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$\textit{ν}$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$\textit{ν}$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.

Netherlands

Jukka John

United States

Germany

Felix Kling

Jelle Koorn

Peter Krack

Juan Rojo

A first determination of the LHC neutrino fluxes from FASER data

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 $${\mathcal {L}}_{{\textrm{pp}}}=250$$ fb $$^{-1},$$ more than $$10^5$$ inclusive muon DIS events, of which up to $$10^4$$ from charm production, are expected at FASER $$\nu .$$ As a representative application, we demonstrate the sensitivity of muon DIS at FASER $$\nu $$ to probe the (intrinsic) charm content of the proton at large-$\textit{x}$. We also provide predictions for event yields of muon DIS for future FASER runs and for the proposed Forward Physics Facility.

Brazil

Reinaldo Francener

Victor Gonçalves

Deep-inelastic scattering at TeV energies with LHC muons

The first direct detection of neutrinos at the LHC not only marks the beginning of a novel collider neutrino program at CERN but also motivates considering additional neutrino detectors to fully exploit the associated physics potential. As the existing forward neutrino detectors are located underground, it is interesting to investigate the feasibility and physics potential of neutrino experiments located at the surface-level. A topographic desk study is performed to identify all points at which the LHC’s neutrino beams exit the earth. The closest location lies about 9 km east of the CMS interaction point, at the bottom of Lake Geneva. Several detectors to be placed at this location are considered, including a water Cherenkov detector and an emulsion detector. The detector designs are outlined at a conceptual level, and projections for their contribution to the LHC forward neutrino program and searches for dark sector particles are presented. However, the dilution of the neutrino flux over distance reduces the neutrino yield significantly, necessitating large and coarse detector designs. We identify the experimental challenges to be overcome by future research, and conclude that at present the physics potential of surface-level detectors is limited in comparison to ones closer to the interaction point, including the proposed Forward Physics Facility.

Switzerland

Japan

Akitaka Ariga

Steven Barwick

Jamie Boyd

Max Fieg

Toni Mäkelä

Camille Vendeuvre

Benjamin Weyer

Detecting LHC neutrinos at surface level

This Letter presents the measurement of the energy-dependent neutrino-nucleon cross section in tungsten and the differential flux of muon neutrinos and antineutrinos. The analysis is performed using proton-proton collision data at a center-of-mass energy of 13.6 TeV and corresponding to an integrated luminosity of <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><mo stretchy="false">(</mo><mn>65.6</mn><mo>±</mo><mn>1.4</mn><mo stretchy="false">)</mo><mtext> </mtext><mtext> </mtext><msup><mrow><mi>fb</mi></mrow><mrow><mo>−</mo><mn>1</mn></mrow></msup></mrow></math>. Using the active electronic components of the FASER detector, <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mn>338.1</mn><mo>±</mo><mn>21.0</mn></math> charged current muon neutrino interaction events are identified, with backgrounds from other processes subtracted. We unfold the neutrino events into a fiducial volume corresponding to the sensitive regions of the FASER detector and interpret the results in two ways: (i) we use the expected neutrino flux to measure the cross section, and (ii) we use the predicted cross section to measure the neutrino flux. Both results are presented in six bins of neutrino energy, achieving the first differential measurement in the TeV range. The observed distributions align with standard model predictions. Using this differential data, we extract the contributions of neutrinos from pion and kaon decays.

