We consider the possible discovery potential for single production of charged heavy leptons via anomalous interactions at the envisaged Compact Linear Collider (CLIC) by taking into account initial state radiation (ISR) and beamstrahlung effects. We calculate the production cross sections and decay widths of charged heavy leptons in the context of anomalous interactions at center of mass energies $\sqrt{s}=1$ and 3 TeV. The signal and corresponding backgrounds are studied in detail for the mass range 300-1900 GeV.
Pair production of tau leptons in two photon collision ${γγ\to τ^{+}τ}^{-}$ is studied at CLIC to test CP violating QED couplings of tau leptons. CP violating effects are investigated using tau pair spin correlations which are observed through the hadronic decay of each $τ$ into $πν$. Competitive bounds with previous works on the electric dipole moment from CP odd terms have been obtained.
We have investigated the electromagnetic moments of the tau lepton in $e^{+}e^{-} \rightarrow e^{+}γ^{*} γ^{*} e^{-}\rightarrow e^{+}τ\barτe^{-}$ process at the CLIC. We have obtained 95% confidence level bounds on the anomalous magnetic and electric dipole moments for various values of the integrated luminosity and the center of mass energy. We have shown that the $e^{+}e^{-} \rightarrow e^{+}γ^{*} γ^{*} e^{-}\rightarrow e^{+}τ\barτe^{-}$ process at the CLIC leads to a remarkable improvement in the existing experimental bounds on the anomalous magnetic and electric dipole moments.
We have examined the anomalous electromagnetic moments of the tau lepton in the processes $e^{-}γ\to ν_eτ\barν_τ$ ($γ$ is the Compton backscattering photon) and $e^{-}e^{+} \to e^{-}γ^* e^{+} \to ν_{e}τ\barν_τe^+$ ($γ^*$ is the Weizsacker-Williams photon) with unpolarized and polarized electron beams at the CLIC. We have obtained 95$\%$ confidence level bounds on the anomalous magnetic and electric dipole moments for various values of the integrated luminosity and center-of-mass energy. Improved constraints of the anomalous magnetic and electric dipole moments have been obtained compared to th…
In this paper, we present detailed studies on the feasibility at $pp$, $e^-p$ and $e^+e^-$ colliders for model-independent sensitivity estimates on the total cross-section and on the anomalous $τ^+τ^-γ$ interaction through the tau pair production channels $pp \to pτ\bar τγp$, $e^-p \to e^- τ\bar τγp$ and $e^+e^- \to e^+ τ\bar τγe^-$ at the $γ^*γ^* \to τ^+τ^-γ$ mode. Measurements of the anomalous couplings of the $τ$-lepton $\tilde{a}_τ$ and $\tilde{d}_τ$ provide an excellent opportunity to probing extensions of the Standard Model. We estimate the sensitivity at the $95\%$ Confidence Level, and…
We establish model independent bounds on the anomalous magnetic and electric dipole moments of the tau-lepton using the two-photon processes $γγ\to τ^+τ^-$ and $γγ\to τ^+τ^-γ$. We use ${\cal L}=10, 50, 100, 300, 500, 1000, 1500, 2000, 3000 \hspace{0.8mm}fb^{-1}$ of data collected with the future $e^+e^-$ linear collider such as the CLIC at $\sqrt{s}=380, 1500, 3000 \hspace{0.8mm}GeV$ and we consider systematic uncertainties of $δ_{sys}=0, 3, 5\hspace{1mm}\%$. Precise bounds at $95\%$ C. L. on the anomalous dipole moments to the tau-lepton $-0.00015\leq \tilde{a}_τ\leq 0.00017$ and $|\tilde{d}_…
We review two commonly used approximate treatments of beamstrahlung at lepton colliders. We discuss the applicability of those approximations and show that they are bound to fail at very high energy $e^+e^-$ colliders such as CLIC. We model the beamstrahlung effects at CLIC by an exact numerical solution to the evolution equation for beamstrahlung and compare to the two approximations. We also discuss the impact of beamstrahlung on precision measurements of new physics parameters. As an illustration we consider Kaluza-Klein lepton production in Universal Extra Dimensions and study the deterior…
This letter presents a study of prospects of searching for excited leptons during RunII of the Fermilab Tevatron. We concentrate on single and pair production of excited electrons in the photonic decay channel in one CDF/DO detector equivalent for 2 fb^{-1}. By the end of RunIIa, the limits should be easily extended beyond those set by LEP and HERA for excited leptons with mass above about 190 GeV.
