A new mechanism for neutrinoless double beta ($\znbb$) decay based on
leptoquark exchange is discussed. Due to the specific helicity structure of the
effective four-fermion interaction this contribution is strongly enhanced
compared to the well-known mass mechanism of $\znbb$ decay. As a result the
corresponding leptoquark parameters are severely constrained from
non-observation of $\znbb$-decay. These constraints are more stringent than
those derived from other experiments.Comment: LaTeX, 6 pages, 1 figur

A. Balysh

B. Kayser

E. Takasugi

H. V. Klapdor-Kleingrothaus

J. Schechter

J.D. Vergados

J.F. Nieves

J.W.F. Valle

K. Grotz

K. Muto

K.S. Babu

M. Doi

M. Hirsch

M. Leurer

M. Moe

R.N. Mohapatra

S. Davidson

S. G. Kovalenko

S. Petcov

W. Buchmüller

W.C. Haxton

English

arXiv

A new mechanism for neutrinoless double beta (0 nu beta beta) decay based on leptoquark exchange is discussed. Because of the specific helicity structure of the effective four-fermion interaction this contribution is strongly enhanced compared to the well-known mass mechanism of 0 nu beta beta decay. As a result the corresponding leptoquark parameters are severely constrained from nonobservation of 0 nu beta beta decay. These constraints are more stringent than those derived from other experiments

Hirsch, Martin

Klapdor-Kleingrothaus, H.V.

Kovalenko, S.G.

