Exploring the nearly degenerate stop region with sbottom decays

Journal of High Energy Physics, Apr 2017

A light stop with mass almost degenerate with the lightest neutralino has important connections with both naturalness and dark matter relic abundance. This region is also very hard to probe at colliders. In this paper, we demonstrate the potential of searching for such stop particles at the LHC from sbottom decays, focusing on two channels with final states \( \mathbf{2}\mathbf{\ell }+{\boldsymbol{\mathsf{E}}}_{\mathbf{\mathsf{T}}}^{\mathbf{\mathsf{miss}}} \) and \( \mathbf{1}\boldsymbol{\mathsf{b}}\mathbf{\mathsf{1}}\mathbf{\ell}+{\boldsymbol{E}}_{\mathbf{T}}^{\mathbf{miss}} \). We found that, if the lightest sbottom has mass around or below 1 TeV and has a significant branching ratio to decay to stop and \( W\left(\tilde{b}\to \tilde{t}W\right) \), a stop almost degenerate with neutralino can be excluded up to about 500-600 GeV at the 13 TeV LHC with 300 fb−1 data. The searches we propose are complementary to other SUSY searches at the LHC and could have the best sensitivity to the stop-bino coannihilation region. Since they involve final states which have already been used in LHC searches, a reinterpretation of the search results already has sensitivity. Further optimization could deliver the full potential of these channels.

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Exploring the nearly degenerate stop region with sbottom decays

Received: January Exploring the nearly degenerate stop region with sbottom decays Haipeng An 0 1 3 6 7 8 9 Jiayin Gu 0 1 3 5 7 8 9 Lian-Tao Wang 0 1 2 3 4 7 8 9 Open Access 0 1 3 7 8 9 c The Authors. 0 1 3 7 8 9 0 Notkestra e 85, Hamburg , D-22607 Germany 1 19B YuquanLu, Chinese Academy of Sciences , Beijing, 100049 China 2 Enrico Fermi Institute, University of Chicago 3 1200 E. California Blvd , Pasadena, CA, 91125 U.S.A 4 Kavli Institute for Cosmological Physics, University of Chicago 5 Center for Future High Energy Physics, Institute of High Energy Physics 6 Walter Burke Institute for Theoretical Physics, California Institute of Technology 7 nal states which have already 8 5640 S Ellis Ave. , Chicago, IL, 60637 U.S.A 9 [54] A. Kobakhidze , N. Liu, L. Wu, J.M. Yang and M. Zhang, Closing up a light stop window in A light stop with mass almost degenerate with the lightest neutralino has important connections with both naturalness and dark matter relic abundance. This region is also very hard to probe at colliders. In this paper, we demonstrate the potential of searching for such stop particles at the LHC from sbottom decays, focusing on two nal states 2` + ETmiss and 1b1` + ETmiss. We found that, if the lightest sbottom has mass around or below 1 TeV and has a signi cant branching ratio to decay to stop and W (~b ! t~W ), a stop almost degenerate with neutralino can be excluded up to about 500{600 GeV at the 13 TeV LHC with 300 fb 1 data. The searches we propose are complementary to other SUSY searches at the LHC and could have the best sensitivity to the stop-bino coannihilation region. Since they involve been used in LHC searches, a reinterpretation of the search results already has sensitivity. Further optimization could deliver the full potential of these channels. sbottom; decays; Supersymmetry Phenomenology 1 Introduction Search channels and backgrounds Simulation procedure and event selection 3.1 2` + ETmiss channel 1b1` + ETmiss channel Reach at the 13 TeV LHC A light stop is essential for the naturalness of supersymmetry (SUSY). The stops have been extensively searched at the LHC. Traditional searches focus on the direct production of a stop pair followed by each stop decaying to the top quark and the lightest neutralino, is the lightest superpartner (LSP). The signal of t~ ! t ,1 while the lightest neutralino these searches often includes large missing transverse momentum (ETmiss) from the LSP. The current LHC bound for R-parity conserving SUSY models on stop mass is around mt~ & 900 GeV, assuming t~ ! t and a su ciently large mass gap between mt~ and m The stops can still be signi cantly lighter than this bound if they are hiding in commW + mb + m m , in which cases it is hard to discriminate the stop signal from standard model (SM) backgrounds or the products of the stop decay is too soft to be identi ed. Based on the stop-neutralino simpli ed model, searching strategies have been proposed to search for stops in these regions [12{62].3 In region is of special interests if the neutralino is mainly composed by the bino B~. The reason is that the annihilation cross section of a pair of B~ is small due to the lack of gauge interaction. Therefore, for B~ to be a thermal dark matter candidate, a charged particle must be nearby to assist the annihilation. This region is thus called stop-bino coannihilation region [64]. This region is also not very well constrained by dark matter direct detection experiments [65]. According to the numerical simulation with micrOmegas4.2 [66], for sub-TeV bino-like dark matter, a mass di erence 30 GeV is required to obtain the measured relic abundance. In the simulations of this work, we x this mass gap to be 30 GeV. The sensitivities of collider search 1In this paper t~ and ~b always denote the lighter mass eigenstates, t~1 and ~b1, unless speci ed otherwise. 2Stop may also decay to the lightest neutralino via an intermediate chargino or heavier neutralino, in which case the bounds on the stop mass is slightly weaker. 