arXiv subjects

Stefan Recksiegel

Publications and source records attributed to Stefan Recksiegel.

At least 19 recordsLinked to original sources

arXiv2016

The last refuge of mixed wino-Higgsino dark matter

We delineate the allowed parameter and mass range for a wino-like dark matter particle containing some Higgsino admixture in the MSSM by analysing the constraints from diffuse gamma-rays from the dwarf spheroidal galaxies, galactic cosmic rays, direct detection and cosmic microwave background anisotropies. A complete calculation of the Sommerfeld effect for the mixed-neutralino case is performed. We find that the combination of direct and indirect searches poses significant restrictions on the thermally produced wino-Higgsino dark matter with correct relic density. For $μ>0$ nearly the entire parameter space considered is excluded, while for $μ<0$ a substantial region is still allowed, provided conservative assumptions on astrophysical uncertainties are adopted.

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arXiv2016

On $D\to X_u l^+ l^-$ within the Standard Model and Frameworks like the Littlest Higgs Model with T Parity

The $D\to X_u l^+ l^-$ transitions -- branching ratios, forward-backward asymmetry $A^c_{FB}$, the CP asymmetry $A^c_{CP}$ and the CP asymmetry in the forward-backward asymmetry $A^{CP}_{FB}$ -- have two sources: for $D^{\pm}$ they represent a pure $ΔC=1$ & $ΔQ=0$ current interaction whereas neutral $D$ mesons can also communicate via their anti-hadron. Standard Model (SM) contributions to BR$(D\to X_u l^+ l^-)$ come primarily from long distance dynamics, which overshadow short distance contributions by several orders of magnitude; still they fall much below the present upper experimental bounds. Even the SM contributions to $A^c_{FB}$, $A^c_{CP}$ and $A^{CP}_{FB}$ are tiny, quite unlike in beauty hadrons. The branching ratios are hardly dented by contributions from the Littlest Higgs Models with T parity (LHT) even in the short distance regime, let alone in the SM long distances dynamics. Yet the asymmetries $A^c_{FB}$, $A^c_{CP}$ and $A^{CP}_{FB}$ in these New Physics models can be enhanced over SM predictions, as they arise purely from short distance dynamics; this can occur in particular for $A^c_{FB}$ and $A^{CP}_{FB}$ which get enhanced by orders of magnitudes. Even such enhancements hardly reach absolute sizes for observable experimental effects for $A^c_{FB}$ and $A^c_{CP}$. However LHT contributions to $A^{CP}_{FB}$ could be measured in experiments like the LHCb and the SuperB Collaboration. These results lead us to draw further conclusions on FCNCs within LHT-like models through some simple scaling arguments that encapsulate the essence of flavour dynamics in and beyond the Standard Model.

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arXiv2016

Quark flavour observables in the Littlest Higgs model with T-parity after LHC Run 1

The Littlest Higgs Model with T-parity (LHT) belongs to the simplest new physics scenarios with new sources of flavour and CP violation. We present a new analysis of quark observables in the LHT model in view of the oncoming flavour precision era. We use all available information on the CKM parameters, lattice QCD input and experimental data on quark flavour observables and corresponding theoretical calculations, taking into account new lower bounds on the symmetry breaking scale and the mirror quark masses from the LHC. We investigate by how much the branching ratios for a number of rare $K$ and $B$ decays are still allowed to depart from their SM values. This includes $K^+\toπ^+ν\barν$, $K_L\toπ^0ν\barν$, $K_L\to μ^+μ^-$, $ε'/ε$, $B\to X_sγ$, $B_{s,d}\toμ^+μ^-$, $B\to K^{(*)}\ell^+\ell^-$, $B\to K^{(*)}ν\barν$. Taking into account the constraints from $ΔF=2$ processes, significant departures from the SM predictions for $K^+\toπ^+ν\barν$ and $K_L\toπ^0ν\barν$ are possible while the effects in $B$ decays are much smaller. In particular, the LHT model favours $\mathcal{B}(B_{s}\toμ^+μ^-) \ge \mathcal{B}(B_{s}\toμ^+μ^-)_{\rm SM}$, which is not supported by the data, and the present anomalies in $B\to K^{(*)}\ell^+\ell^-$ decays cannot be explained in this model. With the recent lattice and large $N$ input the imposition of the $ε'/ε$ constraint implies a significant suppression of the branching ratio for $K_L\toπ^0ν\barν$ with respect to its SM value while allowing only for small modifications for $K^+\toπ^+ν\barν$. Finally, we investigate how the LHT physics could be distinguished from other models by means of indirect measurements and discuss the consequences for quark flavour observables of not finding any LHT state in the coming years.

