Models of neutrino mass generation provide well motivated scenarios of
Beyond-the-Standard-Model physics. The synergy between low energy and high
energy LHC searches facilitates an effective approach to rule out, constrain or
ideally pinpoint such models. In this proceedings report, we provide a brief
overview of scenarios where searches at the LHC can help determine the
mechanism of light neutrino masses and potentially falsify baryogenesis
mechanisms.Comment: Talk presented at CIPANP2015. 9 pages, 3 figure

Deppisch, Frank F.

English

arXiv

Models of neutrino mass generation provide well motivated scenarios of Beyond-the-Standard-Model physics. The synergy between low energy and high energy LHC searches facilitates an effective approach to rule out, constrain or ideally pinpoint such models. In this proceedings report, we provide a brief overview of scenarios where searches at the LHC can help determine the mechanism of light neutrino masses and potentially falsify baryogenesis mechanisms

Deppisch, F

UCL Discovery

CIPANP2015-DeppischProbing Leptonic Models at the LHCFrank F. DeppischDepartment of Physics and AstronomyUniversity College London, London WC1E 6BT, United KingdomModels of neutrino mass generation provide well motivated scenariosof Beyond-the-Standard-Model physics. The synergy between low energyand high energy LHC searches facilitates an effective approach to rule out,constrain or ideally pinpoint such models. In this proceedings report, weprovide a brief overview of scenarios where searches at the LHC can helpdetermine the mechanism of light neutrino masses and potentially falsifybaryogenesis mechanisms.PRESENTED ATTwelfth Conference on the Intersections ofParticle and Nuclear Physics (CIPANP 2015)Vail Colorado, USA, May 19–24, 2015arXiv:1510.01302v1  [hep-ph]  5 Oct 20151 IntroductionWith the discovery of the Higgs boson at the LHC and the determination of itscouplings to fermions, we are tantalizingly close to verifying the mechanism of chargedfermion mass generation. What will remain missing though is an understandingof the light neutrino masses. The observation of neutrino oscillations shows thatneutrinos have finite masses and that individual lepton flavour is violated. Neutrinosare also usually considered to be Majorana particles, an assumption that facilitatesan understanding of their small masses. It is natural to expect that the violationof the individual lepton flavours and, in the case of Majorana neutrinos, the totallepton number will show up in other contexts as well. This for example includes rarelepton flavour violating (LFV) decays of muons and taus and the total lepton numberviolating (LNV) neutrinoless double beta (0νββ) decay.Quite generally, the possible violation of lepton flavour/number should be searchedfor at all energies that are experimentally accessible. This is because the observation ofsuch processes would equally allow us a direct insight into the mechanism of neutrinomass generation. The most popular example is the so called seesaw mechanism (oftype I) in which heavy right-handed Majorana neutrinos N with masses & 1011 GeVare added to the Standard Model (SM). Their Yukawa coupling with the left-handedneutrinos induces the light Majorana masses of light neutrinos after electroweak (EW)symmetry breaking. This motivates the lightness of neutrinos through the breakingof lepton number symmetry at a very high scale [1].Despite its popularity, the default type-I seesaw mechanism has two major phe-nomenological issues: (i) In the expected regime with mN & 1011 GeV, the heavyneutrinos are far too heavy to be probed experimentally; (ii) The heavy neutrinosare sterile, i.e. gauge singlets and they only interact through a small mixing withlight neutrinos. In this short proceedings report, we will briefly review two scenariosthat instead include TeV scale and potentially non-singlet neutrinos and which canbe probed at the LHC. In addition, we will comment on the general impact of theexperimental observation of LNV on baryogenesis models.2 Inverse SeesawIn the standard type-I seesaw model with the (one generation) mass matrix for theleft- and right-handed neutrino, (0 mDmD mN), (1)the mass of the light neutrino ν and its