Perturbative QCD Model Research Articles

Strange quark mass effect on the structure of hybrid stars

We study the strange quark mass effect on the phase diagram of strong interaction and the structure of compact stars with a thermodynamically enhanced perturbative QCD model by matching quark matter onto nuclear matter using the Gibbs conditions. It is found that the mass effect of strange quark matter can obviously stiffen the equation of state of mixed phases and result in more massive hybrid stars (HSs), while that usually lowers the maximum mass of pure quark stars. Given reasonable model parameters, the maximum mass of HSs can reach two times the solar mass and the stars always have mixed-phase core in a considerably wide range of model parameters.

Bulk viscosity of strange quark matter in an enhanced perturbative QCD model

The bulk viscosity is studied in a thermodynamically enhanced perturbative QCD (EPQ) model that includes the running of both the strong coupling and the strange quark mass. Similar to the results in other effective models, the bulk viscosity in the EPQ model is larger by 1--2 orders of magnitude compared with that in the MIT bag model. Comparison among the different models indicates that the similarity in orders is mainly due to the inclusion of interactions between quarks. When the temperature is relatively low and enters the equation of state of quark matter, the bulk viscosity can be enormously enlarged. For the analytical expression of bulk viscosity, a reasonable range of model parameters is found. Moreover, due to the enlarged bulk viscosity, the damping time of a strange star with about 2 times the solar mass can be as short as ${10}^{\ensuremath{-}3}$ second, showing that massive quark stars could reach stability quickly after some radial unstable activities.

Prompt atmospheric neutrino fluxes: perturbative QCD models and nuclear effects

We evaluate the prompt atmospheric neutrino flux at high energies using three different frameworks for calculating the heavy quark production cross section in QCD: NLO perturbative QCD, $k_T$ factorization including low-$x$ resummation, and the dipole model including parton saturation. We use QCD parameters, the value for the charm quark mass and the range for the factorization and renormalization scales that provide the best description of the total charm cross section measured at fixed target experiments, at RHIC and at LHC. Using these parameters we calculate differential cross sections for charm and bottom production and compare with the latest data on forward charm meson production from LHCb at $7$ TeV and at $13$ TeV, finding good agreement with the data. In addition, we investigate the role of nuclear shadowing by including nuclear parton distribution functions (PDF) for the target air nucleus using two different nuclear PDF schemes. Depending on the scheme used, we find the reduction of the flux due to nuclear effects varies from $10\%$ to $50 \%$ at the highest energies. Finally, we compare our results with the IceCube limit on the prompt neutrino flux, which is already providing valuable information about some of the QCD models.

Strange matter and strange stars in a thermodynamically self-consistent perturbation model with running coupling and running strange quark mass

A quark model with running coupling and running strange quark mass, which is thermodynamically self-consistent at both high and lower densities, is presented and applied to study properties of strange quark matter and structure of compact stars. An additional term to the thermodynamic potential density is determined by meeting the fundamental differential equation of thermodynamics. It plays an important role in comparatively lower density and ignorable at extremely high density, acting as a chemical-potential dependent bag constant. In this thermodynamically enhanced perturbative QCD model, strange quark matter still has the possibility of being absolutely stable, while the pure quark star has a sharp surface with a maximum mass as large as about 2 times the solar mass and a maximum radius of about 11 kilometers.

Pair double heavy diquark production in high energy proton–proton collisions

On the basis of perturbative QCD and relativistic quark model we calculate relativistic and bound state corrections in the production processes of a pair of double heavy diquarks. Relativistic factors in the production amplitude connected with the relative motion of heavy quarks and the transformation law of the bound state wave function to the reference frame of the moving S-wave diquark bound states are taken into account. For the gluon and quark propagators entering the amplitudes we use a truncated expansion in relative quark momenta up to the second order. Relativistic corrections to the quark-quark bound state wave functions in the rest frame are considered by means of the Breit-like potential. It turns out that the examined effects significantly decrease nonrelativistic cross sections.

