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Rensselaer Polytechnic Institute, Troy, New York 12180-3590, USA.
Phys Rev Lett. 2006 Sep 15;97(11):112003. doi: 10.1103/PhysRevLett.97.112003.
We report a new measurement of the exclusive electroproduction reaction gamma()p-->pi(0)p to explore the evolution from soft nonperturbative physics to hard processes via the Q(2) dependence of the magnetic (M(1+)), electric (E(1+)), and scalar (S(1+)) multipoles in the N-->Delta transition. 9000 differential cross section data points cover W from threshold to 1.4 GeV/c(2), 4pi center-of-mass solid angle, and Q(2) from 3 to 6 GeV(2)/c(2), the highest yet achieved. It is found that the magnetic form factor G(M)() decreases with Q(2) more steeply than the proton magnetic form factor, the ratio E(1+)/M(1+) is small and negative, indicating strong helicity nonconservation, and the ratio S(1+)/M(1+) is negative, while its magnitude increases with Q(2).
我们报告了对专属电产生反应γ()p→π(0)p的一项新测量，以通过N→Δ跃迁中磁（M(1+)）、电（E(1+)）和标量（S(1+)）多极子的Q(2)依赖性来探索从软非微扰物理到硬过程的演化。9000个微分截面数据点涵盖了从阈值到1.4 GeV/c(2)的W、4π质心立体角以及从3到6 GeV(2)/c(2)的Q(2)，这是迄今所达到的最高值。结果发现，磁形状因子G(M)()随Q(2)的下降比质子磁形状因子更陡峭，E(1+)/M(1+)的比值小且为负，表明强烈的螺旋度不守恒，S(1+)/M(1+)的比值为负，但其大小随Q(2)增加。
