Neutrinoless double beta decay and pseudo-Dirac neutrino mass predictions through inverse seesaw mechanism

0.21 MB·English

by  Ram Lal Awasthi

Neutrinoless double beta decay and pseudo-Dirac neutrino masspredictions
through inverseseesaw mechanism
Ram Lal Awasthi,δ M. K. Parida† and Sudhanwa Patra†
†Center of Excellence in Theoretical and Mathematical Sciences,
Siksha ’O’ Anusandhan University, Bhubaneswar-751030, India.
δHarish-Chandra Research Institute, Chhatnag Road, Jhusi, Allahabad 211019, India.∗
Intheinverseseesawextensionofthestandardmodel,supersymmetricornon-supersymmetric,whilethelight
left-handedneutrinosareMajorana,theheavyright-handedneutrinosarepseudo-Diracfermions.Weshowhow
oneoftheselattercategoryofparticlescancontributequitesignificantlytoneutrinolessdoublebetadecay.The
3 neutrinovirtualitymomentum isfound toplay acrucial role inthenon-standard contributionsleading tothe
1 predictionofthepseudo-Diracfermionmassintherangeof 120MeV−500MeV. WhentheDiracneutrino
0 massmatrixintheinverseseesawformulaissimilartotheup-quarkmassmatrix,characteristicofhighscale
2 quark-leptonsymmetricorigin,thepredictedbranchingratiosforleptonflavorviolatingdecaysarealsofound
tobeclosertotheaccessiblerangeofongoingexperiments.
n
a
J
I. INTRODUCTION: The standard gauge theory of strong, seesawmechanism[17,18],whichrequiresoneRHneutrino
1
weak,andelectromagneticinteractionshasconfrontednumer- as well as an additionalsterile fermionpergeneration, oper-
2
ousexperimentaltestswhilethelastpieceofevidenceonthe atesatTeV scaleandis, therefore,experimentallyverifiable.

] HiggsbosoniscurrentlyunderrigorousscrutinyattheLarge InthisframeworkwhiletheLHlightneutrinosareMajorana
h
HadronCollider(LHC).Inspiteofthese,neutrinooscillation fermions,theRHneutrinosarepseudo-Diracbynaturehaving
p
datauncoveringtinymassesofleft-handed(LH)neutrinoscall heaviermasses.
-
p for physics beyond the standard model (SM) which is most In this letter we show that the inverse seesaw formulaex-
e simply achievedvia canonicalseesaw mechanism[1, 2] that plaining the light neutrino masses and mixings permits the
h
requirestheadditionofoneheavyright-handed(RH)neutrino lightestofthethreepseudo-Diracneutrinosinthemassrange
[
pergenerationprovidedbothLHandRHneutrinosareMajo- (120 500)MeVleadingtonewcontributionsto0νββdecay
1 ranafermions[3]. Severalotherformsofseesawmechanism comp−arableto,ormuchmorethan,thoseduetotheexchanges
v
[5–7]alsorequireMajoranafermions.Quiteinterestingly,on- ofthelightleft-handedneutrinos.Theneutrinovirtualitymo-
4
8 goingexperimentsonneutrinolessdoublebetadecay(0νββ) mentum [19, 20], p 190 MeV, is noted to play a crucial
| | ∼
7 [8] is expectedto resolve the issue between Majorana [3] or rolein suchnew contributions. The originofDirac neutrino
4 Dirac[4]natureoftheneutrino1. Incontrasttothepredicted massmatrixisalsofoundtobeimportantinourestimationsin
1. smallcontributiontothe0νββdecayrateintheSM,therehas predictingleptonflavorviolatingdecaysaccessibletoongoing
0 beenquite significant, orevenmoredominantpredictionsif, experimentalsearches. As our results are also applicable in
3 attheTeVscale,thereisleft-right(LR)gaugetheory[10,11]. the inverseseesaw extensionof the minimalsupersymmetric
1
Even,attemptshavebeenmadetopredictnonstandardcontri- standardmodel(MSSM),theyareconsistentwithgaugecou-
:
v butions to 0νββ decay rate due to the mediation of pseudo- pling unificationat the MSSM-GUT scale, M 2 1016
U
Xi Diracneutrinoswhereeachofthemisconsideredtobeapair GeV. ≃ ×
ofMajorananeutrinos[11,12]. Whilethepossibilityofleft-
r II.THEINVERSESEESAWEXTENSION:Asiscustomaryto
a handed neutrinos being pseudo-Dirac has been shown to be theimplementationofinverseseesawmechanism,weaddtwo
highly challenging [13], contribution of a fourth generation
fermionsingletstoeachgenerationoftheSM,withorwithout
heavypseudo-Diracneutrinoto0νββ hasbeenexploredwith supersymmetry. While we call the first type of singlet a RH
theconditionthatitsmassshouldbegreaterthanMZ/2[14]. neutrino(NR),thesecondtypeofsingletisnamedasasterile
IftheDiracneutrinomassmatrixoccurringinseesawformu- neutrino(S )and,inthe(ν ,Nc,S )basis,the9 9neutrino
lashasitsleft-rightsymmetricorquark-leptonsymmetricori- L L R L ×
massmatrixis[18]
gin, descending from Pati-Salam symmetry [15] or SO(10)
grand unified theory [16] at high scales, then the canonical
0 M 0
D
seesaw scale is toolargeto beexperimentallytestedby high = MT 0 MT , (1)
energyacceleratorsincludingLHC.Alternatively,theinverse Mν  D 
0 M µ
S
 
