PoS(EPS-HEP2011)377 http://pos.sissa.it/ ce. rge asymmetry. Complementary inguish between different models int towards an anomalously large is article summarizes the main classes g picture. cs – HEP 2011 ive Commons Attribution-NonCommercial-ShareAlike Licen [email protected] forward-backwardasymmetry in top-antitop production. Th of physics beyond the standardmeasurements model at that the can Large give rise Hadronand to Collider thereby a will contribute la to allow clarifying to the dist presently puzzlin Several recent measurements at the Tevatron experiments po Copyright owned by the author(s) under the terms of the Creat c July 21-27, 2011 Grenoble, Rhône-Alpes, France International Europhysics Conference on High Energy Physi Susanne Westhoff Institut für Physik (THEP) and Helmholtz-InstitutJohannes Mainz Gutenberg-Universität D-55099 Mainz, Germany E-mail: Top- Asymmetry – A New Physics Overview PoS(EPS-HEP2011)377 ¯ t is t % ) data 5 FB t (1.2) (1.1) → . ¯ t A 0 ¯ t u n a first u ± , width and 8 ¯ t . t 4 , is symmetric →  ¯ : a color-octet t ¯ gure 2. As we q t ) = ( q FB t les exhibit devia- → A channel arise after ¯ lab SM ttX , ) θ ) t e dominant t gg and its invariant mass → FB t a ard-backward excess at ¯ t ¯ A p cos t T ( . p 5 σ d ) mediate vector and axial- γ ul test of the QCD vector ( t µ 8 3 σ A γ rection of the incident proton d T ¯ O t is the antisymmetric tensor of 5 γ )( amplitude that is odd under the CDF and DØ experiments at the ¯ of the top quark [1] and consis- In a theory with CP-conserving u u ¯ (+) t abc a and t tity, arising only at next-to-leading )( ε − T FB t 8 u V asymmetry, defined by 5 o supported by the pattern of e. The process ) ees with the CDF result at the level of l QCD value → A 3 SM) prediction. The present situation rections [3, 4] and increased by elec- = γ the interference of the SM with ge contributions to O T ¯ µ 3 q y. Since a large positive effect in ¯ 5 tX amework of utions of these operators to the process γ T n work in the framework of an effective q t n cross section γ ¯ ¯ u t θ QCD amplitude are → ¯ = ( = ( p cos p d 8 3 A P induce flavor-changing scalar and pseudo-scalar ( O O 2 ory, can be important. 3 σ P d O  , ) θ t and a 3 S cos T , 1 µ O ) 0 t γ Z ¯ t 3 is typically in the range of the momentum transfer in )( = ¯ FB t u uT ) s A a ( )( thus stem from a partonic a T u c t σ µ 3 γ A [6], where the anomaly in the asymmetry is most significant. I ¯ ¯ u tT ¯ t t channel, while flavor-changing chiral currents in the M , = ( = ( s s a agree well with the SM predictions, the asymmetric observab 3 8 S V σ σ ¯ t t O O channel. The corresponding Feynman diagrams are shown in Fi significance. In particular, CDF observes a significant forw = Among the dimension-six four-quark operators mediating th u c t dM σ / A 1 ¯ t t ≡ σ d are the generators of the QCD color octet, and is the scattering angle of the top quark with respect to the di in terms of Mandelstam variables. The operators FB t a ¯ t θ A t T Due to its smallness in QCD, the asymmetry is not only a powerf The anomaly in top-quark pair production persists: both the As the scale of new physics in 1 → ¯ . (New) contributions to u p a Fierz transformation. The operators will discuss in detail, there are three main mediators of lar currents in the tently found higher results compared tois the illustrated standard in model Figure ( 1. While the top-antitop productio vector currents in the interchange of the top with the antitop quark in the final stat spectrum current, but also acouplings, sensitive the forward-backward probe asymmetry of equals a physics charge beyond the SM. [3] is expected to betroweak robust contributions with by at respect most to 20% higher-order [5]. cor u a color triplet. We will distinguish the tree-level contrib Tevatron have measured the forward-backward asymmetry Top-Quark Asymmetry – A New Physics Overview 1. New physics in top-quark pair production large invariant mass, which is less pronouncedreconstructed at data DØ, within but one agr standard deviation.