Wino LSP detection in the light of recent Higgs searches at the LHC

Recent LHC data showed excesses of Higgs-like signals at the Higgs mass of around 125GeV. This may indicate supersymmetric models with relatively heavy scalar fermions to enhance the Higgs mass. The desired mass spectrum is realized in the anomaly-mediated supersymmetry breaking model, in which the Wino can naturally be the lightest superparticle (LSP). We discuss possibilities for confirming such a scenario, particularly detecting signals from Wino LSP at direct detection experiments, indirect searches at neutrino telescopes and at the LHC.

Higgs mass contains very important information about low-energy supersymmetry (SUSY) models, which is well motivated because it provides a viable candidate of dark matter (DM) and also because it realizes the gauge coupling unification. In particular, in the minimal SUSY standard model (MSSM), the lightest Higgs boson cannot be heavier than the Z-boson at the tree level, while a sizable radiative correction may enhance the Higgs mass [1]. The size of the radiative correction depends on the masses (and other parameters) of superparticles. The lightest Higgs mass becomes larger as superparticles (in particular, stops) become heavier. Thus, once the lightest Higgs mass is known, mass scale of superparticles is constrained.
Recently, the ATLAS collaboration reported 3.6σ local excess of the standard model (SM) Higgs-like event at m h ≃ 126 GeV [2]. In addition, the CMS collaboration also showed more than 2σ local excess at m h ≃ 124 GeV [3]. 1 In order to achieve such a value of the lightest Higgs mass in the MSSM, relatively large values of the superparticle masses are required; the typical scale of the sfermion masses to realize m h ≃ 125 GeV is 10 TeV-10 3 TeV [4,5]. Then, if the masses of all the superparticles are of the same order, it is difficult to find experimental signals of low-energy SUSY and the existence of SUSY is hardly confirmed.
Although the sfermion masses are much larger than the electroweak scale, gauginos may be much lighter than sfermions and within the reach of collier and other experiments.
One interesting possibility is the model in which the SUSY breaking scalar masses are from direct coupling to the SUSY breaking field while the gaugino masses are generated by the anomaly-mediation mechanism [6,7]; in this letter, we call such a model as anomalymediated SUSY breaking (AMSB) model. Even in the AMSB model, however, if the pure anomaly-mediation relation holds among the gaugino masses, gluino mass is about 8 times larger than the mass of Wino. Thus, if the Wino mass is a few hundred GeV, which is the lower bound on it from astrophysical and cosmological considerations as will be reviewed later, the gluino mass becomes multi-TeV; with such a heavy gluino, the discovery of the SUSY signal at the LHC becomes challenging because we consider the case that all the squarks are extremely heavy.
Even so, there still exist possibilities of discovering signals of the AMSB scenario.
In particular, in the present framework, the neutral Wino is the lightest superparticle (LSP) and may be DM. In such a case, pair annihilation cross section of the LSP and the scattering cross section of the LSP off the nuclei are both enhanced compared to the Bino LSP case, which has significant implications to direct and indirect detection of DM.
Because the search of the superparticles at the LHC may be difficult, it is important to pursue these possibilities and explore how well we can study the AMSB scenario with these procedures. 2 In this letter, motivated by the recent Higgs searches at the LHC, we discuss the detectability of the signals of AMSB scenario. We pay particular attention to the case of the Wino LSP. We focus on direct/indirect detection of the Wino DM at underground laboratories and neutrino telescopes. We also comment on the LHC reach for the direct Wino production. Since superparticles except for gauginos are heavy, standard methods for SUSY searches may not work. Even in this case, we will show that there are some windows for the confirmation of the SUSY.
