Chapter 4
Study of charged dark fermions at ILC250
In this chapter, we study the 2τ +M ET final state to probe the charged fermions arising in Dark Matter Models at 250 GeV ILC. We perform a detailed signal versus SM background analysis using ILC software package (ILCsoft) and show that charged fermion of 100-124 GeV mass can be probed with significance larger than 5σ using appropriate beam polarisation combinations. Manuscript under review by the collaborators.
such particles are necessary for various reasons. For example, with the experimental ob- servations suggesting that about 27% of the total energy of the universe can be categorised as DM [51,144], some of the recent scenarios have proposed fermionic or scalar DM can- didates with charged fermion partners. An attractive model among the simple scaler DM scenarios is the so-called IHDM [145–149] with an additional scalar doublet compared to the SM, which does not interact with the SM fields other than their gauge interactions.
This inertness owes to their oddness under aZ2 symmetry under which all other particles are considered to be even. The stability of the neutral component of this doublet field is also ensured by the sameZ2symmetry. The DM thus gauge produced can get annihilated before freezing out at a later temperature. The annihilation is typically mediated by the SM Higgs boson in the s-channel or with the gauge bosons in the t-channel. The former can have SM fermions or gauge bosons in the final state, whereas the latter can have only gauge bosons in the final state. Thus, for lighter DM with mass less than MW, the relevant s-channel process is proportional to the square of the effective scalar quartic cou- pling, usually denoted asλL. However, the same coupling controls the nuclear scattering of DM in the direct detection experiments, and therefore are stringently constrained. This severely restricts the annihilation cross section, making the dark matter over-abundant in this region. On the other hand, once kinematically allowed (with dark matter mass larger thanMW), the gauge boson annihilation channels overkill the dark matter making it under abundant in this region of parameter space. Thus, the simple IHDM is pushed to a corner with the push and pull arising from the direct detection constraints on the one hand, and the relic density consideration on the other. However, as shown in a recent work [150] the presence of a charged fermion partner to the inert doublet along with the addition of another (fermionic) dark matter can effortlessly accommodate both the relic density constraint and the direct detection limits. The charged fermion in this case can have mass a few 100 GeV to a TeV compatible with all DM measurements. Apart from this, there are many studies with fermionic or scalar DM candidates along with additional charged fermions suitably chosen to be compatible with all the experimental constraints [151–157]. The other context in which models with heavy fermions arise are supersym- metric models with heavy charginos and neutralinos and the extensions of SM addressing electroweak baryogenesis [158–160]. LHC search for the presence of heavy fermions are
4.1. Introduction 73 mostly limited to those arising in supersymmetric models. ATLAS [161] and CMS [162]
search for chariginos and neutralinos lead to exclusion of about 650-750 GeV assuming massless neutralino, which is considerably relaxed to 125 GeV for the lightest chargino when the mass splitting with the neutralinos is close to Z mass.
In this study, we consider the possible presence of heavy charged fermions (χ±) existing in association with the scalar DM. Being Z2 odd such fermions allow Yukawa couplings with the dark scalar field along with standard fermions. χ± being considered as gauge singlets, the left-handed SM doublet is present in the Yukawa interaction, allowing the heavy fermions to decay directly to the charged leptons. We assume that the couplings to the first two generations of leptons are quite suppressed so as not to run into trouble with the g−2 measurements. Moreover, we consider only diagonal Yukawa interactions, avoiding lepton flavour violating interactions. Thus, we focus on the tau decay of χ± along with large missing energy. With the pair production and decay, the signal of a tau-pair with large missing energy closely mimics the pair production of stau (˜τ) and its decay to tau and neutralino (χ01). CMS search for stau with 35.6 fb−1 data at 13 TeV in this channel has restricted the production cross section of around 3 pb for Mτ˜ ∼100 GeV, which is reduced to around 30 fb for higher masses of 200 GeV at 95% C.L. [163], considering the mass of the missing neutralino to be 50 GeV. In their HL projection with 3000 fb−1 luminosity at 14 TeV centre of mass, ATLAS has presented the exclusion regions in themτ˜−mχ0
1 plane, showing thatmτ˜∼100 GeV with mχ0
1 >60 GeV will not be restricted, whereas restrictions on larger mass regions are more stringent [164]. One may note that these studies have two distinctions with the case proposed in the present study. Firstly, while ˜τ is a scalar, we have a heavy fermion,χ+. This means, the selection criteria tuned to the supersymmetric search may not be applicable as it is.1 Secondly, unlike the 100% branching ratio (BR) of ˜τ toτ, in the present case there are other equally competing channels, which can reduce the BR to one-third. Moreover, it is precisely the low mass regions of around a 100 GeV, which are comparatively difficult at the LHC what we focus on here. We show that the proposed ILC with a baseline centre of mass energy of 250 GeV is quite capable of exploring the presence of χ+ with small luminosity. For specificity, we consider the model studied in Ref.[150]. However the results of this study
1A recast of this search withχ+ channel as the signal is being performed.
do not depend on the details of the model, and can be adapted easily to other similar models with a charged fermion decaying to tau and missing energy. While carrying out a detailed analysis at the 250 GeV ILC with close-to-realistic collider-detector simulation including all the relevant backgrounds considered, we also indicate what may be expected at possible higher energy versions of thee+e−collider. We may add that with low centre of mass energy, the baseline design of ILC is limited in its explorations of particle dynamics beyond the SM. New particles in most BSM scenarios are already restricted by the LHC or will be explored by its HL version, with masses beyond the production threshold of 250 GeV ILC. In this backdrop, the scenario we discuss in this chapter with heavy leptons of mass around 100 GeV comes as an attractive possibility at the ILC. As we demonstrate, ILC will be able to probe its presence within a short period of its commissioning.
We proceed with a brief description of the model in Section 4.2 followed by the in- troduction to the simulation tools, with ILC detector concept and the signal and SM background processes in Section 4.3. Section 4.4 presents the analysis and discussions and then we conclude the results in Section4.5.