Received 13 August 2014; revised 12 September 2014; accepted 12 October 2014

ABSTRACT

The Fourth Generation and Vector Like Quark (VLQ) models are extensions of the Standard Model of particles physics. These models predict the existence of new heavy quarks like heavy top quark with electric charge 2/3 and heavy bottom quark with electric charge −1/3. The and will act the fourth generation quarks. In current work we present a search for a pair production of a fourth generation quark, quark and its antiparticle, followed by their decays to, bosons followed by decays to trilepton plus jets and missing transverse energy in the final state according to the process. We use Monte Carlo simulation techniques Pythia8, MadGraph5 and CalcHEP to simulate this process at both the Large Hadron Collider at CERN (proton-proton collisions) and the Fermilab Tevatron Collider (proton-antiproton collisions). We assume that the quark is a narrow state that always decay to a and bosons plus jets. We select 2 electrons + jets+ missing energy final states with one isolated with high transverse momentum. The three charged leptons plus missing energy in the final state offer the best discovery potential at the hadron colliders for new heavy top quark mass of 500 GeV. We study the possible signals at both the LHC and the Tevatron of new quarks coupled to the third generation quarks in the context of fourth generation and vector like quark models under the assumption of a branching ratios and . Heavy quark pair production gives interesting signals in final states with three charged leptons plus missing energy. Finally, from our analysis the new heavy fourth generation quark can be discovered at both the Tevatron and the LHC with mass 500 GeV.

Keywords:

LHC, Tevatron, Fourth Generation Model, Heavy Top, Monte Carlo Simulation

1. Introduction

The recent results of the SM Higgs boson at the LHC have significantly impacted the prospects and focus of heavy quark searches. In particular, the observation of a new boson by the ATLAS [1] and CMS [2] Collaborations with a mass of ~126 GeV and couplings close to those expected for the SM Higgs boson disfavors [3] fourth-generation models. These models predict a large increase in the production rate for, which is in tension with searches in the and decay Channels [4] [5] .

In this work, we searched for the existence of the new heavy quark top fourth generation quark in the charged trilepton plus jets and missing transverse momentum decay channel. In this channel, the quarks are produced in pairs and the final state contains three leptons that are produced from and bosons decays, and 2 hadrons are produced from decaying the boson and two b jets. We searched for the by analyzing Monte Carlo data generated by MadGraph5, CalcHEP and Pythia8. The discoveries of the top quark at the Tevatron have been many searches for a possible new generation of fermions. Those searches have found no evidence of new fermions beyond the standard model (SM). Searches for these spin-1/2 heavy top-quark partners were performed by the CDF Collaboration in proton-antiproton collisions at center of mass energy 1.96 TeV excluding at 95% confidence level (CL) top-quark partners with masses up to 400 GeV. A previous ATLAS analysis with proton-proton collisions at center of mass energy 7 TeV excludes a with masses up to 420 GeV. Many recent researches suggested the addition of a fourth generation of heavy chiral fermions [6] , which can provide new sources of CP violation that could explain the matter-antimatter asymmetry in the universe. The new weak-isospin doublet contains heavy up-type and down-type quarks that mix with the lighter quarks via an extended CKM matrix. The Vector-Like Quarks appear in many extensions of the SM such as little Higgs or extra dimensional models. In these models, a top-partner quark, for simplicity is referred to here as [7] .

A fourth generation of fermions is interesting since it can accommodate the baryon asymmetry in the universe, induces the heavy neutrino as a candidate for dark matter avoids the need for a light Higgs boson if the mass of the heavy quarks is large, and can relieve the tension in flavour physics results . The distribution of this variable presents a sharp kinematic limit at the W boson mass for production whereas for the signal topology it decreases slowly towards a higher mass value, due to the presence of the two additional invisible particles produced in association with the top-quark pair. A fourth generation of quarks, and would allow indirect bounds on the Higgs boson mass to be relaxed and may result in enough intrinsic matter and anti-matter asymmetry to explain the baryon asymmetry of the universe. Therefore, there is continued theoretical and experimental interest in the search for a fourth generation fermion. Discovery of the Higgs boson and recent measurements of the Higgs production cross section in various channels disfavors the chiral fourth generation.

A top quark fermionic partner which decays into a stable, neutral, weakly interacting particle A0 also appears in other SM extensions, such as little Higgs models or models of Universal Extra Dimensions (UED) with Kaluza-Klein parity. The production cross section at the LHC is predicted to be approximately six times higher for fermionic than for.

