Probing Nonstandard Standard Model Backgrounds with LHC Monojets

TL;DR

This work shows that monojet plus missing energy events at the LHC can constrain nonstandard neutrino interactions (NSI) by modifying neutrino-quark production, potentially mimicking dark matter or large extra dimensions. It first treats NSI as contact operators to derive bounds approaching solar-neutrino hints, then analyzes finite-mass mediator scenarios (e.g., $Z'$ or leptoquarks) to reveal mass-dependent sensitivity, with strong bounds in the $10^2$–$10^3$ GeV range and weaker bounds for very light mediators. The authors demonstrate that collider NSI constraints are complementary to solar data and can be extended via charged-lepton channels, such as multileptons or leptoquark signals, to distinguish NSI from dark matter. They advocate a coordinated collider NSI program across monojets, multileptons, and related channels, noting possible CMS multilepton excesses that could be compatible with NSI values suggested by solar data. Overall, NSI physics can be probed at colliders both in high-scale mediator scenarios and in light-mediator regimes, with implications for dark matter and neutrino phenomenology.

Abstract

Monojet events at colliders have been used to probe models of dark matter and extra dimensions. We point out that these events also probe extensions of the Standard Model modifying neutrino-quark interactions. Such nonstandard interactions (NSI) have been discussed in connection with neutrino oscillation experiments. Assuming first that NSI remain contact at LHC energies, we derive stringent bounds that approach the levels suggested by the Boron-8 solar data. We next explore the possibility that the mediators of the NSI can be produced at colliders. The constraints are found to be strongest for mediator masses in the 10^2-10^3 GeV range, with the best bounds above ~ 200 GeV coming from ATLAS and below from CDF. For mediators with masses below 30 GeV the monojet bounds are weaker than in the contact limit. These results also directly apply to light dark matter searches. Lastly, we discuss how neutrino NSI can be distinguished from dark matter or Kaluza-Klein states with charged lepton searches.

Figures (7)

• Figure 1: The effect of the flavor-diagonal (left) and flavor off-diagonal (right) NSI on the day-time survival probability $P(\nu_{e}\rightarrow \nu_{e})$ of electron neutrinos from the Sun. The thick black curves represent the Standard Model expectations, using the recently measured $\sin^{2}2\theta_{13}\simeq0.1$DayaBayRENO, while the thin red curves represent the result of varying the NSI $\varepsilon$ parameters per electron in the range $[-0.2,0.2]$. The neutrino is taken to be produced at the center of the Sun (a good approximation for the $^8$B neutrinos).
• Figure 2: Feynman diagrams contributing to the monojet signal (\ref{['pp']}), with time flowing from left to right. The shaded blobs denote the NSI contact interaction. At the $7$ TeV LHC the $q\overline{q}$ initial state contributes approximately the $70\%$ of the signal.
• Figure 3: Feynman diagrams contributing to monojet signals in the leptoquark model. Dashed lines denote a leptoquark. The last diagram can dominate for light leptoquark masses, but is subdominant at low energy as it leads to a dimension-8 operator involving a gluon field, two quarks, and two neutrinos.
• Figure 4: Monojet signals in the $Z'$ model of the NSI contact operator. Wavy lines denote a $Z'$.
• Figure 5: Contours of fixed generator-level cross-section in the $Z'$ model. Here it is assumed that the $Z'$ couples equally to $u_{L}$ and a flavor non-conserving neutrino pair. The red-dashed curve illustrates the naïve bound obtained by using a fixed acceptance, corresponding to the contact-operator with veryHighPt cuts. See text for additional details. Actual bounds are shown in Fig. \ref{['TevvsLHC']}.