Praxis Archives – Page 181 of 181

Physicist dies in air crash

Klaus Kinder-Geiger, 35, a leading theorist in relativistic heavy-ion physics, died tragically on 2 September aboard the Swissair New York to Geneva flight which crashed near Nova Scotia.

His Parton Cascade Model has decisively influenced our view of high-energy nuclear reactions, with important implications for future programmes at Brookhaven’s RHIC and CERN’s LHC colliders.

After his thesis on glueball decays at Frankfurt in 1989, Klaus spent postdoctoral years at Duke University and Minnesota, where he developed the Parton Cascade Model. While a Fellow in CERN’s Theory Division from 1994 96, he worked with John Ellis on hadronization theory. He joined Brookhaven’s nuclear theory group in 1996. His recent research also covered the application of the renormalization group to QCD transport theory.

His enthusiasm and vision was an inspiration to his many friends and collaborators, who mourn his untimely death.

Niels Bohr and the 20th century

Niels Bohr (1885-1962) did not coincide entirely with the 20th century, but was nevertheless one of its great motive powers. The meeting, organized by the Niels Bohr Institute in Copenhagen, brought together about 200 eminent physicists at UNESCO’s headquarters in Paris from 2729 May to consider his scientific legacy.

bohr1-10-98 — At the “Niels Bohr and the Evolution of Physics in the 20th Century” meeting, four distinguished scientists were awarded Niels BohrUNESCO gold medals for their important achievements in basic science and their efforts to shorten the bridge between science and society: S Chandrasekhar of the Centre for Liquid Crystal Research, Bangalore, India; Vitaly Ginzburg of Moscow’s P.N. Lebedev Institute; Walter Kohn of the University of California, Sanata Barbara; and Alexander Polyakov of Princeton (pictured).

After presenting the wide variety of Bohr’s seminal ideas and paradigms, speakers turned to the present vitality of these concepts at the frontiers of modern physics, emphasizing also the growing symbioses between physics and biology, and between physics and information theory.

The opening talks on Bohr, by biographer Abraham Pais and by Ove Nathan of the Niels Bohr Institute, recalled the deep involvement of science with the everlasting challenge to express ourselves in a way which is both philosophically correct and reflects profound personal integrity.

Examples are the contest early this century between Bohr and Einstein on the interpretation and understanding of the quantum mechanical concepts of measurement and evidence; and the subsequent contests between Bohr and, in turn, Roosevelt, Churchill (who suggested keeping Bohr under house arrest), and later the UN, in the struggle to prevent world politics degenerating into an atomic arms race. Both of these avenues of confrontation remain strikingly topical, as reflected in the talk by Anton Zeilinger on modern quantum information theory, and in the parallel news of the spread of nuclear arms in the East.

Turning to the present frontiers of science first embraced by Bohr and other monumental personalities of the 1920s, the younger contemporary observer must feel more comfortable with the development of a much wider physics community.

Here an enormous interpersonal web of co-operation and exchange of ideas and resources, aided by public revenue support still driven by the evolutionary spirit of the scientific revolution at the dawn of the century, supports a complex of frontier projects.

This encompasses not only the spectacular “big science” effort as exemplified by the Hubble Space Telescope, CERN’s LHC proton collider, or Japan’s Superkamiokande underground detector, but also the international communities pushing the underlying theoretical understanding beyond the standard models of their respective disciplines.

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And there is modern table-top science “small is beautiful” splitting photon states for quantum teleportation, fixing single atoms in nano-Kelvin states to measure time to an accuracy of 10¹⁹ (to check the time variation of Nature’s “constants”), developing DNA strains to catalyse organic manufacture of photomasks for quantum-level semiconductor chips, and synthetic muscular mini-machines built from multi-polymeric “soft matter” complexes.

bohr2-10-98 — Distinguished physicist, author and Bohr biographer Abraham Pais spoke on “Niels Bohr, man and physicist”.

These specialist subjects are vibrant with excitement, both for the open horizon of learning about the universe of Nature, and the transition from an initial qualitative to a precision quantitative understanding of the quantum world, inorganic and organic.

