arXiv subjects

Roman Pasechnik

Publications and source records attributed to Roman Pasechnik.

At least 127 records · Page 7Linked to original sources

arXiv2017

Phenomenological Review on Quark-Gluon Plasma: Concepts vs. Observations

In this review, we present an up-to-date phenomenological summary of research developments in the physics of the Quark--Gluon Plasma (QGP). A short historical perspective and theoretical motivation for this rapidly developing field of contemporary particle physics is provided. In addition, we introduce and discuss the role of the quantum chromodynamics (QCD) ground state, non-perturbative and lattice QCD results on the QGP properties, as well as the transport models used to make a connection between theory and experiment. The experimental part presents the selected results on bulk observables, hard and penetrating probes obtained in the ultra-relativistic heavy-ion experiments carried out at the Brookhaven National Laboratory Relativistic Heavy Ion Collider (BNL RHIC) and CERN Super Proton Synchrotron (SPS) and Large Hadron Collider (LHC) accelerators. We also give a brief overview of new developments related to the ongoing searches of the QCD critical point and to the collectivity in small ($p+p$ and $p+A$) systems.

hep-ph↗

arXiv2017

Charged scalars from $\mathrm{SU}(3)^3$ theories

Inspired by a recently proposed GUT model based on the trinification ($\mathrm{SU}(3)^3$) gauge group with a global family ($\mathrm{SU}(3)_F$) symmetry, we consider an effective low-energy three Higgs doublet model that may shed light on what underlies the observed fermion mass hierarchies and CKM mixing. We discuss possibilities for charged scalars coming from this model to show in collider experiments and show some interesting benchmark points.

hep-ph↗

arXiv2016

On a radiative origin of the Standard Model from Trinification

In this work, we present a trinification-based grand unified theory incorporating a global $\mathrm{SU}(3)$ family symmetry that after a spontaneous breaking leads to a left-right symmetric model. Already at the classical level, this model can accommodate the matter content and the quark Cabbibo mixing in the Standard Model (SM) with only one Yukawa coupling at the unification scale. Considering the minimal low-energy scenario with the least amount of light states, we show that the resulting effective theory enables dynamical breaking of its gauge group down to that of the SM by means of radiative corrections accounted for by the renormalisation group evolution at one loop. This result paves the way for a consistent explanation of the SM breaking scale and fermion mass hierarchies.

hep-ph↗

arXiv2016

Drell-Yan process in $pA$ collisions: the exact treatment of coherence effects

In this work, we investigate production of Drell-Yan (DY) pairs in proton-nucleus collisions in kinematic regions where the corresponding coherence length does not exceed the nuclear radius, $R_A$, and the quantum coherence effects should be treated with a special care. The results for the nucleus-to-nucleon production ratio available in the literature so far are usually based on the assumption of a very long coherence length (LCL) $l_c\gg R_A$. Since the onset of coherence effects is controlled by the coherence length $l_c$, we estimated its magnitude in various kinematic regions of the DY process and found that the LCL approximation should not be used at small and medium c.m. collision energies ($\sqrt{s} \lesssim 200$ GeV) as well as at large dilepton invariant masses. In order to obtain realistic predictions, we computed for the first time the DY cross section using the generalised color dipole approach based on the rigorous Green function formalism, which naturally incorporates the color transparency and quantum coherence effects and hence allows to estimate the nuclear shadowing with no restrictions on the CL. In addition to the shadowing effect, we studied a complementary effect of initial state interactions (ISI) that causes an additional suppression at large values of the Feynman variable. Numerical results for the nuclear modification factor accounting for the ISI effect and the finite $l_c$ are compared to the data available from the fixed-target FNAL measurements and a good agreement has been found. Besides, we present new predictions for the nuclear suppression as a function of dilepton rapidity and invariant mass in the kinematic regions that can be probed by the RHIC collider as well as by the planned AFTER@LHC and LHCb fixed-target experiments.

