The possibility of detecting single heavy Dirac and Majorana neutrinos at LEP
II is investigated for heavy neutrino masses in the range $M_N=(\sqrt s/2,
\sqrt s)$. We study the process $e^+e^- \longrightarrow \nu_{\ell} \ell q_i
\bar q_j $ as a clear signature for heavy neutrinos. Numerical estimates for
cross sections and distributions for the signal and the background are
calculated and a Monte Carlo reconstruction of final state particles after
hadronization is presented.Comment: 4 pages, 8 figure

Almeida Jr., F. M. L.

Coutinho, Y. A.

Simões, J. A. Martins

Vale, M. A. B. do

Wulck, S.

English

arXiv

The possibility of detecting single heavy Dirac and Majorana neutrinos at LEP II is investigated in detail. We study the process $e^+e^- \longrightarrow \nu_{\ell} \ell q_i \bar q_j $ as a clear signature for heavy neutrinos. Numerical estimates for cross sections and distributions for the signal and the background are calculated and a Monte Carlo reconstruction of final state particles after hadronization is presented

Martins-Simões, J A

Coutinho, Y A

Do Vale, M A B

Almeida Jr, F M L

CERN Document Server

Dirac and Majorana heavy neutrinos at LEP II.F.M.L. Almeida Jr.,Y. A. Coutinhoy,J. A. Martins Simo˜esz, M.A.B. do ValexInstituto de F´ısicaUniversidade Federal do Rio de Janeiro, RJ, BrazilThe possibility of detecting single heavy Dirac and Majorana neutrinos at LEP II is investigated indetail. We study the process e+e− −! ν` ` qi qj as a clear signature for heavy neutrinos. Numericalestimates for cross sections and distributions for the signal and the background are calculated anda Monte Carlo reconstruction of nal state particles after hadronization is presented.PACS: 12.60.-i, 13.85.-t, 14.60.-zThe recently accumulated data at LEP offers a uniqueopportunity for the search of new particles. At centerof mass energies around 200 GeV and integrated lumi-nosities of a few hundreds of inverse picobarns this ex-perimental facility can be employed, for instance, in thesearch of new heavy Dirac and/or Majorana neutrinos.The L3 Collaboration [1] has recently published resultson the search of new possible excited charged and neutralleptons. Their experimental data was compared with aspecific model of new excited leptons interacting with theStandard Model gauge bosons and new limits were found.Single production of new possible charged leptons accord-ing to Standard Model extentions is also under study atthe Delphi Collaboration [2]. A natural question is then:what are the consequences of these results for other mod-els of new heavy leptons? This is particularly impor-tant for new heavy neutrinos. The Super-Kamiokanderesults [3] provide increasing evidence for light neutrinooscillations and non zero neutrino masses. A possibleexplanation of the smallness of neutrino masses is the“see-saw” mechanism, which implies new heavy neutrinostates, with extremely small mixing angles. However,there are theoretical models which decouple the mixingangles from the mass relations [4]. This is the case if inthe general mass matrix one imposes some internal sym-metry that makes the matrix singular. Then the mix-ing parameters are bounded only by its phenomenolog-ical consequences. Another possible scenario for heavyneutrinos is in grand unified extensions of the StandardModel such as SO(10), E6, as well as in mirror models.This new heavy neutrinos could be of the Dirac type. Inthis case we have no simple connection between mixingangles and mass ratios.We are then lead to consider the possibility of newheavy neutrino states of Majorana and Dirac types, witha mixing angle with light neutrinos limited only by it’sphenomenological consequences. There are experimentalbounds on heavy neutrino masses indicating that, if theyexist, their masses must be greater than 80-100 GeV [5,6]and mixings between heavy and light neutrinos are ex-pected to be small. There is some model dependence onthese results, but from radiative corrections [7] there isalso no indication of new physics in this region. The highprecision measurements of the Z properties at LEP/SLCindicates that the mixing of the presently known fermionsand possible new heavy states is known to be small, ofthe order of sin2 mix = 10−2 − 10−3. A recent estimate[8] gives sin2 mix < 0:0052 with 95% C.L. This limitvalue is used throughout this paper for all curves anddistributions. Single production of new heavy neutri-nos, according to some Standard Model extentions, inelectron-positron colliders offers a unique possibility fora search in the neutrino mass region MN = (ps=2;ps).Since there are no new interactions in this kinematicalregion, after mixing the relevant part of the Lagrangianat LEP II energies is given by:Lnc = − g4cW sinmixZµ Nγµ (1− γ5) ν` + h:c:: (1)andLcc = − g2p2sinmixWµ Nγµ (1− γ5) ` + h:c: (2)where ‘ = e,  ,  and N is the