Lepton Number Violation at the LHC Bhupal Dev Washington University - PowerPoint PPT Presentation

Lepton Number Violation at the LHC Bhupal Dev Washington University in St. Louis International Workshop on Baryon and Lepton Number Violation (BLV 2017) Case Western Reserve University, Cleveland May 17, 2017

Why Lepton Number Violation? Non-zero neutrino mass = ⇒ physics beyond the SM neutrinos d s b u c t e µ τ meV eV keV MeV GeV TeV Something beyond the Higgs mechanism?

Seesaw Mechanism A natural way to generate neutrino masses. Break the ( B − L ) -symmetry of the SM. Parametrized by the dim-5 operator ( LLHH ) / Λ . [Weinberg (PRL ’79)] Three tree-level realizations: Type I, II, III seesaw mechanisms. µ ∆ Y N Y Σ Generically predict lepton number and/or (charged) lepton flavor violation. Pertinent question in the LHC era: Can we probe the seesaw mechanism at the LHC (or future colliders)? Experimentally feasible if the seesaw scale is (in)directly accessible. eV Y eV eV

(Minimal) Type-I Seesaw at the LHC SM-singlet heavy Majorana neutrinos. [Minkowski (PLB ’77); Mohapatra, Senjanovi´ c (PRL ’80); Yanagida ’79; Gell-Mann, Ramond, Slansky ’79; Glashow ’80] Same-sign dilepton plus jets without / E T [Keung, Senjanovi´ c (PRL ’83); Datta, Guchait, Pilaftsis (PRD ’94); Han, Zhang (PRL ’06); del Aguila, Aguilar-Saavedra, Pittau (JHEP ’07); · · · ] q W ' q q' N V N W + + V N + q 1 2 -1 | 19.7 fb (8 TeV) N µ 1 ATLAS |V 2 CMS -1 s = 8 TeV, 20.3 fb N µ V -1 10 10 -1 -2 10 CL Expected s -3 95% CL Observed limit 10 CL Expected 1 ± σ -2 10 s CL Expected 2 ± σ 95% CL Expected limit s CL Observed s -4 L3 95% CL Expected limit ± 1 σ 10 DELPHI 95% CL Expected limit 2 ± σ CMS 7 TeV -3 10 -5 10 100 150 200 250 300 350 400 450 500 50 100 150 200 250 300 350 400 450 500 m [GeV] m (GeV) N N [Talks by A. Salvucci and J. Kim]

Type-II Seesaw at the LHC SU ( 2 ) L -triplet scalar (Φ ++ , Φ + , Φ 0 ) . [Schechter, Valle (PRD ’80); Magg, Wetterich (PLB ’80); Cheng, Li (PRD ’80); Lazarides, Shafi, Wetterich (NPB ’81); Mohapatra, Senjanovi´ c (PRD ’81)] Multi-lepton signatures. [Akeroyd, Aoki (PRD ’05); Fileviez Perez, Han, Huang, Li, Wang (PRD ’08); del Aguila, Aguilar-Saavedra (NPB ’09); Melfo, Nemevsek, Nesti, Senjanovi´ c, Zhang (PRD ’12)] CMS Preliminary -1 12.9 fb (13 TeV) 100% Φ ± ± → e ± e ± 100% ± ± e ± ± Φ → µ ± ± 100% Φ → µ ± µ ± ± ± 100% Φ → e ± τ ± 100% Φ ± ± → µ ± τ ± Observed exclusion 95% CL Expected exclusion 95% CL ± ± 100% Φ → τ ± τ ± Associated Production Pair Production Benchmark 1 Combined Benchmark 2 Benchmark 3 Benchmark 4 0 100 200 300 400 500 600 700 800 900 1000 ± ± Mass (GeV) [Talks by A. Salvucci and C. Mills] Φ

