<p>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.</p>

United States

Roshan Mammen Abraham

China

Xiaocong Ai

CERN

John Anders

Switzerland

Claire Antel

Japan

Akitaka Ariga

Tomoko Ariga

Jeremy Atkinson

Germany

Florian U. Bernlochner

Tobias Boeckh

Jamie Boyd

Netherlands

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

Max Fieg

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

Felix Kling

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

Toni Mäkelä

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 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

Jonathan Feng

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

In this paper we study the bounds that can be inferred on New Physics couplings to heavy sterile neutrinos from the recent measurements performed by the Belle collaboration of the angular analysis of $$ B\to {D}^{\ast}\ell {\overline{\nu}}_{\ell } $$ decays, with = , . Indeed, a sterile neutrino may lead to competing $$ B\to {D}^{\ast}\ell \overline{N} $$ decays and Belle might have measured an incoherent sum of these two independent channels. After reviewing the theoretical formalism required to describe this phenomenon in full generality, we first perform a bump hunt in the $$ {M}_{\textrm{miss}}^2 $$ Belle distribution to search for evidences of an additional massive neutrino. We found in such a way a small hint at $$ {M}_{\textrm{miss}}^2 $$ ∼ (354 MeV) . However, the Belle angular analysis is sensitive to masses up to $$ \mathcal{O} $$ (50 MeV), preventing us to further inspect this hint. Nevertheless, we study the potential impact of this additional channel in the allowed mass range on the measured angular distributions and extract model-independent bounds on the new-physics couplings which could mediate such an interaction. In particular, in the mass window here inspected, we obtain the most stringent bounds for vector and left-handed scalar operators to date.

Spain

Marco Fedele

Tim Kretz

Ulrich Nierste

Model independent bounds on heavy sterile neutrinos from the angular distribution of B → D

The Forward Search Experiment (FASER) at CERN’s Large Hadron Collider (LHC) has recently directly detected the first collider neutrinos. Neutrinos play an important role in all FASER analyses, either as signal or background, and it is therefore essential to understand the neutrino event rates. In this study, we update previous simulations and present prescriptions for theoretical predictions of neutrino fluxes and cross sections, together with their associated uncertainties. With these results, we discuss the potential for possible measurements that could be carried out in the coming years with the FASER neutrino data to be collected in LHC Run 3 and Run 4.

Helena Lefebvre

Fulvio Martinelli

Magdalena Munker

Friedemann Neuhaus

Elisa Ruiz-Choliz

Benedikt Vormwald

Di Wang

Yuxiao Wang

Eli Welch

Czechia

Samuel Zahorec

Stefano Zambito

Shunliang Zhang

Neutrino rate predictions for FASER

The first results of the study of high-energy electron neutrino (<math><msub><mi>ν</mi><mi>e</mi></msub></math>) and muon neutrino (<math><msub><mi>ν</mi><mi>μ</mi></msub></math>) charged-current interactions in the <math><mi>FASER</mi><mi>ν</mi></math> emulsion-tungsten detector of the FASER experiment at the LHC are presented. A 128.8 kg subset of the <math><mi>FASER</mi><mi>ν</mi></math> volume was analyzed after exposure to <math><mrow><mn>9.5</mn><mtext> </mtext><mtext> </mtext><msup><mrow><mi>fb</mi></mrow><mrow><mo>−</mo><mn>1</mn></mrow></msup></mrow></math> of <math><msqrt><mi>s</mi></msqrt><mo>=</mo><mn>13.6</mn><mtext> </mtext><mtext> </mtext><mi>TeV</mi></math> <math><mi>p</mi><mi>p</mi></math> data. Four (eight) <math><msub><mi>ν</mi><mi>e</mi></msub></math> (<math><msub><mi>ν</mi><mi>μ</mi></msub></math>) interaction candidate events are observed with a statistical significance of <math><mrow><mn>5.2</mn><mi>σ</mi></mrow></math> (<math><mrow><mn>5.7</mn><mi>σ</mi></mrow></math>). This is the first direct observation of <math><msub><mi>ν</mi><mi>e</mi></msub></math> interactions at a particle collider and includes the highest-energy <math><msub><mi>ν</mi><mi>e</mi></msub></math> and <math><msub><mi>ν</mi><mi>μ</mi></msub></math> ever detected from an artificial source. The interaction cross section per nucleon <math><mi>σ</mi><mo>/</mo><msub><mi>E</mi><mi>ν</mi></msub></math> is measured over an energy range of 560–1740 GeV (520–1760 GeV) for <math><msub><mi>ν</mi><mi>e</mi></msub></math> (<math><msub><mi>ν</mi><mi>μ</mi></msub></math>) to be <math><mrow><mo>(</mo><msubsup><mrow><mn>1.2</mn></mrow><mrow><mo>−</mo><mn>0.7</mn></mrow><mrow><mo>+</mo><mn>0.8</mn></mrow></msubsup><mo>)</mo><mo>×</mo><msup><mrow><mn>10</mn></mrow><mrow><mo>−</mo><mn>38</mn></mrow></msup><mtext> </mtext><mtext> </mtext><msup><mrow><mi>cm</mi></mrow><mrow><mn>2</mn></mrow></msup><mtext> </mtext><msup><mrow><mi>GeV</mi></mrow><mrow><mo>−</mo><mn>1</mn></mrow></msup></mrow></math> [<math><mrow><mo>(</mo><mn>0.5</mn><mo>±</mo><mn>0.2</mn><mo>)</mo><mo>×</mo><msup><mrow><mn>10</mn></mrow><mrow><mo>−</mo><mn>38</mn></mrow></msup><mtext> </mtext><mtext> </mtext><msup><mrow><mi>cm</mi></mrow><mrow><mn>2</mn></mrow></msup><mtext> </mtext><msup><mrow><mi>GeV</mi></mrow><mrow><mo>−</mo><mn>1</mn></mrow></msup></mrow></math>], consistent with standard model predictions. These are the first measurements of neutrino interaction cross sections in those energy ranges.

