Mono-top signature from stop decay at the HE-LHC

Xu-Xu Yang,Hang Zhou,Tian-Peng Tang and Ning Liu,*

1 Department of Physics and Institute of Theoretical Physics,Nanjing Normal University,Nanjing,210023,China

2 School of Microelectronics and Control Engineering,Changzhou University,Changzhou,213164,China

Abstract Searching for the top squark (stop) is a key task to test the naturalness of SUSY.Different from stop pair production,single stop production relies on its electroweak properties and can provide some unique signatures.Following the single production processthe top quark has two decay channels: leptonic channel and hadronic channel.In this paper,we probe the observability of these two channels in a simplified minimal supersymmetric standard model scenario.We find that,at the 27 TeV LHC with the integrated luminosity of L= 15 ab- 1,＜1900 GeV and μ ＜ 750 GeV can be excluded at 2σ through the leptonic mono-top channel,while＜1200 GeV and μ ＜ 350 GeV can be excluded at 2σ through the hadronic channel.

Keywords: top squark,mono-top signature,HE-LHC

1.Introduction

The discovery of Higgs [1,2] made 2012 a landmark year of particle physics.It marks a significant success for the Standard Model(SM)and the efforts of particle physicists for nearly half a century have also been rewarded.This is very inspiring but there are still several dark clouds floating in the sky that make particle physicists disturbed and yearning at the same time,which all imply the existence of new physics and promote physicists to propose new models beyond the SM.Naturalness problem is one of the dark clouds and the weak scale supersymmetry (SUSY) is one of the most promising solutions.SUSY constructs a symmetry between fermions and bosons by introducing superpartners of the SM particles and then cancel the Higgs boson mass quadratic divergence naturally.We can make a bold assumption that μ ＜ 200 GeV and＜1.5TeV within the framework of minimal supersymmetric standard model(MSSM)[3-7].So if we want to test SUSY naturalness,searching for light stops in collider is very important [8-24].

LHC was built initially to search for Higgs boson but now we can also use it to search for new physics,including stops mentioned above.The null results of searching can exclude the low energy range and promote us to search for stops in a relatively high energy scale[25,26].On the basis of analysis on 139 fb-1data from 13 TeV LHC,the current exclusion limits for stop mass have been reached to 400-1250 GeV depending on the mass of LSP at 95% CL[27].While the exclusion limits for charginoand neutralinovary from 180 to 740 GeV depending on a varying LSP mass at 95% CL with 36.1 fb-1[28] and 139 fb-1[29-31] data.Moreover,the large gap between the electroweak energy scale and the Planck energy scale drives the new physics to appear at TeV scale.Therefore,as the LHC cannot cater for the demand for new physics,proposals have been made to build High Energy LHC (HE-LHC),which is designed to run at collision energy of 27 TeV with the integrated luminosity of 15 ab-1.In this work,we propose to search for stops through single stop production via electroweak interaction at the HE-LHC (figure 1).Following the production process,the stop can decay into the SM top quark which then can go through two decay channels: hadronic channel and leptonic channel.The search for stop by these two channels has been studied in[32]at the 14 TeV LHC,we now explore their observability at the 27 TeV LHC.Generally,the stop pair production is considered as the discovery channel,but the single production can manifest the electroweak coupling between stop and neutralino,which can be used as a complementary study to the QCD pair production.After the stop mass can be determined through its pair production in the future,the single production search becomes a good way to identify the natures of electroweakinos.In addition,single production has distinct signatures at the LHC that are not present in pair production,which can serve as further confirmation of the discovery of stop [32-35].

Figure 1.Feynman diagrams for the single stop production at LO.

