Multidifferential study of identified charged hadron distributions in ZZ-tagged jets in proton-proton collisions at s=\sqrt{s}=13 TeV

Jet fragmentation functions are measured for the first time in proton-proton collisions for charged pions, kaons, and protons within jets recoiling against a $Z$ boson. The charged-hadron distributions are studied longitudinally and transversely to the jet direction for jets with transverse momentum 20 $< p_{\textrm{T}} < 100$ GeV and in the pseudorapidity range $2.5 < \eta < 4$. The data sample was collected with the LHCb experiment at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 1.64 fb$^{-1}$. Triple differential distributions as a function of the hadron longitudinal momentum fraction, hadron transverse momentum, and jet transverse momentum are also measured for the first time. This helps constrain transverse-momentum-dependent fragmentation functions. Differences in the shapes and magnitudes of the measured distributions for the different hadron species provide insights into the hadronization process for jets predominantly initiated by light quarks.Comment: All figures and tables, along with machine-readable versions and any supplementary material and additional information, are available at https://cern.ch/lhcbproject/Publications/p/LHCb-PAPER-2022-013.html (LHCb public pages

Study of the B−→Λc+Λˉc−K−B^{-} \to \Lambda_{c}^{+} \bar{\Lambda}_{c}^{-} K^{-} decay

The decay $B^{-} \to \Lambda_{c}^{+} \bar{\Lambda}_{c}^{-} K^{-}$ is studied in proton-proton collisions at a center-of-mass energy of $\sqrt{s}=13$ TeV using data corresponding to an integrated luminosity of 5 $\mathrm{fb}^{-1}$ collected by the LHCb experiment. In the $\Lambda_{c}^+ K^{-}$ system, the $\Xi_{c}(2930)^{0}$ state observed at the BaBar and Belle experiments is resolved into two narrower states, $\Xi_{c}(2923)^{0}$ and $\Xi_{c}(2939)^{0}$, whose masses and widths are measured to be $$m(\Xi_{c}(2923)^{0}) = 2924.5 \pm 0.4 \pm 1.1 \,\mathrm{MeV}, \\ m(\Xi_{c}(2939)^{0}) = 2938.5 \pm 0.9 \pm 2.3 \,\mathrm{MeV}, \\ \Gamma(\Xi_{c}(2923)^{0}) = \phantom{000}4.8 \pm 0.9 \pm 1.5 \,\mathrm{MeV},\\ \Gamma(\Xi_{c}(2939)^{0}) = \phantom{00}11.0 \pm 1.9 \pm 7.5 \,\mathrm{MeV},$$ where the first uncertainties are statistical and the second systematic. The results are consistent with a previous LHCb measurement using a prompt $\Lambda_{c}^{+} K^{-}$ sample. Evidence of a new $\Xi_{c}(2880)^{0}$ state is found with a local significance of $3.8\,\sigma$, whose mass and width are measured to be $2881.8 \pm 3.1 \pm 8.5\,\mathrm{MeV}$ and $12.4 \pm 5.3 \pm 5.8 \,\mathrm{MeV}$, respectively. In addition, evidence of a new decay mode $\Xi_{c}(2790)^{0} \to \Lambda_{c}^{+} K^{-}$ is found with a significance of $3.7\,\sigma$. The relative branching fraction of $B^{-} \to \Lambda_{c}^{+} \bar{\Lambda}_{c}^{-} K^{-}$ with respect to the $B^{-} \to D^{+} D^{-} K^{-}$ decay is measured to be $2.36 \pm 0.11 \pm 0.22 \pm 0.25$, where the first uncertainty is statistical, the second systematic and the third originates from the branching fractions of charm hadron decays.Comment: All figures and tables, along with any supplementary material and additional information, are available at https://cern.ch/lhcbproject/Publications/p/LHCb-PAPER-2022-028.html (LHCb public pages

Measurement of the ratios of branching fractions R(D∗)\mathcal{R}(D^{*}) and R(D0)\mathcal{R}(D^{0})

