Predicting anomalous and couplings via single top production in association with photon at FCC-hh
We study the anomalous FCNC and couplings via signal process including realistic detector effects for both leptonic and hadronic decay channels of the W boson at 100 TeV FCC-hh. The relevant background are considered in the cut based analysis to obtain not only limits on the anomalous and couplings but also branching ratios of and decay channels. We find that the sensitivity to the branching ratio of channel is three order better than the available LHC experimental limits, and it is comparable for the branching ratio of the decay channel with an integrated luminosity of 10 ab at 2 significance level.
One of the a most sensitive probe to search for a new physics beyond the Standard Model (SM) is the top quark with mass of 173.0 0.4 GeV Tanabashi:2018oca () close to electroweak symmetry breaking scale. Flavor changing neutral current interactions involving a top quark, other quark flavors and neutral gauge boson are forbidden at the tree level and are suppressed in a loop level due to Glashow-Iliopoulos-Maiani mechanism Glashow70 (). The predicted SM branching ratios of the top quark FCNC decays to a gluon, photon, or Higgs boson and up-type quarks are expected to be and are out of range for current experimental sensitivity AguilarSaavedra:2004wm (). These branching ratios significantly improved in the certain parameter space of many different models beyond the SM and are close to the current experimental limits ()). Therefore, the possible deviation from SM predictions of FCNC and couplings would imply the existence of new physics beyond the SM. Recently, the exclusion limits on the top quark FCNC couplings have significantly improved by the LHC. The current experimental constraints on the branching ratio of the top quark FCNC decays obtained at the ATLAS and CMS with 95% confidence level (C.L.) are tabulated in Table 1.
Probe of the new physics effects on FCNC top interactions in a model independent way is the effective Lagrangian approach AguilarSaavedra:2008zc (); AguilarSaavedra:2009mx (). In this approach, anomalous FCNC couplings are described by higher-dimensional effective operator independently from the underlying theory. Anomalous FCNC couplings have been extensively studied using this approach in the literature Han:1998tp (); delAguila:1999kfp (); Belyaev:2001hf (); Alan:2002wv (); Cakir:2003cg (); Yang:2004af (); Cakir:2005rf (); Zhang:2008yn (); Cakir:2009rq (); Drobnak:2010by (); Gao:2011fx (); Billur:2013ela (); Agram:2013koa (); Inan:2014mua (); Koksal:2014hba (); Hesari:2014eua (); Khanpour:2014xla (); Sun:2014qoa (); Goldouzian:2014nha (); Degrande:2014tta (); Hesari:2015oya (); Sun:2016kek (); Guo:2016kea (); Liu:2016dag (); Goldouzian:2016mrt (); Zarnecki:2017cmf (); Wang:2017pdg (); Denizli:2017cfx (); TurkCakir:2017rvu (); Cakir:2018ruj ().
where and are the strong and the electromagnetic coupling constants, respectively; are the Gell-Mann matrices with . and are the strength of anomalous FCNC couplings for and , respectively; denotes the left (right) handed projection operators; is the tensor defined as for the FCNC interactions. We assumed no specific chirality for the FCNC interaction vertices, i.e. and in this study.
Phenomenologically searching for FCNC effects involving a top quark are studied in many final states with various sensitivities. Mostly anomalous FCNC couplings are investigated through FCNC decay of top quarks in the processes where large number of top quarks are produced at high energy hadron colliders. However, this situation creates disadvantages, such as separating from generic multijet production by Quantum ChromoDynamics (QCD), especially when determining couplings. Direct single top production in association with a photon is suggested to be powerful probe to search for existence not only vertices but also vertices in hadron colliders. One can expect even further improvements on these bounds with a higher center of mass energy colliders. The Future Circular Collider (FCC) which has the potential to search for a wide parameter range of new physics is the energy frontier collider project currently under consideration FCC (). FCC-hh, is a unique option of FCC, has a design providing proton-proton collisions at the proposed 100 TeV centre-of-mass energy with peak luminosity Mangano:2017tke ().
In this study, we focus on both hadronic and leptonic decays of the final state W in the signal process to investigate the anomalous FCNC ( ) and ( ) couplings at FCC-hh. Details of event selection and cuts on kinematic variables are discussed for the signal and relevant SM background processes in addition to SM background of the same final state signal process. Finally, We conclude with the prediction on the sensitivity of FCC-hh to anomalous FCNC ( ) and ( ) couplings.
