Protonun Boyutu Üzerinde Kopan Fırtına!
14 Temmuz 2010
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KED
Protonun Boyutu Üzerinde Kopan Fırtına!
"The size of the proton"
Deney-Teori Çatışıyor mu?
Bu günlerde fizikte güncel bir tartışmanın içindeyiz. Acaba protonun yarıçapı nedir?
Elbetteki çok küçük parçacıkların yarı çaplarını ölçmek oldukça kolay değildir. Onlara etki eden bir çok çevresel nedenler mevcuttur.
İlk başta şunu bilmeliyiz ki onlar bildiğimiz bilyeler gibi sabit, katı yapıda değildirler. Bir bakıma sıvı, gaz karışımı bazen ise katı etkili yapı gösterirler. Böyle bir yapının yarıçapını ölçek için oldukça hassas davranılması gerekir.
Bu yapılan deneyde 0,0007 femto metre hassasiyet ile ölçümler gerçekleştirilmiştir. Bu ortalama 10-18m hassasiyet demektir ki buda yeni teknolojilerin işin içine girmesi anlamına gelirki tabiki burada ölçüm ve deneyin doğruluğu sorgulaması ve çevresel ve iç etkenlerin oluşması kaçınılmazdır. Daha önceki ölçümler genellikle 10-15m hassasiyetlerindeydi.
Proton: 2 Up + 1 Down kuarktan oluşmaktadır.
Deneyde kullanılan Müonik Hidrojen: bir proton ve bir müon kullanılmıştır. Müon/Muon elektrondan 207 kez daha ağır ve 2 saniye daha önce bozunuma uğrayan bir ekzotik kuzenidir.
Bir laser ışını gönderildiğinde protonun çevresindeki müon bir enerji düzeyinden diğerine geçer.
Bu arada kullanılan Rydberg sabitinide yeniden gözden geçirmek gerekebilir.
Yürüngeler arasında geçiş yapan müon bu sırada Lamb kayması oluşturur. Bunun incelenmesi bizlere bir çok bilgi sağlar.
Sol taraf 2S, sağ taraf ise 2P dir. Yani hidrojen atomu içindeki müonun yörüngelerini göstermektedir.
Lamb Kayması/Shift 1947 yılında iki Ameriklı fizikçi olan Willis Lamb ve Robert Retherford yörüngelerdeki çok küçük enerji farklarında bazı dejenerilik/degenerate olduklarını fark etmişlerdi. Bunun sebebi kuantum çalkalanmasıydı ve Hans Bethe tarafından çok iyi bir şekilde hesaplanarak KEDde kullanılır hale gelmiştir.
Protonuun içindeki kuarkları bir arada tutan gluonlardır ve güçlü nükleer kuvveti oluştururlar.
Neler olmuÅŸ olabilir?
- Deneyin doğruluğu sınanır
- Müon-proton etkileşimleri tekrar hesaplanır
- Kuark-Antikuark etkileşimleri göz önünde bulundurulabilir
- İçsel ve çevresel faktörler yeniden ele alınabilir
- ...
Mehmet Keçeci 14/07/2010 14:25
Konuyu ilk yayınlayanların makalesi.