hep-ph↗

arXiv2016

All one-loop scalar vertices in the effective potential approach

Using the one-loop Coleman-Weinberg effective potential, we derive a general analytic expression for all the derivatives of the effective potential with respect to any number of classical scalar fields. The result is valid for a renormalisable theory in four dimensions with any number of scalars, fermions or gauge bosons. This result corresponds to the zero-external momentum contribution to a general one-loop diagram with N scalar external legs. We illustrate the use of the general result in two simple scalar singlet extensions of the Standard Model, to obtain the dominant contributions to the triple couplings of light scalar particles under the zero external momentum approximation

hep-ph↗

arXiv2016

Interpreting the 750 GeV diphoton signal as technipion

We discuss whether the enhancement in the diphoton final state at $M_{γγ}$ = 750 GeV, observed recently by the ATLAS and CMS Collaborations could be a neutral pseudoscalar technipion $\tildeπ^0$. We considered two distinct minimal models for the dynamical electroweak symmetry breaking. Here we concentrate only on two-flavor vector-like technicolor model and we assume that the two-photon fusion is a dominant production mechanism. We include contributions of $2 \to 1$, $2 \to 2$ and $2 \to 3$ partonic processes. All the mechanisms give similar contributions to the cross section. With the strong Yukawa (technipion-techniquark) coupling $g_{TC} \simeq 20$ we roughly obtain the measured cross section of the "signal". With such value of $g_{TC}$ we get a relatively small total decay width $Γ_{\rm tot}$. We discuss also the size of the signal at lower energies (LHC, Tevatron) for $γγ$ final states, where the enhancement was not observed. We predict a measurable cross section for neutral technipion production associated with one or two soft jets. The technipion signal is compared with the Standard Model diphoton background contributions. We observe the dominance of inelastic-inelastic $γγ$ processes. We predict the signal cross section for purely exclusive $p p \to p p γγ$ procesess at $\sqrt{s}$ = 13 TeV to be about 0.2 fb. Such a cross section would be, however, difficult to measure with the planned integrated luminosity. We conclude that in all considered cases the signal is below the background or/and below the threshold set by statistics although some tension can be seen.

hep-ph↗

arXiv2016

Nuclear effects in Drell-Yan pair production in high-energy $pA$ collisions

The Drell-Yan (DY) process of dilepton pair production off nuclei is not affected by final state interactions, energy loss or absorption. A detailed phenomenological study of this process is thus convenient for investigation of the onset of initial-state effects in proton-nucleus ($pA$) collisions. In this paper, we present a comprehensive analysis of the DY process in $pA$ interactions at RHIC and LHC energies in the color dipole framework. We analyse several effects affecting the nuclear suppression, $R_{pA}<1$, of dilepton pairs, such as the saturation effects, restrictions imposed by energy conservation (the initial-state effective energy loss) and the gluon shadowing, as a function of the rapidity, invariant mass of dileptons and their transverse momenta $p_T$. In this analysis, we take into account besides the $γ^*$ also the $Z^0$ contribution to the production cross section, thus extending the predictions to large dilepton invariant masses. Besides the nuclear attenuation of produced dileptons at large energies and forward rapidities emerging due to the onset of shadowing effects, we predict a strong suppression at large $p_T$, dilepton invariant masses and Feynman $x_F$ caused by the Initial State Interaction effects in kinematic regions where no shadowing is expected. The manifestations of nuclear effects are investigated also in terms of the correlation function in azimuthal angle between the dilepton pair and a forward pion $Δϕ$ for different energies, dilepton rapidites and invariant dilepton masses. We predict that the characteristic double-peak structure of the correlation function around $Δϕ\simeq π$ arises for very forward pions and large-mass dilepton pairs.

hep-ph↗

arXiv2016

Quantum Yang--Mills Dark Energy

In this short review, I discuss basic qualitative characteristics of quantum non-Abelian gauge dynamics in the non-stationary background of the expanding Universe in the framework of the standard Einstein--Yang--Mills formulation. A brief outlook of existing studies of cosmological Yang--Mills fields and their properties will be given. Quantum effects have a profound impact on the gauge field-driven cosmological evolution. In particular, a dynamical formation of the spatially-homogeneous and isotropic gauge field condensate may be responsible for both early and late-time acceleration, as well as for dynamical compensation of non-perturbative quantum vacua contributions to the ground state of the Universe. The main properties of such a condensate in the effective QCD theory at the flat Friedmann--Lemaítre--Robertson--Walker (FLRW) background will be discussed within and beyond perturbation theory. Finally, a phenomenologically consistent dark energy can be induced dynamically as a remnant of the QCD vacua compensation arising from leading-order graviton-mediated corrections to the QCD ground state.