3See also ref. [63] for a recent analysis on the implication of a light stop sector. discussed in this paper are slightly better if the mass gap is smaller. In this compressed region, the stop has two main decay channels, one is the avor-conserving four-body decay through o -shell top quark and W boson (t~ ! bW avor-changing two-body decay to a charm quark (t~ ! c ). The decay rate of the avorchanging channel depends strongly on the avor structure of the squark sector, whereas the rate of the four-body channel depends only on the mixing angle between the left and =bjj ) and the other is the right handed stop. It turns out that with mt~1 makes the stop decay promptly [47]. In this paper, we draw attention to a couple of additional useful search channels using sbottom decays, to further probe this nearly degenerate region. Naturalness prefers the second stop not to be too heavy. Due to the doublet nature of the left handed quarks, the masses of the left handed sbottom is connected to the mass of the left handed stop. The mixing between the left and right handed stops usually makes the mass of the second stop heavier than the left handed sbottom. To minimize the avor violation induced by the squark sector, the mixing between the left and right handed sbottoms is usually assumed to be suppressed by the mass of the bottom quark. Therefore, we can decouple the right handed sbottom in this work. Our search strategy relies on a signi cant mass gap between the lightest stop and sbottom, which we obtain by assuming mt~R is su ciently smaller also assume the winos and the Higgsinos are decoupled and the lightest neutralino is pure bino. The spectrum of the SUSY particles is shown in gure 1. In this simpli ed scenario, the lighter sbottom ~b1 has two decay channels with decay rates ~b1!W t~1 = mW )2)]3=2 where in calculating 2 we neglect the mass of the bottom quark. The stop and sbottom mixing angles are de ned as ~ t2 In the limit m2 m2W , 1 is seemingly enhanced by the factor m~2=m2W due to b the longitudinal contribution. However, the stop mixing angle vanishes if the electroweak 4A large mass gap could also be generated by a very large stop A term even if mt~R mt~L , but with such a large A term also comes the risk of spontaneously breaking SU(3)c. In this scenario, the decay ~b ! t~W would dominate which makes our case even stronger. symmetry is unbroken. Therefore, the stop mixing angle t is secretly proportional to mW . In the limit Atv where At is the A term for the stops. Assuming Ab is suppressed by mb for the sake of avor physics constraints, we have cos b 1. Therefore, 1 and 2 in eq. (1.2) can be simpli ed as The proportionality of 1 to At2 can also be inferred from the goldstone equivalence theorem. The traditional sbottom search based on the sbottom-neutralino simpli ed model assumes the sbottom decays 100% to b and the neutralino. However, as from eq. (1.5) if At is comparable to m~, 1= 2 can be as large as O(100). This region is also favored by the Higgs b mass. On the other hand, in some speci c SUSY breaking models (e.g. gauge mediation models) At is one-loop order suppressed compared to other soft SUSY breaking parameters. In this case, . Therefore, when searching for the signal from sbottoms, it is important to consider both of the two decay channels. We will focus on studying the potential of the sbottom decay channels shown in (a) and (b) of gure 2. We apply relatively straightforward cuts to demonstrate that these channels can lead to interesting reach with an integrated luminosity of 300 fb 1 at the 13 TeV LHC. A more careful optimization of the kinematical selection and more realistic simulation are needed to determine the ultimate reach. This is beyond the scope of this paper. We also present the reach in the more \conventional" sbottom search channel in shown in (c) of gure 2 to illustrate the complementarity between these channels. We would like to emphasize that even in the parameter region in which (c) has a better reach, the new channels (a) and (b) studied in this paper is still useful in the case of a discovery since they directly probe the presence of the stop. The rest of this paper is organized as follows: in section 2, we discuss the main search channels and the corresponding backgrounds. In section 3, we state the selection cuts for symmetric decay of ~b ! t~W , (b) asymmetric decay, and (c) symmetric decay of ~b ! b . each channel and show the results of a few case studies. In section 4, we show the exclusion regions in the parameter space of the 13 TeV LHC with 300 fb 1 data and compare the reaches of di erent channels. The conclusion is drawn in section 5. Search channels and backgrounds With two decay channels ~b ! t~W and ~b ! b , a pair of sbottoms produced at the LHC has three ways to decay, as shown in gure 2. The symmetric decay chain of ~b ! b in gure 2c has already been searched at the LHC in the channel with nal states 2b + ETmiss under the assumption of 100% branching ratio (BR), and sbottom with mass up to 800 GeV are excluded for m . 