The discovery of the top quark [8] which completed the third generation of fundamental fermions in the quark sector of the Standard Model (SM) of particle physics, and searches for heavier quarks have been of particular interest in high-energy physics research. These quarks are often present in new physics models aimed at solving some of the limitations of the SM.

2. The Results

We will present an analysis for pair production and decay of fourth-generation quarks at the both the LHC (CERN) and the Tevatron (Fermi Lab.) with different center of mass energies and analyzing Monte Carlo data of event collisions produced from Pythia8, Madgraph5 and CalcHEP in the context of Fourth Generation and Vector Like Quark models. Pair production of quarks lead to two bosons, boson and two quar- ks in the final state. We will analyze the Monte Carlo data events that contain three charged leptons, jets and missing energy where one of boson decays leptoniclly and the other hadronically and boson decays leptoniclly, finally we get the final state We will calculate the production cross section of pair production, branching ratios, the decay width and different kinematics distributions of trilepton and missing energy in the final state such as transverse momentum, pseudrapidity, and also reconstructed mass of and bosons from final state leptons.

2.1. Production of t’

The resulting observed and expected upper limits on the production cross section are shown in Figure 1 as a function of, assuming and. Figure 1 shows the resulting production cross sections for the signal process of the pair production as a function of the mass at both the LHC for energies 6, 8, 10, 12 and 14 TeV and the Tevatron for 1.96 TeV. The red line represents the prediction of the cross sections of quarks expected at the LHC 14 TeV which is 37 pb at mass 300 GeV while at 500 GeV is 2.43 pb. Also from this figure the productions cross of at the LHC for center of mass energy 8 TeV is 7 pb for mass 300 GeV while at 500 GeV is 0.35 pb. By comparing these results with [9] where e + jets channel happens for the 95% CL observed (expected) lower limits for a quark mass of 490 (540) GeV and for muon + jets channel the corresponding -quark mass limit is 560 (550) GeV. The combined observed (expected) limit from both channels is 570 (590) GeV. A comparable lower limit on the mass of 557 GeV was obtained recently by the CMS Collaboration using a dilepton channel [10] which is the most restrictive yet found and raise the lower limit on the mass of a quark to a region where perturbative calculations for the weak interactions start to fail and non-perturbative effects become significant. Also from the figure the production cross section of the at the Tevatrom for mass 500 GeV is 0.0011 if we compared this with [11] with the top qu- ark mass 172.5 GeV to the NNLO cross section with NNNLO threshold corrections in the and -channels of 3.3 pb [12] . Diboson samples are generated with Pythia8. Their NLO cross sections are 12.3 pb for WW, 3.7 pb for WZ, and 1.4 pb for ZZ production [13] . On the other hand the uncertainty on the cross section for production is taken from the CMS measurement of 154 ± 18 pb at 7 TeV [14] . The cross section only depends on the quark mass. Finally The higher center-of-mass energy of the hadron collisions and the resulting larger production cross section at the Large Hadron Collider (LHC) are expected to improve the statistical precision compared to the Tevatron.

The expected cross section for top quark pair production at 7 TeV in proton-proton collisions is 165 pb for a top mass of 173 GeV [15] . As reference for production we use a Monte-Carlo (MC) simulation sample based on Madgraph5 [16] matched to Pythia8 [17] . The signal sample includes matrix elements describing the generation of up to four additional partons. The most relevant background processes for the trilepton channel in-

clude the production of three real isolated leptons with high transverse momentum (pT). Figure 2 shows the Drell-Yan (DY) production of charged leptons is modeled with Madgraph8 for masses above 10 GeV. Di-boson processes are modeled with Pythia8.

For fourth-generation quark signals, samples are generated with Pythia8 using the CTEQ6.6 PDF set for a range of masses from 200 GeV to 1000 GeV in steps of 100 GeV and the Higgs boson mass is assumed to be 125 GeV. For vector-like signals, samples corresponding to a single quark decaying to, and are generated with Madgraph5 using the CTEQ6L1 PDF set, and interfaced to Pythia8 for the parton shower and fragmentation. Figure 3 shows the transverse momentum of the two charged lepton from Z boson decay.

The main discriminate variable used in this search is the reconstructed Heavy quark mass built from the mass candidate and one of the two b-jet candidates. The reconstruction of the leptonically decaying boson usually yields two solutions, and there are two possible ways to pair the b-jet candidates with the boson candidates to form the heavy quarks. Among the four possible combinations, the one yielding the smallest abso-

lute difference between the two reconstructed heavy quark masses is chosen.

