Scalar Top Quarks Research Articles

On pair production of scalar top quarks in e + e − collisions at ILC and a possibility of their mass reconstruction

We study pair production of scalar top quarks (stop, \( \tilde t_1 \)) in e + e − collisions with the subsequent decay of the top squarks into b quarks and charginos \( \tilde t_1 \to b\tilde \chi _1^ \pm \). We simulate this process by using PYTHIA6.4 for the beam energy 2E b = \( \sqrt 2 \) = 350, 400, 500, 800, 1000 GeV. A set of criteria for physical variables is proposed, which provides good separation of stop signal events from top quark pair production being the main background. These criteria allow us to reconstruct the mass of the top squark with an integrated luminosity of 1000 fb−1 provided that the neutralino mass is known.

Search for the lightest scalar top quark in events with two leptons in [formula omitted] collisions at [formula omitted

We report results of a search for the pair production of the lightest supersymmetric partner of the top quark, t˜1, using a data set corresponding to an integrated luminosity of 1 fb−1 collected by the DØ detector at a pp¯ center-of-mass energy of 1.96 TeV at the Fermilab Tevatron collider. Both scalar top quarks are assumed to decay into a b quark, a charged lepton and a scalar neutrino. The search is performed in the electron plus muon and dielectron final states. The signal topology consists of two isolated leptons, missing transverse energy, and jets. We find no evidence for this process and exclude regions of parameter space in the framework of the minimal supersymmetric standard model.

Polarized tops from stop decays at the LHC

In supersymmetric models, scalar top quarks, or stops, generically have parity-violating couplings to top quarks. As a result, tops produced in stop decays should be polarized. In this paper, we will argue that this effect may be observable at the LHC with realistic integrated luminosities, provided that one of the stops is copiously produced and can decay to a top and a neutralino. We define the ``effective'' stop mixing angle, which determines the degree of top polarization, and discuss the prospects for a measurement of this angle at the LHC. If some information about the neutralino mixing matrix is available, this measurement can be used to constrain the mixing angle in the stop sector, one of the most important ingredients in assessing the naturalness of electroweak symmetry breaking in the MSSM.

Search for admixture of scalar top quarks in the [formula omitted] lepton + jets final state at [formula omitted

A search for pair production of the lightest supersymmetric partner of the top quark, t˜1, is performed in the lepton+jets channel using 0.9 fb−1 of data collected by the DØ experiment. Kinematic differences between t˜1t˜¯1 and the dominant top quark pair production background are used to separate the two processes. First limits from Run II of the Fermilab Tevatron Collider for the scalar top quark decaying to a chargino and a b quark (t˜1→χ˜1+b) are obtained for scalar top quark masses of 130–190 GeV and chargino masses of 90–150 GeV.

Light stop searches at the LHC in events with one hard photon or jet and missing energy

Low energy supersymmetric models provide a solution to the hierarchy problem and also have the necessary ingredients to solve two of the most outstanding issues in cosmology: the origin of the baryon asymmetry and the source of dark matter. In the MSSM, weak scale generation of the baryon asymmetry may be achieved in the presence of light stops, with masses lower than about 130 GeV. Moreover, the proper dark matter density may be obtained in the stop-neutralino co-annihilation region, where the stop-neutralino mass difference is smaller than a few tens of GeV. Searches for scalar top quarks (stops) in pair production processes at the Tevatron and at the Large Hadron Collider (LHC) become very challenging in this region of parameters. At the LHC, however, light stops proceeding from the decay of gluino pairs may be identified, provided the gluino mass is smaller than about 900 GeV. In this article we propose an alternative method for stop searches in the co-annihilation region, based on the search for these particles in events with missing energy plus one hard photon or jet. We show that this method is quite efficient and, when complemented with ongoing Tevatron searches, allows to probe stop masses up to about 160 GeV, fully probing the region of parameters consistent with electroweak baryogenesis in the MSSM.

Search for scalar top quarks in the acoplanar charm jets and missing transverse energy final state in [formula omitted] collisions at [formula omitted

We present a search for the pair production of scalar top quarks, t˜, using 995 pb−1 of data collected in pp¯ collisions with the DØ detector at the Fermilab Tevatron Collider at s=1.96TeV. Both scalar top quarks are assumed to decay into a charm quark and a neutralino (χ˜10), where χ˜10 is the lightest supersymmetric particle. This leads to a final state with two acoplanar charm jets and missing transverse energy. We find the yield of such events to be consistent with the standard model expectation, and exclude sets of t˜ and χ˜10 masses at the 95% C.L. that substantially extend the domain excluded by previous searches.

Search for the lightest scalar top quark in events with two leptons in [formula omitted] collisions at [formula omitted

Data collected by the D0 detector at a pp¯ center-of-mass energy of 1.96 TeV at the Fermilab Tevatron Collider have been used to search for pair production of the lightest supersymmetric partner of the top quark decaying into bℓν˜. The search is performed in the ℓℓ′=eμ and μμ final states. No evidence for this process has been found in data samples of approximately 400 pb−1. The domain in the [M(t˜1),M(ν˜)] plane excluded at the 95% C.L. is substantially extended by this search.

