Measurements of hyperon semileptonic decays at the CERN Super Proton Synchrotron

The charged hyperon beam at the CERN Super Proton Synchrotron (SPS) has been used to collect data on semileptonic decays ofΣ ^?,Ξ ^? andΛ. A magnetic channel selects 100 GeV/c negatively charged particles produced in the forward direction by interaction of the 200 GeV/c SPS proton beam on a BeO target. TheΣ ^? andΞ ^? hyperons are concurrently identified in a DISC ?erenkov counter, and their decay products are analysed by a magnetic spectrometer. Electron-hadron discrimination is achieved by the combined use of lead-glass and lead/scintillator counters, transition radiation detectors, and a ?erenkov counter. In this article we report results on the \(\Sigma ^ - \to \Lambda e^ - \bar v\) decay mode. Measurements of the Λ polarization and of the centre-of-mass distributions (baryon kinetic energy, electron-neutrino correlation, and Dalitz plot distributions) yield the vector to axialvector form factor ratiof ₁/g ₁=+0.034±0.080, in agreement with the value expected from the conserved vector current hypothesis (f ₁/g ₁=0). TheΣ ^?→Λe ^? v→ branching ratio measurement gives a value of (5.41±0.30)×10^?5. The effects of radiative corrections are not included in these results. They are discussed in the text. Results on the otherΞ ^?,Σ ^?, andΛ semileptonic decays are reported in separate articles.     

Quark model analysis of hyperon semileptonic decays

A study of hyperon semileptonic decays has been made in various quark models and compared with the standard Cabibbo analysis which is found to be in good agreement with experiments. Results obtained in a quasirelativistic constituent quark model are in reasonable agreement with the standard Cabibbo analysis and show no sizeableSU(3) symmetry breaking effects. Results in other quark models which take into account theSU(3) symmetry breaking effects in the dominant form factors are similar to the quasirelativistic constituent quark model.     

On the full experiment in weak semileptonic hyperon decays

The possibilities of the realisation of full experiment in weak semileptonic hyperon decays are investigated. Several proposals for the determination of constant form-factors are made.     

Helicity analysis of semileptonic hyperon decays including lepton-mass effects

Using the helicity method we derive complete formulas for the joint angular decay distributions occurring in semileptonic hyperon decays including lepton-mass and polarization effects. Compared to the traditional covariant calculation, the helicity method allows one to organize the calculation of the angular decay distributions in a very compact and efficient way. In the helicity method the angular analysis is of cascade type, i.e. each decay in the decay chain is analyzed in the respective rest system of that particle. Such an approach is ideally suited as input for a Monte Carlo event generation program. As a specific example we take the decay  (l ⁻=e ⁻,μ ⁻) followed by the nonleptonic decay Σ ⁺→p+π ⁰ for which we show a few examples of decay distributions which are generated from a Monte Carlo program based on the formulas presented in this paper. All the results of this paper are also applicable to the semileptonic and nonleptonic decays of ground-state charm and bottom baryons, and to the decays of the top quark.     

The Large Hadron Collider and the Super Proton Synchrotron at CERN as tools to Generate Warm Dense Matter and Non–Ideal Plasmas

The largest accelerator in the world, the Large Hadron Collider (LHC) at CERN, has entered into commissioning phase. It is expected that when this impressive machine will become fully operational, it will generate two counter rotating 7 TeV/c proton beams that will be made to collide, leading to an unprecedented luminosity of 10³⁴ cm^–2s^–1. Total energy stored in each LHC beam is about 362 MJ, sufficient to melt 500 kg copper. Safety of operation is a very critical issue when working with such extremely powerful beams. It is important to know the consequences of an accidental release of the beam energy in order to design protection system for the equipment. For this purpose we have carried out extensive numerical simulations of the interaction of one full LHC beam with copper and graphite targets which are materials of practical importance. Our calculations have shown that the LHC protons will penetrate up to about 35 m in solid copper and 10 m in solid graphite. A very interesting outcome of this work is that the impact of the LHC beam on solid matter will generate Warm Dense Matter (WDM) and Strongly Coupled Plasmas (SCP). The beams for the LHC are pre‐accelerated in the SPS (Super Proton Synchrotron) to 450 GeV/c and transferred to LHC via two beam lines. Several SPS cycles are required to fill the LHC, in one cycle a batch with up to 288 bunches can be accelerated. From the safety point of view it is also very important to study the damage caused to the equipment in case of an accident involving an uncontrolled release of the SPS beam. For this purpose we have also carried out detailed numerical simulations of the impact of the full SPS beam on solid copper and tungsten targets. These simulations have shown that the targets are severely damaged by the beam. It is also interesting to note that also in this case, a large part of the target material is converted into WDM and SCP. This study, therefore, shows that the LHC and the SPS have the potential to be used for studying these important fields of research. However, to achieve this goal, it is necessary to advance this work by designing dedicated experiments. This work is in progress (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Experiments with the CERN SPS hyperon beam