2.2. Event Selection

The event selection is optimized to identify the trileptonic final state which have three charged leptons with high transverse momenta, two electrons from boson decay and muon from boson decay candidates are reconstructed with GeV/c and. We apply a particle-based isolation and require that the flux of reconstructed neutral and charged particles in a cone built around the lepton’s thrust axis. The ambiguity is resolved by selecting the dilepton pair with opposite electric charge (from decay) and highest sum of transverse momenta. The event preselection criteria closely follow those used in recent ATLAS top quark studies and require exactly one isolated electron or muon with large transverse momentum at least two jets among which at least one is identified as originating from a b quark, and large missing transverse momentum. Same-flavor dileptons (ee or) compatible with are removed by requiring GeV For all dilepton channels it is further required that GeV in order to veto low- mass dileptons mostly produced by quarkonium decays. Directions as shown in Figure 5 have transverse momentum GeV and, excluding the transition region. Muon candidates are required to satisfy GeV and. For leptons satisfying these requirements the efficiencies of the relevant single-lepton triggers have reached their plateau values.

A search by the ATLAS Collaboration in the dilepton final state using 1.04 fb-1 of data at TeV obtained a lower 95% CL limit of GeV. This search did not attempt to identify the flavor of the jets, making a more relaxed assumption of, where could be any down type SM quark. A 95% CL limit of GeV [18] assuming, was obtained by the CMS Collaboration using 5.0 fb-1 of data at TeV.

The leptonically decaying boson is reconstructed using the lepton (muon as in Figure 4) and identified as the neutrino. Requiring that the identified as the neutrino invariant mass of the lepton- neutrino system equals the nominal boson mass allows reconstruction of the neutrino longitudinal momentum up to a two-fold ambiguity. In case no real solution exists, the neutrino pseudorapidity is set equal to that of the lepton, since in the kinematic regime of interest for this analysis the decay products of the boson tend to be collinear.

The transverse mass is defined by the formula where is the of the lep-

ton and s the azimuthal angle separation between the lepton and directions, as in Figure 5 for transverse mass distribution of W boson. To take advantage of the kinematic differences that exists between quark and quark decays when GeV. In particular, the hadronically decaying W boson can be reconstructed as a single isolated jet when it is sufficiently boosted, leading to a significantly improved sensitivity in comparison to previous searches. In addition, the result of this search is interpreted in the context of vector-like quark models where and.. In this case the additional signals,

other than, contribute to the signal acceptance and are accounted for in the analysis. Figure 6 shows Mass distributions of pair production of new top at the LHC energies 8, 12, 14 TeV and at the Tevatron for energy 1.96 TeV in the Vector Like Quark mode.

2.3. Branching Ratios of t’

Has a priori three main possible decay modes, , and, with branching ratios that vary Figure 7 shows the measurement of the ratios of branching fractions of using CalcHEP [18] . The quark as a function of Mass of and depend on the weak isospin quantum number of the quark. While all three decay modes can be sizable for a weak isospin singlet, decays to only and are most natural for a doublet. In the case of a triplet, the quark can decay either as a singlet or a doublet depending on its hypercharge. The mass limits obtained for the quark depend on the ranching ratio (BR) assumed. In current work a search for a decaying to is 25% for mass 300 GeV and start to decrease for higher mass of on contrast the decay channel to Higgs boson, starting to increase to reach the maximum value and become the heights one to 60% on the other hand the decay channel act as 10% at 400 GeV and this value of mass is near from the expectation value of which is 500 GeV so we search for in the current work via the decay channels and. CMS has measured the ratio of the Branching Ratios where [16] is the BR of the top decaying to s and where is the branching fraction of the top quark to a boson and a quark. A high-purity sample in the full-leptonic decay mode, corresponding to 2.2 fb-1 was used.

Figure 7 shows the decay channels of and the branching ratios for different values of mass. The energy of the jets is corrected as a function of and observed. Extra corrections to take into account the pile-up contribution are also applied on an event-per-event basis. The number of selected jets which have resulted from the fragmentation and hadronization of quarks. By counting the jets identified as -jets. The heavy flavor content of the selected events (both signal and background) can be determined from the -tagging multiplicity distribution.

