Gauge Mediated Proton Decay in a Renormalizable SUSY SO(10) with Realistic Mass Matrices

Transcription

1 TAUP WUB Gauge Mediated Proton Decay in a Renormalizable SUSY SO10) with Realistic Mass Matrices Yoav Achiman a,b and Marcus Richter b a School of Physics and Astronomy, Tel Aviv University, Tel Aviv, Israel b Department of Physics,University of Wuppertal, D Wuppertal, Germany July 2001 Abstract Proton decay via d=5 operators is excluded by now not only in the framework of SUSY SU5) but also its extensions like SUSY SO10) are on the verge of being inconsistent with d=5 decays. We suggest therefore, as in several recent papers with a lighter M X, to consider gauge boson induced d=6 decays. This is done explicitly in a renormalizable SUSY SO10) with realistic mass matrices. We find that the recently observed large leptonic mixing leads to an enhancement of the nucleon decay channels involving µ s and in particular the µ + π o, µ + π modes. achiman@theorie.physik.uni-wuppertal.de richter@theorie.physik.uni-wuppertal.de

2 Nothing is known directly at present about the supersymmetric SUSY) partners, except for the experimental lower bounds on their masses. The main indication for low energy) SUSY is the unification of the coupling constant of the minimal SUSY Standard Model MSSM) at a high scale GeV. However, this unification speaks even more for SUSY-Grand Unified Theories GUTs) [1], which predict also the observed partial Yukawa unification i.e. m τ = m b ) at those energies. Yet proton decay, the other main prediction of GUTs, was not observed till now [2]. Is this consistent with the expectation from SUSY GUTs? It is generally assumed that the leading proton decay modes in SUSY-GUTs are due to d=5 operators, as the d=4 contributions must be suppressed via a symmetry like R- parity) to avoid a much too fast decay. Those d=5 decay modes are induced using the, not yet observed, SUSY particles and involve therefore many unknown parameters [3]-[5]. As a consequence the predicted rates are highly model dependent. This large freedom does not save SUSY-SU5) from being practically ruled out if the needed threshold corrections are used to limit the mass of the Higgsinos H t ) which mediate proton decay [6]. Even extensions of SUSY SU5) like SUSY SO10) are on the verge of being excluded by similar arguments [7] 1. To save it one must push several parameters to their allowed limit and in particular M Ht is required in many papers to be much larger than M GUT and even larger than M Planck = GeV. This leads to very large couplings in the superpotential or the corresponding non-renormalizable contributions. In this work we will use a suggestion already put forward by several people [8] to suppress not only the d=4 but also the d=5 contribution by a symmetry and consider the gauge mediated d=6 operators. Those involve only known coupling constants and masses and therefore their predicted rates are far more reliable than the d=5 ones. They are the real test of the GUTs because d=5 induced proton decay is allowed in non-gut theories as well. Although d=6 contributions are suppressed by 1/MX 4 they could yet be observable in the near future if the relevant gauge bosons mass M X is somewhat lower than GeV. This gauge-boson induced proton decay does not require non-renormalizable contributions or particles with masses above the GUT scale. We will apply therefore the renormalized see-saw mechanism using Φ 126 to give large masses to the right-handed RH) Majorana neutrinos. This has many advantages, in particular R-parity invariance, that is needed to avoid the catastrophic d=4 contributions, is automatically obeyed in this case. This invariance leads also to a stable neutralino as a natural candidate for dark matter 2. On top of that, a way to lower the GUT scale in terms of a fully renormalizable gauge theory was suggested recently by Aulakh, Bajc, Melfo, Ra sin and Senjanović [9]. They introduce an intermediate Pati-Salam gauge group [10] at a scale M I. Such an intermediate scale 1 See however Refs. [4] [5]. 2 The known problem with such models, i.e. that coupling constants diverge andau pole ) not far above the unification scale, is not relevant here as long as we do not consider physics above the unification scale. 2

