Transverse Momentum Imbalance Research Articles

Monojet signatures from heavy colored particles: future collider sensitivities and theoretical uncertainties

In models with colored particle mathcal {Q} that can decay into a dark matter candidate X, the relevant collider process pprightarrow mathcal {Q}bar{mathcal {Q}}rightarrow Xbar{X},+,jets gives rise to events with significant transverse momentum imbalance. When the masses of mathcal {Q} and X are very close, the relevant signature becomes monojet-like, and Large Hadron Collider (LHC) search limits become much less constraining. In this paper, we study the current and anticipated experimental sensitivity to such particles at the High-Luminosity LHC at sqrt{s}=14 TeV with mathcal {L}=3 ab^{-1} of data and the proposed High-Energy LHC at sqrt{s}=27 TeV with mathcal {L}=15 ab^{-1} of data. We estimate the reach for various Lorentz and QCD color representations of mathcal {Q}. Identifying the nature of mathcal {Q} is very important to understanding the physics behind the monojet signature. Therefore, we also study the dependence of the observables built from the pprightarrow mathcal {Q}bar{mathcal {Q}} + j process on mathcal {Q} itself. Using the state-of-the-art Monte Carlo suites MadGraph5_aMC@NLO+Pythia8 and Sherpa, we find that when these observables are calculated at NLO in QCD with parton shower matching and multijet merging, the residual theoretical uncertainties are comparable to differences observed when varying the quantum numbers of mathcal {Q} itself. We find, however, that the precision achievable with NNLO calculations, where available, can resolve this dilemma.

Measurement of Final-State Correlations in Neutrino Muon-Proton Mesonless Production on Hydrocarbon at ⟨Eν⟩=3 GeV

Final-state kinematic imbalances are measured in mesonless production of ν_{μ}+A→μ^{-}+p+X in the MINERvA tracker. Initial- and final-state nuclear effects are probed using the direction of the μ^{-}-p transverse momentum imbalance and the initial-state momentum of the struck neutron. Differential cross sections are compared to predictions based on current approaches to medium modeling. These models underpredict the cross section at intermediate intranuclear momentum transfers that generally exceed the Fermi momenta. As neutrino interaction models need to correctly incorporate the effect of the nucleus in order to predict neutrino energy resolution in oscillation experiments, this result points to a region of phase space where additional cross section strength is needed in current models, and demonstrates a new technique that would be suitable for use in fine-grained liquid argon detectors where the effect of the nucleus may be even larger.

Study of isolated-photon and jet momentum imbalance in pp and PbPb collisions

In this paper, we study the production of isolated-photon plus a jet in pp and PbPb collisions, which can be used as an important probe to the jet transport property in quark gluon plasma created in heavy ion collisions. Normally, there are two types of observables associated with the production of isolated-photon plus a jet, namely, the azimuthal angular correlation and the transverse momentum imbalance. To understand both observables in the full kinematical region, we need to employ the perturbative QCD calculation, which takes into account the hard splitting of partons, together with the Sudakov resummation formalism, which resums soft gluon splittings. Furthermore, by introducing energy-loss into the system, we calculate the enhancement of the momentum imbalance distribution for AA as compared to pp collisions and make predictions for future unfolded experimental data. In addition, in order to extract the jet transport coefficient more precisely in our numerical calculation, we also distinguish quark jets from gluon jets, since they interact with quark gluon plasma with different strengths. This work provides a reliable theoretical tool for the calculation of the gamma-jet correlation, which can lead us to a more precise extraction of the jet transport coefficient in relativistic heavy-ion collisions.

Search for new physics in final states with an energetic jet or a hadronically decaying W or Z boson and transverse momentum imbalance at s=13 TeV

A search for new physics using events containing an imbalance in transverse momentum and one or more energetic jets arising from initial-state radiation or the hadronic decay of W or Z bosons is presented. A data sample of proton-proton collisions at $\sqrt{s} = $ 13 TeV, collected with the CMS detector at the LHC and corresponding to an integrated luminosity of 35.9 fb$^{-1}$, is used. The observed data are found to be in agreement with the expectation from standard model processes. The results are interpreted as limits on the dark matter production cross section in simplified models with vector, axial-vector, scalar, and pseudoscalar mediators. Interpretations in the context of fermion portal and nonthermal dark matter models are also provided. In addition, the results are interpreted in terms of invisible decays of the Higgs boson and set stringent limits on the fundamental Planck scale in the Arkani-Hamed, Dimopoulos, and Dvali model with large extra spatial dimensions.

