Zong-Guo Si

Mixed QCD and weak corrections to top quark pair production at hadron colliders

Abstract The order α s 2 α mixed QCD and weak corrections to top quark pair production by quark–antiquark annihilation are computed, keeping the full dependence on the t and t ¯ spins. We determine the contributions to the cross section and to single and double top spin asymmetries at the parton level. These results are necessary ingredients for precise standard model predictions of top quark observables, in particular of top-spin induced parity-violating angular correlations and asymmetries at hadron colliders.

Correlative signatures of heavy Majorana neutrinos and doubly-charged Higgs bosons at the Large Hadron Collider

Abstract We explore an intriguing possibility to test the type-II seesaw mechanism of neutrino mass generation at the Large Hadron Collider (LHC). We show that the lepton-number-violating signatures of heavy Majorana neutrinos N i (for i = 1 , 2 , 3 ) and doubly-charged Higgs bosons H ± ± can be closely correlated with each other in a class of TeV-scale type-II seesaw models. Taking the minimal version of such models for example, we calculate the cross sections of p p → l α ± l β ± X processes mediated separately by N 1 and H ± ± , and illustrate their nontrivial correlation at the LHC.

NRQCD factorization for twist-2 light-cone wave-functions of charmonia

We show that the twist-2 light-cone wave-functions of eta(c) and J/psi can be factorized with nonrelativistic QCD (NRQCD) at the one-loop level, where the nonperturbative effects are represented by NRQCD matrix elements. In quarkonium at rest the c- or (c) over bar -quark moves with a small velocity v. The factorization is achieved by expanding the small velocity v. To the leading order of v, the twist-2 light-cone wave-functions of eta(c) and J/psi can be factorized as the product of a perturbative function and an NRQCD matrix element. The perturbative function is calculated at the one-loop level and is free from any soft divergences. Our results can be used for exclusive productions of quarkonia, which are studied in an experiment with two B-factories. (c) 2007 Elsevier B.V. All rights reserved.

Factorization approach for inclusive production of doubly heavy baryon

We study inclusive production of doubly heavy baryon at a e(+)e(-) collider and at hadron colliders through fragmentation. We study the production by factorizing nonpertubative and perturbative effects. In our approach the production can be thought as a two-step process: a pair of heavy quarks can be produced perturbatively and then the pair is transformed into the baryon. The transformation is nonperturbative. Since a heavy quark moves with a small velocity in the baryon in its rest frame, we can use NRQCD to describe the transformation and perform a systematic expansion in the small velocity. At the leading order we find that the baryon can be formed from two states of the heavy-quark pair, one state is with the pair in S-3(1) state and in color 3, another is with the pair in S-1(0) state and in color 6. Two matrix elements are defined for the transformation from the two states, their perturbative coefficients in the contribution to the cross section at a e+e- collider and to the function of heavy quark fragmentation are calculated. Our approach is different than previous approaches where only the pair in S-3(1) state and in color is taken into account. Numerical results for e(+)e(-) colliders at the two B-factories and for hadronic colliders LHC and Tevatron are given

Testing the realistic seesaw model with two heavy Majorana neutrinos at the CERN Large Hadron Collider

In the conventional type-(I+II) seesaw model, the effective mass matrix of three known light neutrinos is given by M(v) = M(L) - M(D)M(R)(-1)M(D)(T) in the leading-order approximation. We propose an intriguing scenario, in which the structural cancellation condition M(D)M(R)(-1)M(D)(T) = 0 is guaranteed by the A(4) x Z(2) flavor symmetry. As a consequence, neutrino masses are mainly generated by the Higgs triplet M(v) = M(L), while the neutrino mixing matrix is non-unitary and takes on the nearly tri-bimaximal pattern. A discriminating feature of this scenario from the pure type-II seesaw model is that the lepton-number-violating signatures induced by the heavy Majorana neutrinos can be discovered at the CERN Large Hadron Collider. We calculate the total cross section of the same-sign dilepton events pp -> l(alpha)(+/-)N(i) -> l(alpha)(+/-)l(beta)(+/-)jj (for i = 1, 2 and alpha, beta = e, mu, tau), and emphasize the significant interference of the contributions from two different heavy Majorana neutrinos. The background from the standard model and the kinematic cuts used to reduce it have been considered. The possible way to distinguish between the signals from heavy Majorana neutrinos and those from doubly-charged Higgs bosons is briefly discussed

TOP QUARK PAIR PRODUCTION AND DECAY INCLUDING SPIN EFFECTS AT HADRON COLLIDERS: PREDICTIONS AT NLO QCD

Top quark-antiquark pairs will be produced copiously at the Tevatron collider and in huge numbers at the LHC. This will make possible detailed investigations of the properties and interactions of this quark flavor. The analysis and interpretation of future data requires precise predictions of the hadronic production of pairs and of their subsequent decays. In this talk the reactions , are considered and results are presented of our calculation6 of the dilepton angular distribution at next-to-leading order QCD, keeping the full dependence on the spins of the intermediate state. The angular distribution is determined for different choices of reference axes that can be identified with the t and spin axes. While the QCD corrections to the leading-order distribution turn out to be small in the case of the LHC, we find them to be sizeable in the case of the Tevatron and find, moreover, the angular distribution to be sensitive to the parton content of the proton.

MIXED QCD AND WEAK CORRECTIONS TO

We report on our computation of the mixed QCD and weak corrections to including t, spin effects, in particular on single top spin asymmetries.

t\bar{t}

Abstract Top quark pair production by (polarized) γγ collisions offers an interesting testing ground of the Standard Model and its extensions. In this Letter we present results for differential cross sections of top quark pair production and decay including QCD radiative corrections. We take into account the full dependence on the top quark spins. We give analytic and numerical results for single and double differential angular distributions of t t ¯ decay products which are due to top quark polarizations and spin correlations in the intermediate state.

PRODUCTION BY QUARK-ANTIQUARK ANNIHILATION

Abstract A strict method to calculate the color rearrangement probability at partonic level in hadronic W + W − decays is proposed. The color effective Hamiltonian H c is constructed from invariant amplitude for the process e + e − → W + W − → q 1 q 2 q 3 q 4 + ng ( n = 0, 1, 2, …) and is used to calculate the cross sections of various color singlets and the color rearrangement probability. The true meaning of the color rearrangement is clarified and its difference from color interference is discussed.

Top quark pair production and decay at a polarized photon collider

Abstract The true meaning of the color flow model (CFM) is elucidated in a strict PQCD formulation, where the model is shown to be only an approximation in describing the exact color structure of the multigluon system.