Search results for "PERTURBATION"
showing 10 items of 811 documents
The width of the omega meson in the nuclear medium
2013
We evaluate the width of the omega meson in nuclear matter. We consider the free decay mode of the omega into three pions, which is dominated by rho IEuro decay, and replace the rho and pi propagators by their medium-modified ones. We also take into account the quasielastic and inelastic processes induced by a vector-baryon interaction dominated by vector meson exchange, as well as the contributions coming from the mechanism with medium-modified K , propagators. We obtain a substantial increase of the omega width in the medium, reaching a value of 121 +/- 10 MeV at normal nuclear matter density for an omega at rest, which comes mainly from omega N -> pi pi N, omega NN -> pi NN processes ass…
Running of the Charm-Quark Mass from HERA Deep-Inelastic Scattering Data
2017
Physics letters / B 775, 233 - 238 (2017). doi:10.1016/j.physletb.2017.11.002
The radiative decay of the Lambda(1405) and its two-pole structure
2007
We evaluate theoretically the radiative decay widths into $\gamma\Lambda$ and $\gamma\Sigma^0$ of the two poles of the $\Lambda(1405)$ found in chiral unitary theories and we find quite different results for each of the two poles. We show that, depending on which reaction is used to measure the $\Lambda(1405)$ radiative decays, one gives more weight to one or the other pole, resulting in quite different shapes in the $\gamma\Lambda(\Sigma^0)$ invariant mass distributions. Our results for the high-energy pole agree with those of the empirical determination of the $\gamma\Lambda$ and $\gamma\Sigma^0$ radiative widths (based on an isobar model fitting of the $K^-p$ atom data), which are someti…
Precision measurement of the ratio of the charged kaon leptonic decay rates
2013
A precision measurement of the ratio RK of the rates of kaon leptonic decays K+- --> e nu and K+- --> mu nu with the full data sample collected by the NA62 experiment at CERN in 2007-2008 is reported. The result, obtained by analysing ~150000 reconstructed K+- --> e nu candidates with 11% background contamination, is RK = (2.488+-0.010)*10^{-5}, in agreement with the Standard Model expectation.
Compton scattering off pions and electromagnetic polarizabilities
2019
The electric ($\alpha_\pi$) and magnetic ($\beta_\pi$) Compton polarizabilities of both the charged and the neutral pion are of fundamental interest in the low-energy sector of quantum chromodynamics (QCD). Pion polarizabilities affect the shape of the $\gamma\pi\to\gamma\pi$ Compton scattering angular distribution at back scattering angles and $\gamma\gamma\to\pi\pi$ absolute cross sections. Theory derivations are given of the $\gamma\pi\to\gamma\pi$ Compton scattering differential cross section, dispersion relations, and sum rules in terms of the polarizabilities. We review experimental charged and neutral polarizability studies and theoretical predictions. The $\pi^0$ polarizabilities we…
$W^{+}W^{-}$ production at the LHC: fiducial cross sections and distributions in NNLO QCD
2016
We consider QCD radiative corrections to $W^+W^-$ production at the LHC and present the first fully differential predictions for this process at next-to-next-to-leading order (NNLO) in perturbation theory. Our computation consistently includes the leptonic decays of the $W$ bosons, taking into account spin correlations, off-shell effects and non-resonant contributions. Detailed predictions are presented for the different-flavour channel $pp\to\mu^+e^-\nu_\mu {\bar \nu}_e+X$ at $\sqrt{s}=8$ and $13$ TeV. In particular, we discuss fiducial cross sections and distributions in the presence of standard selection cuts used in experimental $W^+W^-$ and $H\to W^+W^-$ analyses at the LHC. The inclus…
Experiments at the electron accelerator MAMI
2008
Measurement of ππ Scattering Lengths from and Decays
2011
In the last few years it has become possible to study low energy ππ scattering with high precision in kaon decays, thanks to the high statistics measurements of K ± → π ± π 0 π 0 ( K 3 π ) and K ± → π + π − e ± ν ( K e 4 ) decays performed by the NA48/2 experiment at the CERN SPS. In K ± → π ± π 0 π 0 , at the π + π − threshold, the π 0 π 0 mass spectrum exhibits a Wigner-cusp, from which the scattering lengths a 0 and a 2 are extracted. From the study of K e 4 decays a similar precision, but quite independent measurement of a 0 and a 2 can be obtained. The combination of both measurements provides one of the most stringent tests of Chiral Perturbation Theory.
The longitudinal muon polarization in KL → μ+μ-
1991
17 páginas, 3 figuras, 1 tabla.-- CERN-TH-6035-91 ; UWTHPH-1991-9.
A new measurement of the K±→π±γγ decay at the NA48/2 experiment
2014
Abstract The NA48/2 experiment at CERN collected two data samples with minimum bias trigger conditions in 2003 and 2004. A measurement of the rate and dynamic properties of the rare decay K ± → π ± γ γ from these data sets based on 149 decay candidates with an estimated background of 15.5 ± 0.7 events is reported. The model-independent branching ratio in the kinematic range z = ( m γ γ / m K ) 2 > 0.2 is measured to be B MI ( z > 0.2 ) = ( 0.877 ± 0.089 ) × 10 − 6 , and the branching ratio in the full kinematic range assuming a particular Chiral Perturbation Theory description to be B ( K π γ γ ) = ( 0.910 ± 0.075 ) × 10 − 6 .