High energy charged hyperons produced in proton collisions at large accelerators have a lifetime long enough to be channelled into beams of particles of appreciable intensity. This has opened up a new domain of research in experimental high energy physics. This article reviews recent works at the CERN SPS using a dedicated charged hyperon beam facility.     

Extraction of the CKM matrix element V<Subscript>us</Subscript> from the hyperon semileptonic decays

The chiral constituent quark model with configuration mixing ( c \chi CQM _config , which is successful in explaining the weak vector and axial-vector form factors for the strangeness-changing as well as strangeness-nonchanging hyperon semileptonic decays at Q ² = 0 , has been extended to determine the CKM matrix element V _us for the strangeness-changing decays. The implications of the effect of the SU(3) symmetry breaking, Q²-dependence and radiative corrections on the form factors and V _us have also been investigated. It is found that the results with SU(3) symmetry breaking show considerable improvement over the SU(3) symmetric results when compared with the existing experimental data. The inclusion of the Q²-dependence and radiative corrections in form factors have only a small effect on the prediction of V _us as is expected from the theory.     

Weak radiative hyperon decays in the pole model

We compute the parity conserving and parity violating amplitudes of the weak radiative hyperon decays in a pole model which includes negative parity intermediate states. The matrix elements are determined from experimentally known quantities and also evaluated in a nonrelativistic quark model. We find good numerical agreement with the data for the decay Σ⁺→P+γ and obtain predictions for the other decays where experimental information has been scarce up to now.     

Inclusive semileptonic decays of B mesons

The probability of inclusive semileptonic decay of B mesons is calculated in the constituent-quark model. A compact formula is obtained for the differential decay width of a B meson in terms of the corresponding decay width of a free b quark and the wave function of the internal motion of the quarks in the B meson. Numerical values of semileptonic-decay widths are obtained for a series of models of the wave function. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 10, 734–737 (25 May 1997)     

The absolute magnitude of hyperon nonleptonic decays

With the conventional current-current interaction, using the nonrelativistic quark model for the baryons and the standard PCAC and current algebra techniques for the emitted pion, we discuss hyperon nonleptonic decays. We obtain the octet rule and the absolute magnitude in good agreement with experiment; there is no need here for new (right-handed) currents or for an enhancement factor.     

Weak radiative hyperon decays in the quark model

Weak radiative hyperon decays are considered on the basis of the quark model including one-quark transitions associated with the effective sdγ vertex and two-quark processes s + u → u + d + γ induced by W-boson exchange. The phenomenological model proposed in this study gives values that describe experimental data well and makes predictions for the asymmetry of the decays Λ → n + γ and ξ^? → Σ^? + γ.     

Exclusive semileptonic decays of heavy mesons

Exclusive semileptonic decays of heavy mesons provide interesting information on systems consisting of quarks of unequal mass. We express the formfactors of the hadronic current in terms of relativistic bound state wave functions for which we take the solutions of a relativistic harmonic oscillator potential. The wave function overlap is determined by the quark mass dependent longitudinal momentum distribution and differs from results based on non relativistic wave functions. The semileptonic widths and lepton spectra are calculated using in addition nearest pole dominance for the momentum transfer dependence of the formfactors. We compare our results with recent experimental data. The formfactor calculation also allows an estimate of special nonleptonic transitions. From the CLEO results on \(\bar B^0 \to \pi ^ + \pi ^ -\) and \(\bar B^0 \to D^{* + } + \pi ^ -\) we find for the corresponding Kobayashi-Maskawa matrix element ratio the limit |V _ub /V _cb |?0.3.