We propose two different and complementary observables for singling out possible signals of physics beyond the standard model in the semi-leptonic decays $Λ_b \to Λ_c \ell{\bar ν}_{\ell}$, both with the $τ$ lepton and with a light lepton. The two observables are the partial decay width and a T-odd asymmetry, whose respective sensitivities to scalar and/or pseudo-scalar coupling are calculated as functions of the parameters characterizing new physics. Two different form factors are used. Three particular cases are discussed and analyzed in detail.
In this work, the production processes of heavy neutral scalar and pseudo scalar associated with standard model gauge boson $Z_L$ at future $e^{+}e^{-}$ colliders (ILC and CLIC) are examined. The total and differential cross sections are calculated for the processes in the context of the littlest Higgs model. Also dependence of production processes to littlest Higgs model parameters in the range of compatibility with electroweak precision measurements and decays to lepton flavor violating final states are analyzed. We have found that both heavy scalar and pseudoscalar will be produced in $e^+e…
The tau lepton plays important role in the correlation between the low-energy neutrino oscillation data and the lepton flavor structure in heavy neutrino decay. We investigate the lepton flavor signatures with tau lepton at hadron collider through lepton number violating (LNV) processes. In the Type I Seesaw with U$(1)_{\rm B-L}$ extension, we study the pair production of heavy neutrinos via a $Z'$ resonance. We present a detailed assessment of the search sensitivity to the channels with tau lepton in the subsequent decay of heavy neutrinos. For the benchmark model with $Z'$ only coupled to th…
Heavy Neutral Leptons are naturally present in many well-motivated extensions of the Standard Model. If their mass is of few dozens of GeVs, they can be long-lived and lead to events with displaced vertices, giving rise to promising signatures due to the low background. We revisit the opportunities offered by the LHC to discover these long-lived states via searches with displaced vertices. We study in particular the implication on the parameter space sensitivity when all mixings to active flavors are taken into account.
A recently proposed class of supersymmetric models predicts rather light and nearly mass-degenerate higgsinos, while the other superparticles are significantly heavier. In this paper we study the early LHC phenomenology of a benchmark model of this kind. If the squarks and gluinos, and in particular the lighter stop, are still light enough to be within reach, then evidence for our model can be found in hadronic SUSY searches. Moreover, with dedicated searches it will be possible to distinguish the light higgsino model from generic SUSY models with a bino LSP. Search channels with b-jets and wi…
We investigate the possibility of searching the lepton-flavor-violating (LFV) $τ\rightarrow μγ$ rare decay at the Super Charm-Tau Factory (CTF). By comparing the kinematic distributions of the LFV signal and the standard model (SM) background, we develop an optimized event selection criteria which can significantly reduce the background events. It is concluded that new $2 σ$ upper limit of about $1.9 \times 10^{-9}$ on $Br(τ\rightarrow μγ)$ can be obtained at the CTF, which is beyond the capability of Super-B factory in searching $τ$ lepton rare decay. Within the framework of the scalar leptoq…
Prospective searches for lepton flavor violation (LFV) in rare kaon decays at the existing and future intermediate-energy accelerators are considered. The proposed studies are complementary to LFV searches in muon-decay experiments and offer a unique opportunity to probe models with approximately conserved fermion-generation quantum number with sensitivity superior to that in other processes. Consequently, new searches for LFV in kaon decays are an important and independent part of the general program of searches for lepton flavor violation in the final states with charged leptons.
A model of spin-3/2 vector-like leptons is introduced, which serve as an alternative probe of possible lepton compositeness. Possible signatures at the Large Hadron Collider are explored, and current data are used to constrain the masses of the hypothetical new leptons.
