arXiv subjects

Olaf Kittel

Publications and source records attributed to Olaf Kittel.

18 recordsLinked to original sources

arXiv2012

Measurement of radiative neutralino production

We perform the first experimental study with full detector simulation for the radiative production of neutralinos at the linear collider, at sqrt{s} = 500 GeV and realistic beam polarizations. We consider all relevant backgrounds, like the Standard Model background from radiative neutrino production. The longitudinal polarized beams enhance the signal and simultaneously reduce the background, such that statistical errors are significantly reduced. We find that the photon spectrum from the signal process can be well isolated. The neutralino mass and the cross section can be measured at a few per-cent level, with the largest systematic uncertainties from the measurement of the beam polarization and the beam energy spectrum.

hep-ph↗

arXiv2012

Model-independent WIMP Characterisation using ISR

The prospects of measuring the parameters of WIMP dark matter in a model independent way at the International Linear Collider are investigated. The signal under study is direct WIMP pair production with associated initial state radiation. The analysis is performed in full simulation of the ILD detector concept. With an integrated luminosity of L = 500 fb-1 and realistic beam polarizations the helicity structure of the WIMP couplings to electrons can be determined, and the masses and cross sections can be measured to the percent level. The systematic uncertainties are dominated by the polarization measurement and the luminosity spectrum.

hep-ex↗

arXiv2012

Measurement of CP Violation in the MSSM Neutralino Sector with the ILD

Supersymmetric models provide many new complex phases which lead to CP violating effects in collider experiments. As an example, CP-sensitive triple product asymmetries in neutralino production and subsequent leptonic two-body decays are studied within the Minimal Supersymmetric Standard Model. A full ILD detector simulation has been performed at a center of mass energy of 500GeV, including the relevant Standard Model background processes, a realistic beam energy spectrum, beam backgrounds and a beam polarization of 80% and -60% for the electron and positron beams, respectively. Assuming an integrated luminosity of 500fb-1 collected by the experiment and the performance of the current ILD detector, a relative measurement accuracy of 10% for the CP-sensitive asymmetry can be achieved in the chosen scenario.

hep-ph↗

arXiv2011

CP Violation in Correlated Production and Decay of Unstable Particles

We study resonant CP-violating Einstein--Podolsky--Rosen correlations that may take place in the production and decay of unstable scalar particles at high-energy colliders. We show that as a consequence of unitarity and CPT invariance of the S-matrix, in 2 --> 2 scatterings mediated by mixed scalar particles, at least three linearly independent decay matrices associated with the unstable scalar states are needed to obtain non-zero CP-odd observables that are also odd under C-conjugation. Instead, for the correlated production and decay of two unstable particle systems in 2 --> 4 processes, we find that only two independent decay matrices are sufficient to induce a net non-vanishing CP-violating phenomenon. As an application of this theorem, we present numerical estimates of CP asymmetries for the correlated production and decay of supersymmetric scalar top--anti-top pairs at the LHC, and demonstrate that these could reach values of order one. As a byproduct of our analysis, we develop a novel spinorial trace technique, which enables us to efficiently evaluate lengthy expressions of squared amplitudes describing the resonant scalar transitions.

hep-ph↗

arXiv2011

Testing the CP-violating MSSM in stau decays at the LHC and ILC

We study CP violation in the two-body decay of a scalar tau into a neutralino and a tau, which should be probed at the LHC and ILC. From the normal tau polarization, a CP asymmetry is defined which is sensitive to the CP phases of the trilinear scalar coupling parameter $A_τ$, the gaugino mass parameter $M_1$, and the higgsino mass parameter $μ$ in the stau-neutralino sector of the Minimal Supersymmetric Standard Model. Asymmetries of more than 70% are obtained in scenarios with strong stau mixing. As a result, detectable CP asymmetries in stau decays at the LHC are found, motivating further detailed experimental studies for probing the SUSY CP phases.

hep-ph↗

arXiv2010

How light can the lightest neutralino be?

In this talk we summarize previous work on mass bounds of a light neutralino in the Minimal Supersymmetric Standard Model. We show that without the GUT relation between the gaugino mass parameters M_1 and M_2, the mass of the lightest neutralino is essentially unconstrained by collider bounds and precision observables. We conclude by considering also the astrophysics and cosmology of a light neutralino.

hep-ph↗

arXiv2010

CP violation in sbottom decays

We study CP asymmetries in two-body decays of bottom squarks into charginos and tops. These asymmetries probe the SUSY CP phases of the sbottom and the chargino sector in the Minimal Supersymmetric Standard Model. We identify the MSSM parameter space where the CP asymmetries are sizeable, and analyze the feasibility of their observation at the LHC. As a result, potentially detectable CP asymmetries in sbottom decays are found, which motivates further detailed experimental studies for probing the SUSY CP phases.