360 GeV [67]. With a smaller branching ratio, the reach of this channel is signi cantly weaker. Here our main interest is in the decay chains that involves the stop, namely, the symmetric decay chain in gure 2a and the asymmetric decay chain in gure 2b. As such, we will focus on two channels, one with nal states of two opposite sign leptons and ETmiss (2` + ETmiss), and the other with one hard b-jet, one lepton and ETmiss (1b1` + ETmiss). These two channels are studied in details in section 3, while in section 4 we compare their reaches together with the one of the 2b + ETmiss channel for di erent sbottom branching ratios. The 2` + ETmiss channel is designed to pick up the the symmetric decay chain in gure 2a with both W s decaying leptonically, and should be the optimal search channel if the decay ~b ! t~W dominates. This channel has been searched at the LHC for the searches of sleptons and electroweakinos [68, 69], for which the main background is top quark pair signi cant contribution to the SM backgrounds after imposing our selection cuts. The 1b1` + ETmiss channel is designed for the asymmetric decay chain in but could also pick up some events from the symmetric decay chain in gure 2a with one W decaying hadronically, if the event happens to have a hard b-jet. This channel is similar to the direct search of stop pair in the semileptonic channel [6, 7, 11], where the main backgrounds include tt, tW , W + jets, diboson and ttZ. We expect this channel to be useful if the branching ratios of ~b ! t~W and ~b ! b are comparable. In principle, one could also search in the channel with nal states of one lepton, ETmiss and one or two hard jets with no b-jets (1` + jets + ETmiss), which could come from either gure 2a with one W decaying hadronically or gure 2b if the b-jet is not tagged. While this channel could contain signi cant amount of signal events, the backgrounds are also large and more complicated. In this paper, we focus on the simpler leptonic channels as an initial assessment of the potential of these new decay channels. Simulation procedure and event selection For both signal and backgrounds, the events are generated at parton level using Madgraph5 [70], followed by parton showering with PYTHIA6.4 [71]. The detector simulation is performed with Delphes [72] in which the b-tagging e ciency is from ref. [73]. In particular, the b-tagging e ciency is within 60%{70% for pT in the range 50{300 GeV, while the mis-tag rate is below 15% for a charm jet and below 0.5% for a light jet with pT < 400 GeV. We use the above procedure to generate the events of sbottom pair production and then rescale the cross section to the values from the NLO+NLL calculation in refs. [74, 75].5 For tt, single top and W; Z+jets events, the MLM matching procedure is also employed. For tt events, the total cross section is scaled to the NNLO+NNLL result given in refs. [76, 77]. For single top (including tW ) events, the total cross section is scaled to the NLO results in ref. [78]. For diboson events, the total cross section is scaled to the NLO result in ref. [79]. For ttZ events, we scale the cross section to the central value of the recent measurement in ref. [80]. body decay, t~ ! bW 2` + ETmiss channel We present the details of our collider study in this section, including the selection cuts for each channel and the results of a few case studies. The signal samples listed table 1 by the 1b1` + ETmiss channel. For S1 & S3, we assume the stop only decays to charm and neutralino, t~ ! c ; for S2 & S4, we assume that the stop only goes through the 4correspond to the \best reach" of the two channels, which are shown later in section 4. Selection cuts. For an event to pass the cut, we require it to have ETmiss > 150 GeV and contain exactly 2 leptons with opposite charge. We require the scalar sum of the pT of the two leptons to be larger than 200 GeV. We also apply a b-veto by requiring the event to have no b-jet with pT > 50 GeV. The requirement on pT of b-jets could prevent one 5Jet matching is not performed for signal events as scanning the parameter space with jet matching requires a huge amount of computing power. Since for signal events the hard b-jets and charged leptons are from the decay of the sbottoms, we do not expect the behavior of ISR jets to have a strong e ect on the behavior of the signal. We have also checked with speci c signal benchmark points that this is indeed 1b1` + ETmiss channel (S3 & S4). from removing signal events with soft b-jets from stop decays. We require the invariant mass of the lepton pair (mll) to be larger than 20 GeV to remove potential backgrounds from low mass resonances. If the two leptons have the same avor, we further require their invariant mass to be at least 20 GeV away from the Z boson mass. A stringent cut around the Z resonance helps remove the ZZ background with ZZ ! `+` e ciently removed by the MT2 variable [81, 82] due to the di erent event topology. In order to remove events with a large ETmiss coming from mis-measurements of jet energy, we require that the azumithal angle between the missing transverse momentum and any jet , which cannot be j j > 0:2. Finally, we require the MT2 of the lepton with pT > 50 GeV to satisfy j MET pair to be larger than 150 GeV. The distributions of ETmiss, P plT (scalar sum of the pT of the two leptons) and MT2 are shown in gure 3 for signal S1, S2 and the major backgrounds. In gure 3 one could backgrounds.6 A cut on MT2 with a value much larger than the W mass is very e cient at removing these backgrounds. On the other hand, the ttZ background has additional neutrinos and does not have the endpoint feature. While it has a much smaller cross section, after the MT2 cut we found it to be comparable with other major backgrounds. The numbers of signals and backgrounds after the selection cuts and the corresponding could see that the decay channel of the light stop has a rather small impact on the reach, due to the high jet and lepton threshold we choose to use. 1b1` + ETmiss channel Selection cuts. We require the event to have ETmiss > 350 GeV and contain exactly one lepton, one b-jet with pT > 150 GeV and no additional b-jet with pT > 50 GeV. To remove events with large ETmiss due to mis-measurements of jet energy, we require j MET for any jet with pT > 100 GeV. We require the transverse mass of the lepton MT > 200 GeV in order to remove backgrounds of which the dominate source of missing energy is from the leptonically decaying W (e.g. semileptonic tt). Finally, we require the variable MTW2 reconstructed from the event to be at least 200 GeV. An event is also kept if it does not contain any additional jet for MTW2 to be constructed. 