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arXiv2011

Conclusions from CDF Results on CP Violation in D^0 \to π^+π^-, K^+K^- and Future Tasks

Within the Standard Model (SM) one predicts both direct and indirect CP violation in D^0 \to π^+π^-, K^+K^- transitions, although the effects are tiny: Indirect CP asymmetry cannot exceed O(10^{-4}), probably even O(10^{-5}); direct effects are estimated at not larger than 10^{-4}. At B factories direct and indirect asymmetries have been studied with /τ_{D^0} ~ 1; no CP asymmetry was found with an upper bound of about 1%. CDF has shown intriguing data on CP violation in D^0 \to π^+π^- [K^+K^-] with /τ_{D^0} ~ 2.4 [2.65]. Also, CDF has not seen any CP violation. For direct CP asymmetry, CDF has a sensitivity similar to the combination of the B factories, yet for indirect CP violation it yields a significantly smaller sensitivity of a_{cp}^{ind}=(-0.01 +- 0.06_{stat} +- 0.05_{syst})% due to it being based on longer decay times. New Physics models (NP) like Little Higgs Models with T-Parity (LHT) can produce an indirect CP asymmetry up to 1%; CDF's findings thus cover the upper range of realistic NP predictions ~ 0.1 - 1%. One hopes that LHCb and a Super-Flavour Factory will probe the lower range down to ~0.01%. Such non-ad-hoc NP like LHT cannot enhance direct CP violation significantly over the SM level in D^0 \to π^+π^-, K^+K^- and D^{\pm} \to π^{\pm}K^+K^- transitions, but others might well do so.

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arXiv2010

D^0 -> gamma gamma and D^0 -> mu^+ mu^- Rates on an Unlikely Impact of the Littlest Higgs Model with T-Parity

The decays D^0 -> gamma gamma, $mu^+ mu^- are highly suppressed in the Standard Model (SM) with the lion's share of the rate coming from long distance dynamics; D^0 -> mu^+ mu^- is driven predominantly by D^0 -> gamma gamma -> mu^+ mu^-. Their present experimental bounds are small, yet much larger than SM predictions. New Physics models like the Littlest Higgs models with T parity (LHT) can induce even large indirect CP violation in D^0 transitions. One would guess that LHT has a `fighting chance' to affect these D^0 -> gamma gamma, mu^+ mu^- rates in an observable way. We have found LHT contributions can be much larger than short distance SM amplitude by orders of magnitude. Yet those can barely compete with the long distance SM effects. If D^0 -> gamma gamma, mu^+ mu^- modes are observed at greatly enhanced rates, LHT scenarios will not be candidates for generating such signals. LHT-like frameworks will not yield larger D^0 rightarrow gamma gamma/mu^+ mu^- rates as they are constrained by B and K rare decays.

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arXiv2010

Patterns of Flavour Violation in the Presence of a Fourth Generation of Quarks and Leptons

We calculate a number of observables related to particle-antiparticle mixing and the branching ratios for the most interesting rare and CP-violating K and B decays in the Standard Model (SM3) extended by a fourth generation of quarks and leptons (SM4). A model-independent parameterisation of these observables in terms of gauge-independent functions is adopted, which is useful for studying the breaking of the universality between K, Bd and Bs systems through non-minimal flavour violating interactions. We calculate first the mass differences Delta M_i in the neutral K and B system, the mixing-induced CP asymmetries S_{psi K_S}, S_{psi phi}, S_{phi K_S}, S_{eta' K_S} and epsilon_K. Subsequently, a detailed analysis of K^+ -> pi^+ nu nu, K_L -> pi^0 nu nu, B_{s,d} -> mu^+ mu^-, B -> X_{s,d} nu nu, K_L -> pi^0 l^+ l^-, B -> X_s gamma and B -> X_{s,d} l^+ l^- is presented, and also epsilon'/epsilon is considered. For some of these observables the departures from SM3 predictions can still be spectacular. We discuss how the new mixing parameters (3 angles, 2 CP phases) can be determined using the flavour observables in question. We identify the different hierarchical structures in the SM4 flavour mixing matrix, allowed by phenomenological and theoretical constraints, and define the corresponding generalised Wolfenstein expansion. Most importantly, we show how the characteristic patterns of correlations among the considered flavour observables allow to distinguish this New Physics scenario from supersymmetric flavour models, the Littlest Higgs model with T-parity and the Randall-Sundrum model with custodial protection.

