Top Channel for Early SUSY Discovery at the LHC
Abstract
In recent years many models of supersymmetry have implied a large production rate for events including a high multiplicity of third generation quarks, such as four top quarks. It is arguably the bestmotivated channel for early LHC discovery. A particular example is generic string theories compactified to four dimensions with stabilized moduli which typically have multiTeV squarks and lighter gluinos (below a TeV) with a large pair production rate and large branching ratios to four tops. We update and sharpen the analysis 4top signals and background to 7 TeV LHC energy. For 1 fb integrated luminosity, gluinos up to about 650 GeV in mass can be detected, with larger masses accessible for higher luminosities or at higher energies. More than one signature is likely to be accessible, with one charged lepton plus two or more bjets, and/or samesign dileptons plus bjets being the best channels. A nonStandard Model signal from counting is robust, and provides information on the gluino mass, cross section, and spin.
I Introduction
The Large Hadron Collider (LHC) is likely to accumulate significant amounts of data in 2011. While the detector groups will be sensitive to many ways new physics could appear, it is not possible to focus equally on all possible interesting signatures, so it is valuable to examine wellmotivated channels that may yield results at the initial LHC energies and luminosities. In recent years it has increasingly been recognized that considerations of new physics point toward topquark and bottomquark rich final states at the Large Hadron Collider (LHC), as naturalness of electroweak symmetry breaking (EWSB) typically requires the existence of a top partner to cancel the quadratic divergences in the Standard Model (SM).
Supersymmetry implies the existence of a top partner that cancels quadratic divergences. Supersymmetry also introduces a partner for the gluon, the gluino, in the low energy spectrum. At proton colliders, pair production of gluinos, and consequently their decay products, typically become the main channel of supersymmetric signals. Models with light top partners, are common and they imply that a typical signature of production of the gluino will be multiple top quarks in the final states lightgluino .
In the rest of this paper, we will study this signature of low energy supersymmetry with light gluinos, focusing on the wellmotivated scenario in which the squarks are considerably heavier than the gluino and the third generation squarks are lighter than those of the first two generations. In this case, the gluino will dominantly decay into top and/or bottom quarks. Earlier some of us, along with Acharya, Grajek, and Suruliz 14TeVPaper studied such processes in detail for the 14 TeV LHC. In this paper we update the study for early LHC at 7 TeV, and focus on the significant reach and robustness of a signal with the number of events from 1 fb
This scenario is a generic possibility from the point of view of many SUSY models models . Heavier squark masses are often preferred due to constraints from flavor changing neutral currents and CP violation. Even more generally, when embedding low energy supersymmetry into a string theory, moduli stabilization and cosmological constraints imply that moduli masses and gravitno mass, and consequently scalar masses moduli , must be larger than about 20 TeV modulibound . Then, standard renormalization group (RG) running of scalar masses from the unification scale down to the electroweak scale will push the third generation squark masses significantly lower than those of the other generations. In most cases this turns out to be right handed stop squark. Alternative models leading to multitop final states, and corresponding anaylsis approaches, have been studied OtherStudies (See Ref. 14TeVPaper for a more extensive list.).
The gluino decays via virtual squarks to or . Since the rate for a given diagram scales as the virtual squark mass to the power from the propagator, the lightest squarks dominate. Therefore, we are led to consider decay channels , , and . Decays of multiple top quarks lead to brich and lepton rich final states, and give excellent potential for early discovery. In fact, we show that significant excesses can be observed at the early LHC7 TeV. For example, gluino masses larger than 600 GeV can be discovered in the singlelepton plus 4 bjets channel.
We carry out our study on several benchmark models. To study the reach of gluino pair production, with decays into third generation squarks, a detailed scan of the parameter space involving the gluino mass and LSP mass, for different branching ratios, is performed. We emphasize that the goal of this study is to demonstrate that gluino pair production with decays via third generation squarks provides an ideal channel for early discovery at the LHC, since it leads to lepton and quark rich final states.
