Strange Quark Spin Research Articles

Rise and fall of Λ and Λ¯ global polarization in semi-central heavy-ion collisions at HADES, NICA and RHIC energies from the core-corona model

We compute the $\mathrm{\ensuremath{\Lambda}}$ and $\overline{\mathrm{\ensuremath{\Lambda}}}$ global polarizations in semicentral heavy-ion collisions using the core-corona model where the source of $\mathrm{\ensuremath{\Lambda}}\mathrm{s}$ and $\overline{\mathrm{\ensuremath{\Lambda}}}\mathrm{s}$ is taken as consisting of a high-density core and a less dense corona. We show that the overall properties of the polarization excitation functions can be linked to the relative abundance of $\mathrm{\ensuremath{\Lambda}}\mathrm{s}$ coming from the core versus those coming from the corona. For low collision energies, the former are more abundant whereas for higher energies the latter become more abundant. The main consequence of this reversing of the relative abundance is that both polarizations peak at collision energies $\sqrt{{s}_{NN}}\ensuremath{\lesssim}10$ GeV. The exact positions and heights of these peaks depend not only on this reversal of relative abundances, but also on the centrality class, which is directly related to the quark gluon plasma volume and lifetime, as well as on the relative abundances of $\mathrm{\ensuremath{\Lambda}}\mathrm{s}$ and $\overline{\mathrm{\ensuremath{\Lambda}}}\mathrm{s}$ in the core and corona regions. The intrinsic polarizations are computed from a field-theoretical approach that links the alignment of the strange quark spin with the thermal vorticity and modeling the quark gluon plasma volume and lifetime using a Bjorken expansion scenario. We predict that the $\mathrm{\ensuremath{\Lambda}}$ and $\overline{\mathrm{\ensuremath{\Lambda}}}$ global polarizations should peak at the energy range accessible to NICA and HADES.

Relaxation time for strange quark spin in rotating quark-gluon plasma

Experiments at the Relativistic Heavy Ion Collider (RHIC) have measured the net polarization of $\Lambda$ and $\bar{\Lambda}$ hyperons and attributed it to a coupling between their spin and the vorticity of the fluid created in heavy ion collisions. Equipartition of energy is generally assumed, but the dynamical mechanism which polarizes them has yet to be determined. We consider two such mechanisms: vorticity fluctuations and helicity flip in scatterings between strange quarks and light quarks and gluons. With reasonable parameters both mechanisms lead to equilibration times orders of magnitude too large to be relevant to heavy ion collisions. Our conclusion is that strange quark spin or helicity is unchanged from the time they are created to the time they hadronize. A corollary is that vorticity fluctuations do not affect the hyperon spin either.

Quark contribution to the proton spin from 2+1+1 -flavor lattice QCD

We present the first chiral-continuum extrapolated up, down and strange quark spin contribution to the proton spin using lattice QCD. For the connected contributions, we use eleven ensembles of 2+1+1-flavor of Highly Improved Staggered Quarks (HISQ) generated by the MILC Collaboration. They cover four lattice spacings $a \approx \{0.15,0.12,0.09,0.06\}$ fm and three pion masses, $M_\pi \approx \{315,220,135\}$ MeV, of which two are at the physical pion mass. The disconnected strange calculations are done on seven of these ensembles, covering the four lattice spacings but only one with the physical pion mass. The disconnected light quark calculation was done on six ensembles at two values of $M_\pi \approx \{315,220\}$ MeV. High-statistics estimates on each ensemble for all three quantities allow us to quantify systematic uncertainties and perform a simultaneous chiral-continuum extrapolation in the lattice spacing and the light-quark mass. Our final results are $\Delta u \equiv \langle 1 \rangle_{\Delta u^+} = 0.777(25)(30)$, $\Delta d \equiv \langle 1 \rangle_{\Delta d^+} = -0.438(18)(30)$, and $\Delta s \equiv \langle 1 \rangle_{\Delta s^+} = -0.053(8)$, adding up to a total quark contribution to proton spin of $\sum_{q=u,d,s} (\frac{1}{2} \Delta q) = 0.143(31)(36)$. The second error is the systematic uncertainty associated with the chiral-continuum extrapolation. These results are obtained without model assumptions and are in good agreement with the recent COMPASS analysis $0.13 < \frac{1}{2} \Delta \Sigma < 0.18$, and with the $\Delta q$ obtained from various global analyses of polarized beam or target data.

