The spectral properties of a set of local gauge-invariant composite operators are investigated in the U (1) Higgs model quantized in the 't Hooft R ξ gauge. These operators enable us to give a gauge-invariant description of the spectrum of the theory, thereby surpassing certain incommodities when using the standard elementary fields. The corresponding two-point correlation functions are evaluated at one-loop order and their spectral functions are obtained explicitly. As expected, the above mentioned correlation functions are independent from the gauge parameter ξ, while exhibiting positive spectral densities as well as gauge-invariant pole masses corresponding to the massive photon and Higgs physical excitations.
I. INTRODUCTIONAn essential aspect of gauge theories is that all physical observable quantities have to be gauge-invariant [1,2]. However, in practice, the explicit calculations of the S-matrix elements and corresponding cross sections are done by employing the non-gauge-invariant elementary fields such as the W bosons and the Higgs field of the electroweak theory, giving results in quite accurate agreement with experimental ones.The success of making use of the non-gauge-invariant elementary fields can be traced back to the so called Nielsen identities [3][4][5][6][7] which follow from the Slavnov-Taylor identities encoding the BRST symmetry of quantized gauge theories. The Nielsen identities ensure that the pole masses of both transverse gauge bosons and Higgs field propagators do not depend on the gauge parameters entering the gauge fixing condition, a pivotal property shared by the Smatrix elements. Nevertheless, as one can easily figure out, the use of the non-gauge-invariant fields has its own limitations which show up in several ways. For example, the analysis of the spectral properties of the elementary two-point correlation functions in terms of the Källén-Lehmann ( KL) representation is often plagued by an undesired dependence of the spectral densities on the gauge parameters and/or the densities attaining negative values, obscuring their physical interpretation. Indeed, from e.g. non-perturbative lattice QCD studies, it is well known that not only direct particle-spectrum related properties are hiding in the spectral functions, but at finite temperature also information on transport properties in the quark-gluon plasma etc., see for instance [8][9][10][11][12]. The spectral functions considered are those of gauge-invariant operators. Moreover, it is also known that in certain classes of gauges, the Nielsen identities can suffer from fatal infrared singularities [3,13,14], obscuring what happens with e.g. the pole mass or effective potential governing the Higgs vacuum expectation value in such gauges.A formulation of the properties of the observable excitations in terms of gauge-invariant variables is thus very welcome. Such an endeavour has been addressed by several authors 1 [17-20], who have been able to construct, out of the elementary fields, a set of local gauge-invariant composite operat...