arXiv subjects

Katri Huitu

Publications and source records attributed to Katri Huitu.

At least 19 recordsLinked to original sources

arXiv2025

Freeze-in and Freeze-out in a Right-Handed Neutrino Extended MSSM with a Seesaw Mechanism

We investigate the possibility of saturating the relic density bound with light Higgsinos. When the minimal supersymmetric Standard Model is extended with right-handed neutrino superfields and the seesaw scale is very low, right-handed sneutrinos can be produced via the freeze-in mechanism. In such a case we can have essentially two independent sources for dark matter, the traditional freeze-out of Higgsinos and the freeze-in of right-handed sneutrinos. The heavier of these two will decay to the lighter species with a delay. We rule out such a scenario for all seesaw models as the lifetime of sterile neutrinos produced over-abundantly via Dodelson-Widrow mechanism exceeds the age of the universe and will contribute to the relic density.

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arXiv2025

Boosted Dark Matter Driven by Cosmic Rays and Diffuse Supernova Neutrinos

Direct detection of light dark matter can be significantly enhanced by up-scattering of dark matter with energetic particles in the cosmic ambient. This boosted dark matter flux can reach kinetic energies up to tens of MeV, while the typical kinetic energies of GeV mass dark matter particles in the Milky Way halo are of the order of keV. Dark matter boosted by energetic diffuse supernova background neutrinos can be detected only through nuclear or electron scattering in ground-based detectors requiring a non-zero interaction of dark matter with nucleon or electron, in addition to its interaction with neutrino. However, in the presence of dark matter-nucleon (electron) interaction, the scattering of dark matter with cosmic rays is unavoidable. Thus, we consider boosted dark matter resulting from diffuse supernova neutrinos as well as cosmic protons (electrons) considering both energy-dependent and energy-independent scattering cross-sections between dark matter and standard model particles. We explore this scenario in dark matter detectors such as XENONnT and neutrino detectors like Super-Kamiokande.

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arXiv2024

Revisiting Universal Extra-Dimension Model with Gravity Mediated Decays

We explore the collider phenomenology of the fat-brane realization of the Minimal Universal Extra Dimension (mUED) model, where Standard Model (SM) fields propagate in a small extra dimension while gravity accesses additional large extra dimensions. This configuration allows for gravity-mediated decay (GMD) of Kaluza-Klein (KK) particles, resulting in unique final states with hard photons, jets, massive SM bosons, and large missing transverse energy due to invisible KK gravitons. We derive updated constraints on the model's parameter space by recasting ATLAS mono-photon, di-photon, and multi-jet search results using 139 inverse femtobern of integrated luminosity data. Recognizing that current LHC search strategies are tailored for supersymmetric scenarios and may not fully capture the distinct signatures, we propose optimized strategies using machine learning algorithms to tag boosted SM bosons and enhance signal discrimination against SM backgrounds. These methods improve sensitivity to fat-brane mUED signatures and offer promising prospects for probing this model in future LHC runs.

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arXiv2024

Revisiting the LHC Constraints on Gauge-Mediated Supersymmetry Breaking Scenarios

Supersymmetry (SUSY) addresses several problems of the Standard Model, such as the naturalness problem and gauge coupling unification, and can provide cosmologically viable dark matter candidates. SUSY must be broken at high energy scales with mechanisms like gravity, anomaly, gauge mediation, etc. This paper revisits the Gauge Mediated SUSY Breaking (GMSB) scenarios in the context of data from the Large Hadron Collider (LHC) experiment. The ATLAS mono-photon search at 139 inverse femtobarn integrated luminosity at the 13 TeV LHC, in the context of a simplified General Gauge Mediation (GGM) scenario (which is a phenomenological version of GMSB with an agnostic approach to the nature of the hidden sector), relies on assumptions that do not hold across the entire parameter space. We identify a few crucial assumptions regarding the decay widths of SUSY particles into final states with gravitinos that affect the LHC limits on the masses of the SUSY particles. Our study aims to reinterpret the ATLAS constraints on the gluino-NLSP mass plane, considering all possible decay modes of SUSY particles in a realistic GGM model.

