A search for leptoquarks produced singly and in pairs in proton-proton collisions is presented. We consider the leptoquark (LQ) to be a scalar particle of charge -1/3e coupling to a top quark plus a ...tau lepton ( t τ ) or a bottom quark plus a neutrino ( b ν ), or a vector particle of charge +2/3e, coupling to t ν or b τ . These choices are motivated by models that can explain a series of anomalies observed in the measurement of B meson decays. In this analysis the signatures t τ ν b and t τ ν are probed, using data recorded by the CMS experiment at the CERN LHC at s = 13 TeV and that correspond to an integrated luminosity of 137 fb-1. These signatures have not been previously explored in a dedicated search. The data are found to be in agreement with the standard model prediction. Lower limits at 95% confidence level are set on the LQ mass in the range 0.98–1.73 TeV, depending on the LQ spin and its coupling λ to a lepton and a quark, and assuming equal couplings for the two LQ decay modes considered. These are the most stringent constraints to date on the existence of leptoquarks in this scenario.
GUT Physics in the Era of the LHC Croon, Djuna; Gonzalo, Tomás E.; Graf, Lukas ...
Frontiers in physics,
06/2019, Volume:
7
Journal Article
Peer reviewed
Open access
Grand Unified Theories (GUTs) are one of the most interesting high-energy completions of the Standard Model, because they provide a rich, powerful and elegant group-theoretical framework able to ...resolve a variety of problems remaining in our current understanding of particle physics. They usually act as motivators for many low energy BSM theories, such as left-right symmetric or supersymmetric models, and they serve to fill the gap between the experimentally reachable low energies and the physics in the ultraviolet. In recent years, however, they have fallen slightly from the spotlight, in favor of “simplified” models with more specific phenomenological predictions. The aim of this review is to summarize the state of the art on GUTs and argue for their importance in modern physics. Recent advances in experiments permit to test the predictions of GUTs at different energy scales. First, as GUTs can play a role in the inflationary dynamics of the early Universe, their imprints could be found in the CMB observations by the Planck satellite. Remarkably enough, GUTs could manifest themselves also in terrestrial tests; several planned experiments aim to probe the proton stability and to establish order of magnitude higher bounds on its lifetime. Moreover, the predictions of specific GUT models could be tested even at the LHC thanks to its high energy reach, via searches for exotic states or additional contributions to flavor anomalies.
A search is presented for dark matter in proton-proton collisions at a center-of-mass energy of s=13 TeV using events with at least one high transverse momentum (pT) muon, at least one high-pT jet, ...and large missing transverse momentum. The data were collected with the CMS detector at the CERN LHC in 2016 and 2017, and correspond to an integrated luminosity of 77.4 fb−1. In the examined scenario, a pair of scalar leptoquarks is assumed to be produced. One leptoquark decays to a muon and a jet while the other decays to dark matter and low-pT standard model particles. The signature for signal events would be significant missing transverse momentum from the dark matter in conjunction with a peak at the leptoquark mass in the invariant mass distribution of the highest pT muon and jet. The data are observed to be consistent with the background predicted by the standard model. For the first benchmark scenario considered, dark matter masses up to 500 GeV are excluded for leptoquark masses mLQ≈1400 GeV, and up to 300 GeV for mLQ≈1500 GeV. For the second benchmark scenario, dark matter masses up to 600 GeV are excluded for mLQ≈1400 GeV.
A search is presented for dark matter in proton-proton collisions at a center-of-mass energy of $\sqrt{s} =$ 13 TeV using events with at least one high transverse momentum ($p_\mathrm{T}$) muon, at ...least one high-$p_\mathrm{T}$ jet, and large missing transverse momentum. The data were collected with the CMS detector at the CERN LHC in 2016 and 2017, and correspond to an integrated luminosity of 77.4 fb$^{-1}$. In the examined scenario, a pair of scalar leptoquarks is assumed to be produced. One leptoquark decays to a muon and a jet while the other decays to dark matter and low-$p_\mathrm{T}$ standard model particles. The signature for signal events would be significant missing transverse momentum from the dark matter in conjunction with a peak at the leptoquark mass in the invariant mass distribution of the highest $p_\mathrm{T}$ muon and jet. The data are observed to be consistent with the background predicted by the standard model. For the first benchmark scenario considered, dark matter masses up to 500 GeV are excluded for leptoquark masses $M_\mathrm{LQ}$ $\approx$ 1400 GeV, and up to 300 GeV for $M_\mathrm{LQ}$ $\approx$ 1500 GeV. For the second benchmark scenario, dark matter masses up to 600 GeV are excluded for $M_\mathrm{LQ}$ $\approx$ 1400 GeV.
