Status and Prospects of Top-Quark Physics

TL;DR

The paper surveys the status and prospects of top-quark physics, emphasizing the top quark’s exceptional mass, rapid decay, and central role in precision tests of the Standard Model and potential new physics. It reviews the SM framework for the top quark, indirect constraints from electroweak data, and direct measurements of mass, charge, and Yukawa coupling, as well as production and decay mechanisms at hadron colliders. It discusses the QCD-based factorization approach to top-quark production and the role of parton distribution functions in cross-section predictions, and it highlights the LHC as a prolific top-quark factory with stringent prospects for precision measurements. Overall, top-quark physics provides stringent SM consistency tests and a sensitive probe for beyond-the-Standard-Model phenomena.

Abstract

The top quark is the heaviest elementary particle observed to date. Its large mass of about 173 GeV/c^2 makes the top quark act differently than other elementary fermions, as it decays before it hadronises, passing its spin information on to its decay products. In addition, the top quark plays an important role in higher-order loop corrections to standard model processes, which makes the top quark mass a crucial parameter for precision tests of the electroweak theory. The top quark is also a powerful probe for new phenomena beyond the standard model. During the time of discovery at the Tevatron in 1995 only a few properties of the top quark could be measured. In recent years, since the start of Tevatron Run II, the field of top-quark physics has changed and entered a precision era. This report summarises the latest measurements and studies of top-quark properties and gives prospects for future measurements at the Large Hadron Collider (LHC).

Figures (5)

• Figure 1: Left: Table of lepton and quark properties such as electric charge and mass (in $\rm MeV/c^2$). The top quark is unique amongst all fermions due to its very large mass. (All elementary particles are point-like. The relative size of the drawn spheres merely symbolizes the mass of fermions, but does not scale linearly with the fermion mass.) Right: A fermion (quark or lepton) triangle diagram which potentially could cause an anomaly.
• Figure 2: Left: Virtual top quark loops contributing to the $W$ and $Z$ boson masses. Right: Virtual Higgs boson loops contributing to the $W$ and $Z$ boson masses.
• Figure 3: Left: Blueband plot, showing the indirect determination of the Higgs boson mass from all electroweak precision data together with the 95% C.L. limit on the Higgs boson mass from the direct searches in yellow lep_sm_higgstevatron_sm_higgs. Right: Lines of constant Higgs mass on a plot of $M_W$ vs. $m_t$. The dotted ellipse is the 68% C.L. direct measurement of $M_W$ and $m_t$. The solid ellipse is the 68% C.L. indirect measurement from precision electroweak data.
• Figure 4: History of the limits on or measurements of the top quark mass (updated April 2009 by C. Quigg from priv_comm_cquigg: $(\bullet)$ Indirect bounds on the top-quark mass from precision electroweak data; $(\blacksquare)$ World-average direct measurement of the top-quark mass (including preliminary results); $(\blacktriangle)$ published CDF and $(\blacktriangledown)$ DØ measurements; Lower bounds from $p\bar{p}$ colliders $\rm Sp\bar{p}S$ and the T evatron are shown as dash-dotted and dashed lines, respectively, and lower bounds from $e^+e^-$ colliders (P etra, T ristan, L ep and S lc) are shown as a solid light grey line.
• Figure :