Hadron Spectrum Research Articles

Measurement of the charged-hadron spectra and nuclear modification factor in lead–lead collisions with the ATLAS detector

The ATLAS experiment at the Large Hadron Collider (LHC) measures charged hadron spectra obtained in 2010 and 2011 lead–lead LHC data taking periods with total integrated statistics of 0.15 nb−1. The results are compared to the pp spectra of charged hadrons at the same centre-of-mass energy based on the data sample with integrated luminosity of 4.2 pb−1 obtained by the ATLAS experiment in 2011 and 2013. This allows a detailed comparison of the two collision systems in a wide transverse momentum (0.5<pT<150 GeV) and pseudorapidity (|η|<2) ranges in different centrality intervals of Pb+Pb collision. The nuclear modification factor RAA is presented in detail as a function of centrality, pT and η. It shows a distinct pT-dependence with a pronounced minimum at about 7 GeV. Above 60 GeV, it is consistent with a flat, centrality-dependent, value within the uncertainties. The value is 0.55±0.01(stat)±0.04(mathitsyst.) in the most central collisions. The RAA is observed to be consistent with flat |η| dependence over the whole transverse momentum range in all centrality classes.

Pinning down QCD-matter shear viscosity in ultrarelativistic heavy-ion collisions via EbyE fluctuations using pQCD + saturation + hydrodynamics

We introduce an event-by-event pQCD + saturation + hydro (”EKRT”) framework for high-energy heavy-ion collisions, where we compute the produced fluctuating QCD-matter energy densities from next-to-leading order (NLO) perturbative QCD (pQCD) using saturation to control soft particle production, and describe the space-time evolution of the QCD matter with viscous hydrodynamics, event by event (EbyE). We compare the computed centrality dependence of hadronic multiplicities, pT spectra and flow coefficients vn against LHC and RHIC data. We compare also the computed EbyE probability distributions of relative fluctuations of vn, as well as correlations of 2 and 3 event-plane angles, with LHC data. Our systematic multi-energy and -observable analysis not only tests the initial state calculation and applicability of hydrodynamics, but also makes it possible to constrain the temperature dependence of the shear viscosity-to-entropy ratio, η/s(T), of QCD matter in its different phases. Remarkably, we can describe all these different flow observables and correlations consistently with η/s(T) that is independent of the collision energy.

Jet Hadronization via Recombination of Parton Showers in Vacuum and in Medium

We have studied the hadronization of jet parton showers based on the quark recombination model. This is achieved by letting gluons at the end of the perturbative shower evolution undergo a non-perturbative splitting into quark and antiquark pairs, then applying a Monte-Carlo version of instantaneous quark recombination, and finally subjecting remnant quarks (those which have not found a recombination partner) to Lund string fragmentation. When applied to parton showers from the PYTHIA Monte Carlo generator, the final hadron spectra from our calculation compare quite well to PYTHIA jets that have been hadronized with the default Lund string fragmentation. Modeling the quark gluon plasma produced in heavy ion collisions by a blast wave model, we have further studied medium effects on the hadronization of jet shower partons by also including their recombination with the thermal partons from the quark gluon plasma. We find that the latter leads to a significant enhancement of intermediate transverse momentum pions and protons at both RHIC and LHC. Our results thus suggest that medium modification of jet fragmentation provides a plausible explanation for the enhanced production of intermediate transverse momentum hadrons observed in experiments.

Universal effective hadron dynamics from superconformal algebra

An effective supersymmetric QCD light-front Hamiltonian for hadrons composed of light quarks, which includes a spin-spin interaction between the hadronic constituents, is constructed by embedding superconformal quantum mechanics into AdS space. A specific breaking of conformal symmetry inside the graded algebra determines a unique effective quark-confining potential for light hadrons, as well as remarkable connections between the meson and baryon spectra. The results are consistent with the empirical features of the light-quark hadron spectra, including a universal mass scale for the slopes of the meson and baryon Regge trajectories and a zero-mass pion in the limit of massless quarks. Our analysis is consistently applied to the excitation spectra of the $\pi, \rho, K, K^*$ and $\phi$ meson families as well as to the $N, \Delta, \Lambda, \Sigma, \Sigma^*, \Xi$ and $\Xi^*$ in the baryon sector. We also predict the existence of tetraquarks which are degenerate in mass with baryons with the same angular momentum. The mass of light hadrons is expressed in a universal and frame-independent decomposition in the semiclassical approximation described here.

