We propose a two-component dark matter (DM) scenario by extending the Standard Model with two additional <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>S</mi><mi>U</mi><mo stretchy="false">(</mo><mn>2</mn><msub><mo stretchy="false">)</mo><mi>L</mi></msub></math> doublets, one scalar, and another fermion. To ensure the stability of the DM components, we impose a global <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msub><mi mathvariant="script">Z</mi><mn>2</mn></msub><mo>×</mo><msubsup><mi mathvariant="script">Z</mi><mn>2</mn><mo>′</mo></msubsup></math> symmetry. The lightest neutral states for both the scalar and fermion, which are non-trivially transformed under the extended symmetry, behave as stable two-component DM candidates. While single components are underabundant due to their gauge interactions, in a mass region between <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msub><mi>m</mi><mi>W</mi></msub></math> and 525 GeV for the scalar and a mass below 1200 GeV for the fermion, and the fermion DM conflicts with direct detection limits over the whole parameter space, having two components helps to saturate relic density in the regions with under-abundance. Compliance with direct detection constraints leads to two options, either introducing dim-5 effective operators, or embedding the scenarios into a complete UV theory, which reproduces a type II seesaw model, thus naturally including neutrino masses. We analyze the consequences of this scenario at the LHC.

Canada

Mariana Frank

India

Purusottam Ghosh

United Kingdom

Chayan Majumdar

United States

Supriya Senapati

Exploring two component doublet dark matter

We investigate the sensitivity of future high-energy muon colliders to heavy Majorana neutrinos, considering both opposite-sign (<math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msup><mi>μ</mi><mo>+</mo></msup><msup><mi>μ</mi><mo>−</mo></msup></math>) and same-sign (<math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msup><mi>μ</mi><mo>+</mo></msup><msup><mi>μ</mi><mo>+</mo></msup></math>) collision modes. We focus on <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msup><mi>μ</mi><mo>+</mo></msup><msup><mi>μ</mi><mo>−</mo></msup></math> colliders operating at center-of-mass energies of 1, 3, and 10 TeV, as well as the proposed <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>μ</mi><mi>TRISTAN</mi></math> facility at 2 TeV, and we analyze the production of heavy neutrinos via vector boson fusion in the <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>t</mi></math>-channel, a mechanism that becomes dominant in the multi-TeV regime. We evaluate its exclusion potential in terms of the heavy neutrino mass and the mixing of the heavy neutrino with its Standard Model counterparts, using both cut-based selections and boosted decision trees trained to exploit the distinct kinematic signatures of heavy Majorana neutrino exchanges. Our results demonstrate the complementarity between collider configurations and show that active-sterile mixing angles as small as 0.001 could be probed for neutrino masses up to 100 TeV, an experimentally inaccessible region of the parameter space at current facilities. Altogether, this work establishes the discovery potential of muon colliders for testing super-heavy Majorana neutrinos, complementary to conventional probes, and provides compelling motivation for the next generation of high-energy lepton colliders.

Parham Dehghani

France

Benjamin Fuks

Vector boson fusion signatures of superheavy Majorana neutrinos at muon colliders

We explore constraints on the parameter space of the alternative left-right model originating from the leptonic sector. Our analyses focuses on both lepton-flavor-conserving observables, particularly the anomalous magnetic moment of the muon, and lepton-flavor-violating processes like <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>μ</mi><mo stretchy="false">→</mo><mi>e</mi><mi>γ</mi></math> decay and <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>μ</mi><mo>−</mo><mi>e</mi></math> conversions in nuclei. While contributions to the anomalous magnetic moment fall below the measured values at <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><mn>2</mn><mi>σ</mi></mrow></math>, current and future experimental sensitivities to flavor-violating branching rations of the Standard Model leptons are expected to impose lower bounds on the mass of the peculiar <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>S</mi><mi>U</mi><mo stretchy="false">(</mo><mn>2</mn><msub><mo stretchy="false">)</mo><mi>R</mi></msub></math> gauge boson of the model. This provides complementary constraints relative to existing limits, which are indirect and derived from collider bounds on the mass of the associated neutral gauge boson <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msup><mi>Z</mi><mo>′</mo></msup></math>.

Sumit K. Garg

Poulose Poulose

Leptonic probes of alternative left-right symmetric models

The stability of the standard model (SM) Higgs vacuum is a long-standing issue in particle physics. The SM Higgs quartic coupling parameter is expected to become negative at high scales, potentially generating an unstable vacuum well before reaching the Planck scale. We investigate whether the introduction of vectorlike leptons (VLLs) in six distinct gauge anomaly-independent representations can stabilize the SM vacuum without introducing additional scalar fields. By analyzing the mass spectrum and mixing angles of these VLLs with the SM leptons, we identify conditions under which the Higgs potential remains stable. We demonstrate that there exists a set of allowed but narrowed VLL spectra that can indeed stabilize the SM vacuum. We also study the effect of these VLLs on electroweak precision observables, particularly the oblique parameters <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi mathvariant="double-struck">S</mi></math> and <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi mathvariant="double-struck">T</mi></math>, to ensure the fit within global experimental constraints. Finally, we perform a comprehensive analysis of the parameter space allowed by electroweak precision data and the parameter space required for stability, highlighting cases that accommodate both. This study opens up new avenues for understanding the role of vectorlike leptons in extending the validity of the standard model up to the Planck scale.

