arXiv subjects

Stefan Antusch

Publications and source records attributed to Stefan Antusch.

At least 37 records · Page 2Linked to original sources

arXiv2023

Heavy neutrino-antineutrino oscillations at the FCC-ee

We discuss the impact of heavy neutrino-antineutrino oscillations (NNOs) on heavy neutral lepton (HNL) searches at proposed electron-positron colliders such as the future circular $e^+e^-$ collider (FCC-ee). During the $Z$ pole run, HNLs can be produced alongside a light neutrino or antineutrino that escapes detection and can decay into a charged lepton or antilepton together with an off-shell $W$ boson. In this case, signals of lepton number violation only show up in the final state distributions. We discuss how NNOs, a typical feature of collider-testable low-scale seesaw models where the heavy neutrinos form pseudo-Dirac pairs, modify such final state distributions. For example, the forward-backward asymmetry (FBA) of the reconstructed heavy (anti)neutrinos develops an oscillatory dependence on the HNL lifetime. We show that these oscillations can be resolvable for long-lived HNLs. We also discuss that when the NNOs are not resolvable, they can nevertheless significantly modify the theory predictions for FBAs and observables such as the ratio of the total number of HNL decays into $\ell^-$ over ones into $\ell^+$, in an interval of the angle~$θ$ between the HNL and the beam axis. Our results show that NNOs should be included in collider simulations of HNLs at the FCCee.

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arXiv2023

Singling out SO(10) GUT models using recent PTA results

In this work, we construct promising model building routes towards SO(10) GUT inflation and examine their ability to explain the recent PTA results hinting at a stochastic gravitational wave (GW) background at nanohertz frequencies. We consider a supersymmetric framework within which the so-called doublet-triplet splitting problem is solved without introducing fine-tuning. Additionally, realistic fermion masses and mixings, gauge coupling unification, and cosmic inflation are incorporated by utilizing superfields with representations no higher than the adjoint representation. Among the three possible scenarios, two of these cases require a single adjoint Higgs field, and do not lead to cosmic strings. In contrast, the third scenario featuring two adjoints, can lead to a network of metastable cosmic strings that generates a GW background contribution compatible with the recent PTA findings and testable by various ongoing and upcoming GW observatories.

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arXiv2023

Fully Testable Axion Dark Matter within a Minimal $SU(5)$ GUT

We present a minimal Grand Unified Theory model, based on $SU(5)$ gauge symmetry and a global $U(1)$ Peccei-Quinn symmetry, that predicts the existence of an ultralight axion dark matter within a narrow mass range of $m_a\in[0.1,\,4.7]\,$neV. This mass window is determined through an interplay between gauge coupling unification constraints, partial proton decay lifetime limits, and the need to reproduce the experimentally observed fermion mass spectrum. The entire parameter space of the proposed model will be probed through a synergy between several low-energy experiments that look for proton decay (Hyper-Kamiokande), axion dark matter through axion-photon coupling (ABRACADABRA and DMRadio-GUT) and nucleon electric dipole moments (CASPEr Electric).

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arXiv2023

Decoherence effects on lepton number violation from heavy neutrino-antineutrino oscillations

We study decoherence effects and phase corrections in heavy neutrino-antineutrino oscillations (NNOs), based on quantum field theory with external wave packets. Decoherence damps the oscillation pattern, making it harder to resolve experimentally. Additionally, it enhances lepton number violation (LNV) for processes in symmetry-protected low-scale seesaw models by reducing the destructive interference between mass eigenstates. We discuss a novel time-independent shift in the phase and derive formulae for calculating decoherence effects and the phase shift in the relevant regimes, which are the no dispersion regime and transverse dispersion regime. We find that the phase shift can be neglected in the parameter region under consideration since it is small apart from parameter regions with large damping. In the oscillation formulae, decoherence can be included by an effective damping parameter. We discuss this parameter and present averaged results, which apply to simulations of NNOs in the dilepton-dijet channel at the HL-LHC. We show that including decoherence effects can dramatically change the theoretical prediction for the ratio of LNV over LNC events.

