Shrinking the proton again!

05.10.2017, 21:10 | Wissenschaft | Autor: | Jetzt kommentieren

Scientists from the Max Planck Institute of Quantum Optics, using high precision laser spectroscopy of atomic hydrogen, confirm the surprisingly small value of the proton radius determined from muonic hydrogen.

It was one of the breakthroughs of the year 2010: Laser spectroscopy of muonic hydrogen resulted in a value for the proton charge radius that was significantly smaller, by four standard deviations, than previous determinations using regular hydrogen. This discrepancy and its origin have attracted much attention in the scientific community, with even extensions of the so-called standard model of physics being discussed. Now, a team of scientists from the Laser Spectroscopy Division of Professor Theodor W. Hänsch at the Max Planck Institute of Quantum Optics in Garching has made a new spectroscopic measurement of regular hydrogen. The resulting values for the Rydberg constant and the proton radius are in excellent agreement with the muonic results (Nature 466, 213 (2010)), but disagree by 3.3 standard deviations with the average of the previous determinations from regular hydrogen.

Hydrogen is the simplest of all chemical elements. According to the model proposed by Niels Bohr in 1913, it consists of a single proton and an electron orbiting around it. The theory of quantum electrodynamics predicts the energy levels of this system with 12 digits of precision. Because of this, hydrogen plays a key role in our understanding of nature. Its study allows the determination of fundamental constants such as the Rydberg constant and the proton charge radius.

Hydrogen is thus the ideal subject for testing the laws of nature. This is why a measurement on muonic hydrogen, resulting in a surprisingly small value for the proton charge radius, made big waves in 2010. In that experiment, done at the Paul Scherrer Institute in Villingen, Switzerland, the electron of the hydrogen atom is replaced with its sister particle, the 200-times heavier and short-lived muon. Laser spectroscopy of this muonic hydrogen resulted in a value of the proton radius that was extremely precise, but four percent smaller than previous measurements on regular hydrogen. “Since the muon is 200-times heavier than the electron, it orbits much closer to the proton and ‘feels’ its size,” explains Prof. Randolf Pohl (now at Johannes Gutenberg-Universität Mainz), a member of the MPQ team. “Because of this, the proton radius has a seven orders of magnitude larger influence on the spectral lines than in regular hydrogen. This allows us to determine the proton radius with such a high precision.”

The large discrepancy between the measurements of regular hydrogen and its exotic cousin has sparked many debates about its origin. “However, some of the previous measurements in fact agree with the muonic value. The influence of the proton radius on the energy levels in regular hydrogen is tiny, and even very high precision measurements struggle to resolve it. The discrepancy only becomes significant when all measurements are averaged,” explains Lothar Maisenbacher, one of the graduate students working on the project. “This is why, to solve this ‘proton radius puzzle’, new individual measurements with high precision, and, if possible, using different experimental approaches are necessary.”

In order to determine both the Rydberg constant and the proton charge radius from spectroscopy of regular hydrogen, two different transition frequencies need to be measured. The by far sharpest resonance, the so-called 1S-2S transition, serves as a corner stone in this determination. Its frequency has been measured, in 2011, to 15 digits by the MPQ team (Phys. Rev. Lett. 107, 203001 (2011)). This high precision was made possible not least by the invention of the frequency comb, for which Professor Hänsch was awarded the Nobel Prize in Physics in 2005. For the second frequency measurement needed, the MPQ team chose the so-called 2S-4P transition, which connects the metastable 2S state with the much shorter lived 4P state.

In the experiment, this transition is excited by a laser with a wavelength of 486 nm and the collected fluorescence from the decay of the 4P state serves as a signal. The apparatus used previously for the 1S-2S measurement serves as a source of atoms in the 2S state. Compared to previous experiments, which used room temperature atoms, the atoms probed here have a substantially lower temperature of 5.8 Kelvin and, consequently, a much lower velocity. This, together with especially developed techniques, strongly suppresses the Doppler shift, which constitutes the largest source of uncertainty for this measurement.

