While the proton and the neutron are made up of quarks and gluons, the spin of the proton or the neutron is difficult to reconcile with the spins of quark and gluon components. For more than a decade, physicists have struggled to solve this puzzle.
In 1988, CERN’s European Muon Collaboration (EMC) experiment stunned the world of physics with the announcement that some of the nucleon’s spin was missing. More than a decade later, physicists are still trying to account for the missing spin and the spotlight has moved to the HERMES experiment at Hamburg’s DESY laboratory.
A close look inside the nucleon
The closer you look, the more you see. That may sound like a common sense maxim, but when it comes to nucleons it takes on an interesting twist. In a simple quark-parton model, a nucleon is made up of three quarks and it is the spins of these quarks that are supposed to give the nucleon its spin. That simple notion was disproved when the EMC experiment gave rise to what was quickly dubbed the nucleon spin crisis, announcing that quarks could account for only around 20% of a nucleon’s spin at most.
The EMC result led to several experiments taking a much closer look at what goes on inside nucleons. The three quarks that give a nucleon its identity the valence quarks – are just the beginning of the story. They swim in a “sea” of virtual quarks and antiquarks that are constantly popping in and out of the vacuum. Moreover, gluons flit about inside nucleons, holding the quarks together. All of these can contribute to a nucleon’s spin, and their constant movement generates an intrinsic angular momentum of the nucleon as a whole.
A succession of experiments to pin down the effect ensued at CERN (the NMC and SMC collaborations using muon beams) and at SLAC (with electron beams), and by the late 1990s the spin crisis had transformed into a puzzle: that of finding out how the nucleon’s spin was distributed among its various contributing factors. The CERN and SLAC experiments had confirmed, with greatly improved precision, the original EMC finding that quarks alone could not be responsible for the nucleon’s spin. The next task was to measure the contributions of the individual quark flavours and of gluons. The baton passed to DESY, whose HERA measurement of spin (HERMES) experiment had started to collect data in 1995.
HERMES uses the polarized (spin-oriented) positron or electron beam of DESY’s HERA collider incident on a polarized gas-jet target. This, coupled with the HERMES detector’s powerful particle identification capability, has allowed the collaboration to measure precisely the contributions to nucleon spin of each valence quark flavour. HERMES’ results are in perfect agreement with earlier results and show that the “up” valence quarks spin the same way as the nucleon as a whole, while the “down” valence quarks spin the opposite way.
HERMES has also published a first estimate of the gluon contribution to a nucleon’s spin. HERA’s lepton probes do not see electrically neutral gluons directly, so this is a difficult measurement to make. HERMES has achieved it by exploiting the process of photon-gluon fusion. When an electron scatters from a quark in a nucleon, it does so through the exchange of a virtual photon, and this photon can interact with gluons as they dissociate into quark-antiquark pairs.
In HERMES’ first phase of running, from 1995 to 2000, the experiment showed that gluons do indeed contribute to a nucleon’s spin, spinning in the same direction as the nucleon. Future analyses will use the HERMES detector’s particle identification capabilities to quantify the gluon contribution by studying processes involving charm quarks, which give a precise handle on gluons.
