Standard Model Production Research Articles

Evidence for tWZ production in proton-proton collisions at [formula omitted] TeV in multilepton final states

The first evidence for the standard model production of a top quark in association with a W boson and a Z boson is reported. The measurement is performed in multilepton final states, where the Z boson is reconstructed via its decays to electron or muon pairs. At least one W boson, associated or from top quark decay, decays leptonically, too. The analysed data were recorded by the CMS experiment at the CERN LHC in 2016–2018 in proton-proton collisions at s=13 TeV, and correspond to an integrated luminosity of 138 fb−1. The measured cross section is 354±54(stat)±95(syst) fb, and corresponds to a statistical significance of 3.4 standard deviations.

Lepton Pair Production in the Non-Commutative Standard Model with Polarized $${{e}^{ + }}{{e}^{ - }}$$ Beams

Lepton Pair Production in the Non-Commutative Standard Model with Polarized $${{e}^{ + }}{{e}^{ - }}$$ Beams

Searches for exclusive Higgs and Z boson decays into a vector quarkonium state and a photon using 139 fb^{-1} of ATLAS sqrt{s}=13 TeV proton–proton collision data

Searches for the exclusive decays of Higgs and Z bosons into a vector quarkonium state and a photon are performed in the mu ^+mu ^-,gamma final state with a proton–proton collision data sample corresponding to an integrated luminosity of 139 fb^{-1} collected at sqrt{s}=13 TeV with the ATLAS detector at the CERN Large Hadron Collider. The observed data are compatible with the expected backgrounds. The 95% confidence-level upper limits on the branching fractions of the Higgs boson decays into J/psi ,gamma , psi (2S),gamma , and Upsilon (1S,2S,3S),gamma are found to be 2.0times 10^{-4}, 10.5times 10^{-4}, and (2.5,4.2,3.4)times 10^{-4}, respectively, assuming Standard Model production of the Higgs boson. The corresponding 95% CL upper limits on the branching fractions of the Z boson decays are 1.2times 10^{-6}, 2.4times 10^{-6}, and (1.1,1.3,2.4)times 10^{-6}. An observed 95% CL interval of (-133,175) is obtained for the kappa _c/kappa _gamma ratio of Higgs boson coupling modifiers, and a 95% CL interval of (-37,40) is obtained for kappa _b/kappa _gamma .

Search for supersymmetric sleptons and charginos with the ATLAS detector

A new search for the electroweak production of supersymmetric particles decaying into two leptons with missing transverse momentum is presented. Two simplified models are considered: direct pair production of sleptons decaying into the lightest neutralinos through leptons of the Standard Model (SM) and direct pair production of the lightest charginos decaying into the lightest neutralinos through W bosons of the SM. The analysis targets phase space regions where the difference in mass between the slepton or the lightest chargino and the lightest neutralino is close to or below the mass of the W boson. Such regions with compressed mass spectra have not been covered by any searches conducted so far due to the low cross-section of the supersymmetric signal. Therefore, improved analysis strategies are crucial to separate the supersymmetric signal from the SM backgrounds. A search for an excess of same-flavour lepton pairs in opposite-sign lepton events is made in the direct slepton pair production analysis while a machine learning approach using boosted decision trees is exploited in the chargino pair production analysis and considering both the same-flavour and different-flavour channels. No significant excesses over the expected background are observed using proton-proton collisions data collected by the ATLAS experiment at √s = 13 TeV and exclusion limits at 95% confidence level are set for each considered model. Exclusion limits are also set for selectrons and smuons separately and portions of the region excluded by the search of smuons pair production are expected to be compatible with the g-2 anomaly for small tanβ values.

Search for invisible Higgs-boson decays in events with vector-boson fusion signatures using 139 fb−1 of proton-proton data recorded by the ATLAS experiment

A direct search for Higgs bosons produced via vector-boson fusion and subsequently decaying into invisible particles is reported. The analysis uses 139 fb−1 of pp collision data at a centre-of-mass energy of sqrt{s} = 13 TeV recorded by the ATLAS detector at the LHC. The observed numbers of events are found to be in agreement with the background expectation from Standard Model processes. For a scalar Higgs boson with a mass of 125 GeV and a Standard Model production cross section, an observed upper limit of 0.145 is placed on the branching fraction of its decay into invisible particles at 95% confidence level, with an expected limit of 0.103. These results are interpreted in the context of models where the Higgs boson acts as a portal to dark matter, and limits are set on the scattering cross section of weakly interacting massive particles and nucleons. Invisible decays of additional scalar bosons with masses from 50 GeV to 2 TeV are also studied, and the derived upper limits on the cross section times branching fraction decrease with increasing mass from 1.0 pb for a scalar boson mass of 50 GeV to 0.1 pb at a mass of 2 TeV.

