Probing heavy neutrino oscillation and associated CP violation at future hadron colliders Yongchao Zhang ( « [ ‡ ) Washington University in St. Louis July 26, 2019 Flasy 2019, Hefei based on P. S. B. Dev, R. N. Mohapatra & YCZ, 1904.04787
Seesaw mechanism Minkowski, ’77; Mohapatra & Senjanovic, ’80; Yanagida, ’79; Gell-Mann, Ramond & Slansky, ’79; Glashow, ’80 m ν ≃ − m D M − 1 N m T D At least two heavy right-handed neutrinos (RHNs) to generate the tiny neutrino masses. (“fair-play rule”) Yongchao Zhang (Wustl) RHN mixing & CPV July 26, 2019 2 / 24
Seesaw scenarios In pure type-I seesaw and U ( 1 ) B − L gauge extension of SM, RHN mixing and associated CP violation signatures depend on heavy-light neutrino mixing. Thanks to the discussions with Shun Zhou [Chao, Si, Zheng, Zhou ’09] In the left-right model based on the gauge group SU ( 2 ) L × SU ( 2 ) R × U ( 1 ) B − L : [Pati & Salam, ’74; Mohapatra & Pati, ’75; Senjonavi´ c & Mohapatra, ’75] � u L � � � � u R � � � 2 , 1 , 1 1 , 2 , 1 P Q L = ∈ ← → Q R = ∈ d L d R 3 3 � � � � ν L N R P Ψ L = ∈ ( 2 , 1 , − 1 ) ← → Ψ R = ∈ ( 1 , 2 , − 1 ) e L e R The RHN mixing and CP violation can be measured at colliders. This can be used to directly test TeV-scale leptogenesis at future hadron colliders! Yongchao Zhang (Wustl) RHN mixing & CPV July 26, 2019 3 / 24
Flavor dependence of same-sign dilepton signals ℓ − α W − ℓ − β R N 1 , 2 W R q ′ ¯ q The “smoking-gun” signal of W R and N ! [Keung & Senjanovi´ c, ’83] With only one RHN, or the production and decays of RHNs not interfering coherently: Γ( N → ℓ + jj ) = Γ( N → ℓ − jj ) N ( ℓ ± ℓ ± ) = N ( ℓ + ℓ − ) = ⇒ If we have more than one RHNs, and there are mixing and CPV in the RHN sector [Dev & Mohapatra, ’15; Gluza, Jelinski & Szafron, ’16; Anamiati, Hirsch & Nardi, ’16; Antusch, Cazzato & Fischer, ’17; Das, Dev & Mohapatra, ’17] Γ( N α → ℓ + β jj ) = Γ( N α → ℓ − N ( ℓ ± α ℓ ± β ) � = N ( ℓ + α ℓ − β jj ) , but β ) , N ( ℓ + α ℓ + β ) � = N ( ℓ − α ℓ − β ) (CP-induced effects) Yongchao Zhang (Wustl) RHN mixing & CPV July 26, 2019 4 / 24
RHN mixing and CP violation Some assumptions Only two RHNs N e , µ mixing with each other; the third one N τ does not mix with N e , µ : � � � � � � sin θ R e − i δ R cos θ R N e N 1 = , − sin θ R e i δ R cos θ R N µ N 2 The mass relation M 1 , 2 < M W R (and M 3 > M W R ): on-shell production of RHNs from W R decay: W ± R → ℓ ± α N α Yongchao Zhang (Wustl) RHN mixing & CPV July 26, 2019 5 / 24
