We calculate flavor-changing neutral current (FCNC) processes with top-quark
production via anomalous couplings at LHC energies. We report on progress on
the FCNC processes gu -> tZ, gu -> t gamma and gu -> tg. We go beyond leading
order and include soft-gluon corrections through next-to-next-to-leading order.Comment: 5 pages, 3 figures; presented at DPF 2013, Santa Cruz, California,
  August 13-17, 201

Kidonakis, Nikolaos

Martin, Elwin

English

arXiv

We calculate flavor-changing neutral current (FCNC) processes with top-quark production via anomalous couplings at LHC energies. We report on progress on the FCNC processes gu -\u3e tZ, gu -\u3e t gamma and gu -\u3e tg. We go beyond leading order and include soft-gluon corrections through next-to-next-to-leading order

DigitalCommons@Kennesaw State University

Kennesaw State UniversityDigitalCommons@Kennesaw State UniversityFaculty Publications10-1-2013FCNC Top Quark Production via AnomalousCouplingsNikolaos KidonakisKennesaw State University, nkidonak@kennesaw.eduElwin MartinGeorgia Institute of TechnologyFollow this and additional works at: http://digitalcommons.kennesaw.edu/facpubsPart of the Physics CommonsThis Article is brought to you for free and open access by DigitalCommons@Kennesaw State University. It has been accepted for inclusion in FacultyPublications by an authorized administrator of DigitalCommons@Kennesaw State University. For more information, please contactdigitalcommons@kennesaw.edu.Recommended CitationKidonakis, Nikolaos and Martin, Elwin, "FCNC Top Quark Production via Anomalous Couplings" (2013). Faculty Publications. 3858.http://digitalcommons.kennesaw.edu/facpubs/3858arXiv:1310.0363v1  [hep-ph]  1 Oct 2013DPF2013-101October 2, 2013FCNC Top Quark Production via Anomalous CouplingsNikolaos Kidonakisaand Elwin Martinb1a Kennesaw State University, Kennesaw, GA 30144, USAb Georgia Institute of Technology, Atlanta, GA 30332, USAWe calculate flavor-changing neutral current (FCNC) processes withtop-quark production via anomalous couplings at LHC energies. We re-port on progress on the FCNC processes gu → tZ, gu → tγ and gu → tg.We go beyond leading order and include soft-gluon corrections throughnext-to-next-to-leading order.PRESENTED ATDPF 2013The Meeting of the American Physical SocietyDivision of Particles and FieldsSanta Cruz, California, August 13–17, 20131Speaker1 IntroductionTop quark production can probe various physics beyond the Standard Model, in-cluding flavor-changing neutral currents (FCNC). In various new physics models suchFCNC processes involve anomalous couplings of the top quark.The effective Lagrangian involving anomalous couplings of a t, q pair to elec-troweak bosons is the following:∆L1 =1Λe κtqV t σµν q FµνV + h.c., (1)where Λ is an effective scale which we set equal to the top quark mass, e is theelementary charge, κtqV is the anomalous FCNC coupling, with q a u- or c-quarkand V a photon or Z-boson, F µνV are the usual photon/Z-boson field tensors, andσµν = (i/2)(γµγν − γνγµ) with γµ the Dirac matrices. We note that t-u-Z and t-u-γcouplings numerically dominate the FCNC cross sections, while charm contributionsare small due to the small charm parton densities.Similarly, the Lagrangian that involves anomalous top couplings with gluons is∆L2 =1Λgs κqgt t σµν T a χ q Gaµν + h.c., (2)where gs is the strong coupling, κqgt is the anomalous coupling, Ta are the Gell-Mannmatrices, χ = fL PL + fR PR with PL(PR) the left (right) hand projection operator,and Gaµν is the gluon field tensor.Historically, HERA and Tevatron have looked for these processes and set limits onthe relevant anomalous couplings. More recently, the LHC has been used to search forFCNC processes in the top-quark sector. ATLAS has set limits of κugt/Λ < 6.9×10−3TeV−1 and κcgt/Λ < 1.6× 10−2 TeV−1 [1].In this contribution we study higher-order corrections that arise from soft-gluonemission to FCNC top quark processes. In particular we study the processes gu → tZ,gu → tγ, and gu → tg. The first two have already been studied in some detail in[2] where the one-loop soft-anomalous dimensions were calculated and approximatenext-to-leading order (NLO) and next-to-next-to-leading order (NNLO) analyticalexpressions were derived. Also in [2] the cross sections for FCNC production viagu → tZ and gu → tγ were calculated for Tevatron energy and the higher-ordercorrections were