aa r X i v : . [ h e p - ph ] A p r MADPH–08–1509, NSF–KITP–08–55
The “Top Priority” at the LHC
Tao Han ∗ Department of Physics, University of Wisconsin, Madison, WI 53706KITP, University of California, Santa Barbara, CA 93107 (Dated: November 4, 2018)
Abstract
The LHC will be a top-quark factory. With 80 million pairs of top quarks and an additional34 million single tops produced annually at the designed high luminosity, the properties of thisparticle will be studied to a great accuracy. The fact that the top quark is the heaviest elementaryparticle in the Standard Model with a mass right at the electroweak scale makes it tempting tocontemplate its role in electroweak symmetry breaking, as well as its potential as a window tounknown new physics at the TeV scale. We summarize the expectations for top-quark physics atthe LHC, and outline new physics scenarios in which the top quark is crucially involved.To be published as a chapter in the book of “
Perspectives on the LHC ”, edited by G. Kane andA. Pierce, by World Scientific Publishing Co., 2008. ∗ email: [email protected] . BRIEF INTRODUCTION The top quark plays a special role in the Standard Model (SM) and holds great promisein revealing the secret of new physics beyond the SM. The theoretical considerations includethe following: • With the largest Yukawa coupling y t ∼ m t ∼ v/ √ • The largest contribution to the quadratic divergence of the SM Higgs mass comes fromthe top-quark loop, which implies the immediate need for new physics at the Terascalefor a natural EW theory [2], with SUSY and Little Higgs as prominent examples. • Its heavy mass opens up a larger phase space for its decay to heavy states
W b, Zq, H , ± q , etc. • Its prompt decay much shorter than the QCD scale offers the opportunity to explorethe properties of a “bare quark”, such as its spin, mass, and couplings.Top quarks will be copiously produced at the LHC. The production and decay are wellunderstood in the SM. Therefore, detailed studies of the top-quark physics can be rewardingfor both testing the SM and searching for new physics [3].
II. TOP QUARK IN THE STANDARD MODEL
In the SM, the top quark and its interactions can be described by − L SM = m t ¯ tt + m t v H ¯ tt + g s ¯ tγ µ T a tG aµ + eQ t ¯ tγ µ tA µ (1)+ g cos θ w ¯ tγ µ ( g V + g A γ ) tZ µ + g √ d,s,b X q V tq ¯ tγ µ P L qW − µ + h.c. Besides the well-determined gauge couplings at the electroweak scale, the other measuredparameters of the top quark are listed in Table I.2
ABLE I: Experimental values for the top quark parameters [4]. m t (pole) | V tb | | V ts | | V td | (172.7 ± > .
78 (40 . ± . × − (7 . ± . × − The large top-quark mass is important since it contributes significantly to the electroweakradiative corrections. For instance, the one-loop corrections to the electroweak gauge bosonmass can be cast in the form∆ r = − G F m t √ π tan θ W + 3 G F M W √ π (cid:18) ln m H M Z − (cid:19) . (2)With the m t value in Table I, the best global fit in the SM yields a Higgs mass m H = 89 +38 − GeV [4]. The recent combined result from CDF and D0 at the Tevatron Run II gave thenew value [5] m t = 171 . ± . . (3)The expected accuracy of m t measurement at the LHC is better than 1 GeV [6], with errorsdominated by the systematics.To directly determine the left-handed V - A gauge coupling of the top quark in the weakcharged current, leptonic angular distributions and W polarization information would beneeded [7]. No direct measurements are available yet for the electroweak neutral currentcouplings, g tV = T / − Q t sin θ W , g tA = − T / Q t = +2 /
3, although there are proposalsto study them via the associated production processes t ¯ tγ, t ¯ tZ [8]. The indirect global fitshowever indicate the consistency with these SM predictions [4]. A. Top-Quark Decay in the SM
