Lepton flavor violation induced by a neutral scalar at future lepton - PowerPoint PPT Presentation

Lepton flavor violation induced by a neutral scalar at future lepton colliders Yongchao Zhang Washington University in St. Louis July 7, 2018 ICHEP2018, Seoul based on P. S. B. Dev, R. N. Mohapatra & YCZ, PRL120 (2018)221804 [1711.08430] (see also P. S. B. Dev, R. N. Mohapatra & YCZ, 1803.11167) contributing to CEPC CDR & CLIC CERN Yellow Book

Outline Motivations Effective LFV couplings of a (light) BSM neutral scalar H On-shell production of H at CEPC & ILC Off-shell production of H at CEPC & ILC Prospects and discussions e + e + ℓ + e − α γ, Z γ, Z ℓ − H ℓ − e − ℓ − e + β H ℓ − ℓ − e − e − α α H H ℓ + ℓ + e + e + β β Yongchao Zhang (Wustl) LFV July 7, 2018 2 / 21

Motivation examples: LFV beyond SM muon g − 2 [Carena, Giudice, Wagner ’96; Raidal+ ’08; Wolfgang Altmannshofer, Carena, Crivellin ’16] H τ µ µ h µτ h µτ γ H : beyond SM scalar neutrino mass generation [Dreiner, Nickel, Staub+ ’12; de Gouvea, P. Vogel ’13; Vicente ’15; Lindner, Platscher, Queiroz ’16] charged LFV is always connected to neutrino mass generation by beyond SM scalars. [see the plenary talk by S. Petcov] Yongchao Zhang (Wustl) LFV July 7, 2018 3 / 21

Calling for New Physics... The LFV couplings of the SM Higgs h , e.g. y µτ ; [Blankenburg, Ellis, Isidori ’12; Harnik, Kopp, Zupan ’12] Beyond SM doubly-charged scalars H ±± , e.g. from type-II seesaw; [Fileviez Perez, Han, Huang+ ’08; Rentala, Shepherd, Su ’11; King, Merle, Panizzi ’14] Beyond SM (light) neutral scalar H with LFV couplings h αβ Beyond SM neutral scalar: its mass & the LFV couplings: model-dependent... The most efficient way to probe the LFV couplings: future lepton colliders: CEPC, ILC, FCC-ee, CLIC if the beyond scalar H is hadrophobic and does not mixing sizably with the SM Higgs. Yongchao Zhang (Wustl) LFV July 7, 2018 4 / 21

Well-motivated underlying models RPV SUSY: sneutrinos ( ˜ ν ) [Aulakh, Mohapatra ’82; Hall,Suzuki ’84; Ross, Valle ’85, Barbier+ ’04; Duggan, Evans, Hirschauer ’13] L RPV = 1 2 λ αβγ � L α � L β � E c γ Left-right symmetric models: the SU ( 2 ) R -breaking scalar H 3 [Dev, Mohapatra, YCZ ’16; ’16; ’17; Maiezza, Senjanovi´ c, Vasquez ’16] LFV couplings are generated at tree and/or loop level 2HDM: CP-even or odd (heavy) scalars from the 2nd doublet [Branco+ ’11; Crivellin, Heeck, Stoffer ’15] LFV couplings are induced from small deviation from the lepton-specific structure. Mirror models: singlet scalar connecting the SM leptons to heavy mirror leptons [Hung ’06, ’07; Bu, Liao, Liu ’08; Chang, Chang, Nugroho+ ’16; Hung, Le, Tran+ ’17] LFV couplings arise from the SM-heavy lepton mixing Yongchao Zhang (Wustl) LFV July 7, 2018 5 / 21

Beyond SM neutral Higgs & effective LFV couplings Model-independent effective LFV couplings of H L Y = h αβ ¯ ℓ α, L H ℓ β, R + H . c . . For simplicity, we assume h αβ are real, symmetric, H is CP-even. H might originate from a isospin singlet, doublet or triplet, depending on specific underlying models. 1 Effective Dim-4 couplings � = Effective 4-fermion couplings like Λ 2 (¯ ee )(¯ e µ ) [Kabachenko, Pirogov ’97; Ferreira, Guedes, Santos ’06; Aranda, Flores-Tlalpa, Ramirez-Zavaleta+ ’09; Murakami, Tait ’14; Cho, Shimo ’14] m H < √ s ⇒ on-shell production Yongchao Zhang (Wustl) LFV July 7, 2018 6 / 21

on-shell production e + e − → ℓ ± α ℓ ∓ β + H e + e + e + e + e − ℓ + α γ, Z γ, Z γ, Z ℓ − ℓ − H ℓ − e − e − e − ℓ − e + β H H

