Bound-free Pair Production Research Articles

Optimizing Pb beam losses at the LHCb for maximum luminosity

In addition to the physics program with proton beams, the Large Hadron Collider (LHC) at CERN also provides collisions of fully-stripped Pb beams for about one month per year. When colliding Pb nuclei, electromagnetic interactions are the dominating processes because of the intense Coulomb field produced by the ions. These ’ultra-peripheral’ interactions give rise to ions with a changed magnetic rigidity. This causes losses in the machine that can impose limits on the luminosity. Among them, the bound-free pair production (BFPP) causes a localised power deposition downstream of each collision point, which could induce superconducting magnet quenches if not well controlled. These losses were studied and successfully mitigated for most LHC experiments, however the recent request by LHCb to increase the Pb-Pb luminosity requires a revision of BFPP collisional loss limitations. In this paper, the simulation of BFPP losses from Pb-Pb collisions around LHCb is presented. The loss patterns are discussed for different beam parameters. Finally, a mitigation strategy by means of an orbit bump is studied.

Bound-free Pair Production Mechanism in Pb–\(p\) Collisions at the LHC

Bound-free Pair Production Mechanism in Pb–\(p\) Collisions at the LHC

Bound-free Pair Production Mechanism at the NICA Accelerator Complex

In this work, bound-free pair production (BFPP) cross section calculations are done for NICA collider. In the BFPP mechanisms, after free pair production process, free electron is captured by one of the ions. This causes to the decrement of the ion from the beam, also diminish the intensity of the beam and changes the beam lifetime. BFPP cross section calculations are done for p-Au and p-Bi collisions for NICA collider and it is seen that the obtained results are in the range of a few mbarns. In this work, we used the lowest-order perturbation theory and our calculations are done in QED framework.

Bound-free pair production from nuclear collisions and the steady-state quench limit of the main dipole magnets of the CERN Large Hadron Collider

During its Run 2 (2015-2018), the Large Hadron Collider (LHC) operated at almost twice higher energy, and provided Pb-Pb collisions with an order of magnitude higher luminosity, than in the previous Run 1. In consequence, the power of the secondary beams emitted from the interaction points by the bound-free pair production (BFPP) process increased by a factor ~20, while the propensity of the bending magnets to quench increased with the higher magnetic field. This beam power is about 35 times greater than that contained in the luminosity debris from hadronic interactions and is focused on specific locations that fall naturally inside superconducting magnets. The risk of quenching these magnets has long been recognized as severe and there are operational limitations due to the dynamic heat load that must be evacuated by the cryogenic system. High-luminosity operation was nevertheless possible thanks to orbit bumps that were introduced in the dispersion suppressors around the ATLAS and CMS experiments to prevent quenches by displacing and spreading out these beam losses. Further, in 2015, the BFPP beams were manipulated to induce a controlled quench, thus providing the first direct measurement of the steady-state quench level of an LHC dipole magnet. The same experiment demonstrated the need for new collimators that are being installed around the ALICE experiment to intercept the secondary beams in the future. This paper discusses the experience with BFPP at luminosities very close to the future High Luminosity LHC (HL-LHC) target, gives results on the risk reduction by orbit bumps and presents a detailed analysis of the controlled quench experiment.

Bound-free pair production in relativistic nuclear collisions from the NICA to the HE LHC colliders

We consider the electron-positron pair production in relativistic heavy ion collisions, in which the produced electron is captured by one of the nuclei resulting, thus, in the formation of a hydrogen--like ion. These ions emerge from the collision point and hit the vacuum chamber wall inside superconducting magnets. Therefore, this process may be important for the problems of beam life time and for the quenching the irradiated magnet. A theoretical investigation for such a bound-free pair production (BFPP) at the colliders from NICA to HE LHC is presented. We obtain an approximate universal formula for the total cross section of the process. We compare it with the results of available numerical calculations and estimate that an accuracy of our calculations is better than $30$ % at the energies of the NICA collider and becomes of the order of a few percent for the RHIC and HE LHC colliders. Based on the obtained results, the detailed calculations are performed for future experiments at the NICA collider. We find that the expected BFPP cross sections for the Au$^{79+}$-Au$^{79+}$ and Bi$^{83+}$-Bi$^{83+}$ collisions are in the range from $10$ to $70$ barn, while for the p-Au$^{79+}$ and p-Bi$^{83+}$ collisions they are in the range of a few mbarn.

Polarization effects in bound-free pair production

We present a theoretical study of bound-free electron-positron pair production in the interaction of $\gamma$-rays with bare ions. Special attention is paid to the longitudinal polarization of both the emitted positrons and the produced hydrogen-like ions. To evaluate this polarization we employed exact solutions of the relativistic Dirac equation and treat the electron-photon coupling within the framework of first-order perturbation theory. Detailed calculations have been performed for both, low- and high-Z ions and for a wide range of photon energies. The results of these calculations suggest that bound-free pair production can be a source of strongly polarized positrons and ions.

