Search results for "Perturbation theory"
showing 10 items of 584 documents
Measurement of the Charged-Pion Polarizability
2015
The COMPASS collaboration at CERN has investigated pion Compton scattering, $\pi^-\gamma\rightarrow \pi^-\gamma$, at centre-of-mass energy below 3.5 pion masses. The process is embedded in the reaction $\pi^-\mathrm{Ni}\rightarrow\pi^-\gamma\;\mathrm{Ni}$, which is initiated by 190\,GeV pions impinging on a nickel target. The exchange of quasi-real photons is selected by isolating the sharp Coulomb peak observed at smallest momentum transfers, $Q^2<0.0015$\,(GeV/$c$)$^2$. From a sample of 63\,000 events the pion electric polarisability is determined to be $\alpha_\pi\ =\ (\,2.0\ \pm\ 0.6_{\mbox{\scriptsize stat}}\ \pm\ 0.7_{\mbox{\scriptsize syst}}\,) \times 10^{-4}\,\mbox{fm}^3$ under the …
The full Boltzmann hierarchy for dark matter-massive neutrino interactions
2020
The impact of dark matter-neutrino interactions on the measurement of the cosmological parameters has been investigated in the past in the context of massless neutrinos exclusively. Here we revisit the role of a neutrino-dark matter coupling in light of ongoing cosmological tensions by implementing the full Boltzmann hierarchy for three massive neutrinos. Our tightest 95% CL upper limit on the strength of the interactions, parameterized via $u_\chi =\frac{\sigma_0}{\sigma_{Th}}\left(\frac{m_\chi}{100 \text{GeV}}\right)^{-1}$, is $u_\chi\leq3.34 \cdot 10^{-4}$, arising from a combination of Planck TTTEEE data, Planck lensing data and SDSS BAO data. This upper bound is, as expected, slightly …
Rosenbluth Separation of the π^{0} Electroproduction Cross Section.
2016
We present deeply virtual $\pi^0$ electroproduction cross-section measurements at $x_B$=0.36 and three different $Q^2$--values ranging from 1.5 to 2 GeV$^2$, obtained from experiment E07-007 that ran in the Hall A at Jefferson Lab. The Rosenbluth technique was used to separate the longitudinal and transverse responses. Results demonstrate that the cross section is dominated by its transverse component, and thus is far from the asymptotic limit predicted by perturbative Quantum Chromodynamics. An indication of a non-zero longitudinal contribution is provided by the interference term $\sigma_{LT}$ also measured. Results are compared with several models based on the leading twist approach of G…
Scaling property of variational perturbation expansion for a general anharmonic oscillator with xp-potential
1995
We prove a powerful scaling property for the extremality condition in the recently developed variational perturbation theory which converts divergent perturbation expansions into exponentially fast convergent ones. The proof is given for the energy eigenvalues of an anharmonic oscillator with an arbitrary $x^p$-potential. The scaling property greatly increases the accuracy of the results.
Detecting gravitational waves from cosmological phase transitions with LISA: an update
2020
MC was funded by the Royal Society under the Newton International Fellowship program. GD would like to thank CNPq (Brazil) for financial support. MH was supported by the Science and Technology Facilities Council (grant number ST/P000819/1), and the Academy of Finland (grant number 286769). SJH was supported by the Science and Technology Facilities Council (grant number ST/P000819/1). The work of JK was supported by Department of Energy (DOE) grant DE-SC0019195 and NSF grant PHY-1719642. TK and GS are funded by the Deutsche Forschungsgemeinschaft under Germany's Excellence Strategy - EXC 2121 \Quantum Universe" - 390833306. JMN is supported by Ramon y Cajal Fellowship contract RYC-2017-22986…
Ab initio study on the low-lying excited states of retinal
1997
Ab initio results for the electronic spectrum of all-trans-retinal and its truncated model 3-methyl-all-trans (10-s-cis)-2,4,6,8,10-undecapentaen-1-al are presented. The study includes geometry determination of the ground state. Vertical excitation energies have been computed using multiconfigurational second-order perturbation theory through the CASPT2 formalism. The lowest singlet excited state in gas phase is predicted to be of nπ∗ character. The lowest triplet state corresponds, however, to a ππ∗ state. The most intense feature of the spectrum is due to the strongly dipole-allowed ππ∗ transition, in accordance with the observed maximum in the one-photon spectra. The vertical excitation …
Charm-quark fragmentation fractions and production cross section at midrapidity in pp collisions at the LHC
2022
Physical review / D 105(1), L011103 (2022). doi:10.1103/PhysRevD.105.L011103
DLPNO-MP2 second derivatives for the computation of polarizabilities and NMR shieldings
2021
We present a derivation and efficient implementation of the formally complete analytic second derivatives for the domain-based local pair natural orbital second order Møller–Plesset perturbation theory (MP2) method, applicable to electric or magnetic field-response properties but not yet to harmonic frequencies. We also discuss the occurrence and avoidance of numerical instability issues related to singular linear equation systems and near linear dependences in the projected atomic orbital domains. A series of benchmark calculations on medium-sized systems is performed to assess the effect of the local approximation on calculated nuclear magnetic resonance shieldings and the static dipole …
A Lagrangian framework for deriving triples and quadruples corrections to the CCSD energy.
2014
Using the coupled cluster Lagrangian technique, we have determined perturbative corrections to the coupled cluster singles and doubles (CCSD) energy that converge towards the coupled cluster singles, doubles, and triples (CCSDT) and coupled cluster singles, doubles, triples, and quadruples (CCSDTQ) energies, considering the CCSD state as the unperturbed reference state and the fluctua- tion potential as the perturbation. Since the Lagrangian technique is utilized, the energy corrections satisfy Wigner’s 2n + 1 rule for the cluster amplitudes and the 2n + 2 rule for the Lagrange multi- pliers. The energy corrections define the CCSD perturbation series, CCSD(T–n) and CCSD(TQ–n), which are ter…
Analytic evaluation of first-order properties within the mean-field variant of spin-free exact two-component theory.
2019
We present a scheme for the calculation of energies and analytic energy gradients within spin-free exact two-component (SFX2C) theory in its mean-field variant, which we refer to as SFX2C-mf. In the presented scheme, the Foldy-Wouthuysen transformation is carried out after the spin-free four-component Hartree-Fock treatment such that in electron-correlated calculations only the non-mean-field part of the two-electron interactions is handled in an untransformed manner. The formulation of analytic gradients requires some adjustments in comparison with the nonrelativistic case, i.e., the additional solution of the spin-free Dirac Coulomb coupled-perturbed Hartee-Fock equations together with a …