Generation Leptoquarks Research Articles

Addendum to “Combined constraints on first generation leptoquarks”

In this addendum to the paper by Crivellin and Schnell [Phys. Rev. D 103, 115023 (2021)], we discuss the implications of the recent CMS analysis of lepton flavor universality violation in nonresonant di-lepton pairs for first generation leptoquarks. As CMS finds more electron events than expected from background, this analysis prefers the leptoquark representations ${\stackrel{\texttildelow{}}{S}}_{1},{S}_{2},{S}_{3},{\stackrel{\texttildelow{}}{V}}_{1},{V}_{2}({\ensuremath{\kappa}}_{2}^{\mathrm{RL}}\ensuremath{\ne}0)$ and ${V}_{3}$ which lead to constructive interference with the standard model (SM). In principle the excess could also be (partially) explained by the representations ${\stackrel{\texttildelow{}}{S}}_{2},{V}_{1}({\ensuremath{\kappa}}_{1}^{R}\ensuremath{\ne}0),{V}_{2}({\ensuremath{\kappa}}_{2}^{\mathrm{LR}}\ensuremath{\ne}0),{\stackrel{\texttildelow{}}{V}}_{2}$ which are interfering destructively, as this would still lead to the right effect in bins with high invariant mass where the new physics contribution dominates. However, in these cases large couplings would be required which are excluded by other observables. The representations ${S}_{1},{V}_{1}({\ensuremath{\kappa}}_{1}^{L}\ensuremath{\ne}0)$ cannot improve the fit to the CMS data compared to the SM.

Combined constraints on first generation leptoquarks

In this article we perform a combined analysis of low energy precision constraints and LHC searches for leptoquarks which couple to first generation fermions. Considering all ten leptoquark representations, five scalar and five vector ones, we study at the precision frontier the constraints from $K\to\pi\nu\nu$, $K\to\pi e^+e^-$, $K^0-\bar K^0$ and $D^0-\bar D^0$ mixing, as well as from experiments searching for parity violation (APV and QWEAK). We include LHC searches for $s$-channel single resonant production, pair production and Drell-Yan-like signatures of leptoquarks. Interestingly, we find that the recent non-resonant di-lepton analysis of ATLAS provides stronger bounds than the resonant searches recasted so far to constrain $t$-channel production of leptoquarks. Taking into account all these bounds, we observe that none of the leptoquark representations can address the so-called "Cabibbo angle anomaly" via a direct contribution to super-allowed beta decays.

Phenomenology of TeV-scale scalar leptoquarks in the EFT

We examine new aspects of leptoquark (LQ) phenomenology using effective field theory (EFT). We construct a complete set of leading effective operators involving SU(2) singlets scalar LQ and the SM fields up to dimension six. We show that, while the renormalizable LQ-lepton-quark interaction Lagrangian can address the persistent hints for physics beyond the Standard Model in the B-decays $\bar B \to D^{(*)} \tau \bar\nu$, $\bar B \to \bar K \ell^+ \ell^-$ and in the measured anomalous magnetic moment of the muon, the LQ higher dimensional effective operators may lead to new interesting effects associated with lepton number violation. These include the generation of one-loop sub-eV Majorana neutrino masses, mediation of neutrinoless double-$\beta$ decay and novel LQ collider signals. For the latter, we focus on 3rd generation LQ ($\phi_3$) in a framework with an approximate $Z_3$ generation symmetry, and show that one class of the dimension five LQ operators may give rise to a striking asymmetric same-charge $\phi_3 \phi_3$ pair-production signal, which leads to low background same-sign leptons signals at the LHC. For example, with $M_{\phi_3} \sim 1$ TeV and a new physics scale of $\Lambda \sim 5$ TeV, we expect about $5000$ positively charged $\tau^+ \tau^+$ events via $pp \to \phi_3 \phi_3 \to \tau^+ \tau^+ + 2 \cdot j_b$ ($j_b$=b-jet) at the 13 TeV LHC with an integrated luminosity of 300 fb$^{-1}$. It is interesting to note that, in the LQ EFT framework, the expected same-sign lepton signals have a rate which is several times larger than the QCD LQ-mediated opposite-sign leptons signals, $gg,q \bar q \to \phi_3 \phi_3^* \to \ell^+ \ell^- +X$. We also consider the same-sign charged lepton signals in the LQ EFT framework at higher energy hadron colliders such as a 27 TeV HE-LHC and a 100 TeV FCC-hh.