mixing θ with the heavy neutrino N is givenby mν = −m2D/mN and θ = mD/mN =√mν/mN , respectively. Here, mD is the110-7 10-4 0.1 100 105 108 101110-1210-810-41104108ΜS HGeVLNeutrinoMassHGeVLDmatm2M1M3M2ΜR = ΜS100 101 102 10310-310-210-1100101102103MN (GeV)μ S(keV)ΓN/ΔMN≈0.1 11010-210-4|VℓN 2=10-6Figure 1: Left: Spectrum of neutrino mass eigenstates as a function of the LNVparameter µS = µR with mN = 100 GeV and mD = 1 GeV. The horizontal linedenotes the desired light neutrino mass scale mν ≈ 0.05 eV for the lightest state.Right: Contours of the ratio of the average decay width of sterile heavy neutrinos totheir mass splitting ≈ µS in the inverse seesaw model. The LNV signal is expected tobe unobservably small in the red shaded region with ΓN/∆mN & 10. The horizontalblue lines denote contours of constant active-sterile mixing |VlN |2. Taken from [4].neutrino Dirac mass, expected to be of the order of the EW scale, and mN is anLNV Majorana mass of the right-handed. For an observed light neutrino mass scalemν ≈ 0.1 eV this yields θ ≈ 10−5√GeV/mN . For a GeV to TeV scale heavy neutrinothe mixing is rather small. This will be very different in the inverse seesaw scenario[2] described by the mass matrix 0 mD 0mD µR mN0 mN µS , (2)similarly for the left-handed neutrino, the right-handed neutrino and an additional SMgauge singlet state S. Due to the presence of the small lepton number violating massparameters µR and µS, light neutrino masses are achievable for any θ = mD/mN [3];in the simplest inverse scenario with µR = 0 one has θ ≈ 10−2√keV/µS. The reasonfor this suppression can be understood as the two heavy neutrino states formed by Nand S have opposite CP parities and they combine to form quasi-Dirac neutrinos witha fractional mass splitting of order µS/mN . All lepton number violating observables,such as the light neutrino mass, will be suppressed by this small mass splitting.In order to see the transition between standard and inverse seesaw, we chooseµS = µR in eq. (2). Fixing the other terms as mN = 100 GeV and mD = 1 GeV,Fig. 1 (left) shows that successful light neutrino mass generation occurs for µ ≈210−6 GeV corresponding to the inverse seesaw and µ ≈ 106 GeV corresponding to thenormal high-scale seesaw. The inverse case contains two heavy Majorana neutrinoswith masses mN±µS constituting a quasi-Dirac state, whereas for large µ mN GeVtwo heavy quasi-degenerate Majorana neutrinos with masses µ±mN are formed.3 Heavy Sterile NeutrinosA large number of laboratory searches put constraints on the mixing between sterileand active neutrinos: For mN  1 MeV, sterile neutrinos are being probed in neutrinooscillation experiments. For pure Majorana sterile neutrinos, 0νββ searches providestringent constraints on the mixing with electron neutrinos [5, 6], but these limits areconsiderably weakened for quasi-Dirac neutrinos such as found in the inverse seesawmechanism discussed above. For 1 MeV . mN . 1 GeV, the active-sterile mixing isconstrained by peak searches in leptonic decays of pions and kaons and in beam dumpexperiments. A more coherent overview of experimental searches for sterile neutrinoscan be found in the recent review [4].Regarding LNV at high energy colliders, a general observation can be made inscenarios with approximately conserved lepton number like the above inverse seesawmechanism: Like any LNV observable, the rate of an LNV process will be suppressedby the small mass splitting, but for on-shell resonant production of a heavy neutrino,the suppression is with respect to the neutrino width, ∆mN/ΓN , rather than theabsolute mass or the energy scale of the process. For ∆mN ≈ ΓN it can be resonantlyenhanced [7]. The effect of the suppression is shown in Fig. 1 (right) giving contoursof ΓN/∆mN as a function of the inverse seesaw parameters mN and µS. Within theshaded region the suppression would be too severe to expect an LNV observation.For mN & 100 GeV, one would require a small sterile-active mixing |V`N |2 . 10−4which suppresses the LNV rate as well.In the specific context of the LHC, a Majorana heavy neutrino leads to a LNVsignature with two same-sign leptons plus jets and no missing energy: pp→ W (∗) →N`± → `±`±jj [8, 9]. The CMS and ATLAS collaborations have performed directsearches for the production of heavy neutrinos limiting the mixing to active neutrinos|Ve(µ)N |2 . 