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

1 December 2018

ERC Consolidator grant for Paolo Crivelli

Dr. Paolo Crivelli (ETH Zurich, Department of Physics, Institute for Particle Physics and Astrophysics) has recently been awarded an ERC Consolidator grant for his project "Mu-MASS" aiming at a new precision measurement of the Muonium 1S-2S transition energy, ultimately with an improvement by three orders of magnitude. Muonium is the hydrogen-like atom formed by a positive muon and an electron. Its energy levels are, in contrast to the case of ordinary hydrogen, completely dominated by Quantum-Electro-Dynamics (QED) and precision QED calculations are currently ahead of experiment. It will be the first time that CW two-photon spectroscopy is applied to muonium. Results will improve our knowledge of the muon mass, test bound state QED and search for physics beyond the Standard Model of Particle Physics. Moreover, it will help to resolve the present 'proton radius puzzle' by providing a determination of the Rydberg constant free of finite size effects. While Dr. Crivelli is located at ETH H&oml;nggerberg, he and his team will spend a large amount of time at PSI and collaborate closely, both, with the NUM laboratories for Muon Spin Spectroscopy and for Particle Physics.

27 June 2018

Eccellenza Professorship to Lea Caminada

Dr Lea Caminada, a researcher in the High-Energy Particle Physics group of the Laboratory for Particle Physics (LTP) in NUM, has been awarded a Swiss National Science Foundation (SNF) Eccellenza Professorial Fellowship. Together with the High-Energy Particle Physics group Lea is working for the CMS experiment at the LHC facility at CERN. Her research proposal consists of two parts. In the first one she proposes to investigate the decay of the Higgs boson into charm quarks, which will enable a deeper understanding of it's nature. The second part focuses on the further development of silicon pixel detectors, a speciality of the group at PSI. Lea obtained her PhD in 2010 at the ETH in Zurich. After a 4 year postdoc at LBNL in Berkeley she moved to the University of Zurich in 2014. Since 2016 she has been holding a joint scientist position at PSI and University of Zurich.

27 June 2018

CMS Young Researcher Prize awarded to Lea Caminada

Lea Caminada, a researcher in the High-Energy Particle Physics group of the Laboratory for Particle Physics (LTP) in NUM, has received the annual CMS Young Researcher Prize. This Prize is given once a year to outstanding young physicists who made very significant and sustained contributions to the CMS experiment at the LHC facility at CERN. Dr. Caminada has been recognized for her contribution to the construction, installation and commissioning of the two pixel detectors which were build at PSI for the CMS experiment. Her work also included the measurement of the B-meson production cross section and the observation of the Higgs boson in association with top quarks.

14 January 2019

Detailed polarization measurements of the prompt emission of five gamma-ray bursts

Gamma-ray bursts (GRBs) are the strongest explosions in the Universe since the Big Bang. They are believed to be produced either in the formation of black holes at the end of massive star evolution or the merging of compact objects. Spectral and timing properties of GRBs suggest that the observed bright gamma-rays are produced in the most relativistic jets in the Universe; however, the physical properties (especially the structure and magnetic topologies) of the jets are still not well known, despite several decades of studies. It is widely believed that precise measurements of the polarization properties of GRBs should provide crucial information on the highly relativistic jets. As a result, there have been many reports of GRB polarization measurements with diverse results; however, many such measurements suffer from substantial uncertainties, most of which are systematic. After the first successful measurements by the Gamma-Ray Burst Polarimeter (GAP) and Compton Spectrometer and Imager (COSI) instruments, here we report a statistically meaningful sample of precise polarization measurements, obtained with the dedicated GRB polarimeter POLAR onboard China’s Tiangong-2 space laboratory. Our results suggest that the gamma-ray emission is at most polarized at a level lower than some popular models have predicted, although our results also show intrapulse evolution of the polarization angle. This indicates that the low polarization degrees could be due to an evolving polarization angle during a GRB.
Reference: S.N. Zhang et al, Nature Astronomy, adv. online publication (January 2019)

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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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6 April 2018