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arXiv2023

Characterizing the post-inflationary reheating history, Part II: Multiple interacting daughter fields

We characterize the post-inflationary dynamics of an inflaton $ϕ$ coupled to multiple interacting daughter fields $X_n$ ($n=1,\dots N_d$) through quadratic-quadratic interactions $g_n^ 2ϕ^2 X_n^2$. We assume a monomial inflaton potential $V(ϕ) \propto |ϕ|^p$ ($p \geq 2$) around the minimum. By simulating the system in 2+1-dimensional lattices, we study the post-inflationary evolution of the energy distribution and equation of state, from the end of inflation until a stationary regime is achieved. We show that in this scenario, the energy transferred to the daughter field sector can be larger than $50\%$, surpassing this way the upper bound found previously for single daughter field models. In particular, for $p \geq 4$ the energy at very late times is equally distributed between all fields, and only $100/(N_d + 1) \%$ of the energy remains in the inflaton. We also consider scenarios in which the daughter fields have scale-free interactions $λ_{nm} X_n^2 X_m^2$, including the case of quartic daughter field self-interactions (for $n=m$). We show that these interactions trigger a resonance process during the non-linear regime, which in the single daughter field case already allows to deplete more than $50\%$ of the energy from the inflaton for $p\geq 4$.

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arXiv2023

Quark-lepton Yukawa ratios and nucleon decay in SU(5) GUTs with type-III seesaw

We consider an extension of the Georgi-Glashow SU(5) GUT model by a 45-dimensional scalar and a 24-dimensional fermionic representation, where the latter leads to the generation of the observed light neutrino masses via a combination of a type I and a type III seesaw mechanism. Within this scenario, we investigate the viability of predictions for the ratios between the charged lepton and down-type quark Yukawa couplings, focusing on the second and third family. Such predictions can emerge when the relevant entries of the Yukawa matrices are generated from single joint GUT operators (i.e. under the condition of single operator dominance). We show that three combinations are viable, (i) $y_τ/y_b=3/2$, $y_μ/y_s=9/2$, (ii) $y_τ/y_b=2$, $y_μ/y_s=9/2$, and (iii) $y_τ/y_b=2$, $y_μ/y_s=6$. We extend these possibilities to three toy models, accounting also for the first family masses, and calculate their predictions for various nucleon decay rates. We also analyse how the requirement of gauge coupling unification constrains the masses of potentially light relic states testable at colliders.

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arXiv2022

Characterizing the post-inflationary reheating history, Part I: single daughter field with quadratic-quadratic interaction

We study the evolution of the energy distribution and equation of state of the Universe from the end of inflation until the onset of either radiation domination (RD) or a transient period of matter domination (MD). We use both analytical techniques and lattice simulations. We consider two-field models where the inflaton $Φ$ has a monomial potential after inflation $V(Φ) \propto |Φ- v|^p$ ($p\geq2$), and is coupled to a daughter field $X$ through a quadratic-quadratic interaction $g^2Φ^2 X^2$. We consider two situations, depending on whether the potential has a minimum at $i)$ $v = 0$, or $ii)$ $v > 0$. In the scenario $i)$, the final energy transferred to $X$ is independent of $g^2$ and entirely determined by $p$: it is negligible for $p < 4$, and of order $\sim 50\%$ for $p \geq 4$. The system goes to MD at late times for $p = 2$, while it goes to RD for $p > 2$. In the later case, we can calculate exactly the number of e-folds until RD as a function of $g^2$, and hence predict accurately inflationary observables like the scalar tilt $n_s$ and the tensor-to-scalar ratio $r$. In the scenario $ii)$, the energy is always transferred completely to $X$ for $p>2$, as long as its effective mass $m_X^2 = g^2(Φ-v)^2$ is not negligible. For $p=2$, the final ratio between the energy densities of $X$ and $Φ$ depends strongly on $g^2$. For all $p \ge 2$, the system always goes to MD at late times.

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arXiv2022

Implications of the Zero 1-3 Flavour Mixing Hypothesis: Predictions for $θ_{23}^\mathrm{PMNS}$ and $δ^\mathrm{PMNS}$