6Note that most ZZ background are removed by the lepton invariant mass cut. If this cut is not imposed, a signi cant amount of ZZ background will have MT2 & mW . channel for signal sample S1 and the major backgrounds. P plT is the scalar sum of the pT of the two leptons. To illustrate the usefulness of the variables, the cuts f are removed. The number of events correspond to 300 fb 1 at the 13 TeV LHC. lT > 200 GeV, MT2 > 150 GeVg # of events (300 fb 1) s= b selection cuts for the 2` + ETmiss channel with 300 fb 1 data. The details of signal samples S1 & S2 are listed in table 1. All the generated backgrounds are included in the row \total SM". 1b1l+ETmiss channel 1b1l+ETmiss channel 1b1l+ETmiss channel right ) of the 1b1` + ETmiss channel for signal sample S3 and the major backgrounds. To illustrate the usefulness of the variables, the cuts fETmiss > 350 GeV, pbT > 150 GeV, MT > 200 GeV, MTW2 > 200 GeVg are replaced by looser cuts fETmiss > 200 GeV, pbT > 50 GeV, MT > 150 GeV, MTW2 > stacked on the last bin. The number of events correspond to 300 fb 1 at the 13 TeV LHC. 0 GeVg. For the MTW2 distribution, events for which MTW2 cannot be constructed below 1 TeV are The variable MTW2, proposed in ref. [83], is constructed for dileptonic tt background with one lepton not reconstructed, and has been shown to be useful in suppressing this type of background [7, 9].7 The calculation of MTW2 requires one to identify the two b-jets and to know which one is on the same side as the visible lepton. In practice, one does not have this knowledge and would usually calculate the MTW2 for di erent possible combinations and output the minimum value from these combinations. Here we assume the other bjet is among the three leading non-b-tagged jets. We then choose the combination which minimizes MTW2. The distributions of ETmiss, pb , MT and MTW2 are shown in gure 4 for signal S1 and T the major backgrounds. For the MTW2 distribution, events for which MTW2 > 1 TeV are stacked on the last bin. The usefulness of MTW2 can be seen in gure 4, as the number of 7Other variables have also been proposed for suppressing this background, such as amT2 [84] and topness [85]. As their performances are somewhat similar, for simplicity we only use MTW2 in this paper. s=pb the selection cuts for the 1b1` + ETmiss channel with 300 fb 1 data. The details of signal samples S3 & S4 are listed in table 1. All the generated backgrounds are included in the row \total SM". background events, in particular for tt, falls sharply with MTW2 above the top mass. The for 300 fb 1 data are shown in table 3. For the 1b1` + ETmiss channel, the reach is also not very sensitive to the decay channel of stop. Reach at the 13 TeV LHC We scan over the signal parameter space to determine the reach of the 2` + ETmiss and 1b1` + ETmiss channels at the 13 TeV LHC, assuming an integrated luminosity of 300 fb 1. For comparison, we also include the results of the conventional search channel of the sbottom, 2b + ETmiss, which has the best reach if the dominant decay of sbottom is ~b ! b . To estimate the reach of the 2b + ETmiss channel, we adopt the cuts in ref. [67] for signal region SRA450, which has the best reach if the mass gap between sbottom and neutralino is large. We have checked that the total number of backgrounds after the selection cuts, if normalized to 3:2 fb 1, is in good agreement with ref. [67]. We use the asymptotic formula for the signi cance in ref. [86] (also adopted by refs. [55, 56]), = p2 [(s + b) log (1 + s=b) which reduces to the usual s=pb in the limit b s. While the optimal values of the selection cuts depend on the signal spectrum, for simplicity we x the cuts as in section 3. In particular, for the 2` + ETmiss channel the cuts we choose are relatively conservative to maintain a su ciently large simulated signal sample. A more sophisticated optimization method could further improve the reach of the searches. gure 5, we show the expected exclusion regions for the three channels in the = 30 GeV. On the left panel, the red, blue and green contours indicate the 2-sigma limits of the 2` + ETmiss, 1b1` + ETmiss and 2b + ETmiss channels, respectively, and the corresponding shaded regions m∼(GeV) m∼(GeV) 650 700 750 800 850 900 950 1000 650 700 750 800 850 900 950 1000 dimensional polynomial. limits (left) and 5 reaches (right) in the (m~b; BR(~b ! t~W )) plane mt~ = 400 GeV and mt~ = 30 GeV. The red, blue and green contours indicate the regions excluded by the 2` + ETmiss, 1b1` + ETmiss and 2b + ETmiss channels, respectively. The solid (dashed) curves corresponds to the stop decay (m~b; BR(~b ! t~W )) plane with a grid spacing of (20 GeV, 0.05) and tting the points to a 2are excluded at 95% con dence level (CL). On the right panel, the 5-sigma reaches are shown instead. The contours are obtained by scanning the (m~b; BR(~b ! t~W )) plane with a grid spacing of (20 GeV, 0.05). A t to a 2-dimensional polynomial was performed to reduce the unphysical uctuations due to the statistical