In this paper, we propose two sets of different <math><mi>C</mi><mi>P</mi></math>-sensitive observables inspired by the Higgs production in association with the top quark. We employ a dark matter simplified model that couples a scalar mediator with top quarks. The reconstruction of the kinematic variables is presented at next to leading order (NLO) accuracy for events associated with this massive scalar particle, which is assumed to be vanishing to invisible decays in a detector such as ATLAS. We build these observables by taking advantage of the similarity between the scalar coupling with the top quark and the factorization theorem in the total scattering amplitude, in order to represent the basis in which the phase space is parametrized. A twofold approach employs the direct implementation of the four-momentum phase space measure in building <math><mi>C</mi><mi>P</mi></math> sensitive observables such as <math><msub><mi>b</mi><mn>2</mn></msub></math> for the Higgs, and the spin polarization of the top-quark decays in the narrow width approximation for the employed model. We studied the asymmetries of these distributions to test for any improvement in increasing the exclusion region for the <math><msubsup><mi>g</mi><msub><mi>u</mi><mn>33</mn></msub><mi>S</mi></msubsup><mo>−</mo><msubsup><mi>g</mi><msub><mi>u</mi><mn>33</mn></msub><mi>P</mi></msubsup></math> parameters associated with this vanishing scalar particle. We have found no significant effect in the exclusion limits by using the forward-backward asymmetry distributions and the full-shaped ones. Considering the case of an invisible mediator with mass of <math><mrow><msup><mrow><mn>10</mn></mrow><mrow><mo>−</mo><mn>2</mn></mrow></msup><mtext> </mtext><mtext> </mtext><mi>GeV</mi></mrow></math> for a luminosity of <math><mn>300</mn><mtext> </mtext><mtext> </mtext><msup><mi>fb</mi><mrow><mo>−</mo><mn>1</mn></mrow></msup></math> expected at the end of Run 3, the best limits for <math><msubsup><mi>g</mi><msub><mi>u</mi><mn>33</mn></msub><mi>S</mi></msubsup></math> and <math><msubsup><mi>g</mi><msub><mi>u</mi><mn>33</mn></msub><mi>P</mi></msubsup></math> at NLO accuracy were obtained using the variables <math><msubsup><mover><mi>b</mi><mo>˜</mo></mover><mn>2</mn><mover><mi>y</mi><mo>^</mo></mover></msubsup></math> and <math><msub><mi>b</mi><mn>2</mn></msub></math> respectively, with corresponding limits set to <math><mrow><mo>[</mo><mo>−</mo><mn>0.0425</mn><mo>,</mo><mn>0.0425</mn><mo>]</mo></mrow></math> and <math><mrow><mo>[</mo><mo>−</mo><mn>0.83</mn><mo>,</mo><mn>0.83</mn><mo>]</mo></mrow></math> at 68% CL.

Portugal

E. Chalbaud

Rui M. Silva

António Onofre

Ricardo Gonçalo

United States

Miguel C. N. Fiolhais

Asymmetries in invisible dark matter mediator production associated with <math><mi>t</mi><mover><mi>t</mi><mo>¯</mo></mover></math> final states

We study a simple extension of the Standard Model featuring a dark sector with an ultralight pseudo-Nambu-Goldstone boson as a dark matter candidate. We focus on the mass range <math><mrow><mi>O</mi><mo>(</mo><msup><mrow><mn>10</mn></mrow><mrow><mo>−</mo><mn>20</mn></mrow></msup><mi>–</mi><msup><mrow><mn>10</mn></mrow><mrow><mo>−</mo><mn>10</mn></mrow></msup><mo>)</mo><mtext> </mtext><mtext> </mtext><mi>eV</mi></mrow></math>, relevant for strong gravity applications, and explore its production and evolution in the early Universe. The model is formulated in such a way that dark matter does not couple directly to photons or other Standard Model particles avoiding some of the most stringent cosmological bounds related to axionlike particles. In this work, two different scenarios are considered depending on whether dark matter is produced in a preinflationary or postinflationary regime. We also discuss the effect from emergent topological defects such as cosmic strings and domain walls and estimate the spectrum of stochastic gravitational waves produced by their decay, enabling us to test the model at current and future gravitational-wave experiments.

