120 research outputs found
A perturbed lepton-specific two-Higgs-doublet model facing experimental hints for physics beyond the Standard Model
The BaBar, Belle, and LHCb collaborations have reported evidence for new
physics in and of approximately .
There is also the long lasting discrepancy of about in the anomalous
magnetic moment of the muon, and the branching ratio for is
() above the Standard Model expectation using the HFAG
(PDG) values. Furthermore, CMS found hints for a non-zero decay rate of
. Interestingly, all these observations can be explained by
introducing new scalars. In this article we consider these processes within a
lepton-specific two-Higgs doublet model (i.e. of type X) with additional
non-standard Yukawa couplings. It is found that one can accommodate
with modified Higgs-- couplings. The anomalous
magnetic moment of the muon can be explained if the additional neutral CP-even
Higgs is light (below 100 GeV). Also and can be easily
explained by additional ----Higgs couplings. Combining these --
couplings with a light the decay rate for can be in a testable
range for the LHC. Effects in are also possible, but in this case
a simultaneous explanation of the anomalous magnetic moment of the muon is
difficult due to the unavoidable decay.Comment: 14 pages, 10 figure
Two-pion contribution to hadronic vacuum polarization
We present a detailed analysis of data up to
in the framework of dispersion relations. Starting
from a family of -wave phase shifts, as derived from a previous
Roy-equation analysis of scattering, we write down an extended Omn\`es
representation of the pion vector form factor in terms of a few free parameters
and study to which extent the modern high-statistics data sets can be described
by the resulting fit function that follows from general principles of QCD. We
find that statistically acceptable fits do become possible as soon as potential
uncertainties in the energy calibration are taken into account, providing a
strong cross check on the internal consistency of the data sets, but preferring
a mass of the meson significantly lower than the current PDG average.
In addition to a complete treatment of statistical and systematic errors
propagated from the data, we perform a comprehensive analysis of the systematic
errors in the dispersive representation and derive the consequences for the
two-pion contribution to hadronic vacuum polarization. In a global fit to both
time- and space-like data sets we find and . While the constraints are
thus most stringent for low energies, we obtain uncertainty estimates
throughout the whole energy range that should prove valuable in corroborating
the corresponding contribution to the anomalous magnetic moment of the muon. As
side products, we obtain improved constraints on the -wave,
valuable input for future global analyses of low-energy scattering, as
well as a determination of the pion charge radius, .Comment: 40 pages, 16 figures, 13 tables; version published in JHE
A Dispersive Treatment of Decays
decays offer several reasons of interest: they allow an accurate
measurement of -scattering lengths; they provide the best source for
the determination of some low-energy constants of ChPT; one form factor is
directly related to the chiral anomaly, which can be measured here. We present
a dispersive treatment of decays that provides a resummation of
- and -rescattering effects. The free parameters of the
dispersion relation are fitted to the data of the high-statistics experiments
E865 and NA48/2. The matching to ChPT at NLO and NNLO enables us to determine
the LECs , and . With recently published data from
NA48/2, the LEC can be determined as well. In contrast to a pure chiral
treatment, the dispersion relation describes the observed curvature of one of
the form factors, which we understand as a rescattering effect beyond NNLO.Comment: 86 pages, 21 figures. Draws on and extends arXiv:1412.5171 [hep-ph]
and arXiv:1209.0755 [hep-ph
Isospin Breaking Effects in Decays
In the framework of chiral perturbation theory with photons and leptons, the
one-loop isospin breaking effects in decays due to both the
photonic contribution and the quark and meson mass differences are computed. A
comparison with the isospin breaking corrections applied by recent high
statistics experiments is performed. The calculation can be used to
correct the existing form factor measurements by isospin breaking effects that
have not yet been taken into account in the experimental analysis. Based on the
present work, possible forthcoming experiments on decays could correct
the isospin breaking effects in a more consistent way.Comment: 62 pages, 18 figure
Low-Energy Effective Field Theory below the Electroweak Scale: Operators and Matching
The gauge-invariant operators up to dimension six in the low-energy effective
field theory below the electroweak scale are classified. There are 70 Hermitian
dimension-five and 3631 Hermitian dimension-six operators that conserve baryon
and lepton number, as well as , , and operators. The matching onto these operators from the
Standard Model Effective Field Theory (SMEFT) up to order is
computed at tree level. SMEFT imposes constraints on the coefficients of the
