Neutrino mass physics at lepton colliders

Transcription

1 Neutrino mass physics at lepton colliders Oliver Fischer University of Basel, Switzerland soon: Karlsruhe Institute of Technology, Germany ACFI workshop, July the 19th, 2017, Amherst

2 Motivation for sterile neutrinos ν e ν μ ν τ Shaposhnikov et al. Neutrino oscillations: at least two massive light neutrinos. No renormalisable way in the SM therefore; evidence for new physics. Focus: type I seesaw mechanism. Oliver Fischer Neutrino mass physics at lepton colliders 1 / 24

3 Type I seesaw The naïve version: The simplified version: (1 ν L, 1 ν R ) ( ) 0 m Mass matrix, with m = y m M ν v EW M. Light neutrino mass: m ν = 1 vew 2 y ν 2 2. M R The symmetry protected scenario: Symmetry: for instance lepton number or B L. A (2 ν L, 2 ν R ) example: ( ) ( ) O(yν ) 0 0 MR y ν, M O(y ν ) 0 ε M R m νi = 0 + ε v 2 EW O(y 2 ν ) M 2 R Large ratio of y ν and M R can be compatible with m νi. Oliver Fischer Neutrino mass physics at lepton colliders 2 / 24

4 The Big Picture Neutrino Yukawa coupling yν Y top 10-3 Y e Λ EW m 2 2 ν =Δm atm reactor & LSND anomaly GUT m 2 2 ν =Δm sol ev kev GeV PeV ZeV M GUT M Pl Sterile neutrino mass scale Oliver Fischer Neutrino mass physics at lepton colliders 3 / 24

5 Symmetry Protected Seesaw Scenario Benchmark model for future collider studies, defined in Antusch, OF, JHEP 1505 (2015) 053. Similar to e.g.: Mohapatra, Valle (1986); Shaposhnikov (2007); Gavela, Hambye, Hernandez (2009) Collider phenomenology dominated by two sterile neutrinos N i with protective symmetry, such that L N = 1 2 N1 R M(N2 R )c y να N 1 R φ L α + H.c. Further decoupled sterile neutrinos may exist. v Active-sterile mixing: θ α = y EW να 2 M, θ 2 α θ α 2 The leptonic mixing matrix to leading order in θ α : N e1 N e2 N e3 i 2 θ e 2 1 θ e N µ1 N µ2 N µ3 i 2 θ µ 2 1 θ µ U = N τ1 N τ2 N τ3 i 2 θ τ 2 θ τ i ( 2 ) ( ) θe θµ θτ i 2 1 θ θ2 2 2 Oliver Fischer Neutrino mass physics at lepton colliders 4 / 24

6 Heavy neutrino interactions Charged current (CC): Neutral current (NC): j 0 µ = j ± µ = g 2 θ α l α γ µ ( in 1 + N 2 ) g [ θ 2 N 2 γ µ N 2 + ( ν i γ µ ξ α1 N 1 + ν i γ µ ξ α2 N 2 + H.c) ] 2 c W Higgs boson Yukawa interaction: L Yukawa = 3 i=1 ξ α2 2 M v EW ν i φ 0 ( N 1 + N 2 ) With the mixing parameters: ξ α1 = ( i) Nαβ 2, ξ α2 = i ξ α1 θ β Oliver Fischer Neutrino mass physics at lepton colliders 5 / 24

7 Precision observables in seesaw scenarii Input parameters: M Z, α(m Z ), G F. N µi ν i µ e The Fermi constant: N ej ν j Muon decay ( NN ) ( ee NN ) µµ Fermi constant G F muon decay constant G µ. Tree-level relation: G F = G µ (NN ) ee(nn ) µµ = απ 2s 2 W c 2 W m2 Z Analogous: Observables involving weak decays. Theory prediction for electroweak observables. Oliver Fischer Neutrino mass physics at lepton colliders 6 / 24

8 Constraints on PMNS non-unitarity from precision data Analysis of non-unitarity of the PMNS matrix. 34 precision observables: Electroweak Precision Observables (EWPO), lepton universality, charged lepton flavour violation, CKM unitarity Lots of details in the backup - please ask! Highest posterior density intervals at 90% Bayesian C.L.: ε ee ε µµ ε ττ 0 ε eµ < ε eτ < ε µτ < Antusch, OF, JHEP 1410 (2014) 094 Non-unitarity parameters: ε αβ = θ αθ β. Weak statistical preference for non-zero mixing for ε ee. Oliver Fischer Neutrino mass physics at lepton colliders 7 / 24

