Search

The multiplicity distribution of charged particles produced in the central rapidity ($|\eta|<2.5$) region is calculated for the eikonal and the $U$-matrix unitarization schemes using the AGK cutting rules and compared with the ATLAS 13 TeV data. The data favours the eikonal approach., Comment: 5 pages, 3 figures

We perform the analysis of elastic scattering $pp$ and $\bar{p}p$ data at low momentum transfer $|t|<0.1$ GeV$^2$ within large collider energy interval $\sqrt{s}=50$ GeV - 13 TeV in order to evaluate quantitatively the possible Odderon contribution. We use the two-channel eikonal model, which naturally accounts for the screening of the Odderon amplitude by $C$-even (Pomeron) exchanges., Comment: 17 pages, 8 figures; v2 minor changes, typo corrected, a footnote added, version to be published in Phys. Rev. D

We study the high-energy behavior of the elastic scattering amplitude using two distinct unitarization schemes: the eikonal and the $U$-matrix. Our analysis begins with a formalism involving solely Pomerons, incorporating pion-loop insertions in the Pomeron trajectory representing the nearest singularity generated by $t$-channel unitarity. Subsequently, we explore a scenario that includes the presence of an Odderon. In our analyses, we explore the tension between the TOTEM and the ATLAS measurements for $\sigma_{tot}$ and $d\sigma/dt$ at 7, 8, and 13 TeV, and the subsequent implications for the properties of both the Pomeron and Odderon. Our results show that the Odderon phase factor $\xi_{\Bbb O}= -1$ is favored in both unitarization schemes. More interestingly, this specific phase factor stands as the sole one that aligns with results consistent with a non-zero Odderon coupling., Comment: 15 pages, 2 figures, 4 tables

Indium incorporation into wurtzite (0001)-oriented In$_{x}$Al$_{y}$Ga$_{1-x-y}$N layers grown by plasma-assisted molecular beam epitaxy was studied as a function of the growth temperature (565-635 $^{\circ}$C) and the AlN mole fraction (0.01 < y < 0.27). The layer stoichiometry was determined by Rutherford backscattering spectrometry (RBS). RBS shows that Indium incorporation decreased continuously with increasing growth temperature due to thermally enhanced dissociation of In-N bonds and for increasing AlN mole fractions. High resolution x-ray diffraction and transmission electron microscopy (TEM) measurements did not show evidence of phase separation. The mosaicity of the quaternary layers was found to be mainly determined by the growth temperature and independent on alloy composition within the range studied. However, depending on the AlN mole fraction, nanometer-sized composition fluctuations were detected by TEM. Photoluminescence spectra showed a single broad emission at room temperature, with energy and bandwidth S- and W-shaped temperature dependences typical of exciton localization by alloy inhomogeneities. Cathodo-luminescence measurements demonstrated that the alloy inhomogeneities, responsible of exciton localization, occur on a lateral length scale below 150 nm, which is corroborated by TEM.

We investigate the two-gluon-exchange model of the Pomeron using nonperturbative gluon propagators characterized by a dynamical mass scale. We present the results for an analysis of the available $pp$ differential cross section data at TeV energies which accounts for dynamical gluon masses obtained from a non-linear version of the Schwinger-Dyson equations. We show that our two-gluon exchange model gives a very good description of the LHC data, provided we demand the Reggeization of the scattering amplitude and make a suitable choice for the convolution of proton wave functions., Comment: 9 pages, 6 figures, 2 tables, text improved, references added. Version to be published in Phys. Rev. D

New plasma regimes with high confinement, low core impurity accumulation and small Edge localized mode (ELMs) perturbations have been obtained close to ITER conditions in magnetically confined plasmas from the Joint European torus (JET) tokamak. Such regimes are achieved by means of optimized particle fuelling conditions which trigger a self-organize state with a strong increase in rotation and ion temperature and a decrease of the edge density. An interplay between core and edge plasma regions leads to reduced turbulence levels and outward impurity convection. These results pave the way to an attractive alternative to the standard plasmas considered for fusion energy generation in a tokamak with metallic wall environment such as the ones expected in ITER

We investigate non-inertial effects on $CP$-violating processes using a model, based on the framework of quantum field theory in curved spacetimes, devised to account for the decay of accelerated particles. We show that the $CP$ violation parameter for the decay of accelerated kaons into two pions decreases very slightly as very high accelerations are achieved, implying decreased asymmetry between matter and antimatter in this regime. We discuss the relationship between these results and cosmological processes surrounding matter-antimatter asymmetry and argue that, due to the connection between non-inertial and thermal phenomena established by the Unruh effect, this kind of computation may prove useful in furthering the understanding of thermodynamical effects in curved spacetimes., Comment: 15 pages, 1 table, 4 figures

