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Juan Rojo

Publications and source records attributed to Juan Rojo.

At least 19 recordsLinked to original sources
arXiv2026

Constraints on a Long-Lived X17 Vector Boson with FASER

The ATOMKI anomalies motivate the possibility of a new particle, the X17, with a mass of 17 MeV and weak couplings to nuclei and electrons. Such a particle may be produced at the LHC, travel 480 m in the far-forward direction, and decay to $e^+e^-$ in FASER. In this paper, previous FASER searches for dark photons and axion-like particles (ALPs) are combined and interpreted as searches for a candidate X17 particle, the protophobic vector boson. The searches use data collected from $pp$ collisions at $\sqrt{s} = 13.6$ TeV in LHC Run 3 from 2022 for the dark photon search and from 2022-2023 for the ALP search. No excess events are found above the expected background, and the combined null results exclude a previously-unconstrained "low-coupling window" with electron coupling $6 \times 10^{-5} < ε_e < 1.2 \times 10^{-4}$. A "high-coupling window" remains.

hep-ex↗
arXiv2026

Proton Structure from Neural Simulation-Based Inference at the LHC

The precise determination of the parton distribution functions (PDFs) of the proton is an essential ingredient for LHC analyses, including for those at the upcoming High-Luminosity LHC. So far, PDFs are determined from global fits to binned low-dimensional data obtained from unfolded hard-scattering cross section measurements. In this work we demonstrate for the first time the feasibility of neural simulation-based inference (NSBI) for constraining the proton PDFs using a high-dimensional unbinned data set. Exploiting the full statistical power of unbinned data removes the loss of information inherited by the binning procedure. As a proof-of-concept, we determine the gluon PDF from simulated data of top quark pair production at the LHC with $\sqrt{s}=13$ TeV. Taking into account both experimental and theoretical systematic uncertainties in the detector-level features, we demonstrate how the NSBI pipeline achieves significant improvements in precision compared to low-dimensional binned analyses. Our results illustrate the potential of unbinned inference to reduce the reliance on coarse approximations of uncertainties and their correlations entering PDF determinations, hence contributing to a new paradigm of unbinned detector-level ML-assisted measurements at the LHC.

hep-ph↗
arXiv2026

Event Generation for Neutrino and Muon DIS at the LHC

Neutrinos and muons produced in the forward direction of LHC collisions and detected by the FASER and SND@LHC experiments enable a comprehensive deep-inelastic scattering (DIS) programme, covering both neutral and charged currents, at TeV energies. This programme aims to provide stringent constraints on proton and nuclear structure as well as on hadron fragmentation, and to offer novel insight into neutrino interactions at the highest energies achieved in a laboratory. Realising this potential requires accurate and precise simulations of neutral- and charged-current DIS. We systematically compare the predictions of NLO general-purpose Monte Carlo (MC) event generators for neutrino- and muon-initiated DIS at the LHC (Sherpa, Herwig7, and POWHEG) against analytical structure-function calculations (YADISM) and dedicated neutrino simulation packages (GENIE, NuWro, GiBUU). Within this framework, we quantify the role of higher-order QCD corrections, (nuclear) parton distribution functions (PDFs), parton shower and hadronisation modelling, and QED radiation both for inclusive observables and for the FASER fiducial selections. We also present representative applications to the FASER neutrino programme: comparisons with its most recent data from the emulsion and electronic sub-detectors, a study of strange PDF sensitivity in the dimuon final state, and predictions for charged-hadron production in semi-inclusive DIS. Our analysis highlights the agreement of modern event generators for inclusive and differential DIS observables once common settings are adopted, while differences remain for observables sensitive to the hadronic final state.

hep-ph↗
arXiv2026

Disentangling $V_{ud}$ from New Physics in Beta Decays with the FCC-ee

Superallowed nuclear $β$-decays enable the most precise determination of the CKM element $V_{ud}$, but they are only sensitive to an effective combination of $V_{ud}$ and New Physics contributions to left-handed charged currents, which low-energy data alone cannot disentangle. Here we determine $V_{ud}$ simultaneously with the relevant Wilson coefficients of the Standard Model Effective Field Theory (SMEFT) by combining, within the SMEFiT framework, a dedicated likelihood for superallowed transitions, including radiative and nuclear-structure uncertainties, with LEP and LHC world data and HL-LHC and FCC-ee projections. Current collider data and HL-LHC projections only partially lift the degeneracy between $V_{ud}$ and possible New Physics effects. Instead, the FCC-ee fully decouples $V_{ud}$ from New Physics, recovering SM-level precision while significantly tightening the constraints on the SMEFT Wilson coefficients. This conclusion is robust against variations in flavour assumptions and theory uncertainties. Our results highlight the complementarity between future colliders and low-energy experiments, and pave the way towards a SMEFT-consistent interpretation of first-row CKM unitarity tests and the Cabibbo angle anomaly.

