Cross Section For Hadron Production Research Articles

Overview of hadronization of quarks in proton-proton and e+e− collisions

The so-called hadronization is a non-perturbative QCD phenomenon corresponding to the formation of colourless hadrons from coloured quark con stituents. The hadron formation in point-like e+e− collisions can be described by string models. According to the Lund model, the qq¯ pair production in the scattering is followed by a shower of light partons produced via multiple colour-string breaking, which produce colour singlets in the final states. The probability to obtain a hadron of a given species carrying a certain momentum fraction of the original quark is quantified by the fragmentation functions. They are assumed universal and usually constrained from e+ e− and e−p collisions, and they successfully describe the production of mesons in e+e− and pp colli sions at the colliders. However, recent measurements of pp collision data from the LHC showed a surprising relative enhancement of baryon production com pared to mesons, and model predictions based on string fragmentation do not describe the data. In this talk, an overview of the most recent experimental re sults of hadron production cross section in pp collisions at the LHC compared with e+e− results will be provided. A comparison with novel theoretical models implementing hadronization mechanisms different from the Lund string frag mentation will be also discussed, as well as experimental results obtained in larger systems, like p–Pb and Pb–Pb collisions.

Single inclusive hadron production in DIS at small x: next to leading order corrections

We calculate the one-loop corrections to single inclusive hadron production in Deep Inelastic Scattering (DIS) at small x in the forward rapidity region using the Color Glass Condensate formalism. We show that the divergent parts of the next to leading order (NLO) corrections either cancel among each other or lead to x (rapidity) evolution of the leading order (LO) dipole cross section according to the JIMWLK evolution equation and DGLAP evolution of the parton-hadron fragmentation function. The remaining finite parts constitute the NLO (αs) corrections to the LO single inclusive hadron production cross section in DIS at small x.

Odderon mechanism for transverse single spin asymmetry in the Wandzura-Wilczek approximation

We compute the transverse single spin asymmetry in forward ${p}^{\ensuremath{\uparrow}}p\ensuremath{\rightarrow}hX$ and ${p}^{\ensuremath{\uparrow}}A\ensuremath{\rightarrow}hX$ collisions from the odderon mechanism originally suggested by Kovchegov and Sievert [Phys. Rev. D 86, 034028 (2012)]. Working in the hybrid approach of the color glass condensate effective theory, we first identify the relevant collinear parton distribution function (PDF) of the transversely polarized proton ${p}^{\ensuremath{\uparrow}}$ as the intrinsic twist-3 ${g}_{T}(x)$ distribution. We further argue that the complete polarized cross section also contains contributions from the kinematical and the dynamical twist-3 PDFs, in addition to the intrinsic twist-3 PDF. By restricting to the Wandzura-Wilczek approximation, where the dynamical twist-3 PDFs are dropped, we find that the odderon contribution to the polarized cross section for inclusive hadron production is exactly zero at the next-to-leading order in the strong coupling.

Final-state interaction in the process e+e−→ΛcΛ¯c

We show that the final-state interaction explains the nontrivial near-threshold energy dependence of the cross section of the process ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}{\mathrm{\ensuremath{\Lambda}}}_{c}{\overline{\mathrm{\ensuremath{\Lambda}}}}_{c}$ observed by the Belle and BESIII collaborations. This energy dependence is the result of the mixture of $S$-wave and $D$-wave components of the ${\mathrm{\ensuremath{\Lambda}}}_{c}{\overline{\mathrm{\ensuremath{\Lambda}}}}_{c}$ wave function due to a tensor interaction. The Coulomb potential is important only in the narrow energy region about a few MeV above the threshold of the process. It is shown that the widely used assumption that the impact of the Coulomb interaction on the cross sections of hadron production is reduced to the Sommerfeld-Gamow-Sakharov factor is not correct.

Quark and gluon contents of a lepton at high energies

In high-energy leptonic collisions, such as at a multi-TeV muon collider, the collinear splittings of the electroweak (EW) gauge bosons and leptons are the dominant phenomena, and the scattering processes should thus be formulated in terms of the EW parton distribution functions (EW PDFs). We complete this formalism in the Standard Model to include the QCD sector and evaluate the quark and gluon PDFs inside a lepton at the double-log accuracy. The splittings of the photon and subsequently the quarks and gluons control the quark/gluon PDFs below the EW scale. The massive gauge bosons lead to substantial contributions at high scales. The jet production cross section can reach the order of a few nb (50 pb) in e+e−(μ+μ−) collisions, at the TeV c.m. energies with a moderate acceptance cut, that governs the overall event shape up to about {p}_T^j ∼ 60 GeV. To complete the picture, we also provide an estimation of the total cross section for the photon-induced hadronic production at low partonic energies, which can reach the level of one hundred or a few tens of nb in high-energy electron or muon collisions.