where M is the Dirac massterm ofthe neutrino,and M is
D
theheavyDiracmassmatrixrelatingN andS . Thematri-
1Besides the two distinct possibilities, Dirac or Majorana, very recently R L
cesM andM areingeneral3 3complexinflavorspace
a new hypothesis has been advanced in which neutrinos could be D ×
schizophrenic[9]. whereastheµS is3 3complexsymmetricmatrix.
×
Transformationfromflavortomassbasisanddiagonaliza-
2
tionareachievedthrough where
ν = ∗ ν , (2)
f m
| i V | i
M∗−1µ∗(M M−1)† 0
BT − S D . (6)
† ∗ = ˆ =Diag m ;M , (3) ≃(cid:18) (MDM−1)† (cid:19)≃(cid:18)X†(cid:19)
V MνV Mν { νi ζj}
where ν = (ν˜,ζ )T represents the three light and six
m i j Hence, in the leadingorderapproximation, can be written
| i
heavy mass states, and i and j run over the light and heavy V
as
masseigenstates,respectively. Withµ ,M M,thema-
S D
≪
trix canbeblockdiagonalizedtolightandheavysectors
ν
M
1 1XX† 0 X
MD MD T − 20 1 0 Uν 0 , (7)
mν ≃ M µS M , V ≃ X† 0 1 1X†X(cid:18) 0 UH(cid:19)
(cid:18) (cid:19) (cid:18) (cid:19) − − 2
0 MT  
M . (4)
H ≃ M µS
(cid:18) (cid:19)
whereX =(M M−1),andalltheelementsinthefirstblock
D
where mν has the well known inverse seesaw formula [18] are3 3matrices.
and M is the mass matrix for heavy pseudo-Diracpairs of ×
H
comparablemasseswithsplittingoftheorderof µS. TheµS (II. A) µS from neutrino oscillation data: The inverse see-
term in the Lagrangianbreaksthe leptonic globalsymmetry, saw formula in eqn. (4) predicts light neutrino mass ma-
U(1)L,whichisotherwisepreservedinthestandardmodelin trix in terms of three other matrices, MD, M, and µS. At
thelimitµS → 0renderingalltheLHneutrinostobemass- first we take MD ≃ Mℓ, the charged lepton mass matrix,
less. Hence the small µ should be a natural parameter in which may arise if the SM originates from high scale left-
S
thistheoryinthe’tHooftsense[21]. Theaboveblockdiago- right gauge symmetry, SU(2)L SU(2)R U(1)B−L
× × ×
nalizedmatricesarefurtherdiagonalizedthroughthe PMNS SU(3) MR SM,whereM >> M . Assumingthema-
C R W
matrix, Uν, and a 6 6 unitary matrix UH, respectively, so trix M to−→be diagonal for the sake of simplicity and using
×
that M =diag m ,m ,m = 0.0005,0.1,1.7 GeV,weob-
D e µ τ
{ } { }
tain µ from global fits to the neutrino oscillation data [22]
1 1B∗BT B∗ U 0 S
V ≃(cid:18) −−2BT 1− 21BTB∗(cid:19)(cid:18) 0ν UH(cid:19) , (5) giveninTABLEI
µ (GeV) = X−1 mˆ T XT−1 (8)
S ν
N N
6.71 10−7+1.96 10−7i 1.17 10−8 3.22 10−8i 3.71 10−8 2.03 10−8i
× × − × − × − × − ×
= 1.17 10−8 3.22 10−8i 1.53 10−08 2.22 10−10i 7.0 10−9 2.83 10−9i , (9)
 
