Neutrino Masses from TeV Scale New Physics -- Tests of Neutrino Masses at the LHC Mu-Chun Chen, University of California at Irvine GGI What’s Nu?, June 26, 2012 Theoretical Challenges (i) Absolute mass scale: Why mν << mu,d,e? • seesaw mechanism: most appealing scenario ⇒ MajoranaSeesaw model has been previously shown [11] to induce a non-unitary leptonic mixing matrix. In this work we will explicitly analyze the issue for the other types of Seesaw • UV completions of Weinberg operators HHLL models. φ φ µ ‣ Type-I seesaw: exchange of singlet fermions φ φ ∆ φ φ Minkowski, 1977; Yanagida, 1979; NR ΣR † Y † Y Glashow, 1979; YN N ∆ YΣ Σ Gell-mann, Ramond, Slansky,1979; NR: SU(3)c x SU(2)w x U(1)Y ~(1,1,0) Mohapatra, Senjanovic, 1979; ! ! ! ! Seesaw model has been previously shown [11] to induce a non-unitary leYpt∆onic mixing matrix. In this work we will explicitly analyze the issue for the ot!her types of See!saw models. Figure 1: The three generic realizations of the Seesaw mechanism, depending on the φ φ ‣ Type-II seesaw: exchange of weak triplet scalar nature of the heavy fields exchanged: SM singlet fermions (type I Seesaw) on the left, µ∆ φ φSM triplet scalars (type II Sφeesaw) and SM tripleφt fermions (type III Seesaw) on the Lazarides, 1980; Mohapatra, Senjanovic, 1980 NR right. ΣR YN† YN ∆ YΣ† YΣ SΔee:s aSU(3)w modce lxh SU(2)as beenw pxr eU(1)viousYl y~(1,3,2)shown [11] to induce a non-unitary leptonic mixing ! ! ! ! matrix. In this work we will explicitly analyze the issue for the othYe∆r types of Seesaw ‣ Type-III seesaw: exchangem ofode lweaks. triplet fermion ! ! Figure 1:φThe three genφeric realizations of the Seesaw mechanism, depending on the Foot, Lew, He, Joshi, 1989; Ma, 1998 µ φ φnature of the hea∆vy fields exchanφged: SM singlet fermionφs (type I Seesaw) on the left, SM triplet scalars (type II Seesaw) and SM triplet fermions (type III Seesaw) on the NR right. ΣR ΣR: SU(3)YN† c x SU(2)YNw x U(1)Y ~(1,3,0)∆ YΣ† YΣ ! ! ! ! Y∆ ! ! Mu-Chun Chen, UC Irvine F i g u r e 1Testing: The tNeutrinohree gen Masseseric rea latiz athetio nLHCs of t h e S e e s a w m e c h a n i s m , d e p e n d i n g o n t GGI,he 06/26/2012 2 nature of the heavy fields exchanged: SM singlet fermions (type I Seesaw) on the left, SM triplet scalars (type II Seesaw) and SM triplet fermions (type III Seesaw) on the right. 6 6 6 Theoretical Challenges For a recent review on TeV scale seesaw: M.-C. C., J.R. Huang, arXiv:1105.3188 (i) Absolute mass scale: Why mν << mu,d,e? • seesaw mechanism: most appealing scenario ⇒ Majorana • can originate from GUT scale Physics: • indirect probe through LFV processes at colliders • seesaw scale can also be at TeV (if yukawa ~ 10-6 allowed) • type II, III, inverse seesaw, ..... • TeV scale new physics ⇒ Dirac or Majorana • extra dimension: through small wave function overlap • associated phenomenology in extra dimension [Talk by Renata Zukanovich-Funchal] • extra U(1)’ gauge symmetry • associated Z’ phenomenology • Discrete R-Symmetries • simultaneous solution to mu problem and small Dirac mass Mu-Chun Chen, UC Irvine Testing Neutrino Masses at the LHC GGI, 06/26/2012 3 Theoretical Challenges (ii) Flavor Structure: Why neutrino mixing large while quark mixing small? • seesaw doesn’t explain entire mass matrix w/ 2 large, 1 small mixing angles • family symmetry: there’s a structure, expansion parameter (symmetry effect) • mixing result from dynamics of underlying symmetry • if symmetry breaking at TeV ⇒ signatures at colliders • with SUSY: superpartners charged under family symmetry, can probe (indirectly) flavor sector even for high symmetry breaking scale Mu-Chun Chen, UC Irvine Testing Neutrino Masses at the LHC GGI, 06/26/2012 4 m = 0 ν ⇤ yD, mν 0 mν = 0 ⇥ Type-I Seesaw⇤ at Colliders Minkowski, 1977; Yanagida, 1979; Glashow,Seesaw 1979;mode l has been previously shown [11] to induce a non-unitary leptonic mixing Gell-mann,matrix. IRamond,n this wo Slansky,1979;rk we will exp licitly analyze the issue for the other types of Seesaw MR 100 GeV Mohapatra,models. Senjanovic, 1979; yD, mν 0 ∼ ⇥ φ φ • assuming no new interaction: small neutrino mass from µ φ φ ∆ φ φ mν = 0 m4 ν = 0 MR 100 GeV mD me 10− GeV⌅ N ∼ ⌅ R ΣR ∼ ∼ YN† YN ∆ YΣ† YΣ Introduc•tion same levelCancella tofions & S“un-naturalness”ymmetries Colliders if smallCo ncelectronlusions Yukawa allowed 4 y , m 0 yD, mν 0 ! ! Y ! ! mD me 10− GeV D ν ⇥ ∆ Elec•trRHowe