Validation of energy deposition simulations for proton and heavy ion losses in the CERN Large Hadron Collider

Monte Carlo shower simulations are essential for understanding and predicting the consequences of beam losses in high-energy proton and ion colliders. Shower simulations are routinely used at CERN for estimating the beam-induced energy deposition, radiation damage, and radioactivity in the Large Hadron Collider (LHC). Comparing these shower simulations against beam loss measurements is an important prerequisite for assessing the predictive ability of model calculations. This paper validates fluka simulation predictions of beam loss monitor (BLM) signals against BLM measurements from proton fills at 3.5 and 4 TeV and Pb20882+ ion fills at 1.38A TeV. The paper addresses typical loss scenarios and loss mechanisms encountered in LHC operation, including proton collisions with dust particles liberated into the beams, halo impact on collimators in the betatron cleaning insertion, proton-proton collisions in the interaction points, and dispersive losses due to bound-free pair production in heavy ion collisions. Model predictions and measured signals generally match within a few tens of percent, although systematic differences were found to be as high as a factor of 3 for some regions and source terms.10 MoreReceived 18 March 2019DOI:https://doi.org/10.1103/PhysRevAccelBeams.22.071003Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasBeam lossParticle phenomenaPhysical SystemsHadron collidersMachine protectionTechniquesMonte Carlo methodsAccelerators & Beams

Bound-free electron–positron pair production mechanism at an electron-ion collider

The aims of building electron-ion colliders (EICs) are to reach high collision luminosity and polarization, to search on the structure and the interactions of gluon-matter, to investigate the spin structure of matter, to probe the behavior of parton densities in the nuclear medium, and to study and to test quantum chromodynamics (QCD). In this work, we have calculated the cross-sections of the ion beam loss mechanism at EICs via bound-free pair production (BFPP) mechanism. The BFPP is important for the detector design because of its large cross-sections that will give rise to decrease the luminosity of the accelerators. We have done the cross-section calculations for BFPP in the framework of quantum electrodynamics (QED) by using the lowest-order perturbation theory for three different EICs that are mentioned below.

Pair production in low-energy collisions of uranium nuclei beyond the monopole approximation

A method for calculation of electron-positron pair production in low-energy heavy-ion collisions beyond the monopole approximation is presented. The method is based on the numerical solving of the time-dependent Dirac equation with the full two-center potential. The one-electron wave functions are expanded in the finite basis set constructed on the two-dimensional spatial grid. Employing the developed approach the probabilities of bound-free pair production are calculated for collisions of bare uranium nuclei at the energy near the Coulomb barrier. The obtained results are compared with the corresponding values calculated in the monopole approximation.

Bound-free pair production with a correction term in relativistic heavy-ion collisions

In our previous work, we calculated bound-free electron-positron pair production cross section without a correction term. In this work, bound-free electron-positron pair production with a correction term is considered to calculate the cross section for peripheral relativistic heavy ion collisions. The Dirac wave functions have been used for the leptons and first order corrections are included. It is seen that the results for the production cross sections are considerably smaller than those of the previous calculations. We applied the same method for the calculations of the antihydrogen production cross sections as well.

Experimental concepts of positron spectroscopy at HESR

In this paper we discuss the unique possibilities of studying atomic electron–positron pair production processes in high-resolution experiments at the high-energy storage ring (HESR) at the future FAIR accelerator complex. The two pair-production processes bound-free pair production and negative-continuum dielectronic recombination are contrasted. An overview of previous studies is given and requirements for experimental parameters of a magnetic forward-angle positron spectrometer at the HESR are depicted.

Heavy ion beam loss mechanisms at an electron-ion collider

There are currently several proposals to build a high-luminosity electron-ion collider, to study the spin structure of matter and measure parton densities in heavy nuclei, and to search for gluon saturation and new phenomena like the colored glass condensate. These measurements require operation with heavy-nuclei. We calculate the cross-sections for two important processes that will affect accelerator and detector operations: bound-free pair production, and Coulomb excitation of the nuclei. Both of these reactions have large cross-sections, 28-56 mb, which can lead to beam ion losses, produce beams of particles with altered charge:mass ratio, and produce a large flux of neutrons in zero degree calorimeters. The loss of beam particles limits the sustainable electron-ion luminosity to levels of several times $10^{32}/$cm$^2$/s.

Physics of Ultra-Peripheral Collisions at RHIC and LHC

The creation of electron-positron pairs from the vacuum by strong electromagnetic fields is a fundamental and one of the most fascinating processes of relativistic quantum mechanics. Fields sufficiently strong to induce pair creation can be produced, for example, in collisions of bare ions. The process has been studied intensively both in theory and experiment since the 1980s. In peripheral relativistic heavy-ion collisions electromagnetic fields are very strong and interact with each other for a very short time. In these strong fields, probabilities of various electromagnetic processes such as free pair production, bound-free pair production and excitation are large and increase with the Lorentz factor γ. Measurement of cross sections and other relevant distributions are very important for understanding the strong field effect.

