We investigate the dynamics of the electroweak phase transition within an extended Standard Model framework that includes one real scalar (Φ) and one complex scalar ( ), both of which are SM gauge singlets. The global U(1) symmetry is softly broken to a $$ {\mathcal{Z}}_3 $$ symmetry by the term in the scalar potential. After this U(1) symmetry breaking, the imaginary component of the complex scalar ( ) acts as a pseudo-Nambu-Goldstone boson (pNGB) dark matter candidate, naturally stabilized by the $$ {\mathcal{Z}}_2 $$ symmetry of the scenario. Specially, the spontaneous breaking of the global U(1) symmetry to a discrete $$ {\mathcal{Z}}_3 $$ subgroup can introduce effective cubic terms in the scalar potential, which facilitates a strong first-order phase transition. We analyze both single-step and multi-step first-order phase transitions, identifying the parameter space that satisfies the dark matter relic density constraints, complies with all relevant experimental constraints, and exhibits a strong first-order electroweak phase transition. The interplay of these criteria significantly restricts the model parameter space, often leading to an underabundant relic density. Moreover, we delve into the gravitational wave signatures associated with this framework, offering valuable insights that complement traditional dark matter direct and indirect detection methods.

India

Dilip Ghosh

Koustav Mukherjee

Canada

United States

Shourya Mukherjee

Electroweak phase transition in two scalar singlet model with pNGB dark matter

Exotic spin-0 states with unusual couplings with the gauge and matter fields of the Standard Model are worth exploring at the CERN LHC. Though our approach is largely model independent, we take inspiration from flavor models based on some discrete symmetries which predict a set of a scalar and a pseudoscalar having purely off-diagonal Yukawa interactions with quarks and leptons. In a previous paper, some of us explored how to decipher such exotic scalar and pseudoscalar states whose off-diagonal Yukawa couplings involve light quarks. In this work we follow a complementary path and focus on the Yukawa couplings that necessarily involve a top quark. If one such spin-0 state is lighter than the top quark, then the rare decay of the latter, on account of the high yield of the  $$t\bar{t}$$  <math> <mrow> <mi>t</mi> <mover> <mrow> <mi>t</mi> </mrow> <mrow> <mo>¯</mo> </mrow> </mover> </mrow> </math>   events, could provide a potential hunting ground of those exotic states particularly during the high luminosity phase of the LHC run. We carry out an exhaustive collider analysis of some promising signatures of those exotic states using sophisticated Machine Learning techniques and obtain considerable signal significance.

Gautam Bhattacharyya

Indrani Chakraborty

Tapoja Jha

France

Gourab Saha

Searching for exotic Higgs bosons from top quark decays at the HL-LHC

The current upper limit on  $$N_\textrm{eff}$$  <math> <msub> <mi>N</mi> <mtext>eff</mtext> </msub> </math>   at the time of CMB by Planck 2018 can place stringent constraints in the parameter space of BSM paradigms where their additional interactions may affect neutrino decoupling. Motivated by this fact in this paper we explore the consequences of light gauge boson ( $$Z'$$  <math> <msup> <mi>Z</mi> <mo>′</mo> </msup> </math>  ) emerging from local  $$U(1)_X$$  <math> <mrow> <mi>U</mi> <msub> <mrow> <mo>(</mo> <mn>1</mn> <mo>)</mo> </mrow> <mi>X</mi> </msub> </mrow> </math>   symmetry in  $$N_\textrm{eff}$$  <math> <msub> <mi>N</mi> <mtext>eff</mtext> </msub> </math>   at the time of CMB. First, we analyze the generic  $$U(1)_X$$  <math> <mrow> <mi>U</mi> <msub> <mrow> <mo>(</mo> <mn>1</mn> <mo>)</mo> </mrow> <mi>X</mi> </msub> </mrow> </math>   models with arbitrary charge assignments for the SM fermions and show that, in the context of  $$N_\textrm{eff}$$  <math> <msub> <mi>N</mi> <mtext>eff</mtext> </msub> </math>   the generic  $$U(1)_X$$  <math> <mrow> <mi>U</mi> <msub> <mrow> <mo>(</mo> <mn>1</mn> <mo>)</mo> </mrow> <mi>X</mi> </msub> </mrow> </math>   gauged models can be broadly classified into two categories, depending on the charge assignments of first generation leptons. We then perform a detailed analysis with two specific  $$U(1)_X$$  <math> <mrow> <mi>U</mi> <msub> <mrow> <mo>(</mo> <mn>1</mn> <mo>)</mo> </mrow> <mi>X</mi> </msub> </mrow> </math>   models:  $$U(1)_{B_3-3L_e}$$  <math> <mrow> <mi>U</mi> <msub> <mrow> <mo>(</mo> <mn>1</mn> <mo>)</mo> </mrow> <mrow> <msub> <mi>B</mi> <mn>3</mn> </msub> <mo>-</mo> <mn>3</mn> <msub> <mi>L</mi> <mi>e</mi> </msub> </mrow> </msub> </mrow> </math>   and  $$U(1)_{B_3-3L_\mu }$$  <math> <mrow> <mi>U</mi> <msub> <mrow> <mo>(</mo> <mn>1</mn> <mo>)</mo> </mrow> <mrow> <msub> <mi>B</mi> <mn>3</mn> </msub> <mo>-</mo> <mn>3</mn> <msub> <mi>L</mi> <mi>μ</mi> </msub> </mrow> </msub> </mrow> </math>   and explore the contribution in  $$N_\textrm{eff}$$  <math> <msub> <mi>N</mi> <mtext>eff</mtext> </msub> </math>   due to the presence of  $$Z'$$  <math> <msup> <mi>Z</mi> <mo>′</mo> </msup> </math>   realized in those models. For comparison, we also showcase the constraints from low energy experiments like: Borexino, Xenon 1T, neutrino trident, etc. We show that in a specific parameter space, particularly in the low mass region of  $$Z'$$  <math> <msup> <mi>Z</mi> <mo>′</mo> </msup> </math>  , the bound from  $$N_\textrm{eff}$$  <math> <msub> <mi>N</mi> <mtext>eff</mtext> </msub> </math>   (Planck 2018) is more stringent than the experimental constraints. Additionally, a part of the regions of the same parameter space may also relax the  $$H_0$$  <math> <msub> <mi>H</mi> <mn>0</mn> </msub> </math>   tension.