9 Present Constraints (dominated by LEP) Θ DELPHI (Z pole Θ 2 = θ 2 LHC (Higgs Θ 2 = θ 2 ALEPH (e - e + 4 Θ 2 2 = θ e Precision Θ 2 2 = θ e Precision Θ 2 2 = θ μ Precision Θ 2 2 = θ τ M [GeV] Antusch, OF, JHEP 1505 (2015) 053 Z pole search: limits from Z branching ratios. Abreu et al. Z.Phys. C74 (1997) Higgs decays: Best constraints from h γγ. WW production cross section: δσsm WW = stat syst OPAL collaboration, Abbiendi et al. (2007) Oliver Fischer Neutrino mass physics at lepton colliders 8 / 24

10 Heavy Neutrino Production in electron-positron collisions Circular Collider e Z ν ΣΝN Θ 2 pb Z pole run 90 GeV WW threshold run 160 GeV Higgs physics run 250 GeV top threshold run 350 GeV e + e W N ν e + N Z pole: production via s-channel Z, sensitive to θ 2. At higher energies: t-channel W, production sensitive to θ e 2. At Z pole very large instantaneous luminosities are feasible. Oliver Fischer Neutrino mass physics at lepton colliders 9 / 24

11 Luminosities at lepton colliders (old) Oliver Fischer Neutrino mass physics at lepton colliders 10 / 24

12 Signatures for direct searches Name Final State θ, Z pole θ, s > m Z lepton-dijet l α νjj θ α 2 θ e θ α 2 θ 2 mixed flavour dilepton l α l β νν θ α 2 θ e θ α 2 same flavour dilepton l α l α νν θ 2 θ e 2 dijet ννjj θ 2 θ e 2 invisible νννν θ 2 θ e 2 S. Antusch, E. Cazzato and OF, Int. J. Mod. Phys. A 32 (2017) no.14, θ 2 Measurement of LNV not straightforward. The dependency on the active-sterile mixing is determined by the center-of-mass energy, i.e. by the physics run. For masses below m W lepton isolation becomes an issue. S. Dube, D. Gadkari and A. M. Thalapillil, arxiv: [hep-ph]. Oliver Fischer Neutrino mass physics at lepton colliders 11 / 24

13 Indirect Signatures: Electroweak precision tests Observable LEP precision from CEPC precdr M W [MeV] 33 3 sin 2 θw eff 0.07% 0.01% R b 0.3% 0.08% R c 0.3% 0.07% R inv 0.27% R l 0.1% 0.1% Γ l 0.1% 0.1% σh 0 [nb] FCC-ee: much more ambitious; ILC: no strong Z pole program. ΕΤΤ 10 2 present ILC 10 3 theory FCC ee Ε ee Ε ΜΜ S. Antusch and OF, JHEP 1410 (2014) 094 Measuring the non-unitarity of the PMNS matrix. Improvement required: δ theory and δ syst. Not included: lepton universality tests of W decays Not included: rare charged LFV l decays Oliver Fischer Neutrino mass physics at lepton colliders 12 / 24

14 Indirect signatures: Higgs boson properties 1 Higgs boson branching ratios: New decay channel h νn Large branching ratio possible, modified Br h SM Precision Br h WW 10 3 M. Ruan, Nucl. Part. Phys. Proc , 857 (2016) Brh ΝN Α ΘΑ 2 DELPHI M GeV Antusch, OF, JHEP 1505 (2015) 053 Higgs production: e W ν ν Additional production mechanism at high energies. e + N h Enhanced mono-higgs production cross section. S. Antusch, OF, JHEP 1604 (2016) 189 Oliver Fischer Neutrino mass physics at lepton colliders 13 / 24

15 Exotic signature: displaced vertices I Lifetime O(1 100) ps. Assumption: no SM background for displacements > 0.1 mm. Considered ILC detector SiD as benchmark. S. Antusch, E. Cazzato and OF, JHEP 1612, 007 (2016) A. Blondel et al., Nucl. Part. Phys. Proc (2016) 1883 Oliver Fischer Neutrino mass physics at lepton colliders 14 / 24