− × − × × − × × − ×
3.71 10−8 2.03 10−8i 7.0 10−9 2.83 10−9i 5.50 10−9+5.26 10−11i
 − × − × × − × − × × 
 
where N = (1 − η)Uν and η = 21XX† is a mea- Neutrinooscillationparameters Globallyfittedvalues
sure of unitarity violation. This particular structure of
µS has been derived using, as an example, the nor- ∆m2sol[eV2] 7.58×10−5
mal hierarchical (NH) light neutrino masses mˆdνiag = |∆m2atm|[eV2] 2.35×10−3
diag(0.00127 eV, 0.00885 eV, 0.0495 eV) and non- sin2θ12 0.320
degenerate eigenvalues of M = diag{0.2,2.6,23.7} GeV. sin2θ23 0.427
Similaranalysispredictssomewhatdifferentstructuresofµ
S sin2θ13 0.0246
forinvertedhierarchical(IH)andquasi-degenerate(QD)pat-
tern of the light neutrinosand can furtherbe easily obtained δCP 0.8π
for degenerate M = M = M or, partially-degenerate
1 2 3
M1 = M2 M3 after taking care of the phenomenolog- TABLEI:Masssquareddifferences,mixingangles,andCP-phase
≪
ical bounds η < 2.0 10−3, η < 8.0 10−4, and fromglobalfitstoneutrinooscillationdata[22].
ee µµ
| | × | | ×
η <2.7 10−3.OuransatzwithM =diag(M ,M ,M )
ττ 1 2 3
| | ×
3
gives pseudo-Dirac neutrinos have been discussed in Sec-II. The
half-lifeof0νββ transitionisthenfoundtobe
1.25 10−7 0.005 1.35
η =diag × , , , −1
(cid:18) M12 M22 M32(cid:19) T10/ν2ββ =K0ν meνe,LL+Mζe,eLL 2 , (13)
whereallmassesontherighthandsideareinGeV. (cid:20) (cid:21) (cid:20)(cid:12) (cid:12) (cid:21)
III.NEUTRINOLESSDOUBLEBETADECAYPREDICTIONS where 0ν contains phase s(cid:12)pace factors plus(cid:12)nuclear matrix
K
Two separate contributions due to light and heavy neutrino elementsandmeνe,LL(Mζe,eLL)representstheeffectiveneutrino
exchangesto 0νββ transition becometransparentbywriting mass derived from light neutrino (heavy pseudo-Dirac neu-
theflavoreigenstatesaslinearcombinationoflightandheavy trino)exchangesinthemassbasis. Theanalyticformsofthe
masseigenstates two effective masses have been estimated for this model as
showninTABLE.II:
ν = ν + ζ ,
α αi i αj j
N U
Effectivemass Analyticalexpression
where (0,X)U isa3 6matrix.Thentheweakcharge-
H
U ≃ ×
currentLagrangiancanbeexpressedas
= g Wµℓ¯ γµP ν +h.c. meνe,LL Ne2imνi
LCC √2 L α L α
g
= √2WLµℓ¯αγµPL(Nαiνi+Uαjζj)+h.c., (10) Mζe,eLL (Uej)2 p2M−Mζjζ2j|hpi|2
resultingintwodifferentcategoriesofFeynmanamplitudes:
TABLE II: Effective mass parameter for standard (non-standard)
ν which arises from the Feynman diagram of
• ALL contributionsduetolight(heavypseudo-Dirac)neutrinoexchanges
Fig.1(a)duetoonlylightneutrinoexchanges
