Particles In Proton-proton Collisions Research Articles

Search for long-lived particles using displaced vertices and missing transverse momentum in proton-proton collisions at s=13 TeV

A search for the production of long-lived particles in proton-proton collisions at a center-of-mass energy of 13 TeV at the CERN LHC is presented. The search is based on data collected by the CMS experiment in 2016–2018, corresponding to a total integrated luminosity of 137 fb−1. This search is designed to be sensitive to long-lived particles with mean proper decay lengths between 0.1 and 1000 mm, whose decay products produce a final state with at least one displaced vertex and missing transverse momentum. A machine learning algorithm, which improves the background rejection power by more than an order of magnitude, is applied to improve the sensitivity. The observation is consistent with the standard model background prediction, and the results are used to constrain split supersymmetry (SUSY) and gauge-mediated SUSY breaking models with different gluino mean proper decay lengths and masses. This search is the first CMS search that shows sensitivity to hadronically decaying long-lived particles from signals with mass differences between the gluino and neutralino below 100 GeV. It sets the most stringent limits to date for split-SUSY models and gauge-mediated SUSY breaking models with gluino proper decay length less than 6 mm. © 2024 CERN, for the CMS Collaboration 2024 CERN

Multiplicity Dependencies of Midrapidity Transverse Momentum Distributions of Identified Charged Particles in Proton-Proton Collisions at (s)1/2 = 7 TeV at the LHC

Dependencies of midrapidity pt distributions of the charged pions and kaons, protons and antiprotons on charged-particle multiplicity density (<dNch/dη>) in inelastic proton-proton collisions at (s)1/2 = 7 TeV at the LHC, measured by ALICE Collaboration, are investigated. The simultaneous minimum χ2 fits with the Tsallis function with thermodynamical consistence and the Hagedorn function with included transverse flow have well-described the pt spectra of the particle species in the ten studied groups of charged-particle multiplicity density. The effective temperatures, T, of the Tsallis function with thermodynamical consistence have shown a steady rise with increasing the charged-particle multiplicity in proton-proton collisions at (s)1/2 = 7 TeV, in agreement with the similar result obtained recently in proton-proton collisions at (s)1/2 = 13 TeV at the LHC. The respective T versus <dNch/dη> dependence in proton-proton collisions at (s)1/2 = 7 TeV is reproduced quite well by the simple power function with the same value (≈ 1/3) of the exponent parameter as that extracted in proton-proton collisions at (s)1/2 = 13 TeV. The identical power dependence T~ε1/3 between the initial energy density and effective temperature of the system has been observed in proton-proton collisions at (s)1/2 = 7 and 13 TeV. We have observed that the transverse radial flow emerges at <dNch/dη> ≈ 6 and then increases, becoming substantial at larger multiplicity events in proton-proton collisions at (s)1/2 = 7 TeV. We have estimated, analyzing T0 and ⟨βt⟩ versus <dNch/dη> dependencies, that the possible onset of deconfinement phase transition in proton-proton collisions at (s)1/2 = 7 TeV occurs at <dNch/dη> ≈ 6.1 ± 0.3, which is close to the corresponding recent estimate (<dNch/dη> ≈ 7.1 ± 0.2) in proton-proton collisions at (s)1/2 = 13 TeV. The corresponding critical energy densities for probable onset of deconfinement phase transition in proton-proton collisions at (s)1/2 = 7 and 13 TeV at the LHC have been estimated to be 0.67 ± 0.03 and 0.76 ± 0.02 GeV/fm3, respectively.

Measurement of prompt charged-particle production in pp collisions at sqrt{mathrm{s}} = 13 TeV

The differential cross-section of prompt inclusive production of long-lived charged particles in proton-proton collisions is measured using a data sample recorded by the LHCb experiment at a centre-of-mass energy of sqrt{s} = 13 TeV. The data sample, collected with an unbiased trigger, corresponds to an integrated luminosity of 5.4 nb−1. The differential cross-section is measured as a function of transverse momentum and pseudorapidity in the ranges pT ∈ [80, 10 000) MeV/c and η ∈ [2.0, 4.8) and is determined separately for positively and negatively charged particles. The results are compared with predictions from various hadronic-interaction models.

Search for heavy long-lived multicharged particles in proton-proton collisions at s=13 TeV using the ATLAS detector

A search for heavy long-lived multi-charged particles is performed using the ATLAS detector at the LHC. Data with an integrated luminosity of 36.1 fb$^{-1}$ collected in 2015 and 2016 from proton-proton collisions at $\sqrt{s}$ = 13 TeV are examined. Particles producing anomalously high ionization, consistent with long-lived massive particles with electric charges from |q|=2e to |q|=7e, are searched for. No events are observed, and 95% confidence level cross-section upper limits are interpreted as lower mass limits for a Drell-Yan production model. Multi-charged particles with masses between 50 GeV and 980-1220 GeV (depending on their electric charge) are excluded.

