Top-philic scalar Dark Matter with a vector-like fermionic top partner

Journal of High Energy Physics, Oct 2016

We consider a simple extension of the Standard Model with a scalar top-philic Dark Matter (DM) S coupling, apart from the Higgs portal, exclusively to the right-handed top quark t R and a colored vector-like top partner T with a Yukawa coupling y ST which we call the topVL portal. When the Higgs portal is closed and y ST is perturbative (≲1), T S → (W + b, gt), \( SS\to t\overline{t} \) and \( T\overline{T}\to \left(q\overline{q},\; gg\right) \) provide the dominant (co) annihilation contributions to obtain ΩDM h 2 ≃ 0.12 in light, medium and heavy DM mass range, respectively. However, large \( {y}_{ST}\sim \mathcal{O}(10) \) can make SS → gg dominate via the loop-induced coupling C SSgg in the m S < m t region. In this model it is the C SSgg coupling that generates DM-nucleon scattering in the direct detection, which can be large and simply determined by ΩDM h 2 ≃ 0.12 when SS → gg dominates the DM annihilation. The current LUX results can exclude the SS → gg dominating scenario and XENON-1T experiment may further test y ST ≳ 1, and 0.5 ≲ y ST ≲ 1 may be covered in the future LUX-ZP experiment. The current indirect detection results from Fermi gamma-ray observations can also exclude the SS → gg dominating scenario and are sensitive to the heavy DM mass region, of which the improved sensitivity by one order will push DM mass to be above 400, 600, 1000 GeV for y ST = 0.3, 0.5, 1.0, respectively. \( T\overline{T} \) pair produced at the hadron collider will decay 100% into signal when kinematically open. The latest ATLAS 13 TeV 13.2 fb−1 data can excluded m T between 300 (650) and 1150 (1100) GeV for m S =40 (400) GeV and the exclusion region can reach up to m S ∼ 500 GeV.

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Top-philic scalar Dark Matter with a vector-like fermionic top partner

Received: June Top-philic scalar Dark Matter with a vector-like fermionic top partner Seungwon Baek 0 2 3 Pyungwon Ko 0 1 2 3 Peiwen Wu 0 2 3 to obtain DMh 0 3 Open Access 0 3 c The Authors. 0 3 0 85 Hoegiro , Seoul 02455 , Republic of Korea 1 Quantum Universe Center , KIAS 2 School of Physics , KIAS 3 [9] L. Beck , F. Blekman, D. Dobur, B. Fuks, J. Keaveney and K. Mawatari, Probing top-philic We consider a simple extension of the Standard Model with a scalar top-philic Dark Matter (DM) S coupling, apart from the Higgs portal, exclusively to the right-handed top quark tR and a colored vector-like top partner T with a Yukawa coupling yST which we call the topVL portal. When the Higgs portal is closed and yST is perturbative (. 1), T S ! (W +b; gt), SS ! tt and T T ! (qq; gg) provide the dominant (co)annihilation contributions large yST ' 0:12 in light, medium and heavy DM mass range, respectively. However, O(10) can make SS ! gg dominate via the loop-induced coupling CSSgg in the mS < mt region. In this model it is the CSSgg coupling that generates DM-nucleon scattering in the direct detection, which can be large and simply determined by yST = 0:3; 0:5; 1:0, respectively. T T pair produced at the hadron collider will decay 100% into tt + E= T signal when kinematically open. The latest ATLAS 13 TeV 13.2 fb 1 data can excluded mT between 300 (650) and 1150 (1100) GeV for mS =40 (400) GeV and the exclusion region can reach up to mS fermionic; top; partner - 0:12 when SS ! gg dominates the DM annihilation. The current LUX results can exclude the SS ! gg dominating scenario and XENON-1T experiment may further test yST & 1, and 0:5 . yST . 1 may be covered in the future LUX-ZP experiment. indirect detection results from Fermi gamma-ray observations can also exclude the SS ! gg dominating scenario and are sensitive to the heavy DM mass region, of which the improved sensitivity by one order will push DM mass to be above 400, 600, 1000 GeV for 1 Introduction 2 Thermal relic density 2.1 TopVL and CSSgg coupling 2.3 Interplay between TopVL and Higgs portal 3 5 6 4 Indirect detection Collider search Combined constraints on the model Conclusion The discovery of a new scalar particle at the Large Hadron Collider (LHC) whose properties are similar to those of the Higgs boson predicted in the Standard Model (SM) within the current experimental uncertainties was a huge success