for0νββdecay.
m
ν =G2 2 νi , (11)
ALL F Nei p2 Wediscussbelowthreedifferentcases:
(III.A)Thestandardcontribution.Itiswellknownthatthe
where p 190 MeV represents neutrino virtuality standardcontributionsduetolightneutrinoexchangesarede-
h i ≃
momentum[19,20]. pendenton their allowed mass patterns; normalhierarchical
(NH),invertedhierarchical(IH),orquasi-degenerate(QD),
ζ which arises from the Feynman diagram of
• AFiLg.L1(b)duetoheavypseudo-Diracneutrinos, mν U2 m +U2 e2iαm +U2 e2iβm
ee,LL ≃ e1 ν1 e2 2 e3 3
M
ζ =G2 ( )2 ζj . (12) 0.004eV NH,
ALL F U ej p2 M2
− ζj ⇒|mνee,LL|≃ 0.048eV IH, (14)
0.1eV QD.
n p n p In our case, U and light neutrino exchanges in the
WL − WL − Nei ≃ ei
Nei eL Uej eL massbasisgivesalmostthesamecontributionswhicharepre-
sentedbysolidlinesshowninFig. 2,Fig. 3,andFig. 4.
νi ζj
(III. B) M p: In the inverse seesaw extension under
Nei e−L Uej e−L study,inadζdjiti≫on|to|thestandardeffectivemassparameter,the
WL WL
n p n p additionaleffectivemassparameterfor M p satisfies
| ζj| ≫ | |
(a) (b) M −1 = ( ±)2 −1 . Thisresultsinnewcontributionto
h ζ±i U Mζ±
0νββ transitionhalf-life
FIG.1: Feynmandiagramscontributingtoneutrinolessdoublebeta
decayduetolightneutrinoexchanges(left-panel)andheavypseudo-
−1 2
1 1
Diracneutrinoexchanges(right-panel). T0νββ = p 2
1/2 K0ν |h i| M − M
(cid:20) (cid:21) (cid:12) (cid:18)h ζ+i h ζ−i(cid:19)(cid:12)
The mass eigenstates of heavy pseudo-Dirac neutrinos (cid:12) 2 (cid:12)
are ζ1+,ζ2+,ζ3+;ζ1−,ζ2−,ζ3− with almost degenerate pairs ≃ K0ν(cid:12)(cid:12)|hpi|2 U± 2ek µMSk2k , (cid:12)(cid:12) (15)
(ζ+,ζ−; k=1,2,3) but having small mass difference µ (cid:12) kk(cid:12)
k (cid:0)k (cid:1) S (cid:12) (cid:0) (cid:1) (cid:12)
between the members of the pair and the flavor states where µ and M are(cid:12)the eigenvalues of µ(cid:12) and M, re-
Skk kk (cid:12) S(cid:12)
are (N ,N ,N ;S ,S ,S ). The mixing matrix for these spectively. One exampleofthiscase hasbeenshownin Fig.
1 2 3 1 2 3
4
10 10
1 1
|e |e
e 0.1 e 0.1
M M
Std Std
| |
150 150
0.01 180 0.01 180
250 250
500 500
0.001 0.001
1e-05 0.0001 0.001 0.01 0.1 1 1e-05 0.0001 0.001 0.01 0.1 1
m (eV) m (eV)
1 1
FIG.2:Predictionsofeffectivemass|Mee|in0νββdecaywithDiracneutrinomassMD =chargedleptonmassMlandthediagonalstructure
ofM forNH(IH)patternoflightLHneutrinomassesasshownintheleft(right)panel. Thestandardcontributionisshownbysolidlineand
nonstandardcontributionswithpseudo-DiracneutrinoexchangesofdifferentmassesexpressedinMeVareshownbyotherlines.
