Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Proton size puzzle reinforced!

25.01.2013
International team of scientists confirms surprisingly small proton radius with laser spectroscopy of exotic hydrogen.

The initial results puzzled the world three years ago: the size of the proton (to be precise, its charge radius), measured in exotic hydrogen, in which the electron orbiting the nucleus is replaced by a negatively charged muon, yielded a value significantly smaller than the one from previous investigations of regular hydrogen or electron-proton-scattering.


Photo: Dr. Franz Kottmann, Dr. Randolf Pohl, and Dr. Daniel Covita (from left to right) in front of a superconducting magnet (5 Tesla) in which the experiment is set up: both the myon detectors and the hydrogen target are located inside the magnet. The strong magnetic field is necessary for collimating the muon beam down to a size as small as the diameter of a pencil.
© CREMA-collaboration, MPQ

A new measurement by the same team confirms the value of the electric charge radius and makes it possible for the first time to determine the magnetic radius of the proton via laser spectroscopy of muonic hydrogen (Science, January 25, 2013). The experiments were carried out at the Paul Scherrer Institut (PSI) (Villigen, Switzerland) which is the only research institute in the world providing the necessary amount of muons.

The international collaboration included the Max-Planck-Institute of Quantum Optics (MPQ) in Garching near Munich, the Swiss Federal Institute of Technology ETH Zurich (Switzerland), the University of Fribourg (Switzerland), the Institut für Strahlwerkzeuge (IFSW) of the Universität Stuttgart, Dausinger & Giesen GmbH, Stuttgart, the Universities of Coimbra and Aveiro (Portugal), and the Laboratoire Kastler Brossel (LKB), Paris. The new results fuel the debate as to whether the discrepancies observed can be explained by standard physics, for example an incomplete understanding of the systematic errors that are inherent to all measurements, or whether they are due to new physics.

The hydrogen atom has played a key role in the investigation of the fundamental laws of physics. Its nucleus consists of a single positively charged proton orbited by a negatively charged electron, a model whose success dates from its proposed by Bohr in 1913. The energy levels of this simplest of atoms can be predicted with excellent precision from the theory of quantum electrodynamics.

However, the calculations have to take into account that – in contrast to the point-like electron – the proton is an extended object with a finite size, made of three quarks bound by so-call ‘gluons’. Therefore, the electric charge as well as the magnetism of the proton is distributed over a certain volume. The extended nature of the proton causes a shift of the energy levels in hydrogen. Hence the electric and the magnetic charge radii can be deduced from a measurement of the level shifts.

In 2010, the first results on the spectroscopic determination of the shift of the so-called 2S energy level in muonic hydrogen were published. The exotic atoms were generated by bombarding a target of regular hydrogen with muons from an accelerator at PSI. Muons behave a lot like electrons, except for their mass: muons are 200 times heavier than electrons. The atomic orbit of the muon is therefore much closer to the proton than the electron’s orbit in a regular hydrogen atom. This results in a much larger sensitivity of the muon’s energy level to the proton size and hence to a stronger shift of the energy levels. Measuring the level shifts is very technologically demanding: muonic hydrogen is very short-lived (muons decay after about two millionths of a second), so the light pulses for the excitation of the resonance have to be fired onto the hydrogen target only nanoseconds after the detection of a muon. The new disk laser technology developed by the Institut für Strahlwerkzeuge (IFSW) of the Universität Stuttgart was an important element to fulfil this requirement. The lasers necessary for exciting the resonance were developed by the Max-Planck-Institute of Quantum Optics in cooperation with the Laboratoire Kastler Brossel (Paris). Coimbra, Aveiro and Fribourg universities were responsible for the development of the x-rays detectors.

In the experiment described in the newly published Science article, the energy shift was determined for another transition. This leads to a new measurement of the electric charge radius of the proton. Its value of 0.84087(39) femtometres (1 fm = 0.000 000 000 000 001 metre) is in good agreement with the one published in 2010, but 1.7 times as precise. The discrepancy with existing radius measurements made in regular hydrogen or by electron-proton-scattering, the so-called proton size puzzle, has thus been reaffirmed.