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arXiv2024

On the family discrimination in 331-model

In the so-called 331-models the gauge anomalies cancel only if there are three generations of fermions. This requires one of the quark generations to be in a different representation than the other two. But which generation is treated differently? In this work we study how the choice of differently treated generation effects the quark flavour structure and how the discriminated generation can be deduced from experiments. We study a general model based on $β=-1/\sqrt{3}$, which contains exotic quarks with same electric charges as SM quarks. We take fully into account the effects from exotic quark mixing with the SM quarks, which is often omitted in literature. We will also pay particular attention to $125$ GeV Higgs, and show analytically why its flavour violating couplings between SM quarks are suppressed.

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arXiv2022

Type-II see-saw: searching the LHC elusive low-mass triplet-like Higgses at $e^-e^+$ colliders

While the triplet-like Higgses up to a few hundred GeV masses are already excluded for a vast region of the model parameter space from the LHC searches, strikingly, there is a region of this parameter space that is beyond the reach of the existing LHC searches, and doubly/singly-charged and neutral Higgses as light as 200 GeV or even lighter are still allowed by the LHC data. We study several search strategies targeting different parts of this LHC elusive parameter space at two configurations of $e^-e^+$ colliders -- 500 GeV and 1 TeV centre of mass energies. We find that a vast region of this parameter space could be probed with 5$σ$ discovery with the early $e^-e^+$ colliders' data.

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arXiv2022

Multi-phase critical Higgs boson at colliders

The recently proposed multi-phase criticality principle in Coleman-Weinberg models can provide a new explanation for the hierarchy between the electroweak and new physics scales. When applied to the Standard Model, a Higgs boson as light as the pseudo-Goldstone boson of broken scale invariance occurs. The suppressed mixing between the two light fields still carries information about the large scale of symmetry breaking, albeit up to logarithmic corrections. In this work we probe this scenario with the present LHC data and assess the impact of future lepton and hadron colliders. Our results show that the multi-phase criticality can easily explain the apparent absence of new physics at the energy scales tested in current experiments.

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arXiv2020

Reinterpretation of LHC Results for New Physics: Status and Recommendations after Run 2

We report on the status of efforts to improve the reinterpretation of searches and measurements at the LHC in terms of models for new physics, in the context of the LHC Reinterpretation Forum. We detail current experimental offerings in direct searches for new particles, measurements, technical implementations and Open Data, and provide a set of recommendations for further improving the presentation of LHC results in order to better enable reinterpretation in the future. We also provide a brief description of existing software reinterpretation frameworks and recent global analyses of new physics that make use of the current data.

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arXiv2020

Probing heavy scalars with an effective $Hb\bar bg$ coupling at the LHC

We have explored the prospect of probing a neutral scalar ($H$) produced in association with one $b$-quark and decaying either invisibly or into a pair of $b$-quarks at the LHC with centre of mass energy $\sqrt s = 14$ TeV. In this regard, we adopt an effective theory approach to parameterize a $Hb\bar bg$ vertex arising from a dimension six operator that encompasses the effect of some new physics setting in at a high scale. We concentrate solely on the five-flavor scheme to ascertain the sensitivity of the 14 TeV LHC in probing such an effective coupling as a function of the scalar mass at the highest possible projected luminosity, $3000~{\rm fb}^{-1}$. Through our multivariate analysis using machine learning algorithm we show that staying within the perturbative limit of the Wilson coefficient of the effective interaction, evidence with statistical significance of $3σ$ can be obtained in two different signal regions for $m_H\lesssim 2$ TeV and the scale of new physics $Λ= 3$ TeV.

