Search For Heavy Particles Research Articles

Search for heavy particles in the b -tagged dijet mass distribution with additional b -tagged jets in proton-proton collisions at s=13 TeV with the ATLAS experiment

A search optimized for new heavy particles decaying to two b-quarks and produced in association with additional b-quarks is reported. The sensitivity is improved by b-tagging at least one lower-pT jet in addition to the two highest-pT jets. The data used in this search correspond to an integrated luminosity of 103 fb−1 collected with a dedicated trijet trigger during the 2017 and 2018 s=13 TeV proton-proton collision runs with the ATLAS detector at the LHC. The search looks for resonant peaks in the b-tagged dijet invariant mass spectrum over a smoothly falling background. The background is estimated with an innovative data-driven method based on orthonormal functions. The observed b-tagged dijet invariant mass spectrum is compatible with the background-only hypothesis. Upper limits at 95% confidence level on a heavy vector-boson production cross section times branching ratio to a pair of b-quarks are derived.Received 23 August 2021Accepted 11 November 2021DOI:https://doi.org/10.1103/PhysRevD.105.012001Published 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 ATLAS CollaborationPhysics Subject Headings (PhySH)Research AreasExtensions of Higgs sectorExtensions of gauge sectorParticles & Fields

Search for heavy particles decaying into a top-quark pair in the fully hadronic final state in pp collisions at s=13 TeV with the ATLAS detector

A search for new particles decaying into a pair of top quarks is performed using proton-proton collision data recorded with the ATLAS detector at the Large Hadron Collider at a center-of-mass energy of $\sqrt{s} = $13 TeV corresponding to an integrated luminosity of 36.1 fb$^{-1}$. Events consistent with top-quark pair production and the fully hadronic decay mode of the top quarks are selected by requiring multiple high transverse momentum jets including those containing $b$-hadrons. Two analysis techniques, exploiting dedicated top-quark pair reconstruction in different kinematic regimes, are used to optimize the search sensitivity to new hypothetical particles over a wide mass range. The invariant mass distribution of the two reconstructed top-quark candidates is examined for resonant production of new particles with various spins and decay widths. No significant deviation from the Standard Model prediction is observed and limits are set on the production cross-section times branching fraction for new hypothetical $Z'$ bosons, dark-matter mediators, Kaluza-Klein gravitons and Kaluza-Klein gluons. By comparing with the predicted production cross-sections, the $Z'$ boson in the topcolor-assisted-technicolor model is excluded for masses up to 3.1$-$3.6 TeV, the dark-matter mediators in a simplified framework are excluded in the mass ranges from 0.8 TeV to 0.9 TeV and from 2.0 TeV to 2.2 TeV, and the Kaluza-Klein gluon is excluded for masses up to 3.4 TeV, depending on the decay widths of the particles.

Search for heavy particles decaying into top-quark pairs using lepton-plus-jets events in proton\u2013proton collisions at sqrt{s} = 13\xa0text {TeV} with the ATLAS detector

A search for new heavy particles that decay into top-quark pairs is performed using data collected from proton–proton collisions at a centre-of-mass energy of 13 text {TeV} by the ATLAS detector at the Large Hadron Collider. The integrated luminosity of the data sample is 36.1 fb^{-1}. Events consistent with top-quark pair production are selected by requiring a single isolated charged lepton, missing transverse momentum and jet activity compatible with a hadronic top-quark decay. Jets identified as likely to contain b-hadrons are required to reduce the background from other Standard Model processes. The invariant mass spectrum of the candidate top-quark pairs is examined for local excesses above the background expectation. No significant deviations from the Standard Model predictions are found. Exclusion limits are set on the production cross-section times branching ratio for hypothetical Z' bosons, Kaluza–Kein gluons and Kaluza–Klein gravitons that decay into top-quark pairs.

