ar X iv : h ep - p h / 92 11 32 2 v 1 2 6 N ov 1 99 2 TTP 92 - 37 23 Nov . 1992 LIGHT GLUINOS IN Z 0 DECAYS ?

We point out that an apparent discrepancy between the values of αs(MZ) as determined from low versus high energy experiments can be explained if an electrically neutral coloured fermion exists which slows down the running of the strong coupling constant αs. Alexander von Humboldt Foundation Fellow. Permanent address: Institute of Nuclear Physics, Cracow, Poland, e-mail: [email protected] and [email protected] 20 years after it was born QCD is commonly accepted as the theory of strong interactions [1]. Two years of data taking by LEP experiments contributed a lot to this belief. The impact of eeexperiments on QCD can be compared only to the impact of deep inelastic lepton-hadron scattering (DIS). It is therefore worrying that these two processes seem to imply slightly different values of αs(MZ ), c.f. Table 1 taken from [2]. The results of deep inelastic lepton-nucleon scattering, crystal clear from the theoretical point of view, are systematically below the results of eeanalyses of event shapes. The discrepancy is even more striking when αs(MZ )= 0.112 ± 0.004 as determined from DIS is compared to the result of the improved analysis of event shapes, including resumation of double logs and removing the unpleasant scale dependence [4, 5]. The averaged LEP value obtained from this analysis is αs(MZ)= 0.124 ± 0.005 [2]. The data on e ecross sections for annihilation also indicate higher values of αs, albeit with much larger errors. Process Q [GeV] αs(Q) αs(MZ ) DIS [ν] 5.0 0.193± .019 .018 0.111± .006 DIS [μ] 7.1 0.180± .014 0.113± .005 ee[ev.shapes] PETRA,PEP,TRISTAN 35.0 0.140± .020 0.119± .014 AMY 58.0 0.130± .008 0.122± .007 LEP,SLC 91.2 0.120± .006 0.120± .006 LEP[resumed] 91.2 0.124± .005 0.124± .005 ee[σ] ee[σhad] 34.0 0.157± .018 0.131± .012 Γ(Z → had) 91.2 0.130± .012 0.130± .012 PDG’92 0.1134± .0035 Table 1: Strong coupling constant αs(MZ ) determined from deep inelastic lepton-nucleon and eeannihilations. The experimental values are taken from ref.[2] . A possible and likely explanation of the above mentioned problem is that the theoretical uncertainties and the related systematic errors are underestimated in the analyses [3]. However, an alternative solution exists which should not be overlooked until it is really excluded by experiment. It is possible that an electrically neutral coloured fermion of relatively low (∼ a few GeV) mass slows down the running of αs between the scales accessible to the ‘high energy’ eeand those being probed by ‘low energy’ deep inelasting experiments. An obvious candidate is a light gluino. We are aware of arguments, based on unification and cosmology, disfavouring existence of light gluino; see also [6] . On the other hand we are impressed by the fact that, contrary to widespread belief and earlier claims