arXiv subjects

Vadim Lensky

Publications and source records attributed to Vadim Lensky.

30 records · Page 2Linked to original sources

arXiv2017

Generalized polarizabilities of the nucleon in baryon chiral perturbation theory

The nucleon generalized polarizabilities (GPs), probed in virtual Compton scattering (VCS), describe the spatial distribution of the polarization density in a nucleon. They are accessed experimentally via the process of electron-proton bremsstrahlung ($ep\to epγ$) at electron-beam facilities, such as MIT-Bates, CEBAF (Jefferson Lab), and MAMI (Mainz). We present the calculation of the nucleon GPs and VCS observables at next-to-leading order in baryon chiral perturbation theory (B$χ$PT), and confront the results with the empirical information. At this order our results are predictions, in the sense that all the parameters are well-known from elsewhere. Within the relatively large uncertainties of our calculation we find good agreement with the experimental observations of VCS and the empirical extractions of the GPs. We find large discrepancies with previous chiral calculations---all done in heavy-baryon $χ$PT (HB$χ$PT)---and discuss the differences between B$χ$PT and HB$χ$PT responsible for these discrepancies.

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arXiv2016

Description of light nuclei in pionless effective field theory using the stochastic variational method

We construct a coordinate-space potential based on pionless effective field theory with a Gaussian regulator. Charge-symmetry breaking is included through the Coulomb potential and through two- and three-body contact interactions. Starting with the effective field theory potential, we apply the stochastic variational method to determine the ground states of nuclei with mass number $A\leq 4$. At next-to-next-to-leading order, two out of three independent three-body parameters can be fitted to the three-body binding energies. To fix the remaining one, we look for a simultaneous description of the binding energy of $^4$He and the charge radii of $^3$He and $^4$He. We show that at the order considered we can find an acceptable solution, within the uncertainty of the expansion. We find that the EFT expansion shows good agreement with empirical data within the estimated uncertainty, even for a system as dense as $^4$He.

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arXiv2016

Excitation spectra of solitary waves in scalar field models with polynomial self-interaction

We study excitations of solitary waves -- the kinks -- in scalar models with degree eight polynomial self-interaction in (1+1) dimensions. We perform numerical studies of scattering of two kinks with an exponential asymptotic off each other and analyse the occurring resonance phenomena. We connect these phenomena to the energy exchange between the translational and the vibrational modes of the colliding kinks. We also point out that the interaction of two kinks with power-law asymptotic can lead to a long-range interaction between the two kinks.

hep-th↗

arXiv2016

Predictions of covariant chiral perturbation theory for nucleon polarisabilities and polarised Compton scattering

We update the predictions of the SU(2) baryon chiral perturbation theory for the dipole polarisabilities of the proton, $\{α_{E1},\,β_{M1}\}_p=\{11.2(0.7),\,3.9(0.7)\}\times10^{-4}$fm$^3$, and obtain the corresponding predictions for the quadrupole, dispersive, and spin polarisabilities: $\{α_{E2},\,β_{M2}\}_p=\{17.3(3.9),\,-15.5(3.5)\}\times10^{-4}$fm$^5$, $\{α_{E1ν},\,β_{M1ν}\}_p=\{-1.3(1.0),\,7.1(2.5)\}\times10^{-4}$fm$^5$, and $\{γ_{E1E1},\,γ_{M1M1},\,γ_{E1M2},\,γ_{M1E2}\}_p=\{-3.3(0.8),\,2.9(1.5),\,0.2(0.2),\,1.1(0.3)\}\times10^{-4}$fm$^4$. The results for the scalar polarisabilities are in significant disagreement with semi-empirical analyses based on dispersion relations, however the results for the spin polarisabilities agree remarkably well. Results for proton Compton-scattering multipoles and polarised observables up to the Delta(1232) resonance region are presented too. The asymmetries $Σ_3$ and $Σ_{2x}$ reproduce the experimental data from LEGS and MAMI. Results for $Σ_{2z}$ agree with a recent sum rule evaluation in the forward kinematics. The asymmetry $Σ_{1z}$ near the pion production threshold shows a large sensitivity to chiral dynamics, but no data is available for this observable. We also provide the predictions for the polarisabilities of the neutron: $\{α_{E1},\,β_{M1}\}_n=\{13.7(3.1),\,4.6(2.7)\}\times10^{-4}$fm$^3$, $\{α_{E2},\,β_{M2}\}_n=\{16.2(3.7),\,-15.8(3.6)\}\times10^{-4}$fm$^5$, $\{α_{E1ν},\,β_{M1ν}\}_n=\{0.1(1.0),\,7.2(2.5)\}\times10^{-4}$fm$^5$, and $\{γ_{E1E1},\,γ_{M1M1},\,γ_{E1M2},\,γ_{M1E2}\}_n=\{-4.7(1.1),\,2.9(1.5),\,0.2(0.2),\,1.6(0.4)\}\times10^{-4}$fm$^4$. The neutron dynamical polarisabilities and multipoles are examined too. We also discuss subtleties related to matching dynamical and static polarisabilities.

hep-ph↗

arXiv2015

Kink excitation spectra in the (1+1)-dimensional $φ^8$ model

We study excitation spectra of BPS-saturated topological solutions -- the kinks -- of the $φ^8$ scalar field model in $(1+1)$ dimensions, for three different choices of the model parameters. We demonstrate that some of these kinks have a vibrational mode, apart from the trivial zero (translational) excitation. One of the considered kinks is shown to have three vibrational modes. We perform a numerical calculation of the kink-kink scattering in one of the considered variants of the $φ^8$ model, and find the critical collision velocity $v_{\scriptsize \mbox{cr}}$ that separates the different collision regimes: inelastic bounce of the kinks at $v_{\scriptsize \mbox{in}}\ge v_{\scriptsize \mbox{cr}}$, and capture at $v_{\scriptsize \mbox{in}}<v_{\scriptsize \mbox{cr}}$. We also observe escape windows at some values of $v_{\scriptsize \mbox{in}}<v_{\scriptsize \mbox{cr}}$ where the kinks escape to infinity after bouncing off each other two or more times. We analyse the features of these windows and discuss their relation to the resonant energy exchange between the translational and the vibrational excitations of the colliding kinks.