Type-III Seesaw at the LHC SU ( 2 ) L -triplet fermion (Σ + , Σ 0 , Σ − ) . [Foot, Lew, He, Joshi (ZPC ’89)] Multi-lepton signatures. [Franceschini, Hambye, Strumia (PRD ’08); Li, He (PRD ’09); Arhrib, Bajc, Ghosh, Han, Huang, Puljak, Senjanovi´ c (PRD ’10); Ruiz (JHEP ’15)] P 2 P 2 Σ + Σ ± Z/ γ ∗ /h W ± Σ − Σ 0 P 1 P 1 -1 CMS Preliminary 35.9 fb (13 TeV) Figure 1: Examples of Feynman diagrams for heavy fermion production in the type-III seesaw σ (pp → Σ Σ ) ± σ theo. unc. 95% CL upper limits Observed − 1 10 Expected 1 std deviation 2 std deviation (pb) σ 2 10 − − 3 10 400 500 600 700 800 900 1000 Mass (GeV) Σ [Talk by A. Salvucci]

Outline Low-scale seesaw (mostly focus on type-I) Lepton number violating and conserving signals (both are important) Beyond the minimal seesaw (gauge extensions) Complementarity with low-energy probes (LFV and 0 νββ ) Consequences for leptogenesis

Why low-scale seesaw? In flavor basis { ν c , N } , type-I seesaw mass matrix � � 0 M D M ν = M T M N D Y top Neutrino Yukawa coupling y n m n 2 =D m atm 2 GUT For || M D M − 1 N || ≪ 1, M light ≃ − M D M − 1 N M T 10 - 3 D . ν L EW m n 2 =D m sol 2 Y e 10 - 7 In traditional GUT models, M N ∼ 10 14 GeV. 10 - 9 10 - 11 But in a bottom-up approach, allowed to be reactor & LSND anomaly eV keV GeV PeV ZeV M GUT M Pl anywhere (down to eV-scale). Sterile neutrino mass scale

Why low-scale seesaw? In flavor basis { ν c , N } , type-I seesaw mass matrix � � 0 M D M ν = M T M N D Y top Neutrino Yukawa coupling y n m n 2 =D m atm 2 GUT For || M D M − 1 N || ≪ 1, M light ≃ − M D M − 1 N M T 10 - 3 D . ν L EW m n 2 =D m sol 2 Y e 10 - 7 In traditional GUT models, M N ∼ 10 14 GeV. 10 - 9 10 - 11 But in a bottom-up approach, allowed to be reactor & LSND anomaly eV keV GeV PeV ZeV M GUT M Pl anywhere (down to eV-scale). Sterile neutrino mass scale Suggestive upper limit M N � 10 7 GeV from naturalness arguments. [Vissani (PRD ’98); Clarke, Foot, Volkas (PRD ’15); Bambhaniya, BD, Goswami, Khan, Rodejohann (PRD ’17)] NH IH 20.0 20.0 15.0 15.0 10.0 10.0 7.0 7.0 Im Z m h � 126 GeV Im Z m h � 126 GeV 5.0 5.0 Naturalness � ∆Μ 2 � � 1 TeV � 2 � Naturalness � ∆Μ 2 � � 1 TeV � 2 � m t � 173.2 GeV m t � 173.2 GeV Naturalness � ∆Μ 2 � � 5 TeV � 2 � Naturalness � ∆Μ 2 � � 5 TeV � 2 � 3.0 3.0 Α s � 0.1184 Α s � 0.1184 Naturalness � ∆Μ 2 � � 0.2 TeV � 2 � Naturalness � ∆Μ 2 � � 0.2 TeV � 2 � 2.0 Metastability 2.0 Metastability LFV LFV 1.5 Allowed Region 1.5 Allowed Region Perturbativity Perturbativity 1.0 1.0 10 4 10 6 10 8 10 10 10 4 10 6 10 8 10 10 100 100 M N � GeV � M N � GeV �

