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Conserving the lepton number $L_{e}-L_μ-L_τ$ in the exact solution of a 3-3-1 gauge model with right-handed neutrinos PDF

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Preview Conserving the lepton number $L_{e}-L_μ-L_τ$ in the exact solution of a 3-3-1 gauge model with right-handed neutrinos

Conserving the lepton number L L L in e µ τ − − the exact solution of a 3-3-1 gauge model with right-handed neutrinos 7 0 0 ADRIAN PALCU 2 n Departmentof Theoreticaland ComputationalPhysics -West a J Universityof Timis¸oara,V. PârvanAve. 4, RO -300223Romania 9 1 Abstract v Inthispaperweconsider aplausiblescenariowithconserved leptonnumber 66 L=Le−Lµ−Lτ withintheframeworkoftheexactsolutionofaparticular3- 3-1gaugemodel. Wediscusstheconsequencesofconservingthisgloballeptonic 0 symmetryfromtheviewpointoftheneutrinomassmatrixconstructedviaspecial 1 0 Yukawaterms(involvingtensorproductsamongHiggstriplets).Weprovethatthe 7 actual experimental data can naturally be reproduced by our scenario since soft 0 breakingtermswithrespecttothisleptonsymmetryareproperlyintroduced. Asa / consequence,oursolutionpredictsfortheneutrinosectorthecorrectmasssplitting h p ratio(∆m212/∆m223 ≃0.033),theinvertedmasshierarchy,thecorrectvaluesfor - theobservedmixingangles(sin2θ23 ≃0.5andsin2θ12 =0.31)andtheabsolute p massofthelightestneutrino(m0 ∼ 0.001eV)independentofthebreakingscale e ofthemodel. h PACSnumbers:14.60.St;14.60.Pq;12.60.Fr;12.60.Cn : v Keywords:neutrinomasses,3-3-1models,leptonnumber i X r 1 Introduction a In a recent series of papers (Refs. [1, 2, 3]), the author has developed an original methodforcontructingtheneutrinomassmatrixwithintheframeworkofaparticular 3-3-1 gauge model with no other restrictive additional symmetries (such as the lep- tonnumber). ApropertensorproductamongtheHiggstriplets- designedto recover (successivelythespontaneoussymmetrybreakdown)thewellknownmass-generating Yukawatermsintheunitarygauge-isexploitedwithintheexactsolutionofamodel basedonthegaugegroupSU(3) SU(3) U(1) thatdoesnotcontainparticles c L X ⊗ ⊗ withexoticelectriccharges.Herewemeanbytheexactsolution(seeforfurtherdetails thegeneralprescriptionsinRef.[4])aspecificalgebraicalmethodrelyingonanappro- priateparametrizationin thescalar sectorof themodel. Itprovidesuswith theexact masseigenstatesandmasseigenvaluesforthegaugebosonsandthecharges(boththe electric and neutralones) of all the involvedparticles, justby exactly solvingcertain equations. 1 ThechargedfermionsacquiretheirmassesthroughtraditionalYukawacouplings. Atthe same time, theyessentially determinethe textureof the neutrinomassmatrix, since the same coupling coefficient acts for both the charged lepton and its neutrino partner. All these results can be achieved [1] just by tuning a sole free remaining parameterinthemodel. Regardingtheneutrinosector,thepredictionsclaimedbythis methodinclude[2]: theinvertedmasshierarchy,thebi-maximalmixingand-forthe firsttimeintheliterature,asweknow-theminimalabsolutevalueintheneutrinomass spectrumm 0.0035eV. 0 ≃ Thepriceofdealingwithonlyonefreeparameterresidedinaverylargebreaking scaleofthemodeland,consequently,insomeveryheavynewgaugebosonsthathad