Prospects to search for a new massless neutral gauge boson, the paraphoton, in e+ e- collisions at center-of-mass energies of 0.5 and 1 TeV are studied. The paraphoton naturally appears in models with abelian kinetic mixing. A possible realistic model independent lowest order effective Lagrangian contains magnetic interactions of the paraphoton with the Standard Model fermion fields. These interactions are proportional to the fermion mass and grow with energy, with however very weak paraphoton couplings to ordinary matter. At the ILC, a potentially interesting process to search for the paraphoton is its radiation off top quarks, so that the event topology to be searched for is a pair of acoplanar top quark jets with missing energy. By combining many discriminating features of signal and background events efficient paraphoton event selection was achieved allowing to set limits for the top-paraphoton coupling. Arguments in favor of the missing energy as the paraphoton with spin 1 are discussed.
After recalling the theoretical and experimental status of QCD in the Regge limit and the requirement of high energy scattering process of onium-onium type for testing this limit, we show that the International Linear Collider would be a major step in this field.
We investigate the possibility of detecting hidden vector gauge bosons at ILC linear collider. The study is performed in the framework of hidden sector extension of Standard Model with 3 degenerate dark gauge bosons. By studying the cross section of pair dark gauge boson with photon at initial state radiation we found that at the energy $\sqrt{s} \approx$ 1300 GeV the cross section can be as large as $48fb$, the same order ($\mathcal{O}(fb)$) with the irreducible background of the Standard Model. Hence more methods needed to be done to eliminate the background for this model.
Using the event generator WHIZARD we study in a realistic ILC environment the prospects of measuring properties of sneutrinos that decay invisibly into the lightest neutralino and the neutrino.
This paper discusses some aspects of the estimates of Higgs boson coupling sensitivities for LHC and ILC presented at the Snowmass 2013 meeting. I estimate the measurement accuracies underlying the CMS presentation to Snowmass. I present new fits for the ILC capabilities. I present some joints fits to prospective LHC and ILC data that demonstrates the synergy of the High-Luminosity LHC and ILC programs. Contributed to the proceedings of the Snowmass 2013 Community Summer Study.
An interesting model of dark matter involves a hidden sector decoupled from Standard Model (SM) fields except for some portal interaction. A concrete realization of this is the Hidden Abelian Higgs Model, which gives rise to decays of the SM Higgs boson into a pair of new bosons, called $Z_{\mathrm{d}}$ or dark photons. This note explores prospects for the search for such dark photons at the ILC with $\sqrt{s}=250$ GeV, where the dark photons decay promptly. For the $H\rightarrow Z_{\mathrm{d}}Z_{\mathrm{d}}\rightarrow 4\ell$ ($\ell = e,μ$) final state, it follows closely recent similar search…
For a long time, global fits of the electroweak sector of the Standard Model (SM) have been used to exploit measurements of electroweak precision observables at lepton colliders (LEP, SLC), together with measurements at hadron colliders (Tevatron, LHC), and accurate theoretical predictions at multi-loop level, to constrain free parameters of the SM, such as the Higgs and top masses. Today, all fundamental SM parameters entering these fits are experimentally determined, including information on the Higgs couplings, and the global fits are used as powerful tools to assess the validity of the the…
The prospects for the discovery and exploration of low-energy Supersymmetry at future colliders, the Large Hadron Collider (LHC) and the future international linear electron positron collider (ILC) are summarized. The focus is on the experimental techniques that will be used to discover superpartners and to measure their properties. Special attention is given to the question how the results from both machines could influence each other, in particular when they have overlapping running time.
Prospects on electroweak physics at a future International Linear Collider (ILC) are summarized, including gauge coupling measurements, top quark physics and Higgs physics.
We review the ILC capabilities to explore the electroweak (EW) sector of the SM at high precision and the prospects of unveiling signals of BSM physics, either through the presence of new particles in higher-order corrections or via direct production of extra EW gauge bosons. This includes electroweak precision observables, global fits to the SM Higgs boson mass as well as triple and quartic gauge boson couplings.