hep-ph↗

arXiv2010

Probing SUSY CP Violation in Two-Body Stop Decays at the LHC

We study CP asymmetries in two-body decays of top squarks into neutralinos and sleptons at the LHC. These asymmetries are used to probe the CP phases possibly present in the stop and neutralino sector of the Minimal Supersymmetric Standard Model. Taking into account bounds from experimental electric dipole moment searches, we identify areas in the mSUGRA parameter space where CP asymmetries can be sizeable and discuss the feasibility of their observation at the LHC. As a result, potentially detectable CP asymmetries in stop decays at the LHC are found, motivating further detailed experimental studies for probing SUSY CP phases.

hep-ph↗

arXiv2010

Normal tau polarisation as a sensitive probe of CP violation in chargino decay

CP violation in the spin-spin correlations in chargino production and subsequent two-body decay into a tau and a tau-sneutrino is studied at the ILC. From the normal polarisation of the tau, an asymmetry is defined to test the CP-violating phase of the higgsino mass parameter μ. Asymmetries of more than \pm70% are obtained, also in scenarios with heavy first and second generation sfermions. Bounds on the statistical significances of the CP asymmetries are estimated. As a result, the normal tau polarisation in the chargino decay is one of the most sensitive probes to constrain or measure the phase ϕ_μat the ILC, motivating further detailed experimental studies.

hep-ph↗

arXiv2009

SUSY CP phases and asymmetries at colliders

In the Minimal Supersymmetric Standard Model, physical phases of complex parameters lead to CP violation. We show how triple products of particle momenta or spins can be used to construct asymmetries, that allow us to probe these CP phases. To give specific examples, we discuss the production of neutralinos at the International Linear Collider. For the Large Hadron Collider, we discuss CP asymmetries in squark decays, and in the tri-lepton signal. We find that the CP asymmetries can be as large as 60%.

hep-ph↗

arXiv2008

CP-violating Higgs boson mixing in chargino production at the muon collider

We study the pair-production of charginos in the CP-violating Minimal Supersymmetric Standard Model at center-of-mass energies around the heavy neutral Higgs boson resonances. If these resonances are nearly degenerate, as it can happen in the Higgs decoupling limit, radiatively induced scalar-pseudoscalar transitions can be strongly enhanced. The resulting mixing in the Higgs sector leads to large CP-violating effects, and a change of their mass spectrum. For longitudinally polarized muon beams, we analyze CP asymmetries which are sensitive to the interference of the two heavy neutral Higgs bosons. We present a detailed numerical analysis of the cross sections, chargino branching ratios, and the CP observables. We obtain sizable CP-asymmetries, which would be accessible in future measurements at a muon collider. Especially for intermediate values of the parameter \tanβ, where the largest branching ratios of Higgs bosons into charginos are expected, this process allows to analyze the Higgs sector properties and its interaction to supersymmetric fermions.

hep-ph↗

arXiv2008

Disentangling CP phases in nearly degenerate resonances: neutralino production via Higgs at a muon collider

In the CP-violating Minimal Supersymmetric Standard Model, we study the pair production of neutralinos at center-of-mass energies around the heavy neutral Higgs boson resonances. For longitudinally polarized muon beams, we analyze CP asymmetries which are sensitive to the interference of the two heavy neutral Higgs bosons. Due to radiatively induced scalar-pseudoscalar transitions, the CP asymmetries can be strongly enhanced when the resonances are nearly degenerate, as in the Higgs decoupling limit. The Higgs couplings to the neutralino sector can then be analyzed in the presence of CP violating phases. We present a detailed numerical analysis of the cross sections, neutralino branching ratios, and the CP observables. We find that radiatively induced CP violation in the Higgs sector leads to sizable CP-asymmetries, which are accessible in future measurements at a muon collider. However, we expect that our proposed method should be applicable to other processes with nearly degenerate scalar resonances, even at hadron colliders.

hep-ph↗

arXiv2007

The Role of Beam polarization for Radiative Neutralino Production at the ILC

We analyze the impact of electron and positron beam polarization on radiative neutralino production at the International Linear Collider (ILC). We focus on three different mSUGRA scenarios in turn at the Higgs strahlung threshold, the top pair production threshold, and at $\sqrt{s} =500\GeV$. In these scenarios at the corresponding $\sqrt{s}$, radiative neutralino production is the only supersymmetric production mechanism which is kinematically allowed. The heavier neutralinos, and charginos as well as the sleptons, squarks and gluinos are too heavy to be pair produced. We calculate the signal cross section and also the Standard Model background from radiative neutrino production. For our scenarios, we obtain significances larger than 10 and signal to background ratios between 2% and 5%, if we have electron beam polarization $P_{e^-} = 0.0- 0.8$ and positron beam polarization $P_{e^+} = 0.0 - 0.3$. If we have electron beam polarization of $P_{e^-} = 0.9$, then the signal is observable with $P_{e^+} = 0.0$ but both the significance and the signal to background ratio are significantly improved for $P_{e^+} = 0.3$.