Search for invisible decays of the Higgs boson produced via vector boson fusion in proton-proton collisions at s=13 TeV

A search for invisible decays of the Higgs boson produced via vector boson fusion (VBF) has been performed with 101 fb−1 of proton-proton collisions delivered by the LHC at s=13 TeV and collected by the CMS detector in 2017 and 2018. The sensitivity to the VBF production mechanism is enhanced by constructing two analysis categories, one based on missing transverse momentum and a second based on the properties of jets. In addition to control regions with Z and W boson candidate events, a highly populated control region, based on the production of a photon in association with jets, is used to constrain the dominant irreducible background from the invisible decay of a Z boson produced in association with jets. The results of this search are combined with all previous measurements in the VBF topology, based on data collected in 2012 (at s=8 TeV), 2015, and 2016, corresponding to integrated luminosities of 19.7, 2.3, and 36.3 fb−1, respectively. The observed (expected) upper limit on the invisible branching fraction of the Higgs boson is found to be 0.18 (0.10) at the 95% confidence level, assuming the standard model production cross section. The results are also interpreted in the context of Higgs-portal models.6 MoreReceived 27 January 2022Accepted 14 March 2022DOI:https://doi.org/10.1103/PhysRevD.105.092007Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP3.© 2022 CERN, for the CMS CollaborationPhysics Subject Headings (PhySH)Physical SystemsHiggs bosonsW & Z bosonsTechniquesHadron collidersParticles & Fields

Triple top quark production in standard model

In this paper, the triple top quark production processes have been investigated and calculated in the scope of the Standard Model. The cross-sections for different channels are provided for the proton–proton collision energies of [Formula: see text] and 100 TeV. The importance of the electroweak contribution has been demonstrated. For the main channels, the interference between the gluon and the weak bosons mediated contributions is negative and significant, therefore the complete set of diagrams has to be taken into account. Estimated total rates are about 1.9 fb for [Formula: see text] TeV and 530 fb for [Formula: see text] TeV. A simple estimation of the uncertainty of the calculated cross-sections gives about 20%. The integrated luminosity of 3 ab[Formula: see text] at HL-LHC allows expecting about 5700 events, giving a chance to detect this rare process.

Observation of electroweak production of two jets in association with an isolated photon and missing transverse momentum, and search for a Higgs boson decaying into invisible particles at 13\xa0text {TeV} with the ATLAS detector

This paper presents a measurement of the electroweak production of two jets in association with a Zgamma pair, with the Z boson decaying into two neutrinos. It also presents a search for invisible or partially invisible decays of a Higgs boson with a mass of 125 text {GeV} produced through vector-boson fusion with a photon in the final state. These results use data from LHC proton–proton collisions at sqrt{s} = 13 text {TeV} collected with the ATLAS detector and corresponding to an integrated luminosity of 139 hbox {fb}^{-1}. The event signature, shared by all benchmark processes considered for the measurements and searches, is characterized by a significant amount of unbalanced transverse momentum and a photon in the final state, in addition to a pair of forward jets. Electroweak Zgamma production in association with two jets is observed in this final state with a significance of 5.2 (5.1 expected) standard deviations. The measured fiducial cross-section for this process is 1.31pm 0.29 fb. An observed (expected) upper limit of 0.37 (0.34^{+0.15}_{-0.10}) at 95% confidence level is set on the branching ratio of a 125 text {GeV} Higgs boson to invisible particles, assuming the Standard Model production cross-section. The signature is also interpreted in the context of decays of a Higgs boson into a photon and a dark photon. An observed (expected) 95% CL upper limit on the branching ratio for this decay is set at 0.018 (0.017^{+0.007}_{-0.005}), assuming the Standard Model production cross-section for a 125 text {GeV} Higgs boson.