Nonresonant leptoquark with multigeneration couplings for μμjj and μνjj at the LHC

CMS has recently reported a moderate excess in the $\mu\nu jj$ final state in a second generation Leptoquark search, but they have disregarded it because the excess is not present in the $\mu\mu jj$ final state and because they do not observe the expected resonant peak in the distributions. As a proof of concept we show that a simple Leptoquark model including second and third generation couplings with non-negligible single- and non-resonant production in addition to usual pair production could explain the data: excess ($\mu\nu jj$), lack of excess ($\mu\mu jj$) and missing peak in the distributions; while being in agreement with collider constraints. We take this result and analysis as a starting point of a reconsideration of the ATLAS and CMS second generation Leptoquark searches. We also discuss which would be the consequences and modifications that should be performed in the searches to test if this deviation would correspond to a New Physics signal. We observe that low-energy flavor constraints can be avoided by adding heavier particles to the model.

Search for scalar leptoquarks in pp collisions at = 13 TeV with the ATLAS experiment

An inclusive search for a new-physics signature of lepton-jet resonances has been performed by the ATLAS experiment. Scalar leptoquarks, pair-produced in pp collisions at = 13 TeV at the large hadron collider, have been considered. An integrated luminosity of 3.2 fb−1, corresponding to the full 2015 dataset was used. First (second) generation leptoquarks were sought in events with two electrons (muons) and two or more jets. The observed event yield in each channel is consistent with Standard Model background expectations. The observed (expected) lower limits on the leptoquark mass at 95% confidence level are 1100 and 1050 GeV (1160 and 1040 GeV) for first and second generation leptoquarks, respectively, assuming a branching ratio into a charged lepton and a quark of 100%. Upper limits on the aforementioned branching ratio are also given as a function of leptoquark mass. Compared with the results of earlier ATLAS searches, the sensitivity is increased for leptoquark masses above 860 GeV, and the observed exclusion limits confirm and extend the published results.

Search for pair production of first and second generation leptoquarks in proton-proton collisions ats=8 TeV

A search for pair production of first and second generation leptoquarks is performed in final states containing either two charged leptons and two jets, or one charged lepton, one neutrino and two jets, using proton-proton collision data at sqrt(s) = 8 TeV. The data, corresponding to an integrated luminosity of 19.7 inverse femtobarns were recorded with the CMS detector at the LHC. First-generation scalar leptoquarks with masses less than 1010 (850) GeV are excluded for beta = 1.0 (0.5), where beta is the branching fraction of a leptoquark decaying to a charged lepton and a quark. Similarly, second-generation scalar leptoquarks with masses less than 1080 (760) GeV are excluded for beta = 1.0 (0.5). Mass limits are also set for vector leptoquark production scenarios with anomalous vector couplings, and for R-parity violating supersymmetric scenarios of top squark pair production resulting in similar final-state signatures. These are the most stringent limits placed on the masses of leptoquarks and RPV top squarks to date.

Searches for scalar leptoquarks in pp collisions at [Formula: see text] = 8TeV with the ATLAS detector.

Searches for pair-produced scalar leptoquarks are performed using 20 fb^{-1} of proton–proton collision data provided by the LHC and recorded by the ATLAS detector at sqrt{s}=8 TeV. Events with two electrons (muons) and two or more jets in the final state are used to search for first (second)-generation leptoquarks. The results from two previously published ATLAS analyses are interpreted in terms of third-generation leptoquarks decaying to bnu _{tau }bar{b}bar{nu _{tau }} and tnu _{tau }bar{t}bar{nu _{tau }} final states. No statistically significant excess above the Standard Model expectation is observed in any channel and scalar leptoquarks are excluded at 95 % CL with masses up to m_{mathrm {LQ1}}< 1050 GeV for first-generation leptoquarks, m_{mathrm {LQ2}}< 1000 GeV for second-generation leptoquarks, m_{mathrm {LQ3}}< 625 GeV for third-generation leptoquarks in the bnu _{tau }bar{b}bar{nu _{tau }} channel, and 200 < m_{mathrm {LQ3}}< 640 GeV in the tnu _{tau }bar{t}bar{nu _{tau }} channel.

Non-SUSY Beyond Standard Model Searches: Recent Results from ATLAS and CMS

The Standard Model of particle physics is a sensational success, especially since the discovery of the 125 GeV Higgs boson. However, there are still numerous unanswered questions. Why is the Higgs so light? Do the interactions couplings unify and how can gravity be included? Why three fermion generations? What is dark matter? Theories Beyond the Standard Model (BSM), such as Grand Unified Theories, Extra Dimensions or Technicolour are trying to answer these questions. In these proceedings, we will focus on the most recent results obtained by the ATLAS and CMS experiments at the LHC for BSM searches, excluding Higgs and supersymmetry searches. New results on Dark Matter, heavy narrow-width resonances, new heavy quarks and third generation leptoquarks are presented. A summary of the prospects at 14 TeV and at the High Luminosity LHC period is given.