found to be important. Here we update these calculations for LHCenergies. We also consider soft-gluon corrections for the process gu → tg. For moredetails on soft-gluon resummation and its applications to Standard Model top quarkprocesses, including top-pair and single-top production, see the review in [3].1guutZg ttu ZFigure 1: Lowest-order Feynman diagrams for gu → tZ.2 gu → tZWe begin with FCNC top production via the process gu → tZ. The leading-order(LO) diagrams are shown in Fig. 1.165 170 175 180mt (GeV)45678910σ (pb)LONLO approxNNLO approxgu -> t Z       LHC     S1/2=8 TeV     ktuZ=0.1165 170 175 180mt (GeV)45678910σ (pb)8 TeV7 TeVgu -> t Z       LHC     NNLO approx     ktuZ=0.1Figure 2: (Left) LO, approximate NLO, and approximate NNLO results for gu → tZat 8 TeV LHC energy. (Right) Approximate NNLO results for gu → tZ at 7 and 8TeV LHC energies.In Fig. 2 we show results for the cross section for this process at LHC energiesas a function of top quark mass, mt. We use the MSTW2008 NNLO parton densities[4] and choose κtuZ = 0.1. We also set the factorization and renormalization scalesequal to the top quark mass. The left plot shows the LO and approximate NLOand NNLO cross sections at 8 TeV LHC energy. We see that the higher-order soft-gluon corrections significantly increase the cross section. The right plot compares theapproximate NNLO results at 7 TeV and 8 TeV energies at the LHC.The NLO approximate corrections (i.e. the NLO soft-gluon corrections) providean around 33% increase over the LO cross section at 7 and 8 TeV LHC energies.The sum of the NLO and NNLO approximate corrections provides a total increaseof around 40% over the LO cross section at both 7 and 8 TeV energies, which is2very significant. It is interesting to note that the percentage size of the corrections issimilar to those at Tevatron energy as calculated in [2].3 gu → tγguutγg ttu γFigure 3: Lowest-order Feynman diagrams for gu → tγ.We continue with the process gu → tγ. In Fig. 3 we show the lowest-orderdiagrams for this process. Analytical results for the NLO and NNLO soft-gluoncorrections were given in [2] where numerical results for Tevatron energy were alsoshown.Here we consider this process in pp collisions at the LHC. Again, the NLO soft-gluon corrections provide a considerable increase of the LO cross section, around 25%or more at 7 and 8 TeV LHC energies. However, the NNLO approximate correctionsare small (a few percent) and negative, in contrast to tZ production.4 gu → tgFinally, we study the process gu → tg which we have not considered before. Asmentioned in the Introduction, ATLAS has set limits on the anomalous couplingfor this process [1] and it is important to find how the limits may be affected fromchanges in the cross section due to higher-order corrections. The color structure ofthis process is considerably more complicated than for tZ and tγ production sincehere there is a gluon in the final state. In fact the soft anomalous dimensions are 3×3matrices in color exchange and they are currently being calculated. More results willbe reported elsewhere.5 ConclusionsTop quark production via FCNC processes may proceed via various anomalous cou-plings of the top quark in physics beyond the Standard Model. We have studiedseveral such processes and calculated higher-order corrections through NNLO fromsoft-gluon emission; such corrections are known to dominate the cross section for top3quark Standard Model processes so it is natural to assume the same for FCNC topquark processes. We find that the contributions from these corrections at LHC en-ergies are rather large, up to around 40% for gu → tZ and 25% for gu → tγ. Theprocess gu → tg is theoretically more complicated and is currently under study.ACKNOWLEDGMENTSThis material is based upon work supported by the National Science Foundationunder Grant No. PHY 1212472.References[1] ATLAS Collaboration, Phys. Lett. B 712, 351 (2012) [arXiv:1203.0529 [hep-ex]].[2] N. Kidonakis and A. Belyaev, JHEP 12, 004 (2003) [hep-ph/0310299].[3] N. Kidonakis and B.D. Pecjak, Eur. Phys. J. C 72, 2084 (2012) [arXiv:1108.6063 [hep-ph]].[4] A.D. Martin, W.J. Stirling, R.S. Thorne, and G. Watt, Eur. Phys. J. C 63, 189(2009) [arXiv:0901.0002 [hep-ph]].4

FCNC Top Quark Production via Anomalous Couplings

Abstract

Similar works

Full text

Available Versions

DigitalCommons@Kennesaw State University

CiteSeerX