The ratios of branching fractions $\mathcal{R}(D^{*})\equiv\mathcal{B}(\bar{B}\to D^{*}\tau^{-}\bar{\nu}_{\tau})/\mathcal{B}(\bar{B}\to D^{*}\mu^{-}\bar{\nu}_{\mu})$ and $\mathcal{R}(D^{0})\equiv\mathcal{B}(B^{-}\to D^{0}\tau^{-}\bar{\nu}_{\tau})/\mathcal{B}(B^{-}\to D^{0}\mu^{-}\bar{\nu}_{\mu})$ are measured, assuming isospin symmetry, using a sample of proton-proton collision data corresponding to 3.0 fb${ }^{-1}$ of integrated luminosity recorded by the LHCb experiment during 2011 and 2012. The tau lepton is identified in the decay mode $\tau^{-}\to\mu^{-}\nu_{\tau}\bar{\nu}_{\mu}$. The measured values are $\mathcal{R}(D^{*})=0.281\pm0.018\pm0.024$ and $\mathcal{R}(D^{0})=0.441\pm0.060\pm0.066$, where the first uncertainty is statistical and the second is systematic. The correlation between these measurements is $\rho=-0.43$. Results are consistent with the current average of these quantities and are at a combined 1.9 standard deviations from the predictions based on lepton flavor universality in the Standard Model.Comment: All figures and tables, along with any supplementary material and additional information, are available at https://cern.ch/lhcbproject/Publications/p/LHCb-PAPER-2022-039.html (LHCb public pages

Mechanism of alcohol chemical vapor deposition growth of carbon nanotubes: Catalyst oxidation

Alcohol chemical vapor deposition (ACVD) was established as one of the most promising methods for single-walled carbon nanotube (SWCNT) growth almost two decades ago however the mechanisms behind its success remain elusive. To unveil the mechanism of SWCNT growth via ACVD, we employed density functional tight binding molecular dynamics simulations, supplying ethanol to a Fe nanoparticle. Here we demonstrate the oxidation of the Fe catalyst with varying supply rates of ethanol and how the catalyst composition is controlled by the reaction pathways mediated by the hydroxyl OH radical. Following ethanol dissociation on Fe and subsequent O dissolution, the catalyst becomes oxidized and the mobility and availability of Fe to bond with C are reduced. However, SWCNT growth is promoted via the key reaction pathways of the hydroxyl H; controlling the catalyst composition through the formation and release of H2O and H-2. These reaction pathways also demonstrate how active growth species such as ethylene can be formed preferentially to ethane from ethanol dissociation. This work provides important insight into the mechanism of how the catalyst composition changes during ACVD and can be extended to understand the catalyst nature during other O-assisted SWCNT growth processes such as H2O-assisted supergrowth and CO/CO2-promoted growth. (C) 2022 Elsevier Ltd. All rights reserved

High Temperature Accelerated Stone-Wales Transformation and the Threshold Temperature of IPR-C-60 Formation

The Stone-Wales bond rotation isomerization of nonicosahedral C-60 (C-2v-C-60) into isolated-pentagon rule following icosahedral C-60 (I-h-C-60 or IPR-C-60) is a limiting step in the synthesis of I-h-C-60. However, extensive previous studies indicate that the potential energy barrier of the Stone-Wales bond rotation is between 6 and 8 eV, extremely high to allow for bond rotation at the temperatures used to produce fullerenes conventionally. This is also despite data indicating a possible fullerene road mechanism that necessitates low-temperature annealing. However, these previous investigations often have limiting factors, such as using the harmonic approximation to determine free energies at high temperatures or considering only the reverse I-h-C-60 to C-2v-C-60 transition as a basis. Indeed, when the difference in energy between I-h-C-60 and C-2v-C-60 is accounted for, this barrier is generally reduced by similar to 1.5 eV. Thus, utilizing the recently developed density functional tight binding metadynamics (DFTB-MTD) interface, the effects of temperature on the bond rotation in the conversion of C-2v-C-60 to I-h-C-60 have been investigated. We found that Stone-Wales bond rotations are complex processes with both in-plane and out-of-plane transition states, and which transition path dominates depends on temperature. Our results clearly show that at temperatures of 2000 K, the free energy for a C-2v-C-60 to I-h-C-60 transition is only similar to 4.21 eV and further reduces to similar to 3.77 eV at 3000 K. This translates to transition times of similar to 971 mu s at 2000 K and similar to 34 ns at 3000 K, indicating that defect healing is a fast process at temperatures typical of arc jet or laser ablation experiments. Conversely, below similar to 2000 K, bond rotation becomes prohibitively slow, putting a lower threshold limit on the temperature of fullerene formation and subsequent annealing