Constraints on $$t$$ t -channel leptoquark exchange from LHC contact interaction searches

The t-channel exchange of a first generation leptoquark could contribute to the cross-section for q q-bar to e+e-. The leptoquark is off-shell, so this process can be sensitive to leptoquarks beyond the mass reach of pair production searches at the LHC (currently m_{LQ}> 830 GeV). We attempt to analytically translate ATLAS bounds on $ (\bar{q} \gamma ^\mu q) (\bar{e} \gamma _\mu e) $ contact interactions to the various scalar leptoquarks, and obtain a bound on their quark-lepton coupling of order $\lambda^2 \leq (m_{LQ}/2$ TeV)$^2$. The greatest difficulty in this translation is that the leptoquarks do not induce the contact interaction studied by ATLAS, so the interference with the Standard Model is different. If bounds were quoted on the functional dependance of the cross-section on s-hat, rather than on particular contact interaction models, this difficulty in applying experimental bounds to theoretical models could be circumvented.

Cornering scalar leptoquarks at LHC

We study implications of large lepton-quark-leptoquark couplings for direct leptoquark searches at Large Hadron Collider. We present all existing flavor constraints on the strength of these couplings assuming that leptoquarks under consideration interact exclusively with charged leptons and quarks of the same generation. We find that these leptoquarks can have sizeable couplings to the Standard Model fermions. This insures a self consistency of our study. We discuss the leptoquark production mechanisms at LHC and demonstrate the importance of inclusion of a t-channel pair production and, in particular, a single leptoquark production through a recast of an existing CMS search at LHC for the second generation leptoquark. Our recast yields the best direct limit on Yukawa coupling of the second generation leptoquark that couples to a muon and a strange quark to date.

Searches for the pair production of scalar leptoquarks at CMS

Results are presented for searches for leptoquark (LQ) pair production using 1.8 –5.0 fb−1 of proton–proton collision data at = 7 TeV collected by the CMS detector at the LHC. First– and second–generation scalar leptoquarks are searched for in final states with either two leptons and two jets (ℓℓjj) or one lepton, missing transverse energy (), and two jets (ℓvjj). Third–generation leptoquarks are searched for in the final state with two b–tagged jets and large (vvbb). No significant excess beyond the standard model predictions is found and 95% confidence level (CL) upper limits are set on the scalar leptoquark pair production cross section in each channel. Limits are calculated for a range of leptoquark mass and for a variable branching fraction (β) of the leptoquark to a charge lepton and a quark. These limits are the most stringent to date.

Search for third generation scalar leptoquarks in pp collisions at $ \sqrt{s}=7 $ TeV with the ATLAS detector

AbstractA search for pair-produced third generation scalar leptoquarks is presented, using proton-proton collisions at$ \sqrt{s}=7 $TeV at the LHC. The data were recorded with the ATLAS detector and correspond to an integrated luminosity of 4.7 fb−1. Each leptoquark is assumed to decay to a tau lepton and ab-quark with a branching fraction equal to 100%. No statistically significant excess above the Standard Model expectation is observed. Third generation leptoquarks are therefore excluded at 95% confidence level for masses less than 534GeV.

Search for second generation scalar leptoquarks in pp collisions at $\sqrt{s}=7~\mbox{TeV}$ with the ATLAS detector

AbstractThe results of a search for the production of second generation scalar leptoquarks are presented for final states consisting of either two muons and at least two jets or a muon plus missing transverse momentum and at least two jets. A total of 1.03 fb−1 integrated luminosity of proton-proton collision data produced by the Large Hadron Collider at $\sqrt{s}=7~\mbox{TeV}$ and recorded by the ATLAS detector is used for the search. The event yields in the signal regions are found to be consistent with the Standard Model background expectations. The production of second generation leptoquarks is excluded for a leptoquark mass mLQ&lt;594 (685) GeV at 95 % confidence level, for a branching ratio of 0.5 (1.0) for leptoquark decay to a muon and a quark.

Search for first generation leptoquarks in ep collisions at HERA

A search for first generation scalar and vector leptoquarks produced in ep collisions is performed by the H1 experiment at HERA. The full H1 data sample is used in the analysis, corresponding to an integrated luminosity of 446 pb−1. No evidence for the production of leptoquarks is observed in final states with a large transverse momentum electron or with large missing transverse momentum, and constraints on leptoquark models are derived. For leptoquark couplings of electromagnetic strength λ=0.3, first generation leptoquarks with masses up to 800 GeV are excluded at 95% confidence level.

