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arXiv 1108.4265nucl-th
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1.25 GeV pp和np碰撞中双轻子的产生

Dilepton production in pp and np collisions at 1.25 GeV

B. V. Martemyanov, M. I. Krivoruchenko, Amand Faessler

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摘要：

在核子占优势的模型和$Δ$共振模型下，研究了实验室动能为1.25 GeV的包含反应$pp \rightarrow e^+ e^- X$和$np \rightarrow e^+ e^- X$.这些反应的实验数据最近由HADES协作组报道。在原始模型中，双轻子要么是由核子和$Δ$共振$R \rightarrow N e^+ e^-$的衰变产生的，要么是由$R \to Nπ^0$和$R \to Nη$衰变中产生的$π^0$ -和$η$ -介子的Dalitz衰变产生的。发现$pp \rightarrow e^+ e^- X$反应中的双轻子不变质量分布可以很好地由$R \rightarrow N e^+ e^-$衰变和$R \rightarrow N π^0$，$π^0 \to γe^+e^-$衰变的贡献再现。在共振中，主要的贡献来自$Δ(1232)$，它决定了直接衰变通道$R \rightarrow N e^+ e^-$和π衰变通道。在碰撞$np \rightarrow e^+ e^- X$中，$η$ -介子Dalitz衰变、辐射俘获$np \rightarrow d e^+ e^-$和$np \rightarrow dη$韧致辐射产生了额外的重要贡献。这些机制可以部分地解释强过剩的双轻子在横截面的碰撞$np$与$pp$，范围从7到100倍的双轻子不变质量为0.2至0.5 GeV。

英文摘要：

The inclusive reactions $pp \rightarrow e^+ e^- X$ and $np \rightarrow e^+ e^- X$ at the laboratory kinetic energy of 1.25 GeV are investigated in a model of dominance of nucleon and $Δ$ resonances. Experimental data for these reactions have recently been reported by the HADES Collaboration. In the original model, the dileptons are produced either from the decays of nucleon and $Δ$ resonances $R \rightarrow N e^+ e^-$ or from the Dalitz decays of $π^0$- and $η$-mesons created in the $R \to Nπ^0$ and $R \to Nη$ decays. We found that the distribution of dilepton invariant masses in the $pp \rightarrow e^+ e^- X$ reaction is well reproduced by the contributions of $R \rightarrow N e^+ e^-$ decays and $R \rightarrow N π^0$, $π^0 \to γe^+e^-$ decays. Among the resonances, the predominant contribution comes from the $Δ(1232)$, which determines both the direct decay channel $R \rightarrow N e^+ e^-$ and the pion decay channel. In the collisions $np \rightarrow e^+ e^- X$, additional significant contributions arise from the $η$-meson Dalitz decays, produced in the $np \rightarrow npη$ and $np \rightarrow dη$ reactions, the radiative capture $np \rightarrow d e^+ e^-$, and the $np \rightarrow np e^+ e^-$ bremsstrahlung. These mechanisms may partly explain the strong excess of dileptons in the cross section for collisions of $np$ versus $pp$, which ranges from 7 to 100 times for the dilepton invariant masses of 0.2 to 0.5 GeV.

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