This is a very brief overview of the current status of top-quark physics at hadron colliders, a selection of some mainly theoretical aspects without any claim of completeness

Signer, A.

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DOI 10.1393/ncc/i2012-11212-yColloquia: TOP2011IL NUOVO CIMENTO Vol. 35 C, N. 3 Maggio-Giugno 2012Top physics: An overviewA. SignerIPPP, Department of Physics, University of Durham - Durham DH1 3LE, UKricevuto l’ 1 Marzo 2012pubblicato online il 25 Maggio 2012Summary. — This is a very brief overview of the current status of top-quarkphysics at hadron colliders, a selection of some mainly theoretical aspects withoutany claim of completeness.PACS 14.65.Ha – Top quarks.1. – IntroductionThe special role of the top quark in testing the Standard Model and looking forphysics beyond it has been well documented. In this brief note I will try to give a personaloverview over the field without any claims whatsoever regarding completeness. Given thespecial motivation for top-quark physics, let me split the note into four parts: the breadand butter section that deals with standard topics such as cross section calculations, massdeterminations and spin correlations; the bread only section that—from a top-physicspoint of view—deals with controlling the background; the pure honey section that dealswith looking for BSM physics through the top sector and finally the bread and butterand a hint of honey section, dealing with the forward-backward asymmetry. In manyways this observable is a prototype for a large number of other investigations, where aparticular observable is studied in great detail in the hope of finding a deviation fromthe Standard Model. This procedure is of course not restricted to top-quark physics.However, a combination of two factors make the top sector particularly promising forsuch an attempt. First, the top can be described by perturbative methods in a veryreliable way and the danger of non-perturbative effects diluting deviations from theStandard Model are much smaller than in other areas. Second, in many extensions ofthe Standard Model the top sector plays a special role and, thus, it is natural to expect adeviation to show up first in the top sector. It is a lucky combination of these two factsthat gives us this unique opportunity and we should fully exploit it.2. – Bread and butterThe main processes to be considered are top-pair production as well as t-channel, s-channel and associated single-top production. These processes can be studied at variousc© Societa` Italiana di Fisica 34 A. SIGNERlevels of detail regarding the final state. The simplest approach is to treat the top quarkas a stable particle. Following this approach only observables that are completely inclu-sive in the decay products of the tops can be studied. Going beyond this approximation,the decay of the tops has to be included. This allows for the application of more realisticcuts on the decay products as well as the study of spin correlations. Finally one can goeven further by including so-called non-factorizable corrections that link the productionwith the decay of the tops. The theory status can roughly (!) be summarised as fol-lows: fully differential observables including the decay of the tops are known to NLO inthe strong and electroweak coupling for top pair production [1] and all single-top pro-cesses [2], including non-factorizable corrections in αs, whereas tt¯ pair production forstable tops basically is known at NNLO. All processes have been combined with partonshowers [3]. Furthermore, resummation of large logarithms as well as bound-state effectsin tt¯ production have been studied.So what can we do with these tools in hand? The most important observable of“bread and butter” physics is certainly the top quark mass. It is needed for StandardModel consistency checks and as input for other measurements and calculations. It is bynow well known that the standard measurement of the top mass through the invariantmass of the decay products has certain limitations. First, it is not clear how precisely torelate the measured value to a theoretically well-defined quantity such as the pole massmt. Second, small effects that are either not included or not fully understood such asnon-factorizable corrections and the non-perturbative link between the decay productsand the beam remnants might have an effect that is of the same order as the currentlyclaimed error. Finally, there is a theoretical limitation of the order of ΛQCD to theerror on the determination of the pole mass within a perturbative treatment, due torenormalon ambiguities.In order to avoid some of these issues, alternative top mass measurements can beconsidered. The dependence of the total cross section or any other suitable observableon the top mass can be used to determine the pole mass mt or directly the MS-mass mt.The latter does not suffer from renormalon ambiguities. This should lead to a betterperturbative behaviour of the total cross section expressed in terms of mt compared tomt and, therefore, to a more precise determination. In principle, such a determinationcould (and actually should) also be done for other short-distance masses that are closerto the pole mass (but still free of renormalon ambiguities).With increasing statistics it should also be possible to measure the top-quark massthrough single-top production. These measurement might not lead to the same preci-sion as measurements through top-pair production but could provide important crosschecks. In this context it should be mentioned that the best possible measurement ofthe top-quark mass can be obtained through a threshold scan at a linear collider. Sucha measurement would be free of many of the problems mentioned above and would allowto determine a short-distance top mass with an error δ ≤ 200MeV.Finally let me mention a few other interesting topics related to bread-and-butterphysics. Due to the fast decay of the top its spin information is transferred to the decayproducts. This leads to interesting spin correlations [4] that can be studied in both,top-pair production and single-top processes. Processes involving top quarks might alsobe useful in constraining parton distribution functions. In fact the cross section fortop-pair production has been computed to an accuracy that is better than the spreadcaused by the full spectrum of commonly used parton distributions. Distinguishingbetween single-top and single–anti-top production might also yield useful information.Apart from the mass, the width of the top and its Yukawa coupling yt are veryTOP PHYSICS: AN OVERVIEW 5important quantities. A reasonably precise determination of yt in particular would allowfor a direct test of the Higgs mechanism. Unfortunately, measuring yt through