Strange Quark Distribution Research Articles

Spectra of identified hadrons in Pb-Pb collisions at 2.76 TeV at the CERN Large Hadron Collider

The transverse-momentum distributions of identified hadrons produced in Pb-Pb collisions at the Large Hadron Collider (LHC) are studied in the low and intermediate range for ${p}_{T}<5$ GeV/$c$. All four spectra ($\ensuremath{\pi},K,p,\ensuremath{\Lambda}$) can be well reproduced in the recombination model based on a common thermal parton distribution of light and strange quarks and on shower partons emitted in hard and semihard jets. Two essential parameters are adjusted to fit the data, one being the inverse slope of the thermal distribution and the other revealing the degree of momentum degradation in the medium. Various combinations of thermal and shower-parton components are calculated, showing the dominance of minijets. The effect of minijets is to produce harder baryons than mesons, resulting in their ratio to peak at around ${p}_{T}\ensuremath{\sim}3$ GeV/$c$. A substantial portion of the jet energy is found to be lost to the dense medium before the partons emerge at the surface to undergo hadronization by recombination.

Test of CDF dijet anomaly within the standard model

Dijet anomaly reported by the CDF (Collider Detector at Fermilab) collaboration in 1.96 TeV p-pbar collisions is investigated within the standard model by considering effects of parton distribution functions on various processes: W+dijet, Z+dijet, WW, ZW, and top production. Since the anomalous peak exists in the dijet-mass region of 140 GeV with the p-pbar center-of-mass energy sqrt{s}=1.96 TeV, a relevant momentum fraction x of partons is roughly 0.1. In this x region, recent HERMES semi-inclusive charged-lepton scattering experiment indicated that the strange-quark distribution could be very different from a conventional one, which has been used for many years, based on opposite-sign dimuon measurements in neutrino-induced deep inelastic scattering. We investigated effects of such variations in the strange-quark distribution s(x) on the anomaly. We found that distributions of W+dijets and other process are affected by the strange-quark modifications in wide dijet-mass regions including the 140 GeV one. Since the CDF anomaly was observed in the shoulder region of the dijet-mass distribution, a slight modification of the distribution shape could explain at least partially the CDF excess. Therefore, it is important to consider such effects within the standard model for judging whether the CDF anomaly indicates new physics beyond the standard model. We also show modification effects of the strange-quark distribution in the LHC (Large Hadron Collider) kinematics, where cross sections are sensitive to a smaller-x region of s(x).

Formula omitted] heavy flavor corrections to charged current deep inelastic scattering in Mellin space

We provide a fast and precise Mellin space implementation of the O(αs) heavy flavor Wilson coefficients for charged current deep inelastic scattering processes. They are of importance for the extraction of the strange quark distribution in neutrino–nucleon scattering and the QCD analyses of the HERA charged current data. Errors in the literature are corrected. We also discuss a series of more general parton parameterizations in Mellin space.

Evaluation of charged current neutrino–nucleon interaction differential cross section

The charged current neutrino–nucleon interaction differential cross section are evaluated in the kinematical range 30 < E ν < 300 GeV , 0.1 < x < 0.8 and 0 < y < 1 using QCD inspired Thermodynamic Bag Model (TBM). We also discuss the x and Q 2 dependence of nucleon structure functions F 2 ( x , Q 2 ) and x F 3 ( x , Q 2 ) estimated with statistical approach. The contribution of strange quark distribution function to the cross section is explored and the results obtained have been compared with relevant data from NuTeV and CHORUS experiments.

Parton transverse momentum distribution at the moment of hadronization at RHIC

We extract the transverse momentum distribution of effective partons using the spectra of Ω, Ξ, Λ and ϕ hadrons measured by the STAR Collaboration from Au + Au collisions at \( \sqrt {s_{NN} } \) = 200 GeV at RHIC. The extracted momentum distribution of strange quarks is flatter than that of up/down quarks consistent with hydrodynamics expansion in partonic phase prior to hadronization. Consistency in quark ratios derived from various hadron spectra gives clear evidence for hadron production as suggested by quark coalescence or recombination model.

Production and polarization of the Λ and [formula omitted] hyperons in DIS at COMPASS

Λ and Λ ¯ hyperons were produced at the COMPASS experiment at CERN, using Deep-Inelastic Scattering (DIS) of 160 GeV/c polarized muons on a longitudinally polarized target. The study of Λ and Λ ¯ hyperon in DIS is important for the understanding of the nucleon structure, the mechanisms of hyperon production and the hyperon spin structure. In particular, it may provide valuable information on the unpolarized strange quark distributions s ( x ) and s ¯ ( x ) in the nucleon. The data sample contains about 70 000 Λ and 42 000 Λ ¯ . Large and comparable statistics on both Λ and Λ ¯ hyperons is a distinct feature of the COMPASS experiment. Preliminary results on polarization of Λ and Λ ¯ hyperons and yields of heavy hyperons in DIS are presented.

Nucleon Strangeness: Intrinsic or Extrinsic?