of particle physics community [1, 2]. However, the nature of Dark Matter (DM) which occupies about 26% of the current energy content of the Universe [3] is still a big puzzle. Since the SM cannot provide a suitable candidate for DM, many new physics models have been proposed to accommodate this new kind of matter. The simplest extension of the SM would be a model with a singlet scalar DM S which couples to the SM through the following Higgs portal (HP): 2 SH S2HyH: In the above Lagrangian a discrete Z2 symmetry is assumed under which the DM is odd while the SM particles are even, which ensures stability of the DM. There are only two new parameters in this simple model, namely a DM mass parameter S and a renormalizable quartic coupling SH . The phenomenology of this simple Higgs portal model has been well studied (see for example [4] and the references therein). The more-extended fermionic and vector Higgs portal models were studied in [5, 6]. Apart from DM, top quark may also be a window to new physics beyond the SM. As the heaviest quark in the SM, it has the largest Yukawa coupling to the Higgs boson which implies it may play a special role in the electroweak symmetry breaking (EWSB). Top quark also provides the largest contribution to the running of Higgs quartic coupling H . A small change of top quark mass can signi cantly shift the energy scale where becomes negative [7], which makes the precise measurement of top quark properties very important for new physics studies at high energy scale. Consequently, it is well motivated to connect the DM sector to top quark and a specially interesting scenario is the DM which couples only to the top quark sector. Some top-philic new particle sectors and/or DM models can be found in [8{17] and the references therein. Di erent from previous studies, in this work we consider a top-philic scalar DM model by extending the above Higgs portal model with a vector-like fermionic particle T (topVL) which is also odd under the unbroken discrete Z2 symmetry. We require DM S to couple to T and the right-handed (RH) top quark tR via a Yukawa interaction with coupling yST . The Lagrangian including the Yukawa and new covariant kinetic terms reads: (yST ST tR + h:c:); where D is the SM covariant derivative. While the scenario of vector-like doublet coupling any constraint from the bottom quark sector, we will focus on the tR case in this work. The gauge invariance requires the top partner T to be also SU(2)L singlet and have the same hypercharge as tR. Note that the Z2-odd parity assigned to T forbids it from mixing with the SM top quark, thus the current LHC constraints on heavy vector-like quarks do not apply here [18, 19]. The above Yukawa interaction terms will generate DM annihilation SS ! tt through t-channel and the co-annihilations T S; T T . Since the Higgs portal interaction shown in eq. (1.1) can also provide the SS ! tt process in the s-channel, there will be interference with the topVL portal which can be either constructive or destructive. As we will discuss later, when the Higgs portal interaction is closed by setting SH = 0, the topVL portal can be e ective by itself to obtain the observed thermal relic density ' 0:12. However, interplay with the Higgs portal can shift the topVL portal parameter space due to the interference in SS ! tt and the other annihilation channels provided by Higgs portal. Another feature of this model is that S can couple to the gluon via the 1-loop box diagram with t and T running inside [20]. This e ective coupling CSSgg will provide the DM annihilation SS ! gg and we found that large yST O(10) can make SS ! gg dominate in the mS < mt region. Due to the absence of valence top quark in the nucleon, in this model it is the CSSgg coupling that generates DM-nucleon scattering in the direct detection (DD), which can be large and simply determined by ' 0:12 when SS ! gg dominates the DM annihilation. We found that the current LUX results can exclude the SS ! gg dominating scenario and the expected sensitivity of XENON-1T may further test yST & 1, and 0:5 . yST . 