Haruhi Fujimori

Takumi Kanai

Motoya Nonaka

Kazuaki Okui

First Measurement of <math><msub><mi>ν</mi><mi>e</mi></msub></math> and <math><msub><mi>ν</mi><mi>μ</mi></msub></math> Interaction Cross Sections at the LHC with FASER’s Emulsion Detector

The FASER experiment at the LHC is designed to search for light, weakly-interacting particles produced in proton-proton collisions at the ATLAS interaction point that travel in the far-forward direction. The first results from a search for dark photons decaying to an electron-positron pair, using a dataset corresponding to an integrated luminosity of 27.0 <math><msup><mrow><mtext>fb</mtext></mrow><mrow><mo>−</mo><mn>1</mn></mrow></msup></math> collected at centre-of-mass energy <math><msqrt><mrow><mi>s</mi></mrow></msqrt><mo>=</mo><mn>13.6</mn></math> TeV in 2022 in LHC Run 3, are presented. No events are seen in an almost background-free analysis, yielding world-leading constraints on dark photons with couplings <math><mi>ϵ</mi><mo>∼</mo><mn>2</mn><mo>×</mo><msup><mrow><mn>10</mn></mrow><mrow><mo>−</mo><mn>5</mn></mrow></msup><mo>−</mo><mn>1</mn><mo>×</mo><msup><mrow><mn>10</mn></mrow><mrow><mo>−</mo><mn>4</mn></mrow></msup></math> and masses <math><mo>∼</mo><mn>17</mn><mtext> MeV</mtext><mo>−</mo><mn>70</mn><mtext> MeV</mtext></math>. The analysis is also used to probe the parameter space of a massive gauge boson from a U(1)<math><msub><mrow></mrow><mrow><mi>B</mi><mo>−</mo><mi>L</mi></mrow></msub></math> model, with couplings <math><msub><mrow><mi>g</mi></mrow><mrow><mi>B</mi><mo>−</mo><mi>L</mi></mrow></msub><mo>∼</mo><mn>5</mn><mo>×</mo><msup><mrow><mn>10</mn></mrow><mrow><mo>−</mo><mn>6</mn></mrow></msup><mo>−</mo><mn>2</mn><mo>×</mo><msup><mrow><mn>10</mn></mrow><mrow><mo>−</mo><mn>5</mn></mrow></msup></math> and masses <math><mo>∼</mo><mn>15</mn><mtext> MeV</mtext><mo>−</mo><mn>40</mn><mtext> MeV</mtext></math> excluded for the first time.

Henso Abreu

Monica D'Onofrio

France

Candan Dozen

Poland

Iftah Galon

Sam Meehan

Jakob Salfeld-Nebgen

Sebastian Trojanowski

Search for dark photons with the FASER detector at the LHC

We report the first direct observation of neutrino interactions at a particle collider experiment. Neutrino candidate events are identified in a 13.6 TeV center-of-mass energy <math><mi>p</mi><mi>p</mi></math> collision dataset of <math><mrow><mn>35.4</mn><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 at the Large Hadron Collider. The candidates are required to have a track propagating through the entire length of the FASER detector and be consistent with a muon neutrino charged-current interaction. We infer <math><mrow><msubsup><mrow><mn>153</mn></mrow><mrow><mo>−</mo><mn>13</mn></mrow><mrow><mo>+</mo><mn>12</mn></mrow></msubsup></mrow></math> neutrino interactions with a significance of 16 standard deviations above the background-only hypothesis. These events are consistent with the characteristics expected from neutrino interactions in terms of secondary particle production and spatial distribution, and they imply the observation of both neutrinos and anti-neutrinos with an incident neutrino energy of significantly above 200 GeV.