Ii Signal Cross Sections
In this study, we consider signal processes for searching anomalous FCNC and interactions which denotes in Eq.1. In the production of signal events, the effective Lagrangian with FCNC couplings is implemented to FeynRules package Alloul:2013bka () and embedded into
MadGraph2.5.3_aMC@NLO Alwall:2014hca () as a Universal FeynRules Output (UFO) module Degrande:2011ua (). A. set of Feynman diagrams contributing to signal process at tree level are shown in Fig.1. As seen from Fig.1, three diagrams in the first row contains vertices (black dot) and the four diagrams on the second row contains vertices (red dot). In Fig.2, we show that the total cross sections as a function of and couplings of signal processes which include anomalous FCNC and interactions and SM contribution as well as interference between FCNC vertices and SM. As it can be seen from Fig.2, in the region where the value of the couplings is less than 0.005 (0.0005), () and () couplings contribute at the same rate while contribution of () is larger than () coupling for large coupling region since the quark PDF has the dominant distribution at center of mass energy of 100 TeV. In addition, the anomalous contributions are visible for the value of the couplings bigger than 0.005 (0.0005) compared to SM background for ().
Iii Signal and Background Simulations
In this section, the analysis of signal process including the FCNC and couplings as well as relevant SM backgrounds with experimental conditions of the FCC-hh are given. The number of events events are generated by
MadGraph2.5.3_aMC@NLO Alwall:2014hca (). for each signals (using different coupling values) and relevant backgrounds.These generated events are passed through PYTHIA Sjostrand:2006za () for parton showering and hadronization. The FCC-hh baseline detector configuration embedded into Delphes 3.3.3 via FCC-hh card is used to include fast detector effects deFavereau:2013fsa (). During the production of events, produced jets inside the events are clustered by using FastJet Cacciari:2011ma () with anti- algorithm where a cone radius is = 0.4 Cacciari:2008gp (). Both leptonic () and hadronic () decays of boson are considered in the analysis of the signal. Then, analysis for and final states are performed. The relevant background processes and their corresponding cross sections are
where and . The relevant backgrounds sm, , , , and are considered in final state analysis. In addition to this relevant backgrounds and QCD backgrounds are also included in final states analysis. In order to minimize the effect of experimental issues such as fake photon and mis-tagged b-jet, and are considered as the other backgrounds. Since the light jet could be misidentified as b-jet (or photon) candidate. The and processes are also added as background events since there are more than one b-jet in the each top decays to . The process is an another SM background in our analysis to include any error in the mass reconstruction of Z and W bosons due to possible inaccuracy of the hadronic calorimeter.
In order to distinguish signal from relative backgrounds, different preselection and kinematical cuts are applied separately to hadronic and leptonic channel of boson in the signal process as follows: In the leptonic channel of signal, at least one photon () and one lepton () are required with one isolated b-jet () as a preselection cut. On the other hand, at least one photon ( ) and three jets (), one of them is isolated b-jet (), with no lepton are applied as a preselection cut in the hadronic channel of the signal. By these preselection cuts, not only b-jet rich backgrounds but also multijet backgrounds that contain mis-tagged particles in their events are eliminated for effective analysis of hadronic and leptonic signal channels. Kinematic distributions of the final state particles for leptonic and hadronic channels after pre-selection are given in Fig.3 and Fig.4, respectively. In Fig.3, the transverse momentum distributions of the photon, lepton, b-jet, MET distribution, a separation between a photon and b-jet as well as photon and lepton in the pseudorapidity-azimuthal angle plane are shown. In Fig.4, the transverse momentum distributions of the photon, first leading jet (), b-jet, a separation between a photon and b-jet , photon and as well as photon and second leading jet () in the pseudorapidity-azimuthal angle plane are depicted. Firstly, we applied cut on the transverse momentum of leading photon 50 GeV as well as other kinematical cuts for the final state particles (kinematic-I). As seen from Fig.3 and Fig.4, photons have large momentum because of recoil against the heavy top quark. Thus, leading photon with > 150 GeV (kinematic-II) is required to distinguish signal from backgrounds in both channels as well as other optimal kinematical cuts summarized in Table 2. Two leading light jets are used to reconstruct boson for hadronic channel while lepton and neutrino for leptonic channel. Since four-momentums of the leading and second-leading jets are precisely measured, one can reconstruct mass easily for hadronic channel. However, for the reconstruction of -boson in leptonic channel, one needs to know the longitudinal component of the neutrino momentum () has to be taken into account. The is obtained by missing transverse energy of the neutrino () and energy-momentum conservation in the vertex:
where ; the , and are the energy, transverse and longitudinal momentum components of the leading lepton, respectively. We chose the solution with the smallest absolute value of because the true is about 70% Belyaev:1998dn (). For both leptonic and hadronic channel, constraints on mass range of the reconstructed boson as well as the reconstructed top quark which is the vector sum of the 4-momenta of reconstructed -boson and -tagged jet are used as in Table 2. Effects of cuts defined in Table 2 on the number of events with fb can be seen in Table 3 and Table 4 for leptonic and hadronic channels, respectively. Specially kinematic-I cut set reduces , and backgrounds while selecting high cut (kinematic-II) effects other backgrounds as well. For example, the cut efficiency of kinematic-I after pre-selection is about 28.5 % for signal (), 4.1 % for sm background which has the same final state with signal, 3.6 % for , 0.02 % for , 0.23 % for tt , 13 % for tt and 0.44 % for in the leptonic channel. Applying kinematic-II cut enhance the cut efficiency further one order. In Fig.5, the reconstructed invariant mass distributions of signal ( and on the top and bottom, respectively) and relevant SM background processes for leptonic (on the left) and hadronic channel (on the right) are plotted in the mass window with the pre-selection cut. The sharp signal peaks for both leptonic and hadronic channels are clearly seen above broad relevant backgrounds in the invariant mass distributions. Therefore, we require reconstructed invariant mass window between 135 GeV and 195 GeV to calculate Statistical Significance (SS).
