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Physics Has a Core Problem Physics Has a Core Problem Physicists can solve many puzzles by taking more accurate and careful measurements. Randolf Pohl and his colleagues at the Max Planck Institute of Quantum Optics in Garching, however, actually created a new problem with their precise measurements of the proton radius, because the value they measured differs significantly from the value previously considered to be valid. The difference could point to gaps in physicists’ picture of matter. Measuring with a light ruler: Randolf Pohl and his team used laser spectroscopy to determine the proton radius – and got a surprising result. PHYSICS & ASTRONOMY_Proton Radius TEXT PETER HERGERSBERG he atmosphere at the scien- tific conferences that Ran- dolf Pohl has attended in the past three years has been very lively. And the T physicist from the Max Planck Institute of Quantum Optics is a good part of the reason for this liveliness: the commu- nity of experts that gathers there is working together to solve a puzzle that Pohl and his team created with its mea- surements of the proton radius. Time and time again, speakers present possible solutions and substantiate them with mathematically formulated arguments. In the process, they some- times also cast doubt on theories that for decades have been considered veri- fied. Other speakers try to find weak spots in their fellow scientists’ explana- tions, and present their own calcula- tions to refute others’ hypotheses. In the end, everyone goes back to their desks and their labs to come up with subtle new deliberations to fuel the de- bate at the next meeting. In 2010, the Garching-based physi- cists, in collaboration with an interna- tional team, published a new value for the charge radius of the proton – that is, the nucleus at the core of a hydro- gen atom. The charge radius describes the space in which the positive charge of the nucleus is concentrated. To de- termine this number, the researchers working with Randolf Pohl used a dif- Photo: Axel Griesch ferent method than the one used pre- PHYSICS & ASTRONOMY_Proton Radius viously, and obtained a result that dif- femtometers and 0.875 ± 0.011 fem- nover. Clearly, this is going to cause fers significantly from the figure that tometers, respectively. problems – it would be no different used to be considered valid. So signifi- In other words, the difference be- than if one were to suddenly look for cantly, in fact, that the difference can’t tween the measurements is 0.036 fem- Hamburg’s St. Michael’s church on Mu- be explained by the measurement accu- tometers, or 4 percent. That doesn’t nich’s main square, Marienplatz. racies of the two methods. And to keep sound like much, but in the context of To explain the great difference be- the tension from subsiding, Randolf the given uncertainty of the measuring tween the two measurements, physi- Pohl and his colleagues recently refined accuracy, it is actually a lot. The dis- cists initially scoured the two methods their measurement result further, thus crepancy corresponds to seven com- for systematic errors that might skew making it clear: more precise analyses bined error bars and is thus significant- the result. “It could, of course, be the don’t make the problem go away. ly larger than one would expect if case that we overlooked such an error, For years, Randolf Pohl and his col- different experiments merely yielded but we searched very carefully and leagues thought that their measuring slightly different results in the context found nothing,” says Randolf Pohl, instrument wasn’t sufficiently accu- of statistical fluctuations. whose team took a new approach to rate: they first conducted an experi- measuring the proton radius some 15 ment in 2003 to determine the size of NO SYSTEMATIC ERROR years ago, and published the first result a proton. However, they didn’t discov- WAS FOUND of this work in 2010. er the signal that would give them in- The researchers used laser spectros- formation about it. “But this wasn’t Just how great the difference is between copy to determine how high the ener- due to the inaccuracy of our method, the results and uncertainties of the two gy of a photon has to be to transport an but rather to the fact that we hadn’t different methods of measuring the exotic hydrogen atom from one special expected such a large deviation,” says proton radius can be illustrated by energy state to another. The energy of Randolf Pohl. The researchers had se- transferring them to a map of Germa- some of these states depends on the lected too small a window for their ny. Suppose one were to put the result proton radius, and that of others measurements. obtained by Randolf Pohl’s team in the doesn’t. If the atom is taken from one According to the latest measure- center of Munich, and the competing state that appears to be sensitive to the