In this letter, we use LHC data from the Drell–Yan processes
p
p
→
ℓ
i
ℓ
j
(with
i
≠
j
) to derive model-independent upper limits on lepton-flavor-violating meson decays. Our analysis is based on an ...Effective Field Theory (EFT) approach and it does not require a specific assumption regarding the basis of effective operators. We find that current LHC data (140
fb
-
1
) already provides competitive limits on
B
(
B
→
π
e
τ
)
and
B
(
B
→
π
μ
τ
)
with respect to the ones obtained through experimental searches at the
B
-factories. Moreover, we derive upper limits on several decays that have not been searched for experimentally yet, such as
D
0
→
e
τ
in the charm sector, and various semileptonic decays such as
B
→
ρ
μ
τ
,
B
s
→
K
μ
τ
and
B
s
→
ϕ
μ
τ
. Lastly, we discuss the validity of the EFT description of LHC data and the impact of loop corrections in our analysis.
We consider the evaluation of the
η
π
isospin-violating vector and scalar form factors relying on a systematic application of analyticity and unitarity, combined with chiral expansion results. It is ...argued that the usual analyticity properties do hold (i.e. no anomalous thresholds are present) in spite of the instability of the
η
meson in QCD. Unitarity relates the vector form factor to the
η
π
→
π
π
amplitude: we exploit progress in formulating and solving the Khuri–Treiman equations for
η
→
3
π
and in experimental measurements of the Dalitz plot parameters to evaluate the shape of the
ρ
-meson peak. Observing this peak in the energy distribution of the
τ
→
η
π
ν
decay would be a background-free signature of a second-class amplitude. The scalar form factor is also estimated from a phase dispersive representation using a plausible model for the
η
π
elastic scattering
S
-wave phase shift and a sum rule constraint in the inelastic region. We indicate how a possibly exotic nature of the
a
0
(
980
)
scalar meson manifests itself in a dispersive approach. A remark is finally made on a second-class amplitude in the
τ
→
π
π
ν
decay.
Isospin analysis of charmless B-meson decays Charles, J.; Deschamps, O.; Descotes-Genon, S. ...
The European physical journal. C, Particles and fields,
08/2017, Letnik:
77, Številka:
8
Journal Article
Recenzirano
Odprti dostop
We discuss the determination of the CKM angle
α
using the non-leptonic two-body decays
B
→
π
π
,
B
→
ρ
ρ
and
B
→
ρ
π
using the latest data available. We illustrate the methods used in each case and ...extract the corresponding value of
α
. Combining all these elements, we obtain the determination
α
dir
=
(
86.2
-
4.0
+
4.4
∪
178.4
-
5.1
+
3.9
)
∘
. We assess the uncertainties associated to the breakdown of the isospin hypothesis and the choice of the statistical framework in detail. We also determine the hadronic amplitudes (tree and penguin) describing the QCD dynamics involved in these decays, briefly comparing our results with theoretical expectations. For each observable of interest in the
B
→
π
π
,
B
→
ρ
ρ
and
B
→
ρ
π
systems, we perform an indirect determination based on the constraints from all the other observables available and we discuss the compatibility between indirect and direct determinations. Finally, we review the impact of future improved measurements on the determination of
α
.
We discuss the existence of the light scalar meson K*0(800) (also called κ) in a rigorous way, by showing the presence of a pole in the πK→πK amplitude on the second Riemann sheet. For this purpose, ...we study the domain of validity of two classes of Roy–Steiner representations in the complex energy plane. We prove that one of them is valid in a region sufficiently broad in the imaginary direction. From this representation, we compute the l=0 partial wave in the complex plane neither making any additional approximation nor having model dependence, relying only on experimental data. A scalar resonance with strangeness S=1 is found with the following mass and width: Mκ=658±13 MeV and Γκ=557±24 MeV.
The 331 models constitute an extension of the Standard Model (SM) obtained by enlarging the SM gauge group SU (3)C × SU (2)L × U (1)Y to the group SU (3)C × SU (3)L × U (1)X. We investigate how a ...nonminimal 331 model may embed lepton flavor universality violating contributions to b → sℓℓ processes without introducing lepton flavor violation, as suggested by the recent LHCb measurements of the ratios RK and RK*. We discuss the model-independent scenarios of new physics in b → sℓℓ currently favored by the data that could be accommodated by this model and consider a few phenomenological constraints on this model.
With the aim of generating new constraints on the OZI suppressed couplings of chiral perturbation theory a set of six equations of the Roy and Steiner type for the S- and P-waves of the $\pi K$ ...scattering amplitudes is derived. The range of validity and the multiplicity of the solutions are discussed. Precise numerical solutions are obtained in the range $E\lesssim1$ GeV which make use as input, for the first time, of the most accurate experimental data available at $E\gtrsim1$ GeV for both $\pi K\to\pi K$ and $\pi\pi\to K\overline{K}$ amplitudes. Our main result is the determination of a narrow allowed region for the two S-wave scattering lengths. Present experimental data below 1 GeV are found to be in generally poor agreement with our results. A set of threshold expansion parameters, as well as sub-threshold parameters are computed. For the latter, a matching with the SU(3) chiral expansion at NLO is performed.
