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                                 Abstract:The τ+τ- production has been studied in the data acquired by the Beijing Spectrometer (BES) at the comer-of-mass energy 4.03 GeV. Using a double-tagging technique, the decay τ~±→π~+π~-π~±υ_τhas been investigated. The branching ratios are determined to be Br(τ~±→a_1~±υ_τ→ρ~0π~±υ_τ, ρ~0→π~+π~-) = (7.3±0.5) % and Br(τ~±→ K~*±υτ→K_s~0π~±υτ, K_s~0→π~+π~-) = (6.0 ±1.5) ×10~(-3). From the Dalitz projection distribution, the dominant decay mode of ρπ in a~1 decay has been observed. Meanwhile,we have also found that the S-wave contribution dominates the a_1 decay. Applying the Kuhn model to fit the data, we have obtained the a_1 resonance parameters of m_a1=1.24±0.02 GeV and Γ_a1=0.57±0.07 GeV.
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                        References
	
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