RevModPhys.84.1477】Tests of the standard electroweak model at the energy frontier

Constraints on anomalous hobbs neubauer and

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: interaction Lagrangian with anomalous coupling parameters gV , V , and 1 V is given by (Hagiwara et al., 1987; Hagiwara, Woodside, and Zeppenfeld, 1990)  þ À Leff ¼ igWWV gV ðW W À À W þ W ÞV  WWV 1   À  þÀ þ þ V W W V  þ V W  W  V : 2 MW Note that Leff reduces to LSM for the values  ¼ Z ¼ 0 and g ¼ gZ ¼  ¼ Z ¼ 1. Deviations from the SM val1 1 ues of the coupling parameters are denoted by ÁgV , ÁV , and 1 ÁV . We have assumed that C and P are conserved in the interaction Lagrangian. There is no reason to believe that this assumption is valid unless the physics that leads to anomalous couplings respects these symmetries. It is straightforward to include additional terms that violate C and P, but we refrain from doing so in order to keep the discussion simple. Electromagnetic gauge invariance requires Ág ¼ 0. The 1 W boson magnetic moment W and the electric quadrupole moment QW are related to the coupling parameters by W ¼ e ð1 þ  þ  Þ 2m W and QW ¼ À e ð À  Þ: m2 W The anomalous couplings (aside from g ) are usually 1 assumed to have some dependence on an energy scale (form factors) which suppresses them at large scales to avoid violation of tree-level unitarity in the diboson production amplitude (Baur and Zeppenfeld, 1988; Zeppenfeld and Willenbrock, 1988). The parametrization generally used for the energy dependence of a given coupling parameter is ^ ðsÞ ¼ 0 ; ^ ð1 þ s=Ã2 Þ2 pffiffiffi ^ where s is the partonic center-of-mass collision energy, 0 ^ is the value of the coupling parameter in the limit s ! 0, and à is the ffiffiffi cutoff scale. p ^ The s distribution used in the measurements described in this section is obtained through Monte Carlo simulation of the collision physics. With the substantially increased diboson statistics that will be available at the LHC,ffi anomalous TGC pffiffi ^ searches can be reported as a function of s in diboson decayffi pffiffi ^ channels resuting in fewer than two neutrinos, where the s can be estimated on an event-by-event basis. This approach would lead to improved sensitivity and less dependence on ad hoc form factors as compared to the standard approach. When reporting coupling limits from hadron collider data, the value of à is taken to be close to the hadron collision energy; even large variations (e.g., 50%) of à around this scale have minimal impact on the results. Physically, the scale à can be considered the scale at which the new physics responsible for the anomalous coupling is directly accessible (e.g., through pair production of new particles). This Rev. Mod. Phys., Vol. 84, No. 4, October–December 2012 1487 approach is different from the effective-field-theory approach discussed in Sec. II, where the coefficients of higherdimension operators are constants. While in the same spirit as effective field theory, the effective Lagrangian approach to anomalous couplings is different in practice. In particular, the form facto...
View Full Document

This document was uploaded on 09/28/2013.

Ask a homework question - tutors are online