Meson Ratios Research Articles

Measurements of charm quark production and hadronization at CMS

The study of charm quark hadrons is an important probe into the hadronization of heavy quarks. More specifically, we present results on the production of Λc baryon, the nuclear modification factors (RAA), and the Λc/D0 yield ratios at √SNN = 5.02 TeV in proton-proton (pp) collisions and in different centrality regions in lead-lead (PbPb) collisions, using data recorded with the CMS detector in 2017 and 2018, respectively. The reported RAA for Λc provides useful information regarding the energy loss mechanism and the hadronization processes of charm quark in the quark-gluon plasma. The transverse momentum (pT) dependence of the RAA is similar to that of other charm and beauty hadrons but with its minimum shifted towards higher pT. Comparing the Λc/D0 production ratio in pp and PbPb collisions suggests that coalescence as a hadronization process is not significant for pT > 10 GeV/c. The ratio becomes comparable to the measurements in e+e− collisions for pT > 30 GeV/c. We also present results of the Λc baryon and D0 meson production and their ratios in proton-lead (pPb) collisions at √SNN = 8.16 TeV as a function of pT and final-state multiplicity using the data recorded by the CMS experiment in 2016. We do not observe significant multiplicity dependence for the baryon over meson ratio for charm hadrons. Based on a previous study, the difference between the results from charm quarks and those from light quarks suggests coalescence processes for heavy quarks do not increase further with multiplicity, unlike light quarks.

Singlet vector leptoquark model facing recent LHCb and BABAR measurements

Very recently the LHCb experiment released the first measurement of the ratio R(Λc)=BR(Λb→Λcτν¯τ)/BR(Λb→Λcμν¯μ). Moreover, the BABAR experiment reported a new result of the leptonic decay ratio of Upsilon meson ϒ(3S), namely, Rϒ(3S)=BR(ϒ(3S)→τ+τ−)/BR(ϒ(3S)→μ+μ−). Both measurements are below their corresponding Standard Model predictions (deficit), deviating by ∼1.1σ and ∼1.8σ, respectively. In addition, the LHCb recently presented the first search of the lepton flavor violating decay B0→K⁎0μ±τ∓. Motivated by these new data, in this work we study their impact on the phenomenology of the singlet vector leptoquark (U1) model addressing the hints of lepton flavor universality violation in the semileptonic decays of B mesons (B meson anomalies), by carrying out a global fit analysis. In general, we found that a minimal version of the U1 model with a mass of 2 TeV can successfully explain the B meson anomalies, while being compatible with all other flavor observables and LHC bounds. Interestingly, our study shows that the new observables R(Λc) and Rϒ(3S) generate strong tension, leading to non-trivial effects on the global fit. Future improvements at the LHCb and Belle II experiments would help to understand their complementary. Moreover, we also analyze the impact of the expected sensitivity on flavor observables at Belle II to provide a further test of the U1 model. Finally, we study the minimal assumptions under which the U1 model could, in addition, provide a combined explanation of the anomalous magnetic moment of the muon.

Influence of Cosmic-Ray Spectrum and Hadron—Nucleus Interaction Model on the Properties of High-Energy Atmospheric-Neutrino Fluxes

The neutrino/antineutrino flux ratios ( $${\nu _e}/{\overline \nu _e}$$ and $${\nu _\mu }/{\overline \nu _\mu }$$ ) and the flavor ratio $$\left( {{\nu _\mu } + {{\overline \nu }_\mu }} \right)/\left( {{\nu _e} + {{\overline \nu }_e}} \right)$$ clearly demonstrate the distinctions between the predictions of hadronic-interaction models for the production spectra of pions and kaons, which are the main sources of atmospheric neutrinos in the region of Eν ≲ 500 TeV. The values of the $${\nu /\overline \nu }$$ ratio are sensitive to variations in the π+/π−, π/K, and K+/K− meson ratios, which are determined by the cross sections for inclusive meson-production processes in hadron-nucleus (hA) collisions; the elemental composition of cosmic rays also exerts influence, through the p/n ratio, on the development of hadronic cascades and on the neutrino ratios. The difference in $${\nu /\overline \nu }$$ and in the flavor ratio for a number of hadronic-interaction models is shown on the basis of calculating neutrino fluxes at energies in the range between 102 and 108 GeV. A comparison of the calculated results with new experimental data shows the reliability of this calculation, which, on the whole, reflects correctly the mechanism of atmospheric-neutrino production. The spectra of atmospheric muon neutrinos from calculations based on the Kimel-Mokhov, SIBYLL 2. 1, and EPOS LHC models describe fairly well experimental results. At energies in the range of 1–500 TeV, the curves calculated within these models combined with the Hillas-Gaisser spectrum are close to the best fit to data from measurements in the IceCube experiment.

