Strange News about the Proton

The Oxford group played a leading role in the ATLAS measurement of the strange quark parton density in the proton. The ATLAS W- and Z-boson data indicate that the strange quark is not suppressed at low values of Bjorken x, the momentum that the quark which is struck in a collision takes as a fraction of the proton’s momentum.

The proton is a very dynamic object which is made out of three valence quarks: one "down" quark and "two" up quarks. These valence quarks are glued to each other via the exchange of gluons, the force carrier particles of the strong interaction. It turns out that the valence quarks alone can not explain the measured properties of the proton and that the so called "sea" in the proton plays an essential role to complete the very dynamic picture of the proton (see above). The sea is produced by the gluons in the proton which split continuously into quark and anti-quark pairs or radiate other gluons. The sea quarks can also be quarks from the heavier quark families, e.g strange quarks. This buzzing activity within the proton can be observed at particle colliders.

Figure 1: The ratio of the strange to the down sea quark density.

The strange quark has traditionally been thought to be suppressed with respect to the down and up type quarks in the proton sea, because of its higher mass. There is experimental evidence for this at high values of Bjorken x. However, W and Z production at the LHC probe lower values of Bjorken x, x~0.01 and the longitudinal momentum spectra of these particles shows that the strange sea is contributing more to the cross-section than had been expected. It seems that strangeness is not suppressed at low x, gluons split to u,d,and s quarks with flavour democracy at the LHC. Figure 1 shows the ratio of the strange to the down sea quark density at the LHC. The measured value (vertical line with yellow and green error bands) is compared to theory predictions (bullets).

The Oxford Standard Model Group made important contributions to this result: Prof Cooper-Sarkar performed the QCD parton distribution function fits to the data (HERA+ATLAS W and Z), and Dr Sam Whitehead played a significant role in the ATLAS W boson measurements in the electron channel which were used in the fit.