83 research outputs found
Can we reach the Zeptouniverse with rare K and B_{s,d} decays?
The Large Hadron Collider will directly probe distance scales as short as
10^{-19}m, corresponding to energy scales at the level of a few TeV. In order
to reach even higher resolutions before the advent of future high-energy
colliders, it is necessary to consider indirect probes of New Physics (NP), a
prime example being Delta F=2 neutral meson mixing processes, which are
sensitive to much shorter distance scales. However Delta F=2 processes alone
cannot tell us much about the structure of NP beyond the LHC scales. To
identify for instance the presence of new quark flavour-changing dynamics of a
left-handed (LH) or right-handed (RH) nature, complementary results from Delta
F=1 rare decay processes are vital. We therefore address the important question
of whether NP could be seen up to energy scales as high as 200 TeV,
corresponding to distances as small as 10^{-21}m -- the Zeptouniverse -- in
rare K and B_{s,d} decays, subject to present Delta F=2 constraints and
perturbativity. We focus in particular on a heavy Z' gauge boson. If restricted
to purely LH or RH Z' couplings to quarks, we find that rare K decays, in
particular KL-> pi^0 nu nubar and K^+->pi^+ nu nubar, allow us to probe the
Zeptouniverse. On the other hand rare B_s and B_d decays, which receive
stronger Delta F=2 constraints, allow us to reach about 15 TeV. Allowing for
both LH and RH couplings a loosening of the Delta F=2 constraints is possible,
and we find that the maximal values of M_Z' at which NP effects that are
consistent with perturbative couplings could be found are approximately 2000
TeV for K decays and 160 TeV for rare B_{s,d} decays. Finally we present a
simple idea for an indirect determination of M_Z' that could be realised at the
next linear e^+e^- or mu^+ mu^- collider and with future precise flavour data.Comment: 43 pages, 10 figures, 6 tables; new section on the role of heavy
neutral scalars in reaching Zeptouniverse added; clarifying comments and
references adde
Z-Z' Mixing and Z-Mediated FCNCs in SU(3)_C x SU(3)_L x U(1)_X Models
Most of the existing analyses of FCNC processes in the 331 models, based on
the gauge group SU(3)_C x SU(3)_L x U(1)_X, take only into account tree-level
exchanges of a new heavy neutral gauge boson Z'. However due to the Z-Z' mixing
also corresponding contributions from Z boson are present that are usually
neglected. We calculate the impact of these contributions on Delta F=2
processes and rare K, B_s and B_d decays for different values of a parameter
beta, which distinguishes between various 331 models and for different fermion
representations under the SU(3)_L group. We find a general expression for the
Z-Z' mixing in terms beta, M_Z, M_Z' and tan(bar{beta}), familiar from 2 Higgs
Doublet models, that differs from the one quoted in the literature. We study in
particular the models with beta=+-n/sqrt{3} with n=1,2 which have recently been
investigated by us in the context of new data on B_{s,d}->mu^+ mu^- and
B_d->K^*(K)mu^+ mu^-. We find that these new contributions can indeed be
neglected in the case of Delta F=2 transitions and decays, like
B_d->K^*mu^+mu^-, where they are suppressed by the small vectorial Z coupling
to charged leptons. However the contributions of tree-level Z exchanges to
decays sensitive to axial-vector couplings, like B_{s,d}->mu^+ mu^- and B_d->K
mu^+ mu^-, and those with neutrinos in the final state, like b->s nu bar{nu}
transitions, K^+->pi^+ nu bar{nu} and K_L->pi^0 nu bar{nu} cannot be generally
neglected with size of Z contributions depending on beta, tan(bar{beta}) and
M_Z'. We analyze for the first time the ratio epsilon'/epsilon in these models
including both Z' and Z contributions. Our analysis of electroweak precision
observables within 331 models demonstrates transparently that the interplay of
NP effects in electroweak precision observables and those in flavour
observables could allow in the future to identify the favourite 331 model.Comment: 44 pages, 15 figures, 9 tables; clarifying comments added; version
accepted for publication in JHE
Yukawa coupling and anomalous magnetic moment of the muon: an update for the LHC era
We study the interplay between a soft muon Yukawa coupling generated
radiatively with the trilinear A-terms of the minimal supersymmetric standard
