Study of single top production at high energy electron positron colliders

Eur. Phys. J. C Study of single top production at high energy electron positron colliders J. Fuster 0 I. Garca 0 P. Gomis 0 M. Perell 0 E. Ros 0 M. Vos 0 0 IFIC, University of Valencia-CSIC , Valencia , Spain The effect of single top production on the study of top quark pair production in future high energy electronpositron colliders is evaluated. The rate of the single top quark production process is sizeable throughout a large range of center-of-mass energies and the final state cannot easily be distinguished from the dominant pair production process. We discuss the impact on the top quark mass extraction from a scan through the pair production threshold and the determination of top quark form factors in the continuum. These results advocate for the exploration of the inclusive e+e W +bW b process, that includes both top quark pair and single top quark production. 1 Introduction A high-luminosity, high-energy, linear e+e collider yields excellent opportunities for precision tests of the Standard Model of particle physics. The combination of precisely calculable electroweak production and strict control of the initial state with the relatively benign experimental environment and state-of-the-art detector systems allow for a characterization of Standard Model and new physics processes with a precision that goes well beyond what can be achieved at hadron colliders. Two projects of linear electron-positron colliders are being considered: the International Linear Collider (ILC [1,2]) and the Compact Linear Collider (CLIC [3]). The physics case for a linear e+e machine has been made in great detail in Refs. [411]. The specific case of a multi-TeV e+e collider is discussed in Refs. [1214]. In both cases, the center-of-mass energy will exceed s = 350 GeV, the threshold for top quark pair production. Unlike other quarks, the top quark has never been produced in e+e machines, and therefore a precise measurement of electroweak top quark pair production is missing. The study of top quark properties is therefore one a e-mail: of the most exciting prospects for a future linear collider [15]. Detailed full-simulation studies have been made of the prospects for a precise top quark mass measurement [16,17] and characterization of the t tZ and t t vertices [18]. Single top production, through the e+e W t b, W +tb process depicted in the central panel of Fig. 1, is abundant at e+e colliders that operate at s > 300 GeV. Note that for the t bW decay, this process gives rise to the same W +bW b final state as top pair production. Using MADGRAPH we find ten leading-order diagrams of this type, against two for top quark production. A third group of processes gives rise to the same final state: W W Z , W W h and W W production, with Z / / h bb. At treelevel there are approximately 50 diagrams that produce two W -bosons and two b-quarks that do not involve top quarks. Ultimately, all three processes yield the same set of six-fermion final states. In many studies of the linear collider prospects for top physics, single top quark production has been neglected. Notable exceptions are found in Refs. [19,20]. A fully consistent analysis of the inclusive e+e W +bW b is currently impossible. Even if event generation for the ILC TDR routinely included the full 2 6 matrix element at leading order, higher-order corrections for this process are not available for the extraction of top quark properties and couplings. In the continuum, results for the cross-section of top quark pair production are available at NNLO. Cross-sections for the process W +bW b are for the moment only available at LO, but are required at least at NLO (note that NLO calculations are already available for the LHC). In this note we investigate the impact of single top events in the study of top quark pair production and propose a more inclusive experimental strategy that compares precise predicFig. 1 Feynman diagrams for top quark pair production at a linear collider (e+e Z / tt, left panel), single top production (e+e W tb, W +tb, central panel), and triple gauge boson production (e+e W +W Z , right panel) 150 200 250 300 invariant mass hadronic top candidate [GeV] Fig. 2 Reconstructed invariant mass at thruth level of the hadronic versus the leptonic side of tt W +bW b events. The events cluster around the input top mass value, but one can observe a significant amount of off-shell events 2 Distinguishing single top from top quark pair production As top quark pair production and single top quark production give rise to the same six-fermion final state, the question arises as to how one can distinguish both sources. At a fundamental level the single top and top quark pair production processes are entangled by interference between the different diagrams. No algorithm can ever separate them fully. However, one could hope to use some of the marked features of the e+e t t process to make it stand out among the other processes that give rise to the W +bW b final state. One could then hope to isolate samples that are enriched in top quark pairs or single top quark events. In Fig. 2 we present the invariant mass of the W +b and W b combinations at truth level using e+e t t 200 250 300 energy hadronic top candidate [GeV] Fig. 3 Reconstructed beam energy at thruth level of the hadronic versus the leptonic side of W +bW b events. The center-of-mass energy is 500 GeV, so double-top events cluster at beam energies of 250 GeV. In single-top events, the beam energy follows the diagonal Elep + Ehad = 500 GeV W +bW b events generated with WHIZARD [21,22] at s = 500 GeV, including the effect of initial state radiation and the beam energy spread expected at the ILC. The majority of events is found around mW +b mW b mt . However, in a significant fraction of events one of the W boson b-quark pairs has an invariant far from the top quark mass. The cross is populated by a mixture of t t events with an off-shell top quark and genuine single-top production through diagrams such as that in the central panel of Fig. 1 In Fig. 3 we present the reconstructed beam energy of the W b decaying leptonically versus the W b decaying hadronically. The energy of the W b pair tends to cluster at about half the center-of-mass energy, as expected for double-top events. We observe however a significant fraction of events along the diagonal Elep + Ehad = s. Those are mostly single top events. This figure suggests another potential criterium for partial separation of single and double-top events using the reconstructed beam energy. These figures suggest an (ad-hoc) truth-level categorisation of events according to the number of on-shell top quark candidates, that is used to quantify the non-t t contribution to e+e W +bW b production in the remainder of this paper. We consider the event to correspond to top quark pair production whenever the following relation is satisfied for both W b combinations: mW b mtMC < 15 GeV Events that meet (...truncated)