Search for neutral heavy leptons produced in ZZ decays

Weak isosinglet Neutral Heavy Leptons (νm) have been searched for using data collected by the DELPHI detector corresponding to 3.3 × 106 hadronic Z0 decays at LEP1. Four separate searches have been performed, for short-lived νm production giving monojet or acollinear jet topologies, and for long-lived νm giving detectable secondary vertices or calorimeter clusters. No indication of the existence of these particles has been found, leading to an upper limit for the branching ratio BR(Z0 → νmν̄) of about 1.3 × 10-6 at 95% confidence level for νm masses between 3.5 and 50 GeV/c2. Outside this range the limit weakens rapidly with the νm mass. The results are also interpreted in terms of limits for the single production of excited neutrinos. © Springer-Verlag 1997

Design concepts for the Cherenkov Telescope Array CTA: an advanced facility for ground-based high-energy gamma-ray astronomy

Ground-based gamma-ray astronomy has had a major breakthrough with the impressive results obtained using systems of imaging atmospheric Cherenkov telescopes. Ground-based gamma-ray astronomy has a huge potential in astrophysics, particle physics and cosmology. CTA is an international initiative to build the next generation instrument, with a factor of 5-10 improvement in sensitivity in the 100 GeV-10 TeV range and the extension to energies well below 100 GeV and above 100 TeV. CTA will consist of two arrays (one in the north, one in the south) for full sky coverage and will be operated as open observatory. The design of CTA is based on currently available technology. This document reports on the status and presents the major design concepts of CTA

Multi-messenger observations of a binary neutron star merger

On 2017 August 17 a binary neutron star coalescence candidate (later designated GW170817) with merger time 12:41:04 UTC was observed through gravitational waves by the Advanced LIGO and Advanced Virgo detectors. The Fermi Gamma-ray Burst Monitor independently detected a gamma-ray burst (GRB 170817A) with a time delay of ~1.7 s with respect to the merger time. From the gravitational-wave signal, the source was initially localized to a sky region of 31 deg2 at a luminosity distance of 40+8-8 Mpc and with component masses consistent with neutron stars. The component masses were later measured to be in the range 0.86 to 2.26 Mo. An extensive observing campaign was launched across the electromagnetic spectrum leading to the discovery of a bright optical transient (SSS17a, now with the IAU identification of AT 2017gfo) in NGC 4993 (at ~40 Mpc) less than 11 hours after the merger by the One- Meter, Two Hemisphere (1M2H) team using the 1 m Swope Telescope. The optical transient was independently detected by multiple teams within an hour. Subsequent observations targeted the object and its environment. Early ultraviolet observations revealed a blue transient that faded within 48 hours. Optical and infrared observations showed a redward evolution over ~10 days. Following early non-detections, X-ray and radio emission were discovered at the transient’s position ~9 and ~16 days, respectively, after the merger. Both the X-ray and radio emission likely arise from a physical process that is distinct from the one that generates the UV/optical/near-infrared emission. No ultra-high-energy gamma-rays and no neutrino candidates consistent with the source were found in follow-up searches. These observations support the hypothesis that GW170817 was produced by the merger of two neutron stars in NGC4993 followed by a short gamma-ray burst (GRB 170817A) and a kilonova/macronova powered by the radioactive decay of r-process nuclei synthesized in the ejecta

Discovery of VHE gamma rays from PKS 2005-489

The high-frequency peaked BL Lac PKS 2005-489 (z=0.071) was observed in 2003 and 2004 with the H.E.S.S. stereoscopic array of imaging atmospheric-Cherenkov telescopes in Namibia. A signal was detected at the 6.7 sigma level in the 2004 observations (24.2 hrs live time), but not in the 2003 data set (27.3 hrs live time). PKS 2005-489 is the first blazar independently discovered by H.E.S.S. to be an emitter of VHE photons, and only the second such blazar in the Southern Hemisphere. The integral flux above 200 GeV observed in 2004 is (6.9 +/- 1.0 +/- 1.4) x 10$^{-12}$ cm$^{-2}$ s$^{-1}$, corresponding to ~2.5% of the flux observed from the Crab Nebula. The 99% upper limit on the flux in 2003, I(>200 GeV) < 5.2 x 10$^{-12}$ cm$^{-2}$ s$^{-1}$, is smaller than the flux measured in 2004, suggesting an increased level of activity in 2004. However, the data show no evidence for significant variability on any time scale less than a year. An energy spectrum is measured and is characterized by a very soft power law (photon index = 4.0 +/- 0.4)

Observations of selected AGN with HESS

A sample of selected active galactic nuclei (AGN) was observed in 2003 and 2004 with the High Energy Stereoscopic System (H.E.S.S.), an array of imaging atmospheric-Cherenkov telescopes in Namibia. The redshifts of these candidate very-high-energy (VHE, >100 GeV) gamma-ray emitters range from z=0.00183 to z=0.333. Significant detections were already reported for some of these objects, such as PKS 2155-304 and Markarian 421. Marginal evidence (3.1 sigma) for a signal is found from large-zenith-angle observations of Markarian 501, corresponding to an integral flux of I(>1.65 TeV) = (1.5 +/- 0.6 (stat) +/- 0.3 (syst)) x 10^{-12} cm^{-2} s^{-1} or ~15% of the Crab Nebula flux. Integral flux upper limits for 19 other AGN, based on exposures of ~1 to ~8 hrs live time, and with average energy thresholds between 160 GeV and 610 GeV, range from 0.4% to 5.1% of the Crab Nebula flux. All the upper limits are the most constraining ever reported for these objects