Repositori d'Objectes Digitals per a l'Ensenyament la Recerca i la Cultura

arXiv:hep-ph/9603213v1  4 Mar 1996New Leptoquark Mechanism of Neutrinoless Double Beta DecayM. Hirsch, H.V. Klapdor-Kleingrothaus and S.G. Kovalenko∗Max-Planck-Institut fu¨r Kernphysik, P.O. 10 39 80, D-69029, Heidelberg,Germany∗Joint Institute for Nuclear Research, Dubna, RussiaAbstractA new mechanism for neutrinoless double beta (0νββ) decay basedon leptoquark exchange is discussed. Due to the specific helicity struc-ture of the effective four-fermion interaction this contribution is stronglyenhanced compared to the well-known mass mechanism of 0νββ decay.As a result the corresponding leptoquark parameters are severely con-strained from non-observation of 0νββ-decay. These constraints aremore stringent than those derived from other experiments.PACs: 11.30, 12.30, 13.15, 14.80, 23.40Neutrinoless double beta decay (0νββ) is forbidden in the standard model(SM) of electro-weak interactions since it violates lepton number conserva-tion. Therefore, experimental observation of this exotic process would be anunambiguous signal of physics beyond the SM (see refs. [1]-[4] for reviews).Essential progress in the exploration of 0νββ-decay both from theoreticaland experimental sides has been achieved in the last few years (see, for instance[4] and references therein). The considerably improved experimental lowerbounds on the half lives of various isotopes enhance the potential of 0νββexperiments in testing different concepts of physics beyond the SM such assupersymmetry (SUSY) and leptoquarks (LQ).The SUSY mechanisms of 0νββ-decay were comprehensively investigatedin a series of papers [5]-[9]. It turned out that constraints on certain SUSY-parameters from non-observation of 0νββ-decay [7] are stronger than thosefrom current and near future accelerator and non-accelerator experiments.Therefore, it is useful to investigate other possible contributions of physicsbeyond the SM to 0νββ-decay to obtain 0νββ constraints on the correspond-ing parameters.In this note we present a new mechanism of 0νββ-decay associated withthe leptoquark contribution to the effective low-energy charged current lepton-quark interactions. The diagrams describing this contribution are presentedin fig. 1.1The SM symmetries allow 5 scalar (S) and 5 vector (V µ) LQs with the fol-lowing LQ(SU(3)c⊗SU(2)L⊗U(1)Y ) assignments: S0(3c, 1;−2/3), S˜0(3c, 1;−8/3),S1/2(3¯c, 2;−7/3), S˜1/2(3¯c, 2;−1/3), S1(3c, 3;−2/3), V0(3¯c, 1;−4/3), V˜0(3¯c, 1;−10/3),V1/2(3c, 2;−5/3), V˜1/2(3c, 2; 1/3), V1(3¯c, 3;−4/3), where Y = 2(Qem − T3).The most general form of the renormalizable LQ-quark-lepton interactionsconsistent with SU(3)c ⊗ SU(2)L ⊗ U(1)Y gauge symmetry can be written as[10]LLQ−l−q = λ(R)S0 · ucReR · SR†0 + λ(R)S˜0 · dcReR · S˜†0 + λ(R)S1/2· uRℓL · SR†1/2 ++λ(R)S˜1/2· dRℓL · S˜†1/2 + λ(L)S0 · qcLiτ2ℓL · SL†0 + λ(L)S1/2 · qLiτ2eR · SL†1/2 ++λ(L)S1· qcLiτ2Sˆ†1ℓL + λ(R)V0 · dRγµeR · V R†0µ + λ(R)V˜0 · uRγµeR · V˜ †0µ + (1)+λ(R)V1/2· dcRγµℓL · V R†1/2µ + λ(R)V˜1/2 · ucRγµℓL · V˜ †1/2µ + λ(L)V0 · qLγµℓL · V L†0µ ++λ(L)V1/2· qcLγµeR · V L†1/2µ + λ(L)V1 · qLγµVˆ †1µℓL + h.c.Here q and ℓ are the quark and the lepton doublets. Following [10, 11] wedistinguish S(V )L,R being LQs coupled to the left-handed and right-handedquarks respectively (see, however, the discussion on chiral couplings in [12]).For LQ triplets Φ1 = S1, Vµ1 the notation Φˆ1 = ~τ · ~Φ1 is used.On the same footing the LQ fields couple to the SM Higgs doublet fieldH . A complete list of the renormalizable LQ-Higgs interactions is given in ref.[12]. These new interactions are especially important for 0νββ-decay, sinceafter electro-weak symmetry breaking they lead to mixing between different LQmultiplets. In turn this mixing generates the effective 4-fermion interactionsinvolving right-handed leptonic currents. In combination with the ordinary SMleft-handed charged current interactions the latter produce the contribution to0νββ-decay shown in the diagrams of fig. 1 with large enhancement factors.This type of contribution is absent in the case of decoupled LQ and Higgssectors [12].Under electro-weak symmetry breaking the neutral component of the SMHiggs field acquires a non-zero vacuum expectation value, 〈H0〉, which createsvia LQ-Higgs interaction terms non-diagonal mass matrices for LQ fields withthe same electric charge but from different SU(2)L multiplets. To obtainobservable predictions from the LQ-lepton-quark interaction Lagrangian ineq. (1), the LQ fields (I = S, V ) with non-diagonal mass matrices have tobe rotated to the mass eigenstate basis I ′. This can be done in the standardway: I(Q) = N (I)(Q) ·I ′(Q), where N (I)(Q) are orthogonal matrices such that2N (I)T (QI) · M2I(Q) · N (I)(Q) = Diag{M2In}, with the MIn being the mass ofthe relevant mass eigenstate field I ′.Now it is straightforward to derive the effective 4-fermion ν − u − d − einteraction terms generated by the LQ exchange in the upper parts of thediagrams in fig. 1. After Fierz rearrangement they take the form [12]LeffLQ = (ν¯PRec)[ǫSM2S(u¯PRd) +ǫVM2V(u¯PLd)]− (2)− (ν¯γµPLec)×α(R)SM2S+α(R)VM2V (u¯γµPRd)−√2α(L)SM2S+α(L)VM2V (u¯γµPLd) ,whereǫI = 2−ηI[λ(L)I1λ(R)I˜1/2(θI43(Q(1)I ) + ηI√2θI41(Q(2)I ))− λ(L)I0 λ(R)I˜1/2θI13(Q(1)I )],(3)α(L)I =23 + ηIλ(L)I1/2λ(L)I1 θI24(Q(2)I ), α(R)I =23 + ηIλ(R)I0 λ(R)I˜1/2θI23(Q(1)I ). (4)ηS,V = 1,−1 for scalar and vector LQs. θIkn(Q) is a mixing parameter definedbyθIkn(Q) =∑lN (I)kl (Q)N (I)nl (Q)(MIMIl(Q))2, (5)where N (I)(Q) are mixing matrix elements for