Деркач Денис Александрович

Study of the ψ2(3823) and χc1(3872) states in B+→(J/ψπ+π−)K+ decays

2020 · ARTICLE · en

The decays B+ → J/ψπ+π−K+ are studied using a data set corresponding to an integrated luminosity of 9 fb−1 collected with the LHCb detector in proton-proton collisions between 2011 and 2018. Precise measurements of the ratios of branching fractions with the intermediate ψ2(3823), χc1(3872) and ψ(2S) states are reported. The values are B+→ψ2(3823)K+×ψ2(3823)→J/ψπ+π−B+→χc1(3872)K+×χc1(3872)→J/ψπ+π−=(3.56±0.67±0.11)×10−2,B+→ψ2(3823)K+×ψ2(3823)→J/ψπ+π−B+→ψ(2S)K+×ψ(2S)→J/ψπ+π−=(1.31±0.25±0.04)×10−3,B+→χc1(3872)K+×χc1(3872)→J/ψπ+π−B+→ψ(2S)K+×ψ(2S)→J/ψπ+π−=(3.69±0.07±0.06)×10−2,BB+→ψ2(3823)K+×Bψ2(3823)→J/ψπ+π−BB+→χc1(3872)K+×Bχc1(3872)→J/ψπ+π−=(3.56±0.67±0.11)×10−2,BB+→ψ2(3823)K+×Bψ2(3823)→J/ψπ+π−BB+→ψ(2S)K+×Bψ(2S)→J/ψπ+π−=(1.31±0.25±0.04)×10−3,BB+→χc1(3872)K+×Bχc1(3872)→J/ψπ+π−BB+→ψ(2S)K+×Bψ(2S)→J/ψπ+π−=(3.69±0.07±0.06)×10−2, where the first uncertainty is statistical and the second is systematic. The decay of B+ → ψ2(3823)K+ with ψ2(3823) → J/ψπ+π− is observed for the first time with a significance of 5.1 standard deviations. The mass differences between the ψ2(3823), χc1(3872) and ψ(2S) states are measured to be 𝑚χc1(3872)−𝑚ψ2(3823)=47.50±0.53±0.13MeV/𝑐2,𝑚ψ2(3823)−𝑚ψ2(2S)=137.98±0.53±0.14MeV/𝑐2,𝑚χc1(3872)−𝑚ψ2(2S)=185.49±0.06±0.03MeV/𝑐2,mχc1(3872)−mψ2(3823)=47.50±0.53±0.13MeV/c2,mψ2(3823)−mψ2(2S)=137.98±0.53±0.14MeV/c2,mχc1(3872)−mψ2(2S)=185.49±0.06±0.03MeV/c2, resulting in the most precise determination of the χc1(3872) mass. The width of the ψ2(3823) state is found to be below 5.2 MeV at 90% confidence level. The Breit-Wigner width of the χc1(3872) state is measured to be ΓBWχc1(3872)=0.96+0.19−0.18±0.21MeVΓχc1(3872)BW=0.96−0.18+0.19±0.21MeV which is inconsistent with zero by 5.5 standard deviations.

Observation of structure in the J/ψ-pair mass spectrum

2020 · ARTICLE · en

Using proton-proton collision data at centre-of-mass energies of s=7,8 and 13TeV recorded by the LHCb experiment at the Large Hadron Collider, corresponding to an integrated luminosity of 9fb-1, the invariant mass spectrum of J/ψ pairs is studied. A narrow structure around 6.9GeV/c2 matching the lineshape of a resonance and a broad structure just above twice the J/ψ mass are observed. The deviation of the data from nonresonant J/ψ-pair production is above five standard deviations in the mass region between 6.2 and 7.4GeV/c2, covering predicted masses of states composed of four charm quarks. The mass and natural width of the narrow X(6900) structure are measured assuming a Breit–Wigner lineshape.

ML-assisted versatile approach to Calorimeter R&D

2020 · ARTICLE · en

Advanced detector R&D for both new and ongoing experiments in HEP requires performing computationally intensive and detailed simulations as part of the detector-design optimisation process. We propose a versatile approach to this task that is based on machine learning and can substitute the most computationally intensive steps of the process while retaining the GEANT4 accuracy to details. The approach covers entire detector representation from the event generation to the evaluation of the physics performance. The approach allows the use of arbitrary modules arrangement, different signal and background conditions, tunable reconstruction algorithms, and desired physics performance metrics. While combined with properties of detector and electronics prototypes obtained from beam tests, the approach becomes even more versatile. We focus on the Phase II Upgrade of the LHCb Calorimeter under the requirements on operation at high luminosity. We discuss the general design of the approach and particular estimations, including spatial and energy resolution for the future LHCb Calorimeter setup at different pile-up conditions.