The CMS collaboration reported an intriguing \sim 3 sigma (local) excess at 96 GeV in the light Higgs-boson search in the diphoton decay mode. This mass coincides with a \sim 2 sigma (local) excess in the bb final state at LEP. We present the interpretation of this possible signal as the lightest Higgs boson in the 2 Higgs Doublet Model with an additional real Higgs singlet (N2HDM). It is shown that the type II and type IV (flipped) of the N2HDM can perfectly accommodate both excesses simultaneously, while being in agreement with all experimental and theoretical constraints. The excesses are m…
A precise measurement of the top quark Yukawa coupling is of great importance, since it may shed light on the mechanism of EWSB. We study the prospects of such measurement during the first phase of the ILC at sqrt(s)=500 GeV, focusing in particular on recent theoretical developments as well as the potential benefits of beam polarization. It is shown that both yield improvements that could possibly lead to a measurement competitive with the LHC.
This note presents an analysis of the potential of future high-energy electron-positron colliders to measure the $b$-quark mass. We perform a full-simulation study of the measurement of the ratio of the three-jet rates in events with $b\bar{b}(g)$ and $q\bar{q}(g)$ production, $R_{3}^{bl}$, and assess the dominant uncertainties, including theory and experimental systematic uncertainties. We find that the ILC "Higgs factory" stage, with an integrated luminosity of 2 ab$^{-1}$ at $\sqrt{s}=$ 250 GeV can measure the $b-$quark $bar{MS}$ mass at a scale of 250 GeV ($m_b(250~$GeV$)$) with a precisio…
The physics prospect at future linear $e^{+}e^{-}$ colliders for the study of the Higgs triple self-coupling via the process of $e^{+}e^{-}\to ZHH$ is investigated. In this paper, we calculate the contribution of the new particles predicted by the littlest Higgs model to the cross sections of this process in the future high energy $e^{+}e^{-}$ collider($ILC$). The results show that, in the favorable parameter spaces preferred by the electroweak precision, the deviation of the total cross sections from its $SM$ value varies from a few percent to tens percent, which may be detected at the future…
We explore discrimination of two types of leptonic Yukawa interactions associated with Higgs triplet, $\bar L^c_L ΔL_L$, and with $SU(2)$ singlet doubly charged scalar, $\bar e_R^c k^{++} e_R$. These interactions can be distinguished by measuring the effects of doubly charged scalar boson exchange in the $e^+ e^- \to \ell^+ ell^-$ processes at polarized electron-positron colliders. We study a forward-backward asymmetry of scattering angular distribution to estimate the sensitivity for these effects at the ILC. In addition, we investigate prospects of upper bounds on the Yukawa couplings by com…
In this paper we study the effect of new physics contributions to the top quark pair production ($t\bar{t}$) in a possible future linear collider, such as the International Linear Collider (ILC). The use of a dimension-six gauge invariant effective operator approach allows to compare the prospected results at the ILC with the current ones obtained at the Large Hadron Collider (LHC), both in neutral and charged current processes. We also prove that the use of specific observables, together with a combination of measurements in different polarized beam scenarios and with different center-of-mass…
We study future prospects of the stau which contributes to the Higgs coupling to di-photon. The coupling is sensitive to new physics and planned to be measured at percent levels in future colliders. We show that, if the excess of the coupling is measured to be larger than 4 %, the lightest stau is predicted to be lighter than about 200 GeV by taking vacuum meta-stability conditions into account. Such a stau can be discovered at ILC. Moreover, we show how accurately the stau contribution to the coupling can be reconstructed from the information that is available at ILC. We also argue that, if t…
We investigate the signatures of the recently proposed Higgsless models at future colliders. We focus on tests of the mechanism of partial unitarity restoration in the longitudinal vector boson scattering, which do not depend on any Higgsless model-building details. We study the LHC discovery reach for charged massive vector boson resonances and show that all of the preferred parameter space will be probed with $100 {\rm fb}^{-1}$ of LHC data. We also discuss the prospects for experimental verification of the Higgsless nature of the model at the LHC. In addition, in this talk we present new re…
We study the prospects of searching for leptophilic gauge bosons (LGBs) at the beam dump experiment using $e^\pm$ beams of International $e^+e^-$ Linear Collider (ILC). We consider LGBs in association of $U(1)_{e-μ}$, $U(1)_{e-τ}$, and $U(1)_{μ-τ}$ gauge symmetries, which are assumed to be light and long-lived. Utilizing the energetic electron and positron beams of the ILC, we show that the ILC beam dump experiment can cover the parameter regions which have not been explored before. We also discuss the possibility of distinguishing various models.