Constraints on the LFV couplings: on-shell On-shell production amplitudes depend linearly on the LFV couplings muonium anti-muonium oscillation: (¯ µ e ) ↔ ( µ ¯ e ) ( h e µ ) e − µ − e − µ − H H µ + µ + e + e + Oscillation probablity [Clark, Love ’03] 2 (∆ M ) 2 P = Γ 2 µ + 4 (∆ M ) 2 with the H -induced mass splitting ∆ M = 2 α 3 EM h 2 e µ µ 3 m e m µ , µ = π m 2 m e + m µ H Yongchao Zhang (Wustl) LFV July 7, 2018 8 / 21

Constraints on the LFV couplings: on-shell Electron and muon g − 2 ( h e ℓ , h µℓ ) [Lindner, Platscher, Queiroz ’16] H µ e e h e µ h e µ γ � � m 2 � � ∆ a e ≃ h 2 e µ m e m µ H 2 log − 3 . 16 π 2 m 2 m 2 H µ The value of h e µ to explain ( g − 2 ) µ discrepancy is excluded by the ( g − 2 ) e constraint. ∆ a µ ≡ ∆ a exp − ∆ a th µ = ( 2 . 87 ± 0 . 80 ) × 10 − 9 µ Yongchao Zhang (Wustl) LFV July 7, 2018 9 / 21

Constraints on the LFV couplings: on-shell Bhabha scattering, LEP ee → ℓℓ data ( h e ℓ ) [OPAL ’03; L3 ’03; DELPHI ’05] e − ℓ − H e + ℓ + Effective 4-fermion interaction h 2 ⇒ 1 Fierz transf. e γ µ e )(¯ e ℓ ℓγ µ ℓ ) (¯ e ℓ )(¯ e ℓ ) = = = = = = = Λ 2 (¯ m 2 H If m H � √ s , the LEP limits on the cut-off scale Λ do not apply, and we have to consider the kinetic dependence 1 1 1 → ≃ q 2 − m 2 m 2 − s cos θ/ 2 − m 2 H H H Yongchao Zhang (Wustl) LFV July 7, 2018 10 / 21

SM backgrounds: on-shell Main SM backgronds are particle misidentification for e + e − → ℓ + α ℓ − β + X , ( α � = β ) The mis-identification rate is expected to be small, of order 10 − 3 [Milstene, Fisk, Para ’06; Hammad, Khalil, Un ’16; Yu, Ruan, Boudry+ ’17] Examle: e + e − → Zh → ( e + e − /µ + µ − ) h � e ± µ ∓ + h m H = 50 GeV s = 240 GeV m H = 300 GeV 1000 s = 1 TeV 10 5 ab - 1 1 ab - 1 h e μ = 0.003 h e μ = 0.01 5 Number of Events Number of Events 100 background 1 background signal 0.50 signal 10 0.10 1 0.05 0.1 0.01 50 100 150 200 200 400 600 800 1000 m e μ [ GeV ] m e μ [ GeV ] � � S / S + B = 55 S / S + B = 61 Yongchao Zhang (Wustl) LFV July 7, 2018 11 / 21

CEPC & ILC prospects: on-shell 1 2 ) μ ( g - 2 ) e g - ( → 0.1 ee → μμ muonium oscillation | h e μ | 10 - 2 ILC 10 - 3 CEPC 10 - 4 10 100 1000 m H [ GeV ] Long-dashed, short-dashed, solid lines: 1%, 10%, and 100% of the decay products of H is reconstructible (visible). Shaded regions are excluded. Dotted brown line: central values of muon g − 2 anomaly, green and yellow bands: the 1 σ and 2 σ regions. Yongchao Zhang (Wustl) LFV July 7, 2018 12 / 21

