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New experimental results increase confidence in the Standard Model, including the latest mass measurements for the top quark, W and Z particles, as well as electroweak precision measurements, according to speakers at various invited and contributed sessions of the 1996 APS Division of Particles and Fields (DPF) Meeting, held 10-15 August in Minneapolis, Minnesota. This year's meeting also featured reports on the first observation of W+W-pairs and exciting new results in QCD theory.
New Precision Measurements
More precise measurements of the top quark have been achieved at Fermilab's Tevatron collider merely one year after its momentous experimental discovery was announced. According to Sally Dawson of Brookhaven National Laboratory, who closed the conference with a summary of the year's highlights in particle physics, the new results are viewed by many as a triumph of the Standard Model, although some key questions remain unresolved.
The new mass of the top quark is determined to be 175 +/-6 GeV, an improvement in precision by a factor of two. The 100 candidate events used to calculate a new value for the top quark mass represent the combined inventories of both the CDF and D0 detector groups. The combined data also resulted in a more precise determination of the mass of the W boson, a carrier of the so-called weak force. The new mass, expressed as an average from Fermilab's CDF and D0 detectors and from CERN experiments, was determined to be 80.35 GeV, reducing the overall uncertainty from 160 to 130 MeV/c2.
The mass of the Z boson has been determined as 91.1863 +/-.0020 GeV/c2, as a result of new extremely precise measurements that emerged from the ALEPH, DELPHI, L3 and OPAL experiments at CERN's LEP electron positron collider, as well as the SLD experiment at SLAC. In addition, new measurements from the CLEO experiment at Cornell University helped resolve two possible areas of deviation observed last year at LEP and SLAC in the rate of decay of the Z boson into charm and anti-charm quarks, and in the rate for Z decay into bottom and anti-bottom quarks.
In addition to providing further experimental confirmation for the Standard Model, these improved particle measurements are an important link to finding the as-yet-unobserved Higgs boson, which endows the W and Z bosons with large mass and is also believed to be responsible for breaking the symmetry between the weak and electromagnetic forces. In fact, if the Higgs mass lies within the lower part of the range suggested by the new measurements of the top quark and W particle, it could be observed at the upgraded LEP-II collider at CERN. If the mass is towards the higher range, scientists may have to wait for a future upgrade of the Tevatron or for CERN's Large Hadron Collider, scheduled to begin collecting data in 2006.
Fermilab's Liz Buckley-Geer, who spoke on Thursday morning, reported that the CDF detector also produced measurements of quark or gluon jets at large angles with respect to the proton beams, and with energies approaching half the energy of incoming beam particles. While the data agreed qualitatively with QCD predictions in the production rate, data for the largest-energy jets exceeded predictions by nearly a factor of two. In contrast, the D0 experimental data, agreed more closely with QCD theory. Eventually it was determined that the data and theory could be brought into better agreement by modifying the fraction of the proton momentum carried by energetic gluons.
Last year, scientists at the HERA electron-proton collider in Germany reported that the density of low momentum gluons in the proton was much larger than expected. Using new special detectors, the H1 and ZEUS experiments found that this excess persists down to those gluons carrying as little as one-millionth of the proton's momentum, a striking effect that may provide evidence of a new regime in which perturbative treatment is supplanted with collective effects of multiple-gluon states. In addition, scientists with CERN's LEP collaborations have succeeded in isolating pure samples of quarks or gluons emerging from Z boson decay, which had previously been experimentally indistinguishable. This ability could become an important experimental tool for disentangling the decays of particles which decay predominantly into quarks from the large QCD backgrounds.
The LEP electron-positron collider also significantly increased the energy at which it operates in 1996, from about 135 GeV at the beginning of the year to 162 GeV mid-year. This increase resulted in the first experimental observation of pairs of W bosons, as well as permitting the extension of searches for new particles. A sixfold increase in data samples produced by the Tevatron experiments this year has also substantially increased the range of new particle searches, although none have yet been found.
Sunday evening featured a special plenary session celebrating 100 years of particle physics. SLAC's Martin Perl reflected on the discovery and subsequent impact of the tau lepton on the field, and Robert Wilson of Cornell University compared the dreams of Fermilab with the present-day reality. Robert Sachs of the University of Chicago closed the session with a review of the conception and birth of the DPF itself.
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