Large Missing Transverse Energy Research Articles

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|.

Search for gluino mediated bottom- and top-squark production in multijet final states in pp collisions at 8 TeV

A search for supersymmetry is presented based on events with large missing transverse energy, no isolated electron or muon, and at least three jets with one or more identified as a bottom-quark jet. A simultaneous examination is performed of the numbers of events in exclusive bins of the scalar sum of jet transverse momentum values, missing transverse energy, and bottom-quark jet multiplicity. The sample, corresponding to an integrated luminosity of 19.4 fb−1, consists of proton–proton collision data recorded at a center-of-mass energy of 8 TeV with the CMS detector at the LHC in 2012. The observed numbers of events are found to be consistent with the standard model expectation, which is evaluated with control samples in data. The results are interpreted in the context of two simplified supersymmetric scenarios in which gluino pair production is followed by the decay of each gluino to an undetected lightest supersymmetric particle and either a bottom or top quark–antiquark pair, characteristic of gluino mediated bottom- or top-squark production. Using the production cross section calculated to next-to-leading-order plus next-to-leading-logarithm accuracy, and in the limit of a massless lightest supersymmetric particle, we exclude gluinos with masses below 1170 GeV and 1020 GeV for the two scenarios, respectively.

Dark matter mass constrained by the relic abundance, direct detections, and colliders

We take into account a generic form of a Dirac fermionic dark matter (DM), which communicates with the Standard Model quarks via a scalar mediator in a model-independent way. Four special interaction scenarios are investigated, where one is parity conserving and the other three are parity violating. Three of them result in the $v$ suppressed DM-nucleon cross sections, where $v\ensuremath{\sim}{10}^{\ensuremath{-}3}c$ is the velocity of the DM in the laboratory frame. We constrain the masses of the dark matter and mediator as well as the couplings from the thermal relic abundance and the recent results of the XENON100 direct detection and collider experiments involving the following two channels: (i) monojet plus large missing transverse energy and (ii) dijet. The current monojet constraint is not stronger than that from the requirement of the correct relic density and the null result by the XENON100 direct detection. We find that the dijet resonance measurements can exclude a large part of the parameter space $({m}_{\ensuremath{\chi}},{m}_{Y})$, where the couplings for the mediator coupled to the dark matter and to the quarks are small and have roughly the same magnitude. The constraint from indirect detections and diphoton resonance searches is also briefly discussed.

The lightest massive invisible particles at the LHC

The observation of new physics events with large missing transverse energy at the LHC would potentially serve as evidence for the direct production of dark matter. A crucial step toward verifying such evidence is the measurement of the would-be dark matter mass. If, for example, the invisible particles are found to have masses consistent with zero, it may prove very challenging to ascertain whether light dark matter or neutrinos are being observed. We assume that new invisible particles are pair-produced in a tt¯-like topology and use two MT2-based methods to measure the masses of the particles associated with the missing energy. Instead of simulating events and backgrounds, we estimate the uncertainty associated with measuring the mass of the invisible particle by assuming a fixed value of the uncertainty associated with the location of the MT2 endpoint. We find that if this uncertainty associated with measuring the MT2 endpoints is, quite optimistically, O(1 GeV), the invisible particles must have masses greater than O(10 GeV) so they can be distinguished from massless ones at 95% CL. If the results from the CoGeNT, DAMA/LIBRA, and CRESST experiments have indeed revealed the existence of light dark matter with mass O(10 GeV), our results suggest that it may be difficult for the LHC to distinguish dark matter from neutrinos solely via mass measurements.

LHC missing-transverse-energy constraints on models with universal extra dimensions