Let us first briefly discuss important properties of the AMSB scenario. We assume that the soft SUSY breaking scalar masses are generated by the direct coupling between the scalars and the SUSY breaking hidden sector field, while the gaugino masses are generated by the anomaly mediation mechanism. Adopting the pure AMSB relation, the gaugino masses are given by [6,7] where g a (a = 1-3) are gauge coupling constants of the SM gauge groups, m 3/2 is the Taking account of the radiative correction due to the gauge boson loops, the neutral Wino becomes lighter than the charged one. Therefore, we focus on the case of neutral Wino LSP. In addition, we assume that the LSP (i.e., the neutral Wino) is the dominant component of DM. The Wino LSP accounts for the present DM density for mW ≃ 3 TeV if it is produced only from thermal bath [10]. In the AMSB scenario, however, the Wino LSP can be non-thermally produced from the gravitino or moduli decay [7,11]. If the reheating temperature takes an appropriate value, for example, the decay of gravitino produces the Wino LSP with correct relic density [12], while thermal leptogenesis [13] works successfully [14]. Thus the Wino is a good DM candidate in the present setup.
Hereafter, we assume that the right amount of Wino is somehow produced in the early universe to be DM.
We start with discussing direct detection experiments of DM. The scattering cross section of the Wino LSP off the nucleon significantly depends on µ. Since all scalars except for the lightest Higgs boson are expected to be heavy enough, it is only the lightest Higgs boson that mediates the spin-independent (SI) scattering. The DM-proton scattering cross section is given by [15] where the first and the second term in the bracket are the contributions of SI and spindependent (SD) interaction, respectively. Here mχ 0 is the LSP mass, m N is the mass of the target nucleus, n p (n n ) is the number of proton (neutron) in the target nucleus, J is the total nuclear spin, a p and a n are the effective DM-nucleon SD couplings, and s p(n) are the expectation values of the spin content of the proton and neutron groups within the nucleus. The effective DM-proton coupling, f p , is given by where Tq , m p and m q denote the proton and quark masses, respectively, and f H q is the effective DM-quark coupling obtained by the exchange of the Higgs boson. Since the DM-Higgs coupling is proportional to the magnitude of Wino-Higgsino mixing, 3 the cross section is enhanced if the Wino-Higgsino mixing is large. In Fig. 1 we plot the Wino-proton SI and SD scattering cross section. In this plot we have used following values for the quark contents in the proton [17] : f We have also calculated the detection rate at the IceCube DeepCore, arising from high-energy neutrinos produced by the Wino annihilation at the Galactic Center (GC). We distinguish two event classes following Refs. [20,21,22] : contained muon events and shower events. The contained muons correspond to those emerge inside the instrumental volume through the high-energy neutrino interactions with nucleons. The shower events are caused by charged current interactions of electron and tau neutrinos, and neutral current interactions of all neutrino species. They leave electromagnetic/hadronic shower inside the instrumental volume. The event rate of the contained muons is given by where E νµ is the incident neutrino energy, E µ is the muon energy resulting from the neutrino-proton (neutron) interactions, E th is the threshold energy above which the muon can be detected, dΦ νµ /dE νµ is the neutrino flux at the Earth, dσ (CC) νµp(n) /dE µ denotes the neutrino-proton (neutron) charged current cross section for producing the muon energy with E µ , n p (n n ) is the proton (neutron) number density in the detector material, and V eff is the effective volume for the muon detection. The incident neutrino flux generated by DM annihilation from the GC within cone half angle of θ is given by Here R ⊙ = 8.5 kpc and ρ ⊙ = 0.3 GeV cm −3 , σv is the Wino self-annihilation cross section including the non-perturbative effect [23], and dN ν j /dE ν j is the energy spectrum of the neutrino produced by DM annihilation, which is calculated by the PYTHIA package for the W W final state [24], P j→i is the probability that the ν j at the production is  We have adopted the NFW density profile and considered the neutrino flux from the cone half angle θ = 10 • and θ = 25 • around the GC. As noted in Ref. [22], the sensitivity is maximized for θ ≃ 10 • . For this cone half angle, the flux dependence on the DM density profile is not large [25]. The effective volume for the contained and shower events are set to be 0.04 km 3 and 0.02 km 3 , respectively [20]. The atmospheric background is taken from Ref. [26]. It is seen that the signal-to-noise ratio is at most order one for the Wino mass of a few hundred GeV. We have also checked that the upward muon events expected at the KM3NeT detector [27], assuming the effective area of 1 km 2 and taking account of the energy loss of muons [28], provide similar sensitivities to the DeepCore.