Purusottam Ghosh

Sk Jeesun

Rahul Srivastava

Hubble tension and cosmological imprints of  $$U(1)_X$$  <math> <mrow> <mi>U</mi> <msub> <mrow> <mo>(</mo> <mn>1</mn> <mo>)</mo> </mrow> <mi>X</mi> </msub> </mrow> </math>   gauge symmetry:  $$U(1)_{B_3-3 L_i}$$  <math> <mrow> <mi>U</mi> <msub> <mrow> <mo>(</mo> <mn>1</mn> <mo>)</mo> </mrow> <mrow> <msub> <mi>B</mi> <mn>3</mn> </msub> <mo>-</mo> <mn>3</mn> <msub> <mi>L</mi> <mi>i</mi> </msub> </mrow> </msub> </mrow> </math>   as a case study

The relic density of Dark Matter (DM) in the freeze-in scenario is highly dependent on the evolution history of the universe and changes significantly in a non-standard (NS) cosmological framework prior to Big Bang Nucleosynthesis (BBN). In this scenario, an additional species dominates the energy budget of the universe at early times (before BBN), resulting in a larger cosmological expansion rate at a given temperature compared to the standard radiation-dominated (RD) universe. To investigate the production of DM in the freeze-in scenario, we consider both standard RD and NS cosmological picture before BBN and perform a comparative analysis. We extend the Standard Model (SM) particle content with a SM singlet DM particle χ and an axion-like particle (ALP) a. The interactions between ALP, SM particles, and DM are generated by higher dimensional effective operators. This setup allows the production of DM χ from SM bath through the mediation of ALP, via ALP-portal processes. These interactions involve non-renormalizable operators, leading to ultraviolet (UV) freeze-in, which depends on the reheating temperature (T  RH ) of the early universe. In the NS cosmological scenario, the faster expansion rate suppresses the DM production processes, allowing for enhanced effective couplings between the visible and dark sectors to satisfy the observed DM abundance compared to RD scenario. This improved coupling increases the detection prospects for freeze-in DM via the ALP-portal, which is otherwise challenging to detect in RD universe due to small couplings involved. Using an effective field theory set-up, we show that various ALP searches such as in FASER, DUNE, and SHiP, etc. will be able to probe significant parameter space depending on the different model parameters.