Production cross sections of tetraquark states in elementary hadronic collisions

Inclusive production cross sections of the possible exotic state X(3872) in proton–proton, pion-proton and proton–antiproton collisions are calculated using a statistical based model, which is previously used to describe inclusive charmed and bottomed hadron production cross sections in the low energy region. With the extensions made here the model is capable to include tetraquarks as well, using the diquark picture of tetraquarks. The evaluated cross section ratio of varPsi (2S) and X(3872) at sqrt{s}=7 TeV agrees well with the measured value.

Factorization of e+e\u2212 \u2192 H X cross section, differential in zh, PT and thrust, in the 2-jet limit

Factorizing the cross section for single hadron production in e+e− annihilations is a highly non trivial task when the transverse momentum of the outgoing hadron with respect to the thrust axis is taken into account. We work in a scheme that allows to factorize the e+e−→ H X cross section as a convolution of a calculable hard coefficient and a Transverse Momentum Dependent (TMD) fragmentation function. The result, differential in zh, PT and thrust, will be given to all orders in perturbation theory and explicitly computed to Next to Leading Order (NLO) and Next to Leading Log (NLL) accuracy. The predictions obtained from our computation, applying the simplest and most natural ansatz to model the non-perturbative part of the TMD, are in exceptional agreement with the experimental measurements of the BELLE Collaboration. The factorization scheme we propose relates the TMD parton densities defined in 1-hadron and 2-hadron processes, restoring the possi- bility to perform global phenomenological studies of TMD physics including experimental data from semi-inclusive deep inelastic scattering, Drell-Yan processes, e+e−→ H1H2X and e+e−→ H X annihilations.

Universality-breaking effects in e^+e^- hadronic production processes

Recent BELLE measurements provide the cross section for single hadron production in e^+e^- annihilations, differential in thrust and in the hadron transverse momentum with respect to the thrust axis. Universality breaking effects due to process-dependent soft factors make it very difficult to relate this cross sections to those corresponding to hadron-pair production in e^+e^- annihilations, where transverse momentum dependent (TMD) factorization can be applied. The correspondence between these two cross sections is examined in the framework of the Collins-Soper-Sterman factorization, in the collinear as well as in the TMD approach. We propose a scheme that allows to relate the TMD parton densities defined in 1-hadron and in 2-hadron processes, neatly separating, within the soft and collinear parts, the non-perturbative terms from the contributions that can be calculated perturbatively. The regularization of rapidity divergences introduces cut-offs, the arbitrariness of which will be properly reabsorbed by means of a mechanism closely reminiscent of a gauge transformation. In this way, we restore the possibility to perform global phenomenological studies of TMD physics, simultaneously analyzing data belonging to different hadron classes.

The impact of the LHCb B± production cross-section on parton distribution functions and determining the mass of heavy quarks

In this work, the impact of recent measurements of heavy-flavour production in pp collisions on parton distribution functions (PDFs) and their uncertainties is studied. In this regard, the absolute and normalised cross sections of beauty hadron production measured by LHCb at center-of-mass energy of 7 TeV and 13 TeV are separately included in the next-to-leading order (NLO) global QCD analysis together with the measurements of the inclusive and heavy-flavour production cross sections at HERA. It is illustrated that the heavy-flavour data of the LHCb experiment impose additional constraints on the PDFs, especially on the gluon distribution at low partonic fractions x of the proton momentum. One of the most important results of the present analysis is the significant reduction of the gluon uncertainties in the region less than x=10−4 that can play a crucial rule in many areas of high energy physics investigations.