Search for long-lived particles with displaced vertices in multijet events in proton-proton collisions at s=13 TeV

Results are reported from a search for long-lived particles in proton-proton collisions at $\sqrt{s} =$ 13 TeV delivered by the CERN LHC and collected by the CMS experiment. The data sample, which was recorded during 2015 and 2016, corresponds to an integrated luminosity of 38.5 fb$^{-1}$. This search uses benchmark signal models in which long-lived particles are pair-produced and each decays into two or more quarks, leading to a signal with multiple jets and two displaced vertices composed of many tracks. No events with two well-separated high-track-multiplicity vertices are observed. Upper limits are placed on models of $R$-parity violating supersymmetry in which the long-lived particles are neutralinos or gluinos decaying solely into multijet final states or top squarks decaying solely into dijet final states. For neutralino, gluino, or top squark masses between 800 and 2600 GeV and mean proper decay lengths between 1 and 40 mm, the analysis excludes cross sections above 0.3 fb at 95% confidence level. Gluino and top squark masses are excluded below 2200 and 1400 GeV, respectively, for mean proper decay lengths between 0.6 and 80 mm. A method is provided for extending the results to other models with pair-produced long-lived particles.

Search for supersymmetry in events with a \u03c4 lepton pair and missing transverse momentum in proton-proton collisions at sqrt{s}=13 TeV

A search for the electroweak production of supersymmetric particles in proton-proton collisions at a center-of-mass energy of 13 TeV is presented in final states with a τ lepton pair. Both hadronic and leptonic decay modes are considered for the τ leptons. Scenarios involving the direct pair production of τ sleptons, or their indirect production via the decays of charginos and neutralinos, are investigated. The data correspond to an integrated luminosity of 35.9 fb−1 collected with the CMS detector in 2016. The observed number of events is consistent with the standard model background expectation. The results are interpreted as upper limits on the cross section for τ slepton pair production in different scenarios. The strongest limits are observed in the scenario of a purely left-handed low mass τ slepton decaying to a nearly massless neutralino. Exclusion limits are also set in the context of simplified models of chargino-neutralino and chargino pair production with decays to τ leptons, and range up to 710 and 630 GeV, respectively.

Search for disappearing tracks as a signature of new long-lived particles in proton-proton collisions at sqrt{s}=13 TeV

A search is presented for long-lived charged particles that decay within the CMS detector and produce the signature of a disappearing track. A disappearing track is an isolated track with missing hits in the outer layers of the silicon tracker, little or no energy in associated calorimeter deposits, and no associated hits in the muon detectors. This search uses data collected with the CMS detector in 2015 and 2016 from proton-proton collisions at a center-of-mass energy of 13 TeV at the LHC, corresponding to an integrated luminosity of 38.4 fb−1. The results of the search are interpreted in the context of the anomaly-mediated supersymmetry breaking model. The data are consistent with the background-only hypothesis. Limits are set on the product of the cross section for direct production of charginos and their branching fraction to a neutralino and a pion, as a function of the chargino mass and lifetime. At 95% confidence level, charginos with masses below 715 (695) GeV are excluded for a lifetime of 3 (7) ns, as are charginos with lifetimes from 0.5 to 60 ns for a mass of 505 GeV. These are the most stringent limits using a disappearing track signature on this signal model for chargino lifetimes above ≈0.7 ns.

Experimental research of π0 meson production in U+U collision at 192 GeV

To study properties of quark-gluon plasma(QGP) is one of the main tasks of modern High Energy Physics. One of the signatures of QGP formation is jet quenching, which was observed as a suppression of particle yields (compared to the yield of the same particles in proton-proton collisions) at high transverse momentum region in central collisions of ultra-relativistic heavy nuclei. Jet quenching is the effect of the final state of heavy nuclei collision which is usually explained through parton energy loss in hot and dense medium. Experimentally the jet quenching is studied with nuclear modification factor. This paper presents invariant transverse momentum spectra and nuclear modification factors for π0 mesons in different classes of centrality in U+U collisions at √SNN=192GeV. Spectra and factors are measured in a wide pT range up to 18 GeV/c for different classes of centrality. In central U+U collisions suppression of neutral π-mesons is the same as the suppression of neutral π-mesons in Au+Au collisions at the similar collision energy and similar numbers of participants.

Search for Supersymmetry in pp Collisions at s=13 TeV in the Single-Lepton Final State Using the Sum of Masses of Large-Radius Jets

Results are reported from a search for supersymmetric particles in proton-proton collisions in the final state with a single lepton, multiple jets, including at least one b-tagged jet, and large missing transverse momentum. The search uses a sample of proton-proton collision data at s=13 TeV recorded by the CMS experiment at the LHC, corresponding to an integrated luminosity of 35.9 fb−1. The observed event yields in the signal regions are consistent with those expected from standard model backgrounds. The results are interpreted in the context of simplified models of supersymmetry involving gluino pair production, with gluino decay into either on- or off-mass-shell top squarks. Assuming that the top squarks decay into a top quark plus a stable, weakly interacting neutralino, scenarios with gluino masses up to about 1.9 TeV are excluded at 95% confidence level for neutralino masses up to about 1 TeV.Received 12 May 2017DOI:https://doi.org/10.1103/PhysRevLett.119.151802Published 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.© 2017 CERN, for the CMS CollaborationPhysics Subject Headings (PhySH)Research AreasHypothetical particle physics modelsParticle decaysSupersymmetric modelsPhysical SystemsSuperpartnersTechniquesHadron collidersParticles & Fields