Search for new physics in events with a leptonically decaying Z boson and a large transverse momentum imbalance in proton\u2013proton collisions at sqrt{s} = 13,text {TeV}

A search for new physics in events with a Z boson produced in association with large missing transverse momentum at the LHC is presented. The search is based on the 2016 data sample of proton-proton collisions recorded with the CMS experiment at sqrt{s} = 13,text {TeV} , corresponding to an integrated luminosity of 35.9,text {fb}^{-1}. The results of this search are interpreted in terms of a simplified model of dark matter production via spin-0 or spin-1 mediators, a scenario with a standard-model-like Higgs boson produced in association with the Z boson and decaying invisibly, a model of unparticle production, and a model with large extra spatial dimensions. No significant deviations from the background expectations are found, and limits are set on relevant model parameters, significantly extending the results previously achieved in this channel.

Search for natural supersymmetry in events with top quark pairs and photons in pp collisions at sqrt{s}=8 TeV

Results are presented from a search for natural gauge-mediated supersymmetry (SUSY) in a scenario in which the top squark is the lightest squark, the next-to-lightest SUSY particle is a bino-like neutralino, and the lightest SUSY particle is the gravitino. The strong production of top squark pairs can produce events with pairs of top quarks and neutralinos, with each bino-like neutralino decaying to a photon and a gravitino. The search is performed using a sample of pp collision data accumulated by the CMS experiment at sqrt{s}=8 TeV, corresponding to an integrated luminosity of 19.7 fb−1. The final state consists of a lepton (electron or muon), jets, and one or two photons. The imbalance in transverse momentum in the events is compared with the expected spectrum from standard model processes. No excess event yield is observed beyond the expected background, and the result is interpreted in the context of a general model of gauge-mediated SUSY breaking that leads to exclusion of top squark masses below 650–730 GeV.

Vector boson tagged jets and jet substructure

In these proceedings, we report on recent results related to vector boson-tagged jet production in heavy ion collisions and the related modification of jet substructure, such as jet shapes and jet momentum sharing distributions. Z0-tagging and γ-tagging of jets provides new opportunities to study parton shower formation and propagation in the quark-gluon plasma and has been argued to provide tight constrains on the energy loss of reconstructed jets. We present theoretical predictions for isolated photon-tagged and electroweak boson-tagged jet production in Pb+Pb collisions at √sNN = 5.02 TeV at the LHC, addressing the modification of their transverse momentum and transverse momentum imbalance distributions. Comparison to recent ATLAS and CMS experimental measurements is performed that can shed light on the medium-induced radiative corrections and energy dissipation due to collisional processes of predominantly quark-initiated jets. The modification of parton splitting functions in the QGP further implies that the substructure of jets in heavy ion collisions may differ significantly from the corresponding substructure in proton-proton collisions. Two such observables and the implication of tagging on their evaluation is also discussed.

Search for dark matter produced in association with heavy-flavor quark pairs in proton-proton collisions at sqrt{s}= 13,text{TeV}

A search is presented for an excess of events with heavy-flavor quark pairs ({t}overline{{t}} and {b} overline{{b}} ) and a large imbalance in transverse momentum in data from proton–proton collisions at a center-of-mass energy of 13,text{TeV}. The data correspond to an integrated luminosity of 2.2,text{fb}^{-1} collected with the CMS detector at the CERN LHC. No deviations are observed with respect to standard model predictions. The results are used in the first interpretation of dark matter production in {t}overline{{t}} and {b} overline{{b}} final states in a simplified model. This analysis is also the first to perform a statistical combination of searches for dark matter produced with different heavy-flavor final states. The combination provides exclusions that are stronger than those achieved with individual heavy-flavor final states.

