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1. Measurement of the helicity asymmetry $${\mathbb {E}}$$ for the $$\vec {\gamma }\vec {p} \rightarrow p \pi ^0$$ reaction in the resonance region: The CLAS Collaboration

   https://doi.org/10.1140/epja/s10050-023-01123-3  

   Kim, C. W.; Zachariou, N.; Bashkanov, M.; Briscoe, W. J.; Fegan, S.; Kashevarov, V. L.; Nikonov, K.; Sarantsev, A.; Schmidt, A.; Strakovsky, I. I.; et al (September 2023, The European Physical Journal A)

   The double-spin-polarization observable E for γ p → pπ0 has been measured with the CEBAF Large Acceptance Spectrometer (CLAS) at photon beam energies Eγ from 0.367 to 2.173 GeV (corresponding to center-ofmass energies from 1.240 to 2.200 GeV) for pion center-ofmass angles, cos θc.m. π0 , between − 0.86 and 0.82. These new CLAS measurements cover a broader energy range and have smaller uncertainties compared to previous CBELSA data and provide an important independent check on systematics. These measurements are compared to predictions as well as new global fits from The George Washington University, Mainz, and Bonn-Gatchina groups. Their inclusion in multipole analyses will allow us to refine our understanding of the single-pion production contribution to the Gerasimov-Drell- Hearn sum rule and improve the determination of resonance properties, which will be presented in a future publication. 

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2. Flow Straight and Fast: Learning to Generate and Transfer Data with Rectified Flow

   Xingchao Liu; Chengyue Gong; Qiang Liu (January 2023, International conference on learning representations (ICLR))

   We present rectified flow, a surprisingly simple approach to learning (neural) ordinary differential equation (ODE) models to transport between two empirically observed distributions π0 and π1, hence providing a unified solution to generative modeling and domain transfer, among various other tasks involving distribution transport. The idea of rectified flow is to learn the ODE to follow the straight paths connecting the points drawn from π0 and π1 as much as possible. This is achieved by solving a straightforward nonlinear least squares optimization problem, which can be easily scaled to large models without introducing extra parameters beyond standard supervised learning. The straight paths are special and preferred because they are the shortest paths between two points, and can be simulated exactly without time discretization and hence yield computationally efficient models. We show that the procedure of learning a rectified flow from data, called rectification, turns an arbitrary coupling of π0 and π1 to a new deterministic coupling with provably non-increasing convex transport costs. In addition, recursively applying rectification allows us to obtain a sequence of flows with increasingly straight paths, which can be simulated accurately with coarse time discretization in the inference phase. In empirical studies, we show that rectified flow performs superbly on image generation, image-to-image translation, and domain adaptation. In particular, on image generation and translation, our method yields nearly straight flows that give high quality results even with a single Euler discretization step. 

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3. Earth mover’s distance as a measure of CP violation

   https://doi.org/10.1007/JHEP06(2023)098  

   Davis, Adam; Menzo, Tony; Youssef, Ahmed; Zupan, Jure (June 2023, Journal of High Energy Physics)

   We introduce a new unbinned two sample test statistic sensitive to CP violation utilizing the optimal transport plan associated with the Wasserstein (earth mover’s) distance. The efficacy of the test statistic is shown via two examples of CP asymmetric distributions with varying sample sizes: the Dalitz distributions of B0 → K+π−π0 and of D0 → π+π−π0 decays. The windowed version of the Wasserstein distance test statistic is shown to have comparable sensitivity to CP violation as the commonly used energy test statistic, but also retains information about the localized distributions of CP asymmetry over the Dalitz plot. For large statistic datasets we introduce two modified Wasserstein distance based test statistics — the binned and the sliced Wasserstein distance statistics, which show comparable sensitivity to CP violation, but improved computing time and memory scalings. Finally, general extensions and applications of the introduced statistics are discussed. 

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4. PHENIX results on nuclear modification of $$\pi^0$$-hadron correlations in small and large collision systems

   https://doi.org/10.22323/1.345.0106  

   Connors, Megan Elizabeth (January 2019, International Conference on Hard and Electromagnetic Probes of High-Energy Nuclear Collisions (HardProbes2018))

   Two-particle correlations using high-pT particles allow jet-like measurements at low Q2 where jet reconstruction is challenging. Therefore, such correlations provide critical input to our understanding of how partons interact in a colored medium. PHENIX has measured π0-hadron correlations in multiple collisions systems. The away-side widths in p+Au collisions are broader than those in p+p collisions. In Au+Au collisions both the shape and the yield of the away-side distributions are modified compared to those in p+Au and p+p collisions. The implications of these results to our understanding of energy loss in cold and hot nuclear matter are discussed. 

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5. Search for π0 decays to invisible particles

   https://doi.org/10.1007/JHEP02(2021)201  

   Cortina Gil, E.; Kleimenova, A.; Minucci, E.; Padolski, S.; Petrov, P.; Shaikhiev, A.; Volpe, R.; Numao, T.; Velghe, B.; Bryman, D.; et al (February 2021, Journal of High Energy Physics)

   null (Ed.)

   A bstract The NA62 experiment at the CERN SPS reports a study of a sample of 4 × 10 9 tagged π 0 mesons from K + → π + π 0 ( γ ), searching for the decay of the π 0 to invisible particles. No signal is observed in excess of the expected background fluctuations. An upper limit of 4 . 4 × 10 − 9 is set on the branching ratio at 90% confidence level, improving on previous results by a factor of 60. This result can also be interpreted as a model- independent upper limit on the branching ratio for the decay K + → π + X , where X is a particle escaping detection with mass in the range 0.110–0.155 GeV /c 2 and rest lifetime greater than 100 ps. Model-dependent upper limits are obtained assuming X to be an axion-like particle with dominant fermion couplings or a dark scalar mixing with the Standard Model Higgs boson. 

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