Top Mass Measurement and Bottom Fragmentation at the Lhc *

We show some recent HERWIG results related to the top quark mass reconstruction at the LHC and discuss possible improvements for studies of the bottom quark fragmentation function in the top decay. The large amount of t ¯ t pairs which are expected at the future experiments at the LHC 1 will allow precise measurements of the top-quark properties. For the sake of an improved measurement of the top mass m t , a new approach has been recently suggested 2 , which consists of using the invariant-mass distributions of J/ψ + ℓ pairs, where the J/ψ is produced by the decay of a b-flavoured hadron and ℓ is the lepton coming from the W produced in the top decay t → bW. Even though the branching ratio for the process B → J/ψ is small, it has been shown that it provides a clean signal at the LHC and about 10 3 well-reconstructed final states are expected in one year of high luminosity. The m J/ψℓ distributions can then be compared with a template of shapes parametrized by the top mass and m t can be fitted. The expected experimental uncertainty, which is dominated by the statistical error, has been estimated to be ∆m t ≃ 1 GeV. This is a method which crucially relies on the Monte Carlo description of top production and decay and on the model which is used to simulate the b-quark hadronization. Standard Monte Carlo algorithms 3,4 simulate multiparton radiation in the soft or collinear approximation and leave regions of the phase space completely empty ('dead zones'). In particular, HERWIG simulates the top decay in the top rest frame. We do not have any soft-gluon radiation from the top quark, while the b quark is allowed to radiate gluons in the 'forward' hemisphere 0 < θ g < π/2, with θ g being the emission angle of the soft gluon with respect to the b direction 5. The following parton cascade satisfies the prescription of angular ordering 6. The total