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Search for B0 → ρ0ρ0 and non-resonant B0 → 4π

Abe K., Adachi I., Aihara H., Arinstein K., Aso T., Aulchenko V., Aushev T., Aziz T., Bahinipati S., Bakich A. M., Balagura V., Ban Y., Banerjee S., Barberio E., Bay A., Bedny I., Belous K., Bhardwaj V., Bitenc U., Blyth S., Bondar A., Bozek A., Bracko M., Brodzicka J., Browder T. E., Chang M. C., Chang P., Chao Y., Chen A., Chen K. F., Chen W. T., Cheon B. G., Chiang C. C., Chistov R., Cho I. S., Choi S. K., Choi Y., K. Choi Y., Cole S., Dalseno J., Danilov M., Das A., Dash M., Dragic J., Drutskoy A., Eidelman S., Epifanov D., Fratina S., Fujii H., Fujikawa M., Gabyshev N., Garmash A., Go A., Gokhroo G., Goldenzweig P., Golob B., Grosse Perdekamp M., Guler H., Ha H., Haba J., Hara K., Hara T., Hasegawa Y., Hastings N. C., Hayasaka K., Hayashii H., Hazumi M., Heffernan D., Higuchi T., Hinz L., Hoedlmoser H., Hokuue T., Horii Y., Hoshi Y., Hoshina K., Hou S., Hou W. S., Hsiung Y. B., Hyun H. J., Igarashi Y., Iijima T., Ikado K., Inami K., Ishikawa A., Ishino H., Itoh R., Iwabuchi M., Iwasaki M., Iwasaki Y., Jacoby C., Joshi N. J., Kaga M., Kah D. H., Kaji H., Kajiwara S., Kakuno H.

We search for the decay B0 → ρ0ρ0 and other possible charmless modes with a π+ π- π+ π- final state, including B0 → ρ0f0(980), B0 → f0(980)f0(980), B0 → f0(980)π π, B0 → ρ0π π and non-resonant B0 → 4π. These results are obtained from a data sample containing 520 × 106 BB pairs collected by the Belle detector at the KEKB asymmetric-energy e+e- collider. We measure a branching fraction of (0.9 ± 0.4+0.3 -0.4) × 10-6, or B(B 0 → ρ0ρ0) < 1.6 × 10-6 at the 90% confidence level. The significance including systematic uncertainties is 1.8σ. These values correspond to the final state being longitudinally polarized. We also measure the branching fraction of non-resonant B0 → 4π decay to be (10.2 ± 4.7+2.3 -1.5) × 10-6 with 2.1σ significance, and set the 90% confidence level upper limit B(B0 → 4π) < 17.3 × 10-6. For the other related decays, B0 → ρ0f0(980), B0 → f0(980)f0(980), B0 → f0(980)ρ and B0 → ρ0π π, no significant signals are observed and upper limits on the branching fractions are set.