A closer look at the beam-beam processes at ILC and CLIC

The strength of the electromagnetic fields in the bunch collision at a linear collider will have a significant effect, yielding large numbers of beamstrahlung photons and associated coherent pair production. These effects are limited in the proposed ILC beam parameters which limit the strength of the bunch field to $\Upsilon_{\text{ave}}=0.27$. The CLIC 3 Tev design by comparison has a $\Upsilon_{\text{ave}}=3.34$ yielding huge number of coherent pairs. In terms of the precision physics programs of these proposed colliders there is an imperative to investigate the effect of the strong bunch fields on higher order processes. From the exact wavefunctions used in the calculation of transition rates within the Furry interaction picture, and using appropriate simplifications, a multiplicative factor to the coupling constants was obtained. This indicates a significant variation to the transition rate near threshold energies. Further studies are in progress to calculate the exact effect on expected observables.Comment: 7 pages, 3 figures, LCWS11 (Granada) Proceedings, v

Furry picture transition rates in the intense fields at a lepton collider interaction point

The effect on particle physics processes by intense electromagnetic fields in the charge bunch collisions at future lepton colliders is considered. Since the charge bunch fields are tied to massive sources (the $e^{+}e^{-}$ charges), a reference frame is chosen in which the fields appear to be co-propagating. Solutions of the Dirac equation minimally coupled to the electromagnetic fields reasonably associated with two intense overlapping charge bunches are obtained and found to be a Volkov solution with respect to a null 4-vector whose 3-vector part lies in the common propagation direction. These solutions are used within the Furry interaction picture to calculate the beamstrahlung transition rate for electron radiation due to interaction with the electromagnetic fields of two colliding charge bunches. New analytic expressions are obtained and compared numerically with the beamstrahlung in the electromagnetic field of one charge bunch. The techniques developed will be applied to other collider physics processes in due course.Comment: 7 pages, 5 figures, Phys Lett B preprin

Strong field QED in lepton colliders and electron/laser interactions

Studies of strong field particle physics processes in electron/laser interactions and lepton collider interaction points are reviewed. These processes are defined by the high intensity of the electromagnetic fields involved and the need to take them into account as fully as possible. The main theoretical framework considered is the Furry picture. In this framework, the influence of a background electromagnetic field in the Lagrangian is calculated non perturbatively, involving exact solutions for quantised charged particles in the background field. These "dressed" particles go on to interact perturbatively with other particles. The background field starts to polarise the vacuum, in effect rendering it a dispersive medium. Particles encountering this dispersive vacuum obtain a lifetime, either radiating or decaying into pair particles at a rate dependent on the intensity of the background field. In fact, the intensity of the background field enters into the coupling constant of the strong field QED Lagrangian, influencing all particle processes. A number of new phenomena occur. Particles gain an intensity dependent rest mass shift that accounts for their presence in the dispersive vacuum. Multi photon events involving more than one external field photon occur at each vertex. Higher order processes which exchange a virtual strong field particle, resonate via the lifetimes of the unstable strong field states. Two main arenas of strong field physics are reviewed; those occurring in relativistic electron interactions with intense laser beams, and those occurring in the beam beam physics at the interaction point of colliders. This review outlines the theory, describes its significant novel phenomenology and details the experimental schema required to detect strong field effects and the simulation programs required to model them.Comment: Review article, 56 pages, 29 figures. Version 2 has corrected errata, 1 new reference, 5 updated figure

Incoherent pair background processes with full polarizations at the ILC

Incoherent background pair production processes are studied with respect to full polarizations of all states. Real initial photon polarizations are obtained via a QED calculation of the beamstrahlung process. Virtual photon polarizations are related to the electric field of the colliding bunches at the point of pair production. An explicit expression for the virtual photon polarization vector is developed and found to have no circular polarization component. Pair polarization states are highly dependent on initial state circular polarization and are consequently produced almost unpolarized. The Breit-Wheeler cross-section with full polarizations is calculated and coded into the CAIN pair generator program. Numerical evaluations of the ILC, operating with the seven proposed collider parameter sets, shows that there are 10 - 20% less low energy pairs than previously thought. Collider luminosity, as calculated by CAIN, remains the same

Methods for evaluating physical processes in strong external fields at e+e- colliders: Furry picture and quasi-classical approach

Future linear colliders designs, ILC and CLIC, are expected to be powerful machines for the discovery of Physics Beyond the Standard Model and subsequent precision studies. However, due to the intense beams (high luminosity, high energy), strong electromagnetic fields occur in the beam-beam interaction region. In the context of precision high energy physics, the presence of such strong fields may yield sensitive corrections to the observed electron-positron processes. The Furry picture of quantum states gives a conceptually simple tool to treat physics processes in an external field. A generalization of the quasi-classical operator method (QOM) as an approximation is considered too.Comment: 14 pages, 6 figures, to appear in the conference proceedings of the Corfu Summer Institute 201