Due to the absence of the flavor-changing neutral currents at tree level in the SM (theGlashow-Iliopoulos-Maiani mechanism), the dominant decay channels for a top quark willbe through the weak charged-currents, with the partial width given by [9]Γ( t → W + q ) = | V tq | m t πv (1 − r W ) (1 + 2 r W ) (cid:20) − α s π ( 2 π −
52 ) (cid:21) , (4)where r W = M W /m t . The subsequent decay of W to the final state leptons and light quarksis well understood. Two important features are noted:3 Since | V tb | ≫ | V td | , | V ts | , a top quark will predominantly decay into a b quark. While V ts , V td may not be practically measured via the top-decay processes, effective b -tagging at the Tevatron experiments has served to put a bound on the ratio B ( t → W b ) B ( t → W q ) = | V tb | | V td | + | V ts | + | V tb | , (5)that leads to the lower bound for | V tb | in Table I. • Perhaps the most significant aspect of Eq. (4) is the numerics:Γ( t → W + q ) ≈ . ≈ . × − s > Λ QCD ∼
200 MeV . (6)This implies that a top quark will promptly decay via weak interaction before QCDsets in for hadronization [10]. So no hadronic bound states (such as ¯ tt, ¯ tq , etc. ) wouldbe observed. The properties of a “bare quark” may be accessible for scrutiny.It is interesting to note that in the top-quark rest frame, the longitudinal polarization ofthe W is the dominant mode. The ratio between the two available modes isΓ( t → b L W λ =0 )Γ( t → b L W λ = − ) = m t M W . (7) B. Top-Quark Production in the SM t ¯ t production via QCD Historically, quarks were discovered via their hadronic bound states, most notably forthe charm quark via
J/ψ (¯ cc ) and bottom quark via Υ(¯ bb ). Due to the prompt decay ofthe top quark, its production mechanisms and search strategy are quite different from thetraditional one.The leading processes are the open flavor pair production from the QCD strong interac-tion, as depicted in Fig. 1. The contributing subprocesses are from q ¯ q, gg → t ¯ t. (8)The cross sections have been calculated rather reliably to the next-to-leading order [12] andincluding the threshold resummations [13, 14], as given in Table II.4 P tt x P x P FIG. 1: Top-quark pair production in hadronic collisions via QCD interaction. This figure is takenfrom Ref. [11].TABLE II: Cross sections, at next-to-leading-order in QCD, for top-quark production via thestrong interaction at the Tevatron and the LHC [14]. Also shown is the percentage of the totalcross section from the quark-antiquark-annihilation and gluon-fusion subprocesses. σ NLO (pb) q ¯ q → t ¯ t gg → t ¯ t Tevatron ( √ s = 1 . p ¯ p ) 4 . ±
10% 90% 10%Tevatron ( √ s = 2 . p ¯ p ) 6 . ±
10% 85% 15%LHC ( √ s = 14 TeV pp ) 803 ±
15% 10% 90%
Largely due to the substantial gluon luminosity at higher energies, the t ¯ t production rate isincreased by more than a factor of 100 from the Tevatron to the LHC. Assuming an annualluminosity at the LHC of 10 cm − s − ⇒
100 fb − / year, one expects to have 80 milliontop pairs produced. It is truly a “top factory”. In Fig. 2(a), we plot the invariant massdistribution, which is important to understand when searching for new physics in the t ¯ t channel. Although the majority of the events are produced near the threshold m ( t ¯ t ) ∼ m t ,there is still a substantial cross section even above m ( t ¯ t ) ∼ m ( t ¯ t ) and decay branching fractions of one top decaying hadronically and the otherleptonically have been included.It should be noted that the forward-backward charge asymmetry of the t ¯ t events can begenerated by higher order corrections, reaching 10 −
15% at the partonic level from QCD[15] and 1% from the electroweak [16]. 5
FIG. 2: (a) Invariant mass distribution of t ¯ t at the LHC and (b) integrated cross section versusa minimal cutoff on m ( t ¯ t ). Decay branching fractions of one top decaying hadronically and theother leptonically ( e, µ ) have been included.
