arXiv subjects

Ivan Schmidt

Publications and source records attributed to Ivan Schmidt.

At least 235 records · Page 13Linked to original sources

arXiv2002

An approach to solve Slavnov-Taylor identities in nonsupersymmetric non-Abelian gauge theories

We present a way to solve Slavnov--Taylor identities in a general nonsupersymmetric theory. The solution can be parametrized by a limited number of functions of spacetime coordinates, so that all the effective fields are dressed by these functions via integral convolution. The solution restricts the ghost part of the effective action and gives predictions for the physical part of the effective action.

hep-ph↗

arXiv2002

Proton Stability in Leptoquark Models

We show that in generic leptoquark (LQ) extensions of the standard model lepton and baryon numbers are broken at the level of renormalizable operators. In particular, this may cause fast proton decay unless the leptoquarks are heavy enough. We derive stringent bounds for the 1st generation LQ masses and couplings from the proton stability constraints.

hep-ph↗

arXiv2002

Single Hadronic-Spin Asymmetries in Weak Interaction Processes

We show that measurements of single-spin asymmetries (SSAs) in charged current weak interaction processes such as deep inelastic neutrino scattering on a polarized target and inclusive $W$ production in polarized hadron-hadron collisions discriminate between the two fundamental QCD mechanisms (the Sivers and Collins effects) which have been proposed to explain such time-reversal-odd asymmetries. It has recently been shown that QCD final-state interactions due to gluon exchange between the struck quark and the proton spectators in semi-inclusive deep inelastic lepton scattering will produce non-zero Sivers-type single-spin asymmetries which survive in the Bjorken limit. We show that this QCD final-state interaction produces identical SSAs in charged and neutral current reactions. Furthermore, the contribution of each quark to the SSA from this mechanism is proportional to the contribution of that quark to the polarized baryon's anomalous magnetic moment. In contrast, the Collins effect contribution to SSAs depends on the transversity distribution of quarks in the polarized target. Since the charged current only couples to quarks of one chirality, it cannot sense the transversity distribution of the target, and thus it gives no Collins-type contribution to single-spin correlations.

hep-ph↗

arXiv2002

Understanding the proton spin "puzzle" with a new "minimal" quark model

We investigate the spin structure of the nucleon in an extended Jaffe-Lipkin quark model. In addition to the conventional $3q$ structure, different $(3q)(Q\bar{Q})$ admixtures in the nucleon wavefunction are also taken into account. The contributions to the nucleon spin from various components of the nucleon wavefunction are discussed. The effect due to the Melosh-Wigner rotation is also studied. It is shown that the Jaffe-Lipkin term is only important when antiquarks are negatively polarized. We arrive at a new "minimal" quark model, which is close to the naive quark model, in order to understand the proton spin "puzzle".

hep-ph↗

arXiv2002

Reanalysis of Azimuthal Spin Asymmetries of Meson Electroproduction

The azimuthal spin asymmetries for pion production in semi-inclusive deep inelastic scattering of unpolarized charged lepton beams on longitudinally polarized nucleon targets, are reanalyzed by taking into account an important sign correction to previous formulas. It is found that different approaches of distribution functions and fragmentation functions may lead to distinct predictions on the azimuthal asymmetries measured in the HERMES experiments, thus the available data cannot be considered as a direct measurement of quark transversity distributions, although they still can serve to provide useful information on these distributions and on T-odd fragmentation functions. Predictions of the azimuthal spin asymmetries for kaon production are also presented, with different approaches of distribution and fragmentation functions. The unfavored fragmentation functions cannot be neglected for $K^-$ and $K^0_S$ production in semi-inclusive deep inelastic processes.

hep-ph↗

arXiv2002

Axial vector form factor of nucleons in a light-cone diquark model

The nucleon axial vector form factor is investigated in a light-cone quark spectator diquark model, in which Melosh rotations are applied to both the quark and vector diquark. It is found that this model gives a very good description of available experimental data and the results have very little dependence on the parameters of the model. The relation between the nucleon axial constant and the anomalous magnetic moment of nucleons is also discussed.

hep-ph↗

arXiv2002

Nuclear Effects on the Extraction of $\sin^2θ_W$

We study the impact of nuclear effects on the extraction of the weak-mixing angle $\sin^2θ_W$ from deep inelastic (anti-)neutrino-nucleus scattering, with special emphasis on the recently announced NuTeV Collaboration 3$σ$ deviation of $\sin^2θ_W$ from its standard model value. We have found that nuclear effects, which are very important in electromagnetic deep inelastic scattering (DIS), are quite small in weak charged current DIS. In neutral current DIS processes, which contain the weak mixing angle, we predict that these effects play also an important role and may dramatically affect the value of $\sin^2θ_W$ extracted from the experimental data.

