Top quark

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Top quark
Composition: Elementary particle
Family: Fermion
Group: Quark
Generation: Third
Interaction: Strong, Weak, Electromagnetic force, Gravity
Antiparticle: Top antiquark (t)
Theorized: Makoto Kobayashi and Toshihide Maskawa (1973)
Discovered: CDF and DØ collaborations (1995)
Symbol(s): t
Mass: 171.8–174.4 GeV/c2[1]
Decays into: Bottom quark, strange quark, down quark
Electric charge: +2⁄3 e
Color charge: Yes
Spin: 1⁄2
Flavour in particle physics
v • d • e
Flavour quantum numbers:

Combinations:

Flavour mixing

The top quark is the third-generation up-type quark with a charge of +2⁄3 e.[2] It was discovered in 1995 by the CDF and D0 experiments at Fermilab,[3][4] and is the most massive of known elementary particles. (The Higgs boson, which may be as massive, has not yet been experimentally observed.) Its mass is measured at 173.1±1.3 GeV/c2, about the same weight as an atom of rhenium.[1]

The top quark interacts primarily by the strong interaction but can only decay through the weak force. It almost exclusively decays to a W boson and a bottom quark. The Standard Model predicts its lifetime to be roughly 1×10−25 s; this is about 20 times shorter than the timescale for strong interactions, and therefore it does not hadronize, giving physicists a unique opportunity to study a "bare" quark.

Contents

[edit] History

In 1973,Makoto Kobayashi and Toshihide Maskawa predicted the existence of a third generation of quarks to explain observed CP violations in kaon decay.[citation needed] The new hypothetical particles were labelled t and b for top and bottom respectively. These names mirrored the names of the first generation of quarks (up and down) reflecting the fact that the two were the 'spin up' and 'spin down' component of a weak isospin doublet.[citation needed]

Soon after their prediction experimental searches started to the two new particles in particle colliders. The discovery of the bottom by Leon Lederman's team at Fermilab in 1977, strengthened the belief that their should be a sixth quark, the top. It was known that this quark would be heavier than the bottom requiring more energy the create in particle collisions, but the general expectation was that the sixth quark would soon be found. However, it took another 18 years before the existence of the top was confirmed.[5]

Early searches for the top quark at the Stanford Linear Accelerator Center and DESY in Hamburg can up empty-handed. When in the early eighties the Super Proton Synchrotron (SPS) at CERN discovered the W boson and the Z boson, it was again felt that the discovery of the top was imminent. As the SPS gained compitition from the Tevatron at Fermilab there was still no sign of the missing particle, and it was announced by the group at CERN that the top mass must be at least 41 GeV/c2. After a race between CERN and Fermilab to discover the top, the accelerator at CERN reached its limits without creating a single top pushing the lower bound on its mass up to 77 GeV/c2.[5]

For the foreseeable futute, Tevatron would be the only hadron collider powerful enough to potentially produce the top. In order to be able to confirm a future discovery a second detector D0 was added to the complex (in addition to the Collider Detector at Fermilab (CDF) already present). In October 1992, the two groups found their first hint of the top, with a single creation event that appeared to contain the top. In the following years more evidence was collected and on April 22, 1994 the CDF group published a paper presenting tentative evidence for the existence of a top quark with a mass of about 175 GeV/c2. In the meantime D0 had found no more evidence than the suggestive event in 1992. A year later after having gathered more evidence and a reanalysis of the D0 data (who had been searching for a much lighter top), the two groups jointly reported the discovery of the top with a certainty of 99.9998% at a mass of 176±18 GeV/c2.[6][7][5]

In the years leading up to the top quark discovery, it was realized that certain precision measurements of the electro-weak vector boson masses and couplings are very sensitive to the value of the top quark mass. These effects become much larger for higher values of the top mass and therefore could indirectly see the top quark even if it could not be directly produced in any experiment at the time. The largest effect from the top quark mass was on the T parameter and by 1994 the precision of these indirect measurements had led to a prediction of the top quark mass to be between 145 GeV/c2 and 185 GeV/c2. It is the development of techniques that ultimately allowed such precision calculations that led to Gerardus 't Hooft and Martinus Veltman winning the Nobel Prize in physics in 1999.

After the discovery of the first third-generation quark, an attempt was made to name it "beauty" and the predicted sixth quark "truth"; however, this later gave way to the names bottom and top.

