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.

United States

Roshan Abraham

China

Xiaocong Ai

Switzerland

John Anders

Claire Antel

Japan

Akitaka Ariga

Tomoko Ariga

Jeremy Atkinson

Germany

Florian Bernlochner

Emma Bianchi

Tobias Boeckh

Jamie Boyd

Netherlands

Lydia Brenner

Angela Burger

Franck Cadoux

Roberto Cardella

David Casper

United Kingdom

Charlotte Cavanagh

Xin Chen

Eunhyung Cho

Dhruv Chouhan

Italy

Andrea Coccaro

Stephane Débieux

Monica D’Onofrio

Ansh Desai

Sergey Dmitrievsky

Romania

Radu Dobre

Sinead Eley

Yannick Favre

Deion Fellers

Jonathan Feng

Carlo 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 Lutz

Jack MacDonald

Chiara Magliocca

Toni Mäkelä

Lawson McCoy

Josh McFayden

Andrea Medina

Matteo Milanesio

Théo Moretti

Mitsuhiro Nakamura

Toshiyuki Nakano

Laurie Nevay

Ken Ohashi

Hidetoshi Otono

Lorenzo Paolozzi

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 Valdes Martinez

Svetlana Vasina

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

We propose novel methods to determine the <math><mrow><mi>ϒ</mi><mo>(</mo><mn>4</mn><mi>S</mi><mo>)</mo><mo>→</mo><msup><mrow><mi>B</mi></mrow><mrow><mo>+</mo></mrow></msup><msup><mrow><mi>B</mi></mrow><mrow><mo>−</mo></mrow></msup></mrow></math> and <math><mrow><mi>ϒ</mi><mo>(</mo><mn>4</mn><mi>S</mi><mo>)</mo><mo>→</mo><msup><mrow><mi>B</mi></mrow><mrow><mn>0</mn></mrow></msup><msup><mrow><mover><mrow><mi>B</mi></mrow><mrow><mo>¯</mo></mrow></mover></mrow><mrow><mn>0</mn></mrow></msup></mrow></math> decay rates. The precision to which they and their ratio are known yields at present a limiting uncertainty around 2% in measurements of absolute <math><mi>B</mi></math> decay rates, and thus in a variety of applications, such as precision determinations of elements of the Cabibbo-Kobayashi-Maskawa matrix and flavor-symmetry relations. The new methods we propose are based in one case on exploiting the <math><mi>ϒ</mi><mo>(</mo><mn>5</mn><mi>S</mi><mo>)</mo></math> datasets, in the other case on the different average number of charged tracks in <math><msup><mi>B</mi><mo>±</mo></msup></math> and <math><msup><mi>B</mi><mn>0</mn></msup></math> decays. We estimate future sensitivities using these methods and discuss possible measurements of <math><mrow><msub><mrow><mi>f</mi></mrow><mrow><mi>d</mi></mrow></msub><mo>/</mo><msub><mrow><mi>f</mi></mrow><mrow><mi>u</mi></mrow></msub></mrow></math> at the (HL-)LHC.

Martin Jung

Munira Khan

Greg Landsberg

Zoltan Ligeti

Novel approaches to determine <math><mrow><msup><mrow><mi>B</mi></mrow><mrow><mo>±</mo></mrow></msup></mrow></math> and <math><msup><mi>B</mi><mn>0</mn></msup></math> meson production fractions

We present the first determination of V  cb  from inclusive   <math> <mi>B</mi> <mo>→</mo> <msub> <mi>X</mi> <mi>c</mi> </msub> <mi>ℓ</mi> <msub> <mover> <mi>v</mi> <mo>¯</mo> </mover> <mi>ℓ</mi> </msub> </math>  $$ B\to {X}_c \ell {\overline{v}}_{\ell} $$  using moments of the dilepton invariant mass, q 2. These moments are reparametrization invariant quantities and depend on a reduced set of non-perturbative parameters. This reduced set of parameters opens a new path to extract these parameters up to   <math> <mn>1</mn> <mo>/</mo> <msubsup> <mi>m</mi> <mi>b</mi> <mn>4</mn> </msubsup> </math>  $$ 1/{m}_b^4 $$  purely from data and can potentially reduce the uncertainty on V  cb . In this paper, we present our first determination of V  cb  using this method. Combining the recent measurements of q 2 moments by Belle and Belle II, our default fit gives |V  cb  | = (41.69±0.63)·10 −3. This result presents an important independent cross check of, and is consistent with, the previous state-of-the-art inclusive determinations using lepton energy and hadronic invariant mass moments.

Matteo Fael

Kevin Olschewsky

Eric Persson

Canada

Raynette Tonder

K. Vos

Maximilian Welsch

First extraction of inclusive V  cb  from q 2 moments

<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.

Henso Abreu

Yoav Afik

Jason Arakawa

CERN

David W. Casper

Francesco Cerutti

Candan Dozen

Jonathan L. Feng

Ahmed Ismail

Helena Lefebvre

Sam Meehan

Dimitar Mladenov

Marzio Nessi

Friedemann Neuhaus

Carlo Pandini

Hao Pang

Francesco Pietropaolo

Filippo Resnati

Marta Sabaté-Gilarte

Jakob Salfeld-Nebgen

John Spencer

Poland

Sebastian Trojanowski

Serhan Tufanli

Benedikt Vormwald

Di Wang

Gang Zhang

First neutrino interaction candidates at the LHC

Precise measurements of  $$b\rightarrow c\tau \bar{\nu }$$  <math> <mrow> <mi>b</mi> <mo>→</mo> <mi>c</mi> <mi>τ</mi> <mover> <mrow> <mi>ν</mi> </mrow> <mrow> <mo>¯</mo> </mrow> </mover> </mrow> </math>   decays require large resource-intensive Monte Carlo (MC) samples, which incorporate detailed simulations of detector responses and physics backgrounds. Extracted parameters may be highly sensitive to the underlying theoretical models used in the MC generation. Because new physics (NP) can alter decay distributions and acceptances, the standard practice of fitting NP Wilson coefficients to SM-based measurements of the  $$R(D^{(*)})$$  <math> <mrow> <mi>R</mi> <mo>(</mo> <msup> <mi>D</mi> <mrow> <mo>(</mo> <mrow></mrow> <mo>∗</mo> <mo>)</mo> </mrow> </msup> <mo>)</mo> </mrow> </math>   ratios can be biased. The newly developed Hammer software tool enables efficient reweighting of MC samples to arbitrary NP scenarios or to any hadronic matrix elements. We demonstrate how Hammer allows avoidance of biases through self-consistent fits directly to the NP Wilson coefficients. We also present example analyses that demonstrate the sizeable biases that can otherwise occur from naive NP interpretations of SM-based measurements. The Hammer library is presently interfaced with several existing experimental analysis frameworks and we provide an overview of its structure.

Stephan Duell

Michele Papucci

Dean Robinson

Das ist der HAMMER: consistent new physics interpretations of semileptonic decays