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.
At the LHC combinatorics make it unlikely that we will be able to observe stop pair production with a decay to a semi-leptonic top pair and missing energy for generic supersymmetric mass spectra. Using a Standard-Model top tagger on fully hadronic top decays we can not only extract the stop signal but also measure the top momentum. To illustrate the promise of tagging tops with moderate boost we include a detailed discussion of our HEPTopTagger algorithm.
Top partner searches are one of the key aspects of new physics analyses at the LHC. We correct an earlier statement that supersymmetric top searches based on decays to semileptonic tops are not promising. Reconstructing the direction of the boosted leptonic top quark and correlating it with the measured missing transverse energy vector allows us to reduce the top pair background to an easily manageable level. In addition, reconstructing the full momentum of the leptonic top quark determines the stop mass based on an M_{T2} endpoint.
Top quarks may be produced in large numbers in association with new physics at the LHC. The polarization of these top quarks probes the chiral structure of the new physics. We discuss several kinematic distributions which are sensitive to the polarization of single top quarks and can be used without full event reconstruction. For collimated tops we construct polarization-sensitive observables for both hadronic and leptonic decay modes and plot their distributions. We compute the observable polarization signals from top quarks produced in the on-shell cascade decay of a stop squark into a top q…
Supersymmetric extensions of the Standard Model with highly mixed squark flavours beyond minimal flavour violation provide interesting scenarios of new physics, which have so far received limited attention. We propose a calculable realization of such scenarios in models of gauge mediation augmented with an extra interaction between the messengers and the up type squark. We compute the supersymmetric spectrum and analyze the flavour physics constraints on such models. In a simplified model approach, we show that scenarios with maximal squark flavour mixing result in interesting phenomenological…
Current searches for direct production of scalar top quarks, or stops, in supersymmetry focus on their decays into W boson + b quark + neutralino by way of top + neutralino and bottom + chargino. While the polarization of the top quark depends on the stop mixing, the chargino turns out to be fully polarized when the bottom Yukawa coupling can be neglected relative the top Yukawa coupling. We compute the energy and angular spectra of the charged lepton in the chargino channel, which could serve as the spin-analyzer of the chargino. In addition, we demonstrate the top polarization could have a s…
We study jet substructures of a boosted polarized top quark, which undergoes the hadronic decay $t\to b u\bar d$, in the perturbative QCD framework, focusing on the energy profile and the differential energy profile. These substructures are factorized into the convolution of a hard top-quark decay kernel with a bottom-quark jet function and a $W$-boson jet function, where the latter is further factorized into the convolution of a hard $W$-boson decay kernel with two light-quark jet functions. Computing the hard kernels to leading order in QCD and including the resummation effect in the jet fun…
Stops with the mass nearly degenerate with the top mass, decaying into tops and soft neutralinos, are usually dubbed "stealth stops". Their kinematics looks very similar to that of the standard tops events, which leads to events with little or no excess of missing transverse energy. This complicates the probing of this region of the stop parameter space by hadron colliders, rendering the application of standard searching techniques challenging. In this Snowmass white paper we reanalyze the spin correlation approach to the search of stealth stops, focusing on the feasibility of this search at t…
We study the production of a single charged slepton in association with a top quark in a R-parity violating supersymmetric model with lepton number violating interactions at the Large Hadron Collider. We find that the longitudinal polarization asymmetry of the top quark in such a production mode is significantly different from that in the production of a single top or a top pair in the Standard Model for a wide range of slepton masses. Our signal analysis shows that the top-slepton associate production leads to final states with distinct kinematic signatures, which differ from the Standard Mod…
We study the discovery prospects for stops which decay to a top and a light neutralino. We consider fully hadronic decays of the tops and present an estimate for the reach at various future collider runs. Our results are summarized in Table 1.