We revisit mixing sum rule relations in the lepton and quark sectors under the assumption that the 1-3 elements of the flavour mixing matrices ($V^u_L,V^d_L,V^e_L,V^ν_L$) are zero in the flavour basis. We consider the exact relations resulting from the validity of this "zero 1-3 flavour mixing hypothesis" and analyse their implications based on the current experimental data, including effects from RG running. In particular, we analyse how the existing precise measurement of $θ_{13}^\mathrm{PMNS}$ allows to derive predictions for $θ_{23}^\mathrm{PMNS}$ in models with constrained $θ_{12}^\mathrm{e}$. As examples, we calculate the predictions for $θ_{23}^\mathrm{PMNS}$ which arise in classes of Pati-Salam models and SU(5) GUTs that relate $θ_{12}^\mathrm{e}$ to $θ_{12}^\mathrm{d}$. We also derive a novel "lepton phase sum rule", valid under the additional assumption of small charged lepton mixing contributions. We furthermore point out that, in the context of GUT flavour models, the quark and lepton CP violating phases $δ^\mathrm{CKM}$ and $δ^\mathrm{PMNS}$ can both be predicted from a single imaginary element in the mass matrices.

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arXiv2022

Viable quark-lepton Yukawa ratios and nucleon decay predictions in $SU(5)$ GUTs with type-II seesaw

We investigate the viability of predictive schemes for quark-lepton Yukawa ratios and nucleon decay in non-supersymmetric SU(5) Grand Unified Theories (GUTs) where neutrino masses are generated by a type II seesaw mechanism. The scalar sector of the considered scenario contains 5-, 24- and 45-dimensional representations plus a 15-dimensional representation for realising the type II seesaw. Predictions for the ratios of the quark and lepton Yukawa couplings emerge when the relevant entries of the Yukawa matrices are generated from single joint GUT operators (i.e. under the condition of "single operator dominance"). Focusing on the 2nd and 3rd family and hierarchical Yukawa matrices, we show that only two sets of predictions, $\frac{y_τ}{y_b}=\frac{3}{2}$, $\frac{y_μ}{y_s}=\frac{9}{2}$ and $\frac{y_τ}{y_b}=2$, $\frac{y_μ}{y_s}=6$ are viable. To further investigate both options, we extend the minimal scenarios to two "toy models", including the 1st family of charged fermions, and calculate the models' predictions, e.g. for the nucleon decay rates and for the masses of the light relics that are potentially within reach of colliders.

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arXiv2021

Explaining excesses in four-leptons at the LHC with a double peak from a CP violating Two Higgs Doublet Model

Extended scalar sectors with additional degrees of freedom appear in many scenarios beyond the Standard Model. Heavy scalar resonances that interact with the neutral current could be discovered via broad resonances in the tails of the four-lepton invariant mass spectrum, where the Standard Model background is small and well understood. In this article we consider a recent ATLAS measurement of four-lepton final states, where the data is in excess over the background for invariant masses above 500 GeV. We discuss the possibility that this excess could be interpreted as a "double peak" from the two extra heavy neutral scalars of a CP violating Two Higgs Doublet Model, both coupling to the $Z$ boson. We apply an iterative fitting procedure to find viable model parameters that can match the excess, resulting in a benchmark point where the observed four-lepton invariant mass spectrum can be explained by two scalar particles $H_2$ and $H_3$, with masses of 540 GeV and 631 GeV, respectively, being admixtures of the CP eigenstates. Our explanation predicts additional production processes for $t\bar t$, $W^+W^-$, $4b$ and $γγ$, some of which have cross sections close to the current experimental limits. Our results further imply that the electric dipole moment of the electron should be close to the present bounds.

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arXiv2021

Unveiling Hidden Physics at the LHC

The field of particle physics is at the crossroads. The discovery of a Higgs-like boson completed the Standard Model (SM), but the lacking observation of convincing resonances Beyond the SM (BSM) offers no guidance for the future of particle physics. On the other hand, the motivation for New Physics has not diminished and is, in fact, reinforced by several striking anomalous results in many experiments. Here we summarise the status of the most significant anomalies, including the most recent results for the flavour anomalies, the multi-lepton anomalies at the LHC, the Higgs-like excess at around 96 GeV, and anomalies in neutrino physics, astrophysics, cosmology, and cosmic rays. While the LHC promises up to 4/ab of integrated luminosity and far-reaching physics programmes to unveil BSM physics, we consider the possibility that the latter could be tested with present data, but that systemic shortcomings of the experiments and their search strategies may preclude their discovery for several reasons, including: final states consisting in soft particles only, associated production processes, QCD-like final states, close-by SM resonances, and SUSY scenarios where no missing energy is produced. New search strategies could help to unveil the hidden BSM signatures, devised by making use of the CERN open data as a new testing ground. We discuss the CERN open data with its policies, challenges, and potential usefulness for the community. We showcase the example of the CMS collaboration, which is the only collaboration regularly releasing some of its data. We find it important to stress that individuals using public data for their own research does not imply competition with experimental efforts, but rather provides unique opportunities to give guidance for further BSM searches by the collaborations. Wide access to open data is paramount to fully exploit the LHCs potential.

