arXiv 1111.37132011-11-16hep-exhep-ph

中微子相互作用中的单光子事件

A Search for Single Photon Events in Neutrino Interactions

C. T. Kullenberg, S. R. Mishra

DOI：: 10.1016/j.physletb.2011.11.049

摘要：

我们提出了一个搜索中微子诱导的事件包含一个单一的，排他性的光子使用的数据从NOMAD实验在欧洲核子研究中心SPS的中微子通量的平均能量是$\simeq 25$ GeV。该搜索的动机是由MiniBOONE实验报告的200- 475 MeV能量区域中的过量类电子事件。在NOMAD中，光子通过它们在嵌入磁场中的活性靶中转化为$e^+e^-$来识别。单光子信号的背景主要由中性π介子的不对称衰变所决定，所述中性π介子的不对称衰变产生于相干中微子-核相互作用中，或者产生于中微子-核子中性流深度非弹性散射中，或者产生于基准体积之外的相互作用中。在打开“信号盒”之前，使用对照数据样本{ \it in situ}测定所有三种背景。在信号区，我们观察到{ \bf 155}事件，预测背景为{ \bf 129.2 $\pm$ 8.5 $\pm$ 3.3}。我们将其解释为单光子事件过多的无效证据，并设定了限制。假设假设单光子的动量分布与相干$π^0$衰变的光子的动量分布相似，则测量结果会产生单光子事件的上限，即每个\nm \带电电流事件的{ \boldmath $< 4.0 \times 10^{-4}$ }。将搜索范围缩小到光子与入射中微子近似共线的事件，我们观察到{ \bf 78}事件，其预测背景为{ \bf 76.6 $\pm$ 4.9 $\pm$ 1.9}，得到更严格的上限，{ \boldmath $< 1.6 \times 10^{-4}$ }/\nm \带电电流事件。

英文摘要：

We present a search for neutrino-induced events containing a single, exclusive photon using data from the NOMAD experiment at the CERN SPS where the average energy of the neutrino flux is $\simeq 25$ GeV. The search is motivated by an excess of electron-like events in the 200--475 MeV energy region as reported by the MiniBOONE experiment. In NOMAD, photons are identified via their conversion to $e^+e^-$ in an active target embedded in a magnetic field. The background to the single photon signal is dominated by the asymmetric decay of neutral pions produced either in a coherent neutrino-nucleus interaction, or in a neutrino-nucleon neutral current deep inelastic scattering, or in an interaction occurring outside the fiducial volume. All three backgrounds are determined {\it in situ} using control data samples prior to opening the `signal-box'. In the signal region, we observe {\bf 155} events with a predicted background of {\bf 129.2 $\pm$ 8.5 $\pm$ 3.3}. We interpret this as null evidence for excess of single photon events, and set a limit. Assuming that the hypothetical single photon has a momentum distribution similar to that of a photon from the coherent $π^0$ decay, the measurement yields an upper limit on single photon events, {\boldmath $< 4.0 \times 10^{-4}$} per \nm\ charged current event. Narrowing the search to events where the photon is approximately collinear with the incident neutrino, we observe {\bf 78} events with a predicted background of {\bf 76.6 $\pm$ 4.9 $\pm$ 1.9} yielding a more stringent upper limit, {\boldmath $< 1.6 \times 10^{-4}$} per \nm\ charged current event.

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