Nature 466, 213-216 (8 July 2010) | doi:10.1038/nature09250; Received 22 March 2010; Accepted 1 June 2010
The size of the proton
Randolf Pohl1, Aldo Antognini1, François Nez2, Fernando D. Amaro3, François Biraben2, João M. R. Cardoso3, Daniel S. Covita3,4, Andreas Dax5, Satish Dhawan5, Luis M. P. Fernandes3, Adolf Giesen6,13, Thomas Graf6, Theodor W. Hänsch1, Paul Indelicato2, Lucile Julien2, Cheng-Yang Kao7, Paul Knowles8, Eric-Olivier Le Bigot2, Yi-Wei Liu7, José A. M. Lopes3, Livia Ludhova8, Cristina M. B. Monteiro3, Françoise Mulhauser8,13, Tobias Nebel1, Paul Rabinowitz9, Joaquim M. F. dos Santos3, Lukas A. Schaller8, Karsten Schuhmann10, Catherine Schwob2, David Taqqu11, João F. C. A. Veloso4 & Franz Kottmann12
- Max-Planck-Institut für Quantenoptik, 85748 Garching, Germany
- Laboratoire Kastler Brossel, École Normale Supérieure, CNRS, and Université P. et M. Curie-Paris 6, 75252 Paris, Cedex 05, France
- Departamento de FÃsica, Universidade de Coimbra, 3004-516 Coimbra, Portugal
- I3N, Departamento de FÃsica, Universidade de Aveiro, 3810-193 Aveiro, Portugal
- Physics Department, Yale University, New Haven, Connecticut 06520-8121, USA
- Institut für Strahlwerkzeuge, Universität Stuttgart, 70569 Stuttgart, Germany
- Physics Department, National Tsing Hua University, Hsinchu 300, Taiwan
- Département de Physique, Université de Fribourg, 1700 Fribourg, Switzerland
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544-1009, USA
- Dausinger & Giesen GmbH, Rotebühlstr. 87, 70178 Stuttgart, Germany
- Paul Scherrer Institute, 5232 Villigen-PSI, Switzerland
- Institut für Teilchenphysik, ETH Zürich, 8093 Zürich, Switzerland
- Present addresses: Deutsches Zentrum für Luft- und Raumfahrt e.V. in der Helmholtz-Gemeinschaft, 70569 Stuttgart, Germany (A.G.); International Atomic Energy Agency, A-1400 Vienna, Austria (F.M.).
Correspondence to: Randolf Pohl1 Email:
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Abstract
The proton is the primary building block of the visible Universe, but many of its properties—such as its charge radius and its anomalous magnetic moment—are not well understood. The root-mean-square charge radius, rp, has been determined with an accuracy of 2 per cent (at best) by electron–proton scattering experiments1, 2. The present most accurate value of rp (with an uncertainty of 1 per cent) is given by the CODATA compilation of physical constants3. This value is based mainly on precision spectroscopy of atomic hydrogen4, 5, 6, 7 and calculations of bound-state quantum electrodynamics (QED; refs 8, 9). The accuracy of rp as deduced from electron–proton scattering limits the testing of bound-state QED in atomic hydrogen as well as the determination of the Rydberg constant (currently the most accurately measured fundamental physical constant3). An attractive means to improve the accuracy in the measurement of rp is provided by muonic hydrogen (a proton orbited by a negative muon); its much smaller Bohr radius compared to ordinary atomic hydrogen causes enhancement of effects related to the finite size of the proton. In particular, the Lamb shift10 (the energy difference between the 2S1/2 and 2P1/2 states) is affected by as much as 2 per cent. Here we use pulsed laser spectroscopy to measure a muonic Lamb shift of 49,881.88(76) GHz. On the basis of present calculations11, 12, 13, 14, 15 of fine and hyperfine splittings and QED terms, we find rp = 0.84184(67) fm, which differs by 5.0 standard deviations from the CODATA value3 of 0.8768(69) fm. Our result implies that either the Rydberg constant has to be shifted by −110 kHz/c (4.9 standard deviations), or the calculations of the QED effects in atomic hydrogen or muonic hydrogen atoms are insufficient.
- Max-Planck-Institut für Quantenoptik, 85748 Garching, Germany
- Laboratoire Kastler Brossel, École Normale Supérieure, CNRS, and Université P. et M. Curie-Paris 6, 75252 Paris, Cedex 05, France
- Departamento de FÃsica, Universidade de Coimbra, 3004-516 Coimbra, Portugal
- I3N, Departamento de FÃsica, Universidade de Aveiro, 3810-193 Aveiro, Portugal
- Physics Department, Yale University, New Haven, Connecticut 06520-8121, USA
- Institut für Strahlwerkzeuge, Universität Stuttgart, 70569 Stuttgart, Germany
- Physics Department, National Tsing Hua University, Hsinchu 300, Taiwan
- Département de Physique, Université de Fribourg, 1700 Fribourg, Switzerland
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544-1009, USA
- Dausinger & Giesen GmbH, Rotebühlstr. 87, 70178 Stuttgart, Germany
- Paul Scherrer Institute, 5232 Villigen-PSI, Switzerland
- Institut für Teilchenphysik, ETH Zürich, 8093 Zürich, Switzerland
- Present addresses: Deutsches Zentrum für Luft- und Raumfahrt e.V. in der Helmholtz-Gemeinschaft, 70569 Stuttgart, Germany (A.G.); International Atomic Energy Agency, A-1400 Vienna, Austria (F.M.).