Spectator-induced EM Effects on Charged Meson Ratios in Heavy-ion Collisions

Spectator-induced EM Effects on Charged Meson Ratios in Heavy-ion Collisions

Strange Baryon to Meson Ratio

We present a model to compute baryon and meson transverse momentum distributions, and their ratios, in relativistic heavy-ion collisions. The model allows to compute the probability to form colorless bound states of either two or three quarks as functions of the evolving density during the collision. The qualitative differences of the baryon to meson ratio for different collision energies and for different particle species can be associated to the different density dependent probabilities and to the combinatorial factors which in turn depend on whether the quarks forming the bound states are heavy or light. We compare to experimental data and show that we obtain a good description up to intermediate values of pt.

Strange baryon vs meson ratio in near-side and away-side jets in p$+$p collisions at ALICE using azimuthal correlations

Strange baryon vs meson ratio in near-side and away-side jets in p$+$p collisions at ALICE using azimuthal correlations

Present and future investigations of hadron formation mechanisms in Pb–Pb collisions at the LHC with the ALICE experiment

The present status and the future of hadron production characterization in Pb–Pb collisions at LHC by measuring the p T dependence of baryon over meson ratios is presented. The current RHIC and LHC results showing a strong enhancement of the Λ/K S 0 ratio in Pb–Pb collisions with respect to proton-proton collisions at intermediate p T and their interpretation in terms of coalescence processes are discussed. A crucial extension of these studies to heavy flavor hadrons (D, B mesons and Λ c , Λ b baryons) will be enabled by the foreseen reforging of the ALICE Inner Tracking System (ITS). The main performance objectives and technological ingredients of the ITS upgrade are summarized.

Dynamical fluctuations in baryon–meson ratios

The event-by-event dynamical fluctuations in kaon–proton and proton–pion ratios have been studied as a function of the center-of-mass energies of nucleon–nucleon collisions . Based on the changing phase space volume which apparently is the consequence of the phase transition(s) from hadrons to a quark–gluon plasma at large , the single-particle distribution function f is assumed to be modified. Varying f and the phase space volume are implemented in the grand-canonical ensemble, especially at GeV, so that the hadron resonance gas model, when taking into account the experimental acceptance and the quark phase space occupation factor γ, turns out to be able to reproduce the dynamical fluctuations in and ratios over the entire range of . It seems that the applied thermodynamic statistics could come up with a new modification to the Tsallis statistics. Both non-extensivity and changing the phase space are suggested and utilized in the present work.

Meson spectra andmTscaling inp+p,d+ Au, and Au + Au collisions atsNN=200GeV

The meson spectra provide insight into the particle production mechanism and interaction in the hadronic and quark gluon plasma (QGP) phases. The detailed study of systematics of meson spectra is important also because it acts as an ingredient for estimating the hadronic decay backgrounds in the photon, single lepton, and dilepton spectra which are the penetrating probes of quark gluon plasma. In this work, we parametrize experimentally measured pion spectra and then obtain the spectra of other light mesons using a property known as ${m}_{T}$ scaling. The ${m}_{T}$ scaled spectra for each meson is compared with experimental data for $p+p$, $d$ + Au, and Au + Au systems at $\ensuremath{\surd}{s}_{NN}$ = 200 GeV. The agreement of the ${m}_{T}$ scaled and experimental data shapes are excellent in most cases and their fitted relative normalization gives ratio of meson to pion ${m}_{T}$ spectra. These ratios are useful to obtain the hadronic decay contribution in photonic and leptonic channels but also point to the quantitative changes in the dynamics of the heavy ion collision over $p+p$ collisions. It is shown that, the particles with charm contents behave differently as compared to pions in $d$ + Au systems and particles either with strange or charm contents behave differently from pions in Au + Au systems. For Au + Au system, three centrality classes have been studied which reveal that for the particles, such as kaon and $\ensuremath{\varphi}$, peripheral collision data are better reproduced as compared to central collision data and their relative ratios with pions also increase as the collisions become more central.

