Scientific Highlights 2018

12 September 2018

Solid deuterium surface degradation at ultracold neutron sources

Solid deuterium (sD2) is used as an efficient converter to produce ultracold neutrons (UCN). Itis known that the sD2 must be sufficiently cold, of high purity and mostly in its ortho-state in order to guarantee long lifetimes of UCN in the solid from which they are extracted into vacuum. Also the UCN transparency of the bulk sD2 material must be high because crystal inhomogeneities limit the mean freepath for elastic scattering and reduce the extraction efficiency. Observations at the UCN sources at Paul Scherrer Institute and at Los Alamos National Laboratory consistently show a decrease of the UCN yield with time of operation after initial preparation or later treatment (“conditioning”) of the sD2. We show that, in addition to the quality of the bulk sD2, the quality of its surface is essential. Our observations and simulations support the view that the surface is deteriorating due to a build-up of D2 frost-layers underpulsed operation which leads to strong albedo reflections of UCN and subsequent loss. We report results of UCN yield measurements, temperature and pressure behavior of deuterium during source operation and conditioning, and UCN transport simulations. This, together with optical observations of sD2 frost formation on initially transparent sD2 in offline studies with pulsed heat input at the North Carolina State University UCN source, results in a consistent description of the UCN yield decrease.
Facility: UCN

Reference: A. Anghel et al, European Physical Journal A 54, 148 (2018), featured as EPJ A highlight

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4 June 2018

Observation of ttH Production

The observation of Higgs boson production in association with a top quark-antiquark pair is reported, based on a combined analysis of proton-proton collision data at center-of-mass energies of √s = 7,8, and 13 TeV, corresponding to integrated luminosities of up to 5.1, 19.7, and 35.9 fb-1, respectively. The data were collected with the CMS detector at the CERN LHC. The results of statistically independent searches for Higgs bosons produced in conjunction with a top quark-antiquark pair and decaying to pairs of W bosons, Z bosons, photons, τ leptons, or bottom quark jets are combined to maximize sensitivity. An excess of events is observed, with a significance of 5.2 standard deviations, over the expectation from the background-only hypothesis. The corresponding expected significance from the standard model for a Higgs boson mass of 125.09 GeV is 4.2 standard deviations. The combined best fit signal strength normalized to the standard model prediction is 1.26+0.31-0.26.
Facilities: Particle Physics Facilities

Reference: A.M. Sirunyan et al (CMS collaboration), Physical Review Letters 120, 231801 (2018), Editors' suggestion

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3 May 2018

Searching for New Physics with b → sτ+τ-

In recent years, intriguing hints for the violation of lepton flavor universality (LFU) have been accumulated in semileptonic B decays, both in the charged-current transitions b → cl-ν-l (i.e., RD, RD∗, and RJ/Ψ and the neutral-current transitions b → sl+l- (i.e., RK and RK∗. Hints for LFU violation in RD(∗) and RJ/Ψ point at large deviations from the standard model (SM) in processes involving tau leptons. Moreover, LHCb has reported deviations from the SM expectations in b → sμ+μ- processes as well as in the ratios RK and RK∗, which together point at new physics (NP) affecting muons with a high significance. These hints for NP suggest the possibility of huge LFU-violating effects in b → sτ+τ- transitions. In this Letter, we predict the branching ratios of B → Kτ+τ-, B → Kτ+τ-, and Bs → Φτ+τ-, taking into account NP effects in the Wilson coefficients Cττ9(') and Cττ10('). Assuming a common NP explanation of RD, RD(∗), and RJ/Ψ, we show that a very large enhancement of b → sτ+τ- processes, of around 3 orders of magnitude compared to the SM, can be expected under fairly general assumptions. We find that the branching ratios of Bs → τ+τ-, Bs → Φτ+τ-, and B → K(∗)τ+τ- under these assumptions are in the observable range for LHCb and Belle II.
Reference: B. Capdevila et al, Physical Review Letters 120, 181802 (2018)

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