Primakoff Reaction Research Articles

Design of beam optics for RF-separated kaon and antiproton beams in the M2 beam line of the CERN North Area

The present contribution reviews the principle of RF separation and explains its dependence on different parameters of beam optics and hardware. The first examination of potential showstoppers for the RF-separated beam implementation for Phase-2 of the AMBER experiment in the M2 beam line of the North Area is presented. Different beam optics settings have been examined, providing either focused or parallel beams inside the RF cavities. The separation and transmission capability of the different optics settings for realistic characteristics of RF cavities are discussed and the preliminary results of the potential purity and intensity of the RF-separated beam are presented. These show that a trade-off between the overall beam intensity and the share of the required particle type in the overall beam needs to be established. No showstoppers have been identified for achieving the beam parameters required for AMBER’s kaonic Primakoff reactions, kaon spectroscopy, prompt-photon production and kaon charge-radius programs. However, the high beam intensity requirements of the AMBER Drell–Yan programme cannot be satisfied with an RF-separated beam.

Dispersive analysis of the Primakoff reaction gamma K rightarrow K pi

We provide a dispersion-theoretical representation of the reaction amplitudes gamma Krightarrow K pi in all charge channels, based on modern pion–kaon P-wave phase shift input. Crossed-channel singularities are fixed from phenomenology as far as possible. We demonstrate how the subtraction constants can be matched to a low-energy theorem and radiative couplings of the K^*(892) resonances, thereby providing a model-independent framework for future analyses of high-precision kaon Primakoff data.

Electromagnetic Polarizabilities of Mesons

The Chiral Perturbation Theory (CHPT) has been very successful in describing low-energy hadronic properties in the non-perturbative regime of Quantum Chromodynamics. The results of ChPT, many of which are currently under active experimental investigation, provide stringent predictions of many fundamental properties of hadrons, including quantities such as electromagnetic polarizabilities. Yet, even for the simplest hadronic system, a pion, we still have a broad spectrum of polarizability measurements (MARK II, VENUS, ALEPH, TPC/2g, CELLO, Belle, Crystal Ball). The meson polarizability can be accessed through Compton scattering, so we can measure it through Primakoff reaction. This paper will provide an analysis of the CHPT predictions of the SU(3) meson electromagnetic polarizabilities and outline their relationship to the Primakoff cross section at the kinematics relevant to the planned JLab experiments.

Anomalous decay and scattering processes of the $$\eta $$ η meson

We amend a recent dispersive analysis of the anomalous $\eta$ decay process $\eta\to\pi^+\pi^-\gamma$ by the effects of the $a_2$ tensor meson, the lowest-lying resonance that can contribute in the $\pi\eta$ system. While the net effects on the measured decay spectrum are small, they may be more pronounced for the analogous $\eta'$ decay. There are nonnegligible consequences for the $\eta$ transition form factor, which is an important quantity for the hadronic light-by-light scattering contribution to the muon's anomalous magnetic moment. We predict total and differential cross sections, as well as a marked forward-backward asymmetry, for the crossed process $\gamma\pi^-\to\pi^-\eta$ that could be measured in Primakoff reactions in the future.

Program of COMPASS-II at CERN

COMPASS collaboration has started in 2012 a five-year program of new measurements, which are outlined in the 'COMPASS-II' proposal. The two new major projects of the proposal are measurements of polarized Drell-Yan process in π- scattering off transversely polarized protons and studies of GPDs via measurements of Deeply Virtual Compton Scattering and Hard Exclusive Meson Production in muon scattering off a liquid hydrogen target. In addition, high statistics SIDSI measurements with muon beams and a study of charged pion and kaon polarizabilities via Primakoff reactions with hadron beams are also foreseen as a part of this program.

