We suggest a simplified model that simultaneously addresses the dark matter problem and gives rise to top-quark flavor changing neutral current (FCNC) interactions at the one-loop order. The model consists of two extra <math><mi>S</mi><mi>U</mi><mo>(</mo><mn>2</mn><msub><mo>)</mo><mi>L</mi></msub></math> gauge singlets: a colored mediator of spin zero (<math><mi>S</mi></math>) and a right-handed fermion (<math><mi>χ</mi></math>); both are odd under an ad hoc <math><msub><mi>Z</mi><mn>2</mn></msub></math> symmetry. The right-handed fermion plays the role of the dark matter candidate. In this model, the presence of the two dark sector particles generates one-loop induced FCNC decays of the top quark into light quarks and bosons such as the gluon, the photon, the <math><mi>Z</mi></math> boson, or the Higgs boson. As a case study, we analyze the top-quark FCNC decays into light quarks (<math><mi>u</mi></math> or <math><mi>c</mi></math>) and <math><mi>Z</mi></math> or Higgs bosons. We then study the reliable solutions to the dark matter problem by estimating the regions in the parameter space that are consistent with the Planck measurement of the dark matter relic density. We also revisit the bounds from the searches of dark matter in events with at least one high-<math><msub><mi>p</mi><mi>T</mi></msub></math> jet and large missing transverse energy at the Large Hadron Collider (LHC). We then define four benchmark points that are consistent with the existing constraints from collider experiments and cosmology. We finally estimate, for these benchmark scenarios, the rates of a broad range of channels that can be used to probe the connection between the top FCNC transitions and dark matter, both at the HL-LHC and at a future 100 TeV collider.

South Korea

Adil Jueid

Japan

Shinya Kanemura

Dark matter as the trigger of flavor changing neutral current decays of the top quark

The Zee-Babu model is a minimal realization of radiative neutrino mass generation mechanism at the two-loop level. We study the phenomenology of this model at future multi-TeV muon colliders. After imposing all theoretical and low-energy experimental constraints on the model parameters, we find that the Zee-Babu states are expected not to reside below the TeV scale, making it challenging to probe them at the LHC. We first analyze the production rates for various channels, including multi singly charged and/or doubly charged scalars at muon colliders. For concreteness, we study several benchmark points that satisfy neutrino oscillation data and other constraints and find that most channels have large production rates. We then analyze the discovery reach of the model using two specific channels: the pair production of singly and doubly charged scalars. For the phenomenologically viable scenarios considered in this study, charged scalars with masses up to <math><mrow><mi>O</mi><mo>(</mo><mn>3</mn><mi>–</mi><mn>4</mn><mo>)</mo><mtext> </mtext><mtext> </mtext><mi>TeV</mi></mrow></math> can be probed for the center-of-mass energy of 10 TeV and total luminosity of <math><mrow><mn>10</mn><mtext> </mtext><mtext> </mtext><msup><mrow><mi>ab</mi></mrow><mrow><mo>−</mo><mn>1</mn></mrow></msup></mrow></math>.

Bangladesh

Talal Ahmed Chowdhury

Italy

Salah Nasri

Switzerland

Shaikh Saad

Probing Zee-Babu states at muon colliders

In this work, we delve into the often overlooked cosmological implications of the spontaneous breaking of the non-Abelian discrete groups, specifically focusing on the formation of domain walls in the case of <math><msub><mi>S</mi><mn>4</mn></msub></math> flavor symmetry. In particular, we investigate three interesting breaking patterns of the <math><msub><mi>S</mi><mn>4</mn></msub></math> group and study the structure of the domain walls in the broken phase for three possible residual symmetries. The presentation of domain walls in the case of multiple vacua is usually complicated, which therefore implies that most of the analyses only approximate their presentation. Here, we propose a subtle way to represent the <math><msub><mi>S</mi><mn>4</mn></msub></math> domain wall networks by presenting the vacua in each breaking pattern as vectors with their components corresponding to their coordinates in the flavon space. Then, through the properties of the obtained vectors, we find that the domain wall networks can be represented by Platonic or Archimedean solids where the vertices represent the degenerate vacua while the edges correspond to the domain walls that separate them. As an illustration, we propose a type-II seesaw model that is based on the <math><msub><mi>S</mi><mn>4</mn></msub></math> flavor symmetry, and study its phenomenological implications on the neutrino sector. To solve the domain wall problem within this toy model, we consider an approach based on high-dimensional effective operators induced by gravity that explicitly break the structure of the induced vacua favoring one vacuum over the others.

