Measurements of the production, decay and properties of the top quark: a review

With the full Tevatron Run II and early LHC data samples, the opportunity for furthering our understanding of the properties of the top quark has never been more promising. Although the current knowledge of the top quark comes largely from Tevatron measurements, the experiments at the LHC are poised to probe top-quark production and decay in unprecedented regimes. Although no current top quark measurements conclusively contradict predictions from the standard model, the precision of most measurements remains statistically limited. Additionally, some measurements, most notably A _FB in top quark pair production, show tantalizing hints of beyond-the-Standard-Model dynamics. The top quark sample is growing rapidly at the LHC, with initial results now public. This review examines the current status of top quark measurements in the particular light of searching for evidence of new physics, either through direct searches for beyond the standard model phenomena or indirectly via precise measurements of standard model top quark properties.     

Status and prospects of top-quark physics

The top quark is the heaviest elementary particle observed to date. Its large mass of about 173 GeV/c² 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).     

Properties of the top quark

The top quark was discovered at the CDF and D0 experiments in 1995. As the partner of the bottom quark its properties within the Standard Model are fully defined. Only the mass is a free parameter. The measurement of the top quark mass and the verification of the expected properties have been an important topic of experimental top quark physics since. In this review the recent results on top quark properties obtained by the Tevatron experiments CDF and D0 are summarised. At the advent of the LHC special emphasis is given to the basic measurement methods and the dominating systematic uncertainties.     

Top quark forward-backward asymmetry and same-sign top quark pairs

The top quark forward-backward asymmetry measured at the Tevatron collider shows a large deviation from standard model expectations. Among possible interpretations, a nonuniversal Z' model is of particular interest as it naturally predicts a top quark in the forward region of large rapidity. To reproduce the size of the asymmetry, the couplings of the Z' to standard model quarks must be large, inevitably leading to copious production of same-sign top quark pairs at the energies of the Large Hadron Collider (LHC). We explore the discovery potential for tt and ttj production in early LHC experiments at 7-8 TeV and conclude that if no tt signal is observed with 1 fb?1 of integrated luminosity, then a nonuniversal Z' alone cannot explain the Tevatron forward-backward asymmetry.     

Detecting Toppion via a Flavor-Changing Single Top Quark Production Process

In the framework of topcolor-assisted technicolor (TC2) theory, we study a flavor changing single top quarkproduction process pp(pp) →Ⅱ0t → tc(tc) at upgraded Tevatron and LHC. The results show that with the flavor changingcoupling of neutral toppion to top and charm quark, toppion provides a large te-channel resonance effect. The signal ofsingle top production could be detected at LHC. Otherwise, the narrow peak in the invariant mass distribution could beclearly detected both at upgraded Tevatron and LHC. Therefore, such single top production process provides a uniqueway to test TC2 model via the detection of tc-channel neutral toppion contribution.     

Yukawa corrections to the charged Higgs boson production in association with the top quark at hadron colliders

We calculate the Yukawa corrections of order to charged Higgs boson production in association with a top quark at the Tevatron and the LHC. The corrections are not very sensitive to the mass of the charged Higgs boson and can exceed for low values of , where the contribution of the top quark is large, and high values of where the contribution of the bottom quark becomes large. These Yukawa corrections could be significant for charged Higgs boson searches based on this production process, particularly at the LHC where the cross section is relatively large. Received: 12 October 1999 / Revised version: 3 December 1999 / Published online: 6 April 2000     

Top quark from the Tevatron to LHC

The history of the detection of the heaviest elementary particle, the top quark, at the Tevatron accelerator using the CDF detector is presented. The basic channels of detecting the top quark and the methods of determining its mass are described. The importance and the strategy of measuring the top quark mass in various channels of the \(\mathop {tt}\limits^ - \)-pair decay at the Large Hadron Collider (LHC) with a cm energy of 14 TeV, an operation which is expected to begin in 2007, is discussed. The investigations into the physics of the top quark at the earliest stage of the LHC operation are described.     

