Search for lepton flavor violation in the Higgs boson

Report
Search for tau-e (tau-mu) flavor
mixing at a linear collider
Shinya KANEMURA
(Osaka Univ.)
with
Yoshitaka KUNO, Toshihiko OTA (Osaka Univ)
Masahiro Kuze
(Tokyo Inst. Tech.)
ACFA’07, Nov 11. 2004, at National Taiwan University
Introduction
LFV is a clear signal for physics beyond the SM.
e ⇔μ
μ⇔τ
τ⇔e
Neutrino oscillation may indicate LFV among charged leptons.
In SUSY models, LFV can naturally appear.
Borzumati, Masiero
Hisano et al.
In this talk, we discuss tau-associated LFV in
SUSY models
τ⇔e & τ ⇔μ


The Higgs mediated LFV is proportional to the Yukawa
coupling
⇒ Tau-associated LFV processes.
It is less constraind by current data as compared to theμ⇔e
mixing
μ→eγ
μ→3e
μTi→eTi
τ→μγ
τ→3μ
τ→μη
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)
LFV in SUSY
LFV is induced at one loop due to slepton mixing
Gauge mediation :
Higgs mediation :
Higgs mediation does not decouple in the large MSUSY limit
LFV Yukawa coupling
Slepton mixing induces LFV
in SUSY models.
κij
= Higgs LFV parameter
Babu, Kolda;
Dedes,Ellis,Raidal;
Kitano, Koike, Okada
Consider that MSUSY is as large as O(1) TeV
with a fixed value of |μ|/MSUSY
While gauge mediated LFV is suppressed,
the Higgs-LFV coupling κij can be sufficiently large .
Babu,Kolda;
Brignole, Rossi
mSUSY ~ O(1) TeV
Search for Higgs mediated
τ- e & τ- μ mixing
Tau’s rare decays
τ→eππ (μππ)
τ→eη
(μη)
τ→μe e (μμμ)、 ….
In near future, τ decay searches will improve the upper limit
by 1-2 orders of magnitude.
Other possibilities ?


Higgs decays into a tau-mu or tau-e pair
The DIS process e N (μN) →τX
by a fixed target experiment at a LC (μC)
Higgs boson decay
After the Higgs boson is found, we can consider the
possibility to measure the LFV Higgs couplings directly
from the decay of the Higgs bosons.
 LHC
Assamagan et al; Brignole, Rossi
 LC
Kanemura, Ota, et al.,
PLB599(2004)83.
Search for h →τμ (τe) at LC:



Simple kinematic structure (Esp. Higgssrahlung
process)
Precise measurements:
property (mh,Γ,σ,Br,…) will be thoroughly measured
Less backgrounds
Higgs Production at a LC
~10^5 Higgs produced
Decay branching ratio (h→τμ)
The branching ratio of
10^(-4) – 10^(-3)
is possible.
Signal
The process can be identified by using
Z recoil:
Theτmomentum is reconstructed by
using Ecm, mh, pZ and pμ
It is not required to measure τ
The # of the signal event
11 event for leptonic decay of Z
118 event for hadronic decay
Backgrounds
Feasibility
Resolution of Z momentum
Signal / Fake
118 / 230 events
(Z →jj、 δ=3GeV)
11 / 8 events
(Z→ll, δ=1GeV)
For some specific parameter region,
h → τμ (τe) can be studied at a LC.
No big advantage, although it depends on machine.
Alternative process for search of the Higgs
LFV coupling?
At future ν factories (μ colliders) ,
10^20 muons of energy 50 GeV
(100-500GeV) can be available.
DIS μN→τX process
τ
μ(e)
h, H, A
q
Sher, Turan, PRD69(2004)302
Kanemura, et al, hep-ph/0410044
q
N
X
At a LC (Ecm=500GeV L=10^34/cm^2/s)
10^22 of 250GeV electrons available.
We here consider the LFV DIS processes
eN→τX
by using the electron (positron) beam of a LC
A fixed target experiment option of LC
Cross section in SUSY model
h, H, A
Each sub-process
e q (μq) →τq
is proportional to the d-type
quark masses.
For the energy > 60 GeV,
the total cross section
is enhanced due to
the b-quark sub-process
Eμ=50 GeV 10^(-5)fb
100 GeV 10^(-4)fb
250 GeV 10^(-3)fb
τ
μ(e)
q
q
N
X
CTEQ6L
Energy distribution for each angle
 From the l L 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 large angle with a PT.
E=100 GeV
Lab-frame
μL
θ
Target
E=500 GeV
τR
Signal
Number of taus (case of electron beam)
E=250 GeV, L =10^34 /cm^2/s, ⇒ 10^22 electrons (positrons)
in a SUSY model with |κ3i |^2=0.3×10^(-6): σ=10^(-3) fb
10^5 of τleptons are produced for the target of ρ=10 g/cm^2
Naively, non-obervation of the e N → τ X process may improve the
current upper limit on the e-τ-Φ coupling by around 4-5 orders of
magnitude
We may consider its hadronic products as the signal
τ→(π、ρ, a1, …)+ missings
# of hadrons ≒ 0.3×(# of tau)
τR
νL
π
Bullock, Hagiwara, Martin
Hard hadrons emitted into the same direction as the parent
τ’s
Backgrounds
Hadrons from the target (N) should be softer,
and more unimportant for higher energies of the
initial e or μ beam.
Hard leptons from l N→ l X would be be a fake
signal via mis-ID of l as π. (l = e or μ)



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.
Summary
Possibility of measuring LFV via e N→τX by using
the high energy electron beam of a LC with a fixedtarget.
Ecm=500GeV ⇒ σ=10^(-3) fb
L=10^34/cm^2/s ⇒ 10^22 electrons available
10^5 of taus are produced for ρ=10 g/cm^2
Non-observation of the signal would improve the
current limit on the τ-e-Φ coupling by 10^(4-5).
The signal would be hard hadrons from τ→πν、ρν,
a1ν, .... , which go along the τdirection.
Main background: mis-ID of e from eN→eX.
Background simulation will be done.

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