eta and eta' mesons and dimension 2 gluon condensate <A^2>

The study of light pseudoscalar quark-antiquark bound states in the Dyson-Schwinger approach with the effective QCD coupling enhanced by the interplay of the dimension 2 gluon condensate and dimension 4 gluon condensate , is extended to the eta-eta' complex. We include the effects of the gluon axial anomaly into the Dyson-Schwinger approach to mesons. The calculated masses, mixing and two-photon decay widths of eta and eta' mesons are in agreement with experiment. Also, in a model-independent way, we give the modification of the Gell-Mann--Okubo and Schwinger nonet relations due to the interplay of the gluon anomaly and SU(3) flavor symmetry breaking.Comment: 15 pages, 2 tables, 1 eps figure, revtex

Bypassing the axial anomalies

Many meson processes are related to the U_A(1) axial anomaly, present in the Feynman graphs where fermion loops connect axial vertices with vector vertices. However, the coupling of pseudoscalar mesons to quarks does not have to be formulated via axial vertices. The pseudoscalar coupling is also possible, and this approach is especially natural on the level of the quark substructure of hadrons. In this paper we point out the advantages of calculating these processes using (instead of the anomalous graphs) the Feynman graphs where axial vertices are replaced by pseudoscalar vertices. We elaborate especially the case of the processes related to the Abelian axial anomaly of QED, but we speculate that it seems possible that effects of the non-Abelian axial anomaly of QCD can be accounted for in an analogous way.Comment: 12 pages, 4 figures, uses ws-ijmpa.cls, published in the proceedings of MRST 2005 conference, Utica, New York, 16-18 May 200

Barionski model predvidio neočekivano malu iskustvenu vektorsku stranost nukleona

Most of model considerations of the hidden nucleon strangeness, as well as some preliminary experimental evidence, led to the expectations of relatively sizeable strange vector form factors of the proton. For example, it seemed that the contribution of the fluctuating strange quark-antiquark pairs accounts for as much as one tenth of the proton\u27s magnetic moment. By the same token, baryon models which failed to produce the "vector strangeness" of the nucleon seemed disfavoured. Recently, however, more accurate measurements and more sophisticated data analysis, as well as lattice simulations, revealed that the form factors associated with the vector strangeness of the nucleon are much smaller than thought previously; in fact, due to the experimental uncertainties, the measured strange vector-current proton form factors may be consistent with zero. In the light of that, we re-asses the merit of the baryon models leading to little or no vector strangeness of the nucleon. It is done on the concrete example of the baryon model which essentially amounts to the MIT bag enriched by the diluted instanton liquid.Većina teorijskih razmatranja skrivene stranosti nukleona, kao i neki prethodni eksperimentalni podaci, potaknuli su očekivanja relativno velikih stranih vektorskih faktora oblika protona. Npr., činilo se da doprinos kolebanja parova strani kvark– antikvark iznosi čak desetinu magnetskog momenta protona. Zbog tih su stajališta bili zanemareni barionski modeli koji nisu davali “vektorsku stranost” nukleona. Međutim, nedavnija točnija mjerenja i bolje analize mjernih podataka, kao i simulacije na rešetkama, našli su da su faktori oblika povezani s vektorskom stranosti nukleona mnogo manji nego se prije mislilo. U stvari, uzevši u obzir točnost eksperimentalnih podataka, mjereni strani vektorski faktori oblika mogu biti jednaki nuli. U tom svjetlu obnavljamo razmatranje barionskih modela koji daju malu ili nultu vektorsku stranost nukleona. To smo načinili s posebnim primjerom barionskog modela koji je u biti MIT vreća obogaćena razrijeđenom tekućinom instantona

Izbjegavanje aksijalnih asimetrija i problem jakog CP

Many meson processes are related to the UA(1) axial anomaly, present in the Feynman graphs where fermion loops connect axial vertices with vector vertices. However, the coupling of pseudoscalar mesons to quarks does not have to be formulated via axial vertices. The pseudoscalar coupling is also possible, and this approach is especially natural on the level of the quark substructure of hadrons. In this paper we point out the advantages of calculating these processes using (instead of the anomalous graphs) the graphs where axial vertices are replaced by pseudoscalar vertices. We elaborate especially the case of the processes related to the Abelian axial anomaly of QED, but we speculate that it seems possible that effects of the non-Abelian axial anomaly of QCD can be accounted for in an analogous way.Mnogi su mezonski procesi u svezi s aksijalnom anomalijom UA(1), koja se javlja u Feynmanovim grafovima gdje fermioske petlje povezuju aksijalne s vektorskim vrhovima. Međutim, vezanje pseudoskalarnih mezona na kvarkove ne mora se formulirati aksijalnim vrhovima. Moguće je i pseudoskalarno vezanje, i taj je pristup posebno prirodan na razini kvarkovske podstrukture hadrona. U ovom se radu ukazuje na prednosti u računanju tih procesa primjenom (umjesto anomalnih) onih grafova u kojima su aksijalni vrhovi zamijenjeni pseudoskalarnim vrhovima. Posebno razrađujemo slučaj procesa u svezi s Abelovom aksijalnom anomalijom QED, te smatramo da se čini mogućim objasniti učinke ne-Abelove aksijalne anomalije na sličan način

η\eta and η′\eta' mesons in the Dyson-Schwinger approach at finite temperature

We study the temperature dependence of the pseudoscalar meson properties in a relativistic bound-state approach exhibiting the chiral behavior mandated by QCD. Concretely, we adopt the Dyson-Schwinger approach with a rank-2 separable model interaction. After extending the model to the strange sector and fixing its parameters at zero temperature, T=0, we study the T-dependence of the masses and decay constants of all ground-state mesons in the pseudoscalar nonet. Of chief interest are $\eta$ and $\eta^\prime$. The influence of the QCD axial anomaly on them is successfully obtained through the Witten-Veneziano relation at T=0. The same approach is then extended to T>0, using lattice QCD results for the topological susceptibility. The most conspicuous finding is an increase of the $\eta^\prime$ mass around the chiral restoration temperature $T_{\rm Ch}$, which would suggest a suppression of $\eta^\prime$ production in relativistic heavy-ion collisions. The increase of the $\eta^\prime$ mass may also indicate that the extension of the Witten-Veneziano relation to finite temperatures becomes unreliable around and above $T_{\rm Ch}$. Possibilities of an improved treatment are discussed.Comment: 13 pages, 15 figure