Tobias Blesgen

Dimitar Mladenov

Marzio Nessi

Francesco Pietropaolo

Filippo Resnati

Serhan Tufanli

First Direct Observation of Collider Neutrinos with FASER at the LHC

Using the GAMBIT global fitting framework, we constrain the MSSM with an eV-scale gravitino as the lightest supersymmetric particle, and the six electroweakinos (neutralinos and charginos) as the only other light new states. We combine 15 ATLAS and 12 CMS searches at 13 TeV, along with a large collection of ATLAS and CMS measurements of Standard Model signatures. This model, which we refer to as the  $${{\tilde{G}}}$$  <math> <mover> <mi>G</mi> <mo>~</mo> </mover> </math>  -EWMSSM, exhibits quite varied collider phenomenology due to its many permitted electroweakino production processes and decay modes. Characteristic  $${{\tilde{G}}}$$  <math> <mover> <mi>G</mi> <mo>~</mo> </mover> </math>  -EWMSSM signal events have two or more Standard Model bosons and missing energy due to the escaping gravitinos. While much of the  $${{\tilde{G}}}$$  <math> <mover> <mi>G</mi> <mo>~</mo> </mover> </math>  -EWMSSM parameter space is excluded, we find several viable parameter regions that predict phenomenologically rich scenarios with multiple neutralinos and charginos within the kinematic reach of the LHC during Run 3, or the High Luminosity LHC. In particular, we identify scenarios with Higgsino-dominated electroweakinos as light as 140 GeV that are consistent with our combined set of collider searches and measurements. The full set of  $${{\tilde{G}}}$$  <math> <mover> <mi>G</mi> <mo>~</mo> </mover> </math>  -EWMSSM parameter samples and GAMBIT input files generated for this work is available via Zenodo.

Norway

Viktor Ananyev

Australia

Csaba Balázs

Ankit Beniwal

Lasse Braseth

Andy Buckley

Jonathan Butterworth

Christopher Chang

Canada

Matthias Danninger

Andrew Fowlie

Tomás Gonzalo

Anders Kvellestad

Farvah Mahmoudi

Gregory Martinez

Tomasz Procter

Are Raklev

Pat Scott

Patrick Stöcker

Jeriek Abeele

Martin White

Yang Zhang

Collider constraints on electroweakinos in the presence of a light gravitino

The search for flavour-changing neutral current effects in B-meson decays is a powerful probe of physics beyond the Standard Model. Deviations from SM behaviour are often quantified by extracting the preferred values of the Wilson coefficients of an operator product expansion. We use the FlavBit module of the GAMBIT package to perform a simultaneous global fit of the Wilson coefficients  $$C_7$$  <math> <msub> <mi>C</mi> <mn>7</mn> </msub> </math>  ,  $$C_9$$  <math> <msub> <mi>C</mi> <mn>9</mn> </msub> </math>  , and  $$C_{10}$$  <math> <msub> <mi>C</mi> <mn>10</mn> </msub> </math>   using a combination of all current data on  $$b{\rightarrow }s\mu ^{+}\mu ^{-}$$  <math> <mrow> <mi>b</mi> <mo>→</mo> <mi>s</mi> <msup> <mi>μ</mi> <mo>+</mo> </msup> <msup> <mi>μ</mi> <mo>-</mo> </msup> </mrow> </math>   transitions. We further extend previous analyses by accounting for the correlated theoretical uncertainties at each point in the Wilson coefficient parameter space, rather than deriving the uncertainties from a Standard Model calculation. We find that the best fit deviates from the SM value with a significance of 6.6 $$\sigma $$  <math> <mi>σ</mi> </math>  . The largest deviation is associated with a vector coupling of muons to b and s quarks.

Jihyun Bhom

Marcin Chrzaszcz

A model-independent analysis of  $$b{\rightarrow }s\mu ^{+}\mu ^{-}$$  <math> <mrow> <mi>b</mi> <mo>→</mo> <mi>s</mi> <msup> <mi>μ</mi> <mo>+</mo> </msup> <msup> <mi>μ</mi> <mo>-</mo> </msup> </mrow> </math>  transitions with GAMBIT ’s FlavBit

<math><mi>FASER</mi><mi>ν</mi></math> at the CERN Large Hadron Collider (LHC) is designed to directly detect collider neutrinos for the first time and study their cross sections at TeV energies, where no such measurements currently exist. In 2018, a pilot detector employing emulsion films was installed in the far-forward region of ATLAS, 480 m from the interaction point, and collected <math><mrow><mn>12.2</mn><mtext> </mtext><mtext> </mtext><msup><mrow><mi>fb</mi></mrow><mrow><mo>−</mo><mn>1</mn></mrow></msup></mrow></math> of proton-proton collision data at a center-of-mass energy of 13 TeV. We describe the analysis of this pilot run data and the observation of the first neutrino interaction candidates at the LHC. This milestone paves the way for high-energy neutrino measurements at current and future colliders.

First neutrino interaction candidates at the LHC