Heusch, C A

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Presented at the 16th International Conference on Neutrino Physics, Eilat, Israel, May 29—June 3, 1994* Work supported in part by the US. Department of Energy.irrespective of any conservation law of this been invoked in terms of the ’see-saw'different ’lepton number' (say, vQ——> e+) Fritzsch and Minkowski [l])that have sincechirality, will be prevented from producing a the neutral leptons, as first introduced bya massless neutrino, which cannot change its way, to be compared with ’mixing masses’ forquarks acquire their masses in the conventional(1) element in a scheme where charged leptons andCPT |vi}(7t) —-> n|Vi)(—7t);masses for the known neutrinos can be seen as anconservation, since and quarks, the vexing evidence of very smalllepton number is simply due to helicity symmetry scheme encompassing both leptonsmeaningless: the apparent conservation of establish a credible and plausible higherthe concept of lepton number becomes non—zero neutrino rest masses? By no means: tointeractions. For massless neutrinos, however, the absence of any compelling indication ofimposing some conservation scheme on particle ls all of this a purely academic discussion innumber can be seen as fundamental charges 'families'sectors, so that baryon number and lepton masses which fit more easily into lepton—quarkcompatibility between quark and lepton takes a special symmetry to establish Diracsymmetry group will make us expect some all other fundamental fermions, whereas itfermion spectrum in terms of some higher masses, which would set neutrinos apart fromAny attempt at understanding our basic The more general concept is that of Majoranabelow our measurement sensitivity. probe for the very nature of neutral leptons.accompanying these flavours, if not zero, are acquires another dimension: it permits us toquark/lepton families; but the rest masses The question of neutrino masses thusthree neutrino flavours in a framework of three(2)not seen a convincing breakthrough. We have CIVM> _’ WMIVM)different fronts, the neutrino mass question haseigenstate of C:theoretical as well as experimental effort onand a Maiorana neutrino, which is anNotwithstanding a great deal ofdistinction between our usual Dirac neutrino1. INTRODUCTION number. In fact, for m(v) = 0, there is noexplain the observed low-mass spectrum.W‘W" is likely to prove the preferred discovery channel for TeV-range Majorana masses that mayprovides a viable overall scenario. We review available evidence and argue that the process e`e“—>their Dirac or Majorana character, the existence of heavy neutrino states mixing with the light onesIn the absence of definitive information on either the exact values of lighbneutrino masses orAbstract\l OCR OutputInstitute for Particle Physics, University of California, Santa Cruz, California, U.S.A. '*’ UJand\OPPE Division, CERN, Geneva, SwitzerlandC. A. HeuschHow to Catch a Massive Neutrin0*September 1994SCIPP 94/24{L;-C/fFig. 1, is predicated on the likelihood that two denominator comes straight from experiment. OCR OutputThis process, illustrated by the diagram in matrix element calculation, and theA. Neutrinoless double B decay ln Eq. (7), the numerator is due to a nuclearobserved(7)studied experimental approach to this problem > _ “Nuclcus mv - l LV1 ~—-——Dirac/Majorana confusion’ problem. The besttransition points the way out of the ’practical a Majorana mass [4]vM,N exchanges may mediate a AL = 2 conclusion that at least one neutrino must haveus for neutrinos. Nevertheless, the fact that observed BBN, V decay lifetime, it leads to theNeither of these approaches is accessible to masses) due to helicity suppression; for anyobservation of T—violating decays. is proportional to the neutrino mass (for smalldetailed balance measurements or words, the amplitude for (BBN, V) observationinvariance or noninvariance has been tested bydescribing their coupling to electrons. In otherflip cannot be induced. Traditionally, the