Search for tau-e (tau-mu) flavor mixing at a linear collider

1. 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

2. Introduction • LFV is a clear signal for physics beyond the SM. e ⇔μμ⇔ττ⇔e • Neutrino oscillation may indicate LFV among charged leptons. • In SUSY models, LFV can naturally appear. Borzumati, Masiero Hisano et al.

3. 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 1.2 ×10＾（－11）　　　　　　　 μ→３e1.1×10＾（－12） μTi→eTi6.1 ×10＾（－13） τ→μγ 3.1 ×10＾（－7） τ→３μ 1.4-3.1 ×10＾（－7） τ→μη3.4 ×10＾（－7）　

4. 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

5. Babu, Kolda; Dedes,Ellis,Raidal; Kitano, Koike, Okada LFV Yukawa coupling Slepton mixing induces LFV in SUSY models. κij= Higgs LFV parameter

6. Consider that MSUSY is as large as O(1) TeV with a fixed value of |μ|/MSUSY While gauge mediated LFVis suppressed, the Higgs-LFV coupling κijcan be sufficiently large . Babu,Kolda; Brignole, Rossi mSUSY ～　O(1) TeV

7. Search for Higgs mediated τ- e & τ- μ mixing • Tau’s rare decays τ→eππ　（μππ） τ→eη (μη) τ→μe e (μμμ)、　…. In nearfuture, 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)

8. 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. • LHCAssamagan et al; Brignole, Rossi • LC Kanemura, Ota, et al., PLB599(2004)83. Search for h →τμ (τe) atLC: • Simple kinematic structure (Esp. Higgssrahlung process) • Precise measurements: property (mh,Γ,σ,Br,…) will be thoroughly measured • Less backgrounds

9. Higgs Production at a LC ～10^5 Higgs produced Decay branching ratio (h→τμ) The branching ratio of 10^(-4) – 10^(-3) is possible.

10. Signal The process can be identified by using Z recoil: • Theτmomentum is reconstructed by using Ecm, mh, pZand pμ It is not required to measure τ • The # of the signalevent 11 event for leptonic decay of Z 118 event for hadronic decay

11. Backgrounds

12. 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.

13. 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→τＸprocess Sher, Turan, PRD69(2004)302 Kanemura, et al, hep-ph/0410044 • 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

14. Cross section in SUSY model • 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 GeV10^(-4)fb 250 GeV10^(-3)fb CTEQ6L

15. Energy distribution for each angle • From thelL beam, τR is emitted to the backward direction due to (1 ー cosθCM)nature in the CM frame. • In Lab-frame, tau is emitted forward direction but with large angle with a PT. 2 E＝100 GeV E＝500 GeV

16. 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 producedfor 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-Φcoupling 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) Hard hadrons emitted into the same direction as the parent τ’s Bullock, Hagiwara, Martin

17. 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→ lX 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.

18. Summary • Possibility of measuring LFV via e N→τX by using the high energy electron beam of a LC with a fixed-target. • 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.