The differential cross section for inclusive particle production as a function of energy in proton-proton collisions at a center-of-mass energy of 13 TeV is measured in the very forward region of the CMS detector. The measurement is based on data collected with the CMS apparatus at the LHC, and corresponds to an integrated luminosity of 0.34 µb −1 . The energy is measured in the CASTOR calorimeter, which covers the pseudorapidity region −6.6 < η < −5.2. The results are given as a function of the total energy deposited in CASTOR, as well as of its electromagnetic and hadronic components. The spectra are sensitive to the modeling of multiparton interactions in pp collisions, and provide new constraints for hadronic interaction models used in collider and in high energy cosmic ray physics. The CMS collaboration 12
IntroductionParticle production at very forward rapidities (|η| > 5) in high energy hadronic collisions receives contributions from multiparton interactions (MPI), initial-and final-state radiation (from the hardest parton scattering), fragmentation of the so-called "beam remnants", plus diffraction [1]. The sum of all these particle production mechanisms, typically referred to as the "underlying event" [2], is modeled phenomenologically in hadronic Monte Carlo event generators with parameters tuned from the data [3][4][5]. A good understanding of forward particle production is important for a complete description of the final states in proton-proton (pp) interactions at colliders, as well as to accurately simulate extensive air showers induced in the earth atmosphere by very high energy cosmic rays [6]. In particular, forward charged hadron production has direct impact on the total number of air-shower muons at the ground, whose measurement shows unexplained excesses compared to model predictions [7]. The rapidities covered in the presented measurement (−6.6 < η < −5.2, still well separated from the beam rapidity y ≈ 9.5) are mostly sensitive to the MPI activity [8]. Previous studies of very forward particle production in pp collisions have been carried out at center-of-mass energies of 0.9, 2.76, 7 and 8 TeV by CMS [8, 9], at 7 and 8 TeV by TOTEM [9][10][11], and at 7 TeV by LHCf [12, 13]. The present paper reports new measurements of inclusive energy spectra at a center-of-mass energy of 13 TeV. The data are discussed in terms of the production of electrons and photons (mostly from π 0 decays), as well as hadrons (mostly π ± ) in the very forward direction covered by the CASTOR calorimeter of the CMS experiment at the CERN LHC. CASTOR [14] covers the pseudorapidity region −6.6 < η < −5.2, and can distinguish between electromagnetic and hadronic energy depositions. CASTOR is only installed on the negative z-side of CMS, and hence has acceptance at negative pseudorapidities. Because of CASTOR's very forward location, the data are sensitive to parton interactions at very small and large fractional momenta in the proton, x < 10 −4 and x → 1.-1 -