We study the prospects of the B-L model with an additional $Z'$ boson to be a Higgs boson factory at high-energy and high-luminosity linear electron positron colliders, such as the ILC and CLIC, through the Higgs-strahlung process $e^{+}e^{-}\rightarrow (Z, Z') \to Zh$, including both the resonant and non-resonant effects. We evaluate the total cross section of $Zh$ and we calculate the total number of events for integrated luminosities of 500-2000\hspace{0.8mm}$fb^{-1}$ and center of mass energies between 500 and 3000\hspace{0.8mm}$GeV$. We find that the total number of expected $Zh$ events c…
The prospects for electroweak precision physics at the LHC and the ILC are reviewed. This includes projections for measurements of the effective Z pole weak mixing angle, sin^2 theta_W (eff.), as well as top quark, W boson, and Higgs scalar properties. The upcoming years may also see very precise determinations of sin^2 theta_W (eff.) from lower energies.
The particle discovered in the Higgs boson searches at the LHC with a mass of about 125 GeV is compatible within the present uncertainties with the Higgs boson predicted in the Standard Model (SM), but it could also be identified with one of the neutral Higgs bosons in a variety of Beyond the SM (BSM) theories with an extended Higgs sector. The possibility that an additional Higgs boson (or even more than one) could be lighter than the state that has been detected at 125 GeV occurs generically in many BSM models and has some support from slight excesses that were observed above the background …
In a class of Randall-Sundrum(RS) models, matter fermions and gauge bosons live in a five dimensional bulk, while the Higgs boson lives in a four dimensional visible brane. The Higgs boson can mix with a radion, by a Higgs-radion mixing term. It is difficult to directly discover the heavy Kaluza-Klein(KK) particles at the Large Hadron Collider(LHC), because the mass of the lightest KK particle is expected to be above 10 TeV to satisfy constraints from avor changing neutral currents(FCNC). Instead, the precision measurements of the Higgs sector at the high luminosity LHC(HL-LHC) and the Interna…
The future of high-precision electroweak physics lies in e+e- collider measurements of properties of the Z boson, the W boson, the Higgs boson, and the top quark. We estimate the expected performance of three possible future colliders: the ILC, FCC-ee (formerly known as TLEP), and CEPC. In particular, we present the first estimates of the possible reach of CEPC, China's proposed Circular Electron-Positron Collider, for the oblique parameters S and T and for seven-parameter fits of Higgs couplings. These results allow the physics potential for CEPC to be compared with that of the ILC and FCC-ee…
Prospects for SUSY discoveries and measurements at future colliders LHC and ILC are discussed. The problem of reconstructing the underlying theory and SUSY breaking mechanism is also addressed.
