arXiv subjects

Ivan Schmidt

Publications and source records attributed to Ivan Schmidt.

At least 199 records · Page 11Linked to original sources

arXiv2007

Cronin effect in the Drell-Yan reaction

We explore the mechanism of transverse momentum broadening of fast quarks propagating in nuclei, using Drell-Yan (DY) transverse momentum distributions measured in the experiment E866 at FermiLab with beams of 800 GeV protons. Our theoretical analysis is based on the color dipole approach in the target rest frame, which has provided a successful phenomenological description of a variety of hadronic reactions. The present application is relevant to the regime of short coherence length (SCL), where the spatial extent of the fluctuations of the projectile responsible for the Drell-Yan reaction is short compared to the internucleon spacing. In this limit, momentum broadening comes from initial state interactions and is described as color filtering, i.e. absorption of large-size dipoles leading to diminished transverse separation and hence enhanced transverse momentum. The predictions we present are in good agreement with the E866 data. The interactions leading to the acquisition of transverse momentum arise from the color-dipole cross section determined previously from deep-inelastic scattering on proton targets. Aside from the determination of the color-dipole cross section, no other phenomenological input is needed to explain the experimental results. The mean-square momentum broadening of dileptons determined in a recent separate analysis of the data is likewise well described by our theory. These results confirm that the origin of momentum broadening in DY is the color dipole cross section mediating soft initial state interactions between the parton of the projectile that initiates the reaction and the nucleons of the nucleus, as provided by the color dipole description. Predictions for broadening observables at RHIC are presented.

hep-ph↗

arXiv2007

Connection between the Sivers function and the anomalous magnetic moment

The same light-front wave functions of the proton are involved in both the anomalous magnetic moment of the nucleon and the Sivers function. Using the diquark model, we derive a simple relation between the anomalous magnetic moment and the Sivers function, which should hold in general with good approximation. This relation can be used to provide constraints on the Sivers single spin asymmetries from the data on anomalous magnetic moments. Moreover, the relation can be viewed as a direct connection between the quark orbital angular momentum and the Sivers function.

hep-ph↗

arXiv2007

Production of Polarized Vector Mesons off Nuclei

Using the light-cone QCD dipole formalism we investigate manifestations of color transparency (CT) and coherence length (CL) effects in electroproduction of longitudinally (L) and transversally (T) polarized vector mesons. Motivated by forthcoming data from the HERMES experiment we predict both the A and Q^2 dependence of the L/T- ratios, for rho^0 mesons produced coherently and incoherently off nuclei. For an incoherent reaction the CT and CL effects add up and result in a monotonic A dependence of the L/T-ratio at different values of Q^2. On the contrary, for a coherent process the contraction of the CL with Q^2 causes an effect opposite to that of CT and we expect quite a nontrivial A dependence, especially at Q^2 >> m_V^2.

hep-ph↗

arXiv2007

Color Transparency at Low Energies: Predictions for JLAB

The study of color transparency (CT) in elastic electroproduction of vector mesons off nuclei encounters the problem of the onset of coherence length (CL) effects. The problem of CT-CL separation arises especially at medium energies, corresponding to HERMES experiment, when the coherence length is of the order of the nuclear radius R_A. Only at asymptotic large energies, corresponding to large CL, l_c >> R_A, the CT-CL mixing can be eliminated. On the other hand, the net CT effects can be studied in the kinematic range accessible by the CLAS experiment, since in this case the CL is much smaller than the nuclear radius. Using light-cone quantum chromodynamics (QCD) dipole formalism we investigate manifestations of CT effects in electroproduction of vector mesons. Motivated by expected data from the CLAS experiment at JLab, we predict the A and Q^2 dependence of nuclear transparency for rho^0 mesons produced incoherently off nuclei. We demonstrate that in the CLAS kinematical region the CL effects are weak enough to keep the photon energy at such values as to obtain maximal photon virtualities keeping optimal statistics of the data. This has a clear advantage in comparison with a standard investigation of net CT effects fixing CL.

hep-ph↗

arXiv2007

Flavor separation of the Boer-Mulders function from unpolarized $π^- p$ and $π^- D$ Drell-Yan processes

We show that measuring the $\cos 2ϕ$ angular dependence in unpolarized Drell-Yan processes with $π^-$ beams colliding on proton and deuteron targets can determine the ratio of the Boer-Mulders functions for $d$ and $u$ quarks inside the proton $h_1^{\perp,d}/h_1^{\perp,u}$, which is still lack of theoretical constraint. The comparison of the $\cos 2 ϕ$ asymmetries measured in unpolarized $π^- p$ and $π^- D$ Drell-Yan processes, which are accessible at CERN by the COMPASS collaboration, can help to discriminate whether $h_1^\perp$ effects or QCD vacuum effects are preferred by data.

