Electroweak baryogenesis in the minimal supersymmetric extension of the Standard Model may be realized within the light stop scenario, where the right-handed stop mass remains close to the top-quark mass to allow for a sufficiently strong first order electroweak phase transition. All other supersymmetric scalars are much heavier to comply with the present bounds on the Higgs mass and the electron and neutron electric dipole moments. Heavy third generation scalars render it necessary to resum large logarithm contributions to perform a trustable Higgs mass calculation. We have studied the one--loop RGE improved effective theory below the heavy scalar mass scale and obtained reliable values of the Higgs mass. Moreover, assuming a common mass $\tilde m$ for all heavy scalar particles, and values of all gaugino masses and the Higgsino mass parameter about the weak scale, and imposing gauge coupling unification, a two-loop calculation yields values of the mass $\tilde m$ in the interval between three TeV and six hundred TeV. Furthermore for a stop mass around the top quark mass, this translates into an upper bound on the Higgs mass of about 150 GeV. The Higgs mass bound becomes even stronger, of about 129 GeV, for the range of stop and gaugino masses consistent with electroweak baryogenesis. The collider phenomenology implications of this scenario are discussed in some detail.
It is widely thought that increasing bounds on the gluino mass, which feeds down to the stop mass through renormalization group running, are making a light stop increasingly unlikely. Here we present a counter-example. We examine the case of the Minimal Composite Supersymmetric Standard Model which has a light composite stop. The large anomalous dimension of the stop from strong dynamics pushes the stop mass toward a quasi-fixed point in the infrared, which is smaller than standard estimates by a factor of a large logarithm. The gluino can be about three times heavier than the stop, which is c…
Stop searches in supersymmetric frameworks with $R$-parity conservation usually assume the lightest neutralino to be the lightest supersymmetric particle. In this paper we consider an alternative scenario in which the left-handed tau sneutrino is lighter than neutralinos and stable at collider scales, but possibly unstable at cosmological scales. Moreover the (mostly right-handed) stop $\widetilde t$ is lighter than all electroweakinos, and heavier than the scalars of the third generation doublet, whose charged component, $\widetildeτ$, is heavier than the neutral one, $\widetildeν$. The remai…
In supersymmetric models the mass of the stops can be considered as the naturalness measure of the theory. Roughly, the lighter the stops are, the more natural the theory is. Both, the absence of supersymmetric signals at experiment and the measurement of the Higgs mass, put scenarios with light stops under increasing tension. I will present a supersymmetry breaking mechanism of the Scherk-Schwarz type that, by introducing extra $SU(2)_L$ triplets in the Higgs sector, is able to generate the correct Higgs mass while keeping stops light.
We consider a light supersymmetric top quark (stop) scenario, where a stop and a neutralino are the lightest supersymmetric particles and the stop mass is close to the sum of the neutralino mass and the top quark mass. In this scenario, the top quark and the neutralino coming from the decay of a stop have almost equal velocity vectors, and hence the missing transverse momentum in stop pair production events at colliders is suppressed, which makes the stop search challenging, as in the degenerate scenario where the stop mass and the neutralino mass are nearby. In this paper, we propose a novel …
A light supersymmetric top quark partner (stop) with a mass nearly degenerate with that of the Standard Model (SM) top quark can evade direct searches. The precise measurement of SM top properties such as the cross-section has been suggested to give a handle for this `stealth stop' scenario. We present an estimate of the potential impact a light stop may have on top quark mass measurements. The results indicate that certain light stop models may induce a bias of up to a few GeV, and that this effect can hide the shift in, and hence sensitivity from, cross-section measurements. The studies make…