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arXiv2020

The left-right supersymmetric option at a high-energy upgrade of the LHC

We investigate the possibility that a minimal realization of left-right supersymmetry can be reachable at a high-energy upgrade of the LHC, expected to operate at a center-of-mass energy of 27 TeV. This minimal scenario has a relatively light $SU(2)_R$ doubly-charged Higgs boson, which could decay dominantly into tau-lepton pairs. We explore the associated signals comprised of at least three hadronically-decaying taus, or with at least two hadronic taus and one same-sign-same-flavor charged lepton pair. Our analysis shows that the former signature is challenging to use for getting handles on the signal due to the large corresponding background, and that the latter one can lead to a handful of new physics events in an almost background-free environment. We however find that a signal comprised of three hadronically-decaying tau leptons is likely to be observed at a low luminosity of proton-proton collisions at a 27 TeV upgrade of the LHC.

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arXiv2020

Natural neutrino sector in a 331-model with Froggatt-Nielsen mechanism

The extensions of the Standard Model based on the $SU(3)_c\times SU(3)_L\times U(1)_X$ gauge group (331-models) have been advocated to explain the number of fermion families in nature. It has been recently shown that the Froggatt-Nielsen mechanism, a popular way to explain the mass hierarchy of the charged fermions, can be incorporated into the 331-setting in an economical fashion (FN331). In this work we extend the FN331-model to include three right-handed neutrino singlets. We show that the seesaw mechanism is realized in this model. The scale of the seesaw mechanism is near the $SU(3)_L\times U(1)_X$-breaking scale. The model we present here simultaneously explains the mass hierarchy of all the fermions, including neutrinos, and the number of families.

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arXiv2020

Suppression of scalar mediated FCNCs in a $SU(3)_c\times SU(3)_L\times U(1)_X$-model

The models based on $SU(3)_C\times SU(3)_L\times U(1)_X$ gauge symmetry (331-models) have been advocated to explain the number of fermion families. These models place one quark family to a different representation than the other two. The traditional 331-models are plagued by scalar mediated quark flavour changing neutral currents (FCNC) at tree-level. So far there has been no concrete mechanisms to suppress these FCNCs in 331-models. Recently it has been shown that the Froggatt-Nielsen mechanism can be incorporated into the 331-setting in an economical fashion (FN331-model). The FN331-model explains both the number of fermion families in nature and their mass hierarchy simultaneously. In this work we study the Higgs mediated quark FCNCs in FN331-model. The flavour violating couplings of quarks are suppressed by the ratio of the $SU(2)_L \times U(1)_Y$ and $SU(3)_L\times U(1)_X$ breaking scales. We find that the $SU(3)_L\times U(1)_X$-breaking scale can be as low as 5 TeV in order to pass the flavour bounds.

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arXiv2020

Froggatt-Nielsen mechanism in a model with $SU(3)_c\times SU(3)_L \times U(1)_X$ gauge group

The models with the gauge group $SU(3)_c\times SU(3)_L \times U(1)_X$ (331-models) have been advocated to explain why there are three fermion generations in Nature. As such they can provide partial understanding of the flavour sector. The hierarchy of Yukawa-couplings in the Standard Model is another puzzle which remains without compelling explanation. We propose to use Froggatt-Nielsen -mechanism in a 331-model to explain both fundamental problems. It turns out that no additional representations in the scalar sector are needed to take care of this. The traditional 331-models predict scalar flavour changing neutral currents at tree-level. We show that they are strongly suppressed in our model.

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arXiv2020

Cosmological constraints on light flavons

The Froggatt-Nielsen mechanism is a well-motivated framework for generating the fermion mass hierarchy. This mechanism introduces flavons, complex scalars which are singlet under the Standard Model gauge symmetry and charged under a new global family symmetry. We make use of a leptophilic flavon to produce the charged lepton Yukawa matrix. The real part of the flavon mixes with the Higgs boson and introduces lepton flavour violating interactions which are bounded by experiment. The imaginary part of the flavon, $η$, is a long-lived light particle, whose abundance is restricted by cosmological observations. For $m_η< 2m_e$ where the decay of $η$ to charged leptons is kinematically forbidden, we identify allowed regions of $m_η$ with respect to the vacuum expectation value of the flavon field where all experimental and cosmological constraints are satisfied.