CERN

António P. Morais

Brazil

Vinícius Oliveira

Sweden

Roman Pasechnik

Rui Santos

Exploring the viability of a pseudo-Nambu-Goldstone boson as ultralight dark matter in a mass range relevant for strong gravity applications

Searches for new physics (NP) at particle colliders typically involve multivariate analysis of kinematic distributions of final state particles produced in a decay of a hypothetical NP resonance. Since the pair-production cross-sections mediated by such resonances are strongly suppressed by the NP scale, this analysis becomes less relevant for NP searches for masses of the BSM resonance above 1 TeV. On the other hand, t-channel processes are less sensitive to the mass of the virtual mediator and therefore larger phase-space can be potentially probed as well as the couplings between the NP particles and the Standard Model fields. The fact that transitions between different generations of quarks and leptons may exist, the potential of the search presented in this article can be used, as a reference guide, to enlarge significantly the scope of searches performed at the LHC to flavour off-diagonal channels, in a theoretically consistent approach. In this work, we study non-resonant production of scalar leptoquarks which have been proposed in the literature to provide a potential avenue for radiative generation of neutrino masses, accommodating as well the existing flavour physics data. Final states involving just two muons at the LHC (μ + μ  − ), are used as a well-motivated case study.

João Gonçalves

António Morais

Exploring mixed lepton-quark interactions in non-resonant leptoquark production at the LHC

We use the current   <math> <mi>t</mi> <mover> <mi>t</mi> <mo>¯</mo> </mover> </math>  $$ t\overline{t} $$  experimental analysis to look for Dark Matter (DM) particles hidden in the final state. We present a phenomenological study where we successfully perform the reconstruction of a   <math> <mi>t</mi> <mover> <mi>t</mi> <mo>¯</mo> </mover> </math>  $$ t\overline{t} $$  system in the presence of a scalar mediator Y 0, that couples to both Standard Model (SM) and to DM particles. We use a MadGraph5_aMC@NLO simplified DM model, where signal samples of pp →   <math> <mi>t</mi> <mover> <mi>t</mi> <mo>¯</mo> </mover> <mi>Y</mi> </math>  $$ t\overline{t}Y $$  0 are generated at the Large Hadron Collider (LHC) with both Charge-Parity (CP) -even and CP-odd couplings of Y 0 to the top quarks. Different mass scales for the Y 0 mediator are considered, from the low mass region (~ 0 GeV) to masses close to the Higgs boson mass (125 GeV). The dileptonic final states of the   <math> <mi>t</mi> <mover> <mi>t</mi> <mo>¯</mo> </mover> </math>  $$ t\overline{t} $$  system were used in our analysis. The reconstruction of the   <math> <mi>t</mi> <mover> <mi>t</mi> <mo>¯</mo> </mover> </math>  $$ t\overline{t} $$  system is done with a kinematic fit, without reconstructing the mediator. All relevant SM backgrounds for the dileptonic   <math> <mi>t</mi> <mover> <mi>t</mi> <mo>¯</mo> </mover> </math>  $$ t\overline{t} $$  search at the LHC are considered. Furthermore, CP angular observables were used to probe the CP-nature of the coupling between the mediator and top-quarks, which allowed to set confidence level (CL) limits for those Yukawa couplings as a function of the mediator mass.

Germany

Duarte Azevedo

Rodrigo Capucha

Pedro Chaves

João Martins

Search for an invisible scalar in   <math> <mi>t</mi> <mover> <mi>t</mi> <mo>¯</mo> </mover> </math>  $$ t\overline{t} $$  final states at the LHC

In this work, we propose and explore for the first time a new collider signature of heavy neutral scalars typically found in many distinct classes of multi-Higgs models. This signature, particularly relevant in the context of the Large Hadron Collider (LHC) measurements, is based on a topology with two charged leptons and four jets arising from first and second generation quarks. As an important benchmark scenario of the multi-Higgs models, we focus on a recently proposed Branco-Grimus-Lavoura (BGL) type model enhanced with an Abelian U(1) flavor symmetry and featuring an additional sector of right-handed neutrinos. We discuss how kinematics of the scalar fields in this model can be used to efficiently separate the signal from the dominant backgrounds and explore the discovery potential of the new heavy scalars in the forthcoming LHC runs. The proposed method can be applied for analysis of the statistical significance of heavy scalars’ production at the LHC and future colliders in any multi-Higgs model.

P. M. Ferreira

Antonio P. Morais

Vasileios Vatellis

Collider phenomenology of new neutral scalars in a flavored multi-Higgs model

The discovery potential of both singlet and doublet vector-like leptons (VLLs) at the Large Hadron Collider (LHC) as well as at the not-so-far future muon and electron machines is explored. The focus is on a single production channel for LHC direct searches while double production signatures are proposed for the leptonic colliders. A Deep Learning algorithm to determine the discovery (or exclusion) statistical significance at the LHC is employed. While doublet VLLs can be probed up to masses of 1 TeV, their singlet counterparts have very low cross sections and can hardly be tested beyond a few hundreds of GeV at the LHC. This motivates a physics-case analysis in the context of leptonic colliders where one obtains larger cross sections in VLL double production channels, allowing to probe higher mass regimes otherwise inaccessible even to the LHC high-luminosity upgrade.

