“The thorn in the side of QCD” (Sheldon Glashow about polarized proton-proton cross section)

In this article I would like to shed light on a yet another phenomenon that contradicts QCD. The phenomenon was discovered by the group of Alan D. Krisch, a famous experimental physicist since the 1960s. Krisch published in 2007 an article [1] describing a set of experiments conducted between the late 1970s until the beginning of the 2000s.

In the first experiment, conducted in 1977-8, the Krisch group measured the interaction of polarized proton beam on polarized proton target. When the data appeared, “people were totally astounded.” The data show that protons with parallel spins interact stronger in cases where collision intensity is higher, while protons with anti-parallel spins interact much weaker. Consider the graph below. Here the x-axis is a quantity related to the angle between the incoming proton and the scattered proton. The difference between the lower curve (anti-parallel spins) and the upper curve (parallel spins) becomes significant for x>3. The y-axis is related to the number of protons that are scattered in the specific angle. The scale of the y-axis is logarithmic and for x>4, the interaction of parallel spin protons is about 4 times bigger than that of anti-parallel spins.


According to QCD, the interaction of parallel protons and anti-parallel protons should be the same (I will discuss this below). Krisch mentions that when the data appeared, the Nobel prize laureate Sheldon Glashow described it as “the thorn in the side of QCD.”

Krisch points out that this result contradicts QCD and that the problem persists. About 20 years later, in a summary talk at Blois, 2005, Stan Brodsky called it “one of the unsolved mysteries in hadronic physics.”

Krisch continues his story and tells about several experiments which were conducted during the 1980s and show similar results. After another experiment in 1990 which contradicted QCD again, Krisch concluded with humor that “one result of our experiments was to make them unpopular in some circles.”

Some explanation
What does all this mean? The figure describes experimental results where all protons have the same energy. According to quantum mechanics, particles are waves, and this property is related to the scattering angle. Thus, the effective scattering center takes a smaller volume for a larger scattering angle. The data suggest that if the collision of protons is made in an effective shorter distance then the spin effect is relatively higher. This contradicts QCD, because QCD’s asymptotic freedom implies that forces in short distance become weaker.

In comparison, in Comay’s model this effect is a straightforward result of the duality between the strong forces and the electromagnetic forces. It is well known that the relative interaction strength of magnetic fields (which are created by the spin) becomes much stronger in short distance (proportional to 1/r3). This can be demonstrated, for example, in the Rosenbluth formula, which describes the interaction of an electron and a proton, and both particles have a magnetic dipole.

According to Comay’s model there is a duality between the strong force and the electromagnetic force. Therefore there is a pair of “strong fields” which are dual to the electric and the magnetic fields of an electric charge. The conclusion is that with strong forces we observe effects which are dual to the electromagnetic effects.

This phenomenon is #21 in the list of QCD contradictory phenomena. See the others in the article to continue as usual.

QCD and spin
During the last decade we observe an interesting shift in the scientists’ perception regarding the spin structure of the proton and the neutron. This shift has the potential to produce a storm in particle physics.

When QCD was introduced, physicists thought that the proton and some other baryons contain three quarks which are s-waves only. This means that their wave function is spherical, and they do not carry orbital angular momentum. This perception led to a crisis during the 1960s. Physicists believed that ordinary quantum mechanics cannot explain the structure of several baryons, like the Δ++. As a result, QCD, a fantastic theory with fantastic new assumptions, was invented. Even today students are taught that QCD is justified because of this 1960s crisis.[2]

However, today more and more scientists assimilate that the quarks do carry orbital angular momentum, and they are not s-waves only, even in the most stable case of the proton and the neutron.

In 2004, after puzzling data regarding the quark spin were observed, theoretical physicist Xiangdong Ji of the University of Maryland at College Park in an interview to Peter Weiss from the popular science journal Science News [3] stated: “That’s very disturbing. The finding suggests that scientists may have erred in calculations using fundamental theory to predict quark behavior within neutrons, he says. It might also indicate that orbital motions of particles within neutrons, in addition to those particles’ spins, are more important than previously recognized.” His colleague, Xiaochao Zheng added: “Given that neutrons and protons are sister particles, called nucleons, the new findings apply to both.”

There are also scientists who think that the orbital angular momentum of the quarks is the key to the solution of the proton spin puzzle. “These long-awaited neutron-spin data indicate that, under some conditions, the previously overlooked orbital motions of valence quarks make a major contribution to nucleon spin, comments theorist Gerald A. Miller of the University of Washington in Seattle.”[3]

The fact that the proton’s quarks carry orbital angular momentum is almost trivial, as Comay claimed a long time ago. Physicists knew since the end of the 1940s that every bound particle consisting of more than two fermions cannot stay in s-waves only. You can see, for example, how the structure of the 2-electron Helium atom was discussed 60 years ago.[4] Calculations show that even in this simple case, single particle s-wave does not dominate the electrons’ quantum state.[5]

What might happen if physicists will accept that the proton quarks do carry orbital angular momentum?
If they would, and I’m not sure that they will, it might become very difficult to continue blocking publications that contradict QCD. Today there are two major excuses why QCD must be correct, and no other theory should be examined:
– QCD is the only theory which can explain the baryons Δ++, and Ω.[6] This argument will no longer be valid because the reason that these particles could not be explained was that people thought that their quarks are s-waves only.
– Lattice QCD was able to calculate the mass of the proton very accurately. This argument will collapse, because this calculation is obtained with the wrong assumption that the quark waves are spherical. In fact, it will make every lattice QCD calculation highly suspicious.

Should we be optimistic that this is going to happen any time soon? Based on past experience the obvious answer is “Not Really”, but one must always hope…

[1] Hard collisions of spinning protons: Past, present and future, The European Physical Journal A 31, 417-423 (2007)
[2] See, for example: www.physics.uci.edu/~silverma/MurrayGell-Mann.pdf.
“The way out of the Pauli Exclusion problem for the Δ++ was to add yet another quantum number or “color charges”: red, green or blue to each quark. Making the colors different for the three quarks in a baryon, meant that the three quarks were no longer identical. This is the proton with the three different color quarks, making the proton “colorless” as a whole by antisymmetrizing them. This was introduced in 1972 by GellMann and Harald Fritzsch.”
[3] Topsy Turvy: In neutrons and protons, quarks take wrong turns, (Science News, January 3, 2004)
[4] G.R. Taylor and R.G. Parr, Superposition of configurations: The helium atom, Proc., Natl.Acad. Sci. USA 38, (1952). p.154-160
[5] A.W. Weiss, Configuration Interaction in Simple Atomic Systems, Phys. Rev. 122, (1961). p.1826–1836
[6] A straightforward refutation of this claim can be found here.


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