Search for Exotica and Compositeness at LEP

Transcription

1 Search for Exotica and Compositeness at LEP S. Shevchenko Lauristen Laboratory, California Institute of Technology, Pasadena, CA 9, USA ABSTRACT The energy upgrade at LEP allows new regimes to be explored in the search for New Physics. In this paper the results on searches for Exotica and Compositeness at p s = 3 and 3 GeV are presented. The LEP potential for the above searches is also discussed. I. INTRODUCTION The standard model (SM) of elecroweak and strong interaction has been very successful in describing the experimental data. Nevertheless it seems to be not the ultimate theory because too many facts keep unexplained, e.g. the family structure of quarks and leptons and the explanation of the mass. A search for heavy states in a new mass domain provides a window to catch a glimpse of various types of physics beyond the SM and would significantly influence the development of a new theory if evidence for new states would be found. Here we report on direct searches for fourth generation leptons, excited leptons, and contact interactions using the ALEPH, DELPHI, and OPAL data ( - pb per experiment) at p s = 3 and 3 GeV. We also discuss LEP potential for the above searches and search for leptoquarks. II. NEW HEAVY SEQUENTIAL LEPTONS Electron-positron colliders are well suited for the search for new heavy leptons, with masses up to the kinematic limit m L E beam []. The predicted production cross sections are large and final state particles can be identified cleanly. Heavy neutral and charged leptons that have not so far been observed are predicted by various models []. The sequential fourth generation neutral and charged leptons are the most natural extension. Here we report on a direct search for unstable sequential neutral heavy leptons (heavy neutrinos), L, of the Dirac or Majorana type, and charged heavy leptons, L [3]. Sequential heavy leptons are pair-produced through the s- channel: e + e! =Z!L + L,L L. The following decay modes were considered: () L! `+W via lepton flavour mixing, where ` is e, or, andw is a virtual W boson. Work supported by US Department of Energy Contract No. DE-FG3-9ER7. The only exception is single heavy neutrino production e + e!l e through t-channel W exchange. The cross section for this process depends on the degree of mixing between L and the electron generation. Single L production is not considered in this analysis. In this search we consider that neutral heavy leptons couple to electron, muon, ot tau families and we neglect the possibility of mixing between light leptons. () L!L +W,whereL is associated stable heavy neutrino and assumed to be heavier than the lower mass limit from LEP data ( GeV/c ). (3) L! `+W,where` is e, or, and the decay occurs via lepton flavour mixing. The expected experimental signature in the search for heavy neutrino L L is at least two isolated leptons with the same flavour. For L + L events for both cases, () and (3), is that of event with large, unbalanced transverse momentum, missing energy, and/or isolated lepton. The events in case () are expected to have a larger missing energy than in case (3), because the two heavy neutrinos carry away more energy. No events remained after selection cuts, consistent with expected number of background events from known processes. The 9% C.L. lower limits are shown in Table I and Table II. Table I: 9% C.L. lower mass limit in GeV/c for neutral heavy lepton L. Decay mode Dirac Majorana ALEPH L! ew 3..3 L! W 3..3 L! W 3..3 L! ew L! W OPAL L! ew.. L! W 3.. L! W 7.. To estimate LEP potential for discovering new neutral heavy sequential leptons, a detailed study was done with full detector simulation []. As the production cross section is relatively large ( pb), and there is almost no background after isolation cuts, the sequential neutral heavy lepton could be found up to Table II: 9% C.L. lower mass limit in GeV/c for charged heavy lepton L. Decay mode ALEPH L! L W 3. L! `W. L! L W. OPAL L! L W. L! `W 3.9

2 a mass close to the kinematical limit (near E beam ) with an integrated luminosity of pb (see Figure ). Luminosity, inverse picobarn 3 3 Ecm = 9 GeV, Sigma Discovery Limit Heavy Lepton Mass, GeV Figure : The integrated lumiosity needed for heavy neutrino discovery, as a function of heavy neutrino mass, for center-ofmass energy = 9 GeV. in pairs (e + e! ``) or singly (e + e! ``). In the first case the maximum mass of excited lepton is limited to the beam energy, whereas in the second case it can reach mass region close to the center-of-mass energy. Results of searches for single and pair production of excited leptons at p s = 33 GeV are reported by four LEP experiments []. Excited leptons usually decay radiatively, which gives a signal topologyfor pair production of two leptons and two energetic photons. For excited neutrinos, the radiative decay may be forbidden, and in that case, the excited neutrino decays into an ordinary lepton and a W. The cross section for pair production can be as large as 8 pb, and limits were set on excited lepton masses to nearly kinematical limit, as shown in Table III []. Table III: 9% C.L. lower mass limits in GeV/c for pair produced excited leptons e (`W) () ALEPH...8 DELPHI OPAL Entries/ GeV Entries/ GeV 3 a) c) m eγ [GeV] MC m τγ [GeV] Entries/ GeV Events/ GeV 3 b) d) MC ννγ MC γγγ MC m µγ [GeV] 8 8 m recoil [GeV] Figure : Selected events in single production searches: a) invariant mass of all e combinations; b) invariant mass of all combinations; c) invariant mass of all combinations (after applying kinematic constraints); d) recoil mass of single photons. For each plot, the bin size is comparable to the expected mass resolution. III. EXCITED LEPTONS Excited leptons are natural consequence of models where the standard leptons are composite rather then elementary particles []. At e + e colliders, excited leptons would be produced Single production of excited leptons results in events with two leptons and a photon. The production cross section for this process is suppressed by a factor of /, where the parameter can λ/m l* [GeV - ] λ/m ν* [GeV - ] a) e* µ* τ* ν* m l* [GeV] b) f = f = m ν* [GeV] Figure 3: The upper limit of the coupling constant =m` at 9% C.L. as a function of m`: a) f = f for all excited leptons; b)f = f for excited neutrinos with = f if f =,and = f if f =.Wheref and f refer to SU() and U() free parameters in the effective Lagrangian. The excluded region is above and to the left of the curves.