Why low-scale seesaw? In flavor basis { ν c , N } , type-I seesaw mass matrix � � 0 M D M ν = M T M N D Y top Neutrino Yukawa coupling y n m n 2 =D m atm 2 GUT For || M D M − 1 N || ≪ 1, M light ≃ − M D M − 1 N M T 10 - 3 D . ν L EW m n 2 =D m sol 2 Y e 10 - 7 In traditional GUT models, M N ∼ 10 14 GeV. 10 - 9 10 - 11 But in a bottom-up approach, allowed to be reactor & LSND anomaly eV keV GeV PeV ZeV M GUT M Pl anywhere (down to eV-scale). Sterile neutrino mass scale Suggestive upper limit M N � 10 7 GeV from naturalness arguments. [Vissani (PRD ’98); Clarke, Foot, Volkas (PRD ’15); Bambhaniya, BD, Goswami, Khan, Rodejohann (PRD ’17)] NH IH 20.0 20.0 15.0 15.0 10.0 10.0 7.0 7.0 Im Z m h � 126 GeV Im Z m h � 126 GeV 5.0 5.0 Naturalness � ∆Μ 2 � � 1 TeV � 2 � Naturalness � ∆Μ 2 � � 1 TeV � 2 � m t � 173.2 GeV m t � 173.2 GeV Naturalness � ∆Μ 2 � � 5 TeV � 2 � Naturalness � ∆Μ 2 � � 5 TeV � 2 � 3.0 3.0 Α s � 0.1184 Α s � 0.1184 Naturalness � ∆Μ 2 � � 0.2 TeV � 2 � Naturalness � ∆Μ 2 � � 0.2 TeV � 2 � 2.0 Metastability 2.0 Metastability LFV LFV 1.5 Allowed Region 1.5 Allowed Region Perturbativity Perturbativity 1.0 1.0 10 4 10 6 10 8 10 10 10 4 10 6 10 8 10 10 100 100 M N � GeV � M N � GeV � Similar naturalness arguments in the context of neutral top partners [Batell, McCullough (PRD ’15)] and warped seesaw [Agashe, Hong, Vecchi (PRD ’16)] also predict a low seesaw scale.

Low-scale seesaw with large mixing Naively, active-sterile neutrino mixing is small for low-scale seesaw: � � M ν 100 GeV V lN ≃ M D M − 1 � 10 − 6 ≃ N M N M N ‘Large’ mixing effects possible with special structures of M D and M N . [Pilaftsis (ZPC ’92); Kersten, Smirnov (PRD ’07); Gavela, Hambye, Hernandez, Hernandez (JHEP ’09); Ibarra, Molinaro, Petcov (JHEP ’10); Deppisch, Pilaftsis (PRD ’11); Adhikari, Raychaudhuri (PRD ’11); Mitra, Senjanovi´ c, Vissani (NPB ’12)]

Low-scale seesaw with large mixing Naively, active-sterile neutrino mixing is small for low-scale seesaw: � � M ν 100 GeV V lN ≃ M D M − 1 � 10 − 6 ≃ N M N M N ‘Large’ mixing effects possible with special structures of M D and M N . [Pilaftsis (ZPC ’92); Kersten, Smirnov (PRD ’07); Gavela, Hambye, Hernandez, Hernandez (JHEP ’09); Ibarra, Molinaro, Petcov (JHEP ’10); Deppisch, Pilaftsis (PRD ’11); Adhikari, Raychaudhuri (PRD ’11); Mitra, Senjanovi´ c, Vissani (NPB ’12)] One example: [Kersten, Smirnov (PRD ’07)]     δ 1 ǫ 1 m 1 0 M 1 0  and M N = M D = δ 2 ǫ 2 with ǫ i , δ i ≪ m i . m 2 M 1 0 0    δ 3 ǫ 3 m 3 0 0 M 2 In the limit ǫ i , δ i → 0, all three light neutrino masses vanish at tree-level, while the mixing given by V ij ∼ m i / M j can still be large. The textures can be stabilized by invoking discrete symmetries. [Kersten, Smirnov (PRD ’07); BD, Lee, Mohapatra (PRD ’13)] But LNV is suppressed, as generically expected due to constraints from neutrino oscillation data and 0 νββ . [Abada, Biggio, Bonnet, Gavela, Hambye (JHEP ’07); Ibarra, Molinaro, Petcov (JHEP ’10); Fernandez-Martinez, Hernandez-Garcia, Lopez-Pavon, Lucente (JHEP ’15)]

An Exception For suitable choice of CP phases, resonant enhancement of the LNV amplitude for ∆ m N � Γ N . [Bray, Pilaftsis, Lee (NPB ’07)] 2 ∆ m N � ∆ m N � A LNV ∝ V 2 + O ℓ N ∆ m 2 N + Γ 2 m N N Just like resonant enhancement of CP-asymmetry. V e 1 = V µ 1 = V µ 2 = 0 . 05 , V e 2 = 0 . 05 i