The production at the LHC of exotic excited leptons of charge $Q = +2e$ is considered. Such states are predicted in composite models with extended isospin multiplets ($I_{W}=1$ and $I_{W}=3/2$). The coupling among these doubly charged leptons and Standard Model fermions may occurs either via gauge or contact interactions. In the former case the decay channels are more constrained. We study the production cross section at the LHC of $L^{++}$ ($pp \rightarrow L^{++} \, \ell^{-}$) and focus on the leptonic signature deriving from the subsequent decays $L^{++} \rightarrow W^{+} \ell^{+} \rightarro…
Lepton flavor violating interactions are absent in the standard model but are expected in various beyond standard models. In this work, the potential of the future circular electron-positron collider to probe the four fermion lepton flavor couplings via the $e^{+}e^{-}\rightarrow e^{\pm}τ^{\mp}$ process is revisited by means of an effective field theory approach. We provide constraints at $95\%$ CL on the dimension-six Wilson coefficients including major sources of background processes and considering realistic detector effects at four expected operation energies $\sqrt{s}=157.5$, $162.5$, $24…
This Fifth International WEIN Symposium is devoted to physics beyond the standard model. This talk is about tau lepton physics, but I begin with the question: do we know how to find new physics in the world of elementary particles? This question is interwoven with the various tau physics topics. These topics are: searching for unexpected tau decay modes; searching for additional tau decay mechanisms; radiative tau decays; tau decay modes of the W, B, and D; decay of the Z^0 to tau pairs; searching for CP violation in tau decay; the tau neutrino, dreams and odd ideas in tau physics; and tau res…
We investigate the potential of the Compact Linear Collider (CLIC) to probe the anomalous $W^+W^-γ$ coupling in $γγ$, $γγ^*$ and $γ^*γ^*$ collisions through the processes $γγ\rightarrow W^+ W^-$, $e^+ γ\rightarrow e^+ γ^{*} γ\rightarrow e^+ W^- W^+$ and $e^+ e^-\rightarrow e^+ γ^{*} γ^{*} e^- \rightarrow e^+ W^- W^+ e^-$. We perform leptonic, semi-leptonic and hadronic decays channels for $W^+W^-$ production in the final state. Taking ${\cal L}=1\hspace{0.8mm}{\rm ab^{-1}}@ \sqrt{s}=0.380\hspace{0.8mm}{\rm TeV}$, ${\cal L}=2.5\hspace{0.8mm}{\rm ab^{-1}}@ \sqrt{s}=1.5\hspace{0.8mm}{\rm TeV}$ an…
We analyze the potential of TESLA and CLIC based electron-photon colliders to search for excited spin-1/2 electrons. The production of excited electrons in the resonance channel through the electron-photon collision and their subsequent decays to leptons and electroweak gauge bosons are investigated. We study in detail the three signal channels of excited electrons and the corresponding backgrounds through the reactions e gamma --> e gamma, e gamma --> eZ and e gamma --> nu W. Excited electrons with masses up to about 90% of the available collider energy can be probed down to the coupling f=f'…
We consider possible production of heavy leptons via anomalous interactions at future $ep$ colliders (THERA and LHeC), $e^+e^-$ colliders (ILC and CLIC) and $pp$ collider CERN LHC. The production, backgrounds and signatures of heavy leptons are analyzed. We obtain the upper mass limits of 800 GeV at LHeC, 450 GeV at ILC and 650 GeV at LHC for optimal choices of relevant parameters.
In this work, we study the possible unparticle signatures through tau pair production e+e- -> tau+tau- process at the low energy particle factories. We take into account lepton flavor conserving (s-channel) and violating (t-channel) couplings for the unparticle leptonic interactions. For different values of the scaling parameter d we extract upper limits on the scalar unparticle couplings at the integrated luminosities of 100/fb.yr and 1 ab/fb.yr.