Measuring top polarization is extremely important in understanding any new physics that couples to top quark. Here, we take up the task of extracting polarization of top quark coming from TeV-scale stop decay at the upgraded high luminosity and high energy LHC. We focus on the scenario where the two tops from the stop decay lead to semi-leptonic final state. We take into account all relevant backgrounds and put kinematical cuts to suppress them. We arrive at a signal region where the desired signal can be observed with reasonable significance. Next, we examine, in such signal regions, how well…
From the point of view of the R-parity breaking supersymmetric model, we propose a scalar top (stop) search with longitudinally polarized electron (e-) and positron(e+) beams which will soon be available at the upgraded HERA. Fully polarized e- or e+ beams could produce the stop two times as much as unpolarized beams, while they increase background events due to the process of the standard model by about 30% in comparison with unpolarized ones. We show that right-handed e+ beams at HERA is efficient to produce the stop in the model. With 1 fb**(-1) of integrated luminosity we estimate reach in…
A proton-proton collider with center of mass energy around 100 TeV is the energy frontier machine that is likely to succeed the LHC. One of the primary physics goals will be the continued exploration of weak scale naturalness. Here we focus on the pair-production of stops that decay to a top and a neutralino. Most of the heavy stop parameter space results in highly boosted tops, populating kinematic regimes inaccessible at the LHC. New strategies for boosted top-tagging are needed and a simple, detector-independent tagger can be constructed by requiring a muon inside a jet. Assuming 20% system…
We compute the wall velocity in the MSSM with W, tops and stops contributing to the friction. In a wide range of parameters including those which fulfil the requirements of baryogenesis we find a wall velocity of order v_w=O(0.01) much below the SM value.
Top squark (stop) plays a crucial role in the naturalness of supersymmetry (SUSY) models, which has been extensively searched for in the LHC experiments. Due to the parity-violating couplings, top quark and chargino thus produced from stop decay are expected to be polarized. We investigate the polarization of top quark and chargino, from which we find that the mass differences between neutralino and stop/chargino can affect the degree of polarization of top quark/chargino in the stop decay. We then examine the top polarization effects on kinematics of stop decay in natural SUSY (NSUSY) at the …
We discuss pair production and decays of stops, sbottoms, $τ$-sneutrinos, and staus in $e^+e^-$ annihilation in the energy range $\sqrt{s} = 500 GeV$ to 2 TeV. Numerical predictions within the Minimal Supersymmetric Standard Model (MSSM) for cross sections and decay rates are presented. We study the stop discovery potential for $\sqrt{s} = 500 GeV$ and $10 \fb^{-1}$ integrated luminosity for polarized e^- beams. Moreover, we give an estimate of the error of the soft-breaking stop and sbottom parameters that can be obtained by cross section measurements with polarized e^- beams.
Top polarization is an important probe of new physics that couples to the top sector, and which may be discovered at the 14 TeV LHC. Taking the example of the MSSM, we argue that top polarization measurements can put a constraint on the soft supersymmetry breaking parameter A_t. In light of the recent discovery of a Higgs-like boson of mass ~125 GeV, a large A_t is a prediction of many supersymmetric models. To this end, we develop a *detector level* analysis methodology for extracting polarization information from hadronic tops using boosted jet substructure. We show that with 100 fb^(-1) of …
We show that a large mixing between the right-handed charm and top squarks (i) is allowed by low-energy flavour constraints; (ii) reduces the experimental bound on the stop mass; (iii) has a mild, but beneficial, effect on fine-tuning; (iv) leads to interesting signatures at the LHC not presently investigated by experiments. We estimate the current bound on the stop mass, in presence of flavour mixing, and discuss the new collider signatures. The signal in the t anti-c (c anti-t) + missing E_T channel is large enough that it can be immediately searched for experimentally, while the signature w…
We study an interesting region of phase space at the LHC for pair-produced stops decaying into hadronic top quarks and light neutralinos. After imposing a sizeable cut on the missing transverse energy, which is the key variable for reducing backgrounds, we have found that the two hadronic tops are likely to merge into a single fat jet. We develop a jet-substructure-based strategy to tag the two merged top-jets and utilize the MT2 variable to further reduce the backgrounds. We obtain about a 50% increase to the ratio of the signal over background and a mild increase on the signal discovery sign…
We investigate a potentially large CP violating asymmetry in the decay of a neutral scalar or pseudoscalar Higgs boson into the top-anti-top pair. The source of the CP nonconservation is the complex mixing in the (left-right) stop sector. One of the interesting consequence is the different rates of the Higgs boson decays into CP conjugate polarized states.
We present a phenomenological analysis of production and decays of stops, sbottoms, and staus in e+e- collisions with sqrt(s) = 500 - 800 GeV. We include SUSY-QCD and Yukawa coupling corrections as well as initial state radiation. We show that e- beam polarization is a powerful tool for a determination of the underlying SUSY parameters. Using in addition a polarized e+ beam improves the precision of the parameter determination by about 25%.