United Arab Emirates

Mohamed Amin Loualidi

Morocco

Mohamed Amine Ouahid

Cosmological domain walls from the breaking of <math><msub><mi>S</mi><mn>4</mn></msub></math> flavor symmetry

We explore the potential of the Large Hadron Collider (LHC) in detecting a signal originating from the production of a heavy SU(2) R  charged gauge boson that then decays into a top-bottom quark pair via the mediation of a right-handed neutrino, pp → W  R  → N  R  ℓ → (ℓ  ′  tb)ℓ. Such a channel, that we study in the context of the minimal Left-Right Symmetric Model, contrasts with conventional smoking-gun signatures targeted experimentally and phenomenologically in which only light quarks are involved. We propose a selection strategy aimed at extracting such a top-bottom signal and we estimate the resulting sensitivity of the LHC to the model. Our results demonstrate the potential impact of such a search and we therefore urge the experimental collaborations to carry out a similar analysis in the light of present and future data.

Canada

Mariana Frank

France

Benjamin Fuks

United Kingdom

Sweden

Stefano Moretti

Özer Özdal

A novel search strategy for right-handed charged gauge bosons at the Large Hadron Collider

In dark-matter annihilation channels to hadronic final states, stable particles — such as positrons, photons, antiprotons, and antineutrinos — are produced via complex sequences of phenomena including QED/QCD radiation, hadronisation, and hadron decays. These processes are normally modelled by Monte Carlo (MC) event generators whose limited accuracy imply intrinsic QCD uncertainties on the predictions for indirect-detection experiments like Fermi-LAT, Pamela, IceCube or Ams–02. In this article, we perform a comprehensive analysis of QCD uncertainties, meaning both perturbative and nonperturbative sources of uncertainty are included — estimated via variations of MC renormalization-scale and fragmentation-function parameters, respectively — in antimatter spectra from dark-matter annihilation, based on parametric variations of the Pythia 8 event generator. After performing several retunings of light-quark fragmentation functions, we define a set of variations that span a conservative estimate of the QCD uncertainties. We estimate the effects on antimatter spectra for various annihilation channels and final-state particle species, and discuss their impact on fitted values for the dark-matter mass and thermally-averaged annihilation cross section. We find dramatic impacts which can go up to   <math> <mi>O</mi> </math>  $$ \mathcal{O} $$ (40) GeV for uncertainties on the dark-matter mass and up to   <math> <mi>O</mi> </math>  $$ \mathcal{O} $$ (10%) for the annihilation cross section. We provide the spectra in tabulated form including QCD uncertainties and code snippets to perform fast dark-matter fits, in this github repository.

Netherlands

Jochem Kip

Spain

Roberto Ruiz de Austri

Australia

Peter Skands

The Strong Force meets the Dark Sector: a robust estimate of QCD uncertainties for anti-matter dark matter searches

Abstract The search for dark matter (DM) candidates at high-energy colliders is one of the most promising avenues to understand the nature of this elusive component of the universe. Several searches at the Large Hadron Collider (LHC) have strongly constrained a wide range of simplified models. The combination of the bounds from the LHC with direct-detection experiments exclude the most minimal scalar-singlet DM model. To address this, lepton portal DM models are suitable candidates where DM is predominantly produced at lepton colliders since the DM candidate only interacts with the lepton sector through a mediator that carries a lepton number. In this work, we analyze the production of DM pairs in association with a Higgs boson decaying into two bottom quarks at future muon colliders in the framework of the minimal lepton portal DM model. It is found that the usual cut-based analysis methods fail to probe heavy DM masses for both the resolved (where the decay products of the Higgs boson can be resolved as two well-separated small-R jets) and the merged (where the Higgs boson is clustered as one large-R jet) regimes. We have then built a search strategy based on boosted-decision trees (BDTs). We have optimized the hyperparameters of the BDT model to both have a high signal-to-background ratio and to avoid overtraining effects. We have found very important enhancements of the signal significance with respect to the cut-based analysis by factors of 8–50 depending on the regime (resolved or merged) and the benchmark points. Using this BDT model on a 1D parameter space scan, we found that future muon colliders with  TeV and  can exclude DM masses up to 1 TeV at the 95% confidence level.

