Test Of Chiral Perturbation Theory Research Articles

Compton scattering off pions and electromagnetic polarizabilities

The electric [Formula: see text] and magnetic [Formula: see text] Compton polarizabilities of both the charged and the neutral pion are of fundamental interest in the low-energy sector of quantum chromodynamics (QCD). Pion polarizabilities affect the shape of the [Formula: see text] Compton scattering angular distribution at back scattering angles and [Formula: see text] absolute cross sections. Theory derivations are given for the [Formula: see text] Compton scattering differential cross section, dispersion relations, and sum rules in terms of the polarizabilities. We review experimental charged and neutral polarizability studies and theoretical predictions. The [Formula: see text] polarizabilities were deduced from DESY Crystal Ball [Formula: see text] data, but with large uncertainties. The charged pion polarizabilities were deduced most recently from (1) radiative pion Primakoff scattering [Formula: see text] at CERN COMPASS, (2) two-photon pion pair production [Formula: see text] at SLAC Mark II, and (3) radiative pion photoproduction [Formula: see text] from the proton at MAMI in Mainz. A stringent test of chiral perturbation theory (ChPT) is possible based on comparisons of precision experimental charged pion polarizabilities with ChPT predictions. Only the CERN COMPASS charged pion polarizability measurement has acceptably small uncertainties. Its value [Formula: see text] agrees well with the two-loop ChPT prediction [Formula: see text], strengthening the identification of the pion with the Goldstone boson of chiral symmetry breaking in QCD.

Light meson decays at BESIII

The BESIII experiment has accumulated the world’s largest data samples of 1.3 billion J/ψ events and 0.45 billion ψ(3686) events, which provide a unique opportunity to investigate light meson decays. The η and η′ decays are sensitive tools for investigations of π − π and η − π interactions, symmetry breaking, and serve as a test of chiral perturbation theory. In recent years considerable results on η and η′ decays were achieved at the BESIII experiment. In this proceeding, we present the significant progresses on amplitude analyses of Dalitz decays η' → 3π,η' → ηππ, and radiative decay η' → γπ+π−, η' → γγπ0. Additionally, the $a_0^0\left( {980} \right) - {f_0}\left( {980} \right)$ mixing is an important probe to the nature of those two lightest scalar mesons. The first observation of $a_0^0\left( {980} \right) - {f_0}\left( {980} \right)$ mixing is also presented, which will help to improve the understanding of the nature of $a_0^0\left( {980} \right)$ and f0(980).

Chiral Perturbation Theory tests at NA48/2 and NA62 experiments at CERN

Final results from an analysis of about 400 K± → π±γγ rare decay candidates collected by the NA48/2 and NA62 experiments at CERN during low intensity runs with minimum bias trigger configurations are presented. The results include a model-independent decay rate measurement and fits to Chiral Perturbation Theory (ChPT) descriptions. The data support the ChPT prediction for a cusp in the di-photon invariant mass spectrum at the two pions threshold.

Tests of Chiral perturbation theory with COMPASS

The COMPASS experiment at CERN accesses pion-photon reactions via the Primakoff effect., where high-energetic pions react with the quasi-real photon field surrounding the target nuclei. When a single real photon is produced, pion Compton scattering is accessed and from the measured cross-section shape, the pion polarisability is determined. The COMPASS measurement is in contradiction to the earlier dedicated measurements, and rather in agreement with the theoretical expectation from ChPT. In the same experimental data taking, reactions with neutral and charged pions in the final state are measured and analyzed in the context of chiral perturbation theory.

ChPT tests at NA48 and NA62

Final results from an analysis of about 400 K± → π±γγ rare decay candidates collected by the NA48/2 and NA62 experiments at CERN during low intensity runs with minimum bias trigger configurations are presented. The results include a model-independent decay rate measurement and fits to Chiral Perturbation Theory (ChPT) description. The data support the ChPT prediction for a cusp in the di-photon invariant mass spectrum at the two pion threshold.

Differential cross section and vector analysing power of the p pol p → {pp} s π0 reaction at 353–700 MeV

The reaction ppolp → {pp}sπ0 was studied with the ANKE spectrometer at COSY-Julich using a polarized beam with energies 353, 500, 550 and 700 MeV. The proton pairs {pp}s were detected at low excitation energy Epp < 3 MeV, where S-wave dominates. The angular dependences of vector analysing power Ay and differential cross section dσ/dΩ of the reaction have been obtained for the most of the angular range at 353 MeV and forward angles at the higher beam energies. The partial wave amplitude analysis, done with the 353 MeV results, is important for Chiral Perturbation Theory tests at this energy. The data at higher energies detalize the energy dependence of dσ/dΩ(0°) obtained earlier. It allows to learn about the dynamics of the Δ(1232) resonance excitation in two-nucleon systems.