neutrinoak∼-Scale∼S mayingle tbes within reach⇥ of LHC ! ! NR: SU(3)c x SU(2)w x U(1)Y ~(1,1,0) Figure 1: The three generic realizations of the Seesaw mechanism, depending on the • OnlyW wayhat if m toR ∼ test100 G eseesawV? is by producing RH neutrinos nature of the heavy fields exchanged: SM singlet fermions (type I Seesaw) on the left, −4 MR 100 GeV MR 100 GeV mD ∼ 10 GeV = 100 keV ∼ me SM triplet scalars (type II Seesaw) and SM triplet fermions (type III Seesaw) on the ! ∼ ∼ • YukawaNot tota ll~y uO(10nreason-6a):bl eirrelevant for colliders right. ⇒ RH neutrinos may be within reach of LHC and ILC Yukawa couplings tiny ⇒ irrelevant for colliders 4 4 • RH neutrino production:mD m gaugee 10 interaction− mGeVD me through10− GeVheavy-light mixing Gauge interactions via mixing, e.∼g. ∼ ∼ ∼ l− − 4 4 −m1 10 4 G10eV − GeV−6 ∝ V = mDmR ∼D = 10 mD 610− GeV 6 W V = 100 GeV V = = 10− = 10− N ⇤ MR ∼ 100⇤ GeV MR ∼ 100 GeV Observation at colliders needs V " 0.01 Han, Zhang, PRL 97 (2006); del Aguila, Aguilar-Saavedra, Pittau, J. Phys. Conf. Ser. 53 (2006); Bray, Lee, Pilaftsis, hep-ph/0702294 Han, Zhang, 06; del Aguila, Aguila-Saavedra, • Observable⇒ no way? at colliders: require mixing V > 0.01 Pittau, 06; Bray, Lee, Pilaftsis, 07 Mu-Chun Chen, UC Irvine Testing Neutrino Masses at the LHC GGI, 06/26/2012 5 6 1 1 1 1 If the heavy neutrinos are to be observable at the LHC or the ILC, their mixing angles must not lie far below the upper limit (7) [6,9,10,12,14]: V ! 0.01 . (8) | αi| Using this value, we obtain from Eq. (5) a contribution to the light neutrino mass V 2 M m(i) V 2M = 107 eV | αi| i . (9) ν ∼ | αi| i ! 0.01 " !100 GeV" Thus, to reconcile mν 0.1 eV with the observability of RH neutrinos at the LHC or the ILC, one needs to arran∼ge a cancellation between the contribution from a given RH neutrino and some other contribution at the level of 10−8. The situation improves only slightly if one considers more advanced machines like CLIC or an eγ collider, which could increase the reach in the mixing angle by about an order of magnitude compared to Eq. (8) [7,8,10]. In what follows we will discuss cancellations between the contributions from different RH neutrinos, i.e. we will stay within the framework of the type-I seesaw scenario. One could also consider a cancellation wimthνco=nt0ributions from other mechanisms, for example involving a Higgs triplet (type-II seesaw [⌅40–43]), a fermion triplet (type-III seesaw [36,37]) or a radiatively generated neutrino mass [44,45]. However, in these cases contributions from different, in general unrelated souyrDce,s mhaνve to0cancel, which looks extremely implausible. The left-right symmetric models have bee⇥n suggested as an exception, since there the type-I and type-II seesaw contributions can be related [46]. M 100 GeV R ∼ 2.2 Cancellation of Light Neutrino Masses 4 Let us consider first the necmesDsary maned su10ffi−cienGeVt conditions for an exact cancellation of contributions to the light neutrin∼o mass∼es. In the case of two RH neutrinos, two matrices have to cancel, 4 mD 10− GeV 6 Type-I Seesawm(1) + m (2at) = 0Colliders. V = = 10− (10) ν ν ⇤ MR ∼ 100 GeV Together with Eq. (2) this implies [17,19,20] proportionality of the vectors m" i, • Neutrino mass get contributions from different singlet fermionsT m" 1 = y1m" 0 , m" 2 = y2m" 0 V (>m" 0.01m (1, α, β) ) , (11) If the heavy neutrinos are to be observa≡ble at the LHC or the ILC, their mixing angles • neutrino massandm smallust no NOTt lie fduear b etolow seesaw,the upp ebutr li mcancellationit (7) [6,9,10 among,12,14]: these contributions Buchmuller, Wyler ‘90; Pilaftsis, ‘92 • universality of weaky2 interactiony2 & Z-width: 1 V+αi !2 0=.001 . V < 0.1 (12(8)) M | M| • cancellation atU 10sin-8g1 tlevelhis v 2atolu eget, w e0.1ob teVain neutrinofrom Eq. mass(5) a contribution to the light neutrino mass Therefore, the Dirac mass matrix has the form 2 (i) 2 7 Vαi Mi m Vyαi Myi = 10 eV | | . (9) ν ∼ | 1| 2 ! 0.01 " !100 GeV" mD = m αy1 αy2 . (13) m . Thus, to reconβcyil1e βνy2 0 1 eV with the observability of RH neutrinos at the LHC or the • with 3 singlets:IL Clight, on eneutrinoneeds to amassesrran∼ge a vanishcancella ifti oandn be onlytwee nifthe contribution from a given RH neutrino This result can be generalised to the case o−f 8three neutrinos [18, 21, 22]. The light • Dirac massa matrixnd som ehasoth ranker co n1tribution at the level ofBuchmuller,10 .