56 research outputs found
Lepton flavor violation in minimal flavor violation extensions of the seesaw
A minimal flavour violation hypothesis for leptons can be implemented
essentially in two ways that are compatible with a type-I seesaw structure with
three heavy singlet neutrinos , and that satisfy the requirement of being
predictive, in the sense that all lepton flavor violating (LFV) effects can be
expressed in terms of low energy observables. The first realization is CP
conserving and is based on the flavor group (being and the SU(2) singlet and doublet leptons). The
second realization allows for CP violation and is based on . I review the main features of the two schemes and point out
their different implications for LFV observables.Comment: 5 pages. Invited talk at the Workshop on e+ e- Collisions from \Phi\
to \Psi\ (PHIPSI08)}, September 19-22, 2011, Novosibirsk, Russi
CPV tests with rare kaon decays
The K_S \to pi+/- e+ e- decay mode has been investigated using the data
collected in 2002 by the NA48/1 collaboration. With about 23k signal events and
59k K_L \to pi+ pi- pi0_D normalization decays, the K_S \to pi+ pi- e+ e-
branching ratio was determined. This result is also used to set an upper limit
on the presence of E1 direct emission in the decay amplitude. The CP-violating
asymmetry has been also measured.
We report on measurements of the rare decays K +/- \to pi+/- e+ e- and K+/-
\to pi+/- mu+ mu- . The full NA48/2 data set was analyzed, leading to more than
7200 reconstructed events in the electronic and more than 3000 events in the
muonic channel, the latter exceeding the total existing statistics by a factor
of four. For both channels the selected events are almost background-free. From
these events, we have determined the branching fraction and form factors of
K+/- \to pi+/- e+ e- using different theoretical models. Our results improve
the existing world averages significantly. In addition, we measured the CP
violating asymmetry between K+ and K- in this channel to be less than a few
percent.Comment: 4 pages, 1 figure, To appear in the proceedings of IX International
Conference on Hyperons, Charm and Beauty Hadrons (BEACH2010), Perugia, Italy,
21-26 June 201
The Other Natural Two Higgs Doublet Model
We characterize models where electroweak symmetry breaking is driven by two
light Higgs doublets arising as pseudo-Nambu-Goldstone bosons of new dynamics
above the weak scale. They represent the simplest natural two Higgs doublet
alternative to supersymmetry. We construct their low-energy effective
Lagrangian making only few specific assumptions about the strong sector. These
concern their global symmetries, their patterns of spontaneous breaking and the
sources of explicit breaking. In particular we assume that all the explicit
breaking is associated with the couplings of the strong sector to the Standard
Model fields, that is gauge and (proto)-Yukawa interactions. Under those
assumptions the scalar potential is determined at lowest order by very few free
parameters associated to the top sector. Another crucial property of our
scenarios is the presence of a discrete symmetry, in addition to custodial
SO(4), that controls the -parameter. That can either be simple CP or a
that distinguishes the two Higgs doublets. Among various possibilities we study
in detail models based on SO(6)/SO(4) SO(2), focussing on their
predictions for the structure of the scalar spectrum and the deviations of
their couplings from those of a generic renormalizable two Higgs doublet model.Comment: 54 page
Implications of the LHCb Evidence for Charm CP Violation
The LHCb collaboration recently announced preliminary evidence for CP
violation in D meson decays. We discuss this result in the context of the
standard model (SM), as well as its extensions. In the absence of reliable
methods to evaluate the hadronic matrix elements involved, we can only estimate
qualitatively the magnitude of the non-SM tree level operators required to
generate the observed central value. In the context of an effective theory, we
list the operators that can give rise to the measured CP violation and
investigate constraints on them from other processes.Comment: 7 pages, 3 figures; minor corrections, conclusions unchange
Direct test of time-reversal symmetry in the entangled neutral kaon system at a -factory
We present a novel method to perform a direct T (time reversal) symmetry test
in the neutral kaon system, independent of any CP and/or CPT symmetry tests.
This is based on the comparison of suitable transition probabilities, where the
required interchange of in out states for a given process is obtained
exploiting the Einstein-Podolsky-Rosen correlations of neutral kaon pairs
produced at a -factory. In the time distribution between the two decays,
we compare a reference transition like the one defined by the time ordered
decays with the T -conjugated one defined by . With the use of this and other T conjugated comparisons, the KLOE-2
experiment at DANE could make a significant test
Complete next-to-leading order gluino contributions to b--> s gamma and b--> s g
We present the first complete order alpha_s corrections to the Wilson
coefficients (at the high scale) of the various versions of magnetic and
chromomagnetic operators which are induced by a squark-gluino exchange. For
this matching calculation, we work out the on-shell amplitudes b--> s gamma and
b --> s g, both in the full and in the effective theory up to order alpha_s^2.