Search for dark photons in Higgs boson production via vector boson fusion in proton-proton collisions at sqrt{s} = 13 TeV

A search is presented for a Higgs boson that is produced via vector boson fusion and that decays to an undetected particle and an isolated photon. The search is performed by the CMS collaboration at the LHC, using a data set corresponding to an integrated luminosity of 130 fb−1, recorded at a center-of-mass energy of 13 TeV in 2016–2018. No significant excess of events above the expectation from the standard model background is found. The results are interpreted in the context of a theoretical model in which the undetected particle is a massless dark photon. An upper limit is set on the product of the cross section for production via vector boson fusion and the branching fraction for such a Higgs boson decay, as a function of the Higgs boson mass. For a Higgs boson mass of 125 GeV, assuming the standard model production rates, the observed (expected) 95% confidence level upper limit on the branching fraction is 3.5 (2.8)%. This is the first search for such decays in the vector boson fusion channel. Combination with a previous search for Higgs bosons produced in association with a Z boson results in an observed (expected) upper limit on the branching fraction of 2.9 (2.1)% at 95% confidence level.

Observability of Production of tttt quarks at pp-colliders in new physics models

The Standard Model production of four top quarks in the process pp --> tttt at a center-of-mass energy s1/2=13 Tev. The data collected by the ATLAS detector represents an impressive study potential, with an integrated luminosity of around 139 fb-1. In this manuscript, we present the production process of four top quarks at the LHC as well as some new physics models associated with this process. These models are studied in analysis carried. Some preliminary results are presented, in particular those of a new method for estimating background noise due to false leptons developed.

Anatomy of tthh physics at the HL-LHC

The $tthh$ production at colliders contain rich information on the nature of Higgs boson. In this article, we systematically studied its physics at High-Luminosity Large Hadron Collider (HL-LHC), using exclusive channels with multiple ($\geq 5$) $b$-jets and one lepton ($5b1\ell$), multiple ($\geq 5$) $b$-jets and opposite-sign di-lepton ($5b2\ell$), same-sign di-lepton (SS2$\ell$), multiple leptons (multi-$\ell$), and di-tau resonance ($\tau\tau$). The scenarios analyzed include: (1) the $tthh$ production in Standard Model; (2) the $tthh$ production mediated by anomalous cubic Higgs self-coupling and $tthh$ contact interaction; (3) heavy Higgs ($H$) production with $tt H \to tthh$; and (4) pair production of fermionic top partners ($T$) with $T T \to tthh$. To address the complication of event topologies and the mess of combinatorial backgrounds, a tool of Boosted-Decision-Tree was applied in the analyses. The $5b1\ell$ and SS2$\ell$ analyses define the two most promising channels, resulting in slightly different sensitivities. For non-resonant $tthh$ production, a combination of these exclusive analyses allows for its measurment in the SM with a statistical significance $\sim 0.9\sigma$ (with $S/B > 1 \%$), and may assist partially breaking the sensitivity degeneracy w.r.t. the cubic Higgs self-coupling, a difficulty usually thought to exist in gluon fusion di-Higgs analysis at HL-LHC. These sensitivities were also projected to future hadron colliders at 27 TeV and 100 TeV. For resonant $tthh$ productions, the heavy Higgs boson in type II Two-Higgs-Doublet-Model could be efficiently searched for between the mass thresholds $2 m_h < m_H < 2 m_t$ and even beyond that, for relatively small $\tan\beta$, while the fermionic top partners in composite Higgs models could be probed for up to $\sim 1.5$ TeV and $\sim 1.7$ TeV, for Br$(T\to th)=25\%$ and $50\%$, respectively.

Search for production of four top quarks in final states with same-sign or multiple leptons in proton\u2013proton collisions at sqrt{s}=13,text {TeV}

The standard model (SM) production of four top quarks (text {t} {}{overline{text {t}}} text {t} {}{overline{text {t}}} ) in proton–proton collisions is studied by the CMS Collaboration. The data sample, collected during the 2016–2018 data taking of the LHC, corresponds to an integrated luminosity of 137,text {fb}^{-1} at a center-of-mass energy of 13,text {TeV}. The events are required to contain two same-sign charged leptons (electrons or muons) or at least three leptons, and jets. The observed and expected significances for the text {t} {}{overline{text {t}}} text {t} {}{overline{text {t}}} signal are respectively 2.6 and 2.7 standard deviations, and the text {t} {}{overline{text {t}}} text {t} {}{overline{text {t}}} cross section is measured to be 12.6^{+5.8}_{-5.2},text {fb} . The results are used to constrain the Yukawa coupling of the top quark to the Higgs boson, y_{text {t}}, yielding a limit of |y_{text {t}}/y_{text {t}}^{mathrm {SM}} | < 1.7 at 95% confidence level, where y_{text {t}}^{mathrm {SM}} is the SM value of y_{text {t}}. They are also used to constrain the oblique parameter of the Higgs boson in an effective field theory framework, hat{H}<0.12. Limits are set on the production of a heavy scalar or pseudoscalar boson in Type-II two-Higgs-doublet and simplified dark matter models, with exclusion limits reaching 350–470,text {GeV} and 350–550,text {GeV} for scalar and pseudoscalar bosons, respectively. Upper bounds are also set on couplings of the top quark to new light particles.