Ii Benchmark Models
Three benchmark models are considered which will form the basis for the numerical scan discussed below. The model parameters and relevant decay branching ratios are shown in Table 1. Model A is a simple example of multitop physics. The spectrum would have a stop much lighter than the other squarks, and therefore gluino pair production always produces four tops in the final state. Model B is designed to include the decay channel , which will result if the sbottom is also lighter than the first two generation squarks, and . Model B is observably different than Model A, while somewhat more difficult to discover. These models have a Binolike LSP. In Model C, the Wino is the LSP, and is approximately degenerate with the lightest chargino, which is also Winolike. It is designed to further include a chargino in the decay chain, which allows the decay . Since the charged Wino is approximately degenerate with the wino LSP, it appears only as missing energy; though if one focuses on the signal events the chargino stub Feng:1999fu can probably be seen in the vertex detector.
Branching ratios  

A  1  0  0 
B  0.5  0.5  0 
C  .08  0.22  0.7 
tab:model
The three models are taken as a basis for 3 seperate numerical scans, where and , are varied while the branching ratios are fixed, as shown in Table 1. In particular, scans in model A and model B varied and . while scan in model C varied and .
Iii Signal Isolation and Backgrounds
The relatively large jet and lepton multiplicity associated with multiple top production provide for potentially striking signatures that are easily distinguishable above the expected SM background. By requesting multiple tagged jets and at least one lepton, it is possible to achieve signal significance for 1 fb of integrated luminosity.
The most significant backgrounds from the SM for final states with many jets, several isolated leptons and missing energy, are from top pair production, . The expected crosssection at the LHC for 7TeV centerofmass energy is pb (NLO) SMttAtlas . Also included in the analysis are a set of SM backgrounds involving associated production of gauge bosons with third generation quarks. These contribute less significantly to the backgrounds than , but can contribute to signals with high lepton multiplicity. All background sources considered, and their respective cross sections are given in Table 2. With the exception of the cross section, we increased all SM background cross sections by a factor of 2, to account for possible Kfactor from NLO corrections. Since the relevant backgrounds for the channels considered end up small (Table 2), uncertainties in the cross section are not important.
All background event samples were produced with Madgraph v.4 madgraph , while the parton shower and hadronization were done by Pythia 6.4 pythia . Additional hard jets (up to three) were generated via Madgraph, while the MLM mlm matching scheme implemented in Madgraph was used to match these jets to the ones produced in the Pythia showers. The events were then passed through the PGS4 PGS4 detector simulators with parameters chosen to mimic a generic ATLAS type detector. The btagging efficiency was changed to more closely match the expected efficiencies at ATLAS btagatlas . For jets with , which is typical of the jets in the signal, the efficiency is approximately 60% for tagging a quark.
Process  [fb]  [fb]  [fb]  
34.0  107.8  
7.71  13.3  
42.3  95.4  
14.3  27.6  
7.37  26.6  
1.45  3.94  
2076.7  5905.6  
108.6  377.7  
Model A  403.8  508.1  
Model B  505.2  703.1  
Model C  300.5  420.5 
The signal event samples, for gluino pair production and decay, were produced using Pythia 6.4 and have been passed through the same PGS4 detector simulation. Basic muon isolation was applied to all samples. To reduce the number of backgrounds events are required to pass the L1triggers as defined by PGS. We also display the effect of two possible additional selection cuts, together with the additional requirement GeV,
(1)  
(2) 
in the last two columns of Table 2. The second cut (weaker than the first) is optimal for discovery signatures, such as the samesign dilepton signature, that have relatively small SM backgrounds.
Next, the signal is searched for in multi jet () and multi lepton channels (). All objects are required to have a minimum of 20 GeV. Same sign (SS) and opposite sign (OS) dileptons are separated as they can have different origins and sizes. We will use the possible excess in these channels to assess the discovery potential. Table 3 shows the expected number of events from the SM background as classified according to the number of tagged jets and isolated leptons in the event.
Table 3 shows the expected number of signal events with tagged jets and isolated leptons for the three benchmark models. Model A, which is predominantly a four top signal, has significantly more multilepton and jet events passing selection cuts than Model B and Model C, which have fewer four top events. In Table 3, the signal significance achievable with integrated luminosity is shown. By requesting at least tagged jets it is possible to observe signal significance 5 for events with a single lepton. The onelepton fourbjet channel will prove to be robust and the best channel for discovery.