Astrophysics left a particularly strong impression, with new results from Hubble on the dark matter problem (Martin Rees), and with Superkamiokande’s tentative evidence for neutrino oscillations (John Bahcall) giving the first hints of physics beyond the Standard Model.

The symposium was a memorable experience for participants, achieving its basic but ambitious objective of surveying the evolution of phyics this century. It was in some ways a modern revival of the dramatic meetings held at the beginning of the century, such as the historic 1911 Solvay Conference, but with a small number of insular scientific luminaries replaced by a host of enthusiastic heirs.

Notably absent were the prophets of doom, predicting the imminent end of science. Regrettably, as the symposium would have convinced them otherwise.

SLC perched on a high

slc1-10-98 — By compressing the size of the electron and positron beams at the interaction point (IP), the SLC has substantially increased its luminosity, a measure of the electronpositron collision rate. s_x and s_y are the rms beam half width and height, respectively; their product is a measure of the cross sectional area of the beam at the interaction point.

Just before noon on 8 June, operators at the Stanford Linear Accelerator Center (SLAC) shut down the SLC Stanford Linear Collider for what may have been the last time.

It had been a record-breaking year-long run in which the world’s first-ever linear collider more than tripled its collision rate, or luminosity, and generated over 350,000 massive Z particles for physicists on the SLD experiment collaboration to study. But unless the US Department of Energy provides the funding for an extension, the 199798 run will have been the SLC’s last hurrah.

The data-taking phase of the run had begun slowly in July 1997. Although the peak luminosity matched that of prior years, it was not quite up to the levels of the 1996 run. Accelerator physicists and operators were taking extra time to tune the machine carefully in preparation for the long run.

In October and November the collider really hit its stride, however, often exceeding a hundred hours of successful operations and 10, 000 Zs per week. After recovering from the Christmas shutdown, the machine surpassed even those levels, occasionally delivering over 20,000 Zs per week during the rest of the run.

Under the leadership of Nan Phinney, who was aided by the untiring efforts of Pantaleo Raimondi and Tracy Usher, the SLC performed beyond expectations. Often working through the night, this pair found clever solutions to long-standing problems and helped achieve large luminosity gains without any major hardware upgrades.

With beam profiles at the interaction point as small as 0.65 by 1.5 microns (rms half height and width), physicists finally began to observe the long-awaited phenomenon of “disruption”, in which a bunch is compressed by the macroscopic electromagnetic fields of the bunch it passes through. According to estimates, this enhancement doubled the peak luminosity, leading to the high values attained toward the end of the run. Experimenters marvelled at occasional events in which they could clearly discern the crossing of two Zs produced in a single bunch.

The collider was roaring along, generating 5300 Zs in the previous 24 hours, when a vacuum failure in the positron source forced its premature shutdown a week early. At the time the SLC was spewing out about 300 Zs per hour, or a peak luminosity of about 3×10³⁰ per sq cm per s half its design value.

The number of Zs the SLC produced in the 199798 run is nearly twice that generated in all previous runs, bringing the cumulative total to more than half a million. When combined with electron beam polarization levels of 73%, this bonanza means that SLD physicists will have plenty of work in the coming months extracting the world’s best measurement of the weak mixing angle. And using the SLC’s narrow beams together with what is the world’s most sophisticated vertex detector, the collaboration may also be able to uncover other unique physics results such as measuring the mass difference that determines the frequency of particle-antiparticle mixing in Bs mesons.

Undoubtedly there will be important questions left unanswered, however, and collaboration leaders are already lobbying for a final SLC run to raise the total number of Zs to more than a million. Another run could also allow them to boost its peak luminosity above the design value to 10³¹, Phinney estimates. Such an achievement would help advance the state of the linear collider art in preparation for the TeV-scale linear collider that many now agree is the next major project for high-energy physics.

Whatever else happens, the SLC will go down in history as the machine that proved the feasibility of the linear collider concept. Although outgunned by CERN’s LEP collider ring in terms of the sheer output of Z particles, physicists at SLAC fought back successfully by emphasizing narrow beams and polarization as important tools for doing precision physics measurements. With nearly twice the beam polarization originally planned, the SLC has now essentially achieved the overall performance goals set for it more than a decade ago despite falling a bit short on the luminosity front. And even that shortcoming could be erased in a final run.

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