Boris Krippa

The University of Manchester, Manchester, England, United Kingdom

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Publications (18)58.99 Total impact

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    ABSTRACT: We apply the functional renormalisation group to few-nucleon systems. Our starting point is a local effective action that includes three- and four-nucleon interactions, expressed in terms of nucleon and two-nucleon boson fields. The evolution of the coupling constants in this action is described by a renormalisation group flow. We derive these flow equations both in the limit of exact Wigner SU(4) symmetry and in the realistic case of broken symmetry. In the symmetric limit we find that the renormalisation flow equations decouple, and can be combined into two sets, one of which matches the known results for bosons, and the other result matches the one for fermions with spin degrees only. The equations show universal features in the unitary limit, which is obtained when the two-body scattering length tends to infinity. We calculate the spin-quartet neutron-deuteron scattering length and the deuteron-deuteron scattering lengths in the spin-singlet and quintet channels.
    Physical Review C 07/2012; 87(5). · 3.72 Impact Factor
  • Boris Krippa
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    ABSTRACT: A functional renormalisation group is applied to the few-nucleon problem. We have considered the average effective action with exact SU(4)SU(4) symmetry and derived the flow equations which have been used to calculate the deuteron–deuteron scattering lengths in the singlet and quintet spin–isospin channels.
    Progress in Particle and Nuclear Physics 04/2012; 67(2):526–529. · 2.26 Impact Factor
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    ABSTRACT: We study the convergence of a functional renormalisation group technique by looking at the ratio between the fermion-fermion scattering length and the dimer-dimer scattering length for a system of nonrelativistic fermions. We find that in a systematic expansion in powers of the fields there is a rapid convergence of the result that agrees with know exact results.
    Physical Review A 11/2010; 83. · 3.04 Impact Factor
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    ABSTRACT: We study the ratio between the fermion-fermion scattering length and the dimer-dimer scattering length for systems of nonrelativistic fermions, using the same functional renormalisation technique as previously applied to fermionic matter. 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. Comment: 5 pages, 2 pictures
    Physical Review A 11/2009; · 3.04 Impact Factor
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    Boris Krippa
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    ABSTRACT: We study the exact renormalization group flow for ultracold Fermi gases in the unitary regime. We introduce a pairing field to describe the formation of the Cooper pairs and take a simple ansatz for the effective action. A set of approximate flow equations for effective couplings including boson and fermionic fluctuations is derived. At some value of the running scale, the system undergoes a phase transition to a gapped phase. The values of the energy density, chemical potential, pairing gap and the corresponding proportionality constants relating the interacting and non-interacting Fermi gases are calculated. Standard mean-field results are recovered if we omit the boson loops.
    Journal of Physics A Mathematical and Theoretical 10/2009; 42(46):465002. · 1.77 Impact Factor
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    Boris Krippa
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    ABSTRACT: We study a dynamics of ultracold Fermi-gases near the unitary limit in the framework of Effective Field Theory. It is shown that, while one can obtain a reasonable description of the universal proportionality constant both in the narrow and the broad Feshbach resonance limits, the reguirement of the reparametrisation invariance leads to appearance of the three body forces needed to cancel the otherwise arising off-shell uncertainties. The size of the unsertainties is estimated.
    Physical Review A 07/2007; · 3.04 Impact Factor
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    B. Krippa
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    ABSTRACT: The application of the exact renormalisation group to symmetric as well as asymmetric many-fermion systems with a short-range attractive force is studied. Assuming an ansatz for the effective action with effective bosons, describing pairing effects, a set of approximate flow equations for the effective coupling including boson and fermionic fluctuations has been derived. The phase transition to a phase with broken symmetry is found at a critical value of the running scale. The mean-field results are recovered if boson-loop effects are omitted. The calculations with two different forms of the regulator are shown to lead to similar results. We find that, being quite small in the case of the symmetric many-fermion system the corrections to mean-field approximation become more important with increasing mass asymmetry.
    European Physical Journal A 02/2007; 31(4):734-738. · 2.04 Impact Factor
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    Boris Krippa
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    ABSTRACT: The application of the exact renormalization group to a many-fermion system with a short-range attractive force is studied. We assume a simple ansatz for the effective action with effective bosons, describing pairing effects and derive a set of approximate flow equations for the effective coupling including boson and fermionic fluctuations. The phase transition to a phase with broken symmetry is found at a critical value of the running scale. The mean-field results are recovered if boson-loop effects are omitted. The calculations with two different forms of the regulator were shown to lead to similar results.
    Journal of Physics A General Physics 06/2006; 39(25):8075.
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    Boris Krippa
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    ABSTRACT: The application of the nonperturbative renormalisation group approach to a system with two fermion species is studied. Assuming a simple ansatz for the effective action with effective bosons, describing pairing effects we derive a set of approximate flow equations for the effective coupling including boson and fermionic fluctuations. The case of two fermions with different masses but coinciding Fermi surfaces is considered. The phase transition to a phase with broken symmetry is found at a critical value of the running scale. The large mass difference is found to disfavour the formation of pairs. The mean-field results are recovered if the effects of boson loops are omitted. While the boson fluctuation effects were found to be negligible for large values of pFa they become increasingly important with decreasing pFa thus making the mean field description less accurate.
    Physics Letters B 01/2006; 643(2):104-108. · 4.57 Impact Factor