The LHC has released precise measurements of elastic proton-proton scattering that provide a unique constraint on the asymptotic behavior of the scattering amplitude at high energies. Recent reanalyses of part of these data indicate that the central values of some forward quantities would be different than initially observed. We introduce correlation information between the original and the reanalyzed data sets in a way suitable for a global fitting analysis of all data. The careful treatment of correlated errors leads to much less stringent limits on the $\rho$ uncertainty and sets up the stage for describing the forward data using a scattering amplitude dominated by only crossing-even terms. In the light of these correlated data we determine the parameters of the soft Pomeron from the Regge theory. We use Born-level and eikonalized amplitudes. In the Born-level case we estimate the contribution of the double Pomeron exchange, while in the latter case we investigate the role of the eikonalization in both the one- and two-channel models. The role of the proton-Pomeron vertex form and of the nearest $t$-channel singularity in the Pomeron trajectory receives particular attention. We discuss the implications of our results and present predictions for the total cross section and the $\rho$ parameter in proton-proton collisions at LHC and cosmic ray energies., Comment: 17 pages, 6 figures, 5 tables, minor typos fixed. Version to be published in Phys. Rev. D. arXiv admin note: text overlap with arXiv:1908.01040

An analysis of accelerated kaon decays based on the Unruh effect shows a slight decrease in a CP-violation parameter for very high accelerations, as a consequence of the previously know increase in decay rates for non-inertial systems. Its consequences on the understanding of the relation between thermal and non-inertial phenomena are briefly discussed., Comment: 4 pages, 1 table, 2 figures, to be published in Astronomische Nachrichten. Contribution to the Proceedings of the 9th International Workshop on Astronomy and Relativistic Astrophysics, Mexico City, Mexico, 6-12 September, 2020

We consider the effect of higher twist operators of the Wilson operator product expansion in the structure function $F_{2}(x,Q^{2})$ at small-$x$, taking into account QCD effective charges whose infrared behavior is constrained by a dynamical mass scale. The higher twist corrections are obtained from the renormalon formalism. Our analysis is performed within the conventional framework of next-to-leading order, with the factorization and renormalization scales chosen to be $Q^{2}$. The infrared properties of QCD are treated in the context of the generalized double-asymptotic-scaling approximation. We show that the corrections to $F_{2}$ associated with twist-four and twist-six are both necessary and sufficient for a good description of the deep infrared experimental data., Comment: 19 pages, 6 figures, text improved, references added, gluon distribution function calculated. To be published in Phys. Lett. B

The role of low-$x$ parton dynamics in dictating the high-energy behavior of forward scattering observables at LHC energies is investigated using a QCD-based model with even-under-crossing amplitude dominance at high-energies. We explore the effects of different sets of pre- and post-LHC fine-tuned parton distributions on the forward quantities $\sigma_{tot}$ and $\rho$, from $pp$ and $\bar{p}p$ scattering in the interval 10 GeV - 13 TeV. We also investigate the role of the leading soft contribution, the low-energy cuttoff, and the energy dependence of the semihard form factor on these observables. We show that in all cases investigated the highly restrictive data on $\rho$ parameter at $\sqrt{s}=13$ TeV indicate that a crossing-odd component may play a crucial role in forward elastic scattering at the highest energies. In the Regge language an odd-under-crossing object is called Odderon., Comment: 15 pages, 9 tables, 9 figures. One table and two references added, minor revision and discussions improved

Recently the TOTEM experiment at the LHC has released measurements at $\sqrt{s} = 13$ TeV of the proton-proton total cross section, $\sigma_{tot}$, and the ratio of the real to imaginary parts of the forward elastic amplitude, $\rho$. Since then an intense debate on the $C$-parity asymptotic nature of the scattering amplitude was initiated. We examine the proton-proton and the antiproton-proton forward data above 10 GeV in the context of an eikonal QCD-based model, where nonperturbative effects are readily included via a QCD effective charge. We show that, despite an overall satisfactory description of the forward data is obtained by a model in which the scattering amplitude is dominated by only crossing-even elastic terms, there is evidence that the introduction of a crossing-odd term may improve the agreement with the measurements of $\rho$ at $\sqrt{s} = 13$ TeV. In the Regge language the dominant even(odd)-under-crossing object is the so called Pomeron (Odderon)., Comment: 5 pages, 2 figures, 1 table. Phenomenological approach revised, results and conclusions changed, suggesting now the presence of Odderon effects in forward scattering (once confirmed the TOTEM data at 13 TeV)