Search for supersymmetry in pp collisions at s = 13 $$ \sqrt{s}=13 $$ TeV in the single-lepton final state using the sum of masses of large-radius jets

Results are reported from a search for supersymmetric particles in proton-proton collisions in the final state with a single, high transverse momentum lepton; multiple jets, including at least one b-tagged jet; and large missing transverse momentum. The data sample corresponds to an integrated luminosity of 2.3 fb−1 at $$ \sqrt{s}=13 $$ TeV, recorded by the CMS experiment at the LHC. The search focuses on processes leading to high jet multiplicities, such as gluino pair production with $$ \tilde{\mathrm{g}}\to \mathrm{t}\overline{\mathrm{t}}{\tilde{\chi}}_1^0 $$ . The quantity M J , defined as the sum of the masses of the large-radius jets in the event, is used in conjunction with other kinematic variables to provide discrimination between signal and background and as a key part of the background estimation method. The observed event yields in the signal regions in data are consistent with those expected for standard model backgrounds, estimated from control regions in data. Exclusion limits are obtained for a simplified model corresponding to gluino pair production with three-body decays into top quarks and neutralinos. Gluinos with a mass below 1600 GeV are excluded at a 95% confidence level for scenarios with low $$ {\tilde{\chi}}_1^0 $$ mass, and neutralinos with a mass below 800 GeV are excluded for a gluino mass of about 1300 GeV. For models with two-body gluino decays producing on-shell top squarks, the excluded region is only weakly sensitive to the top squark mass.

Constraints on the pMSSM, AMSB model and on other models from the search for long-lived charged particles in proton-proton collisions at [Formula: see text

Stringent limits are set on the long-lived lepton-like sector of the phenomenological minimal supersymmetric standard model (pMSSM) and the anomaly-mediated supersymmetry breaking (AMSB) model. The limits are derived from the results presented in a recent search for long-lived charged particles in proton–proton collisions, based on data collected by the CMS detector at a centre-of-mass energy of 8 TeV at the Large Hadron Collider. In the pMSSM parameter sub-space considered, 95.9 % of the points predicting charginos with a lifetime of at least 10 ns are excluded. These constraints on the pMSSM are the first obtained at the LHC. Charginos with a lifetime greater than 100 ns and masses up to about 800 GeV in the AMSB model are also excluded. The method described can also be used to set constraints on other models.

Measurement of neutral strange particle production in the underlying event in proton-proton collisions ats=7 TeV

Measurements are presented of the production of primary KS0 and Λ particles in proton-proton collisions at s=7 TeV in the region transverse to the leading charged-particle jet in each event. The average multiplicity and average scalar transverse momentum sum of KS0 and Λ particles measured at pseudorapidities |η|<2 rise with increasing charged-particle jet pT in the range 1–10 GeV/c and saturate in the region 10–50 GeV/c. The rise and saturation of the strange-particle yields and transverse momentum sums in the underlying event are similar to those observed for inclusive charged particles, which confirms the impact-parameter picture of multiple parton interactions. The results are compared to recent tunes of the pythia Monte Carlo event generator. The pythia simulations underestimate the data by 15%–30% for KS0 mesons and by about 50% for Λ baryons, a deficit similar to that observed for the inclusive strange-particle production in non-single-diffractive proton-proton collisions. The constant strange- to charged-particle activity ratios with respect to the leading jet pT and similar trends for mesons and baryons indicate that the multiparton-interaction dynamics is decoupled from parton hadronization, which occurs at a later stage.Received 26 May 2013DOI:https://doi.org/10.1103/PhysRevD.88.052001This article is available under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.© 2013 CERN, for the CMS Collaboration

The construction and testing of the silicon position sensitive modules for the LHCf experiment at CERN

In this paper the design criteria, construction and final performance of the silicon micro-strip modules installed in the LHCf experiment are described. LHCf is an experiment currently placed at CERN in the LHC tunnel. It consists of two small calorimeters each one placed 140 metres away from the ATLAS interaction point. Their purpose is to study very forward production of neutral particles in proton-proton collisions. The silicon modules are installed in one of the two calorimeters and provide precision information on the shower transverse profile.

The LHCf experiment at CERN: motivations and current status

LHCf is an experiment currently installed at CERN at the LHC complex. It consists of two small calorimeters each one placed 140 meters away from the ATLAS interaction point. Their purpose is to study forward production of neutral particles in proton-proton collisions at extremely low angles. The results will provide invaluable inputs to the many air-shower Monte Carlo codes currently used for modeling cosmic rays interactions in the Earth atmosphere. Depending on machine start up, data will be taken from 900 GeV in the centre of mass up to 14 TeV (laboratory equivalent collision energy of 10 17 eV), thus covering an energy range up to and beyond the “knee” of the cosmic ray spectrum.