arXiv subjects

Sacha Davidson

Publications and source records attributed to Sacha Davidson.

At least 55 records · Page 3Linked to original sources

arXiv2011

Reconstructing Seesaws

We explore some aspects of "reconstructing" the heavy singlet sector of supersymmetric type I seesaw models, for two, three or four singlets. We work in the limit where one light neutrino is massless. In an ideal world, where selected coefficients of the TeV-scale effective Lagrangian could be measured with arbitrary accuracy, the two-singlet case can be reconstructed, two three or more singlets can be differentiated, and an inverse seesaw with four singlets can be reconstructed. In a more realistic world, we estimate $\ell_\a \to \ell_\b γ$ expectations with a "Minimal-Flavour-Violation-like" ansatz, which gives a relation between ratios of the three branching ratios. The two singlet model predicts a discrete set of ratios.

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arXiv2011

Non standard neutrino interactions at LEP2 and the LHC

We consider Non-Standard neutrino Interactions (NSI) connecting two neutrinos with two first-generation fermions ($e, u$ or $d$), which we assume to arise at at dimension eight due to New Physics. The coefficient is normalised as $4 εG_F/\sqrt{2}$. We explore signatures of NSI-on-electrons at LEP2, and of NSI-on-quarks at the LHC, treating the NSI as contact interactions at both energies. In models where the coefficients of dangerous dimension six operators are suppressed by cancellations, LEP2 provides interesting bounds on NSI operators ($ε\lsim 10^{-2} - 10^{-3}$), which arise because $\sqrt{s} \sim 200$ GeV, and the cancellation applied at zero momentum transfer. At the LHC, we use the Equivalence Theorem, which relates the longitudinal $W$ to the Higgs, to estimate the rate for $\bar{q} q W^+W^- e_α^+ e_β^-$ induced by NSI. We find that the cross-section is small, but that the outgoing particles have very high $p_T > 400$ GeV, which reduces the issue of backgrounds. In a conservative scenario, we find that the LHC at 14 TeV and with 100 fb$^{-1}$ of data would have a sensitivity to $ε\gsim 3 \times 10^{-3}$.

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arXiv2011

Non-Standard Neutrino Interactions at Colliders

Many extensions of the Standard Model lead to non-standard neutrino interactions (NSI), which can affect the interpretation of current and future neutrino data. We explore an alternative approach to the study of these dimension eight four fermion interactions (of coefficient $4 εG_F/\sqrt{2}$), by bringing into play collider data. In models where coefficients of potential dimension six operators are suppressed by cancellations, LEP2 provides interesting bounds on NSI operators ($ε\lsim 10^{-2} - 10^{-3}$). If NSI are contact interactions at LHC energies, they induce an effective interaction $\bar{q} q W^+W^- e_\a^+ e_\b^-$(according to the Equivalence Theorem), to which the LHC at 14 TeV and with 100 fb$^{-1}$ of data has excellent sensitivity ($ε\gsim 3 \times 10^{-3}$).

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arXiv2011

Leptoquarks decaying to a top quark and a charged lepton at hadron colliders

We study the sensitivity of the Tevatron and the 7 TeV LHC to a leptoquark S coupling to a top quark and a charged lepton L (= e, mu, or tau). For the Tevatron, we focus on the case m_S < m_t, where the leptoquark pair production cross section is large, and the decay is three-body: S --> W b L^{\pm}. We argue that existing Tevatron observations could exclude m_S \lsim 160 GeV. For m_S > m_t, we show that the LHC experiments with low integrated luminosity could be sensitive to such leptoquarks decaying to tl^{\pm} with l= mu or tau.

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arXiv2010

Minimal Flavour Violation for Leptoquarks

Scalar leptoquarks, with baryon and lepton number conserving interactions, could have TeV scale masses, and be produced at colliders or contribute to a wide variety of rare decays. In pursuit of some insight as to the most sensitive search channels, We assume that the leptoquark-lepton-quark coupling can be constructed from the known mass matrices. We estimate the rates for selected rare processes in three cases: leptoquarks carrying lepton and quark flavour, leptoquarks with quark flavour only, and unflavoured leptoquarks. We find that leptoquark decay to top quarks is an interesting search channel.

