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Effet des boucles de quarks legers sur la structure chirale du vide de QCD

Abstract : At low energies, QCD is governed by the spontaneous breakdown of chiral symmetry (SBXS). If the number of light flavours N_f could be increased from QCD (N_f=3), the pattern of SBXS should change due to light-quark loops. From the spectrum of the Euclidean Dirac operator, two chiral order parameters are shown to decrease when N_f rises: the light-quark condensate S=- (most simple order parameter) and the pseudoscalar decay constant F (nonambiguous signal of SBXS). The size of this decrease depends on the Zweig-rule violation in the scalar sector. The spectrum of pseudoscalar mesons (masses and decay constants) provides constraints on some low-energy constants that describe the chiral structure of the vacuum for N_f=2 (m_u,m_d->0) and N_f=3 (m_u,m_d,m_s->0). Further, experimental data in the 0++ sector can be used in a sum rule to get bounds on S(2)-S(3). The presence of the f_0(980) resonance favours a steep decrease of S between 2 and 3 flavours. This could be explained by the vicinity of a critical value of N_f for which the quark condensate would vanish. Combinations of chiral order parameters highly sensitive to such a phase transition can be extracted by studying how inverse moments of the spectrum of the Dirac operator on an Euclidean torus depend on the volume. Lattice simulations of these inverse moments would provide clues about the phase transition S(N_f)->0.
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Contributor : Sebastien Descotes-Genon <>
Submitted on : Wednesday, April 9, 2003 - 8:21:08 PM
Last modification on : Wednesday, September 16, 2020 - 4:07:55 PM
Long-term archiving on: : Tuesday, September 11, 2012 - 8:15:21 PM


  • HAL Id : tel-00002704, version 1



Sebastien Descotes-Genon. Effet des boucles de quarks legers sur la structure chirale du vide de QCD. Physique des Hautes Energies - Expérience [hep-ex]. Université Paris Sud - Paris XI, 2000. Français. ⟨tel-00002704⟩



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