Selection and reconstruction of the top quarks in the all-hadronic decays at a Linear Collider

A method of reconstruction of the top quarks produced in the process E+E- -> t\bar{t} -> 6 jets at a Linear Collider (LC) is proposed. The approach does not involve a kinematic fit, as well as assumptions on the invariant masses of the dijets originating from the decays of W bosons and, therefore, the method is expected to be less sensitive to theoretical and experimental uncertainties on the top-mass measurement than traditional reconstruction methods. For the first time, the reconstruction of the top quarks was investigated using the full LC detector simulation after taking into account the background arising from QCD multi-jet production.Comment: 22 pages, including 13 figures and 3 table

X-ray and neutron diffraction studies of coupled structural phase transitions in DyBaCo2_{2}O5.5_{5.5}

A structural transition at $T\approx 322$ K from the $Pmmm$ to $Pmma$ phase is found to coincide with an anomaly of resistivity. Another structural phase transition doubling the lattice parameter $c$, which has been postulated earlier to accompany a low-temperature magnetic transition in TbBaCo$_{2}$O$_{5.5}$, is observed in a single crystal DbBaCo$_{2}$O$_{5.5}$ by means of the X-ray and neutron diffraction. The low temperature phase does not belong to the space group $Pcca$ that has been chosen earlier as the highest subgroup of the $Pmma$. The transition is of the first order with the temperature hysteresis, between $T\approx 100$ and $T\approx 200$ K, which probably explains anomalous magnetic properties in this temperature range.Comment: 6 pages, 4 figure

LHC as πp\pi p and ππ\pi\pi Collider

We propose an experiment at the LHC with leading neutron production.The latter can be used to extract from it the total $\pi^+ p$ cross-sections. With two leading neutrons we can get access to the total $\pi^+\pi^+$ cross-sections. In this note we give some estimates and discuss related problems and prospects.Comment: 22 pages, 18 figures, 8 tables, to be publishe

Factorization Breaking in Dijet Photoproduction with a Leading Neutron

The production of dijets with a leading neutron in ep-interactions at HERA is calculated in leading order and next-to-leading order of perturbative QCD using a pion-exchange model. Differential cross sections for deep-inelastic scattering (DIS) and photoproduction are presented as a function of several kinematic variables. By comparing the theoretical predictions for DIS dijets to recent H1 data, the pion flux factor together with the parton distribution functions of the pion is determined. The dijet cross sections in photoproduction show factorization breaking if compared to the H1 photoproduction data. The suppression factor is S = 0.48 (0.64) for resolved (global) suppression.Comment: 16 pages, 5 figure

Towards Extraction of π+p\pi^+ p and π+π+\pi^+\pi^+ cross-sections from Charge Exchange Processes at the LHC

We study the possibilities to analyse the data on leading neutrons production at first LHC runs. These data could be used to extract from it $\pi^+ p$ and $\pi^+\pi^+$ cross-sections. In this note we estimate relative contributions of $\pi$, $\rho$ and $a_2$ reggeons to charge exchanges and discuss related problems of measurements.Comment: 8 pages, 9 figures, to be publishe

Elastic π+p\pi^{+}p and π+π+\pi^{+}\pi^{+} scattering at LHC

We discuss the possibility of measuring leading neutron production at the LHC. These data could be used to extract from it $\pi^+ p$ and $\pi^+\pi^+$ cross-sections. In this note we give some estimates for the case of elastic cross-sections and discuss related problems and prospects.Comment: 26 pages, 25 figures, to be published, minor text correction

Gluon shadowing in the Glauber-Gribov model

New data from HERA experiment on (diffractive) deep inelastic scattering has been used to parameterize nucleon and Pomeron structure functions. Within the Gribov theory, the parameterizations were employed to calculate gluon shadowing for various heavy ions and compared our results with predictions from other models. Calculations for d+Au collisions at forward rapidities at ultra-relativistic energies have been made and are compared to RHIC data on the nuclear modification factor. Results for gluon shadowing are also confronted with recent data on the nuclear modification factor at $\sqrt{s} = 17.3$ GeV at various values of the Feynman variable $x_F$, and the energy dependence of the effect is discussed.Comment: To appear in the proceedings of the Workshop for young scientists on the physics of ultrarelativistic nucleus-nucleus collisions, Hot Quarks 2006. To be published in EPJ

Search for contact interactions, large extra dimensions and finite quark radius in ep collisions at HERA

A search for physics beyond the Standard Model has been performed with high-Q^2 neutral current deep inelastic scattering events recorded with the ZEUS detector at HERA. Two data sets, e^+ p \to e^+ X and e^- p \to e^- X, with respective integrated luminosities of 112 pb^-1 and 16 pb^-1, were analyzed. The data reach Q^2 values as high as 40000 GeV^2. No significant deviations from Standard Model predictions were observed. Limits were derived on the effective mass scale in eeqq contact interactions, the ratio of leptoquark mass to the Yukawa coupling for heavy leptoquark models and the mass scale parameter in models with large extra dimensions. The limit on the quark charge radius, in the classical form factor approximation, is 0.85 10^-16 cm.Comment: 28 pages, 4 figures, accepted for publication in Physics Letters

Photoproduction of D∗±D^{*\pm} mesons associated with a leading neutron

The photoproduction of $D^{*\pm} (2010)$ mesons associated with a leading neutron has been observed with the ZEUS detector in $ep$ collisions at HERA using an integrated luminosity of 80 pb$^{-1}$. The neutron carries a large fraction, {$x_L>0.2$}, of the incoming proton beam energy and is detected at very small production angles, {$\theta_n<0.8$ mrad}, an indication of peripheral scattering. The $D^*$ meson is centrally produced with pseudorapidity {$|\eta| 1.9$ GeV}, which is large compared to the average transverse momentum of the neutron of 0.22 GeV. The ratio of neutron-tagged to inclusive $D^*$ production is $8.85\pm 0.93({\rm stat.})^{+0.48}_{-0.61}({\rm syst.})\%$ in the photon-proton center-of-mass energy range {$130 <W<280$ GeV}. The data suggest that the presence of a hard scale enhances the fraction of events with a leading neutron in the final state.Comment: 28 pages, 4 figures, 2 table

Forward jet production in deep inelastic ep scattering and low-x parton dynamics at HERA

Differential inclusive jet cross sections in neutral current deep inelastic ep scattering have been measured with the ZEUS detector. Three phase-space regions have been selected in order to study parton dynamics where the effects of BFKL evolution might be present. The measurements have been compared to the predictions of leading-logarithm parton shower Monte Carlo models and fixed-order perturbative QCD calculations. In the forward region, QCD calculations at order alpha_s^1 underestimate the data up to an order of magnitude at low x. An improved description of the data in this region is obtained by including QCD corrections at order alpha_s^2, which account for the lowest-order t-channel gluon-exchange diagrams, highlighting the importance of such terms in parton dynamics at low x.Comment: 25 pages, 4 figure