Abstract
Measurements of inclusive observables, such as particle multiplicities and momentum spectra, have already delivered important information on soft-inclusive (“minimum-bias”) physics at the Large Hadron Collider. In order to gain a more complete understanding, however, it is necessary to include also observables that probe the structure of the studied events. We argue that forward–backward (FB) correlations and event-shape observables may be particularly useful first steps in this respect. We study the sensitivity of several different types of FB correlations and two event-shape variables—transverse thrust and transverse thrust minor—to various sources of theoretical uncertainty: multiple parton interactions, parton showers, color (re)connections, and hadronization. The power of each observable to furnish constraints on Monte Carlo models is illustrated by including comparisons between several recent, and qualitatively different, Pythia 6 tunes, for pp collisions at $\sqrt{s}=900~\mbox{GeV}$ .
Highlights
The LHC offers a rich cornucopia of opportunities to test and expand the data sets used for MC tuning
The measurements used to constrain physics models of high energy particle collisions came primarily from experiments done at the previous generations of accelerators, such as the SPS, LEP, and the Tevatron
Lar, studies of “minimum-bias” and underlying-event physics For minimum bias (MB), which is the focus of this pahave been widely used to constrain the poorly known non- per, there is no “hard scale”, and all observables refactorizable and non-perturbative aspects of Monte Carlo ceive large non-perturbative corrections
Summary
The LHC offers a rich cornucopia of opportunities to test and expand the data sets used for MC tuning. It is worth noting, that the smooth transition between soft and hard scattering processes is the main reason the PYTHIA modeling can be used for both minimum-bias and underlying-event physics. – New: transverse-momentum-ordered parton showers, including showers off the additional MPI, and a more sophisticated treatment of the beam remnant, in which “string junctions” [24] carry the beam baryon number In both cases, the fundamental MPI cross sections are derived from a Sudakov-like unitarization/resummation of perturbative QCD 2 → 2 scattering [21], normalized to the total inelastic non-diffrative cross section, and regulated at low p⊥ by a smooth dampening factor. Two other significant parameters are the assumed transverse shape of the proton (lumpy or smooth), and the strength of colour reconnections (CR) in the final state, cf. [8]
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