Anomalous Muon Production in e + e-Annihilation as Evidence for Heavy Leptons by PLUTO Collaboration

A. Biicker, J. Biirger, C. Grupen and G. Zech Gesamthochschule Siegen

H. Heyer and K. \\'acker Gesamthochschule \\l'uppcrtal We have measured inclusive muon production in e+e-annihilation for CHS energies between ),6 and 5,0 GeV. Aboye '' GeV the cross SPction cannot be explained by conventional sources like higher order QF.ll processes or inclusive production of thP J/1/1 (3.1), It is, holievet·, r·ompatibel with the pair production of heavy particles of a mass of al1out 1.9 GeV/c 2 • Spin assignment and decay parameters are investigatPd.
-i" on leave from CF.RN -1-+now at CF!IN Several experiments have reported anomalous lepton signals in e+eannihilation as evidence for the existence of heavy leptons 1 • 2 ) and of charmed particles 3 • 4 ). In this and the following letter we present new evidence for the production of heavy leptons, and derive some of their major decay properties.
Inclusive muon spectra were measured at CMS energies IS between 3.6 and 5.0 GeV with the magnetic detector PLUTO at the e+e-storage ring The main conventional sources of twoprong events are the QED processes (1) e+e--r p"'"J.I-, (2) e+e---+ p+p.-y , and (3) e+e--+ P.+Jl-yy. Reaction (1) and part of (2) were removed by requiring an acoplanarity angle of> 10°, The contribution of (2) was further reduced by a cut in the squared missing In case of the multiprongs, the nlisidentified hadrons constitute the main background source. Contributions of e+e--+ e"""e-+ Jl+Jl-were calculated and found to be negligible 7 ). The contribution of +e e + J/• (3.1) + X with subsequent decay J/~ ~ ~+pwas eliminated by a cut in the invariant two-particle mass. These events have been discussed in a previous publication 6 ) A summary of the elimination procedure is given in table I. The final twoprong cross section shows a threshold behaviour with a clear signal above It GeV, being consistent with zero at ).6 GeV. The multiprong cross section is also different from zero, but statistically less significant.
Both are in good agreement with earlier measurements 2 ) Fig:. 2 shows t,h,. muon spectra of the twoprongs for three diffel·ent CHS energies. The cross section has been corrected for trigger and detector acceptance, assuming isotropy, but not for losses due to the missing mass cut: The measured spectra show the triangular upper end characteristic of the )-body decay of a moving object, and are incompatible with the rectangu-lar shape expected from a twobody decay. The velocity of the decavinl! par-ticle increases with energy, as indicated by the shift of the spectra.
In fact, all momentum distributions are consistent with the pair production and decay of a particle of about 1.9 GeV/c 2 mass into one muon, its associated neutrino, aDd a third partiel~ of low mass. w~ remark that this third particle cannot be a 1r 0 or n meson because of the measured low mean photon multiplicity of less than 0.7 per (anomalous) twoprong event. Further evidence against. photons and hadrons accompanying the * The loss due to the missing mass cut is model dependent. On the basis of the observed distributions it is estimated to be about 10~. All branching ratios given below are corrected for 1his loss. As a consequence, we should have found 22!. 5 anomalous p+ p pairs, in contrast to only 6 observed. We will therefore consider spin 0 1 although not completely exclnded, highly improbable, and try to narrow down the Tl1e large x-of tiJP last fit PX<·ln<JPS thP possibility that. we observe the decay of n ll('W hnr.vnn into n('utr·on +- 'fh(•refore, the HSSamption of a 1\!•fl\'.\' lt>ptun !, !)) app<'ars to be the most convincing on(>, The r-A t!eca:v assignment is favnure<l b.\" lht• data, indepPndent of a possiblC' smnll mass H (vL)' but V+ . .\ cannot he excluder! lO) The muon spectrum of the multiprongs at IS ::: ') Gr>\' is shown in fig. "'j, Within its bnger errors i t is consistE>nt with the twoprnng signal, and argues for a common source of both classes. Comparing the two, we ohtain the branching ratiQS for J, decays into 1-prongs n1ul mu J t i prongs a:.;: nn ( for V+A decay.
The quoted errors of the branching ratios arP purely statistical. We estimate the systematic uncertainties to amount to ~ 20~.
All branching ratios are in reasonable agreement expectations for the decay of a sequential heavy with thPoretical lepton 9 ), We cannot experimentally exclude, however, the possibility that our multiprongs contain some contributions from the decay of charmed mesons. In that casP BR(l-prong) will come out somewhat higher, and nn(r) correspondingly lower, In conclusion, our measured twoprong { p + 1-prong) events presPnt new evidence for the pair production of heavy particles of about 1.9 GeV/c 2 mass with subsequent 3-body decay, The origin from pointlike spin 0 particles can be ruled out as highly improbable, The low number nf multi-