LowpTCharm Meson Production at CDF II

I report on the differential production cross-section measurements of D mesons ( D 0 and D ± ) extended to low transverse momentum; I also show the first D 0 signal ever observed in the sample collected at √ s = 900 GeV. This measurement probes a region where perturbative QCD (pQCD) models are not able to describe the heavy-quark production because of the running of the strong coupling constant, α S . The data samples collected at two different center of mass energies, 1.96 TeV and 900 GeV, also give the opportunity to assess the energy dependence of the production cross section. The CDF II experiment can still give us distinguishing results, complementary to B-factories and LHC, thanks to the uniqueness of the combination of energies and collision initial state ( pp ), that are likely to remain the only ones of this kind for several years.

Gaps between jets in double-Pomeron-exchange processes at the LHC

The possibility to measure jet-gap-jet final states in double-Pomeron-exchange events at the LHC is presented. In the context of the ATLAS experiment with additional forward physics detectors, cross sections for different experimental settings and gap definitions are estimated. This is done in the framework of the forward physics Monte Carlo interfaced with a perturbative QCD model that successfully reproduces standard jet-gap-jet cross sections at the Tevatron. The extrapolation to LHC energies follows from the Balitsky-Fadin-Kuraev-Lipatov dynamics, implemented in the model at next-to-leading logarithmic accuracy.

Measurement of prompt J/ψ and beauty hadron production cross sections at mid-rapidity in pp collisions at $ \sqrt{s}=7 $ TeV

AbstractThe ALICE experiment at the LHC has studied J/ψproduction at mid-rapidity in pp collisions at$ \sqrt{s}=7 $TeV through its electron pair decay on a data sample corresponding to an integrated luminosityLint= 5.6 nb−1. The fraction of J/ψfrom the decay of long-lived beauty hadrons was determined for J/ψcandidates with transverse momentumpt>1.3 GeV/cand rapidity |y|<0.9. The cross section for prompt J/ψmesons, i.e. directly produced J/ψand prompt decays of heavier charmonium states such as theψ(2S) and χcresonances, is σprompt J/ψ(pt>1.3 GeV/c,|y|<0.9) = 8.3 ± 0.8 (stat.) ± 1.1 (syst.)$ _{-1.4}^{+1.5 } $(syst.pol.)μb. The cross section for the production of b-hadrons decaying to J/ψwithpt>1.3 GeV/cand |y|<0.9 is$ {\sigma_{{{J \left/ {{\psi \leftarrow {{\mathrm{h}}_{\mathrm{B}}}}} \right.}}}} $(pt> 1.3 GeV/c, |y| < 0.9) = 1.46 ± 0.38 (stat.)$ _{-0.32}^{+0.26 } $(syst.)μb. The results are compared to QCD model predictions. The shape of theptandydistributions of b-quarks predicted by perturbative QCD model calculations are used to extrapolate the measured cross section to derive the$ \mathrm{b}\overline{\mathrm{b}} $pair total cross section and dσ/dyat mid-rapidity.

Predictions forp+Pb at 4.4ATeV to test initial-state nuclear shadowing at energies available at the CERN Large Hadron Collider

Collinear factorized perturbative QCD model predictions are compared for p+Pb at 4.4A TeV to test nuclear shadowing of parton distribution at the Large Hadron Collider (LHC). The nuclear modification factor (NMF), R_{pPb}(y=0,p_T<20 GeV/c) = dn_{p Pb} /(N_{coll}(b)dn_{pp}), is computed with electron-nucleus (e+A) global fit with different nuclear shadow distributions and compared to fixed Q^2 shadow ansatz used in Monte Carlo Heavy Ion Jet Interacting Generator (HIJING) type models. Due to rapid DGLAP reduction of shadowing with increasing Q^2 used in e+A global fit, our results confirm that no significant initial state suppression is expected (R_{pPb} (p_T) = 1 \pm 0.1) in the p_T range 5 to 20 GeV/ c. In contrast, the fixed Q^2 shadowing models assumed in HIJING type models predict in the above p_T range a sizable suppression, R_{pPb} (p_T) = 0.6-0.7 at mid-pseudorapidity that is similar to the color glass condensate (CGC) model predictions. For central (N_{coll} = 12) p+ Pb collisions and at forward pseudorapidity (\eta = 6) the HIJING type models predict smaller values of nuclear modification factors (R_{pPb}(p_T)) than in minimum bias events at mid-pseudorapidity (\eta = 0). Observation of R_{pPb}(p_T= 5-20 GeV/c) less than 0.6 for minimum bias p+A collisions would pose a serious difficulty for separating initial from final state interactions in Pb+Pb collisions at LHC energies.