Search for the doubly heavy Ξ0𝑏𝑐 baryon via decays to D0pK−

2020 · ARTICLE · en

A search for the doubly heavy {\Xi}_{bc}^0Ξbc0​ baryon using its decay to the D^{0}0pK^{–}– final state is performed using proton-proton collision data at a centre-of-mass energy of 13 TeV collected by the LHCb experiment between 2016 and 2018, corresponding to an integrated luminosity of 5.4 fb^{−1}−1. No significant signal is found in the invariant mass range from 6.7 to 7.2 GeV/c^{2}2. Upper limits are set at 95% credibility level on the ratio of the {\Xi}_{bc}^0Ξbc0​ production cross-section times its branching fraction to D^{0}0pK^{−}− relative to that of the {\Lambda}_b^0\to {D}^0{pK}^{-}Λb0​→D0pK− decay. The limits are set as a function of the {\Xi}_{bc}^0Ξbc0​ mass and lifetime hypotheses, in the rapidity range from 2.0 to 4.5 and in the transverse momentum region from 5 to 25 GeV/c. Upper limits range from 1.7 × 10^{−2}−2 to 3.0 × 10^{−1}−1 for the considered {\Xi}_{bc}^0Ξbc0​ mass and lifetime hypotheses.

First branching fraction measurement of the suppressed decay Ξ0c→π−Λ+c

2020 · ARTICLE · en

The Ξc0 baryon is unstable and usually decays into charmless final states by the c→sud¯ transition. It can, however, also disintegrate into a π- meson and a Λc+ baryon via s quark decay or via cs→dc weak scattering. The interplay between the latter two processes governs the size of the branching fraction B(Ξc0→π-Λc+), first measured here to be (0.55±0.02±0.18)%, where the first uncertainty is statistical and second systematic. This result is compatible with the larger of the theoretical predictions that connect models of hyperon decays using partially conserved axial currents and SU(3) symmetry with those involving the heavy-quark expansion and heavy-quark symmetry. In addition, the branching fraction of the normalization channel, B(Ξc+→pK-π+)=(1.135±0.002±0.387)% is measured.

Observation of Enhanced Double Parton Scattering in Proton-Lead Collisions at at √sNN = 8.16 TeV

2020 · ARTICLE · en

Searches for low-mass dimuon resonances

2020 · ARTICLE · en

Searches are performed for a low-mass dimuon resonance, X, produced in proton-proton collisions at a center-of-mass energy of 13 TeV, using a data sample corresponding to an integrated luminosity of 5.1 fb−1−1 and collected with the LHCb detector. The X bosons can either decay promptly or displaced from the proton-proton collision, where in both cases the requirements placed on the event and the assumptions made about the production mechanisms are kept as minimal as possible. The searches for promptly decaying X bosons explore the mass range from near the dimuon threshold up to 60 GeV, with nonnegligible X widths considered above 20 GeV. The searches for displaced X → μ++μ−− decays consider masses up to 3 GeV. None of the searches finds evidence for a signal and 90% confidence-level exclusion limits are placed on the X → μ++μ−− cross sections, each with minimal model dependence. In addition, these results are used to place world-leading constraints on GeV-scale bosons in the two-Higgs-doublet and hidden-valley scenarios.

First observation of the decay 𝐵0→𝐷0𝐷⎯⎯⎯⎯⎯0𝐾+𝜋−

2020 · ARTICLE · en

The first observation of the decay B0→D0¯D0K+π− is reported using proton-proton collision data corresponding to an integrated luminosity of 4.7 fb−1 collected by the LHCb experiment in 2011, 2012 and 2016. The measurement is performed in the full kinematically allowed range of the decay outside of the D*− region. The ratio of the branching fraction relative to that of the control channel B0→D*−D0K+ is measured to be R=(14.2±1.1±1.0)%, where the first uncertainty is statistical and the second is systematic. The absolute branching fraction of B0→D0¯D0K+π− decays is thus determined to be B(B0→D0¯D0K+π−)=(3.50±0.27±0.26±0.30)×10−4, where the third uncertainty is due to the branching fraction of the control channel. This decay mode is expected to provide insights to spectroscopy and the charm-loop contributions in rare semileptonic decays.

Study of the lineshape of the χc1(3872) state

2020 · ARTICLE · en

A study of the lineshape of theχc1ð3872Þstate is made using a data sample corresponding to anintegrated luminosity of3fb−1collected inppcollisions at center-of-mass energies of 7 and 8 TeV withthe LHCb detector. Candidateχc1ð3872Þandψð2SÞmesons fromb-hadron decays are selected in theJ=ψπþπ−decay mode. Describing the lineshape with a Breit-Wigner function, the mass splitting betweentheχc1ð3872Þandψð2SÞstates,Δm, and the width of theχc1ð3872Þstate,ΓBW, are determined to beΔm¼185.5980.0670.068MeV;ΓBW¼1.390.240.10MeV;where the first uncertainty is statistical and the second systematic. Using a Flatt ́e-inspired model, the mode and full width at half maximum of the lineshape are determined to bemode¼3871.69þ0.00þ0.05−0.04−0.13MeV;FWHM¼0.22þ0.07þ0.11−0.06−0.13MeV:An investigation of the analytic structure of the Flatt ́e amplitude reveals a polestructure, which is compatible with a quasiboundD0 ̄D0state but a quasivirtual state is still allowed at thelevel of 2 standard deviations.

Search for CP violation in Ξ+c→pK−π+ decays using model-independent techniques

2020 · ARTICLE · en

A first search for CPCP violation in the Cabibbo-suppressed Ξ+c→pK−π+Ξc+→pK−π+ decay is performed using both a binned and an unbinned model-independent technique in the Dalitz plot. The studies are based on a sample of proton-proton collision data, corresponding to an integrated luminosity of 3.0fb−13.0fb−1, and collected by the LHCb experiment at centre-of-mass energies of 7 and 8TeV8TeV. The data are consistent with the hypothesis of no CPCP violation.