3 be regarded as the compositeness scale. Figure shows invariant mass distribution for all possible lepton-photon combinations in single production searches obtained by. No excess was found, and 9% C.L. upper limits of the coupling constant =m` are shown in Figure 3. Similar results are obtained by other LEP experiments []. IV. QED DEVIATIONS FROM THE () SAMPLE At LEP, the reaction e + e! () provides a clean test of QED. Deviations from the expected differential cross section for the production of two photons could be evidence for new physics due to, for example, e + e contact interactions, which are usually characterised by cut-off parameters + and [8, 9], or by the exchange of a virtual excited electron e [9]. By comparing the measured distribution with QED predictions (see, for example, Figure ), one can thus extract the limits on cutoff parameters and mass of excited electron. (/π) dσ/d cosθ* (pb) OPAL e + e - γγ(γ) < θ γ < < s> = 33 GeV O(α 3 ) QED prediction Λ + = GeV Λ - = GeV Figure : Limits for excited lepton pairs, decaying radiatively at LEP, for center-of-mass energy = 9 GeV cosθ* Figure : The measured angular distributionfor e + e! () events. The QED prediction is shown by full line. The evaluated 9% C.L. limits on the cut-off parameters are indicated by dotted ( + ) and dashed-dotted ( ) lines. The 9% C.L. limits on the cut-off parameters and on the mass of the excited electron are given in Table IV for ALEPH, and OPAL analyses []. Table IV: 9% C.L. limits on QED cut-off parameters + and, and on the mass of excited electron m e,ingev. Figure : Exclusion and discovery limits (at 9 and GeV) in the mass-coupling plane for singly produced excited electron. The LEP potential for discovering excited leptons is shown in Figure and Figure. The productioncross section and selection are discussed in details in [7]. + m e ALEPH OPAL 3 7 At LEP, assuming an integrated luminosity pb at p s = 9 GeV, one can achive 9% C.L. limits on QED cut-off pa- 7

4 rameters and on the mass of the excited electron of -3 GeV. V. () EVENT [] has found event with four photons (Figure 7). The energies and directions of the photons are given in Table V. The event is balanced in the transverse plane and the longitudinal missing momentum is equal to. GeV/c, in agreement with the hypothesis of a missing zero-mass particle. Therefore, the event is consistent with a QED process e + e!, where one of the photons escapes detection at very low polar angle. Table V: Event with four photons at a center-of-mass energy of 3 GeV. p s(gev) Photon Energy (GeV) in a pb sample at p s = 33 GeV in which the energy of all visible photons is above 7 GeV and the energy of an additional collinear photon is above GeV. VI. LEPTOQUARKS AT LEP Leptoquarks are colour triplet bosons of spin or which couple to quark-fermion pairs. They are predicted by many models beyond the Standard Model, e.g. in unifications models SU(), SO(), superstrings E, technicolour and composite models []. At LEP leptoquarks can be pair or single produced. The pair production cross sections for different leptoquarks (see Table VI) of masses between - GeV/c at LEP vary from pb (S )topb(u 3 ) []. Depending on the leptoquark decay products the expected signatures for the leptoquark pairs are: two electrons (muons) and two hadronic jets; one electron(muon), two hadronic jets and missing energy; two taus and two hadronic jets; two hadronic jets and missing energy. Table VI: Leptoquarks quantum numbers e GeV 3. GeV TEC ECAL HCAL 7. GeV. GeV e - LeptoQuark Decay modes Charge Spin T 3 S lq; q /3 S lq /3 lq /3 +! S3 lq; q /3 q -/3 - lq; q /3 +/ R lq; q /3 -/ R lq /3 +/ q -/3 -/ lq; q /3 +/ V lq; q /3 -/ lq /3 +/ Ṽ q -/3 -/ lq; q /3 Ũ lq /3 lq /3 + U 3 lq; q /3 q -/3 - Figure 7: Longitudinal view of the event with four isolated photons recorded by the detector at a center-of-mass energy of 3 GeV. Energy depositions in the ECAL are shown as a polar histogram. There are no hits in the central tracking detector indicating the presence of charged particles. The event has a longitudinal energy imbalance consistent with the production of an unobserved photon of GeV at very low polar angle. The estimated probabilty is.% for finding () event A detailed study on the search for pair produced leptoquarks at LEP with detector has been carried out [3]. Different leptoquarks species and signatures are considered. The main result is that after applying the appropriate selection cuts all the leptoquark decay modes, except the q one, are practically background free. For the latter to background events are expected for integrated luminoaity pb. The estimated discovery mass limits for different leptoquarks are shown in Figure 8 at p s = 7 GeV. 8

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