Using Poisson formula
where and are the signal and total background events at a particular luminosity. The results for the values depending on the couplings and at L=100 fb for leptonic (on the left) and hadronic (on the right) are given in Fig. 6. In this figure, only one coupling ( or ) at a time is varied from its SM value and and discovery ranges are presented. It is clear from Fig. 6 that the FCC-hh would reach =0.0065 (0.005) while =0.0041 (0.0028) at significance for leptonic (hadronic) channel. We also simultaneously vary both anomalous top couplings to find excluded region in - plane. The boundary of , and excluded region in - plane for leptonic (on the left) and hadronic (on the right) channels with an integrated luminosity 10 ab at 100 TeV are plotted in Fig. 7. For both anomalous top couplings at with L=100 fb gives better results than at with L=100 fb as seen in Fig. 7.
One can express results in terms of branching ratios which can be comparable with the results
of other studies. Both FCNC decay widths and total decay width ()) of the top quark are evaluated by
MadGraph2.5.3_aMC@NLO. We calculated the FCNC decay widths and depending on coupling and is defined as
Using Eqs. (4) and (5) and total decay width of the top quark , the FCNC coupling =0.0027 and obtained from Fig. 7 at SS value can be converted to the branching ratio and for hadronic channel with L=10 ab. These branching ratios are at the same order for leptonic channel.
We compare our results on the branching ratios with the current experimental results summarized in Table 1. Based on proton-proton collisions at 8 TeV within the CMS detector at the LHC at an integrated luminosity of 19.8 fb, the limits on the top quark FCNC branching ratios are and at 95% C.L. Khachatryan:2015att (). Our limit on the branching ratio for is three order smaller than the current CMS experimental results and one order better than the projected limits on top FCNC couplings at LHC 14 TeV and HL-LHC reported in Ref. ATLAS:2013hta (), where the expected upper limits on branching ratio are and for an integrated luminosity 300 fb and 3000 fb, respectively. The limits on the FCNC branching ratio of the decay channel are and reported from ATLAS collaboration via single top-quark production with flavor-changing neutral current processes in proton-proton collisions at a centre-of-mass energy of 8 TeV and corresponding to an integrated luminosity of 20.3 fb. The limit on the with the anomalous single top quark production in association with a photon process ranges at the same order as current ATLAS experiment.
Deviations from the SM predictions are often interpreted in terms of anomalous top couplings in the single top production. One can put constraints on each effective operators which could describe these possible deviations. In this paper, anomalous top FCNC couplings and in a model independent way have been investigated via signal process at 100 TeV center of mass energy. The both leptonic and hadronic decay channels of W boson in the final state of the signal are taken into account to obtain sensitivities of the anomalous couplings at FCC-hh including realistic detector effects in the analysis. Using distinctive feature of the prompt photon radiation in the final state of the signal process, the top FCNC interactions can be uncovered from overwhelming relevant SM backgrounds. Thus, high cut with other optimum kinematic cuts requirement are used as a tool to probe sensitivity of the anomalous couplings. With an integrated luminosity of 10 ab and = 100 TeV for a 2 SS value, the sensitivity to the branching ratio of channel is three order better than the available experimental limits, and comparable for the branching ratio of the decay channel.
Acknowledgements.This work was supported by Turkish Atomic Energy Authority (TAEK) under the grant No. 2018TAEK(CERN)A5.H6.F2-20. We acknowledge exciting discussion within the FCC-hh physics analysis meeting. The K.Y. O, A. S. and H. D work partially supported by the Bolu Abant Izzet Baysal University Scientific Research Projects under the Project no: 2018.03.02.1286.
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|Cuts||Leptonic channel||Hadronic channel|
|Pre-selection||, and||, , and no lepton|
|GeV, GeV||GeV, GeV|
|, MET > 30 GeV||, > 0.7|
|> 0.7 and > 0.7||> 0.7 and > 0.7|
|W-reconstruction||80 GeV < < 90 GeV||35 GeV < < 90 GeV|
|Top-reconstruction||135 GeV < < 195 GeV||135 GeV < < 195 GeV|