Measurement of radiative widths of a2(1320) and $ \pi_{2}$ (1670)

The COMPASS Collaboration at CERN has investigated the reaction $\pi^- \gamma \rightarrow \pi^-\pi^-\pi^+$ embedded in the Primakoff reaction of $190~\textrm{GeV}$ pions scattering in the Coulomb field of a lead target, $\pi^- \text{Pb} \rightarrow \pi^-\pi^-\pi^+ \text{Pb}$. Exchange of quasi-real photons is selected by isolating the sharp Coulomb peak observed at momentum transfer below $0.001~(\text{GeV}/c)^2$. Using a partial-wave analysis the amplitudes and relative phases of the $a_2(1320)$ and $\pi_2(1670)$ mesons have been extracted, and the Coulomb and the diffractive contributions have been disentangled. Measuring absolute production cross sections we have determined the radiative width of the $a_2(1320)$ to be $\Gamma_0(a_2(1320) \rightarrow \pi\gamma) = (358 \pm 6_{\textrm{stat}} \pm 42_{\textrm{syst}})~\textrm{keV}$. As the first measurement, $\Gamma_0(\pi_2(1670) \rightarrow \pi\gamma) = (181 \pm 11_{\textrm{stat}} \pm 27_{\textrm{syst}})~\textrm{keV} \cdot (\textrm{BR}^{\textrm{PDG}}_{f_2 \pi}/\textrm{BR}_{f_2 \pi})$ is obtained for the radiative width of the $\pi_2(1670)$, where in this analysis the branching ratio $\textrm{BR}^{\textrm{PDG}}_{f_2 \pi}=0.56$ has been used. We compare these values to previous measurements and theoretical predictions.

Recent results from the COMPASS hadron program

COMPASS is a fixed-target experiment aimed to study the structure and dynamics of hadrons. Using a 190 GeV/ c pion beam, the spectrum of light mesons is studied at four-momentum transfers squared to the target between 0:1 and 1:0 GeV 2 / c 2 . The flagship channel is the diffractive production of π − π − π + final states for which COMPASS has recorded the world’s largest data sample. The determination of properties of known resonances, and the search for new states is also pursued in the π − π 0 π 0 final state, and in centrally produced systems. The structure of light mesons is studied in Primakoff reactions at lowest four-momentum transfers. Using these processes, the polarizability of the pion, the radiative width of the a 2 (1320) and, for the first time, that of the π 2 (1670) have been measured.

THE ANOMALOUS PROCESS γπ → ππ AND ITS IMPACT ON THE π0 TRANSITION FORM FACTOR

The process γπ → ππ, in the limit of vanishing photon and pion energies, is determined by the chiral anomaly. This reaction can be investigated experimentally using Primakoff reactions, as currently done at COMPASS. We derive a dispersive representation that allows one to extract the chiral anomaly from cross-section measurements up to 1 GeV, where effects of the ρ resonance are included model-independently via the ππ P-wave phase shift. We discuss how this amplitude serves as an important input to a dispersion-theoretical analysis of the π0 transition form factor, which in turn is a vital ingredient to the hadronic light-by-light contribution to the anomalous magnetic moment of the muon.

Experimental Program of the Future COMPASS-II Experiment at CERN

Over the last decade the COMPASS experiment has successfully obtained precise results on nucleon structure and hadron spectroscopy, with statistical errors much lower than previously reported. Recently, the new COMPASS-II program, focused on the study of a more complete nucleon description, was approved by CERN. The goal of the program is to access the nucleon structure beyond the collinear approxi-mation, including the quark intrinsic transverse momentum distributions, which are described by the Transverse Momentum Dependent (TMD) Parton Distribution Functions (PDFs) and by Generalised Parton Distributions (GPDs). The COMPASS collaboration proposes to measure the TMD PDFs, for the first time, via the polarised Drell-Yan process. The results will be complementary to those already obtained in polarised Semi-Inclusive Deep Inelastic Scattering (SIDIS). The GPDs can be accessed using deeply virtual Compton scattering and hard exclusive meson production using an unpolarised hydrogen target in a kinematic region not yet covered by any existing experiment. Also unpolarised SIDIS will be studied, by improving the knowledge of the strange quark PDF and obtaining the kaon fragmentation functions. Another topic covered in this COMPASS-II program is the measurement of the pion and kaon polarisabilities using the Primakoff reaction, which corresponds to a test of the chiral perturbative theory.

Upgrade of the COMPASS calorimetric trigger

In 2009 COMPASS performed a short measurement of neutral Primakoff reactions, characterised by highly energetic photons in one of the two electromagnetic calorimeters.A digital trigger was implemented in the existing readout electronicswhich calculates the energy released in the central region of thecalorimeter. In 2012 a long measurement of these processes has beenperformed. In order to extend the kinematic range to lower energeticphotons the trigger system has been upgraded in a way to be moreselective to specific physics channels. The new ADC firmware preserveshit information and provides it to newly developed backplane triggermodules. There hits from all three thousand channels are processedand the trigger decision is made.

