Quantum information approach to high energy interactions

High energy hadron interactions are commonly described by using a probabilistic parton model that ignores quantum entanglement present in the light-cone wave functions. Here, we argue that since a high energy interaction samples an instant snapshot of the hadron wave function, the phases of different Fock state wave functions cannot be measured—therefore the light-cone density matrix has to be traced over these unobservable phases. Performing this trace with the corresponding U ( 1 ) Haar integration measure leads to ‘Haar scrambling’ of the density matrix, and to the emergence of entanglement entropy. This entanglement entropy is determined by the Fock state probability distribution, and is thus directly related to the parton structure functions. As proposed earlier, at large rapidity η the hadron state becomes maximally entangled, and the entanglement entropy is S E ∼ η according to QCD evolution equations. When the phases of Fock state components are controlled, for example in spin asymmetry measurements, the Haar average cannot be performed, and the probabilistic parton description breaks down. This article is part of the theme issue ‘Quantum technologies in particle physics’.

Baryon number in proton-proton and proton-nucleus high energy collisions

New analyses of baryon spectra in proton-proton and proton-carbon collisions at $$\sqrt{s}_\mathrm {_{NN}}=17.3$$ s NN = 17.3 GeV, made in the framework of two phenomenological models are presented. The first model in question is the classic Dual Parton Model by Capella and Tran Thanh Van, the second is the Gluon Exchange Model very recently proposed by the authors. For both studies, the usage of modern experimental data from the CERN SPS eliminates several of the most important limitations inherent to earlier studies of this type. In both studies, the standard mechanism of baryon stopping with preservation of the diquark, proposed by Capella and Tran Thanh Van fails to describe the distribution of non-strange baryons in collisions of the projectile proton with more than one nucleon from the carbon target obtained from experimental data, and the upper limit for the contribution of this mechanism can be established. In both cases, the conclusion is that the projectile diquark must be very often disintegrated. This opens new diagrams not available in proton-proton collisions which lead to the transport of baryon number over long distances in rapidity. The present limitations, and possibility of improvement in both approaches are discussed. The implications of our findings for new measurements are addressed, in particular using antiproton beams.

Twist-2 relation and sum rule for tensor-polarized parton distribution functions of spin-1 hadrons

Sum rules for structure functions and their twist-2 relations have important roles in constraining their magnitudes and x dependencies and in studying higher-twist effects. The Wandzura-Wilczek (WW) relation and the Burkhardt-Cottingham (BC) sum rule are such examples for the polarized structure functions g1 and g2. Recently, new twist-3 and twist-4 parton distribution functions were proposed for spin-1 hadrons, so that it became possible to investigate spin-1 structure functions including higher-twist ones. We show in this work that an analogous twist-2 relation and a sum rule exist for the tensor-polarized parton distribution functions f1LL and fLT, where f1LL is a twist-2 function and fLT is a twist-3 one. Namely, the twist-2 part of fLT is expressed by an integral of f1LL (or b1) and the integral of the function f2LT = (2/3)fLT− f1LL over x vanishes. If the parton-model sum rule for f1LL (b1) is applied by assuming vanishing tensor-polarized antiquark distributions, another sum rule also exists for fLT itself. These relations should be valuable for studying tensor-polarized distribution functions of spin-1 hadrons and for separating twist-2 components from higher-twist terms, as the WW relation and BC sum rule have been used for investigating x dependence and higher-twist effects in g2. In deriving these relations, we indicate that four twist-3 multiparton distribution functions FLT, GLT, $$ {H}_{LL}^{\perp } $$ H LL ⊥ , and HTT exist for tensor-polarized spin-1 hadrons. These multiparton distribution functions are also interesting to probe multiparton correlations in spin-1 hadrons. In the near future, we expect that physics of spin-1 hadrons will become a popular topic, since there are experimental projects to investigate spin structure of the spin-1 deuteron at the Jefferson Laboratory, the Fermilab, the nuclotron-based ion collider facility, the electron-ion colliders in US and China in 2020’s and 2030’s.

On the Shape of Nucleons at High Energies

A scenario of the evolution of the shape of nucleons with increasing energy is described in the framework of an extended parton model, which consistently takes into account the transverse motion of the partons. At the energy E up to LHC, the nucleons have the form of a spheroid which expands as lnE in the transverse directions and grows linearly in E in the longitudinal direction. With a further increase in the energy, a mode of correlated behavior of the partons is established, which stops the longitudinal growth. Simultaneously, the expansion in the transverse directions changes to lnE, and a hollow mostly free of partons is formed inside the nucleons along the central axis in the direction of their motion. Numerical estimates of the corresponding parameters are obtained.

Double prompt J/ψ production at hadron colliders

In this paper, we give a brief review of recent theoretical studies of double prompt [Formula: see text] production at the FNAL Tevatron and the CERN LHC, to which both single- and double-parton-scattering processes contribute. In single parton scattering, we mainly present the results computed within the framework of non-relativistic quantum chromodynamics factorization combined with one of the two approaches to describe the parton inside the (anti)proton, the collinear parton model and the parton Reggeization approach. The contributions from double parton scattering calculated with different values of [Formula: see text] are also discussed. We conclude that, in double prompt [Formula: see text] hadroproduction, there remain some differences between theoretical predictions and experimental measurements.

An analysis of parton distribution functions of the pion and the kaon with the maximum entropy input

We present pion and kaon parton distribution functions from a global QCD analysis of the experimental data within the framework of dynamical parton model. We use the DGLAP equations with parton–parton recombination corrections and the valence input of uniform distribution which maximizes the information entropy. At our input scale $$Q_0^2$$ Q 0 2 , there are no sea quark and gluon distributions. All the sea quarks and gluons of the pion and the kaon are completely generated from the parton splitting processes. The mass-dependent parton splitting kernel is applied for the strange quark distribution in the kaon. The obtained valence quark and sea quark distributions at high $$Q^{2}$$ Q 2 ($$Q^2>5$$ Q 2 > 5 GeV$$^2$$ 2 ) are compatible with the existed experimental measurements. Furthermore, the asymptotic behaviours of parton distribution functions at small and large x have been studied for both the pion and the kaon. Lastly, the first three moments of parton distributions at high $$Q^{2}$$ Q 2 scale are calculated, which are consistent with other theoretical predictions.

Deep inelastic scattering on the quark-gluon plasma

We provide an interpretation of the structure functions of a thermal medium such as the quark-gluon plasma in terms of the scattering of an incoming electron on the medium via the exchange of a spacelike photon. We then focus on the deep-inelastic scattering (DIS) regime, and formulate the corresponding moment sum rules obeyed by the structure functions. Accordingly, these moments are given by the thermal expectation value of twist-two operators, which is computable from first principles in lattice QCD for the first few moments. We also show how lattice QCD calculations can be used to probe how large the photon virtuality needs to be in order for the Bjorken scaling of structure functions to set in. Finally, we provide the parton-model interpretation of the structure functions in the Bjorken limit and test its consistency. As in DIS on the proton, the kinematic variable x is proportional to the longitudinal momentum carried by the partons, however x ranges from zero to infinity. Choosing the parton momentum parametrization to be xT u where u is the fluid four-velocity and T its temperature in the rest frame, the parton distribution function for a plasma of non-interacting quarks is proportional to x log(1 + e−x/2).

Super heavy particle production in high energy heavy ion collision

The estimate based on the parton model is made on the rate of production of Super Heavy Particle ( SHP ) in subthreshold collision of heavy ions at LHC. For the one-month run of lead-lead collision the yield of 16 TeV particle is of the order of 70 per year.