Very high momentum particle identification at the LHC

The anomalies observed at RHIC for the baryon — meson ratios have prompted a number of theoretical works on the nature of the hadrochemistry in the hadronisation stage of the pp collisions and in the evolution of the dense system formed in heavy ion collisions. Although the predictions differ in the theoretical approach, generally a substantial increase in the baryon production is predicted in the range 10–30 GeV/c. We will present the possibilities of a gas ring imaging Cherenkov detector of limited acceptance which would be able to identify track-by-track protons until 24 GeV/c for the ALICE experiment.

PROBING THE SYMMETRY ENERGY AT HIGH BARYON DENSITY WITH HEAVY ION COLLISIONS

The nuclear symmetry energy at densities above saturation density (ρ0 ~ 0.16fm-3) is poorly constrained theoretically and very few relevant experimental data exist. Its study is possible through Heavy Ion Collisions (HIC) at energies E/A > 200 MeV , particularly with beams of neutron-rich radioactive nuclei. The energy range implies that the momentum dependence of the isospin fields, i.e. the difference of the effective masses on protons and neutrons, also has to be investigated before a safe constraint on E sym (ρ) is possible. We discuss the several observables which have been suggested, like n/p emission and their collective flows and the ratio of meson yields with different isospin projection, π-/π+ and K0/K+. We point out several physical mechanisms that should be included in the theoretical models to allow a direct comparison to the more precise experiments which will be able to distinguish the isospin projection of the detected particles: CSR/Lanzhou, FAIR/GSI, RIBF/RIKEN, FRIB/MSU.

Study of strange particle production in pp collisions with the ALICE detector

ALICE is well suited for strange particle production studies since it has very good reconstruction capabilities in the low transverse momentum (pt) region and it also allows the identification to be extended up to quite high pt. Charged strange mesons (K+, K−) are reconstructed via energy loss measurements whereas neutral strange mesons (K0s) and strange hyperons (Λ, Ξ, Ω) are identified via vertex reconstruction. All these particles carry important information: firstly, the measurement of production yields and the particle ratio within the statistical models can help to understand the medium created, and secondly the dynamics at intermediate pt investigated via the baryon over meson ratio (Λ/K0s) allows a better understanding of the hadronization mechanisms and of the underlying event processes. We present these two aspects of the strange particle analysis in pp collisions using simulated data.

Semileptonic decays of charmed and beauty baryons with heavy sterile neutrinos in the final state

We obtain tree-level estimates of various differential branching ratios of heavy baryon decays with massive sterile neutrinos ${\ensuremath{\nu}}_{x}$ in the final state. Generally, charmed baryons are found to be less promising than charmed mesons, in contrast to $b$ hadrons. In the latter case, branching ratios of beauty mesons and baryons into sterile neutrinos are of the same order. As a consequence, at high energies beauty baryons give contribution to sterile neutrino production comparable to the contribution of beauty mesons (up to about 15%). Experimental limits on active-to-sterile mixing are quite strong for neutrinos lighter than $D$ mesons but for heavier neutrinos they are weaker. As an example, for neutrino masses in the range $2\text{ }\text{ }\mathrm{GeV}\ensuremath{\lesssim}{m}_{{\ensuremath{\nu}}_{x}}\ensuremath{\lesssim}2.5\text{ }\text{ }\mathrm{GeV}$, current data imply that the bounds on ${\ensuremath{\Lambda}}_{b}$-hyperon branching ratios into sterile neutrinos are $\mathrm{Br}({\ensuremath{\Lambda}}_{b}\ensuremath{\rightarrow}{\ensuremath{\Lambda}}_{c}+{e}^{\ensuremath{-}}+{\ensuremath{\nu}}_{x})\ensuremath{\lesssim}1.3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}--1.7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$ and $\mathrm{Br}({\ensuremath{\Lambda}}_{b}\ensuremath{\rightarrow}{\ensuremath{\Lambda}}_{c}+{\ensuremath{\mu}}^{\ensuremath{-}}+{\ensuremath{\nu}}_{x})\ensuremath{\lesssim}3.9\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}--1.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}$.

