arXiv subjects

B. Krippa

Publications and source records attributed to B. Krippa.

13 recordsLinked to original sources

arXiv2026

Pseudogap formation in the moderate correlated layered attractive Hubbard model

We consider the layered attractive Hubbard model with a moderate interaction strength at quarter filling. The Green's function, self-energy, and density of states are calculated for relatively large clusters using the fluctuating local field method within the approximation of vanishing effective interaction. The emergence of a pseudogap is demonstrated for the cluster system, and it is shown that integration over the fluctuating field is equivalent to the summation of zero-mode ladder diagrams above critical temperature. For the large layered system, a pseudogap in the density of states is obtained within a cluster scheme that treats the coupling between layers as a static $U(1)$ symmetry-breaking field.

cond-mat.str-el↗

arXiv2009

Nonperturbative Renormalisation Group: Applications to the few and many-body systems

We consider the applications of functional renormalisation group to few and many-body systems. As an application to the few-body dynamics we study the ratio between the fermion-fermion scattering length and the dimer-dimer scattering length for systems of few nonrelativistic fermions. We find a strong dependence on the cutoff function used in the renormalisation flow for a two-body truncation of the action. Adding a simple three-body term substantially reduces this dependence. In the context of many-body physics we study the dynamics of both symmetric and asymmetric many-fermion systems using the same functional renormalisation technique. It is demonstrated that functional renormalisation group gives sensible and reliable results and provides a solid theoretical ground for the future studies. Open questions as well as lines of further developments are discussed.

nucl-th↗

arXiv2004

Exact renormalization group and many-fermion systems

The exact renormalization group methods is applied to many fermion systems with short-range attractive force. The strength of the attractive fermion-fermion interaction is determined from the vacuum scattering length. A set of approximate flow equations is derived including fermionic and bosonic fluctuations. The numerical solutions show a phase transition to a gapped phase. The inclusion of bosonic fluctuations is found to be significant only in the small-gap regime.

hep-ph↗

arXiv2001

A note on the Georgi vector limit at finite temperature/density

The mechanism of chiral symmetry restoration in the context of the Georgi Vector Limit (GVL) at finite temperature/density is discussed. It is suggested that, whereas the system may be driven to the GVL, the fact of reaching the point of restoration does not neceserily mean the complete realization of the GVL since the other mechanisms of restoration may contribute at lower temperatures/densities.

hep-ph↗

arXiv2000

Effective Field Theory at Moderate Nuclear Density

Effective field theory of the in-medium nucleon-nucleon interaction is considered. The effective range parameters are found to be of a natural scale. The low density limit is discussed both in perturbative and nonperturbative situations. In the nonperturbative case the attractive character of the nucleon-nucleon interactions in the $^{1}S_0$ channel leads to the nuclear superfluidity which is analyzed in the framework of the renormalization group. The numerical values of the corresponding energy gap are in agreement with results obtained by more more traditional many-body techniques. The S-wave part of potential energy per particle is calculated for different values of nuclear density. The role of pion and many-body effects is discussed. Problems and challenges in constructing the chiral theory of nuclear matter are outlined.

hep-ph↗

arXiv2000

Effective theory of NN interactions in a separable representation

We consider the effective field theory of the NN system in a separable representation. The pionic part of the effective potential is included nonperturbatively and approximated by a separable potential. The use of a separable representation allows for the explicit solution of the Lippmann-Schwinger equation and a consistent renormalization procedure. The phase shifts in the $^1S_0$ channel are calculated to subleading order.

hep-ph↗

arXiv2000

Chiral properties of hadron correlators in nuclear matter

The constraints imposed by chiral symmetry on hadron correlation functions in nuclear medium are discussed. It is shown that these constraints imply a certain structure for the in-medium hadron correlators and lead to the cancelation of the order $ρm_π$ term in the in-medium nucleon correlator. We also consider the effect of mixing of the chiral partners correlation functions arising from the interaction of nuclear pions with corresponding interpolating currents. It reflects the phenomena of partial restoration of chiral symmetry. The different scenarios of such restoration are briefly discussed.

