LOOKING INSIDE!

Charge distributions in nuclei, experiment and theory.


Neutron distributions are harder to measure because of absorption of the probe!




THE NUCLEON!

Here x = -q2/(2mp(E - E')). See sections 6.8, 6.9 in the text.  Physically, x is the fraction of the proton's total momentum carried by the particular particle that interacted with the electron.  A personal saga of deep inelastic scattering! Some more to think about.





An unfolding puzzle... where does the “spin” of the proton and neutron come from? It contains “orbital” contributions, and spin contributions from gluons, and from virtual pairs! More details here.

Pointlike fundamental particles with charge and intrinsic spin have an intrinsic magnetic moment. That is, they have an intrinsic magnetic field as well as an intrinsic electric field. The basic unit of magnetic moment in such a case is the Bohr magneton, which in gaussian units is μB = (eℏ/2mc). The Dirac equation PREDICTS the magnetic moment of the electron to be μs = -gμB(S/ℏ), with the factor g = 2 precisely.  However, experimentally g differs from 2 in the third decimal place.  This is due to the interaction of the electron with the vacuum.  Calculation of g for charged leptons is therefore a strong test of whether we know what virtual particles dominate the vacuum.


Calculation of electron |g|. One of the great triumphs of physics!
Calculation of g for the muon has proven far more difficult, because of its much more complex couplings to the vacuum.


Every calculation of the muon magnetic moment based on standard field-theoretic techniques, done up to a few years ago, showed a consistent discrepancy between experiment (as of August 2023) and theory. No satisfying explanation was at first evident, which suggested to many that new physics beyond the Standard Model might possibly be involved. However, the latest calculations using the lattice gauge theoretic approach are currently giving good agreement with the latest data. The field-theoretic calculations are so difficult, and involve so many semi-empirical estimates, that the discrepancies were generally not taken seriously, even years ago.





Check this Comment.  And what about the electron??

Just as expected, the problem went away (see red dot) when a powerful method called lattice gauge theory was applied. We will have more to say about this method and what it has to say about the internal structure of baryons and mesons, later in the course.

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