Scientific Highlights 2016

14 November 2016

The deuteron too poses a mystery

The deuteron — one of the simplest atomic nuclei, consisting of just one proton and one neutron — is considerably smaller than previously thought. This finding was arrived at by an international research group that carried out experiments at the Paul Scherrer Institute, PSI. The new result is consistent with a 2010 study by the same group, in which the researchers measured the proton and found a significantly smaller value than previous research using different experimental methods. The result from 2010 formed the basis for what has been known since then as the proton radius puzzle. The new measurement of the deuteron’s size has now given rise to an analogous mystery. It is possible that this will lead to an adjustment of the Rydberg constant, a fundamental quantity in physics. Another possible explanation is that a physical force as yet unknown is at work. For their experiments the researchers used laser spectroscopy to measure so-called muonic deuterium: an artificial atom consisting of a deuteron orbited by an exotic elementary particle known as a muon. The experiments took place at PSI because the world’s most powerful muon source, available here, was needed to produce sufficient muonic deuterium. The researchers have published their new study of the deuteron’s size in the renowned journal Science.
Facility: Particle Physics

Reference: R. Pohl, F. Nez, L.M.P. Fernandes, F.D. Amaro, F. Biraben, J.M.R. Cardoso, D.S. Covita, A. Dax, S. Dhawan, M.Diepold, A. Giesen, A.L. Gouvea, T. Graf, T.W. Hänsch, P. Indelicato, L. Julien, P. Knowles, F. Kottmann, E.-O. Le Bigot, Y.-W. Liu, J.A.M. Lopes, L. Ludhova, C.M.B. Monteiro, F. Mulhauser, T. Nebel, P. Rabinowitz, J.M.F. dos Santos, L.A. Schaller, K. Schuhmann, C. Schwob, D. Taqqu, J.F.C.A. Veloso, A. Antognini Science 12 August 2016: Vol. 353, no. 6300, page 669

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27 September 2016

POLAR experiment successfully launched on Chinese spacecraft

The second Chinese space laboratory satellite Tian Gong 2 was successfully launched from the Jiuquan Satellite Launch Center on September 15th, 2016 at 22:04 BTC (UTC+8h). Among more than ten instruments onboard it also brought to space the only non-Chinese experiment POLAR - the hard X-ray polarimeter. This novel instrument was constructed in collaboration between University of Geneva, the Paul Scherer Institut in Switzerland, the Institute of High Energy Physics CAS in China and the National Center for Nuclear Research in Poland. POLAR is equipped with an array of 1600 plastic scintillators allowing for precise and efficient measurement of the linear polarization from the prompt emission of Gamma Ray Bursts (GRB) – the biggest explosions in the Universe.

The PSI laboratory of particle physics (LTP) has contributed with the initial concept, its verification and validation using the bread board model. In the main project stage we provided the full design, construction and qualification of all electronic subsystems except of power supplies as well as developed the firmware for front-end electronics and central computer.

The POLAR experiment onboard of Tian Gong 2 was switched on the 22nd September at 18:45 BCT. The polarimeter is now undergoing a 21 days long commissioning period followed by a calibration phase. Initial data show that POLAR works fully as expected and its operating conditions are stable. All detection channels are delivering useful data. The PSI POLAR Data Server receives preprocessed telemetry packets from the satellite with a delay of about 24 hours. POLAR onboard Tian Gong 2 is scheduled for two to three years operation in space. It anticipates detections of several tens of very strong GRBs events as well as dozens of intense Solar Flares. POLAR data are expected to provide final explanation for still puzzling physical mechanisms of GRBs. It will also be possible to conduct first precise measurements of the hard X-ray polarization in solar flares.

The first transient event from space was detected by POLAR already on September 23rd. Two other strong transient events were observed on September 25th. Their detection was confirmed by RHESSI satellite - another space mission sensitive to the hard X-rays. RHESSI is devoted mainly for solar observation and is operated by the FHNW in Brugg-Windisch – our collaborators for studies of solar flare eruption mechanisms.
Facility: Particle Physics

23 August 2016

Search for the lepton flavour violating decay μ+→e+γ with the full dataset of the MEG experiment

The final results of the search for the lepton flavour violating decay μ+→e+γ based on the full dataset collected by the MEG experiment at the Paul Scherrer Institut in the period 2009–2013 and totalling 7.5×1014 stopped muons on target are presented. No significant excess of events is observed in the dataset with respect to the expected background and a new upper limit on the branching ratio of this decay of B(μ+→e+γ)<4.2×10−13 (90 % confidence level) is established, which represents the most stringent limit on the existence of this decay to date.
Facility: Particle Physics

Reference: Baldini, A.M., Bao, Y., Baracchini, e. et al, Eur. Phys. J. C 76 (2016), 434

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18 July 2016

POLAR detector developed at the PSI flies into orbit with a Chinese space mission

Researchers working with Wojciech Hajdas at the Paul Scherrer Institute PSI have developed a detector called POLAR. This instrument is expected to search out and investigate so-called gamma ray bursts coming from the depths of the universe. Gamma ray bursts are eruptions of high-energy light that despite being extremely strong remain, up to now, only poorly understood. Among other things, the origin of gamma ray bursts has not been resolved; it is possible that these strong flashes of light are emitted during the formation of black holes. To improve our understanding of gamma ray bursts, POLAR will measure a property of their light. POLAR was realised in cooperation with researchers at the University of Geneva and will be launched into orbit this coming September with a Chinese space mission.
Facility: Particle Physics

More information: here

13 July 2016

Muon polarization in the MEG experiment: predictions and measurements

The MEG experiment makes use of one of the world’s most intense low energy muon beams, in order to search for the lepton flavour violating process μ+→e+γ . We determined the residual beam polarization at the thin stopping target, by measuring the asymmetry of the angular distribution of Michel decay positrons as a function of energy. The initial muon beam polarization at the production is predicted to be Pμ=−1Pμ=−1 by the Standard Model (SM) with massless neutrinos. We estimated our residual muon polarization to be Pμ=−0.86±0.02 (stat) +0.05−0.06 (syst)Pμ=−0.86±0.02 (stat) −0.06+0.05 (syst) at the stopping target, which is consistent with the SM predictions when the depolarizing effects occurring during the muon production, propagation and moderation in the target are taken into account. The knowledge of beam polarization is of fundamental importance in order to model the background of our μ+→e+γ search induced by the muon radiative decay: μ+→e+ν¯μνeγ .
Facility: Particle Physics

Reference: A.M. Baldini et al, European Physical Journal C 76, 223 (2016)

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1 January 2016

Rate of Molecular Transfer of Allyl Alcohol across an AOT Surfactant Layer Using Muon Spin Spectroscopy

The transfer rate of a probe molecule across the interfacial layer of a water-in-oil (w/o) microemulsion was investigated using a combination of transverse field muon spin rotation (TF-μSR), avoided level crossing muon spin resonance (ALC-μSR), and Monte Carlo simulations. Reverse micro-emulsions consist of nanometer-sized water droplets dispersed in an apolar solvent separated by a surfactant monolayer. Although the thermodynamic, static model of these systems has been well described, our understanding of their dynamics is currently incomplete. For example, what is the rate of solute transfer between the aqueous and apolar solvents, and how this is influenced by the structure of the interface? With an appropriate choice of system and probe molecule, μSR offers a unique opportunity to directly probe these interfacial transfer dynamics.
Facility: SμS

Reference: U.A. Jayasooriya et al, Langmuir 32, 664 (2016)

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