Texchanged neutrinos, and UG] a mixing matrixsmall or vanishing neutrino masses, a helicitywith mv the mass eigenstates of all KAhelicity? First of all, remember that for veryeigenvalue of the T operator (Eq. 1), i.e., thedifference between Iv) and CPT Iv) is the (6)L men =lZ¤ea[Uei] mvvil »we utilize the fact that the only observableMajorana character of neutrino masses. How doneutrino mass termSuppose now we want to verify thematrix element and may is some effectiveMASSES Here, MN is the hard-to—evaluate nuclear2. OBSERVABILITY OF MAJORANAdouble beta decay BBHO V.Figure lz The dominant graph expected in neutrinolessbe understood as Majorana masses [3].heavy masses satisfying Eq. 3 are naturally toModel as a low-energy phenomenology. Theclasses of models that contain our Standardthe realm of easier accommodation in broadthe mass question for the 'light’ neutrinos into ws >·· novexperimentally accessible, would tend to put 9 ··2 1 TeV, well above any limit that has beenthe existence of heavy neutrinos with massesE(6) —> SO(10) —> chain [2]. In other words,schemes, notably in decompositions of thenaturally in a number of higher-symmetry (5)A(BBnov l " meff MN·mass. Such heavy neutrino states N occurwhere mf can be a quark or a charged—lepton will be mediated by an amplitude(4)mv ·IT\N = IU? , (A,Z) —> 2e‘ + (A,Z+ 2)the typical fermion mass of a generation via up in two different protons. The ensuing processvM, and that the resulting two u quarks wind(conjectured) heavy one (N) will be related toappropriate nucleus overlap so as to exchange amasses of a light neutrino (v) and ad quarks inside two different neutrons in anmechanism. In this context, the product of thecurrent scattering. OCR Outputelectron, which is a product of VE (mit Ve) charged44Ti(u-—/ u+) 44Cadetector some distance away finds the V9 produces anprocessB decay in a reactor emits an electron and a Ve. Afulfill this requirement. An evaluation of theFigure 3: Neutrino-antineutrino ’oscillation’; neutrononly radioactive targets such as Ti, Se4472REACTOR DETECTORlight ones. Missimer et al. [6] point out thatwhich the u* is to be ejected to appropriately d w‘ VM l wtrestricts the choice of (Z—2,A) nuclei frominto a 1S atomic state of the (Z,A) nucleusrather, the binding energy of the incident tfeffects are not what limits process (Bb)masses will be hopelessly suppressed.well as heavy Majorana masses. The radiativeproportional to their mass; heavy Majoranabut is in principle sensitive, again, to light asprocess occur, for light neutrinos, at a ratesystematic limitations due to radiative effects,I-Ielicity suppression will again make thisabove [5]. Experimentation on process (8a) hasa Majorana mass.are intimately related to the (BBHO V) processVe behave like ve, i.e. it will be a signature for(Sb)u" + (Z,A) —+ tf + (Z—2,A) positron, that will be a sure—fire sign that somequasi·elastically, an electron instead of a(8a)e" + (Z,A) —> e+ + (Z—2,A) distance from the reactor, the Ve flux produces,The processes amounts of Ve. If, in a detector at a sizeableenussion From neutron B decay, a reactor will emit largeB. Elecuon (or muon) capture with positron (u+) oscillation, shown schematically in Fig. 3.can be visualized is the so-called vvdetection. Another process where AL = 2 transitionsBBN, V projects are not in the running for NM C. Neut1·ino—antineutrin0 oscillationsfor the detection of small Majorana masses,We conclude that, for all their usefulness (antimuon) emission.needed to pin this question down. Figure 2: Electron (muon) capture with positronmeasurements on different nuclei would bestate: a whole series of systematiclight neutrino from one caused by a heavy NMway to tell an experimental effect due to avery unreliable and, most of all, there is nothe calculated nuclear matrix elements aremakes for a sharp mass suppression ~ m(N)‘e*<u*>e‘<u‘> - YMdemand on the overlap of the parent d quarlcsMajorana neutrino; but the much heightened stopping muons per second.sensitive also to the exchange of a