Large Missing Transverse Energy Research Articles

Discovery and Exclusion Prospects for Staus Produced by Heavy Higgs Boson Decays at the LHC

In a previous work, we developed a search strategy for staus produced by the decay of the heavy CP-even Higgs bosonHwithin the context of the largetanβregime of the minimal supersymmetric standard model (MSSM) in a scenario of large stau mixing. Here, we study the performance of such search strategy by confronting it with the complementary mixing pattern in which decays of both the CP-even and CP-odd heavy Higgs bosons contribute to the production ofτ~1τ~2∗+c.cpairs. Again, we focus on final states with two opposite-sign tau leptons and large missing transverse energy. We find that our proposed search strategy, although optimized for the large stau mixing scenario, is still quite sensitive to the complementary mixing pattern. We also extend the results reported in the preceding work for the large mixing scenario by including now the exclusion limits at the next run of the LHC and the prospects both for exclusion and discovery in a potential high-luminosity phase. Finally, we discuss the possibility to distinguish the two mixing scenarios when they share the same relevant mass spectrum and both reach the discovery level with our search strategy.

Spotting hidden sectors with Higgs binoculars

We explore signals of new physics with two Higgs bosons and large missing transverse energy at the LHC. Such a signature is characteristic of models for dark matter or other secluded particles that couple to the standard model through an extended scalar sector. Our goal is to provide search strategies and an interpretation framework for this new signature that are applicable to a large class of models. To this end, we define simplified models of hidden sectors leading to two different event topologies: symmetric decay, i.e., pair-produced mediators decaying each into a Higgs plus invisible final state; and di-Higgs resonance, i.e., resonant Higgs-pair production recoiling against a pair of invisible particles. For both scenarios, we optimize the discovery potential by performing a multi-variate analysis of final states with four bottom quarks and missing energy, employing state-of-the-art machine learning algorithms for signal-background discrimination. We determine the parameter space that the LHC can test in both scenarios, thus facilitating an interpretation of our results in terms of complete models. Di-Higgs production with missing energy is competitive with other missing energy searches and thus provides a new opportunity to find hidden particles at the LHC.

Potential discovery of staus through heavy Higgs boson decays at the LHC

In this work we present a new search strategy for the discovery of staus at the LHC in the context of the minimal supersymmetric standard model. The search profits from the large s-channel b-quark annihilation production of the heavy CP-even and CP-odd Higgs bosons (H/A) which can be attained in regions of tan β ≫ 1 that avoid the stringent H/A → τ+τ− searches via decays into stau pairs. We also focus on regions where the staus branching ratios are dominated by the decays into a tau lepton and the lightest neutralino. Thus the experimental signature consists of final states made up of a tau-lepton pair plus large missing transverse energy. We take advantage of the large stau-pair production cross sections via heavy Higgs boson decays, which are between one or two orders of magnitude larger than the usual electroweak production cross sections for staus. A set of basic cuts allow us to obtain significances of the signal over the SM backgrounds at the discovery level (5 standard deviations) in the next LHC run with a center-of-mass energy of 14 TeV and a total integrated luminosity of only 100 fb−1.

Natural SUSY at LHC with right-sneutrino LSP

We study an extension of the minimal supersymmetric standard model (MSSM) with additional right-handed singlet neutrino superfields. While such an extension incorporates a mechanism for the neutrino mass, it also opens up the possibility of having the right-sneutrinos ( tilde{nu} ) as the lightest supersymmetric particle (LSP). In this work, we focus on the viability of rather small (≲500 GeV) higgsino mass parameter (μ), an important ingredient for “naturalness”, in the presence of such a LSP. For simplicity, we assume that the bino and wino mass parameters are much heavier; thus we only consider (almost) pure and compressed higgsino-like states, with small mathcal{O}left(1{0}^{-2}right) gaugino admixture which nevertheless still affect the decay of the low-lying higgsino-like states, thus significantly affecting the proposed signatures at colliders. Considering only prompt decays of the higgino-like states, especially the lightest chargino, we discuss the importance of leptonic channels consisting of up to two leptons with large missing transverse energy to probe this scenario at the Large Hadron Collider (LHC). In addition we also comment on the dark matter predictions for the studied scenario.

Probing the dark sector through mono-Z boson leptonic decays

Collider search for dark matter production has been performed over the years based on high pT standard model signatures balanced by large missing transverse energy. The mono-Z boson production with leptonic decay has a clean signature with the advantage that the decaying electrons and muons can be precisely measured. This signature not only enables reconstruction of the Z boson rest frame, but also makes possible recovery of the underlying production dynamics through the decaying lepton angular distribution. In this work, we exploit full information carried by the leptonic Z boson decays to set limits on coupling strength parameters of the dark sector. We study simplified dark sector models with scalar, vector, and tensor mediators and observe among them different signatures in the distribution of angular coefficients. Specifically, we show that angular coefficients can be used to distinguish different scenarios of the spin-0 and spin-1 models, including the ones with parity-odd and charge conjugation parity-odd operators. To maximize the statistical power, we perform a matrix element method study with a dynamic construction of event likelihood function. We parametrize the test statistic such that sensitivity from the matrix element is quantified through a term measuring the shape difference. Our results show that the shape differences provide significant improvements in the limits, especially for the scalar mediator models. We also present an example application of a matrix-element-kinematic-discriminator, an easier approach that is applicable for experimental data.