Roshan Mammen Abraham

China

Xiaocong Ai

CERN

John Anders

Claire Antel

Tomoko Ariga

Jeremy Atkinson

Florian U. Bernlochner

Tobias Boeckh

Lydia Brenner

Angela Burger

Franck Cadoux

Roberto Cardella

David W. Casper

Charlotte Cavanagh

Xin Chen

Dhruv Chouhan

Italy

Andrea Coccaro

Stephane Débieux

United Kingdom

Monica D’Onofrio

Ansh Desai

Sergey Dmitrievsky

Romania

Radu Dobre

Sinead Eley

Yannick Favre

Deion Fellers

Jonathan L. Feng

Carlo Alberto Fenoglio

Didier Ferrere

Wissal Filali

Elena Firu

Edward Galantay

Ali Garabaglu

Stephen Gibson

Sergio Gonzalez-Sevilla

Yuri Gornushkin

Carl Gwilliam

Daiki Hayakawa

Michael Holzbock

Shih-Chieh Hsu

Zhen Hu

Giuseppe Iacobucci

Tomohiro Inada

Luca Iodice

Sune Jakobsen

Hans Joos

Israel

Enrique Kajomovitz

Hiroaki Kawahara

Alex Keyken

Daniela Köck

Pantelis Kontaxakis

Umut Kose

Rafaella Kotitsa

Susanne Kuehn

Thanushan Kugathasan

Lorne Levinson

Ke Li

Jinfeng Liu

Yi Liu

Margaret S. Lutz

Jack MacDonald

Chiara Magliocca

Lawson McCoy

Josh McFayden

Andrea Pizarro Medina

Matteo Milanesio

Théo Moretti

Mitsuhiro Nakamura

Toshiyuki Nakano

Laurie Nevay

Ken Ohashi

Hidetoshi Otono

Hao Pang

Lorenzo Paolozzi

Pawan Pawan

Brian Petersen

Titi Preda

Markus Prim

Michaela Queitsch-Maitland

Hiroki Rokujo

André Rubbia

Jorge Sabater-Iglesias

Osamu Sato

Paola Scampoli

Kristof Schmieden

Matthias Schott

Anna Sfyrla

Davide Sgalaberna

Mansoora Shamim

Savannah Shively

Yosuke Takubo

Noshin Tarannum

Ondrej Theiner

Eric Torrence

Oscar Ivan Valdes Martinez

Svetlana Vasina

First Measurement of the Muon Neutrino Interaction Cross Section and Flux as a Function of Energy at the LHC with FASER

The recent direct detection of neutrinos at the LHC has opened a new window on high-energy particle physics and highlighted the potential of forward physics for groundbreaking discoveries. In the last year, the physics case for forward physics has continued to grow, and there has been extensive work on defining the Forward Physics Facility and its experiments to realize this physics potential in a timely and cost-effective manner. Following a 2-page Executive Summary, we first present the status of the FPF, beginning with the FPF’s unique potential to shed light on dark matter, new particles, neutrino physics, QCD, and astroparticle physics. We then summarize the current designs for the Facility and its experiments, FASER2, FASER $$\nu $$ 2, FORMOSA, and FLArE.

Poland

Jyotismita Adhikary

Luis Anchordoqui

Alan Barr

Brian Batell

Jianming Bian

Matthew Citron

Albert Roeck

Milind Diwan

Jonathan Feng

Christopher Hill

Yu Jeong

Steven Linden

Kostas Mavrokoridis

Dennis Soldin

Anna Stasto

Sebastian Trojanowski

Matteo Vicenzi

Wenjie Wu

Scientific program for the Forward Physics Facility

With no concrete evidence for nongravitational interactions of dark matter to date, it is natural to wonder whether dark matter couples predominantly to the Standard Model (SM)’s neutrinos. Neutrino interactions (and the possible existence of additional neutrinophilic mediators) are substantially less understood than those of other SM particles, yet this picture will change dramatically in the coming decades with new neutrino sources. One potential new source arises with the construction of a high-energy muon collider (MuCol); due to muons’ instability, a MuCol is a source of high-energy collimated neutrinos. Importantly, since the physics of muon decays (into neutrinos) is very well-understood, this leads to a neutrino flux with systematic uncertainties far smaller than fluxes from conventional high-energy (proton-sourced) neutrino beams. In this work, we study the capabilities of a potential neutrino detector, “<math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><mi>MuCol</mi><mi>ν</mi></mrow></math>”, placed <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mo>∼</mo><mn>100</mn><mtext> </mtext><mtext> </mtext><mi mathvariant="normal">m</mi></math> downstream of the MuCol interaction point. The <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><mi>MuCol</mi><mi>ν</mi></mrow></math> detector would be especially capable of searching for a neutrinophilic mediator <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>ϕ</mi></math> through the mononeutrino scattering process <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msub><mi>ν</mi><mi>μ</mi></msub><mi>N</mi><mo stretchy="false">→</mo><msup><mi>μ</mi><mo>+</mo></msup><mi>ϕ</mi><mi>X</mi></math>, exceeding searches from other terrestrial approaches for <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msub><mi>m</mi><mi>ϕ</mi></msub></math> in the <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><mo>∼</mo><mi>few</mi></mrow></math> MeV- ten GeV range. Even with a 10 kg-yr exposure, <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><mi>MuCol</mi><mi>ν</mi></mrow></math> is capable of searching for well-motivated classes of thermal freeze-out and freeze-in neutrino-portal dark matter.

