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arXiv 1109.2500hep-ph
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正向中子的单横自旋不对称性

Single transverse spin asymmetry of forward neutrons

B. Z. Kopeliovich, I. K. Potashnikova, Ivan Schmidt, J. Soffer

备注：
8 pages, 6 figures. Based on the talk given by B.K. at the Third International Workshop on Transverse Polarization Phenomena, Veli Losinj, Croatia, 29 August - 2 September 2011
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摘要：

本文计算了RHIC能区内$pp$碰撞中相对于极化质子的正向快度的单横自旋不对称性$A_N(t)$。对π介子磁极的吸收修正在自旋翻转和非翻转振幅之间产生相对相位，导致横向自旋不对称，这被发现太小而无法解释在PHENIX实验中观察到的$A_N$的大小。一个更大的贡献，它不会消失在高能量，来自π介子和$a_1$ -Reggeon交换的干扰。$a_1$的非自然奇偶性保证了相当大的相移，尽管幅度被衍射$πp\to a_1 p$横截面的小性强烈抑制。我们用与$1^+S$状态中的$πρ$交换相对应的Regge切割来替换Regge $a_1$极点。这种状态的产生，我们把它当作一个有效极$a$，在衍射$πp$相互作用中形成了$3π$不变质量分布的窄峰。横截面很大，因此可以假设这个状态饱和了轴向电流的谱函数，我们可以通过PCAC Goldberger-Treiman关系和第二温伯格求和规则确定它与核子的耦合。对$A_N$的无参数计算的数值结果与PHENIX数据吻合得很好。

英文摘要：

We calculate the single transverse spin asymmetry $A_N(t)$, for inclusive neutron production in $pp$ collisions at forward rapidities relative to the polarized proton in the energy range of RHIC. Absorptive corrections to the pion pole generate a relative phase between the spin-flip and non-flip amplitudes, leading to a transverse spin asymmetry which is found to be far too small to explain the magnitude of $A_N$ observed in the PHENIX experiment. A larger contribution, which does not vanish at high energies, comes from the interference of pion and $a_1$-Reggeon exchanges. The unnatural parity of $a_1$ guarantees a substantial phase shift, although the magnitude is strongly suppressed by the smallness of diffractive $πp\to a_1 p$ cross section. We replace the Regge $a_1$ pole by the Regge cut corresponding to the $πρ$ exchange in the $1^+S$ state. The production of such a state, which we treat as an effective pole $a$, forms a narrow peak in the $3π$ invariant mass distribution in diffractive $πp$ interactions . The cross section is large, so one can assume that this state saturates the spectral function of the axial current and we can determine its coupling to nucleons via the PCAC Goldberger-Treiman relation and the second Weinberg sum rule. The numerical results of the parameter-free calculation of $A_N$ are in excellent agreement with the PHENIX data.

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