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arXiv2021

Nucleon decay in a minimal non-SUSY GUT with predicted quark-lepton Yukawa ratios

We investigate the predictions for various nucleon decay rates and their ratios in non-supersymmetric SU(5) Grand Unified Theories (GUTs) where the masses of the third and second family down-type quarks and charged leptons each stem dominantly from single GUT operators. Extending the Georgi-Glashow SU(5) model by a 45-dimensional GUT-Higgs representation, the gauge couplings can meet at the high scale $M_\mathrm{GUT}$ and the GUT scale predictions $y_τ/y_b = 3/2$ and $y_μ/y_s = 9/2$ can emerge within single operator dominance. Explaining the observed neutrino masses via the type I seesaw mechanism by adding SU(5) singlet fermion representations and taking their renormalization group effects into account, we show that these predictions can lead to viable low scale second and third family down-type quark and charged lepton masses. To investigate nucleon decay predictions, we extend the minimal scenario to two "toy models" towards explaining quark and lepton masses and mixings with different flavor structure, and perform Markov Chain Monte Carlo (MCMC) analyses confronting the models with the available experimental data. We show that if several nucleon decay channels are observed, the ratios between their partial decay rates can serve as "fingerprints", allowing to separate between GUT models with different flavor structure.

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arXiv2021

Searching for charged lepton flavor violation at $ep$ colliders

We investigate the sensitivity of electron-proton ($ep$) colliders for charged lepton flavor violation (cLFV) in an effective theory approach, considering a general effective Lagrangian for the conversion of an electron into a muon or a tau via the effective coupling to a neutral gauge boson or a neutral scalar field. For the photon, the $Z$ boson and the Higgs particle of the Standard Model, we present the sensitivities of the LHeC for the coefficients of the effective operators, calculated from an analysis at the reconstructed level. As an example model where such flavor changing neutral current (FCNC) operators are generated at loop level, we consider the extension of the Standard Model by sterile neutrinos. We show that the LHeC could already probe the LFV conversion of an electron into a muon beyond the current experimental bounds, and could reach more than an order of magnitude higher sensitivity than the present limits for LFV conversion of an electron into a tau. We discuss that the high sensitivities are possible because the converted charged lepton is dominantly emitted in the backward direction, enabling an efficient separation of the signal from the background.

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arXiv2021

Heavy Neutrino-Antineutrino Oscillations in Quantum Field Theory

It has been proposed that the coherent propagation of long-lived heavy neutrino mass eigenstates can lead to an oscillating rate of lepton number conserving (LNC) and violating (LNV) events, as a function of the distance between the production and displaced decay vertices. We discuss this phenomenon, which we refer to as heavy neutrino-antineutrino oscillations, in the framework of quantum field theory (QFT), using the formalism of external wave packets. General formulae for the oscillation probabilities and the number of expected events are derived and the coherence and localisation conditions that have to be satisfied in order for neutrino-antineutrino oscillations to be observable are discussed. The formulae are then applied to a low scale seesaw scenario, which features two nearly mass degenerate heavy neutrinos that can be sufficiently long lived to produce a displaced vertex when their masses are below the $W$ boson mass. The leading and next-to-leading order oscillation formulae for this scenario are derived. For an example parameter point used in previous studies, the kinematics of the considered LNC/LNV processes are simulated, to check that the coherence and localisation conditions are satisfied. Our results show that the phenomenon of heavy neutrino-antineutrino oscillations can indeed occur in low scale seesaw scenarios and that the previously used leading order formulae, derived with a plane wave approach, provide a good approximation for the considered example parameter point.