2forM = 0.5GeVwherethepredictedeffectivemasspa- Inthenexttwoexamplesweadoptplausibleparametrization
ζ1
rameterisnearly3/2(4)timeslargerthanthestandardpredic- predicting significantly larger contribution to these branch-
tionforNH(IH)case. ingratioswhileretainingthedominantcontributionsto0νββ
(III.C)M p: Inthisregionwheredifferentallowedval- transition.
ζj ≃| |
uesofM areoftheorderofneutrinovirtualitymomentum (IV.A)M M withnon-diagonalM: Wegeneratenon-
ζj D ≃ ℓ
p 190 MeV, the new contributionto neutrinolessdouble diagonalmatrix M to satisfy the existing phenomenological
| | ≃
beta decay due to heavy pseudo-Dirac neutrino exchange is boundsontheelementsofη[23],
foundtobemoredominantthanthestandardcontributionand
η <2.0 10−3, η <3.5 10−5,
the0νββ transitionhalf-lifeisgivenbelow | ee| × | eµ| ×
η <8.0 10−3, η <8.0 10−4,
eτ µµ
| | × | | ×
T0νββ −1= p 2 ± Mζ+ Mζ− 2 |ηµτ|<5.1×10−3, |ηττ|<2.7×10−3. (17)
(cid:20) 1/2 (cid:21)pseudo−KD0iνra(cid:12)(cid:12)c|h i| U p2−Mζ2+ − p2−Mζ2−!(cid:12)(cid:12) Usingtheparametrizationofthetypeusedinref.[24],M can
(cid:12) 2 (cid:12) beexpressedas
≃ K0ν(cid:12)(cid:12)(cid:12)|hpi|2 U± 2ek p2µ−SkMkk2k(cid:12) . (16(cid:12)(cid:12)) 1 −1 † 1 −1
(cid:12) (cid:0) (cid:1) (cid:12) M OTM M OTM
(cid:12) (cid:12) D D
The predicted new v(cid:12)alues of the effective mas(cid:12)s parameters " (cid:18)√2η (cid:19) # " (cid:18)√2η (cid:19) #
arising solely due to pseudo-Dirac neutrino exchanges have
=1 =V†V (18)
beenshowninFig. 2intheleft-pannel(right-pannel)forNH 3
(IH)patternsofthelightneutrinomasses,respectively,where whereO isthematrixdiagonalizing η andV isanarbitrary
| |
Mζ1 = (0.15−0.5)GeV.Itisquiteclearfromtheplotsthat unitarymatrix. Choosing,forthesakeofsimplicity, V = 13
even for Mζ1 = 0.25 GeV or, 0.5 GeV, the new contribu- andwenotethatthatalightestpairwithMζ1 ≃0.16GeV,in
tions are 3-6 times larger than the standard ones. While for thevicinityofneutrinovirtualitymomentum,canbeachieved
thevalueofMζ1 = 0.18GeV,thecontributionisnearly100 by suitable rescaling, e.g. ηαβ → ηαβ/(1500). After this
timeslargershowninFig. 2. Thislargeenhancementoccurs scalingwefind
asM approachesthevicinityoftheneutrinovirtualitymo-
ζ1 1
mentum, p 190 MeV. We point out that such important M(GeV) = OTM (19)
effectsof|ps|eu≃do-Diracneutrinomassesarefoundforthefirst √2η D
(cid:18) (cid:19)
timeinthiswork.
0.092i 14.08i 383.7i
IV.LEPTONFLAVORVIOLATIONWITHDOMINANT0νββ = 0.217 70.36 −80.39 ,
 
