Clockwork Dark Matter

I give a pedagogical discussion of thermal dark matter (DM) within the clockwork mechanism. The clockwork mechanism, which is a natural way to generate small numbers starting from order-one couplings, allows to have a long-lived, but unstable, DM particle that nevertheless has O(1) couplings with electroweak- or TeV-scale states. Remarkably, DM decays on time scales much longer than the age of the Universe and has at the same time sizeable couplings with light states, which therefore allow to produce it thermally within the WIMP paradigm. These new particles with large couplings can be searched for at current or future colliders. I also briefly comment on how this setup can minimally emerge from the deconstruction of an extra dimension in flat spacetime.Comment: 7 pages, 3 figures; Contribution to proceedings of the 52nd Rencontres de Moriond on EW Interactions and Unified Theories, 18-25 March, 2017, La Thuile, Italy. Comments on extra-dimensional constructions adde

Mass Bounds on Light and Heavy Neutrinos from Radiative MFV Leptogenesis

We derive novel limits on the masses of the light and heavy Majorana neutrinos by requiring successful leptogenesis in seesaw models of minimal flavour violation (MFV). Taking properly into account radiative flavour effects and avoiding the limitations due to a no-go theorem on leptonic asymmetries, we find that the mass of the lightest of the observable neutrinos must be smaller than $\sim 0.05$ eV, whilst the Majorana scale of lepton number violation should be higher than $\sim 10^{12}$ GeV. The latter lower bound enables one to probe the existence of possible new scales of MFV, up to energies of $\sim 100$ TeV, in low-energy experiments, such as $\mu \to e\gamma$ and $\mu \to e$ conversion in nuclei. Possible realizations of MFV leptogenesis in Grand Unified Theories are briefly discussed.Comment: 5 pages, 3 figures; journal versio

Cold keV dark matter from decays and scatterings

We explore ways of creating cold keV-scale dark matter by means of decays and scatterings. The main observation is that certain thermal freeze-in processes can lead to a cold dark matter distribution in regions with small available phase space. In this way the free-streaming length of keV particles can be suppressed without decoupling them too much from the Standard Model. In all cases, dark matter needs to be produced together with a heavy particle that carries away most of the initial momentum. For decays, this simply requires an off-diagonal DM coupling to two heavy particles; for scatterings, the coupling of soft DM to two heavy particles needs to be diagonal, in particular in spin space. Decays can thus lead to cold light DM of any spin, while scatterings only work for bosons with specific couplings. We explore a number of simple models and also comment on the connection to the tentative 3.5 keV line.Comment: 19 pages, 6 figures. published versio

Pati-Salam and lepton universality in B decays

Recent hints for lepton-flavor non-universality in $B$-meson decays can be interpreted as hints for the existence of leptoquarks. We show that scalar leptoquarks unavoidably arise in grand unified theories, using the well-known Pati-Salam model as an example. These GUT-motivated leptoquarks can have a number of appealing features including automatic absence of proton decay, purely chiral couplings, and relations between the various leptoquark couplings. We show that $R(K^{(*)})$ can be connected to the neutrino mass matrix that arises via type-II seesaw, resulting in testable lepton flavor violation. In order to also explain $R(D^{(*)})$ one instead has to assume the existence of light right-handed neutrinos, once again with testable predictions in other $B$-meson decays and at the LHC.Comment: 6 pages, contribution to the 2019 EW session of the 54th Rencontres de Morion

Symmetry Improved 2PI Effective Action and the Infrared Divergences of the Standard Model

Resummations of infinite sets of higher-order perturbative contributions are often needed both in thermal field theory and at zero temperature. For instance, the behaviour of the Standard Model (SM) effective potential extrapolated to very high energies is known to be extremely sensitive to higher-order effects. The 2PI effective action provides a systematic approach to consistently perform such resummations. However, one of its major limitations was that its loopwise expansion introduces residual violations of possible global symmetries, thus giving rise to massive Goldstone bosons in the spontaneously broken phase of the theory. We review the recently developed symmetry-improved 2PI formalism for consistently encoding global symmetries in the 2PI approach, and discuss its satisfactory field-theoretical properties. We then apply the formalism to study the infrared divergences of the SM effective potential due to Goldstone bosons, which may affect the stability analyses of the SM. We present quantitative comparisons, for the scalar sector of the SM, with the approximate partial resummation procedure recently developed to address this problem, and show the quantitative discrepancy of the latter with the more complete 2PI approach, thus motivating further studies in this direction.Comment: 19 pages, 14 figures; Contribution to the Proceedings of DISCRETE 2014, London, United Kingdo

Pati-Salam explanations of the B-meson anomalies

We provide a combined explanation of the increasingly tantalizing $B$-meson anomalies, both in $R_{K^{(*)}}$ and $R_{D^{(*)}}$, in the Pati-Salam model with minimal matter content. This well-known model, based on the gauge group $SU(4)_{LC} \times SU(2)_L \times SU(2)_R$, naturally contains a variety of scalar leptoquarks with related and restricted couplings. In particular we show that the seesaw-motivated scalar leptoquark within the representation $(\overline{10}, 3, 1)$ and its right-handed parity partner $(\overline{10}, 1, 3)$ can solve both anomalies while making testable predictions for related observables such as $B \to K\nu\nu$ and $B \to K \mu\tau$. The solution of the $R_{K^{(*)}}$ anomaly alone can be related to a type-II seesaw neutrino mass structure. Explaining also $R_{D^{(*)}}$ requires the existence of a light right-handed neutrino, which constrains the UV structure of the model.Comment: 26 pages, includes discussion of RGEs; to appear in JHE

