Relationship between the quark condensate and low-energy Pi-Pi observables beyond O(p^4)

The two-flavor Gell-Mann-Oakes-Renner ratio is expressed in terms of low-energy pi-pi observables including the O(p^6) double chiral logarithms, computed in Generalized Chiral Perturbation Theory. It is found that their contribution is important and tends to compensate the one from the single chiral logarithms.Comment: contribution to LNF Spring School, Frascati, 15-20 May, 200

Chiral order and fluctuations in multi-flavour QCD

Multi-flavour (N_f>=3) Chiral Perturbation Theory (ChPT) may exhibit instabilities due to vacuum fluctuations of sea q-bar q pairs. Keeping the fluctuations small would require a very precise fine-tuning of the low-energy constants L_4 and L_6 to L_4[crit](M_rho) = - 0.51 * 10^(-3), and L_6[crit](M_rho) = - 0.26 * 10^(-3). A small deviation from these critical values -- like the one suggested by the phenomenology of OZI-rule violation in the scalar channel -- is amplified by huge numerical factors inducing large effects of vacuum fluctuations. This would lead in particular to a strong N_f-dependence of chiral symmetry breaking and a suppression of multi-flavour chiral order parameters. A simple resummation is shown to cure the instability of N_f>=3 ChPT, but it modifies the standard expressions of some O(p^2) and O(p^4) low-energy parameters in terms of observables. On the other hand, for r=m_s/m > 15, the two-flavour condensate is not suppressed, due to the contribution induced by massive vacuum s-bar s pairs. Thanks to the latter, the standard two-flavour ChPT is protected from multi-flavour instabilities and could provide a well-defined expansion scheme in powers of non-strange quark masses.Comment: Published versio

Operation of Quantum Cellular Automaton cells with more than two electrons

We present evidence that operation of QCA (Quantum Cellular Automaton) cells with four dots is possible with an occupancy of 4N+2 electrons per cell (N being an integer). We show that interaction between cells can be described in terms of a revised formula for cell polarization, which is based only on the difference between diagonal occupancies. We validate our conjectures with full quantum simulations of QCA cells for a number of electrons varying from 2 to 6, using the Configuration-Interaction method.Comment: 4 pages, 4 figures included, submitted to AP

Paramagnetic effect of light quark loops on Chiral Symmetry Breaking

We argue that light quark loops produce a paramagnetic suppression of infrared-sensitive order parameters such as , as the number N_f of light fermions increases. The possibly strong dependence of on N_f is related to the observed Zweig rule violation in the scalar channel. Presuming the existence of a chiral phase transition for not too large N_f, we discuss the phenomenological possibilities of separately determining the two-flavour and three-flavour condensates and the quark mass ratio r=2m_s/(m_u+m_d). The issue is closely related to the interpretation of new forthcoming precise pi-pi data at low energy.Comment: 15 pages, Latex, using JHEP.cls (included), 1 PS figur

Subleading contributions to the three-nucleon contact interaction

We obtain a minimal form of the two-derivative three-nucleon contact Lagrangian, by imposing all constraints deriving from discrete symmetries, Fierz identities and Poincare' covariance. The resulting interaction, depending on 13 unknown low-energy constants, leads to a three-nucleon potential which we give in a local form in configuration space. We also consider the leading (no-derivative) four-nucleon interaction and show that there exists only one independent operator.Comment: 11 pages. Three more operators found after correcting some mistaken Fierz relation

Effect of three-nucleon interaction in p-3He elastic scattering

We present a detailed study of the effect of different three-nucleon interaction models in p-3He elastic scattering at low energies. In particular, two models have been considered: one derived from effective field theory at next-to-next-to-leading order and one derived from a more phenomenological point of view -- the so-called Illinois model. The four-nucleon scattering observables are calculated using the Kohn variational principle and the hyperspherical harmonics technique and the results are compared with available experimental data. We have found that the inclusion of either one of the other force model improves the agreement with the experimental data, in particular for the proton vector analyzing power.Comment: 4 pages, 3 figure

The two-nucleon electromagnetic charge operator in chiral effective field theory (χ\chiEFT) up to one loop

The electromagnetic charge operator in a two-nucleon system is derived in chiral effective field theory ($\chi$EFT) up to order $e\, Q$ (or N4LO), where $Q$ denotes the low-momentum scale and $e$ is the electric charge. The specific form of the N3LO and N4LO corrections from, respectively, one-pion-exchange and two-pion-exchange depends on the off-the-energy-shell prescriptions adopted for the non-static terms in the corresponding potentials. We show that different prescriptions lead to unitarily equivalent potentials and accompanying charge operators. Thus, provided a consistent set is adopted, predictions for physical observables will remain unaffected by the non-uniqueness associated with these off-the-energy-shell effects.Comment: 16 pages, 10 figure

Bakamjian-Thomas mass operator for the few-nucleon system from chiral dynamics

We present an exploratory study consisting in the formulation of a relativistic quantum mechanics to describe the few-nucleon system at low energy, starting from the quantum field theoretical chiral Lagrangian involving pions and nucleons. To this aim we construct a Bakamjian-Thomas mass operator and perform a truncation of the Fock space which respects at each stage the relativistic covariance. Such truncation is justified, at sufficiently low energy, in the framework of a systematic chiral expansion. As an illustration we discuss the bound state observables and low-energy phaseshifts of the nucleon-nucleon and pion-nucleon scattering at the leading order of our scheme.Comment: 17 pages, 10 figures. Revised formulation, matches the journal versio

Effect of three nucleon forces in p-3He scattering

The effect of the inclusion of different models of three nucleon (3N) forces in p-3He elastic scattering at low energies is studied. Two models have been considered: one derived from effective field theory at next-to-next-to-leading order and one derived from a more phenomenological point of view -- the so-called Illinois model. The four nucleon scattering observables are calculated using the Kohn variational principle and the hyperspherical harmonic technique and the results are compared with available experimental data. We have found that with the inclusion of both 3N force models the agreement with the experimental data is improved, in particular for the proton vector analyzing power A_y.Comment: 8 pages, 4 figures, talk presented at the 20th International IUPAP Conference on Few-Body Problems in Physics, 20 - 25 August, 2012, Fukuoka, Japa