The CMS collaboration reported a $\sim 3 \, σ$ (local) excess at $96\;$GeV in the search for light Higgs-boson decaying into two photons. This mass coincides with a $\sim 2 \, σ$ (local) excess in the $b\bar b$ final state at LEP. We show an interpretation of these possible signals as the lightest Higgs boson in the 2 Higgs Doublet Model with an additional complex Higgs singlet (2HDMS). The interpretation is in agreement with all experimental and theoretical constraints. We concentrate on the 2HDMS type II, which resembles the Higgs and Yukawa structure of the Next-to Minimal Supersymmetric St…
It is an appealing possibility that the observed dark matter density in the universe can be fully explained by SUSY. The current experimental knowledge indicates that this possibility strongly favors a co-annihilation scenario. In such scenarios, the mass difference between the next-to-lightest SUSY particle (the NLSP) and the lightest one (the LSP) is quite small, which assures that the annihilation cross-section is sufficient not to predict a too large abundance of dark matter. However, the small mass difference also means that observing SUSY becomes hard at hadron colliders, where the obser…
We investigate the prospects of determining the CP nature of the 126 GeV neutral spin-0 (Higgs) boson h, discovered at the LHC, at a future linear collider. We consider the production of h by the Higgsstrahlung process and its subsequent decays to tau leptons. We investigate how precisely a possible pseudoscalar component of h can be detected by the measurement of a suitably defined angular distribution, if all major decay modes of the tau lepton are used. From our numerical simulations, we estimate the expected precision to the scalar-pseudoscalar mixing angle, including estimates of the back…
A high-energy $e^+e^-$ Linear Collider has been considered since a long time as an important complement to the LHC. Unprecedented precision measurements as well as the exploration of so far untouched phase space for direct production of new particles will provide unique information to advance the limits of our understanding of our universe. Within this project, the physics prospects of such a collider as well as their interplay with design of the accelerator and the detectors have been investigated in a quantitative way. This kind of study required a close collaboration between theory and expe…
If the new boson is lying in the narrow mass range between 122 - 127 GeV is confirmed to be a Higgs boson, then stability of the electroweak vacuum in a minimal model with a universal extra dimension (mUED) will require a much lower cutoff for the theory than has been envisaged earlier. We show that this low cutoff would lead to important changes in much of the mUED phenomenology studied till now. In particular, prospects for LHC searches for n = 1 states are rather limited, while resonant n = 2 states may go completely undetected. Prospects for detection at the ILC and CLIC are less affected.
Fermion dark matter (DM) as an admixture of additional singlet and doublet vector like fermions provides an attractive and allowed framework by relic density and direct search constraints within TeV scale, although limited by its discovery potential at the Large Hadron Collider (LHC). An extension of the model with scalar triplet can yield neutrino masses and provide some cushion to the direct search constraint of the DM through pseudo-Dirac mass splitting. This in turn, allow the model to live in a larger region of the parameter space and open the door for detection at LHC, even if slightly. …
The discovery of the Higgs boson along with the vigorous confirmation of the SM coupled with non-appearance of new (SUSY) particles has led to an apparent naturalness crisis. We argue the crisis stems from overestimates of finetuning. A proper evaluation of naturalness in SUSY leads to light higgsinos with mass mu~ 100-300 GeV while other sparticles may inhabit the multi-TeV range at little cost to naturalness. Special signatures like dilepton mass edges in gluino pair events and same-sign dibosons from wino pairs might occur at LHC13. An ILC with \sqrt{s}>2 mu would make the definitive test o…
We study the Dark Matter (DM) discovery prospect and its spin discrimination in the theoretical framework of gauge invariant and renormalizable Higgs portal DM models at the ILC with $\sqrt{s} = 500$ GeV. In such models, the DM pair is produced in association with a $Z$ boson. In case the singlet scalar DM, the mediator is just the SM Higgs boson, whereas for the fermion or vector DM there is an additional singlet scalar mediator that mixes with the SM Higgs boson, which produces significant observable differences. After careful investigation of the signal and backgrounds both at parton level …