3 is needed anyhow to explain the fact that the masses of the heavy Majorana neutrinos required to give the light neutrinos masses consistent with the observed oscillations [13], are considerably smaller than the unification scale 3. In this SUSY-SO10) model, due to the absence of trilinear terms in the superpotential, some particles acquire masses of OMI 2/M GUT ) via mixing. Those particles affect the renormalization group equations in a way that lowers the unification mass. The model can solve the doublet-triplet problem a la Dimopoulos-Wilczek [14] and can suppress naturally the d=5 operators i.e. Higgsino mediated proton decay. M X and/or the unification scale have smaller values also in other recent models [8] and in particular in those using large extra dimensions. As a specially interesting example let us mention here the paper of Hall and Nomura [15] based on the model of Kawamura [16]. The idea is to use a five dimensional SU5) GUT compactified to four dimensions on the orbifold S 1 /Z 2 Z 2 ). This yields the MSSM with doublet-triplet splitting and a vanishing proton decay from d=5 and d=4 operators by a U1) R symmetry. The model gives a compactification scale M c = M X ) somewhat lower than the four dimensional unification scale. This model is not a 4d GUT, but it involves all the properties we use for the fermionic mass matrices and proton decay. When d=6 contributions are discussed in the literature one refers always to the proton decay into e + π 0, that is the dominant decay mode only when the mixing is neglected. This is in contrast with recent papers about d=5 proton decay [7] [3], where the effects of realistic mass matrices are explicitly considered. In this letter we present a SUSY-SO10) model with realistic fermionic mass matrices. We will show that the observed [13] large leptonic mixing leads to the enhancement of the branching ratios of the nucleon decay into muons with respect to those with e + in the final state. The large leptonic mixing will correspond in our model to large quark mixing. Note, that only the difference between the lefthanded H) mixing angles of the quarks is small and the righthanded RH) rotations are unobservable in the framework of the SM or the MSSM. The nucleon decay in GUTs is one of the few observables in which all mixing angles are involved 4. The aim of the model is not to get as many as possible predictions for the known observables of the SM. It is to calculate all the non-observable mixing angles in terms of the observables of the SM in order to predict the proton decay. Our model uses a scheme developed in a series of papers [17] [18]. All mass matrices have a non hermitian Fritzsch texture [19] [20] M = 0 A 0 B 0 C 0 D E. 1) This texture and the contributions of the Higgs representations are fixed by a global U F 1) or Z n ). We use only renormalizable contribution àlá Harvey, Ramond and Reiss [21]. This has some interesting advantages in our opinion, as was mentioned before, compared to the method of Froggatt and Nielsen [22] that uses broken U F 1) s with non-renormalizable 3 This scale is needed also for the invisible-axion [11] and the baryon asymmetry induced via the leptogenesis [12]. 4 Partial information on the non-observable angles can be obtained by looking for RH currents, scalar leptoquark interactions, baryon asymmetry due to leptogenesis e.t.c. 3