2. Single top production via weak interaction
As discussed in the last section, the charged-current weak interaction is responsible forthe rapid decay of the top quark. In fact, it also participates significantly in the productionof the top quark as well [17]. The three classes of production processes, s -channel Drell-Yan, t -channel W b fusion, and associated
W t diagrams, are plotted in Fig. 3. Two remarks arein order: • The single top production is proportional to the quark mixing element | V tb | and thusprovides the direct measurement for it, currently [18] 0 . < | V tb | ≤ • The s -channel and t -channel can be complementary in the search for new physics suchas a W ′ exchange [19].For the production rates [20, 21, 22, 23, 24], the largest of all is the t -channel W b fusion.It is nearly one third of the QCD production of the t ¯ t pair. Once again, it is mainly from theenhancement of the longitudinally polarized W . The total cross sections for these processesat Tevatron [23] and LHC energies [24] are listed in Table III [20, 21, 22]. We see the typicalchange of the production rate from the Tevatron to the LHC: A valence-induced process (DY-6 q tbW q qb tW g tb W FIG. 3: Single top-quark production in hadronic collisions via the charged-current weak interaction.This figure is taken from Ref. [11]. type) is increased by about an order of magnitude; while the gluon- or b -induced processesare enhanced by about a factor of 100. TABLE III: Cross sections, at next-to-leading-order in QCD, for top-quark production via thecharged current weak interaction at the Tevatron and the LHC. σ (pb) s -channel t -channel W t
Tevatron ( √ s = 2 . p ¯ p ) 0 . ±
5% 2 . ±
5% 0 . ± √ s = 14 TeV pp ) 10 . ±
5% 250 ±
5% 75 ±
3. Top quark and Higgs associated production
Of fundamental importance is the measurement of the top-quark Yukawa coupling. Thedirect probe to it at the LHC is via the processes [25] q ¯ q, gg → t ¯ tH. (9)The cross section has been calculated to the next-to-leading-order (NLO) in QCD [26, 27]and the numerics are given in Table IV. The cross section ranges are estimated from theuncertainty of the QCD scale.The production rate at the LHC seems quite feasible for the signal observation. It wasclaimed [28] that a 15% accuracy for the Yukawa coupling measurement may be achievablewith a luminosity of 300 fb − . Indeed, the decay channel H → γγ should be useful forthe search and study in the mass range of 100 < m H <
150 GeV [29, 30]. However, thepotentially large backgounds and the complex event topology, in particular the demand on7
ABLE IV: Total cross section at the NLO in QCD for top-quark and Higgs associated productionat the LHC [27]. m H (GeV) 120 150 180 σ (fb) 634 −
719 334 −
381 194 − the detector performance, make the study of the leading decay H → b ¯ b very challenging[31]. III. NEW PHYSICS IN TOP-QUARK DECAY
The high production rate for the top quarks at the LHC provides a great opportunityto seek out top-quark rare decays and search for new physics Beyond the Standard Model(BSM). Given the annual yield of 80 million t ¯ t events plus 34 million single-top events, onemay hope to search for rare decays with a branching fraction as small as 10 − . A. Charged Current Decay: BSM
The most prominent examples for top-quark decay beyond the SM via charged-currentsmay be the charged Higgs in SUSY or with an extended Higgs sector, and charged technicolorparticles t → H + b, π + T b. (10)Experimental searches have been conducted at the Tevatron [32], and some simulations areperformed for the LHC as well [3, 33]. It is obvious that as long as those channels arekinematically accessible and have a sizable branching fraction, the observation should bestraightforward. In fact, the top decay to a charged Higgs may well be the leading channelfor H ± production.More subtle new physics scenarios may not show up with the above easy signals. It maybe desirable to take a phenomenological approach to parameterize the top-quark interactionsbeyond the SM [7, 34], and experimentally search for the deviations from the SM. Those“anomalous couplings” can be determined in a given theoretical framework, either fromloop-induced processes or from a new flavor structure. One can write the interaction terms8s L CC = g √ (cid:0) ¯ t (1 + δ L ) γ µ P L qW − µ + ¯ tδ R γ µ P R qW − µ (cid:1) + h.c. (11)The expected accuracy of the measurements on δ L,R is about 1% [3, 34], thus testing thetop-quark chiral coupling.