largelyovertakentheir lower experimentalmass limit [5] (arround1TeVfor non-SM bosons).Inordertoimprovethephenomenologicalconsequencesregardingthebreak- ingscale,anewapproachwasproposed:acanonicalseesawmechanismcanariseinthe model,justbyalteringtheparametermatrixofthescalarsectorwithasmallamount(a fine-tuningparameter)[3]. Thissecondparameterallowsfordecouplingtheneutrino phenomenologyfrom the breaking scale issue (generatingthus reasonable masses in thebosonsector), whilekeepingunalteredallthe neutrinomassesandmixingangles achievedintheone-parameterversion[2]. Hereweintendtoimprovetheoriginalone-parametersolutionofthe3-3-1model ofinterest(brieflypresentetdinSec. 2)inadifferentway. Thistime,anewsymmetry istakenintoconsideration,namelythegloballeptonnumberL=L L L (Sec. e µ τ − − 3)anditsimplicationsforourmodelarediscussed. Then,somenewsmallparameters (α,β,γ)areintroducedinthespecialYukawatermsinordertosoftlybreakthisglobal leptonsymmetry.Whentheµ τ interchangesymmetryisinvokedtheparametersget − aparticularratio,namelyγ/β =m(µ)/(τ). Underthesecircumstances,thediagonal- izationofthemassmatrixleadstopredictionsingoodagreementwiththeexperimental values for mass splitting ratio and mixing angles. We also consider as a big success ofourscenariotheminimalabsoluteneutrinomasscomputedindependentofanypa- rameter,dependingonlyonthepreciseaccountforthemixingangles. Afewremarks concerningphenomenologicalaspectsofourmethodaresketchedinthefinalsection (Sec. 4). 2 Preliminaries Inordertogettotheveryessenceoftheresultingphenomenologyofintroducingthe globalleptonsymmetryL=L L L intothe3-3-1modelwithoutexoticelec- e µ τ − − triccharges,westartbypresentingtheparticlecontentofthismodelandtheoriginal mannertogenerateneutrinomassmatrixviaspecialYukawaterms. 2.1 Fermioncontent ofthemodel Thefemionsectorofthepureleft(subscriptL)3-3-1gaugemodelwithright-handed neutrinos(namelymodelD inRef. [6])consistsoftwo distinctsectors: leptonfami- liesandquarkfamilies. Alltheleptongenerationsobeythesamerepresentationwith respecttothegaugegroup: 2 e α f = ν (1,3∗, 1/3) ec (1,1,1) (1) αL  α  ∼ − αL ∼ νc α L   wheree =e,µ,τarethewellknownchargedleptonsfromSM,whileν =ν ,ν ,ν α α e µ τ are their neutrinopartners. We mean by superscriptc the chargeconjugationcarried outbytheoperatorC =iγ2γ0(formoredetailswerefferthereadertoAppendixBin Ref. [4]). Thequarkscome(accordingtotheanomalycancellationrequirementthatpreserves therenormalizability)inthreedistinctgenerationsasfollows: u d i Q = d (3,3,0) Q= u (3,3∗,1/3) (2) iL i   ∼   ∼ D U i L L     (d )c,(d )c (3∗,1,1/3) (u )c,(u )c (3∗,1, 2/3) (3) L iL L iL ∼ ∼ − (U )c (3∗,1, 2/3) (D )c (3∗,1,1/3) (4) L iL ∼ − ∼ with i = 1,2 and capital letters denoting exotic quarks. The numbers in brackets followingthe left-handedfermionfelds (Eqs. (1) - (4)) labelthe representationsand theircharacterswithrespecttothegaugegroupSU(3) SU(3) U(1) . Notethat c L X ⊗ ⊗ thethirdquarkfamilyhastoobeyadifferentrepresentationcomparedtotheothertwo families. ThemainphenomenologyoftheexactsolutionofthismodelcanbefoundinRef. [1]whereaspecialgeneralizedWeinbergtransformationisalsogiveninordertosep- aratetheneutralbosons(A -theelectromagneticone,Z - theWeinbergbosonfrom µ µ Standard Model, and Z′- the new neutral boson of the present theory) that get their µ exacteigenstates.Hereweareconcernedonlywiththeneutrinomassissuesincesome specialcouplingtermsareintroducedinthe Yukawasectoranda globalsymmetryis addedaswell. 