4 contributions 5. The SO10) symmetry as well as U F 1) give relations between the different entries of the mass matrices. Also, using only one large VEV in Φ 126 to give the RH Majorana neutrinos masses the corresponding mass matrix must have the texture [17] M νr = 0 a 0 a b. 2) Because our main interest lies in the nucleon decay, CP violation is neglected and the parameters are taken to be real. We use, as is discussed in [18], one heavy Φ 126 to give the RH neutrinos a mass. To generate the light mass matrices one Φ 126 and pairs of Φ 10 and Φ 120 are used 6. The three fermionic families in 16 i i=1,2,3 and the Higgs representations Φ k transform under the global U1) F as follows: 16 j expiα j θ)16 j 3) Φ k expiβ k θ)φ k. 4) Invariance of the Yukawa coupling terms 16 i Φ k 16 j under U1) F requires α i + α j = β k. Hence, the most general structure of the Yukawa matrices is: 0 x y z 1 0 Y 1) = 10 x ; Y h) = 126 y ; Y 1) = 120 z ; 0 0 x ỹ Y 2) 10 = ; Y 2) 126 = ; Y 2) 120 =. 5) 0 0 x 2 0 x y 2 0 y z 2 0 z 2 0 These Yukawa matrices give explicit expressions for the M d, M u, M e and M ν Dir) mass matrices, in terms of a set of 14 parameters combinations of the Yukawa couplings and the corresponding VEVs). These matrices are diagonalized and fitted to the the known masses and mixing of the quarks and leptons as will be described in the following. We start by taking diagonal mass matrices for the charged fermions at M Z. Than the full two-loop renormalization group equations RGEs) of the Minimal Supersymmetric Standard Model MSSM), with tan β = 5, are used to get the following masses at M GUT = : m u M GUT ) m d M GUT ) m s M GUT ) 1.04 MeV 1.33 MeV 26.5 MeV m c M GUT ) m b M GUT ) m t M GUT ) 302 MeV 1 GeV 129 GeV m e M GUT ) m µ M GUT ) m τ M GUT ) MeV MeV MeV 5 The method of Froggatt and Nielsen is useful to explain the hierarchy in the quark mass matrices but not for the neutrinos. This is because the matrix elements are fixed in this method only up to unknown O1) factors and the see-saw matrix is a product of three matrices. The neutrino matrix elements are given in this case only up to corrections of [O1)] 3 which can be large. 6 The model involves pairs like e.t.c. to avoid breaking of the SUSY at high energies, but only part of those are relevant for the fermionic mass matrices. 4

5 In our model only 12 independent parameters are needed to specify charged fermion matrices at the GUT scale, exactly the number of the underlying masses and CKM mixing angles the CKM matrix changes only slightly from M Z to M GUT ). By fitting those parameters, all the H and RH mixing angles of the quarks and leptons are fixed 7. This procedure involves a set of non-linear equations: U M u U R = M D) u, D M d D R = M D) d, E M e E R = M D) e, U D = V CKM. 6) We found five solutions to those equations all of which have several large mixing angles [23]. These fix the neutrino mass matrices M Dir ν and M νr also up to two parameters and the overall scale M R ). Hence, the see-saw matrix M light ν M Dir) ν M Maj) νr ) 1 ) M Dir) T 7) ν as well as the leptonic H mixing U MNS = E N ν are also known. We varied than the two free parameters and looked for solutions that reproduced the neutrino data with MA-MSW or SMA-MSW for the solar neutrinos [24]. The details will be given in a forthcoming paper [23]. The best fit is obtained for the a MA-MSW solution with the following properties: 1. The Quark H and RH mixing angles at the GUT scale: θ12 u = 0.077, θu 23 = 1.48, θu 13 = θr12 u = 0.045, θu R23 = , θr13 u = θ12 d =0.15, θd 23 = 1.44, θd 13 = θ d R12 = 0.33, θd R23 = , θ d R13 = The mixing angles of the Charged eptons: θ12 l = 1.17, θl 23 =1.44, θl 13 = θr12 l =0.002, θl R23 = 0.003, θl R13 = The Neutrino masses : M R = GeV m νe = ev, m νµ = ev, m ντ = ev. 4. The H eptonic Neutrino) mixing angles: θ ν 12 =0.55, θν 23 =0.74, θν 31 = Using these results one can calculate the proton and neutron decay branching ratios. We use the method of Gavela et al. [25] as it was extended in a series of papers [26] [18] to models with large fermionic mixing especially RH ones). In this work it is once more generalized into a SUSY GUT. 7 If only one parameter is taken to be complex, one can use its phase to account for the observed CP violation and again all mixing angles will be fixed [23]. 5

