Today at the European Physical Society conference on high energy physics, EPS-HEP 2021 the LHCb Collaboration presented results of a measurement consistent with the presence of valence-like charm content in the proton, referred to as intrinsic charm. In order to obtain this result LHCb physicists measured the fraction of proton-proton collisions with a Z boson and a jet coming from a charm quark (Z+c-jet) /(Z+jet), where a jet is a large cone of particles produced by a high energy quark produced in the event.

In the quark model protons are composed of three quarks, u, u and d, named “valence quarks”, carring a large fraction, x, of the proton momentum and interacting by exchange of gluons. Gluons occasionally can convert into quark-antiquark pairs, dd, uu, ss and cc. These quarks are named “sea” quarks and they carry a small fraction of the proton momentum. It has been debated for decades if the proton also contains a cc component at large x behaving like valence quarks and referred to as intrinsic charm.

Measurements of c-hadron production in deep inelastic scattering and in fixed-target experiments have been interpreted both as evidence for and against of existence of intrinsic charm component of the proton at the percent level. Today LHCb physicists announced an important turning point in this research by measuring (Z+c-jet)/(Z+jet) in the forward region of proton-proton collisions, see the leading Feynmann graphs above.

The image above shows (Z+c-jet)/(Z+jet) (grey bands) for three intervals of forward Z rapidity, y(Z). In simple terms, rapidity is related to the angle of the particle with respect to the beam axis. A higher rapidity means the Z boson is travelling closer to the proton beam. The results are compared to the Standard Model predictions without intrinsic charm (No IC – blue points), allowing to vary intinsic charm contribution, hence permitting, (green squares) and predicting intinsic charm with a mean momentum fraction of 1% (red triangles). The observed values are consistent with both the no intinsic charm and intinsic charm hypotheses for the first two y(Z) intervals; however, this is not the case in the forward-most interval where the ratio of the observed to no-intinsic-charm-expected values is 1.82±0.25. Indeed, when including a contribution of intrinsic charm to the calculations, with an overall momentum fraction of 1% (red triangles) at values of x around 0.3, the theory predictions are consistent with the data. Similarly, the NNPDF 3.0 predictions that include intrinsic charm (green squares) are consistent with the data as well.

The LHCb results are consistent with the prediction of effects expected if the proton contains a valence-like charm content, however, these results will need to be incorporated into the global analyses of parton distribution functions of the proton before firm conclusions can be drawn about intrinsic charm. The weight of protons dominates the weight of our human body. Philosophers might find it interesting that the human body contains a charming component… and even an anti-charming component.

Read more in the LHCb EPS presentation and in the forthcoming paper.

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