Following a brief outline of the CLIC project, this talk summarizes some of the principal motivations for an e+ e- collider with E_CM = 3 TeV. It is shown by several examples that CLIC would represent a significant step beyond the LHC and ILC in its capabilities for precision measurements at high energies. It would make possible a complete study of a light Higgs boson, including rare decay modes, and would provide a unique tool to study a heavy Higgs boson. CLIC could also complete the studies of supersymmetric spectra, if sparticles are relatively light, and discover any heavier sparticles. I…
In the minimal supersymmetric standard models, neutralino dark matter with mass of $m_χ\sim m_Z/2\sim 45$ GeV and $m_χ\sim m_h/2\sim 62$ GeV can have the thermal relic abundance $Ω_{χ_1}h^2\simeq 0.120$ via the $Z$- and Higgs-resonant annihilations, respectively, while avoiding all the current constraints. Phenomenology of such scenarios is determined only by three parameters, Bino mass $M_1$, Higgsino mass $μ$, and $\tanβ$, in the limit that all other supersymmetric particles and heavy Higgs bosons are decoupled. In this paper, we comprehensively study the constraints and future prospects of …
Many Standard Model extensions can contribute to four-lepton signals at large colliders. We review the particular case of leptophilic interactions eventually observable at the LHC and the ILC, paying special attention to the addition of a new vector boson coupled to muon minus tau lepton number, $Z'_{μ- τ}$, and emphasizing the prospects at a very large hadron collider with $\sqrt s =$ 100 TeV. We also discuss in this case the new contribution to two-lepton (Drell-Yan) production when the new leptophilic interaction has a non-vanishing kinetic mixing with the SM.
We estimate the sensitivity to the top-quark Yukawa coupling $y_t$ at future $e^+e^-$ colliders. We go beyond the standard approach that focuses on $t\bar t h$ production and consider final states with a Higgs boson but not top quarks. The sensitivity to $y_t$ in such processes comes from the coupling of the Higgs boson to top quarks in loops. Such final states can be produced in significant numbers at center-of-mass energies that will be accessible by all proposed $e^+e^-$ colliders. In a simplified theoretical framework to parametrise deviations from the Standard Model, we find that at FCC-$…
We study the production of a Higgs boson recoiling from a massless invisible system in $e^ + e^ -$ collisions. This is a quite distinctive signature that can arise when the Higgs boson is produced in association with a massless dark photon, which can happen in BSM scenarios foreseeing an extra unbroken $U(1)$ gauge group. Dark photons can indeed acquire effective couplings to the Higgs boson as occurs in models recently proposed to generate exponentially-spread Yukawa couplings. We analyze the signal and corresponding backgrounds for $H\to b\bar{b}$, and estimate ILC and FCC-ee sensitivities i…
Within models containing a very light scalar particle coupled to the 125 GeV Higgs boson, we present the first detailed study of Higgs decays into three of these light scalars. We determine model-independent conditions which the scalar sector after electroweak symmetry breaking has to satisfy in order for the three-body channel to become relevant. Using a specific model -- the real scalar singlet-extension of the Standard Model (SM) -- we then identify scenarios, where the rates of scalar three-body Higgs decays are comparable to or even exceed those of the well-studied two-body channel. All t…
The Circular Electron Positron Collider (CEPC) with a center-of-mass energy $\sqrt{s}$ = 240 GeV is proposed to serve as a Higgs factory, while it can also provide good opportunity for new physics searches at lower energy, which are difficult in hadron colliders but well-motivated by some theory models such as dark matter. This paper presents the sensitivity study of direct stau / smuon production at CEPC using full Monte Carlo (MC) simulation. With the assumption of flat 5% systematic uncertainty, the CEPC has the potential to discover the production of combined left-handed and right-handed s…
The proposed Circular Electron Positron Collider (CEPC), with a center-of-mass energy $\sqrt{s} = 240$ GeV, will primarily serve as a Higgs factory. At the same time, it can offer good opportunities to search for new physics phenomena at low energy, which can be challenging to discover at hadron colliders, but well motivated by some theoretical models developed to explain, e.g., the relic abundance of dark matter. This paper presents sensitivity studies of chargino pair production, considering scenarios for both a Bino-like and a Higgsino-like neutralino as the lightest supersymmetric particle…
