48 research outputs found
b -> d Penguins: CP Violation, General Lower Bounds on the Branching Ratios and Standard Model Tests
With the wealth of new data from the B-factories, b -> d penguin decays
become available for study, in addition to their b -> s counterparts that have
proven an indespensable tool for the exploration of new-physics effects in
flavour physics. A prominent example of the b -> d penguin transitions is . We show that this decay can be charaterized in the
Standard Model by a surface in the observable space of the direct and
mixing-induced CP asymmetries and the branching ratio. The form of this
surface, which is theoretically clean, implies a lower bound for the branching
ratio that has recently been confirmed experimentally. If future measurements
of the CP asymmetries yield a point away from the SM surface, this would be an
interesting signal of new physics. We point out that the hadronic parameters in
that parameterize the position on the SM surface
are related to hadronic parameters in the B -> pi K system. The fact that the
branching ratio of is very close to its lower
bound yields interesting implications for B -> pi K even without knowledge of
the CP asymmetries of . The mechanism that
produces the lower bound for is actually much
more general; we derive lower bounds for various other b -> d penguin-induced
processes, including B -> rho gamma and .
Some of these theoretical lower bounds are very close to the current
experimental upper bounds.Comment: 4 pages, 2 figures. Contribution to the proceedings of EPS-HEP200
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.Comment: 30 pages, 14 figures, 5 tables. Clarifying comments added, matches
published versio
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. The latter originate in the interactions of ordinary quarks and leptons with heavy mirror quarks and leptons that are mediated by new heavy gauge bosons. Also a heavy fermionic top partner is present in this model which communicates with the SM fermions by means of standard (Formula presented.) and (Formula presented.) gauge bosons. We present a new analysis of quark flavour 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 (Formula presented.) , (Formula presented.) , (Formula presented.) , (Formula presented.) , (Formula presented.) , (Formula presented.) , (Formula presented.) , and (Formula presented.). Taking into account the constraints from (Formula presented.) processes, significant departures from the SM predictions for (Formula presented.) and (Formula presented.) are possible, while the effects in B decays are much smaller. In particular, the LHT model favours (Formula presented.) , which is not supported by the data, and the present anomalies in (Formula presented.) decays cannot be explained in this model. With the recent lattice and large N input the imposition of the (Formula presented.) constraint implies a significant suppression of the branching ratio for (Formula presented.) with respect to its SM value while allowing only for small modifications of (Formula presented.). 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. © 2016, The Author(s)
General Lower Bounds for b -> d Penguin Processes
For the exploration of flavour physics, b -> d penguin processes are an
important aspect, with the prominent example of \bar B^0_d -> K^0 \bar K^0. We
recently derived lower bounds for the CP-averaged branching ratio of this
channel in the Standard Model; they were found to be very close to the
corresponding experimental upper limits, thereby suggesting that \bar B^0_d ->
K^0 \bar K^0 should soon be observed. In fact, the BaBar collaboration
subsequently announced the first signals of this transition. Here we point out
that it is also possible to derive lower bounds for \bar B -> \rho \gamma
decays, which are again surprisingly close to the current experimental upper
limits. We show that these bounds are realizations of a general bound that
holds within the Standard Model for b -> d penguin processes, allowing further
applications to decays of the kind B^\pm -> K^{(\ast)\pm} K^{(\ast)} and B^\pm
-> \pi^\pm \ell^+ \ell^-, \rho^\pm \ell^+ \ell^-.Comment: Minor changes, to appear as rapid communication in Phys. Rev
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 , , , , and , paying particular attention to non-MFV contributions
present in the model.
Imposing all available constraints we find that the decay rates for and can be enhanced by at most
50% and 35% relative to the SM values, while and
can be both as high as .
Significant enhancements of the decay rates are also
possible. Simultaneously, the CP-asymmetries and
can be enhanced by an order of magnitude, while the electroweak
penguin effects in turn out to be small, in agreement with the
recent data.Comment: 65 pages, 19 figures. Note added on omitted term in Z-penguin
contribution. For an updated analysis see arXiv:0906.545
B -> pi pi, New Physics in B -> pi K and Implications for Rare K and B Decays
The measured B -> pi pi, pi K branching ratios exhibit puzzling patterns. We
point out that the B -> pi pi hierarchy can be nicely accommodated in the
Standard Model (SM) through non-factorizable hadronic interference effects,
whereas the B -> pi K system may indicate new physics (NP) in the electroweak
(EW) penguin sector. Using the B -> pi pi data and the SU(3) flavour symmetry,
we may fix the hadronic B -> pi K parameters, which allows us to show that any
currently observed feature of the B -> pi K system can be easily explained
through enhanced EW penguins with a large CP-violating NP phase. Restricting
ourselves to a specific scenario, where NP enters only through Z^0 penguins, we
derive links to rare K and B decays, where an enhancement of the K_L-> pi^0 nu
nu_bar rate by one order of magnitude, with BR(K_L -> pi^0 nu nu_bar) > BR(K^+
-> pi^+ nu nu_bar), BR(K_L -> pi^0 e^+ e^-)=O(10^{-10}), (\sin2\beta)_{pi nu
nu_bar} K* mu^+ mu^-,
are the most spectacular effects. We address also other rare K and B decays,
epsilon'/epsilon and B_d -> phi K_S.Comment: 6 pages, LaTeX, reference added and a few typos correced, to appear
in Physical Review Letter