In order to accommodate the observed Higgs boson mass in the CMSSM, the stops must either be very heavy or the mixing in the stop sector must be very large. Lower stop masses, possibly more accessible at the LHC, still give the correct Higgs mass only if the trilinear stop mixing parameter $|A_t|$ is in the multi-TeV range. Recently it has been shown that such large stop mixing leads to an unstable electroweak vacuum which spontaneously breaks charge or colour. In this work we therefore go beyond the CMSSM and investigate the effects of including baryon number violating operators $λ'' \bar{\bf…
We present the calculation of the stop ($\tilde{t}$) lifetime that results from its decay into gravitinos ($\tildeΨ^μ$) in the final state, namely $\tilde{t}\to \tildeΨ^μ + X$, where $X=t, bW, b lν_l$ for the two-, three- and four-body decay modes. The full calculation is obtained using the Feynman rules for massive gravitino, which is compared with the results obtained employing the equivalence theorem, where the longitudinal component of the gravitino is replaced by the goldstino. The stop turns out to be very long-lived in these scenarios, with lifetimes of $\mathcal{O}$($10^8$s, $10^{14}$s…
We have explored the possibilities of scenarios with heavy gluinos and light stops in supersymmetric (SUSY) standard models with extra vector-like multiplets. If we assume the hierarchical structure for soft masses of the minimal supersymmetric standard model (MSSM) scalar fields and extra scalars, the light stop and the observed Higgs boson can be realized. While the stau is the lightest SUSY particle (LSP) in broad parameter space, we have found that the neutralino LSP is realized in the case that the non-zero soft parameters for the MSSM Higgs doublets or the non-universal gaugino masses ar…
The "top-down" approach to minimal supersymmetric standard model (MSSM) phenomenology provides model-dependent indications for a natural stop mass. The approach is based on specific assumptions about the supersymmetry-breaking energy scale and parameters degeneracies. In order to determine robust predictions we update the stop-mass prediction within the MSSM with 25 parameters (MSSM-25) by including electroweak fine-tuning as "naturalness data" during the Bayesian fits of the parameters to experimental data. The approximately prior-independent results show that imposing naturalness, taken here…
We present a study of b-tau Yukawa unified supersymmetric SU(4)_c x SU(2)_L x SU(2)_R model (with mu > 0), which predicts the existence of gluino - neutralino and stop - neutralino coannihilation scenarios compatible with the desired relic LSP neutralino dark matter abundance and other collider constraints. The NLSP gluino or NLSP stop masses vary between 400 GeV to ~ 1 TeV. The NLSP gluinos will be accessible at the 14 TeV LHC, while we hope that the NSLP stop solutions will be probed in future LHC searches. We also identify regions of the parameter space in which the gluino and the lighter s…
New Signatures for a Light Stop at LEP2 in SUSY Models with Spontaneously Broken R-Parity
在一类具有R宇称的超对称模型中，最轻的终止可以产生新的衰变模式，成为第三代费米子$\tilde{t}_1 \rightarrow b + τ$。我们表明，这种衰变可能是占主导地位的或至少相当于普通的R宇称守恒模式$\tilde{t}_1 \rightarrow c + \tildeχ_1^0$，其中$\tildeχ_1^0$表示最轻的中性伴子。新的R宇称破坏衰变模式可以为LEP停止生产提供新的特征。
The discovery of a Higgs boson with a mass of 126 GeV at the LHC when combined with the non-observation of new physics both in direct and indirect searches imposes strong constraints on supersymmetric models and in particular on the top squark sector. The experiments for direct detection of dark matter have provided with yet more constraints on the neutralino LSP mass and its interactions. After imposing limits from the Higgs, flavour and dark matter sectors, we examine the feasibility for a light stop in the context of the pMSSM, in light of current results for stop and other SUSY searches at…
Collider Signatures of Minimal Flavor Mixing from Stop Decay Length Measurements
我们调查的前景，提取超对称耦合的衰变长度测量在大型强子对撞机。具体来说，我们利用的机会，一个轻的和长寿命的停止，这是对生产通过胶子与同号顶夸克。任何观察到的终止衰减长度的有限值都强烈支持这样的模型，即味道仅由标准模型汤川耦合以最小的方式破坏。我们发现，1皮秒的停止寿命，主要由t~ -> cchi ^0衰变，产生宏观横向碰撞参量约为180微米。如果最轻的中性伴子主要是希格西诺或质量非常接近光阻，光阻的寿命甚至会增加，并允许直接观察光阻轨迹和可能的次级顶点。如果中性伴子是NLSP，那么用终止衰变长度测量夸克味破坏也适用于引力伴子LSP。在这种情况下，我们指出了质量<= keV的极轻引力子的t~ -> c chi ^0-> c gamma G~衰变的机会。
If the stop is the next-to-lightest supersymmetric particle (NLSP) and the mass difference to the neutralino is smaller than the top mass, it can decay via flavour-violating decay modes to $c\tildeχ^0_1/u\tildeχ^0_1$ or a four-body decay to $b\tildeχ_1^0f\bar{f}'$, which above the $W$ boson threshold corresponds to the decay to $b\tildeχ_1^0W$. Improving on existing calculations for these decay modes, we analyse the branching ratios (BRs) for the respective decays and show that they can significantly deviate from one.