Poland

Anish Ghoshal

Axion-like particle (ALP) portal freeze-in dark matter confronting ALP search experiments

A color octet isodoublet can have esoteric origins and it complies with minimal flavour violation. In this study, we take a scenario where the well known Type-X Two-Higgs doublet model is augmented with a color octet isodoublet. We shed light on how such a setup can predict the recently observed value for the W-boson mass. The two-loop Barr-Zee contributions to muon  $$g-2$$  <math> <mrow> <mi>g</mi> <mo>-</mo> <mn>2</mn> </mrow> </math>   stemming from the colored scalars are evaluated. It is subsequently found that the parameter space compatible with the observed muon  $$g-2$$  <math> <mrow> <mi>g</mi> <mo>-</mo> <mn>2</mn> </mrow> </math>   gets relaxed w.r.t. what it is in the pure Type-X 2HDM by virtue of the contribution from the colored scalars. The extended parameter region therefore successfully accounts for both the W-mass and muon  $$g-2$$  <math> <mrow> <mi>g</mi> <mo>-</mo> <mn>2</mn> </mrow> </math>   anomalies simultaneously. Finally, a collider signature leading to a  $$\tau ^+ \tau ^- b \overline{b}$$  <math> <mrow> <msup> <mi>τ</mi> <mo>+</mo> </msup> <msup> <mi>τ</mi> <mo>-</mo> </msup> <mi>b</mi> <mover> <mi>b</mi> <mo>¯</mo> </mover> </mrow> </math>   final state is explored at the 14 TeV LHC using both cut-based and multivariate techniques. Such a signal can confirm the existence of both colorless as well colored scalars that are introduced by this framework.

Nabarun Chakrabarty

Muon  $$g-2$$  <math> <mrow> <mi>g</mi> <mo>-</mo> <mn>2</mn> </mrow> </math>   and W-mass in a framework of colored scalars: an LHC perspective

We investigate the phenomenology of a non-thermal dark matter (DM) candidate in the context of flavor models that explain the hierarchy in the masses and mixings of quarks and leptons via the Froggatt-Nielsen (FN) mechanism. A flavor-dependent U(1)FN symmetry explains the fermion mass and mixing hierarchy, and also provides a mechanism for suppressed interactions of the DM, assumed to be a Majorana fermion, with the Standard Model (SM) particles, resulting in its FIMP (feebly interacting massive particle) character. Such feeble interactions are mediated by a flavon field through higher dimensional operators governed by the U(1)FN charges. We point out a natural stabilizing mechanism for the DM within this framework with the choice of half-integer U(1)FN charge n for the DM fermion, along with integer charges for the SM fermions and the flavon field. In this flavon portal scenario, the DM is non-thermally produced from the decay of the flavon in the early universe which becomes a relic through the freeze-in mechanism. We explore the allowed parameter space for this DM candidate from relic abundance by solving the relevant Boltzmann equations. We find that reproducing the correct relic density requires the DM mass to be in the range (100 − 300) keV for n = 7.5 and (3 − 10) MeV for n = 8.5 where n is the U(1)FN charge of the DM fermion.

K. Babu

Shreyashi Chakdar

Nandini Das

FIMP dark matter from flavon portals

In this work, we explore the signals of an  $$S_3$$  <math> <msub> <mi>S</mi> <mn>3</mn> </msub> </math>  -symmetric two Higgs doublet model with two generations of vector-like leptons (VLLs) at the proposed International Linear Collider (ILC). The lightest neutral component of the VLL in this model provides a viable dark matter (DM) candidate satisfying the current relic density data as well as circumventing all direct and indirect DM search constraints. Some representative benchmark points have been selected with low, medium and high DM masses, satisfying all theoretical and experimental constraints of the model and constraints coming from the DM sector. The VLLs (both neutral and charged) will be produced in pair leading to multi-lepton and multi-jet final states. We show that the ILC will prove to be a much more efficient and useful machine to hunt for such signals compared to LHC. Using traditional cut-based analysis as well as sophisticated multivariate analysis, we perform a detailed analysis of some promising channels containing mono-lepton plus di-jet, di-lepton, four-lepton and four jets along with missing transverse energy in the final state at 1 TeV ILC.

Nivedita Ghosh

Santosh Rai

Signals for vector-like leptons in an  $$S_3$$  <math> <msub> <mi>S</mi> <mn>3</mn> </msub> </math>  -symmetric 2HDM at ILC

We study the  $$S_3$$  <math> <msub> <mi>S</mi> <mn>3</mn> </msub> </math>  -symmetric two Higgs doublet model by adding two generations of vector like leptons (VLL) which are odd under a discrete  $$Z_2$$  <math> <msub> <mi>Z</mi> <mn>2</mn> </msub> </math>  -symmetry. The lightest neutral component of the VLL acts as a dark matter (DM) whereas the full VLL set belongs to a dark sector with no mixings allowed with the standard model fermions. We analyse the model in light of dark matter and collider searches. We show that the DM is compatible with the current relic density data as well as satisfying all direct and indirect dark matter search constraints. We choose some representative points in the model parameter space allowed by all aforementioned dark matter constraints and present a detailed collider analysis of multi-lepton signals viz. the mono-lepton, di-lepton, tri-lepton and four-lepton along with missing transverse energy in the final state using both the cut-based analysis and multivariate analysis respectively at the high luminosity 14 TeV LHC run.

