Probing Neutral and Doubly-Charged Scalars at Future Lepton - PowerPoint PPT Presentation

Probing Neutral and Doubly-Charged Scalars at Future Lepton Colliders Fang Xu Collaborators: B. Dev, Y. Zhang Washington University in St. Louis xufang@wustl.edu October 13, 2019 H 3 & H ±± at future lepton colliders Washington University in St. Louis October 13, 2019 1 / 18

Overview Introduction 1 Motivation of the project Future lepton colliders Probing Yukawa couplings of H 3 and H ±± in eµ sector 2 Background & Signals at future lepton colliders Signals of neutral H 3 and doubly-charged H ±± Higgs Invariant mass for the signal and background Yukawa couplings in parameter space Conclusions 3 H 3 & H ±± at future lepton colliders Washington University in St. Louis October 13, 2019 2 / 18

Motivation of the project Many new physics scenarios beyond the Standard Model (SM) often necessitate the existence of new neutral ( H 3 ) and/or doubly-charged ( H ±± ) scalar fields, which might couple to the SM charged leptons through Yukawa interaction: L H 3 ⊃ Y αβ ¯ l α H 3 l β + h . c . (1) L H ++ ⊃ Y αβ l α H ++ l β + h . c . (2) For example, in left-right symmetric model (LRSM), the physical fields H 3 and H ±± comes from the triplet Higgs fields ∆ R : H 3 ≡ Re(∆ 0 ) and H ±± ≡ ∆ ±± R , where R √ � � ∆ + ∆ ++ L , R / 2 L , R √ ∆ L , R = (3) − ∆ + ∆ 0 L , R / 2 L , R L Y ⊃ Y L ,αβ L T L ,α C ∆ L L L ,β + Y R ,αβ L T R ,α C ∆ R L R ,β + h . c . (4) H 3 & H ±± at future lepton colliders Washington University in St. Louis October 13, 2019 3 / 18

Motivation of the project With the characters of H 3 and H ±± , we can explore the discovery prospect of them as well as the magnitude of the corresponding Yukawa couplings. We treat the center-of-mass energy √ s , Yukawa couplings Y αβ and the mass of H 3 and H ±± as parameters to simulate the e + e − collisions at future lepton colliders and to see to what extent the couplings can be probed. For now, we are only working in the electron-muon sector of Yukawa matrices. H 3 & H ±± at future lepton colliders Washington University in St. Louis October 13, 2019 4 / 18

Future lepton colliders Future lepton colliders provide a clean environment for the searches of the neutral and doubly-charged scalars. At LHC, although we can use pair production pp → H ++ H −− to search for the signal of doubly-charged scalars, the magnitude of Yukawa couplings cannot be probed. Table1: The planned center-of-mass energy and expected integrated luminosity for the International Linear Collider (ILC) and two stages of Compact Linear Collider (CLIC) √ s (TeV) L int (ab − 1 ) Collider ILC 1.0 1.0 1.5 2.5 CLIC 3.0 5.0 H 3 & H ±± at future lepton colliders Washington University in St. Louis October 13, 2019 5 / 18

Background & Signals at future lepton colliders At future lepton colliders, there are two kinds of interesting processes which can be used to probe the neutral and doubly-charged Higgs: e + e − → e + e − µ + µ − and e + e − → e + e + µ − µ − / e + e − → e − e − µ + µ + . And we notice that in SM, there is no process which can give a final state of the second type. For the simplest case, we assume only the off-diagonal terms of Yukawa matrices Y eµ are non-zero, which will cause lepton flavor violating (LFV) signals. In this case, the process e + e − → e + e − µ + µ − can be used to probe Yukawa couplings below 0.1 at future lepton colliders. H 3 & H ±± at future lepton colliders Washington University in St. Louis October 13, 2019 6 / 18

Background & Signals at future lepton colliders Fig.1: Feynman diagrams for the SM background H 3 & H ±± at future lepton colliders Washington University in St. Louis October 13, 2019 7 / 18

Background & Signals at future lepton colliders Fig.2: Feynman diagrams for the production of H 3 H 3 & H ±± at future lepton colliders Washington University in St. Louis October 13, 2019 8 / 18

Background & Signals at future lepton colliders Fig.3: Feynman diagrams for the single production of H ++ When √ s � 2 M H ±± , cross section ( ∝ | Y eµ | 2 ) are dominated by the pair production modes ( | Y eµ | independent) for small Yukawa couplings. H 3 & H ±± at future lepton colliders Washington University in St. Louis October 13, 2019 9 / 18

e + e − → e ± µ ∓ H 3 → e ± µ ∓ ( e ∓ µ ± ) e + e − → e ∓ µ ∓ H ±± → e ∓ µ ∓ ( e ± µ ± ) Here we are considering the on-shell single production. The decay branching ratios (BR) of H 3 → e ± µ ∓ are considered to be 50% respectively. The decay branching ratios of H ±± → e ± µ ± are considered to be 100%. In SM, there is no decay of the kind X → eµ (LFV) which can be distinguished from the SM background. The distribution of invariant mass of e ± µ ∓ should have a peak around the mass of H 3 for the signal. The distribution of invariant mass of e ± µ ± should have a peak around the mass of H ±± for the signal. H 3 & H ±± at future lepton colliders Washington University in St. Louis October 13, 2019 10 / 18