Nuclear EMC effect through Λ¯/Λ production in semi-inclusive deep-inelastic scattering processes

We calculate $\mathrm{\ensuremath{\Lambda}}$ and $\overline{\mathrm{\ensuremath{\Lambda}}}$ hadron production cross sections in charged lepton semi-inclusive deep-inelastic scattering off nuclear target ($A$, using iron $\mathrm{Fe}$ as an example) and deuteron ($D$) target. The results show that the ratio $({\overline{\mathrm{\ensuremath{\Lambda}}}}^{A}/{\mathrm{\ensuremath{\Lambda}}}^{A})/({\overline{\mathrm{\ensuremath{\Lambda}}}}^{D}/{\mathrm{\ensuremath{\Lambda}}}^{D})$ is sensitive to the sea quark content of the nucleus. We adopt three different models to take the nuclear EMC effect into account. The ratio $({\overline{\mathrm{\ensuremath{\Lambda}}}}^{A}/{\mathrm{\ensuremath{\Lambda}}}^{A})/({\overline{\mathrm{\ensuremath{\Lambda}}}}^{D}/{\mathrm{\ensuremath{\Lambda}}}^{D})$ is predicted to be different by these different models.

Threshold and Jet Radius Joint Resummation for Single-Inclusive Jet Production.

We present the first threshold and jet radius jointly resummed cross section for single-inclusive hadronic jet production. We work at next-to-leading logarithmic accuracy and our framework allows for a systematic extension beyond the currently achieved precision. Long-standing numerical issues are overcome by performing the resummation directly in momentum space within soft collinear effective theory. We present the first numerical results for the LHC and observe an improved description of the available data. Our results are of immediate relevance for LHC precision phenomenology including the extraction of parton distribution functions and the QCD strong coupling constant.

Cosmic ray air showers from sphalerons

The discovery of the Higgs boson marks a key ingredient to establish the electroweak structure of the Standard Model. Its non-abelian gauge structure gives rise to, yet unobserved, non-perturbative baryon and lepton number violating processes. We propose to use cosmic ray air showers, as measured at the Pierre Auger Observatory, to set a limit on the hadronic production cross section of sphalerons. We identify several observables to discriminate between sphaleron and QCD induced air showers.

Search for metastable heavy charged particles with large ionization energy loss inppcollisions ats=13 TeVusing the ATLAS experiment

This paper presents a search for massive charged long-lived particles produced in pp collisions at $\sqrt{s}=$ 13 TeV at the LHC using the ATLAS experiment. The dataset used corresponds to an integrated luminosity of 3.2 fb$^{-1}$. Many extensions of the Standard Model predict the existence of massive charged long-lived particles, such as $R$-hadrons. These massive particles are expected to be produced with a velocity significantly below the speed of light, and therefore to have a specific ionization higher than any Standard Model particle of unit charge at high momenta. The Pixel subsystem of the ATLAS detector is used to measure the ionization energy loss of reconstructed charged particles and to search for such highly ionizing particles. The search presented here has much greater sensitivity than a similar search performed using the ATLAS detector in the $\sqrt{s}=$ 8 TeV dataset, thanks to the increase in expected signal cross-section due to the higher center-of-mass energy of collisions, to an upgraded detector with a new silicon layer close to the interaction point, and to analysis improvements. No significant deviation from Standard Model background expectations is observed, and lifetime-dependent upper limits on $R$-hadron production cross-sections and masses are set. Gluino $R$-hadrons with lifetimes above 0.4 ns and decaying to $q\bar{q}$ plus a 100 GeV neutralino are excluded at the 95% confidence level, with lower mass limit ranging between 740 GeV and 1590 GeV. In the case of stable $R$-hadrons the lower mass limit at the 95% confidence level is 1570 GeV.

Heavy quarks in proton-nucleus collisions: The hybrid formalism

We explore the quark mass effects on inclusive hadron production in proton-nucleus collisions at high energies. We consider two processes. First, we compute the single inclusive cross-section for production of hadrons with open heavy flavour in the proton forward direction at leading order. Next, in the same kinematics, we calculate the heavy-quark contribution to single inclusive production of light or unidentified hadrons at next-to-leading-order. For both studies we exploit the hybrid formalism, that is the collinear factorisation on the proton side while high-density and high-energy effects are resummed on the side of the nucleus.

Radiative corrections and Monte Carlo generators for physics at flavor factories

I review the state of the art of precision calculations and related Monte Carlo generators used in physics at flavor factories. The review describes the tools relevant for the measurement of the hadron production cross section (via radiative return, energy scan and in γγ scattering), luminosity monitoring, searches for new physics and physics of the τ lepton.

