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Probing Topcolor-Assisted Technicolor from Top-Charm Associated Production at LHC

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Probing Topcolor-Assisted Technicolor from Top-Charm Associated Production at LHC Probing Topcolor-Assisted Technicolor from Top-Charm Associated Production at LHC Junjie Caoa;b, Zhaohua Xiong c, Jin Min Yang b a CCAST (World Laboratory), P.O.Box 8730, Beijing 100080, China b Institute of Theoretical Physics, Academia Sinica, Beijing 100080, China c Graduate School of Science, Hiroshima University, Hiroshima 937-6256, Japan (April 2, 2003) tc bb + +√2KUR∗ KDLc¯RbLπt + h:c: ; (1) We propose to probe the topcolor-assisted technicolor i (TC2) model from the top-charm associated productions at where v 174 GeV, F 50 GeV is the top-pion decay the LHC, which are highly suppressed in the Standard Model. t constant' and the factor' v2 F 2=v reflects the effect Due to the flavor-changing couplings of the top quark with − t the scalars (top-pions and top-Higgs) in TC2 model, the of the mixing between thep top-pions and the would-be top-charm associated productions can occur via both the s- goldstone bosons [8]. KUL, KDL and KUR are the rota- channel and t-channel parton processes by exchanging a scalar tion matrices that transform the weak eigenstates of left- field at the LHC. We examined these processes through Monte handed up-type, down-type and right-handed up-type Carlo simulation and found that they can reach the observ- quarks to the mass eigenstates, respectively. As pointed able level at the LHC in quite a large part of the parameter tc out in [6], the transition between tR and cR, KUR,canbe space of the TC2 model. naturally around 10% 30% without conflict with low energy experimental data∼ and Ks can be parameterized 14.65.Ha, 12.60.Fr, 12.60.Jv as follows: Ktt = Kbb =1;Ktt =1 , The fancy idea of technicolor (TC) provides a possi- UL DL DR − ble mechanism of breaking the electroweak symmetry dy- tc tt 2 tu 2 2 KUR = 1 KUR KUR 2 ; (2) namically. However, it is hard for technicolor to generate q − − ≤ p − the fermion masses, especially the heavy top quark mass. with representing the fraction of top quark mass gen- As a realistic TC model, the topcolor-assisted technicolor erated from TC interactions. Throughout this paper, we (TC2) model [1] combines technicolor with topcolor, with fix Ktc = √2 2 and treat as an input parameter UR − the former mainly responsible for electroweak symmetry in the range 0 0:1. TC2 model also predicts a CP-even ∼ breaking and the latter for generating a major part of scalar ht called top-Higgs [7]. Its couplings to quarks are top quark mass. This model is one of the promising can- similar to that of the neutral top-pion except that the didates of new physics, awaiting to be tested at the up- neutral top-pion is CP-odd. coming CERN Large Hadron Collider (LHC). g Ø Ø As shown in numerous previous studies, the top quark b processes are sensitive to new physics [2]. In some new Ø h Ø ¦ Ø Ø ¼ physics models like supersymmetry, there may emerge Ø Ø some new production and decay mechanisms for the top c c g quark at hadron colliders [3–5]. The study of these new b aµ ´bµ mechanisms will help to reveal the new physics effects. ´ It is noticeable that the TC2 model may have richer FIG. 1. Feynman diagrams contributing to tc¯ associated top-quark phenomenology than other new physics mod- production in TC2 model. els since it treats the top quark differently from other quarks to single out top quark for condensation [1]. In fact, the top-color interaction is flavor non-universal and The above large Yukawa couplings will induce the top- consequently may induce new large flavor-changing (FC) charm associated production pp tc¯ + X through both 0 → interactions [6,7]. One kind of such FC interactions occur the s-channel gg πt ;ht tc¯, as shown in Fig.1(a), → → + between quarks and the top-pion, the pseudo-goldstone and the t-channel bb tc¯ by exchanging a π ,asshown → t boson predicted by TC2 model with mass of a few hun- in Fig.1(b). Compared to the t¯b productions, which is dred GeV, which are given by [1,6] also sensitive to TC2 [6], the tc¯ productions are highly suppressed in the SM and thus its observation would be m v2 F 2 a robust evidence of new physics. t t iKtt Ktt t¯ t π0 = − UR UL∗ L R t The tc¯ production through the s-channel gg h tc¯ L √2Ft p v t has been briefly studied in the literature [7].