We consider the performance of the ATLAS and CMS searches for events with missing transverse energy, which were originally motivated by supersymmetry, in constraining extensions of the Standard Model based on extra dimensions, in which the mass differences between recurrences at the same level are generically smaller than the mass hierarchies in typical supersymmetric models. We consider first a toy model with pair-production of a single vector-like quark U1 decaying into a spin-zero stable particle A1 and jet, exploring the sensitivity of the CMS alphaT and ATLAS meff analysis to U1 mass and the U1-A1 mass difference. For this purpose we use versions of the Delphes generic detector simulation with CMS and ATLAS cards, which have been shown to reproduce the published results of CMS and ATLAS searches for supersymmetry. We then explore the sensitivity of these searches to a specific model with two universal extra dimensions, whose signal is dominated by the pair production of quark recurrences, including searches with leptons. We find that the LHC searches have greater sensitivity to this more realistic model, due partly to the contributions of signatures with leptons, and partly to events with large missing transverse energy generated by the decays of higher-level Kaluza-Klein recurrences. We find that the CMS alphaT analysis with ~5/fb of data at 7 TeV excludes a recurrence scale of 600 GeV at a confidence level above 99%, increasing to 99.9% when combined with the CMS single-lepton search, whereas a recurrence scale of 700 GeV is disfavoured at the 72% confidence level.

Detecting light stop pairs in coannihilation scenarios at the LHC

In this work, we study the light stop pair signals at the large hadron collider (LHC) in three coannihilation scenarios. In order to yield the desired dark matter (DM) relic density, the neutralino can coannihilate with stop, chargino and stau, respectively. Signatures of the first scenario can be probed at the LHC via the associated jet production processes $pp \to j + {\tilde t} {\tilde t}^*$ by tagging an energetic mono-jet and a large missing transverse energy. The signatures of the other two scenarios can be searched via the pair production process $pp \to {\tilde t} {\tilde t}^*$ by tagging energetic b-jets in the final states and a large missing transverse energy. We find that the LHC results at 7 TeV with 5 $\mathrm{fb}^{-1}$ of data can exclude the stop mass up to 220, 380 and 220 GeV for these three scenarios, respectively. While the 20 $\mathrm{fb}^{-1}$ dataset at 8 TeV is considered, the LHC can be expected to exclude the stop mass up to 340, 430 and 370 GeV.

Search for new physics in events with photons, jets, and missing transverse energy in pp collisions at $ \sqrt{s}=7 $ TeV

Abstract A search for physics beyond the standard model involving events with one or more photons, jets, and missing transverse energy has been performed by the CMS experiment. The data sample corresponds to an integrated luminosity of 4.93 fb−1 of proton-proton collisions at $ \sqrt{s}=7 $ TeV, produced at the Large Hadron Collider. No excess of events with large missing transverse energy is observed beyond expectations from standard model processes, and upper limits on the signal production cross sections for new physics processes are set at the 95% confidence level. The results of this search are interpreted in the context of three models of new physics: a general model of gauge-mediated supersymmetry breaking, Simplified Models, and a theory involving universal extra dimensions. In the absence of evidence for new physics, exclusion regions are derived in the parameter spaces of the respective models.

Measurement of the [formula omitted] production cross section in pp collisions at [formula omitted] with lepton + jets final states

A measurement of the tt¯ production cross section in pp collisions at s=7 TeV is presented. The results are based on data corresponding to an integrated luminosity of 2.3 fb−1 collected by the CMS detector at the LHC. Selected events are required to have one isolated, high transverse momentum electron or muon, large missing transverse energy, and hadronic jets, at least one of which must be consistent with having originated from a b quark. The measured cross section is 158.1±2.1(stat.)±10.2(syst.)±3.5(lum.) pb, in agreement with standard model predictions.

Search for Stop Pair Production at the LHC using the CMS Detector

A search for pair production of top squark events in proton-proton collisions at a center-of-mass energy of √s = 8 TeV is performed in a sample with a single isolated electron or muon, jets, large missing transverse energy and large transverse mass. The data sample used corresponds to an integrated luminosity of 19.5 fb−1 collected by the CMS experiment. No significant excess in data is observed above the expectation from standard model processes. In the context of supersymmetric models with pair production of top squarks decaying to either a top quark and the lightest supersymmetric particle (LSP) or a bottom quark and a chargino that subsequently decays to a W boson and the LSP, the results probe top squarks with masses up to about 650 GeV for LSP masses up to about 250 GeV.

Search for Exotic mono-jet and mono-photon signatures with the ATLAS detector

Mono-jet and mono-photon signatures are final states in a variety of scenarios beyond the Standard Model, such as the Large Extra Dimension models, gauge-mediated SUSY breaking scenarios, and models with pair production of Weakly Interacting Massive Particles considered as dark matter candidates. The produced exotic particles do not interact with the detector, resulting in missing transverse energy. The results of searches, performed in the ATLAS experiment at the LHC, for new physics in final states with an energetic jet or photon and large missing transverse energy are presented. The mono-jet search is performed using both 4.6 fb −1 of 7 TeV and 10.5 fb −1 of 8 TeV data, while the mono-photon results correspond to 4.6 fb −1 of 7 TeV data.

