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A measurement of the multi-strange Ξ− and Ω− baryons and their antiparticles by the ALICE experiment at the CERN Large Hadron Collider (LHC) is presented for inelastic proton–proton collisions at a centre-of-mass energy of 7 TeV. The transverse momentum (pT) distributions were studied at mid-rapidity (|y|<0.5) in the range of 0.6<pT<8.5 GeV/c for Ξ− and Ξ¯+ baryons, and in the range of 0.8<pT<5 GeV/c for Ω− and Ω¯+. Baryons and antibaryons were measured as separate particles and we find that the baryon to antibaryon ratio of both particle species is consistent with unity over the entire range of the measurement. The statistical precision of the current data has allowed us to measure a difference between the mean pT of Ξ− (Ξ¯+) and Ω− (Ω¯+). Particle yields, mean pT, and the spectra in the intermediate pT range are not well described by the PYTHIA Perugia 2011 tune Monte Carlo event generator, which has been tuned to reproduce the early LHC data. The discrepancy is largest for Ω− (Ω¯+). This PYTHIA tune approaches the pT spectra of Ξ− and Ξ¯+ baryons below pT<0.85 GeV/c and describes the Ξ− and Ξ¯+ spectra above pT>6.0 GeV/c. We also illustrate the difference between the experimental data and model by comparing the corresponding ratios of (Ω−+Ω¯+)/(Ξ−+Ξ¯+) as a function of transverse mass.
The ALICE Zero Degree Calorimeter system (ZDC) is composed of two identical sets of calorimeters, placed at opposite sides with respect to the interaction point, 114 meters away from it, complemented by two small forward electromagnetic calorimeters (ZEM). Each set of detectors consists of a neutron (ZN) and a proton (ZP) ZDC. They are placed at zero degrees with respect to the LHC axis and allow to detect particles emitted close to beam direction, in particular neutrons and protons emerging from hadronic heavy-ion collisions (spectator nucleons) and those emitted from electromagnetic processes. For neutrons emitted by these two processes, the ZN calorimeters have nearly 100% acceptance.
During the √sNN = 2.76 TeV Pb-Pb data-taking, the ALICE Collaboration studied forward neutron emission with a dedicated trigger, requiring a minimum energy deposition in at least one of the two ZN. By exploiting also the information of the two ZEM calorimeters it has been possible to separate the contributions of electromagnetic and hadronic processes and to study single neutron vs. multiple neutron emission.
The measured cross sections of single and mutual electromagnetic dissociation of Pb nuclei at √sNN = 2.76 TeV, with neutron emission, are σsingle EMD = 187:4 ± 0.2 (stat.)−11.2+13.2 (syst.) b and σmutual EMD = 5.7 ± 0.1 (stat.) ±0.4 (syst.) b, respectively [1]. This is the first measurement of electromagnetic dissociation of 208Pb nuclei at the LHC energies, allowing a test of electromagnetic dissociation theory in a new energy regime. The experimental results are compared to the predictions from a relativistic electromagnetic dissociation model.
In particle collider experiments, elementary particle interactions with large momentum transfer produce quarks and gluons (known as partons) whose evolution is governed by the strong force, as described by the theory of quantum chromodynamics (QCD)1. These partons subsequently emit further partons in a process that can be described as a parton shower2, which culminates in the formation of detectable hadrons. Studying the pattern of the parton shower is one of the key experimental tools for testing QCD. This pattern is expected to depend on the mass of the initiating parton, through a phenomenon known as the dead-cone effect, which predicts a suppression of the gluon spectrum emitted by a heavy quark of mass mQ and energy E, within a cone of angular size mQ/E around the emitter3. Previously, a direct observation of the dead-cone effect in QCD had not been possible, owing to the challenge of reconstructing the cascading quarks and gluons from the experimentally accessible hadrons. We report the direct observation of the QCD dead cone by using new iterative declustering techniques4,5 to reconstruct the parton shower of charm quarks. This result confirms a fundamental feature of QCD. Furthermore, the measurement of a dead-cone angle constitutes a direct experimental observation of the non-zero mass of the charm quark, which is a fundamental constant in the standard model of particle physics.