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arXiv2010

Characterising New Physics Models by Effective Dimensionality of Parameter Space

We show that the dimension of the geometric shape formed by the phenomenologically valid points inside a multi-dimensional parameter space can be used to characterise different new physics models and to define a quantitative measure for the distribution of the points. We explain a simple algorithm to determine the box-counting dimension from a given set of parameter points, and illustrate our method with examples from different models that have recently been studied with respect to precision flavour observables.

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arXiv2010

The Impact of a 4th Generation on Mixing and CP Violation in the Charm System

We study D0-D0 mixing in the presence of a fourth generation of quarks. In particular, we calculate the size of the allowed CP violation which is found at the observable level well beyond anything possible with CKM dynamics. We calculate the semileptonic asymmetry a_SL and the mixing induced CP asymmetry eta_fS_f which are correlated with each other. We also investigate the correlation of eta_fS_f with a number of prominent observables in other mesonic systems like epsilon'/epsilon, Br(K_L -> pi0 nu nu), Br(K+ -> pi+ nu nu), Br(B_s ->mu+ mu-), Br(B_d -> mu+ mu-) and finally S_psi phi in the B_s system. We identify a clear pattern of flavour and CP violation predicted by the SM4 model: While simultaneous large 4G effects in the K and D systems are possible, accompanying large NP effects in the B_d system are disfavoured. However this behaviour is not as pronounced as found for the LHT and RSc models. In contrast to this, sizeable CP violating effects in the B_s system are possible unless extreme effects in eta_fS_f are found, and Br(B_s ->mu+ mu-) can be strongly enhanced regardless of the situation in the D system. We find that, on the other hand, S_psi phi > 0.2 combined with the measured epsilon'/epsilon significantly diminishes 4G effects within the D system.

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arXiv2010

FCNC Processes in the Littlest Higgs Model with T-Parity: an Update

We update our 2006-2007 results for FCNC processes in the Littlest Higgs model with T-parity (LHT). The removal of the logarithmic UV cutoff dependence in our previous results through a new contribution to the Z^0-penguin diagrams identified by Goto et al. and del Aguila et al., while making the deviations from the SM expectations in the quark sector less spectacular, still allows for sizable new physics effects in K -> pi nu anti-nu and K_L -> pi^0 l^+ l^- decays and in the CP-asymmetry S_{psi phi} with the latter unaffected by the new contribution. We extend our analysis by a study of the fine-tuning required to fit the data on epsilon_K and by the inclusion of the decay K_L -> mu^+ mu^-. A number of correlations can distinguish this model from the custodially protected Randall-Sundrum model analysed recently. We also reconsider lepton flavour violating decays, including now a discussion of fine-tuning. While the l_i -> l_j gamma decays are unaffected by the removal of the logarithmic cutoff dependence, the branching ratios for decays with three leptons in the final state, like mu -> 3 e are lowered by almost an order of magnitude. In spite of this, the pattern of lepton flavour violation in the LHT model can still be distinguished from the one in supersymmetric models.

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arXiv2010

The Littlest Higgs Model with T-Parity Facing CP-Violation in B_s - anti-B_s Mixing

The non-minimal flavour violating interactions of mirror quarks and new heavy gauge bosons in the Littlest Higgs model with T-parity (LHT) give rise to naturally large CP-violating effects in the B_s system. In view of a large new CP phase in B_s - anti-B_s mixing hinted by the CDF and D0 data and the recent UTfit analysis, we update our 2006 analysis of particle-antiparticle mixing and rare K and B decays in the LHT model, using the most recent values of a number of input parameters and performing a more careful error analysis. We find that the CP-asymmetry S_{psi phi} can easily reach values ~ 0.15-0.20, compared to the SM value ~ 0.04, while higher values are rather unlikely though not excluded. Large enhancements are also possible in the branching ratios for K_L -> pi0 nu anti-nu, K+ -> pi+ nu anti-nu and K_L -> pi0 l+ l- with much more modest effects in B_{s,d} -> mu+ mu-. We perform a detailed study of correlations between the latter decays and S_{psi phi} as well as of the correlation between S_{psi phi} and S_{psi K_S}. We also point out that the possible tension between epsilon_K and the tree level CKM determination recently hinted by various analyses can easily be resolved in the LHT model.

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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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arXiv2009

Flavour Physics in the Littlest Higgs Model with T-Parity: Effects in the K, B_d/s and D systems

The Littlest Higgs Model with T parity (LHT) is an interesting alternative model for New Physics at the TeV scale. Although Flavour Physics was not the reason for creating the LHT model, significant effects (such as large CP violation where not predicted by the SM) can be created without violating existing experimental bounds. We study the B-, K- and especially the D-sector.

