Color reconnection and flow-like patterns in pp collisions

Data collected at the LHC are confronted with the possible existence of flow in pp collisions. Here we present a study inside the framework of PYTHIA 8, showing that it contains implicit flow-like effects coming from multiple hard subcollisions and color string formation between initial and final partons from independent hard scatterings: color reconnection. We present studies with strange hadron observables in pp collisions at 7 TeV. Studies have been done both for minimum bias and multiplicity intervals in events with and without color reconnection to isolate the flow-like effect.


Introduction
The existence of something that is called flow has been taken as a proof of collective behavior of partons and hadrons. In central heavy ion collisions the transverse pressure gradient causes a transverse hydrodynamic expansion. This effect is transmitted to hadrons via a boost by the local velocity field. The transverse flow shifts the emitted particles to higher transverse momentum p T , affecting the baryon to meson ratio as a function of p T which exhibits an enhancement of the ratio and its position is pushed to higher momenta when one goes from peripheral to central Pb-Pb collisions [1]. In pp collisions at √ 7 TeV, the behavior of different ratios of particles could be related with flow. The ratios (p + + p − )/(π + + π − ) and (Λ 0 +Λ 0 )/(2 × K 0 s ) exhibits a small enhancement, and it looks like there is no or little energy dependence [2] which is qualitatively well reproduced by PYTHIA version 8.17 [3] tune 4C [4], even though in general PYTHIA underestimates the production of strange particles [5]. The aim of this work is to show that the enhancement in the baryon to meson ratio is attributed to color reconnection, a mechanism present in PYTHIA 8.

Color reconnection
Color reconnection (CR) is a microscopic mechanism in which final partons are connected by color string, in such a way that the total string length becomes as short as possible [6]. One string connecting two partons follows the movement of the partonic end points. The effect of this movement is a common boost of the string fragments. With CR two partons from independent hard scattering at mid-rapidity can color reconnect and make a large transverse boost (Fig. 1).  3. Studies to isolate the flow-like effect.
The parameter which controls CR is the reconnection range (RR) which enters in the probability to merge a hard scalep T system with one of a harder scale, tune 4C uses the value RR = 1.5 which gives a good description of mean transverse momentum as a function of multiplicity.

Comparison with and without CR.
To study the effect we simulate MB events with PYTHIA 8, tune 4C, with (RR = 1.5) and without (RR = 0) CR. Left side of Fig. 2 shows the evolution of the p T spectra of Λ for ranges of the number of Multiple Parton Interactions (MPI), from this it is observed a harder spectra when CR are included. Also shown in the right side of Fig. 2 different baryon to meson ratios as well as heavy to light meson ratios as a function of p T , all of them shown a bump around 2.5−3 GeV/c which diminishes when CR is turned off. The position of the maximum is shifted to larger p T values when CR is included, however after p T > 7 Gev/c, the particles ratio are unaffected by CR. A qualitatively similar evolution of the spectral shapes and particles ratios with collision centralities has been observed in heavy ion collisions [7]. MPI and Particle ratios. In reference [8] We shown that the baryon to meson ratios are enhanced in events with increased MPI activity. In events with more than 20 MPI's the peak is also pushed  p T as a function of multiplicity. At LHC energies the CMS collaboration has published p T spectra for pions, kaons and protons as a function of the track multiplicity. They found that p T increases with multiplicity, this can only be described by PYTHIA when CR is included. In Fig. 3 we observe an increase of p T with multiplicity when CR is turned on, while it is essentially flat when color reconnection is turned off. The stronger increase of p T for heavy hadrons mimics a flow-like effect due to the string boost. Energy dependence. If the center of mass energy is increased in the simulations the MPI activity increases and the peak is shift further out in p T and the magnitude increases. So the effect of CR becomes more prominent at higher energies.

Summary
The CR scheme used in the PYTHIA 8 event generator exhibits a qualitatively new feature that has potentially important consequence on our understanding of the details of the pp collisions. The "flow" mechanism present in PYTHIA 8 does not require thermalization or a medium to be formed. This could lead us to make predictions that are different from hydrodynamics for small systems as pp and p-Pb.