VEPP-2M Collider Research Articles

Beams of electrons, gammas and fast neutrons at BINP

An overview of test beam facilities at Budker Institute of Nuclear Physics (BINP) is presented. Facility for generation of extracted beams of electrons was designed at BINP for testing of detector prototypes in HEP field. The electron beam energy is from 100[Formula: see text]MeV to 3.5[Formula: see text]GeV. Since 2011, the facility has been used successfully for testing various detector prototypes. On the base of VEPP–4M collider, it is possible to obtain gamma beams too. The energy scale and resolution of calorimeters can be calibrated using edges of Compton and bremsstrahlung spectra. A facility for boron–neutron capture therapy (BNCT) was upgraded to perform radiation tests with fast neutrons with the integral flux up to [Formula: see text][Formula: see text]n/cm2. In 2022, the experiment on the study of the radiation hardness of optical fibers for the laser calibration system of electromagnetic calorimeter and the CMS detector was carried out. It has been demonstrated for the first time that at the BINP it is possible to operate with such doses. It could be further used for the wide range of radiation test tasks related with the development of facilities for HEP.

VEPP-4M linear optics correction using orbit response matrices

This paper presents procedures and techniques for correcting the linear optics of the VEPP-4M collider. The accelerator structure investigation is based on the measurement and analysis of orbit response matrices (ORM) and optical functions reconstructed from turn-by-turn (TbT) data. Having no technical description of quadrupole lenses and corrector magnets, we have calibrated their current dependencies of magnetic strengths and fields, respectively. Corresponding to the reference orbit, quadrupole shifts in each plane have been obtained by using the beam-based alignment (BBA) technique. The accelerator optics has been corrected and calibrated at the injection energy of 1.9 GeV. The software combining automated optical functions measurement and correction has been developed and successfully implemented at VEPP-4M.

Measurement of the branching fraction of J/ψ → ρπ at KEDR

We present the study of the decay J/ψ → ρπ. The results are based on of 5.2 million J/ψ events collected by the KEDR detector at the VEPP-4M collider. The branching fractions are measured to be mathcal{B} (J/ψ → ρπ) = (2.072 ± 0.017 ± 0.062) ∙ 10−2 and mathcal{B} (J/ψ → π+π−π0) = (1.878 ± 0.013 ± 0.051) ∙ 10−2, where the first uncertainties are statistical and the second systematic. Our results are more precise than the previous relative measurements.

VEPP-4M Collider Operation at High Energy

VEPP-4M is an electron positron collider equipped with the universal KEDR detector for HEP experiments in the beam energy range from 1 GeV to 6 GeV. A unique feature of VEPP-4M is the high precision beam energy calibration by resonant polarization technique which allows conducting of interesting experiments despite the low luminosity of the collider. Recently we have started new luminosity acquisition run above 2 GeV. The hadron cross section was measured from 2.3 GeV to 3.5 GeV has been done. The luminosity run for gamma-gamma physics has been started. The luminosity at ψ(1S)-meson has been obtained. For the beam energy calibration the laser polarimeter is used. The paper discusses recent results from VEPP-4M collider.

The high-resolution particle tracking triple-GEM detector for the Test Beam facility at VEPP-4M collider

Test Beam facility is an experimental facility of the VEPP-4M collider uses Brems-strahlung gamma-rays from the movable beam scraper inserted in the halo of the circulating electron beam for test experiments with HEP detector components. Measurements taken at this installation require high-resolution low-mass tracking detectors to precisely determine particle trajectories. The high-resolution GEM-based tracking detector developed for this facility is described in this paper.

The GEM-based detector for tracking the Compton-scattered photons in the Laser Polarimeter facility at VEPP4-M collider

The Laser Polarimeter is being developed at VEPP-4M collider for beam energy calibration by Resonant Depolarization technique. The essential part of this facility is a photon-tracking detector measuring coordinates of backscattered gammas. The detector is based on triple-GEM cascade with pad readout. Its design and the first results of the measurements with radioactive source are described in the paper.

Measurement of the VEPP-4M Collider Energy Spread in the Entire Energy Range

The energy spread of the VEPP-4M electron–positron collider at the Budker Institute of Nuclear Physics was measured in the 1100–4750 MeV range (i.e., in the entire operating energy range of the collider) in two ways: based on the beam length and based on the envelope of betatron oscillations. The influence of the Touschek effect on this parameter was studied in the 1–1.5 GeV range. The results obtained using the two different methods agree well.

The triple-GEM detector for the Laser Polarimeter facility at VEPP4-M collider

The Laser Polarimeter is being developed at VEPP-4M collider for beam energy calibration by Resonant Depolarization technique. The essential part of this facility is a photon-tracking detector measuring coordinates of backscattered gammas.