16 Exotic signature: displaced vertices II Schematic of the detector components sensitivities: active sterile mixing x rd 100 events 10 events 1event x xres mw Μ system HCAL ECAL Tracker Vertex detector Inner region heavy neutrino mass Sensitivities: θ θ θ FCC-ee M [GeV] 10-9 CEPC M [GeV] 10-9 ILC M [GeV] Ecm mz; Ecm 250 GeV; Ecm 350 GeV; Ecm 500 GeV; Conventional search 95 C.L. Oliver Fischer Neutrino mass physics at lepton colliders 15 / 24

17 Summary: FCC-ee sensitivities Θ Conventional Z pole Θ 2 = θ 2 Displaced vertex Θ 2 = θ 2 Higgs branching Θ 2 = θ 2 Θ 2 2 = θ e WW production cross Θ 2 2 = θ e Θ 2 2 = θ e Θ 2 = θ 2 2 e + θ μ M [GeV] Θ 2 = θ τ 2 "Unprotected" type-i seesaw Displaced vertex searches test θ for M m W. EWPOs test θ up to M 60 TeV with O(1) Yukawa couplings. Oliver Fischer Neutrino mass physics at lepton colliders 16 / 24

18 Promising search strategies at electron-proton colliders Oliver Fischer Neutrino mass physics at lepton colliders 17 / 24

19 Heavy neutrino production at electron-proton colliders W (q) t W (γ) t Leading order production of heavy neutrino mass eigenstate. W (q) t : dominant at lower center-of-mass energies. W (γ) t : relevant for larger masses. Oliver Fischer Neutrino mass physics at lepton colliders 18 / 24

20 Production cross sections (W (q) t ) Σ N Θe 2 pb M 10 GeV M 50 GeV M 100 GeV LHeC Σ N Θe 2 pb M 10 GeV M 50 GeV M 100 GeV M 500 GeV FCC-eh For 60 GeV as benchmark for the electron beam E e : σ νn increase of 30% for E e 100 GeV. Increased by 80% when including polarisation. Consider 1 ab 1 (for FCC-eh and LHeC). Oliver Fischer Neutrino mass physics at lepton colliders 19 / 24

21 Signal channels from W (q) t Name Final State θ α Dependency LFV lepton-trijet jjjl α θ e θ α 2 θ 2 jet-dilepton jl α l + β ν θ e θ α 2 trijet jjjν θ e 2 monojet jννν θ e 2 θ 2 ( ) LFV (and LNV) signature for α e, β α, and γ α, β Unambiguous lepton-number-violating final states, e.g. e + jjj. More (and more complex) signatures from W (γ) t. Oliver Fischer Neutrino mass physics at lepton colliders 20 / 24

22 Lepton-flavour-violating signatures l α - (α=μ τ) Θ = θ α θ τ / θ Θ τ - μ + ν Θ = θ θ τ / θ 10-5 µ eγ Dashed lines denote the LHeC Antusch, Cazzato, OF; [ ] M [GeV] Very sensitive tests of combinations θ e θ α. Upper bounds: θ e θ µ from µ eγ (MEG); θ e θ τ from precision data. Requires θ α > θ e for sizeable branching ratios. Oliver Fischer Neutrino mass physics at lepton colliders 21 / 24

23 Sensitivities: summary At one-sigma confidence level. ep and pp at parton level pp ep ee FCC S. Antusch, E. Cazzato, OF; The combination of ee with pp and ep colliders provides complementary tests for the neutrino mass mechanism.

24 Summary Present constraints: active-sterile mixing θ Search strategies at lepton colliders: Direct: lepton-dijet, dilepton, dijets Higgs: production and decay channels EWPO: measurement of PMNS non-unitarity Displaced vertices: best sensitivity for M < m W pp/ep: LFV dilepton-dijet/lepton-trijet & displaced vertices Synergy: The combination of direct and indirect signatures at ee/pp/ep will allow to test model specific predictions. Testing the origin of neutrino masses. Oliver Fischer Neutrino mass physics at lepton colliders 22 / 24

25 Conclusion CEPC CDR writeup this year FCC-ee CDR writeup next year ILC & LHeC seeking additional physics motivation (neutrinos NOT included yet) Extremely important input for future collider projects! Oliver Fischer Neutrino mass physics at lepton colliders 23 / 24