Recent data from LHC13 by the TOTEM Collaboration have indicated an unexpected decrease in the value of the $\rho$ parameter and a $\sigma_{tot}$ value in agreement with the trend of previous measurements at 7 and 8 TeV. These data at 13 TeV are not simultaneously described by the predictions from Pomeron models selected by the COMPETE Collaboration, but show agreement with the maximal Odderon dominance, as recently demonstrated by Martynov and Nicolescu. Here we present a detailed analysis on the applicability of Pomeron dominance by means of a general class of forward scattering amplitude, consisting of even-under-crossing leading contributions associated with single, double and triple poles in the complex angular momentum plane. We carry out fits to $pp$ and $\bar{p}p$ data in the interval 5 GeV - 13 TeV. The data set comprises all the accelerator data below 7 TeV and we consider two independent ensembles by adding either only the TOTEM data or TOTEM and ATLAS data at the LHC energy region. In the data reductions to each ensemble the uncertainty regions are evaluated with both one and two standard deviation ($\sim$ 68 \% and $\sim$ 95 \% CL, respectively). Besides the general analytic model, we investigate four particular cases of interest, three of them typical of outstanding models in the literature. We conclude that, within the experimental and theoretical uncertainties and both ensembles, the general model and three particular cases are not able to describe the $\sigma_{tot}$ and $\rho$ data at 13 TeV simultaneously. However, if the discrepancies between the TOTEM and ATLAS data are not resolved, one Pomeron model, associated with double and triple poles and with only 7 free parameters, seems not to be excluded by the complete set of experimental information presently available., Comment: 27 pages, 10 figures, 5 tables, minor corrections and one paragraph added in the Introduction; Accepted for publication in Phys. Rev. D

Considering, as a limit case, an approximately flat pion distribution amplitude, which is determined from the hardest, in momentum space, solution of the Bethe-Salpeter equation for the pion wave function, we compute the pion transition form factor $F_{\pi\gamma\gamma^{*}}(Q^{2})$ and the pion form factor $F_{\pi}(Q^2)$, taking into account the LO as well as NLO form of the hard coefficient function entering the leading-twist factorization formula. We also compute the exclusive photoproduction of pions pairs at high energies, $\gamma \gamma \to \pi^{+}\pi^{-}$, where perturbative QCD can be used to compute the hard scattering matrix elements. We verify that the existent data for exclusive pion production can be reasonably described as a function of such flat distribution amplitude., Comment: 9 pages, 5 figures, extended discussion, new references, version to appear in J. Phys. G

Recent data from LHC13 by the TOTEM Collaboration on $\sigma_{tot}$ and $\rho$ have indicated disagreement with all the Pomeron model predictions by the COMPETE Collaboration (2002). On the other hand, as recently demonstrated by Martynov and Nicolescu (MN), the new $\sigma_{tot}$ datum and the unexpected decrease in the $\rho$ value are well described by the maximal Odderon dominance at the highest energies. Here, we discuss the applicability of Pomeron dominance through fits to the \textit{most complete set} of forward data from $pp$ and $\bar{p}p$ scattering. We consider an analytic parametrization for $\sigma_{tot}(s)$ consisting of non-degenerated Regge trajectories for even and odd amplitudes (as in the MN analysis) and two Pomeron components associated with double and triple poles in the complex angular momentum plane. The $\rho$ parameter is analytically determined by means of dispersion relations. We carry out fits to $pp$ and $\bar{p}p$ data on $\sigma_{tot}$ and $\rho$ in the interval 5 GeV - 13 TeV (as in the MN analysis). Two novel aspects of our analysis are: (1) the dataset comprises all the accelerator data below 7 TeV and we consider \textit{three independent ensembles} by adding: either only the TOTEM data (as in the MN analysis), or only the ATLAS data, or both sets; (2) in the data reductions to each ensemble, uncertainty regions are evaluated through error propagation from the fit parameters, with 90 \% CL. We argument that, within the uncertainties, this analytic model corresponding to soft Pomeron dominance, does not seem to be excluded by the \textit{complete} set of experimental data presently available., Comment: 10 pages, 4 figures, 1 table. Two paragraphs and four references added. Accepted for publication in Phys. Lett. B

