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arXiv 0810.1167nucl-th
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冲激近似的崩溃及其后果:低Q^2问题

Breakdown of the impulse approximation and its consequences: the low-Q^2 problem

Artur M. Ankowski

摘要:

中微子散射数据和标准的截面计算结果在低Q ^2(四个动量转移平方)区域显示出差异。这些计算依赖于一个被称为冲量近似的假设,即中微子散射的原子核可以被描述为一个独立核子的集合,因此只有一个核子参与相互作用。由电子散射可知,只有当转移动量|q| >400 MeV/c时,这种图象才是有效的。对于较低的|q|,探测原子核的空间分辨率较低,只有少数核子参与散射,因此使用脉冲近似是不合理的。这意味着当|q| <400 MeV/c时,截面的标准计算是不可靠的。我表明,低动量相互作用的准弹性中微子截面的贡献不能减少到19%以下的任何选择的光束能量在几个GeV的区域。此外,低Q^2中微子事件也只有在转移动量很低的情况下才会发生。因此,低Q^2区域的差异是由蒙特卡罗模拟中常用的模型对低|q|相互作用的不恰当描述引起的。

英文摘要:

Neutrino scattering data and the standard calculations of the cross section show a discrepancy in the low-Q^2 (four-momentum transfer squared) region. The calculations rely on the assumption, called the impulse approximation, that the nucleus the neutrino scatters off can be described as a collection of independent nucleons and therefore only one nucleon takes part in the interaction. It is known from electron scattering that such picture is valid only when transferred momentum |q|>400 MeV/c. For lower |q|'s, the nucleus is probed with a lower spatial resolution and a few nucleons are involved in the scattering, so the use of the impulse approximation is unjustified. It means that the standard calculations of the cross sections are unreliable for |q|<400 MeV/c. I show that the contribution of low-momentum interactions to the quasielastic neutrino cross section cannot be reduced below ~19% by any selection of the beam energy in the few-GeV region. Moreover, low-Q^2 neutrino events happen only when transferred momentum is low too. Hence the discrepancy in the low-Q^2 region is caused by the inappropriate description of low-|q| interactions in the models commonly used in Monte Carlo simulations.

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