Felipe Freitas

Deep learning searches for vector-like leptons at the LHC and electron/muon colliders

In this paper, we use the associated production of top-quark pairs (  <math> <mi>t</mi> <mover> <mi>t</mi> <mo>¯</mo> </mover> </math>  $$ t\overline{t} $$ ) with a generic scalar boson (ϕ) at the LHC (pp →   <math> <mi>t</mi> <mover> <mi>t</mi> <mo>¯</mo> </mover> <mi>ϕ</mi> </math>  $$ t\overline{t}\phi $$ ) to explore the sensitivity of a large set of observables to the sign of the CP mixing angle (α), present in the coupling between the scalar boson and the top quarks. The mass of the scalar boson is set to m  ϕ  = 125 GeV (the Standard Model Higgs boson mass) and its coupling to top-quarks is varied such that α = 0°, 22.5°, 45.0°, 67.5°, 90.0°, 135.0° and 180.0°. Dileptonic final states of the   <math> <mi>t</mi> <mover> <mi>t</mi> <mo>¯</mo> </mover> <mi>ϕ</mi> </math>  $$ t\overline{t}\phi $$  system are used (pp → bℓ + ν  ℓ    <math> <mover> <mi>b</mi> <mo>¯</mo> </mover> </math>  $$ \overline{b} $$  ℓ  −    <math> <mover> <mi>ν</mi> <mo>¯</mo> </mover> </math>  $$ \overline{\nu} $$   ℓ  b   <math> <mover> <mi>b</mi> <mo>¯</mo> </mover> </math>  $$ \overline{b} $$ ), where the scalar boson is expected to decay according to ϕ →   <math> <mi>b</mi> <mover> <mi>b</mi> <mo>¯</mo> </mover> </math>  $$ b\overline{b} $$ . A new method to reconstruct the scalar mass, originally designed for the low mass regime is used, improving the resolution of the Higgs mass by roughly a factor of two. A full phenomenological analysis is performed using Standard Model (SM) background and signal events generated with MadGraph5_aMC@NLO, in turn reconstructed using a kinematical fit. The most sensitive CP-observables are selected to compute Confidence Level (CL) limits as a function of the sign of the top quark Yukawa couplings to the ϕ boson. We also explore the sensitivity to interference terms using differential distributions and angular asymmetries. Given the significant difference between the pure scalar (σ 0+) and pure pseudo-scalar (σ 0− ) production cross section values, it is unlikely the   <math> <mi>t</mi> <mover> <mi>t</mi> <mo>¯</mo> </mover> <mi>ϕ</mi> </math>  $$ t\overline{t}\phi $$  channel alone will be sensitive to the sign of the CP-mixing angle or interference terms, even at the end of the LHC. Using the   <math> <msubsup> <mi>b</mi> <mn>2</mn> <mrow> <mi>t</mi> <mover> <mi>t</mi> <mo>¯</mo> </mover> <mi>ϕ</mi> </mrow> </msubsup> </math>  $$ {b}_2^{t\overline{t}\phi } $$  and   <math> <msubsup> <mi>b</mi> <mn>4</mn> <mrow> <mi>t</mi> <mover> <mi>t</mi> <mo>¯</mo> </mover> <mi>ϕ</mi> </mrow> </msubsup> </math>  $$ {b}_4^{t\overline{t}\phi } $$  variables, exclusion limits at 95% CL for the CP-even and CP-odd components of the top quark Yukawa couplings are expected to be set to   <math> <mover> <mi>κ</mi> <mo>~</mo> </mover> </math>  $$ \overset{\sim }{\kappa } $$  ∈ [-0.698,+0.698] and |κ| ∈ [0.878,1.04], respectively, at the end of the High Luminosity phase of the LHC (HL-LHC) by using the dileptonic decay channel alone.