Hadron spectroscopy with COMPASS at CERN

The aim of the COMPASS hadron program is to study the light-quark hadron spectrum, and in particular, to search for evidence of hybrids and glueballs. COMPASS is a fixed-target experiment at the SPS at CERN and features a two-stage spectrometer with high momentum resolution, large acceptance, particle identification and calorimetry. A short pilot run in 2004 resulted in the observation of a spin-exotic state with JPC=1−+ consistent with the debated π1(1600). In addition, Coulomb production at low momentum transfer data provide a test of chiral perturbation theory. During 2008 and 2009, a world leading data set was collected with the hadron beam, which is currently being analysed. The large statistics allows for a thorough decomposition of the data into partial waves. The COMPASS hadron data span over a broad range of channels and shed light on several different aspects of quantum chromodynamics.

Hadron Spectroscopy with COMPASS at CERN

The aim of the COMPASS hadron programme is to study the light-quark hadron spectrum, and in particular, to search for evidence of hybrids and glueballs. COMPASS is a fixed-target experiment at the CERN SPS that features a two-stage spectrometer with high momentum resolution and wide acceptance. It also provides particle identification and calorimetry. A short pilot run in 2004 resulted in the observation of a spin-exotic state with JPC = 1−+ consistent with the debated π1(1600) resonance. In addition, Coulomb production data at low momentum transfer provide a test of Chiral Perturbation Theory. During 2008 and 2009, a world leading data set was collected with hadron beam which is currently being analysed. The unprecedentedly large number of events allows for a thorough decomposition of the data into spin states. The COMPASS hadron data span over a broad range of channels and shed light on several different aspects of QCD.

Light Meson Decay Studies at Hadron Machines

Studies of light meson decays are important tools to perform precision tests of Chiral Perturbation Theory, to determine transition form factors and to test Standard Model predictions. Experiments using hadronic probes to produce light mesons contribute significantly to these investigations. This presentation focuses on the light meson decay program at the WASA facility at COSY. Highlights of the π 0 and η meson studies are discussed.

Determination of ππ scattering lengths from measurement of [formula omitted] atom lifetime

The DIRAC experiment at CERN has achieved a sizeable production of π + π − atoms and has significantly improved the precision on its lifetime determination. From a sample of 21 227 atomic pairs, a 4% measurement of the S-wave ππ scattering length difference | a 0 − a 2 | = ( 0.2533 − 0.0078 + 0.0080 | stat | syst − 0.0073 + 0.0078 ) M π + − 1 has been attained, providing an important test of Chiral Perturbation Theory.

Measurement of the vector analyzing power of the reaction at intermediate energies at ANKE/COSY

The reaction where {pp}s is a proton pair with an excitation energy Epp < 3 MeV, has been observed with the ANKE spectrometer at COSY-Jülich using a polarized beam with energies 353, 500, 550 and 700 MeV. The data have been processed to obtain the vector analyzing power Ay of the reaction. The setup acceptance covers most of the angular range at 353 MeV and forward angles at the higher beam energies, allowing one to obtain the Ay angular dependence. From the results of the analysis at 353 MeV, one can extract information on the pion d-wave contribution, which is important for Chiral Perturbation Theory tests at this energy. At higher energies the results are compared with a phenomenological model and this gives additional information about Δ-nucleon dynamics.

Measurement of ππ Scattering Lengths from [formula omitted] and [formula omitted] Decays

In the last few years it has become possible to study low energy ππ scattering with high precision in kaon decays, thanks to the high statistics measurements of K ± → π ± π 0 π 0 ( K 3 π ) and K ± → π + π − e ± ν ( K e 4 ) decays performed by the NA48/2 experiment at the CERN SPS. In K ± → π ± π 0 π 0 , at the π + π − threshold, the π 0 π 0 mass spectrum exhibits a Wigner-cusp, from which the scattering lengths a 0 and a 2 are extracted. From the study of K e 4 decays a similar precision, but quite independent measurement of a 0 and a 2 can be obtained. The combination of both measurements provides one of the most stringent tests of Chiral Perturbation Theory.

Chiral Dynamics in Photopion Physics: Theory, Experiment, and Future Studies at the HIγS Facility

We review photopion experiments on the nucleon in the near-threshold region. We compare the results with the predictions of the low-energy theorems of quantum chromodynamics calculated using chiral perturbation theory (ChPT), which is based on the spontaneous breaking of chiral symmetry as well as explicit breaking due to the finite quark masses. As a result of the vanishing of the threshold amplitudes in the chiral limit, the experiments are difficult to perform because the cross sections are small. Nevertheless, the field is now mature in terms of accuracy and sensitivity. We also discuss the accomplishments and limitations of past measurements, as well as future planned experiments at Mainz and HIγS. The technical developments required for the HIγS facility are emphasized. Finally, we show that future experiments will provide even more accurate tests of ChPT and will be sensitive to isospin-breaking dynamics due to the mass difference between the up and down quarks.

