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Probing Neutral and Doubly-Charged Scalars at FutureLepton Colliders
Fang Xu
Collaborators: B. Dev, Y. Zhang
Washington University in St. Louis
October 13, 2019
Washington University in St. Louis H3 & H±± at future lepton colliders October 13, 2019 1 / 18
Overview
1 IntroductionMotivation of the projectFuture lepton colliders
2 Probing Yukawa couplings of H3 and H±± in eµ sectorBackground & Signals at future lepton collidersSignals of neutral H3 and doubly-charged H±± HiggsInvariant mass for the signal and backgroundYukawa couplings in parameter space
3 Conclusions
Washington University in St. Louis H3 & H±± at future lepton colliders October 13, 2019 2 / 18
Motivation of the project
Many new physics scenarios beyond the Standard Model (SM) oftennecessitate the existence of new neutral (H3) and/or doubly-charged(H±±) scalar fields, which might couple to the SM charged leptonsthrough Yukawa interaction:
LH3 ⊃ Yαβ l̄αH3lβ + h.c. (1)
LH++ ⊃ YαβlαH++lβ + h.c. (2)
For example, in left-right symmetric model (LRSM), the physicalfields H3 and H±± comes from the triplet Higgs fields ∆R:H3 ≡ Re(∆0) and H±±
R ≡ ∆±±R , where
∆L,R =
(∆+
L,R/√
2 ∆++L,R
∆0L,R −∆+
L,R/√
2
)(3)
LY ⊃ YL,αβLTL,αC∆LLL,β + YR,αβL
TR,αC∆RLR,β + h.c. (4)
Washington University in St. Louis H3 & H±± at future lepton colliders October 13, 2019 3 / 18
Motivation of the project
With the characters of H3 and H±±, we can explore the discoveryprospect of them as well as the magnitude of the correspondingYukawa couplings.
We treat the center-of-mass energy√s, Yukawa couplings Yαβ and
the mass of H3 and H±± as parameters to simulate the e+e−
collisions at future lepton colliders and to see to what extent thecouplings can be probed.
For now, we are only working in the electron-muon sector of Yukawamatrices.
Washington University in St. Louis H3 & H±± at future lepton colliders October 13, 2019 4 / 18
Future lepton colliders
Future lepton colliders provide a clean environment for the searches ofthe neutral and doubly-charged scalars.
At LHC, although we can use pair production pp→ H++H−− tosearch for the signal of doubly-charged scalars, the magnitude ofYukawa couplings cannot be probed.
Table1: The planned center-of-mass energy and expected integratedluminosity for the International Linear Collider (ILC) and two stages of
Compact Linear Collider (CLIC)
Collider√s (TeV) Lint (ab−1)
ILC 1.0 1.0
CLIC1.5 2.53.0 5.0
Washington University in St. Louis H3 & H±± at future lepton colliders October 13, 2019 5 / 18
Background & Signals at future lepton colliders
At future lepton colliders, there are two kinds of interesting processeswhich 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 finalstate of the second type.
For the simplest case, we assume only the off-diagonal terms ofYukawa matrices Yeµ are non-zero, which will cause lepton flavorviolating (LFV) signals.
In this case, the process e+e− → e+e−µ+µ− can be used to probeYukawa couplings below 0.1 at future lepton colliders.
Washington University in St. Louis H3 & H±± at future lepton colliders October 13, 2019 6 / 18
Background & Signals at future lepton colliders
Fig.1: Feynman diagrams for the SM background
Washington University in St. Louis H3 & H±± at future lepton colliders October 13, 2019 7 / 18
Background & Signals at future lepton colliders
Fig.2: Feynman diagrams for the production of H3
Washington University in St. Louis H3 & H±± at future lepton colliders October 13, 2019 8 / 18
Background & Signals at future lepton colliders
Fig.3: Feynman diagrams for the single production of H++
When√s & 2MH±± , cross section (∝ | Yeµ |2) are dominated by the pair
production modes (| Yeµ | independent) for small Yukawa couplings.Washington University in St. Louis H3 & H±± at future lepton colliders October 13, 2019 9 / 18
e+e− → e±µ∓H3 → 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 H3 → e±µ∓ are considered to be50% respectively.
The decay branching ratios of H±± → e±µ± are considered to be100%.
In SM, there is no decay of the kind X → eµ (LFV) which can bedistinguished from the SM background.
The distribution of invariant mass of e±µ∓ should have a peakaround the mass of H3 for the signal.
The distribution of invariant mass of e±µ± should have a peakaround the mass of H±± for the signal.
Washington University in St. Louis H3 & H±± at future lepton colliders October 13, 2019 10 / 18
Invariant mass for the signal and background
SM Background H3 Signal
0 200 400 600 800 10000.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
Me+ μ- [GeV]
σ[pb/bin]
Yeμ = 0.2
s = 1TeV
MH3 = 800GeV
SM Background H3 Signal
0 200 400 600 800 10000.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
Me- μ+ [GeV]
σ[pb/bin]
Yeμ = 0.2
s = 1TeV
MH3 = 800GeV
Fig.4: Distributions of invariant mass Me+µ− (left) and Me−µ+ (right) at√s = 1TeV, | Yeµ |= 0.2, mass of neutral Higgs MH3
= 800GeV.
