New Precision Measurement of Top Quark Mass

Photo of DZero detector

DZero detector

The following news releases on a new precision measurement of the top quark mass are being made public today by the journal Nature (where the work appears in the June 10, 2004 issue) and Fermi National Accelerator Laboratory (where the measurement was made using the DZero detector at the Tevatron accelerator). Several Brookhaven Lab scientists who are collaborators on the DZero experiment are co-authors on the Nature paper*. The Brookhaven physicists were involved in identifying and selecting the events for the top quark mass measurement, and checking the validity of the new measurement.

* Brookhaven Lab co-authors include: David Adams, Stephen Kahn, Jon Kotcher, Abid Patwa, Serban Protopopescu, Srinivasan Rajagopalan, Scott Snyder, and Andre Turcot, all formerly or currently members of the Brookhaven Lab Physics Department, and Kin Yip of the Collider-Accelerator Department.

For more information about Brookhaven Lab_s role or to speak with a Brookhaven Lab scientist, contact: Karen McNulty Walsh, 631 344-8350, or Mona S. Rowe, 631 344-5056,

Featured press release entry distributed by Nature:

Physics: The mass of the top quark (pp638-642; N&V)

The discovery of the top quark in 1995 provided dramatic confirmation of the essential validity of the standard model of particle physics. But there_s a catch — the standard model contains parameters whose origins are still unknown and which cannot be predicted, but whose values are constrained by their interactions with other particles and fields. Something else is afoot.

For example, the mass of the top quark is related to the mass of the long-hypothesized but still undetected Higgs boson. Properties of the (equally hypothetical) field associated with this particle would help explain why matter is, not to put too fine a point on it, _massive._ In principle, the top quark is point-like and should have no mass; yet, through its interactions with the Higgs field, the physical mass of the top quark appears to be about that of a gold nucleus.

Because it is so heavy, the top quark provides an unusually sensitive tool for investigating the Higgs field. Well-constrained values for the top-quark mass will enable researchers to predict the mass of the Higgs particle, eliminating as untenable various theoretical fixes for deficiencies in the standard model, while supporting others. Much, therefore, rides on precision measurements of the top quark.

This is where the multinational DØ [DZero] collaboration comes in: they have measured the mass of the top quark with unprecedented precision. Their value, 178 ± 4.3 GeV/c2 (where GeV/c2 represents gigaelectron volts divided by the square of the speed of light), is important because it raises the upper limit on the Higgs mass from 219 to 251 GeV/c2. The most likely value of the mass is now 117 GeV/c2; the previous value of 96 GeV/c2 falls in a range now ruled out by experiment. Further improvements in precision are to be expected from the Tevatron at Fermilab, and from the Large Hadron Collider at CERN (the European nuclear research laboratory at Geneva) when it becomes operational after 2007.

Georg Weiglein discusses the research in an accompanying News and Views article.

CONTACT: Greg Landsberg (Brown University, Providence, RI, USA) Tel: +1 401 863 1464, E-mail:

Georg Weiglein (University of Durham, UK) Tel: +44 191 334 3520, E-mail:

Press release distributed by Fermilab: Contact: Kurt Riesselmann, Public Affairs, Fermilab, 630-840-3351

Fermilab Results Change Higgs Mass Estimate

Batavia, Ill.-Scientists at the Department of Energy_s Fermi National Accelerator Laboratory today (June 9) announced new results that change the best estimate of the mass of the postulated Higgs boson from approximately 96 GeV/c2 to 117 GeV/c2. Compared to the previous value, the new value is in better agreement with direct searches - such as those conducted by CERN experiments - that excluded a mass below 114 GeV/c2. In a paper to appear in the June 10 issue of Nature magazine, physicists of Fermilab_s DZero experiment report on results obtained by applying a new analysis technique to data obtained from 1992 to 1996 during Collider Run I at the Fermilab Tevatron, the world_s highest-energy particle accelerator.

"These important results demonstrate how our scientists are applying new techniques to existing data, producing new estimates for the mass of the Higgs boson," said Dr. Raymond L. Orbach, Director of the Department of Energy_s Office of Science, which funds Fermilab. "We eagerly await the next round of results from the vast quantities of data that are generated today at the Fermilab Tevatron."

Particle physicists predict the existence of a heavy particle, the Higgs boson, which interacts with particles of matter to give them their characteristic masses. However, despite attempts at particle accelerator experiments to observe the Higgs, no one has yet detected its existence. While scientists have never observed the Higgs itself, they can predict its mass and other characteristics by making precise observations of known particles with which it presumably interacts. In particular, the masses of the top quark, discovered at Fermilab in 1995, and the W boson together constrain the mass of the Higgs. A precise measurement of the top quark mass not only lets experimenters home in on the energy level of particle collisions required to produce a Higgs, but also reveals whether the boson_s existence is consistent with experimental data. The indirect indication of the Higgs mass guides particle physicists in their ongoing hunt for direct observation and confirmation of the particle_s existence.

Scientists from nearly 40 U.S. universities and 40 foreign institutions contributed to this measurement. The DZero paper employs a novel technique for data analysis, based on ideas first developed in the 1980s by physicist Kunitaka Kondo of Japan_s Waseda University, a collaborator on Fermilab_s CDF experiment, and independently by Richard Dalitz and Gary Goldstein at Oxford.

"This analysis technique allows us to extract more information from each top-quark event that occurred in our detector," said DZero cospokesperson John Womersley. "The result yields a greatly improved precision, of 5.3 GeV/c2, in the DZero top mass measurement. The new measurement is comparable to the precision of all previous top quark mass measurements put together. When this new result is combined with all other measurements from both the DZero and CDF experiments, the new world average for the top mass becomes 178.0 4.3 GeV/c2."

The new value of the top quark mass shifts the best estimate of the Higgs mass from 96 to 117 GeV/c2, and the upper bound on the Higgs mass at the 95 percent confidence level rises from 219 to 251 GeV/c2.

The impact of the new value of the top mass goes beyond the Higgs boson, said DZero co-spokesperson Gerald C. Blazey, a physicist from Northern Illinois University.

"Various supersymmetric theories of particle physics make predictions for the Higgs mass," Blazey said. "The improved knowledge of the Higgs mass will help further refine these theories."

Particle theorists predict a new class of heavier, "supersymmetric" partners for all of the known matter particles of the Standard Model, the theory of the fundamental particles and forces of nature. Searching for evidence of supersymmetry is among the key objectives for the CDF and DZero experiments during Run II at Fermilab_s Tevatron.

Fermilab Director Michael Witherell noted that DZero_s result does not end the story of precision measurements of the top quark mass.

"The two collider detectors, CDF and DZero, are recording large amounts of data in Run II," Witherell said. "The CDF collaboration has recently reported preliminary new measurements of the top mass based on Run II data. The precision of the world average will improve further when their results are final. Over the next few years, both experiments will make increasingly precise measurements of the top quark mass."

Fermilab is a national laboratory funded by the Office of Science of the U.S. Department of Energy, operated by Universities Research Association, Inc. For a list of DZero member institutions, please go to .

Tags: physics

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