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Link to publication Citation for published version (APA):Abreu, P., Boudinov, E., Holthuizen, D. J., Kjaer, N. J., Kluit, P. M., Mulders, M. P., ... van Eldik, J. E. (1997). Search for neutral heavy leptons produced in $Z$ decays. Zeitschrift für Physik. C, Particles and Fields, 74, 57. DOI: 10.1007/s002880050370 General rightsIt is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons). Disclaimer/Complaints regulationsIf you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: http://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible.Download date: 09 May 2018 Z. Phys. C 74, 57-71 (1997) ZEITSCHRIFT FÜR PHYSIK C Abstract. Weak isosinglet Neutral Heavy Leptons (ν m ) have been searched for using data collected by the DEL-PHI detector corresponding to 3.3 × 10 6 hadronic Z 0 decays at LEP1. Four separate searches have been performed, for short-lived ν m production giving monojet or acollinear jet topologies, and for long-lived ν m giving detectable secondary vertices or calorimeter clusters. No indication of the existence of these particles has been found, leading to an upper limit for the branching ratio BR(Z 0 → ν m ν) of about 1.3 × 10 −6 at 95% confidence level for ν m masses between 3.5 and 50 GeV/c 2 . Outside this range the limit weakens rapidly with the ν m mass. The results are also interpreted in terms of limits for the single production of excited neutrinos.

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We present the first results of next-to-leading order QCD corrections to three jet heavy quark production at LEP including mass effects. Among other applications, this calculation can be used to extract the bottom quark mass from LEP data, and therefore to test the running of masses as predicted by QCD. 12.15.Ff, 12.38.Bx, 12.38.Qk, 13.38.Dg, 13.87.Ce, 14.65.Fy The decay width of the Z gauge boson into three jets has already been computed at the leading order (LO) including complete quark mass effects [1][2][3] where it has been shown that mass effects could be as large as 1% to 6%, depending on the value of the mass and the jet resolution parameter y c . In fact, these effects had already been seen in the experimental tests of the flavor independence of the strong coupling constant [4][5][6][7][8]. In view of that we proposed [3], together with the DELPHI collaboration [9], the possibility of using the ratio [3,6,7] as a means to extract the bottom quark mass from LEP data. In this equation Γ q 3j (y c )/Γ q is the three-jet fraction of Z decays into the quark q and y c is the jet resolution parameter.Since the measurement of R bd 3 is done far away from the threshold of b quark production, it will allow, for the first time, to test the running of a quark mass as predicted by QCD. However, in [3] we also discussed that the leading order calculation does not distinguish among the different definitions of the quark mass, perturbative pole mass, M b , running mass at M b , or running mass at m Z . Therefore in order to correctly take into account mass effects it is necessary to perform a complete next-to-leading order (NLO) calculation of three jet ratios including quark masses [10][11][12].In this letter we sketch the main points of this calculation, leaving the details of the complete calculation for other publications [13,14], and we present the results that have been used by the DELPHI collaboration to measure the running mass of the bottom quark at µ = m Z [15,16].In the last years the most popular definitions of jets are based on the so-called jet clustering algorithms. These algorithms can be applied at the parton level in the theoretical calculations and also to the bunch of real particles observed at experiment. In the jet-clustering algorithms jets are defined as follows: starting from a bunch of particles with momenta p i one computes, for example, a quantity like y ij = 2 min(E 2 i , E 2 j )/s (1 − cos θ ij ) for all pairs (i, j) of particles. Then one takes the minimum of all y ij and if it satisfies that it is smaller than a given quantity y c (the resolution parameter, y-cut) the two particles which define this y ij are regarded as belonging to the same jet, therefore, they are recombined into a new pseudoparticle by defining the four-momentum of the pseudoparticle according to some rule, for example, p k = p i + p j . After this first step one has a bunch of pseudoparticles and the algorithm can be applied again and again until all the pseudoparticles satisfy y ij > y c . The number of pseudop...