uncertainties of the simulations. We also checked manually that the tted curves are in good agreement with the grid of data points. For the solid curves, we assume the stop only decays to charm and neutralino, t~ ! c ; for the dashed curves, we assume that the stop only goes through the 4-body impact on the reach. The complementarity of di erent channels is well demonstrated in gure 5. The 2` + ETmiss (2b + ETmiss) channel has the best reach if the decay ~b ! t~W (~b ! b ) is dominant, and the 1b1` + ETmiss channel has a better reach if the branching ratio of the two decay channels are comparable. We also found that the 2b + ETmiss channel has rather good reaches, comparable to the reach of the 1b1` + ETmiss channel even for 0:5. Nevertheless, the 1b1` + ETmiss channel could still signi cantly improve the overall signi cance (of all channels combined) and impose constraints on the To determine the bounds on masses of sbottom and stop, we also show the 2-sigma mt~) plane in gure 6 for BR(~b ! t~W ) = 0:9 = 30 GeV. Similar to mt~) plane with a grid spacing of (20 GeV, 30 GeV) and tting the points to a 2-dimensional polynomial. A few benchmarks BR(˜b→ ˜tW)=0.5, 5σ reach at 300 fb-1 ∼mt400 ∼ b350 ∼mt400 ∼ b350 ∼mt400 ∼ b350 ∼mt400 ∼ b350 ∼= ∼= ∼= mb∼ (GeV) ∼= ∼= ∼= ∼= ∼= mb∼ (GeV) ∼= ∼= mb∼ (GeV) mb∼ (GeV) 13 TeV LHC with 300 fb 1 data, assuming mt~ limits (left) and 5 reaches (right) in the (m~b; m~ b mt~) plane from the = 30 GeV. The top (bottom) panel assumes corresponds to the stop decay t~ ! c the 2` + ETmiss, 1b1` + ETmiss and 2b + ETmiss channels, respectively. The solid (dashed) curves =bjj ) with 100% BR. The dotted diagonal plane with a grid spacing of (20 GeV, 30 GeV) and tting the points to a 2-dimensional polynomial. and stop masses up to the 1b1`+ETmiss and 2b+ETmiss channels have comparable reaches, with the 1b1`+ETmiss However, it should be noted that the 2b + ETmiss channel does not direct constrain mt~, and = 30 GeV. For the 1b1`+ETmiss channel, stop masses up to 500 GeV can be excluded for m~b . 900 GeV. It should also be noted that in obtaining the constraints we have assumed a su cient mass gap between the sbottom and the stop. If the mass gap is small, the search strategy can required to determine the collider reach in this region. mt~ . mW . Further studies are Comparing to the reach of the direct stop search, the recent results from the ATLAS mono-jet search has excluded stop masses below 323 GeV with 3:2 fb 1 data at p s = 5 GeV [59]. This already surpasses the constraints from the 8 TeV run [25, 26]. CMS conducted a search with 2:3 fb 1 data at p s = 13 TeV using the In both searches, the bounds on stop mass are also signi cantly weaker for slightly larger values of m~ m . In ref. [35], it is estimated that the high luminosity LHC with 3000 fb 1 500 GeV, assuming the stop is in the coannihilation region. While the constraints from mono-jet searches do not rely on the properties of sbottom and are hence more robust, the search with sbottom decays could potentially have a much better reach. The two searches are also complementary; if a signi cant excess is found in the 2` + ETmiss or 1b1` + ETmiss channel, one may also expect a mild excess in the mono-jet search if the excess comes from a light stop in the coannihiliation region. A light stop with mass almost degenerate with the lightest neutralino is an appealing SUSY scenario. It could evade the bounds of traditional stop searches and hence reduce the tension between naturalness and the current LHC results, while also having interesting implications for bino dark matter. In this paper, we propose a novel way of probing such stop particles by searching for it from sbottom decays, under the assumptions that the sbottom is not too heavy and has a signi cant branching ratio of decaying into a stop and a W boson (~b ! t~W ). Such assumptions are favored by naturalness and Higgs mass considerations. In this scenario, the constraints on the masses of stop and sbottom from the traditional searches are weak. We show that a dedicated search for a sbottom pair with one or both sbottom decaying to stop and W at the 13 TeV LHC could impose strong constraint if the decay ~b ! t~W is dominant, the channel with reach, and can exclude stop masses up to below 1 TeV; if the sbottom decays to either t~W or b 600 GeV with 300 fb 1 data if the sbottom is nal states 2` + ETmiss has the best with comparable branching ratios, only reach up to channel of sbottom with the channel with nal states 1b1` + ETmiss has a better reach and could exclude the stop with mass up to 500 GeV with 300 fb 1 data if the sbottom is below 900 GeV. While the results rely on the properties of the sbottom, the reaches are potentially much better than the one from direct searches of stop with mono-jet + ETmiss nal states, which could 500 GeV with 3000 fb 1 data at p s = 14 TeV. The traditional search nal states 2b + ETmiss is also complementary to the ones we propose. Together, these searches can cover a wide range of model parameter space and provide valuable information on the status of SUSY. There are other interesting scenarios not explored in this paper but may worth further investigation. It is possible that the chargino or second neutralino are lighter than the sbottom, making its decay more complicated [87]. In this case, searching for the asymmetric decay chains with one sbottom decaying to t~W , the other decaying to t or b 2 could be useful. For larger values of the stop-neutralino mass gap, the stop decay products become more visible and it