Revisiting the LHC reach in the displaced region of the minimal left-right symmetric model
We revisit discovery prospects for a long-lived sterile neutrino at the Large Hadron Collider (LHC) in the context of left-right symmetric theories. We focus on a displaced vertex search strategy sensitive to (GeV) neutrino masses produced via a right-handed boson. Both on-shell and off-shell Drell-Yan production of are considered. We estimate the reach as a function of and . With TeV and 300/fb of integrated luminosity, the LHC can probe neutrino masses as high as GeV for just below 6 TeV. The reach goes up to 11.5 TeV with 3000/fb and GeV. This represents an improvement of a factor of 2 in sensitivity with respect to earlier work.
The origin of the mechanism responsible for providing mass to the neutrinos in the Standard Model (SM) remains unknown. Left-right symmetric theories Pati and Salam (1974); Mohapatra and Pati (1975) are popular candidates to explain light neutrino masses via the so-called see-saw mechanism Minkowski (1977), that additionally address the origin of parity violation in the SM weak sector.
In the minimal left-right framework, parity is broken spontaneously together with the new right-handed weak interaction, generating Majorana masses for both light and right-handed neutrinos . Production and decay of the right-handed or sterile neutrino depends mostly on the unknown mass of the new, heavy right-handed gauge boson, . For , the golden channel is the Keung-Sejanović process (KS) Keung and Senjanovic (1983) where production proceeds through a via .
Recently the authors in Ref. Nemevšek et al. (2018) have systematically classified all the signatures resulting from the KS process into four regions, based on the mass of the sterile neutrino . These regions are: standard, in which GeV, leading to the well studied same sign leptons plus jets () signature Keung and Senjanovic (1983) (for a recent review, see Cai et al. (2018) and references therein); the merged region in which the final state is a prompt lepton and a neutrino jet (); the displaced region where the neutrino appears at a visible distance from the primary vertex leading to a displaced vertex signature; and the invisible region where escapes the detector and leads to events with missing transverse momenta.
Several experimental searches at the LHC target the KS process in the standard region, in a regime where decays promptly. Most recently, the ATLAS Aaboud et al. (2019) and CMS Sirunyan et al. (2018a) collaborations provide stringent limits on the mass of , for masses close to the TeV scale. masses below TeV are already excluded by dijet resonance searches Aaboud et al. (2017a). Existing searches for a new heavy boson decaying to leptons and missing transverse momenta Khachatryan et al. (2017); Aaboud et al. (2018) can constrain part of the invisible region Nemevšek et al. (2018). No dedicated experimental searches targeting the merged and displaced regions exists yet, when has a mass below the electroweak scale 111Although we note that the CMS search for right-handed neutrinos in events with three prompt charged leptons in the final state Sirunyan et al. (2018b) is sensitive to masses below GeV, where the focus is on a simplified model that extends the SM fermionic sector with only one right-handed neutrino..
For sterile neutrino masses below the electroweak scale, becomes a long-lived particle that can be detected via its displaced decay to a lepton and jets Helo et al. (2014); Nemevsek et al. (2011). There is a rapidly growing interest in the study of the LHC capabilities to those heavy neutrino signatures, and heavy neutral leptons in general Abada et al. (2019) (see also comunity () for a community white paper). Long-lived particle signatures in left-right symmetric theories include displaced diphoton jets Bhupal Dev et al. (2017), displaced jets Nemevšek et al. (2018), displaced neutrino jets Mitra et al. (2016) and displaced vertices Helo et al. (2014); Castillo-Felisola et al. (2015, 2015); Cottin et al. (2018a).
The purpose of this paper is to revisit the sensitivity estimates we made in Cottin et al. (2018a) for the displaced KS region. In Cottin et al. (2018a), we focused on the on-shell production of . The authors in Nemevšek et al. (2018) have demonstrated that for TeV, the KS process is dominated by off-shell production via (see Figure 3 of Nemevšek et al. (2018)). Considering this contribution will give an enhancement in the cross-section and therefore in sensitivity of our displaced strategy. Note that in Cottin et al. (2018a) we considered a zero background, multitrack displaced vertex search strategy, as opposed to the recognition of displaced jets performed in Nemevšek et al. (2018). We compare and discuss on the complementarity of both strategies and overlaps with the other KS regions in the regime GeV.
Ii Simulations of production and decay
The minimal left-right symmetric extension of the SM Pati and Salam (1974); Mohapatra and Pati (1975); Mohapatra and Senjanovic (1981) has gauge group . This model contains a right-handed gauge boson and three right-handed Majorana neutrinos, with lightest state . In what follows, we consider only one neutrino in the kinematic region of interest (, and GeV) and we restrict, for simplicity, our discussion to sterile neutrino mixing with the electron sector only.
