On-shell Couplings Research Articles

Vector leptoquarks beyond tree level

Models with massive vector leptoquarks, resulting from an $SU(4)$ gauge symmetry spontaneously broken at the TeV scale, are of great phenomenological interest given the current "anomalies" in semileptonic $B$ decays. We analyze the relations between low- and high-energy observables in such class of models to next-to-leading order accuracy in the $SU(4)$ gauge coupling $g_4$. For large values of $g_4$, motivated by recent $B$-physics data, one-loop corrections are sizeable. The main effect is an enhanced contribution at low-energy, at fixed on-shell couplings. This result has important implications for current and future high-energy searches of vector leptoquark models.

Higgs mechanism and loop-induced decays of a scalar into twoZbosons

We discuss general on-shell couplings of a scalar with two $Z$ bosons using an operator analysis. In addition to the operator that originated from the Higgs mechanism, two dimension-five operators, one $CP$ even and one $CP$ odd, are generated only at the loop level. Simple formulas are derived for the differential decay distributions when the $Z$ pair subsequently decays into four leptons by computing the helicity amplitudes, from which it is shown the $CP$-odd operator merely induces a phase shift in the azimuthal angular distribution between the two decay planes of the $Z$ bosons. We also investigate new physics scenarios giving rise to loop-induced decays of a scalar into the $ZZ$ pair, and argue that the total decay width of such a scalar would be an order-of-magnitude smaller than that of a Higgs boson, should such decays be observed in the early running of the LHC. Therefore, the total decay width alone is a strong indicator of the Higgs nature, or the lack thereof, of a scalar resonance in $ZZ$ final states. In addition, we study the possibility of using the azimuthal angular distribution to disentangle effects among all three operators.

Tensor Reggeons from warped space at the LHC

The hierarchy problem can be addressed by extending the four-dimen\-sional space-time to include an extra compact spatial dimension with non-trivial ``warped metric, as first suggested by Randall and Sundrum. If the Randall-Sundrum framework is realized in string theory, the effective value of the string scale in the vicinity of the infrared boundary should be in the TeV domain. The most attractive models of this type embed the Standard Model particles as zero-modes of five-dimensional fields. In such models, Regge excitations of the Standard Model states should appear around the TeV scale. We construct a toy model that describes tensor (spin-2) excitations of the Standard Model gauge bosons, and their on-shell couplings with light matter and gauge fields, within this framework. We use this toy model to predict the phenomenologically important features of the tensor Regge gluon, such as its mass, production cross section at the LHC, and decay patterns.

The large N limit of C/ZN and supergravity

The /N orbifold of type II string theory has localized tachyons with m2 ranging from −1+1/N to −2/N in units of 2/α'. We show that by restricting attention to the lightest tachyons it is possible to take a zero-slope limit where N is taken to infinity while Nα' is held fixed. This is done by applying Buscher duality in the angular direction of the cone to obtain a supergravity solution on which the tachyons are gravitational instabilities. In this picture, supergravity provides a natural off-shell description of the tachyonic interactions. For example, the three-point couplings can be read off easily (to leading order in 1/N) from the supergravity action, and are in agreement with the on-shell couplings computed using CFT techniques.

Calculation of the QED couplingα^(MZ) in the modified minimal-subtraction scheme

I calculate the QED coupling, alpha, directly in the MS-bar scheme using an unsubtracted dispersion relation for the three light quarks, and perturbative QCD for charm and bottom quarks. Compact analytical expressions are presented, making this approach particularly suitable for electroweak fits. After alpha^(-1) (m_tau) = 133.513 +- 0.025 is obtained in a first step, I perform a 4-loop renormalization group evolution with 3-loop matching conditions to arrive at alpha^(-1) (M_Z) = 127.934 +- 0.026 for alpha_s (M_Z) = 0.120. The corresponding hadronic contribution to the on-shell coupling is Delta alpha_had^(5) (M_Z) = 0.02779 +- 0.00019. The error is mainly from m_c, and from experimental uncertainties in e^+ e^- annihilation into unflavored and strange hadrons and tau decay data.

Tree-level(π,K)amplitude and analyticity

We consider the tree-level amplitude, describing all 3 channels of the binary (pi ,K)-reaction, as a meromorphic polynomially bounded function of 3 dependent complex variables. Relying systematically on the Mittag-Leffler theorem, we construct 3 convergent partial fraction expansions, each one being applied in the corresponding domain. Noting, that the mutual intersections of those domains are nonempty, we realize the analytical continuation. It is shown that the necessary conditions to make such a continuation feasible, are the following: 1) The only parameters completely determining the amplitude are the on-shell couplings and masses; 2) These parameters are restricted by a certain (infinite) system of bootstrap equations; 3) The full cross-symmetric amplitude takes the typically dual form, the Pomeron contribution being taken into account; 4)This latter contribution corresponds to a nonresonant background, which, in turn, is expressed in terms of cross-channel resonances parameters. It is demonstrated also, that the Chiral Symmetry provides a unique scale for the mentioned parameters, the resonance saturation effect appearing as a direct consequence of the above results.

Pion coupling constants in nuclei and the isobar-hole model

We discuss the relevance of the on-shell pion-nucleus coupling constants to modern theories of pion and isobar degrees of freedom in nuclei. The excitation of specific transitions is first considered and the on-shell coupling constant is shown to carry other information than the familiar Gamow-Teller constant measured in β-decay; we present a critical discussion of the scarce existing determinations which are at considerable variance with our theoretical expectations. Next, the sum rule for the “effective” pion-nucleus coupling constant is calculated and predicted to be drastically reduced by δ-hole renormalization of the pion propagator besides the Lorentz-Lorenz effect. Dispersion relations strongly favour the validity of the model.