hep-ph↗
arXiv2026

Electromagnetic Shower Reconstruction and Identification in FASER's Emulsion Detector for LHC Forward Neutrino Measurements

We present methods for electromagnetic shower reconstruction and identification in the FASERnu emulsion detector using 100 GeV and 200 GeV electron test-beam data from the CERN SPS H4 beamline. The reconstruction employs a clustering-based algorithm without energy-dependent tuning to determine shower axes. A multi-level identification chain comprising track pre-selection, a cut-based selection, and a BDT classifier achieves combined background rejection rates of 99.99% (100 GeV) and 99.94% (200 GeV). The method reaches total reconstruction and identification efficiencies of 58.9% (100 GeV) and 70.8% (200 GeV) evaluated from simulated samples. Energy reconstruction using the total number of reconstructed segments as the calorimetric estimator yields relative biases of +0.6% (100 GeV) and -0.8% (200 GeV), with resolutions of 25.4% and 22.6%, respectively. Systematic uncertainties on the energy reconstruction are dominated by variations in emulsion film detection efficiency, with totals of (+10.9%/-8.2%) at 100 GeV and (+10.3%/-6.9%) at 200 GeV. The methodology provides a validated framework for electron neutrino identification with the FASERnu detector at the LHC.

hep-ex↗
arXiv2026

On the construction of K-contact non-Sasakian Smale-Barden manifolds

In the breakthrough paper \cite{Mu-jems}, it is constructed the first example of a simply connected compact $5$-manifold (aka.\ Smale-Barden manifold) which admits a K-contact structure but does not carry a Sasakian structure. In this work we clarify some aspects of the construction of \cite{Mu-jems}, determining explicitly the number $N$ of symplectic surfaces needed to have an isotropy locus that produce a $5$-manifold that is K-contact but not Sasakian. Also, in order to analyse the geography problem of determining which Smale-Barden manifolds admit K-contact but not Sasakian structures, we refine and generalize the constructions of symplectic surfaces in a symplectic $4$-manifold with transversal intersections giving rise to such manifolds.

math.DG↗
arXiv2026

New Physics Reach through Precision at Future Colliders: a Multi-Pronged Approach

We present projections for the sensitivity of future high-energy colliders to new physics through precision measurements of the Standard Model (SM) interactions, focusing on near-term electron-positron facilities: FCC-ee, LEP3, and the Linear Collider Facility. We interpret these projections in three complementary frameworks: Higgs coupling modifiers, effective Higgs and electroweak couplings, and global SMEFT fits. The SMEFT analysis includes renormalisation-group evolution, linear/quadratic contributions, and NLO corrections to EFT cross sections where available. By matching the EFT to UV-complete models, we also quantify the sensitivity of future colliders to representative benchmark scenarios, including composite Higgs models and single-particle SM extensions. In parallel, we release an updated version of the open-source SMEFiT framework, enabling the results presented here to be fully reproduced, extended, and customised.

hep-ph↗
arXiv2026

Parton distribution functions and theory parameters: an NNPDF perspective

Parton Distribution Functions (PDFs) are a key ingredient in theoretical predictions for Large Hadron Collider (LHC) observables and play a central role in the extraction of precision Standard Model (SM) and Beyond the SM (BSM) parameters from LHC data. Recent analyses demonstrate that the determination of fundamental SM parameters such as $α_s(m_Z)$, $m_W$, $m_t$, and $\sin^2θ_W$ is strongly influenced by the choice of input PDFs. In this contribution, we present the status and challenges of PDF determination from the NNPDF perspective, both in stand-alone fits and in joint extractions with (B)SM parameters. We place particular emphasis on results for $α_s(m_Z)$, $m_t$, and Wilson coefficients in the SM Effective Field Theory (SMEFT) framework.

hep-ph↗
arXiv2025

Extractions of the strong coupling from collider data without PDF refitting are biased

We present an explicit demonstration that a determination of the strong coupling constant $α_s(m_Z)$ from deep-inelastic scattering and hadron collider data without a simultaneous determination of the parton distribution functions (PDFs) leads to a biased result for both the central value and the uncertainty, even in the ideal scenario (closure test) where there are no internal tensions between datasets and where theoretical calculations describe perfectly the experimental measurements. Specifically, we show that a determination of $α_s(m_Z)$ from a single process leads in general to a result that differs from the global best fit more than the value of $α_s(m_Z)$ that is actually favoured by this process.