1 may be covered in the future LUX-ZP experiment. The collider search for this model can be performed through the pair production T T which is dominated by the QCD processes. The top partner will decay 100% into top quark from the latest ATLAS 13 TeV 13.2 fb 1 data. We found that mT can be excluded between We note that a similar model was analyzed in ref. [21] where DM couples only to light quarks uR or dR. Our model is phenomenologically distinguished from theirs in several aspects. For example, when the Higgs portal interaction is turned o , the DM annihilation model, while s-wave is allowed in our case. Also in the absence of the Higgs portal, DM the nucleon occurs at the tree-level in their case, while it occurs only via one-loop processes in our scenario. The LHC signature in our model is also di erent from which neglected the Higgs portal interaction, we considered the interplay between the topVL and Higgs portal. Other models where DM interacts with the SM leptons were considered in [22{25]. This paper is organized as follows. In section 2 we study the various mechanisms in this top-philic scalar DM model to obtain the observed thermal relic density and the interplay among these mechanisms. In section 3 we investigate the CSSgg contribution to the DM direct detection through the loop process. In section 4 we discuss the current constraints on this model from Fermi gamma-ray observations of dwarf galaxies and line spectrum. In section 5 we study the collider signal of this model based on the latest ATLAS 13 TeV 13.2 fb 1 data. We present the combined results in section 6 and nally conclude in section 7. Thermal relic density The DM annihilation in this top-philic model can occur mainly via three di erent interactions: Higgs portal, topVL portal and the e ective CSSgg coupling. Since the Higgs portal mechanism has been well studied in other works, we will rst focus on the topVL portal by it is only e ective with large yST & 1 and in the mS < mt region where the topVL portal is not su cient. Then we turn on the CSSgg coupling to see its contribution to the DM annihilation compared to SS ! tt and co-annihilations. Finally we will bring the Higgs portal contribution back by setting SH = where 0(mS) is the proper SH in the Higgs portal for mS to obtain the observed relic density, while r is some fractions such as 0:1; 0:2; 0:5; 1:0 to control the Higgs portal strength. With these settings we are able to see the interplay between these two portals which can be either constructive or destructive in di erent parameter space. DM in this model can annihilate via the topVL portal into tt nal state. Both the DM and top partner can co-exist in thermal equilibrium in the early Universe when m = mS . Tf with Tf the temperature at freeze out [26]. This allows co-annihilations T S; T T ! SM which can become important when SS ! tt is kinematically closed or not e cient. We implemented this top-philic DM model with FeynRules [27] and used micromegas [28, 29] to calculate the DM thermal relic density. yST =0.3, 0.5, 1.0. Here we set SH ; CSSgg=0. In gure 1 we show contours with DMh2 = 0:12 which was measured by Planck [30], in be close to 1 to annihilate e ciently. The co-annihilation processes become important as we can see from the fact that r 1. However, larger yST can alleviate this tension to some 0.5, 1.0. When SS ! tt becomes kinematically open, the production of on-shell tt can enhance the annihilation signi cantly. In order not to annihilate too fast the mass ratio r in this case needs to deviate from 1 more than the mS < mt case. This is especially 1.0. When DM mass becomes even heavier, the total annihilation cross section will receive overall suppression from the heavy propagator and/or smaller phase space, in which case the mass ratio r also needs to be close to 1. In this regime the co-annihilation processes become important again. Again, larger yST provides the topVL portal more room to cope drops back to the value in the mS < mt range. TopVL and CSSgg coupling Now we study the e ective CSSgg coupling between DM and gluon which has been calculated in [20]. However, since the top quark mass is heavy, we should not use the approximated result in the limit mt mS; mT . Instead, we used the full expression of CSSgg the same plane of (mS; r) as in jCSSggj(yS2T =8) 1 depending on mS and r = mT =mS. Left panel: on gure 1; right panel: for xed r=1.0, 1.1, 1.5, 2.0, 5, 10. Note that here we do not include the constraints from DM h2 = 0:12 and only show the general features of loop coupling CSSgg. include the