abstract

The ratio \(\varepsilon '/\varepsilon \) measures the size of the direct CP violation in \(K_{\mathrm {L}}\to \pi \pi \) decays \((\varepsilon ^\prime )\) relative to the indirect one described by \(\varepsilon \), and is very sensitive to new sources of CP violation. As such, it played a prominent role in particle physics already for 45 years. Due to the smallness of \(\varepsilon '/\varepsilon \), its measurement required heroic efforts in the 1980s and the 1990s on both sides of the Atlantic with final results presented by NA48 and KTeV collaborations at the beginning of this millennium. On the other hand, even 45 years after the first calculation of \(\varepsilon '/\varepsilon \), we do not know to which degree the Standard Model agrees with this data and how large is the room left for New Physics (NP) contributions to this ratio. This is due to significant non-perturbative (hadronic) uncertainties accompanied by partial cancellation between the QCD penguin contributions and electroweak penguin contributions. In addition to the calculation of hadronic matrix elements of the relevant operators including isospin breaking effects and QED corrections, it is crucial to accurately evaluate the Wilson coefficients of the relevant operators. While the significant control over the latter short-distance effects has been achieved already in the early 1990s, with several improvements since then, different views on the non-perturbative contributions to \(\varepsilon '/\varepsilon \) have been expressed by different authors over last thirty years. In fact, even at the dawn of the 2020s, the uncertainty in the room left for NP contributions to \(\varepsilon '/\varepsilon \) is still very significant, which I find to be very exciting. My own work on \(\varepsilon '/\varepsilon \) started in 1983 and involved both perturbative and non-perturbative calculations. This writing is a non-technical recollection of the steps which led to the present status of \(\varepsilon '/\varepsilon \) including several historical remarks not known to everybody. The present status of the \(\Delta I=1/2\) rule is also summarized.

abstract

Gamow–Teller transitions in nuclei tell us that the nucleon’s axial charge \(g_{\mathrm {A}}^{(3)}\) is quenched in large nuclei by about 20%. This result tells us that the spin structure of the nucleon is modified in nuclei and disfavours models of the medium dependence of parton structure based only on nucleon short-range correlations in nuclei. For polarized photoproduction, the Gerasimov–Drell–Hearn integral is expected to be strongly enhanced in medium.

abstract

Present study is an attempt to have a detailed look into event-by-event (e-by-e) pseudorapidity fluctuations of the relativistic charged particles produced in \(^{28}\)Si–nucleus interactions at incident momenta 4.5 \(A\) and 14.5 \(A\) GeV/\(c\). The method used in the present study makes use of a kinematic variable which is derived in terms of the average pseudorapidity and the total number of particles produced in a single event. The multiplicity and pseudorapidity dependence of these fluctuations have also been studied. The results obtained for the experimental data are compared with a HIJING simulation.

abstract

In this paper, a hidden extra symmetry of conformally invariant Lagrangians occuring in physics is pointed out. This symmetry is most apparent in a metric-independent, i.e. in a Palatini-like presentation of the variational problem. In such a presentation, the usual conformal weight of fields can be encoded as local dilatation group gauge charges. The conventional conformal invariance of Lagrangians is then equivalent to dilatation gauge invariance. The claim of the paper is that the most commonly occurring conformally invariant Lagrangians turning up in physics are not only invariant to local dilatation gauge transformations, but they are also invariant to any change of the dilatation gauge connection, meaning an additional algebraic symmetry property. In terms of dimensional analysis and differential geometry, this additional symmetry means complete insensitivity of the Lagrangian to the choice of the parallel transport rule of local measurement units throughout spacetime.