Measurement of the total cross section of uranium-uranium collisions atsNN=192.8GeV

Heavy ion cross sections totaling several hundred barns have been calculated previously for the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC). These total cross sections are more than an order of magnitude larger than the geometric ion-ion cross sections primarily due to Bound-Free Pair Production (BFPP) and Electro-Magnetic Dissociation (EMD). Apart from a general interest in verifying the calculations experimentally, an accurate prediction of the losses created in the heavy ion collisions is of practical interest for the LHC, where some collision products are lost in cryogenically cooled magnets and have the potential to quench these magnets. In the 2012 RHIC run uranium ions collided with each other at $\sqrt{s_{NN}} = 192.8$ GeV with nearly all beam losses due to collisions. This allows for the measurement of the total cross section, which agrees with the calculated cross section within the experimental error.

Angular distribution of high-energye+e−photoproduction close to the end of the spectrum at large momentum transfer

We consider the differential cross section of electron-positron pair production by a high-energy photon in a strong Coulomb field close to the end of the electron or positron spectrum. When the momentum transfer largely exceeds the electron mass, the cross section is obtained analytically in a compact form. Coulomb corrections essentially modify the cross section even for moderate values of the nuclear charge number $Z$. In the same kinematical region, the angular distribution for bound-free pair production, bremsstrahlung, and photorecombination is also obtained.

Bound–free pair production in ultra–relativistic ion collisions at the LHC collider: analytic approach to the total and differential cross sections

A theoretical investigation of the bound-free electron-positron pair production in relativistic heavy ion collisions is presented. Special attention is paid to the positrons emitted under large angles with respect to the beam direction. The measurement of these positrons in coincidence with the down--charged ions is in principle feasible by LHC experiments. In order to provide reliable estimates for such measurements, we employ the equivalent photon approximation together with the Sauter approach and derive simple analytic expressions for the differential pair--production cross section, which compare favorably to the results of available numerical calculations. Based on the analytic expressions, detailed calculations are performed for collisions of bare Pb$^{82+}$ ions, taking typical experimental conditions of the LHC experiments into account. We find that the expected count rate strongly depends on the experimental parameters and may be significantly enhanced by increasing the positron-detector acceptance cone.

Interference effects in bound-free pair production in relativistic collisions of nuclei with molecules

We consider bound-free electron-positron pair production in extreme relativistic collisions of a highly charged nucleus and a molecule. We show that, in such collisions, pronounced interference effects can arise in the emission pattern of the created positrons that are caused by the coherent interaction of the lepton field with the nuclei of the molecule.

Bound–bound pair production in relativistic collisions

Electron–positron pair production is considered in the relativistic collision of a nucleus and an anti-nucleus, in which both leptons are created in bound states of the corresponding nucleus–lepton system. Compared to free and bound-free pair production, this process is shown to display a qualitatively different dependency both on the impact energy and on the charges of the colliding particles. Interestingly, at high impact energies the cross section for this process is found to be larger (due to a bigger statistical weight) than that for the analogous atomic process of non-radiative electron capture, although the latter does not involve the creation of new particles.

Simultaneous electron capture and bound-free pair production in relativistic collisions of heavy nuclei with atoms

We consider a two-step process that may occur in relativistic ion-atom collisions. In this process, within a single ion-atom collision, an atomic electron is captured by the ion via the radiative and/or nonradiative capture channels, while the colliding nuclei of the atom and ion simultaneously produce an electron-positron pair with the electron created in a bound state of the ion. As a result, the ion picks up two electrons and a positron is emitted. We calculate the total cross section for this process in collisions of incident $1--4\text{ }\text{GeV}/\text{u}$ ${\text{U}}^{92+}$ projectiles with various atomic targets and discuss its dependence on the collision energy and target atomic number. We also analyze whether this process can compete with electron recombination proceeding via the excitation of the negative-continuum states, in which an atomic electron incident on a bare nucleus is captured into a bound state of the ion by transferring the energy excess to a negative-continuum electron with the latter undergoing a transition into a bound state. According to our estimates, the electron recombination dominates the two-step process in collisions with light and intermediately heavy atoms $({Z}_{a}\ensuremath{\lesssim}50)$, while in collisions with heavy atoms the latter leads to much larger cross sections.

Bound-freee+e−pair creation with a linearly polarized laser field and a nuclear field

The process of bound-free pair production of electrons and positrons in combined laser and Coulomb fields is investigated. It is assumed that an ion collides at relativistic speed with an intense x-ray laser beam of linear polarization. The process proceeds nonlinearly due to simultaneous absorption of a few laser photons. The capture of the electron into the ground state and the $L$ shell is considered. The scaling of the total rate, the angular distributions of the emitted positrons, and a comparison to the competing free-free channel are surveyed. Numerical results of pair production rates for parameters for the planned x-ray free-electron lasers at DESY and SLAC are presented. We find that pair production with these laser facilities can become observable in the near future.