ALICE perspectives for the study of charm and beauty energy loss at the LHC

At LHC energy, heavy quarks will be abundantly produced and the design of the ALICE detector will allow us to study their production using several channels. The expected heavy-quark in-medium energy loss in nucleus-nucleus collisions at the LHC is calculated within a model, that is compared to the available heavy-quark quenching measurements at RHIC. The nuclear modification factors and heavy-to-light ratios of charm and beauty mesons are considered. The capability of the ALICE experiment for addressing this phenomenology is discussed.

Charged Meson Rapidity Distributions in CentralAu+AuCollisions atsNN=200 GeV

We have measured rapidity densities dN/dy of pions and kaons over a broad rapidity range (-0.1 < y < 3.5) for central Au+Au collisions at sqrt(snn) = 200 GeV. These data have significant implications for the chemistry and dynamics of the dense system that is initially created in the collisions. The full phase-space yields are 1660 +/- 15 +/- 133 (pi+), 1683 +/- 16 +/- 135 (pi-), 286 +/- 5 +/- 23 (K+) and 242 +/- 4 +/- 19 (K-). The systematics of the strange to non--strange meson ratios are found to track the variation of the baryo-chemical potential with rapidity and energy. Landau--Carruthers hydrodynamic is found to describe the bulk transport of the pions in the longitudinal direction.

Rapidity dependence of charged hadron production in central Au + Au collisions at with BRAHMS

We have measured the rapidity distributions dN/dy of π±, K± and in central Au+Au collisions at . The average rapidity loss per participant nucleon is 2.0 ± 0.2 units of rapidity. The strange to non-strange meson ratios K/π are found to track variations of the baryo-chemical potential in energy and rapidity.

Heavy quarks and QCD matter

I present recent results on the theory of QCD matter production in high energy heavy ion collisions and on the interactions of heavy quarks in such environment. The centrality and rapidity dependence of hadron production is evaluated in semi--classical approach. The energy loss of heavy quarks in matter is computed. The heavy--to--light meson ratio (e.g., D/pion) at moderate transverse momenta is demonstrated to be both sensitive to the density of color charges in the medium and infrared stable.

MASS EFFECTS IN THE QUARK–GLUON DECAYS OF HEAVY PARAQUARKONIA

Quark–gluon decays of heavy paraquarkonia [Formula: see text] are investigated with account of the masses of final quarks. The decay widths and the energy distributions of the final quarks and gluons are calculated in dependence on the relative quark masses. The strong collinear enhancement of the gluon energy distribution at the end of the spectrum is shown to take place in all such decays of ηc and ηb mesons except the decay [Formula: see text]. The total decay width is shown to have an essential dependence on the final quark masses. The corresponding branching ratios of ηc and ηb mesons are numerically estimated with a good agreement of [Formula: see text] with experimental data on ηc decays.

ON AN INCLUSIVE DETERMINATION OF HADRONIC WIDTH/MASS RATIOS

An inclusive determination of the (Γ/M) ratios of mesons and baryons is made from the ansatz that there exists a short breathing mode for the temporary dissociation of a hadron into a pair of “quasifree” quarks (q) and/or diquarks (D) with significantly lower masses than their constituent values which subsequently hadronize with certainty. The coupling scheme for mesons to these [Formula: see text] and DD* pairs (which yields Γ directly) is determined by Schwinger’s partial symmetry for quarks (which “subsumes” the Sakurai gauge principle for ρ, ω, ɸ couplings to [Formula: see text] pairs). A similar construction holds for baryon couplings to quark(q)-diquark(D) pairs, with the same overall coupling constant(gs) for both systems. Its tentative identification with the QCD value yields a surprisingly good pattern of agreement with data for both hadron types, using just two inputs (mud and ms). The Γ/M ratios (R) are expressible as αs(M2) f(J, M/mq), and indicate discreteness in R iff the standard Regge slope patterns in M2 are taken into account.

Right-handed current effect on inclusive decays of theB meson

Recent experimental values for the inclusive semileptonic decay branching ratio of theB meson are low compared to the spectator model expectation. That branching ratio is investigated in the right-handed current model. The right-handed current could enhance the nonleptonic decay mode, thereby suppressing the semileptonic branching ratio. Then, the quark mixing matrix of the right-handed sector is restricted to be of one special type and the right-handed neutrino has to be the heavy Dirac one.