A review of experimental results for the measurement of the strange quark distributions in the nucleon, is given. Contributions of the strange quarks to the nucleon mass, electromagnetic form factors and spin, are discussed.

Determining the strange and antistrange quark distributions of the nucleon

The difference between the strange and antistrange quark distributions, δs(x) = s(x)–s̄ (x), and the combination of light quark sea and strange quark sea, Δ(x) = d̄(x) + ū(x) – s(x) –s̄ (x), are originated from non-perturbative processes and can be calculated using non-perturbative models of the nucleon. We report calculations of δs(x) and Δ(x) using the meson cloud model. Combining our calculations of Δ(x) with relatively well known light antiquark distributions obtained from global analysis of available experimental data, we estimate the total strange sea distributions of the nucleon.

Strange sea distributions of the nucleon

The strange and antistrange quark distributions of the nucleon are less constrained by experimental data than the non-strange quark sea. The combination of light quark sea distributions, , originates mainly from non-perturbative processes and can be calculated using non-perturbative models of the nucleon. We have calculated Δ(x) using the meson cloud model, which, when combined with the relatively well-known non-strange light antiquark distributions obtained from global analysis of the available experimental data, enables us to make new estimates of the total strange sea distributions of the nucleon and the strange sea suppression factor.

Longitudinal polarization of hyperons in the forward region in polarizedppcollisions

We study the longitudinal polarization of hyperons and anti-hyperons at forward pseudorapidity, $2.5<\eta<4$, in singly polarized $pp$ collisions at RHIC energies by using different parameterizations of the polarized parton densities and different models for the polarized fragmentation functions. The results show that the $\Sigma^+$ polarization is able to distinguish different pictures on spin transfer in high energy fragmentation processes; and the polarization of $\Lambda$ and $\bar\Lambda$ hyperons can provide sensitivity to the helicity distribution of strange sea quarks. The influence from beam remnant to hyperon polarization in the forward region is also discussed.

Determination of strange sea distributions from νN deep inelastic scattering

We present an analysis of the nucleon strange sea extracted from a global Parton Distribution Function fit including the neutrino and anti-neutrino dimuon data by the CCFR and NuTeV Collaborations, the inclusive charged lepton–nucleon Deep Inelastic Scattering and Drell–Yan data. The (anti-)neutrino induced dimuon analysis is constrained by the semileptonic charmed-hadron branching ratio Bμ=(8.8±0.5)%, determined from the inclusive charmed hadron measurements performed by the FNAL-E531 and CHORUS neutrino emulsion experiments. Our analysis yields a strange sea suppression factor κ(20 GeV2)=0.62±0.04(exp.)±0.03(QCD), the most precise value available, an x-distribution of total strange sea that is slightly softer than the non-strange sea, and an asymmetry between strange and anti-strange quark distributions consistent with zero (integrated over x it is equal to S−(20 GeV2)=0.0013±0.0009(exp.)±0.0002(QCD)).

Unexpected nuclear effects in v-Fe interactions and MINERvA's program to measure v scattering off a range of nuclei

Recent studies of nuclear effects in v -Fe data by members of the CTEQ collaboration suggest that nuclear effects as seen by neutrinos are very different compared to those from μ/e-Fe scattering. The CTEQ analysis extracted a set of iron PDFs and showed that under reasonable assumptions it is possible to constrain the valence, light sea, and strange quark distributions. The CTEQ iron PDFs were then used to compute xBj -dependent and Q2 -dependent nuclear correction factors for iron structure functions, which are required in global analyses of free nucleon PDFs. Comparing these results with nuclear correction factors from neutrino-nucleus scattering models and correction factors for μ/e-Fe scattering, CTEQ found that, except for very high xBj, the correction factors differ in both shape and magnitude from the correction factors of the models and charged-lepton scattering.The MINERvA experiment will measure v -A scattering off of He, C, Fe and Pb to systematically study these nuclear effects in neutrino interactions with high statistics. The poster will summarize the CTEQ results and outline the MINERvA program for measuring nuclear effects.

Nuclear parton distribution functions from neutrino deep inelastic scattering

We study nuclear effects in charged current deep inelastic neutrino-iron scattering in the framework of a ${\ensuremath{\chi}}^{2}$ analysis of parton distribution functions (PDFs). We extract a set of iron PDFs and show that under reasonable assumptions it is possible to constrain the valence, light sea, and strange quark distributions. Our iron PDFs are used to compute ${x}_{Bj}$-dependent and ${Q}^{2}$-dependent nuclear correction factors for iron structure functions which are required in global analyses of free nucleon PDFs. We compare our results with nuclear correction factors from neutrino-nucleus scattering models and correction factors for ${\ensuremath{\ell}}^{\ifmmode\pm\else\textpm\fi{}}$-iron scattering. We find that, except for very high ${x}_{Bj}$, our correction factors differ in both shape and magnitude from the correction factors of the models and charged-lepton scattering.

