How Strange is the Proton "Sea" By: Professor Philip Roos

Around that same time and well into the 70's and 80's, particle physics research studies of high energy reactions identified a myriad of new particles and indicated the presence of pointlike particles in our "fundamental" nucleons. The combination of experiments and theoretical insight and progress has led to our current understanding of hadronic (nucleons and mesons) structure as made up of pointlike particles called quarks, of which there are six types, or flavors. The strong interaction between these hadrons arises from the exchange of quanta of energy called gluons. The fundamental theory which describes the interaction of quarks and gluons is called Quantum Chromodynamics (QCD). The strong force, as described by QCD, is complicated and not amenable to simple solutions. In fact there is currently only one approach that would seem to provide a method for solving problems, such as the internal structure of a nucleon, although the available computational power is still insufficient to obtain a realistic solution. For this reason our knowledge of the internal structure of the nucleon or other hadrons comes primarily from the interpretation of experimental results.

These theoretical developments, technological advancements, and the costs of doing experimental science have led to a concentration of research at a few accelerator laboratories. Currently there are approximately 7 nuclear physics laboratories in the US, and only two of these are major facilities. Consistent with this, the scale of the experimental effort has increased enormously, in terms of manpower, time, and costs. A typical nuclear physics experiment at one of the major facilities today would involve 50 or more physicists, run continuously for several months, and cost well into the millions of dollars - all in the name of progress.

With these changes there has also been a broadening of the nuclear physics community in terms of their research interests. Some nuclear physicists have continued to study the properties of nuclei, pushing to the limits of stability, in terms of the proton or neutron excess, as well as the limits of angular momentum and deformation. Others have chosen to venture into relativistic heavy ion collisions at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven in search of matter that would have been present in the very early universe. Still others, such as members our experimental group at Maryland, have chosen to focus on understanding the structure and interaction of hadrons in terms of their quark and gluon degrees of freedom, and to understand QCD in the regime relevant to nuclear physics, a regime in which the strong interaction cannot be treated perturbatively.

One of the two major U.S. nuclear physics facilities at which QCD related research takes place is the Thomas Jefferson National Accelerator Facility in Newport News, VA. The CEBAF accelerator at Jefferson Laboratory currently can deliver electron beams in excess of 100 microamperes (~6x10^14 electrons/ second) to three experimental halls with energies up to 6 GeV. These electrons have a deBroglie wavelength of 0.2 fm
(2 x10^-16 m) allowing experiments to probe structure down to a small fraction of the size of the nucleon. (Currently a proposal has been submitted to the DoE to double the energy of the accelerator to 12 GeV which would then permit extensive searches for new particles which, if discovered, will lead to a further understanding of QCD).

The University of Maryland Experimental Nuclear Physics Group has focused their efforts over the past decade at the Jefferson Lab and much of the research has been focused on understanding the structure of the nucleon. We have participated in scattering studies to measure the electric and magnetic structure of the nucleon - particularly the electric form factor of the proton and neutron (Professors Kelly and Chang and Ph.D. student Nikolai Savvinov have played important roles in this work). Such measurements determine the charge density and magnetization density of the proton, but do not provide explicit information on the quarks and gluons that comprise the nucleon.

Important to furthering our understanding of QCD is the understanding of the underlying structure of the proton. In the simplest QCD based theoretical model the nucleons consist of three quarks, referred to as valence quarks, and only the up and down flavors of quarks (the lightest of the six quarks) are present. For example, in this simple model the proton would consist of two up quarks and one down quark, and the neutron as two down and one up quark. Such a simple model had some remarkable predictive successes (spin of the nucleon, magnetic moment), but we know that it cannot be correct. For example, from many years of studying the proton, we know that there is a pi-meson (pi-mesons consist of a quark-antiquark pair) cloud around the nucleon which indicates that there must be significant 4 quark-1 antiquark components in the nucleon ground state wave function. The additional quark-antiquark pairs are referred to as "sea" quarks. Also in recent years measurements with polarized leptons (mu-mesons and electrons) have shown that the spin of the nucleon is not due to the three valence nucleons - perhaps less than 25% arises from that component of the ground state wave function. This work led to speculation that the nucleon may well have significant components from other quark flavors, and since the next lightest quark is the strange quark, one might expect significant strange quark-anti strange quark pairs present in the ground state. (Note that we must have the quark-anti quark pairing so that the proton ground state has no strangeness - an empirical fact.)

The determination of the strange quark content of the nucleon ground state is the goal of a major experimental effort by members of our group. The so-called G0 experiment at Jefferson Lab utilizes the weak interaction to learn about how strange quarks contribute to the proton's charge and magnetism. Professors Betsy Beise, Research Scientist Herbert Breuer, graduate student Jianglai Liu, and I, as Deputy Spokesman, have been playing major roles in this G0 experiment. Additionally, over the past four years during the construction phase, approximately 9 University of Maryland undergraduate students have made contributions to this research effort. Shown in the picture are Ken Rossato and Kristen Kiriluk who worked this summer on the detector support structure. The wooden "box" is a prototype Cerenkov detector that they constructed to check the design of the support system.

The G0 experiment will measure the parity-violating asymmetry in elastic electron- nucleon scattering for a range of scattering angles or momentum transfers. Specifically, we will measure the difference in the elastic scattering of positive helicity electrons (spin of electron aligned with its velocity) and negative helicity electrons (anti-aligned), the difference arising from parity violation due to the weak interaction. These asymmetries are sensitive to the interference of the weak and electromagnetic amplitudes. From the measured asymmetries, therefore, the weak form factors of the nucleons can be determined. The vector weak form factors, electric and magnetic, are analogs of the familiar charge and magnetic elastic form factors of the nucleons. By measuring the asymmetry for both forward and backward scattering angles the G0 experiment will be able to separate the electric and magnetic form factors. Furthermore, by measuring a range of momentum transfers, information will be available on the spatial distribution in the nucleon.

Once these data are obtained the G0 data will be combined with earlier data for elastic scattering to compare the electromagnetic and neutral weak vector form factors. This will permit one to extract the contributions of each of the three main quark flavors present in the nucleon: u, d and s. The direct relation between the sets of form factors is possible because in the Standard Model the couplings of photons and Z's to the quarks are precisely known (the electromagnetic and neutral weak charges, respectively). As noted above, by measuring asymmetries at both forward and backward scattering angles, the contributions of each quark flavor to the charge and magnetic form factors can be determined.

The difficulty of the experiment, and the reason for the large investment of manpower and money, is due to the fact that the asymmetries are a few parts per million. The proposed accuracy of the G0 experiment is a few parts in 10^7. Clearly, based on counting statistics alone, such a experiment necessitates the measurement of something of the order of 10^14 elastic scattering counts. A typical experiment at Jefferson Lab might take data at a rate of a few to ten kHz, so any attempt to carry out the experiment using the conventional equipment would take far too much time. Therefore to carry out the G0 experiment, dedicated equipment had to be constructed. A picture of the G0 experiment installed in Hall C at Jefferson Lab is shown in the picture.

The heart of the experiment is a magnetic spectrometer consisting of an eight sector superconducting toroidal magnet. This magnet will focus recoil protons (forward angle scattered electron measurement) or electrons (backward measurement) from a 20 cm long liquid hydrogen or deuterium target to pairs of plastic scintillator detectors.