Dark Matter and Collider Searches in  $$S_3$$  <math> <msub> <mi>S</mi> <mn>3</mn> </msub> </math>  -Symmetric 2HDM with Vector Like Lepton

Color-octet scalars arise in various Grand Unification scenarios and also in other models of new physics. They are also postulated for minimal flavour violation. Purely phenomenological imprints of such scalars are therefore worth looking at. Motivated by this, we perform a complete one-loop calculation of the  $$H^+ \rightarrow W^+ Z (\gamma )$$  <math> <mrow> <msup> <mi>H</mi> <mo>+</mo> </msup> <mo>→</mo> <msup> <mi>W</mi> <mo>+</mo> </msup> <mi>Z</mi> <mrow> <mo>(</mo> <mi>γ</mi> <mo>)</mo> </mrow> </mrow> </math>   decay in a two Higgs doublet model augmented by a color-octet  $$SU(2)_L$$  <math> <mrow> <mi>S</mi> <mi>U</mi> <msub> <mrow> <mo>(</mo> <mn>2</mn> <mo>)</mo> </mrow> <mi>L</mi> </msub> </mrow> </math>   scalar doublet. The computation is conveniently segregated into colorless and colored components. The color-octet part of the amplitude, being scaled by the color-factor, provides an overall enhancement to the form factors. Crucial constraints from perturbative unitarity, positivity of the scalar potential, oblique parameters, Higgs signal strengths and direct search of a charged Higgs and color-octet scalars are folded-in into the analysis. Sensitivity of the loop-induced  $$H^+ \rightarrow W^+ Z (\gamma )$$  <math> <mrow> <msup> <mi>H</mi> <mo>+</mo> </msup> <mo>→</mo> <msup> <mi>W</mi> <mo>+</mo> </msup> <mi>Z</mi> <mrow> <mo>(</mo> <mi>γ</mi> <mo>)</mo> </mrow> </mrow> </math>   vertex to other model parameters is elucidated. Finally, the prospect of observing a loop-induced  $$H^+ \rightarrow W^+ Z (\gamma )$$  <math> <mrow> <msup> <mi>H</mi> <mo>+</mo> </msup> <mo>→</mo> <msup> <mi>W</mi> <mo>+</mo> </msup> <mi>Z</mi> <mrow> <mo>(</mo> <mi>γ</mi> <mo>)</mo> </mrow> </mrow> </math>   interaction at the future hadronic collisions is also discussed.

Taiwan

Fingerprinting the contribution of colored scalars to the  $$H^+ W^- Z(\gamma )$$  <math> <mrow> <msup> <mi>H</mi> <mo>+</mo> </msup> <msup> <mi>W</mi> <mo>-</mo> </msup> <mi>Z</mi> <mrow> <mo>(</mo> <mi>γ</mi> <mo>)</mo> </mrow> </mrow> </math>   vertex

The flavorful $$Z^\prime $$ <math><msup><mi>Z</mi><mo>′</mo></msup></math>  model with its couplings restricted to the left-handed second generation leptons and third generation quarks can potentially resolve the observed anomalies in $$R_K$$ <math><msub><mi>R</mi><mi>K</mi></msub></math>  and $$R_{K^*}$$ <math><msub><mi>R</mi><msup><mi>K</mi><mo>∗</mo></msup></msub></math> . After examining the current limits on this model from various low-energy processes, we probe this scenario at 14 TeV high-luminosity run of the LHC using two complementary channels: one governed by the coupling of $$Z'$$ <math><msup><mi>Z</mi><mo>′</mo></msup></math>  to b-quarks and the other to muons. We also discuss the implications of the latest LHC high mass resonance searches in the dimuon channel on the model parameter space of our interest.

Siddharth Dwivedi

Adam Falkowski

Associated $$Z^\prime $$ <math><msup><mi>Z</mi><mo>′</mo></msup></math>  production in the flavorful U(1) scenario for $$R_{K^{(*)}}$$ <math><msub><mi>R</mi><msup><mi>K</mi><mrow><mo>(</mo><mrow></mrow><mo>∗</mo><mo>)</mo></mrow></msup></msub></math>