A New System for Measuring the Beam Position in the Electron– Positron Transport Channel from the VEPP-3 Storage Ring of the VEPP-4M Collider

A new system for measuring the position of the beam in a pulsed transport channel from the VEPP-3 storage ring to the VEPP-4M collider is discussed, which provides measurement of the position and intensity of an electron or positron beam in one span. For reliable operation of the experimental complex, it is necessary to continuously monitor the beam trajectory in a non-destructive manner, as well as to measure the possible losses of the beam charge. In 2018, new sensors and new electronics of the beam position measurement system were developed, manufactured, and installed in the transportation channel, which provide much greater accuracy compared to the old system. In the new system, it was possible to almost completely eliminate interference and to achieve measurement accuracy better than 0.02 mm, which is more than enough for optimal tuning of the channel optics and achieving high bypass efficiency. The design of the sensor, the structure and the main features of the construction of electronics are described, the accuracy of measurements is analyzed, and some results of the operation of the new system in the channel are given.

Crossing integer spin resonance with conservation of beam polarization

A method proposed to preserve the electron beam polarization at the VEPP-4M collider during acceleration with crossing the integer (imperfection) spin resonance at energy E=1763 MeV has been successfully applied. It is based on full decompensation of the $ 0.6\times3.3$ Tesla$\times$meter integral of the KEDR detector longitudinal magnetic field due to the anti-solenoids 'switched-off'.

Particle detection efficiency of the KEDR detector ASHIPH system

The particle identification system ASHIPH (Aerogel, SHifter, PHotomultiplier) has been working in the KEDR experiment at VEPP-4M collider (Budker INP, Novosibirsk) since 2014. The system consists of 160 counters arranged in two layers and covers the 96% of the solid angle. Two layer system permits to use different approaches for π/K separation. Three such approaches are described. The status of the system is presented. Main system parameters such as BhaBha suppression factor, π/K separation power, relativistic particle detection efficiency were measured with help of cosmic particles and experimental data collected at the peak of J/ψ-meson. π/K separation in the momentum range from 0.95 to 1.45GeV/c is better than 4σ. Monte-Carlo simulation procedures are described and results are presented as well.

Observation of Saw-Tooth effect orbit at VEPP-4M collider

We study the relative position of the electron and positron closed orbits in the VEPP-4M single storage ring collider in experiments on orbit precision monitoring. A difference in the orbits can affect the accuracy of several fundamental experiments, e.g. precise comparison of the electron and positron spin frequencies (the CPT invariance test). In this case, the spin precession frequencies of particles should be compared within at least 5·10−9. The distinction of frequencies depends on the features of the radial orbits. Ideally, the difference in the electron and positron orbits is set only by distributed radiation losses of particle energy. The corresponding contribution to the total orbit distortions is called the Saw-Tooth effect orbit. Another example of possible precision experiment at VEPP-4M is search for the light speed anisotropy (LSA). In this case, it is necessary to ensure a stability of the difference in the radial orbits of electrons and positrons at a level of 1 μm.

Study of the spatial resolution of low-material GEM tracking detectors

The spatial resolution of GEM based tracking detectors has been simulated and measured. The simulation includes the GEANT4 based transport of high energy electrons with careful accounting for atomic relaxation processes including emission of fluorescent photons and Auger electrons and custom post-processing, including accounting for diffusion, gas amplification fluctuations, the distribution of signals on readout electrodes, electronics noise and a particular algorithm of the final coordinate calculation (center of gravity). The simulation demonstrates that a minimum of the spatial resolution of about 10 μm can be achieved with strip pitches from 250 μm to 300 μm. For larger pitches the resolution is quickly degrading reaching 80-100 μm at a pitch of 500 μm. The spatial resolution of low-material triple-GEM detectors for the DEUTRON facility at the VEPP-3 storage ring is measured at the extracted beam facility of the VEPP-4M collider. The amount of material in these detectors is reduced by etching the copper of the GEMs electrodes and using a readout structure on a thin kapton foil rather than on a glass fibre plate. The exact amount of material in one DEUTRON detector is measured by studying multiple scattering of 100 MeV electrons in it. The result of these measurements is X/X0 = 2.4×10−3 corresponding to a thickness of the copper layers of the GEM foils of 3 μm. The spatial resolution of one DEUTRON detector is measured with 500 MeV electrons and the measured value is equal to 35 ± 1 μm for orthogonal tracks.

Energy scale calibration of the KEDR tagging system

The KEDR detector tagging system is a symmetric focusing magnetic spectrometer of small-angle electrons born in the interaction region of the VEPP-4M collider. Using this system, which is designed for studying the two-photon processes, the scattered electrons and positrons energy is measured with a resolution of ΔE/E0= 0.03–0.6% where E0 is the beam energy. Two different methods are employed for calibrating the tagging system energy scale: the first one involves the electron/positron tagging by the energy of the singlebremsstrahlung photon measured in the BGO calorimeter, and the second one is based on the determination of the recoil electrons spectrum edge in the Compton backscattering of monochromatic laser radiation. The technical implementation and the present status of the calibration system are discussed in this paper.

