In this talk, we present the latest study of e+e− → t¯t, based on a detailed simulation of the ILD detector concept, which assumes a centre-of-mass energy of √s = 500 GeV and a luminosity of L = 500fb−1, equality shared between the incoming beam polarisations of P e−,e+ = (±0.8,∓0.3). The study comprises the cross sections, the forward-backward asymmetry and the slope of the helicity angle asymmetry. The vector and axial vector couplings are separately determined for the photon and the Z component. The tensorial CP-conserving coupling can be also extracted by assuming the other couplings to be the SM values. We show that the sensitivity to new physics would be dramatically improved with respect to what is expected from LHC for electroweak couplings

Amjad, M. S.

Frisson, T.

Kou, E.

Pöschl, R.

Richard, F.

Rouëné, J.

English

International audienceIn this talk, we present the latest study of e + e − → t ̄ t , based on a detailed simulation of the ILD detector concept, which assumes a centre-of-mass energy of √ s = 500 GeV and a luminosity of L =500fb − 1 , equality shared between the incoming beam polarisations of P e − ,e + =( ± 0 . 8 , ∓ 0 . 3). The study comprises the cross sections, the forward-backward asymmetry and the slope of the helicity angle asymmetry. The vector and axial vector couplings are separately determined for the photon and the Z component. The tensorial CP -conserving coupling can be also extracted by assuming the other couplings to be the SM values. We show that the sensitivity to new physics would be dramatically improved with respect to what is expected from LHC for electroweak coupling

S. Amjad, M.

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Top EW couplings at linear colliders

