1 Yurii Maravin, SMU/CLEO Snowmass 2001 Experimental Aspects of  physics at CLEO-c 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.  The tau is the heaviest of the leptons  can provide important input on a number of fundamental question in particle physics:

2 Yurii Maravin, SMU/CLEO Snowmass 2001 Measurements of masses of tau and its neutrino Mass of tau lepton: MeV (BES ’91) 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 CL (Aleph ’98) it is difficult to measure better in high energy experiments. Fundamental parameter in the Standard Model, CLEO-c will have sensitivity to M  in the range 1-5 MeV.

3 Yurii Maravin, SMU/CLEO Snowmass 2001 Tau decay branching fractions Decay Br(Decay), %         e                                               0.10 Dominant Cabibbo-favored decay modes (world average values) Lepton Universality tests (discussed later), CLEO-c can measure branching fractions of many tau decay modes very precisely!  tests of CVC: Possible inconsistencies:  CVC predicts B(   ) = (24.64  0.29)%,  different from experiment by (0.76  0.32)%,  important state, accounts for 70% hadronic correction to BNL E821 a  measurement.

4 Yurii Maravin, SMU/CLEO Snowmass 2001 Tests of Lepton Universality Precision measurements of certain tau decay rates are important tests of  universality: g  and g  – charged weak couplings,  w and   – weak and EM radiative corrections. In Standard Model the ratio is exactly 1. (Transverse W boson couplings) CLEO-c can measure very precise mass of tau and B(  e e  ) m  measurement can permit testing of lepton universality to level.

5 Yurii Maravin, SMU/CLEO Snowmass 2001 Tests of Lepton Universality (Longitudinal W boson couplings) H is very well known, test is limited by B(   ) measurement (1%), CLEO-c can measure B(   ) very precisely due to distinct kinematics of the decay. CLEO-c can play an important role in testing lepton universality to level

6 Yurii Maravin, SMU/CLEO Snowmass 2001 Measurement of Michel parameters. Shape of the lepton energy spectrum is sensitive to non-Standard Model physics. It can be descried by Michel parameters:    and  measurement require knowledge of  spin (spin-correlation): at threshold spin correlations are different than at Y(4S)  measure  and  with different spin structure than BaBar and Belle,  unique opportunity for CLEO-c to measure  in       non-zero value of  would indicate a scalar current (charged Higgs). CLEO-c BaBar/Belle —  =0 —  =0.2

7 Yurii Maravin, SMU/CLEO Snowmass 2001 Hadronic dynamics of  decays. Production of light hadronic systemsKnowledge about strong dynamics CLEO-c has good opportunities to advance the understanding of light vector and axial meson resonance parameters and decay dynamics: CLEO-c has unique opportunities: studies of Wess-Zumino anomalous three meson coupling, measurements of non-perturbative parameters to aid the extraction of  s, tests of sum rules, searches for second class currents in   and   … tests of chiral perturbation theory (excellent acceptance at low q 2 ), measurement of vacuum polarization corrections to the muon magnetic moment (low systematic error)

8 Yurii Maravin, SMU/CLEO Snowmass 2001 Searches for New Physics Most interesting tau decays modes to search for non-Standard Model: Search for unknown massive neutrino:  - x, search for weakly interacting spin 0 or 1 neutral particle:  e - G, search for anomalous couplings in radiative decays:  e e   and   , search for CP violation in tau lepton decays. These are briefly discussed in the next several slides:

9 Yurii Maravin, SMU/CLEO Snowmass 2001 Search for  -   - X At threshold, momentum of  is monochromatic  direct information on the mass of neutrino M X M x = 600 MeV, Br(  X )=0.2% Not sensitive to tau neutrino mass, sensitive to exotic massive neutrino, large tau mass  large range of M X can be explored

10 Yurii Maravin, SMU/CLEO Snowmass 2001 Search for  -  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 Yurii Maravin, SMU/CLEO Snowmass 2001 Radiative tau decays Radiative tau decays are sensitive to non-SM couplings.  CLEO-c has unique opportunities due to small or absent initial and final state radiation.  CLEO II measured only a small  e  region: huge background from radiative tau pair production. High sensitivity in all  e  region, for 0.25 pb -1, the sensitivity to anomalous signal is ~10 -4.

12 Yurii Maravin, SMU/CLEO Snowmass 2001 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:  B-factories are better due to high luminosity (10 -2 level with 500 fb -1 ), Current CLEO result (preliminary): (assuming X~Y)  Search for CPV using spin correlations of tau leptons in              is another choice. CLEO-c is unique due to the different spin correlations near tau threshold  sensitivity to the different CP-violating effects.

13 Yurii Maravin, SMU/CLEO Snowmass 2001 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  in     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)