First Measurement of Chiral Dynamics inπ−γ→π−π−π+

The COMPASS Collaboration at CERN has investigated the π- γ → π- π- π+ reaction at center-of-momentum energy below five pion masses, sqrt[s]<5m(π), embedded in the Primakoff reaction of 190 GeV pions impinging on a lead target. Exchange of quasireal photons is selected by isolating the sharp Coulomb peak observed at smallest momentum transfers, t'<0.001 GeV2/c2. Using partial-wave analysis techniques, the scattering intensity of Coulomb production described in terms of chiral dynamics and its dependence on the 3π-invariant mass m(3π)=sqrt[s] were extracted. The absolute cross section was determined in seven bins of sqrt[s] with an overall precision of 20%. At leading order, the result is found to be in good agreement with the prediction of chiral perturbation theory over the whole energy range investigated.

The Primakoff reaction study for pion polarizability measurement at COMPASS

The electromagnetic structure of charged pions can be described by the electric (απ) and magnetic (βπ) polarizabilities that depend on the rigidity of pion’s internal structure as a composite particle. It is shown that the values of απ and βπ can be precisely measured via the Primakoff reaction π− + (A, Z) → π− + (A, Z) + γ in the COMPASS experiment at CERN.

Plans for a measurement of pion polarizabilities at COMPASS

The pion electromagnetic structure can be probed in π − + ( A , Z ) → π − + ( A , Z ) + γ Compton scattering in inverse kinematics (Primakoff reaction) and described by the electric ( α π ) and the magnetic ( β π ) polarizabilities that depend on the rigidity of pion's internal structure as a composite particle. Values for pion polarizabilities can be extracted from the comparison of the differential cross section for scattering of point-like pions with the measured cross section. The opportunity to measure pion polarizability via the Primakoff reaction at the COMPASS experiment was studied with a π − beam of 190 GeV. The obtained results are used for preparation of the new measurement.

Pion polarizabilities measurement at COMPASS

The electromagnetic structure of pions is probed in π− + (A, Z) π− + (A, Z) + γ Compton scattering in inverse kinematics (Primakoff reaction) and described by the electric (απ) and the magnetic (βπ) polarizabilities that depend on the rigidity of pion's internal structure as a composite particle. Values for pion polarizabilities can be extracted from the comparison of the differential cross section for scattering of point like pions with the measured cross section. The pion polarizability measurement was performed with a π− beam of 190 GeV. The high beam intensity, the good spectrometer resolution, the high rate capability, the high acceptance and the possibility to use pion and muon beams, unique to the COMPASS experiment, provide the tools to measure precisely the pion polarizabilities in the Primakoff reaction.

Measurement of electric and magnetic polarizabilities with Primakoff reaction at COMPASS

The COMPASS spectrometer is well suited to perform precise measurements of the pion polarizabilities via the Primakoff reaction π(Z,A)→π+(Z,A)+ψ. The electric\((\bar \alpha _\pi )\) and magnetic\((\bar \beta _\pi )\) polarizabilities characterize the response of the response of the pion quark substructure to the electromagnetic field of the ψ during the πψ scattering. The results of a simulation for the foreseen 2004 setup are presented. The measurement of the pion polarizabilities allows for a test of the chiral perturbation theory (χPT) predictions.

Physics projects of COMPASS with hadron beams

COMPASS, a new state-of-the-art spectrometer to be installed at the CERN Superprotonsynchrotron for experiments with muon and hadron beams, will be exposed to hadron beams with intensities up to 10 8/sec and energies up to 280 GeV. The physics goals are to study the rare production of charmed hadrons, including doubly charmed baryons, in inelastic interactions, with particular interest in their semileptonic decays; to search for glueballs and hybrids in central and diffractive production. Predictions of chiral perturbation theory will be tested in Primakoff reactions. The spectrometer shall be equipped with excellent particle identification and tracking, with calorimetry, dedicated triggers and fast read-out. A significant improvement of light hadron spectroscopy — compared to previous measurements —can be achieved already in the initial phase of the experiment.