Belgium

Chiara Arina

France

Benjamin Fuks

Italy

Luca Panizzi

United States

Michael Baker

South Africa

Alan Cornell

Germany

Jan Heisig

United Kingdom

Benedikt Maier

Portugal

Rute Pedro

Dominique Trischuk

Diyar Agin

Alexandre Arbey

Giorgio Arcadi

Emanuele Bagnaschi

Kehang Bai

Brazil

Disha Bhatia

Mathias Becker

Alexander Belyaev

Ferdinand Benoit

Monika Blanke

Canada

Jackson Burzynski

Jonathan Butterworth

Antimo Cagnotta

China

Lorenzo Calibbi

Linda Carpenter

Spain

Xabier Cid Vidal

Emanuele Copello

Louie Corpe

Francesco D’Eramo

Aldo Deandrea

Australia

Aman Desai

Caterina Doglioni

South Korea

Sunil Dogra

Mathias Garny

Mark Goodsell

Norway

Sohaib Hassan

Philip Harris

Julia Harz

Alejandro Ibarra

Alberto Iorio

Felix Kahlhoefer

Deepak Kar

Egypt

Shaaban Khalil

Valery Khoze

Pyungwon Ko

Sabine Kraml

Greg Landsberg

Andre Lessa

Laura Lopez-Honorez

Alberto Mariotti

Vasiliki Mitsou

Kirtimaan Mohan

Chang-Seong Moon

Colombia

Alexander Moreno Briceno

Maria Moreno Llacer

Leandre Munoz-Aillaud

Taylor Murphy

Anele Ncube

Wandile Nzuza

Clarisse Prat

Lena Rathmann

Thobani Sangweni

Dipan Sengupta

William Shepherd

Sukanya Sinha

Tim Tait

Andrea Thamm

Michel Tytgat

Zirui Wang

David Yu

Shin-Shan Yu

Publisher Erratum: $\textit{t}$-channel dark matter models – a whitepaper

This report, summarising work achieved in the context of the LHC Dark Matter Working Group, investigates the phenomenology of $\textit{t}$-channel dark matter models, spanning minimal setups with a single dark matter candidate and mediator to more complex constructions closer to UV-complete models. For each considered class of models, we examine collider, cosmological and astrophysical implications. In addition, we explore scenarios with either promptly decaying or long-lived particles, as well as featuring diverse dark matter production mechanisms in the early universe. By providing a unified analysis framework, numerical tools and guidelines, this work aims to support future experimental and theoretical efforts in exploring $\textit{t}$-channel dark matter models at colliders and in cosmology.

$\textit{t}$-channel dark matter models – a whitepaper

We study the possibility of observing a light pseudo-scalar a at LHCb. We target the mass region  $$2.5\,\mathrm{GeV}\lesssim m_a\lesssim 60\,\mathrm{GeV}$$  <math> <mrow> <mn>2.5</mn> <mspace width="0.166667em"></mspace> <mi>GeV</mi> <mo>≲</mo> <msub> <mi>m</mi> <mi>a</mi> </msub> <mo>≲</mo> <mn>60</mn> <mspace width="0.166667em"></mspace> <mi>GeV</mi> </mrow> </math>   and various decay channels, some of which have never been considered before: muon pairs, tau pairs, D meson pairs, and di-photon. We interpret the results in the context of models of 4D Composite Higgs and Partial Compositeness in particular.

Sweden

Diogo Buarque Franzosi

Giacomo Cacciapaglia

Gabriele Ferretti

Thomas Flacke

CERN

Carlos Vázquez Sierra

Exploring new possibilities to discover a light pseudo-scalar at LHCb

A model that can simultaneously explain Dark Matter relic density and the apparent matter anti-matter imbalance of the universe has been recently proposed. The model requires b-hadron branching fractions to Dark Matter at the per mille level. The b-hadrons decay to a dark sector baryon,  $$\psi _{\mathrm{DS}}$$  <math> <msub> <mi>ψ</mi> <mi>DS</mi> </msub> </math>  , which has a mass in the region  $$940 ~\mathrm{MeV}/c^2\le m_{\psi _{\mathrm{DS}}}\le 4430~\mathrm{MeV}/c^2$$  <math> <mrow> <mn>940</mn> <mspace width="3.33333pt"></mspace> <mi>MeV</mi> <mo>/</mo> <msup> <mi>c</mi> <mn>2</mn> </msup> <mo>≤</mo> <msub> <mi>m</mi> <msub> <mi>ψ</mi> <mi>DS</mi> </msub> </msub> <mo>≤</mo> <mn>4430</mn> <mspace width="3.33333pt"></mspace> <mi>MeV</mi> <mo>/</mo> <msup> <mi>c</mi> <mn>2</mn> </msup> </mrow> </math>  . In this paper, we discuss the sensitivity of the LHCb experiment to search for this dark baryon, covering different types of topology and giving prospects for Runs 3 and 4 of the LHC, as well as for the proposed Upgrade II. We show that the LHCb experiment can cover the entire mass range of the hypothetical dark baryon.