Opportunities and challenges of Standard Model production cross section measurements in proton-proton collisions at √s=8 TeV using CMS Open Data

The CMS Open Data project offers new opportunities to measure cross sections of standard model (SM) processes which have not been probed so far. We evaluate the challenges and the opportunities of the CMS Open Data project in the view of cross section measurements. In particular, we reevaluate the SM cross sections of the production of W bosons, Z bosons, top-quark pairs and WZ dibosons in several decay channels at a center of mass energy of 8 TeV with an integrated luminosity of 1.8 fb−1. These cross sections were previously measured by the ATLAS and CMS Collaborations and are used to validate our analysis and calibration strategy. The results indicate the achievable level of precision for future measurements using the CMS Open Data performed by scientists who are not members of the LHC Collaborations and hence lack detailed knowledge of experimental and detector related effects and their handling.

Search for the production of four top quarks in the single-lepton and opposite-sign dilepton final states in proton-proton collisions at sqrt{s} = 13 TeV

A search for the standard model production of four top quarks (pp → mathrm{t}overline{mathrm{t}}mathrm{t}overline{mathrm{t}} ) is reported using single-lepton plus jets and opposite-sign dilepton plus jets signatures. Proton-proton collisions are recorded with the CMS detector at the LHC at a center-of-mass energy of 13 TeV in a sample corresponding to an integrated luminosity of 35.8 fb−1. A multivariate analysis exploiting global event and jet properties is used to discriminate mathrm{t}overline{mathrm{t}}mathrm{t}overline{mathrm{t}} from mathrm{t}overline{mathrm{t}} production. No significant deviation is observed from the predicted background. An upper limit is set on the cross section for mathrm{t}overline{mathrm{t}}mathrm{t}overline{mathrm{t}} production in the standard model of 48 fb at 95% confidence level. When combined with a previous measurement by the CMS experiment from an analysis of other final states, the observed signal significance is 1.4 standard deviations, and the combined cross section measurement is {13}_{-9}^{+11} fb. The result is also interpreted in the framework of effective field theory.

Search for dark photons in decays of Higgs bosons produced in association with Z bosons in proton-proton collisions at sqrt{s} = 13 TeV

A search is presented for a Higgs boson that is produced in association with a Z boson and that decays to an undetected particle together with an isolated photon. The search is performed by the CMS Collaboration at the Large Hadron Collider using a data set corresponding to an integrated luminosity of 137 fb−1 recorded at a center-of-mass energy of 13 TeV. No significant excess of events above the expectation from the standard model background is found. The results are interpreted in the context of a theoretical model in which the undetected particle is a massless dark photon. An upper limit is set on the product of the cross section for associated Higgs and Z boson production and the branching fraction for such a Higgs boson decay, as a function of the Higgs boson mass. For a mass of 125 GeV, assuming the standard model production cross section, this corresponds to an observed (expected) upper limit on this branching fraction of 4.6 (3.6)% at 95% confidence level. These are the first limits on Higgs boson decays to final states that include an undetected massless dark photon.

Search for invisible decays of a Higgs boson produced through vector boson fusion in proton-proton collisions at [formula omitted

A search for invisible decays of a Higgs boson is performed using proton-proton collision data collected with the CMS detector at the LHC in 2016 at a center-of-mass energy s=13TeV, corresponding to an integrated luminosity of 35.9fb−1. The search targets the production of a Higgs boson via vector boson fusion. The data are found to be in agreement with the background contributions from standard model processes. An observed (expected) upper limit of 0.33(0.25), at 95% confidence level, is placed on the branching fraction of the Higgs boson decay to invisible particles, assuming standard model production rates and a Higgs boson mass of 125.09 GeV. Results from a combination of this analysis and other direct searches for invisible decays of the Higgs boson, performed using data collected at s=7, 8, and 13 TeV, are presented. An observed (expected) upper limit of 0.19(0.15), at 95% confidence level, is set on the branching fraction of invisible decays of the Higgs boson. The combined limit represents the most stringent bound on the invisible branching fraction of the Higgs boson reported to date. This result is also interpreted in the context of Higgs-portal dark matter models, in which upper bounds are placed on the spin-independent dark-matter-nucleon scattering cross section.