6 The effective baryon number violating agrangian of SO10) is eff = A 1 εαβγ ū Cγ γµ u β )ē+ ) µd α + A2 εαβγ ū Cγ γµ u β )ē+ ) R µd α R + A 3 εαβγ ū Cγ γµ u β + ) µ ) µd α + A4 εαβγ ū Cγ γµ u β + ) µ ) R µd α R + A 5 εαβγ ū Cγ γµ u β )ē+ ) µs α + A6 εαβγ ū Cγ γµ u β )ē+ ) R µs α R ) ) + A 7 εαβγ ū Cγ γ µ u β + ) µ γ µ s α + A8 εαβγ ū Cγ γ µ u β + ) µ Rγ µ s α R ) ) + A 9 εαβγ ū Cγ γ µ d β C ) ν erγ µ d α R + A10 εαβγ ū Cγ γ µ d β C ) ν µrγ µ d α R ) ) + A 11 εαβγ ū Cγ γ µ d β C ) ν erγ µ s α R + A12 εαβγ ū Cγ γ µ d β C ) ν µrγ µ s α R + A 13 εαβγ ū Cγ γµ s β C ) ν ) er µd α R + A14 εαβγ ū Cγ γµ s β C ) ν ) µr µd α R + A 15 εαβγ ū Cγ γµ d β C ) ν ) τr µd α R + A16 εαβγ ū Cγ γµ d β C ) ν ) τr µs α R ) + A 17 εαβγ ū Cγ γ µ s β C ) ν τrγ µ d α R + terms with two s quarks ) + terms with c,b and t quarks ) + terms with τ + and,r νc e,µ,τ ) + h.c. 8) 9) where A i are functions of the mixing angles given in Ref. [18]. The partial decay rate for a given process nucleon meson + antilepton is expressed as follows [25]: Γ j = 1 16π m2 nucl ρ j S 2 A 2 A 2 l A l M l 2 + A R 2 r A r M r 2), 10) where M l and M r are the hadronic transition matrix elements for the relevant decay process. l and r denote the chirality of the corresponding antilepton [26]. A l and A r are the relevant coefficients of the effective agrangian 8). A, A and A R are factors which result from the renormalization of the four fermion operators, see Ref. [23]. Using all this we obtain the nucleon decay branching ratios for our best solution. These are presented in the following tables compared with the d=6 nucleon decays without mixing. 6

7 proton % % neutron % % decay channel no mixing A-MSW decay channel no mixing A-MSW p e + π n e + π p µ + π n µ + π 30.0 p e + K n e + ρ p µ + K n µ + ρ 4.6 p e + η n ν C e π p µ + η 0.6 n ν C e K p e + ρ n ν C e η p µ + ρ n ν C µ π p e + ω n ν C µ K p µ + ω 8.1 n ν C η 0.2 µ p ν C e π n ν C e ρ p ν C µ π n ν C e ω p ν C µ K n ν C µ ρ0 0.8 p ν C e ρ n ν C µ ω 2.6 p ν C e K n ν C τ π p ν C µ ρ n ν C τ K One sees clearly that the nucleon decay rates into muons are strongly enhanced. Also decays into e + K 0 and νk 0 are not negligible. The other solutions in the MA-MSW case give similar results. Since we use the most general Yukawa matrices we expect our results to be generic in the large mixing scenario. The absolute decay rates are much less reliable than the branching ratios given above. They depend not only on the unknown value of M X, but also on the uncertain hadronic matrix elements [27], the short distance enhancement factors e.t.c. Recent estimates [28] of the gauge induced proton decay lifetime are around 10 τ = GeV ) 4 yrs. M X To observe gauge mediated proton decay in the near future one needs surely lighter X- bosons than Gev. This is a natural feature of several recent models as indicated before. Some of the new nucleon decay experiments and especially ICARUS [29] are well suitable to look for decays into µ s and ν s. Therefore, if the contributions of the d=5 operators to the nucleon decay are really suppressed the search for the µ + π o, µ + π modes should not be neglected. The enhancement of the muon branching ratios is a unique feature of our model because the decay mode p e + π o is not negligible also in the d = 5 induced decays [4]. In view of the fact that this enhancement is the effect of the large observed leptonic mixing on the d = 6 nucleon decay, we suggest that the observation of a considerable rate for the decay p µ + π o will be a clear indication for a gauge mediated proton decay. One can say in general, that the branching ratios of the nucleon decay can teach us about the fundamental mass matrices as they depend on all mixing angles. The present huge freedom in the mass matrices would then be strongly restricted and one could better understand the origin of the fermionic masses. 7

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