Search for supersymmetry in events withb-quark jets and missing transverse energy inppcollisions at 7 TeV

Results are presented from a search for physics beyond the standard model based on events with large missing transverse energy, at least three jets, and at least one, two, or three b-quark jets. The study is performed using a sample of proton-proton collision data collected at sqrt(s) = 7 TeV with the CMS detector at the LHC in 2011. The integrated luminosity of the sample is 4.98 inverse femtobarns. The observed number of events is found to be consistent with the standard model expectation, which is evaluated using control samples in the data. The results are used to constrain cross sections for the production of supersymmetric particles decaying to b-quark-enriched final states in the context of simplified model spectra.

Search forZγevents with large missing transverse energy inpp¯collisions ats=1.96 TeV

We present the first search for supersymmetry (SUSY) in Zgamma final states with large missing transverse energy using data corresponding to an integrated luminosity of 6.2 fb-1 collected with the D0 experiment in ppbar collisions at sqrt(s)=1.96 TeV. This signature is predicted in gauge-mediated SUSY-breaking models, where the lightest neutralino is the next-to-lightest supersymmetric particle (NLSP) and is produced in pairs, possibly through decay from heavier supersymmetric particles. The NLSP can decay either to a Z boson or a photon and an associated gravitino that escapes detection. We exclude this model at the 95% C.L. for SUSY breaking scales of Lambda < 87 TeV, corresponding to neutralino masses of < 151 GeV.

Gauge-boson production with multiple jets near threshold

Signatures of new physics beyond the Standard Model are often characterized by large missing transverse energy ($\not E_T$) produced in association with multiple jets. The dominant Standard Model background to such processes comes from gauge-boson production in association with jets. A standard search strategy involves looking for an excess in the $m_{eff}$ distribution, where $m_{eff}= \not E_T +\sum_{J} p^T_J$ and $p^T_J$ denotes the transverse momentum of the $J$-th jet. The region of large $m_{eff}$ is dominated by jet production near threshold, giving rise to large Sudakov logarithms that can change the magnitude and shape of the $m_{eff}$ distribution. We present an effective theory framework for the resummation of such threshold logarithms. We perform an analysis for exclusive jet production using the N-jettiness global event shape, which allows theoretical control to also be maintained over large logarithms induced by vetoing additional jets. As a first step, we give explicit numerical results with next-to-leading-log (NLL) resummation for $pp \to \gamma + 2$ jets in the large $m_{eff}$ region.

Dipole moment dark matter at the LHC

Monojet and monophoton final states with large missing transverse energy (E̸T) are important for dark matter (DM) searches at colliders. We present analytic expressions for the differential cross sections for the parton-level processes, qq¯(qg)→g(q)χχ¯ and qq¯→γχχ¯, for a neutral DM particle with a magnetic dipole moment (MDM) or an electric dipole moment (EDM). We collectively call such DM candidates dipole moment dark matter (DMDM). We also provide monojet cross sections for scalar, vector and axial-vector interactions. We then use ATLAS/CMS monojet+E̸T data and CMS monophoton+E̸T data to constrain DMDM. We find that 7 TeV LHC bounds on the MDM DM–proton scattering cross section are about six orders of magnitude weaker than on the conventional spin-independent cross section.

Probing axino LSP from diphoton events with large missing transverse energy

In a supersymmetry model with an axino as the lightest supersymmetric particle (LSP) and a Bino as the next LSP (NLSP), supersymmetric particle production ends up with including two Binos, followed by each Binoʼs decaying into a photon and an axino. Final states are diphoton with large missing energy. In a benchmark scenario, we have comprehensively studied the implication of γγ+E̸(T) data from the ALEPH, CDF II, ATLAS and CMS experiments. No excess over the standard model backgrounds can be explained in this model if the Bino NLSP decays outside the detector. Long life time of the Bino is possible because of high Peccei–Quinn symmetry breaking scale fa. The ALEPH and CDF II data put a very strong bound on fa for light Bino case with mB˜≲150 GeV: the narrow hadronic axion window around fa∼106 GeV is completely closed. The recent ATLAS and CMS data show very interesting bound on fa≳105 GeV for the Bino mass below 700 GeV. This is already stronger than the previous laboratory bounds.

