arXiv subjects

Simone Biondini

Publications and source records attributed to Simone Biondini.

31 records · Page 2Linked to original sources

arXiv2020

Dark matter and Standard Model reheating from conformal GUT inflation

Spontaneous breaking of conformal symmetry has been widely exploited in successful model building of both inflationary cosmology and particle physics phenomenology. Conformal Grand Unified Theory (CGUT) inflation provides the same scalar tilt and tensor-to-scalar ratio as of Starobinsky and Higgs inflation. Moreover, it predicts a proton life time compatible with the current experimental bound. In this paper, we extend CGUT to account for the production of dark matter and the reheating of the Standard Model. To this end, we introduce a hidden sector directly coupled to the inflaton, whereas the reheating of the visible sector is realized through a portal coupling between the dark particles and the Higgs boson. The masses and interactions of the dark particles and the Higgs boson are determined by the form of the conformal potential and the non-vanishing VEV of the inflaton. We provide benchmark points in the parameter space of the model that give the observed dark matter relic density and reheating temperatures compatible with the Big Bang nucleosynthesis.

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arXiv2020

Perturbative unitarity bounds for fermions composite models

Perturbative unitarity is a powerful tool for inferring the range of validity of a given effective field theory. Here, we study such a bound in the parameter space of dimension-5 and dimension-6 effective operators that arise in a scenario of fermion compositeness. These operators are routinely used in experimental searches at the LHC to constraint contact and gauge interactions between ordinary Standard Model fermions and excited states of mass $M$. We derive the unitarity bound for the production process of an excited neutrino, then we implement such bound and compare it with the recent experimental exclusion curves for Run 2, the High-Luminosity and High-Energy configurations of the LHC. The results also apply to the searches where a generic single excited state is produced via dimension-6 contact interactions. The unitarity bound, so far overlooked in these effective models, is quite compelling and can serve as a guide for exploring the parameter space ($M,Λ$) in addition to the standard request $M \le Λ$.

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arXiv2019

Scalar dark matter coannihilating with a coloured fermion

We analyse the phenomenology of a simplified model for a real scalar dark matter candidate interacting with quarks via a coloured fermionic mediator. In the coannihilation regime, the dark matter abundance is controlled by the dynamics of the coloured fermions which can be significantly affected by non-perturbative effects. We employ a non-relativistic effective field theory approach which allows us to systematically treat the Sommerfeld effect and bound-state formation in the early Universe. The parameter space compatible with the dark matter relic abundance is confronted with direct, indirect and collider searches. A substantial part of the parameter space, with dark matter masses up to 18 TeV, is already excluded by XENON1T. Most of the remaining thermal relics can be probed by a future Darwin-like experiment, when taking properly into account the running of the relevant couplings for the direct detection processes.

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arXiv2018

Bound-state effects for dark matter with Higgs-like mediators

In this paper we study the impact of a scalar exchange on the dark matter relic abundance by solving a plasma-modified Schroedinger equation. A simplified model is considered where a Majorana dark matter fermion is embedded in a U(1)$'$ extension of the Standard Model and couples with a dark Higgs via a Yukawa interaction. We find that the dark-Higgs exchange can increase the overclosure bounds significantly. For the largest (smallest) value of the Yukawa coupling examined in this work, the dark matter mass is lifted from 5 TeV (0.55 TeV) to 27 TeV (0.70 TeV).

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arXiv2018

Status of rates and rate equations for thermal leptogenesis

In many realizations of leptogenesis, heavy right-handed neutrinos play the main role in the generation of an imbalance between matter and antimatter in the early Universe. Hence, it is relevant to address quantitatively their dynamics in a hot and dense environment by taking into account the various thermal aspects of the problem at hand. The strong washout regime offers an interesting framework to carry out calculations systematically and reduce theoretical uncertainties. Indeed, any matter-antimatter asymmetry generated when the temperature of the hot plasma $T$ exceeds the right-handed neutrino mass scale $M$ is efficiently erased, and one can focus on the temperature window $T \ll M$. We review recent progresses in the thermal field theoretic derivation of the key ingredients for the leptogenesis mechanism: the right-handed neutrino production rate, the CP asymmetry in the heavy-neutrino decays and the washout rates. The derivation of evolution equations for the heavy-neutrino and lepton-asymmetry number densities, their rigorous formulation and applicability are also discussed.

