I breafly discuss very recent progress in the theoretical description of

Standard Model single top production at hadron colliders

Francesco, Tramontano

I breafly discuss very recent progress in the theoretical description of
Standard Model single top production at hadron colliders

Francesco Tramontano

Archivio della ricerca - Università degli studi di Napoli Federico II

SNSN-323-63January 18, 2017Single top theoryFrancesco TramontanoDepartimento di Fisica “E. Pancini”,Universita` di Napoli Federico II and INFN, sezione di Napoli,via Cintia I-80126 Napoli, ItalyI breafly discuss very recent progress in the theoretical description ofStandard Model single top production at hadron colliders.PRESENTED AT9th International Workshop on Top Quark PhysicsOlomouc, Czech Republic, September 19–23, 2016arXiv:1701.04230v2  [hep-ph]  17 Jan 20171 IntroductionMany things have changed since the work of Willenbrock and Dicus in 1986 [1] thatshowed the relevance of single top physics at hadron colliders using tree level diagrams.In this presentation I’ll review the progress of the last year, since the end of TOP2015.Furthermore, I’ll just present a selection of results, I apologise, there has been indeeda lot of progress recently. The reader will note that “single top” production in hadroncollisions is more and more an important tool that triggers progress in high energyphysics. In the next three subsections I’ll report about new computations for t-channel, tW associated production and s-channel respectively.2 t-channelIn ref. [2] the authors report about the computation of the Next-to-Next-to LeadingOrder (NNLO) QCD corrections to the t-channel single top production and decay.The computation has been performed using the structure function approach neglect-ing the color transfer among the two fermion lines, furthermore, the merging of thecorrections for the production and decay of the top quark has been worked out inthe narrow width approximation. From this differential computation one expects thehighest precision on distributions that are inclusive or insensitive to the invariant massof the reconstructed top, like the light jet transverse momentum and pseudo-rapidity.Furthermore, such computations might be relevant for pdf studies. Adopting thefollowing definition for the NNLO cross section:δσNNLO =1Γ0t[dσ2dΓ0t + dσ1(dΓ1t −Γ1tΓ0tdΓ0t)(1)+ dσ0(dΓ2t −Γ2tΓ0tdΓ0t −Γ1tΓ0t(dΓ1t −Γ1tΓ0tdΓ0t))]the results for the total cross section at LHC at 13TeV obtained using CT14 pdf andusing mt = 173.2 as central scale are reported in Table 1. One can observe the niceinclusive [pb] LO NLO NNLOt quark 143.7+8.1%−10% 138.0+2.9%−1.7% 134.3+1.0%−0.5%t quark 85.8+8.3%−10% 81.8+3.0%−1.6% 79.3+1.0%−0.6%Table 1: Inclusive cross sections for top (anti-)quark pro- duction at 13 TeV at variousorders in QCD.features of the higher order corrections that give a small contribution and reduce the1Figure 1: Ratios of fiducial cross sections of top anti-quark to top quark productionwith decay at 13 TeV as a function of the pseudorapidity of the charged lepton.theoretical error. The impact of higher order corrections on the lepton charge ratiois shown in Figure 1.Another important step forward has been done improving the matching of partonshower to matrix elements that involve a top-quark resonance. The problem of thematching of the Next to Leading Order (NLO) computation to a Parton Shower (PS)in presence of internal resonances is of prominent importance for the whole physicsprogram at the LHC. Leading collaborations have started to address this problemstudying t-channel single top production.With the general subtraction schemes like Catani and Seymour or Frixione, Kun-szt and Signer, only the ingoing and outgoing particle momenta are relevant and thecounter events have the primary condition to adsorb the momenta of the extra radi-ated parton among the particles of the Born process, irrespective of the presence ofinternal resonances. This situation deteriorates the efficiency of the infrared cancella-tion. In a NLO+PS computation in presence of resonances there are even more severeproblems. In the case of the POWHEG method for example, to work correctly, theratio among the real emission and underlining Born matrix elements must becomelarge only in the collinear limit, while, if there are resonances, the general mappingsthat are employed to attach the radiation can produce unphysical distortions in theSudakov factor. Last, but not less important, PS has to be instructed about whenpreserve resonance mass during the shower to avoid arbitrary shifts of the resonanceinvariant mass resulting in an unphysical distortion of the top line shape.Solutions to all the problems listed above have been proposed by two groups.The solution found in POWHEG in [3] is based on a division of the contributions to2the cross section following all the possible resonance histories obtained introducingappropriate multiplicative factors for all the contributions. The other fundamentalstep is the introduction of resonance aware mappings, in such a way to minimisethe mismatch among real and subtractions. A crucial aspect here is the delicateseparation of the real contribution into resonance histories that preserve collinearfactorisation and allow for subtractions computed in the resonance frame.Similarly, the solution found in MG5 aMC@NLO [4] is based on the divisionof the contribution following the possible resonance histories performed combininginput from the amplitude structure, the FKS singular region and information on thekinematical configuration. Then a remapping of the event kinematics is performed tomatch the resonance invariant mass, resorting on numerical