Thermal dileptons from coarse-grained transport as fireball probes at SIS energies

Utilizing a coarse-graining method to convert hadronic transport simulations of Au+Au collisions at SIS energies into local temperature, baryon and pion densities, we compute the pertinent radiation of thermal dileptons based on an in-medium $\rho$ spectral function that describes available spectra at ultrarelativistic collision energies. In particular, we analyze how far the resulting yields and slopes of the invariant-mass spectra can probe the lifetime and temperatures of the fireball. We find that dilepton radiation sets in after the initial overlap phase of the colliding nuclei of about 7 fm/c, and lasts for about 13 fm/c. This duration closely coincides with the development of the transverse collectivity of the baryons, thus establishing a direct correlation between hadronic collective effects and thermal EM radiation, and supporting a near local equilibration of the system. This fireball "lifetime" is substantially smaller than the typical 20-30 fm/c that naive considerations of the density evolution alone would suggest. We furthermore find that the total dilepton yield radiated into the invariant-mass window of $M=0.3-0.7$ GeV/$c^{2}$, normalized to the number of charged pions, follows a relation to the lifetime found earlier in the (ultra-) relativistic regime of heavy-ion collisions, and thus corroborates the versatility of this tool. The spectral slopes of the invariant-mass spectra above the $\phi$ mass provide a thermometer of the hottest phases of the collision, and agree well with the maximal temperatures extracted from the coarse-grained hadron spectra.

Production of secondary particles and nuclei in cosmic rays collisions with the interstellar gas using the FLUKA code

The measured fluxes of secondary particles produced by the interactions of Cosmic Rays (CRs) with the astronomical environment play a crucial role in understanding the physics of CR transport. In this work we present a comprehensive calculation of the secondary hadron, lepton, gamma-ray and neutrino yields produced by the inelastic interactions between several species of stable or long-lived cosmic rays projectiles (p, D, T, 3He, 4He, 6Li, 7Li, 9Be, 10Be, 10B, 11B, 12C, 13C, 14C, 14N, 15N, 16O, 17O, 18O, 20Ne, 24Mg and 28Si) and different target gas nuclei (p, 4He, 12C, 14N, 16O, 20Ne, 24Mg, 28Si and 40Ar). The yields are calculated using FLUKA, a simulation package designed to compute the energy distributions of secondary products with large accuracy in a wide energy range. The present results provide, for the first time, a complete and self-consistent set of all the relevant inclusive cross sections regarding the whole spectrum of secondary products in nuclear collisions. We cover, for the projectiles, a kinetic energy range extending from 0.1 GeV/n up to 100 TeV/n in the lab frame. In order to show the importance of our results for multi-messenger studies about the physics of CR propagation, we evaluate the propagated spectra of Galactic secondary nuclei, leptons, and gamma rays produced by the interactions of CRs with the interstellar gas, exploiting the numerical codes DRAGON and GammaSky. We show that, adopting our cross section database, we are able to provide a good fit of a complete sample of CR observables, including: leptonic and hadronic spectra measured at Earth, the local interstellar spectra measured by Voyager, and the gamma-ray emissivities from Fermi-LAT collaboration. We also show a set of gamma-ray and neutrino full-sky maps and spectra.

Jet fragmentation via recombination of parton showers

We propose to model hadronization of parton showers in QCD jets through a hybrid approach involving quark recombination and string fragmentation. This is achieved by allowing gluons at the end of the perturbative shower evolution to undergo a non-perturbative splitting into quark and antiquark pairs, then applying a Monte-Carlo version of instantaneous quark recombination, and finally subjecting remnant quarks (those which have not found a recombination partner) to Lund string fragmentation. When applied to parton showers from the PYTHIA Monte Carlo event generator, the final hadron spectra from our calculation compare quite well to PYTHIA jets that have been hadronized with the default Lund string fragmentation. Our new approach opens up the possibility to generalize hadronization to jets embedded in a quark gluon plasma.

