arXiv subjects

Selma Uhlig

Publications and source records attributed to Selma Uhlig.

13 recordsLinked to original sources

arXiv2010

Littlest Higgs Model with T-Parity Confronting the New Data on D^0-\bar D^0 Mixing

Motivated by the first experimental evidence of meson oscillations in the D system, we study D^0 - \bar D^0 mixing in the Littlest Higgs model with T-parity, we investigate its role in constraining the model parameters and its impact on the most interesting flavour observables. We find that the experimental data are potentially strongly constraining but at present limited by large theoretical uncertainties in the long-distance Standard Model contribution to D^0 - \bar D^0 mixing.

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arXiv2010

Rare and CP-Violating K and B Decays in the Littlest Higgs Model with T-Parity

We calculate the most interesting rare and CP-violating K and B decays in the Littlest Higgs model with T-parity. We give a collection of Feynman rules including v^2/f^2 contributions that are presented here for the first time and could turn out to be useful also for applications outside flavour physics. We adopt a model-independent parameterization of rare decays in terms of the gauge independent functions X_i,Y_i,Z_i (i=K,d,s), which is in particular useful for the study of the breaking of the universality between K, B_d and B_s systems through non-MFV interactions. Performing the calculation in the unitary and 't Hooft-Feynman gauge, we find that the final result contains a divergence which signals some sensitivity to the ultraviolet completion of the theory. Including an estimate of this contribution, we calculate the branching ratios for the decays $K^+\toπ^+ν\barν$, $K_L\toπ^0ν\barν$, $B_{s,d}\to μ^+μ^-$, $B\to X_{s,d}ν\barν$, $K_L \toπ^0\ell^+\ell^-$ and $B\to X_{s,d}\ell^+\ell^-$, paying particular attention to non-MFV contributions present in the model. Imposing all available constraints we find that the decay rates for $B_{s,d} \to μ^+ μ^-$ and $B \to X_{s,d} ν\bar ν$ can be enhanced by at most 50% and 35% relative to the SM values, while $Br(K^+\toπ^+ν\barν)$ and $Br(K_L\toπ^0ν\barν)$ can be both as high as $5 \cdot 10^{-10}$. Significant enhancements of the decay rates $K_L \toπ^0\ell^+\ell^-$ are also possible. Simultaneously, the CP-asymmetries $S_{ψϕ}$ and $A^s_\text{SL}$ can be enhanced by an order of magnitude, while the electroweak penguin effects in $B \to πK$ turn out to be small, in agreement with the recent data.

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arXiv2010

Correlations between epsilon'/epsilon and Rare K Decays in the Littlest Higgs Model with T-Parity

We calculate the CP-violating ratio epsilon'/epsilon in the Littlest Higgs model with T-parity (LHT) and investigate its correlations with the branching ratios for K_L -> pi^0 nu {bar nu}, K_L -> pi^0 l^+ l^- and K^+ -> pi^+ nu {bar nu}. The resulting correlations are rather strong in the case of K_L decays, but less pronounced in the case of K^+ -> pi^+ nu {bar nu}. Unfortunately, they are subject to large hadronic uncertainties present in epsilon'/epsilon, whose theoretical prediction in the Standard Model (SM) is reviewed and updated here. With the matrix elements of Q_6 (gluon penguin) and Q_8 (electroweak penguin) evaluated in the large-N limit and m_s^MS(2 GeV) = 100 MeV from lattice QCD, (epsilon'/epsilon)_SM turns out to be close to the data so that significant departures of Br(K_L -> pi^0 nu {bar nu}) and Br(K_L -> pi^0 l^+ l^-) from the SM expectations are unlikely, while Br(K^+ -> pi^+ nu {bar nu}) can be enhanced even by a factor 5. On the other hand, modest departures of the relevant hadronic matrix elements from their large-N values allow for a consistent description of epsilon'/epsilon within the LHT model accompanied by large enhancements of Br(K_L -> pi^0 nu {bar nu}) and Br(K_L -> pi^0 l^+ l^-), but only modest enhancements of Br(K^+ -> pi^+ nu {bar nu}).

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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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arXiv2008

Bounding the Minimal 331 Model through the Decay B -> X_s gamma

We study the decay B -> X_s gamma within the framework of the minimal 331 model, taking into account both new experimental and theoretical developments that allow us to update and improve on an existing ten year old analysis. In contrast to several other flavor changing observables that are modified already at tree level from a new Z' gauge boson, we have only one loop contributions in this case. Nevertheless, these are interesting, as they may be enhanced and can shed light on the charged gauge boson and Higgs sector of the model. Numerically, we find that the Higgs sector, which is well approximated by a 2 Higgs doublet model (2HDM), dominates, since the gauge contributions are already very strongly constrained. With respect to B -> X_s gamma, the signal of the minimal 331 model is therefore nearly identical to the 2HDM one, which allows us to obtain a lower bound on the charged Higgs mass. Further, we observe, in analogy to the 2HDM model, that the branching fraction can be rather strongly increased for small values of tan beta. Also, we find that B -> X_s gamma has no impact on the bounds obtained on rare K and B decays in an earlier analysis.