hep-ph↗
arXiv2025

Letter of Intent: The Forward Physics Facility

The Forward Physics Facility (FPF) is a proposed extension of the HL-LHC program designed to exploit the unique scientific opportunities offered by the intense flux of high energy neutrinos, and possibly new particles, in the far-forward direction. Located in a well-shielded cavern 627 m downstream of one of the LHC interaction points, the facility will support a broad and ambitious physics program that significantly expands the discovery potential of the HL-LHC. Equipped with four complementary detectors -- FLArE, FASER$ν$2, FASER2, and FORMOSA -- the FPF will enable breakthrough measurements that will advance our understanding of neutrino physics, quantum chromodynamics, and astroparticle physics, and will search for dark matter and other new particles. With this Letter of Intent, we propose the construction of the FPF cavern and the construction, integration, and installation of its experiments. We summarize the physics case, the facility design, the layout and components of the detectors, as well as the envisioned collaboration structure, cost estimate, and implementation timeline.

hep-ex↗
arXiv2025

Deep-Inelastic Scattering at TeV Energies with LHC Muons

The LHC far-forward experiments FASER and SND@LHC have pioneered the detection of TeV-energy neutrinos produced in hard-scattering proton-proton collisions at the LHC. In addition to neutrinos, an intense flux of TeV-energy muons reaches these detectors, representing a dominant background for both neutrino studies and beyond the Standard Model searches. Here we demonstrate that this forward muon flux enables a comprehensive neutral-current deep-inelastic scattering (DIS) program at FASER with a strong kinematical overlap with the Electron Ion Collider. For the Run 3 luminosity of $\mathcal{L}_{\rm pp}=250$~fb$^{-1}$, more than $10^5$ inclusive muon DIS events, of which up to $10^4$ from charm production, are expected at FASER$ν$. As a representative application, we demonstrate the sensitivity of muon DIS at FASER$ν$ to probe the (intrinsic) charm content of the proton at large-$x$. We also provide predictions for event yields of muon DIS for future FASER runs and for the proposed Forward Physics Facility.

hep-ph↗
arXiv2025

A Determination of $α_s(m_Z)$ at ${\bf aN^3LO_{QCD}}\otimes {\bf {NLO}_{QED}}$ Accuracy from a Global PDF Analysis

We present a determination of the strong coupling $α_s(m_Z)$ from a global dataset including both fixed-target and collider data from deep-inelastic scattering and a variety of hadronic processes, with a simultaneous determination of parton distribution functions (PDFs) based on the NNPDF4.0 methodology. This determination is performed at NNLO and approximate N$^3$LO (aN$^3$LO) perturbative QCD accuracy, including QED corrections and a photon PDF up to NLO accuracy. We extract $α_s$ using two independent methodologies, both of which take into account the cross-correlation between $α_s$ and the PDFs. The two methodologies are validated by closure tests that allow us to detect and remove or correct for several sources of bias, and lead to mutually consistent results. We account for all correlated experimental uncertainties, as well as correlated theoretical uncertainties related to missing higher order perturbative corrections (MHOUs). We study the perturbative convergence of our results and the impact of QED corrections. We assess individual sources of uncertainty, specifically MHOUs and the value of the top quark mass. We provide a detailed appraisal of methodological choices, including the choice of input dataset, the form of solution of evolution equation, the treatment of the experimental covariance matrix, and the details of Monte Carlo data generation. We find $α_s(m_Z)=0.1194^{+0.0007}_{-0.0014}$ at aN$^3$LO$_{\rm QCD}\otimes {\rm NLO}_{\rm QED}$ accuracy, consistent with the latest PDG average and with recent lattice results.

hep-ph↗
arXiv2025

Latest neutrino results from the FASER experiment and their implications for forward hadron production

The muon puzzle -- an excess of muons relative to simulation predictions in ultra-high-energy cosmic-ray air showers -- has been reported by many experiments. This suggests that forward particle production in hadronic interactions is not fully understood. Some of the scenarios proposed to resolve this predict reduced production of forward neutral pions and enhanced production of forward kaons (or other particles). The FASER experiment at the LHC is located 480 m downstream of the ATLAS interaction point and is sensitive to neutrinos and muons, which are the decay products of forward charged pions and kaons. In this study, the latest measurements of electron and muon neutrino fluxes are presented using the data corresponding to 9.5 $\mathrm{fb^{-1}}$ and 65.6 $\mathrm{fb^{-1}}$ of proton-proton collisions with $\sqrt{s}=13.6~\mathrm{TeV}$ by the FASER$ν$ and the FASER electronic detector, respectively. These fluxes are compared with predictions from recent hadronic interaction models, including EPOS-LHCr, SIBYLL 2.3e, and QGSJET 3. The predictions are generally consistent with the measured fluxes from FASER, although some discrepancies appear in certain energy bins. More precise flux measurements with additional data will follow soon, enabling validation of pion, kaon, and charm meson production with finer energy binning, reduced uncertainties, and multi-differential analyses.