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arXiv2019

Neutrinos, vacuum stability and triple Higgs coupling in SMASH

We perform a phenomenological analysis of the observable consequences on the extended scalar sector of the SMASH (Standard Model - Axion - Seesaw - Higgs portal inflation) framework. We solve the vacuum metastability problem in a suitable region of SMASH scalar parameter spaces and discuss the one-loop correction to triple Higgs coupling $λ_{HHH}$. We also find that the correct neutrino masses and mass squared differences and baryonic asymmetry of the universe can arise from this model and consider running of the Yukawa couplings of the model. In fact, we perform a full two-loop renormalization group analysis of the SMASH model.

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arXiv2019

Dark matter and Collider signals in supersymmetric $U(1)^\prime$ models with non-universal $Z^\prime$ couplings

We analyse supersymmetric models augmented by an extra $U(1)$ gauge group. To avoid anomalies in these models without introducing exotics, we allow for family-dependent $U(1)^\prime$ charges, and choose a simple form for these, dependent on one $U(1)^\prime$ charge parameter only. With this choice, $Z^\prime$ decays into di-taus but not di-leptons, weakening considerably the constraints on its mass. In the supersymmetric sector, the effect is to lower the singlino mass, allowing it to be the dark matter candidate. We investigate the dark matter constraints and collider implications of such models, with mostly singlino, or mostly higgsinos, or a mixture of the two as lightest supersymmetric particles. In these scenarios, $Z^\prime$ decays significantly into chargino or neutralino pairs, and thus indirectly into final state leptons. We devise benchmarks which, with adequate cuts, can yield signals visible at the high-luminosity LHC.

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arXiv2019

Probing pseudo-Goldstone dark matter at the LHC

Pseudo-Goldstone dark matter coupled to the Standard Model via the Higgs portal offers an attractive framework for phenomenologically viable pseudo-scalar dark matter. It enjoys natural suppression of the direct detection rate due to the vanishing of the relevant (tree level) Goldstone boson vertex at zero momentum transfer, which makes light WIMP-like dark matter consistent with the strong current bounds. In this work, we explore prospects of detecting pseudo-Goldstone dark matter at the LHC, focusing on the vector boson fusion (VBF) channel with missing energy. We find that, in substantial regions of parameter space, relatively light dark matter ($m_χ< 100$ GeV) can be discovered in the high luminosity run as long as it is produced in decays of the Higgs-like bosons.

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arXiv2019

Same-sign trilepton signal for stop quark in the presence of sneutrino dark matter

We have explored a minimal supersymmetric standard model scenario extended by one pair of gauge singlets per generation. In the model light neutrino masses and their mixings are generated via inverse seesaw mechanism. In such a scenario, a right-handed sneutrino can be the lightest supersymmetric particle and a cold Dark Matter (DM) candidate. If Casas-Ibarra parametrisation is imposed on the Dirac neutrino Yukawa coupling matrix ($Y_ν$) to fit the neutrino oscillation data, the resulting $Y_ν$ is highly constrained from the lepton flavor violating (LFV) decay constraints. The smallness of $Y_ν$ requires the sneutrino DM to co-annihilate with other sparticle(s) in order to satisfy DM relic density constraint. We have studied sneutrino co-annihilation with wino and observed that this sneutrino-wino compressed parameter space gives rise to a novel same-sign trilepton signal for the stop quark, which is more effective than the conventional stop search channels in the present framework. We have shown that the choice of neutrino mass hierarchy strongly affects the signal event rate, making it easier to probe the scenario with inverted mass hierarchy.

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