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STANFORD (SLAC) Asymmetry Parameter Using a Dictate Where This Quark Can Sit If the Polarized Electron Beam Is a Straight­ Standard Model Is to Survive Intact

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STANFORD (SLAC) Asymmetry Parameter Using a Dictate Where This Quark Can Sit If the Polarized Electron Beam Is a Straight­ Standard Model Is to Survive Intact Physics monitor The SLD determination of the confirmed, consistency arguments STANFORD (SLAC) asymmetry parameter using a dictate where this quark can sit if the polarized electron beam is a straight­ Standard Model is to survive intact. Precision electroweak forward measurement whose errors Using LEP data alone, the top mass result are still dominated by counting is 165 ± 22 GeV, while a global fit to statistics. The systematic error in this all world data yields 178 + 9 + 17-20 measurement (which has dropped to GeV. Precision testing of the electroweak 1.7% from 3.6% in 1992) is due Excitement is high as the next sector of the Standard Model has mainly to the uncertainties in measur­ round of SLD experiments begins intensified with the recent publica­ ing the SLC polarization using a this June. With at least 100,000 tion* of results from the SLD collabo­ Compton polarimeter, situated close events expected and a beam polari­ ration's 1993 run on the Stanford to the SLC interaction point. zation of about 75 percent, the error Linear Collider, SLC. Using a highly But the 1993 SLD run was not on the final result should be halved. polarized electron beam colliding with without its surprises. Testing the spin With LEP precision improving too, an unpolarized positron beam, SLD transport through the SLC arc to the next year will certainly be inter­ physicists measured the left-right evaluate an expected chromatic esting for electroweak physics. asymmetry at the Z boson resonance effect on the polarization measure­ * K. Abe et al, SLAC-PUB-6456 (April with dramatically improved accuracy ment, an SLD group was confronted 1994), submitted to Physical Review over 1992. Combining the two by a large discrepancy between the Letters. measurements yields the world's polarizations measured with the most accurate single determination of Compton polarimeter and with a 2 the weak mixing angle, sin 9w = diagnostic Moller polarimeter at the 0.2294 ±0.0010. end of the two-mile SLAC linac. Standard Model parameters (The weak mixing angle dictates Finally, after some furious head- how the two neutral carrier particles scratching last fall, these physicists The Standard Model - an amalgam of of the underlying electroweak theory stumbled across a Russian paper the current electroweak and quark combine to form the physical photon, that helped resolve the quandary. pictures of basic physics - is increas­ which carries the electromagnetic The momenta of electrons in the ingly constrained on all sides by new force and is massless, and the Z, inner shells of the iron and cobalt precision results. which carries the weak nuclear force, atoms in the Moller target had In addition to the new electroweak and at 91 GeV is the heaviest known spuriously increased the apparent asymmetry measurement from the particle.) beam polarization by about 10% - an SLD detector at Stanford's SLC The sixfold improvement in accu­ effect previously ignored in Moller linear electron-positron collider, fresh racy is due to increased luminosity polarimetry. With this finally under­ constraints come from several other (50,000 Zs compared to 11,000 in stood, the discrepancy vanished and directions. At CERN's LEP electron- 1992), higher polarization (63% the SLD collaboration published its positron collider, some 3 million Zs compared to 22%) and much im­ electroweak results. were collected in 1993, adding to the 2 proved systematic errors. Use of The value of sin 6w extracted from 5 million seen from 1989-92, while at "flat" SLC beams (2.6 by 0.8 micro­ the combined 1992-93 SLD asymme­ Fermilab's Tevatron proton-anti- meters at the collision point) is try measurements is smaller (by proton collider, the CDF experiment largely responsible for the luminosity more than two standard deviations) has set a new value for the mass of improvement. An almost threefold than the present LEP value, 0.2322 ± the W, the electrically-charged increase in the electron beam polari­ 0.0005, an average of 29 different counterpart of the neutral Z, at 80.38 zation resulted from an intensive measurements based on some ± 0.23 GeV, and the companion DO effort involving SLAC, Wisconsin and 8 million Zs seen from 1989-3. experiment provides a new 'floor' of Berkeley physicists to develop high- This offset between the LEP and 131 GeV below which the top quark polarization photocathodes using SLD electroweak results is reflected mass cannot go. gallium arsenide crystals with on the corresponding interpretation in (The June issue will include a major "strained lattices" (July/August 1993, the context of the Standard Model. feature of the Standard Model by page 5). With the sixth ('top') quark not yet physics writer Christine Sutton.) CERN Courier, May 1994 11 .
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