01.11.2013

Researchers Find Proverbial Needle in Haystack

Particle physicists identify rare Z0 events in HERA data

Particle physicists from the international ZEUS collaboration have found the proverbial needle in a haystack. They identified a mere 15 Z⁰ bosons in over 500 million collision events at DESY’s former electron–proton accelerator HERA. Z⁰ particles are carriers of the weak nuclear force, one of four fundamental forces in the universe. Although Z⁰ bosons have been intensely studied at other particle accelerators, they have never been directly observed in electron–proton collisions before. Z⁰ production is the rarest process ever measured at HERA.

An international team of particle physicists identified 15 Z0 bosons in over 500 million particle collision events at DESY’s former HERA accelerator. (Right) The Z0 bosons manifest themselves in a peak in the data that were recorded with the ZEUS detector over a time span of 15 years (1992–2007). The production of Z0 bosons, which were discovered at the research centre CERN (Switzerland) in 1983, was a very rare process in HERA’s electron/positron–proton collisions. (Left) Z0 particles are one type of force carriers in the Standard Model of particle physics. Z and W bosons mediate the weak nuclear force. Photons and gluons are carriers of the electromagnetic and strong nuclear forces, respectively. The Higgs boson is associated with the Higgs field, which explains the masses of the other bosons. (Credit: Stefan Schmitt/DESY)

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.

Z⁰bosons and the weak force

The Z⁰ boson is an elementary particle that, together with the W bosons, mediates the weak force, which is responsible for the radioactive decay of atomic nuclei and other processes.

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.