In addition, the new measurement allows a determination of the magnetic radius of the proton for the first time by laser spectroscopy of muonic hydrogen. This results in a value of 0.87(6) femtometres, in agreement with all previous measurements. Though the precision is, at present, of the same order as in other experiments, laser spectroscopy of muonic hydrogen has the potential of achieving a much better accuracy in the determination of the magnetic proton radius in the future.

Physicists around the world are actively seeking a solution to the proton size puzzle. Previous measurements in regular hydrogen and by electron-proton-scattering are being reanalyzed and even repeated. Theorists of various disciplines suggested ways to explain the discrepancy. Very interesting proposals explain the discrepancies by physics beyond the standard model. Other explanations suggest a proton structure of higher complexity than assumed today which only reveals itself under the influence of the heavy muon. New measurements are needed to check on these possibilities. Muon-proton-scattering experiments are being developed at PSI, new precision measurements at the electron accelerator in Mainz are being considered, and the PSI team plans to measure, for the first time ever, laser spectroscopy of the muonic helium atom in the course of this year. The required modifications of the current laser system are being investigated in the frame of the project “Thin-disk laser for muonic atoms spectroscopy” which (financed by the Swiss National Science Foundation (SNSF) and the Deutsche Forschungsgemeinschaft (DFG)) is carried out at the ETH Zürich (Prof. Dr. Klaus Kirch, Dr. Aldo Antognini) and at the IFSW (Prof. Dr. Thomas Graf, Dr. Andreas Voß). The Project “Muonic Helium” is also generously supported by the European Research Council (ERC) by an ERC Starting Grant held by Dr. Randolf Pohl from the MPQ in Garching.

Olivia Meyer-Streng

Original publication:
Aldo Antognini, François Nez, Karsten Schuhmann, Fernando D. Amaro, François Biraben, João M. R. Cardoso, Daniel S. Covita, Andreas Dax, Satish Dhawan, Marc Diepold, Luis M. P. Fernandes, Adolf Giesen, Andrea L. Gouvea, Thomas Graf, Theodor W. Hänsch, Paul Indelicato, Lucile Julien, Cheng-Yang Kao, Paul Knowles, Franz Kottmann, Eric-Olivier Le Bigot, Yi-Wei Liu, José A. M. Lopes, Livia Ludhova, Cristina M. B. Monteiro, Françoise Mulhauser, Tobias Nebel, Paul Rabinowitz, Joaquim M. F. dos Santos, Lukas A. Schaller, Catherine Schwob, David Taqqu, João F. C. A. Veloso, Jan Vogelsang, Randolf Pohl
Proton structure from the measurement of 2S − 2P transition frequencies of muonic hydrogen

Science, January 25, 2013

The experiment was the collaborative success of many institutes from various countries: Max-Planck-Institute of Quantum Optics, Garching, Germany, Institute for Particle Physics, ETH Zurich, Switzerland, Laboratoire Kastler Brossel, École Normale Supérieure, CNRS, and Université P. et M. Curie, Paris, France, Dausinger & Giesen GmbH, Stuttgart, Germany, Departamento de Física, Universidade de Coimbra, Portugal, Departamento de Física, Universidade de Aveiro, Portugal, Physics Department, Yale University, New Haven, USA, Institut für Strahlwerkzeuge, Universität Stuttgart, Germany, Physics Department, National Tsing Hua University, Hsinchu, Taiwan, Département de Physique, Université de Fribourg, Switzerland, Department of Chemistry, Princeton University, Princeton, USA, Paul Scherrer Institute, Villigen, Switzerland, Ludwig-MaximiliansUniversität, Munich, Germany