The scenario of two mass-degenerate Higgs bosons within the general two-Higgs-doublet model (2HDM) is revisited. We focus on the global picture when two $CP$-even Higgs bosons of $h$ and $H$ are nearly mass-degenerate. A global fit to the signal strength of the 125 GeV Higgs measured at the LHC is performed. Based on the best-fit result of the 2HDM mixing angles $(α,β)$, theoretical constraints, charged and $CP$-odd Higgs boson direct search constraints and the electroweak precision constraints are imposed to the 2HDM parameter space. We present the signal predictions of the $(4b\,, 2b\,2γ)$ c…
Experiments at the future colliders LHC and ILC will, for the first time, explore electroweak reactions at energies in the TeV region, i.e. far beyond the masses of the W and Z bosons. In exclusive processes without the emission of massive gauge bosons double-logarithmic contributions, so-called Sudakov logarithms, will then become relevant. Precise theoretical predictions require the knowledge of the complete logarithmic two-loop corrections in the high energy limit, thus exceeding the approximation through the leading powers of logarithms known so far. This PhD thesis is focussed on the calc…
Neutrinos in the Standard Model (SM) are considered to be massless which is in contradiction with the evidence from the neutrino oscillation data. These experiments established that at least two SM neutrinos have non-zero masses and that individual lepton numbers are violated. This is strong evidence of new physics beyond the SM that should be responsible for generating non zero mass for the neutrinos. In this work, we study the collider phenomenology of an extension of the SM where neutrinos are generated radiatively at three-loop. We show that the production of same-sign dilepton at lepton c…
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.
The Georgi-Machacek (GM) model is one of many Beyond Standard Model scenarios with an extended scalar sector which can group under the custodial $\rm SU(2)_C$ symmetry into a fiveplet, a triplet, and two singlets. The heavy charged custodial fiveplet Higgs $\rm H_5^{\pm}$ are typical particles in GM model which couple to the electroweak gauge bosons, therefore provide a good testing ground for the detection of the $\rm H^\pm W^\mp Z$ vertex. The neutral custodial fiveplet Higgs $\rm H_5^{0}$ in the GM model has the same mass with $\rm H_5^{\pm}$ and couples to the electroweak gauge bosons $\rm…
A new particle with mass ~ 125 GeV that resembles the Higgs boson has recently been discovered by ATLAS and CMS. We propose a low-energy gamma-gamma collider as a cost- and time-efficient option for a Higgs factory capable of studying this particle in detail. In the past, this option has been suggested as a possible application of the CLIC two-beam accelerator technology (the CLIC Higgs Experiment, CLICHE) or as an option for the ILC. Here we propose a design based on a pair of \sim 10 GeV recirculating Linacs (Small Accelerator for Photon-Photon Higgs production using Recirculating Electrons,…
We discuss the prospects for a consistent, nonlinear, partially massless (PM), gauge symmetry of bimetric gravity (BMG). Just as for single metric massive gravity, we show that consistency of BMG relies on it having a PM extension; we then argue that it cannot.
In this note, I examine the possible consequences for high energy colliders of a Z-prime interpretation of the LHCb anomaly observed in the K*mu+mu- final state. Two examples are elaborated in the framework of the so-called 331 model. In the first one it is shown that LEP2 provides the tightest lower mass limit for the Z-prime boson, above 8 TeV, while in the second one the lower mass limit is set by ATLAS/CMS to about 3 TeV. It is then shown that precision measurements at ILC 500 GeV can fully explore the underlying structure of the model by measuring the fermion final states separately: lept…
The recent discovery of the Higgs-like resonance at $125\,\rm{GeV}$ has opened up new avenues in the search for beyond standard model physics. Hints of such extensions could manifest themselves as modifications in the Higgs-fermion couplings and other Higgs related observables. In this work, we study aspects of a class of models where the light fermion masses are radiatively generated. Specifically, we consider models where the light fermion masses, partially or completely, arise from chiral violation in the soft supersymmetry-breaking sector. In these models, the radiatively generated Higgs-f…