Multilayer graphene sunk growth on Cu(111) surface

The controllable growth of multilayer graphene is a challenging research topic. Prior results show graphene adlayers can grow beneath pre-existing graphene layers on a Cu(111). The conventional inverted-wedding-cake (IWC) model used to describe this incurs an energy disadvantage due to deformation in the overlying graphene. We propose an alternative theoretical model, the sunk growth mode, for understanding multilayer graphene growth on Cu substrates. Extensive density functional theory (DFT) calculations show that multilayer graphene grown via this sunk mode is energetically favourable compared to the on-terrace growth mode for Cu(111). These results reveal that graphene underlayers tend to grow in a sunk growth mode, minimizing deformation in the overlayers, reducing deformation energy. Further density functional tight binding-molecular dynamic (DFTBMD) simulations on Cu(111) substrates yield sunken structures consistent with our sunk growth mode. Moreover, AFM investigations of experimentally grown multilayer graphene on polycrystaline Cu show that while friction data indicates multiple graphene edges in the sample, the topological height measurement indicates flat graphitic sheets, further confirming our sunk growth mode. This discovery provides a novel and more reasonable model for the &quot;underlayer&quot; growth of multilayer graphene and can be extended to a general theory for the multilayer graphene growth on various substrates

Borophene with Large Holes

In two-dimensional (2D) borophene, the structural transition from triangular lattice to hexagonal lattice with an increase in vacancy concentration is a basic principle of constructing various borophene isomers. Here, by performing an extensive structural search of 4239 borophene isomers with both hexagonal holes (HHs) and large holes (LHs), we show that the structural transformation from triangular lattice to borophene with large holes is energetically more favorable. Borophene isomers with LHs are more stable than those with only HHs at high vacancy concentrations (&gt;20%) and are just slightly less stable than those with only HHs at low vacancy concentrations. This discovery greatly expands the family of 2D borophene and opens a route for synthesizing new borophene isomers

Role of Graphitic Bowls in Temperature Dependent Fullerene Formation

Fullerenes are used extensively in organic electronics as electron acceptors among other uses; however, there are still several key mysteries regarding their formation such as the importance of graphitic intermediates and the thermokinetics of initial cage formation. To this end, we have conducted density functional tight binding molecular dynamics (DFTB-MD) calculations on disintegrated Ih-C60 to investigate the formation mechanisms of fullerenes at high temperature conditions. From the results of these DFTB-MD calculations we were able to develop a thermokinetic model to describe the free energies and kinetics of fullerene formation at a range of temperatures. Direct observation of the mechanism revealed fullerenes readily forming in nanosecond times between 2000 and 3000 K but were hindered above this temperature window. Analysis revealed temperature dependent formation mechanisms where at low temperatures (&lt;2750K) flat graphitic bowls play an important part as metastable intermediates while highly curved bowls follow a direct fast transformation. Meanwhile at higher temperatures (&gt;2750 K), flat bowls become the transitory structure between chains and fullerene. Free energy analysis from our thermokinetic model shows this change in graphitic bowls to being transitory hinders fullerene formation at high temperatures compared to lower temperatures, essentially kinetically trapping C60 as chain networks. This investigation gives new key insights into the formation mechanisms of C60 fullerenes and highlights important intermediates while also illuminating the temperature window for fullerene formation, facilitating better optimization of experimental methods

Theory of sigma bond resonance in flat boron materials

Here, the authors present a resonance theory to describe the bonding configuration of flat boron materials without quantum calculation. Like aromaticity theory in carbon, it allows to intuitively understand the stability and properties of boron-related material