First measurement of chiral dynamics in π- γ → π- π- π+

Chiral symmetry breaking: Current experimental status and prospects

Chiral symmetry, linked to the smallness of the quark masses compared to the QCD bound states, and its breaking pattern are exploited in effective field theory to describe a multitude of phenomena by a few low-energy constants. Those concern light-meson dynamics and decays, their couplings to photons and meson-nucleon interactions. Special emphasis is given to the pion properties, in terms of pion-pion low-energy scattering, the pion polarizability and the chiral anomaly, which describes the coupling of three pions to a photon. These properties are studied by the COMPASS collaboration at CERN since first data taking with pion beams in the year 2004, and several following campaigns. In the framework of the upcoming AMBER collaboration, it is planned to extend the studies to the kaon sector.

Structural plasticity of substrate selection by activation-induced cytidine deaminase as a regulator of its genome-wide mutagenic activity

Activation-induced cytidine deaminase (AID) mediates somatic hypermutation and class-switch recombination of antibodies. Computational-biochemical and crystallography analyses of AID have identified three surface grooves for binding single-stranded DNA (ssDNA). Functional studies have also found evidence for RNA-binding motifs on AID. Although AID and the related apolipoprotein B mRNA-editing enzyme catalytic polypeptide-like (APOBEC) enzymes share a conserved core, AID uniquely features multiple substrate-binding motifs on its surface. Here we suggest that combinatorial deployment of AID's multiple ssDNA- or RNA-binding motifs yields many substrate-binding modes that can accommodate ssDNA, RNA, or DNA/RNA substrates of diverse structures. We also suggest that AID oligomerization generates yet additional novel substrate-binding modes. We propose that this plasticity in substrate choice is an evolved aspect of AID's structure that contributes to the regulation of its differential mutagenic activity at various loci.

COMPASS: meson spectroscopy and low-energy meson dynamics

Within the broad physics programme of the COMPASS collaboration at CERN SPS, soft reactions of high-energy hadron beams on different nuclear targets are investigated. Aiming at a better understanding of the strong interaction, novel results range from tests of chiral symmetry breaking to properties of diffractively produced meson resonances studied in their multi-particle decays, such as the a1(1420) with unusual properties. The talk will highlight the challenges of the employed experimental techniques, and as well of the analysis methods, that are developed to a large extent along with the analysis of the world’s largest data sets in the field.

Measurement of the charged-pion polarizability

The COMPASS collaboration at CERN has investigated pion Compton scattering, π(-)γ→π(-)γ, at center-of-mass energy below 3.5 pion masses. The process is embedded in the reaction π(-)Ni→π(-)γNi, which is initiated by 190 GeV pions impinging on a nickel target. The exchange of quasireal photons is selected by isolating the sharp Coulomb peak observed at smallest momentum transfers, Q(2)<0.0015  (GeV/c)(2). From a sample of 63,000 events, the pion electric polarizability is determined to be α(π)=(2.0±0.6(stat)±0.7(syst))×10(-4)  fm(3) under the assumption α(π)=-β(π), which relates the electric and magnetic dipole polarizabilities. It is the most precise measurement of this fundamental low-energy parameter of strong interaction that has been addressed since long by various methods with conflicting outcomes. While this result is in tension with previous dedicated measurements, it is found in agreement with the expectation from chiral perturbation theory. An additional measurement replacing pions by muons, for which the cross-section behavior is unambiguously known, was performed for an independent estimate of the systematic uncertainty.