(QCD), In the the forward-backward fr asymmetry is aorder suppressed in powers quan of the strong coupling constant [2]. The smal tions of up to 3 Here in its initial state and therefore does not yield an asymmetr required to explain the discrepancy, ita is new most particle likely exchanged due at to tree level. This statement is als where resonance effects, which are not covered in an effective the at high invariant mass theory [7, 8]. approach to classifying those interference effects, one ca transition, the relevant operators that interfere with the PoS(EPS-HEP2011)377 . A t to g ¯ t u (2.1) g ect to arises % Σ , l and FB 2 ­ ¯ G distribu- ut A 3.3 q A ¯ g t M 15.2 t g / A t M production, g ¯ t q A t g lab for high invariant mass Σ L % ­ ¯ , > t FB ) 15 entral measured values [1]. 1.8 2 A ) H FB t 2 G A may have flavor-universal ¯ ( t M 2 t ¯ t t − M M ¯ t 2 t > ics (NP) in top-antitop produc- L TeV. The inclusive asymmetry in the Σ ≈ is displayed for the total cross section % M cross section and its ­ ¯ t ] ( FB G ¯ 4 ass, we give examples of concrete 48 t 2 A 3.3 must exhibit axial-vector couplings . H exp t ) to , dubbed axigluons, gener- ry prediction of the asymmetry with M channel, a color-singlet vector in the hysics and collider observables have 1 ¯ t A t t O , f q A s g / 8 t on the magnitude of xigluon with the QCD gluon (INT) and he g . M ( llowing contributions to 0 2 ark pair production. SM [ ) , besides its bin j ≫ q A O ∈ tt ll ¯ + g t L G l t ¯ . The asymmetry in the laboratory frame is denoted t ( Σ t ¯ ) t % ll t t FB ­ ¯ M ) M A ∼ FB 42 2.3 H A FB t ( 3 A for NP s ( and by the signs of the quark couplings σ > ¯ t ) t ¯ to quarks in the t Σ denotes the strong coupling constant of QCD. The sign t j s ­ ¯ q A + channel, both with flavor-changing couplings → tt l g g 2.1 % L dM ¯ , u q / t FB q 2 % G σ A 19.6 H d A t M ( g 15.8 − q A ¯ t 2 g t > L M tt is determined by the magnitude of the axigluon mass with resp 2 s GeV g  dM  INT a ∼ tt σ Σ d 0.07fb H INT a σ is the charged lepton asymmetry. Numbers correspond to the c l FB theo tot ∆ ∆ A tt Σ D0, D0, CDF 7.5pb is the mass of the axigluon and , and Top-antitop production at the Tevatron. The ratio G lab 45 TeV, as well as for the dilepton channel, ) . M 0 FB t

0.5 1.5 1.0 Color-octet vector bosons with axial-vector couplings In the following, we will review the three classes of new phys

A > exp SM and its invariant mass distribution O O  ( ¯ t ¯ t t channel or a color-triplet scalar in the t ate a charge asymmetry at tree level.the The interference interference of of an an a axigluon with itself (NP) yield the fo tion in terms of QCD. Furtherto strong be constraints respected from flavor when p constructingrealizations a of viable NP that model. yield a For consistent each picture cl of top-qu 2. s channel: color-octet vectors (axigluons) vector boson with axial-vector couplings by In order to generate a positive asymmetry, a light axigluon o couplings to quarks, whereas a heavy axigluon with quarks. tion. Our main focus will beits on measurement, models while that preserving reconcile the the good theo description of t where parton frame is shown for the lepton + jets channel, of different sign to light and top quarks. A strong constrain of the interference term M the momentum transfer in the process Figure 1: σ Top-Quark Asymmetry – A New Physics Overview t PoS(EPS-HEP2011)377 , . C T ) ∗ 3 ( 2TeV ottom and rk cou- ¯ t 1 and a t SU & S = G production, vertex. The s A t M ¯ ¯ t g q g t . The massive − Zq R g ) = 3 s uark asymmetry at ( g q A with the SM gluon. g ¯ t t SU ¯ u u × interfere with the charge- → L ¯ u ) u 2TeV, 3 ( pt of [11], an ex- MS have carried out searches = 500GeV [10]. Axigluon contri- 200GeV [10]. Constraints from parameter should be kept in mind [10]. flavor-changing neutral currents SU G T ion. The agreement with the QCD & & fects can be confined to the up-type air production is their simultaneous M the couplings to light quarks, while versal couplings, the misalignment n resonance, but requires new states G G us rather mild, but reach the level of erved as resonances in d in the oblique parameters lisions at a center-of-mass energy of rks [10]. 