A method to measure vacuum birefringence at FCC-ee

It is well-known that the Heisenberg-Euler-Schwinger effective Lagrangian predicts that a vacuum with a strong static electromagnetic field turns birefringent. We propose a scheme that can be implemented at the planned FCC-ee, to measure the nonlinear effect of vacuum birefringence in electrodynamics arising from QED corrections. Our scheme employs a pulsed laser to create Compton backscattered photons off a high energy electron beam, with the FCC-ee as a particularly interesting example. These photons will pass through a strong static magnetic field, which changes the state of polarization of the radiation - an effect proportional to the photon energy. This change will be measured by the use of an aligned single-crystal, where a large difference in the pair production cross-sections can be achieved. In the proposed experimental setup the birefringence effect gives rise to a difference in the number of pairs created in the analyzing crystal, stemming from the fact that the initial laser light has a varying state of polarization, achieved with a rotating quarter wave plate. Evidence for the vacuum birefringent effect will be seen as a distinct peak in the Fourier transform spectrum of the pair-production rate signal. This tell-tale signal can be significantly above background with only few hours of measurement, in particular at high energies.Comment: Presented by UIU at the International Symposium on "New Horizons in Fundamental Physics: From Neutrons Nuclei via Superheavy Elements and Supercritical Fields to Neutron Stars and Cosmic Rays," held to honor Walter Greiner on his 80th birthday at Makutsi Safari Farm, South Africa, November 23-29, 201

Detector tests and readiness for 2014, CERN NA63

As discussed in our previous report to the SPSC (CERN-SPSC-2013-025/SPSC-SR-122), in week 47 of 2013 we planned to conduct a test of our 4 mimosa detectors to verify that they are operational, that tracking algorithms function and that their efficiency is sufficient to conduct the experiment proposed on positron production from single diamond crystals. That test was performed, our detectors function as desired and we are thus ready to install the equipment in SPS H4 to do the proposed positron production experiment

Status for 2017, CERN NA63

In the NA63 experiment of May-June 2017 the purpose was to look for the effect of classical radiation reaction as described in \cite{DiPiazza20171} but for 50 GeV positrons aligned to the (110) plane of silicon crystals of varying thicknesses in the range 1-6 mm. The data which was taken during the run shows promising results, but the final verdict awaits accurate calculations of the theoretically expected spectra, and a final analysis. For the 2016 results on quantum radiation reaction obtained by CERN NA63, we have submitted a manuscript, which is presently under refereeing \cite{Wist17}

Technical Design Report, CERN NA63

We summarize the status and plans for the 2015 run for the CERN NA63 collaboration. First, we review the motivation for our proposed experiments with the scheduled Ar18+ beam of Feb/Mar 2015. We also show some slightly updated versions of our calculations that better reflect the experimental situation. Then we turn to a detailed description of the detector setup, focusing on the deployment of the MIMOSA detectors

Status and plans for 2015, CERN NA63

We summarize the status and plans for the future for the CERN NA63 collaboration. A systematic study of the structured target 'resonance' appearing from radiation emission by electrons passing two amorphous foils positioned with separations in the range $10-20000~\mu$m was performed in our latest run. The recently published results \cite{Ande14} confirm a previously obtained result \cite{Ande12a} that by this method, the formation length - of macroscopic dimensions up to 0.5 mm - for the generation of MeV-GeV radiation from multi-hundred GeV electrons can be \emph{directly} measured. In fact the results obtained now allow a distinction between competing theories \cite{Blan97a,Baie99b}, showing that it is unlikely that the correction-term introduced by Blankenbecler holds true \cite{Blan97b}. Our investigations of the LPM effect in low-$Z$ targets have been completed and published \cite{Ande13}. The overall conclusion for these studies is that the formalism of Migdal is sufficient to describe the experimental data - for any $Z$ - within the statistical uncertainty. Preparing for beam time in 2015, we have refined and updated our theoretical analyses of bremsstrahlung and delta-electron emission from heavy, and medium-heavy, ultrarelativistic nuclei. We can thus conclude that - although not quite as interesting as for Pb projectiles - projectiles of Ar and/or Xe are suitable for the investigation, and for establishing the method to eventually measure the charge distribution of short-lived ($ct\gtrsim3$m) fragments \cite{Mikk14}. Furthermore, we have prepared a new simulation code to determine the theoretically expected number of positrons produced by electrons traversing an aligned diamond target. This code, in a previous version, compares very well to radiation emission observed from sub-GeV electrons passing a crystalline undulator \cite{Wist14} - previously also the subject of investigations by CERN NA63