Implementing the exact kinematical constraint in the saturation formalism

We revisit the issue of the large negative next-to-leading order (NLO) cross section for single inclusive hadron production in $pA$ collisions in the saturation formalism. By implementing the exact kinematical constraint in the modified dipole splitting functions, two additional positive NLO correction terms are obtained. In the asymptotic large $k_\perp$ limit, we analytically show that these two terms become as large as the negative NLO contributions found in our previous calculation. Furthermore, the numerical results demonstrate that the applicable regime of the saturation formalism can be extended to a larger $k_\perp$ window, where the exact matching between the saturation formalism (in the asymptotic $k_\perp$ regime) and the collinear factorization calculations will have to be performed separately. In addition, after significantly improving the numerical accuracy of the NLO correction, we obtain excellent agreement with the LHC and RHIC data for forward hadron productions.

Jet quenching phenomenology from soft-collinear effective theory with Glauber gluons.

We present the first application of a recently developed effective theory of jet propagation in matter, soft-collinear effective theory with Glauber gluons (SCET_{G}), to inclusive hadron suppression in nucleus-nucleus collisions at RHIC and the LHC. SCET_{G}-based splitting kernels allow us to go beyond the traditional energy loss approximation and unify the treatment of vacuum and medium-induced parton showers. In the soft gluon emission limit, we establish a simple analytic relation between the QCD evolution and energy loss approaches to jet quenching. We quantify the uncertainties associated with the implementation of the in-medim modification of hadron production cross sections and show that the coupling between the jet and the medium can be constrained with better than 10% accuracy.

Study of the hadronisation process from single hadron and hadron-pair production in SIDIS at COMPASS

Hadron production in hard scattering reactions is described by the hadronization mechanism which combines quarks into final-state hadrons. Within the theoretical framework of leading-twist collinear QCD, the cross section for hadron production in semi-inclusive deep-inelastic scattering can be factorized into a hard scattering cross section describing the hard interaction at the quark level calculable in perturbative QED, and non-perturbative universal functions: parton distribution functions which reflect the quark structure of initial-state hadrons and collinear fragmentation functions which encode details on the hadronization process. In the last decades, a major effort has been achieved on theoretical and experimental levels and allowed to constraint, with very high precision, parton distribution functions except strange quark distribution, which still carries large uncertainties. Fragmentation functions, however, remain at a very preliminary stage of study with a growing interest in a more accurate and precise knowledge. Nowadays, while pion fragmentation functions are known with a limited precision and kaon fragmentation functions are poorly known, the situation for dihadron fragmentation functions turns out to be behind the schedule and no studies or measurements have been yet performed.Measuring both single hadron and hadron pair multiplicities in semi-inclusive deep inelastic scattering at COMPASS represent a fundamental contribution towards a better understanding of the hadronization process and a first measurement for hadron pair multiplicities in SIDIS. These measurements will be presented and discussed.

Recent COMPASS results on the nucleon longitudinal spin structure and QCD fits

The latest measurements of the proton longitudinal spin structure function, ɡ 1 p , in the deep inelastic (DIS) regime are presented. They improve the statistical accuracy of the existing data and extend the kinematic domain to a lower value of x and higher values of Q2 . A global NLO QCD fit of all ɡ 1 world data on the proton, deuteron and neutron has been achieved. The results give a quantification of the quark spin contribution to the nucleon spin, 0.26 )2 in MS scheme. The errors are dominated by the uncertainty on the shape of the functional forms assumed in the fit. A new verification of the fundamental Bjorken sum rule is obtained at a 9% level, using only COMPASS ɡ 1 proton and deuteron measurements. Preliminary results of a reevaluation of the gluon polarization Δɡ /ɡ are presented. The analysis is based on double spin asymmetry of high-p T hadron production cross-sections in the DIS regime. A positive value of 〈Δɡ /ɡ 〉 = 0.113 ± 0.038 ± 0.035 is obtained at leading order at x ~ 0.1. In parallel, the double spin asymmetry in the photoproduction regime is also studied. Finally, preliminary results on quark fragmentation functions into pions extracted from a LO fit of pion multiplicities in semi-inclusive DIS are presented.

FRAGMENTATION FUNCTIONS AT BABAR

Inclusive hadron production cross section in e+e- annihilation shed light on fundamental questions of hadronization and fragmentation processes. We present measurements of inclusive spectra of charged pions, kaons, and protons (antiprotons) produced in e+e- collisions at the center-of-mass energy of 10.54 GeV, and tests of QCD predictions and hadronization models. We also report the results on the measurement of the azimuthal modulation induced by the Collins effect in inclusive production of charged pion pairs e+e- → ππX, where the two pions are produced in opposite hemispheres. These data allows the determination of the polarized Collins fragmentation functions.