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DOI 10.1393/ncc/i2014-11736-0Colloquia: LC13IL NUOVO CIMENTO Vol. 37 C, N. 2 Marzo-Aprile 2014Top EW couplings at Linear CollidersM. S. Amjad, T. Frisson, E. Kou, R. Po¨schl, F. Richard and J. Roue¨ne´LAL, Laboratoire de l’Acce´le´rateur Line´aire, Universite´ Paris-Sud 11, CNRS/IN2P3(UMR 8607) - 91405 Orsay, Francericevuto il 17 Febbraio 2014Summary. — In this talk, we present the latest study of e+e− → tt¯, based ona detailed simulation of the ILD detector concept, which assumes a centre-of-massenergy of√s = 500GeV and a luminosity of L = 500 fb−1, equality shared betweenthe incoming beam polarisations of Pe−,e+ = (±0.8,∓0.3). The study comprisesthe cross sections, the forward-backward asymmetry and the slope of the helicityangle asymmetry. The vector and axial vector couplings are separately determinedfor the photon and the Z component. The tensorial CP -conserving coupling can bealso extracted by assuming the other couplings to be the SM values. We show thatthe sensitivity to new physics would be dramatically improved with respect to whatis expected from LHC for electroweak couplings.PACS 14.65.Ha – Top quarks.PACS 13.66.Jn – Precision measurements in e−e+ interactions.1. – IntroductionThe International Linear Collider project (ILC), as proposed in Japan, has a baselinestaged in four phases: Higgs threshold ( 250GeV); Top-quark threshold ( 350GeV);Top-Higgs threshold ( 500GeV); Vector-Vector mode (above  800GeV). While thefirst stage is presently the most appealing with the recent discovery of a Higgs-like boson,the next steps are essential ingredients of the program. In particular, the precisionmeasurements of top properties to search for a signal beyond the Standard Model (SM)is one of the three pillars of linear collider projects together with the Higgs and W bosonprecision measurements. The fact that the top quark is the only matter field in the SMwhich has mass near the electroweak scale makes us wonder about a special role of thetop quark in the electroweak symmetry breaking. The top physics programs at ILC canbe summarized in brief as follows:i) Top mass, width and Yukawa coupling : At the pair production threshold, thetheoretical relation between the cross section line shape and the top quark mass allowsc© Societa` Italiana di Fisica 5556 M. S. AMJAD, T. FRISSON, E. KOU, R. PO¨SCHL, F. RICHARD and J. ROUE¨NE´for precise measurements of the top quark mass mt as well as of other properties, suchas its total width Γt and couplings, most notably the strong coupling αs.The total top-pair production cross section is quite well studied theoretically. Inparticular various QCD corrections have been computed at a very high precision (up toNNLL). Since the top kinetic energy is of the order of the top quark width, electroweakeffects, which also include finite-lifetime and interference contributions, are crucial as well.This makes the cross section dependent on the experimental prescription concerning thereconstructed final state. Further developments in theoretical predictions for differentialcross sections, such as the top momentum distribution or asymmetries, are foreseen.The measurement of the top-Yukawa coupling is in principle also possible from thetop threshold, but the precision is strongly limited by the fact that Higgs effects representcorrections suppressed by the inverse square of the Higgs mass.ii) Top Electroweak Couplings: The measurements of top quark production are ofhigh relevance for the discovery of new physics. At the ILC tt¯ pairs would be copiouslyproduced, about 570 kEvents for an integrated luminosity of 500 fb−1, which correspondsto a few years of running. The advantage of searching for physics beyond the StandardModel (BSM) in top physics is that it allows eliminating nearly entirely the backgroundfrom other SM processes. The high precision expected will allow for stringent tests ofnew physics up to several 10TeV, where maybe also a new particle such as a Z ′ couldappear. The ILC is capable of solving the puzzle on AFB and ALR, which is a heritageof LEP and SLC, and shedding light on the deviations observed in thed asymmetrymeasurements at the Tevatron.In this talk, we discuss the latest simulation result for the item ii) and the futureprospect.2. – Top electroweak couplingsIn e+e− → tt¯ (see fig. 1), one often uses the following parameterization of