Searching for Top Squark at Hadron Colliders

In this talk we briefly summarise our recent study (hep-ph/0007165) on searching for top squark at hadron colliders. The light top-squark (stop) if produced in hadron colliders in the form of the t1t1 pair and decaying through the likely decay chain t1→X+b followed by X→X0ff′, can mimic closely a top quark event when the mass of the stop is close to that of the top quark. Because of the much lower production rate, the stop event can be buried under the top quark event sample. In order to uncover the stop event, specific selection cuts need to be applied. Through Monte Carlo simulation with suitable kinematic cuts, we found that such stop event can be extracted from the top quark sample and detected by the top counting experiments in the upcoming upgraded Tevatron and LHC. However, because of the small statistics of the Run 1 of the Tevatron, the stop signal remains hidden at Run 1.     

NNLL threshold resummation for top-pair and single-top production

I discuss threshold resummation at NNLL accuracy in the standard moment-space approach in perturbative QCD for top-pair and single-top production. For top quark pair production I present new approximate NNLO results for the total cross section and for the top quark transverse momentum and rapidity distributions at 8 TeV LHC energy. I discuss the accuracy of the soft-gluon approximation and show that the NLO and NNLO approximate results from resummation are practically indistinguishable from exact NLO and partial NNLO results. For single top production I present new approximate NNLO results for the total cross sections in all three channels at the LHC and also for the top quark transverse momentum distributions in t-channel production and in top-quark associated production with a W boson. For both \(t\bar t\) and single-top production the agreement of theoretical results with LHC and Tevatron data is excellent.     

Probing top anomalous couplings at the Tevatron and the Large Hadron Collider

Chromomagnetic and chromoelectric dipole interactions of the top quark are studied in a model-independent framework. Limits are set on the scale of new physics that might lead to such contributions using latest Tevatron measurements of the t[`(t)]t{\bar t} cross-section. It is demonstrated that the invariant mass distribution is a sensitive probe. Prospects at the LHC are examined. It is shown that, for unitarized amplitudes, an increase in the LHC energy is of little importance, while the accumulation of luminosity plays a crucial role.     

Quantum effects on t\rightarrow H^{+}\,b in the MSSM: a window to “virtual” supersymmetry?

We analyze the one-loop effects (strong and electroweak) on the unconventional top quark decay mode within the MSSM. The results are presented in the on-shell renormalization scheme with a physically well motivated definition of . The study of this process at the quantum level is useful to unravel the potential supersymmetric nature of the charged Higgs emerging from that decay. As compared with the standard mode , the corrections to are large, slowly decoupling and persist at a sizeable level even for all sparticle masses well above the LEP 200 discovery range. As a matter of fact, the potential size of the SUSY effects, which amount to corrections of several ten percent, could counterbalance the standard QCD corrections and even make them to appear with the “wrong” sign. Therefore, if the charged Higgs decay of the top quark is kinematically allowed – a possibility which is not excluded by the existing measurements of the branching ratio at the Tevatron – it could be an invaluable laboratory to search for “virtual” supersymmetry. While a first significant test of these effects could possibly be performed at the upgraded Tevatron, a more precise verification would most likely be carried out in future experiments at the LHC. Received: 18 April 1997 / Revised version: 18 June 1997     

Top quark physics at hadron colliders

The top quark, discovered at the FERMILAB TEVATRON collider in 1995, is the heaviest known elementary particle. Today, ten years later, still relatively little is known about its properties. The strong and weak interactions of the top quark are not nearly as well studied as those of the other quarks and leptons. The strong interaction is most directly measured in top quark pair production. The weak interaction is measured in top quark decay and single top quark production, which remains thus far unobserved. The large top-quark mass of about 175 GeV/c² suggests that it may play a special role in nature. It behaves differently from all other quarks due to its large mass and its correspondingly short lifetime. The top quark decays before it hadronises, passing its spin information on to its decay products. Therefore, it is possible to measure observables that depend on the top quark spin, providing a unique environment for tests of the Standard Model and for searches for physics beyond the Standard Model. This report summarises the latest measurements and studies of top quark properties and rare decays from the TEVATRON in Run II. With more than 1 fb^-1 of luminosity delivered to each experiment, CDF and DO, top quark physics at the TEVATRON is at a turning point from first studies to precision measurements with sensitivity to new physics. An outlook onto top quark physics at the Large Hadron Collider (LHC) at CERN, planned to begin operation in the year 2007, is also given.     