An iterative method for analysis of hadron ratios and Spectra in relativistic heavy-ion collisions

A new iteration method is proposed for analyzing both the multiplicities and the transverse momentum spectra measured within a small rapidity interval with low momentum cut-off without assuming the invariance of the rapidity distribution under the Lorentz-boost and is applied to the hadron data measured by the ALICE collaboration for Pb+Pb collisions at \(\sqrt {^sNN} \) = 2.76 TeV. In order to correctly consider the resonance contribution only to the small rapidity interval measured, we only consider ratios involving only those hadrons whose transverse momentum spectrum is available. In spite of the small number of ratios considered, the quality of fitting both of the ratios and the transverse momentum spectra is excellent. Also, the calculated ratios involving strange baryons with the fitted parameters agree with the data surprisingly well.

Light-Front Holography, Color Confinement, and Supersymmetric Features of QCD

Light-Front Quantization provides a physical, frame-independent formalism for hadron dynamics and structure. Observables such as structure functions, transverse momentum distributions, and distribution amplitudes are defined from the hadronic light-front wavefunctions. One obtains new insights into the hadronic spectrum, light-front wavefunctions, and the functional form of the QCD running coupling in the nonperturbative domain using light-front holography -- the duality between the front form and AdS$_5$, the space of isometries of the conformal group. In addition, superconformal algebra leads to remarkable supersymmetric relations between mesons and baryons of the same parity. The mass scale $\kappa$ underlying confinement and hadron masses can be connected to the parameter $\Lambda_{\overline {MS}}$ in the QCD running coupling by matching the nonperturbative dynamics, as described by the effective conformal theory mapped to the light-front and its embedding in AdS space, to the perturbative QCD regime. The result is an effective coupling defined at all momenta. This matching of the high and low momentum transfer regimes determines a scale $Q_0$ which sets the interface between perturbative and nonperturbative hadron dynamics. The use of $Q_0$ to resolve the factorization scale uncertainty for structure functions and distribution amplitudes, in combination with the principle of maximal conformality (PMC) for setting the renormalization scales, can greatly improve the precision of perturbative QCD predictions for collider phenomenology. The absence of vacuum excitations of the front-form vacuum has important consequences for the cosmological constant. I also discuss evidence that the antishadowing of nuclear structure functions is flavor dependent, and why shadowing and antishadowing phenomena may be incompatible with sum rules for nuclear parton distribution functions.

Connecting physical resonant amplitudes and lattice QCD

We present a determination of the isovector, $P$-wave $\pi\pi$ scattering phase shift obtained by extrapolating recent lattice QCD results from the Hadron Spectrum Collaboration using $m_\pi =236$ MeV. The finite volume spectra are described using extensions of L\"uscher's method to determine the infinite volume Unitarized Chiral Perturbation Theory scattering amplitude. We exploit the pion mass dependence of this effective theory to obtain the scattering amplitude at $m_\pi= 140$ MeV. The scattering phase shift is found to be in good agreement with experiment up to center of mass energies of 1.2 GeV. The analytic continuation of the scattering amplitude to the complex plane yields a $\rho$-resonance pole at $E_\rho= \left[755(2)(1)(^{20}_{02})-\frac{i}{2}\,129(3)(1)(^{7}_{1})\right]~{\rm MeV}$. The techniques presented illustrate a possible pathway towards connecting lattice QCD observables of few-body, strongly interacting systems to experimentally accessible quantities.