Very high energy gamma rays from the composite SNR G0.9+0.1

Very high energy (> 100 GeV) gamma-ray emission has been detected for the first time from the composite supernova remnant G0.9+0.1 using the H.E.S.S. instrument. The source is detected with a significance of 13 sigma, and a photon flux above 200 GeV of (5.7+/-0.7 stat +/- 1.2 sys) * 10^-12 cm^-2 s^-1, making it one of the weakest sources ever detected at TeV energies. The photon spectrum is compatible with a power law (dN/dE \propto E^-Gamma) with photon index Gamma = 2.40 +/- 0.11 stat +/- 0.20 sys. The gamma-ray emission appears to originate in the plerionic core of the remnant, rather than the shell, and can be plausibly explained as inverse Compton scattering of relativistic electrons

The HESS survey of the inner galaxy in very high energy gamma rays

We report on a survey of the inner part of the Galactic Plane in very high energy gamma-rays, with the H.E.S.S. Cherenkov telescope system. The Galactic Plane between +-30deg in longitude and +-3deg in latitude relative to the Galactic Centre was observed in 500 pointings for a total of 230 hours, reaching an average flux sensitivity of 2% of the Crab Nebula at energies above 200 GeV. Fourteen previously unknown sources were detected at a significance level greater than 4 sigma after accounting for all trials involved in the search. Initial results on the eight most significant of these sources were already reported elsewhere. Here we present detailed spectral and morphological information for all the new sources, along with a discussion on possible counterparts in other wavelength bands. The distribution in Galactic latitude of the detected sources appears to be consistent with a scale height in the Galactic disk for the parent population smaller than 100 pc, consistent with expectations for supernova remnants and/or pulsar wind nebulae

Determination of vertical bar V-cb vertical bar from the semileptonic decay B-0-&gt;D*(-)l(+)nu

Semileptonic decays B --&gt; D*(-)l(+)nu X were selected from a sample of 3.1 million hadronic Z decays collected by the DELPHI detector at LEP. A topological search for semileptonic B decays to resonant and non-resonant D*(-)pi(+) states was performed and the ratio of the branching fractions: Br(B --&gt; D*(-)l(+)nu X)/Br(B --&gt; D*(-)l(+)nu X) + Br(B-0 --&gt; D*(-)l(+)nu) = 0.19 +/- 0.10(stat) +/- 0.06(syst) was determined. Taking into account this contribution, the differential decay width of B-0 --&gt; D*(-)l(+)nu was measured as a function of the momentum transfer from the B to the D*(-) in two separate analyses, using exclusive and inclusive methods of D*(-) reconstruction. The distributions were fitted over the full momentum transfer range to extract the product of /V-cb/ times the normalization of the decay form factor F(q(max)(2)): F(q(max)(2))/V-cb/ = (35.4 +/- 1.9(stat) +/- 2.4(syst)) . 10(-3). The value of /V-cb/ was computed using theoretical calculations of F(q(max?2), giving: /V-cb/ = (38.9 +/- 2.0(stat) +/- 2.6(syst) +/- 1.7(theory)) . 10(-3). The total branching fraction Br(B-0 --&gt; D*(-)l(-)nu) was determined to be: Br(B-0 --&gt; D*(-)l(+)nu) = (5.52 +/- 0.17(stat) +/- 0.68(syst))%

Measurement of the quark and gluon fragmentation functions in Z0 hadronic decays

The transverse, longitudinal and asymmetric components of the fragmentation function are measured from the inclusive charged particles produced in e(+)e(-) collisions at LEP. As in deep inelastic scattering, these data are important for tests of QCD. The transverse sigma(T) and longitudinal sigma(L) components of the total hadronic cross section sigma(tot) are evaluated from the measured fragmentation functions. They are found to be a sigma(T)/sigma(tot) = 0.949 +/- 0.001(stat.) +/- 0.007(syst.) and sigma(L)/sigma(tot) = 0.051 +/- 0.001(stat.) +/- 0.007(syst.) respectively. The strong coupling constant is calculated from sigma(L)/sigma(tot) in next-to-leading order of perturbative QCD, giving alpha(s)(M-Z) = 0.120 +/- 0.002(stat.) +/- 0.013(syst.) +/- 0.007(scale). Including non-perturbative power corrections :leads to alpha(s)(M-Z) = 0.101 +/- 0.002(stat.) +/- 0.013(syst.) +/- 0.007(scale). The measured transverse and longitudinal components of the fragmentation function are used to estimate the mean charged multiplicity, [n(ch)] = 21.21 +/- 0.01(stat.)+/- 0.20(syst.) The fragmentation functions and multiplicities in bb and light quark events are compared. The measured transverse and longitudinal components of the fragmentation function allow the gluon fragmentation function to be evaluated