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Register a new userAn unambiguous test of positivity at lepton colliders
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The diphoton channel at lepton colliders, $e^+e^- (mu^+mu^-) to gamma gamma$, has a remarkable feature that the leading new physics contribution comes only from dimension-eight operators. This contribution is subject to a set of positivity bounds, derived from fundamental principles of Quantum Field Theory, such as unitarity, locality and analyticity. These positivity bounds are thus applicable to the most direct observable -- the diphoton cross sections. This unique feature provides a clear, robust, and unambiguous test of these principles. We estimate the capability of various future lepton colliders in probing the dimension-eight operators and testing the positivity bounds in this channel. We show that positivity bounds can lift certain degeneracies among the effective operators and significantly change the perspectives of a global analysis. We also perform a combined analysis of the $gammagamma/Zgamma/ZZ$ processes in the high energy limit and point out the important interplay among them.
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Various types of electroweak-interacting particles, which have non-trivial charges under the $mathrm{SU}(2)_L times mathrm{U}(1)_Y$ gauge symmetry, appear in various extensions of the Standard Model. These particles are good targets of future lepton colliders, such as the International Linear Collider (ILC), the Compact LInear Collider (CLIC) and the Future Circular Collider of electrons and positrons (FCC-ee). An advantage of the experiments is that, even if their beam energies are below the threshold of the production of the new particles, quantum effects of the particles can be detected through high precision measurements. We estimate the capability of future lepton colliders to probe electroweak-interacting particles through the quantum effects, with particular focus on the wino, the Higgsino and the so-called minimal dark matters, and found that a particle whose mass is greater than the beam energy by 100-1000 GeV is detectable by measuring di-fermion production cross sections with $O(0.1)$% accuracy. In addition, with the use of the same analysis, we also discuss the sensitivity of the future colliders to model independent higher dimensional operators, and found that the cutoff scales corresponding to the operators can be probed up to a few ten TeV.
New physics close to the electroweak scale is well motivated by a number of theoretical arguments. However, colliders, most notably the Large Hadron Collider (LHC), have failed to deliver evidence for physics beyond the Standard Model. One possibility for how new electroweak-scale particles could have evaded detection so far is if they carry only electroweak charge, i.e. are color neutral. Future $e^+e^-$ colliders are prime tools to study such new physics. Here, we investigate the sensitivity of $e^+e^-$ colliders to scalar partners of the charged leptons, known as sleptons in supersymmetric extensions of the Standard Model. In order to allow such scalar lepton partners to decay, we consider models with an additional neutral fermion, which in supersymmetric models corresponds to a neutralino. We demonstrate that future $e^+e^-$ colliders would be able to probe most of the kinematically accessible parameter space, i.e. where the mass of the scalar lepton partner is less than half of the colliders center-of-mass energy, with only a few days of data. Besides constraining more general models, this would allow to probe some well motivated dark matter scenarios in the Minimal Supersymmetric Standard Model, in particular the incredible bulk and stau co-annihilation scenarios.
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.
In composite Higgs (CH) models, large mixings between the top quark and the new strongly interacting sector are required to generate its sizeable Yukawa coupling. Precise measurements involving top as well as left-handed bottom quarks therefore offer an interesting opportunity to probe such new physics scenarios. We study the impact of third-generation-quark pair production at future lepton colliders, translating prospective effective-field-theory sensitivities into the CH parameter space. Our results show that one can probe a significant fraction of the natural CH parameter space through the top portal, especially at TeV centre-of-mass energies.
The Standard Model (SM) is characterized by a well-defined pattern of couplings of the gauge bosons to the fermions. Any deviation from this description would correspond to a definitive hint for beyond SM physics. These deviations would manifest in terms of different yields for the final states in a direct search. In this letter, we develop a universal framework to extract the extent to which a direct search can probe these differences. Our strategy is effectively demonstrated for a heavy neutral $Z$ decaying into a pair of electrons and muons with different branching fractions. We note the implications of the drastically different reconstructions (especially in the HL-LHC) on the computation of discovery significance as well as the extraction of non-universality. The potential of the current and future $pp$ colliders, quantifying the extent of the asymmetry (in yields) to which they are sensitive, is explored. While the HL-LHC phase offers an optimistic but a restricted picture, it naturally paves the way for the FCC-hh machine. The latter being sensitive not only to heavier masses but also to minor departures from universality in the couplings for the lighter masses.
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