Search for pair production of first or second generation leptoquarks in proton-proton collisions ats=7 TeVusing the ATLAS detector at the LHC

This paper describes searches for the pair production of first or second generation scalar leptoquarks using 35/pb of proton-proton collision data recorded by the ATLAS detector at sqrt(s)=7 TeV. Leptoquarks are seached in events with two oppositely charged muons or electrons and at least two jets, and in events with one muon or electron, missing transverse momentum and at least two jets. After event selection, the observed yields are consistent with the predicted backgrounds. Leptoquark production is excluded at the 95% CL for masses mLQ<376 (319) GeV and mLQ<422 (362) GeV for first and second generation scalar leptoquarks, respectively, when assuming the branching fraction of a leptoquark to a charged lepton is equal to 1.0 (0.5).

Search for pair production of second generation scalar leptoquarks

We report on a search for the pair production of second generation scalar leptoquarks (LQ) in pp¯ collisions at the center of mass energy s=1.96 TeV using a data set corresponding to an integrated luminosity of 1.0 fb−1 collected with the DØ experiment at the Fermilab Tevatron Collider. Topologies arising from the LQLQ¯→μqνq and LQLQ¯→μqμq decay modes are investigated. No excess of data over the standard model prediction is observed and upper limits on the leptoquark pair production cross section are derived at the 95% C.L. as a function of the leptoquark mass and the branching fraction β for the decay LQ→μq. These are interpreted as lower limits on the leptoquark mass as a function of β. For β=1(0.5), scalar second generation leptoquarks with masses up to 316 GeV (270 GeV) are excluded.

Search for Third Generation Scalar Leptoquarks Decaying intoτb

We have searched for third generation leptoquarks (LQ3) using 1.05 fb(-1) of data collected with the D0 detector at the Fermilab Tevatron Collider operating at sqrt[s]=1.96 TeV. We set a 95% C.L. lower limit of 210 GeV on the mass of a scalar LQ3 state decaying solely to a b quark and a tau lepton.

Executive function measures in healthy relatives of bipolar and schizophrenia probands

We report on a search for second generation leptoquarks (LQ2) which decay into a muon plus quark in pp¯ collisions at a center-of-mass energy of s=1.96TeV in the D0 detector using an integrated luminosity of about 300 pb−1. No evidence for a leptoquark signal is observed and an upper bound on the product of the cross section for single leptoquark production times branching fraction β into a quark and a muon was determined for second generation scalar leptoquarks as a function of the leptoquark mass. This result has been combined with a previously published D0 search for leptoquark pair production to obtain leptoquark mass limits as a function of the leptoquark–muon–quark coupling, λ. Assuming λ=1, lower limits on the mass of a second generation scalar leptoquark coupling to a u quark and a muon are mLQ2>274GeV and mLQ2>226GeV for β=1 and β=1/2, respectively.

Search for single production of scalar leptoquarks in pp¯ collisions decaying into muons and quarks with the D0 detector

We report on a search for second generation leptoquarks LQ_2 which decay into a muon plus quark in p\bar{p} collisions at a center-of-mass energy of sqrt{s} = 1.96 TeV in the D0 detector using an integrated luminosity of about 300 pb-1. No evidence for a leptoquark signal is observed and an upper bound on the product of the cross section for single leptoquark production times branching fraction beta into a quark and a muon was determined for second generation scalar leptoquarks as a function of the leptoquark mass. This result has been combined with a previously published D0 search for leptoquark pair production to obtain leptoquark mass limits as a function of the leptoquark-muon-quark coupling, lambda. Assuming lambda=1, lower limits on the mass of a second generation scalar leptoquark are m{LQ_2}>274 GeV and m{LQ_2}>226 GeV for beta=1 and beta=1/2, respectively.

Probing leptoquark production at IceCube

We emphasize the inelasticity distribution of events detected at the IceCube neutrino telescope as an important tool for revealing new physics. This is possible because the unique energy resolution at this facility allows to separately assign the energy fractions for emergent muons and taus in neutrino interactions. As a particular example, we explore the possibility of probing second and third generation leptoquark parameter space (coupling and mass). We show that production of leptoquarks with masses (greater-or-similar sign)250 GeV and diagonal generation couplings of O(1) can be directly tested if the cosmic neutrino flux is at the Waxman-Bahcall level.