DECAYRATE:Wehaveclearlyshownthatthepredictednon- − −
0.074 8.866 320.5
standardcontributionsto neutrinolessdoublebetadecayrate  
 
are dominant for the lightest allowed pseudo-Dirac neutrino where
massM (0.15 0.5)GeV.However,becauseofthediag-
ζ1 ≃ − 0.5874i 0.5446 0.5987
onalnatureofM andassumedstructureofM,thebranching
D
ratios for lepton flavor violating (LFV) decays, µ e+γ, O = 0.4284i 0.8368 0.3409 .
→ −
τ e+γ,andτ µ+γareassmallastheSMpredictions. 0.6866i 0.0562 0.7248
→ → − − 
 
5
10 10
1 1
|e |e
e 0.1 e 0.1
M M
| | Std
Std
|η|/1000 |η|/1000
0.01 |η|/1500 0.01 |η|/1500
|η|/2000 |η|/2000
|η|/3000 |η|/3000
0.001 0.001
1e-05 0.0001 0.001 0.01 0.1 1 1e-05 0.0001 0.001 0.01 0.1 1
m (eV) m (eV)
1 1
FIG.3:SameasFig.2butnowwithnon-diagonalstructureofM andreducedvaluesofnonunitaritymatricesηasdescribedinthetext.
With the allowed mass eigenvalues for the heavy pseudo- eqn.(8). Weobtainforn=4
Dirac neutrinos, M = diag 0.159,72.0,506.4 GeV, the
ζ
{ } M(GeV)
predicted branching ratios for lepton flavor violating decays
are[25] 5.9 3.45i 0.2 60.72i 5.15 1760i
− − − −
= 10.44+2.13i 60.9 0.5i 598.0 12.16i ,
 
− − − − −
Br(µ e+γ)=1.56 10−26,  7.08−5.09i 70.86−0.18i 1547−4.15i 
Br(τ →e+γ)=5.79×10−27,   (22)
→ ×
Br(τ µ+γ)=1.10 10−18. (20) µ (eV)
S
→ ×
3.42 0.51i 1.92+5.55i 0.28 1.92i
− − − −
Althoughallthethreebranchingratiosaremuchsmallerthan = 1.92+5.55i 39.1+5.68i 12.1+0.11i .(23)
 
their correspondingexperimental upper limits [27], they are − −
0.28 1.92i 12.1+0.11i 4.10 0.68i
considerably larger than the SM predictions. However we  − − − 
 
note below that with M similar to M , the up-quarkmass Our predictionson numericalvalues of the effective mass
D u
matrix, a phenomenonunderlyingthe possible origin of SM parameterfor 0νββ are shownin Fig. 4 forNH, IH andQD
from Pati-Salam [15] or SO(10) model, LFV decays have cases. ForNHlightneutrinoswefindthatthepredictedvalue
much larger predicted values, accessible to ongoing experi- of M is increasedby a factor3 for η = η /3, corre-
ee max
| | | |
mentalsearches,whilesimilarpredictionsondominant0νββ sponding to lightest pair M = 131 MeV, while the incre-
ζ1
decayaremaintained. ment is 10 times for η = η /4 and M = 152 MeV,
| |max ζ1
and 30 times for η = η /5 and M = 169 MeV. We
(IV. B) MD Mu and GUT connection: In this case | |max ζ1
≃ find that the enhancement survives as long as lightest pair
Dirac neutrino mass matrix is approximated to be up-quark
M 120 350MeV.FortheIHlightneutrinomassesthe
mass matrix, which originates if the high scale symme- ζ1 ≃ −
resultsaresimilarasshownontherightpanelofFig.4. The
try is Pati-Salam or SO(10) GUT, SU(2) SU(2)
L R
× × branching ratios for lepton flavor violating decays predicted
SU(4) or SO(10) MR SM. Using running masses
C −→ inthisscenariowithMζ =(0.152,39.5,2426)GeVare
(m ,m ,m ) = (0.00233,1.275,160) GeV and Cabbibo-
u c t
Kobayashi-Maskawamixingmatrix,V [28], Br(µ e+γ)=3.6 10−13,
CKM
→ ×
Br(τ e+γ)=4.2 10−14,
→ ×
M (GeV) M =V Mˆ VT Br(τ µ+γ)=3.3 10−12, (24)
D ≃ u CKM u CKM → ×
0.067 0.004i 0.302 0.022i 0.55 0.53i while the present experimental limits at 90% C.L. on these
− − −
= 0.302 0.022i 1.48 0.0i 6.534 0.001i .(21) branching ratios are Br(µ e+γ) 1.2 10−11,
 − − −  Br(τ e+γ) 3.3 10−8→,andBr(τ ≤ µ+γ)× 4.4
0.55 0.53i 6.534 0.0009i 159.72+0.0i
→ ≤ × → ≤ ×
 − −  10−8 [27]. The projected reach of sensitivity in the future
 