hep-ph↗

arXiv2007

Transverse spin effects of sea quarks in unpolarized nucleons

We calculate the non-zero Boer-Mulders functions of sea quarks inside the proton in a meson-baryon fluctuation model. The results show that the transverse spin effects of sea quarks in an unpolarized nucleon are sizable. Using the obtained antiquark Boer-Mulders functions, we estimate the $\cos 2 ϕ$ asymmetries in the unpolarized $pp$ and $p D$ Drell-Yan processes at FNAL E866/NuSea experiments. The prediction for the $\cos 2 ϕ$ asymmetries in the unpolarized $pp$ Drell-Yan process at the BNL Relativistic Heavy Ion Collider (RHIC) is also given.

hep-ph↗

arXiv2007

Nuclear Broadening of Transverse Momentum in Drell-Yan Reactions

Data for Drell-Yan (DY) processes on nuclei are currently available from fixed target experiments up to the highest energy of $\sqrt{s}=40\GeV$. The bulk of the data cover the range of short coherence length, where the amplitudes of the DY reaction on different nucleons do not interfere. In this regime, DY processes provide direct information about broadening of the transverse momentum of the projectile parton experiencing initial-state multiple interactions. We revise a previous analysis of data from the E772 experiment and perform a new analysis of broadening including data from the E866 experiment at Fermilab. We conclude that the observed broadening is about twice as large as the one found previously. This helps to settle controversies that arose from a comparison of the original determination of broadening with data from other experiments and reactions.

hep-ph↗

arXiv2006

Towards the two-loop Lcc vertex in Landau gauge

We are interested in the structure of the Lcc vertex in the Yang-Mills theory, where c is the ghost field and L the corresponding BRST auxiliary field. This vertex can give us information on other vertices, and the possible conformal structure of the theory should be reflected in the structure of this vertex. There are five two-loop contributions to the Lcc vertex in the Yang-Mills theory. We present here calculation of the first of the five contributions. The calculation has been performed in the position space. One main feature of the result is that it does not depend on any scale, ultraviolet or infrared. The result is expressed in terms of logarithms and Davydychev integral J(1,1,1) that are functions of the ratios of the intervals between points of effective fields in the position space. To perform the calculation we apply Gegenbauer polynomial technique and uniqueness method.

hep-th↗

arXiv2006

Dipole description of inclusive particle production

The effects of multiple interactions in colliding particles (e.g. in nucleus-nucleus collisions) are modeled using the light-cone dipole approach. Guided by the abelian analogue of multi-photon interactions in the production of a pair of charged particles, we relate the inclusive cross section of quark pair production with the cross sections of interaction of a QCD dipole with either the beam or the target.

hep-ph↗

arXiv2006

Time Evolution of Jets and Perturbative Color Neutralization

In-medium production of leading hadrons in hard reactions, carrying the main fraction of the jet momentum, involves two stages: (i) the parton originated from the hard process propagates through the medium radiating gluons due to the initial hard collision, as well as to multiple interactions in the medium; (ii) perturbative color neutralization, e.g. picking up an anti-colored parton produced perturbatively, followed by evolution and attenuation of the (pre)hadron in the medium. The color neutralization (or production) length for leading hadrons is controlled by coherence, energy conservation and Sudakov suppression. The pT-broadening is a sensitive and model independent probe for the production length. The color neutralization time is expected to shrink with rising hard scale. In particular, we found a very fast energy dissipation by a highly virtual parton: half of the jet energy is radiated during the first Fermi. Energy conservation makes the production of leading hadrons at longer times difficult.

hep-ph↗

arXiv2006

Glue drops inside hadrons

We present experimental evidences for the existence of a semi-hard scale in light hadrons. This includes the suppression of gluon radiation that is seen in high mass hadron diffraction; the weak energy dependence of hadronic total cross sections; the small value of the Pomeron trajectory slope measured in photoproduction of J/Psi; the weakness of gluon shadowing in nuclei; shortage of gluons in the proton revealed by an unusual behavior of the proton structure function in the soft limit, and the enhanced intrinsic transverse momentum of quarks and gluons, which considerably exceeds the inverse hadronic size. All these observations suggest that gluons in hadrons are located within spots of a small size relative to the confinement radius.