constraints from coupling CSSgg. concentrate on how CSSgg contributes to the DM annihilation in the light DM mass range. The value of CSSgg depends on fmS; r; yST g and in the limit mt mS; mT it has a simple expression CSSgg (yS2T =8) [6m2S(r2 1)2] 1. The complete expression can be found in [20]. In the following, we extract the overall factor depending on yST and de ne r. The left panel of gure 2 shows how CS0Sgg varies with mS; mT on the same plane of 10 for better understanding. Note that here we used the full expression of CSSgg since the DM mass region we consider include the case mS; mT < mt. Moreover, gure 2 does not DM h2 = 0:12 and only shows the general features of loop for very small mS and can be very small xed r, the value of CS0Sgg will increase with increasing mS rst and then drop, except for the r = 1 degenerate case where CS0Sgg will approach a constant. Larger maximum CS0Sgg is obtained for smaller r and the point where CS0Sgg starts to drop occurs at larger mS. For a xed DM mass mS the larger mass ratio r will decrease CS0Sgg, especially for large mS. These features suggest that in gure 1 when SS ! tt is e cient, where DM mass is moderate and r is relatively large, CSSgg is generally suppressed and we checked that SS ! tt in this region occupies almost 100% of the DM annihilation (see gure 3 below). However, when SS ! tt is kinematically closed or not e cient, SS ! gg may play an important role, which can be more signi cant in the mS < mt region where gg nal state receives much smaller phase space suppression compared to co-annihilation. We should not forget that yS2T is an overall factor in the full CSSgg which implies that the curve with larger yST in gure 1 can result in larger SS ! gg contribution. In gure 3 we show the contributions to the DM annihilation from di erent channels, solid line) starts to dominate the annihilation when kinematically open. With even heavier DM mass of several hundreds of GeV, it is the co-annihilation channel T T ! qq; gg that dominates the contribution. However, larger yST can help SS ! tt dominate a wider DM mass range. As for the mS < mt region, co-annihilations T S ! W +b; gt have the largest contributions in most cases, while with larger yST > 1 the SS ! gg (cyan solid line) can will dominate in most of the mS < mt and mS > mt region, respectively. This can be depends only on yST , which means SS ! gg can bene t more from large yST > 1 than the co-annihilation ST . On the contrary, the contribution from gluon channel SS ! gg is is basically the same as those in gure 1 where we manually turned o gluon channel to show how the topVL portal itself generates 2 = 0:12. In this case, for each point in gure 3, one can estimate its loop coupling CS0Sgg by comparing gure 1 and gure 2, since a point with fmS; r; yST g read from with fmS; rg and thus the corresponding CS0Sgg. gure 1 can be used to estimate its location in gure 2 0(mS) is the proper SH in the Higgs portal for mS to obtain the observed relic density, while r is chosen to be 0:1; 0:2; 0:5; 1:0. The vertical axis shows the ratio of the modi ed topVL+HPh2 to topVLh2(= 0:12) in gure 1. Interplay between TopVL and Higgs portal Now we study the interplay between the topVL and Higgs portal. The interference happens between the t=u-channel processes SS ! T ! tt in the topVL portal and the s-channel ! tt in the Higgs portal. However, considering that SS ! h occupies a small branch fraction in the Higgs portal annihilation (below 10%, see gure 2 in ref [4]), we would expect generally constructive contributions to the total annihilation cross section from other channels provided by the Higgs portal. For each model point SH = gure 1 with di erent DM mass mS, we set the Higgs portal coupling to be 0(mS) r where 0(mS) is the proper SH in the Higgs portal for mS to obtain the observed relic density, and r is chosen to be 0:1; 0:2; 0:5; 1:0 to control the Higgs portal strength. If the modi ed relic density is larger than those in gure 1 (which is 0.12), then there must be destructive interference from SS ! tt between the topVL and Higgs portal resulting in a decreased total annihilation cross section. However, if the relic density becomes smaller we can not claim the interference is constructive since the Higgs portal also provides other channels which will increase the annihilation cross section. Note that here we consider yST . 