Interpreting the [formula omitted] anomaly in the colored Zee–Babu model

We consider the feasibility of interpreting the RK(⁎) anomaly in the colored Zee–Babu model. The model generates neutrino masses at two loops with the help of a scalar leptoquark S∼(3,3,−13) and a scalar diquark ω∼(6,1,−23), and contributes to the transition b→sℓ−ℓ+ via the exchange of a leptoquark S at tree level. Under constraints from lepton flavor violating (LFV) and flavor changing neutral current (FCNC) processes, and direct collider searches for heavy particles, we acquire certain parameter space that can accommodate the RK(⁎) anomaly for both normal (NH) and inverted (IH) hierarchies of neutrino masses. We further examine the LFV decays of the B meson, and find a strong correlation with the neutrino mass hierarchy, i.e., Br(B+→K+μ±τ∓)≳Br(B+→K+μ±e∓)≈Br(B+→K+τ±e∓) for NH, while Br(B+→K+μ±τ∓)≪Br(B+→K+μ±e∓)≈Br(B+→K+τ±e∓) for IH. Among these decays, only B+→K+μ±e∓ in the case of NH is promising at the LHCb RUN II, while for IH all LFV decays are hard to detect in the near future.

Kaluza-Klein gravitons at LHC2

In this work we study constraints from new searches for heavy particles at the LHC on the allowed masses and couplings of a KK Graviton in a holographic composite Higgs model. Keeping new electroweak states heavy such that electroweak precision tests are satisfied, we control the mass of the lightest KK graviton using a brane kinetic term. With this we study KK graviton masses from 0.5-3 TeV. In our analysis we also employ Little Randall-Sundrum (RS) Models, characterised by a lower UV scale in the 5D model which in turn implies modified couplings to massless bulk fields. Viewing this scenario as a strongly coupled 4D theory with a composite Higgs boson, the KK graviton is interpreted as a composite spin-2 state and the varying UV scale corresponds to a varying intermediate scale between the cutoff of the low energy effective theory and the Planck scale. We find that KK gravitons with masses in the range 0.5-3 TeV are compatible with current collider constraints, where the most promising channels for detecting these states are the di-photon and ZZ channels. A detection is more likely in the little RS models, in which the dual gauge theory has a larger number of colours than in traditional RS models.

Searches for long lived heavy particles at LHCb

Its forward acceptance and excellent vertex resolution allow LHCb to perform competitive searches for heavy particles beyond the Standard Model. A search for Hidden Valley particles with the LHCb detector using 35.8 pb-1 of data at s=7 TeV is reported and prospects for searches of the stau particle presented.

Search for diphoton resonances in the mass range from 150 to 850 GeV in pp collisions at [formula omitted

Results are presented of a search for heavy particles decaying into two photons. The analysis is based on a 19.7fb−1 sample of proton–proton collisions at s=8TeV collected with the CMS detector at the CERN LHC. The diphoton mass spectrum from 150 to 850GeV is used to search for an excess of events over the background. The search is extended to new resonances with natural widths of up to 10% of the mass value. No evidence for new particle production is observed and limits at 95% confidence level on the production cross section times branching fraction to diphotons are determined. These limits are interpreted in terms of two-Higgs-doublet model parameters.

Parton densities from LHC vector boson production at small and large transverse momenta

The parton densities of the proton are of fundamental importance not only for our description of hadronic and nuclear structure, but also for reliable predictions for new heavy-particle searches at colliders. At the large partonic momentum fractions required for the production of these particles, the parton distribution functions\char22{}in particular, that of the gluon\char22{}are unfortunately still badly constrained. In this paper, we investigate the possibility to improve on their determination with new data coming from electroweak vector boson production at large transverse momenta at the LHC with center-of-mass energies of 7, 8, or 14 TeV. We demonstrate that this process is dominated by quark-gluon scattering, that theoretical predictions can be reliably made on the basis of next-to-leading order perturbation theory and its resummation, and that these data should thus be used in global fits. We also point out that the nonperturbative parameters determined from Tevatron Run 1 $Z$-boson data at low ${p}_{T}$ describe very well the new LHC data at $\sqrt{s}=7\text{ }\text{ }\mathrm{TeV}$.

Searches for new physics with tau leptons at the ATLAS detector

Searches for heavy particles decaying to a pair of tau leptons, and searches for pair produced third- generation scalar leptoquarks decaying to a b-jet and a tau lepton are reported. The studies use the full data sample recorded in 2011 at √ s = 7TeV centre-of-mass energy by the ATLAS experiment at the LHC.

