Christophorus Grab

Last Name

Grab

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Christophorus

ORCID

0000-0002-6182-3380

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Publications 1 - 10 of 708

Study of J/ψ meson production inside jets in pp collisions at s=8TeV
CMS Collaboration; Sirunyan, Albert M.; Backhaus, Malte; et al. (2020)
Physics Letters B
A study of the production of prompt J/ψ mesons contained in jets in proton-proton collisions at s=8TeV is presented. The analysis is based on data corresponding to an integrated luminosity of 19.1 fb−1 collected with the CMS detector at the LHC. For events with at least one observed jet, the angular separation between the J/ψ meson and the jet is used to test whether the J/ψ meson is part of the jet. The analysis shows that most prompt J/ψ mesons having energy above 15 GeV and rapidity |y|<1 are contained in jets with pseudorapidity |ηjet|<1. The differential distributions of the probability to have a J/ψ meson contained in a jet as a function of jet energy for a fixed J/ψ energy fraction are compared to a theoretical model using the fragmenting jet function approach. The data agree best with fragmenting jet function calculations that use a long-distance matrix element parameter set in which prompt J/ψ mesons are predicted to be unpolarized. This technique demonstrates a new way to test predictions for prompt J/ψ production using nonrelativistic quantum chromodynamics.
Measurement of the Z/𝛾∗→𝜏𝜏 cross section in pp collisions at √s=13 TeV and validation of 𝜏 lepton analysis techniques
CMS Collaboration; Sirunyan, Albert M.; Backhaus, Malte; et al. (2018)
The European Physical Journal C
A measurement is presented of the Z/γ∗→ττ cross section in pp collisions at s√=13 TeV, using data recorded by the CMS experiment at the LHC, corresponding to an integrated luminosity of 2.3 fb−1. The product of the inclusive cross section and branching fraction is measured to be σ(pp→Z/γ∗+X)B(Z/γ∗→ττ)=1848±12(stat)±67 (syst \,+\,lumi) pb, in agreement with the standard model expectation, computed at next-to-next-to-leading order accuracy in perturbative quantum chromodynamics. The measurement is used to validate new analysis techniques relevant for future measurements of τ lepton production. The measurement also provides the reconstruction efficiency and energy scale for τ decays to hadrons+ντ final states, determined with respective relative uncertainties of 2.2 and 0.9%.
Azimuthal anisotropy of dijet events in PbPb collisions at √SNN = 5.02 TeV
The CMS collaboration; Tumasyan, Armen; Erdmann, Wolfram; et al. (2023)
Journal of High Energy Physics
The path-length dependent parton energy loss within the dense partonic medium created in lead-lead collisions at a nucleon-nucleon center-of-mass energy of √sNN = 5.02 TeV is studied by determining the azimuthal anisotropies for dijets with high transverse momentum. The data were collected by the CMS experiment in 2018 and correspond to an integrated luminosity of 1.69 nb⁻¹. For events containing back-to-back jets, correlations in relative azimuthal angle and pseudorapidity (η) between jets and hadrons, and between two hadrons, are constructed. The anisotropies are expressed as the Fourier expansion coefficients vₙ, n = 2–4 of these azimuthal distributions. The dijet vₙ values are extracted from long-range (1.5 < |∆η| < 2.5) components of these correlations, which suppresses the background contributions from jet fragmentation processes. Positive dijet v₂ values are observed which increase from central to more peripheral events, while the v₃ and v₄ values are consistent with zero within experimental uncertainties.
Search for resonant production of strongly coupled dark matter in proton-proton collisions at 13 TeV
CMS collaboration; Tumasyan, Armen; Adam, Wolfgang; et al. (2022)
Journal of High Energy Physics
The first collider search for dark matter arising from a strongly coupled hidden sector is presented and uses a data sample corresponding to 138 fb−1, collected with the CMS detector at the CERN LHC, at s = 13 TeV. The hidden sector is hypothesized to couple to the standard model (SM) via a heavy leptophobic Z′ mediator produced as a resonance in proton-proton collisions. The mediator decay results in two “semivisible” jets, containing both visible matter and invisible dark matter. The final state therefore includes moderate missing energy aligned with one of the jets, a signature ignored by most dark matter searches. No structure in the dijet transverse mass spectra compatible with the signal is observed. Assuming the Z′ boson has a universal coupling of 0.25 to the SM quarks, an inclusive search, relevant to any model that exhibits this kinematic behavior, excludes mediator masses of 1.5–4.0 TeV at 95% confidence level, depending on the other signal model parameters. To enhance the sensitivity of the search for this particular class of hidden sector models, a boosted decision tree (BDT) is trained using jet substructure variables to distinguish between semivisible jets and SM jets from background processes. When the BDT is employed to identify each jet in the dijet system as semivisible, the mediator mass exclusion increases to 5.1 TeV, for wider ranges of the other signal model parameters. These limits exclude a wide range of strongly coupled hidden sector models for the first time. [Figure not available: see fulltext.]
Measurement of Energy Correlators inside Jets and Determination of the Strong Coupling α_s(m_Z)
CMS Collaboration; Hayrapetyan, A.; Erdmann, Wolfram; et al. (2024)
Physical Review Letters
