Production and decay of up-type and down-type new heavy quarks through anomalous interactions at the LHC

# Production and decay of up-type and down-type new heavy quarks through anomalous interactions at the LHC

İ. Turk Çakır Istanbul Aydın University, Application and Research Center for Advanced Studies, 34295, Istanbul, Turkey    S. Kuday Istanbul Aydın University, Application and Research Center for Advanced Studies, 34295, Istanbul, Turkey    O. Çakır Istanbul Aydın University, Application and Research Center for Advanced Studies, 34295, Istanbul, Turkey Ankara University, Department of Physics, 06100, Ankara, Turkey
###### Abstract

We study the process (where and ) through the anomalous interactions of the new heavy quarks at the LHC. Considering the present limits on the masses and mixings, the signatures of the heavy quark anomalous interactions are discussed and analysed at the LHC for the center of mass energy of TeV. An important sensitivity to anomalous couplings TeV, TeV, TeV and TeV, TeV, TeV for the mass of 750 GeV of the new heavy quarks and can be reached for an integrated luminosity of fb.

## I introduction

The standard model (SM) of the strong and electroweak interactions describes succesfully the phenomena of particle physics. However, there are many unanswered questions suggesting the SM to be an effective theory. In order to answer some of the problems with the SM, additional new fermions can be accommodated in many models beyond the SM (see Refs. Holdom2009 (), Atre2009 (), Atre2011 (), Chakdar2013 () and references therein). The new heavy quarks could also be produced in pairs at the LHC with center of mass energy of TeV. However, due to the expected smallness of the mixing between the new heavy quarks and known quarks, the decay modes can be quite different from the one relevant to charged weak interactions. A new symmetry beyond the SM is expected to explain the smallness of these mixings. The arguments given in Ref. Fritzsch1999 () for anomalous interactions of the top quark are more valid for the new heavy quarks and due to their expected larger masses than the top quark.

The ATLAS experiment ATLAS-3 () and CMS experiment CMS-5 () have searched for the fourth generation of quarks and set limits on the mass of GeV and GeV at TeV. The pair production of new heavy quarks have been searched by the ATLAS experiment Aad:2014efa (), ATLAS:2012qe () and the GeV mass limits are set at TeV. The CMS experiment have excluded masses below 557 GeVCMS-6 () . The vector-like quarks have been searched by the ATLAS experiment Aad:2011yn (), Aad:2012uu () and set bounds as GeV for charged current channel and 760 GeV for neutral current channel at TeV. The CMS experiment CMS-7 (), CMS-8 () have set the lower bounds on the mass of GeV at TeV. Some of the final states in the searches of new phenomena ATLAS2014 () and excited quarks Aad:2013rna () can also be considered in relation with the new heavy quarks.

The anomalous resonant productions of the fourth family quarks have been studied in Refs. Ciftci:2008tc (); Cakir2009 () at the LHC with TeV. sThe possible single productions of fourth generation quarks via anomalous interactions at Tevatron have also been studied in Refs. Arik:2002sg (); Sahin (). The parameter space for the mixing of the fourth generation quarks have been presented in Ref. Bobrowski (). The CP violating flavour changing neutral current processes of the fourth generation quarks have been analysed in Ref. Eilam2009 (), and the large mixing between fourth generation and first three generations have been excluded under the proposed fit conditions. Investigation of the parameter space favoured by the precision electroweak data have been performed for the fourth SM family fermions in Ref. OPUCEM ().

In this work, we present the analysis of anomalous productions and decays of new heavy quarks and at the LHC. We have performed the fast simulation for the signal and background. Any observations of the invariant mass peak in the range of GeV and excess in the events with the final states originating from and can be interpreted as the signal for the new heavy quarks and via the anomalous interactions.

## Ii Heavy Quarks Anomalous Interactıons

A general theory that has the standard model (SM) as its low energy limit can be written as a series in with operators obeying the required symmetries. The effective Lagrangian for the anomalous interactions among the heavy quarks ( or ), ordinary quarks , and the gauge bosons can be written explicitly:

 L=∑qi=u,c,tκqiγΛQqige¯t′σμνqiFμν+∑qi=u,c,tκqiz2Λgz¯t′σμνqiZμν+∑qi=u,c,tκqig2Λgs¯t′σμνλaqiGμνa
 +∑qi=d,s,bκqiγΛQqige¯¯b′σμνqiFμν+∑qi=d,s,bκqiz2Λgz¯¯b′σμνqiZμν+∑qi=d,s,bκqig2Λgs¯¯b′σμνλaqiGμνa+h.c. (1)

where , and are the field strength tensors of the gauge bosons; ; are the Gell-Mann matrices; is the electric charge of the quark (); , and are the electromagnetic, neutral weak and the strong coupling constants, respectively. , where is the weak mixing angle. is the anomalous coupling with photon; is for the boson, and is the coupling with gluon. Finally, is the cutoff scale for the new interactions.

