Search

In the post-LHC era, the muon collider represents a frontier project capable of providing high-energy and high-luminosity leptonic collisions among future lepton-lepton particle accelerators. In addition, it provides significantly cleaner final states than those produced in hadron collisions. With this expectation in mind, in this article, we research the sensitivity of the anomalous neutral gauge boson couplings $Z\gamma\gamma\gamma$, $ZZ\gamma \gamma$, and $ZZZ\gamma$ defined by dimension-8 operators, through the $\mu^+\mu^- \to \mu^+\mu^-Z\gamma$ signal, with the $Z$-boson decaying to neutrino pair. The projections of new physics at the future muon collider with the center-of-mass energy of $\sqrt{s}=10$ TeV, integrated luminosity of ${\cal L}=10$ $\rm ab^{-1}$, and systematic uncertainties of $\delta_{sys}=0\%, 3\%, 5\%$, for extraction of expected sensitivity on the anomalous $f_ {T,j}/\Lambda^4$ couplings at $95\%$ confidence level, are of the order of ${\cal O}(10^{-4}- 10^{-3})$. Compared with the research of the ATLAS and CMS Collaborations on the anomalous quartic gauge couplings, we find that the high-luminosity future muon collider could have better sensitivity.

The presence of multi-boson self-interactions is implied by the non-Abelian gauge structure of the Standard Model (SM). Precise measurements of these interactions allow not only testing the nature of the SM but also new physics contribution arising from the beyond SM. The investigation of these interactions can be approached in a model-independent manner using an effective theory approach, which forms the main motivation of this study. In this paper, we examine the anomalous neutral quartic gauge couplings through the process $\gamma \gamma \rightarrow Z Z$ at the Compact Linear Collider (CLIC) with the center-of-mass energy of $\sqrt{s}=3$ TeV, integrated luminosities of ${\cal L}=5$ $\rm ab^{-1}$. The anomalous neutral quartic gauge couplings is implemented into FeynRules to generate a UFO module inserted into Madgraph to generate both background and signal events. These events are then passed through Pythia 8 for parton showering and Delphes to include realistic detector effects. We obtain that the sensitivities on the anomalous quartic neutral gauge couplings with $95\%$ Confidence Level are given as: $f_{T0}/\Lambda^{4}=[-1.06; 1.08]\times 10^{-3}$ ${\rm TeV^{-4}}$, $f_{T1}/\Lambda^{4}=[-1.06; 1.08]\times 10^{-3}$ ${\rm TeV^{-4}}$,$f_{T2}/\Lambda^{4}=[-1.06; 1.08]\times 10^{-3}$ ${\rm TeV^{-4}}$,$f_{T0}/\Lambda^{4}=[-1.06; 1.08]\times 10^{-3}$ ${\rm TeV^{-4}}$, $f_{T5}/\Lambda^{4}=[-4.08; 4.08]\times 10^{-4}$ ${\rm TeV^{-4}}$ and $f_{T8}/\Lambda^{4}=[-1.10; 1.10]\times10^{-4}$ ${\rm TeV^{-4}}$. Our results on the anomalous quartic neutral gauge couplings are set more stringent sensitivity with respect to the recent experimental limits., Comment: 21 pages, 6 figures

We have studied the sensitivity to flavor changing neutral current interaction of top and Higgs boson at the future circular collider in the hadron-hadron collision mode (FCC-hh). Our main concerns are the processes of $pp\rightarrow th$ (FCNC production) and $pp\rightarrow t\bar{t}$ (one top FCNC decay) which contributes to the single lepton, at least three jets and the missing energy transverse in the final state. On the one hand FCC-hh offers very high luminosity and large cross section for these signal processes, on the other hand signal can be distinguished from background which needs application of ingenious methods. Here, we have inspired and followed the searches at the LHC using our analysis path and enhanced attainable limits obtained on the possible top-Higgs FCNC couplings phenomenologically. We obtain new limits which are beyond the current experimental limits obtained from different channels and processes at the LHC. The potential discovery or exclusion limits on branching ratios for $tqH$ FCNC interactions can be set $BR(\eta_{uc})_{\mathrm{disc}}=9.08\times10^{-6}$ or $BR(\eta_{uc})_{\mathrm{exc}}=2.78\times10^{-6}$ at an integrated luminosity of 30 ab$^{-1}$ , respectively. Our results are compatible with the other channels already studied at FCC-hh., Comment: 15 pages, 29 figures