Number of Background Events ()
Standard Model  

Number of Signal Events ()
Model A  Model B  Model C  




Significance
Model A  Model B  Model C  




Iv Scan and Results
For each model (a fixed and ), we simulated of data using Pythia and PGS. Then we searched for the models over the backgrounds for the selection cuts in Eqs. 12 in each of the jet and lepton () channels. A statistical significance in a channel is defined as where is the number of signal(background) events expected to be in the channel for one of the two selection cuts in Eqs. 12. Thus, if for any of the significances, , the model can be considered discoverable at . In Figures 1 we plot contours, for the channels
In the first two channels cut1 is used, and in the last three channel, the weaker cut2, is used. As is evident from Table 3, the backgrounds for are significantly smaller than the backgrounds for , and therefore it is not beneficial to combine them into the inclusive channel . The channels we used in this study maximize the significance.
In all case the  channel provides the best channel for discovery. But, the SSdilepton channel can be a competitive mode for discovery. It is important that the 4top final state will give signatures in several channels if it appears in any. Finding a second predicted channel would be valuable confirmation. If two or more channels are present a combined significance would be a useful construct and facilitate a claim of discovery.
V Summary
We have studied the signatures of low energy supersymmetry in multitop and/or multi production at 7 TeV LHC, and associated Standard Model backgrounds. Results are presented in terms of discovery reaches for . In recent years a number of models have been proposed that lead to such final states. The required spectrum, heavy squarks with the third generation somewhat lighter than the first two and light gluino, satisfies the existing experimental constraints better and can be motivated on very general theoretical grounds. In addition, it has been realized that generic string theories compactified to 4 dimensions and satisfying phenomenological constraints typically lead to such final states (as briefly described in the introduction). Thus such final states have emerged as an unusually wellmotivated discovery channel at LHC. We focus on gluino pair production in supersymmetric theories both because of the strong theoretical motivations and because of the well defined nature of the such models. At 7 TeV LHC with the reach can be over 600 GeV (up to about 650 GeV) gluino mass. Discovery reach at higher luminosity can be scaled from our result straightforwardly. Precise discovery reach at a different energy requires a different full study, such as the case of TeV studied in Ref. 14TeVPaper . However, we can roughly estimate for TeV, the reach in gluino mass can be enhanced by about a factor of . Top reconstruction was studied in 14TeVPaper and is difficult, but counting leptons and jets excess for discovery is robust. The size of the counting signal provides information on the gluino cross section, which in turn is correlated with the gluino spin. Addition kinematical distributions could also help to enhance the discovery reach. More careful analysis, preferably with data driven approaches, will be necessary to understand the background distribution in detail. We urge experimentalists to focus attention on these channels.
Vi Acknowledgments
We thank Bobby Acharya, Daniel Feldman, Brent Nelson and Aaron Pierce for discussion. E.K. is grateful to the String Vacuum Project for travel support and for a String Vacuum Project Graduate Fellowship funded through NSF grant PHY/0917807. This work was supported by the DOE Grant #DEFG0295ER40899. L.T.W. is supported by the NSF under grant PHY 0756966, and by a DOE Early Career award under grant #DESC0003930.
While this work was in preparation for posting Gregoire:2011ka appeared which explores similar issues.
References
 (1) Models looking at light gluinos at 7 TeV but not focusing on mutliple tops have been studied, for example see: D. Feldman, G. Kane, R. Lu, B. D. Nelson, Phys. Lett. B687, 363370 (2010). [arXiv:1002.2430 [hepph]]. E. Izaguirre, M. Manhart, J. G. Wacker, JHEP 1012, 030 (2010). [arXiv:1003.3886 [hepph]]. G. F. Giudice, T. Han, K. Wang, L. T. Wang, Phys. Rev. D81, 115011 (2010). [arXiv:1004.4902 [hepph]]. D. S. M. Alves, E. Izaguirre, J. G. Wacker, [arXiv:1008.0407 [hepph]]. N. Chen, D. Feldman, Z. Liu, P. Nath and G. Peim, arXiv:1011.1246 [hepph].
 (2) B. S. Acharya, P. Grajek, G. L. Kane, E. Kuflik, K. Suruliz, L. T. Wang, [arXiv:0901.3367 [hepph]].