Small visible energy scalar top iterative discriminant analysis

Light scalar top quarks with a small mass difference with respect to the neutralino mass are of particular cosmological interest. This study uses an iterative discriminant analysis method to optimize the expected selection efficiency at the international linear collider (ILC).

Phenomenology of top partners at the ILC

The assumption of a new symmetry provides a nice explanation of the existence of dark matter and an elegant way to avoid the electroweak constraints. This symmetry often requires the pair production of new particles at colliders and it guarantees that cascade decays down to the lightest particle give rise to missing energy plus jets and leptons. For a long time, supersymmetry with the conserved R-parity was the only candidate for such signals. However, any new physics with this type of new symmetry may show up with similar signals and the discrimination between different models at the LHC is quite challenging. In this paper, we address the problem of discrimination between different models, more concretely, in the little Higgs theory with T-parity (LHT) and the supersymmetric theory with R-parity. We concentrate on the pair production of heavy top partners, e.g., T-odd quarks in LHT and the scalar top quarks in the MSSM at linear colliders (LC).

Search for the pair production of scalar top quarks in the acoplanar charm jet final state in collisions at

A search for the pair production of scalar top quarks, t˜, has been performed in 360 pb−1 of data from pp¯ collisions at a center-of-mass energy of 1.96 TeV, collected by the DØ detector at the Fermilab Tevatron collider. The t˜ decay mode considered is t˜→cχ˜10, where χ˜10 is the lightest supersymmetric particle. The topology analyzed therefore consists of a pair of acoplanar heavy-flavor jets with missing transverse energy. The data and standard model expectation are in agreement, and a 95% C.L. exclusion domain in the (mt˜,mχ˜10) plane has been determined, extending the domain excluded by previous experiments.

Analyzing the scalar top coannihilation region at the International Linear Collider

The minimal supersymmetric standard model opens the possibility of electroweak baryogenesis provided that the light scalar top quark (stop) is lighter than the top quark. In addition, the lightest neutralino is an ideal candidate to explain the existence of dark matter. For a light stop with mass close to the lightest neutralino, the stop-neutralino coannihilation mechanism becomes efficient, thus rendering the predicted dark matter density compatible with observations. Such a stop may however remain elusive at hadron colliders. Here it is shown that a future linear collider provides a unique opportunity to detect and study the light stop. The production of stops with small stop-neutralino mass differences is studied in a detailed experimental analysis with a realistic detector simulation including a CCD vertex detector for flavor tagging. Furthermore, the linear collider, by precision measurements of superpartner masses and mixing angles, also allows to determine the dark matter relic density with an accuracy comparable to recent astrophysical observations.

Search for bosonic stop decays in R-parity violating supersymmetry in e+pe+p collisions at HERA

A search for scalar top quarks in R-parity violating supersymmetry is performed in e+p collisions at HERA using the H1 detector. The data, taken at s=319GeV and 301 GeV, correspond to an integrated luminosity of 106pb−1. The resonant production of scalar top quarks t˜ in positron quark fusion via an R-parity violating Yukawa coupling λ′ is considered with the subsequent bosonic stop decay t˜→b˜W. The R-parity violating decay of the sbottom quark b˜→dν¯e and leptonic and hadronic W decays are considered. No evidence for stop production is found in the search for bosonic stop decays nor in a search for the direct R-parity violating decay t˜→eq. Mass dependent limits on λ′ are obtained in the framework of the minimal supersymmetric Standard Model. Stop quarks with masses up to 275GeV can be excluded at the 95% confidence level for a Yukawa coupling of electromagnetic strength.

Search for bosonic stop decays in R-parity violating supersymmetry in [formula omitted] collisions at HERA

A search for scalar top quarks in R-parity violating supersymmetry is performed in e+p collisions at HERA using the H1 detector. The data, taken at s=319GeV and 301 GeV, correspond to an integrated luminosity of 106pb−1. The resonant production of scalar top quarks t˜ in positron quark fusion via an R-parity violating Yukawa coupling λ′ is considered with the subsequent bosonic stop decay t˜→b˜W. The R-parity violating decay of the sbottom quark b˜→dν¯e and leptonic and hadronic W decays are considered. No evidence for stop production is found in the search for bosonic stop decays nor in a search for the direct R-parity violating decay t˜→eq. Mass dependent limits on λ′ are obtained in the framework of the minimal supersymmetric Standard Model. Stop quarks with masses up to 275GeV can be excluded at the 95% confidence level for a Yukawa coupling of electromagnetic strength.