In this study, we investigate the process $pp \rightarrow W^{\pm} \gamma$ for the physics potential of the FCC-hh with $\sqrt{s}=100$ TeV to examine the anomalous $WW\gamma$ couplings defined by three CP-conserving and two CP-violating effective operators of dimension-6. The analysis containing the realistic detector effects is carried out in the mode where $W^{\pm}$ bosons in the final state decay into the leptonic channel. The best sensitivities obtained from the process $pp \rightarrow W^{\pm} \gamma$ on the anomalous couplings $C_{WWW}/ \Lambda^{2}$ and $C_{W,B}/ \Lambda^{2}$ determined by CP-conserving effective Lagrangians are $[-0.01; 0.01]$ TeV$^{-2}$ and $[-0.88; 0.88]$ TeV$^{-2}$, while $C_{\tilde{W}WW}/ \Lambda^{2}$ and $C_{\tilde{W}}/ \Lambda^{2}$ couplings defined by CP-violating effective Lagrangian are obtained as $[-0.03; 0.03]$ TeV$^{-2}$ and $[-0.47; 0.47]$ TeV$^{-2}$ at the FCC-hh with $\sqrt{s}=100$ TeV, $L_{int}=30$ ab$^{-1}$. However, if the systematic uncertainty is included, we obtain reduced sensitivities on the anomalous $WW\gamma$ coupling . The results are compared for assumed systematics of $5\%$ and $10\%$., Comment: 17 pages, 6 Figures and Tables IV

Gauge boson self-couplings are completely defined by the non-Abelian gauge symmetry of the Standard Model (SM). For this reason, a direct search for these couplings is extremely significant in understanding the gauge structure of the SM. The possible deviation from the SM predictions of gauge boson self-couplings would be a sign of the presence of new physics beyond the SM. In this work, we study the sensitivities on the anomalous couplings defined by dimension-8 operators related to the $ZZ\gamma\gamma$ and $Z\gamma\gamma\gamma$ quartic vertices through the process $e^+e^- \to Z\gamma\gamma$ with Z-boson decaying to charged leptons at the Compact Linear Collider (CLIC). We analyze the center-of-mass energy of 3 TeV, integrated luminosities of ${\cal L}=1, 4, 5$ $\rm ab^{-1}$, systematic uncertainties of $\delta_{sys}=0\%, 3\%, 5\%$, unpolarized and polarized electron beams and considering the Initial State Radiation and Beamstrahlung effects for extraction of expected sensitivity on the anomalous $f_ {T,j}/\Lambda^4$ couplings at $95\%$ confidence level, which are especially sensitive to the $Z\gamma\gamma$ channel. It is clear from the results that we expect better limits on the couplings if the systematic error is improved. The best limits obtained on the anomalous quartic couplings for the process $e^+e^- \to Z\gamma\gamma$ with $\sqrt{s}= 3$ TeV, ${\cal L}=5$ $\rm ab^{-1}$, and $\delta_{sys}=0\%$ can be approximately improved up to about 1.1 times better than the limits obtained with $\delta_{sys}=5\%$. Our sensitivities on the anomalous quartic couplings can set more stringent sensitivity by two orders of magnitude concerning the best sensitivity derived from the current experimental limits. Finally, with initial electron beam polarization, the sensitivity of the anomalous quartic couplings improves by almost a factor of 1.2., Comment: 34 pages, 15 figures

We investigate the potential of the HL-LHC for discovering new physics effects via the same-sign top pair signatures. We focus on the semi leptonic (electron and muon) decay of the top quarks and study the reach for a simplified model approach where top quark flavor changing could occur through a neutral scalar exchange. A relatively smaller background contribution and clean signature are the advantages of the leptonic decay mode of the same-sign $W$ bosons in the same-sign production processes of top quark pairs. Assuming the FCNC between top quark, up type quark and scalar boson from the new physics interactions the branchings could be excluded of the order ${\cal O}(10^{-4})$. We use angular observables of the same-sign lepton pairs and the top quark kinematics in the process which provide the possibility of separation of new physics signal from the SM backgrounds using machine learning teqniques. We find that the same-sign top quark pair production is quite capable of testing the top-Higgs FCNCs at the HL-LHC., Comment: 10 pages, 20 figures

