Understanding the proton’s weak side

Research from the Qweak experiment provides a precision measurement of the proton’s weak charge. narrows the search for new physics.

In the QTor magnetic spectrometer, a magnet separates high-energy electrons, which are elastically scattered from protons. Photo courtesy of the Laboratory for Nuclear Science.
In the QTor magnetic spectrometer, a magnet separates high-energy electrons, which are elastically scattered from protons. Photo courtesy of the Laboratory for Nuclear Science.

By conducting Qweak experiment, scientists have set new standards in precision tests of the Standard Model, a highly successful theory of fundamental particles and their interactions. The experiment provides a new result of providing a precise test of the weak force, one of four fundamental forces in nature.

The Qweak experiment has set new gauges in exactness trial of the Standard Model, an exceptionally effective hypothesis of basic particles and their connections. It straightforwardly examined the material science just came to at the most astounding vitality molecule quickening agents.

While the weak force is difficult to observe directly, its influence can be felt in our everyday world. For example, it initiates the chain of reactions that power the sun and it provides a mechanism for radioactive decays that partially heat the Earth’s core and that also enable doctors to detect disease inside the body without surgery.

MIT scientists including Stanley Kowalski, a professor of physics and researcher in the Laboratory for Nuclear Science (LNS), who has pioneered parity violation studies over the past four decades starting in 1980, at the MIT-Bates Linear Accelerator Center, a part of LNS. Other MIT contributors to the work included postdocs W. Deconinck, Jean-Francoise Rajotte, and Rupesh Silwal. Fang Gao, a physics graduate student, analyzed Qweak data for her Ph.D. thesis. Several MIT undergraduates also worked on many aspects of this experiment.

Measured Qweak asymmetry. Results of other experiments are also shown at higher Q. Image courtesy of the researchers.
Measured Qweak asymmetry. Results of other experiments are also shown at higher Q.
Image courtesy of the researchers.

Via experiment, scientists uncovered one of the powerless power’s mysteries: the exact quality of its grasp on the proton. They did this by estimating the proton’s powerless charge to high exactness, which they tested utilizing great pillars accessible at the Continuous Electron Beam Accelerator Facility, a Department of Energy Office of Science User Facility.

To quantify proton’s weak charge, scientists directed an intense beam of electrons onto a target containing cold liquid hydrogen, and the electrons scattered from this target were detected in a precise, custom-built measuring apparatus.

The key to the Qweak experiment is that the electrons in the beam were highly polarized — prepared prior to acceleration to be mostly “spinning” in one direction, parallel or anti-parallel to the beam direction. With the direction of polarization rapidly reversed in a controlled manner, the experimenters were able to latch onto the weak interaction’s unique property of parity (akin to mirror symmetry) violation, in order to isolate its tiny effects to high precision: a different scattering rate by about 2 parts in 10 million was measured for the two beam polarization states.

The proton’s weak charge was found to be QWp=0.0719±0.0045. This turns out to be in excellent agreement with predictions of the Standard Model, which takes into account all known subatomic particles and the forces that act on them. Because the proton’s weak charge is so precisely predicted in this model, the new Qweak result provides insight into predictions of hitherto unobserved heavy particles, such as those that may be produced by the Large Hadron Collider (LHC) at CERN in Europe or future high-energy particle accelerators.

Timothy J. Hallman, associate director for nuclear physics of the U.S. Department of Energy Office of Science said, “This very challenging experimental result is yet another clue in the worldwide search for new physics beyond our current understanding. There is ample evidence the Standard Model of Particle physics provides only an incomplete description of nature’s phenomena, but where the breakthrough will come remains elusive.”

Anne Kinney, assistant director for the Mathematical and Physical Sciences Directorate at the National Science Foundation said, “After more than a decade of careful work, Qweak not only informed the Standard Model, it showed that extreme precision can enable moderate-energy experiments to achieve results on par with the largest accelerators available to science. Such precision will be important in the hunt for physics beyond the Standard Model, where new particle effects would likely appear as extremely tiny deviations.”

Greg Smith, Jefferson Lab senior staff scientist, and Qweak project manager said, “It’s complementary information. So, if they find evidence for new physics in the future at the LHC, we can help identify what it might be, from the limits that we’re setting already in this paper.”

Kowalski says, “Qweak provides a very precise measurement of the weak charge of the proton, probing interesting new physics at the highest energies.”

The Qweak Collaboration consists of about 100 scientists and more than 20 institutions. The experiment was funded by the U.S. Department of Energy Office of Science, the National Science Foundation, the Natural Sciences and Engineering Research Council of Canada, and the Canadian Foundation for Innovation, with matching and in-kind contributions from a number of the collaborating institutions. The MIT effort was funded by a grant from the U.S. Department of Energy.

This result, published recently in Nature, also constrains possibilities for new particles and forces beyond our present knowledge.