522 research outputs found
Implications of the non-universal Z boson in FCNC mediated rare decays
We analyze the effect of the non-universal boson in the rare decays , and decays. These
decays involve the FCNC mediated transitions, and are found to be
very small in the standard model. The smallness of these decays in the standard
model makes them sensitive probe for new physics. We find an enhancement of at
least an order in these branching ratios because of the non-universal
coupling.Comment: 15 pages, 4 figures, minor changes in the text, references added, to
appear in PR
Interplay of magnetism and band topology in EuCaMgBi (x=0, 0.5) from first principles study
Recent discovery of the time reversal symmetry breaking magnetic Weyl
semimetals has created a huge surge of activities in the field of quantum
topological materials. In this work, we have studied systematically the ground
state magnetic order, electronic structure and the interplay between the
magnetic order and band topology in one such materials, EuMgBi (EMB)
and its Ca doped variant using first principles method within the framework of
density functional theory (DFT). The detailed investigation unravels the
existence of different topological phases in this single material which can be
tuned by an external probe such as magnetic field or chemical substitution. Our
DFT calculations including Coulomb correlation (U) and spin-orbit (SO)
interaction within GGA+U+SO approximation confirms that the magnetic ground
state of EMB is A-type Antiferromagnetic (A-AFM) with Eu magnetic moments
aligned along the crystallographic or direction. Although the ground
state of EMB is A-AFM, the Ferromagnetic (FM) state lies very close in energy.
We observe a single pair of Weyl points connecting valence and conduction band
very close to the Fermi level (FL) along -A direction in the FM state
of EuMgBi with Eu moments aligned along crystallographic direction.
On doping 50\% Ca at Eu sites, we observe single pair of Weyl points moving
closer to the FL which is highly desirable for application purposes. Further we
observe that the separation between the Weyl points in the pair decreases in
doped compound compared to that in the parent compound which has direct
consequence on anomalous Hall conductivity (AHC). Our first principles
calculation of AHC shows high peak values exactly at these Weyl points and the
peak height decreases when we dope the system with Ca. Therefore, Ca doping can
be a good external handle to tune AHC in this system.Comment: 9 pages, 12 figure
Effect of FCNC mediated Z boson on lepton flavor violating decays
We study the three body lepton flavor violating (LFV) decays , and the semileptonic decay in the flavor changing neutral current (FCNC) mediated boson
model. We also calculate the branching ratios for LFV leptonic B decays,
, , and the
conversion of muon to electron in Ti nucleus. The new physics parameter space
is constrained by using the experimental limits on and
. We find that the branching ratios for and processes could be as large as and . For other LFV B decays the branching ratios are found to be too
small to be observed in the near future.Comment: 15 pages, 8 figures, typos corrected, one more section added, version
to appear in EPJ
Slow Magnetic Relaxation and Electron Delocalization in an S = 9/2 Iron(II/III) Complex with Two Crystallographically Inequivalent Iron Sites
The magnetic, electronic, and Mössbauer spectral properties of Fe 2L(µ-OAc)2ClO4, 1, where L is the dianion of the tetraimino-diphenolate macrocyclic ligand, H2L, indicate that 1 is a class III mixed valence iron(II/III) complex with an electron that is fully delocalized between two crystallographically inequivalent iron sites to yield a [Fe2]V cationic configuration with a St 9/2 ground state. Fits of the dc magnetic susceptibility between 2 and 300K and of the isofield variable-temperature magnetization of 1 yield an isotropic magnetic exchange parameter, J, of -32(2) cm-1 for an electron transfer parameter, B, of 950 cm-1, a zero-field uniaxial D9/2 parameter of -0.9(1) cm-1, and g 1.95(5). In agreement with the presence of uniaxial magnetic anisotropy, ac susceptibility measurements reveal that 1 is a single-molecule magnet at low temperature with a single molecule magnetic effective relaxation barrier, Ueff, of 9.8 cm-1. At 5.25 K the Mössbauer spectra of 1 exhibit two spectral components, assigned to the two crystallographically inequivalent iron sites with a static effective hyperfine field; as the temperature increases from 7 to 310 K, the spectra exhibit increasingly rapid relaxation of the hyperfine field on the iron-57 Larmor precession time of 5 x 10-8 s. A fit of the temperature dependence of the average effective hyperfine field yields |D9/2| 0.9 cm-1. An Arrhenius plot of the logarithm of the relaxation frequency between 5 and 85 K yields a relaxation barrier of 17 cm-1