B. Neutral Current Decay: BSM
Although there are no Flavor-Changing Neutral Currents (FCNC) at tree level in theSM, theories beyond the SM quite often have new flavor structure, most notably for SUSYand technicolor models. New symmetries or some alignment mechanisms will have to beutilized in order to avoid excessive FCNC. It is nevertheless prudent to keep in mind thepossible new decay modes of the top quark such as the SUSY decay channel t → ˜ t ˜ χ . (12)Generically, FCNCs can always be generated at loop level. It has been shown that theinteresting decay modes t → Zc, Hc, γc, gc (13)are highly suppressed [35, 36] with branching fractions typically 10 − − − in the SM, and10 − − − in the MSSM. It has been shown that the branching fractions can be enhancedsignificantly in theories beyond the SM and MSSM, reaching above 10 − and even as highas 1% [37].One may again take the effective operator approach to parameterize the interactions.After the electroweak symmetry breaking, one can write them as [38, 39, 40] L NC = g θ w X τ = ± ,q = c,u κ τ ¯ tγ µ P τ qZ µ + h.c. (14)+ g s X q = c,u κ gq Λ ¯ tσ µν T a tG aµν + eQ t X q = c,u κ γq Λ ¯ tσ µν tA µν + h.c. (15)The sensitivities for the anomalous couplings have been studied at the LHC by the ATLASCollaboration [41], as listed in Table V 9 ABLE V: 95% C.L. sensitivity of the branching fractions for the top-quark decays via FCNCcouplings at the LHC [41]. Channel 10 fb −
100 fb − t → Zq . × − . × − t → γq . × − . × − t → gq . × − . × − IV. TOP QUARKS IN RESONANT PRODUCTION
The most striking signal of new physics in the top-quark sector is the resonant productionvia a heavy intermediate state X . With some proper treatment to identify the top decayproducts, it is possible to reconstruct the resonant kinematics. One may thus envision fullyexploring its properties in the c.m. frame. A. X → t ¯ t, t ¯ b Immediate examples of the resonant states include Higgs bosons [42], new gauge bosons[43], Kaluza-Klein excitations of gluons [44] and gravitons [45], Technicolor-like dynamicalstates [1, 3, 46] etc.The signal can be generically written as σ ( pp → X → t ¯ t ) = X ij Z dx dx f i ( M X , x ) f j ( M X , x ) × π (2 J + 1) s Γ( X → ij ) B ( X → t ¯ t ) M X . (16)Thus the observation of this class of signals depends on the branching fraction of X → t ¯ t aswell as its coupling to the initial state partons. Figure 4 quantifies the observability for abosonic resonance (spin 0,1,2) for a mass up to 2 TeV at the LHC [47] via q ¯ q, gg → X → t ¯ t .The vertical axis gives the normalization factors ( ω ) for the cross section rates needed toreach a 5 σ signal with a luminosity of 10 fb − . The normalization ω = 1 defines thebenchmark for the spin 0, 1 and 2 resonances. They correspond to the SM-like Higgs boson,a Z ′ with electroweak coupling strength and left (L) or right (R) chiral couplings to SMfermions, and the Randall-Sundrum graviton ˜ h with the couplings scaled with a cutoff scale10 IG. 4: Normalization factor versus the resonance mass for the scalar (dashed) with a width-mass ratio of 20%, vector (dot-dashed) with 5%, and graviton (solid) 2%, respectively. The regionabove each curve represents values of ω that give 5 σ or greater statistical significance with 10 fb − integrated luminosity. as Λ − for ˜ hq ¯ q , and (Λ ln( M ∗ pl / Λ)) − for ˜ hgg , respectively. We see that a Z ′ or a gravitonshould be easy to observe, but a Higgs-like broad scalar will be difficult to identify in the t ¯ t channel.It is of critical importance to reconstruct the c.m. frame of the resonant particle, wherethe fundamental properties of the particle can be best studied. It was demonstrated [47]that with the semi-leptonic decays of the two top quarks, one can effectively reconstruct theevents in the c.m. frame. This relies on using the M W constraint to determine the missingneutrino momentum, while it is necessary to also make use of m t to break the two-foldambiguity for two possible p z ( ν ) solutions. Parity and CP asymmetries [48] can be studied.Top-quark pair events at the high invariant mass are obviously important to search forand study new physics. In this new territory there comes a new complication: When thetop quark is very energetic, γ = E/m t ∼
10, its decay products may be too collimatedto be individually resolved by the detector − recall that the granularity of the hadroniccalorimeter at the LHC is roughly ∆ η × ∆ φ ∼ . × .