new heavy neutrino. ForDirac neutrinos we impose lepton number conservationand for Majorana neutrinos we must allow lepton num-ber violation. There is some model (singlets, doublets,mirror neutrinos) dependence on the N-N-Z vertex butE-mail: marroqui@if.ufrj.bryE-mail: yara@if.ufrj.brzE-mail: simoes@.if.ufrj.brxE-mail: aline@if.ufrj.br1for the light-to-heavy neutrino vertex this dependencedisappears [9].We are interested in the process e+e− −! ` ‘ qi q¯jsince it gives a clear signature for heavy neutrinos andhas a higher cross section than the pure leptonic finalstates. As we will shown below in this paper, it also al-lows a clear separation between the signal and the Stan-dard Model background.In Fig. 1 we show the first order Feynmam diagramsthat display the exchange of a single new heavy neutrinofor the above process. In Fig. 2 we show the corre-sponding Standard Model diagrams that contribute forthe same final state. We must also include in the Stan-dard Model calculation, which is our background, all thediagrams of Fig. 1 with the heavy neutrinos replaced bylight neutrinos. The single heavy Dirac neutrino for theelectron family dominates the associated muon (and tau)family production since in the first case we have s and tchannel exchanges, whereas in the last cases we have onlys channel contribution. For single heavy Majorana onehas to sum over final neutrino and anti-neutrino produc-tion and do the correct sum over the three lepton fami-lies. Another point to be taken into account is the factthat the final state light neutrino is an experimentallyundetected particle. So we must sum over all possiblecombinations whenever necessary.In the search for new particles a fundamental point tobe clarified is the relation between the heavy neutrinosignal and the Standard Model background. This pointwas recently studied for future electron-positron collid-ers [9] atps =500 GeV and new electron-muon colliders[10,11] where single production of new heavy neutrinoswas shown to be more important than pair production[12,13].In the present work we have done a detailed study ofthe process e+e− −! ` ‘ qi q¯j . Calculations for crosssections and distributions are straightforward, althoughrather lengthly. We have at our disposal efficient alge-braic programs like CompHep [14] that can perform thiskind of calculations. Hadronization of quarks can also bedone with the well known program Phytia [15].The total cross section for e+e− −! ` ‘ qi q¯j at LEPII energy ofps = 190 GeV is shown in Fig. 3. From thisfigure on we have summed over the final state chargedleptons e−, e+, −, + and employed the general de-tector cuts E` > 20 GeV and −0:95 < cos` < 0:95,where ` is the angle of the final charged lepton relativeto the initial electron. The Standard Model backgroundclearly dominates the signal. For both Dirac and Ma-jorana production we have a maximum near MN =120GeV. We turn now our attention to distributions andcuts that can improve the signal to background ratio. Inorder to make our calculations closer to experiment, wehave hadronized all final state quarks. In the next figureswe have hadronized the quarks using the Pythia program[15]. All distributions are shown for the Majorana heavyneutrino. For the Dirac case we have a very similar pat-tern.In Fig. 4 we display the invariant charged lepton +neutrino mass distribution. We note that the left scaleapplies to the Standard Model background and the rightone applies to the signal curves for MN =100, 120, 150GeV. The background events are strongly concentratedat the W mass value. This suggests a first cut in theregion 75 GeV < Minv(charged lepton + neutrino) > 85 GeV.Another interesting variable is the total invariant vis-ible charged lepton + hadrons mass distribution shownin Fig. 5. The background has its maximum at 160GeV and the signal is peaked at the heavy neutrino massMN =120 GeV. An important point for this distributioncomes from the fact that in the models we are consider-ing the heavy neutrino has a very narrow width. If thisdistribution is shown with large bins, the signal is spreadand lost. If the bin is narrow, the signal is very clear.Fig. 5 shows this effect for 1 GeV and 5 GeV bins.The invariant hadronic mass is also peaked at theW mass for the background since the main contribu-tion comes from the diagram shown in Fig. 2e andspreads for the signal. We have then used the cut 70GeV < Mhadrons < 90 GeV. Another useful cut comesfrom the angular correlation between `, and the recon-structed final state hadronic angle relative to the initialelectron, hadrons. In the next figures we have chosen thevalue (cos ` − 1)2 + (cos hadrons + 1)2 > (0:6)2.In Figs. 6, 7 and 8 we show the invariant visible mass(charged leptons + hadrons) versus missing (neutrino)energy for background and signal (in arbitrary units) forMN =100, 120, 150 GeV. In