Search for Pair Production of Light Scalar Top Quarks inpp¯Collisions ats=1.8 TeV

Using 85.2±3.6 pb−1 of pp¯ collisions collected at s=1.8 TeV with the D0 detector at Fermilab’s Tevatron Collider, we present the results of a search for direct pair production of scalar top quarks (t˜), the supersymmetric partners of the top quark. We examined events containing two or more jets and missing transverse energy, the signature of light scalar top quark decays to charm quarks and neutralinos. After selections, we observe 27 events while expecting 31.1±6.4 events from known standard model processes. Comparing these results to next-to-leading-order production cross sections, we exclude a significant region of t˜ and neutralino phase space. In particular, we exclude the t˜ mass mt˜<122 GeV/c2 for a neutralino mass of 45 GeV/c2.Received 9 February 2004DOI:https://doi.org/10.1103/PhysRevLett.93.011801©2004 American Physical Society

Search for pair production of scalar top quarks in R-parity violating decay modes in pp collisions at square root of s=1.8 TeV.

We present the results of a search for pair production of scalar top quarks (t(1)) in an R-parity violating supersymmetry scenario in 106 pb(-1) of pp collisions at square root of s=1.8 TeV collected by the Collider Detector at Fermilab. In this mode each t(1) decays into a tau lepton and a b quark. We search for events with two tau's, one decaying leptonically (e or mu) and one decaying hadronically, and two jets. No candidate events pass our final selection criteria. We set a 95% confidence level lower limit on the t(1) mass at 122 GeV/c(2) for Br(t(1)-->tau b)=1.

Search for 3- and 4-body decays of the scalar top quark in [formula omitted] collisions at [formula omitted] 1.8 TeV

We have searched for the signature of 3- and 4-body decays of pair-produced scalar top quarks (stop) in the inclusive final state containing an electron, a muon, and significant missing transverse energy using a sample of pp̄ events corresponding to 108.3 pb−1 of data collected with the DØ detector at Fermilab. The search is done in the framework of the minimal supersymmetric standard model assuming that the neutralino (χ̃01) is the lightest supersymmetric particle and is stable. No evidence for a signal is found and we derive cross-section upper limits as a function of stop (t̃) and neutralino masses in different decay scenarios leading to the bℓνχ̃01 final state.

Search for top and bottom squarks

Searches for the lightest scalar top quark {bar t}{sub 1} and scalar bottom quark {bar b}{sub 1} performed at LEP2 with a center-of-mass energy of up to {radical}s = 209 GeV and at Tevatron using data collected at {radical}s = 1.8 TeV during Run I and at {radical}s = 1.96 TeV during Run II, are discussed. Different possible decay modes were considered and no evidence for any such signal was observed. Exclusion limits were extracted.

Neutral Higgs boson masses of the MSSM at the one-loop level in an explicitCPviolation scenario

The neutral Higgs sector of the minimal supersymmetric standard model (MSSM) in explicit CP violation scenario is investigated at the one-loop level. Within the context of the effective potential formalism, the masses of the neutral Higgs bosons are calculated at the one-loop level by taking into account the contributions of the following loops of ordinary particles and superpartners: top quark, the scalar top quarks, bottom quark, the scalar bottom quarks, tau lepton, the scalar tau leptons, $W$ boson, the charged Higgs boson, the charginos, $Z$ boson, the scalar and pseudoscalar Higgs bosons, and the neutralinos. Our calculation is an improvement in the sense that both the terms which are quartic in the electroweak coupling constants into account, and the pseudoscalar Higgs loop contribution are explicitly included.

Signatures of baryogenesis in the minimal supersymmetric standard model

We reexamine the electroweak baryogenesis within the context of the minimal supersymmetric standard model, studying its potential collider signatures. We find that this mechanism of baryogenesis does not give a new $\mathrm{CP}$ violating signal at the B factories. The first circumstantial evidence may come from enhanced ${B}_{s}$ or ${B}_{d}$ mixing. If a light right-handed scalar top quark and Higgs boson are found as required, a linear collider represents the best possibility for confirming the scenario.

Search for scalar top and scalar bottom quarks at LEP

Searches for a scalar top quark and a scalar bottom quark have been performed using a data sample of 438 pb−1 at centre-of-mass energies of s=192–209 GeV collected with the OPAL detector at LEP. No evidence for a signal was found. The 95% confidence level lower limit on the scalar top quark mass is 97.6 GeV if the mixing angle between the supersymmetric partners of the left- and right-handed states of the top quark is zero. When the scalar top quark decouples from the Z0 boson, the lower limit is 95.7 GeV. These limits were obtained assuming that the scalar top quark decays into a charm quark and the lightest neutralino, and that the mass difference between the scalar top quark and the lightest neutralino is larger than 10 GeV. The complementary decay mode of the scalar top quark decaying into a bottom quark, a charged lepton and a scalar neutrino has also been studied. The lower limit on the scalar top quark mass is 96.0 GeV for this decay mode, if the mass difference between the scalar top quark and the scalar neutrino is greater than 10 GeV and if the mixing angle of the scalar top quark is zero. From a search for the scalar bottom quark, a mass limit of 96.9 GeV was obtained if the mass difference between the scalar bottom quark and the lightest neutralino is larger than 10 GeV.