Measurement of the Nucleon Strange-Antistrange Asymmetry at Next-to-Leading Order in QCD from NuTeV Dimuon Data

We present a new measurement of the difference between the nucleon strange and antistrange quark distributions from dimuon events recorded by the NuTeV experiment at Fermilab. This analysis is the first to use a complete next to leading order QCD description of charm production from neutrino scattering. Dimuon events in neutrino deep inelastic scattering allow direct and independent study of the strange and antistrange content of the nucleon. We find a positive strange asymmetry with a significance of 1.6sigma. We also report a new measurement of the charm mass.

Longitudinal polarisation of Λ and Λ̄ hyperons in lepton–nucleon deep-inelastic scattering

We consider models for the spin transfers to Λ and Λ hyperons produced in lepton–nucleon deep-inelastic scattering. We make predictions for longitudinal Λ and Λ spin transfers for the COMPASS experiment and for HERA, and for the spin transfer to Λ hyperons produced at JLAB. We demonstrate that accurate measurements of the spin transfers to Λ and Λ hyperons with COMPASS kinematics have the potential to probe the intrinsic strangeness in the nucleon. We show that a measurement of Λ polarisation could provide a clean probe of the spin transfer from s quarks and provides a new possibility to measure the antistrange quark distribution function. COMPASS data in a domain of x that has not been studied previously will provide valuable extra information to fix models for the nucleon spin structure. The spin transfer to Λ hyperons, which could be measured by the COMPASS experiment, would provide a new tool to distinguish between the SU(6) and Burkardt–Jaffe (BJ) models for baryon spin structure. In the case of the HERA electron–proton collider experiments with longitudinally-polarised electrons, the separation between the target and current fragmentation mechanisms is more clear. It provides a complementary probe of the strange quark distribution and helps distinguish between the SU(6) and BJ models for the Λ and Λ spin structure. Finally, we show that the spin transfer to Λ hyperons measured in a JLAB experiment would be dominated by the spin transfer of the intrinsic polarised-strangeness in the remnant nucleon, providing an independent way to check our model predictions.

The strange parton distribution of the nucleon: global analysis and applications

The strangeness degrees of freedom in the parton structure of the nucleon are explored in the global analysis framework, using the new CTEQ6.5 implementation of the general mass perturbative QCD formalism of Collins. We systematically determine the constraining power of available hard scattering experimental data on the magnitude and shape of the strange quark and anti-quark parton distributions. We find that current data favor a distinct shape of the strange sea compared to the isoscalar non-strange sea. A new reference parton distribution set, CTEQ6.5S0, and representative sets spanning the allowed ranges of magnitude and shape of the strange distributions, are presented. Some applications to physical processes of current interest in hadron collider phenomenology are discussed.

Ds Asymmetry in Photoproduction

By adopting two models of strange and antistrange quark distributions inside nucleon, the light-cone meson-baryon fluctuation model and the effective chiral quark model, we calculate the [Formula: see text] asymmetry in photoproduction in the framework of heavy-quark recombination mechanism. We find that the effect of asymmetry of strange sea to the Ds asymmetry is considerable and depending on the different models. Therefore, we expect that with the further study in electroproduction, e.g. at HERA and CEBAF, the experimental measurements on the [Formula: see text] asymmetry may impose a strong restriction on the strange-antistrange distribution asymmetry models.

Formula omitted] asymmetry in photoproduction

Considering of the possible difference in strange and antistrange quark distributions inside nucleon, we investigate the Ds+–Ds− asymmetry in photoproduction in the framework of heavy-quark recombination mechanism. We adopt two distribution models of strange sea, those are the light-cone meson–baryon fluctuation model and the effective chiral quark model. Our results show that the asymmetry induced by the strange quark distributions is distinct, which is measurable in experiments. And, there are evident differences between the predictions of our calculation and previous estimation. Therefore, the experimental measurements on the Ds+–Ds− asymmetry may impose a unique restriction on the strange–antistrange distribution asymmetry models.

Quark asymmetries in the proton from a model for parton densities

Based on quantum fluctuations in momentum and of the proton into meson-baryon pairs, we develop a physical model for the nonperturbative $x$ shape of parton density functions in the proton. The model describes the proton structure function and gives a natural explanation of observed quark asymmetries, such as the difference between the anti-up and anti-down sea quark distributions and between the up and down valence distributions. An asymmetry in the momentum distribution of strange and antistrange quarks in the nucleon is found to reduce the NuTeV anomaly to a level which does not give a significant indication of physics beyond the standard model.

Effect of asymmetric strange–antistrange sea to the NuTeV anomaly

We calculate the strange quark and antiquark distributions of the nucleon by using the effective chiral quark model, and find that the strange–antistrange asymmetry can bring a contribution of about 60–100% to the NuTeV deviation of sin2θw from the standard value measured in other electroweak processes. The results are insensitive to different inputs. The light-flavor quark asymmetry of d¯–u¯ is also investigated and found to be consistent with the experimental measurements. Therefore the chiral quark model provides a successful picture to understand the NuTeV anomaly, as well as the light-flavor quark asymmetry and the proton spin problem in previous studies.