might be useful to look at channels with multiple b-jets or multiple leptons [56], or try to tag the charm quark from stop decay [23{25]. On the other hand, if the mass gap is smaller, the stop decay could exhibit displaced vertex, which can help reduce SM background in both the mono-jet search and the search with sbottom decays. It is also complementary to search for the lighter stop from the decays of the heavier Our study serves as a proof of concept. A search carried out by the LHC experimental groups is desired to fully determine the reach of the proposed channels. Since the 2`+ETmiss channel has been used to search for sleptons and electroweakinos, and the conventional search of stop in the semileptonic channel is very similar to the 1b1` + ETmiss channel we studied, reinterpretation of those search results can already lead to interesting reach. At the same time, optimizing the searches with these new channels in mind is needed to realize their full potential. While the current data is still not very constraining, in the future it is straightforward to interpret the results of conventional searches in these two channels in terms of constraints on the scenario studied in this paper. If deviations from the SM is observed, it is non-trivial to discriminate di erent new physics scenarios that leads to similar signals, and the comparisons between di erent search channels are important. Acknowledgments We would like to thank Zhen Liu for useful discussions. HA is supported by the Walter Burke Institute at Caltech and by DOE Grant de-sc0011632. JG is supported by the International Postdoctoral Exchange Fellowship Program between the O ce of the National Administrative Committee of Postdoctoral Researchers of China (ONACPR) and DESY. JG would also like to express a special thanks to the Mainz Institute for Theoretical Physics (MITP) for its hospitality and support. LTW is supported by DOE grant DE-SC0013642. Open Access. This article is distributed under the terms of the Creative Commons Attribution License (CC-BY 4.0), which permits any use, distribution and reproduction in any medium, provided the original author(s) and source are credited. state in proton-proton collisions at p 12:9/fb, CMS-PAS-SUS-16-029 (2016). missing transverse momentum in p ATLAS-CONF-2016-050 (2016). [1] CMS collaboration, Search for direct top squark pair production in the single lepton nal [2] CMS collaboration, Search for direct top squark pair production in the fully hadronic tagging in pp collisions at p [3] CMS collaboration, Search for supersymmetry in the all-hadronic nal state using top quark [4] ATLAS collaboration, Search for top squarks in nal states with one isolated lepton, jets and s = 8 TeV, JHEP 07 (2016) 027 [Erratum s = 8 TeV, Eur. Phys. J. C 76 (2016) 460 Jets + Emiss Final State at p [5] ATLAS collaboration, Search for the Supersymmetric Partner of the Top Quark in the [6] ATLAS collaboration, ATLAS Run 1 searches for direct pair production of third-generation squarks at the Large Hadron Collider, Eur. Phys. J. C 75 (2015) 510 [Erratum ibid. C 76 [7] CMS collaboration, Search for direct pair production of scalar top quarks in the single- and [8] CMS collaboration, Search for direct pair production of supersymmetric top quarks decaying (2016) 153] [arXiv:1506.08616] [INSPIRE]. dilepton channels in proton-proton collisions at p JHEP 09 (2016) 056] [arXiv:1602.03169] [INSPIRE]. [arXiv:1603.00765] [INSPIRE]. nal states in pp collisions at p [9] CMS collaboration, Search for direct top squark pair production in the single lepton nal [10] CMS collaboration, Search for supersymmetry in the multijet and missing transverse nal state, in proceedings of the 51st Rencontres de Moriond on EW Interactions and Uni ed Theories, La Thuile, Italy, 12{19 March 2016 [arXiv:1605.05762] [INSPIRE]. [11] ATLAS collaboration, Search for top squarks in nal states with one isolated lepton, jets and missing transverse momentum in p Phys. Rev. D 94 (2016) 052009 [arXiv:1606.03903] [INSPIRE]. [arXiv:1205.5808] [INSPIRE]. [13] A. Brandenburg, Z.G. Si and P. Uwer, QCD corrected spin analyzing power of jets in decays of polarized top quarks, Phys. Lett. B 539 (2002) 235 [hep-ph/0205023] [INSPIRE]. [arXiv:1012.3093] [INSPIRE]. nal states, Phys. Lett. B 702 (2011) 16 [arXiv:1103.1871] [INSPIRE]. [20] ATLAS collaboration, Measurements of spin correlation in top-antitop quark events from [21] CMS collaboration, Measurements of tt spin correlations and top-quark polarization using [17] D0 collaboration, V.M. Abazov et al., Measurement of spin correlation in tt production using a matrix element approach, Phys. Rev. Lett. 107 (2011) 032001 [arXiv:1104.5194] section in the lepton + jets channel in proton-antiproton collisions at p s = 1:96 TeV, Phys. Rev. D 84 (2011) 012008 [arXiv:1101.0124] [INSPIRE]. [19] ATLAS collaboration, Observation of spin correlation in tt events from pp collisions at [arXiv:1203.4081] [INSPIRE]. proton-proton collisions at p 112016 [arXiv:1407.4314] [INSPIRE]. nal states in pp collisions at p [arXiv:1311.3924] [INSPIRE]. [22] ATLAS collaboration, Measurement of Spin Correlation in Top-Antitop Quark Events and Search for Top Squark Pair Production in pp Collisions at p s = 8 TeV Using the ATLAS Detector, Phys. Rev. Lett. 114 (2015) 142001 [arXiv:1412.4742] [INSPIRE]. [23] A. Choudhury and A. Datta, New limits on top squark NLSP from LHC 4:7 fb 1 data, Mod. Phys. Lett. A 27 (2012) 1250188 [arXiv:1207.1846] [INSPIRE]. violating scalar top quark signal at the LHC, Phys. Rev. D 89 (2014) 015003 [arXiv:1308.6484] [INSPIRE]. [25] ATLAS collaboration, Search for pair-produced third-generation squarks decaying via charm quarks or in compressed supersymmetric scenarios in pp collisions at p s = 8 TeV with the ATLAS detector, Phys. Rev. D 90 (2014) 052008 [arXiv:1407.0608] [INSPIRE]. [26] CMS collaboration, Search for top squark pair production in compressed-mass-spectrum scenarios in proton-proton collisions at p (2017) 403 [arXiv:1605.08993] [INSPIRE]. s = 8 TeV using the T variable, Phys. Lett. B 767 Rev. D 85 (2012) 055021 [arXiv:1111.4467] [INSPIRE]. Phys. Rev. D 86 (2012) 035024 [arXiv:1201.5714] [INSPIRE]. Rev. D 87 (2013) 095016 [arXiv:1212.4856] [INSPIRE]. Phys. J. C 73 (2013) 2370 [arXiv:1212.6847] [INSPIRE]. [arXiv:1310.0077] [INSPIRE]. 