“Another source of uncertainty in this experiment is the so-called quantum interference,” explains Lothar Maisenbacher. “If we could probe a single, isolated transition, the shape of the resulting spectral line would be symmetric. However, in our case there are two other upper states that can be excited by the laser, called 4P 1/2 und 4P 3/2. This results in a slightly asymmetric shape of the spectral lines, making the determination of the line center more challenging. While this is a very small effect, it plays a big role for us because we determine the line center with such a high precision of almost one part in 10,000 of the line width.”

To describe the influence of the quantum interference, the scientists performed sophisticated numerical simulations, which are in very good agreement with the experimental results. “In our case, however, a specially derived, simple fit function is sufficient to remove the effects of quantum interference,” emphasizes Vitaly Andreev, also a graduate student on the project. “We use this fit function for our data evaluation. In this way, the simulation is only needed for small corrections on the order of 1 kHz.”

With this, the MPQ team managed to determine the frequency of the 2S-4P transition with an uncertainty of 2.3 kHz. This corresponds to a fractional uncertainty of 4 parts in 10 to the 12, making this the second-best spectroscopy measurement of hydrogen after the aforementioned 1S-2S transition measurement. Combining these results, the Rydberg constant and the proton size are determined to be R∞ = 10973731.568076(96) m to the minus 1 and r = 0.8335(95) fm, respectively.

“Our measurement is almost as precise as all previous measurements on regular hydrogen combined,” summarizes Prof. Thomas Udem, the project leader. “We are in good agreement with the values from muonic hydrogen, but disagree by 3.3 standard deviations with the hydrogen world data, for both the Rydberg constant and the proton radius. To find the causes of these discrepancies, additional measurements with perhaps even higher precision are needed. After all, one should keep in mind that many new discoveries first showed up as discrepancies.” Olivia Meyer-Streng

Figure caption:
This photo shows the vacuum chamber used to measure the 2S-4P transition frequency in atomic hydrogen. The purple glow in the back stems from the microwave discharge that dissociates hydrogen molecules into hydrogen atoms. The blue light in the front is fluorescence from the ultraviolet laser that excites the atoms to the 2S state. The turquoise blue glow is stray light from the laser system used to measure the frequency of the 2S-4P transition. (Photo: MPQ)

Original publication:
Axel Beyer, Lothar Maisenbacher, Arthur Matveev, Randolf Pohl, Ksenia Khabarova,
Alexey Grinin, Tobias Lamour, Dylan C. Yost, Theodor W. Hänsch, Nikolai Kolachevsky, Thomas Udem
The Rydberg constant and proton size from atomic hydrogen
Science, 6 October 2017


Lothar Maisenbacher
Max Planck Institute of Quantum Optics
Hans-Kopfermann-Str. 1
85748 Garching, Germany
Phone: +49 (0)89 / 32 905 - 295

Prof. Dr. Thomas Udem
Max Planck Institute of Quantum Optics
Hans-Kopfermann-Str. 1
85748 Garching, Germany
Phone: +49 (0)89 / 32 905 - 282

Prof. Dr. Theodor W. Hänsch
Chair of Experimental Physics
Ludwig-Maximilians-Universität, Munich
Max Planck Institute of Quantum Optics
Hans-Kopfermann-Str. 1
85748 Garching, Germany
Phone: +49 (0)89 / 32 905 - 702 /-712

Dr. Olivia Meyer-Streng
Press & Public Relations
Max Planck Institute of Quantum Optics
Hans-Kopfermann-Str. 1, 80748 Garching
Phone: +49 (0)89 / 32 905 - 213