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arXiv2017

Leptogenesis and composite heavy neutrinos with gauge mediated interactions

Leptogenesis is an appealing framework to account for the baryon asymmetry in the universe. To this end physics beyond the Standard Model is demanded. In this paper we investigate the possibility to attain successful leptogenesis with composite Majorana neutrinos. We work in the framework of effective gauge mediated and contact interactions without any reference to an underlying compositeness theory. This approach is the one adopted in all current experimental searches for composite fermions at colliders. In the case of gauge mediated interactions, we calculate the CP asymmetry in heavy composite neutrino decays. Both the direct and indirect CP asymmetry are derived and resonant leptogenesis is also discussed. We find that the Sakharov conditions can be met and, for some choice of the parameters, the correct order of magnitude of the baryon asymmetry is reproduced.

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arXiv2017

Momentum anisotropy effects for quarkonium in a weakly-coupled quark-gluon plasma below the melting temperature

In the early stages of heavy-ion collisions, the hot QCD matter expands more longitudinally than transversely. This imbalance causes the system to become rapidly colder in the longitudinal direction and a local momentum anisotropy appears. In this paper, we study the heavy-quarkonium spectrum in the presence of a small plasma anisotropy. We work in the framework of pNRQCD at finite temperature. We inspect arrangements of non-relativistic and thermal scales complementary to those considered in the literature. In particular, we consider temperatures larger and Debye masses smaller than the binding energy, which is a temperature range relevant for presently running LHC experiments. In this setting we compute the leading thermal corrections to the binding energy and the thermal width induced by quarkonium gluo-dissociation.

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arXiv2016

Effective field theories for heavy Majorana neutrinos in a thermal bath

Heavy Majorana neutrinos enter in many scenarios of physics beyond the Standard Model and play a special role in the leptogenesis framework as origin of the baryon asymmetry in the universe. In this thesis, we develop an effective field theory for non-relativistic Majorana particles. We apply the effective field theory so obtained to address calculations in a hot medium which models the early stages of the universe evolution. In particular, we apply it to the case of a heavy Majorana neutrino decaying in a hot plasma of Standard Model particles, whose temperature is much smaller than the mass of the Majorana neutrino but still much larger than the electroweak scale, and the derivation of the neutrino thermal width is revisited in an EFT fashion. Assuming the same hierarchy between heavy neutrino masses and the temperature, we compute systematically thermal corrections to the direct and indirect CP asymmetries in the Majorana neutrino decays. These are key ingredients entering the equations that describe the thermodynamic evolution of the induced lepton-number asymmetry eventually leading to the baryon asymmetry in the universe. We consider the case of two Majorana neutrinos with nearly degenerate masses, that allows for a resonant enhancement of the CP asymmetry, and a hierarchical spectrum with one heavy neutrino much lighter than the other neutrino species. Flavour effects are also taken into account in the derivation of the CP asymmetries at finite temperature.

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arXiv2016

CP asymmetry in heavy Majorana neutrino decays at finite temperature: the hierarchical case

We consider the simplest realization of leptogenesis with one heavy Majorana neutrino species much lighter than the other ones. In this scenario, when the temperature of the early universe is smaller than the lightest Majorana neutrino mass, we compute at first order in the Standard Model couplings and, for each coupling, at leading order in the termperature the CP asymmetry in the decays of the lightest neutrino into leptons and anti-leptons. We perform the calculation using a hierarchy of two effective field theories organized as expansions in the inverse of the heavy-neutrino masses. In the ultimate effective field theory, leading thermal corrections proportional to the Higgs self coupling and the gauge couplings are encoded in one single operator of dimension five, whereas corrections proportional to the top Yukawa coupling are encoded in four operators of dimension seven, which we compute.