the ttVvertex (V being a neutral gauge boson) [1, 2]:ΓttVμ (k2, q, q¯) =(1)−ie{γμ(FV1V (k2) + γ5FV1A(k2))+σμν2mt(q + q¯)μ(iFV2V + γ5FV2A(k2))}.Within the SM, i.e. V = γ, Z, the form factors at the Born level are given byF γ,SM1V = −23, F γ,SM1A = 0, FZ,SM1V = −14swcw(1− 83s2w),(2)FZ,SM1A =14swcw, F γ,SM2V = Qt(g − 2)2,where sw and cw are the sine and cosine of the Weinberg angle θW , respectively.The top quarks decay predominantly into t → W±b, followed by the decay of W±into lepton plus neutrino or a quark anti-quark paris. In the present study, we focus onthe “lepton+jets” final state, i.e. e+e− → tt¯→ l±νbb¯q′q¯.Top production and decays are very different from the other fermions, due to theemergence of many interesting angular correlations, which can be used to extract newphysics information [3]. In this work, we introduce three independent observables:TOP EW COUPLINGS AT LINEAR COLLIDERS 57e−e+Vtt¯•Fig. 1. – The Feynman diagram of the e+e− → tt¯ process. V can be a neutral gauge boson fromthe SM (i.e. γ/Z) or from new physics (Z′ etc.). The new physics contributions can enter alsoin the • as a radiative correction.– The cross section.– The forward backward asymmetry AtFB . We count the number of events in thehemispheres of the detector with respect to the polar angle θ of the t quark, i.e.(3) AtFB =N(cos θ > 0)−N(cos θ < 0)N(cos θ > 0) + N(cos θ < 0).– The slope of the distribution of the helicity angle.Using the fact that, in the rest frame of the t quark, the angle of the lepton fromthe W boson is distributed linearly, i.e.:(4)1ΓdΓd cos θhel=1 + λtcosθhel2=12+ (2FR − 1)cosθhel2with λt = 1 for tR and λt = −1 for tL, we can measure its slope. Note that wedefine the angle θhel in the rest frame of the t quark with the z-axis defined by thedirection of motion of the t quark in the laboratory [4].The ILC will allow for polarized electron and positron beams. Thus, with the use ofpolarized beams, we can double the above observable for each configuration; i.e. totalof six observables are available. Thus, one can, in principle, determine the six CP -conserving form factors. On the other hand, we have to keep in mind that near thett¯ threshold, the observables depend always on the sum F1V + F2V . Therefore, a fulldisentangling of the form factors will be imprecise for energies below about 1TeV. Hence,in the present study either the four form factors F1V,1A are varied simultaneously, whilethe two F2V are kept at their SM values, or vice versa. Throughout this article, theCP -violating form factors FX2A will be kept at their SM values.3. – ResultThe detailed results on the reconstruction efficiency, AtFB and θhel are given in theoriginal paper [5]. The achievable precision of the form factor determination are sum-marized in table I and fig. 2 and are compared with results of earlier studies for a linearcollider as published in the TESLA TDR [6] as well as with precisions obtained in asimulation study for the LHC. Note, that in the LHC and TESLA studies only one form58 M. S. AMJAD, T. FRISSON, E. KOU, R. PO¨SCHL, F. RICHARD and J. ROUE¨NE´Table I. – Sensitivities achievable at 68.3% CL for CP -conserving form factors eFX1V,A at theLHC and at linear e+e− colliders. Note that the form factors appearing here are related to theform factors defined in eq. (1) via eFX1V = −(FX1V +FX2V ), eFX2V = FX2V , eFX1A = −FX1A, eFX2A = −iFX2A.The assumed luminosity samples and, for e+e− colliders, the beam polarization, are indicated.In the LHC studies and in earlier studies for a linear e+e− collider as published in the TESLATDR [6] study, only one coupling at a time is allowed to deviate from its SM value. In thepresent study, denoted as ILC DBD, either the four form factors eF1 or the two form factors eF2are allowed to vary independently. The sensitivities are based on statistical errors only.Coupling SM value LHC [7] e+e− [6] e+e− [ILC DBD ]L = 300 fb−1 L = 300 fb−1 L = 500 fb−1P,P ′ = −0.8, 0 P,P ′ = ±0.8,∓0.3Δ eF γ1V 0.66+0.043−0.041−−+0.002−0.002Δ eFZ1V 0.23+0.240−0.620+0.004−0.004+0.002−0.002Δ eFZ1A -0.59+0.052−0.060+0.009−0.013+0.006−0.006Δ eF γ2V 0.015+0.038−0.035+0.004−0.004+0.001−0.001Δ eFZ2V 0.018+0.270−0.190+0.004−0.004+0.002−0.002factor was varied at a time, while, in the present study, two or four form factors are variedsimultaneously. From the comparison between the numbers, it is justified to assume thatthe measurements at an electron-positron