Search for Heavy Stops with Merged Top Jets

We study an interesting region of phase space at the LHC for pair-produced stops decaying into hadronic top quarks and light neutralinos. After imposing a sizeable cut on the missing transverse energy, which is the key variable for reducing backgrounds, we have found that the two hadronic tops are likely to merge into a single fat jet. We develop a jet-substructure-based strategy to tag the two merged top-jets and utilize the MT2 variable to further reduce the backgrounds. We obtain about a 50% increase to the ratio of the signal over background and a mild increase on the signal discovery significance, based on a signal with a 1.2 TeV stop and a 100 GeV neutralino, for the 13 TeV LHC with 100 fb$^{-1}$. The general event kinematics could also occur and be explored for other new physics signatures with large missing transverse energy.

Measurement of the [Formula: see text] cross section in pp collisions at [Formula: see text] TeVand limits on anomalous gauge couplings.

A measurement of the W boson pair production cross section in proton-proton collisions at sqrt{s} = 8 TeV is presented. The data collected with the CMS detector at the LHC correspond to an integrated luminosity of 19.4,text {fb}^text {-1}. The {{mathrm{W} }^{+} }mathrm{W}^{-} candidates are selected from events with two charged leptons, electrons or muons, and large missing transverse energy. The measured {{mathrm{W} }^{+} }mathrm{W}^{-} cross section is 60.1pm 0.9,text {(stat)} pm 3.2,text {(exp)} pm 3.1,text {(theo)} pm 1.6,text {(lumi)} text {,pb} = 60.1pm 4.8text {,pb} , consistent with the standard model prediction. The {{mathrm{W} }^{+} }mathrm{W}^{-} cross sections are also measured in two different fiducial phase space regions. The normalized differential cross section is measured as a function of kinematic variables of the final-state charged leptons and compared with several perturbative QCD predictions. Limits on anomalous gauge couplings associated with dimension-six operators are also given in the framework of an effective field theory. The corresponding 95 % confidence level intervals are -5.7< c_{mathrm {WWW}}/Lambda ^2 < 5.9,mathrm{TeV}^{-2}, -11.4< c_{mathrm {W}}/Lambda ^2 < 5.4,mathrm{TeV}^{-2}, -29.2< c_{mathrm {B}}/Lambda ^2 < 23.9,mathrm{TeV}^{-2}, in the HISZ basis.

Search for supersymmetry in electroweak production with photons and large missing transverse energy in pp collisions at s=8TeV

Results are reported from a search for supersymmetry with gauge-mediated supersymmetry breaking in electroweak production. Final states with photons and large missing transverse energy (MET) were examined. The data sample was collected in pp collisions at sqrt(s) = 8 TeV with the CMS detector at the LHC and corresponds to 7.4 inverse femtobarns. The analysis focuses on scenarios in which the lightest neutralino has bino- or wino-like components, resulting in decays to photons and gravitinos, where the gravitinos escape undetected. The data were obtained using a specially designed trigger with dedicated low thresholds, providing good sensitivity to signatures with photons, MET, and low hadronic energy. No excess of events over the standard model expectation is observed. The results are interpreted using the model of general gauge mediation. With the wino mass fixed at 10 GeV above that of the bino, wino masses below 710 GeV are excluded at 95% confidence level. Constraints are also set in the context of two simplified models, for which the analysis sets the lowest cross section limits on the electroweak production of supersymmetric particles.

Search for a light charged Higgs boson decaying to c s ¯ $$ \mathrm{c}\overline{\mathrm{s}} $$ in pp collisions at s = 8 $$ \sqrt{s}=8 $$ TeV

A search for a light charged Higgs boson, originating from the decay of a top quark and subsequently decaying into a charm quark and a strange antiquark, is presented. The data used in the analysis correspond to an integrated luminosity of 19.7 inverse femtobarns recorded in proton-proton collisions at sqrt(s) = 8 TeV by the CMS experiment at the LHC. The search is performed in the process t t-bar to W+/- b H-/+ b-bar, where the W boson decays to a lepton (electron or muon) and a neutrino. The decays lead to a final state comprising an isolated lepton, at least four jets and large missing transverse energy. No significant deviation is observed in the data with respect to the standard model predictions, and model-independent upper limits are set on the branching fraction B(t to H+ b), ranging from 1.2 to 6.5% for a charged Higgs boson with mass between 90 and 160 GeV, under the assumption that B(H+ to c s-bar) = 100%.