We consider theories where lepton flavor is violated, in particular concentrating on the four fermion operator consisting of three electrons and a tau. Strong constraints are available from existing searches for tau -> eee, requiring the scale of the contact interaction to be less than ~(9 TeV)^-2. We reexamine this type of physics, assuming that the particles responsible are heavy (with masses greater than ~TeV) such that a contact interaction description continues to be applicable at the energies for a future e+e- collider. We find that the process e+e- -> e tau can be a very sensitive probe…
We study the lepton portal dark matter (DM) model in which the relic abundance is determined by the portal coupling among the Majorana fermion DM candidate $χ$, the singlet charged scalar mediator $S^\pm$ and the Standard Model (SM) right-handed lepton. The direct and indirect searches are not sensitive to this model. This article studies the lepton portal coupling as well as the scalar portal coupling (between $S^\pm$ and SM Higgs boson), as the latter is generally allowed in the Lagrangian. The inclusion of scalar portal coupling not only significantly enhances the LHC reach via the $gg\to h…
We analyze the lepton flavor violating decays $τ\rightarrow Pl$ ($P=π,η,η';\;l=e,μ$) in the scenario of the minimal R-symmetric supersymmetric standard model. The prediction on the branching ratios BR$(τ\rightarrow P e)$ and BR$(τ\rightarrow P μ)$ is affected by the mass insertion parameters $δ^{13}$ and $δ^{23}$, respectively. These parameters are constrained by the experimental bounds on the branching ratios BR($τ\rightarrow e (μ) γ$) and BR($τ\rightarrow 3e(μ)$). The result shows $Z$ penguin dominates the prediction on BR($τ\rightarrow Pl$) in a large region of the parameter space. The bran…
We discuss the potential of $τ$-charm and B factories for the search of new physics through the study of rare $τ$ decays. We consider decays that involve the violation of lepton flavour conservation. Such decays bear a close relationship to the physics of neutrino mass and the properties of the lepton sector of the electroweak theory.
Probing first-generation doublet vector-like leptons via single production at the 3 TeV CLIC
Henan Institute of Science and Technology（河南科技学院）；Zhongyuan University of Science and Technology（中原工学院）；University of Southampton（南安普顿大学）；Uppsala University（乌普萨拉大学）
本研究在3 TeV CLIC上通过单产生过程，针对第一代二重态类矢量轻子开展信号与本底模拟及截断分析，获得不同积分亮度下的排除与发现灵敏度，可在宽质量范围灵敏探测该类轻子。
We investigate the sensitivity of electron-proton ($ep$) colliders for charged lepton flavor violation (cLFV) in an effective theory approach, considering a general effective Lagrangian for the conversion of an electron into a muon or a tau via the effective coupling to a neutral gauge boson or a neutral scalar field. For the photon, the $Z$ boson and the Higgs particle of the Standard Model, we present the sensitivities of the LHeC for the coefficients of the effective operators, calculated from an analysis at the reconstructed level. As an example model where such flavor changing neutral cur…
We consider the lepton-number-violating processes in $Λ_b$/$Λ_c$ decays mediated by on-shell GeV-scale sterile neutrino. We calculate the branching ratio for the following processes: $Λ_b^{0} \rightarrow \mathcal{B}^+ μ^{-}μ^- π^+$, where $\mathcal{B}^+$ is $Λ_c^+$ or proton, and $Λ_c^{+} \rightarrow Λμ^{+}μ^+ π^-$, as function of the mass of the sterile neutrino $m_N$ and the heavy-light mixing coefficient of the extended Pontecorvo-Maki-Nakagawa-Sakata (PMNS) matrix $|U_{μN}|^2$. The effect of finite detector size is included in our calculation. After comparing the theoretical effective bran…
Heavy long-lived multi-charged leptons (MCLs) are predicted by various new physics models. These hypothetical MCLs can form bound states, due to their high electric charges and long life times. In this work, we propose a novel strategy of searching for MCLs through their bound state productions and decays. By utilizing LHC-8 TeV data in searching for resonances in the diphoton channel, we exclude the masses of isospin singlet heavy leptons with electric charge $|q|\geq 6$ (in units of electron charge) lower than $\sim$1.2 TeV, which are much stronger than the corresponding 8 TeV LHC bounds fro…
A weak singlet charged scalar exists in many new physics models beyond the Standard Model. The discovery potential of the singlet charged scalar is explored at future lepton colliders, e.g. the CEPC, ILC-350 and ILC-500. We demonstrate that one can discover the singlet charged scalar up to 118 GeV at the CEPC with an integrated luminosity of $5~\mathrm{ab}^{-1}$. At the ILC-350 and the ILC-500 with an integrated luminosity of $1~\mathrm{ab}^{-1}$ such a discovery limit can be further improved to 136 GeV and 160 GeV, respectively.