Phenomenology of light sneutrino dark matter in cMSSM/mSUGRA with inverse seesaw

We study the possibility of a light Dark Matter (DM) within a constrained Minimal Supersymmetric Standard Model (cMSSM) framework augmented by a SM singlet-pair sector to account for the non-zero neutrino masses by inverse seesaw mechanism. Working within a 'hybrid' scenario with the MSSM sector fixed at high scale and the singlet neutrino sector at low scale, we find that, contrary to the case of the usual cMSSM where the neutralino DM cannot be very light, we can have a light sneutrino DM with mass below 100 GeV satisfying all the current experimental constraints from cosmology, collider as well as low-energy experiments. We also note that the supersymmetric inverse seesaw mechanism with sneutrino as the lightest supersymmetric partner can have enhanced same-sign dilepton final states with large missing transverse energy (mET) coming from the gluino- and squark-pair as well as the squark-gluino associated productions and their cascade decay through charginos. We present a collider study for the same-sign dilepton+jets+mET signal in this scenario and propose some distinctions with the usual cMSSM. We also comment on the implications of such a light DM scenario on the invisible decay width of an 125 GeV Higgs boson.

Search for the standard model Higgs boson produced in association with aW±boson with7.5 fb−1integrated luminosity at CDF

We present a search for the standard model Higgs boson produced in association with a $W^{\pm}$ boson. This search uses data corresponding to an integrated luminosity of 7.5 fb$^{-1}$ collected by the CDF detector at the Tevatron. We select $WH \to \ell\nu b \bar{b}$ candidate events with two jets, large missing transverse energy, and exactly one charged lepton. We further require that at least one jet be identified to originate from a bottom quark. Discrimination between the signal and the large background is achieved through the use of a Bayesian artificial neural network. The number of tagged events and their distributions are consistent with the standard model expectations. We observe no evidence for a Higgs boson signal and set 95% C.L. upper limits on the $WH$ production cross section times the branching ratio to decay to $b\bar b$ pairs, $\sigma(p\bar p \rightarrow W^{\pm} H) \times {\cal B}(H\rightarrow b\bar b)$, relative to the rate predicted by the standard model. For the Higgs boson mass range of 100 GeV/c$^2$ to 150 GeV/c$^2$ we set observed (expected) upper limits from 1.34 (1.83) to 38.8 (23.4). For 115 GeV/c$^2$ the upper limit is 3.64 (2.78). The combination of the present search with an independent analysis that selects events with three jets yields more stringent limits ranging from 1.12 (1.79) to 34.4 (21.6) in the same mass range. For 115 GeV/c$^2$ the upper limit is 2.65 (2.60).

Search for Higgs boson production in oppositely charged dilepton and missing energy events inpp¯collisions ats=1.96 TeV

We present a search for the standard model Higgs boson using events with two oppositely charged leptons and large missing transverse energy as expected in $H\rightarrow WW$ decays. The events are selected from data corresponding to 8.6 \ifb\ of integrated luminosity in $p \bar{p}$ collisions at $\sqrt{s}=1.96$ TeV collected with the D0 detector at the Fermilab Tevatron Collider. No significant excess above the standard model background expectation in the Higgs boson mass range this search is sensitive to is observed, and upper limits on the Higgs boson production cross section are derived.

Reach the bottom line of the sbottom search

Abstract We propose a new search strategy for directly-produced sbottoms at the LHC with a small mass splitting between the sbottom and its decayed stable neutralino. Our search strategy is based on boosting sbottoms through an energetic initial state radiation jet. In the final state, we require a large missing transverse energy and one or two b-jets besides the initial state radiation jet. We also define a few kinematic variables to further increase the discovery reach. For the case that the sbottom mainly decays into the bottom quark and the stable neutralino, we have found that even for a mass splitting as small as 10 GeV sbottoms with masses up to around 400 GeV can be excluded at the 95 % confidence level with 20 inverse femtobarn data at the 8 TeV LHC.