26 Thank you for your attention.

27 Backup I - EWPO Experimental results and SM predictions for the EWPO, and the modification, to first order in the non-unitarity parameters ε αα = θ αθ β. (formulae for M m Z ) Prediction in MUV SM Prediction Experiment [R l ] SM (1 0.15(ε ee + ε µµ )) (11) (25) [R b ] SM ( (ε ee + ε µµ )) (4) (66) [R [ c ] SM ] (1 0.06(ε ee + ε µµ )) (6) (30) σ 0 had SM (1 0.25(ε ee + ε µµ ) 0.27ε τ )/nb (15) (37) [R inv ] SM ( (ε ee + ε µµ ) ε τ ) (10) 5.942(16) [M W ] SM (1 0.11(ε ee + ε µµ ))/GeV (11) (15) [Γ lept ] SM (1 0.59(ε ee + ε µµ ))/MeV (12) (86) [(s l,lep W,eff )2 ] SM ( (ε ee + ε µµ )) (1) (21) [(s l,had W,eff )2 ] SM ( (ε ee + ε µµ )) (1) (27) Minimal Unitarity Violation scheme: Antusch et al.; JHEP 0610 (2006) 084. Oliver Fischer Neutrino mass physics at lepton colliders 23 / 24

28 Backup II - lepton universality Modification due to sterile neutrinos (formulae for M m Z ): (NN R αβ = ) αα (NN ) ββ 2 (ε αα ε ββ ). Rµe l Rτµ l Reµ W Rτµ W Process Γ(τ ν τ µ ν µ ) Γ(τ ν τ e ν e ) Γ(τ ν τ e ν e ) Γ(µ ν µ e ν e ) Γ(W e ν e ) Γ(W µ ν µ ) Γ(W τ ν τ ) Γ(W µ ν e ) Bound (14) (21) (93) 1.032(11) Rµe π Rτµ π Rτµ K Rτe K Process Γ(π µ ν µ ) Γ(π e ν e ) Γ(τ ν τ π) Γ(π µ ν µ ) Γ(τ Kν τ ) Γ(K µ ν µ ) Γ(τ Kν τ ) Γ(K e ν e ) Bound (16) (31) (72) 1.018(42) Oliver Fischer Neutrino mass physics at lepton colliders 23 / 24

29 Backup III - CKM unitarity constraint Current world averages: V ud = (15), V ub = (15) Vij th 2 = V exp ij 2 (1 + f process (ε αα )), Vud th 2 = V exp,β ud 2 (NN ) µµ. For the kaon decay processes we have: Vus th 2 = Vus exp,k e 2 (NN ) µµ, Vus th 2 = Vus exp,k µ 2 (NN ) ee. Process V us f + (0) K L πeν (6) K L πµν (6) K S πeν (13) K ± πeν (11) K ± πµν (14) Average (5) Processes involving tau leptons: Process f process (ε) V us B(τ Kν) B(τ πν) ε µµ (13) τ Kν ε ee + ε µµ ε ττ (22) τ l, τ s 0.2ε ee 0.9ε µµ 0.2ε ττ (22) Oliver Fischer Neutrino mass physics at lepton colliders 23 / 24

30 Backup IV - lepton flavour violation Present experimental limits at 90% C.L.: Process MUV Prediction Bound Constraint on ε αβ µ eγ ε µe ε µe < τ eγ ε τe ε τe < τ µγ ε τµ ε τµ < Estimated sensitivities of planned experiments at 90% C.L.: Process MUV Prediction Bound Sensitivity Br τe ε τe ε τe Br τµ ε τµ ε τµ Br µeee ε µe ε µe Rµe Ti ε µe ε µe R Ti µe yields a sensitivity to m νr up to 0.3 PeV. Oliver Fischer Neutrino mass physics at lepton colliders 23 / 24

31 Backup V - precision estimates for EWPOs Observable ILC FCC-ee CEPC CEPC R l R inv R b M W [MeV] s 2,l eff σh 0 [nb] n.a Γ l [MeV] n.a Reference Ruan (2014) scaled Private communication. Assumption: CEPC produces Z bosons, compared to the Z Uncertainties scaled: δ CEPC = δ FCC ee 10. Oliver Fischer Neutrino mass physics at lepton colliders 24 / 24