π-核子碰撞中的极化与双电子各向异性研究
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"π-核子碰撞中的极化和双核各向异性"
这篇研究论文探讨了π-核子碰撞中的极化效应以及由此产生的双电子（dilepton）角分布各向异性。作者们深入研究了πN→Ne+e-这一特定反应，其中π代表π介子，N代表核子（如质子或中子），e-表示电子。通过使用自旋5/2的重子共振的一致有效相互作用模型，他们能够消除计算过程中非物理自由度的影响，并计算出不同中间重子共振的各向异性系数。
研究中，各向异性系数被定义为反应过程的角分布对方向的依赖性，这可以揭示出参与反应的粒子的内在性质，例如自旋和奇偶性。具体来说，作者计算了与N（1520）和N（1440）共振相关的各向异性系数，这两个共振态是HADES（High Acceptance Di-Electron Spectrometer，高接受度双电子谱仪）合作组在该能量范围内实验观测的关键因素。
HADES协作组的实验数据对于理解π-核子碰撞中的微观物理过程至关重要。通过比较理论计算与实验数据，可以更准确地解析出反应中重子共振的贡献。各向异性系数为解析这一过程中的共振效应提供了有力的约束条件，有助于科学家们更深入地理解这些高能碰撞下的基本粒子行为。
该研究发表于《Physics Letters B》期刊，属于开放获取文章，意味着公众可以免费访问并阅读其全部内容。文章的接收、修订和接受日期分别在2016年的5月、10月和11月，最终在线发布于2016年11月15日。编辑为V. Metag，研究领域涵盖了核物理学和高能物理，特别是强子的极化性质及其在碰撞中产生的各向异性现象，这是理解基本粒子相互作用和核物质性质的重要途径。
Physics Letters B 764 (2017) 282–288
Contents lists available at ScienceDirect
Physics Letters B
www.elsevier.com/locate/physletb
Polarization and dilepton anisotropy in pion–nucleon collisions
Enrico Speranza
a,b,∗
, Miklós Zétényi
c,d,e
, Bengt Friman
a
a
GSI Helmholtzzentrum für Schwerionenforschung GmbH, D-64291 Darmstadt, Germany
b
Technische Universität Darmstadt, D-64289 Darmstadt, Germany
c
Wigner Research Center for Physics, H-1121 Budapest, Hungary
d
Extreme Matter Institute EMMI, GSI, D-64291 Darmstadt, Germany
e
Institut für Theoretische Physik, Goethe-Universität, D-60438 Frankfurt am Main, Germany
a r t i c l e i n f o a b s t r a c t
Article history:
Received
31 May 2016
Received
in revised form 31 October 2016
Accepted
10 November 2016
Available
online 15 November 2016
Editor:
V. Metag
Hadronic polarization and the related anisotropy of the dilepton angular distribution are studied for
the reaction π N → Ne
+
e
−
. We employ consistent effective interactions for baryon resonances up to
spin-5/2, where non-physical degrees of freedom are eliminated, to compute the anisotropy coefficients
for isolated intermediate baryon resonances. It is shown that the spin and parity of the intermediate
baryon resonance is reflected in the angular dependence of the anisotropy coefficient. We then compute
the anisotropy coefficient including the N(1520) and N(1440) resonances, which are essential at the
collision energy of the recent data obtained by the HADES Collaboration on this reaction. We conclude
that the anisotropy coefficient provides useful constraints for unraveling the resonance contributions to
this process.
© 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license
(http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP
3
.
1. Introduction
Dilepton production in hadronic reactions provides information
on the electromagnetic properties of hadrons. Leptons are also im-
portant
probes of nuclear collisions, since their mean-free path in
nuclear matter is much larger than nuclear sizes. Hence, they can
carry information on the conditions that prevail during the brief,
highly compressed stages of the reaction. Dileptons are produced
in a variety of different elementary processes. Multiply differen-
tial
cross sections for dilepton production can provide information
needed to disentangle the production channels.
Independently
of the specific reaction, dileptons originate from
the decay of virtual photons. According to the vector meson dom-
inance
hypothesis, hadrons couple to the electromagnetic field via
a neutral vector meson, which subsequently converts into a pho-
ton.
Although vector meson dominance does not provide an ac-
curate
description of the electromagnetic coupling for all hadrons,
it provides a tenable first approximation. This implies that dilep-
ton
production in nuclear collisions can furnish information on the
in-medium spectral functions of vector mesons.
A
detailed understanding of elementary hadronic reactions is
an important prerequisite for studies of nuclear collisions. While
*
Corresponding author.
E-mail
address: e.speranza@gsi.de (E. Speranza).
a lot of effort has been invested in the study of dilepton produc-
tion
in nucleon–nucleon collisions both in experiment and theory,
pion–nucleon collisions are less explored. The HADES Collabora-
tion has
recently studied pion-induced reactions, including dilep-
ton
production. First preliminary data have been presented at the
NSTAR2015 conference [1]. The aim of the present paper is to ex-
plore
the reaction π N → R → Ne
+
e
−
, where R is the intermediate
baryon resonance, in terms of effective Lagrangian models at the
center-of-momentum (CM) energy of the HADES experiment. In
particular we study the angular distribution of the produced dilep-
tons.
The
general expression for the angular distribution of dileptons
originating from the decay of a virtual photon is given by [2–4]:
dσ
d
e
∝1 +λ
θ
cos
2
θ
e
+λ
θφ
sin 2θ
e
cos φ
e
+λ
φ
sin
2
θ
e
cos 2φ
e
, (1)
where θ
e
and φ
e
are the polar and azimuthal angles of one of the
two leptons in the rest frame of the photon. The anisotropy coef-
ficients
λ
θ
, λ
θφ
and λ
φ
depend on the choice of the quantization
axis. We use the so-called helicity frame where the polarization
axis is chosen along the momentum of the virtual photon in the
CM frame [5].
As
we discuss in Section 2, in the reaction π N → R → Ne
+
e
−
,
the anisotropy coefficients depend on the quantum numbers of the
intermediate baryon resonance and on the scattering angle θ
γ
∗
of
http://dx.doi.org/10.1016/j.physletb.2016.11.015
0370-2693/
© 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by
SCOAP
3
.
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