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Jiang, Zhiheng,Zhang, Victor,Huang, Yi-Jiun,Achkar, Joseph,Piester, Dirk,Lin, Shinn-Yan,Wu, Wenjun,Naumov, Andrey,Yang, Sung-hoon,Nawrocki, Jerzy,Sesia, Ilaria,Schlunegger, Christian,Yang, Zhiqiang,Fu BUREAU INTERNATIONAL DES POIDS ET MESURES 2018 METROLOGIA -BERLIN- Vol.55 No.5
<P>Two-way satellite time and frequency transfer (TWSTFT) is a primary technique for the generation of coordinated universal time (UTC). About 20 timing laboratories around the world continuously operate TWSTFT using satellite time and ranging equipment (SATRE<SUP>19</SUP>) modems for remote time and frequency comparisons in this context. The precision of the SATRE TWSTFT as observed today is limited by an apparent daily variation pattern (diurnal) in the TWSTFT results. The observed peak-to-peak variation have been found as high as 2 ns in some cases. Investigations into the origins of the diurnals have so far provided no complete understanding about the cause of the diurnals. One major contributor to the diurnals, however, could be related to properties of the receive part in the modem. In 2014 and 2015, it was demonstrated that bypassing the receive part and the use of software-defined radio (SDR) receivers in TWSTFT ground stations (SDR TWSTFT) instead could considerably reduce both the diurnals and the measurement noise.</P> <P>In 2016, the International Bureau of Weights and Measures (BIPM) and the Consultative Committee for Time and Frequency (CCTF) working group (WG) on TWSTFT launched a pilot study on the application of SDR receivers in the TWSTFT network for UTC computation.</P> <P>The first results of the pilot study were reported to the CCTF WG on TWSTFT annual meeting in May 2017, demonstrating that SDR TWSTFT shows superior performance compared to that of SATRE TWSTFT for practically all links between participating stations. In particular, for continental TWSTFT links, in which the strongest diurnals appear, the use of SDR TWSTFT results in a significant suppression of the diurnals by a factor of between two and three. For the very long inter-continental links, e.g. the Europe-to-USA links where the diurnals are less pronounced, SDR TWSTFT achieved a smaller but still significant gain of 30%. These findings are supported by an evaluation of some of the links with an alternate technique based on GPS signals (GPS IPPP) as reported in this paper.</P> <P>Stimulated by these results, the WG on TWSTFT prepared a recommendation for the 21st CCTF meeting, which proposed the introduction of SDR TWSTFT in UTC generation. With CCTF approval of the recommendation, a roadmap was developed for the implementation of SDR TWSTFT in UTC generation. In accordance with the roadmap, most of the stations that participated in the pilot study have updated the SDR TWSTFT settings to facilitate the use of SDR TWSTFT data in UTC generation. In addition, the BIPM conducted a final evaluation to validate the long-term stability of SDR TWSTFT links, made test runs using the BIPM standard software for the calculation of UTC, now including SDR TWSTFT data, and started to calculate SDR TWSTFT time links as backup from October 2017. The use of SDR TWSTFT in UTC generation will begin in 2018.</P>
Aad, G.,Abbott, B.,Abbott, D.C.,Abed Abud, A.,Abeling, K.,Abhayasinghe, D.K.,Abidi, S.H.,AbouZeid, O.S.,Abraham, N.L.,Abramowicz, H.,Abreu, H.,Abulaiti, Y.,Acharya, B.S.,Achkar, B.,Adachi, S.,Adam, L. North-Holland Pub. Co 2020 Physics letters. Section B Vol.801 No.-
<P><B>Abstract</B></P> <P>A search for long-lived particles decaying into an oppositely charged lepton pair, <I>μμ</I>, <I>ee</I>, or <I>eμ</I>, is presented using 32.8 <SUP> fb − 1 </SUP> of <I>pp</I> collision data collected at s = 13 TeV by the ATLAS detector at the LHC. Candidate leptons are required to form a vertex, within the inner tracking volume of ATLAS, displaced from the primary <I>pp</I> interaction region. No lepton pairs with an invariant mass greater than 12 GeV are observed, consistent with the background expectations derived from data. The detection efficiencies for generic resonances with lifetimes (<I>cτ</I>) of 100–1000 mm decaying into a dilepton pair with masses between 0.1–1.0 TeV are presented as a function of <SUB> p T </SUB> and decay radius of the resonances to allow the extraction of upper limits on the cross sections for theoretical models. The result is also interpreted in a supersymmetric model in which the lightest neutralino, produced via squark–antisquark production, decays into <SUP> ℓ + </SUP> <SUP> ℓ ′ − </SUP> ν ( ℓ , <SUP> ℓ ′ </SUP> = e , <I>μ</I>) with a finite lifetime due to the presence of R-parity violating couplings. Cross-section limits are presented for specific squark and neutralino masses. For a 700 GeV squark, neutralinos with masses of 50–500 GeV and mean proper lifetimes corresponding to <I>cτ</I> values between 1 mm to 6 m are excluded. For a 1.6 TeV squark, <I>cτ</I> values between 3 mm to 1 m are excluded for 1.3 TeV neutralinos.</P>
Aad, G.,Abbott, B.,Abbott, D.C.,Abed Abud, A.,Abeling, K.,Abhayasinghe, D.K.,Abidi, S.H.,AbouZeid, O.S.,Abraham, N.L.,Abramowicz, H.,Abreu, H.,Abulaiti, Y.,Acharya, B.S.,Achkar, B.,Adachi, S.,Adam, L. North-Holland Pub. Co 2020 Physics letters. Section B Vol.801 No.-
<P><B>Abstract</B></P> <P>A search for non-resonant Higgs boson pair production, as predicted by the Standard Model, is presented, where one of the Higgs bosons decays via the H → b b channel and the other via one of the H → W <SUP> W ⁎ </SUP> / Z <SUP> Z ⁎ </SUP> / τ τ channels. The analysis selection requires events to have at least two <I>b</I>-tagged jets and exactly two leptons (electrons or muons) with opposite electric charge in the final state. Candidate events consistent with Higgs boson pair production are selected using a multi-class neural network discriminant. The analysis uses 139 fb<SUP>−1</SUP> of <I>pp</I> collision data recorded at a centre-of-mass energy of 13 TeV by the ATLAS detector at the Large Hadron Collider. An observed (expected) upper limit of 1.2 ( 0.9 − 0.3 + 0.4 ) pb is set on the non-resonant Higgs boson pair production cross-section at 95% confidence level, which is equivalent to 40 ( 29 − 9 + 14 ) times the value predicted in the Standard Model.</P>