Constraints on the new particle in $Sigma^+to pmu^+mu^-$.pdf

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Constraints on the new particle in $Sigma^+to pmu^+mu^-$.pdf

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Constraintsonthenewparticlein$Sigma^topmu^mu^-$

a r X i v : h e p - p h / 0 5 0 9 1 7 5 v 2 2 6 O c t 2 0 0 5 Constraints on the new particle in Σ+ → pμ+μ? C. Q. Geng and Y. K. Hsiao Department of Physics, National Tsing Hua University, Hsinchu, Taiwan 300 (Dated: February 2, 2008) Abstract The HyperCP collaboration has presented the branching ratio of Σ+ → pμ+μ? to be (8.6+6.6 ?5.4 ± 5.5) × 10?8 and suggested a new boson P 0 with a mass of 214.3 ± 0.5 MeV to induce the flavor changing transition of s → dμ+μ?. We demonstrate that to explain the data, the new particle cannot be a scalar but pseudoscalar based on the direct constraints from K+ → π+μ+μ? and KL → μ+μ?, respectively. Moreover, we determine that the decay width of the pseudoscalar should be in the range of 10?7 MeV with the lifetime of 10?14 sec. PACS numbers: 11.30.Er, 13.25.Hw 1 According to the report of the HyperCP collaboration [1], the observation of three events for the decay Σ+ → pμ+μ? reveals the possibilities of new physics, as the branching ratio Br(Σ+ → pμ+μ?) = (8.6+6.6 ?5.4± 5.5)× 10?8 is claimed to be larger than the prediction within the Standard Model [1, 2, 3]. The analysis in Ref. [1] has found an unexpectedly narrow dimuon distribution, which can not be explained by the form factors used to deform the phase space due to their mildly momentum dependences [2, 3]. The plausible explanation can be the threshold effect which is induced as mμ+μ? = (pμ+ + pμ?) is approaching to the pole of some unknown intermediate boson, suggesting a two-body Σ+ → pP 0, P 0 → μ+μ? decay shown in Fig. 1a, with the P 0 mass being mP 0 = 214.3 ± 0.5 MeV [1] and the branching ratio [1] Br(Σ+ → pP 0, P 0 → μ+μ?) = (3.1+2.4 ?1.9 ± 1.5)× 10?8. (1) If the effect is true, the flavor-changing neutral current (FCNC) of the s → d transition is discovered at tree level. Clearly, the most important task is to check the reality of the experiment. Note that the observed events are only three and the physical properties of this unknown particle remain ambiguous. Neve