Resonance results from the ALICE detector in 7 TeV pp collisions at the LHC

Author(s): Gangadharan, D | Abstract: The study of resonances in √s = 7 TeV pp collisions provides a test of QCD in a new energy domain as well as a baseline for heavy-ion collisions. The resonances K *(892)0, φ(1020), Λ(1520), Σ(1385) ±, and Ξ(1530) 0 are reconstructed at mid-rapidity from their hadronic decay using data collected by the ALICE detector. We compare the φ(1020) and Σ(1385) ± p T spectra to perturbative QCD models such as PHOJET and different PYTHIA tunes. We also search for the Φ(1860) pentaquark for which we find no evidence.

Forward Neutral-Pion Transverse Single-Spin Asymmetries inp+pCollisions ats=200 GeV

We report precision measurements of the Feynman x (xF) dependence, and first measurements of the transverse momentum (pT) dependence, of transverse single-spin asymmetries for the production of pi0 mesons from polarized proton collisions at sqrt[s] = 200 GeV. The xF dependence of the results is in fair agreement with perturbative QCD model calculations that identify orbital motion of quarks and gluons within the proton as the origin of the spin effects. Results for the pT dependence at fixed xF are not consistent with these same perturbative QCD-based calculations.

Exclusive vector mesons and DVCS at HERA

The latest results on exclusive vector meson production and deeply virtual photon scattering (DVCS) from HERA are reviewed. The measurements are compared with perturbative QCD (pQCD) models.

How important are next-to-leading order models in predicting strange particle spectra in p+p collisions at STAR?

STAR has measured a variety of strange particle species in p + p collisions at $\sqrt{s}$ = 200 GeV. These high statistics data are ideal for comparing to existing leading- and next-to-leading order perturbative QCD (pQCD) models. Next-to-leading (NLO) models have been successful in describing inclusive hadron production using parameterized fragmentation functions (FF) for quarks and gluons. However, in order to describe identied strange particle spectra at NLO, knowledge of flavor separated FF is essential. Such FF have recently been parameterized using data by the OPAL experiment and allow for the first time to perform NLO calculation for strange baryons. In fact, comparing the STAR Lambda data with these calculations allow to put a constraint on the gluon fragmentation function. We show that the Leading-order (LO) event generator PYTHIA has to be tuned significantly to reproduce the STAR identified strange particle data. In particular, it fails to describe the observed enhancement of baryon-to-meson ratio at intermediate pT (2-6 GeV/c). In heavy-ion (HI) collisions this observable has been extensively compared with models and shows a strong dependency on collision centrality or parton density. In the HI context the observed enhancement has been explained by recent approaches in terms of parton coalescense and recombination models.

High-| t| Diffraction and Deeply Virtual Compton Scattering

At the HERA ep collider the diffractive productions of J / ψ mesons and photons were measured. The differential J / ψ cross section is studied in the range 2 | t | 30 GeV 2 , where | t | is the square of the 4-momentum transferred at the proton vertex. The production of photons in the process γ ∗ p → γ Y , where Y denotes the proton dissociative system or the elastically scattered proton, is measured in two different kinematical regimes: i) in photoproduction at large transverse momentum p t γ > 2 GeV and ii) in electroproduction at 4 Q 2 80 GeV 2 . The measurements are compared with perturbative QCD models based on BFKL and DGLAP evolution and with Colour Dipole models.

γ*γ* SCATTERING VIA SECONDARY REGGEON EXCHANGE IN QCD

We summarize the results on the high energy behavior of quark–antiquark exchange in γ*γ* elastic scattering. The ladder diagrams, summed in the double logarithmic approximation, provide a perturbative QCD model for secondary reggeon exchange.

