Roman Pasechnik

Beyond ΛCDM: Problems, solutions, and the road ahead

Despite its continued observational successes, there is a persistent (and growing) interest in extending cosmology beyond the standard model, ΛCDM. This is motivated by a range of apparently serious theoretical issues, involving such questions as the cosmological constant problem, the particle nature of dark matter, the validity of general relativity on large scales, the existence of anomalies in the CMB and on small scales, and the predictivity and testability of the inflationary paradigm. In this paper, we summarize the current status of ΛCDM as a physical theory, and review investigations into possible alternatives along a number of different lines, with a particular focus on highlighting the most promising directions. While the fundamental problems are proving reluctant to yield, the study of alternative cosmologies has led to considerable progress, with much more to come if hopes about forthcoming high-precision observations and new theoretical ideas are fulfilled.

Bjorken sum rule and perturbative QCD frontier on the move

The reasonableness of the use of perturbative QCD notions in the region close to the scale of hadronization, i.e., below 1GeV is under study. First, the interplay between higher orders of pQCD expansion and higher-twist contributions in the analysis of recent Jefferson Lab (JLab) data on the generalized Bjorken sum rule function Γ1p-n(Q2) at 0.1

Search for technipions in exclusive production of diphotons with large invariant masses at the LHC

We focus on exclusive production of neutral technipion (pi) over tilde (0) in pp collisions at the LHC, i.e. on pp -> pp (pi) over tilde (0) reaction. The dependence of the cross section on parameters of recently proposed vector-like Technicolor model is studied. Characteristic features of the differential distributions are discussed. For not too large technipion masses the diphoton decay channel has the dominant branching fraction. This is also the main reason for an enhanced production of neutral technipions in gamma gamma-fusion reaction. We discuss potential backgrounds of the QCD and QED origin to the pp -> pp ((pi) over tilde (0) -> gamma gamma) process at large invariant gamma gamma masses. We conclude that compared to inclusive case the signal-to-background ratio in the considered exclusive reaction is very favorable which thereby could serve as a good probe for Technicolor dynamics searches at the LHC

QCD diffractive mechanism of exclusive W+W- pair production at high energies

We discuss new diffractive mechanism of central exclusive production of W+W- pairs in proton-proton collisions at the LHC. We include diagrams with intermediate virtual Higgs boson as well as quark box diagrams. Several observables related to this process are calculated. Predictions for the total cross section and differential distributions in W-boson rapidity and transverse momentum as well as WW invariant mass are presented. We also show results for different polarization states of the final W-+/- bosons. We compare the contribution of the gamma gamma -> W+W- mechanism considered in the literature with the contribution of the diffractive mechanism through the gg -> W(+)W(-)subprocess for the different observables. The phase space integrated diffractive contribution when separated is only a small fraction of fb compared to 115.4 fb of the gamma gamma-contribution without absorption. The latter contribution dominates at small four-momentum transfers squared in the proton lines and in a broad range of W+W- invariant masses. This offers a possibility of efficient searches for anomalous triple-boson (gamma WW) and quartic-boson (gamma gamma WW) couplings and testing models beyond the Standard Model. We discuss shortly also the pp -> pp gamma gamma process, where the box contribution is very similar to that for W+W- and compare our results with recent CDF data. Nice agreement has been achieved without additional free parameters

Central exclusive quark-antiquark dijet and standard model Higgs boson production in proton-(anti)proton collisions

We consider the central exclusive production of qq pairs and Higgs boson in proton-proton collisions at LHC. The amplitude for the process is derived within the k -factorization approach and considered in different kinematical asymptotics, in particular, in the important high quark transverse momenta and massless quark limits. Quark helicity and spin-projection amplitudes in two different frames are shown in extenso. Rapidity distributions, quark jet p - distributions, invariant qq mass distributions, angular azimuthal correlations between outgoing protons and jets are presented. Irreducible bb background to the central exclusive Higgs boson production is analyzed in detail, in particular, how to impose cuts to maximize signal-to-background ratio.