Hadronic resonances are used to probe the hadron gas produced in the late stage of heavy-ion collisions since they decay on the same timescale, of the order of 1 to 10 fm/c, as the decoupling time of the system. In the hadron gas, (pseudo)elastic scatterings among the products of resonances that decayed before the kinetic freeze-out and regeneration processes counteract each other, the net effect depending on the resonance lifetime, the duration of the hadronic phase, and the hadronic cross sections at play. In this context, the Σ(1385)± particle is of particular interest as models predict that regeneration dominates over rescattering despite its relatively short lifetime of about 5.5 fm/c. The first measurement of the Σ(1385)± resonance production at midrapidity in Pb-Pb collisions at sNN−−−√=5.02 TeV with the ALICE detector is presented in this Letter. The resonances are reconstructed via their hadronic decay channel, Λπ, as a function of the transverse momentum (pT) and the collision centrality. The results are discussed in comparison with the measured yield of pions and with expectations from the statistical hadronization model as well as commonly employed event generators, including PYTHIA8/Angantyr and EPOS3 coupled to the UrQMD hadronic cascade afterburner. None of the models can describe the data. For Σ(1385)±, a similar behaviour as K∗(892)0 is observed in data unlike the predictions of EPOS3 with afterburner.
Six new and four known species of the genus Axonchium Cobb, 1920 are described and illustrated from the Western Ghats of India. Axonchium indicum sp. nov. has a 1.2–1.4 mm long body, offset lip region, 8–9 µm long odontostyle, expanded part of pharynx 39–49 % of total neck length, anterior uterine sac 0.9–1.6 times the mid-body diameter long, and a bluntly conoid tail. Axonchium microspiculum sp. nov. has a 1.1–1.22 mm long body, offset lip region, 7–8 µm long odontostyle, 20–23 µm long spicules, two weakly developed ventromedian supplements, and obtusely rounded tail. Axonchium nilgiriense sp. nov. has a 1.4–1.6 mm long body, offset lip region, 8–9 µm long odontostyle, vagina strongly bent posteriad, 37–41 µm long spicules, 3–4 ventromedian supplements, and bluntly conoid tail. Axonchium paracingulatum sp. nov. has a 2.5–2.8 mm long body, offset lip region, 10–11 µm long odontostyle, vaginal lumen highly expanded in the middle, 69 µm long spicules, and broadly conoid tail with rounded terminus. Axonchium tropicum sp. nov. has a 1.7–2.0 mm long body, offset lip region, 11–12 µm long odontostyle, anterior uterine sac 2.3–4.0 times the mid-body diameter long, 40 µm long spicules, 4 widely spaced ventromedian supplements, and broadly rounded tail. Axonchium uniqum sp. nov. has a 1.7–2.0 mm long body, offset lip region, 10–11 µm long odontostyle, anterior uterine sac 2.5–2.8 times mid-body diameter long, 68 µm long and slender spicules, and broadly conoid tail. Axonchium nitidum, A. saccatum, A. transkeiense and A. vallum are redescribed based on specimens collected from several localities. A diagnostic key to the identification of the valid species of this genus is provided.
Five new and four known species of the genus Dorylaimellus Cobb, 1913 are described and illustrated from the Western Ghats of India. Dorylaimellus attenuatus sp. nov. has a 0.73–0.84 mm long body; lip region rounded, offset; odontostyle attenuated with indistinct lumen, odontophore 9–10 µm long; expanded part of pharynx 39–48% of neck length; female genital system amphidelphic and tail long filiform. Dorylaimellus cylindricaudatus sp. nov. has a 1.0–1.15 mm long body; lip region rounded, offset; odontostyle 4 µm long, odontophore 11–12 µm long; expanded part of pharynx 47–56% of neck length; female genital system amphidelphic, and tail elongate-cylindrical with rounded terminus. Dorylaimellus karnatakensis sp. nov. has a 1.2–1.3 mm long body; lip region rounded, offset; odontostyle 5 µm long, odontophore 12–13 µm long; expanded part of pharynx 45–53% of neck length; female genital system amphidelphic; spicules 22 µm long, ventromedian supplements four, and tail elongate-conoid dorsally convex. Dorylaimellus kasplateauensis sp. nov. has a 0.68–0.83 mm long body, lip region with a weak perioral disc; odontostyle 6–7 µm long, odontophore 11–14 µm long; expanded part of pharynx 38–43% of neck length; female genital system amphidelphic, and tail elongate arcuate conoid. Dorylaimellus tropicus sp. nov. has a 0.6–0.7 mm long body; lip region rounded, offset; odontostyle 4–5 µm long, odontophore 8–10 µm long; hemizonid present; expanded part of pharynx 43–52% of neck length; female genital system amphidelphic and tail elongate-filiform, sudden tapering, ending with rounded tip. Dorylaimellus andrassyi, D. discocephalus, D. belondirelloides, D. chakpilus are redescribed based on the specimens collected from several localities of the Western Ghats.