Heavy-flavour production in high-energy d-Au and p-Pb collisions

Soft-hadron measurements in high-energy collisions of small systems like p-Pb and d-Au show peculiar qualitative features (long-range rapidity correlations, flattening of the $p_T$-spectra with increasing hadron mass and centrality, non-vanishing Fourier harmonics in the azimuthal particle distributions) suggestive of the formation of a strongly-interacting medium displaying a collective behaviour, with a hydrodynamic flow as a response to the pressure gradients in the initial conditions. Hard observables (high-$p_T$ jet and hadron spectra) on the other hand, within the current large systematic uncertainties, appear only midly modified with the respect to the benchmark case of minimum-bias p-p collisions. What should one expect for heavy-flavour particles, initially produced in hard processes but tending, in the nucleus-nucleus case, to approach kinetic equilibrium with the rest of the medium? This is the issue we address in the present study, showing how the current experimental findings are compatible with a picture in which the formation of a hot medium even in proton-nucleus collisions modifies the propagation and hadronization of heavy-flavour particles.

Hadron Physics from Superconformal Quantum Mechanics and Its Light-Front Holographic Embedding

The complex nonperturbative color-confining dynamics of QCD is well captured in a semiclassical effective theory based on superconformal quantum mechanics and its extension to the light-front. I describe here how this new approach to hadron physics incorporates confinement, the appearance of nearly massless pseudoscalar particles, and Regge spectroscopy consistent with experiment. It also gives remarkable connections between the meson and baryon spectrum across the light and heavy-light hadron spectrum. I also briefly discuss how higher spin states are consistently described in this framework by the holographic embedding of the superconformal theory in a higher dimensional semiclassical gravity theory.

Significance of nonperturbative input to the transverse momentum dependent gluon density for hard processes at the LHC

We study the role of the non-perturbative input to the transverse momentum dependent (TMD) gluon density in hard processes at the LHC. We derive the input TMD gluon distribution at low scale mu0^2 ~ 1 GeV^2 from the fit of the inclusive hadron spectra measured at low transverse momenta in pp collisions at the LHC and demonstrate that the best description of these spectra for larger hadron transverse momenta can be achieved by matching the derived TMD gluon distribution with the exact solution of the Balitsky-Fadin-Kuraev-Lipatov (BFKL) equation obtained at low x and small gluon transverse momenta outside the saturation region. Then, we extend the input TMD gluon density to higher mu^2 numerically using the Catani-Ciafoloni-Fiorani-Marchesini (CCFM) gluon evolution equation. A special attention is put to the phenomenological applications of obtained TMD gluon density to some LHC processes, which are sensitive to the gluon content of a proton.

Impressions of the Meson Spectrum: Hybrids &amp; Exotics, present and future

It has long been expected that the spectrum of hadrons in QCD would be far richer and extensive than experiment has so far revealed. While there have been experimental hints of this richness for some time, it is really only in the last few years that dramatic progress has been seen in the exploration both experimentally and in calculations on the lattice. Precision studies enabled by new technology both with detectors and high performance computations are converging on an understanding of the spectrum in strong coupling QCD. These methodologies are laying the foundation for a decade of potential discovery that electro and photoproduction experiments at Jefferson Lab, which when combined with key results on $B$ and charmonium decays from both $e^+e^-$ and $pp$ colliders, should turn mere impressions of the light meson spectrum into a high definition picture.

Heavy [formula omitted] free energy from hadronic states

Within the spirit of the Hadron Resonance Gas model, we study a representation of the heavy Q‾Q free energy at temperatures below the phase transition in terms of the string and heavy-light hadrons. We discuss the string breaking phenomenon and the relevance of avoided crossings between the fundamental string and the hadron spectrum. Good agreement with lattice data is achieved.

The P̅ANDA physics program

The understanding of the QCD in the non-perturbative regime, is one of the key issues to have a complete picture of strong interactions. Recent findings of new and unexpected resonances, with unresolved properties, show that the hadron spectrum is not yet completely understood. This is also underlined by the ongoing discussion on multi- quark states, and on other exotic states with gluonic degrees of freedom. The PANDA experiment, one of the biggest enterprises at the FAIR facility, aims at exploring this field thanks to the gluon rich environment offered by the annihilation of antiprotons. A general overview of the PANDA physics program is given in this paper.