Despite small contributions from other background processes, there is a non-negligible probability that at least one jet from a decay is either missed because it failed the acceptance or because it was not reconstructed, and another jet is chosen instead from ISR. This will be referred to as “jet misassignment”. Figure 8 shows the transverse momentum distribution of the boson at the LHC energies 8, 12, 14 TeV and at the Tevatron for energy 1.96 TeV in the Vector Like Quark.

Both charged particles candidates associated to secondary vertices as well as the residual energy flux from the neutral component due to pile-up. Figure 9 shows the pseudorapidity distribution of the boson via at the LHC energies 8, 12, 14 TeV and at the Tevatron for energy 1.96 TeV in the Vector Like Quark.

We require the jets to have GeV and. Jet candidates are further required to be separated from the selected leptons by in order to remove jets which were clustered with real isolated leptons.

The missing transverse energy Figure 10 of this process comes from decay of and we require GeV for final state. The largest background contribution comes from DY in the same-flavor channels and it is directly estimated from data while the contribution from other processes is estimated from simulation. The distribution of the angle between the leptons is used for this purpose. The measurement requires the correct identification of quark and light quarks, and correct association to its parent top. The is the associated high-pT object jet or electrons and including contributions from selected muon. Background from multi-jet production is suppressed by the requirement (20) GeV in the electrons channel.

Events were none or only one top is correctly reconstructed correspond to background events or to events where at least one of the jets is missed and the top is reconstructed with a wrong choice of jet.

2.4. Background of the Standard Model

Figure 11 shows the SM backgrounds in this search come from like-sign lepton pairs is production, and smaller sources include, like-sign, , and production. High purity sample of dilepton events where the main background contamination is expected to be from Drell-Yan and single-top quark events.

After event preselection the main background is production with lesser contributions from the production of a boson in association with jets (jets) and multi-jet events. Small contributions arise from single top-quark, jets and diboson production.

Multi-jet events contribute to the selected sample mostly via the misidentification of a jet or a photon as an e- lectron, or via the presence of a non-prompt lepton, e.g. from a semileptonic - or -hadron decay.

2.5. Decay Width of t’

Figure 12 shows the total decay width of the quark using events with quark pair candidates reconstructed in the final state with three charged leptons and four hadronic jets. The quark mass and the mass of the hadronically decaying boson are reconstructed for each event and compared with distributions derived from simulated signal and background samples to extract the quark width and the energy.

We reconstruct quark decays in the topology of, and. Events with a W bo-

son decaying into a charged lepton (electron or muon) and a neutrino and the other boson decaying into a pair of jets defines the lepton + jets channel.

For quark the partial decay widths are:

Being a kinematical function.

3. Conclusion

In this work we presented the production of a fourth generation heavy quark together with its antiparticle at both the Tevatron and the LHC using Monte Carlo event generators Pythia8, MadGraph5 and CalcHEP assuming a significant branching ratios for decays into in the context of Fourth Generation and Vector Like Quark models under the assumption of a branching ratios and. This process has very good sensitivity to production. The pair production of quarks gives final state with three leptons with branching ratios not too small. Signals in the trilepton final state involve the leptonic decay of and one boson and hadronic decay for another boson. We study the possible signals at both the LHC and the Tevatron of new quarks coupled to the third generation. Heavy quark pair production gives interesting signals in final states with three charged leptons plus missing energy. We presented an analysis for the Monte Carlo events collisions at mass of quark 500 GeV for events contain the trilepton + missing energy + jets in final state which characterized by a high transverse momentum for dielectron, isolated muon, large missing transverse momentum and four jets. The potential discovery of the new quarks in decay channels was obtained containing a and boson, especially in the final state with one charged lepton and four -tagged jets. It is shown that with the combination of the different channels the new top quarks can be identified and their charged and neutral decays established. The trilepton final state offers a good balance between signal branching ratio in pair production and SM background. Finally, from our analysis the new heavy fourth generation quark can be discovered at both the Tevatron and the LHC with mass 500 GeV.

Acknowledgements

It is a pleasure to thank Prof. Torbjörn Sjostrand, Lund Univ. Sweden for useful discussions of Pythia8 and Prof. Johan Alwall, Stanford Univ. USA for useful discussions of MadGraph5/MadEvent. The work by Maxim Yu. Khlopov was supported by the Ministry of Education and Science of Russian Federation, Project 3.472.2014/K and in the part related with various forms of dark matter by the Grant RFBR 14-22-03048.

References