Same-sign charge asymmetry (SSCA) Define the same-sign charge asymmetry (SSCA) α ℓ − N ( ℓ + α ℓ + β ) − N ( ℓ − β ) A αβ ≡ α ℓ + β ) + N ( ℓ − α ℓ − N ( ℓ + β ) σ ( pp → W + α ℓ + β ) − σ ( pp → W − α ℓ − R ) R ( ℓ + R ) R ( ℓ − β ) = σ ( pp → W + α ℓ + β ) + σ ( pp → W − R ) R ( ℓ − α ℓ − R ) R ( ℓ + β ) Combing both the three-body decays of N α through the gauge couplings to W R boson (1 − BR y ) and two-body decays of N α through the Yukawa couplings via heavy-light neutrino mixing ( BR y ) 1 α ℓ ± α ℓ ± R ( ℓ ± β ) ≃ ( 1 − BR y ) R ( ℓ ± β ) + 4 BR y B αβ � �� � � �� � 3-body decay ctrb. 2-body decay ctrb. Yongchao Zhang (Wustl) RHN mixing & CPV July 26, 2019 6 / 24
3-body and 2-body decay contributions Three-body decays N α → ℓ ± β jj , in the limit of Γ 1 = Γ 2 , with x ≡ ∆ E N / Γ avg [normalization condition � α ℓ ± α,β = e ,µ R ( ℓ ± β ) = 1] � � R ( e ± µ ± ) = R ( µ ± e ± ) ≃ 1 1 − cos 2 δ R ± x sin 2 δ R 4 sin 2 2 θ R , 1 + x 2 R ( e ± e ± ) ≃ R ( µ ± µ ± ) ≃ 1 2 − R ( e ± µ ± ) , β W ∓ ( α = e , µ , β = e , µ, τ ) Two-body decays N α → ℓ ± � � B e β = 1 N ( ℓ ± α ℓ ± R ( e ± e ± ) + R ( e ± µ ± ) β ) ∝ 2 B e β � B αβ = Γ( N α → ℓ ± β W ∓ ) / Γ( N α → ℓ ± β W ∓ ) β In most of the parameter space of interest, the dependence of SSCAs on θ R and δ R is negligible. Yongchao Zhang (Wustl) RHN mixing & CPV July 26, 2019 7 / 24
Some comments A ee , µµ depend both on θ R and δ R , while A e µ depends only on δ R . We expect the relation, in the limit of ( 1 − BR y ) ≫ BR y , � � θ R = π 4 , δ R + π A e µ ( δ R ) = A ee , µµ . 2 A ee , µµ, e µ can be used to determine the RHN mixing angle θ R and CP phase δ R at future colliders. If the two-body decay dominates, the CP-induced SSCAs will be suppressed. If the three-body decay dominates, the TeV-scale leptogenesis efficiency will be suppressed. Yongchao Zhang (Wustl) RHN mixing & CPV July 26, 2019 8 / 24
Dominant (reducible) backgrounds Figure: From ATLAS, 1809.11105, with L = 36 . 1 fb − 1 . [See also CMS, 1803.11116; Mitra, Ruiz, Scott & Spannowsky, ’16; Nemevsek, Nesti & Popara, ’18] Yongchao Zhang (Wustl) RHN mixing & CPV July 26, 2019 9 / 24
Production cross sections s = 14 TeV 1 ± → ℓ ± ℓ ± jj ) W R + 0.100 σ ( pp → W R 0.010 W R - 0.001 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 W R mass [ TeV ] Figure: Using a conservative k -factor of 1.1. Mitra, Ruiz, Scott & Spannowsky, ’16 Even if there is no CPV in the RHN sector, we can still expect non-zero SSCAs: σ ( pp → W + R ) > σ ( pp → W − R ) Yongchao Zhang (Wustl) RHN mixing & CPV July 26, 2019 10 / 24
The proton PDF uncertainties are more important... 1.0 1.0 w / CPV [ ee ] w / CPV [ ee, μμ ] C P V w [ μ μ ] / 0.8 w / o CPV [ ee, μμ & e μ ] 0.8 w / o CPV [ ee, μμ & e μ ] current LHC limit current LHC limit 0.6 0.6 w / CPV [ e μ ] V e μ ] P [ C w / 0.4 0.4 αβ αβ 0.2 0.2 0.0 0.0 - 0.2 - 0.2 LHC14 ( BR y = 0 ) LHC14 ( BR y = 1 / 2 ) - 0.4 - 0.4 5.0 5.2 5.4 5.6 5.8 6.0 5.0 5.2 5.4 5.6 5.8 6.0 W R mass [ TeV ] W R mass [ TeV ] Figure: Using NNPDF3.1 and θ R = δ R = π/ 4. Left : BR y = 0 and Right : BR y = 1 / 2. The proton parton energy fraction s ≃ M 2 x 1 x 2 = ˆ s W R � 0 . 1 for M W R � 5 TeV s We need a higher-energy collider! Yongchao Zhang (Wustl) RHN mixing & CPV July 26, 2019 11 / 24
