Search for long-lived heavy neutral leptons with displaced vertices in proton-proton collisions at $\sqrt{s}$ =13 TeV

The collaboration
CMS-EXO-20-009, 2022.

Abstract
A search for heavy neutral leptons (HNLs), the right-handed Dirac or Majorana neutrinos, is performed in final states with three charged leptons (electrons or muons) using proton-proton collision data collected by the CMS experiment at $\sqrt{s}$ = 13 TeV at the CERN LHC. The data correspond to an integrated luminosity of 138 fb$^{-1}$. The HNLs could be produced through mixing with standard model neutrinos $\nu$. For small values of the HNL mass ($\lt$20 GeV) and the square of the HNL-$\nu$ mixing parameter ( 10$^{-7}$-10$^{-2}$), the decay length of these particles can be large enough so that the secondary vertex of the HNL decay can be resolved with the CMS silicon tracker. The selected final state consists of one lepton emerging from the primary proton-proton collision vertex, and two leptons forming a displaced, secondary vertex. No significant deviations from the standard model expectations are observed, and constraints are obtained on the HNL mass and coupling strength parameters, excluding previously unexplored regions of parameter space in the mass range 1-20 GeV and squared mixing parameter values as low as 10$^{-7}$.

• #### Data and background yields ($eeX$)

Data from Table 3, also in Figure 8

10.17182/hepdata.115355.v1/t1

Number of predicted and observed events in the $eeX$ final states. The quoted uncertainties include statistical and systematic uncertainties.

• #### Data and background yields ($\mu\mu X$)

Data from Table 4, also in Figure 8

10.17182/hepdata.115355.v1/t2

Number of predicted and observed events in the $\mu\mu X$ final states. The quoted uncertainties include statistical and systematic uncertainties.

• #### Predicted signal yields ($eeX$)

Data from Table 5 (also in Figure 8) and Additional Table 1

10.17182/hepdata.115355.v1/t3

Number of predicted signal events in the $eeX$ final states, for several benchmark signal hypotheses for Majorana HNL. The quoted...

• #### Predicted signal yields ($\mu\mu X$)

Data from Table 6 (also in Figure 8) and Additional Table 2

10.17182/hepdata.115355.v1/t4

Number of predicted signal events in the $\mu\mu X$ final states, for several benchmark signal hypotheses for Majorana HNL. The...

• #### Limits on Majorana HNL with electron coupling

Data from Figure 9a

10.17182/hepdata.115355.v1/t5

The 95% CL limits on $|V_{Ne}|^2$ as a function of the HNL mass for a Majorana HNL. Values of $-1$...

• #### Limits on Majorana HNL with muon coupling

Data from Figure 9b

10.17182/hepdata.115355.v1/t6

The 95% CL limits on $|V_{N\mu}|^2$ as a function of the HNL mass for a Majorana HNL. Values of $-1$...

• #### Limits on Dirac HNL with electron coupling

Data from Figure 10a

10.17182/hepdata.115355.v1/t7

The 95% CL limits on $|V_{Ne}|^2$ as a function of the HNL mass for a Dirac HNL. Values of $-1$...

• #### Limits on Dirac HNL with muon coupling

Data from Figure 10b

10.17182/hepdata.115355.v1/t8

The 95% CL limits on $|V_{N\mu}|^2$ as a function of the HNL mass for a Dirac HNL. Values of $-1$...

• #### Covariance matrix ($eeX$)

10.17182/hepdata.115355.v1/t9

Covariance matrix for the background prediction in the signal region bins for the $eeX$ final states. The bin labels specify...

• #### Covariance matrix ($\mu\mu X$)

10.17182/hepdata.115355.v1/t10

Covariance matrix for the background prediction in the signal region bins for the $\mu\mu X$ final states. The bin labels...

• #### Correlation matrix ($eeX$)

10.17182/hepdata.115355.v1/t11

Correlation matrix for the background prediction the signal region bins for the $eeX$ final states. The bin labels specify the...

• #### Correlation matrix ($\mu\mu X$)

10.17182/hepdata.115355.v1/t12

Correlation matrix for the background prediction the signal region bins for the $\mu\mu X$ final states. The bin labels specify...

• #### Mean lifetime and cross section

10.17182/hepdata.115355.v1/t13

Mean lifetime and production cross section for HNLs of Majorana nature, assuming the decay to three charged leptons and coupling...