Cosmic Ray Interaction Models: an Overview

I review the state-of-the-art concerning the treatment of high energy cosmic ray interactions in the atmosphere, discussing in some detail the underlying physical concepts and the possibilities to constrain the latter by current and future measurements at the Large Hadron Collider. The relation of basic characteristics of hadronic interactions to the properties of nuclear-electromagnetic cascades induced by primary cosmic rays in the atmosphere is addressed. Experimental studies of high energy cosmic rays (CR) are traditionally performed using the so-called extensive air shower (EAS) techniques: the properties of the primary CR particles are reconstructed from measured characteristics of nuclear-electro-magnetic cascades induced by their interactions in the atmosphere. This naturally implies the importance of detailed Monte Carlo simulations of EAS development, particularly, of its backbone - the cascade of nuclear interactions of both the primary particles and of the secondary hadrons produced. Thus, the very success of experimental studies depends crucially on the validity of hadronic interaction models used in the analysis. Typically, one chooses between two main experimental procedures (1). In the first case, one deals with the information obtained with scintillation detectors positioned at ground. The energy of the primary particle is reconstructed from the measured lateral density of charged particles (mostly, elec- trons and positrons) while the particle type is inferred from the relative fraction of muons, compared to all charged particles at ground. Alternatively, one may study the longitudinal EAS development by measuring fluorescence light produced by excited air molecules at different heights in the atmo- sphere, for which purpose dedicated fluorescence telescopes are employed. In the latter case, the primary energy is related to the total amount of fluorescence light emitted. In turn, the particle type may be determined from the measured position of the so-called shower maximum Xmax - the depth in the atmosphere (in g/cm 2 ) where the number of ionizing particles reaches its maximal value (which is thus the brightest spot on the shower image). Not surprisingly, the observables used to determine the primary particle type - the lateral muon density ρμ and the EAS maximum position Xmax - appear to be very sensitive to details of high energy hadronic interactions. More precisely, Xmax depends strongly on the properties of the primary particle interaction with air nuclei: the inelastic cross section and the forward spectra of secondary hadrons produced. In turn, the EAS muon content is formed in a multi-step cascade process, driven mostly

Charged particle production in Pb-Pb and p-Pb collisions measured by the ATLAS detector

The ATLAS experiment at the Large Hadron Collider measures charged hadron spectra in Pb+Pb and p+Pb collisions. The results are compared to the pp spectra of charged hadrons at the same centre-of-mass energy. Charged hadron distributions from Pb+Pb are compared to charged particle cross-sections in pp collisions at √s= 2.76 TeV, reference crosssection for p+Pb at √s NN = 5.02 TeV is reconstructed using √s = 2.76 TeV and 7 TeV pp results. These allow for a detailed comparison of the collision systems in a wide transverse momentum and rapidity ranges in different centrality intervals. The nuclear modification factors RAA and RpPb are presented as a function of centrality, pT, η The charged particle RAA is found to vary significantly as a function of transverse momentum, and shows a pronounced minimum at about 7 GeV. Above 60 GeV, RAA is consistent with a flat, centrality-dependent, value within the uncertainties. RpPb results show strong rapidity dependence in the region of so-called Cronin peak at about 2 GeV and above pT of 10 GeV show anomalous enhancement.

Appearance of Hagedorn limiting temperature in microscopic model calculations

One of important consequences of Hagedorn statistical bootstrap model is the prediction of limiting temperature Tcrit for hadron systems colloquially known as Hagedorn temperature. According to Hagedorn, this effect should be observed in hadron spectra obtained in infinite equilibrated nuclear matter rather than in relativistic heavy-ion collisions. We present results of microscopic model calculations for the infinite nuclear matter, simulated by a box with periodic boundary conditions. The limiting temperature indeed appears in the model calculations. Its origin is traced to strings and many-body decays of resonances.

Highly-anisotropic Hydrodynamics for Central Collisions

The framework of leading-order anisotropic hydrodynamics is supplemented with realistic equation of state and self-consistent freeze-out prescription. The model is applied to central proton-nucleus collisions. The results are compared to those obtained within standard Israel-Stewart second-order viscous hydrodynamics. It is shown that the resulting hadron spectra are highly-sensitive to the hydrodynamic approach that has been used.