model (MSSM) and the anomalous magnetic moment of the muon. In the absence of a
tree-level muon Yukawa coupling the lightest smuon mass is predicted to be in
the range between 750 GeV and 2700 GeV at 2 sigma, if the bino mass M_1 is
below 1 TeV. Therefore, a detection of a smuon (in conjunction with a sub-TeV
bino) at the LHC would directly imply a non-zero muon Yukawa coupling in the
MSSM superpotential. Inclusion of slepton flavor mixing could in principle
lower the mass of one smuon-like slepton below 750 GeV. However, the
experimental bounds on radiative lepton decays instead strengthen the lower
mass bound, with larger effects for smaller M_1, We also extend the analysis to
the electron case and find that a light selectron close to the current
experimental search limit may prove the MSSM electron Yukawa coupling to be
non-zero.Comment: 6 pages, 2 figures, references added, version accepted for
publication in PR
and in the Standard Model: Status and Perspectives
In view of the recent start of the NA62 experiment at CERN that is expected
to measure the branching ratio with a precision of 10%,
we summarise the present status of this promising decay within the Standard
Model (SM). We do likewise for the closely related ,
which will be measured by the KOTO experiment around 2020. As the perturbative
QCD and electroweak corrections in both decays are under full control, the
dominant uncertainties within the SM presently originate from the CKM
parameters , and . We show this dependence with the
help of analytic expressions as well as accurate interpolating formulae.
Unfortunately a clarification of the discrepancies between inclusive and
exclusive determinations of and from tree-level decays will
likely require results from the Belle II experiment available at the end of
this decade. Thus we investigate whether higher precision on both branching
ratios is achievable by determining , and by means of
other observables that are already precisely measured. In this context
and , together with the expected progress in
QCD lattice calculations will play a prominent role. We find
and
, which is
more precise than using averages of the present tree-level values of ,
and . Furthermore, we point out the correlation between
,
and within the SM, that is
only very weakly dependent on other CKM parameters. Finally, we also update the
ratio in the SM and present its correlation with
.Comment: 33 pages, 8 figures, 5 tables; v2: new correlation presented and
discussion extended in section 3.2, analysis in section 5 updated, typos
corrected; matches version published in JHE
BSM models facing the recent LHCb data: A first look
During last decade a number of detailed analyses of flavour observables and
of their correlations within more than a dozen specific BSM models have been
performed at the TUM. One of the goals of these analyses was to investigate
which model is capable of obtaining large mixing induced CP asymmetry in the
B_s system, S_{psi phi}, and to find out what this would imply for other
flavour observables. In this context also the rare decays B_{s,d}->mu^+ mu^-
have been considered. In some models their branching ratios can be enhanced by
orders of magnitude above the SM expectations. The recent data on S_{psi phi}
and B_{s,d}-> mu^+ mu^- from the LHCb put an end to these very optimistic hopes
modifying significantly the allowed patterns of deviations from SM predictions
for flavour observables in concrete BSM models. We make a first
semi-quantitative assessment of the most important modifications in the
predictions of the BSM models in question including also recently analyzed
models and taking into account the most recent lattice input. For some BSM
models the LHCb data turned out to be a relief. On the other hand the SM,
models with CMFV and MFV models without flavour blind phases appear to have
significant difficulties in describing all Delta F=2 observables in B_{s,d} and
K^0 meson systems simultaneously. We propose to regard the stringent CMFV
relations between various observables as standard candels of flavour physics.
The pattern of deviations from these relations may help in identifying the
correct NP scenario.Comment: 45 pages, 7 figures; updated talk presented by A.J.B. at the Cracow
Epiphany Conference "On Present and Future of B-Physics", Cracow, Poland,
January 8-11, 201
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