Chinese Physics C (HEP & NP) Vol. 32, No. 9, Sep., 2008

Single top production associated with a neutral scalar at LHC in topcolor-assisted

LIU Guo-Li(7Iw)1 ZHANG Huan-Jun(܋)2;1) 1 (Department of Physics, Zhengzhou University, Henan 450052, China) 2 (Department of Physics, Henan Normal University, Xinxiang 453007, China)

Abstract The topcolor-assisted technicolor (TC2) model predicts a number of neutral scalars like the top-pion 0 0 (πt ) and the top-Higgs (ht ). These scalars have flavor-changing neutral-current (FCNC) couplings, among which the top-charm transition couplings may be sizable. Such FCNC couplings induce single top productions associated with a neutral scalar at the CERN (LHC) through the parton → 0 → 0 processes cg tπt and cg tht . In this paper we examine these productions and find their production rates can exceed the 3σ sensitivity of the LHC in a large part of parameter space. Since in the Standard Model and the minimal supersymmetric model such rare productions have unobservably small production rates at the LHC, these rare processes will serve as a good probe for the TC2 model.

Key words top quark, technicolor, LHC

PACS 14.65.Ha, 12.60.Fr, 12.60.Jv

As the heaviest in the Standard Model ated with a neutral scalar at the LHC through the 0 0 (SM), the top quark may be a sensitive probe of new parton processes cg tπt and cg tht . In this pa- [1] → → physics . So far there remain plenty of room for new per we examine these productions and figure out if physics in top quark sector due to the small statis- their rates can exceed the 3σ sensitivity of the LHC. tics of the top quark events at the Fermilab Tevatron Since in the SM and the MSSM such rare produc- collider[2]. Since the upcoming Large Hadron Collider tions have unobservably small production rates at the (LHC) at CERN will produce top quarks copiously LHC, these rare processes will serve as a probe for the and allow to scrutinize the top quark nature, the new TC2 model if their TC2 predictions can be above the physics related to the top quark will be either uncov- 3σ sensitivity. ered or stringently constrained. Before our calculations we recapitulate the basics One of the properties of the top quark in the of TC2 model. The TC2 model[8] combines techni- Standard Model (SM) is its extremely small flavor- color interaction with topcolor interaction, with the changing neutral-current (FCNC)[3] interactions due former being responsible for electroweak symmetry to the GIM mechanism. Thus, the observation of breaking and the latter for generating large top quark any FCNC top quark process would be a robust evi- mass. The top quark mass is generated from two dence for new physics beyond the SM. Actually, the sources, one is from the extended technicolor (propor- FCNC top quark interactions can be significantly en- tional to ) and the other from the topcolor (propor- hanced in some new physics models, such as the popu- tional to 1 ). So the mass matrix of up-type quarks [4—6] − lar minimal supersymmetric model (MSSM) and is composed of both the extended technicolor and the the topcolor-assisted technicolor (TC2) model[7]. topcolor contributions. The diagonalization of this The TC2 model predicts a number of neutral mass matrix will induce FCNC top quark interactions scalar bosons like the top-pions and top-Higgs at the in the Yukawa couplings which involve the compos- weak scale[8]. These scalars have FCNC top couplings ite scalars respectively from topcolor and technicolor at tree-level, among which the top-charm FCNC cou- condensations. plings are most significant. Such anomalous FCNC The top-charm FCNC couplings with the top-pion couplings will induce single top productions associ- and top-Higgs can be written as[7]

Received 8 November 2007 1) E-mail: [email protected] 697 — 699 698 Chinese Physics C (HEP & NP) Vol. 32