→ But→ a +√2Ktt Kbb t¯ b π+ + iKtc Ktt t¯ c π0 UR∗ DL R L t UR UL∗ L R t detailed Monte Carlo study of its observability, with 1 the consideration of the SM backgrouds, is still lacking. s-andt-channel productions, one sees that for a com- Given the great importance of the LHC phenomenology, mon scalar mass the s-channel rate is higher than the such a study of observability is absolutely necessary. Fur- t-channel rate only in the range from mt to 400 GeV. thermore, the tc¯ production via the t-channel bb tc¯, From this figure one can also see that the cross section which is more important when the top-pion is heavy,→ has of pp π0 tc¯ is at least a couple of times larger than → t → not been studied before. In this work we will give a that of pp h tc¯ for m 0 = m . t πt ht comparative study of both processes. We will not only → → study the production rates, but also perform a Monte Carlo simulation to show explicitly the observability at - the LHC. Our result shows that both the s-channel and LHC: pp→tc+X t-channel processes can reach the observable level at the TC2 Model LHC in quite a large part of the parameter space. There- ε = 0.03 fore, looking for the top-charm associated productions at 10 4 the LHC will serve as an important probe for the TC2 model. The spin- and color-averaged amplitudes of the s- channel and t-channel processes are given by (fb) 3 σ 10 2 2 2 2 π0 1 m v F αs bb- t 2 = t t (2 2)(1 )2 → s 2 −2 tc- |M | 64 2Ft v π − − gg 2 2 2 → gg π 0 sˆ mt (ˆs mt ) 2 c ; (3) 10 → t → 1 2 2 − 2 2 tc- m (ˆs m )2 + m Γ h → × t π0 π0 0 t πt tc- − 2 2 2 2 1 m v F αs ht 2 = t t (2 2)(1 )2 s 2 −2 |M | 64 2Ft v π − − 100 250 400 550 700 850 1000 2 m (GeV) sˆ m2(ˆs m2) scalar c t t ; (4) 2 2 2 2− 2 2 × mt (ˆs m ) + m Γ − ht ht ht FIG. 2. Cross sections of top-charm associated production 2 2 2 2 π+ m v F as a function of corresponding scalar mass for different parton t 2 = t − t (2 2)(1 )2 t 2 2 processes at the LHC. |M | 2Ft v − − tˆ(tˆ m2) − t ; (5) ˆ 2 2 ×(t m + ) Under the assumption that the top quark decays via − π the normal weak interactions to Wb, the final state of tc¯ where α = g2=(4π)withg denoting strong coupling s s s production is Wbc¯. We look for events with the leptonic constant,s ˆ and tˆ are patron level Mandelstam variables. decay of the W , W `ν¯ (` = e or µ) and thus the signa- 0 Γπ (Γht ) is the width of neutral top-pion (top-higgs) → t ture of tc¯ production is an energetic charged lepton, one which can be calculated by considering all its decay b-quark jet, one light c-quark jet, plus missing E from modes. c (R) are loop functions defined by c (R)= T 1;2 1 the neutrino. The potential SM backgrounds are the sin- 1 ln (1 Rx(1 x)) 4 dx − − and c (R)= 2+(1 )c (R). Note 0 x 2 R 1 gle top productions, top pair (tt¯) productions, and Wb¯b, − π0 − R t ht there is no interference between Ms and Ms due to Wcc¯, Wcj and Wjj productions. These backgrounds 0 different CP property of πt and ht. In our numerical cal- have been studied extensively in [11]. In order to use the culation, we use the CTEQ5L parton distribution func- background results in [11], in our analysis we applied the tions [9] with Q = √s=ˆ 2. same selection cuts as in [11]. In Fig.2 we plotted the two-body tc¯ production cross First, we assumed silicon vertex tagging of the b-quark section versus corresponding scalar mass for both the s- jet with 60% efficiency and the probability of 0.5% (15%) channel and t-channel processes. The charge conjugate for a light quark (c-quark) jet to be mis-identified as a production channel, i.e., the tc¯ production, is also in- b-jet, which can reduce the background Wjj efficiently. cluded in our analysis throughout this paper. We required the reconstructed top quark mass M(bW ) We see from Fig.2 that for the t-channel process the to lie within the mass range M(bW ) mt < 20 GeV, production rate drops monotonously with the increase which can reduce all the non-top| backgrounds− | efficiently. of the charged top-pion mass. For the s-channel pro- To simulate the detector acceptance, we made a se- cesses the production rates are maximum when the neu- ries of basic cuts on the transverse momentum (pT ), the tral scalars lie in the range of m + m m 0 2m .
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