Probing fluctuating protons using forward neutrons in soft and hard inelastic proton-nucleus scattering

We present a model for the distribution of the number of forward neutrons emitted in soft (minimum bias) and hard inelastic proton-nucleus (<a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mrow><a:mi>p</a:mi><a:mi>A</a:mi></a:mrow></a:math>) scattering at the CERN Large Hadron Collider. It is based on the Gribov-Glauber model for the distribution over the number of inelastic collisions (wounded nucleons) combined with a parametrization of cross section (color) fluctuations in the projectile proton, which depend on the parton momentum fraction <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:msub><b:mi>x</b:mi><b:mi>p</b:mi></b:msub></b:math>, and the assumption of independent neutron emissions. We observe that, while the effect of soft color fluctuations is modest, a reduction of the interaction strength with increasing <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:msub><c:mi>x</c:mi><c:mi>p</c:mi></c:msub></c:math> provides the dominant effect. It allows us to qualitatively describe the trend of the ATLAS data on the zero degree calorimeter (ZDC) energy spectra of forward neutrons emitted in dijet production in inelastic <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"><d:mrow><d:mi>p</d:mi><d:mi>A</d:mi></d:mrow></d:math> scattering at <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"><e:mrow><e:msqrt><e:msub><e:mi>s</e:mi><e:mrow><e:mi>N</e:mi><e:mi>N</e:mi></e:mrow></e:msub></e:msqrt><e:mo>=</e:mo><e:mn>8.16</e:mn></e:mrow></e:math> TeV.