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arXiv2010

Constraints on two-lepton, two quark operators

Physics from beyond the Standard Model, such as leptoquarks, can induce four fermion operators involving a quark, an anti-quark, a lepton and an anti-lepton. We update the (flavour dependent) constraints on the coefficients of such interactions, arising from collider searches for contact interactions, meson decays and other rare processes. We then make naive estimates for the magnitude of the coefficients, as could arise in texture models or from inverse hierarchies in the kinetic term coefficients. These estimates suggest that rare Kaon decays could be a good place to look for such operators.

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arXiv2010

Lepton flavour violating Higgs and tau to mu gamma

We update phenomenological constraints on a Two Higgs Doublet Model with lepton flavour non-conserving Yukawa couplings. We review that $\tan β$ is ambiguous in such "Type III" models, and define it from the $τ$ Yukawa coupling. The neutral scalars $ϕ$ could be searched for at hadron colliders in $ ϕ\to τ\barμ$, and are constrained by the rare decay $τ\to μγ$. The Feynman diagrams for the collider process, with Higgs production via gluon fusion, are similar to the two-loop "Barr-Zee" diagrams which contribute to $τ\to μγ$. Some "tuning" is required to obtain a collider cross-section of order the Standard Model expectation for $σ(gg \to h_{SM} \to τ^+ τ^-)$, while agreeing with the current bound from $τ\to μγ$.

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arXiv2008

CP Violation in the SUSY Seesaw: Leptogenesis and Low Energy

We suppose that the baryon asymmetry is produced by thermal leptogenesis (with flavour effects), at temperatures $\sim 10^{9} - 10^{10}$ GeV, in the supersymmetric seesaw with universal and real soft terms. The parameter space is restricted by assuming that $l_α\to l_βγ$ processes will be seen in upcoming experiments. We study the sensitivity of the baryon asymmetry to the phases of the lepton mixing matrix, and find that leptogenesis can work for any value of the phases. We also estimate the contribution to the electric dipole moment of the electron, arising from the seesaw, and find that it is (just) beyond the sensitivity of next generation experiments ($\lsim 10^{-29} e$ cm). The fourteen dimensional parameter space is efficiently explored with a Monte Carlo Markov Chain, which concentrates on the regions of interest.

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arXiv2008

Leptogenesis

Leptogenesis is a class of scenarios where the baryon asymmetry of the Universe is produced from a lepton asymmetry generated in the decays of a heavy sterile neutrino. We explain the motivation for leptogenesis. We review the basic mechanism, and describe subclasses of models. We then focus on recent developments in the understanding of leptogenesis: finite temperature effects, spectator processes, and in particular the significance of flavor physics.

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arXiv2008

Solving the flavour problem with hierarchical fermion wave functions

We investigate the flavour structure of generic extensions of the SM where quark and lepton mass hierarchies and the suppression of flavour-changing transitions originate only by the normalization constants of the fermion kinetic terms. We show that in such scenarios the contributions to quark FCNC transitions from dimension-six effective operators are sufficiently suppressed without (or with modest) fine tuning in the effective scale of new physics. The most serious challenge to this type of scenarios appears in the lepton sector, thanks to the stringent bounds on LFV. The phenomenological consequences of this scenarios in view of improved experimental data on quark and lepton FCNC transitions, and its differences with respect to the Minimal Flavour Violation hypothesis are also discussed.

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arXiv2007

Insensitivity of flavoured leptogenesis to low energy CP violation

If the baryon asymmetry of the Universe is produced by leptogenesis, CP violation is required in the lepton sector. In the seesaw extension of the Standard Model with three hierarchical right-handed neutrinos, we show that the baryon asymmetry is insensitive to the PMNS phases: thermal leptogenesis can work for any value of the observable phases. This result was well-known when there are no flavour effects in leptogenesis; we show that it remains true when flavour effects are included.

