Search results for "coupling constant"
showing 10 items of 262 documents
Spin-dipole nuclear matrix elements for double beta decays and astro-neutrinos
2014
Spin-dipole (SD) nuclear matrix elements (NMEs) M±(SD2) for unique first forbidden β±2−→0+ ground-state-to-ground-state transitions are studied by using effective microscopic two-nucleon interactions in realistic single-particle model spaces. The observed values of the NMEs Mexp±(SD2) are compared with the values of the single-quasiparticle NMEs Mqp±(SD2) without nucleon spin–isospin (στ) correlation and the QRPA NMEs MQRPA±(SD2) with the στ correlation. The observed SD matrix elements are found to be reduced by the factor k≈0.2 with respect to Mqp±(SD2) and by the factor kNM≈0.5 with respect to MQRPA±(SD2). We then infer that the SD NME is reduced considerably partly by the nucleon στ corr…
K+Λ photoproduction amplitudes incorporating absorptive effects and hypernuclear formation
1989
Incorporating final-state absorptive effects, we have reexamined the γp→K+Λ elementary process to obtain amplitudes which are used for describing photoproduction of hypernuclei. Our model not only resolves the persisting trouble of the small KNΛ coupling constant, but also yields agreement with cross-section data at higher energies. Using our amplitudes, we calculate angular distributions for the reaction16O(γ, K+)Λ16N. While the forward cross-section increases with increasing energy, the total cross-section is found to remain almost constant fromEγ=1.2 GeV through 2 GeV, which suggests that rather low photon energies hold promise for exploration of hypernuclear excitations.
Three-loop relation of quark $$\overline {MS} $$ and pole masses
1990
We calculate, exactly, the next-to-leading correction to the relation between the $$\overline {MS} $$ quark mass, $$\bar m$$ , and the scheme-independent pole mass,M, and obtain $$\begin{gathered} \frac{M}{{\bar m(M)}} \approx 1 + \frac{4}{3}\frac{{\bar \alpha _s (M)}}{\pi } + \left[ {16.11 - 1.04\sum\limits_{i = 1}^{N_F - 1} {(1 - M_i /M)} } \right] \hfill \\ \cdot \left( {\frac{{\bar \alpha _s (M)}}{\pi }} \right)^2 + 0(\bar \alpha _s^3 (M)), \hfill \\ \end{gathered} $$ as an accurate approximation forN F−1 light quarks of massesM i <M. Combining this new result with known three-loop results for $$\overline {MS} $$ coupling constant and mass renormalization, we relate the pole mass to the…
Two-loop calculation of the anomalous dimension of the axial current with static heavy quarks
1992
Abstract A method to perform two-loop calculations in Eichten's effective field theory for heavy quarks is developed. The anomalous dimension of the axial current for static heavy quarks is calculated at two loops. For N = 3, we get γ A (2) = − 1 36 [ 127 2 +28ξ(2)−5N f ] . This result is very important to understand completely the physical significance of the lattice measurement of the decay constant of the B meson. The two-loop correction generated by γ A (2) turns out to be small, less than 1% for four quark flavours, so that the value of the decay constant of the B meson does not need in practice renormalization group improvement. As extra results, the self-energy renormalization consta…
Infra-Red Asymptotic Dynamics of Gauge Invariant Charged Fields: QED versus QCD
1999
The freedom one has in constructing locally gauge invariant charged fields in gauge theories is analyzed in full detail and exploited to construct, in QED, an electron field whose two-point function W(p), up to the fourth order in the coupling constant, is normalized with on-shell normalization conditions and is, nonetheless, infra-red finite; as a consequence the radiative corrections vanish on the mass shell $p^2=\mu^2$ and the free field singularity is dominant, although, in contrast to quantum field theories with mass gap, the eigenvalue $\mu^2$ of the mass operator is not isolated. The same construction, carried out for the quark in QCD, is not sufficient for cancellation of infra-red …
Antiscreening of the Ampère force in QED and QCD plasmas
2013
The static forces between electric charges and currents are modified at the loop level by the presence of a plasma. While electric charges are screened, currents are not. The effective coupling constant at long distances is enhanced in both cases as compared to the vacuum, and by different amounts, a clear sign that Lorentz symmetry is broken. We investigate these effects quantitatively, first in a QED plasma and secondly using non-perturbative simulations of QCD with two light degenerate flavors of quarks.
Impact of the quenching of gA on the sensitivity of 0νββ experiments
2017
Detection of the neutrinoless $\ensuremath{\beta}\ensuremath{\beta}$ ($0\ensuremath{\nu}\ensuremath{\beta}\ensuremath{\beta}$) decay is of high priority in the particle- and neutrino-physics communities. The detectability of this decay mode is strongly influenced by the value of the weak axial-vector coupling constant ${g}_{A}$. The recent nuclear-model analyses of $\ensuremath{\beta}$ and $\ensuremath{\beta}\ensuremath{\beta}$ decays suggest that the value of ${g}_{A}$ could be dramatically quenched, reaching ratios of ${g}_{A}^{\mathrm{free}}/{g}_{A}\ensuremath{\approx}4$, where ${g}_{A}^{\mathrm{free}}=1.27$ is the free, neutron-decay, value of ${g}_{A}$. The effects of this quenching ap…
Evidence for the spin-0 nature of the Higgs boson using ATLAS data
2013
We acknowledge the support of ANPCyT, Argentina; YerPhl, Armenia; ARC, Australia; BMWF and FWF, Austria; ANAS, Azerbaijan; SSTC, Belarus; CNPq and FAPESP, Brazil; NSERC, NRC and CFI, Canada; CERN; CONICYT, Chile; CAS, MOST and NSFC, China; COLCIENCIAS, Colombia; MSMT CR, MPO CR and VSC CR, Czech Republic; DNRF, DNSRC and Lundbeck Foundation, Denmark; EPLANET, ERC and NSRF, European Union; IN2P3-CNRS, CEA-DSM/IRFU, France; GNSF, Georgia; BMBF, DFG, HGF, MPG and AvH Foundation, Germany; GSRT and NSRF, Greece; ISF, MINERVA, GIF, DIP and Benoziyo Center, Israel; INFN, Italy; MEXT and JSPS, Japan; CNRST, Morocco; FOM and NWO, Netherlands; BRF and RCN, Norway; MNiSW, Poland; GRICES and FCT, Portu…
Use of a running coupling in the NLO calculation of forward hadron production
2018
We address and solve a puzzle raised by a recent calculation [1] of the cross-section for particle production in proton-nucleus collisions to next-to-leading order: the numerical results show an un- reasonably large dependence upon the choice of a prescription for the QCD running coupling, which spoils the predictive power of the calculation. Specifically, the results obtained with a prescription formulated in the transverse coordinate space differ by one to two orders of magnitude from those obtained with a prescription in momentum space. We show that this discrepancy is an artefact of the interplay between the asymptotic freedom of QCD and the Fourier transform from coordinate space to mo…
QCD analysis of the tau hadronic width
1992
32 páginas, 1 figura, 4 tablas.-- CERN-TH-6070-91 ; NUHEP-TH-91-8 ; PM-91-8.