Constraints on the nonuniversal Z^\prime couplings from B\to\pi K, \pi K^{\ast} and \rho K Decays
Motivated by the large difference between the direct CP asymmetries
and , we
combine the up-to-date experimental information on ,
and decays to pursue possible solutions with the nonuniversal
model. Detailed analyses of the relative impacts of different
types of couplings are presented in four specific cases. Numerically, we find
that the new coupling parameters, and with a common
nontrivial new weak phase , which are relevant to the
contributions to the electroweak penguin sector
and , are crucial to the observed " puzzle". Furthermore,
they are found to be definitely unequal and opposite in sign. We also find that
can put a strong constraint on the new
couplings, which implies the contributions to the coefficient of
QCD penguins operator involving the parameter required.Comment: 27 pages, 6 figures. References and a note adde
Can there be any new physics in b -> d penguins
We analyze the possibility of observing new physics effects in the
penguin amplitudes. For this purpose, we consider the decay mode , which has only penguin contributions. Using the QCD
factorization approach, we find very tiny CP violating effects in the standard
model for this process. Furthermore, we show that the minimal supersymmetric
standard model with mass insertion and R-parity violating supersymmetric
model can provide substantial CP violation effects. Observation of sizable CP
violation in this mode would be a clear signal of new physics.Comment: Published versio
Double-Lepton Polarization Asymmetries and Branching Ratio in B \rar K_{0}^{*}(1430) l^+ l^- transition from Universal Extra Dimension Model
We investigate the B \rar K_{0}^{*}(1430) l^+ l^- transition in the
Applequist-Cheng-Dobrescu model in the presence of a universal extra dimension.
In particular, we calculate double lepton polarization asymmetries and
branching ratio related to this channel and compare the obtained results with
the predictions of the standard model. Our analysis of the considered
observables in terms of radius of the compactified extra-dimension as the
new parameter of the model show a considerable discrepancy between the
predictions of two models in low values.Comment: 12 Pages, 15 Figures and 1 Tabl
Goldstone Bosons in Effective Theories with Spontaneously Broken Flavour Symmetry
The Flavour Symmetry of the Standard Model (SM) gauge sector is broken by the
fermion Yukawa couplings. Promoting the Yukawa matrices to scalar spurion
fields, one can break the flavour symmetry spontaneously by giving appropriate
vacuum expectation values (VEVs) to the spurion fields, and one encounters
Goldstone modes for every broken flavour symmetry generator. In this paper, we
point out various aspects related to the possible dynamical interpretation of
the Goldstone bosons: (i) In an effective-theory framework with local flavour
symmetry, the Goldstone fields represent the longitudinal modes for massive
gauge bosons. The spectrum of the latter follows the sequence of
flavour-symmetry breaking related to the hierarchies in Yukawa couplings and
flavour mixing angles. (ii) Gauge anomalies can be consistently treated by
adding higher-dimensional operators. (iii) Leaving the U(1) factors of the
flavour symmetry group as global symmetries, the respective Goldstone modes
behave as axions which can be used to resolve the strong CP problem by a
modified Peccei-Quinn mechanism. (iv) The dynamical picture of flavour symmetry
breaking implies new sources of flavour-changing neutral currents, which arise
from integrating out heavy scalar spurion fields and heavy gauge bosons. The
coefficients of the effective operators follow the minimal-flavour violation
principle.Comment: 27 pages, abstract and introduction extended, more detailed
discussion of heavy gauge boson spectrum and auxiliary heavy fermions,
outline restructured. Matches version to be published in JHE
Unparticles-Higgs Interplay
We show that scalar unparticles coupled to the Standard Model Higgs at the
renormalizable level can have a dramatic impact in the breaking of the
electroweak symmetry already at tree level. In particular one can get the
proper electroweak scale without the need of a Higgs mass term in the
Lagrangian. By studying the mixed unparticle-Higgs propagator and spectral
function we also show how unparticles can shift the Higgs mass away from its
Standard Model value, \lambda v^2, and influence other Higgs boson properties.
Conversely, we study in some detail how electroweak symmetry breaking affects
the unparticle sector by breaking its conformal symmetry and generating a mass
gap. We also show that, for Higgs masses above that gap, unparticles can
increase quite significantly the Higgs width.Comment: 14 pages, 7 figures, typos correcte
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