Searching for anomalous top quark production at the early LHC

We present a detailed study of the anomalous top quark production with subsequent decay at the LHC induced by model-independent flavor-changing neutral-current couplings, incorporating the complete next-to-leading order QCD effects. Our results show that, taking into account the current limits from the Tevatron, the LHC with √s=7 TeV may discover the anomalous coupling at 5σ level for a very low integrated luminosity of 61 pb?1. The discovery potentials for the anomalous couplings at the LHC are examined in detail. We also discuss the possibility of using the charge ratio to distinguish the tug and tcg couplings.     

Hadronic top-quark pair production in association with a hard jet at next-to-leading order QCD: phenomenological studies for the Tevatron and the LHC

We report on the calculation of the next-to-leading order QCD corrections to the production of top–antitop-quark pairs in association with a hard jet at the Tevatron and at the LHC. Results for integrated and differential cross sections are presented. We find a significant reduction of the scale dependence. In most cases the corrections are below 20% indicating that the perturbative expansion is well under control. Moreover, the forward–backward charge asymmetry of the top quark, which is analyzed at the Tevatron, is studied at next-to-leading order. We find large corrections, suggesting that the definition of the observable has to be refined.     

Forward-backward asymmetry in top quark semileptonic decay

The semileptonic decay of the top quark t → bW⁺ → bl⁺ν_l is analyzed in the rest system of the W. The forward-backward asymmetry of the lepton l⁺ with respect to the heavy quark direction is defined and computed. It is argued that this observable will be an ideal tool to study top quark properties at Tevatron and LHC. Higher order QCD corrections are calculated and their structure is elucidated in some detail.     

Probing the top quark flavor-changing couplings at CEPC

We propose to study the flavor properties of the top quark at the future Circular Electron Positron Collider(CEPC) in China.We systematically consider the full set of 56 real parameters that characterize the flavor-changing neutral interactions of the top quark,which can be tested at CEPC in the single top production channel.Compared with the current bounds from the LEP2 data and the projected limits at the high-luminosity LHC,we find that CEPC could improve the limits of the four-fermion flavor-changing coefficients by one to two orders of magnitude,and would also provide similar sensitivity for the two-fermion flavor-changing coefficients.Overall,CEPC could explore a large fraction of currently allowed parameter space that will not be covered by the LHC upgrade.We show that the c-jet tagging capacity at CEPC could further improve its sensitivity to top-charm flavor-changing couplings.If a signal is observed,the kinematic distribution as well as the c-jet tagging could be exploited to pinpoint the various flavor-changing couplings,providing valuable information about the flavor properties of the top quark.     

Top quark production from black holes at the CERN LHC

LHC is expected to be a top quark factory. If the fundamental Planck scale is near a TeV, then we also expect the top quarks to be produced from black holes via Hawking radiation. In this Letter we calculate the cross sections for top quark production from black holes at the LHC and compare it with the direct top quark cross section via parton fusion processes at next-to-next-to-leading order (NNLO). We find that the top quark production from black holes can be larger or smaller than the pQCD predictions at NNLO depending upon the Planck mass and black hole mass. Hence the observation of very high rates for massive particle production (top quarks, Higgs or supersymmetry) at the LHC may be an useful signature for black hole production.     

Physics prospects at the hadron colliders

I start with a brief introduction to the elementary particles and their interactions, Higgs mechanism and supersymmetry. The major physics objectives of the Tevatron and LHC colliders are identified. The status and prospects of the top quark, charged Higgs boson and superparticle searches are discussed in detail, while those of the neutral Higgs boson(s) are covered in a parallel talk by R.J.N. Phillips at this workshop.     

Physics prospects at the hadron colliders

I start with a brief introduction to the elementary particles and their interactions, Higgs mechanism and supersymmetry. The major physics objectives of the Tevatron and LHC colliders are identified. The status and prospects of the top quark, charged Higgs boson and superparticle searches are discussed in detail, while those of the neutral Higgs boson(s) are covered in a parallel talk by R.J.N. Phillips at this workshop.     

Top quark production asymmetries A(FB)t and A(FB)ℓ

A large forward-backward asymmetry is seen in both the top quark rapidity distribution A(FB)(t) and in the rapidity distribution of charged leptons A(FB)(?) from top quarks produced at the Tevatron. We study the kinematic and dynamic aspects of the relationship of the two observables arising from the spin correlation between the charged lepton and the top quark with different polarization states. We emphasize the value of both measurements, and we conclude that a new physics model which produces more right-handed than left-handed top quarks is favored by the present data.