collider lead to a spectacular improvement and,thanks to the γ/Z0 interference, an e+e− collider can fix the sign of the form factors. Atthe LHC the t quark couples either to the photon or to the Z0. In that case the crosssection is proportional to, e.g., (FZ1V )2 + (FZ1A)2. The precision expected at the LHCcannot exclude a sign flip of either FZ1V or of FZ1A. On the one hand, the LEP boundscan exclude a sign flip for FZ1A which renders a much better precision for F˜Z1A comparedwith F˜Z1V . Clearly, the precisions which can be obtained at the LHC are to be revisitedin the light ofreal LHC data.4. – Prospect: issue of theoretical uncertaintiesWe present a comprehensive analysis of the tt¯ quark production using the semi-leptonic decay channel. Results are given for a centre-of-mass energy of√s = 500GeVand an integrated luminosity of L = 500 fb−1 shared equally between the beam polar-izations. P = ±0.8 and P ′ = ∓0.3. The sensitivity to the form factors as obtainedin the present study for the ILC reaches to a per mil level, which would allow for theverification of new physics predictions. While further refinement of the experimentalanalysis and detailed study of the systematic error should be done, it is becoming clearTOP EW COUPLINGS AT LINEAR COLLIDERS 591VγF~1VZF~1AZF~2VγF~2VZF~Uncertainty-310-210-1101 ILC (preliminary)LHC (hep-ph/0601112)Fig. 2. – Comparison of statistical precisions on CP -conserving form factors expected at theLHC, taken from [7] and at the ILC. The LHC results assume an integrated luminosity of L =300 fb−1. The results for ILC assume an integrated luminosity of L = 500 fb−1 at √s = 500GeVand a beam polarization P = ±0.8, P ′ = ∓0.3.that the urgent issue to be resolved in this program is actually reducing the theoreticaluncertainties. The QCD corrections for e+e− → tt¯ are known up to N3LO [8-10] andthe estimated theoretical uncertainties are at a per mil level. On the other hand, theelectroweak corrections are known only at one-loop level [11,12], and the estimated errorsin the cross section and in the forward-backward asymmetry are, respectively, about 5%and 10%, which exceeds the experimental precision. An advance of the theoretical effortstowards this direction is most appreciable. On the other hand, knowing the origin of thetheoretical uncertainties, we may also look for new observables which receive milder elec-troweak corrections. Indeed, as mentioned earlier, the unique angular correlations of thetop production and decay may be used to find such new observables.∗ ∗ ∗We would like to thank to the organizers for the invitation to LC13 at ETC* in Trento.The work of E.K. was supported in part by the ANR contract “LFV-CPV-LHC” ANR-NT09-508531.REFERENCES[1] Juste A., Kiyo Y., Petriello F., Teubner T., Agashe K., Batra P., Baur U.,Berger C. F. et al., hep-ph/0601112.[2] Hollik W., Illana J. I., Rigolin S., Schappacher C. and Stockinger D., Nucl.Phys. B, 551 (1999) 3; 557 (1999) 407(E) [hep-ph/9812298].[3] Parke S. J. and Shadmi Y., Phys. Lett. B, 387 (1996) 199 [hep-ph/9606419].60 M. S. AMJAD, T. FRISSON, E. KOU, R. PO¨SCHL, F. RICHARD and J. ROUE¨NE´[4] Berger E. L., Cao Q.-H., Chen C.-R., Yu J.-H. and Zhang H., Phys. Rev. Lett., 108(2012) 072002 [arXiv:1201.1790 [hep-ph]].[5] Amjad M. S., Boronat M., Frisson T., Garcia I., Poschl R., Ros E., Richard F.,Rouene J. et al., arXiv:1307.8102.[6] Aguilar-Saavedra J. A. et al. (ECFA/DESY LC Physics Working GroupCollaboration), hep-ph/0106315.[7] Juste A., Kiyo Y., Petriello F., Teubner T., Agashe K., Batra P., Baur U.,Berger C. F. et al., hep-ph/0601112.[8] Kiyo Y., Maier A., Maierhofer P. and Marquard P., Nucl. Phys. B, 823 (2009) 269[arXiv:0907.2120 [hep-ph]].[9] Bernreuther W., Bonciani R., Gehrmann T., Heinesch R., Leineweber T.,Mastrolia P. and Remiddi E., Nucl. Phys. B, 750 (2006) 83 [hep-ph/0604031].[10] Hoang A. H., Mateu V. and Mohammad Zebarjad S., Nucl. Phys. B, 813 (2009) 349[arXiv:0807.4173 [hep-ph]].[11] Fleischer J., Leike A., Riemann T. and Werthenbach A., Eur. Phys. J. C, 31 (2003)37 [hep-ph/0302259].[12] Khiem P. H., Fujimoto J., Ishikawa T., Kaneko T., Kato K., Kurihara Y., ShimizuY., Ueda T. et al., Eur. Phys. J. C, 73 (2013) 2400 [arXiv:1211.1112 [hep-ph]].

Top EW couplings at Linear Colliders

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Top EW couplings at Linear Colliders

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