Experimental data on , , and , provided by different collaborations, show sizable deviations from the standard model predictions. To describe these anomalies, many new physics scenarios have been ...proposed. One of them is the leptoquark model, which introduces the simultaneous coupling of vector and scalar leptoquarks to quarks and leptons. To look for similar possible anomalies in the baryonic sector, we investigate the effects of a vector leptoquark on various physical quantities related to the tree-level decays ( ), which proceed via transitions at the quark level. We calculate the differential branching ratio, forward-backward asymmetry, and longitudinal polarizations of leptons and baryons at the and lepton channels in the leptoquark model and compare their behavior to the predictions of the SM in terms of . In the calculations, we use the form factors calculated in full QCD as the main input and account for all errors coming from the form factors and model parameters. We observe that at the channel, the fit solution to data related to the leptoquark model sweeps some regions out of the SM band; nevertheless, the fit has a considerable intersection with the SM predictions. The type solution gives roughly the same results as the SM on . At the channel, the leptoquark model gives results that are consistent with the SM predictions and existing experimental data on the behavior of with respect to . Concerning the behavior of the , the two types of fits for and the predictions at the channel in the leptoquark model give exactly the same results as the SM. We also investigate the behavior of the parameter with respect to and the value of in both the vector leptoquark and SM models. Both fit solutions lead to results that deviate considerably from the SM predictions for and . Future experimental data on and , made available by measurements of the channel, will be particularly helpful. Any experimental deviations from the SM predictions in this channel would emphasize the importance of tree-level hadronic weak transitions as good probes of new physics effects beyond the SM.
A new search signature for excited leptons is explored. Excited muons are sought in the channel pp → * → jet jet , assuming both the production and decay occur via a contact interaction. The analysis ...is based on 20.3 fb−1 of pp collision data at a centre-of-mass energy of s = 8 TeV taken with the ATLAS detector at the large hadron collider. No evidence of excited muons is found, and limits are set at the 95% confidence level on the cross section times branching ratio as a function of the excited-muon mass m * . For m * between 1.3 and 3.0 TeV, the upper limit on B ( * → q q ¯ ) is between 0.6 and 1 fb. Limits on B are converted to lower bounds on the compositeness scale Λ. In the limiting case Λ = m * , excited muons with a mass below 2.8 TeV are excluded. With the same model assumptions, these limits at larger * masses improve upon previous limits from traditional searches based on the gauge-mediated decay * → γ .
Using a data sample of 448×106 ψ(3686) events collected with the BESIII detector operating at the BEPCII storage ring, the decays ψ(3686)→γη and ψ(3686)→γπ0 are observed with a statistical ...significance of 7.3σ and 6.7σ, respectively. The branching fractions are measured to be B(ψ(3686)→γη)=(0.85±0.18±0.05)×10−6 and B(ψ(3686)→γπ0)=(0.95±0.16±0.05)×10−6. In addition, we measure the branching fraction of ψ(3686)→γη′ to be B(ψ(3686)→γη′)=(125.1±2.2±6.2)×10−6, which represents an improvement of precision over previous results.
In this study, a new search signature for excited leptons is explored. Excited muons are sought in the channel ${pp}\to \mu {\mu }^{* }\to \mu \mu \ {\rm{jet}}\;{\rm{jet}}$, assuming both the ...production and decay occur via a contact interaction. The analysis is based on 20.3 fb–1 of pp collision data at a centre-of-mass energy of $\sqrt{s}\;=\;8\;{\rm{TeV}}$ taken with the ATLAS detector at the large hadron collider. No evidence of excited muons is found, and limits are set at the 95% confidence level on the cross section times branching ratio as a function of the excited-muon mass ${m}_{{\mu }^{* }}$. For ${m}_{{\mu }^{* }}$ between 1.3 and 3.0 TeV, the upper limit on $\sigma B({\mu }^{* }\to \mu q\bar{q}$) is between 0.6 and 1 fb. Limits on $\sigma B$ are converted to lower bounds on the compositeness scale Λ. In the limiting case ${\rm{\Lambda }}={m}_{{\mu }^{* }}$, excited muons with a mass below 2.8 TeV are excluded. With the same model assumptions, these limits at larger ${\mu }^{* }$ masses improve upon previous limits from traditional searches based on the gauge-mediated decay ${\mu }^{* }\to \mu \gamma $.
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