchecked toward a black hole. The result was striking: the behavior of neutrinos significantly altered the fate of the stars, with the effect most pronounced for stars between 16 and 30 solar masses — precisely the mass range where predictions have been most uncertain.

Gogilashvili describes the moment the full set of simulations was laid side by side: a whole range of stars flipped from exploding to failing once flavor conversion was included. Seeing such a clear pattern across so many stars, she says, demonstrated that neutrino flavor conversion is a process that simply cannot be left out of models attempting to explain how massive stars end their lives. In other words, the quantum identity of ghost particles — a property invisible to telescopes — may act as a hidden switch that tips the balance between a brilliant supernova and a silent gravitational collapse.

The findings, published in the journal Physical Review D under the title Neutrino Flavor Conversion Shapes the Rate of Failed Core-collapse Supernovae, carry immediate consequences for one of the field’s most persistent puzzles: the supernova rate problem. Astronomers observe significantly fewer supernovae in the universe than theoretical models of stellar evolution predict. One possible explanation is that some massive stars die quietly. If a star collapses directly into a black hole without a luminous explosion, or if its faint outburst is obscured by dust, it effectively disappears from observational counts. The new results suggest that neutrino flavor conversion provides a concrete physical mechanism that could make such failed supernovae more common than previously thought, potentially closing part of the gap between what telescopes see and what theory expects.

Beyond reconciling counts, the work offers a practical tool: if flavor conversion can be modeled reliably, astrophysicists may be better positioned to predict, given a star’s mass and internal structure, whether it will end as a neutron star or a black hole. That predictive power matters for interpreting the next galactic supernova, for understanding the demographics of stellar-mass black holes detected through gravitational waves, and for mapping how often stars in the 16-to-30-solar-mass range quietly vanish. The researchers emphasize that future models of supernovae, neutron stars, and black holes should incorporate neutrino flavor conversion as a standard ingredient rather than an optional refinement.

The study also underscores how much remains to be learned about neutrinos themselves. Because these particles are almost unaffected by electromagnetic and other forces, they carry direct information about processes deep within stars and about the earliest moments of the universe. Flavor conversion occurs when neutrinos interact with different types of matter, and in the extreme densities of a collapsing core, neutrinos interact so strongly with one another that their flavor evolution becomes a collective, nonlinear phenomenon. Untangling this quantum many-body problem in realistic stellar environments remains one of the great computational challenges, and the Copenhagen framework represents an early but consequential step toward that goal.

There is, ultimately, a more intimate stake in these questions. Massive stars forge heavy elements during their lives and scatter them across the galaxy when they explode, seeding the raw material from which planets — and life — later form. As Tamborra points out, studying how massive stars live and die is also a study of the origins of the elements that make up the universe and ourselves. Whether a distant star ends in fire or in darkness may depend on the quantum flip of a ghost particle, and that same process helped write the chemical story of our own existence. The fate of dying stars, it turns out, is written in a language we are only beginning to read.

Subject of Research: The role of neutrino flavor conversion in determining whether massive stars undergo successful core-collapse supernovae or fail and collapse into black holes.

Article Title: Supernova or black hole? Ghost particles’ “flavor” may determine the fate of dying stars

Article References: Supernova or black hole? Ghost particles’ “flavor” may determine the fate of dying stars. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: neutrinos, neutrino flavor conversion, supernovae, black holes, core-collapse, neutron stars, stellar evolution, failed supernovae, Niels Bohr Institute, Physical Review D, astrophysics, ghost particles