On jet mass distributions in Z+jet and dijet processes at the LHC

The mass distribution of jets produced in hard processes at the LHC plays an important role in several jet substructure related studies involving both Standard Model and BSM physics, especially in the context of boosted heavy particle searches. We compute analytically the jet-mass distribution for both Z+jet and dijet processes, for QCD jets defined in the anti-k_t algorithm with an arbitrary radius R, to next-to-leading logarithmic accuracy and match our resummed calculation to full leading-order results. We note the important role played by initial state radiation (ISR) and non-global logarithms explicitly computed here for the first time for hadron collider observables, as well as the jet radius dependence of these effects. We also compare our results to standard Monte Carlo event generators and discuss directions for further studies and phenomenology.

Recombination algorithms and jet substructure: Pruning as a tool for heavy particle searches

We discuss jet substructure in recombination algorithms for QCD jets and single jets from heavy particle decays. We demonstrate that the jet algorithm can introduce significant systematic effects into the substructure. By characterizing these systematic effects and the substructure from QCD, splash-in, and heavy particle decays, we identify a technique, pruning, to better identify heavy particle decays into single jets and distinguish them from QCD jets. Pruning removes protojets typical of soft, wide angle radiation, improves the mass resolution of jets reconstructing a heavy particle decay, and decreases the QCD background. We show that pruning provides significant improvements over unpruned jets in identifying top quarks and W bosons and separating them from a QCD background, and may be useful in a search for heavy particles.

Techniques for improved heavy particle searches with jet substructure

We present a generic method for improving the effectiveness of heavy particle searches in hadronic channels at the Large Hadron Collider. By selectively removing, or pruning, protojets from the substructure provided by a k{sub T}-type jet algorithm, we improve the mass resolution for heavy decays and decrease the QCD background. We show that the protojets removed are typical of soft radiation and underlying event contributions, and atypical of accurately reconstructed heavy particles.

SEARCH FOR HEAVY PARTICLES OF POSITIVE SINGLE CHARGE IN HYDROGEN AND DEUTERIUM

在电磁分离器上用测量粒子荷质比的方法, 在氢和氘样品中寻找正的单电荷重粒子. 实验中用吸收法排除杂质本底. 用特制的薄窗正比计数管记录单个粒子, 并且测量粒子的能量。实验扫描了质量从65 到220 质子质量的重质量区域. 扫描的结果给出: 天然水中正的单电荷重粒子浓度的上限, 在质量65 到148质子质量范围内及148 到220 质子质量范围内, 分别小于2 x 10**-17, 重粒子/ 氢原子及5 x 10**-17 重粒子/ 氢原子。

Search for heavy particles in cosmic rays

A search for heavy, long-lived particles has been made in the cosmic rays. The observations may possibly be explained in terms of known particles, but they appear to be more easily explained in terms of a flux ⪆ 6.10 −9 cm −2 sec −1 sr −1 if singly charged particles having mass ⪆ 6 m p .

Search for Massive Particles in Cosmic Rays

A search for heavy particles with a mass greater than that of a nucleon, the existence of which has been suggested by higher-symmetry schemes, was performed with an apparatus set up at Echo Lake, Colorado (elevation 10 600 ft). The search was sensitive to strongly interacting particles with masses in the range 5-15 BeV, with no restriction imposed on their electric charge. The method used was to measure the time interval between the arrival of strongly interacting particles and accompanying air shower particles. This information, coupled with information from a measurement of the particle's energy and range of absorption in a total absorption spectrometer, enabled a distinction to be made between massive elementary particles, nucleons, and nuclei. In an operating period of 1542 h and with an aperture of 0.78 ${\mathrm{m}}^{2}$ sr, one delayed event was found whose behavior in the total absorption spectrometer was atypical of a nucleon or nucleus. If one considers this event to represent the arrival of a massive particle, then its mass, calculated assuming that its production occurred 1 km above the apparatus, is approximately 6.5 BeV. This one event corresponds to a flux of the order of ${10}^{\ensuremath{-}10}$ ${({\mathrm{cm}}^{2}\phantom{\rule{0ex}{0ex}}\mathrm{s}\mathrm{e}\mathrm{c}\phantom{\rule{0ex}{0ex}}\mathrm{s}\mathrm{r})}^{\ensuremath{-}1}$, where a correction for detection efficiency has been included. As there is also an 8% probability that this event was a nucleon, we do not regard this as significant evidence for the existence of a massive elementary particle, but rather as setting an effective upper limit to the flux of such particles.