Possible compensation of the QCD vacuum contribution to the dark energy

We suggest one of the possible ways to compensate the large negative quantum-topological QCD contribution to the vacuum energy density of the Universe by means of a positive constant contribution from a cosmological Yang-Mills field. An important role of the exact particular solution for the Yang-Mills field corresponding to the finite-time instantons is discussed. An interesting connection of the compensation mechanism to the color confinement in the framework of instanton { � �

Chiral-symmetric technicolor with standard model Higgs boson

Most of the traditional technicolor-based models are known to be in a strong tension with the electroweak precision tests. We show that this serious issue is naturally cured in strongly coupled sectors with chiral-symmetric vectorlike gauge interactions in the framework of the gauged linear sigma model. We discuss possible phenomenological implications of such a nonstandard chiral-symmetric technicolor scenario in its simplest formulation preserving the standard model (SM) Higgs mechanism. For this purpose, we assume the existence of an extra technifermion sector confined under extra SU(3)(TC) at the energy scales reachable at the LHC, Lambda(TC) similar to 0: 1-1 TeV and interacting with the SM gauge bosons in a chiral-symmetric (vectorlike) way. In the framework of this scenario, the SM Higgs vacuum expectation value acquires a natural interpretation in terms of the condensate of technifermions in confinement in the nearly conformal limit. We study the influence of the lowest-lying composite physical states, namely, technipions, technisigma, and constituent technifermions, on the Higgs sector properties in the SM and other observables at the LHC. We find that the predicted Higgs boson signal strengths in gamma gamma, vector-boson VV*, and fermion f (f) over bar decay channels can be sensitive to the new strongly coupled dynamics and are consistent with the current SM-like Higgs boson observations in the limit of relatively small Higgs-technisigma mixing. At the same time, the chiral-symmetric technicolor provides us with rich technipion phenomenology at the LHC, and its major implications are discussed in detail. (Less)

Dark Energy from graviton-mediated interactions in the QCD vacuum

Adopting the hypothesis about the exact cancellation of vacuum condensates contributions to the ground state energy in particle physics to the leading order in graviton-mediated interactions, we argue that the observable cosmological constant can be dynamically induced by an uncompensated quantum gravity correction to them after the QCD phase transition epoch. To start with, we demonstrate a possible cancellation of the quark-gluon condensate contribution to the total vacuum energy density of the Universe at temperatures T < 100 MeV without taking into account the graviton-mediated effects. In order to incorporate the latter, we then calculate the leading-order quantum correction to the classical Einstein equations due to metric fluctuations induced by the non-perturbative vacuum fluctuations of the gluon and quark fields in the quasiclassical approximation. It has been demonstrated that such a correction to the vacuum energy density has a form eΛ ~ GΛQCD6, where G is the gravitational constant, and ΛQCD is the QCD scale parameter. We analyze capabilities of this approach based on the synthesis between quantum gravity in quasiclassical approximation and theory of non-perturbative QCD vacuum for quantitative explanation of the observed Dark Energy density.

Drell-Yan phenomenology in the color dipole picture revisited

An extensive phenomenological study of the Drell-Yan (DY) process in pp collisions at various energies is performed in the color dipole framework. Besides previously studied γ∗ production we also include the Z0 contribution relevant at large dilepton invariant masses. We investigate the DY cross section differential in invariant mass, rapidity and transverse momentum of the dilepton pair in pp collisions at the RHIC and LHC. We consider three different phenomenological models for the dipole cross section and find a reasonable agreement with the available data. As a further test of the color dipole formalism, we also study the correlation function in azimuthal angle between the dilepton pair and a forward pion Δφ for different energies, dilepton rapidities and invariant masses. The characteristic double-peak structure of the correlation function around Δφ≃π found for very forward pions and low-mass dilepton pairs is sensitive to the saturation effects and can be tested by future DY measurements in pp collisions. (Less)

Diffractive gauge bosons production beyond QCD factorization

We discuss single diffractive gauge boson d (gamma*, W+/-, Z) production in proton-proton collisions at different (Large Hadron Collider and Relativistic Heavy Ion Collider) energies within the color dipole approach. The calculations are performed for gauge bosons produced at forward rapidities. The diffractive cross section is predicted as a function of the fractional momentum and invariant mass of the lepton pair. We found a dramatic breakdown of the diffractive QCD factorization caused by an interplay of hard and soft interactions. Data from the CDF experiment on diffractive production of W and Z are well explained in a parameter-free way. DOI: 10.1103/PhysRevD.86.114039