Washington University in St. Louis H3 & H±± at future lepton colliders October 13, 2019 11 / 18
Invariant mass for the signal and background
SM Background HR++ Signal
0 200 400 600 800 10000.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
Me+ μ+ [GeV]
σ[pb/bin]
Yeμ = 0.2
s = 1TeV
MHR±± = 800GeV
SM Background HR-- Signal
0 200 400 600 800 10000.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
Me- μ- [GeV]
σ[pb/bin]
Yeμ = 0.2
s = 1TeV
MHR±± = 800GeV
Fig.5: Distributions of invariant mass Me+µ+ (left) and Me−µ− (right) at√s = 1TeV, | Yeµ |= 0.2, mass of doubly-charged Higgs MH±±
R= 800GeV.
Washington University in St. Louis H3 & H±± at future lepton colliders October 13, 2019 12 / 18
Yukawa couplings in parameter space
Signal significance: N ≥ 3 for a good confidence level
N =S√S +B
(5)
where S and B are the number of events for the signal and SMbackground.
Signal significance can be improved through choosing cut properly.This will allow us to probe Yukawa couplings in a larger region ofparameter space.
For H3, we choose the cut to be Me±µ∓ ≥ 500GeV (ILC 1TeV),600GeV (CLIC 1.5TeV) and 700GeV (CLIC 3TeV)
For H±±, we choose the cut to be Me±µ± ≥ 500GeV (ILC 1TeV),750GeV (CLIC 1.5TeV) and 1500GeV (CLIC 3TeV).
Washington University in St. Louis H3 & H±± at future lepton colliders October 13, 2019 13 / 18
Yukawa couplings in parameter space
ILC 1TeV
CLIC 3TeV
CLIC 1.5TeVBR(H3→e±μ∓) = 50%
before cutafter cut
1.0 1.5 2.0 2.5 3.0
0.05
0.10
0.50
1
MH3 [TeV]
Yeμ
(g-2)emuonium o
scillationee→μμ (LEP
)
Fig.6: Yukawa couplings as a function of neutral Higgs mass MH3at ILC (1TeV,
1ab−1), CLIC (1.5TeV, 2.5ab−1 & 3TeV, 5ab−1) when N = 3.
Washington University in St. Louis H3 & H±± at future lepton colliders October 13, 2019 14 / 18
Yukawa couplings in parameter space
LHC13TeV
ILC 1TeV
CLIC 3TeV
CLIC 1.5TeV
BR(HL±±→e±μ±) = 100%
before cutafter cut
1.0 1.5 2.0 2.5 3.0
0.1
0.5
1
5
MHL±± [TeV]
Yeμ
ee→μμ (LEP)
(g-2)μ
(g-2)e
Pairproduction
Pairproduction
Fig.7: Yukawa couplings as a function of doubly-charged Higgs mass MH±±L
at
ILC (1TeV, 1ab−1), CLIC (1.5TeV, 2.5ab−1 & 3TeV, 5ab−1) when N = 3.
Washington University in St. Louis H3 & H±± at future lepton colliders October 13, 2019 15 / 18
Yukawa couplings in parameter space
LHC13TeV
ILC 1TeV
CLIC 3TeVCLIC 1.5TeV
BR(HR±±→e±μ±) = 100%
before cutafter cut
1.0 1.5 2.0 2.5 3.0
0.1
0.5
1
5
MHR±± [TeV]
Yeμ
ee→μμ (LEP)
(g-2)μ(g-2
)e
Pairproduction
Pairproduction
Fig.8: Yukawa couplings as a function of doubly-charged Higgs mass MH±±R
at
ILC (1TeV, 1ab−1), CLIC (1.5TeV, 2.5ab−1 & 3TeV, 5ab−1) when N = 3.
Washington University in St. Louis H3 & H±± at future lepton colliders October 13, 2019 16 / 18
Conclusions
At ILC 1TeV stage, it is hard to probe the doubly-charged Higgs whilewe still have some sensitive region for the search of neutral Higgsaround | Yeµ |∼ 0.1
At CLIC 1.5TeV and 3TeV stages, both neutral and doubly-chargedHiggs have some sensitive region in the parameter space. But the pairproduction modes of doubly-charged Higgs will prevent us fromprobing the Yukawa couplings at the region MH±± .
√s/2
For now, we have just considered the off-diagonal terms in theYukawa coupling matrices of neutral and doubly-charged Higgs fields.The next step is to include all the elements in the eµ sector of Yukawacoupling matrices, in which another process e+e− → e±e±µ∓µ∓ willalso give a signal that SM does not have. And this process would bea good platform for the search of neutral and doubly-charged Higgs.
Washington University in St. Louis H3 & H±± at future lepton colliders October 13, 2019 17 / 18
Thank You !
Washington University in St. Louis H3 & H±± at future lepton colliders October 13, 2019 18 / 18