An investigation of screwiness in hadronic final states from DELPHI

A recent theoretical model by Andersson et al. proposes that soft gluons order themselves in the form of a helix at the end of the QCD cascades. The Authors of the model present a measure of the rapidity-azimuthal angle correlation, which they call screwiness. We searched for such a signal in DELPHI data, and found no evidence for screwiness. ∗On leave of absence from University of Helsinki, Finland 1 The screwiness model In QCD, two colour charges moving apart from each other, will produce further colour radiation, gluons. At the end of the cascade one enters into a non-perturbative region, where perturbative QCD breaks up due to the small momentum transfer and hence large strong coupling constant αs. There are non-perturbative models to describe the transition regime between the perturbative region and final state hadrons, in which the largest contributing Feynman diagrams are chosen based on the coherence of the gluons. The gluon emissions are ordered by the transverse momentum in the dipole model [1], and by emission angle in the Webber-Marchesini model [2]. The azimuthal angle of the gluons is taken to be uniformly distributed ignoring possible spin effects. In a recent theoretical paper [3], it is studied what happens at the end of the QCD cascades. At the end of the cascade, the soft gluons have a transverse momentum which is of the same order as the mass of the emitter, leading to a situation in which a large recoil may spoil the original coherence. With the assumptions that: • the effective coupling ᾱ is large enough so that the number of emitted gluons is as large as possible; • the emissions fulfil helicity conservation; the Authors show that the closest packing of soft gluons at the end of the cascades is obtained by using the extra degree of freedom provided by the azimuthal angle and arranging the gluons along a helix. The helix is described by the parameter τ = dy/dφ, where y is rapidity and φ is the azimuthal angle with respect to the original parton direction. The correlation between the azimuthal angle and rapidity can be tested by using a Fourier power spectrum, which is called screwiness S in Ref. [3] and defined as: