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Probing Top Quark Polarization Effects in Topcolor-Assisted Technicolor Model

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Probing Top Quark Polarization Effects in Topcolor-Assisted Technicolor Model 1 31 ò 1 11 Ï p U Ô n Ø Ô n Vol. 31, No. 11 2007 c 11 HIGH ENERGY PHYSICS AND NUCLEAR PHYSICS Nov., 2007 Probing Top Quark Polarization Effects in Topcolor-Assisted Technicolor Model * GU Qin-Zhong1;2;1) JIANG Feng-Chun3 WANG Xue-Lei2 YANG Hua4 1 (Shangqiu Teachers College, Shangqiu 476000, China) 2 (Henan Normal University, Xinxiang 453007, China) 3 (Zhengzhou University of Light Industry, Zhengzhou 450002, China) 4 (Information Engineering University, Zhengzhou 450004, China) Abstract We study the polarization effects of the top-quark on the t¯t production cross sections at hadron colliders in the topcolor-assisted technicolor model . The MRS set A0 parton distributions and the helicity projection operators methods are used in the calculation. It is shown that the polarization effects of the top quark are too small to produce enough identified signals at the Tevatron while they can be large enough to be detected at the LHC with reasonable values of the parameters. The polarization effects at the LHC can reach 16%, therefore, they provide feasible tests of the topcolor-assisted technicolor model. Key words topcolor-assisted technicolor model, electroweak symmetry breaking, polarization 1 Introduction a center-of-mass energy (c.m.) 2TeV, which is now engaged in RUN II, has the significant potential to So far, the most unclear part of the Standard discover a light Higgs boson with mass up to about Model (SM) is its symmetry breaking sector. Study- Mh 6 130GeV in the SM or MSSM. However, it will ing the electroweak symmetry breaking (EWSB) have very little capability to determine the overarch- mechanism will be one of the most important tasks ing model that governs the EWSB if a Higgs or Higgs- in particle physics. On the theoretical aspect, some like candidate is observed. The proton-proton collider new physics models beyond the SM (such as the LHC, with 14TeV c.m. energy, will have a consid- minimal supersymmetric standard model (MSSM), erably expanded capability to discover and measure the topcolor-assisted technicolor (TC2)[1] model, the almost all the quantum properties of a SM Higgs of top sea-saw model[2] and extra dimensions[3] have any mass or several of the MSSM Higgs bosons over [5] been proposed. These fancy ideas can explain the the entire MSSM parameter space . On the other EWSB, and at the same time, avoid the shortcom- hand, the LHC also has the capability to detect the ings of triviality and unnaturalness arising from the signals of other new heavy Higgs-like particles in the elementary Higgs field in the SM[4]. On the experi- new physics models related to the EWSB (such as the mental aspect, the planned high energy and luminos- top-pions in the TC2 model). ity colliders are extremely well-suited to study the Among the various new physics models, the forefront problem of the EWSB. The hadron collid- topcolor-assisted technicolor(TC2) model is an at- ers play an important role among the various col- tractive idea that gives a reasonable explanation of liders. The proton-antiproton collider Tevatron with the EWSB and heavy top quark mass, furthermore, Received 25 January 2007, Revised 6 February 2007 * Supported by National Natural Science Foundation of China (10375017) 1) E-mail: [email protected] 1010 | 1015 1 11 Ï §µºÚ9Ï<óÚ.eº§4zAïÄ 1011 it is consistent with the current experiment. In the different polarized intermediate states can interfere. TC2 model, the topcolor interaction makes small con- For lighter quarks, this effect may be smeared heavily tribution to the EWSB, and gives rise to the main by the hadronization process. However, in the case part of the top quark mass (1 )m with a model of the top quark, because of its mass, the decay rate − t dependent parameter 0:03 6 6 0:1. The technicolor is much faster than the other quarks. The top quark interaction plays a main role in the breaking of elec- decays before the hadronization effect has the time to troweak gauge symmetry. To account for the explicit smear its polarization information at production[7, 8]. breaking of quark and lepton flavor symmetries, the This allows us to neglect the effect of hadronization. extended technicolor(ETC) was invented. The ETC Therefore, in this sense, this polarization effect also interaction gives rise to the masses of the ordinary provides a unique possibility of direct observation of fermions including a very small portion of the top the spin of top quark. quark mass mt. This kind of model predicts three On the other hand, studies of the contributions 0 CP odd top-pions (Πt , Πt ) and one CP even top- of s-channel new heavy gauge bosons and new vec- 0 Higgs-boson (ht ) with large Yukawa couplings to the tor resonances in TC models of masses ranging from third family. There are three significant characters in 600GeV to 1TeV to t¯t productions at the Tevatron the TC2 model: (1) There exist three typical physical have been given in Ref. [9], which show that the ef- particles, the top-pions, and the observation of top- fects are experimentally detectable. Another charac- pions can be regarded as the direct evidence of the teristic feature of the TC models is that most TC TC2 model. (2) Topcolor interaction is non-universal models predict certain PGBs of masses below 1TeV, and therefore does not possess a GIM mechanism. and the properties of the PGBs are different in dif- This is an essential feature of this kind of models due ferent models. Therefore, studying the effects of the to the need to single out the top quark for conden- PGB contributions in t¯t productions at high energy sation. Such non-universal gauge interaction results colliders can serve as good tests of TC models. The in the new flavor-changing coupling vertices (such as: effects of color-octet technipions 0a(a = 1; ;8) on ··· 0 ¯ Q Πt t¯c) when one writes the interaction in the quark tt productions at the Tevatron have been studied in mass eigen-basis. (3) The Yukawa couplings of the Ref. [10], and it shows that 0a can make impor- top-pions to the quarks are proportional to the quark tant contributions via the gluonQ fusion process due masses, therefore, the top quark should have the large to the large PGB-gluon-gluon coupling contributed coupling to the top-pions and the processes involving by the techniquark triangle-loop, and such effect can top quark should be very important. These charac- be tested by measuring the differential cross section. ters of the TC2 model can help us to search for the A more complete study of the PGB effects in the t¯t signals of the TC2 model and to distinguish the TC2 productions at the Tevatron and the LHC in the top- model from other new physics models. color-assisted multiscale TC model has been made in On one hand, the large mass of the top quark mea- Ref. [11] in which the contributions from the color- sured at Fermilab[6] allows to probe physics at high singlet technipion and the top-pion are included as energies where new physics is expected to appear as well, and the total effects are shown to be large well, and the top quark is expected to decay keeping enough to be experimentally detected. As a compar- its polarization[7]. Due to the confinement pheno- ison, in this paper, we extend the study of Ref. [11] 0 mena of the strong interaction, the spin of a quark is to include top-higgs (ht ) , furthermore, we also con- in general very difficult to measure directly. Even if sider the polarization effects of the top quark both at the quark is produced in a particular polarized state, the Tevatron and the LHC, and the MRS set A0 par- the spin information is quickly averaged out by the ton distributions[12] and helicity projection operators hadronization process. One of the important effects methods[13] are used in the calculation. Our calcu- of the polarizations of unstable particles is that the lation will show that, with reasonable values of the 1012 p U Ô n Ø Ô n ( HEP & NP ) 1 31 ò m L t K tt K tt∗ 0 parameters in TC2 model and the expected system- = tanβ i UR UL tLtRΠt + vw atic errors in the t¯t cross section measurements at the p2K tt∗K bb t b Π+ + LHC, experimentally testing TC2 model is possible, UR DL R L t K tc K tt∗ 0 p K tc∗K bb + therefore, the top quark polarization effect provides i UR UL tLcRΠt + 2 UR DLcRbLΠt + a feasible test of TC2 model. m∗ b 0 K tt K tt∗ 0 i bLbRΠt + UR UL tLtRht + mt m∗ 2 Calculations b 0 K tc K tt∗ 0 m bLbRht + UR UL tLcRht +h:c: : (2) t Where tanβ = (v =v )2 1, v 60|100GeV is the In this paper, we shall take into account the tree- w t − t ≈ p SM top-pion decay constant, vw=246GeV is the EWSB level SM amplitude Mtree and the PGBs contributed scale, Kij are the matrix elements of the unitary amplitude MPGB described in Ref. [14] and in Fig. 1. U;D matrix KU;D, from which the Cabibbo-Kobayashi- Maskawa (CKM) matrix can be derived as V = −1 tt KULKDL. Their values can be written as: KUL = bb tt tc 2 KDL 1, KUR = 1 ", KUR = p2" " . The ≈ ∗ − − mass mb is a part of b-quark mass which is in- duced by the instanton, and it can be estimated as[1]: 3κm m∗ = t 6:6κGeV with κ 1 to 10−1 as in QCD.
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