Contact:
Dr. Randolf Pohl
Max-Planck-Institute of Quantum Optics
Hans-Kopfermann-Straße 1
85748 Garching
Phone: +49 (0)89 / 32905 -281
Fax: +49 (0)89 / 32905 -200
E-mail: randolf.pohl@mpq.mpg.de
http://www.mpq.mpg.de/~rnp/
Dr. Aldo Antognini
ETH Zürich
CH-8093 Zürich
Phone: +41 (0)56 310 4614
+41 (0)44 633 2031
E-mail: aldo@phys.ethz.ch
https://muhy.web.psi.ch/wiki/
Prof. Dr. Theodor W. Hänsch
Chair of Experimental Physics,
Ludwig-Maximilians-Universität, Munich
Max-Planck-Institute of Quantum Optics
Hans-Kopfermann-Straße 1,
85748 Garching
Phone: +49 (0)89 / 32905 -702/712
Fax: +49 (0)89 / 32905 -312
E-mail: t.w.haensch@mpq.mpg.de
Prof. Dr. Thomas Graf
Universität Stuttgart
Institut für Strahlwerkzeuge
Pfaffenwaldring 43
D-70569 Stuttgart
Phone: +49 (0)711 68566840
E-mail: graf@ifsw.uni-stuttgart.de
Dr. Franz Kottmann
Paul Scherrer Institut
CH-5232 Villigen
Phone: +41 (0) 56 310 3502
E-mail: franz.kottmann@psi.ch
Karsten Schuhmann
ETH Zürich
CH-8093 Zürich
Phone: +41 (0)44 633 2031
E-mail: skarsten@phys.ethz.ch
and
Daunsinger & Giesen GmbH
Rotebühlstrasse 87
D-70178 Stuttgart

Dr. Olivia Meyer-Streng | Max-Planck-Institut
Further information:
http://www.mpq.mpg.de

More articles from Physics and Astronomy:

nachricht Water without windows: Capturing water vapor inside an electron microscope
13.12.2017 | Okinawa Institute of Science and Technology (OIST) Graduate University

nachricht Columbia engineers create artificial graphene in a nanofabricated semiconductor structure
13.12.2017 | Columbia University School of Engineering and Applied Science

All articles from Physics and Astronomy >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Long-lived storage of a photonic qubit for worldwide teleportation

MPQ scientists achieve long storage times for photonic quantum bits which break the lower bound for direct teleportation in a global quantum network.

Concerning the development of quantum memories for the realization of global quantum networks, scientists of the Quantum Dynamics Division led by Professor...

Im Focus: Electromagnetic water cloak eliminates drag and wake

Detailed calculations show water cloaks are feasible with today's technology

Researchers have developed a water cloaking concept based on electromagnetic forces that could eliminate an object's wake, greatly reducing its drag while...

Im Focus: Scientists channel graphene to understand filtration and ion transport into cells

Tiny pores at a cell's entryway act as miniature bouncers, letting in some electrically charged atoms--ions--but blocking others. Operating as exquisitely sensitive filters, these "ion channels" play a critical role in biological functions such as muscle contraction and the firing of brain cells.

To rapidly transport the right ions through the cell membrane, the tiny channels rely on a complex interplay between the ions and surrounding molecules,...

Im Focus: Towards data storage at the single molecule level

The miniaturization of the current technology of storage media is hindered by fundamental limits of quantum mechanics. A new approach consists in using so-called spin-crossover molecules as the smallest possible storage unit. Similar to normal hard drives, these special molecules can save information via their magnetic state. A research team from Kiel University has now managed to successfully place a new class of spin-crossover molecules onto a surface and to improve the molecule’s storage capacity. The storage density of conventional hard drives could therefore theoretically be increased by more than one hundred fold. The study has been published in the scientific journal Nano Letters.

Over the past few years, the building blocks of storage media have gotten ever smaller. But further miniaturization of the current technology is hindered by...

Im Focus: Successful Mechanical Testing of Nanowires

With innovative experiments, researchers at the Helmholtz-Zentrums Geesthacht and the Technical University Hamburg unravel why tiny metallic structures are extremely strong

Light-weight and simultaneously strong – porous metallic nanomaterials promise interesting applications as, for instance, for future aeroplanes with enhanced...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

See, understand and experience the work of the future

11.12.2017 | Event News

Innovative strategies to tackle parasitic worms

08.12.2017 | Event News

AKL’18: The opportunities and challenges of digitalization in the laser industry

07.12.2017 | Event News

 
Latest News

A whole-body approach to understanding chemosensory cells

13.12.2017 | Health and Medicine

Water without windows: Capturing water vapor inside an electron microscope

13.12.2017 | Physics and Astronomy

Cellular Self-Digestion Process Triggers Autoimmune Disease

13.12.2017 | Life Sciences

VideoLinks
B2B-VideoLinks
More VideoLinks >>>