Angular de-correlation of forward di-hadrons: on the optimal $p_t$ range to probe gluon saturation

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An important phenomenological consequence of the phenomenon of gluon saturation is the suppression of back-to-back hadron and jet pairs produced in the forward region of pA collisions. We present a new calculation of this process within the dilute-dense formalism of the color glass condensate (CGC) effective theory. Following Ref. [1], we collide a large-$x$, dilute probe, described in terms of parton distribution functions, off a dense target, described in terms of transverse momentum dependent (TMD) gluon distributions, whose small-x evolution we calculate using rcBK evolution. We then couple this cross section with the formalism introduced in Ref. [2] to implement in an analytically-controlled way the radiation of soft gluons (Sudakov resummation) from the initial and the final state.

We first apply this CGC+Sudakov framework to the production of forward di-jets at high-$p_t$ in pp and pA collisions. We use the same kinematic cuts and collision energy as in a recent analysis by the ATLAS collaboration [3]. We find that the suppression of the di-jet yield at $\Delta\phi=\pi$, predicted by gluon saturation, is essentially washed out by a strong effect of broadening induced by the radiation of soft gluons. Compatibly with ATLAS data, we find that the nuclear modification factor for forward high-$p_t$ dijets is at best of order $R_{pA}\approx 0.9$.

Motivated by this result, we propose to study the production of forward dihadrons (pions) at lower, though moderate, $p_t$ ($\approx 10$ GeV). We find that, in this case, the suppression of back-to-back pairs genuinely predicted by gluon saturation survives the broadening due to the Sudakov resummation. We indeed obtain $R_{pA} \approx 0.75$ at $\Delta\phi=\pi$ using kinematic cuts compatible with the proposed FoCal upgrade of the ALICE detector. We conclude that forward dihadrons at high $p_t$ provide a sensitive probe of gluon saturation at LHC.

[1] J. L. Albacete, G. Giacalone, C. Marquet and M. Matas,…