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Quantum chromodynamics (QCDs) is the strongly interacting part of the Standard Model. It is supposed to describe all of nuclear physics; and yet, almost 50 years after the discovery of gluons and quarks, we are only just beginning to understand how QCD builds the basic bricks for nuclei: neutrons and protons, and the pions that bind them together. QCD is characterised by two emergent phenomena: confinement and dynamical chiral symmetry breaking (DCSB). They have far-reaching consequences, expressed with great force in the character of the pion; and pion properties, in turn, suggest that confinement and DCSB are intimately connected. Indeed, since the pion is both a Nambu–Goldstone boson and a quark–antiquark bound-state, it holds a unique position in nature and, consequently, developing an understanding of its properties is critical to revealing some very basic features of the Standard Model. We describe experimental progress toward meeting this challenge that has been made using electromagnetic probes, highlighting both dramatic improvements in the precision of charged-pion form factor data that have been achieved in the past decade and new results on the neutral-pion transition form factor, both of which challenge existing notions of pion structure. We also provide a theoretical context for these empirical advances, which begins with an explanation of how DCSB works to guarantee that the pion is un-naturally light; but also, nevertheless, ensures that the pion is the best object to study in order to reveal the mechanisms that generate nearly all the mass of hadrons. In canvassing advances in these areas, our discussion unifies many aspects of pion structure and interactions, connecting the charged-pion elastic form factor, the neutral-pion transition form factor and the pion's leading-twist parton distribution amplitude. It also sketches novel ways in which experimental and theoretical studies of the charged-kaon electromagnetic form factor can provide significant contributions. Importantly, it appears that recent predictions for the large-Q2 behaviour of the charged-pion form factor can be tested by experiments planned at the upgraded 12 GeV Jefferson Laboratory. Those experiments will extend precise charged-pion form factor data up to momentum transfers that it now appears may be large enough to serve in validating factorisation theorems in QCD. If so, they may expose the transition between the non-perturbative and perturbative domains and thereby reach a goal that has driven hadro-particle physics for around 35 years. [ABSTRACT FROM AUTHOR]

Taking into account the effects of shadowing and jet quenching, the large transverse momentum distribution of K+, π+ and K+/π+ ratios at originating from resolved photoproduction processes is calculated based on perturbative quantum chromodynamics. It is found that the contribution of K+ and π+ produced by photoproduction processes is evident. The K+/π+ ratios in Au–Au collisions show an obvious enhancement compared with p–p collisions. The numerical results indicate that the photoproduction processes are good modification for kaon and pion production. [ABSTRACT FROM AUTHOR]

The title compound, C19H22N2O2, is a Schiff base. Its mol­ecular conformation is stabilized by an O—H⋯N hydrogen bond. The crystal packing is characterized by C—H⋯π and π–π inter­actions. [ABSTRACT FROM AUTHOR]

In the title complex, [Cu(C6H6O2)(C10H9N3)(H2O)]·3H2O, the Cu atom has a distorted cis-square-pyramidal geometry formed by two O atoms of a bidentate cyclo­butane-1,1-dicarboxyl­ate anion, two N atoms of a bidentate di-2-pyridylamine (bpa) mol­ecule and a water O atom. In the crystal structure, the complex mol­ecules are connected by hydrogen bonds through the water mol­ecules, and by π–π stacking inter­actions between bpa ligands. [ABSTRACT FROM AUTHOR]

The title compound, C17H16Cl2N2, adopts a `vault' conformation. The mol­ecules are linked by a pair of C—H⋯N hydrogen bonds into a centrosymmetric dimer with an R22(18) ring. These dimers are linked by a weak centrosymmetric π–π inter­action into a chain along the [001] direction, and these chains are connected by van der Waals inter­actions, resulting in a three-dimensional network structure. [ABSTRACT FROM AUTHOR]

In the crystal structure of the title compound [systematic name: diaqua­bis(6-methyl-2,2-dioxo-1,2,3-oxathia­zin-4-olato-κ O4)bis­(3-methyl­pyridine-κ N)nickel(II)], [Ni(C4H4NO4S)2(C6H7N)2(H2O)2], the NiII centre resides on a centre of symmetry and has a distorted octa­hedral geometry. The basal plane is formed by two carbonyl O atoms of two monodentate trans-oriented acesulfamate ligands and two trans aqua ligands. The axial positions in the octa­hedron are occupied by two N atoms of two trans pyridine ligands. Mol­ecules are stacked in columns running along the a axis. There are π–π stacking inter­actions between the mol­ecules in each column, with a distance of 3.623 (2) Å between the centroids of the pyridine rings. There are also O—H⋯O inter­actions between the columns. [ABSTRACT FROM AUTHOR]