Kevin J. Kelly

Neutrino-portal dark matter detection prospects at a future muon collider

The first FASER search for a light, long-lived particle decaying into a pair of photons is reported. The search uses LHC proton-proton collision data at $$ \sqrt{s} $$ = 13 6 TeV collected in 2022 and 2023, corresponding to an integrated luminosity of 57 7 fb 1 . A model with axion-like particles (ALPs) dominantly coupled to weak gauge bosons is the primary target. Signal events are characterised by high-energy deposits in the electromagnetic calorimeter and no signal in the veto scintillators. One event is observed, compared to a background expectation of 0 44 ± 0 39 events, which is entirely dominated by neutrino interactions. World-leading constraints on ALPs are obtained for masses up to 300 MeV and couplings to the Standard Model W gauge boson, , around 10 4 GeV 1 , testing a previously unexplored region of parameter space. Other new particle models that lead to the same experimental signature, including ALPs coupled to gluons or photons, U(1) gauge bosons, up-philic scalars, and a Type-I two-Higgs doublet model, are also considered for interpretation, and new constraints on previously viable parameter space are presented in this paper.

Roshan Abraham

Florian Bernlochner

Emma Bianchi

David Casper

Eunhyung Cho

Carlo Fenoglio

Margaret Lutz

Andrea Medina

Oscar Valdes Martinez

Shining light on the dark sector: search for axion-like particles and other new physics in photonic final states with FASER

The LHC produces an intense beam of highly energetic neutrinos of all three flavors in the forward direction, and the Forward Physics Facility (FPF) has been proposed to house a suite of experiments taking advantage of this opportunity. In this study, we investigate the FPF’s potential to probe the neutrino electromagnetic properties, including neutrino millicharge, magnetic moment, and charge radius. We find that, due to the large flux of tau neutrinos at the LHC, the FPF detectors will be able to provide more sensitive constraints on the tau neutrino magnetic moment and millicharge than previous measurements at DONUT, by searching for excess in low recoil energy electron scattering events. We also find that, by precisely measuring the rate of neutral current deep inelastic scattering events, the FPF detectors have the potential to obtain the strongest experimental bounds on the neutrino charge radius for the electron neutrino, and one of the leading bounds for the muon neutrino flavor. The same signature could also be used to measure the weak mixing angle, and we estimate that <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><msup><mrow><mi>sin</mi></mrow><mrow><mn>2</mn></mrow></msup><msub><mrow><mi>θ</mi></mrow><mrow><mi>W</mi></mrow></msub></mrow></math> could be measured to about 3% precision at a scale <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>Q</mi><mo>∼</mo><mn>10</mn><mtext> </mtext><mtext> </mtext><mi>GeV</mi></math>, shedding new light on the longstanding NuTeV anomaly.

Canada

Saeid Foroughi-Abari

Yu-Dai Tsai

Neutrino electromagnetic properties and the weak mixing angle at the LHC Forward Physics Facility

