We set up the framework for the calculation of electric dipole moments (EDMs) of light nuclei using the systematic expansion provided by chiral effective field theory (EFT). We take into account parity (P ) and time-reversal (T ) violation which, at the quark-gluon level, originates from the QCD vacuum angle and dimension-six operators capturing physics beyond the Standard Model. We argue that EDMs of light nuclei can be expressed in terms of six low-energy constants that appear in the P -and T -violating nuclear potential and electric current. As examples, we calculate the EDMs of the deuteron, the triton, and 3 He in leading order in the EFT expansion.

We study the two-alpha-particle (αα) system in an Effective Field Theory (EFT) for halo-like systems. We propose a power counting that incorporates the subtle interplay of strong and electromagnetic forces leading to a narrow resonance at an energy of about 0.1 MeV. We investigate the EFT expansion in detail, and compare its results with existing low-energy αα phase shifts and previously determined effective-range parameters. Good description of the data is obtained with a surprising amount of fine-tuning. This scenario can be viewed as an expansion around the limit where, when electromagnetic interactions are turned off, the 8 Be ground state is at threshold and exhibits conformal invariance. We also discuss possible extensions to systems with more than two alpha particles.

The radiative neutron capture on lithium-7 is calculated model independently using a low-energy halo effective field theory. The cross section is expressed in terms of scattering parameters directly related to the S-matrix elements. It depends on the poorly known p-wave effective range parameter r(1). This constitutes the largest uncertainty in traditional model calculations. It is explicitly demonstrated by comparing with potential model calculations. A single parameter fit describes the low-energy data extremely well and yields r(1)≈-1.47  fm(-1).

We present a relativistic procedure for the chiral expansion of the two-pion exchange component of the N N potential, which emphasizes the role of intermediate πN subamplitudes. The relationship between power counting in πN and N N processes is discussed and results are expressed directly in terms of observable subthreshold coefficients. Interactions are determined by one-and two-loop diagrams, involving pions, nucleons, and other degrees of freedom, frozen into empirical subthreshold coefficients. The full evaluation of these diagrams produces amplitudes containing many different loop integrals. Their simplification by means of relations among these integrals leads to a set of intermediate results. Subsequent truncation to O(q 4 ) yields the relativistic potential, which depends on six loop integrals, representing bubble, triangle, crossed box, and box diagrams. The bubble and triangle integrals are the same as in πN scattering and we have shown that they also determine the chiral structures of box and crossed box integrals. Relativistic threshold effects make our results to be not equivalent with those of the heavy baryon approach. Performing a formal expansion of our results in inverse powers of the nucleon mass, even in regions where this expansion is not valid, we recover most of the standard heavy baryon results. The main differences are due to the Goldberger-Treiman discrepancy and terms of O(q 3 ), possibly associated with the iteration of the one-pion exchange potential.

Abstract. Using halo effective field theory, we provide a model-independent calculation of the radiative neutron capture on lithium-7 over an energy range where the contribution from the 3 + resonance becomes important. We also present power counting arguments that establish a hierarchy for electromagnetic oneand two-body currents. One finds that a satisfactory description of the capture reaction, in the present single-particle approximation, requires a resonance width about three times larger than the experimentally quoted value.

We have recently performed a relativistic O(q 4 ) chiral expansion of the two-pion exchange N N potential, and here we explore its configuration space content. Interactions are determined by three families of diagrams, two of which involve just gA and fπ, whereas the third one depends on empirical coefficients fixed by subthreshold πN data. In this sense, the calculation has no adjusted parameters and gives rise to predictions, which are tested against phenomenological potentials. The dynamical structure of the eight leading non-relativistic components of the interaction is investigated and, in most cases, found to be clearly dominated by a well defined class of diagrams. In particular, the central isovector and spin-orbit, spin-spin, and tensor isoscalar terms are almost completely fixed by just gA and fπ. The convergence of the chiral series in powers of the ratio (pion mass/nucleon mass) is studied as a function of the internucleon distance and, for r > 1 fm, found to be adequate for most components of the potential. An important exception is the dominant central isoscalar term, where the convergence is evident only for r > 2.5 fm. Finally, we compare the spatial behavior of the functions that enter the relativistic and heavy baryon formulations of the interaction and find that, in the region of physical interest, they differ by about 5%. * higa@if.usp.br † robilotta@if.usp.br ‡ carocha@usjt.br symmetric or not, yields the very same OPEP. Chiral symmetry is thus irrelevant for this part of the force, as for all single pion processes.The very opposite happens with the next layer of the interaction, the two-pion exchange potential (TPEP). This component is closely related to the πN scattering amplitude and chiral symmetry becomes extremely important. In the 1960s, no perturbative treatment for strong interactions was available [2] and potentials were constructed which incorporated πN information by means of dispersion relations [3]. In the same decade, chiral symmetry was being developed in a different framework and, with the help of current algebra techniques, low energy theorems for many pionic amplitudes were derived. Applications of chiral symmetry to N N interactions [4], three-body forces [5], and exchange currents [6] began to be performed in the 1970s. At the end of this decade, Weinberg [7] outlined a research programme based on the idea of ChPT. In the 1980s this theory was fully developed for the meson sector [8] and began to be used in the study of meson-baryon interactions [9].The systematic use of ChPT in the study of nuclear forces began in the early 1990s, through the works of Weinberg [10] and Ordóñez and van Kolck [11], followed by other authors [12,13]. These early attempts to construct a chiral TPEP considered only pion and nucleon degrees of freedom and gave rise to poor descriptions of N N data. Realistic potentials require other degrees of freedom, which were introduced in the form of deltas [14], hidden within πN subthreshold coefficients [15,16], or incorporated into low energy constants...

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We investigate the long-range interactions between two neutrons utilizing recent data on the neutron static and dynamic electric and magnetic dipole polarizabilities. The resulting long-range potentials are used to make quantitative comparisons between the collisions of a neutron with a neutron and a neutron with a proton. We also assess the importance of the first pion production threshold and first excited state of the nucleon, the ∆-resonance (J π = + 3/2, I = 3/2). We found both dynamical effects to be quite relevant for distances r between ∼ 50 fm up to ∼ 10 3 fm in the nn system, the neutron-wall system and in the wall-neutron-wall system, reaching the expected asymptotic limit beyond that. Relevance of our findings to the confinement of ultra cold neutrons inside bottles is discussed.