Improved parton showers in ep interactions

A scheme for including the first-order matrix element in ep parton showers is presented. This is an important improvement since it applies to hard emission where the leading-log approximation is less reliable. The contributions from the initial-and final-state showers are treated on an equal footing, solving the potential problem of double-counting of emission. Kinematical constraints, relating the dynamics of the two separate showers, are also solved in this context. The choice of splitting variable in initial-state showers is furthermore discussed.     

A model for parton showers in QCD

A Monte Carlo model for the development of parton jets in QCD is described. Explicit low-order calculations are supplemented by leading logarithmic approximations for higher orders.     

Leading log approximation for parton showers

We correct and extend the leading log parton shower formalism, which is widely used to compute the predictions of QCD for high-energy experiments, by including the information carried by the polarization of the partons. For observables that do not depend on the spins of the final state particles, this introduces a correlation between the azimuthal angles that is not included in current calculations. It also allows one to compute parton showers for observables that do not depend on the final state spins.     

A model for initial state parton showers

We present a detailed model for exclusive properties of initial state parton showers. A numerically efficient algorithm is obtained by tracing the parton showers backwards, i.e. start with the hard scattering partons and then successively reconstruct preceding branchings in falling sequence of spacelike virtualities Q² and rising sequence of parton energies. We show how the Altarelli-Parisi equations can be recast in a form suitable for this, and also discuss the kinematics of the branchings. The complete model is implemented in a Monte Carlo program, and some first results are presented.     

QCD next-to-leading-order predictions matched to parton showers for vector-like quark models

Vector-like quarks are featured by a wealth of beyond the Standard Model theories and are consequently an important goal of many LHC searches for new physics. Those searches, as well as most related phenomenological studies, however, rely on predictions evaluated at the leading-order accuracy in QCD and consider well-defined simplified benchmark scenarios. Adopting an effective bottom-up approach, we compute next-to-leading-order predictions for vector-like-quark pair production and single production in association with jets, with a weak or with a Higgs boson in a general new physics setup. We additionally compute vector-like-quark contributions to the production of a pair of Standard Model bosons at the same level of accuracy. For all processes under consideration, we focus both on total cross sections and on differential distributions, most these calculations being performed for the first time in our field. As a result, our work paves the way to precise extraction of experimental limits on vector-like quarks thanks to an accurate control of the shapes of the relevant observables and emphasise the extra handles that could be provided by novel vector-like-quark probes never envisaged so far.     

The average number of partons per clan in rapidity intervals in parton showers

The dependence of the average number of partons per clan on virtuality and rapidity variables is analytically predicted in the framework of the Generalized Simplified Parton Shower model, based on the idea that clans are genuine elementary subprocesses. The obtained results are found to be qualitatively consistent with experimental trends. This study extends previous results on the behavior of the average number of clans in virtuality and rapidity and shows how important physical quantities can be calculated analytically in a model based on essentials of QCD allowing local violations of the energy-momentum conservation law, still requiring its global validity.     

Consequences of parton saturation and string percolation on the development of cosmic ray showers

At high gluon or string densities, gluon saturation or the strong interaction among strings, either forming color ropes or giving rise to string percolation, induces a strong suppression in the particle multiplicities produced at high energy. This suppression implies important modifications on cosmic ray shower development. In particular, it is shown that it affects the depth of maximum, the elongation rate, and the behavior of the number of muons at energies about 10(17)--10(18) eV. The existing cosmic ray data point out in the same direction.     

Comparative study of various algorithms for the merging of parton showers and matrix elements in hadronic collisions

We compare different procedures for combining fixed-order tree-level matrix-element generators with parton showers. We use the case of W-production at the Tevatron and the LHC to compare different implementations of the so-called CKKW and MLM schemes using different matrix-element generators and different parton cascades. We find that although similar results are obtained in all cases, there are important differences.     

Delayed muons in extensive air showers and double-front showers

The results of a long-term experiment performed in the period between 1995 and 2006 with the aid of the MUON-T underground (20 mwe) scintillation facility arranged at the Tien Shan mountain research station at an altitude of 3340 m above sea level are presented. The time distribution of delayed muons with an energy in excess of 5 GeV in extensive air showers of energy not lower than 106 GeV with respect to the shower front was obtained with a high statistical significance in the delay interval between 30 and 150 ns. An effect of the geomagnetic field in detecting delayed muons in extensive air showers was discovered. This effect leads to the asymmetry of their appearance with respect to the north-south direction. The connection between delayed muons and extensive air showers featuring two fronts separated by a time interval of several tens of to two hundred nanoseconds is discussed. This connection gives sufficient grounds to assume that delayed muons originate from the decays of pions and kaons produced in the second, delayed, front of extensive air showers.     

Neutron multiplicity: Local hadronic showers and extensive air showers

A detailed analysis of events with neutron multiplicity M = 3−30 on neutron monitors (NMs) in Barentsburg (Spitsbergen), Baksan (North Caucasus), and Apatity (Murmansk region) is performed on the data obtained using a unique new data collection system. It is now possible for the first time to register local showers in cosmic rays on NMs and to investigate their structure with a high precision. Multiplicity on NMs is simulated using the GEANT4 Monte Carlo toolkit. Local hadronic cascades in the atmosphere with transverse sizes of 1 to 6 m were observed for the first time using the NM data. NM multiplicity generated by EAS hadronic cascades is studied on the data set on coupling NMs with the CARPET EAS facility.     

Parton showers in leptoproduction events

We present a model that includes the production of arbitrarily many jets in lepton-hadron events, using the leading log formalism for parton shower evolution. The main problem encountered here, which has not previously been illuminated by studies ofe ⁺ e ^? annihilation or Drell-Yan/Z ⁰/W ^± production, is the choice of kinematics in the space-like shower evolution. In our preferred solution, the standard definition of Bjorkenx is preserved during the construction of initial and final state showers—a nontrivial constraint. The resulting model is described in detail, including some first investigations of its properties.