figures 6a, 7a and 8a wehave done only the general detector cuts. For MN =100GeV the signal is already separated from the backgroundbut for higher masses this is no longer possible. The moregeneral cuts discussed above can improve the signal tobackground ratio. Besides these cuts we have done in Fig.6b the cut Echarged lepton < 40 GeV. The background isclearly below the signal. In Fig. 7b, for MN =120 GeV,besides the above cuts we have done Echarged lepton < 60GeV. Here again the signal dominates the background.In Fig. 8b, forMN =150 GeV, we have employed 40 GeV< Echarged lepton < 80 GeV.In Tables I and II we give the total number of eventsfor Majorana and Dirac heavy neutrinos for an integratedluminosity of 200 pb−1. This is shown for the general de-tector cuts and for the more restrictive cuts discussedabove. They clearly do not affect the signal. Finally,from Figs. 6b, 7b and 8b we must do additional cuts inthe final state missing energy E= and final visible energyMvisible, in order to reduce even further the backgroundto an acceptable level and increase the signal to back-ground ratio.The present work shows that the recent LEP II datacan test the possibility of new Dirac and/or Majorananeutrinos with mass in the region MN = (ps=2;ps)and mixing angles with light neutrinos in the range10−2 − 10−3. This was calculated for an integrated lu-minosity of 200 pb−1. The process e+e− −! “charged2lepton + missing energy + hadrons” can give a clearsignature for heavy neutrinos. For the models that weconsidered an important point is that the heavy neutrinowidth is very narrow and distributions must be done withnarrow bins.Acknowledgments: This work was partially supportedby the following Brazilian agencies: CNPq, FUJB,FAPERJ and FINEP.[1] M. Acciarri et al., hep-ex/0011068.[2] M.A.B. do Vale, L.S. de Paula and F.M.L. Almeida Jr.,DELPHI 2000-170 PHYS 882 (2000).[3] A. Mann, hep-ex/9912007.[4] C. Jarlskog, Phys. Lett. B 241 (1990) 579;D. Tommasini, G. Barenboim, J. Bernabeu and C.Jarslkog, Nuc. Phys. B 444 (1994) 451;W. Buchmu¨ller, C. Creub and P. Minkovski, Phys. Lett.B267 (1991) 355.[5] K. Zuber, Phys. Rep. 305 (1998) 295.[6] D.E. Groom et al, Eur. Phys. Jour. C 15 (2000) 1.[7] P. Langacker, in Precision test of the Standard Model,ed. P. Langacker, World Scientic, Singapore, 1995, pg.883;P. Bamert et al., Phys. Rev D 54 (1996) 4275.[8] F.M.L. Almeida Jr., Y.A. Coutinho, J.A. MartinsSim~oes, M.A.B. do Vale, Phys. Rev. D 62 (2000) 075004.[9] F.M.L. Almeida Jr., Y.A. Coutinho, J.A. MartinsSim~oes, M.A.B. do Vale, Phys. Rev. D in press, hep-ph/0008201.[10] F.M.L. Almeida Jr., Y.A. Coutinho, J.A. MartinsSim~oes, M.A.B. do Vale, Phys. Lett. B 494 (2000) 273.[11] G. Cvetic and C.S. Kim Phys. Lett. B 461 (1999) 248;ibid B 471 (2000) 471E.[12] F.M.L. Almeida Jr., J.H. Lopes, J.A. Martins Sim~oes andC.M. Porto Phys. Rev. D 44 (1991) 2836 ;F.M.L. Almeida Jr., J.H. Lopes, J.A. Martins Sim~oes, P.P. Queiroz Filho and A. J. Ramalho Phys. Rev. D 51(1994) 5990.[13] A. Djouadi Z. Phys. C 63 (1994) 317;G. Azuelos, A. Djouadi Z. Phys. C 63 (1994) 327.[14] A. Pukhov, E. Boss, M. Dubinin, V. Edneral, V. Ilyin,D. Kovalenko, A. Krykov, V. Savrin, S. Shichanin and A.Semenov, "CompHEP"- a package for evaluation of Feyn-man diagrams and integration over multi-particle phasespace. Preprint INP MSU 98-41/542, hep-ph/9908288.[15] T. Sjo¨strand, Comp. Phys. Commun. 82 (1994) 74.Figure Captions1. Signal Feynman diagrams for heavy Majorana andDirac neutrino contribution to e+e− −! ` ‘ qi q¯j .2. Standard model Feynman diagrams for e+e− −!` ‘ qi q¯j .3. Total cross section for the Standard Model back-ground, Dirac and Majorana heavy neutrinos atps = 190 GeV and sin2 mix < 0:0052 with 95%.4. Invariant mass distribution for the system chargedlepton + neutrino.5. Invariant mass distribution for the final visible par-ticles (charged lepton + hadrons), for MN= 120GeV. The first figure is done with 5 GeV bins andthe second one is done with 1 GeV bins. Both arein arbitrary units.6. Invariant visible mass (charged leptons + hadrons)versus missing energy (neutrino) for backgroundand signal for MN = 100 GeV (in arbitrary units).Fig. 6a was done with the general detector cutsand Fig. 6b was done with the additional cuts asdiscussed in the text.7. Same as figure 6 for MN = 120 GeV.8. Same as figure 6 for MN = 150 GeV.TABLE I. Expected number of events for Majorana] Detector cutsMN=100 GeV 21.2MN=120 GeV 29.6MN=150 GeV 18.8TABLE II. Expected number of events for Dirac n] Detector cutsMN=100 GeV 31.1MN=120 GeV 42.6MN=150 GeV 26.53e–WN l–W qjqie+ νe(a)__ e–NWνll–e+ Wqjqi(b)__e–e+ZνlNl–Wqjqi(c)__e–Ze–Ne+ Wqjqiνl(d)__Figure 1e–e+Z,γqiqiqjWνll–(a)e–Z,γe–qiqi qjWe+νe(b)e–e+Z,γl–l–νlWqjqi(c)e–Z,γe–We+νeWqjqi(d)e–e+Z,γ WWqjqiνll–(e)e–Z,γe–e+e–νeWqjqi(f)Figure 2____________

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