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arXiv2016

CP asymmetry in heavy Majorana neutrino decays at finite temperature: the nearly degenerate case

In a model where Majorana neutrinos heavier than the electroweak scale couple to Standard Model Higgs bosons and leptons, we compute systematically thermal corrections to the direct and indirect CP asymmetries in the Majorana neutrino decays. These are key ingredients entering the equations that describe the thermodynamic evolution of the induced lepton-number asymmetry eventually leading to the baryon asymmetry in the universe. We compute the thermal corrections in an effective field theory framework that assumes the temperature smaller than the masses of the Majorana neutrinos and larger than the electroweak scale, and we provide the leading corrections in an expansion of the temperature over the mass. In this work, we consider the case of two Majorana neutrinos with nearly degenerate masses.

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arXiv2014

Effective field theories: from Cosmology to Quark Gluon Plasma

Cosmology and particle physics come across a tight connection in the attempt to reproduce and understand quantitatively the results of experimental findings. Indeed, the quark gluon plasma (QGP) found at colliders and the baryon asymmetry provided by the WMAP collaboration are examples where to apply field theoretical techniques in issues relevant for Cosmology. In the simplest leptogenesis framework, heavy Majorana neutrinos are at the origin of the baryon asymmetry. The non-relativistic regime appears to be relevant during the lepton asymmetry generation where the interactions among particles occur in a thermal medium. We discuss the development of an effective field theory (EFT) for non-relativistic Majorana particles to address calculations at finite temperature. We show an application of such a method to the case of a heavy Majorana neutrino decaying in a hot and dense plasma of Standard Model (SM) particles. These techniques are analogous to those widely used for the investigation of heavy-ion collisions at colliders by exploiting hard probes. Finally we sketch some commonalities between Majorana neutrinos and bound state of heavy quarks in medium.

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arXiv2014

Searching Doubly Charged Leptons at Present and Future Colliders

The production at the LHC of exotic excited leptons of charge $Q = +2e$ is considered. Such states are predicted in composite models with extended isospin multiplets ($I_{W}=1$ and $I_{W}=3/2$). The coupling among these doubly charged leptons and Standard Model fermions may occurs either via gauge or contact interactions. In the former case the decay channels are more constrained. We study the production cross section at the LHC of $L^{++}$ ($pp \rightarrow L^{++} \, \ell^{-}$) and focus on the leptonic signature deriving from the subsequent decays $L^{++} \rightarrow W^{+} \ell^{+} \rightarrow \ell^{+} \ \ell^{+} \, ν_l $. The invariant mass distribution of the like-sign dilepton exhibits a sharp end-point corresponding to excited doubly charged lepton mass $m^{*}$. A preliminary study for the production of doubly charged leptons at the future linear colliders, by considering the process $e^{-} e^{-} \rightarrow L_{e}^{--} \, ν_{e}$, is carried out. Both the contact and gauge interaction mechanisms are investigated and compared.

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arXiv2013

An effective field theory for non-relativistic Majorana neutrinos

Heavy Majorana neutrinos enter in many scenarios of physics beyond the Standard Model: in the original seesaw mechanism they provide a natural explanation for the small masses of the Standard Model neutrinos and in the simplest leptogenesis framework they are at the origin of the baryonic matter of the universe. In this paper, we develop an effective field theory for non-relativistic Majorana particles, which is analogous to the heavy-quark effective theory. Then, we apply it to the case of a heavy Majorana neutrino decaying in a hot and dense plasma of Standard Model particles, whose temperature is much smaller than the mass of the Majorana neutrino but still much larger than the electroweak scale. The neutrino width gets zero-temperature contributions that can be computed from in-vacuum matrix elements, and thermal corrections. Only the latter will be addressed. Symmetry and power counting arguments made manifest by the effective field theory restrict the form of the thermal corrections and simplify their calculation. The final result agrees with recent determinations obtained with different methods. The effective field theory presented here is suitable to be used for a variety of different models involving non-relativistic Majorana fermions.

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