heavy currently existing muon factories with > 10In principle, the BBN V decay would be observability will beyond the capabilities ofmass m(vM) 2 1 eV. R-parity breaking supersymmetric models putbut estimates in left—right symmetric models orconclusion that there is at least one vM withheavy intermediate neutrinos can contribute,coincidence that tqge = 10y would lead to the24the Standard Model. Again, both light andthe edge of decay's upper limit (1 > 24 y), thegraph of Fig. 2 and appropriate extensions ofof Ge led to the observation of an event at76can then be done in terms of the vM exchangeExample: If the ongoing search for BBW V decaybelow 1 TeV even for the futuristic LHC®LEl“ evaluated. OCR Outputto some 200 GeV masses for I-{ERA, to well linear colliders for which process (11) was firstexplicitly, this discovery potential is limited concentrate on another option provided by theand decay possible; however, as Fig. 6 shows machines, so that we will, in the last section,make the discovery of s-channel production realistic discovery chance with foreseeableand that tight reconstruction cuts may now mass range of our interest, however, there is nothat cross—sections are not hopelessly small, appropriate production distributions. For thenarrow-width approximation have shown process for an NLC scenario, and givedecays. Detailed calculations [8,9] using the Buchmuller and Greub [8] considered thiscan be well reconstructed from its hadronic (11)e+e‘—> NMV.such as e+ pf, e+ F, or e+ W' — where the W‘can lead to spectacular final-state signatures, the processThis also holds for neutrino production ine' p —> vM X (10)F. Production in e+e‘ collidersprocessindicate futuristic higher-energy scenarios.emerge with the ’wrong’ electric charge. Thethe inaccessibility of TeV-mass states. Broken linessignature: the scattered charged lepton willlheavy N states: the full line is for HE and showsbe possible with a good experimental Figure 6: Cross-section for s-channelpgoduction ofm,[(kVlcurrent interaction such as shown in Fig. 5 will200 400 600 800 NX!)(or a possible LI—IC®LEP upgrade), a charged.(XllIn electron—proton colliders such as I-{ERAE. Production in high-energy ep scattering.0lE \ \\characterize the lowest-order process.Standard Model. It takes a box diagram toconversion: the process is `doubly forbidden’ in theFigure 4: Muonium—antimuonium spontaneous 3 ·'___ _ —VS=3l4C•eV umu-—-· QE \ ·IS=450GcV-·—--w/S=l300GcV LEPXLHCe' p —·—> N XThe final·state pattem is spectacular.states can result from charged-current interactions.Figure 5: Deeply inelastic ep scattering: heavy NMpropagators for large masses is evident.small m(vM ); the suppression in theof two Majorana neutrinos, vulnerable at very Q ihadronsmakes this process, mediated by the exchangeby Ni et al. [7]. Double helicity suppression Vorvr"M· NM /4 Wwhich has been investigated experimentally ll`. T(9)(we') —><u‘€*> ,direct production from lepton beams.process (see Fig. 4)the region of our primary interest, when usingALi = 2 transitions are the signatures for thepenetrating to the 1-10 TeV level indicated asFor completeness, we mention that twoscenario. In other words, we see no chance ofmrcd U mot OCR OutputI I—>jet + jetmasses(13)—>e (u ,1 )+jctIn Eq. 14, mmd stands for the heavy neutrino‘*T T +Pln1iss(14)<N> G = (N)l2Gis S ———-—— 16TC inmdl·l—ll_(T_)+p.Ln1issexchange of a heavy Majorana neutrino N,The resulting cross-section is [11], forc e —>W W —>c tt {I ]+pLmiSS/ -\ \ I flavours.mixing of light (e) and heavy neutrinothe W’W‘system, such asmatrices Ueu that are responsible for thethere are spectacular decay channels forWi. The couplings in Fig. 7 show the mixingwill repress backgrounds efficiently;easy back-to—back final-state kinematics a pair of longitudinally polarized (’scalar’)couplings into our hands; the scattering of two left-handed electrons intoeasily, thus putting the choice of chiral the cross—section for Wi Wi production is fromhigh degrees of polarization can be reachednotation of Fig. 