## Iii Decay Widths and Branchıngs

For the decay channels where , we use the effective Lagrangian to calculate the anomalous decay widths

 (2)
 Γ(Q′→γq)=12(κqγΛ)2αeQ2qm3Q′λ0 (3)
 Γ(Q′→Zq)=116(κqZΛ)2αem3Q′sin2θWcos2θWλZ√λr (4)

with

 λ0=1−3m2q/m2Q′+3m4q/m4Q′−m6q/m6Q′ (5)
 λr=1+m4W/m4Q′+m4q/m4Q′−2m2W/m2Q′−2m2q/m2Q′−2m2Wm2q/m4Q′ (6)
 λZ=2−m2Z/m2Q′−4m2q/m2Q′+2m4q/m4Q′−6mqm2Z/m3Q′−m2Zm2t/m4Q′−m4Z/m4Q′ (7)

The anomalous decay widths in different channels are proportional to , and they are assumed to be dominant for TeV over the charged current channels. In this case, if we take all the anomalous coupling equal then the branching ratios will be nearly independent of . We have used three parametrizations sets entitled PI, PII and PIII. For the PI parametrization, we assume the constant value TeV , and PII has the parameters TeV with . For PIII we take the couplings TeV with the same value of . The index is the generation number.

Table 1 and Table 2 present the decay width and branching ratios of the new heavy quark through anomalous interactions for the parametrizations PI, PII and PIII, respectively. Taking the anomalous coupling TeV we calculate the decay width GeV and GeV for GeV and GeV, respectively. The branching into channel is the largest and branching into channel is the smallest for equal anomalous couplings with the parametrization PI. On the other hand, PII and PIII parametrizations give higher branching ratios into () than () channels due to factor in the parametrizations.

For the new heavy quark the decay witdh and branching ratios are presented in Table 3 and Table 4 for the parametrizations PI, PII and PIII, respectively. We calculate the decay width, by taking the anomalous coupling TeV, GeV and GeV for GeV and GeV, respectively. The branching for is the largest (30%) and its the smallest for (0.2%) channel for equal anomalous couplings with the parametrization PI. For PII and PIII parametrizations the branching ratios into () are larger than () channels. The and decay widths are about the same values for PII and PIII parametrizations.

## Iv The Cross Sectıons

In order to study the new heavy quark productions at the LHC, we have used effective anomalous interaction vertices and implemented these vertices into the CalcHEP package CalcHEP (). In all of the numerical calculations, the parton distribution function are set to the CTEQ6L parametrization CTEQ6L (). The new heavy quarks can be produced through its anomalous couplings to the ordinary quarks and neutral vector bosons as shown in Fig. 1.

Total cross sections for the productions of new heavy quarks and are given in Table 5 and Table 6 for the parametrizations PI, PII and PIII, at the center of mass energy of 8 TeV and 13 TeV. For an illustration, taking the mass of new heavy quarks as 700 GeV the cross section of production is calculated as 8.50 pb (10.03 pb) for the parametrization PIII at TeV. It can be seen from Table 5 and Table 6, the cross sections decreases while the mass of the new heavy quark increases. The cross section for production is larger than the production with a factor of 1.2-1.8 (0.7-1.0) for PI (PII and PIII) parametrization depending on the considered mass range at TeV. The general behaviours of the production cross sections depending on the mass of heavy quarks are presented in Fig. 2 and Fig. 3 for different parametrizations.

### iv.1 Analysis of the process pp→W+bV+X (V=g,Z,γ) for t′ signal

The signal process () includes the exchange both in the -channel and -channel. The -channel contribution to the signal process would appear itself as resonance around the mass value in the invariant mass. The -channel gives the non-resonant contribution. We consider that the boson decays into lepton+missing transverse momentum with the branching ratio and boson decays into dilepton with the branching . In our analyses, we consider the signal in the , and channels, where . However, if one takes the hadronic decays the signal will be enhanced by a factor of

We have obtained the cross sections by using the cuts pseudorapidity and transverse momentum GeV for jets and photon, in Table 7 (Table8, Table 9) for PI (PII, PIII) parametrizations, respectively. It appears from signal significance calculations that the optimized transverse momentum cut is >100 GeV for analyses.

The backgrounds for the final state (where photon, jet and boson) are given in Table 10. We apply the following cuts to the final state photon and jets as and GeV. For the background cross section estimates, we assume the efficiency for b-tagging to be , and the rejection ratios for quark jets and for light quark jets since they are assumed to be mistagged as -jets.

In order to find the discovery limits we use the statistical significance as

 SS=√2[(S+B)ln(1+SB)−S] (8)

where and are the numbers of the signal and background events, respectively. In Figs. 4- 6, the integrated luminosity required to reach significance for the signal of anomalous interactions is shown for parametrization PI, PII and PIII at the LHC with TeV. It is seen from these figures that the channel requires more integrated luminosity than the other channels. By requiring the signal significance , the contour plots of and mass of quark are presented in Fig. 7. The results show that one can discover the quark anomalous couplings down to 0.1 TeV in the channel for =750 GeV.

### iv.2 Analysis of the process pp→bV+X (V=g,Z,γ) for b′ signal

The signal process () includes the new heavy quark exchange both in the -channel and -channel. The - channel contributes to the signal process as resonance around the mass value in the invariant mass, while the - channel contributes to the non-resonant behaviour. For this process, we consider the leptonic decays of boson. In the analyses, we consider the signal to be , and .

We have obtained the cross sections by using the pseudorapidity cuts and transverse momentum cuts GeV for jets and photon, in Table 11 (Table 12, Table 13 for PI (PII, PIII) parametrizations, respectively. It appears from signal significance calculation that the optimized transverse momentum cut is >200 GeV for analyses.

The backgrounds for the final state (where photon, jet and boson) are given in 14. We apply the following cuts to the final state photon and jets as and GeV. It can be noted that the background cross section decreases as the cuts increases. We assume the efficiency for b-tagging to be , and the rejection ratios for quark jets and for light quark jets.