lundi 29 juin 2015

Two scalars to rule the m(ass for almost) all (particles)? / Deux scalaires pour gouverner (presque) toutes les masses

The advanced art of massware in electroweak and QCD quantum vacua/ L'art subtil de la génération de masse dans les vides quantiques électrofaibles et chromodynamiques
This post is a follow-up to this one. / Ce billet fait écho à celui-ci vieux de plus d'un an.
In the standard model the masses of elementary particles arise from the Higgs field acting on the originally massless particles. When applied to the visible matter of the universe this explanation remains unsatisfactory as long as we consider the vacuum as an empty space. The QCD vacuum contains a condensate of up and down quarks. Condensate means that the q pairs are correlated via inter-quark forces mediated by gluon exchanges. As part of the vacuum structure the q pairs have to be in a scalar-isoscalar configuration. This suggests that the vacuum condensate may be described in terms of a scalar-isoscalar particle, |σ>=(|uu̅>+|dd̅>)/√2, providing the σ field. These two descriptions in terms of a vacuum condensate or a σ field are essentially equivalent and are the bases of the Nambu–Jona-Lasinio (NJL) model [28] and the linear σ model (LσM), [9] respectively. Furthermore, it is possible write down a bosonized version of the NJL model where the vacuum condensate is replaced by the vacuum expectation value of the σ field. 
In the QCD vacuum the largest part of the mass M of an originally massless quark, up (u) or down (d), is generated independent of the presence of the Higgs field and amounts to M = 326 MeV [1]. The Higgs field only adds a small additional part to the total constituent-quark mass leading to m u = 331 MeV and m d = 335 MeV for the up and down quark, respectively [1]. These constituent quarks are the building blocks of the nucleon in a similar way as the nucleons are in case of nuclei. Quantitatively, we obtain the experimental masses of the nucleons after including a binding energy of 19.6 MeV and 20.5 MeV per constituent quark for the proton and neutron, respectively, again in analogy to the nuclear case where the binding energies are 2.83 MeV per nucleon for 31 H and 2.57 MeV per nucleon for 3He. 
In the present work we extend our previous [1] investigation by exploring in more detail the rules according to which the effects of electroweak (EW) and strong-interaction symmetry breaking combine in order to generate the masses of hadrons. As a test of the concept, the mass of the π meson is precisely predicted on an absolute scale. In the strange-quark sector the Higgs boson is responsible for about 1/3 of the constituent quark mass, so that effects of the interplay of the two components of mass generation become essential. Progress is made by taking into account the predicted second σ meson, σ′(1344) = |ss̅> [7]. It is found that the coupling constant of the s-quark coupling to the σ′ meson is larger than the corresponding quantity of the u and d quarks coupling to the σ meson by a factor of √2. This leads to a considerable increase of the constituent quark masses in the strange-quark sector in comparison with the ones in the non-strange sector already in the chiral limit, i.e. without the effects of the Higgs boson. There is an additional sizable increase of the mass generation mediated by the Higgs boson due to a∼24 times stronger coupling of the s quark to the Higgs boson in comparison to the u and d quarks. In addition to the progress made in [1] as described above this paper contains a History of the subject from Schwinger’s seminal work of 1957 [10] to the discovery of the Brout-Englert-Higgs (BEH) mechanism, with emphasis on the Nobel prize awarded to Nambu in 2008. This is the reason why paper [1] has been published as a supplement of the Nobel lectures of Englert [11] and Higgs [12]...

The masses of constituent quarks are composed of the masses Mq predicted for the chiral limit and the mass of the respective current quark m0q provided by the Higgs boson (EW interaction) alone. For scalar mesons the sum of Mq and m0q leads to a zero-order approximation for the constituent-quark mass mq, but there are dynamical effects described by the NJL model which modify the simple relation mq=Mq+m0q  , except for the non-strange sector where this relation is a good approximation. Similar results are obtained for the octet baryons. A difference between the scalar mesons and the octet baryons is that that for scalar mesons binding energies do not play a rôle whereas they are of importance in case of octet baryons... 

(Submitted on 1 Jun 2015)

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