The determination of αS from τ decays revisited Davier, M.; Descotes-Genon, S.; Höcker, A. ...
The European physical journal. C, Particles and fields,
08/2008, Letnik:
56, Številka:
3
Journal Article
Recenzirano
Odprti dostop
We revisit the determination of
α
S
(
m
τ
2
) using a fit to inclusive
τ
hadronic spectral moments in light of (1) the recent calculation of the fourth-order perturbative coefficient
K
4
in the ...expansion of the Adler function, (2) new precision measurements from BABAR of e
+
e
−
annihilation cross sections, which decrease the uncertainty in the separation of vector and axial-vector spectral functions, and (3) improved results from BABAR and Belle on
τ
branching fractions involving kaons. We estimate that the fourth-order perturbative prediction reduces the theoretical uncertainty, introduced by the truncation of the series, by 20% with respect to earlier determinations. We discuss to some detail the perturbative prediction of two different methods: fixed-order perturbation theory (FOPT) and contour-improved perturbative theory (CIPT). The corresponding theoretical uncertainties are studied at the
τ
and
Z
mass scales. The CIPT method is found to be more stable with respect to the missing higher order contributions and to renormalization scale variations. It is also shown that FOPT suffers from convergence problems along the complex integration contour. Nonperturbative contributions extracted from the most inclusive fit are small, in agreement with earlier determinations. Systematic effects from quark-hadron duality violation are estimated with simple models and found to be within the quoted systematic errors. The fit based on CIPT gives
α
S
(
m
τ
2
)=0.344±0.005±0.007, where the first error is experimental and the second theoretical. After evolution to
M
Z
we obtain
α
S
(
M
Z
2
)=0.1212±0.0005±0.0008±0.0005, where the errors are respectively experimental, theoretical and due to the evolution. The result is in agreement with the corresponding N
3
LO value derived from essentially the
Z
width in the global electroweak fit. The
α
S
(
M
Z
2
) determination from
τ
decays is the most precise one to date.
We focus on the chiral Lagrangian couplings describing radiative corrections to weak semi-leptonic decays and relate them to the decay amplitude of a lepton, computed by Braaten and Li at one loop in ...the standard model. For this purpose, we follow a two-step procedure. A first matching, from the standard model to Fermi theory, yields a relevant set of counterterms. The latter are related to chiral couplings thanks to a second matching, from Fermi theory to the chiral Lagrangian, which is performed using the spurion method. We show that the chiral couplings of physical relevance obey integral representations in a closed form, expressed in terms of QCD chiral correlators and vertex functions. We deduce exact relations among the couplings, as well as numerical estimates which go beyond the usual \(\log(M_Z/M_\rho)\) approximation.
Chiral symmetry breaking may exhibit significantly different patterns in two chiral limits: Nf=2 massless flavours (mu=md=0, ms physical) and Nf=3 massless flavours (mu=md=ms=0). Such a difference ...may arise due to vacuum fluctuations of ss̄ pairs related to the violation of the Zweig rule in the scalar sector, and it could yield numerical competition between contributions counted as leading and next-to-leading order in the chiral expansions of the observables. We recall and extend resummed chiral perturbation theory (ReχPT), a framework that we introduced previously to deal with such instabilities: it requires a more careful definition of the relevant observables and their one-loop chiral expansions. We analyse the amplitudes for low-energy ππ and πK scatterings within ReχPT, which we match in subthreshold regions with dispersive representations obtained from the solutions of the Roy and Roy–Steiner equations. Using a frequentist approach, we constrain the quark mass ratio as well as the quark condensate and the pseudoscalar decay constant in the Nf=3 chiral limit. The results mildly favour significant contributions of vacuum fluctuations suppressing the Nf=3 quark condensate compared to its Nf=2 counterpart.
“Minimal Flavour Violation” for leptoquarks Davidson, Sacha; Descotes-Genon, Sébastien
The journal of high energy physics,
11/2010, Letnik:
2010, Številka:
11
Journal Article
Recenzirano
Odprti dostop
Scalar leptoquarks, with baryon and lepton number conserving interactions, could have TeV scale masses, and be produced at colliders or contribute to a wide variety of rare decays. In pursuit of some ...insight as to the most sensitive search channels, we assume that the leptoquark-lepton-quark coupling can be constructed from the known mass matrices. We estimate the rates for selected rare processes in three cases: leptoquarks carrying lepton and quark flavour, leptoquarks with quark flavour only, and unflavoured leptoquarks. We find that leptoquark decay to top quarks is an interesting search channel.
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