[edit] Production and decay

As of 2008, Fermilab's Tevatron is the only place in the world where top quarks can be produced. Tevatron is an accelerator complex which collides protons and antiprotons at center-of-momentum energy of 1.96 TeV. There are two main top-production processes:

  • Pair production via strong interactions. This process was first observed simultaneously by two experimental collaboration at Fermilab, CDF and D0 in 1995.
  • Single production via weak interactions. In December 2006, three-sigma evidence for this production process was reported by the D0 Collaboration at Fermilab. In March 2009, both CDF and D0 announced discovery of single-top production.[8]

The top quark is expected to decay to a W boson and a down-type quark (down, strange or bottom). In the standard model, the branching fraction for t → W + q is predicted to be |Vtq|2, where Vtq is an element in the CKM matrix. The predictions for the branching ratios of the top quark are then B(t → W + d) ≈ 0.006%, B(t → W + s) ≈ 0.17% and B(t → W + b) ≈ 99.8%.

[edit] Top quark mass and relationship to the Higgs boson

The Standard Model describes fermion masses through the Higgs mechanism. The Higgs boson has a Yukawa coupling to the left- and right-handed top quarks. After electroweak symmetry breaking (when the Higgs acquires a vacuum expectation value), the left- and right-handed components mix, becoming a mass term.

\mathcal{L} = y_t h q u^c \rightarrow \frac{y_t v}{\sqrt{2}}( 1 + h^0/v) u u^c

The top quark Yukawa coupling has a value of y_t = \sqrt{2} m_t/v \simeq 1, where v=246~{\rm GeV} is the value of the Higgs vacuum expectation value.

[edit] Yukawa couplings

In the Standard Model, all of the quark and lepton Yukawa couplings are small compared to the top quark Yukawa coupling. Understanding this hierarchy in the fermion masses is an open problem in theoretical physics. Yukawa couplings are not constants and their values change depending on what energy scale (distance scale) at which they are measured. The dynamics of Yukawa couplings are determined by the renormalization group equation.

One of the prevailing views in particle physics is that the size of the top quark Yukawa coupling is determined by the renormalization group, leading to the "quasi-infrared fixed point."

The Yukawa couplings of the up, down, charm, strange and bottom quarks, are hypothesized to have small values at the extremely high energy scale of grand unification, 1015 GeV. They increase in value at lower energy scales, at which the quark masses are generated by the Higgs. The slight growth is due to corrections from the QCD coupling. The corrections from the Yukawa couplings are negligible for the lower mass quarks.

If, however, a quark Yukawa coupling has a large value at very high energies, its Yukawa corrections will evolve and cancel against the QCD corrections. This is known as a (quasi-) infrared fixed point. No matter what the initial starting value of the coupling is, if it is sufficiently large it will reach this fixed point value. The corresponding quark mass is then predicted.

The top quark Yukawa coupling lies very near the infrared fixed point of the Standard Model. The renormalization group equation is:

\mu \frac{\partial}{\partial\mu} y_t \approx \frac{y_t}{16\pi^2}\left(\frac{9}{2}y_t^2 - 8 g_3^2- \frac{9}{4}g_2^2 - \frac{17}{20} g_1^2 \right),

where g3 is the color gauge coupling and g2 is the weak isospin gauge coupling. This equation describes how the Yukawa coupling changes with energy scale μ. Solutions to this equation for large initial values yt cause the right-hand side of the equation to quickly approach zero, locking yt to the QCD coupling g3. The value of the fixed point is fairly precisely determined in the Standard Model, leading to a top quark mass of 230  GeV. However, if there is more than one Higgs doublet, the mass value will be reduced by Higgs mixing angle effects in an unpredicted way.

In the minimal supersymmetric extension of the Standard Model (the MSSM), there are two Higgs doublets and the renormalization group equation for the top quark Yukawa coupling is slightly modified:

\mu \frac{\partial}{\partial\mu} y_t \approx \frac{y_t}{16\pi^2}\left(6y_t^2 +y_b^2- \frac{16}{3} g_3^2- 3g_2^2 -\frac{13}{15} g_1^2 \right),

where yb is the bottom quark Yukawa coupling. This leads to a fixed point where the top mass is smaller, 170–200 GeV. The uncertainty in this prediction arises because the bottom quark Yukawa coupling can be amplified in the MSSM. Some theorists believe this is supporting evidence for the MSSM.

The quasi-infrared fixed point has subsequently formed the basis of top quark condensation theories of electroweak symmetry breaking in which the Higgs boson is composite at extremely short distance scales, composed of a pair of top and anti-top quarks.