nucl-th↗

arXiv2000

Toward Chiral Theory of NN interactions in nuclear matter

The effective chiral theory of the in-medium NN interactions is considered. The shallow bound states, which complicate the effective field theory analysis in vacuum do not exist in matter. We show that the next-to-leading order terms in the chiral expansion of the effective Lagrangian can be interpreted as corrections so that the expansion is systematic. The Low Energy Effective Constants of this Lagrangian are found to satisfy the concept of naturalness. The potential energy per particle is calculated. The problems and challenges in constructing the chiral theory of nuclear matter are outlined.

nucl-th↗

arXiv1999

Chiral NN interaction in nuclear matter

The effective field theory of NN interactions in nuclear matter is considered. Due to the Pauli principle the effective NN amplitude is not affected by the shallow bound states. We show that the next-to-leading order terms in the chiral expansion of the effective NN potential can be interpreted as corrections so the expansion is systematic. The value of potential energy per particle is calculated and some issues concerning the chiral effective theory of nuclear matter are outlined.

nucl-th↗

arXiv1999

Chiral NN interactions in nuclear matter

We consider an effective field theory of NN system in nuclear medium. The shallow bound states, which complicate the effective field theory analysis and lead to the large scattering length in the vacuum case do not exist in matter. We study whether the chiral expansion of the effective potential can be truncated in such situation. The cutoff regularization is used to render the solution of the Bethe-Goldstone equation finite. We show that the next-to-leading order terms in the chiral expansion of the effective NN potential can indeed be interpreted as corrections so that the truncation of the expansion is justified. However, it is pointed out that it is still useful to treat the problem nonperturbatively since it may allow the consideration of the nuclear systems with the density smaller that the normal nuclear matter one. The possible directions of constructing the chiral theory of NN interaction in medium are outlined.

hep-ph↗

arXiv1998

What Chiral Symmetry Can Tell Us About Hadron Correlators in Matter

The constraints imposed by chiral symmetry on hadron correlation functions in nuclear medium are discussed. It is shown that these constraints imply some structure of the in-medium hadron correlators, lead to the cancellation of the order $ρm_π$ term in the in-medium nucleon correlator and result in the effect of mixing of the chiral partners correlators, reflecting the phenomena of partial restoration of chiral symmetry. The different scenarios of such restorations are briefly discussed.

nucl-th↗

arXiv1998

Chiral Dynamics of Low-Energy Kaon-Baryon Interactions with Explicit Resonance

The processes involving low energy $\bar{K}N$ and $Yπ$ interactions (where $Y= Σ$ or $Λ$) are studied in the framework of heavy baryon chiral perturbation theory with the $Λ$(1405) resonance appearing as an independent field. The leading and next-to-leading terms in the chiral expansion are taken into account. We show that an approach which explicitly includes the $Λ$(1405) resonance as an elementary quantum field gives reasonable descriptions of both the threshold branching ratios and the energy dependence of total cross sections.

nucl-th↗

arXiv1998

Chiral Dynamics of the low-energy Kaon-Baryon interactions

The processes involving $K^{-}p, KN, Σπ, Λπ,Ση, Λη$ coupled channels are studied in the nonperturbative chiral approach. An effective potential is constructed using a chiral meson-baryon Lagrangian at lowest order. This potential is iterated to all orders with the Lippmann-Schwinger equation. A reasonable fit of the experimental data is obtained. It is pointed out, however, that due to a strong sensitivity of the results to the value of the cut-off, such an approach should be viewed as a rather phenomenological way to fit the experimental data since there is no small expansion parameter allowing for truncation of the chiral expansion of the effective potential at some given order. A possible way to construct the consistent chiral expansion is briefly discussed

hep-ph↗