161 [arXiv:1404.0682] [INSPIRE]. states in proton-proton collisions at p Rev. Lett. 114 (2015) 201801 [arXiv:1502.01721] [INSPIRE]. [37] CMS collaboration, Searches for third-generation squark production in fully hadronic nal supersymmetry at the LHC, Phys. Rev. D 93 (2016) 035003 [arXiv:1505.06006] [INSPIRE]. Phys. Rev. Lett. 113 (2014) 201803 [arXiv:1407.1043] [INSPIRE]. [40] ATLAS collaboration, Measurement of the tt production cross-section using e events with [41] K. Rolbiecki and J. Tattersall, Re ning light stop exclusion limits with W +W sections, Phys. Lett. B 750 (2015) 247 [arXiv:1505.05523] [INSPIRE]. [42] B. Dutta et al., Probing compressed top squark scenarios at the LHC at 14 TeV, Phys. Rev. b-tagged jets in pp collisions at p 74 (2014) 3109 [arXiv:1406.5375] [INSPIRE]. D 90 (2014) 095022 [arXiv:1312.1348] [INSPIRE]. Lett. 72 (1994) 2324 [hep-ph/9310209] [INSPIRE]. Rev. D 49 (1994) 4595 [hep-ph/9312213] [INSPIRE]. (2008) 075002 [arXiv:0801.0237] [INSPIRE]. [arXiv:1504.01740] [INSPIRE]. [43] M. Drees and M.M. Nojiri, A new signal for scalar top bound state production, Phys. Rev. [44] M. Drees and M.M. Nojiri, Production and decay of scalar stoponium bound states, Phys. Decay, JHEP 08 (2013) 085 [arXiv:1304.3148] [INSPIRE]. mass variables, JHEP 05 (2015) 040 [arXiv:1411.0664] [INSPIRE]. of things to come, Phys. Rev. D 87 (2013) 035016 [arXiv:1205.5805] [INSPIRE]. Phys. Rev. D 91 (2015) 094007 [arXiv:1307.1553] [INSPIRE]. [52] H. An and L.-T. Wang, Opening up the compressed region of top squark searches at 13 TeV LHC, Phys. Rev. Lett. 115 (2015) 181602 [arXiv:1506.00653] [INSPIRE]. High-Momentum Recoils, JHEP 03 (2016) 151 [arXiv:1506.07885] [INSPIRE]. natural SUSY at LHC, Phys. Lett. B 755 (2016) 76 [arXiv:1511.02371] [INSPIRE]. Semileptonic Decays, JHEP 05 (2016) 036 [arXiv:1604.00007] [INSPIRE]. 11 (2016) 181 [arXiv:1607.06547] [INSPIRE]. scenarios with a new method of event reconstruction, Phys. Rev. D 95 (2017) 035031 [arXiv:1607.08307] [INSPIRE]. Supersymmetry at LHC RUN-II, Phys. Rev. D 92 (2015) 095021 [arXiv:1509.02530] [59] ATLAS collaboration, Search for new phenomena in nal states with an energetic jet and large missing transverse momentum in pp collisions at p s = 13 TeV using the ATLAS detector, Phys. Rev. D 94 (2016) 032005 [arXiv:1604.07773] [INSPIRE]. states with missing transverse momentum and at least one jet using the T variable, submitted to Eur. Phys. J. C (2016) [arXiv:1611.00338] [INSPIRE]. Channel, Phys. Rev. D 94 (2016) 075009 [arXiv:1604.03938] [INSPIRE]. [62] D. Goncalves, K. Sakurai and M. Takeuchi, Tagging a monotop signature in natural SUSY, Phys. Rev. D 95 (2017) 015030 [arXiv:1610.06179] [INSPIRE]. [63] A. Pierce and B. Shakya, Implications of a Stop Sector Signal at the LHC, arXiv:1611.00771 [INSPIRE]. CMSSM, Astropart. Phys. 18 (2003) 395 [hep-ph/0112113] [INSPIRE]. theory methods for dark matter direct detection, Phys. Rev. D 93 (2016) 095008 [arXiv:1511.05964] [INSPIRE]. candidates, Comput. Phys. Commun. 192 (2015) 322 [arXiv:1407.6129] [INSPIRE]. [67] ATLAS collaboration, Search for bottom squark pair production in proton-proton collisions [arXiv:1606.08772] [INSPIRE]. [68] ATLAS collaboration, Search for direct production of charginos, neutralinos and sleptons in nal states with two leptons and missing transverse momentum in pp collisions at sleptons decaying to leptons and W, Z and Higgs bosons in pp collisions at 8 TeV, Eur. Phys. J. C 74 (2014) 3036 [arXiv:1405.7570] [INSPIRE]. di erential cross sections and their matching to parton shower simulations, JHEP 07 (2014) 079 [arXiv:1405.0301] [INSPIRE]. [72] DELPHES 3 collaboration, J. de Favereau et al., DELPHES 3, A modular framework for fast simulation of a generic collider experiment, JHEP 02 (2014) 057 [arXiv:1307.6346] ATL-PHYS-PUB-2015-022 (2015). [73] ATLAS collaboration, Expected performance of the ATLAS b-tagging algorithms in Run-2, [74] C. Borschensky et al., Squark and gluino production cross sections in pp collisions at https://twiki.cern.ch/twiki/bin/view/LHCPhysics/SUSYCrossSections13TeVstopsbottom. [77] M.J. Costa, NNLO + NNLL top-quark-pair cross sections. ATLAS-CMS recommended predictions for top-quark-pair cross sections using the Top++v2.0 program (M. Czakon, A. Mitov, 2013), (2015) https://twiki.cern.ch/twiki/bin/view/LHCPhysics/TtbarNNLO. [78] O.M. Kind, NLO single-top channel cross sections. ATLAS-CMS recommended predictions for single-top cross sections using the Hathor v2.1 program, (2016) https://twiki.cern.ch/twiki/bin/view/LHCPhysics/SingleTopRefXsec. 