1, which means the CSSgg contribution is negligible in most cases, especially for the DM mass ranges discussed here (mt < mS < 1TeV). The vertical axis shows the ratio of topVLh2(=0:12) in can see that when r is small (e.g. 0:1; 0:2; 0:5) there are DM mass wide ranges where the topVL and Higgs portal. However, for larger r the other annihilation channels in the Higgs portal increases the total cross section signi cantly and results in an underproduced relic density. Here we use the s-wave annihilation amplitude as an example to demonstrate the interference pattern, in which case one can set the relative velocity vrel between the two annihilating DM to be zero to simplify the calculation. iMtt = iMt + iMu + iMH:P: = ut( iyST PL) (P1 from the topVL portal and the s-channel from the Higgs portal. ut; vt are the Dirac spinors of the top quark pair, and PL; PR are the projection operators. v 246 GeV is the vacuum expectation value in the SM. The momenta of the two scalar DM in the initial state are mt) = 0, the above Mtt can be simpli ed into with mS > mt, one can clearly see the opposite sign between these two portals which causes the destructive interference. Meanwhile, the mass ratio r varying with mS shown in gure 1 also determines the interference strength and pattern in gure 4 as mS increases. Nevertheless, since SS ! tt only occupies a small branch fraction (< 10%) in the Higgs portal [4], other annihilation channels in the Higgs portal when r becomes larger will increase the nal h vi and produce a reduced relic abundance. These features can also be gure 5 on the same plane as gure 1, but including contributions from both topVL and Higgs portal to get the correct relic density. For relatively large r (e.g. 0.9), the signi cant contribution from the Higgs portal requires the mass ratio r in topVL portal to further deviate from 1 in order not to annihilate too fast. For smaller r (e.g. 0.5), however, the parameter shift is relatively small and r can be reduced due to destructive Direct detection Since the DM direct detection in Higgs portal models has been studied intensively in the CSSgg loop induced scattering. The real scalar DM-nucleon elastic scattering cross section in terms of Wilson coe cient based on DM-parton e ective operators can be found in [20]. Here we capture some of the relevant points. We start with the e ective Lagrangian of the interactions between the real scalar DM s is the strong couplings constant and GA is the eld strength tensor of the gluon eld. The spin-independent (SI) coupling of the real scalar S with a nucleon can be Le = (N) = fN S2N N; fN =mN = the target nucleus with mass mtar can be expressed as where f (N) is the quark mass fraction de ned as fT(Nq) Tq hN jmqqqjN i=mN [31{34] and f (N) [35]. Finally the SI scattering cross section of the real scalar with Since there is no valence top quark in the nucleon, we only need to consider the gluon contribution here. Consequently, the loop coupling CSSgg plays a unique role in the direct detection when Higgs portal is turned o . In gure 6 the magenta solid, dash and dot lines are the current LUX bound [36] and anticipated sensitivity of XENON-1T and LUX-ZP [37], respectively. The solid red, green, and CSSgg contribution to obtain the observed relic density. One can see that the relaxed r due to on-shell produced SS ! tt will suppress CSSgg and thus pSI, especially for large SS ! gg with large yST > 1 can increase rapidly and dominate over co-annihilation in some range of mS < mt where SS ! tt is mostly kinematically unavailable or ine cient. In this case, since the scattering process occurs via the crossed diagrams of the DM annihilation, CSSgg (and thus fN ) is independent of yST and xed to the proper value depending on mS dominate in a wide range of mS < mt (see mass (see eq. (3.5)). We found that the current LUX results can exclude this SS ! gg dominating scenario for any su ciently large yST , although the perturbative yST . 