Amanda Gray

Originally called Truth, the Top quark was predicted by the Standard Model. (The standard model does not require that quarks come in pairs, nor recognize a deep symmetry that forces the number of leptons and quarks to be the same, but nonetheless physists believe these things to be likely.) The discovery of the tau lepton in 1975 increased the number of known leptons to five, which in turn kicked off the search for the third pair of quarks, Top and Bottom, since only the first four quarks had been discovered by that time: Up and Down in 1968, Strange in 1951 and Charm in 1974. [1] When the Bottom quark was discovered in 1977 at Fermilab [2], it was expected that Top would be found quickly after -- but it took 18 years of sifting through data before the CDF and D0 experiments were able to identify it. [3]

The first evidence for top quark production in proton-antiproton collisions was presented in April 1994, based on data collected at the Fermilab Tevatron collider using the Collider Detector at Fermilab (CDF). [4] The CDF collaboration reported actual observation of the Top Quark in a research paper submitted 24 Feb 1995 to Physical Review Letters, and presented its results at a seminar on 2 Mar 1995 at Fermilab. [5]

"All direct measurements of top quark production and decay have been made by the CDF and D∅ experiments at the Fermilab Tevatron collider in [proton / anti-proton] collisions."[6]

When protons and anti-protons collide, top anti-top (t-tbar) pairs can be produced. The t-tbar pair is identified by what it decays into. [7] Three decay modes have been identified so far:

(1) "All-hadronic" or "all-jets": Each W-boson decays into a pair of quarks. This is "the most common decay mode, but somewhat difficult to distinguish from non-top particle decays... A new mass measurement (F. Abe et al., 15 Sept. [1997] Physical Review Letters) by the CDF group is based on the first identification of all-hadronic top decays."

(2) "dilepton": Each W-boson decays into a lepton (such as an electron or muon) and neutrino. D∅ has reported calculations of the top mass based on the dilepton events.

(3) "lepton-plus-jets": one W-boson decays into a lepton (electron or muon) and neutrino, the other decays into two quarks, which subsequently produce a "jet," or spray of particles. [8]

In addition to t-tbar pairs, top quarks can be produced as single top quarks through the weak interaction. "Single top production is predicted to have a lower cross-section and a more challenging event signature, and has not yet been observed". [9]

Top Mass and the Higgs Boson

It is expected that the Top quark has more mass than the Higgs boson, which is predicted by both the standard model and the minimal supersymmetric model, but has not yet been discovered. The precision with which the the Top quark is measured determines how well the Higgs mass can be predicted; given good predicted mass for the Higgs boson, experimenters "would know its cross-section and how it would decay, allowing their searches... to be more precise." [10] The most recent measurement of the Top quark is: 173.5 [+2.7/-2.6 (stat.) +/- 2.5 (JES) +/- 1.7 (syst.)]GeV/c^2 . The corresponding M_Higgs fit is 94 +54/-35 GeV/c^2. [11] This value is less than has been expected recently, which means that the Higgs boson should be observable by our current accelerators.

How Does Top Spin?

The Top quark has a couple qualities that make it a very interesting thing to study. Its lifespan is so short, it does not have time to couple with anything else, but merely decays, so its decay products retain its spin. Its large size puts it "close to the scale of electroweak symmetry breaking," [12] so experimental information about the top quark can have big implications for electroweak fits in the Standard Model." [9] The Standard Model has "specific predictions for the direction that the decay products of the Top travel," so the observation of a prohibited spin, or spins that do not occur as often as predicted, would indicate that "some physics beyond the Standard Model is happening." [10] "The measurement of the top quark pair production cross section in proton-antiproton collisions at s^(1/2) = 1.96 TeV is a test of quantum chromodynamics and could potentially be sensitive to new physics beyond the standard model." [13]

1. Worldwide discoveries that led to the Standard Model

2. Discoveries at Fermilab - The Bottom Quark

3. Nova: The Elegant Universe: Smashing Pictures

4. First Evidence for the Top Quark, Fermilab-pub-94/-097-E.

5. Observation of the Top Quark, Fermilab-pub-95/022-E.

6. The Top Quark (Rev.) (pdf, 14 pages) (from Standard Model) [From Reviews, Tables, and Plots in the 2004 Review of Particle Physics, S. Eidelman et al., Phys. Lett. B 592, 1 (2004) (bibtex). Available online at the Particle Data Group site.]

7. Highlights of the Analysis, [1].

8. Newly Identified Top-Quark Decay Modes, American Institute of Physics. See also: preprint of "Measurement of the Top Quark Mass Using Dilepton Events," B. Abbott et al. (The D0 Collaboration), Fermilab-Pub-97/172-E.

9. Top Quark Mass Measurements at the Tevatron, Martijn Mulders

10. The Top Quark: worth its weight in gold, by Florencia Canelli, University of Rochester, DØ Experiment. See also: Spin Issues in t-tbar Production and Decay, Gregory Mahlon.

11. Measurement of the Top Quark Mass with In Situ Determination of the Jet Energy Scale (April 2005), Arguin, Jean-Francois, et al.
See also: A New Top Mass Measurement and Why You Should Care About It

12. Top Quark Physics, M. Beneke, I., et al. (Mar 2000).
See also: "the majority of physicists believe Higgs/EWSB is the most important physics for the field to study in the coming years." The Young Physicists Panel (YPP) presents Results of the Survey on the Future of HEP, p.22 (Aug 2001)

13. Top Quark Pair Production at The Tevatron, Jason Nielsen (May 2005).

Contact: grayau.washington.edu