gr-qc↗

arXiv2016

Dynamic colour screening in diffractive deep inelastic scattering

We present a novel Monte-Carlo implementation of dynamic colour screening via multiple exchanges of semi-soft gluons as a basic QCD mechanism to understand diffractive electron-proton scattering at the HERA collider. Based on the kinematics of individual events in the standard QCD description of deep inelastic scattering at the parton level, which at low $x$ is dominantly gluon-initiated, the probability is evaluated for additional exchanges of softer gluons resulting in an overall colour singlet exchange leading to a forward proton and a rapidity gap as the characteristic observables for diffractive scattering. The probability depends on the impact parameter of the soft exchanges and varies with the transverse size of the hard scattering subsystem and is therefore influenced by different QCD effects. We account for matrix elements and parton shower evolution either via conventional DGLAP $\log Q^2$-evolution with collinear factorisation or CCFM small-$x$ evolution with $k_\perp$-factorisation and discuss the sensitivity to the gluon density distribution in the proton and the importance of large log\,$x$-contributions. The overall result is that, with only two model parameters which have theoretically motivated values, a satisfactory description of the observed diffractive cross-section at HERA is obtained in a wide kinematical range.

hep-ph↗

arXiv2016

Hard hadronic diffraction is not hard

Hadronic diffractive processes characterised by a hard scale (hard diffraction) contain a nontrivial interplay of hard and soft, nonperturbative interactions, which breaks down factorisation of short and long distances. On the contrary to the expectations based on the factorization hypothesis, assuming that hard diffraction is a higher twist, these processes should be classified as a leading twist. We overview various implications of this important observation for diffractive radiation of Abelian (Drell-Yan, gauge bosons, Higgs boson) and non-Abelian (heavy flavors) particles, as well as direct coalescence into the Higgs boson of the non-perturbative intrinsic heavy flavour component of the hadronic wave function.

hep-ph↗

arXiv2016

Can the diphoton enhancement at 750 GeV be due to a neutral technipion?

We discuss a scenario in which the diphoton enhancement at $M_{γγ}$ = 750 GeV, observed by the ATLAS and CMS Collaborations, is a neutral technipion $\tildeπ^0$. We consider two distinct minimal models for the dynamical electroweak symmetry breaking. In a first one, two-flavor vector-like technicolor (VTC) model, we assume that the two-photon fusion is a dominant production mechanism. We include $γγ\to {\tilde π}^0$ and production of technipion associated with one or two jets. All the considered mechanisms give similar contributions. With the strong Yukawa (technipion-techniquark) coupling $g_{TC}$ = 10 - 20 we obtain the measured cross section of the "signal". With such values of $g_{TC}$ we get a relatively small $Γ_{\rm tot}$. In a second approach, one-family walking technicolor (WTC) model, the isoscalar technipion is produced dominantly via the gluon-gluon fusion. We also discuss the size of the signal at lower energies (LHC, Tevatron) for $γγ$ (VTC) and jet-jet (WTC) final states and check consistency with the existing experimental data. We predict a measurable cross section for ${\tilde π}^0$ production associated with one or two soft jets. The technipion signal in both models is compared with the SM background diphoton contributions. We observe the dominance of inelastic-inelastic processes for $γγ$ induced processes. In the VTC scenario, we predict the signal cross section for purely exclusive $p p \to p p γγ$ processes at $\sqrt{s}$ = 13 TeV to be about 0.2 fb. Such a cross section would be, however, difficult to measure with the planned integrated luminosity. In all considered cases the signal is below the background or/and below the threshold set by statistics.

hep-ph↗

arXiv2016

Composite scalar Dark Matter from vector-like $SU(2)$ confinement

A toy-model with $SU(2)_{\rm TC}$ dynamics confined at high scales $Λ_{\rm TC}\gg 100$ GeV enables to construct Dirac UV completion from the original chiral multiplets predicting a vector-like nature of their weak interactions consistent with electroweak precision tests. In this work, we investigate a potential of the lightest scalar baryon-like (T-baryon) state $B^0=UD$ with mass $m_B\gtrsim 1$ TeV predicted by the simplest two-flavor vector-like confinement model as a Dark Matter (DM) candidate. We show that two different scenarios with the T-baryon relic abundance formation before and after the electroweak (EW) phase transition epoch lead to symmetric (or mixed) and asymmetric DM, respectively. Such a DM candidate evades existing direct DM detection constraints since its vector coupling to $Z$ boson absents at tree level, while one-loop gauge boson mediated contribution is shown to be vanishingly small close to the threshold. The dominating spin-independent (SI) T-baryon--nucleon scattering goes via tree-level Higgs boson exchange in the $t$-channel. The corresponding bound on the effective T-baryon--Higgs coupling has been extracted from the recent LUX data and turns out to be consistent with naive expectations from the light technipion case $m_{\tilde π}\ll Λ_{\rm TC}$. The latter provides the most stringent phenomenological constraint on strongly-coupled $SU(2)_{\rm TC}$ dynamics so far. Future prospects for direct and indirect scalar T-baryon DM searches in astrophysics as well as in collider measurements have been discussed.