Energy correlators that describe energy-weighted distances between two or three particles in a hadronic jet are measured using an event sample of √s = 13 TeV proton-proton collisions collected by the CMS experiment and corresponding to an integrated luminosity of 36.3 fb⁻¹. The measured distributions are consistent with the trends in the simulation that reveal two key features of the strong interaction: confinement and asymptotic freedom. By comparing the ratio of the measured three- and two-particle energy correlator distributions with theoretical calculations that resum collinear emissions at approximate next-to-next-to-leading-logarithmic accuracy matched to a next-to-leading-order calculation, the strong coupling is determined at the Z boson mass: α_S(m_Z) = 0.1229₋₀.₀₀₅₀⁺⁰.⁰⁰⁴⁰, the most precise α_S(m_Z) value obtained using jet substructure observables.
A general search for new phenomena in ep scattering at HERA
H1 Collaboration; Aktas, Adil; Baumgartner, Simon; et al. (2004)
Physics Letters B
Technical design of the phase I Mu3e experiment
Arndt, Kirk; Augustin, Heiko; Baesso, Paolo; et al. (2021)
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment
The Mu3e experiment aims to find or exclude the lepton flavour violating decay μ→eee at branching fractions above 10−16. A first phase of the experiment using an existing beamline at the Paul Scherrer Institute (PSI) is designed to reach a single event sensitivity of 2⋅10−15. We present an overview of all aspects of the technical design and expected performance of the phase I Mu3e detector. The high rate of up to 10(8) muon decays per second and the low momenta of the decay electrons and positrons pose a unique set of challenges, which we tackle using an ultra thin tracking detector based on high-voltage monolithic active pixel sensors combined with scintillating fibres and tiles for precise timing measurements.
Electron and photon reconstruction and identification with the CMS experiment at the CERN LHC
CMS Collaboration; Sirunyan, Albert M.; Backhaus, Malte; et al. (2021)
Journal of Instrumentation
The performance is presented of the reconstruction and identification algorithms for electrons and photons with the CMS experiment at the LHC. The reported results are based on proton-proton collision data collected at a center-of-mass energy of 13 TeV and recorded in 2016–2018, corresponding to an integrated luminosity of 136 fb^-1. Results obtained from lead-lead collision data collected at √(sNN)=5.02 TeV are also presented. Innovative techniques are used to reconstruct the electron and photon signals in the detector and to optimize the energy resolution. Events with electrons and photons in the final state are used to measure the energy resolution and energy scale uncertainty in the recorded events. The measured energy resolution for electrons produced in Z boson decays in proton-proton collision data ranges from 2 to 5%, depending on electron pseudorapidity and energy loss through bremsstrahlung in the detector material. The energy scale in the same range of energies is measured with an uncertainty smaller than 0.1 (0.3)% in the barrel (endcap) region in proton-proton collisions and better than 1 (3)% in the barrel (endcap) region in heavy ion collisions. The timing resolution for electrons from Z boson decays with the full 2016–2018 proton-proton collision data set is measured to be 200 ps.
Observation of τ Lepton Pair Production in Ultraperipheral Pb-Pb Collisions at √sNN = 5.02 TeV
CMS Collaboration; Tumasyan, Armen; Erdmann, Wolfram; et al. (2023)
Physical Review Letters
We present an observation of photon-photon production of τ lepton pairs in ultraperipheral lead-lead collisions. The measurement is based on a data sample with an integrated luminosity of 404μb⁻¹ collected by the CMS experiment at a center-of-mass energy per nucleon pair of √sNN = 5.02 TeV. The γγ → τ⁺τ⁻ process is observed for τ⁺τ⁻ events with a muon and three charged hadrons in the final state. The measured fiducial cross section is σ(γγ → τ⁺τ⁻) = 4.8 ± 0.6(stat) ± 0.5(syst) μb, where the second (third) term corresponds to the statistical (systematic) uncertainty in σ(γγ → τ⁺τ⁻) in agreement with leading-order QED predictions. Using σ(γγ → τ⁺τ⁻), we estimate a model-dependent value of the anomalous magnetic moment of the τ lepton of aτ = 0.001₋₀.₀₈₉⁺⁰.⁰⁵⁵.
Observation of the Ξ⁻b→ψ(2S)Ξ− decay and studies of the Ξb(5945)⁰ baryon in proton-proton collisions at √s = 13 TeV
CMS Collaboration; Hayrapetyan, A.; Erdmann, Wolfram; et al. (2024)
Physical Review D
The first observation of the decay Ξ⁻b → ψ(2S)Ξ⁻ and measurement of the branching ratio of Ξ⁻b → ψ(2S)Ξ⁻ to Ξ⁻b → J/ψΞ⁻ are presented. The J/ψ and ψ(2S) mesons are reconstructed using their dimuon decay modes. The results are based on proton-proton colliding beam data from the LHC collected by the CMS experiment at √s = 13 TeV in 2016–2018, corresponding to an integrated luminosity of 140 fb⁻¹. The branching fraction ratio is measured to be B(Ξ⁻b → ψ(2S)Ξ⁻)/B(Ξ⁻b → J/ψΞ⁻)=0.84⁺⁰.²¹₋₀.₁₉(stat)±0.10(syst)±0.02(B), where the last uncertainty comes from the uncertainties in the branching fractions of the charmonium states. New measurements of the Ξb(5945)0 baryon mass and natural width are also presented, using the Ξ⁻bπ⁺ final state, where the Ξ⁻b baryon is reconstructed through the decays J/ψΞ⁻, ψ(2S)Ξ⁻, J/ψΛK⁻, and J/ψΣ0K⁻. Finally, the fraction of Ξ⁻b baryons produced from Ξb(5945)⁰ decays is determined.

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Christophorus Grab

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