Kaynak: http://www.nature.com/nature/journal/v466/n7303/full/nature09250.html
Proton is smaller than we thought
Jul 7, 2010
The radius of the proton is significantly smaller than previously thought, say physicists who have measured it to the best accuracy yet. The surprising result was obtained by studying "muonic" hydrogen in which the electron is replaced by a much heavier muon. The finding could mean that physicists need to rethink how they apply the theory of quantum electrodynamics (QED) – or even that the theory itself needs a major overhaul.
A proton contains three charged quarks bound by the strong force and its radius is defined as the distance at which the charge density drops below a certain value. The radius has been measured in two main ways – by scattering electrons from hydrogen and by looking very closely at the difference between certain energy levels of the hydrogen atom called the Lamb shift. Until recently the best estimate of the proton radius was 0.877 femtometres with an uncertainty of 0.007 fm
This Lamb shift is a result of the interactions between the electron and the constituent quarks of the proton as described by QED. These interactions are slightly different for electrons occupying the 2S and 2P energy levels and the resulting energy shift depends in part on the radius of the proton.
The heavier the better
However, in muonic hydrogen the Lamb shift is much more dependent on the proton radius because the much heavier muon spends more time very near to – and often within – the proton itself.
Now an international team led by Randolf Pohl at the Max Planck Institute for Quantum Optics in Garching, Germany has measured the Lamb shift in muonic hydrogen for the first time and found the proton radius to be 0.8418 fm with uncertainty 0.0007 fm. While this is by far the most precise measurement to date, it is in striking disagreement with previous measurements, being well outside the error bars of earlier results.
The team measured the shift using a proton accelerator at the Paul Scherrer Institute in Switzerland to create a beam of muons, which was then fired at hydrogen gas. Whenever a muon collides with a hydrogen molecule, it knocks the molecule apart and replaces the electron to create muonic hydrogen. About 1% of the time the muon finds itself in the 2S state, where it can be excited to the 2P state by absorbing a photon from a laser pulse. The 2P state then decays with the emission of an X-ray.
Complicated calculation
By counting the number of such X-rays while scanning the frequency of the laser pulse, the team could make a very precise measurement of the photon energy required to drive the 2S-2P transition. This is then fed into a complicated QED calculation to obtain the radius of the proton.
Pohl told physicsworld.com that the team has been working on the measurement for the past 12 years and got the first inklings of the anomalous result about six years ago. Since then, the researchers have reviewed, repeated and improved their measurements so that they are confident that the results are correct.
According to Jeff Flowers of the UK's National Physical Laboratory there are three possible explanations for the discrepancy. The most likely is that QED is correct, but has been misapplied in what he describes as a "very difficult calculation". Alternatively there is a problem with the experiment – but Flowers, who was not involved in the measurement, believes that Pohl's team has done an excellent job. The least likely – but most exciting explanation – according to Flowers is that there is something wrong with QED.
'Big philosophical change for physicists'
While QED rests on a weak mathematical foundation, it has been extremely successful in predicting the outcome of experiments. "Changing QED would be big philosophical change for physicists", says Flowers.
The result has already caused a flurry of experimental and theoretical activity, with some physicists carefully redoing Lamb shift calculations and others trying to improve electron-based measurements of the proton radius.
Meanwhile, Pohl's team will repeat its experiment and do a new series of measurements on muonic helium to measure the radius of the helium nucleus.
Kaynak: http://physicsworld.com/cws/article/news/43128