hep-ph↗

arXiv2016

The Drell-Yan process as a testing ground for parton distributions up to LHC

The Drell-Yan massive dilepton production in hadron-hadron collisions provides a unique tool, complementary to Deep Inelastic Scattering, for improving our understanding of hadronic substructure and in particular for testing parton distributions. We will consider measurements of the differential and double-differential Drell-Yan cross sections from FNAL Tevatron up to CERN LHC energies and they will be compared to the predictions of perturbative QCD calculations using most recent sets (CT14 and MMHT14) of parton distribution functions, as well as those provided by the statistical approach.

hep-ph↗

arXiv2016

Beyond $Λ$CDM: 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, $Λ$CDM. 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 $Λ$CDM 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.

astro-ph.CO↗

arXiv2015

Diffractive Higgsstrahlung

We consider single-diffractive (SD) Higgs production in association with heavy flavour in proton-proton collisions at the LHC. The main focus of our study is a reliable estimate of SD/inclusive ratio, not a precision computation of the cross sections. The calculations are performed within the framework of the phenomenological dipole approach, which includes by default the absorptive corrections, i.e. the gap survival effects at the amplitude level. The dominant mechanism is the diffractive production of heavy quarks, which radiate a Higgs boson (Higgsstrahlung). Although diffractive production of $t$-quarks is grossly suppressed as $1/m_t^2$, the large Higgs-top coupling compensates this smallness and the Higgsstrahlung by $t$-quarks becomes the dominant contribution at large Higgs boson transverse momenta. We computed the basic observables such as the transverse momentum and rapidity distributions of the diffractively produced Higgs boson in association with the bottom and top quark pair. Finally, we discuss a potential relevance of the diffractive Higgsstrahlung in comparison to the Higgsstrahlung off intrinsic heavy flavor at forward Higgs boson rapidities.

hep-ph↗

arXiv2015

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 have also included the $Z^0$ 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 RHIC and LHC. We consider three different phenomenological models for the dipole cross section and found 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 rapidites and invariant masses. The characteristic double-peak structure of the correlation function around $Δϕ\simeq π$ 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.

hep-ph↗

arXiv2015

Quasi-classical Gravity effect on neutrino oscillations in a gravitational field of an heavy astrophysical object

In the framework of quantum field theory, a graviton interacts locally with a quantum state having definite mass, i.e. the gravitational mass eigenstate, while a weak boson interacts with a state having definite flavor, i.e. the flavor eigenstate. An interaction of a neutrino with an energetic graviton may trigger the collapse of the neutrino to a definite mass eigenstate with probability expressed in terms of PMNS mixing matrix elements. Thus, gravitons would induce quantum decoherence of a coherent neutrino flavor state similarly to how weak bosons induce quantum decoherence of a neutrino in a definite mass state. We demonstrate that such an essentially quantum gravity effect may have strong consequences for neutrino oscillation phenomena in astrophysics due to relatively large scattering cross sections of relativistic neutrinos undergoing large-angle radiation of energetic gravitons in gravitational field of a classical massive source (i.e. the quasi-classical case of gravitational Bethe-Heitler scattering). This graviton-induced {\it decoherence} is compared to {\it decoherence} due to propagation in the presence of the Earth matter effect. Based on this study, we propose a new technique for the indirect detection of energetic gravitons by measuring the flavor composition of astrophysical neutrinos.

hep-ph↗

arXiv2015

Diffractive Bremsstrahlung in Hadronic Collisions

Production of heavy photons (Drell-Yan), gauge bosons, Higgs bosons, heavy flavors, which is treated within the QCD parton model as a result of hard parton-parton collision, can be considered as a bremsstrahlung process in the target rest frame. In this review, we discuss the basic features of the diffractive channels of these processes in the framework of color dipole approach. The main observation is a dramatic breakdown of diffractive QCD factorisation due to the interplay between soft and hard interactions, which dominates these processes. This observation is crucial for phenomenological studies of diffractive reactions in high-energy hadronic collisions.

hep-ph↗