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Anders Gardestig

Publications and source records attributed to Anders Gardestig.

8 recordsLinked to original sources
arXiv2009

Consistency between renormalization group running of chiral operator and counting rule -- Case of chiral pion production operator --

In nuclear chiral perturbation theory (ChPT), an operator is defined in a space with a cutoff which may be varied within a certain range. The operator runs as a result of the variation of the cutoff [renormalization group (RG) running]. In order for ChPT to be useful, the operator should run in a way consistent with the counting rule; that is, the running of chiral counter terms have to be of natural size. We vary the cutoff using the Wilsonian renormalization group (WRG) equation, and examine this consistency. As an example, we study the s-wave pion production operator for NN\to d pi, derived in ChPT. We demonstrate that the WRG running does not generate any chiral-symmetry-violating (CSV) interaction, provided that we start with an operator which does not contain a CSV term. We analytically show how the counter terms are generated in the WRG running in case of the infinitesimal cutoff reduction. Based on the analytic result, we argue a range of the cutoff variation for which the running of the counter terms is of natural size. Then, we numerically confirm this.

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arXiv2009

Extracting the neutron-neutron scattering length -- recent developments

The experimental and theoretical issues and challenges for extracting the neutron-neutron scattering length are discussed. Particular emphasis is placed on recent results and their impact on the field. Comments are made regarding current experimental and theoretical possibilities.

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arXiv2007

Renormalization group analysis of the chiral pion production operator for NN\to dπ

We are interested in the consistency between the cutoff, chiral symmetry, and the power counting. For this purpose, we apply the Wilsonian renormalization group (RG) to an operator and then decrease the Wilsonian cutoff. As an example, we study the s-wave pion production operator for NN\to dπ, derived in chiral perturbation theory. We find that the renormalized part of the RG effective operator is accurately absorbed by chiral counter terms of higher order with natural coefficients. Thus, the use of the (sharp) cutoff regularization does not require us to introduce chiral-symmetry-breaking counter terms, at least in the case of the NN\to dπreaction.

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arXiv2006

Extraction of a_nn from pi- d -> n n gamma

I present a calculation of the pi- d -> n n gamma reaction to third order in chiral perturbation theory. The short-distance physics of this reaction can be constrained by relating it to several important low-energy weak reactions. The theoretical error in a_nn extracted from this reaction can thus be reduced by a factor larger than three to +-0.05 fm.

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arXiv1999

Multi-pion production in the d d -> alpha X reaction

A simple model, based on two parallel and independent N N -> d pi processes, has recently been proposed for two-pion production in the d d -> alpha X reaction. It reproduces all observed features, including the sharp peak structure in momentum distributions (the ABC effect) and the strong oscillations in the deuteron vector and tensor analyzing powers. This model is now extended to describe also four-pion production with the same basic mechanism, but with two n p -> d pi pi processes as input. The calculations of the high missing mass spectra are within about 30% of the experimental data for beam energies in the range 1.9 < T_d < 2.4 GeV.

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arXiv1998

Structure in two_pion production in the d d -> alpha X reaction

A model for the d d -> alpha pi pi reaction, based on two parallel N N -> d pi reactions, is extended to incorporate a complete set of input amplitudes. While low-energy cross sections are underestimated, the rich structure observed in the alpha-particle momentum distributions for 0.8 < T_d < 1.9 GeV (the ABC effect) is extraordinarily well reproduced. In addition, a recent measurement of deuteron analyzing powers agrees quite well with our predictions, both in frequency and magnitude of the oscillations.

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arXiv1996

Threshold Production of the Hypertriton

The cross section for threshold production of the hypertriton in the reaction pd->^3_ΛH K^+ is calculated in a two-step model and compared to the break-up process pd -> d\LambdaK^+. The latter process is shown to be dominant already at 2 MeV above threshold. The amplitude squared at threshold for the pd -> ^3_ΛH K^+ reaction is |f|^2 = 1.0 nb/sr.

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