We carry out a study in the approach of a dimension-8 effective field theory on the anomalous quartic $WW\gamma\gamma$ couplings that arise from the process $pp \to W\gamma\gamma$ at the Future Circular Collider-hadron hadron (FCC-hh). The process is sensitive to the $WW\gamma\gamma$ vertex, and future projections on the anomalous $f_ {M,i}/\Lambda^4$ and $ f_ {T,j}/\Lambda^4$ couplings can be put by measurements of $W\gamma\gamma$ production. We illustrate the physics opportunities and reach of the FCC-hh operating for a high integrated luminosity scenario of ${\cal L}=1, 5, 10, 30$ ${\rm ab}^{-1}$ of proton-proton collisions at a center-of-mass energy of $\sqrt{s}=100$ TeV and systematic uncertainties of $\delta_{sys}=0\%, 3\%, 5\%, 10\%$. Production cross-sections of the signal and the sensitivity constraints on the anomalous $f_ {M,i}/\Lambda^4$ and $ f_ {T,j}/\Lambda^4$ couplings at $95\%$ C.L. are identified through the leptonic and hadronic decay modes of the $W$-boson. The FCC-hh sensitivity on these anomalous couplings is expected might reach up to the order of magnitude ${\cal O}(10^{-4}-10^1)$. These sensitivity perspectives indicate that the $pp \to W\gamma\gamma$ channel at the FCC-hh has a greater potential to search for the anomalous quartic $WW\gamma\gamma$ couplings and to search for new physics than present colliders., Comment: 29 pages, 8 figures

We study single production of exotic vectorlike $Y$ quark with electric charge $|Q_{Y}|=4/3$ and its subsequent decay at the High Luminosity LHC (HL-LHC). Most of the vector like quark (VLQ) decays have the electroweak $W$ bosons in the intermediate state. Besides their direct productions singly or pairs, the $W$-bosons are involved in decay chains as a result of the decay of a top quark which contributes to the background. This is particularly the case since vectorlike $Y$ quark, which is estimated to be produced with a high cross-section, can only decay via a $W$ boson and a down type quark ($d,s,b$). We calculate the cross sections of signal (for different couplings and mass values) and relevant Standard Model (SM) backgrounds. After a fast simulation of signal and background events, estimations of the sensitivity to the parameters (mass range 1000-2500 GeV for coupling value $\kappa_{Y}=0.5$, and mass range 500-2000 GeV for coupling values $\kappa_{Y}=0.3$ and $\kappa_{Y}=0.15$) have been presented at the HL-LHC with center of mass energy $\sqrt{s}=14$ TeV and integrated luminosity projections of 300 fb$^{-1}$, 1000 fb$^{-1}$ and 3000 fb$^{-1}$., Comment: 14 pages, 21 figures

The Large Hadron electron Collider (LHeC) is designed to move the field of deep inelastic scattering (DIS) to the energy and intensity frontier of particle physics. Exploiting energy recovery technology, it collides a novel, intense electron beam with a proton or ion beam from the High Luminosity--Large Hadron Collider (HL-LHC). The accelerator and interaction region are designed for concurrent electron-proton and proton-proton operation. This report represents an update of the Conceptual Design Report (CDR) of the LHeC, published in 2012. It comprises new results on parton structure of the proton and heavier nuclei, QCD dynamics, electroweak and top-quark physics. It is shown how the LHeC will open a new chapter of nuclear particle physics in extending the accessible kinematic range in lepton-nucleus scattering by several orders of magnitude. Due to enhanced luminosity, large energy and the cleanliness of the hadronic final states, the LHeC has a strong Higgs physics programme and its own discovery potential for new physics. Building on the 2012 CDR, the report represents a detailed updated design of the energy recovery electron linac (ERL) including new lattice, magnet, superconducting radio frequency technology and further components. Challenges of energy recovery are described and the lower energy, high current, 3-turn ERL facility, PERLE at Orsay, is presented which uses the LHeC characteristics serving as a development facility for the design and operation of the LHeC. An updated detector design is presented corresponding to the acceptance, resolution and calibration goals which arise from the Higgs and parton density function physics programmes. The paper also presents novel results on the Future Circular Collider in electron-hadron mode, FCC-eh, which utilises the same ERL technology to further extend the reach of DIS to even higher centre-of-mass energies., Comment: 373 pages, many figures, to be published by J. Phys. G