Revisiting scalar and pseudoscalar couplings with nucleons

Certain dark matter interactions with nuclei are mediated possibly by a scalar or pseudoscalar Higgs boson. The estimation of the corresponding cross sections requires a correct evaluation of the couplings between the scalar or pseudoscalar Higgs boson and the nucleons. Progress has been made in two aspects relevant to this study in the past few years. First, recent lattice calculations show that the strange-quark sigma term σs and the strange-quark content in the nucleon are much smaller than what are expected previously. Second, lattice and model analyses imply sizable SU(3) breaking effects in the determination on the axial-vector coupling constant $ g_A^8 $ that in turn affect the extraction of the isosinglet coupling $ g_A^0 $ and the strange quark spin component Δs from polarized deep inelastic scattering experiments. Based on these new developments, we re-evaluate the relevant nucleon matrix elements and compute the scalar and pseudoscalar couplings of the proton and neutron. We also find that the strange quark contribution in both types of couplings is smaller than previously thought.

Transverse strange quark spin structure of the nucleon

We investigate the transverse quark spin densities of the nucleon with the lowest moment within the framework of the SU(3) chiral quark-soliton model, emphasizing the strange quark spin density. Based on previous results of the vector and tensor form factors, we are able to determine the impact-parameter dependent probability densities of transversely polarized quarks in an unpolarized nucleon as well as those of unpolarized quarks in a transversely polarized nucleon. We find that the present numerical results for the transverse spin densities of the up and down quarks are in good agreement with those of the lattice calculation. We predict the transvere spin densities of the strange quark. It turns out that the polarized strange quark is noticeably distorted in an unpolarized proton.

Strangeness in the Nucleon

Strangeness content inside the nucleon has been one of the most important topics for understanding structure of the nucleon. Using electron scattering off nucleon targets, the strangeness component and its electric, magnetic and spin properties have been studied. Neutrino nucleon scattering is ideal to measure the axial structure of the nucleon, so that strange quark spin, Δs, can be directly determined. Expected intensive neutrino beams in the near future will allow determination of Δs with precision of δ ( Δ s ) ∼ 0.03 .

Study of strange quark in the nucleon with neutrino scattering

Spin structure of the nucleon is one of the current topics of particle physics. It provides a test of QCD which is a part of standard model. Contribution of the strange quark spin to the nucleon spin can be studied with neutrino scattering at medium energies.

The strange quark spin of the proton in semi-inclusive Λ leptoproduction

Considering the possible interpretation of the Ellis-Jaffe sum rule violation that the strange quark in the polarized proton is remarkably polarized in the opposite direction of the proton spin, we investigate its implication in the semi-inclusive Λ production in deep inelastic process with the electron beam unpolarized and proton target polarized longitudinally. As a result, we find that the measurement of the Λ polarization in the process considered can provide clean information about the strange quark spin distribution in the proton.

Combined analysis of world data on nucleon spin structure functions

We analyse the proton, deuteron and neutron spin dependent structure functions at fixed Q 2 = 5 GeV 2. The experimental asymmetries for the proton, neutron and deuteron are compared and found to be consistent. The first moment of the neutron structure function is evaluated using all available data. We find that the Bjorken sum rule is confirmed within an experimental uncertainty of 17%. The quark spin contribution to the nucleon spin is small, and the strange quark spin content is different from zero.

Axial-vector current and quark spin content of the proton in an effective theory

The background field theory is applied to study the internal spin structure of the proton in terms of both the large N c expansion and derivative expansion. A new flavor-independent axial-vector current has been derived. Due to the higher order of this current in the large N c expansion and the higher order in the derivative expansion a small quark spin content is expected and a nonzero strange quark spin content is revealed from the large N c expansion. The numerical results are in agreement with the data.

Generalized Goldberger-Treiman relation and the quark part of the proton spin

Goldberger-Treiman relations for the third, eight and singlet (in a generalized version) axial-vector currents are used to estimate the corresponding weak coupling constants. We take into account SU(3) and isospin mass breaking, as well as π-η-η′ mixing, correcting for some previously encountered inconsistencies. The values obtained suggest a nonvanishing quark “sea” contribution and a nonzero strange quark spin content of the proton.