September 16, 2005  


The Physicist’s Guide to the Z Particle

Before the Z-pole program began, electroweak data at the time predicted the mass of the top quark within a large band (vertical green line on left). With data from SLC and LEP flowing in each year, the possible mass range for the top quark narrowed significantly (red band from left to right). The Tevatron at Fermilab directly searched for the top at increasing energies (black line) and finally found it (black dots) in 1994, exactly at the central value of the range predicted by Z-pole data.

(Graphic courtesy of SciArts)

By Heather Rock Woods

Physicists now have an authoritative guide to everything they know about the Z particle, from A (accelerating antiparticles to annihilation) to Z (zillions of Z bosons).

‘Precision Electroweak Measurements on the Z Resonance’ is the definitive document on the physics of Z particles at high energies intensively studied by major experiments at SLAC and CERN during the 1990s. The Z resonance, also called the Z pole, is the energy where Zs are directly produced.

The final review paper, submitted to Physics Reports on September 7, contains the world’s best measurements of the width and mass of the Z particle, as well as hundreds of other characteristics that paint a precision portrait of the heaviest carrier of the weak force (the Z particle) and of the electroweak force itself. The research accurately predicted the mass of the top quark before it was discovered at Fermilab in 1994, and currently gives the best constraint on the mass of the postulated Higgs particle that gives mass to other particles. The experiments also determined that there are only three light neutrino species, conclusively settling a longstanding question for particle physics and cosmology.

“This paper is meant to be the definitive reference work on the entire Z-pole program of electroweak physics, which produced outstanding results. This is the first time that the LEP experiments at CERN and the SLD experiment at SLAC are submitting a jointly written review to a refereed journal,” said Peter Rowson (SLD), co-leader of the SLD electroweak group who is coordinating the SLD side of the paper.

“Every one of us participating in this work enjoys very much collaborating with colleagues from experiments which are otherwise our strongest competitors,” said Martin Gruenewald (University College, Dublin), a member of the L3 collaboration at LEP and chair of the LEP electroweak working group. He organized the paper’s editorial process.

Preliminary Results

The first preliminary results published jointly came out in 1995. A series of such preliminary papers continued to be published as groups completed their analyses. The final publication gives the final data, plots and global fits from all the analyses, as well as historical information and descriptions of the machines and detectors. The paper is suitable for a graduate student in the field.

“The strengths of the SLD and LEP experiments were complementary: Together they made for a powerful program,” Rowson said.

The Z boson carries the weak force like its lighter, more familiar cousin—the charged W boson involved in radioactive decays. At a hefty 91 GeV, the neutral Z boson acts only over short distances. The Z particle also works asymmetrically, interacting more often with left-handed particles than with right-handed ones. In contrast, photons carrying the electromagnetic force are perfectly symmetric, and Ws are completely asymmetric, interacting only with left-handed particles.

Z Particle Discoveries

CERN discovered the Z particle in 1983, 16 years after theorists developing the Standard Model of particles and their interactions predicted its existence. From 1989 to 1995, four detectors at the LEP machine (ALEPH, DELPHI, L3 and OPAL) collected 17 million Z particles. Meanwhile, the Stanford Linear Collider (SLC) smashed polarized electrons into positrons (the electron’s antiparticle) to produce the massive Z particles. The Mark II detector saw its first Z in 1989. The SLD detector took over in 1991 and accumulated 600,000 Z particles by 1998. SLD and LEP measurements put the Standard Model on solid footing.

While LEP collected 30 times more events, allowing many important measurements, SLD exploited its polarized electron beam and uniquely precise vertex detection system—the SLD vertex detector and the tiny SLC beam—to attain other significant results the other experiments could not do. Vertex detection allowed physicists to identify the heavy charm and bottom quarks, and to study their electroweak interactions. With electron bunches polarized to be left-spinning or right-spinning, the asymmetry in how the left and right electrons behaved when coupling to the Z gave a key measurement called the weak mixing angle.

“For some measurements, polarization is absolutely critical,” Rowson said. “Because we had the polarized beam, we were able to do the single best measurement of any experiment of the weak mixing angle, which tells you about the electroweak force, and is the most powerful single tool for predicting the Higgs mass.”

Complicated Data Analysis Takes Time

The final paper includes 2,501 researchers from multiple countries. In addition to Rowson and Gruenewald, the chapter editors on the writing committee were: Richard Kellogg (OPAL - University of Maryland), Klaus Moenig (DELPHI - DESY), Guenter Quast (ALEPH - University of Karlsruhe), Mike Roney (OPAL - University of Victoria) and Pippa Wells (OPAL - CERN).

When an experiment stops taking data, the analysis work still continues strongly. Most of the analyses were published several years ago, but the more complicated ones took more time.

“The speed of publication depends on the cleanliness of analyses and the cohesion of collaborations,” Rowson said. “People went on to other things, like BABAR, and couldn’t devote full time to SLD analysis.”

The results in the final SLD/LEP paper are relevant today in their own right and for new projects such as the International Linear Collider (ILC), the proposed next-generation linear collider to succeed the SLC. Using the ILC, physicists hope to understand supersymmetry, dark matter and the elusive Higgs particle, which SLC and LEP constrained so elegantly.

RELATED: View a Time Line for Electroweak Physics (in PDF format)





The Stanford Linear Accelerator Center is managed by Stanford University for the US Department of Energy

Last update Tuesday September 20, 2005 by Chip Dalby