Search for supersymmetry in events with at least one photon, missing transverse momentum, and large transverse event activity in proton-proton collisions at sqrt{s}=13 TeV

A search for physics beyond the standard model in final states with at least one photon, large transverse momentum imbalance, and large total transverse event activity is presented. Such topologies can be produced in gauge-mediated supersymmetry models in which pair-produced gluinos or squarks decay to photons and gravitinos via short-lived neutralinos. The data sample corresponds to an integrated luminosity of 35.9 fb−1 of proton-proton collisions at sqrt{s}=13 TeV recorded by the CMS experiment at the LHC in 2016. No significant excess of events above the expected standard model background is observed. The data are interpreted in simplified models of gluino and squark pair production, in which gluinos or squarks decay via neutralinos to photons. Gluino masses of up to 1.50-2.00 TeV and squark masses up to 1.30-1.65 TeV are excluded at 95% confidence level, depending on the neutralino mass and branching fraction.

Search for new phenomena with the M_{mathrm {T2}} variable in the all-hadronic final state produced in proton\u2013proton collisions at sqrt{s} = 13,text {TeV}

A search for new phenomena is performed using events with jets and significant transverse momentum imbalance, as inferred through the M_{mathrm {T2}} variable. The results are based on a sample of proton–proton collisions collected in 2016 at a center-of-mass energy of 13,text {TeV} with the CMS detector and corresponding to an integrated luminosity of 35.9,text {fb}^text {-1}. No excess event yield is observed above the predicted standard model background, and the results are interpreted as exclusion limits at 95% confidence level on the masses of predicted particles in a variety of simplified models of R-parity conserving supersymmetry. Depending on the details of the model, 95% confidence level lower limits on the gluino (light-flavor squark) masses are placed up to 2025 (1550),text {GeV}. Mass limits as high as 1070 (1175),text {GeV} are set on the masses of top (bottom) squarks. Information is provided to enable re-interpretation of these results, including model-independent limits on the number of non-standard model events for a set of simplified, inclusive search regions.

Search for direct production of supersymmetric partners of the top quark in the all-jets final state in proton-proton collisions at sqrt{s}=13 TeV

A search for direct production of top squark pairs in events with jets and large transverse momentum imbalance is presented. The data are based on proton-proton collisions at a center-of-mass energy of 13 TeV, collected with the CMS detector in 2016 at the CERN LHC, and correspond to an integrated luminosity of 35.9 fb−1. The search considers a variety of R-parity conserving supersymmetric models, including ones for which the top squark and neutralino masses are nearly degenerate. Specialized jet reconstruction tools are developed to exploit the unique characteristics of the signal topologies. With no significant excess of events observed above the standard model expectations, upper limits are set on the direct top squark pair production cross section in the context of simplified supersymmetric models for various decay hypotheses. Models with larger differences in mass between the top squark and neutralino are probed for masses up to 1040 and 500 GeV, respectively, whereas models with a more compressed mass hierarchy are probed up to 660 and 610 GeV, respectively. The smallest mass difference probed is for masses near to 550 and 540 GeV, respectively.

Search for top squark pair production in pp collisions at sqrt{s}=13 TeV using single lepton events

A search for top squark pair production in pp collisions at sqrt{s}=13 TeV is performed using events with a single isolated electron or muon, jets, and a large transverse momentum imbalance. The results are based on data collected in 2016 with the CMS detector at the LHC, corresponding to an integrated luminosity of 35.9 fb−1. No significant excess of events is observed above the expectation from standard model processes. Exclusion limits are set in the context of supersymmetric models of pair production of top squarks that decay either to a top quark and a neutralino or to a bottom quark and a chargino. Depending on the details of the model, we exclude top squarks with masses as high as 1120 GeV. Detailed information is also provided to facilitate theoretical interpretations in other scenarios of physics beyond the standard model.

