Search for lepton flavor violating
μＮ→τＸ reaction with
high energy muons
Shinya KANEMURA
(Osaka Univ.)
with
Yoshitaka KUNO, Masahiro KUZE, Toshihiko OTA
TAU ‘04, Sep 16. 2004, Nara, JAPAN
Introduction
 LFV is a clear signal for physics beyond the SM.
 Neutrino oscillation may indicate the possibility
of LFV in the charged lepton sector.
 In new physics models, LFV naturally appears.
 SUSY (slepton mixing)
Borzumati, Masiero
Hisano et al.
 Zee type models for the ν mass
Zee
 Models of dynamical flavor violation
(Topcolor, Top seesaw etc)
Hill et al.
Experimenal bounds on
LFV processes
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process
μ→eγ
μ→３e
μTi→eTi
τ→μγ
τ→３μ
τ→μη
branching ratio
1.2 ×10＾（－11）
1.1 ×10＾（－12）
6.1 ×10＾（－13）
3.1 ×10＾（－7）
1.4-3.1 ×10＾（－7）
3.4 ×10＾（－7）
Present experimental bounds on the tau associated
processes are milder than those on the e-μLFV.
In this talk, we consider tau-associated LFV
 The discovery of large mixing between νμandντ may
be related to large LFV in the μ-τsector
 In SUSY models, the Higgs mediated LFV can
contribute to the tau-associated process with the
enhancement of the tau lepton mass.
Constraints on theτ-μeffective
couplings from current data
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
Scalar coupling
τ→μππ Λ～2.6 TeV
Pseudo-scalar coupling τ→μη
～12 TeV
Vector
τ→μφ ～14 TeV
Pseudo-vector
τ→μπ ～11 TeV
Black, Han, He, Sher, 2002
LFV in SUSY
In SUSY model, effects of slepton mixing can induce LFV
via loop diagrams
gauge mediation =
(pseudo) vector coupling
tensor coupling
Higgs mediation = (pseudo) scalar coupling
∝ lepton mass : → τ-associated process
LFV Yukawa coupling
Slepton mixing induce LFV
in SUSY models.
κij
= Higgs LFV parameter
Babu, Kolda;
Dedes,Ellis,Raidal;
Kitano, et al.
Decoupling property
 Gauge mediation (Dim=5) :
decouple for large MSUSY
 Higgs mediation (Dim=4)
Does not decouple in the large MSUSY limit
Consider that MSUSY is as large as O(1) TeV
with a fixed value of |μ|/MSUSY
A sufficiently large Higgs mediated LFV coupling can be realized
in a SUSY model, with the suppressed gauge mediated LFV.
Babu,Kolda;
Brignole, Rossi
For mA=150GeV and tanβ=60,
Alternative process for the search
of the Higgs LFV coupling
 Future τ decay search may improve the upper
limit by one or two orders of magnitude.
 Do we have another way to measure the Higgs
LFV coupling?
 At future neutrino factories (muon colliders),
Energy 50 GeV (100-500GeV)
10^20 muons can be available.
 We here consider
the DIS process μＮ→τＸ
from such intense muon beam.
The DIS process μN→τX
μL
τR
h, H, A
q
q
N
X
At either a neutrino factory or a muon collider
 High energy muon beam (Eμ＝２０－３００ GeV)
 Intensity （10＾20 muons/year）
CERN SPS muon beam
S.N. Gninenko, et al.,
CERN-SPSC-2004-016
SPSC-EOI-004
SPS muon beam 10-100GeV
Tau detection by NOMAD
Quasi-Elastic scattering of μＮ→τＮ
↓
τ→μνν
Details will be presented at the SPSC Villars Meeting
22-28 Sept’04
The cross section of μＮ→τＸ
Effective scalar coupling
（using limit fromτ→μππ）
σ ＜～ 0.5 fb
Sher
⇒ 10^6×ρ[g/cm^3] tau’s
from intensity 10^20 muons
Pseudo-scalar coupling
(using limit from τ→μη)
σ ＜～ 10^(-4) fb
In SUSY,
scalar coupling
= pseudo-scalar coupling
The cross section is
10^(-4)－10^(-5) smaller than
the scalar coupling case
Enhancement of the SUSY cross section
 Each sub-process
μq→τq
is proportional to the quark
masses because of the
Yukawa coupling.
 For the energy > 50 GeV,
the hadronic cross section
is enhanced due to
the b-quark sub-process
Eμ＝50 GeV 10^(-5)fb
100 GeV 10^(-4)fb
300 GeV 10^(-3)fb
 Importance of higher energy
beam than 50 GeV
CTEQ6L
Angular distribution
Higgs mediation
→ chirality flipped
2
→ （1－cosθCM）
Lab-frame
μL
τR
θ
Target
Lab-frame
Energy distribution for each angle
 From theμL beam, τR is emitted to the backward direction
due to (1 ー cosθCM) 2nature in the CM frame.
 In Lab-frame, tau is emitted forward direction but with
relatively large angle with a PT.
Eμ＝50 GeV
Eμ＝100 GeV
Eμ＝500 GeV
Signal
 Number of tau for L =10^20 muons in a SUSY model
with |κ32|^2=0.3×10^(-6):
Eμ＝50 GeV
100×ρ[g/cm^3] ofτleptons
100 GeV
1000
500 GeV
50000
 We can consider its hadronic products as the signal
τ→(π、ρ, a1, …)＋ missings
 Hard hadrons emitted into the same direction as the
parent τ’s
τR ⇒ backward νL + forward π,ρ、….
τR
νL
π
Bullock, Hagiwara, Martin
 # of hard hadrons
≒ 0.3 × (# of tau)
Backgrounds
 Hadrons from the target (N) should be softer,
and more unimportant for higher energies of the
initial muon beam.
 Hard muons from μN→μX may be a fake signal
via mis-ID of μas π.
 Rate of mis-ID
 Emitted to forwad direction without large PT due to the
Rutherford scattering
1/sin^4(θcM/2)
⇒ PT cuts
 Other factors to reduce the fake
 Realistic Monte Carlo simulation is necessary to
see the feasibility
Summary
 We discussed the possibility of measuring LFV via
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μN→τX by the intense high energy beam.
Non-observation of the signal can improve the present
limit on the scalar LFV coupling by ～10^6.
In the SUSY model (scalar coupling=p-scalar coupling),
100-10000 tau leptons can be produced for Eμ=50-500
GeV.
For Eμ > 50 GeV, the cross section is enhanced due to
the b-quark sub-process.
The signal is hard hadrons from τ→πν、ρν,
a1ν, .... , which go along the τdirection.
Main background: mis-ID of μ from μN→μX.
Different distribution: PT cut may be effective.
Realistic background simulation should be done.
Note added
 In the similar way, we can consider search
for e-τconversion via the DIS process of e
N →τ X.
 At a linear collider (E=500GeV L=10^34/cm^2/s)
10^22 electrons of E=250GeV available.
The constraint on the (eτqq) coupling can
be improved via e N →τ X by 10^8 as
compared to that by τ→eππ.