In this paper, a study is carried out on the $e^-p \to e^-\gamma^* p \to p W^+\gamma \nu_e$ production to probe quartic $W^+W^-\gamma\gamma$ couplings using 10, 100 ${\rm fb^{-1}}$ of $e^-p$ collisions data at $\sqrt{s}$= 1.30, 1.98 GeV at the Large Hadron electron Collider (LHeC) and 100, 1000 ${\rm fb^{-1}}$ with $\sqrt{s}$= 3.46, 5.29 GeV at the Future Circular Collider-hadron electron (FCC-he). Production cross-sections are determined for both at leptonic and hadronic decay channel of the $W$-boson. With the data from future $e^-p$ colliders, it is possible to obtain sensitivity measures at $95\%$ C.L. on the anomalous $f_ {M,i}/\Lambda^4$ and $ f_ {T,i}/\Lambda^4$ couplings which are competitive with the limits obtained by the LHC, as well as with others limits reported in the literature. The production mode $e^-p \to e^-\gamma^* p \to p W^+\gamma \nu_e $ in $e^-p$ collisions offers a window for study the quartic $W^+W^-\gamma\gamma$ electroweak bosons couplings at the LHeC and the FCC-he, which provides a much cleaner collision environment than the LHC., Comment: 27 pages , 10 figures

We explore the physics expected sensitivity at the Large Hadron electron Collider (LHeC) and the Future Circular Collider-hadron electron (FCC-he) to search for the anomalous quartic $WW\gamma\gamma$ couplings in single $W$-boson production in association with a photon. We study the process $ep \to e^-\gamma^* p \to eW\gamma q'X$ via the subprocess $\gamma^* q \to W\gamma q'$. The center-of-mass energies and luminosities of the LHeC are assumed to be $\sqrt{s}=1.30, 1.98$ TeV, ${\cal L}=10-100$ ${\rm fb^{-1} }$ and for the FCC-he $\sqrt{s}=3.46, 5.29$ TeV and ${\cal L}=100-1000$ ${\rm fb^{-1} }$. With these energies and luminosities, we estimate bounds on the anomalous quartic $WW\gamma\gamma$ couplings at $95\%$ C.L., which can be an order of magnitude stringent than the experimental limits report by ATLAS and CMS Collaborations at the LHC., Comment: 28 pages, 10 figures. arXiv admin note: text overlap with arXiv:1211.3100 by other authors

We conducted a study on measuring $W^+W^-$ production and on the sensitivity limits at $95\%$ Confidence Level on thirteen anomalous couplings obtained by dimension-8 operators which are related to the anomalous quartic $WW\gamma\gamma$ couplings. We consider the main $e^-p \to e^-\gamma^*\gamma^*p \to e^-W^+W^-p$ reaction with the sub-process $\gamma^*\gamma^* \to W^+W^-$ at the Large Hadron electron Collider (LHeC) and the Future Circular Collider-hadron electron (FCC-he). For the LHeC, energies of the $e^-$ beams are taken to be $E_e =60$ and 140 GeV and the energy of the $p$ beams is taken to be $E_p = 7$ TeV. For the FCC-he, energies of the $e^-$ beams are taken to be $E_e =60$ and 140 GeV and the energy of the $p$ beams is taken to be $E_p = 50$ TeV, respectively. It is interesting to notice that the LHeC and the FCC-he will lead to model-independent limits on the anomalous quartic $WW\gamma\gamma$ couplings which are one order of magnitude stringent than the CMS Collaboration limits, in addition to being competitive with other limits reported in the literature., Comment: 28 pages, 10 Figures and 13 Tables

The quartic gauge boson couplings that identify the strengths of the gauge boson self-interactions are exactly determined by the non-Abelian gauge nature of the Standard Model. The examination of these couplings at $ep$ collisions with high center-of-mass energy and high integrated luminosity provides an important opportunity to test the validity of the Standard Model and the existence of new physics beyond the Standard Model. The quartic gauge boson couplings can contribute directly to multi-boson production at colliders. Therefore, we examine the potential of the process $ep \rightarrow \nu_{e}W^{+}W^{-} j$ at the Large Hadron Electron Collider and the Future Circular Collider-hadron electron to study non-standard $WWWW$ couplings in a model independent way by means of the effective Lagrangian approach. We present an investigation on measuring $W^{+}W^{-}$ production in pure leptonic and semileptonic decay channels. In addition, we calculate the sensitivity limits at $95\%$ Confidence Level on the anomalous $\frac{f_{M0}}{\Lambda^{4}}$, $\frac{f_{M1}}{\Lambda^{4}}$, $\frac{f_{M7}}{\Lambda^{4}}$, $\frac{f_{S0}}{\Lambda^{4}}$, $\frac{f_{S1}}{\Lambda^{4}}$, $\frac{f_{T0}}{\Lambda^{4}}$, $\frac{f_{T1}}{\Lambda^{4}}$ and $\frac{f_{T2}}{\Lambda^{4}}$ couplings obtained by dimension-8 operators through the process $ep \rightarrow \nu_{e}W^{+}W^{-} j$ for the Large Hadron Electron Collider and the Future Circular Hadron Electron Collider's different center-of-mass energies and integrated luminosities. Our results show that with the process $ep \rightarrow \nu_{e}W^{+}W^{-} j$ at the Large Hadron Electron Collider and the Future Circular Collider-hadron electron the sensitivity estimated on the anomalous $WWWW$ couplings can be importantly strengthened., Comment: 33 pages, 9 figures and 25 tables