CP-violation, asymmetries and interferences in   <math> <mi>t</mi> <mover> <mi>t</mi> <mo>¯</mo> </mover> <mi>ϕ</mi> </math>  $$ t\overline{t}\phi $$

In this paper we propose a new reconstruction method to explore the low mass region in the associated production of top-quark pairs (  <math> <mi>t</mi> <mover> <mi>t</mi> <mo>¯</mo> </mover> </math>  $$ t\overline{t} $$ ) with a generic scalar boson (ϕ) at the LHC. The new method of mass reconstruction shows an improved resolution of at least a factor of two in the low mass region when compared to previous methods, without the loss of sensitivity of previous analyses. It turns out that it also leads to an improvement of the mass reconstruction of the 125 GeV Higgs for the same production process. We use an effective Lagrangian to describe a scalar with a generic Yukawa coupling to the top quarks. A full phenomenological analysis was performed, using Standard Model background and signal events generated with MadGraph5_aMC@NLO and reconstructed using a kinematic fit. The use of CP-sensitive variables allows then to maximize the distinction between CP-even and CP-odd components of the Yukawa couplings. Confidence Levels (CLs) for the exclusion of ϕ bosons with mixed CP (both CP-even and CP-odd components) were determined as a function of the top Yukawa couplings to the ϕ boson. The mass range analysed starts slightly above the ϒΥ mass up to 40 GeV, although the analysis can be used for an arbitrary mass. If no new light scalar is found, exclusion limits at 95% CL for the absolute value of the CP-even and CP-odd Yukawa are derived. Finally, we analyse how these limits constrain the parameter space of the complex two-Higgs doublet model (C2HDM).

Emanuel Gouveia

Light Higgs searches in   <math> <mi>t</mi> <mover> <mi>t</mi> <mo>¯</mo> </mover> <mi>ϕ</mi> </math>  $$ t\overline{t}\phi $$  production at the LHC

The minimal   ${U}(1)_{\rm{{B-L}}}$   extension of the Standard Model (B-L-SM) offers an explanation for neutrino mass generation via a seesaw mechanism; it also offers two new physics states, namely an extra Higgs boson and a new   ${U}(1)_{\rm{{B-L}}}$   gauge boson. The emergence of a second Higgs particle as well as a new   ${U}(1)_{\rm{{B-L}}}$   gauge boson, both linked to the breaking of a local   ${U}(1)_{\rm{{B-L}}}$   symmetry, makes the B-L-SM rather constrained by direct searches in Large Hadron Collider (LHC) experiments. We investigate the phenomenological status of the B-L-SM by confronting the new physics predictions with the LHC and electroweak precision data. Taking into account the current bounds from direct LHC searches, we demonstrate that the prediction for the muon   ${U}(1)_{\rm{{B-L}}}$   anomaly in the B-L-SM yields at most a contribution of approximately   ${U}(1)_{\rm{{B-L}}}$   , which represents a tension of   ${U}(1)_{\rm{{B-L}}}$   standard deviations, with the current   ${U}(1)_{\rm{{B-L}}}$   uncertainty, by means of a   ${U}(1)_{\rm{{B-L}}}$   boson if its mass is in the range of   ${U}(1)_{\rm{{B-L}}}$   to   ${U}(1)_{\rm{{B-L}}}$  , within the reach of future LHC runs. This means that the B-L-SM, with heavy yet allowed   ${U}(1)_{\rm{{B-L}}}$   boson mass range, in practice, does not resolve the tension between the observed anomaly in the muon   ${U}(1)_{\rm{{B-L}}}$   and the theoretical prediction in the Standard Model. Such a heavy   ${U}(1)_{\rm{{B-L}}}$   boson also implies that the minimal value for the new Higgs mass is of the order of 400 GeV.

Felipe F. Freitas

Carlos A.R. Herdeiro

António P. Morais

Eugen Radu

Nicolas Sanchis-Gual

What can a heavy Boson do to the Muon anomaly and to a new Higgs boson mass?  

In this paper we explore CP discrimination in the associated production of top-quark pairs ( $$ t\overline{t} $$ ) with a generic scalar boson ( ) at the LHC. We probe the CP-sensitivity of several observables for a varying scalar boson mass and CP-number, either CP-even ( = ) or CP-odd ( = ), using dileptonic final states of the $$ t\overline{t}\phi $$ system, with $$ b\overline{b} $$ . We show that CP-searches are virtually impossible for boson masses above a few hundred GeV in this channel. A full phenomenological analysis was performed, using Standard Model background and signal events generated with MadGraph5_aMC@NLO and reconstructed using a kinematic fit. The most sensitive CP-observables are used to compute Confidence Levels (CLs), as a function of luminosity, for the exclusion of different signal hypotheses with scalar and pseudoscalar boson masses that range from = 40 GeV up to 200 GeV. We finalize by analysing the impact of a measurement (or limit) of the CP-violating angle in the parameter space of a complex two-Higgs doublet model known as the C2HDM.

Scalar mass dependence of angular variables in