Proposal for VEPP-4M beam energy measurement using magnetic spectrometer with Compton calibration and photon detector

A method for circular e−/e+ accelerator beam energy measurement is proposed. A coordinate of an electron (or a positron) in a focusing magnetic spectrometer built in a circular accelerator depends on its energy, two spectrometer parameters, and the circulating beam energy. The spectrometer parameters can be determined using minimum electron energies from Compton backscattering with two laser wavelengths, and a coordinate detector for photons. The photon detector is calibrated separately at well-known beam energy. The VEPP-4M collider has appropriate equipment for the method implementation: built-in focusing magnetic spectrometer, Compton calibration system with two lasers and a photon coordinate detector. Thus, the proposed technique could be implemented with minimum efforts; tests and further upgrade are planned. The beam energy can be defined with expected uncertainty better than 10−4.

Energy scale calibration of KEDR detector tagging system

The tagging system of the KEDR detector is a symmetrical focusing magnetic spectrometer for electrons and positrons scattered at small angles; it is embedded into the lattice of the VEPP-4M collider and intended for two-photon processes study. It measures scattered electron energy with resolution Δ E/E0=0.03% … 0.6% (E0 is the beam energy). For precise energy scale calibration two methods are used: tagging of bremsstrahlung electron/positron by the photon energy measured by BGO calorimeter, and direct calibration using Compton backscattering spectrum edge. This report covers design and current status of the calibration system.

High resolution micro-pattern gas detectors for particle physics

Micro-pattern gaseous detectors (MPGDs) allow operation at very high background particle flux with high efficiency and spatial resolution. This combination of parameters determines the main application of these detectors in particle physics experiments: precise tracking in the areas close to the beam and in the end-cap regions of general-purpose detectors. MPGDs of different configurations have been developed and are under development for several experiments in the Budker INP. The system of eight two-coordinate detectors based on a cascade of Gas Electron Multipliers (GEM) is working in the KEDR experiment at the VEPP-4M collider in the tagging system that detects electrons and positrons that lost their energy in two-photon interactions and left the equilibrium orbit due to a dedicated magnetic system. Another set of cascaded GEM detectors is developed for the almost-real Photon Tagging System (PTS) of the DEUTRON facility at the VEPP-3 storage ring. The PTS contains three very light detectors with very high spatial resolution (below 50 μm). Dedicated detectors based on cascaded GEMs are developed for the extracted electron beam facility at the VEPP-4M collider. These devices will allow precise particle tracking with minimal multiple scattering due to very low material content. An upgrade of the coordinate system of the CMD-3 detector at the VEPP-2000 collider is proposed on the basis of the resistive micro-WELL (μ-rWELL). A research activity on this subject has just started.

Development of high-resolution GEM-based detector for the extracted electron beam facility at the VEPP-4M collider

The goal of the extracted beam facility at the VEPP-4M collider is to test prototypes of new detectors for particle physics. Measurements taken at this installation require high-resolution low-mass tracking detectors to precisely determine particle trajectories. The high-resolution GEM-based tracking detector developed for this facility is described in this paper.

Limits of a spatial resolution of the cascaded GEM based detectors

Spatial resolution of tracking detectors based on GEM cascades is determined in the simulation and measured. The simulation includes GEANT4 implemented transport of high energy electrons with careful accounting for atomic relaxation processes including emission of fluorescent photons and Auger electrons and custom post-processing taking into account diffusion, gas amplification fluctuations, the distribution of signals over readout electrodes, electronics noise and particular algorithm of final coordinate calculation (centre-of-gravity algorithm). The simulation demonstrates that the minimum of the spatial resolution of about 10–20 μm can be achieved with a gas mixture of Ar-CO2 (75%–25%) at a strip pitch in the range from 250 μm to 300 μm. At a larger pitch the resolution quickly degrades reaching 70–100 μm at a pitch of 450–500 μm. The reasons of such behavior are discussed and corresponding hypothesis is tested. Particularly, the effect of electron cloud modification due to a GEM operation is considered using the ANSYS and Garfield++ simulation programs. The detection efficiency and spatial resolution of low-material triple-GEM detectors for the DEUTERON facility at BINP are measured at the extracted beam facility of the VEPP-4M collider. One-coordinate resolution of two detectors for the DEUTERON facility is measured with a 2 GeV electron beam. The determined values of the detectors' spatial resolution is equal to 46.6 ± 0.1 μm and 38.5 ± 0.2 μm for orthogonal tracks in two detectors, respectively.

Investigation into Cherenkov light scattering and refraction on aerogel surface

The work concerns the development of aerogel radiators for RICH detectors. Aerogel tiles with a refractive index of 1.05 were tested with a RICH prototype on the electron beam on the VEPP-4M collider. It has been shown that polishing with silk tissue yields good surface quality, the amount of light loss at this surface being about 5–7%. The Cherenkov angle resolution was measured for a tile in two conditions: with a clean exit face and with a polished exit face. The number of photons detected was 13.3 and 12.7 for the clean and polished surfaces, respectively. The Cherenkov angle resolution for the polished surface is 55% worse, which can be explained with the forward scattering on the polished surface. A tile with a crack inside was also tested. The experimental data show that the Cherenkov angle resolution is the same for tracks crossing the crack area and in a crack-free area.