low-energy effective theory, which can be checked experimentally to determine
whether the electroweak gauge symmetry is broken by a single fundamental scalar
doublet as in SMEFT. Our results, when combined with the one-loop anomalous
dimensions of the low-energy theory and the one-loop anomalous dimensions of
SMEFT, allow one to compute the low-energy implications of new physics to
leading-log accuracy, and combine them consistently with high-energy LHC
constraints.Comment: 44 pages, 22 tables; version published in JHE
Dispersive approach to hadronic light-by-light scattering
Based on dispersion theory, we present a formalism for a model-independent
evaluation of the hadronic light-by-light contribution to the anomalous
magnetic moment of the muon. In particular, we comment on the definition of the
pion pole in this framework and provide a master formula that relates the
effect from pi pi intermediate states to the partial waves for the process
gamma^* gamma^* --> pi pi. All contributions are expressed in terms of on-shell
form factors and scattering amplitudes, and as such amenable to an experimental
determination.Comment: 33 pages, 4 figures; version accepted for publication in JHEP,
improved presentation, including non-diagonal kernels for the S-wave
Rescattering effects in the hadronic-light-by-light contribution to the anomalous magnetic moment of the muon
We present a first model-independent calculation of intermediate
states in the hadronic-light-by-light (HLbL) contribution to the anomalous
magnetic moment of the muon that goes beyond the scalar QED pion
loop. To this end we combine a recently developed dispersive description of the
HLbL tensor with a partial-wave expansion and demonstrate that the known
scalar-QED result is recovered after partial-wave resummation. Using dispersive
fits to high-statistics data for the pion vector form factor, we provide an
evaluation of the full pion box, . We then construct suitable input for the
helicity partial waves based on a pion-pole left-hand cut and show that for the
dominant charged-pion contribution this representation is consistent with the
two-loop chiral prediction and the COMPASS measurement for the pion
polarizability. This allows us to reliably estimate -wave rescattering
effects to the full pion box and leads to our final estimate for the sum of
these two contributions: .Comment: 7 pages, 1 figure; version to appear in PR
Virtual photon-photon scattering
Based on analyticity, unitarity, and Lorentz invariance the contribution from
hadronic vacuum polarization to the anomalous magnetic moment of the muon is
directly related to the cross section of e^+e^- --> hadrons. We review the main
difficulties that impede such an approach for light-by-light scattering and
identify the required ingredients from experiment. Amongst those, the most
critical one is the scattering of two virtual photons into meson pairs. We
analyze the analytic structure of the process gamma^* gamma^* --> pi pi and
show that the usual Muskhelishvili-Omnes representation can be amended in such
a way as to remain valid even in the presence of anomalous thresholds.Comment: 5 pages, 3 figures, Proceedings for the International Workshop on
e^+e^- collisions from phi to psi 2013, Rome, Italy, September 9-12, 201
Dispersion relation for hadronic light-by-light scattering: two-pion contributions
In this third paper of a series dedicated to a dispersive treatment of the
hadronic light-by-light (HLbL) tensor, we derive a partial-wave formulation for
two-pion intermediate states in the HLbL contribution to the anomalous magnetic
moment of the muon , including a detailed discussion of the
unitarity relation for arbitrary partial waves. We show that obtaining a final
expression free from unphysical helicity partial waves is a subtle issue, which
we thoroughly clarify. As a by-product, we obtain a set of sum rules that could
be used to constrain future calculations of . We
validate the formalism extensively using the pion-box contribution, defined by
two-pion intermediate states with a pion-pole left-hand cut, and demonstrate
how the full known result is reproduced when resumming the partial waves. Using
dispersive fits to high-statistics data for the pion vector form factor, we
provide an evaluation of the full pion box,
. As an application of the
partial-wave formalism, we present a first calculation of -rescattering
effects in HLbL scattering, with helicity partial
waves constructed dispersively using phase shifts derived from the
inverse-amplitude method. In this way, the isospin- part of our calculation
can be interpreted as the contribution of the to HLbL scattering in
. We argue that the contribution due to charged-pion rescattering
implements corrections related to the corresponding pion polarizability and
show that these are moderate. Our final result for the sum of pion-box
contribution and its -wave rescattering corrections reads
.Comment: 70 pages, 14 figures, Mathematica notebook with full expressions for
the basis change included as supplementary material. Version accepted for
publication in JHE
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