2 ken to the diagonal subgroup h QCD-like couplings to M M asymmetry anomaly is a light axigluon luon corrections to the ouplings to light quarks and top quarks ¯ t t , fixed. ints from the ¯ t A t u g production arise from flavor physics and elec- g q A ¯ t S 4 g t causes significant distortions in the invariant mass 20% [10, 12]. The effect is limited by the bounds ¯ channels of the process u u Figure 2: = u NP s to the chiral product σ NP C ) + 3 ¯ t t , and ( t > SM , ) . A heavy axigluon with s | ut SU A t FB t g ′ g A | boson width, its hadronic cross section and its coupling to b ( Z ¯ = u Z u | meson mixing then constrains the mass of an axigluon with qua q A g D | − t ¯ t D t A 10% evades flavor and collider constraints and yields a top-q g = G G u A production cross section. Even though the axigluon does not M g ¯ t / t G Γ ¯ 7TeV, as well as for anomalies in the dijet angular distribut u u Indirect constraints on axigluon effects in When increasing the top-quark couplings, however, constra A broad class of concrete axigluon models relies on the conce A serious limiting factor of axigluon effects in top-quark p 2 = s spectrum, in particular close to the resonance region. Top-Quark Asymmetry – A New Physics Overview troweak precision observables. Inof the the case axigluon of couplings(FCNC) flavor at with non-uni tree the level. In Yukawa the couplings absencequark of sector induces a [9, flavor 10]. theory, FCNC ef Interference of new particles in the predictions strongly constrains the mass of an axigluon wit [9, 10]. In general, dijetkeeping bounds the can relevant product be for evaded the by asymmetry, decreasing from the quarks at LEP yield a lower bound on the axigluon mass of tension of the QCD gauge group In this framework, the origin ofto the be axigluon of restricts the equal c strength, symmetric QCD amplitude, the NP term occur first at the two-loopseveral level. 100GeV The for strong mass axial-vector constraints couplings are to th top qua contribution to dijet productionfor at resonances the√ in LHC. the Both dijet spectrum ATLAS and from proton-proton C col butions to the electroweak gauge boson propagators, encode electroweak precision observables are due to one-loop axig axigluon arises once the product group is spontaneously bro plings of QCD strength to be beyond the electroweak scale, precise measurements of the width high invariant masses of up to from dijet production. Light axigluons should have been obs as well as dijet production. Ain broad the axigluon width decay can [13]. hide an An attractive axigluo solution to the top PoS(EPS-HEP2011)377 ) by ′ (3.1) cross Z gauge ¯ t . L t − try [19]. ) ¯ t t 2 t 100GeV 2 parameter ( / ′ − M 2 ′ ) Z 2 Z u ′ ) constrains the model is thus 2 SU Z M models. R , ′ ( ¯ t ′ M L t 4 Z Z INT channel, a flavor- g s − − ( σ s ¯ t 2 2 t 10 ) boson, contributes to gauge bosons, which R ¯ M , u ′ × Z ( u L 5 g coupling, which induces W are required to achieve a . ( ′ 3 lor, top-quark condensates R channel yields a Rutherford , Z , connected to ∼ ′ t ut L < g Z − ut L NP s L mixing [17]. A large top-quark − g b y and mixings of quarks can be σ , as observed by CDF. Since the c to the cross section prohibits large orated as Kaluza-Klein excitations ay out is to protect the model from in the D t strongly suppressed for the UV- coupling, breaking both the chiral − uza-Klein [15]. Moving the − R , L , − NP ut R s u ut L d 2 avor observables [16]. g overlap of their wave function profiles. ) g σ D on: heavy quarks typically reside in the avor anarchy, it is therefore not possible ut R ouplings, however, requires an additional ′ n. The IR-localized Kaluza-Klein gluons g 2 3, and a broad width [14]. It can induce a Z / via the interference term M 1 + ( ¯ t t 2 − in analogy with the SM = ) σ t ′ A ut L t 5 Z g g ( bosons arise from meson mixing. The coupling to a 2 s = ′ couplings stem from same-sign top production. The ′ g . Contrary to new particles in the Z q A ) · ′ Z ′ flavor symmetry. The forward-backward asymmetry in g b Z NP a Z − 30%. processes by CMS excludes the region of the ut R σ boson with a large − ∼ L ) t , is negative, large couplings ≈ t ′ 2 ( tt j Z − to 95% confidence level [20]. The abelian − , which restricts the flavor structure of viable INT INT > NP s a couplings. The coupling L R ) u , SU → σ ′ σ ¯ u t FB t L t Z FB t g A pp A , , -channel exchange of (electrically neutral) coupling and provide a consistent explanation of the asymme ( R mixing. The resulting strong bound t ¯ 2 , u ) u L ut R d g g and ut R B ′ channel does not appear as a resonance in the spectrum of the g 2 Z tt t − M d + ( boson with flavor-changing