10−2 − 10−1 for mN . 500 GeV at√s = 8 TeV [10]. During the ongoingrun II of the LHC, the limits could be improved to apply to about a TeV. In addi-tion to the basic s-channel production, it is also very worthwhile to consider otherproduction modes and decay scenarios: Electroweak t-channel processes of the formpp→ W ∗γ∗ → N`±jj can for example give a better sensitivity for higher mN values.Furthermore, searches for displaced vertices can considerably improve the sensitivityfor heavy neutrinos lighter than the W boson [11].31 2 3 4 50123MWR (TeV)MN(TeV) Br(μ→e )=5.7⨯10-13γBr(μ→eee)=10-12(M Δ-- =MWR)LHC5σ 90%LHCexcl.T0νββ =2⨯1025yT0νββ =2⨯1026y0.5 1.0 1.50.00.51.01.52.02.53.0MN [TeV]sinθ LRN [10-3 ] eejjBr(WR→eN)=7% 5% 3% 1%Br(N→eqq)=5% 10% 20% 30%40%50%60%Figure 2: Left: Sensitivity of the LHC to production and decay of heavy neutrinos viaright-handed currents in the manifest LRSM. The solid blue contours give the signalsignificance of 5σ and 90% at the LHC with 14 TeV and L = 300 fb−1. The greenand red contours show the sensitivity of 0νββ and LFV searches as denoted. Right:Fitting the CMS eejj excess (red band) in the parameter plane of the heavy neutrinomass MN and the light-heavy mixing sin θNLR. The other LRSM parameters are chosenin concordance with other excesses at 2 TeV as described in the text. Taken from[15].4 Left-Right SymmetryOne of the simplest options to extend the above sterile neutrino scenario is an addi-tional, broken U(1)′ gauge symmetry under which the heavy neutrinos are charged.Under favourable parameter conditions, heavy neutrinos can then be pair-producedabundantly and be probed even for very small mixing with the active neutrinos [12].Another popular option are left-right symmetric models (LRSMs); the minimal LRSMextends the SM gauge symmetry to SU(2)L × SU(2)R × U(1)B−L [13]. Leptons areassigned to doublets L = (ν, `)L and R = (N, `)R under SU(2)L and SU(2)R, re-spectively. The Higgs sector of the minimal LRSM consists of a bidoublet and twotriplets ∆L,R. The VEV vR of the neutral component of ∆R breaks the gauge sym-metry SU(2)R × U(1)B−L to U(1)Y and gives masses to the RH gauge bosons WR,ZR and the right-handed neutrinos N . LRSMs provide a simple ultraviolet completeseesaw mechanism with the key properties built in: The presence of right-handedneutrinos is a necessary ingredient and the LNV seesaw scale can be identified withthe breaking scale of the SU(2)R symmetry.With regard to LHC searches, the right-handed current interactions in the LRSM4can lead to a significant enhancement of the LNV signal. Even for negligible left-right neutrino mixing, heavy neutrinos can be directly produced via s-channel WRexchange [8]. The potential to discover LFV and LNV at the LHC in this scenariohas for example been analyzed in [16]. Fig. 2 (left) compares the sensitivity of suchsearches with the sensitivity of 0νββ and low energy LFV experiments assumingequality of the SU(2)L and SU(2)R gauge couplings, gR = gL.Both ATLAS and CMS have reported excesses in searches for dibosons, dijets ande±e∓jj using the LHC run I data, situated at invariant masses near 2 TeV. While farfrom being statistically significant, their coincidence at a resonant mass of 2 TeV isintriguing. Among several interpretations put forward in the literature, the excessescould be understood as a hint for WR production with mWR ≈ 2 TeV, an SU(2)Rgauge coupling gR ≈ 0.6gL and a W − WR mixing of sin θWLR ≈ 1.5 × 10−3 [17].Following through in this scenario, the CMS excess in e±e∓jj [14] can be tentativelyinterpreted as the production of a heavy, quasi-Dirac neutrino as discussed above.This would allow to connect the LHC searches with neutrino physics, specificallythe heavy neutrino mass and the strength of its mixing with the light neutrinos,cf. Fig. 2 (right).5 Lepton Number Violation and BaryogenesisThe observed matter-antimatter asymmetry of the universe cannot be understoodwith SM physics. A large number of possible mechanisms to generate the observedasymmetry have been proposed in the literature. A particularly interesting scenario inour context is