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    ABSTRACT: We study the application of the exact renormalisation group to a many-fermion system with a short-range attractive force. We introduce a boson field to describe pairing effects, and take a simple ansatz for the effective action. We derive a set of approximate flow equations for the effective coupling including boson and fermionic fluctuations. The initial conditions are obtained by renormalising the interaction to fit the scattering length in vacuum. At some critical value of the running scale, the numerical solutions show a phase transition to a gapped phase. Standard results are recovered if we omit the boson loops. When boson fluctuations are included, we find that their contributions are significant only in the small-gap regime.
    Physics Letters B 01/2005; · 4.57 Impact Factor
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    ABSTRACT: Effective field theory requires all observables to be independent of the representation used for the quantum field operators. It means that off-shell properties of the interactions should not lead to any observable effects. We analyse this issue in the context of many-body approaches to nuclear matter, where it should be possible to shift the contributions of lowest order in purely off-shell two-body interactions into three-body forces. We show that none of the commonly used truncations of the two-body scattering amplitude such as the ladder, Brueckner-Hartree-Fock or parquet approximations respect this requirement. Comment: 5 pages, RevTeX, 6 figures
    Physical Review C 08/2002; · 3.72 Impact Factor
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    Boris Krippa, Michael C. Birse
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    ABSTRACT: In response to Kim's comment (nucl-th/9903040) on the sum rules for pion-baryon coupling constants obtained in hep-ph/9512259 and hep-ph/9606471, we point out that our treatment of the continuum is consistent with duality and with the fact that the correlator in the presence of an external meson or field should be represented by a double dispersion relation. Comment: 5 pages (RevTeX), reply to nucl-th/9903040
    Physical Review C 04/1999; · 3.72 Impact Factor
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    Boris Krippa
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    ABSTRACT: The constraints imposed by chiral symmetry on hadron correlation functions in nuclear medium are discussed. It is shown that these constraints implies the certain structure of the in-medium hadron correlators and lead to the cancellation of the order $\rho m_\pi$ term in the in-medium nucleon correlator. We also consider the effect of mixing of the chiral partners correlaton 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.
    Nuclear Physics A 12/1998; · 1.53 Impact Factor
  • Boris Krippa
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    ABSTRACT: We study the KNΣ and KNΛ coupling constants using QCD sum rules. We consider the correlators of two interpolating baryon fields taken with kaon in initial state. The parts of the correlators with Dirac structure are used, keeping all terms up to dimension 5 in the OPE and including continuum contributions on the phenomenological side. The ratios of these sum rules to baryon mass sum rules yield stable results with values for the couplings of gKNΣ≃3.6±2 and gKNΛ≃10±6. The sources of uncertainty are discussed.
    Physics Letters B 01/1998; · 4.57 Impact Factor
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    Boris Krippa
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    ABSTRACT: The effect of mixing of the vector and axial vector correlation functions in the nuclear medium arising from the interaction of nuclear pions with corresponding interpolating currents is considered. It is shown that the difference between ρ and a1 meson correlators gets smaller with increasing nuclear density. This reflects partial restoration of chiral symmetry whereas the individual correlators could either decrease or increase with density depending on the model used for the phenomenological spectral function.
    Physics Letters B 01/1998; · 4.57 Impact Factor
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    Michael C. Birse, Boris Krippa
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    ABSTRACT: We evaluate the pi-N-N, pi-Sigma-Sigma and pi-Sigma-Lambda coupling constants using QCD sum rules based on pion-to-vacuum matrix elements of correlators of two interpolating baryon fields. The parts of the correlators with Dirac structure k-slash gamma_5 are used, keeping all terms up to dimension 5 in the OPE and including continuum contributions on the phenomenological side. The ratios of these sum rules to baryon mass sum rules yield stable results with values for the couplings of g_{pi NN}=12+/-5, g_{pi Sigma Sigma}=7+/-4 and g_{pi Sigma Lambda}=6+/-3. The sources of uncertainty are discussed.
    Physical Review C 07/1996; · 3.72 Impact Factor
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    Michael C. Birse, Boris Krippa
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    ABSTRACT: We evaluate the πN coupling constant using a QCD sum rule based on the pion-to-vacuum matrix element of the correlator of two interpolating nucleon fields. The part of the correlator with Dirac structure kγ5 is used, keeping all terms up to dimension 5 in the OPE and including continuum contributions on the phenomenological side. The ratio of this sum rule to the nucleon sum rule involving condensates of odd dimension yields stable resultswith values of gπn in the range 12±5. The sources of uncertainty are discussed.
    Physics Letters B 01/1996; · 4.57 Impact Factor
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    Michael C. Birse, Boris Krippa
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    ABSTRACT: The quark condensate in nuclear matter contains a term of order ϱmπ, arising from the contribution of low-momentum virtual pions to the πN sigma commutator. Standard treatments of QCD sum rules for a nucleon in matter generate a similar term in the nucleon effective mass, although this is inconsistent with chiral perturbation theory. We show how an improved treatment of pionic contributions on the phenomenological side of the sum rules can cancel out this unwanted piece. Our results also show that factorisation ansatz for the four-quark condensate cannot be valid in matter.
    Physics Letters B 01/1996; · 4.57 Impact Factor

Publication Stats

175 Citations
58.99 Total Impact Points

Institutions

  • 1996–2012
    • The University of Manchester
      • • Theoretical Physics Division
      • • School of Physics and Astronomy
      Manchester, England, United Kingdom
  • 2009
    • Institute for Theoretical and Experimental Physics
      Moskva, Moscow, Russia
  • 2006
    • University of Surrey
      • Department of Physics
      Guildford, ENG, United Kingdom
  • 1998
    • Russian Academy of Sciences
      • Institute of Nuclear Research
      Moscow, Moscow, Russia
    • University of Amsterdam
      Amsterdamo, North Holland, Netherlands