Conclusions & Outlook on the CMBL setup, measurements and simulation

The Mu3e experiment plans for a rst engineering run in 2019. In order to reach the nal sensitivity, a staged approach is foreseen with dierent detector setups requiring a stepwise increase in muon beam rates. The initial phase I using the CMBL will be carried out in theπE5 area at PSI. The shared use together with the MEG II experiment necessiated a challenging beam line design with the main constraints imposed by the limited available space and the requirement for the highest intensity available.

A solution to t into the front part of theπE5 area, while still having a high∼8·10⁷ µ⁺/s rate at the injection to the Mu3e solenoid was found in the presented CMBL design and rst commissionig runs have proved very succesful. The CMBL allows for a fully indepen-dent operation of the front and the rear part of the experimental area, which means that work on the MEG II setup is not compromised by Mu3e beam times. With the current CMBL design all beam line elements can in principle stay in place and only the ASL mag-net has to be replaced by the BTS when switching between the Mu3e and the MEG II setups. Furthermore, an economic solution concerning the beam elements for the CMBL was found using spare elements, that had already been used in other experiments. As a result of the 2014/2015 beam time the vacuum chambers of the ASL and the ASK and the yoke of the latter dipole were modied to enhance the acceptance and transmission of the CMBL. The investigations that led to these modiactions were based on an accurate G4BL simulation of the last part of the beam line, which made use of realistic eld maps and a reconstruction of the transverse phase space that showed a good agreement with the 2014/2015 test beam setup. The 2016 test beam with the modied ASL and ASK vacuum chambers yielded a signicantly increased nal rate of∼7.8·10⁷ µ⁺/s for the inner collimator in place and∼8.4·10⁷ µ⁺/s with the inner collimator taken out, all measured at 2.2 mA proton current. However these rates are still less than what was expected from the previous simulation. This can partially be explained with the missing momentum corre-lation, which led to an underestimated beam spread from dispersion at the QSO doublet.

It is also possible that the actual optimum setting has not yet been found, since time constraints had an inuence on the schedule during the 2016 beam time.

The 250 mm diameter vacuum-pipe of the QSO doublet represents the smallest diameter aperture that follows the longest distance without active focussing in the entireπE5 beam line. A possible replacement with the initially planned QSN doublet, that has a larger cross-shaped vacuum chamber is not feasible in the current setup due to transverse spatial constraints imposed by the concrete shielding walls.

The beam contamination, that was observed in the 2016 test beam requires further inves-tigation and can be improved either via a modied beam line tune or a dierent collimator setup. The beam tune for the CMBL however requires comparable Triplet II values to minimize the beam losses at the injection to the QSO doublet. Therefore the available data has to be further analyzed and a new collimator position further DS determined for Mu3e. The proposed new collimator should in addition not be round, but rather have an asymmetric design to cut vertically and to be less restrictive in the horizontal direction.

The nal rate can therefore be expected to lie between the rates that were measured in 2016 with the collimator in and out.

The simulation of the Mu3e beam line together with the Mu3e solenoid showed a maxi-mum transmission to the 19 mm stopping target of 68 %. The normalization of all quoted rates to 2.2 mA reects the status of the proton current during most of the beam times.

However, already in 2016 the PSI HIPA accelerator complex recieved the permission to operate at 2.4 mA with possible further increased intensities foreseen in the near future [71], which will directly increase the muon rates. Furthermore, for all measurements, that were presented, the muon production target had a length of 40 mm. However most recent measurements in 2017 using a former production target with a length of 60 mm, show an enhanced rate at the intermediate focus position of ∼30 %. Therefore surface muon rates at the centre of the Mu3e solenoid close to 10⁸ µ⁺/s can be expected for the run using the 60 mm long TgE.

The Mu3e innermost pixel detector requires shielding from muons that do not hit the cen-tre of the target. Figure 2.93(a) can be used to evaluate a suitable collimator position, by studying the dierence in envelopes representing the entire beam and the subset of muons that end on the Mu3e target.