07 (2011) 018 [arXiv:1105.0020] [INSPIRE]. [80] ATLAS collaboration, Measurement of the ttZ and ttW production cross sections in nal states using 3:2 fb 1 of pp collisions at p s = 13 TeV with the ATLAS detector, Eur. Phys. J. C 77 (2017) 40 [arXiv:1609.01599] [INSPIRE]. [81] C.G. Lester and D.J. Summers, Measuring masses of semiinvisibly decaying particles pair produced at hadron colliders, Phys. Lett. B 463 (1999) 99 [hep-ph/9906349] [INSPIRE]. [82] A. Barr, C. Lester and P. Stephens, A variable for measuring masses at hadron colliders when missing energy is expected; mT 2: the truth behind the glamour, J. Phys. G 29 (2003) 2343 [hep-ph/0304226] [INSPIRE]. JHEP 07 (2012) 110 [arXiv:1203.4813] [INSPIRE]. [84] ATLAS collaboration, Search for top squark pair production in nal states with one isolated lepton, jets and missing transverse momentum in p detector, JHEP 11 (2014) 118 [arXiv:1407.0583] [INSPIRE]. s = 8 TeV pp collisions with the ATLAS JHEP 04 (2007) 070 [hep-ph/0702038] [INSPIRE]. [12] M. Jezabek, Top quark physics, Nucl. Phys. Proc. Suppl. B 37 (1994) 197 [hep-ph/9406411] [14] Z. Han, A. Katz, D. Krohn and M. Reece, (Light) Stop Signs, JHEP 08 (2012) 083 [15] CDF collaboration, T. Aaltonen et al., Measurement of tt Spin Correlation in pp Collisions Using the CDF II Detector at the Tevatron, Phys. Rev. D 83 (2011) 031104 [16] D0 collaboration, V.M. Abazov et al., Measurement of spin correlation in tt production using [18] D0 collaboration, V.M. Abazov et al., Measurement of the top quark pair production cross [24] G. Belanger, D. Ghosh, R. Godbole, M. Guchait and D. Sengupta, Probing the avor [29] M.A. Ajaib, T. Li and Q. Sha , Stop-Neutralino Coannihilation in the Light of LHC, Phys. [30] M. Drees, M. Hanussek and J.S. Kim, Light Stop Searches at the LHC with Monojet Events, [27] M. Carena, A. Freitas and C.E.M. Wagner, Light Stop Searches at the LHC in Events with One Hard Photon or Jet and Missing Energy, JHEP 10 (2008) 109 [arXiv:0808.2298] [28] S. Bornhauser, M. Drees, S. Grab and J.S. Kim, Light Stop Searches at the LHC in Events with two b-Jets and Missing Energy, Phys. Rev. D 83 (2011) 035008 [arXiv:1011.5508] [31] H. Dreiner, M. Kramer and J. Tattersall, Exploring QCD uncertainties when setting limits on compressed supersymmetric spectra, Phys. Rev. D 87 (2013) 035006 [arXiv:1211.4981] [33] A. Delgado, G.F. Giudice, G. Isidori, M. Pierini and A. Strumia, The light stop window, Eur. [34] T. Cohen et al., A Comparison of Future Proton Colliders Using SUSY Simpli ed Models: A Snowmass Whitepaper, in proceedings of the Community Summer Study 2013: Snowmass on the Mississippi (CSS2013), Minneapolis, MN, U.S.A., 29 July{6 August 2013 [35] M. Low and L.-T. Wang, Neutralino dark matter at 14 TeV and 100 TeV, JHEP 08 (2014) [36] G. Ferretti, R. Franceschini, C. Petersson and R. Torre, Spot the stop with a b-tag, Phys. [38] K.-i. Hikasa, J. Li, L. Wu and J.M. Yang, Single top squark production as a probe of natural [39] M. Czakon, A. Mitov, M. Papucci, J.T. Ruderman and A. Weiler, Closing the stop gap, [45] S.P. Martin, Diphoton decays of stoponium at the Large Hadron Collider, Phys. Rev. D 77 [46] B. Batell and S. Jung, Probing Light Stops with Stoponium, JHEP 07 (2015) 061 [47] R. Grober, M.M. Muhlleitner, E. Popenda and A. Wlotzka, Light Stop Decays: Implications for LHC Searches, Eur. Phys. J. C 75 (2015) 420 [arXiv:1408.4662] [INSPIRE]. [48] Y. Bai, H.-C. Cheng, J. Gallicchio and J. Gu, A Toolkit of the Stop Search via the Chargino [49] W.S. Cho et al., Improving the sensitivity of stop searches with on-shell constrained invariant [50] D.S.M. Alves, M.R. Buckley, P.J. Fox, J.D. Lykken and C.-T. Yu, Stops and E= T : The shape [56] H.-C. Cheng, L. Li and Q. Qin, Second Stop and Sbottom Searches with a Stealth Stop, JHEP [57] P. Jackson, C. Rogan and M. Santoni, Sparticles in motion: Analyzing compressed SUSY [58] B. Kaufman, P. Nath, B.D. Nelson and A.B. Spisak, Light Stops and Observation of [64] J.R. Ellis, K.A. Olive and Y. Santoso, Calculations of neutralino stop coannihilation in the [65] A. Berlin, D.S. Robertson, M.P. Solon and K.M. Zurek, Bino variations: E ective eld [66] G. Belanger, F. Boudjema, A. Pukhov and A. Semenov, MicrOMEGAs4.1: two dark matter [69] CMS collaboration, Searches for electroweak production of charginos, neutralinos and [70] J. Alwall et al., The automated computation of tree-level and next-to-leading order [71] T. Sjostrand, S. Mrenna and P.Z. Skands, PYTHIA 6.4 Physics and Manual, JHEP 05 [75] S. Padhi, Susycrosssections13tevstopsbottom, [76] M. Czakon and A. Mitov, Top++: A Program for the Calculation of the Top-Pair Cross-Section at Hadron Colliders, Comput. Phys. Commun. 185 (2014) 2930 [79] J.M. Campbell, R.K. Ellis and C. Williams, Vector boson pair production at the LHC, JHEP [83] Y. Bai, H.-C. Cheng, J. Gallicchio and J. Gu, Stop the Top Background of the Stop Search, [85] M.L. Graesser and J. Shelton, Hunting Mixed Top Squark Decays, Phys. Rev. Lett. 111 (2013) 121802 [arXiv:1212.4495] [INSPIRE]. [86] G. Cowan, K. Cranmer, E. Gross and O. Vitells, Asymptotic formulae for likelihood-based tests of new physics, Eur. Phys. J. C 71 (2011) 1554 [Erratum ibid. C 73 (2013) 2501] [87] T. Han, S. Su, Y. Wu, B. Zhang and H. Zhang, Sbottom discovery via mixed decays at the [88] M. Perelstein and C. Spethmann, A Collider signature of the supersymmetric golden region, [89] D. Ghosh, Boosted dibosons from mixed heavy top squarks, Phys. Rev. D 88 (2013) 115013 [90] CMS collaboration, Search for top-squark pairs decaying into Higgs or Z bosons in pp s = 8 TeV, Phys. Lett. B 736 (2014) 371 [arXiv:1405.3886] [INSPIRE].


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Exploring the nearly degenerate stop region with sbottom decays, Journal of High Energy Physics, 2017, DOI: 10.1007/JHEP04(2017)084