We present an updated set of parameters for the PYTHIA 8 event generator. We reevaluate the constraints imposed by LEP and SLD on hadronization, in particular with regard to heavy-quark fragmentation and strangeness production. For hadron collisions, we combine the updated fragmentation parameters with the new NNPDF2.3 LO PDF set. We use minimum-bias, Drell–Yan, and underlying-event data from the LHC to constrain the initial-state-radiation and multi-parton-interaction parameters, combined with data from SPS and the Tevatron to constrain the energy scaling. Several distributions show significant improvements with respect to the current defaults, for both $$ee$$
 and $$pp$$
 collisions, though we emphasize that interesting discrepancies remain in particular for strange particles and baryons. The updated parameters are available as an option starting from PYTHIA 8.185, by setting Tune:ee = 7 and Tune:pp = 14.

/pdf/tuning-pythia-8-1-the-monash-2013-tune-3pdoticvpp.pdf

Tuning PYTHIA 8.1: the Monash 2013 tune

We present an updated set of parameters for the PYTHIA 8 event generator. We reevaluate the constraints imposed by LEP and SLD on hadronization, in particular with regard to heavy-quark fragmentation and strangeness production. For hadron collisions, we combine the updated fragmentation parameters with a new NNPDF2.3 LO PDF set. We use minimum-bias, Drell-Yan, and underlying-event data from the LHC to constrain the initial-state-radiation and multi-parton-interaction parameters, combined with data from SPS and the Tevatron to constrain the energy scaling. Several distributions show significant improvements with respect to the current defaults, for both ee and pp collisions, though we emphasize that interesting discrepancies remain in particular for strange particles and baryons. The updated parameters are available as an option starting from PYTHIA 8.185, by setting Tune:ee = 7 and Tune:pp = 14.

/pdf/tuning-pythia-8-1-the-monash-2013-tune-4b599x3syv.pdf

Tuning PYTHIA 8.1: the Monash 2013 Tune

At sufficiently high temperature and energy density, nuclear matter undergoes a transition to a phase in which quarks and gluons are not confined: the quark-gluon plasma (QGP). Such an exotic state of strongly interacting quantum chromodynamics matter is produced in the laboratory in heavy nuclei high-energy collisions, where an enhanced production of strange hadrons is observed. Strangeness enhancement, originally proposed as a signature of QGP formation in nuclear collisions, is more pronounced for multi-strange baryons. Several effects typical of heavy-ion phenomenology have been observed in high-multiplicity proton-proton (pp) collisions, but the enhanced production of multi-strange particles has not been reported so far. Here we present the first observation of strangeness enhancement in high-multiplicity proton-proton collisions. We find that the integrated yields of strange and multi-strange particles, relative to pions, increases significantly with the event charged-particle multiplicity. The measurements are in remarkable agreement with the p-Pb collision results, indicating that the phenomenon is related to the final system created in the collision. In high-multiplicity events strangeness production reaches values similar to those observed in Pb-Pb collisions, where a QGP is formed.

/pdf/enhanced-production-of-multi-strange-hadrons-in-high-1g851lyf9u.pdf

Enhanced production of multi-strange hadrons in high-multiplicity proton-proton collisions

http://repositorio.unicamp.br/jspui/bitstream/REPOSIP/82014/1/000330556000006.pdf

Multiplicity dependence of pion, kaon, proton and lambda production in p-Pb collisions at √SNN = 5.02 TeV

New physics has traditionally been expected in the high-pT region at high-energy collider experiments. If new particles are light and weakly coupled, however, this focus may be completely misguided: light particles are typically highly concentrated within a few mrad of the beam line, allowing sensitive searches with small detectors, and even extremely weakly coupled particles may be produced in large numbers there. We propose a new experiment, forward search experiment, or FASER, which would be placed downstream of the ATLAS or CMS interaction point (IP) in the very forward region and operated concurrently there. Two representative on-axis locations are studied: a far location, 400 m from the IP and just off the beam tunnel, and a near location, just 150 m from the IP and right behind the TAN neutral particle absorber. For each location, we examine leading neutrino- and beam-induced backgrounds. As a concrete example of light, weakly coupled particles, we consider dark photons produced through light meson decay and proton bremsstrahlung. We find that even a relatively small and inexpensive cylindrical detector, with a radius of ∼10  cm and length of 5–10  m, depending on the location, can discover dark photons in a large and unprobed region of parameter space with dark photon mass mA′∼10–500  MeV and kinetic mixing parameter e∼10-6-10-3. FASER will clearly also be sensitive to many other forms of new physics. We conclude with a discussion of topics for further study that will be essential for understanding FASER’s feasibility, optimizing its design, and realizing its discovery potential.

ForwArd Search ExpeRiment at the LHC

EPOS is a Monte-Carlo event generator for minimum bias hadronic interac- tions, used for both heavy ion interactions and cosmic ray air shower simulations. Since the last public release in 2009, the LHC experiments have provided a number of very inter- esting data sets comprising minimum bias p-p, p-Pb and Pb-Pb interactions. We describe the changes required to the model to reproduce in detail the new data available from LHC and the consequences in the interpretation of these data. In particular we discuss the effect of the collective hadronization in p-p scattering. A different parametrization of flow has been introduced in the case of a small volume with high density of thermalized matter (core) reached in p-p compared to large volume produced in heavy ion collisions. Both parametrizations depend only on the geometry and the amount of secondary particles en- tering in the core and not on the beam mass or energy. The transition between the two flow regimes can be tested with p-Pb data. EPOS LHC is able to reproduce all minimum bias results for all particles with transverse momentum from pt = 0 to a few GeV/c.

/pdf/epos-lhc-test-of-collective-hadronization-with-data-measured-2jes7fgrgq.pdf

E. Yatsenko

Papers

EPOS LHC: Test of collective hadronization with data measured at the CERN Large Hadron Collider