1 is beyond the current LUX sensitivity. However, the future XENON-1T experiment may be capable of detecting yST & 1 for DM mass below around 100 GeV, while the LUX-ZP experiment may further cover the smaller yST & 0:5. Indirect detection Recently the sensitivity of DM indirect detection has been close to the canonical thermal annihilation cross section. In today's Universe, DM S in our model mainly annihilate into tt when mS > mt, while SS ! gg is the dominant annihilation channel for mS < mt. Here we consider the updated results of Fermi gamma-ray observations of continuous spectrum from dwarf galaxies [38] as well as the line spectrum from the Galactic center region [39]. We do not consider the constraints from charged cosmic particles such as positron and anti-proton due to the relatively large uncertainties of their propagation models. We rst recall some main points of the analysis method based on the results of dwarf galaxy observations [40]. The number of photon events observed can be divided into two independent factors: one corresponding to the particle physics process and one describing the astrophysical information of the dwarf galaxies. The expected number of signal events can be expressed as from the CSSgg coupling, with SH =0. The magenta solid, dash and dot lines are the current LUX bound [36] and anticipated sensitivity of XENON-1T and LUX-ZP [37], respectively. The solid red, CSSgg contribution to obtain the observed relic density. where Ae is detector's e ective area and Tobs is the exposure time. The J factor contains the astrophysical information of the DM distribution and is de ned by where the integration is performed along the line of sight in a direction and over a solid For self-conjugate DM particles the particle physics part is de ned as X Bf dE photons emitted N ;f = R m where m is the DM mass and h Avi is the total velocity-averaged cross section of DM annihilation into SM particles in today's Universe. The f denotes the annihilation channels spectrum and the integration from threshold energy Eth to m gives the total number of Since the constraint on h vitt is not given in Fermi dwarf galaxies results [38], we constraints on h vigg are obtained in [21, 41] in a similar way. In our model, both tt and gg channels will contribute to the nal gamma-ray spectrum, thus both the h vitt and trum from dwarf galaxies [38] (left panel) and the line spectrum from the Galactic center region [39] (right panel). The bands re ect the uncertainties in the obtained bounds from the modeling of DM halo pro le imposed in the Fermi reports [38, 39]. The samples plotted are the same as in h vigg bounds will put constraints on the cross section h vitt + h vigg. We also notice that the contribution from SS ! tt is always negligibly small, which is di erent from the light quark scenario in [21]. As for the implementation of line spectrum observations, constraints can be obtained which is generated from the same diagram as the e ective SSgg coupling by where Qt is the top quark electric charge in term of jej. In gure 7 we show the indirect detection constraints on the samples of gure 3 which include the topVL and CSSgg contribution to obtain the observed relic density. In both panels, the bands re ect the uncertainties in the obtained bounds from the modeling of DM halo pro le imposed in the Fermi reports [38, 39]. Similar to the LUX bound in direct detection, current Fermi results from both dwarf and Galactic center observations can cover SS ! gg dominating scenario and exclude some DM mass range depending on the chosen DM pro le. Moreover, given the fact that the limits from Fermi-LAT based on 6 years gure 1 therein) have increased by an order of magnitude compared to 4 years data [43] and it is expected that Fermi-LAT will keep on accumulating data in the next two years claimed by the o cial website [44], we are motivated to consider the future sensitivity improvement by one order of magnitude. Based on this assumption, we can see a large part of the SS ! tt dominating region may also be excluded, pushing DM mass DM mass mS < mt, however, perturbative yST . 