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arXiv2014

Gold-plated moments of nucleon structure functions in baryon chiral perturbation theory

We obtain leading- and next-to-leading order predictions of chiral perturbation theory for several prominent moments of nucleon structure functions. These free-parameter free results turn out to be in overall agreement with the available empirical information on nearly all of the considered moments, in the region of low-momentum transfer ($Q^2 < 0.3$ GeV$^2$). Especially surprising is the situation for the spin polarizability $δ_{LT}$, which thus far was not reproducible in chiral perturbation theory for proton and neutron simultaneously. This problem, known as the "$δ_{LT}$ puzzle," is not seen in the present calculation.

hep-ph↗

arXiv2014

Proton polarisabilities from Compton data using Covariant Chiral EFT

We present a fit of the spin-independent electromagnetic polarisabilities of the proton to low-energy Compton scattering data in the framework of covariant baryon chiral effective field theory. Using the Baldin sum rule to constrain their sum, we obtain $α=[10.6\pm0.25$(stat)$\pm0.2$(Baldin)$\pm0.4$(theory)$]\times10^{-4}$fm$^3$ and $β=[3.2\mp0.25$(stat)$\pm0.2$(Baldin)$\mp0.4$(theory)$]\times10^{-4}$fm$^3$, in excellent agreement with other chiral extractions of the same quantities.

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arXiv2013

Chiral perturbation theory of muonic hydrogen Lamb shift: polarizability contribution

The proton polarizability effect in the muonic-hydrogen Lamb shift comes out as a prediction of baryon chiral perturbation theory at leading order and our calculation yields for it: $ΔE^{(\mathrm{pol})} (2P-2S) = 8^{+3}_{-1}\, μ$eV. This result is consistent with most of evaluations based on dispersive sum rules, but is about a factor of two smaller than the recent result obtained in {\em heavy-baryon} chiral perturbation theory. We also find that the effect of $Δ(1232)$-resonance excitation on the Lamb-shift is suppressed, as is the entire contribution of the magnetic polarizability; the electric polarizability dominates. Our results reaffirm the point of view that the proton structure effects, beyond the charge radius, are too small to resolve the `proton radius puzzle'.

hep-ph↗

arXiv2012

What different variants of chiral EFT predict for the proton Compton differential cross section - and why

We compare the predictions of different variants of chiral effective field theory for the gamma-p elastic scattering differential cross section. We pay particular attention to the role of pion loops, and the impact that a heavy-baryon expansion has on the behavior of those loops. We also correct erroneous results for these loops that were published in Phys. Rev. C 67, 055202 (2003) [ arXiv:nucl-th/0212024 ].

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arXiv2011

Coupled-channel effective field theory and proton-$^7$Li scattering

We apply the renormalisation group (RG) to analyse scattering by short-range forces in systems with coupled channels. For two S-wave channels, we find three fixed points, corresponding to systems with zero, one or two bound or virtual states at threshold. We use the RG to determine the power countings for the resulting effective field theories. In the case of a single low-energy state, the resulting theory takes the form of an effective-range expansion in the strongly interacting channel. We also extend the analysis to include the effects of the Coulomb interaction between charged particles. The approach is then applied to the coupled $p+{^7}$Li and $n+{^7}$Be channels which couple to a $J^P=2^-$ state of $^8$Be very close to the $n+{^7}$Be threshold. At next-to-leading order, we are able to get a good description of the $p+{^7}$Li phase shift and the ${^7}$Be(n,p)${^7}$Li cross section using four parameters. Fits at one order higher are similarly good but the available data are not sufficient to determine all five parameters uniquely.

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arXiv2011

Heavy-particle formalism with Foldy-Wouthuysen representation

Utilizing the Foldy-Wouthuysen representation, we use a bottom-up approach to construct heavy-baryon Lagrangian terms, without employing a relativistic Lagrangian as the starting point. The couplings obtained this way feature a straightforward $1/m$ expansion, which ensures Lorentz invariance order by order in effective field theories. We illustrate possible applications with two examples in the context of chiral effective field theory: the pion-nucleon coupling, which reproduces the results in the literature, and the pion-nucleon-delta coupling, which does not employ the Rarita-Schwinger field for describing the delta isobar, and hence does not invoke any spurious degrees of freedom. In particular, we point out that one of the subleading $πN Δ$ couplings used in the literature is, in fact, redundant, and discuss the implications of this. We also show that this redundant term should be dropped if one wants to use low-energy constants fitted from $πN$ scattering in calculations of $NN\to NNπ$ reactions.

hep-ph↗

arXiv2008

Manifestly-covariant chiral PT calculation of nucleon Compton scattering

We compute the Compton scattering off the nucleons in the framework of manifestly covariant baryon chiral perturbation theory (B$χ$PT). The results for observables differ substantially from the corresponding calculations in heavy-baryon chiral perturbation theory (HB$χ$PT), most appreciably in the forward kinematics. We verify that the covariant $p^3$ result fulfills the forward-Compton-scattering sum rules. We also explore the effect of the $Δ$(1232) resonance at order $p^4/\varDelta$, with $\varDelta\approx 300$ MeV, the resonance excitation energy. We find that the substantial effect of the $Δ$-excitation on the nucleon polarizabilities can naturally be accommodated in the manifestly covariant calculation.

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