Measurement of D*+/- Meson Production and F2c in Deep-Inelastic Scattering at HERA

How charming is the proton?

Protons, we have all heard, are made of quarks. In fact, we all know they are made of three quarks, two being 'up' or 'u' quarks and the third being a 'down' or 'd' quark. When we look at protons using a strong enough microscope, and HERA is the world's most powerful electron microscope, we would expect to be able to see those quarks, and indeed we do. However, the microscope also reveals some puzzles: The theory that explains how quarks interact with each other tells us that this 'something else' is the gluons which carry the force between the quarks. This theory, called Quantum Chromo-Dynamics or QCD for short, says that quarks can only stick together
to form protons by continually swapping gluons. It also says that the unexpected sea of quarks arises because sometimes the gluons that are being exchanged between quarks split to form quark/anti-quark pairs. So can we test these ideas? Yes we can!
QCD says that some of the quark/anti-quark pairs produced when gluons split are u/anti-u and d/anti-d pairs, but that we should also find both 'strange' and 'charmed', s/anti-s and c/anti-c, pairs in the mix. So if we can't find these in the proton, we had better scrap QCD. Strange quarks are hard to find, but charmed quarks form D* mesons, and these leave a characteristic signature in the microscope pictures taken by HERA.
This picture shows the fraction of c and anti-c quarks seen in the proton (red points) and the QCD DGLAP prediction at different values of x, the proportion of the proton's momentum carried by the quarks, and for various values of the magnification at which the proton is studied, which increases as Q2 gets larger.

The experiment described in this paper involved searching through millions of pictures taken using the H1 'camera' at HERA and looking for those in which charm quarks were found in the proton. Amazingly, the pictures reveal that about a quarter of the quarks in the proton are c/anti-c pairs; F2c is about 1/4 . So is this what QCD expects?
This question is a difficult one. A few years ago we would have said 'No, it's too many'! That is because we can only work out approximately how many c quarks we expect to see, and calculations, based on the 'DGLAP' approximation, give fewer charm quarks than observed, as is shown in the picture above. Recent calculations using the more accurate 'CCFM' approximation give results closer to what we have measured. So what have we learned?

Where do we go from here? More precise measurements and better calculations will be needed to test our ideas about how quarks, charmed or otherwise, are held together inside the proton.