Impact of CLAS and COMPASS data on polarized parton densities and higher twist

We have reanalyzed the world data on inclusive polarized deep inelastic scattering (DIS) including the very precise CLAS proton and deuteron data, as well as the latest COMPASS data on the asymmetry $A_1^d$, and have studied the impact of these data on polarized parton densities and higher twist effects. We demonstrate that the low $Q^2$ CLAS data improve essentially our knowledge of higher twist corrections to the spin structure function $g_1$, while the large $Q^2$ COMPASS data influence mainly the strange quark density. In our new analysis we find that a negative polarized gluon density, or one that changes sign as a function of $x$, cannot be ruled out on the basis of the present DIS data.

Figure 1

Effect of CLAS data on the higher twist values.

Figure 2

Impact of CLAS data on the uncertainties for $\mathrm{NLO}(\overline{\mathrm{MS}})$ polarized quark and gluon densities.

Figure 3

Comparison between the higher twist values corresponding to 5 and 7 $x$ bins.

Figure 4

Comparison between polarized $\mathrm{NLO}(\overline{\mathrm{MS}})$ LSS’06 strange quark and gluon densities corresponding to fits of the data using 5 and 7 $x$ bins for higher twist.

Figure 5

Comparison of our $\mathrm{NLO}(\overline{\mathrm{MS}})$ results for $A_1^d$ (a) and $g_1^d$ (b) corresponding to $\Delta G>0$ and $\Delta G<0$ with the new COMPASS data at measured $x$ and $Q^2$ values. Error bars represent the total (statistical and systematic) errors.

Figure 6

Effect of new COMPASS data on the $\mathrm{NLO}(\overline{\mathrm{MS}})$ LSS’06 polarized parton densities.

Figure 7

Effect of new COMPASS data on the higher twist values (a). Comparison between HT values corresponding to the fits with $\Delta G>0$ and $\Delta G<0$ (b).

Figure 8

Strange quark sea densities $x\Delta s(x)$ corresponding to the fits with $\Delta G>0$, $\Delta G<0$ and changing-in-sign $x\Delta G$.

Figure 9

Comparison between our strange quark sea and gluon densities corresponding to $\Delta G>0$ and those obtained by COMPASS [6].

Figure 10

Comparison between our strange quark sea and gluon densities corresponding to $\Delta G<0$ and those obtained by COMPASS [6].

Figure 11

The uncertainties for the strange quark sea and gluon densities corresponding to a negative gluon polarization.

Figure 12

Evolution in $Q^2$ of oscillating-in-sign gluon density.

Figure 13

Comparison between the experimental data and $\mathrm{NLO}(\overline{\mathrm{MS}})$ curves for the gluon polarization $\Delta G(x)/G(x)$ at $Q^2=3{\mathrm{GeV}}^2$ corresponding to $\Delta G>0$, $\Delta G<0$ and an oscillating-in-sign $x\Delta G$. Error bars represent the total (statistical and systematic) errors. The horizontal bar on each point shows the $x$-range of the measurement.