Boosting Hto boverline{b} with machine learning

High-pT Higgs production at hadron colliders provides a direct probe of the internal structure of the gg → H loop with the Hto boverline{b} decay offering the most statistics due to the large branching ratio. Despite the overwhelming QCD background, recent advances in jet substructure have put the observation of the ggto Hto boverline{b} channel at the LHC within the realm of possibility. In order to enhance the sensitivity to this process, we develop a two-stream convolutional neural network, with one stream acting on jet information and one using global event properties. The neural network significantly increases the discovery potential of a Higgs signal, both for high-pT Standard Model production as well for possible beyond the Standard Model contributions. Unlike most studies for boosted hadronically decaying massive particles, the boosted Higgs search is unique because double b-tagging rejects nearly all background processes that do not have two hard prongs. In this context — which goes beyond state-of-the-art two-prong tagging — the network is studied to identify the origin of the additional information leading to the increased significance. The procedures described here are also applicable to related final states where they can be used to identify additional sources of discrimination power that are not being exploited by current techniques.

Searches for exclusive Higgs and Z boson decays into J/ψ γ, ψ(2S) γ, and ϒ(nS) γ at [formula omitted] with the ATLAS detector

Searches for the exclusive decays of the Higgs and Z bosons into a J/ψ, ψ(2S), or ϒ(nS) (n=1,2,3) meson and a photon are performed with a pp collision data sample corresponding to an integrated luminosity of 36.1fb−1 collected at s=13TeV with the ATLAS detector at the CERN Large Hadron Collider. No significant excess of events is observed above the expected backgrounds, and 95% confidence-level upper limits on the branching fractions of the Higgs boson decays to J/ψγ, ψ(2S)γ, and ϒ(nS)γ of 3.5×10−4, 2.0×10−3, and (4.9,5.9,5.7)×10−4, respectively, are obtained assuming Standard Model production. The corresponding 95% confidence-level upper limits for the branching fractions of the Z boson decays are 2.3×10−6, 4.5×10−6 and (2.8,1.7,4.8)×10−6, respectively.

Search for an exotic decay of the Higgs boson to a pair of light pseudoscalars in the final state with two b quarks and two τ leptons in proton–proton collisions at [formula omitted

A search for an exotic decay of the Higgs boson to a pair of light pseudoscalar bosons is performed for the first time in the final state with two b quarks and two τ leptons. The search is motivated in the context of models of physics beyond the standard model (SM), such as two Higgs doublet models extended with a complex scalar singlet (2HDM + S), which include the next-to-minimal supersymmetric SM (NMSSM). The results are based on a data set of proton–proton collisions corresponding to an integrated luminosity of 35.9fb−1, accumulated by the CMS experiment at the LHC in 2016 at a center-of-mass energy of 13TeV. Masses of the pseudoscalar boson between 15 and 60GeV are probed, and no excess of events above the SM expectation is observed. Upper limits between 3 and 12% are set on the branching fraction B(h→aa→2τ2b) assuming the SM production of the Higgs boson. Upper limits are also set on the branching fraction of the Higgs boson to two light pseudoscalar bosons in different 2HDM + S scenarios. Assuming the SM production cross section for the Higgs boson, the upper limit on this quantity is as low as 20% for a mass of the pseudoscalar of 40GeV in the NMSSM.

Evidence for associated production of a Higgs boson with a top quark pair in final states with electrons, muons, and hadronically decaying \u03c4 leptons at sqrt{s}=13 TeV

Results of a search for the standard model Higgs boson produced in association with a top quark pair ( mathrm{t}overline{mathrm{t}}mathrm{H} ) in final states with electrons, muons, and hadronically decaying τ leptons are presented. The analyzed data set corresponds to an integrated luminosity of 35.9 fb−1 recorded in proton-proton collisions at sqrt{s}=13 TeV by the CMS experiment in 2016. The sensitivity of the search is improved by using matrix element and machine learning methods to separate the signal from backgrounds. The measured signal rate amounts to 1.23− 0.43+ 0.45 times the production rate expected in the standard model, with an observed (expected) significance of 3.2σ (2.8σ), which represents evidence for mathrm{t}overline{mathrm{t}}mathrm{H} production in those final states. An upper limit on the signal rate of 2.1 times the standard model production rate is set at 95% confidence level.