1 can easily evade the constraints from the current gamma-ray observation. Since the top partner T carries the color charge, pp ! T T can have sizable production cross section at the LHC. The T T pair will decay through on-shell or o -shell top quarks plus DM particles which nally result in hadronic or leptonic nal states with missing energy. In the collider study of our model, we considered the latest ATLAS 13.2 fb 1 data at 13 TeV of stop searches with 1` + jets + Emiss signals [45] which shows an improvement 850 GeV) in the exclusion capability compared to the 8 TeV 20.3 fb 1 data 710 GeV), especially in the mt~1 m ~01 > mt region with small m ~01 . Since the decay chain of our model is similar to the stop case and the production cross section of fermionic particle T is generally larger than the scalar t~1, we would expect an even higher excluded mT . We use FeynRules [27] to implement this top-philic model into MadGraph5 [46] to generate the parton level events, followed by PYTHIA6 [47] to perform the parton shower. Then we use CheckMate [48, 49], which has encoded Delphes [50, 51], to simulate the collider response and obtain the cut e ciency . Then the number of signal events are calculated as Nsig = L at 13 TeV in [45] and use top++2.0 [52] to calculate where L = 13:2 fb 1 is the ATLAS integrated luminosity is the production cross section of pp ! T T at 13 TeV. (pp ! T T ) at next-to-next-to-leading order (NNLO) including also the next-to-next-to-leading logarithmic (NNLL) contributions. We vary the factorization and renormalization scale between (0:5; 2)mT to estimate the 1 theoretical . CheckMate will use this and the number of generated simulation events NMC to calculate the total uncertainty of signal event number Nsig. Then CheckMATE de nes the following quantity: where No9b5s is the model independent limits at 95% Con dence Level (C.L.) on the number of new physics signal events given in the experimental reports. Then a model can be considered to be excluded at the 95% C.L. if rCM > 1. This rCM-limit is usually weaker than the Nsig in a more conservative manner. More details can be found in [48, 49]. There are seven Signal Regions (SRs) de ned in [45], of which the SR1 and tN high are directly relevant to our case due to the similarity between our process pp ! T T ; T ! tS and the stop case pp ! t~1t~1; t~1 ! t ~01. While both assuming 100% branching fraction (Br), SR1 focuses on small mass splitting between t~1 and ~01 in which case the decay products are fully resolved, while tN high targets larger mass splitting leading to highly boosted top quarks and close-by jets. Since the rst step of decay products are tt + Emiss, the dominant SM background processes include tt; W t; tt + Z(! ) and W + jets. And because all SRs de ned in [45] are required to have exactly one signal lepton, for the W bosons produced in the tt; W t events in the considered SRs, they can both decay leptonically with one of the two leptons being lost (including not identi ed, not reconstructed, or removed in the overlap removal procedure), or one of them decays leptonically and the other decays through a hadronically decaying lepton. Other smaller SM backgrounds include dibosons, tt + W; Z + jets and multijet events. Figure 8. Figure 15 of the latest ATLAS 13.2 fb 1 analysis with 1` + jets + ETmiss signals [45] we use for our validation. We rst checked the reliability of our implementation of [45] into CheckMATE. We chose several supersymmetry (SUSY) samples on the exclusion bound of [45] and compare Nsig = L we calculated to the No9b5s given in [45]. Here (pp ! t~1t~1) is calculated by Prospino2 [53] and the cut e ciency is obtained from CheckMATE. To be consistent among di erent models, here we used only the to calculate a center value Nsig for comparison with No9b5s while neglecting the Nsig which depends on NMC and may be quite di erent when switching from SUSY studies to other new models. Since [45] does not provide the detailed cut- ow information, we would consider our implementation to be reliable if our Nsig is close to No9b5s in several di erent SRs simultaneously. Here we borrow gure 15 of [45] and show it in gure 8. Although only the left of our implementation. Our validation results are shown in table 1 and the largest relative di erences are about 20% due to the small quantity No9b5s. However, for SR1 and tN high very small m ~01 and 1:5% for moderate m ~01 . We did not consider gure 16 in [45] since the decay chains there are quite di erent from our model. We did not consider the in [45] either, since they focus on the searches for new (pseudo-)scalar produced through fermion fusion pp ! + tt where (A) is a new (pseudo-)scalar. Now we turn to our top-philic model and in gure 9 we show the rCM-limit calculated The colored region satis es mT mS > mt which