ments by Pohl’s team, the charge radi- measurement in the center of Ham- proton radius to one that is not sensi- us of the proton is 0.84087 femtome- burg. Then the uncertainty of the Mu- tive to it, then the radius can be calcu- ters (one femtometer is a millionth of a nich value would correspond to the dis- lated. However, this requires physicists millionth of a millimeter) and the mea- tance between the two Munich neigh- to know all the other effects that influ- suring uncertainty here is just ± 0.00039 borhoods Pasing and Trudering. The ence the position of the states. femtometers. With the previously com- Hamburg measurement, in contrast, In ordinary hydrogen, where one mon measuring method, two indepen- would be so imprecise that the actual electron whizzes around the proton, dent groups recently determined that value might very probably even lie the influence of the proton radius is the charge radius must be 0.879 ± 0.009 somewhere between Flensburg and Ha- very small because the lightweight elec- left: Final preparations for the measurement: Randolf Pohl calibrates the apparatus through which muons are directed into a vessel containing hydrogen. A superconducting magnet in the silver cylinder produces the strong magnetic field required for this. right: The green light produces intense red light in a titanium-sapphire crystal. This is subsequently converted to invisible infrared light and used to perform spectroscopy of muonic hydrogen. Photos: MPI of Quantum Optics 48 MaxPlanckResearch 3 | 13 Experiment design: A proton beam strikes Proton beam a carbon disk, producing negative pions Muon beam that decay into muons in a magnetic bot- Pion beam tle (cyclotron trap). The muons are slowed down and directed through a curved channel to the hydrogen vessel, which is located in a superconducting magnet. This part of the apparatus would fill a spacious living room. The extremely intense laser beam is directed into the Cyclotron trap hydrogen vessel. The researchers pro- Superconducting duce this beam a few meters away in the magnet and hydrogen vessel Titanium-sapphire “laser hut,” which is shielded from the Raman cell laser muon beam. There, diode lasers pump two parallel ytterbium-YAG disk lasers with energy. Upon entering the hydrogen vessel, muons produce a signal that, in less than 200 nanoseconds, causes the disk Water cell laser to emit intense light pulses. Their frequency is doubled in order to pump Titanium-sapphire Titanium-sapphire a titanium-sapphire laser. Finally, in amplifier oscillator the Raman cell, the laser’s pulses are converted into light of the appropriate color to allow the muonic hydrogen atom in the hydrogen vessel to be exam- Disk laser Disk laser ined. The entire process lasts less than Diode laser one microsecond. tron usually hangs out far away from Due to their greater mass, muons are of the experiments with muonic hy- the nucleus. In an exotic variant of the located closer to the nucleus and are drogen. Some skeptics had speculated, element, in contrast, in which a muon, thus more sensitive to the proton radi- for example, whether Pohl’s team rather than an electron, orbits the nu- us than are normal electrons. This might, without realizing it, have ob- cleus, the effect is far greater. makes it possible to determine the served negatively charged muonic hy- Physicists produce muonic hydro- charge radius with great accuracy – or drogen ions that contain a muon and gen at the Paul Scherrer Institute in Vil- in any case more accurately than pre- an electron, or molecules composed of ligen, Switzerland, where they can use vious experiments permitted, which two protons and a muon. However, the world’s strongest muon beam. The had put the proton radius at approxi- physicists from Paris performed calcu- muons come about when protons are mately 0.88 femtometers: spectroscopy lations that have since undermined the shot at a carbon disk in a particle accel- of normal hydrogen and electron scat- basis for this suspicion: even if both of erator. Using magnetic fields, the re- tering measurements. For the latter, these exotic entities are created, they searchers direct the muons into a ves- scientists shoot electrons at hydrogen don’t remain stable long enough to sel containing hydrogen gas, which nuclei and observe how they are de- permit examination. halts the exotic elementary particles. flected by the protons. In the meantime, the team working Some muons are captured by hydrogen with the Max Planck physicists used molecules, displace their electrons and THE MUON EXPERIMENTS spectroscopy of muonic hydrogen to form muonic hydrogen atoms. DISREGARD EXOTIC SPECIMENS measure not only the charge radius of Now physicists have to step up the the proton, but also the magnetic radi- pace with their laser experiments that What favors the measurements on mu- us.
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