2.2 The resulting neutrino massmatrix ThemassesofthefermionsinthemodelaregeneratedbytheYukawaLagrangian. In ourapproach[1]itlookslike: G f¯ (φ(ρ)ec +Sfc )+h.c. (5) αβ αL βL βL with S = φ−1 φ(η) φ(χ)+φ(χ) φ(η) (1,6, 2/3) and G as the cou- αβ ⊗ ⊗ ∼ − plingcoefficientsofthe leptonsector. The Yukawasectorreliesonthe scalartriplets φ(ρ) (1,3∗,2(cid:0)/3),φ(χ),φ(η) (1,3∗, 1(cid:1)/3) (see for details of constructingthe ∼ ∼ − Higgssectorin3-3-1modelswithnoexoticelectricchargesRef. [1]). Werecallthat (cid:8) (cid:9) theexactsolutionforthe3-3-1modelofinteresthereleadstotheone-parameterma- trix η = 1 η2 diag a/2cos2θ ,1 a,a 1 tan2θ /2 which determines − 0 W − − W (cid:0) (cid:1) (cid:2) (cid:0) (cid:1) (cid:3) 3 theVEVsalignmentintheHiggssector φ(i) = ηi φ ,i = 1,2,3(successivelythe h i SSB),accordingtothegeneralprescriptionsofthemethodshowninSec. 4.1ofRef. (cid:10) (cid:11) [4]. Obviously,θ istheWeinberganglefromSM. W NotethattheHiggsfieldφplaystheroleofthe”norm”forthegeometrizedscalar sector and it is therefore a main factor in all the VEVs. It is appropriate for one to consider it as the homologueof the neutral scalar field of the Standard Model, since their vacuumexpectationvaluessupplythe massesfor the particle of the modelthey actin. ThechargedleptonsgettheirmassesviatraditionalYukawacouplingsasonecan easily observe in Eq.(5): m(e) = A φ(ρ) , m(µ) = B φ(ρ) , m(τ) = C φ(ρ) . Obviously, G = A, G = B, G = C, but we specify in advance that our ee µµ ττ (cid:10) (cid:11) (cid:10) (cid:11) (cid:10) (cid:11) notationsinclude: G = D, G = E, G = F forthe off-diagonalterms in the eµ eτ µτ flavorbasis. Neutrino mixingis expressedby ν (x) = 3 U ν (x), where α = e,µ,ν αL i=1 αi iL labelthe flavor space (flavor eigenstates)while i = 1,2,3denotethe massive physi- P caleigenstates. WeconsiderthroughoutthepaperthephysicalneutrinosasMajorana fields,i.e. νc (x)=ν (x). TheneutrinomasstermintheYukawasectoryieldsthen: iL iL 1 = ν¯ Mνc +H.c (6) −LY 2 L L T withν = ν ν ν wherethe superscriptT standsfor”transposed”. The L e µ τ L mixing matrix U that diagonalizesthe mass matrix in the mannerU+MU = m δ ij j (cid:0) (cid:1) hasinthestandardparametrizationtheform: c c s c s e−iδ 2 3 2 3 3 U = s c c s s eiδ c c s s s eiδ c s (7) 2 1 2 1 3 1 2 2 3 1 3 1  − − −  s s c c s eiδ s c s s c eiδ c c 2 1 2 1 3 1 2 2 3 1 3 1 − − −   with natural substitutions: sinθ = s , sinθ = s , sinθ = s , cosθ = c , 23 1 12 2 13 3 23 1 cosθ =c ,cosθ =c forthemixingangles,andδfortheCPphase. 12 2 13 3 Ourprocedureprovides(aftertheSSB)thefollowingnewsymmetricmassmatrix fortheneutrinosinvolvedinthemodel: A D E φ(η) φ(χ) M =4 D B F (8)   φ E F C (cid:10) h(cid:11)(cid:10)i (cid:11)   Thenexttaskistodiagonalizethematrix(8)inordertogetthephysicaleigenstates ofthemassiveneutrinos.Thisprocedurewilllead(withintheCase1inRef.