methods to compute thenew Jacobian factor so that there is no need of further analytic integrations.The two collaborations have independently compared their different generatorsfor the same process to understand the effects of the new treatment of the radia-tion. In POWHEG-BOX-RES the authors simulated pp collisions at 8TeV, using theMSTW2008NLO pdf set and assuming the following jet cuts: pT > 25, |η| < 4.5adopting the kt-algorithm with R=0.5. The top quark distributions are given in Fig-ure 2. They are obtained writing the Hardest Radiation (RES-HR ) of the event orboth the hardest radiation from the production and the one from the decay (RES-AR).One observe good agreement among the two. In Figure 3 there is the reconstructedFigure 2: pt (left) and rapidity (right) distributions of the top quark produced in8TeV proton-proton collisions generated with POWHEG-BOX-VRES.top mass distributions. On the left the All Radiation generation (RES-AR) is com-pared to the one with hard radiation generated only in the production process andthe top decay described by the PS. A shift of about 1GeV is found. On the right sidethe AR and HR generators are compared and a less pronounced difference is foundfor the top line shape.With MG5 aMC@NLO the authors had the same setup plus a cut on the recon-structed top mass 140GeV < M(W+, Jb) < 200GeV. In their analysis, the authors3Figure 3: Comparisons of the reconstructed top quark mass obtained with differentgenerators for 8TeV proton-proton collisions generated with POWHEG-BOX-VRES.made a detailed study of the dependance of the predictions on the variation of thetechnical parameters that in turn is useful to understand the remaining associateduncertainties.In Figure 4 and 5 the invariant mass of the the reconstructed top quark and themass of the primary b-jet are shown respectively. One observe how the PS smears andflatten the sharp peak of fixed order prediction in the first and that the b-jet massresults much harder in NLO+PS wrt fixed order NLO. The reader can find manydistributions and details in the original works.3 tW associated productionAs for the tW associated production, recently the POWHEG-BOX-VRES collabo-ration applied the new method for the treatment of the resonances histories to thecomputation of the bb plus four leptons production at the LHC, and so completingthe description of tt pair production and decay at NLO+PS including all the nonresonant and off shell contributions that produce the same final state [5]. This newgenerator is also an ideal generator for tW associated production where all the effectsof the interference with the tt production are consistently included.In Figure 6 are shown the cross sections for exactly 1-bjet above a pt threshold,inclusive in the left panel and exclusive in the right panel. One can note an increas-ingly important single top contribution at small pt jet threshold. A detailed study ofWt and HWt associated production at NLO+PS has also been presented in [6].4Figure 4: invariant mass distribution of the the reconstructed top quark generatedwith MG5 aMC@NLO.4 s-channelBased on the results for the soft gluon resummation of Kidonakis [7, 8, 9] in ref. [10]Alekhin et al. implemented an approximated NNLO s-channel single top cross section.The authors then extracted the value of the top mass from single top cross sectionsand tt production fitting the best experimental determinations. Note that for thes-channel, Tevatron results are the ones with the highest precision. The exercise hasbeen performed for three different pdf sets and shows that slightly more consistentvalues of the top mass are obtained with the ABM sets. Note however that theexperimental cross sections determinations are correlated to the assumptions on thetop mass value and that this correlation is not available for all the measurements andhas not been taken into account in the work of ref. [10] ∗.∗We thank D. Hirschbu¨hl and S. Moch for clarifications on this point.5Figure 5: Mass distribution of the primary b-jet in single top events generated withMG5 aMC@NLO.5 conclusionAfter at least thirty years of computations aiming to describe the production of asingle top quark in hadronic collisions, a lot of progress has been done. Thanks tothe relative simplicity of the hard process and the large amount of available data,the theoretical description of single top production at hadron colliders is an excellentlaboratory to study new ideas on how to treat the radiation. At the same time in thenear future, single top observables could be competitive on pdf studies and top massdeterminations and so trigger further developments.References[1] S. S. D. Willenbrock and D. A. Dicus, Phys. Rev. D 34, 155, (1986).[2] E. L. Berger, J. Gao, C.-P. Yuan and H. X. Zhu, Phys. Rev. D 94 no.7, 071501,(2016)[3] T. Jezˇo and P. Nason, JHEP 1512 065, (2015)6Figure 6: Cross sections for exactly 1-bjet above a pt threshold, inclusive in the leftpanel and exclusive in the right panel, produced with the bb4l POWHEG-BOX-VRESgenerator.[4] R. Frederix, S. Frixione, A. S. Papanastasiou, S. Prestel and P. Torrielli, JHEP1606 027, (2016)[5] T. Jezˇo, J. M. Lindert, P. Nason, C. Oleari and S. Pozzorini, Eur. Phys. J. C 76no.12, 691, (2016)[6] F. Demartin, B. Maier, F. Maltoni, K. Mawatari and M. Zaro, arXiv:1607.05862[hep-ph].[7] N. Kidonakis, Phys. Rev. D 74, 114012, (2006).[8] N. Kidonakis, Phys. Rev. D 75, 071501, (2007).[9] N. Kidonakis, Phys. Rev. D 81, 054028, (2010).[10] S. Alekhin, S. Moch and S. Thier, Phys. Lett. B 763 341, (2016)7

Single top theory

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Single top theory

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