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arXiv2021

Testing CP Properties of Extra Higgs States at the HL-LHC

Extra Higgs states appear in various scenarios beyond the current Standard Model of elementary particles. If discovered at the LHC or future colliders, the question will arise whether CP is violated or conserved in the extended scalar sector. An unambiguous probe of (indirect) CP violation would be the observation that one of the extra Higgs particles is an admixture of a CP-even and a CP-odd state. We discuss the possibility to discover scalar CP violation in this way at the high-luminosity (HL) phase of the LHC. We focus on the Two-Higgs Doublet Model of type I, where we investigate its currently allowed parameter region. Considering a benchmark point that is compatible with the current constraints but within reach of the HL-LHC, we study the prospects of determining the CP property of an extra neutral Higgs state $H$ via the angular distribution of final states in the decay $H \to τ\barτ$. The analysis is performed at the reconstructed level, making use of a Boosted Decision Tree for efficient signal-background separation and a shape analysis for rejecting a purely CP-even or odd nature of $H$.

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arXiv2020

Energy distribution and equation of state of the early Universe: matching the end of inflation and the onset of radiation domination

We study the energy distribution and equation of state of the universe between the end of inflation and the onset of radiation domination (RD), considering observationally consistent single-field inflationary scenarios, with a potential 'flattening' at large field values, and a monomial shape $V(ϕ) \propto |ϕ|^p$ around the origin. As a proxy for (p)reheating, we include a quadratic interaction $g^2ϕ^2X^2$ between the inflaton $ϕ$ and a light scalar 'daughter' field $X$, with $g^2>0$. We capture the non-perturbative and non-linear nature of the system dynamics with lattice simulations, obtaining that: $i)$ the final energy transferred to $X$ depends only on $p$, not on $g^2$, ; $ii)$ the final transfer of energy is always negligible for $2 \leq p < 4$, and of order $\sim 50\%$ for $p \geq 4$; $iii)$ the system goes at late times to matter-domination for $p = 2$, and always to RD for $p > 2$. In the latter case we calculate the number of e-folds until RD, significantly reducing the uncertainty in the inflationary observables $n_s$ and $r$.

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arXiv2020

Employing nucleon decay as a fingerprint of SUSY GUT models using \texttt{SusyTCProton}

While the observation of nucleon decay would be a smoking gun of Grand Unified Theories (GUTs) in general, the ratios between the decay rates of the various channels carry rich information about the specific GUT model realization. To investigate this fingerprint of GUT models in the context of supersymmetric (SUSY) GUTs, we present the software tool \texttt{SusyTCProton}, which is an extension of the module \texttt{SusyTC} to be used with the \texttt{REAP} package. It allows to calculate nucleon decay rates from the relevant dimension five GUT operators specified at the GUT scale, including the full loop-dressing at the SUSY scale. As an application, we investigate the fingerprints of two example GUT toy models with different flavor structures, performing an MCMC analysis to include the experimental uncertainties for the charged fermion masses and CKM mixing parameters. While both toy models provide equally good fits to the low energy data, we show how they could be distinguished via their predictions of ratios for nucleon decay rates. Together with \texttt{SusyTCProton} we also make the additional module \texttt{ProtonDecay} public. It can be used independently from \texttt{REAP} and allows to calculate nucleon decay rates from given $D=5$ and $D=6$ operator coefficients (accepting the required SUSY input for the $D=5$ case in SLHA format). The $D=6$ functionality can also be used to calculate nucleon decay in non-SUSY GUTs.

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arXiv2020

Probing $Z^\prime$ Mediated Charged Lepton Flavor Violation with Taus at the LHeC

While charged lepton flavor violation (cLFV) with taus is often expected to be largest in many extensions of the Standard Model (SM), it is currently much less constrained than cLFV with electrons and muons. We study the sensitivity of the LHeC to $e$-$τ$ (and $e$-$μ$) conversion processes $p e^- \to τ^- + j$ (and $p e^- \to μ^- + j$) mediated by a $Z'$ with flavor-violating couplings to charged leptons in the $t$-channel. Compared to current tests at the LHC, where cLFV decays of the $Z'$ (produced in the s-channel) are searched for, the LHeC has sensitivity to much higher $Z'$ masses, up to O(10) TeV. For cLFV with taus, we find that the LHeC sensitivity from the process $p e^- \to τ^- + j$ can exceed the current limits from collider and non-collider experiments in the whole considered $Z'$ mass range (above $500$ GeV) by more than two orders of magnitude. In particular for extensions of the SM with a heavy $Z'$, where direct production at colliders is kinematically suppressed, $e-τ$ conversion at LHeC provides an exciting new discovery channel for this type of new physics.

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