Evidence for $x$-dependent proton color fluctuations in $pA$ collisions at the CERN Large Hadron Collider

The centrality dependence of forward jet production in $pA$ collisions at the Large Hadron Collider (LHC) has been found to grossly violate the Glauber model prediction in a way that depends on the $x$ in the proton. We argue that this modification pattern provides the first experimental evidence for $x$-dependent proton color fluctuation effects. On average, parton configurations in the projectile proton containing a parton with large $x$ interact with a nuclear target with a significantly smaller than average cross section and have smaller than average size. We implement the effects of fluctuations of the interaction strength and, using the ATLAS analysis of how hadron production at backward rapidities depends on the number of wounded nucleons, make quantitative predictions for the centrality dependence of the jet production rate as a function of the $x$-dependent interaction strength $\sigma \left(x\right)$. We find that $\sigma \left(x\right)\sim 0.6〈\sigma 〉$ gives a good description of the data at $x=0.6$. These findings support an explanation of the European Muon Collaboration effect as arising from the suppression of small-size nucleon configurations in the nucleus.

Figure 1

Probability distributions of $\nu$ proton-nucleon collisions in all $pA$ collisions and in those selected by different $\mathrm{\Sigma }{E}_T$, or centrality, ranges. The inset shows the distributions on a log scale.

Figure 2

Probability of a hard process corresponding to a small $\lambda =〈{\sigma }_{\mathrm{in}}\left(x\right)〉/〈{\sigma }_{\mathrm{in}}〉$ trigger selection relative to that for a generic hard processes, as given in Eq. (5) for ${\omega }_{\sigma }\left(x\right)=0.1,0.2$. $R_{\nu }^{\mathrm{hard}}=1$ is the expectation of the Glauber model.

Figure 3

$R_{\nu }^{\mathrm{hard}}$ for $pA$ collisions with $x=E_{\mathrm{jet}}/E_p=0.6$, and $\lambda =0.6$ for centrality bins extracted from the ATLAS data [5] and using $\nu$ distributions given by the CF model [17]. Errors are combined statistical and systematic errors. The solid line is the Glauber model expectation.

Figure 4

$R_{\nu }^{\mathrm{hard}}$ for different centralities as a function of $\lambda$.