Search for new phenomena with multiple charged leptons in proton\u2013proton collisions at sqrt{s}= 13,text {TeV}

Results are reported from a search for physics beyond the standard model in final states with at least three charged leptons, in any combination of electrons or muons. The data sample corresponds to an integrated luminosity of 2.3,text {fb}^{-1} of proton–proton collisions at sqrt{s} =13,text {TeV}, recorded by the CMS experiment at the LHC in 2015. Two jets are required in each event, providing good sensitivity to strong production of gluinos and squarks. The search regions, sensitive to a range of different new physics scenarios, are defined using the number of jets tagged as originating from bottom quarks, the sum of the magnitudes of the transverse momenta of the jets, the imbalance in the overall transverse momentum in the event, and the invariant mass of opposite-sign, same-flavor lepton pairs. The event yields observed in data are consistent with the expected background contributions from standard model processes. These results are used to derive limits in terms of R-parity conserving simplified models of supersymmetry that describe strong production of gluinos and squarks. Model-independent limits are presented to facilitate the reinterpretation of the results in a broad range of scenarios for physics beyond the standard model.

Search for supersymmetry in the all-hadronic final state using top quark tagging in pp collisions at s=13 TeV

A search is presented for supersymmetry in all-hadronic events with missing transverse momentum and tagged top quarks. The data sample was collected by the CMS detector at the LHC and corresponds to an integrated luminosity of 2.3 inverse femtobarns of proton-proton collisions at a center-of-mass energy of 13 TeV. Search regions are defined using the properties of reconstructed jets, the multiplicity of bottom and top quark candidates, and an imbalance in transverse momentum. With no statistically significant excess of events observed beyond the expected contributions from the standard model, we set exclusion limits at 95% confidence level on the masses of new particles in the context of simplified models of direct and gluino-mediated top squark production. For direct top squark production with decays to a top quark and a neutralino, top squark masses up to 740 GeV and neutralino masses up to 240 GeV are excluded. Gluino masses up to 1550 GeV and neutralino masses up to 900 GeV are excluded for a gluino-mediated production case, where each of the pair-produced gluinos decays to a top-antitop quark pair and a neutralino.

Vector-boson-tagged jet production in heavy ion collisions at energies available at the CERN Large Hadron Collider

Vector boson-tagged jet production in collisions of heavy nuclei opens new opportunities to study parton shower formation and propagation in strongly interacting matter. It has been argued to provide a golden channel that can constrain the energy loss of jets in the quark-gluon plasma created in heavy ion reactions. We present theoretical results for isolated photon-tagged and $Z^0$ boson-tagged jet production in Pb+Pb collisions with $\sqrt{s_{NN}} = 5.02$ TeV at the LHC. Specifically, we evaluate the transverse momentum imbalance ${\rm x_{JV}}$ distribution and nuclear modification factor ${\rm I_{AA}}$ of tagged jets and compare our theoretical calculations to recent experimental measurements by ATLAS and CMS collaborations. Our analysis, which includes both collisional and radiative energy losses, sheds light on their relative importance versus the strength of jet-medium interactions and helps quantify the amount of out-of-cone radiation of predominantly prompt quark-initiated jets.

Searches for pair production of third-generation squarks in sqrt{s}=13,text {TeV} pp collisions

Searches are presented for direct production of top or bottom squark pairs in proton–proton collisions at the CERN LHC. Two searches, based on complementary techniques, are performed in all-jet final states that are characterized by a significant imbalance in transverse momentum. An additional search requires the presence of a charged lepton isolated from other activity in the event. The data were collected in 2015 at a centre-of-mass energy of 13,text {TeV} with the CMS detector and correspond to an integrated luminosity of 2.3,text {fb}^{-1}. No statistically significant excess of events is found beyond the expected contribution from standard model processes. Exclusion limits are set in the context of simplified models of top or bottom squark pair production. Models with top and bottom squark masses up to 830 and 890,text {GeV}, respectively, are probed for light neutralinos. For models with top squark masses of 675,text {GeV}, neutralino masses up to 260,text {GeV} are excluded at 95% confidence level.