Search for direct production of pairs of stops at [formula omitted] by the CMS experiment

We present the results of the search for the production of pairs of stops with final states consisting of a single isolated lepton, large missing transverse energy and at least four jets, performed by the CMS experiment. The data sample studied corresponds to an integrated luminosity of 19.5 fb-1 of proton-proton collisions at a center of mass energy of s=8 TeV. No excess above the standard model background expectation is observed. The results are used to exclude stop masses up to 650 GeV, for neutralino masses below 250 GeV, within the context of supersymmetric models that predict production of pairs of stops that decay either to a top quark and a neutralino or to a bottom quark and a chargino.

ATLAS Z excess in minimal supersymmetric standard model

Recently the ATLAS collaboration reported a 3 sigma excess in the search for the events containing a dilepton pair from a Z boson and large missing transverse energy. Although the excess is not sufficiently significant yet, it is quite tempting to explain this excess by a well-motivated model beyond the standard model. In this paper we study a possibility of the minimal supersymmetric standard model (MSSM) for this excess. Especially, we focus on the MSSM spectrum where the sfermions are heavier than the gauginos and Higgsinos. We show that the excess can be explained by the reasonable MSSM mass spectrum.

Search for the production of dark matter in association with top-quark pairs in the single-lepton final state in proton-proton collisions at s = 8 $$ \sqrt{s}=8 $$ TeV

A search is presented for particle dark matter produced in association with a pair of top quarks in pp collisions at a centre-of-mass energy of sqrt(s) = 8 TeV. The data were collected with the CMS detector at the LHC and correspond to an integrated luminosity of 19.7 inverse femtobarns. This search requires the presence of one lepton, multiple jets, and large missing transverse energy. No excess of events is found above the SM expectation, and upper limits are derived on the production cross section. Interpreting the findings in the context of a scalar contact interaction between fermionic dark matter particles and top quarks, lower limits on the interaction scale are set. These limits are also interpreted in terms of the dark matter-nucleon scattering cross sections for the spin-independent scalar operator and they complement direct searches for dark matter particles in the low mass region.

Search for physics beyond the standard model in final states with a lepton and missing transverse energy in proton-proton collisions ats=8 TeV

A search for new physics in proton-proton collisions having final states with an electron or muon and missing transverse energy is presented. The analysis uses data collected in 2012 with the CMS detector, at an LHC center-of-mass energy of 8 TeV, and corresponding to an integrated luminosity of 19.7 inverse femtobarns. No significant deviation of the transverse mass distribution of the charged lepton-neutrino system from the standard model prediction is found. Mass exclusion limits of up to 3.28 TeV at a 95% confidence level for a W' boson with the same couplings as that of the standard model W boson are determined. Results are also derived in the framework of split universal extra dimensions, and exclusion limits on Kaluza-Klein W[KK,2] states are found. The final state with large missing transverse energy also enables a search for dark matter production with a recoiling W boson, with limits set on the mass and the production cross section of potential candidates. Finally, limits are established for a model including interference between a left-handed W' boson and the standard model W boson, and for a compositeness model.

Hiding missing energy in missing energy

Searches for supersymmetry (SUSY) often rely on a combination of hard physics objects (jets, leptons) along with large missing transverse energy to separate New Physics from Standard Model hard processes. We consider a class of ``double-invisible'' SUSY scenarios: where squarks, stops and sbottoms have a three-body decay into two (rather than one) invisible final-state particles. This occurs naturally when the LSP carries an additional conserved quantum number under which other superpartners are not charged. In these topologies, the available energy is diluted into invisible particles, reducing the observed missing energy and visible energy. This can lead to sizable changes in the sensitivity of existing searches, dramatically changing the qualitative constraints on superpartners. In particular, for m_LSP>160 GeV, we find no robust constraints from the LHC at any squark mass for any generation, while for lighter LSPs we find significant reductions in constraints. If confirmed by a full reanalysis from the collaborations, such scenarios allow for the possibility of significantly more natural SUSY models. While not realized in the MSSM, such phenomenology occurs naturally in models with mixed sneutrinos, Dirac gauginos and NMSSM-like models.