produces on-shell top quarks in the decay chain and is also the region studied in gure 15 of [45] ( gure 8 in this paper). We found that the latest ATLAS 13 TeV search can exclude a wide range of mT between 300 500 GeV for this top-philic DM model. This is an obviously wider region compared to the constraints on SUSY stop, where mt~1 (m ~01 ) up to 850 (250) GeV can be covered. We expect that the ongoing LHC Run-2 accumulating more data will extend this boundary. signal regions, where SR1 and tN high are the most important two which have the decay chain Br(t~1 ! t ~01) = 100% mimicking our case Br(T ! tS) = 100%. conservative exclusion at 95% C.L. if rCM > 1. Nsig)=No9b5s calculated by CheckMate which can be used to claim a Combined constraints on the model Finally we combine the results from thermal relic density, direct detection, indirect detection and collider search in gure 10, where we choose yST = 0:5; 1:0 given their perturbativity and possibility of detection indicated from both black solid lines include the topVL and CSSgg contribution to obtain DMh2 = 0:12. The light pink, orange regions correspond to the excluded parameter space by ATLAS 13 TeV 13.2 fb 1 data [45] and Fermi dwarf results h vigg, while grey yellow and green (cyan) reXENON-1T (LUX-ZP) experiments. One can clearly see the complementarity between ditribution to obtain DMh2 = 0:12. The light pick and orange regions correspond to the excluded parameter space by ATLAS 13 TeV 13.2 fb 1 data [45] and Fermi dwarf results h vigg, while grey yellow and green (cyan) regions correspond to the assumed Fermi dwarf 10 times improvement h vigg=10 and future XENON-1T (LUX-ZP) experiments. rect and indirect detection in the light and heavy DM mass range, while the collider search result is independent of yST since the top partner T has only one decay mode T ! St. We expect that a large portion of the parameter space will be covered by both the future direct and indirect experiments. In this work we studied a scalar top-philic DM S coupling, apart from the Higgs portal, exclusively to the right-handed top quark tR and a colored vector-like top partner T with Yukawa coupling yST which we call the topVL portal. When the Higgs portal is closed and yST is perturbative (. 1), T S ! (W +b; gt), SS ! tt and T T ! (qq; gg) provide the dominant contributions to obtain range, respectively. However, large yST DMh2 ' 0:12 in light, medium and heavy DM mass O(10) can make SS ! gg dominate via the loop-induced coupling CSSgg in the mS < mt region. Due to the absence of valence top quark in the nucleon, in this model it is the CSSgg extend this boundary. coupling that generates DM-nucleon scattering which can be large when SS ! gg dominates the DM annihilation. We found that the the current LUX results can exclude the SS ! gg dominating scenario. The expected sensitivity of XENON-1T may further test yST & 1, and 0:5 . yST . 1 may be covered in the future LUX-ZP experiment. The indirect detection can play a complementary role in this model. The current results from Fermi gamma-ray observations on both continuous spectrum from dwarf galaxies and line spectrum from Galactic center can also exclude the SS ! gg dominating scenario, and are just about to test the heavy DM mass region mS > mt. One order of magnitude of sensitivity improvement can push DM mass to be heavier than about 400, 600, 1000 GeV for yST = 0:3; 0:5; 1:0, respectively. The colored top partner T can be produced in pair at the hadron colliders such as LHC. from the latest ATLAS 13 TeV 13.2 fb 1 data. We found that mT can be excluded between 500 GeV. We expect the ongoing LHC Run-2 accumulating more data will Peiwen Wu would like to thank Liangliang Shang, Yang Zhang and Yilei Tang for helpful discussions. We thank the Korea Institute for Advanced Study for providing computing resources (KIAS Center for Advanced Computation Abacus System) for this work. This work is supported in part by National Research Foundation of Korea (NRF) Research Grant NRF-2015R1A2A1A05001869 (SB,PK). 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. 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Seungwon Baek, Pyungwon Ko, Peiwen Wu. Top-philic scalar Dark Matter with a vector-like fermionic top partner, Journal of High Energy Physics, 2016, 117, DOI: 10.1007/JHEP10(2016)117