We study the collider phenomenology of a simplified model containing a right-handed $W$ in which the $W_R$ couples predominantly to the third generation in the quark sector. The model also includes a light Majorana neutrino, with $M_1\sim {\cal O}(100)$ GeV, giving rise to lepton-number-violating signatures that are visible at the LHC. Taking into account all the searches from the LHC and Tevatron, we find that this $W_R$ can still be as light as $M_R \sim 300$ GeV. We show that this type of new physics, and others like it, can be detected at the LHC using final states with three same-sign sam…
We study constraints on the models of bosonic super-weakly interacting particle (super-WIMP) dark matter (DM) with DM masses $m_X \sim \mathcal{O}(1 - 100)$ keV from leptonic decays $M\rightarrow \ell \barν_\ell + X$, where $M=B^\pm, D^\pm, D_s^\pm$ is a heavy meson state. We focus on two cases where $X$ denotes either a light pseudoscalar (axion-like), or a light vector state that couples to the standard model (SM) through kinetic mixing. We note that for a small DM mass these decays are separately sensitive to DM couplings to quarks, but not its mass.
The determination of scalar lepton and gaugino masses is an important part of the programme of spectroscopic studies of Supersymmetry at a high energy e+e- linear collider. In this article we present results of a study of the processes: e+e- -> eR eR -> e+e- chi0 chi, e+e- -> muR muR -> mu mu- chi0 chi0, e+e- -> eL eL -> e e chi0 chi0 and e+e- -> snu_e snu_e -> e e chi+ chi-in two Supersymmetric benchmark scenarios at 3 TeV and 1.4 TeV at CLIC. We characterize the detector performance, lepton energy resolution and boson mass resolution. We report the accuracy of the production cross section me…
We explore the prospect of Yukawa production of a light boson which can exist in an extended Higgs sector. A particularly interesting case is the light pseudoscalar in Type-X two Higgs doublet model which can explain the anomalous magnetic moment of muon at large $\tanβ$. Considering ILC $Higgs$ $factory$ with $\sqrt{s}=250$ GeV, we show that the available parameter space can be fully examined by the (tau) Yukawa process at $5σ$. We also demonstrate the mass reconstruction of such a light particle which helps to sizably minimize the background events.
We present a strategy for searching for heavy neutrinos at the Large Hadron Collider using the MoEDAL Experiment's MAPP detector. We hypothesize the heavy neutrino to be a member of a fourth generation lepton doublet, with the electric dipole moment (EDM) introduced within a dimension-five operator. In this model the heavy neutrino is produced in association with a heavy lepton. According to our current experimental and theoretical understanding, the electric dipole moment of this heavy neutrino may be as high as $10^{-15}$ $e$ cm. Taking advantage of the sensitivity of MoEDAL detector, we exa…
Semileptonic $B$ meson decays into the four lightest excited charmed meson states ($D_0^*$, $D_1^*$, $D_1$, and $D_2^*$) and their counterparts with $s$ quarks are investigated, including the full lepton mass dependence. We derive the standard model predictions for the differential branching fractions, as well as predictions for the ratios of the semi-tauonic and light lepton semileptonic branching fractions. These can be systematically improved using future measurements of the total or differential semileptonic rates to $e$ and $μ$, as well as the two-body hadronic branching fractions with a …
We present here a study of the impact of a heavy neutrino (or heavy neutral lepton) on leptonic and semileptonic kaon decays. We used a simplified model consisting of 3 light neutrinos responsible for neutrino oscillations and a heavy sterile neutrino. We found that it can lead to large deviations from the Standard Model predictions for leptonic decays, in conflict with experimental measurements of $K_{e2}$ and the lepton universality test $R_K$. This allows to derive new constraints on the leptonic mixing for heavy sterile neutrinos. No tension was found when considering the semileptonic deca…
The observation of lepton flavor violation indicates new physics beyond the Standard Model. Lepton colliders are ideal facilities to probe charged lepton flavor violation (CLFV) signals induced by new physics at high energy. In this work we perform a comprehensive study of the sensitivity of future lepton colliders to charged lepton flavor violation. We consider the most general renormalizable Lagrangian coupling two leptons to new bosonic particles, involving both $ΔL=0$ and $ΔL=2$ interactions. The CLFV processes are introduced by the exchange of off-shell new particles at tree-level. We fin…