hep-ph↗

arXiv2002

Phenomenological Relation between Distribution and Fragmentation Functions

We study the relation between the quark distribution function q(x) and the fragmentation function D_q(z) based on a general form D_q(x) = C(z)z^{alpha}q(z) for valence and sea quarks. By adopting two known parametrizations of quark distributions for the proton, we find three simple options for the fragmentation functions that can provide a good description of the available experimental data on proton production in e^+ e^- inelastic annihilation. These three options support the revised Gribov-Lipatov relation D_q(z) = z q(z) at z --> 1, as an approximate relation for the connection between distribution and fragmentation functions. The three options differ in the sea contributions and lead to distinct predictions for antiproton production in the reaction p+p --> p-bar+X, thus they are distinguishable in future experiments at RHIC-BNL.

hep-ph↗

arXiv2002

Initial-State Interactions and Single-Spin Asymmetries in Drell-Yan Processes

We show that the initial-state interactions from gluon exchange between the incoming quark and the target spectator system lead to leading-twist single-spin asymmetries in the Drell-Yan process. The QCD initial-state interactions produce a $T-$odd spin-correlation between the target spin and the virtual photon production plane which is not power-law suppressed in the Drell-Yan scaling limit. The origin of the single-spin asymmetry in $πp^\uparrow \to \ell^+ \ell^- X$ is a phase difference between two amplitudes coupling the proton target with $J^z_p = \pm {1\over 2}$ to the same final-state, the same amplitudes which are necessary to produce a nonzero proton anomalous magnetic moment. The calculation requires the overlap of target light-front wavefunctions differing by one unit of orbital angular momentum projection $L_z;$ thus the SSA in the Drell-Yan reaction provides a direct measure of orbital angular momentum in the QCD bound state. The single-spin asymmetry predicted for the Drell-Yan process $πp^\uparrow \to \ell^+ \ell^- X$ is similar to the single-spin asymmetries in deep inelastic semi-inclusive leptoproduction $\ell p^\uparrow \to \ell' πX$ which arises from the final-state rescattering of the outgoing quark.

hep-ph↗

arXiv2002

Classical sum rules and spin correlations in photoabsorption and photoproduction processes

In this paper we study the possibility of generalizing the classical photoabsorption ($γa \to b c$) sum rules, to processes $b c \to γa$ and crossed helicity amplitudes. In the first case, using detailed balance, the sum rule is written as $\int_{ν_{th}}^\infty {\frac{dν}ν} KΔσ_{Born} (ν)=0$ where $K$ is a kinematical constant which depends only on the mass of the particles and the center of mass energy. For other crossed helicity amplitudes, we show that there is a range of values of $s$ and $t$ for which the differential cross section for the process $γb \to a c$ or $a c \to γb$ in which the helicities of the photon and particle $a$ have specific values, is equal to the differential cross section for the process in which one of these two helicities is reversed (parallel-antiparallel spin correlation).

hep-ph↗

arXiv2002

Final-State Interactions and Single-Spin Asymmetries in Semi-Inclusive Deep Inelastic Scattering

Recent measurements from the HERMES and SMC collaborations show a remarkably large azimuthal single-spin asymmetries A_{UL} and A_{UT} of the proton in semi-inclusive pion leptoproduction. We show that final-state interactions from gluon exchange between the outgoing quark and the target spectator system lead to single-spin asymmetries in deep inelastic lepton-proton scattering at leading twist in perturbative QCD; i.e., the rescattering corrections are not power-law suppressed at large photon virtuality Q^2 at fixed x_{bj}. The existence of such single-spin asymmetries requires a phase difference between two amplitudes coupling the proton target with J^z_p = + 1/2 and -1/2 to the same final state, the same amplitudes which are necessary to produce a nonzero proton anomalous magnetic moment. We show that the exchange of gauge particles between the outgoing quark and the proton spectators produces a Coulomb-like complex phase which depends on the angular momentum L_z of the proton's constituents and is thus distinct for different proton spin amplitudes. The single-spin asymmetry which arises from such final-state interactions does not factorize into a product of distribution function and fragmentation function, and it is not related to the transversity distribution delta q(x,Q) which correlates transversely polarized quarks with the spin of the transversely polarized target nucleon.

hep-ph↗

arXiv2002

Electromagnetic Form Factors of Nucleons in a Light-cone Diquark Model

We investigate the electromagnetic form factors of nucleons within a simple relativistic quark spectator-diquark model using the light-cone formalism. Melosh rotations are applied to both quark and vector diquark. It is shown that the difference between vector and scalar spectator diquarks reproduces the right electric form factor of neutrons, and both the form factors $G_E(Q^2)$ and $G_M(Q^2)$ of the proton and neutron agree with experimental data well up to $Q^2=2 ~\rm{GeV}^2$ in this simple model.