1. This is a generic problem relevantto any fast-moving top quarks from heavy particle decays [44, 47, 49] (see the next sections).11he interesting questions to be addressed may include: • To what extent can we tell a “fat top-jet” from a massive QCD jet due to showering? • To what extent can we tell a “fat W -jet” from a massive QCD jet? • Can we make use of a non-isolated lepton inside the top-jet ( bℓν ) for the top-quarkidentification and reconstruction? • Can we do b -tagging for the highly boosted top events?These practical issues would become critical to understand the events and thus for newphysics searches. Detailed studies including the detector effects will be needed to reachquantitative conclusions. B. T → tZ, tH, bW In many theories beyond the SM, there is a top-quark partner. These are commonlymotivated by the “naturalness” argument, the need to cancel the quadratic divergence inthe Higgs mass radiative correction, most severely from the top-quark loop. Besides thescalar top quark in SUSY, the most notable example is the Little Higgs theory [50]. If thereis no discrete symmetry, the top partner T will decay to SM particles in the final state,leading to fully a reconstructable fermionic resonance.It was pointed out [51] that the single T production via the weak charged-current maysurpass the pair production via the QCD interaction due to the longitudinal gauge bosonenhancement for the former and the phase space suppression for the latter. This is shown inFig. 5. Subsequent simulations [52] performed by the ATLAS collaboration demonstratedthe clear observability for the signals above the backgrounds at the LHC for T → tZ, bW with a mass M T = 1 TeV, as seen in Fig. 6. V. TOP-RICH EVENTS FOR NEW PHYSICS
Although the top-quark partner is strongly motivated for a natural electroweak theory,it often results in excessively large corrections to the low energy electroweak observables. Inorder to better fit the low energy measurements, a discrete symmetry is often introduced,12
IG. 5: Production of the top-quark partner T in pair and singly at the LHC versus its mass. TheYukawa coupling ratio λ /λ has been taken to be 2 (upper dotted curve) 1 (solid) and 1/2 (lowerdotted), respectively. The T ¯ T pair production via QCD includes an NLO K -factor (dashed curve). such as the R-parity in SUSY, KK-parity in UED, and T-parity in LH [53]. The imme-diate consequence for collider phenomenology is the appearance of a new stable particlethat may provide the cold dark matter candidate, and results in missing energy in colliderexperiments. A. T ¯ T pair production The top partner has similar quantum numbers to the top quark, and thus is commonlyassigned as a color triplet. This leads to their production in QCD q ¯ q, gg → T ¯ T . (17)The production cross section is shown in Fig. 7 for both spin-0 and spin-1/2 top partners.Although there is a difference of a factor of 8 or so (4 from spin state counting and the Alternatively, the breaking of the R-parity [54] or the T-parity [55] would lead to different collider phe-nomenology [56]. nvariant Mass (GeV)0 500 1000 1500 2000 - E v en t s / G e V / f b ATLAS
Invariant Mass (GeV)0 500 1000 1500 2000 - E v en t s / G e V / f b ATLAS
FIG. 6: Observability for the decays (a) T → tZ and (b) T → bW at the ATLAS [52]. rest from threshold effects) in the cross sections, it is still challenging to tell a scalar and afermionic partner apart [57, 58, 59] due to the lack of definitive features.Due to the additional discrete symmetry, the top partner cannot decay to a SM particlealone. Consequently, T → tA , leading to t ¯ t pair production plus large mixing energy. Thecrucial parameter to characterize the kinematical features is the mass difference ∆ M T A = m T − m A . For ∆ M T A ≫ m t , the top quark as a decay product will be energetic andqualitatively different from the SM background. But if ∆ M T A ≈ m t , then the two willhave very little difference, making the signal difficult to separate out. Depending on thetop-quark decay, we present two classes of signals. t ¯ t pure hadronic decay For both t ¯ t to decay hadronically [58, 60], the signal will be 6 jets plus missing energy.While it has the largest decay rate, the backgrounds would be substantial as well. Withjudicious acceptance cuts, the signal observability for ∆ M T A >
200 GeV was established,as seen in Fig. 8. Possible measurements of the absolute mass scale and its spin of the toppartner were considered [57, 58], but the determination remains difficult.14
00 800 1000 1200 1400 m T (GeV) P r odu c ti on c r o ss - s ec ti on ( pb ) QCD top production
Fermionic T productionScalar T production ttZ