isBr(τ e+γ), Br(τ µ+γ) 10−9 andspecifically
→ → ≤
At first using the phenomenologicalbounds from eqn. (17) Br(µ e+γ) 10−19[27].
→ ≤
andsaturatingouransatzforη = η /n,n=1,..5,we Thepredictednonstandardcontributionsto0νββtransition
αβ max
| |
searchformatrix M througheqn. (18)whichgivesµ from are shown in the left-panel for NH and in the it right-panel
S
6
1 10
1
0.1
0.1
|e |e
e 0.01 e
M Std M
| | 0.01 Std
η η
0.001 η/3 η/3
η/4 0.001 η/4
η/5 η/5
0.0001 0.0001
1e-05 0.0001 0.001 0.01 0.1 1 1e-05 0.0001 0.001 0.01 0.1 1
m (eV) m (eV)
1 1
FIG.4: Theeffectivemassparameter, |Mee|,predictionfor0νββ decayduetolight(Solidline)andpseudo-Dirac(dashedlines)neutrino
exchangewhereMD ≃Muandη=|ηmax|/n, n=1,3,4,5asdiscussedinthetext.
for IH case of Fig. 4. In view of the M -dependent en- tributions to 0νββ decay rates, even far exceeding the stan-
ζ1
hancementsof0νββ decayratesdiscussedaboveitistempt- dard contributions. The Dirac neutrino mass possibly origi-
ing to search for the possibility of the lightest pseudo-Dirac natingfromhighscalePati-SalamsymmetryorSO(10)grand
neutrinomasswhichweperformbythereplacementM2 unification,playsacrucialroleindeterminingdominantcon-
ζ1 →
M2 +iM Γ , whereΓ correspondstoplausiblevalueof tributionsto 0νββ decay ratessimultaneouslywith LFV de-
ζ1 ζ1 1 1
widthoftheparticle. Using,forexample,Γ 0.1keV,our cays with predicted branching ratios accessible to on going
1
predictionsarepresentedbysolidcurveinFig.≃5forNHlight search experiments. The underlying mechanism provides
neutrinomasseswheretheresonantbehaviorisclearlyexhib- three distinct platforms for its falsifiability (i) 0νββ decay
itedaroundM =190MeV. rates, (ii) determination of light pseudo-Diracneutrino mass
ζ1
M 120 500MeV,and(iii)thethreepredictedbranching
ζ1 ≃ −
ratiosofeqn.(24).Asallourresultsareapplicableinthecase
1
of inverse seesaw extended supersymmetric standard model,
Non-Std
theyarealsoconsistentwithgaugecouplingunificationatthe
0.1 MSSM-GUTscale,MU 2 1016 GeV.ThePati-Salamor
≃ ×
L SO(10)completionof themodeldiscussedin Sec. IV.B will
V
e bereportedelsewhereinfuturepublication[26].
H 0.01
Èe ACKNOWLEDGEMENT:RamLalAwasthiacknowledgesthe
e
M hospitalityatCenterofExcellenceinTheoreticalandMathe-
StdHNHL
È0.001 maticalSciences,SOAUniversitywherethepresentworkhas
beencompleted.
10-4
170. 180. 190. 200. 210. 220. 230.
M HMeVL
Ζ
1
∗ Electronicaddress: [email protected],[email protected],paridamk@soauni
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