Higgs doublet decay as the origin of the baryon asymmetry

We consider a question which curiously had not been properly considered so far: in the standard seesaw model what is the minimum value the mass of a right-handed (RH) neutrino must have for allowing successful leptogenesis via CP-violating decays? To answer this question requires to take into account a number of thermal effects. We show that, for low RH neutrino masses and thanks to these effects, leptogenesis turns out to proceed efficiently from the decay of the Standard Model (SM) scalar doublet components into a RH neutrino and a lepton. Such decays produce the asymmetry at low temperatures, slightly before sphaleron decoupling. If the RH neutrino has thermalized prior from producing the asymmetry, this mechanism turns out to lead to the bound $m_N>2$ GeV. If, instead, the RH neutrinos have not thermalized, leptogenesis from these decays is enhanced further and can be easily successful, even at lower scales. This Higgs-decay leptogenesis new mechanism works without requiring an interplay of flavor effects and/or cancellations of large Yukawa couplings in the neutrino mass matrix. Last but not least, such a scenario turns out to be testable, from direct production of the RH neutrino(s).Comment: 6 pages, 5 figures. Accepted for PRL. References adde

Dark matter and observable Lepton Flavour Violation

Seesaw models with leptonic symmetries allow right-handed (RH) neutrino masses at the electroweak scale, or even lower, at the same time having large Yukawa couplings with the Standard Model leptons, thus yielding observable effects at current or near-future lepton-flavour-violation (LFV) experiments. These models have been previously considered also in connection to low-scale leptogenesis, but the combination of observable LFV and successful leptogenesis has appeared to be difficult to achieve unless the leptonic symmetry is embedded into a larger one. In this paper, instead, we follow a different route and consider a possible connection between large LFV rates and Dark Matter (DM). We present a model in which the same leptonic symmetry responsible for the large Yukawa couplings guarantees the stability of the DM candidate, identified as the lightest of the RH neutrinos. The spontaneous breaking of this symmetry, caused by a Majoron-like field, also provides a mechanism to produce the observed relic density via the decays of the latter. The phenomenological implications of the model are discussed, finding that large LFV rates, observable in the near-future $\mu \to e$ conversion experiments, require the DM mass to be in the keV range. Moreover, the active-neutrino coupling to the Majoron-like scalar field could be probed in future detections of supernova neutrino bursts.Comment: 10 pages, 8 figures; extended discussion, version to appear in PR

Symmetry Improved CJT Effective Action

The formalism introduced by Cornwall, Jackiw and Tomboulis (CJT) provides a systematic approach to consistently resumming non-perturbative effects in Quantum Thermal Field Theory. One major limitation of the CJT effective action is that its loopwise expansion introduces residual violations of possible global symmetries, thus giving rise to massive Goldstone bosons in the spontaneously broken phase of the theory. In this paper we develop a novel symmetry-improved CJT formalism for consistently encoding global symmetries in a loopwise expansion. In our formalism, the extremal solutions of the fields and propagators to a loopwise truncated CJT effective action are subject to additional constraints given by the Ward identities due to global symmetries. By considering a simple O(2) scalar model, we show that, unlike other methods, our approach satisfies a number of important field-theoretic properties. In particular, we find that the Goldstone boson resulting from spontaneous symmetry breaking of O(2) is massless and the phase transition is a second order one, already in the Hartree-Fock approximation. After taking the sunset diagrams into account, we show how our approach properly describes the threshold properties of the massless Goldstone boson and the Higgs particle in the loops. Finally, assuming minimal modifications to the Hartree-Fock approximated CJT effective action, we calculate the corresponding symmetry-improved CJT effective potential and discuss the conditions for its uniqueness for scalar-field values away from its minimum.Comment: 31 pages, 8 figures. Comments on thermodynamic consistency added. Version published in Nuclear Physics

Baryogenesis from L-violating Higgs-doublet decay in the density-matrix formalism

We compute in the density-matrix formalism the baryon asymmetry generated by the decay of the Higgs doublet into a right-handed (RH) neutrino and a Standard Model lepton. The emphasis is put on the baryon asymmetry produced by the total lepton-number violating decay. From the derivation of the corresponding evolution equations, and from their integration, we find that this contribution is fully relevant for large parts of the parameter space. This confirms the results found recently in the CP-violating decay formalism with thermal corrections and shows in particular that the lepton-number violating processes are important not only for high-scale leptogenesis but also when the RH-neutrino masses are in the GeV range. For large values of the Yukawa couplings, we also find that the strong washout is generically much milder for this total lepton-number violating part than for the usual RH-neutrino oscillation flavour part.Comment: 12 pages, 10 figure