Near-threshold boson pair production in the model of smeared-mass unstable particles
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ELEMENTARY PARTICLES AND FIELDS Theory
Near-Threshold Boson Pair Production in the Model of Smeared-Mass Unstable Particles* V. I. Kuksa1)** and R. S. Pasechnik2)*** Received October 21, 2009; in final form, January 11, 2010
Abstract—Near-threshold production of boson pairs is considered within the framework of the model of unstable particles with smeared mass. We describe the principal aspects of the model and consider the strategy of calculations including the radiative corrections. The results of calculations are in good agreement with LEP II data and Monte-Carlo simulations. Suggested approach significantly simplifies calculations with respect to the standard perturbative one. DOI: 10.1134/S1063778810090140
1. INTRODUCTION Near-threshold production of the unstable particles (UPs) is the most suitable process to observe the finite-width effects (FWEs). These effects are closely connected with the instability and depend on the width and energy of the products E − Ethr with Ethr being the threshold energy. It equals to the sum of masses of the final state particles. The measurements of boson-pair production at the threshold (e.g., LEP II experiments) have provided us with an important information about the masses of bosons and non-Abelian triple gauge-boson couplings. To extract the exact information from the boson-pair production we have to calculate the radiative corrections (RCs), which give a noticeable contribution to the corresponding cross section. Ideally, one would like to have the full RCs to the processes e+ e− → ZZ, ZH, W + W − → f 4f . In practice, this problem is very complicated and cannot be considered analytically. For discussion of the LEP II situation and strategy it is useful to distinguish three levels of sophistication in description of the boson-pair production [1, 2]: (1) On-shell boson-pair production, e+ e− → ZZ, ZH, W + W − , with subsequent on-shell boson decays. All O(α) RCs to these processes are well known. ∗
The text was submitted by the authors in English. Institute of Physics, Southern Federal University, Rostovon-Don, Russia. 2) High Energy Physics, Department of Physics and Astronomy, Uppsala University, Sweden. ** E-mail: [email protected] *** E-mail: [email protected] 1)
(2) Off-shell production of boson pairs which then decay into four fermions. Full set of the RCs is very bulky for the analytical observation and analysis. (3) Total process e+ e− → 4f with an account of the complete set of the O(α) corrections. This problem leads to the additional diagrams with the same final states, and complete electroweak (EW) O(α) corrections are described by many thousands of diagrams. On-shell W -pair production was considered in [1– 3], where the cross section of the process e+ e− → W + W − was given. At the tree level, this process is described by two s-channel diagrams with Z, γexchange and one t-channel diagram with ν exchange. The complete O(α) radiative corrections, comprising the virtual one-loop corrections and realphoton bremsstrahlung, were calculated and represented in [4–
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