Mohamed Belfkir

Boosting dark matter searches at muon colliders with machine learning: The mono-Higgs channel as a case study

We conduct a comprehensive analysis of the potential for discovering lepton flavor violation (LFV) phenomena associated with the <math><mi>Z</mi></math> and Higgs bosons at the LHeC and FCC-he. Our thorough investigation highlights the exceptional suitability of electron-proton colliders for probing these rare new physics signals, due to their distinct advantages, such as negligible pileups, minimal QCD backgrounds, electron-beam polarization <math><msub><mi>P</mi><mi>e</mi></msub></math>, and the ability to distinguish charged-current from neutral-current processes. In our analysis of LFV of the <math><mi>Z</mi></math> boson, we employ an innovative indirect probe, utilizing the <math><mi>t</mi></math>-channel mediation of the <math><mi>Z</mi></math> boson in the process <math><mi>p</mi><msup><mi>e</mi><mo>−</mo></msup><mo>→</mo><mi>j</mi><msup><mi>τ</mi><mo>−</mo></msup></math>. For LFV in the Higgs sector, we examine direct observations of the on-shell decays of <math><mi>H</mi><mo>→</mo><msup><mi>e</mi><mo>+</mo></msup><msup><mi>τ</mi><mo>−</mo></msup></math> and <math><mi>H</mi><mo>→</mo><msup><mi>μ</mi><mo>±</mo></msup><msup><mi>τ</mi><mo>∓</mo></msup></math> through the charged-current production of <math><mi>H</mi></math>. Focusing on <math><mi>H</mi><mo>→</mo><msup><mi>e</mi><mo>+</mo></msup><msup><mi>τ</mi><mo>−</mo></msup></math> is highly efficient due to the absence of positron-related backgrounds in the charged-current modes at electron-proton colliders. Through a dedicated signal-to-background analysis using the boosted decision tree algorithm, we show that the LHeC with a total integrated luminosity of <math><mn>1</mn><mtext> </mtext><msup><mrow><mi>ab</mi></mrow><mrow><mo>−</mo><mn>1</mn></mrow></msup></math> can establish significantly lower <math><mn>2</mn><mi>σ</mi></math> bounds than the HL-LHC with <math><mn>3</mn><mtext> </mtext><msup><mrow><mi>ab</mi></mrow><mrow><mo>−</mo><mn>1</mn></mrow></msup></math>. Specifically, we find <math><mi>Br</mi><mo>(</mo><mi>Z</mi><mo>→</mo><mi>e</mi><mi>τ</mi><mo>)</mo><mo>&lt;</mo><mn>2.2</mn><mo>×</mo><msup><mn>10</mn><mrow><mo>−</mo><mn>7</mn></mrow></msup></math>, <math><mi>Br</mi><mo>(</mo><mi>H</mi><mo>→</mo><mi>e</mi><mi>τ</mi><mo>)</mo><mo>&lt;</mo><mn>1.7</mn><mo>×</mo><msup><mn>10</mn><mrow><mo>−</mo><mn>4</mn></mrow></msup></math>, and <math><mi>Br</mi><mo>(</mo><mi>H</mi><mo>→</mo><mi>μ</mi><mi>τ</mi><mo>)</mo><mo>&lt;</mo><mn>1.0</mn><mo>×</mo><msup><mn>10</mn><mrow><mo>−</mo><mn>4</mn></mrow></msup></math> when we apply the optimal cut on the boosted decision tree output. Furthermore, our study reveals the exceptional precision of the FCC-he in measuring the LFV signatures of the <math><mi>Z</mi></math> and Higgs bosons, indicating a promising potential for future discoveries in this fascinating field.

Jinheung Kim

Soojin Lee

Jeonghyeon Song

Daohan Wang

Exploring lepton flavor violation phenomena of the <math><mi>Z</mi></math> and Higgs bosons at electron-proton colliders

Lepton portal dark matter (DM) models are a class of models where the DM candidates solely couple to charged leptons through a mediator carrying a lepton number. These models are very interesting since they avoid constraints from direct detection experiments even for coupling of order <math><mi>O</mi><mo>(</mo><mn>1</mn><mo>)</mo></math>, they have small annihilation cross sections, and can be probed efficiently at lepton colliders. In this work, we consider a minimal lepton portal DM model that consists of extending the standard model with two <math><mi>S</mi><mi>U</mi><mo>(</mo><mn>2</mn><msub><mo>)</mo><mi>L</mi></msub></math> singlets: a charged scalar singlet and an electrically neutral right-handed fermion. We systematically study the production mechanisms of DM at multi-TeV muon colliders. After considering all the possible theoretical and experimental constraints and studying the phenomenology of lepton flavor violation and DM in the muonphilic scenario, we analyze the production rates of 54 channels (26 channels for prompt DM production and 28 channels for charged scalar production) at multi-TeV muon colliders. Finally, we discuss the possible collider signatures of some channels and the corresponding backgrounds. We find that at least nine channels for DM production can be very efficient in testing DM with masses up to about 1 TeV.