hep-ph↗

arXiv2002

Electromagnetic Pulse from Final Gravitational Stellar Collapse

We employ an effective gravitational stellar final collapse model which contains the relevant physics involved in this complex phenomena: spherical radical infall in the Schwarzschild metric of the homogeneous core of an advanced star, giant magnetic dipole moment, magnetohydrodynamic material response and realistic equations of state (EOS). The electromagnetic pulse is computed both for medium size cores undergoing hydrodynamic bounce and large size cores undergoing black hole formation. We clearly show that there must exist two classes of neutron stars, separated by maximum allowable masses: those that collapsed as solitary stars (dynamical mass limit) and those that collapsed in binary systems allowing mass accretion (static neutron star mass). Our results show that the electromagnetic pulse spectrum associated with black hole formation is a universal signature, independent of the nuclear EOS. Our results also predict that there must exist black holes whose masses are less than the static neutron star stability limit.

astro-ph↗

arXiv2001

The (muon^-,muon^+) conversion in nuclei as a probe of new physics

A detailed study of the muonic analogue of neutrinoless double beta decay, (muon^-,muon^+) conversion, has been carried out for the A=44 nuclear system. We studied several lepton number violating (LNV) mechanisms potentially triggering this process: exchange by light and heavy Majorana neutrinos as well as exchange by supersymmetric particles participating in R-parity violating interactions. The nuclear structure has been taken into account within the renormalized Quasiparticle Random Phase Approximation method. To our knowledge, this is the first realistic treatment of nuclear structure aspects of the (muon^-,muon^+) conversion. We estimated the rate of this process utilizing the existing experimental constraints on the parameters of the underlying LNV interactions and conclude that the (muon^-,muon^+) conversion is hardly detectable in the near future experiments.

hep-ph↗

arXiv2001

Resummation of the hadronic tau decay width with the modified Borel transform method

A modified Borel transform of the Adler function is used to resum the hadronic tau decay width ratio. In contrast to the ordinary Borel transform, the integrand of the Borel integral is renormalization--scale invariant. We use an ansatz which explicitly accounts for the structure of the leading infrared renormalon. Further, we use judiciously chosen conformal transformations for the Borel variable, in order to map sufficiently away from the origin the other ultraviolet and infrared renormalon singularities. In addition, we apply Pade approximants for the corresponding truncated perturbation series of the modified Borel transform, in order to further accelerate the convergence. Comparing the results with the presently available experimental data on the tau hadronic decay width ratio, we obtain $α_s(M^z) = 0.1192 +- 0.0007_{exp.} +- 0.0010_{EW+CKM} +- 0.0009_{th.} +- 0.0003_{evol.}$. These predictions virtually agree with those of our previous resummations where we used ordinary Borel transforms instead.

hep-ph↗

arXiv2001

Azimuthal Spin Asymmetries of Pion Electroproduction

Azimuthal spin asymmetries, both for charged and neutral pion production in semi-inclusive deep inelastic scattering of unpolarized charged lepton beams on longitudinally and transversely polarized nucleon targets, are analyzed and calculated. Various assumptions and approximations in the quark distributions and fragmentation functions often used in these calculations are studied in detail. It is found that different approaches to the distribution and fragmentation functions may lead to quite different predictions on the azimuthal asymmetries measured in the HERMES experiments, thus their effects should be taken into account before using the available data as a measurement of quark transversity distributions. It is also found that the unfavored quark to pion fragmentation functions must be taken into account for $π^-$ production from a proton target, although they can be neglected for $π^+$ and $π^0$ production. Pion production from a proton target is suitable to study the $u$ quark transversity distribution, whereas a combination of pion production from both proton and neutron targets can measure the flavor structure of quark transversity distributions.

hep-ph↗

arXiv2001

Photo-production of Nucleon Resonances and Nucleon Spin Structure Function in the Resonance Region

The photo-production of nucleon resonances is calculated based on a chiral constituent quark model including both relativistic corrections H{rel} and two-body exchange currents, and it is shown that these effects play an important role. We also calculate the first moment of the nucleon spin structure function g1 (x,Q^2) in the resonance region, and obtain a sign-changing point around Q^2 ~ 0.27 {GeV}^2 for the proton.

hep-ph↗

arXiv2001

Helicity and Transversity Distribution of Nucleon and $Λ$-Hyperon from $Λ$ Fragmentation

It is shown that $Λ$-hyperon fragmentation in charged lepton deep inelastic scattering (DIS) on a polarized nucleon target can provide sensitive information concerning the quark helicity and transversity distributions for both nucleon and $Λ$-hyperon at large $x$. Numerical predictions are given for the spin transfers of the produced $Λ$, when the target nucleon is polarized either longitudinally or transversely, and with the nucleon and $Λ$ quark distributions evaluated both in an SU(6) quark-spectator-diquark model and in a perturbative QCD (pQCD) based model. It is also shown that the predicted spin transfers have different behaviors for proton and neutron targets, and this can provide sensitive tests of different predictions for the quark helicity and transversity distributions of the $d$ valence quark of the proton at large $x$.

hep-ph↗