Proton-proton collisions at energy-frontier facilities produce an intense flux of high-energy light particles, including neutrinos, in the forward direction. At the LHC, these particles are currently being studied with the far-forward experiments FASER/FASER and SND@LHC, while new dedicated experiments have been proposed in the context of a Forward Physics Facility (FPF) operating at the HL-LHC. Here we present a first quantitative exploration of the reach for neutrino, QCD, and BSM physics of far-forward experiments integrated within the proposed Future Circular Collider (FCC) project as part of its proton-proton collision program (FCC-hh) at $$ \sqrt{s} $$ ≃ 100 TeV. We find that 10 electron/muon neutrinos and 10 tau neutrinos could be detected, an increase of several orders of magnitude compared to (HL-)LHC yields. We study the impact of neutrino DIS measurements at the FPF@FCC to constrain the unpolarised and spin partonic structure of the nucleon and assess their sensitivity to nuclear dynamics down to ∼ 10 with neutrinos produced in proton-lead collisions. We demonstrate that the FPF@FCC could measure the neutrino charge radius for and and reach down to five times the SM value for . We fingerprint the BSM sensitivity of the FPF@FCC for a variety of models, including dark Higgs bosons, relaxion-type scenarios, quirks, and millicharged particles, finding that these experiments would be able to discover LLPs with masses as large as 50 GeV and couplings as small as 10 8 , and quirks with masses up to 10 TeV. Our study highlights the remarkable opportunities made possible by integrating far-forward experiments into the FCC project, and it provides new motivation for the FPF at the HL-LHC as an essential precedent to optimize the forward physics experiments that will enable the FCC to achieve its full physics potential.

Jinmian Li

Junle Pei

Tanjona Rabemananjara

FPF@FCC: neutrino, QCD, and BSM physics opportunities with far-forward experiments at a 100 TeV Proton Collider

Heavy neutral leptons (HNLs) are motivated by attempts to explain neutrino masses and dark matter. If their masses are in the MeV to several GeV range, HNLs are light enough to be copiously produced at collider and accelerator facilities, but also heavy enough to decay to visible particles on length scales that can be observed in particle detectors. Previous studies evaluating the sensitivities of experiments have often focused on simple but not particularly well-motivated models in which the HNL mixes with only one active neutrino flavor. In this work, we accurately simulate models for HNL masses between 100 MeV and 10 GeV and arbitrary couplings to <math><mi>e</mi></math>, <math><mi>μ</mi></math>, and <math><mi>τ</mi></math> leptons. We include over 150 HNL production channels and over 100 HNL decay modes, including all of the processes that can be dominant in some region of the general parameter space. The result is HNLCalc, a user-friendly, fast, and flexible library to compute the properties of HNL models. As examples, we implement HNLCalc to extend the FORESEE package to evaluate the prospects for HNL discovery at forward LHC experiments. We present sensitivity reaches for FASER and FASER2 in five benchmark scenarios with coupling ratios <math><mrow><mo>|</mo><msub><mrow><mi>U</mi></mrow><mrow><mi>e</mi></mrow></msub><msup><mrow><mo>|</mo></mrow><mn>2</mn></msup><mo>∶</mo><mo>|</mo><msub><mrow><mi>U</mi></mrow><mrow><mi>μ</mi></mrow></msub><msup><mrow><mo>|</mo></mrow><mn>2</mn></msup><mo>∶</mo><mo>|</mo><msub><mrow><mi>U</mi></mrow><mrow><mi>τ</mi></mrow></msub><msup><mrow><mo>|</mo></mrow><mn>2</mn></msup><mo>=</mo><mn>1</mn><mo>∶</mo><mn>0</mn><mo>∶</mo><mn>0</mn></mrow></math>, <math><mrow><mn>0</mn><mo>∶</mo><mn>1</mn><mo>∶</mo><mn>0</mn></mrow></math>, <math><mrow><mn>0</mn><mo>∶</mo><mn>0</mn><mo>∶</mo><mn>1</mn></mrow></math>, <math><mrow><mn>0</mn><mo>∶</mo><mn>1</mn><mo>∶</mo><mn>1</mn></mrow></math>, and <math><mrow><mn>1</mn><mo>∶</mo><mn>1</mn><mo>∶</mo><mn>1</mn></mrow></math>, where the latter two have not been studied previously. Comparing these to current constraints, we identify regions of parameter space with significant discovery prospects.

Alec Hewitt

Daniel La Rocco

Simulating heavy neutral leptons with general couplings at collider and fixed target experiments