The most difficult part of the calculation is the evaluation of the two-loop
diagrams in the full theory; these can be split into two classes: a) diagrams
with one gluino and a virtual gluon; b) diagrams with two gluinos or with one
gluino and a four-squark vertex. Accordingly, the Wilson coefficients can be
split into a part a) and a part b). While part b) of the Wilson coefficients is
presented in this paper for the first time, part a) was given in (Bobeth et
al.). We checked their results for the coefficients of the magnetic operators
and found perfect agreement. Moreover, we work out the renormalization
procedure in great detail. Our results for the complete next-to-leading order
Wilson coefficients are fully analytic, but far too long to be printed. We
therefore publish them in the form of a C++ program. They constitute a crucial
building block for the phenomenological next-to-leading logarithmic analysis of
the branching ratio Bbar --> X_s gamma in a supersymmetric model beyond minimal
flavor violation.Comment: 38 pages, including c++ cod
Effective Theory of Resonant Leptogenesis in the Closed-Time-Path Approach
We describe mixing scalar particles and Majorana fermions using
Closed-Time-Path methods. From the Kadanoff-Baym equations, we obtain the
charge asymmetry, that is generated from decays and inverse decays of the
mixing particles. Within one single formalism, we thereby treat Leptogenesis
from oscillations and recover as well the standard results for the asymmetry in
Resonant Leptogenesis, which apply when the oscillation frequency is much
larger than the decay rate. Analytic solutions for two mixing neutral particles
in a constant-temperature background illustrate our results qualitatively. We
also perform the modification of the kinetic equations that is necessary in
order to take account of the expansion of the Universe and the washout of the
asymmetry.Comment: 46 pages, typos corrected, clarifying details adde
Color-Octet-Electroweak-Doublet Scalars and the CDF Dijet Anomaly
We study the phenomenology of color-octet scalars in the (8, 2)1/2
representation in the context of the 3.2\sigma excess, in the dijet invariant
mass spectrum of the W+jj final state, recently observed by the CDF
collaboration. We consider the region of parameter space with a sizable mass
splitting between the charged and neutral color-octet scalars and consistent
with electroweak precision data. We implement the principle of Minimal Flavor
Violation (MFV) in order to suppress FCNC currents and reduce the number of
free parameters. The excess in the W+jj channel corresponds to the charged
current decay of the heavier neutral octet scalar into its lighter charged
partner which decays into the two jets. In the MFV scenario, the production of
the neutral color-octet is dominated by gluon fusion due to the Yukawa
suppression of production via initial state quarks. As a result, no visible
excess is expected in the \gamma+jj channel due to Yukawa and CKM suppression.
Contributions to the Z+jj final state are suppressed for a mass spectrum where
the decay of the heavier color-octet to this final state is mediated by an
off-shell neutral color-octet partner. MFV allows one to control fraction of
bottom quarks in the final state jets by a single ratio of two free parameters.Comment: 14 pages, 6 figures, typos corrected, references added, text and
figures modified in some places for better clarity, version to appear in
Physics Letters
Interplay of t --> b W Decay and B_q Meson Mixing in Minimal Flavor Violating Models
Precise measurements of the top quark decay properties at hadron colliders
offer interesting new possibilities of testing the standard model. At the same
time, recent intriguing experimental results concerning CP violation in the B_d
and B_s systems have stimulated many studies of physics beyond the standard
model. We investigate anomalous t W d_j interactions as a possible source of
new effects in B_{d,s} - bar B_{d,s} oscillations within a model independent
approach based on the assumptions of Minimal Flavor Violation. After matching
our effective operators onto the low-energy effective Lagrangian describing
B_{d,s} meson mixing and evolving it down to the B-mass scale, we extract the
preferred ranges of the anomalous t W d_j interactions at the weak scale. These
values are then compared to previously considered constraints coming from the
rare radiative B --> X_s gamma decay. Finally, we reconsider the associated
effects in the t --> b W decays and find that the W helicity fractions F_{L,+}
can deviate by as much as 15%, 30% from their standard model values,
respectively. The deviations in F_L in particular, can reach the level of
expected precision measurements at the LHC.Comment: 7 pages, 3 figures, published versio
Probing anomalous tWb interactions with rare B decays
Precision studies of top quark properties are currently underway at the LHC
and Tevatron colliders with the prospect of probing anomalous t-W -b
interactions. In the mean time, recent experimental results for the B_{d,s} -
\bar{B}_{d,s} oscillation observables, the branching ratio B_s->\mu^+\mu^-, as
well as the forward - backward asymmetry in B->K\starl^+l^-, accompanied by the
accurate theoretical predictions for the relevant observables obtained within
the SM motivate a combined study of these observables in the presence of
anomalous t - W - b vertices. We investigate contributions of such anomalous
couplings to the B->X_s l^+l^- decay mode, and combining them with the
modifications of the B_{d,s} - \bar{B}_d,s, B->X_s{\gamma} observables, we
determine indirect bounds on the real and imaginary parts of the anomalous t -
W - b interactions. We find these to be mostly superior to present direct
constraints coming from top decay and production measurements at the LHC and
Tevatron. Finally, we predict the allowed effects in the branching ratios of
the B_s->\mu^+\mu^- and B->K^{(*)}\nu \bar{\nu}, as well as the
forward-backward asymmetry in B->K^*l^+l^-. We find that improved knowledge of
these observables in the future could further constrain some of the anomalous
tWb interactions.Comment: 14 page
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