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The Double-Polarization Observable G for the N(1440)½+ Resonance (Briscoe, Strakovsky) The N(1440)½+ (Roper) resonance has been a puzzle resulting in differing and at times controversial solutions. It is commonly thought of as a quark-model state, representing a radial excitation of the nucleon ground state. On the other hand, its energy is hard to obtain in quark models and its Breit-Wigner parameters differ substantially from its pole position. Some partial-wave analyses have indicated the presence of two poles [Arn91, Arn95], suggesting two resonances, although other interpretations are possible [Ede64, Cut90]. It has also been suggested that the Roper resonance is a candidate for a hybrid (non-qqq) state [Bar83, Li91]. With the polarized photon beam and target at MAMI in conjunction with the Crystal Ball, the quantity G can be examined over a large angular range across the Roper resonance. Calculations suggest that G is very sensitive to the properties of the N(1440) resonance [Bec00], as can be seen in Fig. 2, and so very significant new constraints can be placed on the interpretation of the resonance. Measurement of the double-polarization parameter G in the region of maximum sensitivity will be done during MAMI-B running in 2004.
Figure 2. Results of calculations including (solid line) and excluding (dashed line) the Roper [Bec00].
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