No Arabic abstract
We present studies that show how multi-parton interaction and color reconnection affect the hadro-chemistry in proton-proton (pp) collisions with special focus on the production of resonances using the PYTHIA8 event generator. We find that color reconnection suppresses the relative production of meson resonances such as $rho_{0}$ and K*, providing an alternative explanation for the K*/K decrease observed in proton-proton collisions as a function of multiplicity by the ALICE collaboration. Detailed studies of the underlying mechanism causing meson resonance suppression indicate that color reconnection leads to shorter, less energetic strings whose fragmentation is less likely to produce more massive hadrons for a given quark content, therefore reducing ratios such as K*/K and $rho_0/pi$ in high-multiplicity pp collisions. In addition, we have also studied the effects of allowing string junctions to form and found that these may also contribute to resonance suppression.
Herwig 7 is a general-purpose Monte Carlo generator of particle collisions comprising both hard perturbative as well as soft phenomenological physics. Herwig is therefore capable to describe the entire final state of hadronized particles in a collision event. A spacetime topology of a parton system entering hadronization is fully described and tested for the first time. A combination of information from particles momenta and spacetime positions is utilized to minimize a boost-invariant distance measure of the parton system. We present a reasonable agreement of the model with a selection of experimental data and conclude that spacetime event topology can be meaningfully used in the further development.
New physics at the weak scale that can couple to quarks typically gives rise to unacceptably large flavor changing neutral currents. An attractive way to avoid this problem is to impose the principal of minimal flavor violation (MFV). Recently it was noted that in MFV only scalars with the same gauge quantum numbers as the standard model Higgs doublet or color octet scalars with the same weak quantum numbers as the Higgs doublet can couple to quarks. In this paper we compute the one-loop rate for production of a single color octet scalar through gluon fusion at the LHC, which can become greater than the tree level pair production rate for octet scalar masses around a TeV. We also calculate the precision electroweak constraint from Z decays to a b and anti-b quark; this constraint on color octet mass and Yukawa coupling affects the allowed range for single octet scalar production through gluon fusion.
Color fluctuations in hadron-hadron collisions are responsible for the presence of inelastic diffraction and lead to distinctive differences between the Gribov picture of high energy scattering and the low energy Glauber picture. We find that color fluctuations give a larger contribution to the fluctuations of the number of wounded nucleons than the fluctuations of the number of nucleons at a given impact parameter. The two contributions for the impact parameter averaged fluctuations are comparable. As a result, standard procedures for selecting peripheral (central) collisions lead to selection of configurations in the projectile which interact with smaller (larger) than average strength. We suggest that studies of pA collisions with a hard trigger may allow to observe effects of color fluctuations.
The process of stop-chargino production at LHC has been calculated in the Minimal Supersymmetric Standard Model at the complete electroweak one-loop level, assuming a mSUGRA symmetry breaking scheme. Several properties of the angular and invariant mass distributions of the basic bottom-gluon to stop-chargino amplitudes have been derived. For a meaningful collection of different benchmark points the overall electroweak one-loop effects are at most of the order of a few percent. At the realistically expected LHC accuracy, the main supersymmetric electroweak features of the process can be therefore essentially derived in this theoretical scheme from the simple Born level expressions.
Full one-loop electroweak-corrections for an $e^-e^+rightarrow t bar{t}$ process associated with sequential $trightarrow b mu u_mu$ decay are discussed. At the one-loop level, the spin-polarization effects of the initial electron and positron beams are included in the total and differential cross sections. A narrow-width approximation is used to treat the top-quark production and decay while including full spin correlations between them. We observed that the radiative corrections due to the weak interaction have a large polarization dependence on both the total and differential cross sections. Therefore, experimental observables that depend on angular distributions such as the forward-backward asymmetry of the top production angle must be treated carefully including radiative corrections. We also observed that the energy distribution of bottom quarks is majorly affected by the radiative corrections.