arXiv subjects

Ch. Berger

Publications and source records attributed to Ch. Berger.

7 recordsLinked to original sources

arXiv2014

Photon structure function revisited

The flux of papers from electron positron colliders containing data on the photon structure function ended naturally around 2005. It is thus timely to review the theoretical basis and confront the predictions with a summary of the experimental results. The discussion will focus on the increase of the structure function with x (for x away from the boundaries) and its rise with log Q**2, both characteristics beeing dramatically different from hadronic structure functions. The agreement of the experimental observations with the theoretical calculations of the real and virtual photon structure is a striking success of QCD. It also allows a new determination of the QCD coupling constant which very well agrees with the value quoted in the literature.

hep-ph↗

arXiv2012

Flow of entropy in the evolution of the B^0 anti B^0 system: Upper bound on CP violation from unidirectionality

We have previously studied the time-dependence of a B anti B meson mixture in terms of its density matrix. The requirement that the absolute value of the Stokes vector zeta(t) should evolve monotonically from its initial value zeta(0)=0 to its final value zeta(infinity)=1 was shown to lead to an upper bound on the CP violating overlap . In the present note we consider an entropy variable S as an alternative measure of mixing. We show that exactly the same upper bound emerges from the requirement that the flow of entropy is unidirectional (dS/dt<0). We compare the entropic current dS/dt with and without CP violation and identify certain physical features that appear when the bound on is violated.

hep-ph↗

arXiv2011

An upper limit on CP violation in the $B^0_s-\bar{B}^0_s$ system

In a previous publication we noted that the time dependence of an incoherent $B^0-\bar{B}^0$ mixture undergoes a qualitative change when the magnitude of CP violation $δ$ exceeds a critical value. Requiring, on physical grounds, that the system evolve from an initial incoherent state to a final pure state in a monotonic way, yields a new upper limit for $δ$. The recent measurement of the wrong charge semileptonic asymmetry of $B_s^0$ mesons presented by the D0 collaboration is outside this bound by one standard deviation. If this result is confirmed it implies the existence of a new quantum mechanical oscillation phenomenon.

hep-ph↗

arXiv2009

PCAC and coherent pion production by low energy neutrinos

Coherent pi^+ and pi^0 production in low energy neutrino reactions is discussed in the framework of the partially conserved axial vector current theory (PCAC). The role of lepton mass effects in suppressing the pi^+ production is emphasized. Instead of using models of pion nucleus scattering, the available data on pion Carbon scattering are implemented for an analysis of the PCAC prediction. Our results agree well with the published upper limits for pi^+ production but are much below the recent MiniBooNE result for pi^0 production.

hep-ph↗

arXiv2008

Lepton Mass Effects in Single Pion Production by Neutrinos

We reconsider the Feynman-Kislinger-Ravndal model applied to neutrino-excitation of baryon resonances. The effects of lepton mass are included, using the formalism of Kuzmin, Lyubushkin and Naumov. In addition we take account of the pion-pole contribution to the hadronic axial vector current. Application of this new formalism to the reaction nu(mu) + p --> mu + Delta at E(nu) approx 1 GeV gives a suppressed cross section at small angles, in agreement with the screening correction in Adler's forward scattering theorem. Application to the process nu(tau) + p --> tau + Delta at E(nu) approx 7 GeV leads to the prediction of right-handed tau polarization for forward-going leptons, in line with a calculation based on an isobar model. Our formalism represents an improved version of the Rein-Sehgal model, incorporating lepton mass effects in a manner consistent with PCAC.

hep-ph↗

arXiv2007

CP Violation and Arrows of Time Evolution of a Neutral $K$ or $B$ Meson from an Incoherent to a Coherent State

We study the evolution of a neutral $K$ meson prepared as an incoherent equal mixture of $K^0$ and $\bar{K^0}$. Denoting the density matrix by $ρ(t) = {1/2} N(t) [\1 + \vecζ(t) \cdot \vecσ ] $, the norm of the state $N(t)$ is found to decrease monotonically from one to zero, while the magnitude of the Stokes vector $|\vecζ(t)|$ increases monotonically from zero to one. This property qualifies these observables as arrows of time. Requiring monotonic behaviour of $N(t)$ for arbitrary values of $γ_L, γ_S$ and $Δm$ yields a bound on the CP-violating overlap $δ= \braket{K_L}{K_S}$, which is similar to, but weaker than, the known unitarity bound. A similar requirement on $|\vecζ(t)|$ yields a new bound, $δ^2 < {1/2} (\frac{Δγ}{Δm}) \sinh (\frac{3π}{4} \frac{Δγ}{Δm})$ which is particularly effective in limiting the CP-violating overlap in the $B^0$-$\bar{B^0}$ system. We obtain the Stokes parameter $ζ_3(t)$ which shows how the average strangeness of the beam evolves from zero to $δ$. The evolution of the Stokes vector from $|\vecζ| = 0$ to $|\vecζ| = 1$ has a resemblance to an order parameter of a system undergoing spontaneous symmetry breaking.

hep-ph↗

arXiv1995

The Mellin Transform Technique for the Extraction of the Gluon Density

A new method is presented to determine the gluon density in the proton from jet production in deeply inelastic scattering. By using the technique of Mellin transforms not only for the solution of the scale evolution equation of the parton densities but also for the evaluation of scattering cross sections, the gluon density can be extracted in next-to-leading order QCD. The method described in this paper is, however, more general, and can be used in situations where a repeated fast numerical evaluation of scattering cross sections for varying parton distribution functions is required.

hep-ph↗