The recent data by the TOTEM Collaboration on $\sigma_{tot}$ and $\rho$ at 13 TeV, have shown agreement with a leading Odderon contribution at the highest energies, as demonstrated in the very recent analysis by Martynov and Nicolescu (MN). In order to investigate the same dataset by means of Pomeron dominance, we introduce a general class of forward scattering amplitude, with leading contributions even under crossing, associated with simple, double and triple poles in the complex angular momentum plane. For the lower energy region, we consider the usual non-degenerated Regge trajectories, with even and odd symmetry. The analytic connection between $\sigma_{tot}$ and $\rho$ is obtained by means of dispersion relations and we carry out fits to $pp$ and $\bar{p}p$ data in the interval $\sqrt{s}=5$ GeV - 13 TeV; following MN we consider only the TOTEM data at the LHC energy region. From the fits, we conclude that the general analytic model, as well as some particular cases representing standard parameterizations, are not able to describe satisfactorily the $\sigma_{tot}$ and $\rho$ data at 13 TeV. Further analyses in course and some perspectives are outlined., Comment: 4 pages, 1 table, 1 figure, misprints corrected, minor changes in the text, four references added, one sentence and one reference updated. Poster presented at the XIV International Workshop on Hadron Physics, Florian\'opolis, Brazil, March 2018

We study infrared contributions to semihard parton-parton interactions by considering an effective charge whose finite infrared behavior is constrained by a dynamical mass scale. Using an eikonal QCD-based model in order to connect this semihard parton-level dynamics to the hadron-hadron scattering, we obtain predictions for the proton-proton ($pp$) and antiproton-proton ($\bar{p}p$) total cross sections, $\sigma_{tot}^{pp,\bar{p}p}$, and the ratios of the real to imaginary part of the forward scattering amplitude, $\rho^{pp,\bar{p}p}$. We discuss the theoretical aspects of this formalism and consider the phenomenological implications of a class of energy-dependent form factors in the high-energy behavior of the forward amplitude. We introduce integral dispersion relations specially tailored to relate the real and imaginary parts of eikonals with energy-dependent form factors. Our results, obtained using a group of updated sets of parton distribution functions (PDFs), are consistent with the recent data from the TOTEM, AUGER and Telescope Array experiments., Comment: 29 pages, 6 figures, 3 tables; version to be published in Phys. Rev. D

We consider the effect of an energy-dependent dipole form factor in the high-energy behavior of the forward amplitude. The connection between the semihard parton-level dynamics and the hadron-hadron scattering is established by an eikonal QCD-based model. Our results for the proton-proton ($pp$) and antiproton-proton ($\bar{p}p$) total cross sections, $\sigma_{tot}^{pp,\bar{p}p}(s)$, obtained using the CTEQ6L1 parton distribution function, are consistent with the recent data from the TOTEM experiment., Comment: 7 pages, 1 figure; Contribution to Proceedings of XIIIth International Workshop on Hadron Physics, Angra dos Reis, Brazil, 22-27 March, 2015

The pion distribution amplitude (DA) can be related to the fundamental QCD Green's functions as a function of the quark self-energy and the quark-pion vertex, which in turn are associated with the pion wave function through the Bethe-Salpeter equation. Considering the extreme hard asymptotic behavior in momentum space allowed for a pseudoscalar wave function, which is limited by its normalization condition, we compute the pion DA and its second moment. From the resulting amplitude, representing the field theoretical upper limit on the DA behavior, we calculate the photon-pion transition form factor $F_{\pi\gamma\gamma^{\ast}}(Q^{2})$. The resulting upper limit on the pion transition form factor is compared with existing data published by CLEO, BaBar and Belle collaborations., Comment: 10 pages, 2 figures; version to appear in J. Phys. G

We discuss observations of InN nanowires (NWs) by plan-view high-resolution transmission electron microscopy (TEM). The main difficulties arise from suitable methods available for plan-view specimen preparation. We explore different approaches and find that the best results are obtained using a refined preparation method based on the conventional procedure for plan-view TEM of thin films, specifically modified for the NW morphology. The fundamental aspects of such a preparation are the initial mechanical stabilization of the NWs and the minimization of the ion-milling process after dimpling the samples until perforation. The combined analysis by plan-view and cross-sectional TEM of the NWs allows determination of the degree of strain relaxation and reveals the formation of an unintentional shell layer (2-3 nm thick) around the InN NWs. The shell layer is composed of bcc-In2O3 nanocrystals with a preferred orientation with respect to the wurtzite InN: In2O3 [111] || InN [0001] and In2O3 <110> || InN <11-20>.