No-Go Theorem for Nonstandard Explanations of the τ → KSπντ CP Asymmetry

The CP asymmetry in τ → KSπντ, as measured by the BABAR collaboration, differs from the standard model prediction by 2.8 σ. Most nonstandard interactions do not allow for the required strong phase needed to produce a nonvanishing CP asymmetry, leaving only new tensor interactions as a possible mechanism. We demonstrate that, contrary to previous assumptions in the literature, the crucial interference between vector and tensor phases is suppressed by at least 2 orders of magnitude due to Watson’s final-state- interaction theorem. Furthermore, we find that the strength of the relevant CP-violating tensor interaction is strongly constrained by bounds from the neutron electric dipole moment and D—D¯. These observations together imply that it is extremely difficult to explain the current τ → KSπντ measurement in terms of physics beyond the standard model originating in the ultraviolet.
Reference: V. Cirigliano et al, Physical Review Letters 120, 141803 (2018)

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5 December 2017

The Charpak-Ritz Prize 2018 is awarded to Roland Paul Horisberger

The Charpak-Ritz Prize 2018, jointly awarded by the French Physical Society and the Swiss Physical Society, has been bestowed to Roland Paul Horisberger for his numerous contributions to the development of precision silicon vertex detectors for particle physics experiments as well as for the application of these technologies in X-ray photon sciences. His ground breaking work for the silicon microstrip detector for DELPHI at LEP, the H1-central vertex detector at HERA and the pixel detector for CMS have been crucial for the excellent physics performances and discoveries of these experiments. He successfully transferred these novel technologies, like for instance the PILATUS pixel detectors, to the field of X-ray photon sciences and such enabled a large number of new scientific results. With his group at PSI he has created the foundations that allowed in 2007 the creation of a spin-off company that now is the market leader in single photon counting pixel detectors at synchrotrons world wide.
Facility: Particle Physics

Reference: SPS Site

5 December 2017

Jean-Baptiste Mosset winner of PSI Founder Fellowship

Jean-Baptiste Mosset from the Laboratory of Particle Physics is the winner of a PSI Founder Fellowship and plans now to commercialise a neutron detector to spot plutonium and uranium. With his new technology, less expensive and more efficient neutron detectors could be developed. In the next 18 months, Mosset wants to further develop his prototype and find out if demand for this technology exists in industry.
Facility: Particle Physics

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14 November 2017

Search for Axionlike Dark Matter through Nuclear Spin Precession in Electric and Magnetic Fields

We report on a search for ultralow-mass axionlike dark matter by analyzing the ratio of the spin-precession frequencies of stored ultracold neutrons and 199Hg atoms for an axion-induced oscillating electric dipole moment of the neutron and an axion-wind spin-precession effect. No signal consistent with dark matter is observed for the axion mass range 10-24 ≤ ma ≤ 10-17eV. Our null result sets the first laboratory constraints on the coupling of axion dark matter to gluons, which improve on astrophysical limits by up to 3 orders of magnitude, and also improves on previous laboratory constraints on the axion coupling to nucleons by up to a factor of 40.
Facility: Particle Physics

Reference: C. Abel et al, Physical Review X 7, 041034 (2017)

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20 September 2017

Commissioning and first performance studies of the new CMS pixel detector

In the previous months the new CMS pixel detector was brought into operation. The detector was moved from PSI to CERN and installed in February, followed by an intensive period of commissioning and calibration. This process mostly involved the adjustment of many operational parameters which influence the detector performance, e.g. the individual pixel thresholds had to be optimized for each pixel in order to achieve the best possible detector resolution – a challenging task, as there are 80 million pixels which operate at a readout speed of 40 MHz and have to cope with rates of up to 500 MHz/cm2 in the inner part of the detector. The detector consists of 4 cylindrical barrel layers located at radii between 3 cm and 16 cm around the LHC collision point. The inner most layer is equipped with a newly developed readout chip (PROC600) which has a very different readout architecture compared to the readout chip used in the other layers. As the inner layer has to deal with the highest rates it took a considerable time to understand and learn how to efficiently operate this part of the detector.