KLOE results with kaons

This report presents the recent results on K mesons from the KLOE experiment at the DAFNE e + e − collider working at the ϕ peak. They include V u s determinations, the test of the unitarity of the first row of the CKM matrix and the related experimental measurements. Tests of lepton universality from leptonic and semileptonic decays will be also discussed. Limits on new physics or tests of chiral perturbation theory are also assessed by the study of rarer kaon decays.

Energy dependence of the delta resonance: Chiral dynamics in action

There is an important connection between the low energy theorems of QCD and the energy dependence of the Delta resonance in pi-N scattering, as well as the closely related gamma^{*} N -> pi N reaction. The resonance shape is due not only to the strong pi-N interaction in the p wave but the small interaction in the s wave; the latter is due to spontaneous chiral symmetry breaking in QCD (i.e. the Nambu-Goldstone nature of the pion). A brief overview of experimental tests of chiral perturbation theory and chiral based models is presented

Experimental tests of Chiral Perturbation Theory

In this paper the current status of the threshold photo- and electroproduction of neutral pions as test of Chiral Perturbation Theory is summarized. The combination of differential cross-section data with polarized photon asymmetries allows to determine a complete set of s- and p-wave amplitudes at threshold for the photoproduction case. These extracted amplitudes are in good agreement with predictions and fits of Heavy Baryon Chiral Perturbation Theory. On the other hand, new data on pion electroproduction shows a significant deviation from the predicted cross-sections at a four-momentum transfer of Q 2 = 0.05 GeV2/c 2. For the coherent pion production from the deuteron the photoproduction data agree with Chiral Perturbation Theory. The former reported severe disagreement between the electroproduction s-wave amplitude at a four-momentum transfer of Q 2 = 0.1 GeV2/c 2 and first calculations seems to be resolved now.

NA48 results on neutral kaon and hyperon rare decays

The NA48 Collaboration has performed an extensive program of kaon and hyperon rare decays using the data collected during the period 1997–2001. This program includes new tests of chiral perturbation theory, new measurements of theη mass andK s lifetime and the possibility to measure the Cabibbo angle using Ξ beta decays.

Tests of chiral perturbation theory in Ks rare decays at NA48

The NA48 collaboration has performed an extensive study of rare kaon decays. In particular tests of Chiral Perturbation Theory have been done, using data from the Re( $\varepsilon'/\varepsilon$ ) runs in 1998 and 1999 for KL → π o γ γ and from the NA48/1 High Intensity for KS → γ γ and for the first observation of KS → π o γ γ PACS: 13.20.Eb Decays of K mesons – 14.40.Aq π, K and η mesons

Double π0 photoproduction off the proton at threshold

The reaction γp→π0π0p has been measured using the TAPS BaF2 calorimeter at the tagged photon facility of the Mainz Microtron accelerator. Chiral perturbation theory (ChPT) predicts that close to threshold this channel is significantly enhanced compared to double pion final states with charged pions. In contrast to other reaction channels, the lower order tree terms are strongly suppressed in 2π0 photoproduction. The consequence is the dominance of pion loops in the 2π0 channel close to threshold—a result that opens new prospects for the test of ChPT and in particular its inherent loop terms. The present measurement is the first which is sensitive enough for a conclusive comparison with the ChPT calculation and is in agreement with its prediction. The data also show good agreement with a calculation in the unitary chiral approach.

New measurement of the η → π 0 γγ branching ratio and the slope parameter for η → 3π 0 decay with the crystal ball

We have measured the quadratic slope parameter α for η → 3π 0 decay. Our final result is α=−0.031(4). We have also obtained the branching ratio (BR) for the double radiative decay η → π 0 γγ. Our preliminary result is BR(η → π 0 γγ=3.2(9)×10−4. These new measurements were performed using the recent (1998) high-statistics sample of 1.9 × 107 η produced at the AGS at BNL in π − p → nη close to threshold. The η decay into neutrals was detected with the Crystal Ball multiphoton spectrometer. Both measurements provide important tests of chiral perturbation theory (χPTh). Our result for α is four times more precise than the present world data and disagrees with current χPTh calculations by about four standard deviations. Our branching ratio for η → π 0 γγ is about half the current value and is in agreement with the χPTh predictions.