Status and plans for 2016, CERN NA63

In the NA63 experiment of May 2015 the purpose was primarily to measure the photon spectrum of channeled electrons in diamond with relevance to using diamond as a pair production target for future linear colliders and secondarily to look for the effect of radiation reaction as described in [Di Piazza et al.(2015)Di Piazza, Wistisen, and Uggerhøj]. The setup was designed to be able to be efficient from photon energies as low as ~40MeV. This was estimated based on requiring the energy of a deflected particle in the magnet to stay within the final detector ’M6’, see figure 1, i.e. a deflection of ~1cm at ’M6’ should correspond to 40 MeV. This was, however, not achieved in the experiment. A large drop off of efficiency is seen already at 1.5GeV as seen in figure 4. We have since investigated the cause of this and made a detailed simulation of the whole setup. The most significant effect for the large difference in the expected and achieved lower energy cut off is due to the fact that Bethe-Heitler pair production tends to give one of the produced particles a large fraction of the energy and the subsequently large multiple Coulomb scattering of the low energy particle of the produced pair within the ’Converter foil’ (see figure 1) is so large that it escapes the detectors ’M3’ to ’M6’. This can be partly alleviated by making a more compact setup, i.e. moving the last 4 MIMOSA detectors closer together and possibly acquiring a smaller magnet. However, due to the small size of the MIMOSA detectors it becomes difficult to measure energies below 1GeV. This is not the relevant energy regime for the investigation of a pair production scheme and we therefore wish to focus on the investigation of classical radiation reaction for the run in the summer of 2016. The 100GeV electron data which was taken during the 2015 run shows very promising results in this regard and a redesigned experiment based on what we learned during the 2015 run are expected to give results which can test the Landau Lifshitz equation of classical radiation reaction [Landau(2013)]

Status for 2016 and plans for 2017, CERN NA63

In the NA63 experiment of May 2016 the purpose was to look for the effect of radiation reaction as described in \cite{DiPiazza20171} but for positrons. The 180 GeV positron data which was taken during the run shows promising results, despite the fact that essentially half the useful time for pure data-taking, 5 days, was lost due to an incident in the PS. We are in the process of finalizing a manuscript to submitted, with the aim of publishing these results on quantum radiation reaction. For the next beam-time, in 2017, we will look at a different parameter range to enter the regime of classical radiation reaction instead of quantum radiation reaction which was seen in the 2016 experiment

Status and plans for 2014, CERN NA63

We summarize the status and plans for the future for the CERN NA63 collaboration.\n\nA systematic study of the structured target 'resonance' appearing from radiation emission by electrons passing two amorphous foils positioned with separations in the range $10-20000~\mu$m was performed in September 2012. The results - recently submitted for publication - confirm a previously obtained result \cite{Ande12a} that by this method, the formation length - of macroscopic dimensions up to 0.5 mm - for the generation of MeV-GeV radiation from multi-hundred GeV electrons can be \emph{directly} measured. In fact the results obtained allow a distinction between competing theories \cite{Blan97a,Baie99b}, showing that it is unlikely that the correction-term introduced by Blankenbecler holds true \cite{Blan97b}.\n\nFurthermore, with a substantially improved setup compared to the run in 2010 (where the deconvolution of synchrotron radiation prevented results in the most interesting regime below 0.5 GeV), we investigated again the impact of the Landau-Pomeranchuk-Migdal (LPM) effect with 178 GeV electrons, in particular for low-$Z$ targets where a discrepancy between experiment and theory might turn up. Measurements with 20 GeV electrons in a Cu target shows no indication of the 'kink-like' structure seen in Migdal's theory (the most widely used) for photon energies around 300 MeV. The absence of this structure is in agreement with simulations, and is due to the 'smearing' of the effect from multi-photon emission. These results have been accepted for publication.\n\nA short test measurement of the efficiency of production for positrons originating from electrons impinging on an axially aligned diamond crystal was also performed, where the aim in the run proposed for 2014 is to measure the production angles and energies by means of so-called MIMOSA detectors arranged in a magnetic spectrometer configuration with a permanent-magnet-based magnetic dipole. For the 2012 run, however, the track-reconstruction algorithm yields too few events, most likely due to a too low efficiency of each detector. This was not realised at the time of the test, that nevertheless was useful in establishing the functionality of the rest of the setup as well as the alignment of the diamond $\langle100\rangle$ crystal.\\\n\nFor the future, we propose to measure the production angles and energies of positrons produced by 10-50 GeV electrons penetrating a diamond crystal along the $\langle100\rangle$ axis. Furthermore, we ask for beam time with ultra relativistic heavy ions, as soon as CERN is able to deliver unbunched beams of these, to investigate nuclear size effects in a number of emission processes

Status for 2018, CERN NA63

In the NA63 experiment of April 2018 the purpose was to look for the effect of the derivative term, the so-called Schott-term, in classical radiation reaction as described in \cite{DiPiazza20171}. Data was taken for 20, 40 and 80 GeV electrons and positrons aligned to the $\langle100\rangle$ axis of a diamond crystal of thickness 1.5 mm, as well as for 40 and 80 GeV electrons on a 1.0 mm thick diamond aligned to the $\langle100\rangle$ axis. The data which was taken during the run shows encouraging results, but await a thorough analysis. For the 2017 data, the analysis is still ongoing, but expected to be finished within the coming few months. The results look very promising, but no final conclusion can be drawn at the present stage. For the 2016 results on quantum radiation reaction obtained by CERN NA63, the results have been published in Nature Communications \cite{Wist17}

Status for 2019, CERN NA63

In the NA63 experiment of April 2018 data was taken for 20, 40 and 80 GeV electrons and positrons aligned to the h100i axis of a diamond crystal of thickness 1.5 mm, as well as for 40 and 80 GeV electrons on a 1.0 mm thick diamond aligned to the h100i axis. Off-line analysis has shown that the direction of the crystallographic axis cannot be found with certainty for the 2018 data for 20 GeV, and thus this data set has been discarded. However, the comparison between data and theoretical simulations performed for several angular settings for the 40 and 80 GeV data show remarkable agreement with calculations including the Landau Lifshitz force with a few quantum corrections. A draft of a paper for submission will be finished within the coming month. For the 2017 data, the analysis is finished, shows good results, and a draft of a paper for submission will be finished within the coming month