 (3) R. Barbieri, G. R. Dvali and L. J. Hall, Phys. Lett. B 377, 76 (1996) [arXiv:hepph/9512388]. A. G. Cohen, D. B. Kaplan and A. E. Nelson, Phys. Lett. B 388, 588 (1996) [arXiv:hepph/9607394]. P. Langacker, G. Paz, L. T. Wang and I. Yavin, Phys. Rev. Lett. 100, 041802 (2008) [arXiv:0710.1632 [hepph]]. L. L. Everett, I. W. Kim, P. Ouyang and K. M. Zurek, JHEP 0808, 102 (2008) [arXiv:0806.2330 [hepph]]. B. S. Acharya, K. Bobkov, G. L. Kane, J. Shao and P. Kumar, Phys. Rev. D 78, 065038 (2008) [arXiv:0801.0478 [hepph]]. J. J. Heckman and C. Vafa, arXiv:0809.3452 [hepph]. R. Sundrum, arXiv:0909.5430 [hepth]. R. Barbieri, E. Bertuzzo, M. Farina, P. Lodone and D. Pappadopulo, JHEP 1008, 024 (2010) [arXiv:1004.2256 [hepph]].
 (4) B. S. Acharya, G. Kane, E. Kuflik, [arXiv:1006.3272 [hepph]].
 (5) S.Weinberg, Phys. Rev. Lett. 48, 17761779 (1982). J. R. Ellis, D. V. Nanopoulos, M. Quiros, Phys. Lett. B174, 176 (1986). M. Kawasaki and T. Moroi, Prog. Theor. Phys. 93, 879 (1995) [arXiv:hepph/9403364].
 (6) M. Gerbush, T. J. Khoo, D. J. Phalen, A. Pierce and D. TuckerSmith, Phys. Rev. D 77, 095003 (2008) [arXiv:0710.3133 [hepph]]. B. Lillie, J. Shu and T. M. P. Tait, JHEP 0804, 087 (2008) [arXiv:0712.3057 [hepph]]. A. Pomarol, J. Serra, Phys. Rev. D78, 074026 (2008). [arXiv:0806.3247 [hepph]]. T. Plehn and T. M. P. Tait, J. Phys. G 36, 075001 (2009) [arXiv:0810.3919 [hepph]]. K. Kumar, T. M. P. Tait and R. VegaMorales, JHEP 0905, 022 (2009) [arXiv:0901.3808 [hepph]]. Y. Bai and B. A. Dobrescu, arXiv:1012.5814 [hepph].
 (7) J. L. Feng, T. Moroi, L. Randall, M. Strassler, S. f. Su Phys. Rev. Lett. 83, 17311734 (1999). [hepph/9904250].
 (8) T. A. Collaboration, arXiv:1012.1792 [hepex].
 (9) J. Alwall, P. Demin, S. de Visscher, R. Frederix, M. Herquet, F. Maltoni, T. Plehn, D. Rainwater, T. Stelzer, JHEP 0709, 028 (2007). [arXiv:0706.2334 [hepph]].
 (10) T. Sjostrand, S. Mrenna and P. Skands, JHEP 0605, 026 (2006) [arXiv:hepph/0603175].
 (11) M. L. Mangano, M. Moretti, F. Piccinini, A. D. Polosa, JHEP 0307, 001 (2003). [hepph/0206293]. S. Hoeche, F. Krauss, N. Lavesson, L. Lonnblad, M. Mangano, A. Schalicke, S. Schumann [hepph/0602031]. J. Alwall, S. de Visscher, F. Maltoni, JHEP 0902, 017 (2009). [arXiv:0810.5350 [hepph]].

(12)
John Conway, “http://www.physics.ucdavis.edu/conway/rese
arch/software/pgs/pgs4general.htm”’  (13) G. Aad et al. [The ATLAS Collaboration], arXiv:0901.0512 [hepex]. B. Altunkaynak, M. Holmes, P. Nath, B. Nelson, G. Peim, [arXiv:1008.3423 [hepph]].
 (14) T. Gregoire, E. Katz, V. Sanz, [arXiv:1101.1294 [hepph]].