Quelle: idw

Weitere Nachrichten zum Thema
  • BildReduced sea ice disturbs balance of greenhouse gases (18.02.2013, 17:10)
    The widespread reduction in Arctic sea ice is causing significant changes to the balance of greenhouse gases in the atmosphere. This is shown in a new study conducted by researchers from Lund University in Sweden, among others.According to the...
  • BildRegierbarkeit in Megastädten – Wiederbelebung schrumpfender Städte (22.08.2012, 23:10)
    Auf dem Internationalen Geographenkongress (IGC) diskutieren vom 26.-30. August an der Universität zu Köln mehr als 2.300 Geographen aus aller Welt über Kernthemen der Menschheit. Am Dienstag, den 28.08.2012, stehen die Themen „Urbanisierung und...
  • BildMaster Planning - Paradigm Shift in Urban Planning? (04.10.2011, 13:10)
    Two Master Courses at HFT Stuttgart are celebrating their 10th anniversary in 2011: ‘International Project Management’ and ‘Urban Planning’. This gave us the idea to conduct a joint Symposium and we have chosen a topic which integrates approaches...
  • BildVice-Chancellor der University of Oxford besucht die Ruperto Carola (30.08.2011, 19:10)
    Der Vice-Chancellor der University of Oxford, Prof. Andrew Hamilton, PhD, besucht am 6. September 2011 die Universität Heidelberg im Rahmen seines ersten Deutschlandbesuchs als Leiter der britischen Hochschule.PressemitteilungHeidelberg, 30....
  • BildGrowing by Shrinking? Konsum, Glück und Lebensqualität (06.09.2010, 12:01)
    Konsum wird in der heutigen Gesellschaft als glücksfördernd betrachtet. Dabei vernachlässigen die Verbraucher allerdings die nachhaltige Qualität der Alltagsprodukte und verlagern die Folgen von Umweltbelastungen durch den Massenkonsum auf die...
  • BildTobias Zielony ist neuer Professor für künstlerische Fotografie an der Kunsthochschule für Medien Kö (14.10.2009, 11:00)
    Tobias Zielony übernimmt zum Wintersemester 2009/10 die Professur für Künstlerische Fotografie an der Kunsthochschule für Medien Köln (KHM). Er folgt Jürgen Klauke, der von 1994 an bis Februar 2009 das Lehrgebiet vertrat.Zielony wurde 1973 in...
  • BildShrinking glaciers reveal hidden forests and a warmer climate (04.12.2008, 18:00)
    Uniquely old tree remains have recently been uncovered by the thawing of the rapidly shrinking Kårsa Glacier west of Abisko in Lapland, in northernmost Sweden. The finds show that in the last 7,000 years it has probably never been so warm as...
  • BildClimate changes creating green and flowering mountains (15.05.2008, 11:00)
    Our mountains are growing greener. At the border between woods and bare mountain, trees that require warm temperatures, such as oak, elm, maple, and black alder, have become established for the first time in 8,000 years. This is shown in current...
  • BildDZA Vortragsreihe - 18.01.2006 - 17:30 - Long-term care in Germany and Japan. A comparative perspect (11.01.2007, 12:00)
    In der alternswissenschaftlichen und alternspolitischen Vortragsreihe des Deutschen Zentrums für Altersfragen (DZA), Berlin, wird Dr. Paul Talcott (MPI für Gesellschaftsforschung, Köln; Emory University, Atlanta, GA) am 18.01.2007 um 17:30 zum...
  • BildUni Kassel entwickelt Visionen urbaner Entwicklungen (04.10.2005, 14:00)
    Kassel. Die Pfeiffer-Stiftung für Architektur an der Universität Kassel initiiert gemeinsam mit dem Fachbereich Architektur, Stadtplanung, Landschaftsplanung unter dem Titel "documenta urbana - Auf dem Weg zur Stadt der Zukunft" - eine Reihe von...

Ähnliche Themen in den JuraForen

    Kommentar schreiben

    38 - S/ieb en =

    Bisherige Kommentare zur Nachricht (0)

    (Keine Kommentare vorhanden)

    Fragen Sie einen Anwalt!
    Anwälte sind gerade online.
    Schnelle Antwort auf Ihre Rechtsfrage.


    Kostenlose aktuelle Urteile und Rechtstipps per E-Mail:

    Top 10 Orte in der Anwaltssuche


    Durchsuchen Sie hier nach bestimmten Begriffen:

    © 2003-2017 — Alle Rechte vorbehalten. Keine Vervielfältigung, Verbreitung oder Nutzung für kommerzielle Zwecke.