Drell-Yan production of proceeds via , with further decaying displaced to . This production may be dominantly off-shell Ruiz (2017); Nemevšek et al. (2018). The authors in Nemevšek et al. (2018) have shown that the off-shell enhancement can be significant for light . They also provide a dedicated event generator, the KS Event Generator (KSEG) Popara (), that considers off-shell production as well as light (or heavy) . It also appropriately deals with narrow resonances, as is the case of a long-lived . This is explained in Appendix C of Nemevšek et al. (2018).
We generate parton-level events with KSEG Popara () for a grid of masses covering GeV and TeV. Following Nemevšek et al. (2018), the NLO corrections to the cross-section are taken into account by considering a constant factor of 1.3. Events are hadronized with Pythia 8 Sjöstrand et al. (2015).
The charged decay products are identified to come from a common displaced vertex. A detailed detector response as a function of the displaced vertex invariant mass and number of tracks is applied to each vertex passing the selection criteria Cottin et al. (2018a), described in the following section.
Iii Displaced vertex reach
The displaced vertex search strategy used in this article is the same one proposed in our earlier works in Refs. Cottin et al. (2018a, b) 222With the exception of the “trackless jet cut” defined in Cottin et al. (2018a). We originally kept this cut to follow as closely as possible the selections made in the ATLAS search Aad et al. (2015); Aaboud et al. (2017b). This cut is not necessary and has no effect in the limits presented in this work., which is sensitive to decays occurring in the inner trackers of the LHC detectors. Events are triggered by a prompt electron with GeV, and additional cuts are imposed in the selection of the displaced tracks and vertex.
The displaced vertices are selected by reconstructing tracks with transverse impact parameter bigger than 2 mm, and with GeV. The vertex position must be within 4 mm and 300 mm (tracker acceptance), and must have more than three tracks. The invariant mass of the displaced vertex (which is calculated assuming all tracks have the mass of the pion) is required to be GeV.
Figure 1 shows the CL reach at the 13 TeV LHC in the mass plane , based on signal sensitivity to 3 events in 300 and 3000 fb. The green region corresponds to the limit extracted from our past work in Ref. Cottin et al. (2018a) 333The on-shell production cross-sections used in Fig. 4 of Cottin et al. (2018a) were underestimated by a factor of 3. This was corrected and an errata version should appear soon. We thank Goran Popara for hinting this may come from charge combinations.. When considering off-shell production, the reach goes up to GeV for GeV at 300/fb. With 3000/fb, we are able to reach up to GeV, for GeV.
We also overlay in Figure 1 (with dashed orange lines) the estimated reach with standard leptons + missing transverse momenta searches. These were estimated for the invisible region in Nemevšek et al. (2018) in their Fig. 8 (right panel) and 9. Note that the projection with 3000/fb in Fig. 8 of Nemevšek et al. (2018) was performed for a center of mass energy of 14 TeV, but we still show this for comparison. We see that at high luminosity, this displaced search strategy turns out to be in fact more competitive than searches for invisible decays.
We have revisited LHC prospects with displaced vertices in the search for light, long-lived sterile neutrinos. We focused on the displaced region of the minimal left-right symmetric model, where the long-lived neutrino has a mass below the electroweak scale. By considering that off-shell production of a dominates at (GeV) neutrino masses (as shown in Ref. Nemevšek et al. (2018)), we show that the high luminosity LHC is able to probe sterile neutrino masses up to 47 GeV, for a boson mass of 11.5 TeV. This newly calculated reach turns out to be more competitive than the one projected with standard lepton + missing energy searches. This becomes of great importance when assessing the complementarity of different strategies, as backgrounds for more standard searches will continue to grow at higher center of mass energies, while for displaced vertex searches they will remain low. Further improvements and optimizations of current displaced searches at the LHC look promising for searches for light sterile neutrinos in the left-right model.
Acknowledgements.We thank Goran Popara, Miha Nemevšek and Fabrizio Nesti for clarifying comments. We thank Goran Popara for providing a working version of the KS event generator and help and advice with its usage. We thank Richard Ruiz for early correspondence related to this work. G.C. would also like to thank the Physics and Astronomy Department at University of La Serena for hospitality offered while working on this project. G.C. is supported by the Ministry of Science and Technology of Taiwan under grant No. MOST-106-2811-M-002-035. J.C.H. is supported by Chile grant Fondecyt No. 1161463. M. H. was funded by Spanish MICINN grant FPA2017-85216-P and SEV-2014-0398 (from the Ministerio de Economía, Industria y Competitividad), as well as PROMETEO/2018/165 (from the Generalitat Valenciana).
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