Although initially planned solely for the Mu3e experiment, the CMBL has already been used by several experiments:

ˆ the AlCap Collaboration, a combined Mu2e/Comet test experiment to study muon capture for µ→econversion experiments [72]

ˆ Hyperne splitting inµH andµ³He⁺ [73]

ˆ MuX, measuring the charge radius of Radium [74]

Once the beam positron background situation has been solved with a new collimator design the CMBL will provide an excellent environment for those experiments that require a low background, benetting from the two bending magnets and far distance to the collimator, which reduces the Michel positron background from muons decaying in ight andγs from particle stops in the collimator.

In conclusion it can be stated, that the CMBL successfully passed its rst commissioning tests and the beam could be characterized at three distinct measurement locations, which will also be benetial to futureπE5 users. Several issues concerning the beam line could be identied and improvements applied. In the future the CMBL oers a high rate surface muon beam, with a potential low background from Michels and γs, to Mu3e, enabling a three orders of magnituded more sensitive measurement than the current experimental limit, and enabling the possibilities for other experiments at PSI.

Furthermore, the simulation studies turned out to be a powerful tool showing good agreement with the 2014/2015 test beam results. The long version of the full beam line starting from TgE promises to signicantly contribute to a deeper understanding of the complex πE5 beam line and show potential ways to enhance the beam rates for all experiments inπE5.

3. The MEG II Scintillation Target

The general requirements for the MEG target are still valid for the MEG II target and are:

ˆ High muon stopping eciency

ˆ Minimization of annihilation-in-ight (AIF) or Radiative Muon Decay (RMD) photon production in the target

ˆ Minimization of Multiple Scattering and Bremsstrahlung production from the e⁺ leaving the target after muon decay

ˆ Consideration of the e⁺ angular asymmetry due to the muon polarization

ˆ Allow thee⁺ decay vertex reconstruction and e⁺ direction at the target plane to be determined

ˆ Dimensionally stable and remotely movable target

To satisfy these requirements for MEG a thin, low-Z, low density, elliptical layered struc-ture of Polyethylene (PE) and Polyester (PET) was chosen as a target and placed at∼20.5°

to the beam-axis. In MEG II these essential features are also required, however, based on the upgrade performance of the detectors more stringent requirements must be met con-cerning the stopping rate, positron multiple scattering and photon background production, leading to a new optimized target design.

A study of the optimal beam momentum and target characteristics was undertaken as outlined in the MEG upgrade proposal [4]. The baseline solution chosen was the combina-tion of a surface muon beam of 28 MeV/c together with a 140µm thick PE (CH2) target placed at 15° to the beam-axis, rather than a sub-surface muon beam of 25 MeV/c. This reduction in thickness and angle for example reduces the multiple scattering in the MEG target by a factor 1.5.

Further candidate targets have since been studied and are summarized in table 3.1. Each target has an equivalent thickness to the baseline solution of a 140 µm of CH₂, so achie-ving an equivalent stopping eciency. Apparent from table 3.1 is that all candidate targets have similar properties with the multiple scattering estimates varying less than 10 % from the average. The equivalent thickness in radiation lengths however does vary by as much as 25 % from the average, with dierent materials outperforming each other in dierent categories. Beryllium shows an overall good performance though from the thickness & size required as well as the safety aspect is not so favoured. Diamond which is mechanically stable and known to be more radiation resistant as well as having scintillation properties has however, the second largest radiation length equivalent thickness and is currently not commercially available in the size required for MEG II. The scintillation target (Scint.

PVT) shows mid-range performance characteristics though with the substantial benet of online beam monitoring capabilities comprising of full information about the beam prole as well as the relative intensity at the centre of the COBRA solenoid. The most promising

Table 3.1.: The table lists the possible target candidates that were considered for MEG II and their properties based on calculations. All candidate targets have an 15° inclination angle and are preceeded by a 350µm Mylar degrader. The data was provided by [75, 76]

.

candidate for a scintillation target is BC400 from Saint-Gobain [77], which is only available with thicknesses down to 150µmin a sucent size for MEG II. A scintillation target made from such a thin slab of BC400 was tested during two beam times - at an intermediate focus position of the πE5 beamline at the end of 2015 and used as a stopping target in COBRA during the Pre-Engineering Run in 2016.