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arXiv2007

Minimal Lepton Flavour Violation and Leptogenesis with exclusively low-energy CP Violation

We study the implications of a successful leptogenesis within the framework of Minimal Lepton Flavour Violation combined with radiative resonant leptogenesis and the PMNS matrix being the only source of CP violation, which can be obtained provided flavour effects are taken into account. We find that the right amount of the baryon asymmetry of the universe can be generated under the conditions of a normal hierarchy of the light neutrino masses, a non-vanishing Majorana phase, sin(theta_{13})>0.13 and m_{nu,lightest}<0.04 eV. If this is fulfilled, we find strong correlations among ratios of charged LFV processes.

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arXiv2007

Leptogenesis with exclusively low-energy CP Violation in the Context of Minimal Lepton Flavour Violation

We analyze lepton flavour violation (LFV) and the generation of the observed baryon-antibaryon asymmetry of the Universe (BAU) within a generalized minimal lepton flavour violation framework with three quasi-degenerate heavy Majorana neutrinos. The BAU which is obtained through radiative resonant leptogenesis can successfully be generated widely independent of the Majorana scale in this scenario and flavour effects are found to be relevant. Then we discuss the specific case in which CP violation is exclusively present at low-energies (a real R matrix) in the flavour sensitive temperature regime. Successful leptogenesis in this case leads to strong constraints on low-energy neutrino parameters.

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arXiv2007

Waiting for Precise Measurements of K^+->pi^+ nu nu and K_L->pi^0 nu nu

In view of future plans for accurate measurements of the theoretically clean branching ratios Br(K+ -> pi+ nu nu) and Br(KL -> pi0 nu nu), that should take place in the next decade, we collect the relevant formulae for quantities of interest and analyze their theoretical and parametric uncertainties. We point out that in addition to the angle beta in the unitarity triangle (UT) also the angle gamma can in principle be determined from these decays with respectable precision and emphasize in this context the importance of the recent NNLO QCD calculation of the charm contribution to K+ -> pi+ nu nu and of the improved estimate of the long distance contribution by means of chiral perturbation theory. In addition to known expressions we present several new ones that should allow transparent tests of the Standard Model (SM) and of its extensions. While our presentation is centered around the SM, we also discuss models with minimal flavour violation and scenarios with new complex phases in decay amplitudes and meson mixing. We give a brief review of existing results within specific extensions of the SM, in particular the Littlest Higgs Model with T-parity, Z' models, the MSSM and a model with one universal extra dimension. We derive a new "golden" relation between B and K systems that involves (beta,gamma) and Br(KL -> pi0 nu nu) and investigate the virtues of (R_t,beta), (R_b,gamma), (beta,gamma) and (etabar,gamma) strategies for the UT in the context of K -> pi nu nu decays with the goal of testing the SM and its extensions.

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arXiv2007

Another Look at Minimal Lepton Flavour Violation, l_i -> l_j gamma, Leptogenesis, and the Ratio M_nu/ Lambda_LFV

We analyze lepton flavour violation (LFV), as well as generation of the observed baryon-antibaryon asymmetry of the Universe (BAU) within a generalized minimal lepton flavour violation (MLFV) framework where we allow for CP violation both at low and high energies. The generation of BAU is obtained through radiative resonant leptogenesis (RRL), where starting with three exactly degenerate right-handed neutrinos at Lambda_GUT, we demonstrate explicitly within the SM and the MSSM that the splittings between their masses at the see-saw scale M_nu, generated by renormalization group effects, are sufficient for a successful leptogenesis for M_nu even as low as 10^6 GeV. The inclusion of flavour effects plays an important role in this result and can lead to the observed BAU even in the absence of CP violation beyond the PMNS phases. The absence of a stringent lower bound on M_nu in this type of leptogenesis allows to easily satisfy present and near future upper bounds on mu -> e gamma and other charged lepton flavour violating (LFV) processes even for Lambda_LFV = O(1 TeV). We find, that the MLFV framework in the presence of heavy right-handed neutrinos and leptogenesis is not as predictive as MFV in the quark sector and point out that without a specific MLFV model, there is a rich spectrum of possibilities for charged LFV processes and for their correlation with low energy neutrino physics and the LHC physics, even if the constraint from the observed BAU is taken into account. While certain qualitative features of our analysis confirm findings of Cirigliano et al., at the quantitative level we find phenomenologically important differences. We explain the origin of these differences.