hep-ph↗

arXiv2007

Polarization aspects in radiative neutralino production

We study the impact of beam polarization on radiative neutralino production e+e- \to χ^0_1 χ^0_1 photon at the International Linear Collider. We show that longitudinal polarized beams significantly enhance the signal and simultaneously reduce the Standard Model background from radiative neutrino production e+e- \to ν\barνphoton. We point out that the radiative production of neutralinos could be the only accessible SUSY particles, if neutralinos, charginos, sleptons, as well as squarks and gluinos are too heavy to be pair-produced in the first stage of the ILC at sqrt{s} = 500 GeV.

hep-ph↗

arXiv2006

Discovery potential of radiative neutralino production at the ILC

We study radiative neutralino production e^+e^- \to \tildeχ^0_1 \tildeχ^0_1γat the linear collider with longitudinally polarised beams. We consider the Standard Model background from radiative neutrino production e^+e^- \to ν\barνγ, and the supersymmetric radiative production of sneutrinos e^+e^- \to \tildeν\tildeν^\ast γ, which can be a background for invisible sneutrino decays. We give the complete tree-level formulas for the amplitudes and matrix elements squared. In the Minimal Supersymmetric Standard Model, we study the dependence of the cross sections on the beam polarisations, on the parameters of the neutralino sector, and on the selectron masses. We show that for bino-like neutralinos longitudinal polarised beams enhance the signal and simultaneously reduce the background, such that statistics is significantly enhanced. We point out that there are parameter regions where radiative neutralino production is the only channel to study SUSY particles, since heavier neutralinos, charginos and sleptons are too heavy to be pair-produced in the first stage of the linear collider with \sqrt{s}=500 GeV.

hep-ph↗

arXiv2005

Higgs boson interference in mu^+ mu^- ->chargino_i chargino_j with longitudinally polarized beams

We study chargino production at a muon collider with longitudinally polarized beams and center of mass energies around the heavy neutral Higgs boson resonances. We show that the interference of the CP even and CP odd Higgs bosons can be analyzed using the energy distributions of the lepton or W boson from the chargino two-body decays \tildeχ^\pm_j\to\ell^\mp\tildeν_\ell or \tildeχ^\pm_j \to W^\pm \neutralino_1, respectively. The energy distributions depend on the longitudinal polarization of the decaying chargino, which are correlated to the muon beam polarizations. We define asymmetries in these energy distributions which allow a determination of the H and A couplings to the charginos and in particular of their relative phase. We analyze the asymmetries, cross sections and branching ratios in CP conserving Minimal Supersymmetric Standard Model scenarios. For nearly degenerate Higgs bosons we find maximal asymmetries which can be measured with high statistical significance.

hep-ph↗

arXiv2005

Flavor-singlet hybrid baryons may already have been discovered

The splittings between the spin 1/2 and spin 3/2 iso-singlet baryons Lambda_s(1405) and Lambda_s(1520), and their charmed counterparts Lambda_c(2593) and Lambda_c(2625), have been a theoretical conundrum. Here we investigate the possibility that the QCD binding of color octets comprised of three quarks in a flavor singlet configuration is stronger than previously envisaged, allowing these states to be interpreted as hybrids consisting of three quarks plus a valence gluon (udsg) and (udcg). A fit of their mass separation allows the mass prediction of the strange and charmed flavor octet and decuplet hybrid baryons and the prediction of the mass separation of the beauty hybrids. Such hybrid states come in parity-doubled pairs with the even parity state lighter by about 300 MeV. Existing data accommodates either parity assignment for the observed states and the existence of the required unobserved partners at either higher or lower mass. We discuss difficulties with and strategies for observing the other states under the two cases. A corollary of the strong-binding-in-flavor-singlet-channel hypothesis is that the H-dibaryon may be very long lived or stable with m_H ~< 2 GeV.

hep-ph↗

arXiv2000

Masses of Flavor Singlet Hybrid Baryons

We study the possibility that four iso-singlet baryons $Λ_s(1405)$ $J^P=1/2^{-}$, $Λ_s(1520)$ $J^P=3/2^{-}$, $Λ_c(2593)$ $J^P=1/2^{-}$ and $Λ_c(2625)$ $J^P=3/2^{-}$ are hybrids: three quark one gluon states (udsg). We calculate the mass separations of the candidates, using a degeneracy-lifting hyperfine interaction from an effective single colored gluon exchange between the constituents. The correct ordering of masses is obtained (contrary to the case for the conventional interpretation as 3 quarks with L=1) and the splittings are plausible. The parity of these states is not measured, only assumed to be negative. In the hybrid picture, the lightest states are parity even and the parity odd counterparts lie about 300 MeV higher. Thus the hybrid ansatz predicts that either the parity of the $Λ(1405)$ etc is positive, or that there are undiscovered positive parity states about 300 MeV lower. We also remark that in this picture, the H-dibaryon mass may be around 1.5 GeV.

hep-ph↗