We assume a simplified scenario of the [36-40] in our research in which the only sparticles are the stop and charginos.Bino,winos and higgsinos constitute the electroweakino sector where the neutralinosare formed as mass eigenstates of the neutral components.Their mass matrix can be written as

from which we can infer that the neutralinos can be treated as bino-,wino- or higgsino-like with the mass parameters M1,M2or μ,if the mass of Z boson can be neglected.In a similar way,the mass matrix of charginos

leads wino-or higgsino-like charginos with a negligible W mass.Through unitary matrices N,U and V,the above two mass matrices (1) and (2) can be diagonalized.A nearly degerate higgsino-like LSP (the lightest SUSY particle) can thus be obtained with U12～ 1,V12～sgn(μ),N13,14,23=-N24～V11,U11,N11,12,21,22～ 0 and μ ? M1,2.Moreover,we settanβ=50 andso that the top squark is righthanded,which can be seen from the mass-squared matrix of top squark

with

Table 1.A cut flow analysis of the cross sections (in unit of fb) for the leptonic channel events at the HE-LHC.

where Atis the trilinear parameter.The branching ratio ofis assumed to be 50% [35].Because the mass splitting betweenand neutralino is very small,the leptons fromdecay are so soft that they will give a unique signature which is identified as transverse missing energy at the collider.In addition,with the conserved R-parity,the lightest neutralinomay serve as a dark matter particle.And for a wino-or higgsinolike LSP,the mass of the LSP should be of TeV scale to account for the observed dark matter relic density,due to the large annihilation rates[41].While for a light higgsino-like LSP in the natural MSSM,alternative schemes have been proposed to produce the required relic density through the standard thermally way,such as making an axion-higgsino admixture as the dark matter candidate [42,43].

We use the following softwares for Monte-Carlo simulation: MadGraph5_aMC@NLO for event generation at parton level,Pythia [44] for hadronization and parton showering,Delphes [45] for detector simulation and CheckMATE2 [46]as a processor connecting the above tools.And we also include the effects of QCD corrections at next-to-leading order by applying a K-factor of 1.4 [33,47,48].SUSYHIT is used to calculate the sparticles mass spectrum.To cluster jets,we use anti-ktalgorithm with a cone radius ΔR=0.4 [49].

This paper is organized as follows.In section 2,we present the study of the leptonic channel from single stop production at the HE-LHC.And section 3 is on the hadronic channel.In section 4,we draw a conclusion on our work.

2.Mono-top signature of leptonic channel from single stop production at the HE-LHC

In this section,we investigate the mono-top signature of leptonic channel from single stop production:

The dominant background for this process is the SMproduction from semi-or full-leptonic decay of top quark,since thesystem of limited jet energy resolution and missing leptons can be miss-tagged asAnother background comes from the single top production due to the undetected final leptons.We neglect some possible backgrounds,such as diboson production,because their cross sections are relatively small.

Figure 2.Normalized distributions of N(l),N(b),pT(b1),,and HT3 for the leptonic channel at 27 TeV LHC.The benchmark point is set as μ=200 GeV and =1000 GeV.

Figure 3.Normalized distributions of N(b),pT(b1),and HT3 for hadronic channel events at 27 TeV LHC.The benchmark point is set as μ=200 GeV and=1000 GeV.

The signature of equation(7)consists of a hard b jet and a charged lepton plus missing transverse energy,from which we find several kinematic variables that can be used to separate signal and background,including the number of lepton N?and b-jet Nb,the transverse momentum of the leading b-jet pT(b1),missing transverse energyand the transverse mass of lepton plus missing energyWe also construct the variable HT3[50] as the scalar sum of the transverse momentum of the third to fifth jet,to suppress thebackground,since the signal has less hard jets.We present the distributions of N?,Nb,pT(b1),and HT3for the signal and background events at the 27 TeV LHC.The benchmark point is μ=200 GeV and=1000 GeV.As shown in figure 2,lepton number of the signal events is more likely to center around N?=1 while the backgrounds around N?=0.Because of the boost effect of the b-jet from the stop decay,the leading b jets in signal events tend to be harder than that in the background events.The distributions ofandin the larger region for the signal are natural consequences as a result of the massivewhich are well separated from the backgrounds.