[1],which istheonlyacceptableonefromthephenomenologicalpointofview)tothefollowing genericsolution: 2a 1 2sin2θ m =f [θ ,θ ,θ ,m(e),m(µ),m(τ)] − W (9) i i 12 23 13 (cid:18)√1−a(cid:19)p cos2θW with i = 1,2,3. In these expressions f s are analytical functions depending on the i mixinganglesandthechargedleptonmassesinaparticularwaytobedetermined. 4 = 3 Conserving the lepton number L L L L e µ τ − − InthefollowingweassumethattheleptonnumberL = L L L isconserved e µ τ − − (orapproximatelyconserved,assuggestedbyalotofpapersconcerningthisissue[7] -[21])inthe3-3-1modelpresentedabove. Wefirstanalyzetheconsequencesofthis globalsymmetryfor the neutrinosector andthenthe modificationsprovidedby soft- breaking terms with respect to this symmetry. The modifications occur in the mass matrixduetosomenewsmallparametersintroducedintheYukawasector. 3.1 Neutrino massspectrum Atthispoint,ifoneimposesfortheleptonsectoranadditionalglobalsymmetrygiven bytheleptonnumberL=L L L ,certaincouplingcoefficientsintheYukawa e µ τ − − Lagrangiangetsuppresed,namelyA=B =C =F =0,sincethefollowingassigne- mentfortheleptonnumberholds:L(f )=L(e )=1,L(f )=L(µ )= 1and eL R µL R − L(f )=L(τ )= 1,whileallthescalarfieldscarryzeroleptonnumber. τL R − Hence,themassmatrixwiththeexactglobalLsymmetrybecomes: 0 D E 2a 1 2sin2θ M = D 0 0 − W φ (10)  E 0 0 (cid:18)√1−a(cid:19)p cos2θW h i   Theconcreteformsoff s remaintobe computedbysolvingthefollowingsetof i equations: f = 2c c s D+2s c s E 1 1 2 2 1 2 2 − 0= c s2D+c s2D s c2E+s s2E  − 1 2 1 2 − 1 2 1 2  0=cf1c222=D002==−c1sccc1122ssss22212DDDD+++−scc2121sscs11222cEEE2s−2Es1c22E (11) 0=s c D+c c E obtainedstraightforwardlyfromE0q=. (s1110sf)223vD=ia+U0c+11Ms22UE =diag(m1,m2,m3). Thelines 3, 6, 7 and 8 in Eqs. (11) express the same condition, namely: D = Ecotθ . 23 − Thelines2and4inthesetofequations(11)arefulfiledsimultaneouslyifandonlyif cos2θ =sin2θ (maximalsolarmixingangle). 12 12 Under these circumstances, taking into consideration the maximal atmospheric mixingangletoo,thesolutionreads: 2a 1 2sin2θ m = m =√2D − W φ (12) 1 | 2| (cid:18)√1−a(cid:19)p cos2θW h i m =0 (13) 3 5 givingrisetoaµ τ interchangesymmetry[22]-[30]. − 3.2 Introducing softbreaking terms We exploit in the following the consequencesof assuming some soft breaking terms withrespecttothegloballeptonnumberL,byintroducingthefreeparametersα,β,γ intheYukawaLangrangian: G f¯ (φ(ρ)ec +αSfc )+h.c. ee eL L eL G f¯ (φ(ρ)µc +βSfc )+h.c. (14)  µµ µL L µL  Gττf¯τL(φ(ρ)τLc +γSfτcL)+h.c. Obviously,α, β, γ [0,1). A small nonzeroF = βGµτ couplingcoefficientis also ∈ allowed.Now,themassmatrixoftheneutrinosectorstands: αA D D 2a 1 2sin2θ M = D βB βF − W φ (15)  D βF γC (cid:18)√1−a(cid:19)p cos2θW h i   InordertoremaininthespiritoftheleptonsymmetryL = L L L which e µ τ − − alsofavorstheinterchangesymmetryµ τ,onecanapproximate: − βB =γC (16) ∼ Hence,oneofthethreenewparametershasdisappeared. Thetworemainingones finally will have to fulfil a particular ratio in order to be compatible with the phe- nomenologicaldata, so one remains with only one parameter to be tuned apart from themainone(ainourmodel). Thenewresultingmassmatrix αA D D 2a 1 2sin2θ M(ν)= D βB βF − W φ (17)  D βF βB (cid:18)√1−a(cid:19)p cos2θW h i   isnowtobediagonalized.ThistaskwasalreadycarriedoutinRef. [2]forthegeneral