Prospects @ HE-LHC √ s = 27 TeV 1.0 1.0 C P V e e w [ ] / 0.8 0.8 w / CPV [ ee, μμ ] w / CPV [ μμ ] current LHC limit current LHC limit 0.6 0.6 w / o CPV [ ee, μμ & e μ ] w / o CPV [ ee, μμ & e μ ] αβ αβ 0.4 0.4 e ] V μ P [ C w / 0.2 0.2 w / CPV [ e μ ] 0.0 0.0 HE - LHC ( BR y = 0 ) HE - LHC ( BR y = 1 / 2 ) - 0.2 - 0.2 5.0 5.5 6.0 6.5 7.0 7.5 8.0 5.0 5.5 6.0 6.5 7.0 7.5 8.0 W R mass [ TeV ] W R mass [ TeV ] Figure: Left : BR y = 0 and Right : BR y = 1 / 2. One could measure the RHN mixing and CPV at future high energy colliders by using the SSCA signals. The maximal CPV case ( θ R = δ R = π/ 4) can be measured at √ s = 27 TeV, for a W R mass up to 7.2 TeV. We need only O ( 100 fb − 1 ) of data to have at least 100 events of both ℓ + ℓ + and ℓ − ℓ − at HE-LHC for a W R mass of 5 TeV. Yongchao Zhang (Wustl) RHN mixing & CPV July 26, 2019 12 / 24
Prospects @ FCC-hh/SPPC √ s = 100 TeV 1.0 1.0 w / CPV [ ee ] 0.8 0.8 , μ ] e e μ P V C [ w / μ ] current LHC limit current LHC limit P V [ μ 0.6 0.6 C w / e μ ] e μ ] & & αβ 0.4 μ αβ 0.4 μ , μ , μ e e e e V [ V [ C P C P o o w w / / 0.2 0.2 w / CPV [ e μ ] ] 0.0 0.0 e μ V [ P C w / FCC - hh ( BR y = 0 ) FCC - hh ( BR y = 1 / 2 ) - 0.2 - 0.2 5 10 15 20 25 30 5 10 15 20 25 30 W R mass [ TeV ] W R mass [ TeV ] Figure: Left : BR y = 0 and Right : BR y = 1 / 2. One could measure the RHN mixing and CPV at future high energy colliders by using the SSCA signals. The maximal CPV case ( θ R = δ R = π/ 4) can be measured at √ s = 100 TeV, for a W R mass up to 26 TeV. We need only O ( 100 fb − 1 ) of data to have at least 100 events of both ℓ + ℓ + and ℓ − ℓ − at FCC-hh/SPPC for a W R mass of 10 TeV. Yongchao Zhang (Wustl) RHN mixing & CPV July 26, 2019 13 / 24
Expected SSCAs: benchmark points 1.0 1.0 ee e μ ee e μ 0.8 0.8 μμ w / o CPV μμ 0.6 0.6 w / o CPV αβ αβ 0.4 0.4 0.2 0.2 0.0 θ R = π / 4 θ R = π / 4 0.0 HL - LHC [ W R = 5 TeV ] FCC - hh [ W R = 15 TeV ] - 0.2 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 δ R / π δ R / π The A e µ does not depend on θ R , thus one can use A e µ to first determine the phase δ R , up to a twofold ambiguity. Then one can use A ee , µµ to determine the mixing angle θ R (and potentially remove the ambiguity of δ R ). By comparing the A ee and A µµ data, we can get information on the BRs of 3- and 2-body decays of RHNs. Yongchao Zhang (Wustl) RHN mixing & CPV July 26, 2019 14 / 24
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