Strategies for probing nonminimal dark sectors at colliders: The interplay between cuts and kinematic distributions

In this paper, we examine the strategies and prospects for distinguishing between traditional dark-matter models and models with non-minimal dark sectors --- including models of Dynamical Dark Matter (DDM) --- at hadron colliders. For concreteness, we focus on events with two hadronic jets and large missing transverse energy at the Large Hadron Collider (LHC). As we discuss, simple "bump-hunting" searches are not sufficient; probing non-minimal dark sectors typically requires an analysis of the actual shapes of the distributions of relevant kinematic variables. We therefore begin by identifying those kinematic variables whose distributions are particularly suited to this task. However, as we demonstrate, this then leads to a number of additional subtleties, since cuts imposed on the data for the purpose of background reduction can at the same time have the unintended consequence of distorting these distributions in unexpected ways, thereby obscuring signals of new physics. We therefore proceed to study the correlations between several of the most popular relevant kinematic variables currently on the market, and investigate how imposing cuts on one or more of these variables can impact the distributions of others. Finally, we combine our results in order to assess the prospects for distinguishing non-minimal dark sectors in this channel at the upgraded LHC.

Measurement of the W+W− cross section in pp collisions at $\sqrt{s} = 7\mbox{ TeV}$ and limits on anomalous WWγ and WWZ couplings

A measurement of W+W− production in pp collisions at $\sqrt{s} = 7~\mathrm{TeV}$ is presented. The data were collected with the CMS detector at the LHC, and correspond to an integrated luminosity of 4.92±0.11 fb−1. The W+W− candidates consist of two oppositely charged leptons, electrons or muons, accompanied by large missing transverse energy. The W+W− production cross section is measured to be 52.4±2.0 (stat.)±4.5 (syst.)±1.2 (lum.) pb. This measurement is consistent with the standard model prediction of 47.0±2.0 pb at next-to-leading order. Stringent limits on the WWγ and WWZ anomalous triple gauge-boson couplings are set.

Search for Higgs boson production in oppositely charged dilepton and missing energy final states in9.7 fb−1ofpp¯collisions ats=1.96 TeV

We present a search for Higgs boson in final states with two oppositely charged leptons and large missing transverse energy as expected in H -> WW -> lvlv decays. The events are selected from the full Run II data sample of 9.7 fb-1 of ppbar collisions collected with the D0 detector at the Fermilab Tevatron Collider at sqrt s = 1.96 TeV. To validate our search methodology, we measure the non-resonant W W production cross section and find sigma_WW = 11.6 +/- 0.7 pb, in agreement with the standard model prediction. In the Higgs boson search, no significant excess above the background expectation is observed. Upper limits at the 95% confidence level on the Higgs boson production cross section are therefore derived. Within the standard model, the Higgs boson mass range 159 < MH < 176 GeV is excluded while the expected exclusion sensitivity is 156 < MH < 172 GeV. For a mass hypothesis of MH = 125 GeV, we exclude Higgs boson production cross sections 4.1 times larger than the standard model expectation, which is compatible with the presence of a Higgs boson at this mass. Within a theoretical framework with a fourth generation of fermions, the mass range 125 < MH < 218 GeV is excluded. The search results are also interpreted in the context of fermiophobic Higgs boson couplings, which yields an exclusion of fermiophobic Higgs boson production cross sections 3.1 times larger than the expectation for MH = 125 GeV.

Probing Dark Matter at the LHC Using Vector Boson Fusion Processes

Vector boson fusion processes at the Large Hadron Collider (LHC) provide a unique opportunity to search for new physics with electroweak couplings. A feasibility study for the search of supersymmetric dark matter in the final state of two vector boson fusion jets and large missing transverse energy is presented at 14 TeV. Prospects for determining the dark matter relic density are studied for the cases of wino and bino-Higgsino dark matter. The LHC could probe wino dark matter with mass up to approximately 600 GeV with a luminosity of 1000 fb(-1).

Searches for Physics Beyond the Standard Model in Monojets- and Monophoton-Events with the ATLAS Detector

Various models for new physics not described by the Standard Model predict event topologies with large missing transverse energy due to invisible new particles. Such events can be identified in the detector if they are accompanied by an energetic photon or a jet with high transverse energy. This article presents results from searches for new physics with both signatures in proton-proton collision data with the ATLAS detector at the LHC. The focus is on the analyses using the full 2011 data set with an integrated luminosity of at a center of mass energy of TeV but the article also includes an update of the monojet analysis with of data recorded in 2012 at TeV. The results are translated into exclusion limits on parameters of different theoretical models.

Single Top Quark Measurements at the Tevatron

This paper reports the measurement of electroweak production of single top quarks in lepton plus jets final state by the DØ and CDF collaborations. At Fermilab's 1.96 TeV proton-antiproton collider, events are selected from several inverse femtobarns of data in the W+jets topology consistent with the signature of an isolated charged lepton (electron or muon), large missing transverse energy from the W boson decay and two or more jets, at least one of them is required to be identified as originating from a bottom quark. Sophisticated multivariate analysis techniques are employed to separate the small single top quark signal from background. Both experiments measure the single top production cross section in s-channel, t-channel and s + t-channel. The data is also used to extract limits on the CKM matrix element |Vtb|.