case,nowwejusthavetoinsertthenewdiagonalentriesinthesolutionobtainedthere (Eqs. (13)inRef. [2])andtoimposethemaximalatmosphericmixingangleatleast inthenumerators,whilededenominatorswillbecomputedinthefinalstageofsolv- ingthemassissuebyinsertingasuitableapproximationforthemaximalatmospheric mixingangle.Hence,aninterestingmasssplitisobtained: sin2θ 1 sin2θ f = βB 12 +αA − 12 , 1 − 1 2sin2θ 1 2sin2θ 1 2sin2θ 23 12 (cid:0) 12(cid:1) − − − (cid:0) (cid:1)(cid:0) (cid:1) (cid:0) (cid:1) sin2θ sin2θ 12 12 f =βB +αA , 2 1 2sin2θ 1 2sin2θ 1 2sin2θ 23 12 12 − − − (cid:0) (cid:1)(cid:0) (cid:1) (cid:0) (cid:1) f =βB. (18) 3 6 Evidently,itisrequiredaγ5transformationperformedonthefirstneutrinofieldin ordertogetthesignchangeforitsmass(m ). 1 Themassspectrumintheneutrinosectorbecomes: βm(µ)sin2θ αm(e) 1 sin2θ 2a 1 2sin2θ m = 12 − 12 − W, | 1| " 1−2sin2θ23 1−2sin2θ12 − 1−(cid:0)2sin2θ12 (cid:1)#(cid:18)√1−a(cid:19)p cos2θW (cid:0) (cid:1)(cid:0) (cid:1) (cid:0) (cid:1) βm(µ)sin2θ αm(e)sin2θ 2a 1 2sin2θ 12 12 W m = + − , 2 " 1−2sin2θ23 1−2sin2θ12 1−2sin2θ12 #(cid:18)√1−a(cid:19)p cos2θW (cid:0) (cid:1)(cid:0) (cid:1) (cid:0) (cid:1) 2a 1 2sin2θ m =βm(µ) − W (19) 3 (cid:18)√1−a(cid:19)p cos2θW 3.3 Phenomenological predictions The physical relevant magnitudes for the neutrino oscillations are the mass squared differencesfor solar and atmosphericneutrinos, defined as: ∆m2 = m2 m2 and 12 2 − 1 ∆m2 =m2 m2. Theyresultfromtheaboveexpressions(Eqs. (19)): 23 3− 2 m(e)m(µ)sin2θ 4a2 1 2sin2θ ∆m2 =2αβ 12 − W (20) 12 ∼ 1−2sin2θ12 2 1−2sin2θ23 (cid:18)1−a(cid:19)(cid:18) cos4θW (cid:19) (cid:0) (cid:1) (cid:0) (cid:1) m2(µ)sin4θ 4a2 1 2sin2θ ∆m2 =β2 12 − W (21) 23 ∼ 1−2sin2θ12 2 1−2sin2θ23 2 (cid:18)1−a(cid:19)(cid:18) cos4θW (cid:19) Themasssp(cid:0)littingratiode(cid:1)fin(cid:0)edasr =∆m(cid:1)2 /∆m2 yieldsinourscenario: ∆ 12 23 α m(e) 1 2sin2θ r =2 − 23 (22) ∆ β m(µ) sin2θ (cid:18) (cid:19)(cid:18) (cid:19)(cid:18) 12 (cid:19) As inallthe caseswhereourmethodofgeneratingneutrinomassesis employed, the mass splitting ratio comes out independently of the breaking scale in the theory. The latter is determined by the parameter a which is not involved in formula (22). Once again, we have got an important conclusion: the breaking scale of the model (andconsequently,thebosonmassspectrum)andtheneutrinophenomenologydonot influenceoneanother(thesamehappensinRef. [3],butadifferentstrategybasedon a seesaw mechanismwas employedtherein). Therefore,as soon as new data regard- ingtheprecisionmeasurementsforthemassofthenewneutralboson(otherthanthe WeinbergbosonfromSM)areavailable,onecanestablishtheparametera. Asfaras we know [5], the lower limit is m(Z′) 1.5TeV, that claims in our solution for the ≥ 3-3-1mode:la 0.06and φ 1TeV[1,3]. ≤ h i≥ The next step is to implement the specific values for the atmospheric and solar mixinganglesintoformula(22)andestimatehowtheycaninfluencethemasssplitting 7 ratio. For instance, if the plausible sin2θ = 0.499 and sin2θ = 0.31 are taken 23 12 into account, and assuming the charged lepton masses [5] m(e) = 0.511 MeV and m(µ)= 106MeV,oneobtainesr 0.033(veryclosetothevaluesuppliedbydata) ∆ ≃ if α =530.5 (23) β Thisestimate