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arXiv2009

CP Violation in D0 - anti-D0 Oscillations: General Considerations and Applications to the Littlest Higgs Model with T-Parity

The observed D0 - anti-D0 oscillations provide a new stage in our search for New Physics in heavy flavour dynamics. The theoretical verdict on the observed values of x_D and y_D remains ambiguous: while they could be totally generated by Standard Model dynamics, they could also contain a sizable or even leading contribution from New Physics. Those oscillations are likely to enhance the observability of CP violation as clear manifestations of New Physics. We present general formulae for D0 - anti-D0 oscillations, concentrating on the case of negligible direct CP violation. In particular we derive a general formula for the time-dependent mixing-induced CP asymmetry in decays to a CP eigenstate and its correlation with the semileptonic CP asymmetry a_SL(D0) in D0(t) -> l nu K. We apply our formalism to the Littlest Higgs model with T-parity, using the time-dependent CP asymmetry in D -> K_S phi as an example. We find observable effects at a level well beyond anything possible with CKM dynamics. Comparisons with CP violation in the K and B systems offer an excellent test of this scenario and reveal the specific pattern of flavour and CP violation in the D0 - anti-D0 system predicted by this model. We discuss a number of charm decays that could potentially offer an insight in the dynamics of CP violation in D decays. We also apply our formalism to B_s - anti-B_s mixing.

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arXiv2007

On Puzzles and Non-Puzzles in B -> pi pi, pi K Decays

Recently, we have seen interesting progress in the exploration of CP violation in B^0_d -> pi^+ pi^-: the measurements of mixing-induced CP violation by the BaBar and Belle collaborations are now in good agreement with each other, whereas the picture of direct CP violation is still unclear. Using the branching ratio and direct CP asymmetry of B^0_d -> pi^- K^+, this situation can be clarified. We predict A_CP^dir(B_d -> pi^+ pi^-) = -0.24+-0.04, which favours the BaBar result, and extract gamma=(70.0^{+3.8}_{-4.3})deg, which agrees with the unitarity triangle fits. Extending our analysis to other B -> pi K modes and B^0_s -> K^+ K^- with the help of the SU(3) flavour symmetry and plausible dynamical assumptions, we find that all observables with colour-suppressed electroweak penguin contributions are measured in excellent agreement with the Standard Model. As far as the ratios R_{c,n} of the charged and neutral B -> pi K branching ratios are concerned, which are sizeably affected by electroweak penguin contributions, our Standard-Model predictions have almost unchanged central values, but significantly reduced errors. Since the new data have moved quite a bit towards these results, the "B -> pi K puzzle" for the CP-conserving quantities has been significantly reduced. However, the mixing-induced CP violation of B^0_d -> pi^0 K_S does look puzzling; if confirmed by future measurements, this effect could be accommodated through a modified electroweak penguin sector with a large CP-violating new-physics phase. Finally, we point out that the established difference between the direct CP asymmetries of B^+- -> pi^0 K^+- and B_d -> pi^-+ K^+- appears to be generated by hadronic and not by new physics.

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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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arXiv2005

New Aspects of B -> pi pi, pi K and their Implications for Rare Decays

We analyse the B -> pi pi, pi K modes in the light of the most recent B-factory data, and obtain the following new results: (i) the B0 -> pi+ pi-, pi- K+ modes prefer gamma=(74+-6)deg, which - together with |V_ub/V_cb| - allows us to determine the ``true'' unitarity triangle and to search for CP-violating new-physics contributions to B0_d-\bar B0_d mixing; (ii) the B -> pi K puzzle reflected in particular by the low experimental value of the ratio R_n of the neutral B -> pi K rates persists and still favours new physics in the electroweak penguin sector with a new CP-violating phase phi ~ -90deg, although now also phi ~ +90deg can bring us rather close to the data; (iii) the mixing-induced B0 -> pi0 K_S CP asymmetry is a sensitive probe of the sign of this phase, and would currently favour phi ~ +90deg, as well as the direct CP asymmetry of B+- -> pi0 K+-, which suffers, however, from large hadronic uncertainties; (iv) we investigate the sensitivity of our B -> pi K analysis to large non-factorizable SU(3)-breaking effects and find that their impact is surprisingly small so that it is indeed exciting to speculate on new physics; (v) assuming that new physics enters through Z0 penguins, we study the interplay between B -> pi K and rare B, K decays and point out that the most recent B-factory constraints for the latter have interesting implications, bringing us to a few scenarios for the future evolution of the data, where also the mixing-induced CP violation in B0 -> pi0 K_S plays a prominent role.

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