t t¯ Hproduction seems to be extremely difficult.Let me finish this section by stressing the impressive progress that has been madein computing next-to-leading–order corrections to ever more complicated processes. Forthe processes p p → t t¯ X with X ∈ {H,Z, γ, jet} NLO corrections have been known for awhile. Very recently, they also have been computed for p p → t t¯ j j and p p → W+ bW− b¯.For an overview with references to the various calculations see, e.g., ref. [5].3. – Bread onlyIn order to be able to make detailed studies of top quarks it is important to disentangletop events from background processes. A recent development is to identify boostedhadronically decaying top quarks through so-called top tagging [6]. Apart from thisdirect identification of top events there has also been very impressive progress in thecalculation of next-to-leading–order processes for background processes, in particularthe production of a vector boson with up to four jets [7].4. – Pure honeyAs mentioned in the introduction, one of the main motivations for detailed studies ofthe top sector is the hope to find physics beyond the Standard Model. In order to makesuch tests quantitative we need a way to describe BSM physics.One way to do this is to parameterise our ignorance of what is beyond the StandardModel. This can be done through an effective-theory approach and/or using generalisedvertices with anomalous couplings. This approach is very similar to what is done fortriple gauge-boson couplings. The idea is to devise an observable that is very sensitiveto a particular anomalous coupling and/or BSM operator and try to constrain (or evenbetter: measure) its coefficient. The advantage of this approach is that we can writedown all operators up to a certain dimension (usually all dimension 6 operators areconsidered) and thereby systematically cover all possibilities. One disadvantage is thatthese operators or anomalous couplings usually lead to violation of unitarity for largeenergies. At a hadron collider, the partonic energy covers a fairly large range. In orderto suppress an artificially large effect from the high-energy tail of the partonic centre-of-mass energy it is often necessary to introduce ad hoc form factors. In other words, thisapproach is actually only valid as long as the deviation from the Standard Model is nottoo large and the BSM effect can be treated as a perturbation.Another option is to consider explicit BSM models. A concrete model contains moreinformation that an effective-theory approach can give and can be extended to the high-energy range. If such a model consist of introducing new heavy particles there is of coursea direct connection to the effective-theory approach in that the BSM operators and theircoefficients can be computed by integrating out the heavy fields. However, we can alsothink of possibilities for new physics that do not necessarily involve such new heavy fieldsand which, therefore, can not be covered by the effective-theory approach. An obviousdisadvantage of considering explicit models is that there is simply too much freedomand a systematic approach is virtually impossible in the absence of clear guidance fromexperimental data.6 A. SIGNER5. – Bread and butter and a hint of honeyIt is certainly fair to say that the new measurements of AFB , the forward-backwardasymmetry, by CDF [8] and D0 [9] have caused the biggest excitement in top-quarkphysics since its discovery. While it is clear that both measurements lie above the pre-dicted Standard Model value, it is far less clear, whether the effect depends on theinvariant mass of the top pair. There are also some issues regarding the reliability ofthe theoretical prediction. Since the effect vanishes at tree-level in QCD, the currentlyavailable one-loop calculation is actually a leading-order calculation for AFB [10]. A fullNLO calculation for AFB is not yet available but there are several indications that thecorrections are not expected to be unusually large and, therefore, can not explain the ten-sion between the data and the theoretical prediction. One of the main arguments alongthese lines is due to partial results from resummation of logarithms. On the other hand,electroweak corrections turn out to be surprisingly large and increase the theoreticalvalue of AFB , albeit by far not enough to be in agreement with the measurement.This tension has led to a flurry of activity in studying extensions of the StandardModel that result in an increased AFB [11]. The difficulty in constructing such modelsis to ensure that other quantities, such as the cross section for top-pair production andnotably same-sign top production is not affected too much. Typically, these models areruled out before the ink on the paper is dry. In particular same sign top productionhas not been observed at the LHC and this alone has killed a large number of proposedmodels. There is some debate on whether or not there is room for BSM contributionsin the total cross section for t t¯ production, but in any case this presents another strongconstraint on BSM scenarios. Rather than getting completely carried away with BSMscenarios it is essential that “more boring” explanations are also investigated.Unfortunately, a cross-check of the Tevatron measurements at the LHC is not straight-forward. There is a quantity very similar to AFB , the charge asymmetry, which is causedby precisely the same effect and can be measured at a p p collider. However, because theeffect is limited to the q q¯ initial state this is difficult at the LHC, where the g g initialstate dominates. Currently, the measurements of the charge asymmetry are consistentwith zero (and with the Standard Model value). With additional cuts to enhance the q q¯contribution it should be possible to make more refined tests of the charge asymmetryin the future. Even so, it is perfectly possible that in top physics we will end up with asituation that is remarkably similar to LEP electroweak global fits: a very consistent pic-ture with one ∼ 3σ outlier which, as coincidence would have it, is the forward-backwardasymmetry in both cases.REFERENCES[1] Uwer P., talk presented at this Conference.[2] Motylinski P., these proceedings.[3] Re E., these proceedings.[4] Parke S., these proceedings.[5] Weinzierl S., arXiv:1201.4025 [hep-ph].[6] Spannowsky M., these proceedings.[7] Maitre D., these proceedings.[8] Aaltonen T. et al. (CDF Collaboration), Phys. Rev. D, 83 (2011) 112003.[9] Abazov V. M. et al. (D0 Collaboration), Phys. Rev. D, 84 (2011) 112005.[10] Pecjak B., these proceedings.[11] Aguilar-Saavedra J. A., these proceedings.

Top physics: An overview

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Top physics: An overview

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