110 research outputs found

### Gluon masses without seagull divergences

The study of dynamical gluon mass generation at the level of Schwinger-Dyson
equation involves a delicate interplay between various field-theoretic
mechanisms The underlying local gauge invariance remains intact by resorting to
the well-known Schwinger mechanism, which is assumed to be realized by
longitudinally coupled bound state poles, produced by the non-perturbative
dynamics of the theory. These poles are subsequently included into the
Schwinger-Dyson equation of the gluon propagator through the three-gluon
vertex, generating a non-vanishing gluon mass, which, however, is expressed in
terms of divergent seagull integrals. In this talk we explain how such
divergences can be eliminated completely by virtue of a characteristic
identity, valid in dimensional regularization. The ability to trigger this
identity depends, in turn, on the details of the three-gluon vertex employed,
and in particular, on the exact way the bound state poles are incorporated. A
concrete example of a vertex that triggers the aforementioned identity is
constructed, the ensuing cancellation of all seagull divergences is explicitly
demonstrated, and a finite gluon mass is obtained. Due to the multitude of
conditions that must be simultaneously satisfied, this construction appears to
be exclusively realized within the PT-BFM framework. The resulting system of
integral equations gives rise to a gluon mass that displays power-law running
and an effective charge which, due to the presence of the gluon mass, freezes
in the infrared at a finite (non-vanishing) value.Comment: 12 pages, 5 figures. Talk presented at the International Workshop on
QCD Green's Functions, Confinement, and Phenomenology - QCD-TNT09, September
07 - 11 2009, ECT* Trento, Ital

### The Pinch Technique Approach to the Physics of Unstable Particles

The consistent description of unstable particles within the framework of
perturbative gauge field theories necessitates the definition and resummation
of off-shell Green's functions, which must respect several crucial physical
requirements. We present the solution to this problem at one-loop, using the
pinch technique.Comment: 11 pages, uses revtex, 7 Figures in separate ps file, contribution to
the 1998 Corfu Summer Institute on Elementary Particle Physics (JHEP
proceedings

### The dual gauge fixing property of the S matrix

The $S$-matrix is known to be independent of the gauge fixing parameter to
all orders in perturbation theory. In this paper by employing the pinch
technique we prove at one loop a stronger version of this independence. In
particular we show that one can use a gauge fixing parameter for the gauge
bosons inside quantum loops which is different from that used for the bosons
outside loops, and the $S$-matrix is independent from both. Possible
phenomenological applications of this result are briefly discussed.Comment: 17 pages, Late

### Chiral fermions and gauge-fixing in five-dimensional theories

We study in detail the issue of gauge-fixing in theories with one universal
extra dimension, i.e. theories where both bosons and fermions display
Kaluza-Klein (KK) excitations. The extra dimension is compactified using the
standard orbifold construction for a massless chiral fermion. We carry out the
gauge-fixing procedure at the level of the five-dimensional theory and
determine the tree-level propagators and interaction vertices needed for
performing perturbative calculations with the effective four-dimensional theory
resulting after the compactification. The gauge-independence of the tree-level
S-matrix involving massive KK modes is verified using specific examples. In
order to obtain massive fermionic zero modes one has to enlarge the theory by
introducing a set of mirror fermions, a construction which is carried out in
detail. Finally, the gauge-independence of the tree-level S-matrix involving
the resulting new mass-eigenstates is proved by resorting to generalized
current conservation equations.Comment: 10 pages, 5 figures, revtex and axodra

### Gauge invariant Ansatz for a special three-gluon vertex

We construct a general Ansatz for the three-particle vertex describing the
interaction of one background and two quantum gluons, by simultaneously solving
the Ward and Slavnov-Taylor identities it satisfies. This vertex is known to be
essential for the gauge-invariant truncation of the Schwinger-Dyson equations
of QCD, based on the pinch technique and the background field method. A key
step in this construction is the formal derivation of a set of crucial
constraints (shown to be valid to all orders), relating the various form
factors of the ghost Green's functions appearing in the aforementioned
Slavnov-Taylor identity. When inserted into the Schwinger-Dyson equation for
the gluon propagator, this vertex gives rise to a number of highly non-trivial
cancellations, which are absolutely indispensable for the self-consistency of
the entire approach.Comment: 26 pages, 4 figures; v3: more typos correcte

### Infrared properties of the gluon mass equation

The gauge-invariant generation of a dynamical, momentum-dependent gluon mass
is intimately connected with the presence of non-perturbative massless poles in
the vertices of the theory, which trigger the well-known Schwinger mechanism.
In the deep infrared the integral equation that governs this effective gluon
mass assumes a particularly simple form, which may be derived following two
seemingly different, but ultimately equivalent procedures. In particular, it
may be obtained either as a deviation from a special identity that enforces the
masslessness of the gluon in the absence of massless poles, or as a direct
consequence of the appearance of a non-vanishing bound-state wave function,
associated with the details of the actual formation of these massless poles. In
this presentation we demonstrate that, due to profound relations between the
various ingredients, the two versions of the gluon mass equation are in fact
absolutely identical.Comment: 12 pages, 5 figures. Talk presented by DB at the International
Workshop on QCD Green's Functions, Confinement, and Phenomenology - QCD-TNT
II, September 05-09 2011, ECT* Trento, Ital

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