hep-ph↗

arXiv2006

Pion pair production in $e^+e^-$ annihilation

We present an analysis of the process $e^+e^-\to V^*\toππγ$, where $V=γ$ or $V=Z$ boson, in the kinematical region where $\sqrt{s}$, the c.m. energy of the $e^+e^-$ pair, is large but much below the $Z$-pole. The subprocess $V^*\to ππγ$ can be described by the convolution of the hard scattering coefficient $V\to q \bar{q} γ$ and the general distribution amplitude of two pions $q\bar{q}\to ππ$. In the case of neutral pion production, $V=γ$ is the dominant process, which can therefore be used to access the GPAs of the pion, especially their $C$-even parts. The $γZ$ interference term provides an alternative approach to extract the weak mixing angle $\sin θ_W$ through measuring the helicity asymmetry in the process $e^+e^-\to π^0π^0γ$. In the case of charged pion pair production, the Bremsstrahlung process dominates and its interference with $e^+e^-\to γ^ \star \to π^+π^-γ$ can be applied to study the process $γ^ \star \to ππγ$ at the amplitude level.

hep-ph↗

arXiv2006

Diffraction in QCD

This lecture presents a short review of the main features of diffractive processes and QCD inspired models. It includes the following topics: (1) Quantum mechanics of diffraction: general properties; (2) Color dipole description of diffraction; (3) Color transparency; (4) Soft diffraction in hard reactions: DIS, Drell-Yan, Higgs production; (5) Why Pomerons interact weakly; (6) Small gluonic spots in the proton; (7) Diffraction near the unitarity bound: the Goulianos-Schlein "puzzle"; (8) Diffraction on nuclei: diffractive Color Glass; (9) CGC and gluon shadowing.

hep-ph↗

arXiv2006

Relation between Lepton Flavor Violating Processes

We demonstrate the existence of model independent one-to-one correspondence relations between different lepton flavor violating processes (LFV). Applying the criterion of naturalness, based on the idea of "custodial symmetry", we show that all the LFV processes, independently of specific mechanisms behind them, but with the same external leptons, have a priori comparable amplitudes modulo their kinematics and involved form factors.

hep-ph↗

arXiv2006

Diffractive Higgs Production from Intrinsic Heavy Flavors in the Proton

We propose a novel mechanism for exclusive diffractive Higgs production $pp \to p H p $ in which the Higgs boson carries a significant fraction of the projectile proton momentum. This mechanism will provide a clear experimental signal for Higgs production due to the small background in this kinematic region. The key assumption underlying our analysis is the presence of intrinsic heavy flavor components of the proton bound state, whose existence at high light-cone momentum fraction $x$ has growing experimental and theoretical support. We also discuss the implications of this picture for exclusive diffractive quarkonium and other channels.

hep-ph↗

arXiv2006

Generalized Distribution Amplitudes at the $Z$ pole

We investigate the exclusive two-pion production at the $Z$-pole through the process $e^+e^-\to Z \to ππγ$. In the kinematical region where the invariant mass of the two pions is much smaller than the mass of the $Z$ boson, the process can be factorized into the convolution of a hard coefficient and a soft matrix element, the generalized distribution amplitude of two pions. We calculate the differential cross-section of charged pion pair production and neutral pion pair production and find the former to be much larger than the later. We show that combining the measurements of charged pion pair production and neutral pion pair production at the $Z$-pole provides a convenient approach to access the $C$-odd part of the $2π$ distribution amplitudes.

hep-ph↗

arXiv2006

On the Effective Action of Dressed Mean Fields for N = 4 Super-Yang-Mills Theory

On the basis of the general considerations such as $R$-operation and Slavnov-Taylor identity we show that the effective action, being understood as Legendre transform of the logarithm of the path integral, possesses particular structure in ${\cal N} = 4$ supersymmetric Yang-Mills theory for kernels of the effective action expressed in terms of the dressed effective fields. These dressed effective fields have been introduced in our previous papers as actual variables of the effective action. The concept of dressed effective fields naturally appears in the framework of solution to Slavnov-Taylor identity. The particularity of the structure is independence of these kernels on the ultraviolet regularization scale $Λ$. These kernels are functions of mutual spacetime distances and of the gauge coupling. The fact that $β$ function in this theory vanishes is used significantly.

math-ph↗

arXiv2005

Physics of Nuclear Antishadowing

Shadowing and antishadowing of the electromagnetic nuclear structure functions are produced by the coherence of multiscattering quark nuclear processes. This picture leads to substantially different antishadowing for charged and neutral current processes, particularly in anti-neutrino reactions, thus affecting the extraction of the weak-mixing angle $\sin^2θ_W$.

hep-ph↗