Observation of the cosmic ray moon shadowing effect with the ARGO-YBJ experiment

Cosmic rays are hampered by the Moon and a deficit in its direction is expected (the so-called Moon shadow). The Moon shadow is an important tool to determine the performance of an air shower array. Indeed, the westward displacement of the shadow center, due to the bending effect of the geomagnetic field on the propagation of cosmic rays, allows the setting of the absolute rigidity scale of the primary particles inducing the showers recorded by the detector. In addition, the shape of the shadow permits to determine the detector point spread function, while the position of the deficit at high energies allows the evaluation of its absolute pointing accuracy. In this paper we present the observation of the cosmic ray Moon shadowing effect carried out by the ARGO-YBJ experiment in the multi-TeV energy region with high statistical significance (55 standard deviations). By means of an accurate Monte Carlo simulation of the cosmic rays propagation in the Earth-Moon system, we have studied separately the effect of the geomagnetic field and of the detector point spread function on the observed shadow. The angular resolution as a function of the particle multiplicity and the pointing accuracy have been obtained. The primary energy of detected showers has been estimated by measuring the westward displacement as a function of the particle multiplicity, thus calibrating the relation between shower size and cosmic ray energy. The stability of the detector on a monthly basis has been checked by monitoring the position and the deficit of the Moon shadow. Finally, we have studied with high statistical accuracy the shadowing effect in the ''day/night’’ time looking for possible effect induced by the solar wind

Highlights from the ARGO-YBJ experiment

The ARGO-YBJ experiment at YangBaJing in Tibet (4300 m a.s.l.) has been taking data with its full layout since October 2007. Here we present a few significant results obtained in gamma-ray astronomy and cosmic-ray physics. Emphasis is placed on the analysis of gamma-ray emission from point-like sources (Crab Nebula, MRK 421), on the preliminary limit on the antiproton/proton flux ratio, on the large-scale cosmic-ray anisotropy and on the proton–air cross-section. The performance of the detector is also discussed, and the perspectives of the experiment are outlined

Réparation de l'ADN et mutagenèse dans les mitochondries des vertébrés : preuve de l'asymétrie de l'héritage des brins d'ADN

International audienceA variety of endogenous and exogenous factors induce chemical and structural alterations in cellular DNA in addition to the errors occurring throughout DNA synthesis. These types of DNA damage are cytotoxic, miscoding or both and are believed to be at the origin of cancer and other age-related diseases. A human cell, aside from nuclear DNA, contains thousands of copies of mitochondrial DNA (mtDNA), a double-stranded, circular molecule of 16,569 bp. It has been proposed that mtDNA is a critical target of reactive oxygen species: by-products of oxidative phosphorylation that are generated in the organelle during aerobic respiration. Indeed, oxidative damage to mtDNA is more extensive and persistent as compared to that to nuclear DNA. Although transversions are the hallmark of mutations induced by reactive oxygen species, paradoxically, the majority of mtDNA mutations that occur during ageing and cancer are transitions. Furthermore, these mutations show a striking strand orientation bias: T→C/G→A transitions preferentially occur on the light strand, whereas C→T/A→G on the heavy strand of mtDNA. Here, we propose that the majority of mtDNA progenies, created after multiple rounds of DNA replication, are derived from the heavy strand only, owing to asymmetric replication of the DNA strand anchored to the inner membrane via the D-loop structure.Divers facteurs endogènes et exogènes induisent des altérations chimiques et structurelles dans l'ADN cellulaire, en plus des erreurs qui se produisent tout au long de la synthèse de l'ADN. Ces types de dommages à l'ADN sont cytotoxiques, dus à un mauvais codage ou aux deux, et on pense qu'ils sont à l'origine du cancer et d'autres maladies liées à l'âge. Une cellule humaine, outre l'ADN nucléaire, contient des milliers de copies de l'ADN mitochondrial (ADNmt), une molécule circulaire double brin de 16 569 pb. Il a été proposé que l'ADNmt soit une cible critique des espèces réactives de l'oxygène : des sous-produits de la phosphorylation oxydative qui sont générés dans l'organelle pendant la respiration aérobie. En effet, les dommages oxydatifs de l'ADNmt sont plus étendus et plus persistants que ceux de l'ADN nucléaire. Bien que les transversions soient la marque des mutations induites par les espèces réactives de l'oxygène, paradoxalement, la majorité des mutations de l'ADNmt qui se produisent au cours du vieillissement et du cancer sont des transitions. De plus, ces mutations présentent un biais d'orientation des brins frappant : A→G/G→A transitions se produisent de préférence sur le brin léger, tandis que T→C/A→G sur le brin lourd de l'ADNmt. Ici, nous proposons que la majorité des descendants de l'ADNmt, créés après plusieurs cycles de réplication de l'ADN, soient dérivés du brin lourd uniquement, en raison de la réplication asymétrique du brin d'ADN ancré à la membrane interne via la structure en boucle D.Traduit avec www.DeepL.com/Translator (version gratuite