7, the largest contribution toavailable;going Wi bosons, we found that, in thethere are plentiful electron sourcescombinations for incoming electrons and outsome unique advantages:By investigating all viable helicitypromising, the above channel (see Fig. 7) showslikely extensions.Our motivation is this: if cross-sections areall reactions of the Standard Model and itstheories [12—14].These signatures stand out from practicallythe framework of left—right symmetricafter previous work had concentrated mostly ona neutrino mixing matrix.the cross-sections for a broad parameter space,where the couplings are of the kind gp Um and Um isand Minkowski [11] have recently evaluateddiscovery channel for heavy neutrinos. Heusch Figure 7: The 'elastic’ process e' e` -—> Wi Wi,has a fair chance of being the decisiveer (pz)e‘e' —> W’W‘ (12)($1. P3)Specifically, we maintain that the processmass energies of a few hundred MeV.(€2· P4)last performed over 30 years ago at centre~of—eL (pl)ation with two incident electron beams [10],possibility to revive precision experimentconsiderable intensity have opened up theguidance of highly polarized electron beams of er (pz)technology as well as in the production and 1""’ (82- P4) ~‘Recent advances in linear acceleratorCOLLISIONS ($1- Ps)3. MASSIVE NEUTRINOS IN e‘e"their existence - which would help the OCR Outputhermetic detector, so that detection even of amethod for direct or indirect observation ofmissing transverse momentum observed in athe TeV mass range. We do not know of anothercorrelation angles of leptons and jets, and onfavour the existence of such neutral leptons inenergy seen in the calorimeter, on theI-Iigher-symmetry schemes appear toand energy permit severe cuts on the totalneutrinos with Majorana mass.two escaping neutrinos. Their unobserved p_Lpromising method for the discovery of massivehave two W` in the final state, together withproduction of two back—to-back W` bosons is aprocesses within the Standard Model that willIt is our contention that the elasticrecent studies [15,16] have looked into all thethis competition in some detail. Two thoroughCONCLUSIONtherefore of the greatest importance to knowwith the possibly marginal signal. lt ispopulate all detector elements in competitionStandard Model physics will, of course,discussed in Ref. lll].fb‘l, for the ran *e of neutrino mass matrix choices10-10cmsto healthy counting rates.3435 ‘2 'l an NLC-type machine of a typical luminosity 10-100rates, for ’TLC’ (~/s : 1 TeV) scenarios with ffi the process e"e" ——> Wi WE in the energy range ofthis leads to marginally acceptable event Figure H: Energy dependence of the cross·section for0.5 TeV) luminosities of 1010cms33-34 ‘2 °lrJ? (¤•Vllower limits on nm), in Fig. 8. For ’NLC’ (~/s as 1.0 is U\U"This cross—section is plotted for the upper and(16). (JN) =1tb—\/LMP IU"{) {L 0.5TeV 24 NH2h|l•!¤1¢T‘1 TeV, we then haveincrease with energy. Numerically, for mred :most prominent feature is 0 ~ sz, its strong Q 1¤°but should be reasonably good up to 2 mred. Itsm(W) S Vs S 2m,-ed,The cross—section Eq. 14 is valid in the regime Z 1¤'n|s(2-s0)><10‘(N)m·•¤1¢r•'°’available evidence,flavour universality. We find, from theon limits of lepton flavour violation and leptonwhich has to be culled from existing evidence10*0* moic(,. (15)confidence.(N) = X(ll)2permit its detection with a high degree ofparameter back-to—back W` emission in process (12) willproblem: the tight kinematical constraints ofcalculational ease. 11is the neutrino mixing(N)few events should not present a seriousand we take it to be ml-ed == 1 TeV, for039 (1993).G. lngelman and ]. Rathsman, DESY 93B381 (1992) 109; and DESY 92—023 (1992).W. Buchmuller and C. Greub, Nucl. Phys.2716.B. Ni et al., Phys. Rev. Lett. 59 (1987)1313;A. Halprin, Phys. Rev. Lett. 48 (1982)PR·-94-09 (1994).].H. Missimer et al., PSI preprint PSl—Nucl. Part. Sci. 31 (1981) 145.H. Primakoff and S.P. Rosen, Ann. Rev.Nucl. Part. Phys. 12 (1984) 409.W.C. 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