Lepton jets and low-mass sterile neutrinos at hadron colliders

Sterile neutrinos, if they exist, are potential harbingers for physics beyond the Standard Model. They have the capacity to shed light on our flavor sector, grand unification frameworks, dark matter sector and origins of baryon antibaryon asymmetry. There have been a few seminal studies that have broached the subject of sterile neutrinos with low, electroweak-scale masses (i.e. ${\mathrm{\Lambda }}_{\mathrm{QCD}}\ll m_{N_R}\ll m_{W^{\pm }}$) and investigated their reach at hadron colliders using lepton jets. These preliminary studies nevertheless assume background-free scenarios after certain selection criteria which are overly optimistic and untenable in realistic situations. These lead to incorrect projections. The unique signal topology and challenging hadronic environment also make this mass-scale regime ripe for a careful investigation. With the above motivations, we attempt to perform the first systematic study of low, electroweak-scale, right-handed neutrinos at hadron colliders, in this unique signal topology. There are currently no active searches at hadron colliders for sterile neutrino states in this mass range, and we frame the study in the context of the 13 TeV high-luminosity Large Hadron Collider and the proposed FCC-hh/SppC 100 TeV $pp$-collider.

Figure 1

Current constraints on sterile-active mixing ($|{\mathcal{U}}_{ln}|$) from the ATLAS and CMS collaborations [59, 60, 61]. The relevant final states being searched for are like-sign leptons with associated jets ($l^{\pm }{l}^{\pm }jj$) in all the present analyses. Few of the CMS limits go all the way to intermediate masses of around 50 GeV. The preliminary limit from CMS for $\sqrt{s}=13\mathrm{TeV}$ with $2.3{\mathrm{fb}}^{-1}$ of data [62] is not shown.

Figure 2

In the mass regime of interest, $m_b<m_{N_R}\ll m_{W^{\pm }}$, the main production channel at hadron colliders is through single-$W^{\pm }$ production and decay. Since $m_{N_R}/m_{W^{\pm }}\ll 1$ the leptons from the $N_R$ decay are collimated and form a displaced lepton jet in the relevant parameter space [63]. The lepton from the initial $W^{\pm }$ decay is detected as a prompt lepton.

Figure 3

A possible background from heavy flavor decays. Here, a $J/\psi$ resonance is produced in association with a $W^{\pm }$. The decay products from the boosted $J/\psi$ fake a lepton jet while the $W^{\pm }$ furnishes a prompt lepton. A relatively significant fraction of such events could still survive after a naive selection. They must therefore be accounted for more carefully while making an optimized analysis.

Figure 4

A prototypical background topology that may arise from $t\overline{t}$ events. Even vetoing for hadronic activity and imposing isolation requirements, potentially a large fraction of such events could contaminate the signal region.

Figure 5

The invariant mass of the $\mu$-Jet muons is shown for two signal points and for the combined background ($t\overline{t}$ and $W+\text{jets}$) for a 13 TeV collider.

Figure 6

The production cross section for a sterile, right-handed neutrino in association with a lepton, generated via on-shell $W^{\pm }$ decay. The plot is for $m_{N_R}=8\mathrm{GeV}$ and the variation is shown as a function of the mixing. The two curves are for the 13 TeV (red) and 100 TeV (green) cases.

Figure 7

Depicted on top is the azimuthal angle between the missing transverse energy (MET) and the $\mu$-Jet for signal and the combined background ($t\overline{t}$ and $W+\text{jets}$) for a 13 TeV collider. As expected the $\mathrm{\Delta }{\varphi }_{\text{muon jet}\text{−}\mathrm{MET}}$ peaks at a lower value for the signal while showing no particular preference for any value for the backgrounds. Shown on the bottom is the isolation of the $\mu$-Jet for the signal and combined background. The $\mu$-Jet for the backgrounds is much lesser isolated due to busy hadronic activity. Both of these are obtained after the selections S0 through S2 have been imposed.

Figure 8

Exclusion contours for the 13 TeV LHC (red) and the proposed 100 TeV FCC-hh/SppC (green), for similar selection criteria, assuming a signal efficiency of 100%. For the same integrated luminosity of $300{\mathrm{fb}}^{-1}$, and for a similar cut based analysis, the LHC is already competent with a 100 TeV collider. Constraints from electroweak precision data (EWPT) [67, 68, 69, 70, 71] and LEP data (L3 and DELPHI collaborations) [72, 73] are also shown for comparison.