The Large Hadron Electron Collider and the Future Circular Collider-hadron electron with high center-of-mass energy and luminosity allow to better understand the Standard Model and to examine new physics beyond the Standard Model in the electroweak sector. Multi-boson processes permit for a measurement of the gauge boson self-interactions of the Standard Model that can be used to determine the anomalous gauge boson couplings. For this purpose, we present a study of the process $ep \rightarrow \nu_{e} \gamma \gamma j$ at the Large Hadron Electron Collider with center-of-mass energies of 1.30, 1.98 TeV and at the Future Circular Collider-hadron electron with center-of-mass energies of 7.07, 10 TeV to interpret the anomalous quartic $WW\gamma\gamma$ gauge couplings using a model independent way in the framework of effective field theory. We obtain the sensitivity limits at $95\%$ Confidence Level on 13 different anomalous couplings arising from dimension-8 operators. The best limit in $f_{Mi}/\Lambda^{4}$ ($i=0,1,2,3,4,5,7$) parameters is obtained for $f_{M2}/\Lambda^{4}$ parameter while the best sensitivity derived on $f_{Ti}/\Lambda^{4}$ ($i=0,1,2,5,6,7$) parameters is obtained for $f_{T5}/\Lambda^{4}$. In addition, this study is the first report on the anomalous quartic couplings determined by effective Lagrangians at $ep$ colliders., Comment: 28 Pages, 15 Figures and 7 Tables

We study the potential of the next $ep$ collider, namely LHeC, with two options $\sqrt{s}=1.3$ TeV $\sqrt{s}=1.98$ TeV, to search for excited spin-1/2 and spin-3/2 neutrinos. We calculate the single production cross section of excited spin-1/2 and spin-3/2 neutrinos according to their effective currents describing their interactions between gauge bosons and SM leptons. We choose the $\nu^{\star}\rightarrow eW$ decay mode of excited neutrinos and $W\rightarrow jj$ decay mode of $W$-boson for the analysis. We put some kinematical cuts for the final state detectable particles and plot the invariant mass distributions for signal and the corresponding backgrounds. In order to obtain accessible limits for excited neutrino couplings, we show the $f-f^{\prime}$ and $c_{iV}-c_{iA}$ contour plots for excited spin-1/2 and excited spin-3/2 neutrinos, respectively., Comment: 15 pages, 8 figures, 5 Tables

The self-couplings of the electroweak gauge bosons are completely specified by the non-Abelian gauge nature of the Standard Model (SM). The direct study of these couplings provides a significant opportunity to test the validity of the SM and the existence of new physics beyond the SM up to the high energy scale. For this reason, we investigate the potential of the processes $\gamma\gamma\rightarrow ZZ$, $e^{-}\gamma\rightarrow e^{-}\gamma^{*}\gamma \rightarrow e^{-}Z\, Z$ and $e^{+}e^{-} \rightarrow e^{+}\gamma^{*} \gamma^{*} e^{-} \rightarrow e^{+}\, Z\, Z\, e^{-}$ to examine the anomalous quartic couplings of $ZZ\gamma\gamma$ vertex at the Compact Linear Collider (CLIC) with center-of-mass energy $3$ TeV. We calculate $95\%$ confidence level sensitivities on the dimension-8 parameters with various values of the integrated luminosity. We show that the best bounds on the anomalous $\frac{f_{M2}}{\Lambda^{4}}$, $\frac{f_{M3}}{\Lambda^{4}}$, $\frac{f_{T0}}{\Lambda^4}$ and $\frac{f_{T9}}{\Lambda^4}$ couplings arise from $\gamma\gamma\rightarrow ZZ$ process among those three processes at center-of-mass energy of 3 TeV and integrated luminosity of $L_{int}=2000$ fb$^{-1}$ are found to be $[-3.30;3.30]\times 10 ^{-3}$ TeV$^{-4}$, $[-1.20;1.20]\times 10 ^{-2}$ TeV$^{-4}$, $[-3.40;3.40]\times 10 ^{-3}$ TeV$^{-4}$ and $[-1.80;1.80]\times 10 ^{-3}$ TeV$^{-4}$, respectively., Comment: 20 pages, 7 figures, 2 Tables, published version