A search for new phenomena in pp collisions at sqrt{s} = 13,text {TeV} in final states with missing transverse momentum and at least one jet using the alpha _{mathrm {T}} variable

A search for new phenomena is performed in final states containing one or more jets and an imbalance in transverse momentum in pp collisions at a centre-of-mass energy of 13,text {TeV}. The analysed data sample, recorded with the CMS detector at the CERN LHC, corresponds to an integrated luminosity of 2.3,text {fb}^{-1}. Several kinematic variables are employed to suppress the dominant background, multijet production, as well as to discriminate between other standard model and new physics processes. The search provides sensitivity to a broad range of new-physics models that yield a stable weakly interacting massive particle. The number of observed candidate events is found to agree with the expected contributions from standard model processes, and the result is interpreted in the mass parameter space of fourteen simplified supersymmetric models that assume the pair production of gluinos or squarks and a range of decay modes. For models that assume gluino pair production, masses up to 1575 and 975,text {GeV} are excluded for gluinos and neutralinos, respectively. For models involving the pair production of top squarks and compressed mass spectra, top squark masses up to 400,text {GeV} are excluded.

Associated production of a dilepton and a ϒ(J/ψ) at the LHC as a probe of gluon transverse momentum dependent distributions

We discuss the impact on the study of gluon transverse momentum dependent distributions (TMDs) of the associated production of a lepton pair and a ϒ (or a J/ψ) in unpolarised proton–proton collisions, pp→Qℓℓ¯X, at LHC energies, where one can assume that such final states are dominantly induced by gluon fusion. If the transverse momentum of the quarkonium–dilepton system – namely, the transverse momentum imbalance of the quarkonium state and the lepton pair – is small, the corresponding cross sections can be calculated within the framework of TMD factorisation. Using the helicity formalism, we show in detail how these cross sections are connected to the moments of two independent TMDs: the distribution of unpolarised gluons, f1g, and the distribution of linearly polarised gluons, h1⊥g. We complete our exhaustive derivation of these general relations with a phenomenological analysis of the feasibility of the TMD extraction, as well as some outlooks.

Search for top squark pair production in compressed-mass-spectrum scenarios in proton–proton collisions at s=8 TeV using the αT variable

An inclusive search is performed for supersymmetry in final states containing jets and an apparent imbalance in transverse momentum, pt[miss], due to the production of unobserved weakly interacting particles in pp collisions at a centre-of-mass energy of 8 TeV. The data, recorded with the CMS detector at the CERN LHC, correspond to an integrated luminosity of 18.5 inverse femtobarns. The dimensionless kinematic variable alpha[T] is used to discriminate between events with genuine pt[miss] associated with unobserved particles and spurious values of pt[miss] arising from jet energy mismeasurements. No excess of event yields above the expected standard model backgrounds is observed. The results are interpreted in terms of constraints on the parameter space of several simplified models of supersymmetry that assume the pair production of top squarks. The search provides sensitivity to a broad range of top squark decay modes, including the two-body decay top squark to c chi0, where c is a charm quark and chi0 is the lightest neutralino, as well as the four-body decay top squark to b f f' chi0, where b is a bottom quark and f and f' are fermions produced in the decay of an intermediate off-shell W boson. These modes dominate in scenarios in which the top squark and lightest neutralino are nearly degenerate in mass. For these modes, top squarks with masses as large as 260 and 230 GeV are excluded, respectively, for the two- and four-body decays.

A search for new phenomena in pp collisions at $\sqrt {s} = 13$ TeV in final states with missing transverse momentum and at least one jet using the $α_{T}$ variable

A search for new phenomena is performed in final states containing one or more jets and an imbalance in transverse momentum in pp collisions at a centre-of-mass energy of 13TeV. The analysed data sample, recorded with the CMS detector at the CERN LHC, corresponds to an integrated luminosity of 2.3 $fb^{−1}$. Several kinematic variables are employed to suppress the dominant background, multijet production, as well as to discriminate between other standard model and new physics processes. The search provides sensitivity to a broad range of new-physics models that yield a stable weakly interacting massive particle. The number of observed candidate events is found to agree with the expected contributions from standard model processes, and the result is interpreted in the mass parameter space of fourteen simplified supersymmetric models that assume the pair production of gluinos or squarks and a range of decay modes. For models that assume gluino pair production, masses up to 1575 and 975GeV are excluded for gluinos and neutralinos, respectively. For models involving the pair production of top squarks and compressed mass spectra, top squark masses up to 400GeV are excluded.