CEPC & ILC prospects: on-shell 1 ( g - 2 ) e ⟶ 0.1 ee → ττ | h e τ | 10 - 2 ILC 10 - 3 CEPC 10 - 4 10 100 1000 m H [ GeV ] Long-dashed, short-dashed, solid lines: 1%, 10%, and 100% of the decay products of H is reconstructible (visible). Shaded regions are excluded. Yongchao Zhang (Wustl) LFV July 7, 2018 13 / 21

CEPC & ILC prospects: on-shell 1 d e d u l c x e 2 ) μ g - ( 0.1 ( g - 2 ) μ ILC | h μτ | 10 - 2 CEPC 10 - 3 10 100 1000 m H [ GeV ] Long-dashed, short-dashed, solid lines: 1%, 10%, and 100% of the decay products of H is reconstructible (visible). Shaded regions are excluded. Dotted brown line: central values of muon g − 2 anomaly, green and yellow bands: the 1 σ and 2 σ regions. The muon g − 2 discrepancy can be directly tested at CEPC via the searches of ee → µτ + H Yongchao Zhang (Wustl) LFV July 7, 2018 14 / 21

off-shell production e + e − → ℓ ± α ℓ ∓ β ℓ − ℓ − e − e − α α H H ℓ + ℓ + e + e + β β ...at resonance when m H ≃ √ s might also be mediated by a (light) gauge boson Z ′ with LFV couplings [Heeck ’16]

Constraints on the LFV couplings: off-shell Off-shell production amplitudes depend quadratically on the LFV couplings 3-body LFV decays of muon and tauon, e.g. [Sher, Yuan ’91] ee h e τ | 2 m 5 | h † Γ( τ − → e + e − e − ) ≃ 1 τ , ( δ = 2 ) 3072 π 3 m 4 δ H 2-body LFV decays of muon and tauon, e.g. [Harnik, Kopp, Zupan ’12] Γ( τ → e γ ) = α EM m 5 � | c L | 2 + | c R | 2 � c L = c R ≃ h † ee h e τ τ , . 64 π 4 24 m 2 H h ee , e µ, e τ contribute to ( g − 2 ) e & LEP ee → ℓℓ data, [DELPHI ’05; Hou, Wong ’95] | h † ee h e τ | ⇒ ee → e τ | h † e µ h e τ | ⇒ ee → µτ ( t -channel ) Yongchao Zhang (Wustl) LFV July 7, 2018 16 / 21

Constraints on the LFV couplings: off-shell constraints [ GeV − 2 ] process current data µ − → e − e + e − < 10 − 12 | h † ee h e µ | / m 2 H < 6 . 6 × 10 − 11 τ − → e − e + e − < 2 . 7 × 10 − 8 | h † ee h e τ | / m 2 H < 2 . 6 × 10 − 8 τ − → µ − e + e − < 1 . 8 × 10 − 8 | h † ee h µτ | / m 2 H < 1 . 5 × 10 − 8 τ − → µ + e − e − < 1 . 5 × 10 − 8 | h † e µ h e τ | / m 2 H < 1 . 9 × 10 − 8 τ − → e − γ < 3 . 3 × 10 − 8 ee h e τ | / m 2 H < 1 . 0 × 10 − 6 | h † τ − → µ − γ < 4 . 4 × 10 − 8 e µ h e τ | / m 2 H < 1 . 2 × 10 − 6 | h † | h † ee h e τ | / m 2 H < 1 . 1 × 10 − 7 < 5 . 0 × 10 − 13 ( g − 2 ) e | h † e µ h e τ | / m 2 H < 1 . 0 × 10 − 8 | h † ee h e τ | / m 2 H < 1 . 4 × 10 − 7 ee → ee , ττ Λ > 5 . 7 & 6 . 3 TeV | h † e µ h e τ | / m 2 H < 1 . 3 × 10 − 7 ee → µµ, ττ Λ > 5 . 7 & 7 . 9 TeV The µ → 3 e limit is so strong that the it leaves no hope to see any signal in the channel ee → e µ at CEPC & ILC. Yongchao Zhang (Wustl) LFV July 7, 2018 17 / 21