After the above discussion,we select the events with the following cuts: (1) exact one lepton in the final state is required;(2)we require at least one b-jet and that the leading b-jet satisfies pT(b1) ＞ 200 GeV; (3)andGeV are required; (4) HT3＜ 200 GeV is required to further suppress thebackground; (5) to finally reduce multi-jet events in thebackground,we apply a cut as the minimum azimuthal angle between any of the jets and the missing transverse momentumWe present the cutflow based on the above event cuts in table 1 for the leptonic channel of single stop production as well as for the SM backgrounds.As we can see from the table,the most effective cut is by the largewhich can suppress single-top andbackgrounds by aboutTo estimate the statistical significance α,we usewhere S (B) is the events number after the above cuts applied.In figure 4(a),we present the 2σ exclusion limits for the leptonic channel on the plane of higgsino mass parameter μ versus stop massfrom which we can find that＜1900 GeV and μ ＜ 750 GeV can be excluded at 2σ level.

Figure 4.The statistical significance for the processes: (a) leptonic channel and (b) hadronic channel,on the plane of stop massversus the higgsino mass parameter μ at the HE-LHC.

Table 2.A cut flow analysis of the cross sections(in unit of fb)for the hadronic channel events at the HE-LHC.Note that S/B for this channel is tiny due to the large SM backgrounds and the sensitivity is worse than that of the leptonic channel,as shown in figure 4.

3.Mono-top signature of hadronic channel from single stop production at the HE-LHC

In this section,we investigate the mono-top signature of hadronic channel from single stop production:

In figure 3,we present the distributions of Nb,pT(b1),and HT3.From the kinematic distributions,we can find that signal has more events than background centered around Nb=1.Similar to the case of leptonic channel,the boost of the leading b-jet from the stop decay leads to a largerfor the signal and the massiveleads to a largerof the signal events than the backgrounds.HT3can also be used to distinguish between signal and background due to less hard jets in the signal.

Then we use the following cuts after the above analysis on the kinematic distributions:(1)we require no leptons in the final states; (2) we require at least one b-jet and the leading b-jet satisfeis pT(b1) ＞ 200 GeV; (3)＞700 GeV is required;(4) we require pT(j1) ＞ 650 GeV; (5) HT3＜ 350 GeV is required;(6)where Δφ(j,is defnied in the same way as in the last section.In table 2,we can see that the largerequirement can suppress single-top andbackgrounds most effectively by aboutwith about 10%signal events surviving.

The same formulaα=is adopted to evaluate the statistical significance and the contour on the plane of μ versusis shown in figure 4(b)to present the 2σ exclusion limits for the hadronic channel.The higgsino mass parameter and stop mass can be excluded at 2σ to＜1200 GeV and μ ＜ 350 GeV,respectively.It should be noted due to the large background for the hadronic channel,the sensitivity of this channel is worse than that of the leptonic channel,as can be seen from the contours in figure 4.

In both cases of leptonic and hadronic channels,the systematic uncertainties caused by high pile-up will lead to lower statistical significance and we can resort to real performance of upgraded detectors.And new developed analysis methods including the machine-learning ones[51-54]may be used to improve our results in such searches at hadron colliders.

Finally we mention that,as a complete research,we have also perform studies on the search for another decay mode of the stop,which is the mono-b channel [5,55]:The stop mass can be excluded below 1.9 TeV at 27 TeV LHC with L= 15 ab-1and below 3.9 TeV at the 100 TeV hadron colliders such as FCC-hh or SPPC with L= 3000 fb-1.

4.Conclusion

In this work,we investigate the sensitivity of the leptonic and hadronic channel from single stop production in a simplified MSSM scenario at the 27 TeV LHC with the integrated luminosity of L= 15 ab-1.As a complementary study to the traditional pair production,the single stop production presents some unique signatures from which we construct several useful kinematic variables to distinguish the signal from backgrounds.2σ exclusion limits for both cases are presented:＜1900 GeV and μ ＜ 750 GeV for the leptonic channel,while＜1200 GeV and μ ＜ 350 GeV for the hadronic channel.

Acknowledgments

Xu-Xu Yang would like to thank Yuchao Gu for helpful discussions.This work was supported by the National Natural Science Foundation of China (NNSFC) under Grant No.11 705 093,and by the Jiangsu Planned Projects for Postdoctoral Research Funds,Grant No.2019K197.