Experimental Aspects of t physics at CLEO-c

1. Experimental Aspects of t physics at CLEO-c The tau is the heaviest of the leptons  can provide important input on a number of fundamental question in particle physics: measurements of fundamental quantities, tests of weak couplings and lepton universality, studies of strong and weak interaction physics, direct searches for non-Standard Model physics.

2. Measurements of masses of tau and its neutrino Mass of tau lepton: 1776.96 MeV (BES ’91) +0.18+0.25 -0.21-0.17 Fundamental parameter in the Standard Model, important in lepton universality tests (discussed later), Can be obtained from kinematics (CLEO/Argus): ±1 MeV systematic error, best to scan a threshold region, can be done with ±0.1 MeV overall error. Mass of tau neutrino: <18.2 MeV@95% CL (Aleph ’98) Fundamental parameter in the Standard Model, it is difficult to measure better in high energy experiments. CLEO-c will have sensitivity to Mn in the range 1-5 MeV.

3. Tau decay branching fractions CLEO-c can measure branching fractions of many tau decay modes very precisely! Dominant Cabibbo-favored decay modes (world average values) Lepton Universality tests (discussed later), Decay Br(Decay), % ntm-nm17.37 0.07 nte-nm 17.83  0.06 tests of CVC: ntp- 11.59  0.12 ntp-p0 25.40  0.14 Possible inconsistencies: ntp-p+p- 9.18  0.11 • CVC predicts B(tpp0n) = (24.640.29)%, • different from experiment by (0.760.32)%, • important state, accounts for 70% hadronic correction to BNL E821 am measurement. ntp-p0p0 9.13  0.14 ntp-p+p-p0 4.20  0.08 ntp-3p0 1.08  0.10 . . . . . .

4. Tests of Lepton Universality (Transverse W boson couplings) Precision measurements of certain tau decay rates are important tests of  universality: gt and gm – charged weak couplings, dw and dg – weak and EM radiative corrections. In Standard Model the ratio is exactly 1. CLEO-c can measure very precise mass of tau and B(tenent) mt measurement can permit testing of lepton universality to 10-3 level.

5. Tests of Lepton Universality (Longitudinal W boson couplings) H is very well known, test is limited by B(tpn) measurement (1%), CLEO-c can measure B(tpn) very precisely due to distinct kinematics of the decay. CLEO-c can play an important role in testing lepton universality to 10-3 level

6. Measurement of Michel parameters. Shape of the lepton energy spectrum is sensitive to non-Standard Model physics. It can be descried by Michel parameters: r, h, x, d. • x and d measurement require knowledge of t spin (spin-correlation): CLEO-c • at threshold spin correlations are different than at Y(4S)  measure x and d with different spin structure than BaBar and Belle, — h=0 — h=0.2 BaBar/Belle • unique opportunity for CLEO-c to measure h in tmnmnt, •non-zero value of h would indicate a scalar current (charged Higgs).

7. Hadronic dynamics of t decays. Production of light hadronic systems Knowledge about strong dynamics CLEO-c has good opportunities to advance the understanding of light vector and axial meson resonance parameters and decay dynamics: studies of Wess-Zumino anomalous three meson coupling, measurements of non-perturbative parameters to aid the extraction of as, tests of sum rules, searches for second class currents in tphnt and t4pnt … CLEO-c has unique opportunities: • tests of chiral perturbation theory (excellent acceptance at low q2), • measurement of vacuum polarization corrections to the muon magnetic moment (low systematic error)

8. Searches for New Physics Most interesting tau decays modes to search for non-Standard Model: Search for unknown massive neutrino: tp-nx, search for weakly interacting spin 0 or 1 neutral particle: t  e-G, search for anomalous couplings in radiative decays: tenentg and tpntg, search for CP violation in tau lepton decays. These are briefly discussed in the next several slides:

9. Search for t-p-nX At threshold, momentum of p is monochromatic  direct information on the mass of neutrino MnX • Not sensitive to tau neutrino mass, • sensitive to exotic massive neutrino, • large tau mass  large range of MnX can be explored Mnx = 600 MeV, Br(tpnX)=0.2%

10. Search for t- e-G Similar to the massive neutrino search, we can search for weakly interacting particle G: t e G • Familon (Goldstone boson providing lepton flavor conservation ) • Monochromatic-like electron energy indicates two-body decay.

11. Radiative tau decays Radiative tau decays are sensitive to non-SM couplings. • CLEO II measured only a small qeg region: huge background from radiative tau pair production. • CLEO-c has unique opportunities due to small or absent initial and final state radiation. High sensitivity in all qeg region, • for 0.25 pb-1, the sensitivity to anomalous signal is ~10-4.

12. CP Violation Tau is the most massive lepton  CP violating effects could be enhanced (MHDM) • In MHDM: X,Y and Z are complex couplings to up-, down-quarks, and leptons  CP can be violated. • Most sensitive decay mode: tKpn Current CLEO result (preliminary): (assuming X~Y) B-factories are better due to high luminosity (10-2 level with 500 fb-1), • Search for CPV using spin correlations of tau leptons in t+t-(p+p0n)(p-p0n) is another choice. CLEO-c is unique due to the different spin correlations near tau threshold  sensitivity to the different CP-violating effects.

13. Conclusion CLEO-c is uniquely poised to make several important measurements in tau physics: Precise measurement of the tau lepton mass, improved constraints on the tau neutrino mass, precise measurement of key branching fractions, measurement of the Michel parameter hin tmnmnt, searches for exotic phenomena in tau decay. CLEO-c will provide the complementary results to the asymmetric B-factories. For more information, please look at CLEO-c and CESR-c: A New Frontier of Weak and Strong Interactions (CLNS 01/1742)