LTP: Laboratory for Particle Physics

The Laboratory of Particle Physics (LTP) at the Paul Scherrer Institute pursues fundamental research, addressing the most up to date questions in modern physics. read more

Latest Scientific Highlights

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

24. August 2016

Laser spectroscopy of muonic deuterium

The deuteron is the simplest compound nucleus, composed of one proton and one neutron. Deuteron properties such as the root-mean-square charge radius rd and the polarizability serve as important benchmarks for understanding the nuclear forces and structure. Muonic deuterium μd is the exotic atom formed by a deuteron and a negative muon μ-. We measured three 2S-2P transitions in μd and obtain rd = 2.12562(78) fm, which is 2.7 times more accurate but 7.5σ smaller than the CODATA-2010 value rd = 2.1424(21) fm. The μd value is also 3.5σ smaller than the rd value from electronic deuterium spectroscopy. The smaller rd, when combined with the electronic isotope shift, yields a “small” proton radius rp, similar to the one from muonic hydrogen, amplifying the proton radius puzzle.
Facility: Particle Physics

Reference: Pohl, R., Nez, F., Fernandes, L., et al., Science 353 (2016), 669

Read full article: here

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)

Read full article: here