By separately scattering right- and left-handed electrons off quarks in a deuterium target, researchers have improved, by about a factor of five, on a classic result of mirror-symmetry breaking from 35 years ago. See Letter p.67
Symmetry makes the world go round. Scientific theories of the physics of elementary particles stem from simple symmetries that dictate the fundamental forces governing our Universe. Sometimes symmetries are broken, and that can have profound implications. An important case is the reflection, or right–left mirror, symmetry known as parity. On page 67 of this issue, an international team at the Thomas Jefferson National Accelerator Facility in Newport News, Virginia, reports1 measurements of parity-symmetry breaking that confirm expectations and that unambiguously separate the electron and (much smaller) quark parity-violating interactions. The small quark parity violation can be used as a sensitive probe of new interactions or to measure subtle nuclear effects.
Elementary particles such as electrons and quarks (which make up protons and neutrons) carry intrinsic angular momentum called spin and act much like spinning tops. By convention, particles spinning clockwise with respect to their direction of motion are said to be left-handed, whereas their mirror images — those spinning anticlockwise — are right-handed. Parity symmetry swaps left and right, just as a mirror does.
Gravity, electromagnetism and strong nuclear forces all respect parity; that is, they are symmetrical (unchanged) under left–right interchanges. However, in 1956, Tsung-Dao Lee and Chen-Ning Yang conjectured2 that the weak forces responsible for nuclear decays and neutrino interactions might violate parity. Subsequent experiments not only confirmed that feature, but also found that parity violation was maximal: only left-handed particles experienced the weak interaction; right-handed particles were not affected by the weak forces that were known then. Antiparticles, such as antielectrons and antiquarks, exhibited the opposite preference — only their right-handed components participated in weak interactions. For the revolutionary idea of parity violation, Lee and Yang received the physics Nobel prize in 1957.
Beyond parity violation, small differences between the weak interactions of left-handed particles and those of right-handed antiparticles, known as CP violation or matter–antimatter asymmetry, were subsequently observed3. Today, some as yet undiscovered form of CP violation is thought to be responsible for the dominance of matter over antimatter throughout the Universe — a feature responsible for our very existence. Symmetry violation can, indeed, have profound consequences.
Apart from parity violation, electromagnetic and weak interactions are quite similar. Both can be viewed as exchanges of packets (quanta) of energy called bosons. Electromagnetism is mediated by massless photons, whereas heavy, charged W bosons mediate weak interactions. Although some sort of electroweak unification, jointly describing both interactions, seemed natural4, parity violation caused problems. In 1961, it was shown5 that unification was possible if, in addition to charged W bosons, another heavy neutral boson, now called the Z boson, also existed. Unfortunately, even then, parity violation made it difficult to accommodate or relate elementary-particle masses. The problem was solved in 1967, when it was demonstrated6 how the introduction of symmetry breaking through the Higgs mechanism could be used to provide mass. A predicted remnant of that mechanism — the Higgs boson — was detected in 2012 at CERN, Europe's high-energy physics laboratory near Geneva, Switzerland, and François Englert and Peter Higgs were awarded last year's Nobel Prize in Physics for the theoretical work on the Higgs mechanism.
In the early 1970s, support for the existence of the Z boson was observed in neutrino-scattering experiments7. But follow-up studies proved inconclusive, in that they did not confirm the parity-violating predictions of electroweak unification. Then an experiment8,9 called E122, conducted at the SLAC National Accelerator Laboratory in Menlo Park, California, measured a small parity-violating difference between the scattering of right- and left-handed electrons on up and down quarks in a target of deuterium atoms. The up and down quarks are the lightest of the six possible types of quark, and make up all nuclei. This result unequivocally confirmed the parity-violating predictions of electroweak unification. For their work on electroweak unification and its implications, Sheldon Lee Glashow5, Abdus Salam10 and Steven Weinberg6 received the physics Nobel prize in 1979.
During the 35 years since E122 was completed, better sources of right- and left-handed electrons have been developed, experimental techniques have improved and more-intense electron beams have become available. Parity violation has been used for the precise measurement of parameters that describe the electroweak interaction and to investigate nuclear properties. But the parity-violating difference measured in the E122 experiment has not been improved on — until now.
In their study, the Jefferson Lab team decided to redo the SLAC E122 experiment. The researchers worked at lower energy but with much higher intensity and polarization (degree of handedness). As a result, they improved on some aspects of parity-violating differences between the scattering of right- and left-handed electrons on up and down quarks by about a factor of five. With their higher statistics, they were able to untangle the two parity-violating effects: the dominant effect due to electron parity violation, which had already been clearly measured in E122, and a much smaller parity-violating effect attributable to the quarks in the deuterium nuclei, which was beyond the sensitivity of the SLAC experiment.
Why measure such small effects, and so precisely? Perhaps, like mountain-climbing enthusiasts, physicists study them because they are there and represent challenges. However, unlike mountains, in the case of parity-violating effects sometimes smaller is better. Testing the tiny quark parity-violation prediction is a nice example: a deviation from expectations could signal the presence of a new tiny effect. Indeed, the team's measurement probes some types of additional parity-violating effects that could be as much as 30 times weaker than ordinary weak forces. Precision studies also provide access to small nuclear effects that are hard to probe in other ways. An example is the breaking of charge symmetry (the interchange of up and down quarks in deuterium).
Parity-violating polarized electron scattering experiments are expected to continue at the Jefferson Lab, using higher-energy electrons and better particle-detection systems, after upgrades to the facility, now in progress, are completed. One can anticipate better measurements of electroweak parameters, more-refined nuclear-physics studies and improved searches for new interactions.
A great accomplishment can lead to the demise of a scientific endeavour. A good example is the race to put a man on the Moon. That goal started more than 50 years ago and was a spectacular success, but further undertakings ended after the mission was accomplished. Fortunately, electron-scattering studies of parity violation did not suffer that fate. Following the success of E122 at SLAC, the programme changed direction, but improvements in technical expertise and accelerator facilities continued. The Jefferson Lab has taken leadership in polarized-electron scattering initiatives. As long as these initiatives address frontier questions and interesting goals, they should prosper and grow.
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Physical Review C (2015)