hep-ex↗
arXiv2025

Higgs trilinear coupling in the standard model effective field theory at the high luminosity LHC and the FCC-ee

Motivated by the updated HL-LHC projections for Higgs pair production from ATLAS and CMS and by the release of the FCC-ee Feasibility Study, we critically revisit the sensitivity of the global SMEFT analysis to deformations of the Higgs self-coupling modifier $κ_3$. To this end, we quantify the impact of SMEFT operators modifying double Higgs production at the LHC and single Higgs production, including loop corrections, at the FCC-ee, and include Renormalisation Group Evolution throughout. We demonstrate that significantly improving on the legacy HL-LHC constraints on $κ_3$ at the FCC-ee is not possible without the $\sqrt{s}=365$ GeV run; that individual and marginalised determinations are similar at the HL-LHC while differing by up to a factor 3 at the FCC-ee; and that quadratic EFT corrections cannot be neglected. Overall, the combination of HL-LHC and FCC-ee data offers unique potential to pin down the Higgs self-coupling with $\sim$$15\%$ precision.

hep-ph↗
arXiv2025

Parton distributions confront LHC Run II data: a quantitative appraisal

We present a systematic comparison of theoretical predictions and various high-precision experimental measurements, specifically of differential cross sections performed by the LHC run II for Drell-Yan gauge boson, top-quark pair, single-inclusive jet and di-jet production, and by HERA for single-inclusive jet and di-jet production. Theoretical predictions are computed at next-to-next-to-leading order (NNLO) accuracy in perturbative Quantum Chromodynamics. The most widely employed sets of Parton Distribution Functions (PDFs) are used, and PDF, strong coupling, and missing higher order uncertainties are taken into account. We quantitatively assess the predictive power of each PDF set and the contribution of the different sources of experimental and theoretical uncertainty to the agreement between data and predictions. We show that control over all of these aspects is crucial to precision physics studies, such as the determination of Standard Model parameters at the LHC.

hep-ph↗
arXiv2025

A First Determination of the LHC Neutrino Fluxes from FASER Data

The detection of TeV neutrinos from the LHC by the far-forward detectors FASER and SND@LHC enables a plethora of novel physics opportunities. Among these, the measurement of the flavour, energy, and rapidity dependence of the LHC forward neutrino fluxes provides unique constraints on theoretical predictions of forward particle production in hadronic collisions. We demonstrate that neutrino event yield measurements at FASER from Run 3 and at its HL-LHC upgrades enable a theory-agnostic extraction of the LHC forward neutrino fluxes. We exploit the equivalence of the problem with the determination of parton distributions from deep-inelastic structure functions to apply the NNPDF approach, based on machine learning regression and the Monte Carlo replica method, to LHC neutrino fluxes. The resulting NN$ν$flux methodology is validated through closure tests and applied to a first extraction of the LHC muon neutrino flux from the FASER 2024 data. We show how NN$ν$flux can discriminate between event generators of forward hadron production; scrutinize a possible intrinsic charm component in the proton; and constrain BSM scenarios with enhanced decays of neutral hadrons into neutrinos.

hep-ph↗
arXiv2025

Combination of aN$^3$LO PDFs and implications for Higgs production cross-sections at the LHC

We discuss how the two existing approximate N$^3$LO (aN$^3$LO) sets of parton distributions (PDFs) from the MSHT20 and NNPDF4.0 series can be combined for LHC phenomenology, both in the pure QCD case and for the QCD$\otimes$QED sets that include the photon PDF. Using the resulting combinations, we present predictions for the total inclusive cross-section for Higgs production in gluon fusion, vector boson fusion, and associated production at the LHC Run-3. For the gluon fusion and vector boson fusion channels, the corrections that arise when using correctly matched aN$^3$LO PDFs with N$^3$LO cross section calculations, compared to using NNLO PDFs, are significant, in many cases larger than the PDF uncertainty, and generally larger than the differences between the two aN$^3$LO PDF sets entering the combination. The combined aN$^3$LO PDF sets, MSHT20xNNPDF40_an3lo and MSHT20xNNPDF40_an3lo_qed, are made publicly available in the LHAPDF format and can be readily used for LHC phenomenology.

hep-ph↗