suggeststhat the addedmatrix responsableforthe softviolationof the initialconservedLsymmetrycanbeactuallyconsideredasasmallperturbationofthe form:δM =εdiag(1,1,1). Themethodpresentedaboveallowsonetoestimatethesumoftheabsolutemasses intheneutrinosector: 3 2βm(µ)sin2θ 2a 1 2sin2θ m 12 − W (24) Xi=1 i ≃ 1−2sin2θ12 1−2sin2θ23 (cid:18)√1−a(cid:19)p cos2θW Thisisexpe(cid:0)rimentallyrest(cid:1)ri(cid:0)ctedto: 3 m(cid:1) 1eV,ifwetakeintoconsideration i=1 i ∼ theTroitsk[31]andMainz[32,33]experiments. Ontheotherhand,combiningEqs. P (19)and(23)oneobtaines: 3 2sin2θ m = 12 m (25) i 1 2sin2θ 1 2sin2θ 0 Xi=1 − 12 − 23 withminimalneutrinomassm(cid:0)0 = m3 . This(cid:1)le(cid:0)ads(withthea(cid:1)boveconsideredvalues formixingangles)tom 0.001eV. 0 ≃ 4 Concluding remarks Inthispaperwehavedevelopedastrategythatcombinestheexactsolutionofapartic- ular3-3-1gaugemodelwithpossiblegloballeptonicsymmetryL = L L L . e µ τ − − Whenthisadditionalsymmetryisrigourouslyconserved,theneutrinomassspectrum exhibitsan invertedhierarchywith two degeneratemasses and the third one equalto zero.Atthesametimethissymmetryrestrictsthemixinganglestothebi-maximalset- tingonly. Thisstateofaffairscanbenaturallyovertakenifonedealswiththeapprox- imate globalleptonicsymmetry,by introducingsomesmall termsthatsoftly violates thissymmetryintheYukawasector. Theresultsareamazing: thecorrectpredictions regardingthemasssplittingratio∆m2 /∆m2 0.033andtheobservedvaluesfor themixinganglessin2θ 0.5ands1i2n2θ 2=3 ≃0.31ariseindependentlyofthemain 23 12 ≃ parameter a in the 3-3-1 model of interest. At the same time, the minimal absolute mass in the neutrino sector can be computed based on an exact formula depending onlyontheaccurateaccountforthemixingangles.Inourapproximation,thescenario leadstotheminimalmassintheneutrinospectrum: m(ν ) = 0.001eV,butthisorder 3 ofmagnitudecandecreaseiftheatmosphericanglegetscloser-to-maximalvalues. All the results regardingthe mass squared differences and the mixing angles are achieved just by tuning a second small parameter, let it be α or β. There are many 8 advantagesofthepresentmethodoverthepreviousonesfollowedbytheauthorinRefs. [2]and[3]respectively.Intheone-parametercase[2]withnoleptonnumberconserved andnoµ τ interchangesymmetry,theneutrinophenomenologyrequiredaverylarge − breakingscale and a radiativemechanismwas set to supplypossible deviationsfrom the bi-maximalmixing. In Ref. [3] a second parameterwas introduced(that time in theHiggssector)accompaniedbyseekforacanonicalseesawinterpretationinorderto getaconsiderableautonomyfortheneutrinophenomenologyfromthebreakingscale issue. Here, althougha second parameteris introducedtoo, there is no need for any seesaw mechanism. Instead, finally a µ τ interchange symmetry remains. Along − with all the old valuablepredictionssuppliedby the exactsolutionof ourmodel, the approachbasedonthissecondparameterexplainestheneutrinomassphenomenology and, in addition, successfully passes the difficult challenge of predicting the correct solarandatmosphericmixingangles. References [1] A.Palcu,Mod.Phys.Lett.A21,1203(2006). [2] A.Palcu,Mod.Phys.Lett.A21,2027(2006). 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