We constrain the masses of scalar-tops (stop) by analyzing the new precision Tevatron measurement of the W-boson mass and the LHC/Tevatron indications of a Higgs boson of mass 125.5+-1 GeV. Our study adopts Natural SUSY with low fine-tuning, which has multi-TeV first and second generation squarks and a light Higgsino mixing parameter mu=150 GeV. An effective Lagrangian calculation is made of mh to 3-loops using the H3m program with weak scale SUSY parameters obtained from RGE evolution from the GUT scale in the Natural SUSY scenario. The SUSY radiative corrections to the Higgs mass imply maxim…
The QCD corrections to the top-quark pair production via both polarized and unpolarized gluon fusion in $pp$ collisions are calculated in the Minimal Supersymmetric Model(MSSM). We find the MSSM QCD corrections can reach 4% and may be observable in future precise experiments. Furthermore, we studied the CP violation in the MSSM, our results show that the CP violating parameter is sensitive to the masses of SUSY particles (It becomes zero, when the c.m. energy is less than twice the masses of both gluino and stop quarks.) and may reach $10^{-3}$.
The first chapter serves as an introduction to the MSSM. I briefly review the model and explain why stops and sbottoms provide an interesting playground for exploring and testing this model. In the second chapter I discuss stop and sbottom production at lepton colliders. I give the analytical formulae for the production cross sections at e+e- and mu+mu- colliders including (general) beam polarization. A numerical analysis shows the cross sections that can be expected at LEP2, an e+e- Linear Collider, and a Muon Collider. In particular, I address the topics of what can be gained by using polari…
We study pair production of scalar top quarks in polarized photon-photon collisions with the subsequent decay of the top squarks into b-quarks and charginos. We simulate this process by using PYTHIA6.4 for an electron beam energy 2E_beam =1000 GeV. A set of criteria for physical variables is proposed which leads to a 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 provided that the neutralino mass is known.
The uncertainty of $t\bar{t}$ production cross section measurement at LHC is at a-few-percent level which still allows the stop pair production $\tilde{t}\tilde{t}^{*}$ with identical final states $2b+\ell+nj+{E}_{T}$. In this paper, we attempt to use the existing measurement of $W$-polarization in top quark decay to improve the distinction between stop and top quark states. We apply the ATLAS $χ^2$ method of $W$-polarization measurement in semi-leptonic $t\bar{t}$ final state to semi-leptonic stop pair samples and study its prediction. We find that the faked top events from stop mostly contri…
We survey the expected polarization of the top produced in the decay of a scalar top quark, $\tilde t \rightarrow {\tilde t}χ_i^0, i =1-2$. The phenomenology is quite interesting, since the expected polarization depends both on the mixing in the stop and neutralino sectors and on the mass differences between the stop and the neutralino. We find that a mixed stop behaves almost like a right-handed stop due to the larger hypercharge that enters the stop/top/gaugino coupling and that these polarisation effects disappear, when $m_{\tilde t_1} \approx m_t+m_{\tildeχ^0_i}$. After a discussion on the…
We calculate squared helicity amplitudes for non-diagonal and mixed squark pair production at hadron colliders, taking into account not only loop-induced QCD diagrams, but also previously unconsidered electroweak channels, which turn out to be dominant. Mixing effects are included for both top and bottom squarks. Numerical results are presented for several SUSY benchmark scenarios at both the CERN LHC and the Fermilab Tevatron, including the possibilities of light stops or sbottoms. The latter should be easily observed at the Tevatron in associated production of stops and sbottoms for a large …
In most of the models beyond the Standard Model, the top quark is expected to be polarized when produced in the decay of some heavier particle, like the gluino or the stop. The polarization is constructed, in an experiment or in simulations, through the distribution of top decay products. Here, we propose an estimator of top quark polarization that depends only on the kinematics of it's mother particle, apart from its decay couplings to top quarks, and is given in terms of the top polarization expected in the rest frame of the decaying particle. This estimator allows one to estimate the top po…
Observables sensitive to top quark polarization are important for characterizing or even discovering new physics. The most powerful spin analyzer in top decay is the down-type fermion from the W, which in the case of leptonic decay allows for very clean measurements. However, in many applications it is useful to measure the polarization of hadronically decaying top quarks. Usually it is assumed that at most 50% of the spin analyzing power can be recovered in this case. This paper introduces a simple and truly optimal hadronic spin analyzer, with a power of 64% at leading-order. The improvement…
