Axion assisted Schwinger effect

We point out an enhancement of the pair production rate of charged fermions in a strong electric field in the presence of time dependent classical axion-like background field, which we call axion assisted Schwinger effect. While the standard Schwinger production rate is proportional to $$ \exp \left(-\pi \left({m}^2+{p}_T^2\right)/E\right) $$ exp − π m 2 + p T 2 / E , with m and pT denoting the fermion mass and its momentum transverse to the electric field E, the axion assisted Schwinger effect can be enhanced at large momenta to exp(−πm2/E). The origin of this enhancement is a coupling between the fermion spin and its momentum, induced by the axion velocity. As a non-trivial validation of our result, we show its invariance under field redefinitions associated with a chiral rotation and successfully reproduce the chiral anomaly equation in the presence of helical electric and magnetic fields. We comment on implications of this result for axion cosmology, focussing on axion inflation and axion dark matter detection.

Spectra and mean multiplicities of $$\pi ^{-}$$ in central $${}^{40}$$Ar+$${}^{45}$$Sc collisions at 13A, 19A, 30A, 40A, 75A and 150$$A\,\text{ Ge }\text{ V }\!/\!\textit{c}$$ beam momenta measured by the NA61/SHINE spectrometer at the CERN SPS

The physics goal of the strong interaction program of the NA61/SHINE experiment at the CERN Super Proton Synchrotron (SPS) is to study the phase diagram of hadronic matter by a scan of particle production in collisions of nuclei with various sizes at a set of energies covering the SPS energy range. This paper presents differential inclusive spectra of transverse momentum, transverse mass and rapidity of $$\pi ^{-}$$ π - mesons produced in central$${}^{40}$$ 40 Ar+$${}^{45}$$ 45 Sc collisions at beam momenta of 13A, 19A, 30A, 40A, 75A and 150$$A\,\text{ Ge }\text{ V }\!/\!\textit{c}$$ A Ge V / c . Energy and system size dependence of parameters of these distributions – mean transverse mass, the inverse slope parameter of transverse mass spectra, width of the rapidity distribution and mean multiplicity – are presented and discussed. Furthermore, the dependence of the ratio of the mean number of produced pions to the mean number of wounded nucleons on the collision energy was derived. The results are compared to predictions of several models.

Charged-pion production in $$\mathbf {Au+Au}$$ collisions at $$\sqrt{\mathbf {s}_{\mathbf {NN}}} = 2.4~{\mathbf {GeV}}$$

We present high-statistic data on charged-pion emission from Au + Au collisions at $$\sqrt{s_{\mathrm{NN}}} = 2.4~\hbox {GeV}$$ s NN = 2.4 GeV (corresponding to $$E_{beam} = 1.23~\hbox {A GeV}$$ E beam = 1.23 A GeV ) in four centrality classes in the range 0–40% of the most central collisions. The data are analyzed as a function of transverse momentum, transverse mass, rapidity, and polar angle. Pion multiplicity per participating nucleon decreases moderately with increasing centrality. The polar angular distributions are found to be non-isotropic even for the most central event class. Our results on pion multiplicity fit well into the general trend of the available world data, but undershoot by $$2.5~\sigma $$ 2.5 σ data from the FOPI experiment measured at slightly lower beam energy. We compare our data to state-of-the-art transport model calculations (PHSD, IQMD, PHQMD, GiBUU and SMASH) and find substantial differences between the measurement and the results of these calculations.

Soft interactions in jet quenching

We study the collisional aspects of jet quenching in a high-energy nuclear collision, especially in the final state pion gas. The jet has a large energy, and acquires momentum transverse to its axis more effectively by multiple soft collisions than by few hard scatterings (as known from analogous systems such as J/ψ production at Hera). Such regime of large E and small momentum transfer corresponds to Regge kinematics and is characteristically dominated by the pomeron. From this insight we estimate the jet quenching parameter in the hadron medium (largely a pion gas) at the end of the collision, which is naturally small and increases with temperature in line with the gas density and compare it to the jet quenching parameter obtained within the quark–gluon plasma (QGP) phase in widely known perturbative approximations. The physics in the quark–gluon plasma/liquid phase is less obvious, and here we revisit a couple of simple estimates that suggest indeed that the pomeron-mediated interactions are very relevant and should be included in analysis of the jet quenching parameter. Finally, since the occasional hard collisions produce features characteristic of a Lèvy flight in the [Formula: see text] plane perpendicular to the jet axis, we suggest one- and two-particle q⊥correlations as interesting experimental probes sensitive to the nature (softness versus hardness) of the interactions of a jet inside the QGP.

Analysis of Δ0(1232) production in collisions of protons with carbon nuclei at 4.2 GeV/c

The production of Δ0(1232)-resonances in p+12C collisions at 4.2 GeV/c was analyzed with 4π acceptance. The mass distribution of Δ0(1232) was reconstructed using an angular criterion. The fraction of charged π −-mesons coming from Δ0(1232) decay was estimated and compared to those obtained in earlier works. The momentum, transverse momentum, kinetic energy, and rapidity distributions as well as invariant cross sections of Δ0(1232)-resonances were reconstructed in the laboratory frame. The mean kinematical characteristics of the reconstructed Δ0(1232) were compared to those of participant protons in experiment and within some of the models. The freeze-out temperature of Δ0(1232) estimated in the present analysis was compared with those obtained using different methods for Δ(1232) produced with other sets of colliding nuclei at various incident energies. The relative number of nucleons excited to Δ0 (1232) at freeze-out conditions in p+12C collisions was estimated.