leptogenesis [18]. In its original formulation, the out-of-equilibrium andCP violating decay of the heavy Majorana neutrinos in the type-I seesaw mechanismcreate a lepton asymmetry which is then converted into a baryon asymmetry through(B + L) violating EW sphaleron processes [19].The presence of LNV is a crucial ingredient in leptogenesis. Furthermore, theobservation of LNV would have important consequences on the viability of baryoge-nesis models in general; specifically, it is possible to falsify a large class of high-scalebaryogenesis scenarios if LNV was observed at the LHC [20]. For example if a reso-nant LNV process with the signature pp→ l±l±jj is observed, its LHC cross sectionσLHC is related to the induced lepton asymmetry washout rate ΓW/H (relative to theexpansion of the universe) [20],log10ΓWH& 6.9 + 0.6(MXTeV− 1)+ log10σLHCfb. (3)Here MX is the mass of the hypothetically observed resonance. If ΓW/H  1,the dilution of a primordial net lepton number density, understood to be producedin a baryogenesis mechanism at a higher scale, is highly effective and the lepton50 1 2 3 4 510-810-610-410-2100102MX (TeV)σ LHC(fb)ΓW /H=110-21021041061081010 u ud du d5 7 9 11 μeγ τ ℓγ μeqq102104106108101010121014T[GeV]-Λ- λ-λcurrentfutureLFV0νββEW scaleLHC reachFigure 3: Left: Lepton number washout rate in the early universe as a function of ahypothetically observed LNV scale MX and the LHC cross section σLHC (solid bluecontours). The red dashed curves denote typically expected cross sections for gaugestrength interactions. Right: Temperature intervals where the given LNV and LFVoperators are in equilibrium assuming that the corresponding process is observed atthe current or future experimental sensitivity. Taken from [20, 21].asymmetry would be washed out before it can be converted by sphaleron processes.This is illustrated in Fig. 3 (left). Observation of LNV at the LHC would thereforerule out or strongly constrain baryogenesis scenarios above the scale MX .A similar argument can be applied to non-standard mechanisms mediating 0νββdecay and low energy LFV processes [21]: if observed, the corresponding processeswould be in equilibrium in certain temperature ranges. This is shown in Fig. 3 (right)where the coloured bars denote the efficient equilibration temperatures assuming therelevant observable is seen at the current (left bar) or expected future (right bar)sensitivity. In the case of the 7,9,11-dimensional effective operators O7,9,11 mediating0νββ decay, an electron lepton asymmetry present at higher energies would be washedout. Observation of LFV via 6-dimensional LFV operators at compatible scales wouldallow to extend the argument to other flavours than the electron.6 ConclusionsModels of neutrino mass generation provide well motivated scenarios of BSM physics.The synergy between low energy and high intensity searches on the one hand and6high energy LHC searches on the other are an effective approach to rule out, con-strain or ideally pinpoint such models. In this report, we have briefly reviewed a fewphenomenological scenarios where the LHC can help us to find out whether the lightneutrino masses (and maybe the matter-antimatter asymmetry as well) are generatedin a mechanism close to the electroweak scale or beyond.ACKNOWLEDGEMENTSThe author would like to thank the organizers of CIPANP2015 for the opportunityto contribute to the conference and the proceedings.References[1] P. Minkowski, Phys.Lett. B67, 421 (1977); M. Gell-Mann, P. Ramond, andR. Slansky, (1979), Print-80-0576 (CERN); R. N. Mohapatra and G. Senjanovic´,Phys. Rev. Lett. 44, 912 (1980); J. Schechter and J. W. F. Valle, Phys. Rev.D22, 2227 (1980); J. Schechter and J. W. F. Valle, Phys. Rev. D25, 774 (1982).[2] R. N. Mohapatra and J. W. F. Valle, Phys. Rev. D34, 1642 (1986).[3] M. C. Gonzalez-Garcia and J. W. F. Valle, Phys. Lett. B216, 360 (1989).[4] F. F. Deppisch, P. S. Bhupal Dev, and A. Pilaftsis, New J. Phys. 17, 075019(2015), 1502.06541.[5] S. Kovalenko, Z. Lu, and I. 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Probing Leptonic Models at the LHC

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Probing Leptonic Models at the LHC

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