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arXiv2006

Another Look at the Flavour Structure of the Littlest Higgs Model with T-Parity

We discuss the mixing matrix V_Hd that describes the charged and neutral current interactions between ordinary down-quarks and up- and down-mirror quarks in the Littlest Higgs Model with T-parity (LHT). We point out that this matrix in addition to three mixing angles contains three physical complex phases and not only one as used in the present literature. We explain the reason for the presence of two additional phases, propose a new standard parameterization of V_Hd and briefly comment on the relevance of these new phases for the phenomenology of FCNC processes in the LHT model. In a separate paper we present a detailed numerical analysis, including these new phases, of K and B physics, with particular attention to the most interesting rare decays.

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arXiv2006

Rare K and B Decays in the Littlest Higgs Model without T-Parity

We analyze rare K and B decays in the Littlest Higgs (LH) model without T-parity. We find that the final result for the Z^0-penguin contribution contains a divergence that is generated by the one-loop radiative corrections to the currents corresponding to the dynamically broken generators. Including an estimate of these logarithmically enhanced terms, we calculate the branching ratios for the decays K^+ -> pi^+ nu bar nu, K_L -> pi^0 nu bar nu, B_{s,d} -> mu^+ mu^- and B -> X_{s,d} nu bar nu. We find that for the high energy scale f=O(2-3) TeV, as required by the electroweak precision studies, the enhancement of all branching ratios amounts to at most 15% over the SM values. On the technical side we identify a number of errors in the existing Feynman rules in the LH model without T-parity that could have some impact on other analyses present in the literature. Calculating penguin and box diagrams in the unitary gauge, we find divergences in both contributions that are cancelled in the sum except for the divergence mentioned above.

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arXiv2005

Particle-Antiparticle Mixing, ε_K and the Unitarity Triangle in the Littlest Higgs Model

We calculate the K^{0}-\bar{K}^{0}, B_{d,s}^{0}-\bar{B}_{d,s}^{0} mixing mass differences ΔM_K, ΔM_{d,s} and the CP-violating parameter ε_{K} in the Littlest Higgs (LH) model. For f/v as low as 5 and the Yukawa parameter x_L<0.8, the enhancement of ΔM_{d} amounts to at most 20%. Similar comments apply to ΔM_s and ε_{K}. The correction to ΔM_{K} is negligible. The dominant new contribution in this parameter range, calculated here for the first time, comes from the box diagrams with (W_L^\pm,W_H^\pm) exchanges and ordinary quarks that are only suppressed by the mass of W_H^\pm but do not involve explicit O(v^2/f^2) factors. This contribution is strictly positive. The explicit O(v^2/f^2) corrections to the SM diagrams with ordinary quarks and two W_L^\pm exchanges have to be combined with the box diagrams with a single heavy T quark exchange for the GIM mechanism to work. These O(v^2/f^2) corrections are found to be of the same order of magnitude as the (W_L^\pm,W_H^\pm) contribution but only for x_L approaching 0.8 they can compete with it. We point out that for x_L>0.85 box diagrams with two T exchanges have to be included. Although formally O(v^4/f^4), this contribution is dominant for x_L \approx 1 due to non-decoupling of T that becomes fully effective only at this order. We emphasize, that the concept of the unitarity triangle is still useful in the LH model, in spite of the O(v^2/f^2) corrections to the CKM unitarity involving only ordinary quarks. We demonstrate the cancellation of the divergences in box diagrams that appear when one uses the unitary gauge for W_L^\pm and W_H^\pm.

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arXiv2005

Non-Decoupling Effects of the Heavy T in the B^0_{d,s}-\bar B^0_{d,s} Mixing and Rare K and B Decays

We point out that in the case of a heavy top quark T, present in the Littlest Higgs model (LH), and the t-T mixing parameter x_L > 0.90 the contribution to B^0_{d,s}-\bar B^0_{d,s} mixing from box diagrams with two T exchanges cannot be neglected. Although formally O(v^4/f^4) with v = 246 GeV and f > 1 TeV, this contribution increases linearly with x_T = m_T^2/M^2_W and with x_T = O(f^2/v^2) constitutes effectively an O(v^2/f^2) correction. For x_L ~ 1, this contribution turns out to be more important than the genuine O(v^2/f^2) corrections. In particular it is larger than the recently calculated O(v^2/f^2) contribution of box diagrams with a single T exchange that increases only logarithmically with x_T. For x_L = 0.95 and f/v = 5,10,15, the short distance function S governing the B^0_{d,s}-\bar B^0_{d,s} mixing mass differences ΔM_{d,s} receives 56%, 15% and 7% enhancements relative to its Standard Model (SM) value, implying a suppression of the CKM element V_td and an enhancement of ΔM_s. The short distance functions X and Y, relevant for rare K and B decays, increase only logarithmically with x_T. With the suppressed V_td, K->πν\barνand B_d->μ^+μ^- decays are only insignificantly modified with respect to the SM, while the branching ratio Br(B_s->μ^+μ^-) receives 66%, 19% and 9% enhancements for x_L = 0.95 and f/v = 5,10,15, respectively. Similar enhancement is found for Br(B_s->μ\barμ)/Br(B_d->μ\barμ).

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