After this intense phase of commissioning and calibration the pixel detector has been included in the CMS data acquisition and first performance studies were done. The two most important performance parameters are the hit efficiency and the position resolution.

The efficiency is higher than in the previous pixel detector: It is above 99%, except for the inner layer, where high-rate related data losses limit the efficiency to 97% at the highest LHC collision rates. The achieved position resolutions are very good, too, and illustrated in the two plots: The detector can measure the position of charged particles with an accuracy of ~12 μm in the r-phi direction and ~22 μm in the z-direction.
Facility: Particle Physics

14 September 2017

teaser picture

The hard worker from Val Mesolcina

Matter and Material Research Using Muons Particle Physics

For Aldo Antognini, physics and conviviality are in the blood

PSI researcher Aldo Antognini has received more than 2.2 million Swiss francs from the EU for his latest experiment. He wants to find out how magnetism is distributed in the proton. The particle physicist will be able to apply not only his scientific and technical talents, but his social flair as well.

19 June 2017

Comparison of ultracold neutron sources for fundamental physics measurements

Ultracold neutrons (UCNs) are key for precision studies of fundamental parameters of the neutron and in searches for new charge-parity-violating processes or exotic interactions beyond the Standard Model of particle physics. The most prominent example is the search for a permanent electric-dipole moment of the neutron (nEDM). We have performed an experimental comparison of the leading UCN sources currently operating. We have used a “standard” UCN storage bottle with a volume of 32 liters, comparable in size to nEDM experiments, which allows us to compare the UCN density available at a given beam port.
Facility: Particle Physics

Reference: Bison, G., Daum, M., Kirch K., et al, Phys. Rev. C 95 (2017), 045503-1

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16 March 2017

High-resolution non-destructive three-dimensional imaging of integrated circuits

Modern nanoelectronics has advanced to a point at which it is impossible to image entire devices and their interconnections non- destructively because of their small feature sizes and the complex three-dimensional structures resulting from their integration on a chip. This metrology gap implies a lack of direct feedback between design and manufacturing processes, and hampers quality control during production, shipment and use. Here we demonstrate that X-ray ptychography - a high-resolution coherent diffractive imaging technique - can create three-dimensional images of integrated circuits of known and unknown designs with a lateral resolution in all directions down to 14.6 nanometres. We obtained detailed device geometries and corresponding elemental maps, and show how the devices are integrated with each other to form the chip. Our experiments represent a major advance in chip inspection and reverse engineering over the traditional destructive electron microscopy and ion milling techniques. Foreseeable developments in X-ray sources, optics and detectors, as well as adoption of an instrument geometry optimized for planar rather than cylindrical samples, could lead to a thousand-fold increase in efficiency, with concomitant reductions in scan times and voxel sizes.
Facility: SLS

Reference: M. Holler et al, Nature 543, 402 (2017)

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1 March 2017

Silicon pixel barrel detector successfully installed in the CMS experiment

Middle of February the upgraded CMS silicon pixel barrel detector has been moved from PSI to CERN and was successfully installed in the CMS experiment. The new pixel detector is part of the so-called phase1-upgrade of the CMS detector, located at a distance of only a few centimetres away from the interaction point and able to cope with the harsh particle environment expected due to the increased luminosity of the LHC collider.
The installation of the upgraded pixel detector so far crowns the work of approximately 15 year of a collaborative R&D effort led by the High Energy Group of the Laboratory for Particle Physics. After the installation of the previous pixel detector in 2008 the work concentrated on the design of a series of radiation tolerant pixel readout chips with low pixel thresholds, low noise behaviour and high pixel hit rate capabilities of up to 600 MHz/cm2 and the development and construction of a very light, low material budget detector mechanics.
The performance of the chips and related readout electronics was regularly tested at the πE1 beamline which offers with its high momentum and high rate pion beam similar conditions as the hadronic particle environment close to the interaction point of the CMS experiment. The silicon pixel technology developed at PSI for the first CMS silicon vertex detector was successfully transferred to industry and led in 2007 to the foundation of the spin-off company DECTRIS which fabricates and sells x-ray counting pixel detectors all over the world.
Facility: Particle Physics