Kivanc Y. Cingiloglu

Stability of the standard model vacuum with vectorlike leptons: A critical examination

We investigate the impact of incorporating vector-like leptons into the Standard Model, aiming to address longstanding puzzles related to the anomalous magnetic moments of the muon and electron while maintaining consistency with neutrino masses and mixings. We find that among the various representations of vector-like leptons permitted by the Standard Model gauge symmetry, only weak doublets and singlets offer satisfactory solutions, all associated with a significantly constrained parameter space. Our analysis delves into the associated parameter space, identifying representative benchmark scenarios suitable for collider studies. These setups yield a distinctive six-lepton signature whose associated signals can easily be distinguished from the Standard Model background, providing a clear signal indicative of new physics models featuring vector-like leptons. Our work hence sheds light on the potential implications of vector-like leptons in resolving discrepancies inherent to the Standard Model, while also offering insights into experimental avenues for further exploration.

Collider imprint of vector-like leptons in light of anomalous magnetic moment and neutrino data

We study the interaction between the inert Higgs doublet (IDM) dark matter and a vector-like SU(2) triplet lepton (VLL), both of which are Z 2-odd. The vector current of the VLL with the Z-boson rules out a fermionic or two-component dark matter scenario. However, a compressed mass spectrum and a sufficiently small Yukawa coupling allows co-annihilation and late decay of the VLL into the IDM sector, affecting the relic density of the pseudoscalar dark matter. The same two factors enable displaced decay of the VLL states, providing novel signatures involving hadronically quiet displaced multi-lepton final states. Such signatures to probe the model are studied at the 14 and 27 TeV LHC, as well as the 100 TeV FCC-hh. In addition to being detectable at the CMS/ATLAS experiments, if the new particles have sub-100 GeV masses, signals can also be seen at the proposed MATHUSLA detector.

South Korea

Priyotosh Bandyopadhyay

Snehashis Parashar

Chandrima Sen

Interplay of inert doublet and vector-like lepton triplet with displaced vertices at the LHC/FCC and MATHUSLA

We examine flavour-changing neutral-current decays of the top quark, <math><mi>t</mi><mo>→</mo><mi>q</mi><mi>γ</mi></math>, <math><mi>t</mi><mo>→</mo><mi>q</mi><mi>Z</mi></math>, <math><mi>t</mi><mo>→</mo><mi>q</mi><mi>H</mi></math>, and <math><mi>t</mi><mo>→</mo><mi>q</mi><mi>g</mi></math> (with <math><mi>q</mi><mo>=</mo><mi>u</mi><mo>,</mo><mi>c</mi></math>), in the Alternative Left-Right Model, a left right-symmetric model featuring exotic quarks and light bosons. These decays have a very small probability of occurring within the Standard Model, but they can be enhanced in this model through the presence of the exotic states. While associated signals may be detected directly at the LHC, rare decays have the advantage of offering means to probe new particles indirectly, through loop-contributions. We perform a comprehensive analysis of the model's parameter space to demonstrate the possible existence of enhancements in the corresponding branching ratios, of 10$^{6}$ for the branching ratios <math><mi>B</mi><mo>(</mo><mi>t</mi><mo>→</mo><mi>u</mi><mi>Z</mi><mo>)</mo></math> and <math><mi>B</mi><mo>(</mo><mi>t</mi><mo>→</mo><mi>u</mi><mi>H</mi><mo>)</mo></math>, and in the range of <math><msup><mrow><mn>10</mn></mrow><mrow><mn>2</mn></mrow></msup><mo>−</mo><msup><mrow><mn>10</mn></mrow><mrow><mn>4</mn></mrow></msup></math> for the other decays, relative to the Standard Model. We subsequently determine the preferred parameter space regions of the model in terms of potential of being reached in the near future.

Flavour-changing top quark decays in the alternative left-right model

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.

Adil Jueid

Sweden

Stefano Moretti

Özer Özdal

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

We present a comprehensive analysis of the vacuum stability of the two-Higgs-doublet model, for both type-I and type-II, augmented by vectorlike quarks in singlet, doublet, or triplet representations. We review the model briefly before introducing the extra fermionic states and their interactions, and impose restrictions on the parameters coming from both theoretical considerations and experimental bounds. We then study the renormalization group equation evolution of the parameters of the model in order to isolate the parameter regions that satisfy vacuum stability requirements. We then add the electroweak precision observables to ensure that the resulting parameter space is consistent with the data. We include complete expressions for the renormalization group equations and the <math><mi>S</mi></math> and <math><mi>T</mi></math> parameters used. Finally, we summarize the effects of various vectorlike quark representations on the parameter space. We indicate the regions constrained, highlighting the differences between representations in type-I and type-II, and pinpoint the effects of the interplay between the extended model and the additional fermions.

Vacuum stability and electroweak precision in the two-Higgs-doublet model with vectorlike quarks

We consider a minimal extension of the Standard Model in warped extra dimensions, with fields propagating in the bulk including a bulk SM-like Higgs doublet. We show that the Higgs can acquire a non-trivial oscillatory VEV, strongly localized towards the TeV brane, but such that its value at that brane could be highly suppressed due to its oscillatory behaviour. Within the minimal Randall-Sundrum metric background, this oscillatory VEV can alleviate the bounds coming from oblique precision electroweak parameters, such that the KK gluon mass can be around 3 TeV (instead of ∼8 TeV for the usual non-oscillatory bulk Higgs). We also discuss the stability of the configuration as well as the naturalness of the model parameters.