Bottom-flavored hadrons from top-quark decay at next-to-leading order in the general-mass variable-flavor-number scheme

Abstract

We study the scaled-energy (x B ) distribution of bottom-flavored hadrons (B) inclusively produced in top-quark decays at next-to-leading order (NLO) in the general-mass variable-flavor-number scheme endowed with realistic, nonperturbative fragmentation functions that are obtained through a global fit to e + e − data from CERN LEP1 and SLAC SLC exploiting their universality and scaling violations. Specifically, we study the effects of gluon fragmentation and finite bottomquark and B-hadron masses. We find the NLO corrections to be significant. Gluon fragmentation leads to an appreciable reduction in the partial decay width at low values of x B . Hadron masses are responsible for the low-x B threshold, while the bottom-quark mass is of minor importance. Neglecting the latter, we also study the doubly differential distribution d 2 Γ/(dx B d cos θ) of the partial width of the decay t → bW + → Bℓ + ν ℓ + X, where θ is the decay angle of the charged lepton in the W -boson rest frame.PACS numbers: 12.38.Bx, 13.85.Ni, 14.40.Nd, 14.65.Ha Among other things, the CERN Large Hadron Collider (LHC) is a superlative top factory, producing about 90 million top-quark pairs per year of running at design energy √ S = 14 TeV and design luminosity L = 10 34 cm −2 s −1 in each of the four experiments [1]. This will allow us to determine the properties of the top quark, such as its mass m t , total decay width Γ t , branching fractions, and elements V tq of the Cabibbo-Kobayashi-Maskawa (CKM) [2] quark mixing matrix, with unprecedented precision. Due to its large mass, the top quark decays so rapidly that it has no time to hadronize and passes on its full spin information to its decay products. If it were not for the confinement of color, the top quark could, therefore, be considered as a free particle. Due to |V tb | ≈ 1, top quarks almost exclusively decay to bottom quarks, via t → bW + .On the other hand, bottom quarks hadronize, via b → B + X, before they decay, so that the decay process t → BW + + X is of prime importance, and it is an urgent task to predict its partial decay width as realistically and reliably as possible. Of particular interest are the distribution in the scaled B-hadron energy x B in the top-quark rest frame, and, in the case of leptonic W -boson decays W + → ℓ + ν ℓ , the one in the charged-lepton decay angle θ in the W -boson rest frame. In fact, the x B distribution provides direct access to the B-hadron fragmentation functions (FFs), and the cos θ distribution allows us to analyze the W -boson polarization and so to further constrain the B-hadron FFs by exploiting x B distributions for all the W -boson polarization states. B mesons are, for instance, cleanly identified by their decays to J/ψ mesons, which are easy to tag through their spectacular decays to e + e − and µ + µ − pairs.The theoretical aspects of top-quark physics at the LHC are nicely summarized in a recent review paper [3]. In the approximation of treating the bottom quark as a stable final-state particle that does not hadronize, the QCD co...

$\mathcal{O}(\alpha_{s})$ corrections to the B-hadron energy distribution of the top decay in the general two Higgs doublet model considering GM-VFN scheme

“…Our results in Refs. [4,7] and the present work both will be able to test the universality of the B-hadron fragmentation functions and provide a clean method to gauge the normalization of Monte Carlo event generators.…”

“…We have studied these dominant decay modes along the lines of Ref. [4]. In the theories beyond the standard model including the two-Higgs-doublet, the top quarks also decay into a charged Higgs and a bottom quark thus it may be useful to also use t → H + + B events for a cross check.…”

“…where λ(x, y, z) = (x − y − z) 2 − 4yz is the Källén function and for model I, one has 4) and for model II…”

“…In Ref. [4], we studied the B-meson energy distribution produced in top decay considering both the bottom quark and the gluon fragmentations. We also studied the angular distribution of the W-boson decay products in the decay chain t → bW + → Bl + ν l + X.…”

QCD analysis of light charged Higgs production through polarized top quark decay in two different frames

“…In [28][29][30][31][32], we investigated the energy distribution of B-mesons produced in polarized and unpolarized top quark decays in the SM. By ignoring the interference contribution between the SM top decay, and the 2HDM top decay into charged Higgs, in each top decay (polarized or unpolarized) to obtain the total distribution of the B-hadron energy two contributions due to the decay modes t → bH + (in the 2HDM) and t → bW + (in the SM) should be summed up.…”