Current searches for the light top squark (stop) mostly focus on the decay channels of $\tilde{t} \rightarrow t χ_1^0$ or $\tilde{t} \rightarrow b χ_1^\pm \rightarrow bW χ_1^0$, leading to $t\bar{t}/bbWW+\met$ final states for stop pair productions at the LHC. However, in supersymmetric scenarios with light neutralinos and charginos other than the neutralino lightest supersymmetric particle (LSP), more than one decay mode of the stop could be dominant. While those new decay modes could significantly weaken the current stop search limits at the LHC, they also offer alternative discovery channel…
We discuss the fine-tuning issue within the MSSM framework. Following the idea that the fine-tuning can measure effects of some missing mechanism, we impose non-universal gaugino masses at the GUT scalem and explore the low scale implications. We realize that the fine-tuning parametrized with Delta_EW can be as low as zero. We consider the stop mass with a special importance and focus on the mass scales as m_stop <=700 GeV, which are excluded by current experiments when the stop decays into a neutralino along with a top quark or a chargino along with a b quark. We find that the stop mass can b…
We employ the ATLAS search results for events containing jets and large missing transverse momentum, corresponding to an integrated luminosity of 1 fb^{-1}, to investigate the constrained minimal supersymmetric model (CMSSM) with b-τYukawa coupling unification. In this scenario, one of the stops is the next-to-lightest supersymmetric particle (NLSP), which co-annihilates with the lightest (LSP) neutralino to yield the desired dark matter relic abundance. The NLSP stop, here taken to be lighter than the top quark, is slightly (<~ 20%-30%) heavier than the LSP neutralino, and it primarily decays…
Recently ATLAS reported a $3.3σ$ excess in the stop search with $\ell+jets+E_T^{miss}$ channel. We try to interpret the signal by a light stop pair production in the MSSM. We find: (1) simple models where stop decays into a higgsino or a bino are not favored. (2) an extension of them can explain the data at $2σ$ level without conflicting with the other search channels. A surviving possibility includes a light stop and a light higgsino, which is expected in a natural SUSY scenario.
Understanding the extent to which experimental searches are sensitive to Light Stops (LST) scenarios is essential to resolve questions about naturalness, electroweak baryogenesis and Dark Matter. In this paper we characterize the reach on LST scenarios in two ways. We extend experimental searches to cover specific gaps in the LST parameter space, showing for the first time that assuming a single decay channel one can exclude the region of $m_{\tilde{t}}<m_{top}$, which in its turn excludes electroweak baryogenesis in MSSM. Also, we explore the extent to which searches are weakened in a more ge…
Light Stop Decay in the MSSM with Minimal Flavour Violation
在超对称的情况下，光停止粒子$\tilde{t}_1$和最轻的超对称粒子（LSP）的质量差很小，假设是最轻的中性伴子，味道改变中性电流衰变$\tilde{t}_1 \to c \tildeχ_1^0$可以是主要的衰变通道，并可以超过四体停止衰变的某些参数值。在最小风味破坏（MFV）的框架中，这种衰减被CKM抑制，从而诱导长的停止寿命。在大型强子对撞机上的终止衰变长度测量可以用来测试那些通过标准模型汤川耦合的具有最小味道的模型。衰变宽度在一段时间以前已经通过一个近似公式给出，该公式考虑了MFV标度的主要参数。本文在MFV框架下计算了单圈衰变宽度。与近似结果的比较表明，由于近似衰减公式中忽略了非对数项，大MFV尺度的偏差为10%。分支比的差异可以忽略不计。这些大的木材必须加以修整。通过求解重整化群方程组进行重正化。精确的单回路结果和树水平的味道变化的中性电流衰减，其中包括的mummed的图表的比较，表明，remation效应是重要的，应该考虑。
Non-minimal supersymmetric models that predict a tree-level Higgs mass above the Minimal Supersymmetric Standard Model (MSSM) bound are well motivated by naturalness considerations. Indirect constraints on the stop sector parameters of such models are significantly relaxed compared to the MSSM; in particular, both stops can have weak-scale masses. We revisit the stop-catalyzed electroweak baryogenesis (EWB) scenario in this context. We find that the LHC measurements of the Higgs boson production and decay rates already rule out the possibility of stop-catalyzed EWB. We also introduce a gauge-i…
