First measurement of Ωc0 production in pp collisions at s=13 TeV

The inclusive production of the charm–strange baryon 0 c is measured for the first time via its hadronic √ decay into −π+ at midrapidity (|y| <0.5) in proton–proton (pp) collisions at the centre-of-mass energy s =13 TeV with the ALICE detector at the LHC. The transverse momentum (pT) differential cross section multiplied by the branching ratio is presented in the interval 2 < pT < 12 GeV/c. The pT dependence of the 0 c-baryon production relative to the prompt D0-meson and to the prompt 0 c-baryon production is compared to various models that take different hadronisation mechanisms into consideration. In the measured pT interval, the ratio of the pT-integrated cross sections of 0 c and prompt + c baryons multiplied by the −π+ branching ratio is found to be larger by a factor of about 20 with a significance of about 4σ when compared to e+e− collisions

Elliptic flow of charged particles at midrapidity relative to the spectator plane in Pb–Pb and Xe–Xe collisions

Measurements of the elliptic flow coefficient relative to the collision plane defined by the spectator neutrons v2{ SP} in collisions of Pb ions at center-of-mass energy per nucleon–nucleon pair √ 2.76 TeV and Xe ions at √ sNN = sNN =5.44 TeV are reported. The results are presented for charged particles produced at midrapidity as a function of centrality and transverse momentum for the 5–70% and 0.2–6 GeV/c ranges, respectively. The ratio between v2{ SP} and the elliptic flow coefficient relative to the participant plane v2{4}, estimated using four-particle correlations, deviates by up to 20% from unity depending on centrality. This observation differs strongly from the magnitude of the corresponding eccentricity ratios predicted by the TRENTo and the elliptic power models of initial state fluctuations that are tuned to describe the participant plane anisotropies. The differences can be interpreted as a decorrelation of the neutron spectator plane and the reaction plane because of fragmentation of the remnants from the colliding nuclei, which points to an incompleteness of current models describing the initial state fluctuations. A significant transverse momentum dependence of the ratio v2{ SP}/v2{4} is observed in all but the most central collisions, which may help to understand whether momentum anisotropies at low and intermediate transverse momentum have a common origin in initial state f luctuations. The ratios of v2{ SP} and v2{4} to the corresponding initial state eccentricities for Xe–Xe and Pb–Pb collisions at similar initial entropy density show a difference of (7.0 ±0.9)%with an additional variation of +1.8% when including RHIC data in the TRENTo parameter extraction. These observations provide new experimental constraints for viscous effects in the hydrodynamic modeling of the expanding quark–gluon plasma produced in heavy-ion collisions at the LHC

Burnett function expansions with a bi-Maxwellian weight function for electron swarm physics

In the solution of Boltzmann's equation by polynomial expansion techniques it is important to choose the weight function as close as possible to the actual distribution function in order to ensure rapid convergence. In the case of electron motion through neutral gases in the presence of external electric and magnetic fields, the so-called moment method has had considerable success. The method is essentially a polynomial expansion of the electron velocity distribution function about a Maxwellian weight function at some arbitrary temperature. By choosing the temperature carefully in order to approximate the actual distribution adequate convergence can usually be obtained. However when the interactions between the electrons and the molecules is `soft' and/or reactive processes cause a significant increase in the population of the high energy tail of the distribution function, convergence of the expansion rapidly deteriorates and may not be achieved. In this article we investigate the use of a bi-Maxwellian weight function to improve convergence by the use of a model interaction between the electrons and molecules. The idea being that a Maxwellian at the lower temperature should be sufficient to characterise the electrons in the bulk of the distribution, while a second Maxwellian of smaller amplitude but at a some what higher temperature is used to characterised the electrons in the high energy tail of the distribution

Flux and reactive contributions to electron transport in methane

A previously developed theoretical analysis (Nolan et al. 1997) is applied to the study of electron transport in methane for reduced electricfields in the range 1 to 1000 Td. The technique of analysis identifies the flux and reactive components of the measurable transport, without resort to the two-term approximation. A comparison of the results of the Monte Carlo method with those of a multiterm Boltzmann equation analysis (Ness and Robson 1986) shows good agreement. The sensitivity of the modelled electron transport to post-ionisation energy partitioning is studied by comparison of three ionisation energy partitioning regimes at moderate (300 Td) and high (1000 Td) values of the reduced electricfield

Transport coefficients for electrons in water vapor: definition, measurement, and calculation

Comparison of experimental and theoretical transport data for electron swarms in water vapour over a wide range of fields provides a rigorous test of (e(-), H(2)O) scattering cross sections over a correspondingly broad range of energies. That like should be compared with like is axiomatic, but the definition of transport coefficients at high fields, when non-conservative processes are significant, has long been contentious. This paper revisits and distills the most essential aspects of the definition and calculation of transport coefficients, giving numerical results for the drift velocity and ionisation coefficient of electrons in water vapour. In particular, the relationship between the theoretically calculated bulk drift velocities of [K. F. Ness and R. E. Robson, Phys. Rev. A 38, 1446 (1988)] and the experimental "arrival time spectra" drift velocity data of Hasegawa et al. [J. Phys. D 40(8), 2495 (2007)] is established. This enables the Hasegawa et al. data to be reconciliated with the previous literature, and facilitates selection of the best (e(-), H(2)O) cross section set

Visualization of ion and electron velocity distribution functions in electric and magnetic fields

The Boltzmann equation for ions and electrons in gases subject to arbitrarily oriented electric and magnetic fields is solved by a recently developed unified 'multiterm' theory (White et al 1999a), and attention is focused on portrayal of the velocity distribution function. In particular, we take 'slices' in velocity space to elucidate the effects of changing orientation angle, magnetic field strength and the ion to gas molecule mass ratio. The first results for electrons in CO2 and ion swarms in electric and magnetic fields are presented in this way. The implications of symmetries are discussed

Development of swarm transport theory in radio-frequency electric and crossed electric and magnetic fields

The advancements associated with modern day technology demands incorporation of the best physical understanding and the most accurate modelling of charged particle motion in gases. In recent times there have been major advances in the fundamental swarm transport theory, and this is the subject of the present paper. We start from 1986, when “multi-term” solutions of Boltzmann’s equation for static electric fields had been developed to a sophisticated level, and proceed through to the present day, where the theory is motivated far more by application to industrial processes, which involve both electric and magnetic fields , either static or time varying. We present a unified time-dependent multi-term solution of Boltzmann’s equation, emphasising the common methods and techniques underlying the treatment of all these situations, whether they be for electron or ion swarms. New and significant numerical results are presented to highlight the rich and diverse range of phenomena which are observed

Computation of electron and ion transport properties in gases

A unified multi-term solution of Boltzmann's equation is used to investigate transport properties and velocity distribution functions of charged particles in gases under the influence of an electric field. The effects of the charged-particle to neutral-particle mass ratio on the convergence of the polynomials expansion techniques employed are addressed physically

Computation of electron and ion transport properties in gases

A unified multi-term solution of Boltzmann's equation is used to investigate transport properties and velocity distribution functions of charged particles in gases under the influence of an electric field. The effects of the charged-particle to neutral-particle mass ratio on the convergence of the polynomials expansion techniques employed are addressed physically