Alexandre Brea Rodríguez

Veronika Chobanova

Saúl López Soliño

Diego Martínez Santos

Titus Mombächer

Claire Prouvé

Emilio Rodríguez Fernández

Prospects on searches for baryonic Dark Matter produced in  $${\varvec{b}}$$  <math> <mrow> <mi>b</mi> </mrow> </math>  -hadron decays at LHCb

This document presents the physics case and ancillary studies for the proposed CODEX-b long-lived particle (LLP) detector, as well as for a smaller proof-of-concept demonstrator detector, CODEX- $$\beta $$  <math> <mi>β</mi> </math>  , to be operated during Run 3 of the LHC. Our development of the CODEX-b physics case synthesizes ‘top-down’ and ‘bottom-up’ theoretical approaches, providing a detailed survey of both minimal and complete models featuring LLPs. Several of these models have not been studied previously, and for some others we amend studies from previous literature: In particular, for gluon and fermion-coupled axion-like particles. We moreover present updated simulations of expected backgrounds in CODEX-b’s actively shielded environment, including the effects of shielding propagation uncertainties, high-energy tails and variation in the shielding design. Initial results are also included from a background measurement and calibration campaign. A design overview is presented for the CODEX- $$\beta $$  <math> <mi>β</mi> </math>   demonstrator detector, which will enable background calibration and detector design studies. Finally, we lay out brief studies of various design drivers of the CODEX-b experiment and potential extensions of the baseline design, including the physics case for a calorimeter element, precision timing, event tagging within LHCb, and precision low-momentum tracking.

Giulio Aielli

Eli Ben-Haim

Roberto Cardarelli

Matthew Charles

Victor Coco

Hungary

Biplab Dey

Raphael Dumps

Jared Evans

George Gibbons

Olivier Dortz

Vladimir Gligorov

Philip Ilten

Simon Knapen

Jongho Lee

Saul López Soliño

Benjamin Nachman

Michele Papucci

Francesco Polci

Robin Quessard

Harikrishnan Ramani

Dean Robinson

Heinrich Schindler

Michael Sokoloff

Paul Swallow

Riccardo Vari

Nigel Watson

Mike Williams

Expression of interest for the CODEX-b detector

We study the potential of the LHCb experiment to discover, for the first time, the <math><mrow><msup><mrow><mi>μ</mi></mrow><mrow><mo>+</mo></mrow></msup><msup><mrow><mi>μ</mi></mrow><mrow><mo>−</mo></mrow></msup></mrow></math> true muonium bound state. We propose a search for the vector <math><mrow><msup><mrow><mn>1</mn></mrow><mrow><mn>3</mn></mrow></msup><msub><mrow><mi>S</mi></mrow><mrow><mn>1</mn></mrow></msub></mrow></math> state, <math><mi>TM</mi></math>, which kinetically mixes with the photon and dominantly decays to <math><mrow><msup><mrow><mi>e</mi></mrow><mrow><mo>+</mo></mrow></msup><msup><mrow><mi>e</mi></mrow><mrow><mo>−</mo></mrow></msup></mrow></math>. We demonstrate that a search for <math><mrow><mi>η</mi><mo>→</mo><mi>γ</mi><mi>TM</mi></mrow></math>, <math><mrow><mi>TM</mi><mo>→</mo><msup><mrow><mi>e</mi></mrow><mrow><mo>+</mo></mrow></msup><msup><mrow><mi>e</mi></mrow><mrow><mo>−</mo></mrow></msup></mrow></math> in a displaced vertex can exceed a significance of 5 standard deviations assuming statistical uncertainties. We present two possible searches: an inclusive search for the <math><mrow><msup><mrow><mi>e</mi></mrow><mrow><mo>+</mo></mrow></msup><msup><mrow><mi>e</mi></mrow><mrow><mo>−</mo></mrow></msup></mrow></math> vertex, and an exclusive search which requires an additional photon and a reconstruction of the <math><mi>η</mi></math> mass.