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arXiv2007

Flavoured Leptogenesis

Thermal leptogenesis, in the seesaw model, is a popular mechanism for generating the Baryon Asymmetry of the Universe. It was noticed recently, that including lepton flavour can modify significantly the results. These proceedings aim to discuss why and when flavour matters, in the thermal leptogenesis scenario for hierarchical right-handed neutrinos. No Boltzmann Equations are introduced.

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arXiv2007

The smallest neutrino mass

We consider models where one Majorana neutrino is massless at tree level (like the see saw with two right-handed neutrinos), and compute the contribution to its mass m generated by two-loop quantum corrections. The result is m \sim 10^{-13}eV in the SM and m \sim 10^{-10} eV (tanBeta/10)^4 in the MSSM, compatible with the restricted range suggested by Affleck-Dine baryogenesis.

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arXiv2006

Various definitions of Minimal Flavour Violation for Leptons

Neutrino masses imply the violation of lepton flavour and new physics beyond the Standard Model. However, flavour change has only been observed in oscillations. In analogy with the quark sector, we could deduce the existence of a principle of Minimal Flavour Violation also for Leptons. Such an extension is not straightforward, since the mechanisms generating neutrino masses are unknown and many scenarios can be envisaged. Thus, we explore some possible definitions of MFVL and propose a notion that can include many models. We build an R-parity violating neutrino mass model in agreement with our preferred definition of MFVL, and show that flavour violating processes are not neccessarily controlled by the MNS mixing matrix.

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arXiv2005

Basis-independent methods for the two-Higgs-doublet model

In the most general two-Higgs-doublet model (2HDM), unitary transformations between the two Higgs fields do not change the functional form of the Lagrangian. All physical observables of the model must therefore be independent of such transformations (i.e., independent of the Lagrangian basis choice for the Higgs fields). We exhibit a set of basis-independent quantities that determine all tree-level Higgs couplings and masses. Some examples of the basis-independent treatment of 2HDM discrete symmetries are presented. We also note that the ratio of the neutral Higgs field vacuum expectation values, tan(beta), is not a meaningful parameter in general, as it is basis-dependent. Implications for the more specialized 2HDMs (e.g., the Higgs sector of the MSSM and the so-called Type-I and Type-II 2HDMs) are explored.

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arXiv2005

From transition magnetic moments to majorana neutrino masses

It is well known that a majorana mass induces a (small) transition magnetic moment. The converse is also true; in this paper we estimate the loop contribution of transition magnetic moments $[μ]_{αβ}$ to the neutrino mass matrix $[m]_{αβ}$. We show that for hierarchical neutrino masses, the contribution of $[μ]_{e τ}$ to $[m]_{e τ}$ can exceed the experimental value of $[m]_{e τ}$.

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arXiv2004

Parametrizations of the Seesaw, or, can the Seesaw be tested?

This proceedings contains a review, followed by a more speculative discussion. I review different coordinate choices on the 21-dimensional parameter space of the seesaw, and which of these 21 quantities are observable. In MSUGRA, there is a 1-1 correspondance between the parameters, and the interactions of light (s)particles. However, not all of the 21 can be extracted from data, so the answer to the title question is ``no''. How to parametrise the remaining unknowns is confusing--different choices seem to give contradictory results (for instance, to the question ``does the Baryon Asymmetry depend on the CHOOZ angle?''). I speculate on possible resolutions of the puzzle.

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arXiv2004

Weak-scale implications of thermal leptogenesis in SUSY

Thermal leptogenesis is an attractive mechanism for generating the baryon asymmetry of the Universe. However, in supersymmetric models, the parameter space is severely restricted by the gravitino bound on the reheat temperature $T_{RH}$. Using a parametrisation of the seesaw in terms of weak-scale inputs, the low-energy footprints of thermal leptogenesis are discussed.

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