152 research outputs found
Rapports d'embranchement et résonances dans les désintégrations des mésons B en particules charmées dans BaBar et flux cosmique de positons dans AMS
In this document, ten years of activities in the BaBar experiment, as well as my research project in the AMS experiment, are presented. We describe the experimental setup related to BaBar and the main results obtained by this experiment. My research in BaBar is focused on the study of the B -> Dbar(*) D(*) K final states. There are 22 final states which present very interesting properties. First of all, they constitute a non negligible fraction of the B meson decays and a measurement of the branching fractions is presented. This measurement allows to study the isospin relationships between all the final states. These decays give also insights on the angles of the unitary triangle related to the CP violation. The B -> Dbar(*) D(*) K events contain many resonances, and some of them are still poorly known and remain mysterious. We describe a study performed on resonances such as the X(3872) or Ds1*(2700), using either invariant mass distributions or Dalitz plot analysis. Finally, my research project on AMS is presented, and we explain the interest of measuring the positron fraction as well as the positron spectrum. Discrepancies with respect to the predictions could be the sign of nearby pulsars, or could constitute also the indirect observation of dark matter annihilation.Je prĂ©sente dix annĂ©es d'activitĂ© passĂ©es sur l'expĂ©rience BaBar ainsi que mon projet de recherche sur l'expĂ©rience AMS. Je dĂ©cris les installations expĂ©rimentales relatives Ă BaBar et les principaux rĂ©sultats apportĂ©s par l'expĂ©rience. Ensuite je dĂ©taille mes sujets de recherche qui sont liĂ©s Ă l'Ă©tude des Ă©tats finals B -> Dbar(*) D(*) K. Ces Ă©tats finals sont au nombre de 22 et prĂ©sentent des propriĂ©tĂ©s trĂšs intĂ©ressantes Ă mesurer. Tout d'abord, ils constituent une fraction non nĂ©gligeable des dĂ©sintĂ©grations des mĂ©sons B et une mesure des rapports d'embranchement est ainsi prĂ©sentĂ©e. Cette mesure permet une Ă©tude des relations d'isospin entre tous les Ă©tats finals. Ces dĂ©sintĂ©grations permettent Ă©galement de retirer des informations utiles sur les angles du triangle d'unitaritĂ©, qui sont liĂ©s Ă la violation de CP. Les Ă©vĂ©nements B -> Dbar(*) D(*) K contiennent de nombreuses rĂ©sonances, dont certaines sont encore mal connues et demeurent mystĂ©rieuses. Je dĂ©cris l'Ă©tude qui est faite sur des rĂ©sonances telles que le X(3872) ou le Ds1*(2700), en utilisant des distributions de masses invariantes ou des diagrammes de Dalitz. Enfin, mon projet de recherche sur AMS est prĂ©sentĂ©, et j'explique l'intĂ©rĂȘt d'une mesure de la fraction de positons, et Ă terme, du flux de positons. Des dĂ©viations de ces mesures par rapport aux prĂ©dictions pourraient ĂȘtre le signe de pulsars proches, mais Ă©galement pourraient constituer une observation indirecte de l'annihilation de matiĂšre noire
Detection of variable VHE gamma-ray emission from the extra-galactic gamma-ray binary LMC P3
Context. Recently, the high-energy (HE, 0.1-100 GeV) -ray emission
from the object LMC P3 in the Large Magellanic Cloud (LMC) has been discovered
to be modulated with a 10.3-day period, making it the first extra-galactic
-ray binary.
Aims. This work aims at the detection of very-high-energy (VHE, >100 GeV)
-ray emission and the search for modulation of the VHE signal with the
orbital period of the binary system.
Methods. LMC P3 has been observed with the High Energy Stereoscopic System
(H.E.S.S.); the acceptance-corrected exposure time is 100 h. The data set has
been folded with the known orbital period of the system in order to test for
variability of the emission. Energy spectra are obtained for the orbit-averaged
data set, and for the orbital phase bin around the VHE maximum.
Results. VHE -ray emission is detected with a statistical
significance of 6.4 . The data clearly show variability which is
phase-locked to the orbital period of the system. Periodicity cannot be deduced
from the H.E.S.S. data set alone. The orbit-averaged luminosity in the
TeV energy range is erg/s. A luminosity of erg/s is reached during 20% of the orbit. HE and VHE
-ray emissions are anti-correlated. LMC P3 is the most luminous
-ray binary known so far.Comment: 5 pages, 3 figures, 1 table, accepted for publication in A&
Measurements of the absolute branching fractions of B± âk±Xc c
A study of the two-body decays B±âXcÂŻcK±, where XcÂŻc refers to one charmonium state, is reported by the BABAR Collaboration using a data sample of 424 fbâ1. The absolute determination of branching fractions for these decays are significantly improved compared to previous BABAR measurements. Evidence is found for the decay B+âX(3872)K+ at the 3Ï level. The absolute branching fraction B[B+âX(3872)K+]=[2.1±0.6(stat)±0.3(syst)]Ă10â4 is measured for the first time. It follows that B[X(3872)âJ/ÏÏ+Ïâ]=(4.1±1.3)%, supporting the hypothesis of a molecular component for this resonance.publishedVersio
Light meson spectroscopy from Dalitz plot analyses of ηc decays to η0 K+Kâ , η0 Ï + Ï â , and Î·Ï + Ï â produced in two-photon interactions
We study the processes γγâηcâηâČK+Kâ, ηâČÏ+Ïâ, and ηÏ+Ïâ using a data sample of 519ââfbâ1 recorded with the BABAR detector operating at the SLAC PEP-II asymmetric-energy e+eâ collider at center-of-mass energies at and near the ΄(nS) (n=2, 3, 4) resonances. This is the first observation of the decay ηcâηâČK+Kâ and we measure the branching fraction Î(ηcâηâČK+Kâ)/(Î(ηcâηâČÏ+Ïâ)=0.644±0.039stat±0.032sys. Significant interference is observed between γγâηcâηÏ+Ïâ and the nonresonant two-photon process γγâηÏ+Ïâ. A Dalitz plot analysis is performed of ηc decays to ηâČK+Kâ, ηâČÏ+Ïâ, and ηÏ+Ïâ. Combined with our previous analysis of ηcâKÂŻKÏ, we measure the Kâ0(1430) parameters and the ratio between its ηâČK and ÏK couplings. The decay ηcâηâČÏ+Ïâ is dominated by the f0(2100) resonance, also observed in J/Ï radiative decays. A new a0(1700)âÎ·Ï resonance is observed in the ηcâηÏ+Ïâ channel. We also compare ηc decays to η and ηâČ final states in association with scalar mesons as they relate to the identification of the scalar glueball.publishedVersio