2 2 (1 )mt v Ft scale µF chosen to be µR = µF = mt + MS (MS de- FCNC = − − L √2Ft p v × notes the top-pion mass or the top-Higgs mass). tt∗ tc ¯ 0 tt∗ tc ¯ 0 The parameters involved in our calculations are (iKUL KURtLcRπt +KULKURtLcRht +h.c.), (1) the masses of the top-pions and top-Higgs, the pa- tc rameter KUR, the top-pion decay constant Ft and the where the factor v2 F 2/v (v 174 GeV) reflects − t ≈ parameter  which parametrizes the portion of the the effect of the pmixing between the top-pions and extended-technicolor contribution to the top quark [9] the would-be Goldstone bosons . KUL and KUR are mass. In our numerical calculations, we take Ft = the rotation matrices that transform respectively the tt tt 50 GeV,  = 0.1, KUL = 1, KUR = 0.9 and re- weak eigenstates of the left-handed and right-handed tc tain KUR as a free parameter with a value less than up-type quarks to their mass eigenstates, which can √2 2 = 0.43. The top quark mass is taken as [7] − [2] be parametrized as mt = 170.9 GeV . 0 tt tt mt For the masses of the neutral top-pion and top- KUL 1, KUR mt = 1 , ≈ ≈ − (2) Higgs, current constraints are rather weak. Theoret- tc tt 2 2 KUR 6 1 (KUR) = √2  , ically the top-pion masses are model-dependent and − − [8] 0 p are usually of a few hundred GeV . The top-Higgs with m denoting the topcolor contribution to the top t mass, as analysed in Ref. [7], has a lower bound of quark mass. In Eq. (1) we neglected the mixing be- about 2m , which however is an approximate analy- tween up quark and top quark. t sis and the mass below t¯t threshold is also possible[11]. As shown in Fig. 1, these FCNC interactions in Experimentally, the neutral top-pion mass can be Eq. (1) induce single top productions associated with constrained if we assume the degeneracy of neutral a top-pion or a top-Higgs at the LHC through the and charged top-pion masses (the mass splitting be- parton processes cg tπ0 or cg th0. The ampli- → t → t tween the neutral top-pion and the charged top-pion tude for cg tπ0 is given by → t comes only from the electroweak interactions and 2 2 1 mt v Ft tt∗ tc thus should be small). The charged top-pion mass is = gs − KURKURu¯t + M − √2 Ft p v × constrained from the absence of t πt b, which gives [12] → a lower bound of 165 GeV , and also from Rb data, 1 µ 1 µ (p + p )γ + γ (p p +m ) P u , [13]  sˆ 6 c 6 g uˆ m2 6 t− 6 g t  R c which yields a lower bound of about 250 GeV . In − t (3) our numerical results we will show the dependence on the masses of neutral top-pion and top-Higgs. where PR = (1+γ5)/2, and pt,c,g are the momenta of In the following we present some results for the top quark, charm quark and gluon, respectively. hadronic production cross section via cg tπ0. These → t results are also applicable to the production through cg th0, with the top-pion mass replaced by the → t top-Higgs mass.

Fig. 2. Hadronic cross section for the produc- → 0 Fig. 1. Feynman diagrams for parton-level pro- tion via cg tπt at the LHC versus the top- → 0 → 0 pion mass. cess cg tπt and cg tht . Figure 2 shows that the hadronic cross section ver- tc The hadronic cross section at the LHC is obtained sus the top-pion mass for different values of KUR. We by convoluting the parton cross section with the par- see that the cross section increases with the increasing tc ton distribution functions. In our calculations we use KUR since the cross section is simply proportional to [10] tc 2 CTEQ6L to generate the parton distributions with (KUR) as shown in Eq. (3). As the top-pion mass in- the renormalization scale µR and the factorization creases, the cross section decreases. The cross section No. 9 LIU Guo-Li et alµSingle top production associated with a neutral scalar at LHC in topcolor-assisted technicolor 699 is about several hundreds fb in most of the parameter also inaccessible at the LHC. So these rare processes space. will serve as a good probe for the TC2 model. Due to the large QCD backgrounds at the LHC, for the productions of PP tφ0 + X (φ0 is a neu- → tral scalar and can be top-pion or top-Higgs) we search for the final states from the subsequent decays t Wb `νb (` = e,µ) and φ0 bb.¯ So the main → → → SM background is the production of t¯t and Wbb¯jj, where one light jet is mis-identified as a b-jet while the other light jet is not detected if it goes along the pipeline or its transverse momentum is too small. The observability of the signal at the LHC has been investigated in the effective Lagrangian approach[14]. Assuming a luminosity of 100 fb−1, we know from Table 2, Table 4 and Eq. (4) in Ref. [14] Fig. 3. The contour of 3σ sensitivity (200 fb) that the 3σ sensitivity for the production of PP → for the cross section of the production via φ0 → 0 Ktc t + X is about 200 fb. Although this sensitivity is cg tπt at the LHC in the plane of UR based on the effective Lagrangian approach and may versus the top-pion mass. not be perfectly applicable to a specified model, we can take them as a rough criteria to estimate the ob- In conclusion, we examined the single top pro- servability of these channels. ductions associated with a neutral scalor (top-pion To show the observability of the production of or top-Higgs) at the LHC in topcolor-assisted tech- PP tπ0 + X, we plot in Fig. 3 the contour of the nicolor model. We found that their production rates → t cross section of the 3σ sensitivity (200 fb) in the plane can exceed the 3σ sensitivity of the LHC in a large tc 0 part of parameter space. Since in the Standard of KUR versus mπt . We see that in a large part of the parameter space the cross section can exceed the 3σ Model and the minimal supersymmetric model such sensitivity. As we mentioned earlier, the production rare productions have unobservably small production PP th+X is unobservably small in the SM due to rates at the LHC, these rare processes will serve as → the extremely suppressed ht¯c coupling. In the MSSM a good probe for the topcolor-assisted technicolor the PP th+X has a rate lower than 10 fb[6] and thus model. →

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