The Stimulated Breit-Wheeler Process as a Source of Background e+e- Pairs at the ILC

Passage of beamstrahlung photons through the bunch fields at the interaction point of the ILC determines background pair production. The number of background pairs per bunch crossing due to the Breit-Wheeler, Bethe-Heitler and Landau-Lifshitz processes is well-known. However, the Breit-Wheeler process also takes place in and is modified by the bunch fields. A full QED calculation of this stimulated Breit-Wheeler process reveals cross-section resonances due to the virtual particle reaching the mass shell. The one-loop electron self-energy in the bunch field is also calculated and included as a radiative correction. The bunch field is considered to be a constant crossed electromagnetic field with associated bunch field photons. Resonance is found to occur whenever the energy of contributed bunch field photons is equal to the beamstrahlung photon energy. The stimulated Breit-Wheeler cross-section exceeds the ordinary Breit-Wheeler cross-section by several orders of magnitude and a significantly different pair background may result

Measuring the Boiling Point of the Vacuum of Quantum Electrodynamics

It is a long-standing non-trivial prediction of quantum electrodynamics that its vacuum is unstable in the background of a static, spatially uniform electric field and, in principle, sparks with spontaneous emission of electron-positron pairs. However, an experimental verification of this prediction seems out of reach because a sizeable rate for spontaneous pair production requires an extraordinarily strong electric field strength $|\mathbf E|$ of order the Schwinger critical field, ${\rm E}_c=m_e^2/e\simeq 1.3\times 10^{18}\ {\rm V/m}$, where $m_e$ is the electron mass and $e$ is its charge. Here, we show that the measurement of the rate of pair production due to the decays of high-energy bremsstrahlung photons in a high-intensity laser field allows for the experimental determination of the Schwinger critical field and thus the boiling point of the vacuum of quantum electrodynamics.Comment: 8 pages, 6 figures; extended discussion; corrected typos; corrected figure 5 and new figure 6; conclusions unchange

Effects of a high-dose 24-h infusion of tranexamic acid on death and thromboembolic events in patients with acute gastrointestinal bleeding (HALT-IT): an international randomised, double-blind, placebo-controlled trial

Background: Tranexamic acid reduces surgical bleeding and reduces death due to bleeding in patients with trauma. Meta-analyses of small trials show that tranexamic acid might decrease deaths from gastrointestinal bleeding. We aimed to assess the effects of tranexamic acid in patients with gastrointestinal bleeding. Methods: We did an international, multicentre, randomised, placebo-controlled trial in 164 hospitals in 15 countries. Patients were enrolled if the responsible clinician was uncertain whether to use tranexamic acid, were aged above the minimum age considered an adult in their country (either aged 16 years and older or aged 18 years and older), and had significant (defined as at risk of bleeding to death) upper or lower gastrointestinal bleeding. Patients were randomly assigned by selection of a numbered treatment pack from a box containing eight packs that were identical apart from the pack number. Patients received either a loading dose of 1 g tranexamic acid, which was added to 100 mL infusion bag of 0·9% sodium chloride and infused by slow intravenous injection over 10 min, followed by a maintenance dose of 3 g tranexamic acid added to 1 L of any isotonic intravenous solution and infused at 125 mg/h for 24 h, or placebo (sodium chloride 0·9%). Patients, caregivers, and those assessing outcomes were masked to allocation. The primary outcome was death due to bleeding within 5 days of randomisation; analysis excluded patients who received neither dose of the allocated treatment and those for whom outcome data on death were unavailable. This trial was registered with Current Controlled Trials, ISRCTN11225767, and ClinicalTrials.gov, NCT01658124. Findings: Between July 4, 2013, and June 21, 2019, we randomly allocated 12 009 patients to receive tranexamic acid (5994, 49·9%) or matching placebo (6015, 50·1%), of whom 11 952 (99·5%) received the first dose of the allocated treatment. Death due to bleeding within 5 days of randomisation occurred in 222 (4%) of 5956 patients in the tranexamic acid group and in 226 (4%) of 5981 patients in the placebo group (risk ratio [RR] 0·99, 95% CI 0·82–1·18). Arterial thromboembolic events (myocardial infarction or stroke) were similar in the tranexamic acid group and placebo group (42 [0·7%] of 5952 vs 46 [0·8%] of 5977; 0·92; 0·60 to 1·39). Venous thromboembolic events (deep vein thrombosis or pulmonary embolism) were higher in tranexamic acid group than in the placebo group (48 [0·8%] of 5952 vs 26 [0·4%] of 5977; RR 1·85; 95% CI 1·15 to 2·98). Interpretation: We found that tranexamic acid did not reduce death from gastrointestinal bleeding. On the basis of our results, tranexamic acid should not be used for the treatment of gastrointestinal bleeding outside the context of a randomised trial