Letters to the editor

Completing the proton picture • Proton radius still puzzles • Adjusting cosmic-ray origins • The music of physics • Too much particle physics?

Completing the proton picture

Colourful complexity

Your article “The proton laid bare” (CERN Courier May/June 2019 p38) covered beautifully and to a large extent the subject under the given heading. For completeness, we in the COMPASS collaboration at CERN wish to add the following additional information.

A state-of-the-art description of the field should address the structure of the proton also in terms of its transverse degrees of freedom. These enter with the same relevance as the described longitudinal structure, and complete the 3D picture of the nucleon.

COMPASS has a world-leading role in this field. During the past 20 years it contributed numerous results on the study of helicity distributions, the gluon contribution to the spin of the proton, and the 3D proton structure using polarised Drell-Yan and hard semi-inclusive or exclusive processes, as well as transversity and transverse-momentum-dependent PDFs.

Finally, in terms of addressing “The proton spin crisis” (May/June, p40), the currently operational COMPASS experiment is certainly to be mentioned alongside work at Jefferson Laboratory.

Oleg Denisov, INFN and Jan Friedrich, TUM, COMPASS spokespersons.

Proton radius still puzzles

Your article “Solving the proton-radius puzzle” (CERN Courier May/June 2019 p42) mentioned some recent extractions of the proton charge radius from electronic hydrogen spectroscopy. These are more consistent with the value extracted from muonic hydrogen, 0.8409(4) fm, than the most recent CODATA value of 0.8751(61) fm.

Unfortunately, another recent extraction from electronic hydrogen spectroscopy by Fleurbaey et al. (Phys. Rev. Lett. 120 183001) was omitted in the article; the value they find is 0.877(13) fm, which is more consistent with CODATA than muonic hydrogen.

Even more exciting, we now have two different groups, one in Paris and one in Garching, that have measured the same 1S–3S transition but extract different values of the proton charge radius. Thus, the proton-radius puzzle still stands.

Gil Paz, Wayne State University, Michigan, US.

Adjusting cosmic-ray origins

I recently read your very nice article on IceCube from last year (CERN Courier September 2018 p7), but found the title “IceCube neutrino points to origin of cosmic rays” to be very misleading. By far, the majority of cosmic rays that we detect on Earth are from our own galaxy, and are much lower energy than those that IceCube is sensitive to. There are many things that we don’t understand about the origin of cosmic rays in our own galaxy, for example whether the heaviest r-process cosmic rays originate in supernovae or in binary neutron star mergers, and these results do not address those. This does not in any sense diminish the importance of the IceCube and gamma-ray results. But in the future it would be better to qualify it by saying “extra-galactic” cosmic rays.

Robert Binns, Washington University in St. Louis, US.

The music of physics

In an obituary of Nobel prize-winning chemist Manfred Eigen published recently in Nature, we learned that he remained an impressive amateur pianist, “sometimes playing Mozart’s concertos after scientific meetings.” It reminded me of Werner Heisenberg’s piano proficiency described in this magazine (CERN Courier Jan/Feb 2005 p41). To celebrate Heisenberg’s 60th birthday, a small orchestra was brought together to accompany him in performing Mozart’s Piano Concerto, K488.

Former CERN Director-General Victor Weisskopf could even act as a conductor, as he did during his retirement celebration at MIT in 1974. For Weisskopf, music and physics served as sanctuary: “When life is hard, there are two things which make it worth living: Mozart and quantum mechanics”. Reflecting on the phenomenon in an essay, Weisskopf wrote: True enough, music is “irrational” in the sense there is no “objective” way to prove what musical passage is right or wrong. But the structure of music is related to structure in science, especially in mathematics. I refer to symmetry, repetition of a passage in a different key, inversions of tunes and many other topological features. No wonder scientists are attracted by the fugues of Bach.

Min-Liang, National Chung Hsing University, Taiwan.

Too much particle physics?

I noticed that the Sciencewatch section has disappeared from the CERN Courier.

I have read about all the changes that are being introduced in the magazine (CERN Courier December 2018 p56), but I am still left wondering about this decision. I think it was an interesting and enriching section. Without it, while still remaining very interesting, the magazine feels more “arid” to me. Does it all have to be particle physics 100%, even for the Courier?

Alexandre Sole, The Open University.

Editor’s reply

You are not alone in questioning Sciencewatch’s retirement. As another reader put it: “There’s something wonderful in reading about the subtleties of electroweak symmetry breaking one minute, then turning the page to find a story about the sex lives of lizards.” What can we say? Only that all feedback is taken on board as we strive to keep CERN Courier relevant for a seventh decade and beyond.