We study the flavor changing neutral current couplings of new heavy quarks through the Z' models at the LHC. We calculate the cross sections for the signal and the corresponding standard model background processes. Considering the present limits on the mass of new heavy quarks and the Z' boson, we performed an analysis to investigate the parameter space (mixing and mass) through different Z' models. For an FCNC mixing parameter x=0.1 and the Z' mass M_{Z'}=2000 GeV, and new heavy quark mass m_{t'}=700 GeV at the LHC with \sqrt{s}=13 TeV, we find the cross section for single production of new heavy quarks associated with top quarks as 5.8 fb, 3.3 fb, 1.5 fb and 1.2 fb within the Z'_{\eta} , Z'_{\psi} , Z'_{LP} and Z'_{\chi} models, respectively. It is shown that the sensitivity would benefit from the flavor tagging., Comment: 17 pages, 18 figures, update. arXiv admin note: text overlap with arXiv:1003.3156

We study the anomalous $WW\gamma$ couplings at the Compact Linear Collider through the processes $e^{+}e^{-}\to W^+W^-$, $e^{-}e^{+} \to e^{-} \gamma^{*} e^{+} \to e^{+} \nu_{e} W^-$ and $e^{-}e^{+}\to e^{-} \gamma^{*} \gamma^{*} e^{+} \to e^{-} W^+ W^- e^{+} $ $ (\gamma^{*}$ is the Weizsacker-Williams photon). We give the 95\% confidence level limits for unpolarized and polarized electron (positron) beam on the anomalous couplings for various values of the integrated luminosities and center-of-mass energies. We show that the obtained limits on the anomalous couplings through these processes can highly improve the current experimental limits. In addition, our limits with beam polarization are approximately two times better than the unpolarized case., Comment: Tables and references added

New physics models predict the possibility of extra neutral gauge bosons ($Z'$) associated with an extra $U(1)'$ gauge symmetry. We study the couplings of the Higgs boson to the $Z$ boson and $Z'$ boson predicted by the new physics models. The couplings of the $Z'$ boson to quarks can also be investigated through the $Z'q\bar{q}$ interactions. The accessible ranges of the parameter space have been searched for processes $pp\rightarrow HZX$ and $pp\rightarrow HHZX$ at the LHC with $\sqrt{s}=14$ TeV., Comment: 10 pages, 10 figures

We study the pair production of new heavy leptons within a new $U(1)'$ symmetry extension of the Standard Model. Because of the new symmetry, the production and decay modes of the new heavy leptons would be different from those of three families of the standard model. The pair production cross sections depending on the mixing parameter and the mass of heavy leptons have been calculated for the center of mass energies of $0.5$ TeV, $1$ TeV and $3$ TeV. The accessible ranges of the parameters have been obtained for different luminosity projections at linear colliders. We find the sensitivity to the range of mixing parameter $-1

We investigate the forward-backward asymmetries in the pair production of fourth family fermions through the new Z' interactions in the e+e- collisions. The Z' boson having family universal couplings can contribute to the pair production of fourth family fermions via the s-channel exchange. The linear colliders will provide a clean environment for the physics of Z' boson to measure its couplings precisely. The effects of the Z' boson to the asymmetries are shown to be important in some parameter regions for different Z' models. Among these parameters, the invariant mass distribution m_FFbar will be an important measurement to constrain the Z' models. Providing the fourth family fermion exist in an accesssible mass range, a chi^2 analysis can also be used to probe the Z' models at linear collider energies., Comment: 17 pages, 10 figures, 7 tables

The pair production of tau sneutrinos in $e^{+}e^{-}$ collisions and their subsequent decays are studied in a framework of the supersymmetric extension of the standard model. We present an analysis for the parameter space (BR vs. mass) which could be explored at the future high energy $e^{+}e^{-}$ colliders., Comment: 14 pages, 11 figures

We study the pair production of $\tilde{l}_{4}$, which is the supersymmetric partner of the fourth family charged lepton, at the $e^{+}e^{-}$ colliders. It is shown that the investigation of this process at ILC/CLIC will give opportunity to differentiate the MSSM with three and four families., Comment: 6 pages, 2 figures