The LHCb Collaboration recently gave an update on testing lepton flavour universality with $B^+ \rightarrow K^+ \ell^+ \ell^-$, in which a 3.1 standard deviations from the standard model prediction was observed. The g-2 experiment also reports a 3.3 standard deviations from the standard model on muon anomalous magnetic moment measurement. These deviations could be explained by introducing new particles including leptoquarks. In this paper, we show the possibility to search for heavy spin-1 leptoquarks at a future TeV scale muon collider by performing studies from three channels: 1) same flavou…
The Higgs-portal lepton flavor violation is studied in a vectorlike lepton model. For evading the constraints from rare $Z\to \ell^\pm_i \ell^\mp_j$ decays, we introduce two triplet vectorlike leptons, $(1,3)_{-1}$ and $(1,3)_{0}$. The resultant branching ratio for $h\to μτ$ can be up to $10^{-4}$ when the constraints from the invisible $Z$ decays are applied. As a result, the signal strength for $ττ$ channel has a $12\%$ deviation from the standard model prediction, while the muon $g-2$ is two-order of magnitude smaller than the data and $BR(τ\to μγ)$ is of order of $10^{-12}$. A predicted do…
The possibility to simulate lepton number violating supersymmetric models has been introduced into the recently updated PYTHIA event generator, now containing 1278 decay channels of SUSY particles into SM particles via lepton number violating interactions. This generator has been used in combination with the ATLFAST detector simulation to study the impact of lepton number violation (LV) on event topologies in the ATLAS detector, and trigger menus designed for LV-SUSY are proposed based on very general considerations. In addition, a rather preliminary analysis is presented on the possibility fo…
The Compact Linear Collider (CLIC) was proposed as the next energy-frontier infrastructure at CERN, to study e$^+$e$^-$ collisions at three centre-of-mass energy stages: 380 GeV, 1.5 TeV and 3 TeV. The main goal of its high-energy stages is to search for the new physics beyond the Standard Model (SM). The Inert Doublet Model (IDM) is one of the simplest SM extensions and introduces four new scalar particles: H$^\pm$, A and H; the lightest, H, is stable and hence a natural dark matter (DM) candidate. A set of benchmark points is considered, which are consistent with current theoretical and expe…
The effective Majorana mass which determines the rate of the neutrinoless double beta decay, |<m>|, is considered in the case of three-neutrino mixing and massive Majorana neutrinos. Assuming a rather precise determination of the parameters characterizing the neutrino oscillation solutions of the solar and atmospheric neutrino problems has been made, we discuss the information a measurement of |<m>| > (0.005 - 0.010) eV can provide on the value of the lightest neutrino mass and on the CP-violation in the lepton sector. The implications of combining a measurement of |<m>| with future measuremen…
The virtual production of axion-like particles (ALPs) in the light-by-light scattering at the CLIC collider is studied. Both differential and total cross sections are calculated, assuming interaction of the ALP with photons via CP-odd term in the Lagrangian. The 95\% C.L. exclusion regions for the ALP mass and its coupling constant are given. By comparing our results with existing collider bounds, we see that the ALP search at the CLIC has a great physics potential of searching for the ALPs, especially, in the mass region 1 TeV -- 2.4 TeV, with the collision energy $\sqrt{s} = 3000$ GeV and in…
After a summary of the motivations for searching flavour violating decays and electric dipole moments in the leptonic sector, I briefly sketch the present status of the limits on lepton-slepton misalignement and discuss the potentialities of the planned experimental improvements. Candidates for theories beyond the Standard Model have to satisfy all these experimental constraints; I will discuss in particular how the supersymmetric SM extended with the see-saw faces the flavour problem.
We discuss single heavy neutrino production e+ e- -> N nu -> l W nu, l = e, mu, tau, at a future high energy collider like CLIC, with a centre of mass energy of 3 TeV. This process could allow to detect heavy neutrinos with masses of 1-2 TeV if their coupling to the electron V_eN is in the range 0.004-0.01. We study the dependence of the limits on the heavy neutrino mass and emphasise the crucial role of lepton flavour in the discovery of a positive signal at CLIC energy. We present strategies to determine heavy neutrino properties once they are discovered, namely their Dirac or Majorana chara…