FIG. 7: Leading order total cross section for the top partner T ¯ T production at the LHC versusits mass [57]. Both spin-0 and spin-1/2 top partners are included. The QCD t ¯ t and the SM t ¯ tZ backgrounds are indicated by the horizontal lines. t ¯ t semi-leptonic decay If one of the t ¯ t decays hadronically and the other decays leptonically, the signal may becleaner. It turns out that if the mass difference ∆ M T A is sizable, then requiring large missingtransverse energy may be sufficient to suppress the background. However, if ∆ M T A ∼ m t , then the E/ T for the signal is not much different from the background. On the otherhand, the fact that the t ¯ t kinematics can be fully reconstructed in the SM implies thatthe reconstruction for the signal events would be distinctive due to the large missing mass.Indeed, the reconstructed m rt based on the E/ T will be far away from the true m t , and mostlyresult in an unphysical value. If we impose | m t − m rt | >
110 GeV , (18)we can reach optimal signal identification. The summary plot for the statistical significance(the number of σ ) is given in Fig. 9 at the LHC with an integrated luminosity of 100 fb − ,where the left panel is for a fermionic T , and the right is a scalar ˜ t , both decaying to t + amissing particle. 15
00 400 500 600 700 800 900 1000m t' m n FIG. 8: Contour in m ˜ t − m N for ˜ t → tN for the statistical significance of a scalar ˜ t at the LHCwith an integrated luminosity of 100 fb − . Purely hadronic decays are considered. B. Exotic top signatures
Searching for exotic events related to the top quark can be rewarding. First, there existsa variety of natural electroweak models with distinctive top partners that should not beoverlooked for collider phenomenology. Second, potentially large couplings of the top quarkto new physics may result in multiple top quarks from new particle decays. Finally, theexotic events have less SM background contamination, and thus may stand out for discoveryeven at the early phase of the LHC. We briefly list a few recent examples. • Multiple top quarks and b -quarks in the final state may help to search for new heavyparticles in the electroweak sector and can be distinctive from the SM backgrounds[61]. • Heavy top partners and other KK fermions in the RS model may lead to uniquetop-quark and W -boson signatures [62]. • New exotic colored states may predominantly couple to heavy quarks and thus lead16 T M AH
10 5 1 0.1FERMIONIC TOP PARTNER400 600 800 1000 1200 140040020060080010001200 MT M AH
10 5 1 0.1 1400400 600 800 1000 120040020060080010001200 SCALAR TOP PARTNER
FIG. 9: Contour in m T − m A for T → tA for the statistical significance at the LHC with anintegrated luminosity of 100 fb − . Left panel is for a fermionic T , and the right is a scalar ˜ t , bothdecaying to a top plus a missing particle. to multiple top quarks in the final state [63]. • Composite models for the right-handed top-quark may lead to t ¯ tt ¯ t signals at the LHC[64]. • Like-sign top quark pairs may indicate new dynamics [65].
VI. SUMMARY AND OUTLOOK
The LHC will be a true top-quark factory. With 80 million top-quark pairs plus 34 millionsingle tops produced annually at the designed high luminosity, the properties of this particlewill be studied to a great accuracy and the deep questions related to the top quark at theTerascale will be explored to an unprecedented level. Theoretical arguments indicate that itis highly likely that new physics associated with the top quark beyond the SM will show upat the LHC. This article only touches upon the surface of the rich top quark physics, andis focused on possible new physics beyond the SM in the top-quark sector. The layout ofthis article has been largely motivated by experimental signatures for the LHC. Interesting17ignatures covered here include • Rare decays of the top quark to new light states, or to SM particles via the chargedand neutral currents through virtual effects of new physics. • Top quark pair production via the decay of a new heavy resonance, resulting in fullyreconstructable kinematics for detailed studies. • Top quark pair production via the decay of pairly produced top partners, usuallyassociated with two other missing particles, making the signal identification and theproperty studies challenging. • Multiple top quarks, b quarks, and W ± ’s coming from theories of electroweak symme-try breaking or an extended top-quark sector.The physics associated with top quarks is rich, far-reaching, and exciting. It opens upgolden opportunities for new physics searches, while brings in new challenges as well. Itshould be of high priority in the LHC program for both theorists and experimentalists. Acknowledgments
I thank Gordy Kane and Aaron Pierce for inviting me to write on this subject, whichI consider a very important and exciting part of the LHC physics program. I would alsolike to thank Vernon Barger, Tim Tait and Lian-Tao Wang for reading and commenting onthe draft. This work was supported in part by the US DOE under contract No. DE-FG02-95ER40896 and in part by the Wisconsin Alumni Research Foundation. The work at theKITP was supported by the National Science Foundation under Grant No. PHY05-51164. [1] For a review on new strong dynamics related to the top quark, see e.g. , C. T. Hill andE. H. Simmons,
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