Abstract
We have recently established a new method for measuring the mass of unstable particles produced at hadron colliders based on the analysis of the energy distribution of a massless product from their two-body decays. The central ingredient of our proposal is the remarkable result that, for an unpolarized decaying particle, the location of the peak in the energy distribution of the observed decay product is identical to the (fixed) value of the energy that this particle would have in the rest-frame of the decaying particle, which, in turn, is a simple function of the involved masses. In addition, we utilized the property that this energy distribution is symmetric around the location of peak when energy is plotted on a logarithmic scale. The general strategy was demonstrated in several specific cases, including both beyond the standard model particles, as well as for the top quark. In the present work, we generalize this method to the case of a massive decay product from a two-body decay; this procedure is far from trivial because (in general) both the above-mentioned properties are no longer valid. Nonetheless, we propose a suitably modified parametrization of the energy distribution that was used successfully for the massless case, which can deal with the massive case as well. We test this parametrization on concrete examples of energy spectra of Z bosons from the decay of a heavier supersymmetric partner of top quark (stop) into a Z boson and a lighter stop. After establishing the accuracy of this parametrization, we study a realistic application for the same process, but now including dominant backgrounds and using foreseeable statistics at LHC14, in order to determine the performance of this method for an actual mass measurement. The upshot of our present and previous work is that, in spite of energy being a Lorentz-variant quantity, its distribution emerges as a powerful tool for mass measurement at hadron colliders.
Highlights
Many such kinematics-based techniques have long been proposed, starting with the simplest case where the decay products are all visible and the complete and unambiguous reconstruction of the decaying particle four-momentum is possible on an event-by-event basis
As is well-known, the measurement might not be straightforward, especially when new physics particles are under study
These mass measurement methods are often tailored for specific processes, which implies that, in spite of tremendous efforts, there is no method that can work in all cases
Summary
We first revisit (in section 2.1) the derivation of the fitting function that we proposed for massless visible particles in Ref. [71]. We focus on the energy spectrum of particle d, whereas only mass information of particle D is relevant to the subsequent discussion. With this simple set-up, it is well-known that the energy and momentum of the visible daughter particle d in the rest frame of the mother particle are expressed in terms of the three masses mM , mD, and md as: Ed∗. Throughout this paper, we assume that the mother particles are either scalar or produced in an unpolarized manner so that cos θd∗ has a flat distribution
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