Some of the produced charged Winos may travel through several layers of inner trackers and their track may be reconstructed. In the ATLAS experiment, for example, the charged Wino track can be reconstructed with almost 100 % efficiency ifW ± hits the 3rd layer of the semiconducter tracker (SCT) before it decays [40]. Then, because of the smallness of cτW ± compared to the detector size,W ± decays before going through the whole detector.
Such a charged Wino is identified as a high p T track which disappears in the middle of the detector. Such a signal does not exist in the SM, and hence is a smoking gun evidence of the production ofW ± .
The Wino pair can be produced by the Drell-Yan process at the LHC. However, there is no high p T jet nor track in the final state in such an event, and hence the event cannot be recorded. In order to trigger on the Wino production events, one can use the event with high p T jet; such a jet can be from the initial state radiation. Then, at the parton level, the Wino production processes relevant for the present study are the following: qq →W +W − g, gq →W +W − q, gq →W +W −q , qq ′ →W ±W 0 g, gq →W ±W 0 q ′ , gq →W ±W 0q′ .
We calculate the cross section of the process pp →WW j; we perform the parton level calculation, and we approximate the p T of jet by that of final-state quark or gluon. In the calculation of the cross section, the helicity amplitude package HELAS [41] and the CT10 parton distribution functions [42] are used. For the phase space integration, we use the BASES package [43]. In the calculation, we require that the transverse momentum of the jet be larger than 170, 270, and 370 GeV, and that at least one charged Wino travels more than 44.3 cm which is the distance to the 3rd layer of SCT from the beam pipe in the ATLAS detector [44].
In Fig. 3, the cross section is plotted as a function of the Wino mass. In the high luminosity run with L = 2 × 10 33 cm −2 s −1 , for example, the so-called j370 trigger is planned to be available, which requires a jet with p T > 370 GeV [44]. Then, requiring 10 events with p T > 370 GeV for the discovery, for example, Wino mass smaller than 270 GeV p T > 170GeV p T > 270GeV p T > 370GeV The transverse momentum of j is required to be larger than 170, 270, and 370 GeV from above.
we have assumed that the background is negligible. Thus, in particular when µ is large, the LHC experiment still have a chance to cover the parameter region which has not been excluded yet by the current direct and indirect DM searches. If the p T of the jet for the trigger can be reduced, the LHC can cover the region with larger Wino mass.
So far, we have assumed the pure AMSB relation among gaugino masses. However, as we have mentioned, such a relation may be largely affected by the Higgs and Higgsino loop diagrams. With such an effect, the gluino mass may become ∼ 1 TeV even when the Wino mass is a few GeV. In such a case, the conventional procedures of the SUSY search using the missing energy distribution may work.
In summary, motivated by the recent report on the Higgs searches at the LHC, which indicated excesses of Higgs-like events at around m h ≃ 125 GeV, we have investigated prospects for confirmation of the AMSB scenario, particularly the detection of Wino LSP.
We have considered the situation that the scalars except for gauginos and Higgsinos are heavy enough so that they cannot be produced at colliders. Even in this unfortunate case, the Wino DM may be detected through direct/indirect detection experiments. Direct detection efficiency crucially depends on the Higgsino mass, and if the Wino and Higgsino masses happen to be close, future experiments may find their signals. The neutrino telescopes such as IceCube DeepCore and KM3NeT also have a potential to discover the Wino LSP through the observation muon and/or shower events induced by high-energy neutrinos from DM annihilation at GC.