Cross sections and transverse single-spin asymmetries in forward neutral-pion production from proton collisions at sqrt[s]=200 GeV.

Measurements of the production of forward high-energy pi(0) mesons from transversely polarized proton collisions at sqrt[s]=200 GeV are reported. The cross section is generally consistent with next-to-leading order perturbative QCD calculations. The analyzing power is small at x(F) below about 0.3, and becomes positive and large at higher x(F), similar to the trend in data at sqrt[s]< or =20 GeV. The analyzing power is in qualitative agreement with perturbative QCD model expectations. This is the first significant spin result seen for particles produced with p(T)>1 GeV/c at a polarized proton collider.

Diffractive photoproduction of ρ mesons with large momentum transfer at HERA

The diffractive photoproduction of rho mesons, e p \to e rho Y, with large momentum transfer squared at the proton vertex, |t|, is studied with the H1 detector at HERA using an integrated luminosity of 20.1 pb^{-1}. The photon-proton centre of mass energy spans the range 75 < W < 95 GeV, the photon virtuality is restricted to Q^2 < 0.01 GeV^2 and the mass M_Y of the proton remnant is below 5 GeV. The t dependence of the cross section is measured for the range 1.5 < |t| < 10.0 GeV^2 and is well described by a power law, dsigma/ d|t| \propto |t|^{-n}. The spin density matrix elements, which provide information on the helicity structure of the interaction, are extracted using measurements of angular distributions of the rho decay products. The data indicate a violation of s-channel helicity conservation, with contributions from both single and double helicity-flip being observed. The results are compared to the predictions of perturbative QCD models.

Diffractive photoproduction of J/ψ mesons with large momentum transfer at HERA

The diffractive photoproduction of J/ψ mesons is measured with the H1 detector at the ep collider HERA using an integrated luminosity of 78 pb−1. The differential cross section dσ(γp→J/ψY)/dt is studied in the range 2<|t|<30 GeV2, where t is the square of the four-momentum transferred at the proton vertex. The cross section is also presented as a function of the photon–proton centre-of-mass energy Wγp in three t intervals, spanning the range 50<Wγp<200 GeV. A fast rise of the cross section with Wγp is observed for each t range and the slope for the effective linear Pomeron trajectory is measured to be α′=−0.0135±0.0074(stat.)±0.0051(syst.) GeV−2. The measurements are compared with perturbative QCD models based on BFKL and DGLAP evolution. The data are found to be compatible with s-channel helicity conservation.

A measurement of the t dependence of the helicity structure of diffractive ρ meson electroproduction at HERA

The helicity structure of the diffractive electroproduction of ρ mesons, e+ p→ e+ ρ+ Y, is studied in a previously unexplored region of large four-momentum transfer squared at the proton vertex, t: 0< t′<3 GeV 2, where t′=| t|−| t| min. The data used are collected with the H1 detector at HERA in the kinematic domain 2.5< Q 2<60 GeV 2, 40< W<120 GeV. No t dependence of the r 04 00 spin density matrix element is found. A significant t dependent helicity non-conservation from the virtual photon to the ρ meson is observed for the spin density matrix element combinations r 5 00+2 r 5 11 and r 1 00+2 r 1 11. These t dependences are consistently described by a perturbative QCD model based on the exchange of two gluons.

Diffractiveφand ρ production in a perturbative QCD model

The elastic leptoproduction of $\ensuremath{\varphi}$ measured by the H1 Collaboration at DESY HERA is described by a perturbative QCD model, based on open $s\overline{s}$ production and parton hadron duality, proposed by Martin et al. We observe that both the total cross section and the ratio of the longitudinal and transverse cross sections are well reproduced with an effective strange-quark mass ${m}_{s}\ensuremath{\sim}370\ensuremath{-}400 \mathrm{MeV}$ for various gluon distribution functions. The possible connection of the effective mass and the momentum dependent dynamical mass associated with dynamical breaking of chiral symmetry is discussed. The $\ensuremath{\rho}$ leptoproduction data are also well reproduced with an effective quark mass $\ensuremath{\sim}315 \mathrm{MeV}.$