[edit] Properties

  • At the current Tevatron energy of 1.96 TeV, top/anti-top pairs are produced with a cross section of about 7 picobarns (pb). The Standard Model prediction (at next-to-leading order with mt = 175 GeV/c2 is 6.7–7.5 picobarns.
  • Combining measurements from both CDF and D0, the most recent estimation of the top quark mass is 173.1±1.3 GeV/c2.[1]
  • Production of single top quarks through weak vector bosons is predicted in the Standard Model and has a cross section of 0.9 pb in the s-channel and 2.0 pb in the t-channel. On 8 December 2006, the D0 collaboration announced it had seen evidence for single top production at the 3 sigma level, measuring an s + t channel cross section of 4.9 pb.[9] A preprint article submitted to Physical Review Letters is available from the arXiv.org preprint server.[10] In March 2009, D0 and CDF released twin preprint papers on evidence for single top production at the 5 sigma level in the same s + t channel.[11][12]
  • The W bosons from top quark decays carry polarization from the parent particle, hence pose themselves as a unique probe to top polarization.
  • In the Standard Model, top quark is predicted to have a spin of 1⁄2 and charge +2⁄3. A first measurement of the top quark charge has been published, resulting in approximately 90% confidence limit that the top quark charge is indeed +2⁄3.[13]

[edit] Notes

  1. ^ a b c CDF Collaboration (2009). "Top Quark Mass". Fermilab. http://www-cdf.fnal.gov/physics/new/top/public_mass.html. Retrieved on 2009-04-02. 
  2. ^ S. Willenbrock, H.B Prosper (ed), B. Danilov (ed.) (2003). "The Standard Model and the top quark". Techniques and concepts of high-energy physics XII. NATO Science Series. 123. Kluwer Academic. ISBN 1402015909. http://books.google.com/books?id=HXm6M_YUzoYC&pg=PA1&lpg=PA1&dq=quark+%22standard+model%22&source=web&ots=sLXA9M6IBk&sig=Z6IEyScnONBbJKb0cOETBz9kzlI&hl=en#PPA1,M1. 
  3. ^ S. Abachi et al. (1995). "Search for High Mass Top Quark Production in pp Collisions at sqrt[s] = 1.8 TeV". Physical Review Letters 74: 2422–2426. doi:10.1103/PhysRevLett.74.2422. 
  4. ^ F. Abe et al. (1995). "Observation of Top Quark Production in pp Collisions with the Collider Detector at Fermilab". Physical Review Letters 74: 2626–2631. doi:10.1103/PhysRevLett.74.2626. 
  5. ^ a b c Tony M. Liss; Paul L. Tipton (September 1997). "The Discovery of the Top Quark". Scientific American: 54–59. 
  6. ^ Abe, F. (1995). "Observation of Top Quark Production in p[over ¯]p Collisions with the Collider Detector at Fermilab". Physical Review Letters 74: 2626. doi:10.1103/PhysRevLett.74.2626.  edit
  7. ^ Abachi, S. (1995). "Observation of the Top Quark". Physical Review Letters 74: 2632. doi:10.1103/PhysRevLett.74.2632.  edit.
  8. ^ Fermilab. "Fermilab collider experiments discover rare single top quark". http://www.fnal.gov/pub/presspass/press_releases/Single-Top-Quark-March2009.html. Retrieved on 2009-03-10. 
  9. ^ Fermilab (2006). "DZero finds evidence of rare single top quark". http://www.fnal.gov/pub/presspass/press_releases/Single-Top.html. Retrieved on 2009-04-03. 
  10. ^ V.M. Abazov et al. (D0 Collaboration) (2006). "Evidence for production of single top quarks and first direct measurement of |Vtb|". arΧiv: hep-ex/0612052 [hep-ex]. 
  11. ^ T. Aaltonen et al. (CDF Collaboration) (2009). "First Observation of Electroweak Single Top Quark Production". arΧiv: 0903.0885v1 [hep-ex]. 
  12. ^ V.M. Abazov et al. (D0 Collaboration) (2009). "Observation of Single Top Quark Production". arΧiv: 0903.0850v1 [hep-ex]. 
  13. ^ V.M. Abazov et al. (D0 Collaboration) (2006). "Experimental discrimination between charge 2⁄3 e top quark and charge 4⁄3 e exotic quark production scenarios". arΧiv: hep-ex/0608044 [hep-ex]. 

[edit] References

[edit] External links

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