Search for rare or forbidden decays of the D0 meson
We present a search for nine lepton-number-violating and three lepton-flavor-violating neutral charm decays of the type D0âh'âhââ'+â+ and D0âh'âh+â'±ââ, where h and hâČ represent a K or Ï meson and â and ââČ an electron or muon. The analysis is based on 468 fbâ1 of e+eâ annihilation data collected at or close to the ΄(4S) resonance with the BABAR detector at the SLAC National Accelerator Laboratory. No significant signal is observed for any of the twelve modes, and we establish 90% confidence level upper limits on the branching fractions in the range (1.0â30.6)Ă10â7. The limits are between 1 and 3 orders of magnitude more stringent than previous measurements.publishedVersio
Astronomy outreach in Namibia : H.E.S.S. and beyond
Astronomy plays a major role in the scientific landscape of Namibia. Because of its excellent sky conditions, Namibia is home to ground-based observatories like the High Energy Spectroscopic System (H.E.S.S.), in operation since 2002. Located near the Gamsberg mountain, H.E.S.S. performs groundbreaking science by detecting very-high-energy gamma rays from astronomical objects. The fascinating stories behind many of them are featured regularly in the "Source of the Month", a blog-like format intended for the general public with more than 170 features to date. In addition to other online communication via social media, H.E.S.S. outreach activities have been covered locally, e.g. through 'open days' and guided tours on the site itself. An overview of the H.E.S.S. outreach activities are presented in this contribution, along with discussions relating to the current landscape of astronomy outreach and education in Namibia. There has also been significant activity in the country in recent months, whereby astronomy is being used to further sustainable development via human capacity-building. Finally, as we take into account the future prospects of radio astronomy in the country, momentum for a wider range of astrophysics research is clearly building â this presents a great opportunity for the astronomy community to come together to capitalise on this movement and support astronomy outreach, with the overarching aim to advance sustainable development in Namibia
Detection of new Extreme BL Lac objects with H.E.S.S. and Swift XRT
Extreme high synchrotron peaked blazars (EHBLs) are amongst the most powerful accelerators found in nature. Usually the synchrotron peak frequency of an EHBL is above 10 Hz, i.e., lies in the range of medium to hard X-rays making them ideal sources to study particle acceleration and radiative processes. EHBL objects are commonly observed at energies beyond several TeV, making them powerful probes of gamma-ray absorption in the intergalactic medium. During the last decade, several attempts have been made to increase the number of EHBL detected at TeV energies and probe their spectral characteristics. Here we report new detections of EHBLs in the TeV energy regime, each at a redshift of less than 0.2, by the High Energy Stereoscopic System (H.E.S.S.). Also, we report on X-ray observations of these EHBLs candidates with Swift-XRT. In conjunction with the very high energy observations, this allows us to probe the radiation mechanisms and the underlying particle acceleration processes
Observation of burst activity from SGR1935+2154 associated to first galactic FRB with H.E.S.S.
Fast radio bursts (FRB) are enigmatic powerful single radio pulses with durations of several milliseconds and high brightness temperatures suggesting coherent emission mechanism. For the time being a number of extragalactic FRBs have been detected in the high-frequency radio band including repeating ones. The most plausible explanation for these phenomena is magnetar hyperflares. The first observational evidence of this scenario was obtained in April 2020 when an FRB was detected from the direction of the Galactic magnetar and soft gamma repeater SGR1935+2154. The FRB was preceded with a number of soft gamma-ray bursts observed by Swift-BAT satellite, which triggered the follow-up program of the H.E.S.S. imaging atmospheric Cherenkov telescopes (IACTs). H.E.S.S. has observed SGR1935+2154 over a 2 hour window few hours prior to the FRB detection by STARE2 and CHIME. The observations overlapped with other X-ray bursts from the magnetar detected by INTEGRAL and Swift-BAT, thus providing first observations of a magnetar in a flaring state in the very-high energy domain. We present the analysis of these observations, discuss the obtained results and prospects of the H.E.S.S. follow-up program for soft gamma repeaters and anomalous X-ray pulsars
b-tagging in DELPHI at LEP
Abstract: The standard method used for tagging b-hadrons in the DELPHI experiment at the CERN LEP Collider is discussed in detail. The main ingredient of b-tagging is the impact parameters of tracks, which relies mostly on the vertex detector. Additional information, such as the mass of particles associated to a secondary vertex, significantly improves the selection efficiency and the background suppression. The paper describes various discriminating variables used for the tagging and the procedure of their combination. In addition, applications of b-tagging to some physics analyses, which depend crucially on the performance and reliability of b-tagging, are described briefly
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