Invariant mass for the signal and background SM Background H 3 Signal SM Background H 3 Signal s = 1TeV s = 1TeV 0.14 0.14 M H 3 = 800GeV M H 3 = 800GeV 0.12 0.12 0.10 0.10 σ [ pb / bin ] σ [ pb / bin ] 0.08 0.08  Y e μ  = 0.2  Y e μ  = 0.2 0.06 0.06 0.04 0.04 0.02 0.02 0.00 0.00 0 200 400 600 800 1000 0 200 400 600 800 1000 M e + μ - [ GeV ] M e - μ + [ GeV ] Fig.4: Distributions of invariant mass M e + µ − (left) and M e − µ + (right) at √ s = 1 TeV, | Y eµ | = 0 . 2 , mass of neutral Higgs M H 3 = 800 GeV. H 3 & H ±± at future lepton colliders Washington University in St. Louis October 13, 2019 11 / 18

Invariant mass for the signal and background ++ Signal -- Signal SM Background H R SM Background H R s = 1TeV s = 1TeV 0.14 0.14 ±± = 800GeV ±± = 800GeV M H R M H R 0.12 0.12 0.10 0.10 σ [ pb / bin ] σ [ pb / bin ] 0.08 0.08 0.06 0.06  Y e μ  = 0.2  Y e μ  = 0.2 0.04 0.04 0.02 0.02 0.00 0.00 0 200 400 600 800 1000 0 200 400 600 800 1000 M e + μ + [ GeV ] M e - μ - [ GeV ] Fig.5: Distributions of invariant mass M e + µ + (left) and M e − µ − (right) at √ s = 1 TeV, | Y eµ | = 0 . 2 , mass of doubly-charged Higgs M H ±± = 800 GeV. R H 3 & H ±± at future lepton colliders Washington University in St. Louis October 13, 2019 12 / 18

Yukawa couplings in parameter space Signal significance: N ≥ 3 for a good confidence level S N = √ (5) S + B where S and B are the number of events for the signal and SM background. Signal significance can be improved through choosing cut properly. This will allow us to probe Yukawa couplings in a larger region of parameter space. For H 3 , we choose the cut to be M e ± µ ∓ ≥ 500GeV (ILC 1TeV), 600GeV (CLIC 1.5TeV) and 700GeV (CLIC 3TeV) For H ±± , we choose the cut to be M e ± µ ± ≥ 500GeV (ILC 1TeV), 750GeV (CLIC 1.5TeV) and 1500GeV (CLIC 3TeV). H 3 & H ±± at future lepton colliders Washington University in St. Louis October 13, 2019 13 / 18

Yukawa couplings in parameter space BR ( H 3 → e ± μ ∓ ) = 50 % ee →μμ ( LEP ) 1 CLIC 1.5TeV muonium oscillation ILC 1TeV 2 ) e g - ( 0.50 CLIC 3TeV  Y e μ  0.10 0.05 before cut after cut 1.0 1.5 2.0 2.5 3.0 M H 3 [ TeV ] Fig.6: Yukawa couplings as a function of neutral Higgs mass M H 3 at ILC (1TeV, 1ab − 1 ), CLIC (1.5TeV, 2.5ab − 1 & 3TeV, 5ab − 1 ) when N = 3 . H 3 & H ±± at future lepton colliders Washington University in St. Louis October 13, 2019 14 / 18

Yukawa couplings in parameter space 2 ) e g 5 - ( BR ( H L ±± → e ± μ ± ) = 100 % 2 g ) - ( μ ILC 1TeV CLIC 1.5TeV LHC 13TeV CLIC 3TeV 1  Y e μ  P E ) L ( μ 0.5 μ e e → Pair production Pair production before cut 0.1 after cut 1.0 1.5 2.0 2.5 3.0 ±± [ TeV ] M H L Fig.7: Yukawa couplings as a function of doubly-charged Higgs mass M H ±± at L ILC (1TeV, 1ab − 1 ), CLIC (1.5TeV, 2.5ab − 1 & 3TeV, 5ab − 1 ) when N = 3 . H 3 & H ±± at future lepton colliders Washington University in St. Louis October 13, 2019 15 / 18

Yukawa couplings in parameter space 2 ) e g 5 - ( BR ( H R ±± → e ± μ ± ) = 100 % 2 g ) - μ ( ILC 1TeV CLIC 1.5TeV CLIC 3TeV LHC 13TeV 1  Y e μ  P E ) L ( μ 0.5 μ e e → Pair production Pair production before cut 0.1 after cut 1.0 1.5 2.0 2.5 3.0 ±± [ TeV ] M H R Fig.8: Yukawa couplings as a function of doubly-charged Higgs mass M H ±± at R ILC (1TeV, 1ab − 1 ), CLIC (1.5TeV, 2.5ab − 1 & 3TeV, 5ab − 1 ) when N = 3 . H 3 & H ±± at future lepton colliders Washington University in St. Louis October 13, 2019 16 / 18