couplings → B ′ 2 − ) t Z pp ut L g ( asymmetry of 2 s ¯ t to be below a few 100GeV. The contribution flavor symmetries. In models of topcolor-assisted technico t g flavor symmetry, is constrained by effects in 400GeV, universal couplings · ′ M U Z U a ) ) ≈ M 3 3 ∼ ( G ( M Severe direct constraints on A color-singlet vector boson, referred to as In Randall-Sundrum models, axigluons are naturally incorp Indirect constraints on flavor-violating SU SU INT a mass σ ′ and this setup is induced by the asymmetry could thus be due to a ruled out by same-sign top productionsame-sign at the tops LHC. by A a possible w non-abelian generate the desired large space that is compatible with top-antitop production in the following way, recent search for of the gluon. Inexplained by models their with localization in flavor theinfra-red anarchy, warped (IR), the extra light dimensi mass quarks hierarch in the ultra-violet (UV) regio Axial-vector couplings to thelocalized heavy light quarks. top Without quarks relaxing theto are principle generate large, of a fl bu sizeable forward-backwardlight asymmetry quarks from towards the Kal IR to increaseflavor their protection axial-vector mechanism c to inhibit too large effects in fl 3. t channel: color-singlet vectors ( exhibit axial-vector couplings to quarks, depending on the large high- Z section. However, the Rutherford enhancement of Top-Quark Asymmetry – A New Physics Overview with flavor-diagonal couplings violating boson in the The exchange of a enhancement of the topinterference asymmetry with at the high SM invariant gluon, mass [17] the tree-level effects in [18] suggests large right-chiral symmetry of the SM relates the positive asymmetry from the NP term PoS(EPS-HEP2011)377 s (4.1) (5.1) %. The is also a [27, 28]. ) spectrum ¯ t 2 6 t ¯ t S t − M M 1 . ( and % t ) , however, has an- η 3 . 11 should show up as a S . 3 ) 0 / channel. The effect on 4 ) ± u 1 , 30 6 1TeV . is possible and will help to ( 1 e clarified by exploring corre- s to explore the O = ( may originate from an extended = ( 6 = 3 s, there exist two scalar fields that S um distributions. One can probe metry is not observable in proton- S refore generically smaller than for G me-sign top production and thereby SM , which implies flavor-diagonal cou- scalars. The color sextet igin of the quark and antiquark in- ) 0]. An improved measurement of the , M f. Eq. (1.2)) and a color sextet. Their U η t ymmetry [31]. 3 ) lets . The color triplet / A 3 4 ects it from the dominant flavor-diagonal ( consistent with zero [26]. The results are ( ) 200GeV [21]. uu 1 η t , SU = tics are expected to shrink to A by a factor of about two. It might be difficult 3 → ′ 6 , W S ) ) SM to the forward-backward asymmetry is therefore = ( | | without setting further cuts on M ) 6 ¯ ¯ t t 3 6 η t η t → η η S S -channel exchange is the absence of same-sign tops symmetry further prevents the scalars from inducing | | A A u ( production proceeds via the U uu < < ) ¯ t : | | ) t 3 t t Y ( S ) η η 30% for 1 production via an asymmetry based on pseudo rapidities in | | ( ( ( ¯ 3 SU t ≈ boson enhances the tail of the spectrum. The chiral coupling U t N N ) S ′ > L Z ) ) − 2 ) )+ | | ( FB t ¯ ¯ t t A η η ( | | SU , > > C typically enhance ) | | t t 3 ( FB t η η | | A ( ( SU N N ( [25], ) = 2 η t / -channel dijet production via A θ s tan ( ln − The origin of the Tevatron forward-backward asymmetry can b A promising road to discriminate between different models i The exchange of a color triplet Besides the vector bosons discussed in the previous section spectrum of the forward-backward asymmetry at the Tevatron ¯ t = t -channel NP [23]. A clear advantage of the of top-pair production at the LHC [27, 29]: an axigluon of distinguish between models that yield the same inclusive as Higgs sector in models with grand unification [24]. 5. Model distinction at the LHC lated observables at the LHC.proton While a collisions forward-backward due asym toside the the protons symmetric can initialcharge-asymmetric be state, contributions determined to the by or their different moment plings to quarks. A significant contribution of η allows for a large asymmetry Top-Quark Asymmetry – A New Physics Overview carry “flavor isospin”. 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