Search results for "hep-th"
showing 10 items of 757 documents
Renormalization and Scale Evolution of the Soft-Quark Soft Function
2020
Soft functions defined in terms of matrix elements of soft fields dressed by Wilson lines are central components of factorization theorems for cross sections and decay rates in collider and heavy-quark physics. While in many cases the relevant soft functions are defined in terms of gluon operators, at subleading order in power counting soft functions containing quark fields appear. We present a detailed discussion of the properties of the soft-quark soft function consisting of a quark propagator dressed by two finite-length Wilson lines connecting at one point. This function enters in the factorization theorem for the Higgs-boson decay amplitude of the $h\to\gamma\gamma$ process mediated by…
Electromagnetic form factors of the nucleon in a relativistic quark pair creation model
2001
We study the effects of the | qqq q\bar{q} > component of the hadronic wave function on the description of the electromagnetic structure of the nucleon. Starting with a qqq baryonic wave function which describes the baryonic and mesonic low energy spectrum, the extra q\bar{q} pair is generated through a relativistic version of the 3P_0 model. It is shown that this model leads to a renormalization of the quark mass that allows one to construct a conserved electromagnetic current. We conclude that these dynamical relativistic corrections play an important role in reproducing the Q2 dependence of the electromagnetic form factors at low Q^2.
From Tree Unitarity to Top Quark Physics in 5D Higgsless Models
2005
In five dimensional models of Higgsless electroweak symmetry breaking, tree level unitarity in gauge boson scattering is restored by the exchange of gauge boson Kaluza-Klein modes instead of a Higgs boson. Unitarity of scattering amplitudes involving top quarks requires also the Kaluza-Klein modes of the third family quarks. It is shown that the relevant unitarity cancellations are consistent with gauge symmetry breaking by boundary conditions. These results are used to constrain the couplings of the top quark to Kaluza-Klein modes and the implications for collider phenomenology are discussed.
Nucleon Form Factors at high q2 within constituent quark models
2000
The nucleon form factors are calculated using a non-relativistic description in terms of constituent quarks. The emphasis is put on the reliability of present numerical methods used to solve the three-body problem in order to correctly reproduce the expected asymptotic behavior of form factors. Nucleon wave functions obtained in the hyperspherical formalism or employing Faddeev equations have been considered. While a q**(-8) behavior is expected at high q for a quark-quark force behaving like 1/r at short distances, it is found that the hypercentral approximation in the hyperspherical formalism (K=0) leads to a q**(-7) behavior. An infinite set of waves is required to get the correct behavi…
New method for determining the quark-gluon vertex
2014
We present a novel nonperturbative approach for calculating the form factors of the quark-gluon vertex, in a general covariant gauge. The key ingredient of this method is the exact all-order relation connecting the conventional quark-gluon vertex with the corresponding vertex of the background field method, which is Abelian-like. When this latter relation is combined with the standard gauge technique, supplemented by a crucial set of transverse Ward identities, it allows the approximate determination of the nonperturbative behavior of all twelve form factors comprising the quark-gluon vertex, for arbitrary values of the momenta. The actual implementation of this procedure is carried out in …
New Angle on the Strong CP and Chiral Symmetry Problems from a Rotating Mass Matrix
2007
It is shown that when the mass matrix changes in orientation (i.e. rotates) in generation space for a changing energy scale, the masses of the lower generations are not given just by its eigenvalues. In particular, these masses need not be zero even when the eigenvalues are zero. In that case, the strong CP problem can be avoided by removing the unwanted theta term by a chiral transformation not in contradiction with the nonvanishing quark masses experimentally observed. Similarly, a rotating mass matrix may shed new light on the problem of chhiral symmetry breaking. That the fermion mass matrix may so rotate with the scale has been suggested before as a possible explanation for up-down fer…
Heavy quark decomposition of the S matrix and its relation to the pinch technique.
1995
We propose a decomposition of the S-matrix into individually gauge invariant sub-amplitudes, which are kinematically akin to propagators, vertices, boxes, etc. This decompsition is obtained by considering limits of the S-matrix when some or all of the external particles have masses larger than any other physical scale. We show at the one-loop level that the effective gluon self-energy so defined is physically equivalent to the corresponding gauge independent self-energy obtained in the framework of the pinch technique. The generalization of this procedure to arbitrary gluonic $n$-point functions is briefly discussed.
Hadron correlators and the structure of the quark propagator
1994
The structure of the quark propagator of $QCD$ in a confining background is not known. We make an Ansatz for it, as hinted by a particular mechanism for confinement, and analyze its implications in the meson and baryon correlators. We connect the various terms in the K\"allen-Lehmann representation of the quark propagator with appropriate combinations of hadron correlators, which may ultimately be calculated in lattice $QCD$. Furthermore, using the positivity of the path integral measure for vector like theories, we reanalyze some mass inequalities in our formalism. A curiosity of the analysis is that, the exotic components of the propagator (axial and tensor), produce terms in the hadron c…
Extended van Royen-Weisskopf formalism for lepton-antilepton meson decay widths within non-relativistic quark models
2001
The classical van Royen-Weisskopf formula for the decay width of a meson into a lepton-antilepton pair is modified in order to include non-zero quark momentum contributions within the meson as well as relativistic effects. Besides, a phenomenological electromagnetic density for quarks is introduced. The meson wave functions are obtained from two different models: a chiral constituent quark model and a quark potential model including instanton effects. The modified van Royen-Weisskopf formula is found to improve systematically the results for the widths, giving an overall good description of all known decays.
A solution to the strong CP problem transforming the theta angle to the KM CP-violating phase
2010
It is shown that in the scheme with a rotating fermion mass matrix (i.e. one with a scale-dependent orientation in generation space) suggested earlier for explaining fermion mixing and mass hierarchy, the theta angle term in the QCD action of topological origin can be eliminated by chiral transformations, while giving still nonzero masses to all quarks. Instead, the effects of such transformations get transmitted by the rotation to the CKM matrix as the KM phase giving, for theta of order unity, a Jarlskog invariant typically of order 10(-5), as experimentally observed. Strong and weak CP violations appear then as just two facets of the same phenomenon.