the scalar I = S and vectorI = V LQ fields with electric charges Q = −1/3,−2/3. Common mass scalesMS of scalar and MV of vector LQs are introduced for convenience.Following the well known procedure [2] one can find the LQ contribution tothe 0νββ-decay matrix element for the diagrams in fig. 1. The LQ exchangesectors of these diagrams are described by the point-like 4-fermion interactionsspecified by the effective Lagrangian in eq. (2). Their bottom parts are theSM charged current interactions. The final formula for the inverse half-life of0νββ-decay readsT−11/2(0νββ) = |MGT |22G2F[C˜1a2 + C4b2R + 2C5b2L](6)witha =ǫSM2S+ǫVM2V, bL,R =α(L,R)SM2S+α(L,R)VM2V , C˜1 = C1M(ν)1 / (meR)MGT − α2MF2(7)3In eq. (6) the coefficients Cn are defined following [2]; me and R are theelectron mass and nuclear radius. We kept only the dominant terms in eq. (6),neglecting, particularly, terms proportional to the neutrino mass mν which weassume to be very small and put mν = 0 in eq. (6). Also mixed terms, suchas a · bL/R, are not accounted for, since these are expected to only slightlyaffect our numerical limits. The new matrix elementM(ν)1 was introduced andcalculated in ref. [9] within the pn-QRPA framework. Calculating Ci withinthe same approach [13] for the particular case of 76Ge we have a completeset of nuclear structure coefficients in eq. (6) (all in units of inverse years):|MGT |2C˜1 = 1.63×10−10, |MGT |2C4 = 1.36×10−13, |MGT |2C5 = 4.44×10−9.Now we are ready to derive constraints on the LQ parameters a, bL,R ineq. (6). We use the result from the Heidelberg-Moscow 76Ge experiment [14]T 0νββ1/2 (76Ge, 0+ → 0+) > 7.4× 1024 years 90% c.l.Assuming no spurious cancellations between the different terms in eq. (6)we derive the following constraints on the effective LQ parameters:ǫI ≤ 2.4× 10−9(MI100GeV)2, (8)α(L)I ≤ 2.3× 10−10(MI100GeV)2, (9)α(R)I ≤ 8.3× 10−8(MI100GeV)2. (10)Recall I = S, V .It is interesting to compare these constraints with the corresponding con-straints from other processes [11]. Consider the helicity-suppressed decayπ → eν which is extremely sensitive to the first two scalar-pseudoscalar termsin eq. (2), leading to a helicity-unsuppressed amplitude [11]. The follow-ing constraint from π → eν-decay data was obtained in ref. [12]: ǫI ≤5×10−7 (MI/100GeV)2. Apparently, the corresponding constraints from 0νββ-decay in eq. (8) are more stringent by about two orders of magnitude. Thisconfirms that 0νββ-decay is a powerful probe of physics beyond the standardmodel.In summary, non-observation of 0νββ decay can provide stringent boundson parameters of extensions of the standard model. Moreover, the 0νββ decaybounds on some of these fundamental parameters can be much more stringentthan those from other experiments. Previously such a conclusion was obtainedfor the case of the R-parity violating supersymmetric contribution to 0νββ-4decay [7]-[9]. In this letter we have shown that the leptoquark mechanismallows similar conclusions.ACKNOWLEDGMENTSWe thank V.A. Bednyakov, D.I. Kazakov for helpful discussions. M.H.would like to thank the Deutsche Forschungsgemeinschaft for financial supportby grants kl 253/8-1 and 446 JAP-113/101/0.References[1] W.C. Haxton and G.J. Stephenson, Progr. Part. Nucl. Phys. 12 (1984)409; K. Grotz and H.V. Klapdor-Kleingrothaus, The Weak Interactionin Nuclear, Particle and Astrophysics, Adam Hilger, Bristol, New York,1990;R.N. Mohapatra and P.B. Pal, Massive Neutrinos in Physics and Astro-physics, World Scientific, Singapore, 1991; M. Moe and P. Vogel, AnnualReview of Nucl. and Part. Science 44 (1994) 247.[2] M. Doi, T. Kotani and E. Takasugi, Progr. Theor. Phys. Suppl. 83 (1985)1;[3] J. D. Vergados, Phys. Report, 133 (1986) 1; J.W.F. Valle, Prog. Part.Nucl. Phys. 26 (1991) 91.[4] Proc. of the Int. Workshop on Double Beta Decay and Related Topics,Trento, Italy, April 24-May 5, 1995, ed. by H.V. Klapdor-Kleingrothausand S. Stoica, World Scientific, Singapore, 1996[5] R.N. Mohapatra, Phys.Rev. D34 (1986) 3457.[6] J.D Vergados, Phys.Lett. B184 (1987) 55.[7] M. Hirsch, H.V. Klapdor-Kleingrothaus and S.G. Kovalenko, Phys.Lett.B352 (1995) 1; Phys. Rev. Lett., 75 (1995) 17; Phys. Rev. D53 (1996)1329.[8] K.S. Babu and R.N. Mohapatra, Phys.Rev.Lett., 75 (1995)2276;[9] M. Hirsch, H.V. Klapdor-Kleingrothaus and S.G. Kovalenko, hep-ph/9512237, to appear in Phys. Lett. B (1996).5[10] W. Buchmu¨ller, R. Ru¨ckl and D. Wyler, Phys.Lett. B191 (1987) 442.[11] S. Davidson, D. Bailey and A. Campbell, Z.Phys. C61 (1994) 613; M.Leurer, Phys.Rev.Lett. 71 (1993) 1324; Phys.Rev. D50 (1994) 536.[12] M. Hirsch, H.V. Klapdor-Kleingrothaus, S.G. Kovalenko, hep-ph/9602305(submitted to Phys.Lett. B)[13] K. Muto, E. Bender and H.V. Klapdor, Z. Phys. A334 (1989) 177,187[14] HEIDELBERG-MOSCOW Collaboration: A. Balysh et al., Phys.Lett. B356 (1995) 450.Figure CaptionsFig.1 Feynman graphs for the leptoquark-induced mechanism of 0νββ decay.S and V µ stand symbolically for a) Q = −1/3 (upper part) and b)Q = 2/3 (lower part) scalar and vector LQs.6Figure 1d uuedeµν = ν _W-S, Vd uued eµν = ν _W-S, V

New leptoquark mechanism of neutrinoless double beta decay

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New leptoquark mechanism of neutrinoless double<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>β</mml:mi></mml:math>decay

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New Leptoquark Mechanism of Neutrinoless Double Beta Decay

New Leptoquark Mechanism of Neutrinoless Double Beta Decay

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