Together with their colleagues from the international H1 collaboration, the ZEUS scientists also studied the exchange of virtual Z0 bosons between weakly interacting particles. The researchers found that both the production of real Z0 bosons and the exchange of virtual Z0 bosons at HERA were in excellent agreement with the Standard Model of particle physics. The researchers presented their analyses in a series of scientific papers, published in the journals Physical Letters B, Journal of High Energy Physics and Physical Review D.
There are two ways for Z0 bosons to appear at particle accelerators like HERA. They can either emerge as real particles from the fireballs created in particle collisions or they can be exchanged as short-lived fluctuations, called virtual particles, between ordinary particles that are coupled by the weak force.
Production of real Z0 bosons
Since Z0 bosons are heavy particles and require a large amount of energy for their creation, they were seldomly produced in HERA’s electron–proton and positron–proton collisions.
To increase their chances of detecting Z0 bosons, the ZEUS scientists concentrated their analysis on collision events, in which the proton stayed intact. “If you include all events, you end up with too many other processes where the proton breaks, so you can no longer find those few Z0 bosons that you are looking for,” says DESY researcher and ZEUS Physics Chairwoman Katarzyna Wichmann, who took part in the data analysis.
Using this analysis strategy and a calorimeter that quantified particle energies with an exceptional energy resolution, the particle physicists were able to conclude that 15 real Z0 bosons had been created in the half-billion collision events recorded by the ZEUS detector. These numbers are comparable to the identification of only two individuals in Germany’s entire population.
Exchange of virtual Z0 bosons
Furthermore, the ZEUS researchers teamed up with scientists from the H1 collaboration to study virtual Z0 bosons that are exchanged when electrons or positrons interact with protons. However, due to their electric charges, electrons or positrons and the quarks inside protons also exchange photons, the carrier particles of the electromagnetic force. At small energy transfers in the collision process, the electromagnetic force dominates the weak force.
To extract the much smaller contribution of the Z0-mediated weak force, the researchers must therefore zero in on large energy transfers in the scattering process. “At high energy transfers, the interaction probability depends on whether electrons or positrons are used in the experiment,” says DESY scientist Stefan Schmitt, spokesperson of the H1 collaboration. “The difference in the behaviour of electrons and positrons cannot stem from the exchange of photons because the electromagnetic force does not have that effect.”
The H1 and ZEUS collaborations both measured the asymmetry between electron– proton and positron–proton reactions as evidence of the weak force and the exchange of virtual Z0 particles.
Testing the Standard Model
The scientists used their results to put one of physics’ most successful theories to the test – the Standard Model of particle physics, which describes the behavior of all known particles and their mutual interactions, excluding gravity.
“The detection of Z0 events with very low probability, measured over HERA’s 15-year life span, created the opportunity to check the details of our current theories very precisely,” says DESY’s ZEUS group leader Achim Geiser. So far, the researchers have found their experiments to match the predictions by the Standard Model.
However, this may not be the end of the story. “Not all aspects of the weak interaction between Z0 bosons and quarks have been fully tested yet,” Schmitt says. “We’re currently using our data to check for deviations of these weak couplings from the Standard Model.” Often in research, deviations hint at unknown effects and unexpected, new science.
Original Publications
ZEUS Collaboration; Production of Z0 bosons in elastic and quasi-elastic ep collisions at HERA; Phys. Lett. B 718, 915–921 (2013); DOI: 10.1016/j.physletb.2012.11.051.
H1 Collaboration; Inclusive deep inelastic scattering at high Q2 with longitudinally polarised lepton beams at HERA; JHEP 09 061 (2012); DOI: 10.1007/JHEP09(2012)061.
ZEUS Collaboration; Measurement of high-Q2 neutral current deep inelastic e+p scattering cross sections with a longitudinally polarized positron beam at HERA; Phys. Rev. D 87, 052014 (2013); DOI: 10.1103/PhysRevD.87.052014.