Lepton portal dark matter at muon colliders: Total rates and generic features for phenomenologically viable scenarios

We analyse scalar leptoquark pair production at the LHC with predictions including t-channel lepton exchange contributions up to next-to-leading order (NLO) in QCD. In particular, we calculate NLO-QCD predictions for off-diagonal production channels, i.e. channels that involve two different leptoquark eigenstates and are driven solely by diagrams involving Standard Model leptons in the t-channel at leading order, as opposed to diagonal channels where a pair of the same leptoquark eigenstate is produced. We find that reliable theoretical predictions for both channels require NLO accuracy. The relative importance of the off-diagonal modes depends strongly on the considered scenario. In a generic model involving R 2 and S 3 leptoquarks, at large values of the Yukawa couplings off-diagonal contributions initiated by valence quarks can be up to an order of magnitude higher than the diagonal production. However, we also find that in phenomenologically viable scenarios addressing the flavour anomalies off-diagonal production is generally negligible, with a few exceptions of 10%–30% of the total rate depending on the treatment of the charm density in the proton.

Germany

Christoph Borschensky

CERN

Anna Kulesza

Scalar leptoquarks at the LHC and flavour anomalies: a comparison of pair-production modes at NLO-QCD

In the type-I two-Higgs-doublet model, existing theoretical and experimental constraints still permit a light charged Higgs boson with a mass below the top quark mass. We present a complete road map for the light charged Higgs boson at the LHC through a comprehensive phenomenology study, focusing on the normal scenario where the lighter <math><mi>C</mi><mi>P</mi></math>-even Higgs boson is the observed Higgs boson. In type I, it is challenging to simultaneously accommodate the light mass of the charged Higgs boson and the constraints from theoretical stabilities, electroweak precision data, Higgs data, <math><mi>b</mi><mo>→</mo><mi>s</mi><mi>γ</mi></math>, and direct search bounds. Consequently, the parameter space is extremely curtailed, which predicts somewhat definite phenomenological signatures. We find that the mass of the pseudoscalar Higgs boson, <math><msub><mi>M</mi><mi>A</mi></msub></math>, is the most crucial factor in the phenomenology of the charged Higgs boson. If <math><msub><mi>M</mi><mi>A</mi></msub></math> is light, the charged Higgs boson decays mainly into <math><mi>A</mi><msup><mi>W</mi><mo>±</mo></msup></math>. When <math><msub><mi>M</mi><mi>A</mi></msub></math> is above the <math><mi>A</mi><msup><mi>W</mi><mo>±</mo></msup></math> threshold, the dominant decay mode is into <math><msup><mi>τ</mi><mo>±</mo></msup><mi>ν</mi></math>. Over the entire viable parameter space, we study all the possible production and decay modes of charged Higgs bosons at the LHC and suggest three efficient channels: (i) <math><mi>p</mi><mi>p</mi><mo>→</mo><msup><mi>H</mi><mo>+</mo></msup><msup><mi>H</mi><mo>−</mo></msup><mo>→</mo><mo>[</mo><mi>τ</mi><mi>ν</mi><mo>]</mo><mo>[</mo><mi>τ</mi><mi>ν</mi><mo>]</mo></math>, (ii) <math><mrow><mi>p</mi><mi>p</mi><mo>→</mo><mi>H</mi><mi>A</mi><mo>/</mo><mi>H</mi><mi>H</mi><mo>/</mo><mi>A</mi><mi>A</mi><mo>→</mo><msup><mrow><mi>H</mi></mrow><mrow><mo>±</mo></mrow></msup><msup><mrow><mi>W</mi></mrow><mrow><mo>∓</mo></mrow></msup><msup><mrow><mi>H</mi></mrow><mrow><mo>±</mo></mrow></msup><msup><mrow><mi>W</mi></mrow><mrow><mo>∓</mo></mrow></msup><mo>→</mo><mo>[</mo><mi>τ</mi><mi>ν</mi><mo>]</mo><mo>[</mo><mi>τ</mi><mi>ν</mi><mo>]</mo><mi>W</mi><mi>W</mi></mrow></math>, and (iii) <math><mi>p</mi><mi>p</mi><mo>→</mo><msup><mi>H</mi><mo>+</mo></msup><msup><mi>H</mi><mo>−</mo></msup><mo>→</mo><mo>[</mo><mi>b</mi><mover><mi>b</mi><mo>¯</mo></mover><mi>W</mi><mo>]</mo><mo>[</mo><mi>b</mi><mover><mi>b</mi><mo>¯</mo></mover><mi>W</mi><mo>]</mo></math>. Based on sophisticated signal-background analyses including detector simulation, we show that the significance of the first final state is large and that of the second one is marginal (around 3), but that the third one suffers from enormous <math><mi>t</mi><mover><mi>t</mi><mo>¯</mo></mover></math> related backgrounds.

Taiwan

Kingman Cheung

China

Chih-Ting Lu

Comprehensive study of the light charged Higgs boson in the type-I two-Higgs-doublet model