Kaon beams and experiments at CERN
9 DECEMBER, 2023 | By Lau Gatignon
The recent history of kaon physics experiments at the SPS started in 1980 in the office of the later Nobel Prize winner Jack Steinberger with a small team of physicists including our Niels Doble, Heiner Wahl, Don Cundy and Italo Mannelli. They discussed the design of the NA31 experiment that aimed to discover the existence of the so-called Direct CP Violation. CP is the product of the operators C and P, where C is replacing all charge quantum numbers (electric charge, strangeness, etc.) by their opposite and P inverts all space coordinates (i.e. looking in a mirror). The combination CP of these two operators was believed to conserve symmetry, but in 1964 Cronin, Fitch, Christenson and Turlay discovered that CP was not conserved in the weak interaction and for that the first three got the Nobel prize in 1980.
Neutral kaons come in two variants: KS and KL. In case CP is conserved, the short-lived neutral kaon, called KS, must always decay into two pions and the 600 times longer lived KL into 3 or more particles. Cronin et al discovered that in a few permille of the cases KL also decay into two pions. The neutral strange particles directly produced in the strong interaction, are not KS and KL but K0 and K0. The different decay channels and lifetimes of KS and KL were initially explained by symmetric and anti-symmetric mixing of K0 and K0 itself, the so-called Direct CP violation. In fact this then predicts a difference in the decay rates of particles and antiparticles! The mechanism of Direct CP violation could therefore contribute to the matter-anti-matter asymmetry in the uni-verse (knowing of course that ordinary matter is not made of kaons but of protons, neutrons and electrons). In the neutral kaon system.
Direct CP violation in the K0 system manifests itself by a difference from 1 of the double ratio R of KL decaying into either two charged or two neutral pions and KS decaying into the same final states. Theory says that R = 1 – 6ε’/ε, where e is the parameter describing CP violation through mixing and ε’ the one describing Direct CP violation; ε is as small as 2.228 10-3. The experiments obviously need both a KL beam and a KS beam and the detectors must count the decay rates into two pions with extreme precision.
In KS, almost all decays are into two pions and there is therefore almost no background from other modes, but the KL decays are mostly (more than 99.7%) into other decay modes that may mimic pp decay, e.g. by missing a decay particle. The experiments need relatively high beam intensities to collect sufficient statistics and therefore particles produced by interactions of different protons in the target may sometimes look like decay products of a single kaon. This pile-up is one of the major systematic backgrounds in all kaon experiments. Obviously, the detectors and the beam lines must work together in perfect coordination, hence our very close involvement in these experiments. Another complication is that at the typical kaon energies used (about 100 GeV) the average KS decay length is about 6 m and the average KL decay length 3 km. Therefore, the longitudinal distribution of the decay points (vertices) is very different for KS and KL.
NA31 solved the latter problem by putting the KS target, together with the last MBN beam magnet and another MBN sweeping magnet on a train. The train ‘served’ 41 station positions separated by 1.2 metres (it took about a minute for the trip from one station to the next and was therefore called XTGV). The resulting KS decay vertex distribution was a saw-tooth distribution close enough to the almost flat KL decay distribution. This, together with the decision not to use a spectrometer magnet for the charged decays, essentially eliminated the need of Monte Carlo simulations. The experiment design was very elegant and largely driven by the beam design. A technical complication was the fact that the train had to run inside the big blue vacuum tank. Inside vacuum the conductivity of air has a maximum around 10-1 mbar and this complicates the functioning of the magnets and the Fiscs before the target.
Therefore, a good vacuum was required in spite of the outgassing of the magnets. The experiment, including the 100 m long blue vacuum tank, was located in the H4 beam in EHN1. The experiment was a success and published the first observation of Direct CP violation with 3σ precision (still considered sufficient at that time): ε’/ε = (2.0 ± 0.7) 10-3. Unfortunately, the E731 experiment in Fermilab found a result consistent with zero and both experiments decided to go for even more precise measurements. By the way, in that period also 5s started to become the norm required to claim discovery.
At CERN the next experiment was NA48. It included several lessons learnt from NA31. Pileup turned out to be the largest systematic uncertainty in NA31 and therefore the KS and KL beams had this time to operate simultaneously. For the detector there should be no bias between the acceptance for the charged and neutral decays and the beam lines has to be as collinear as possible.
Nevertheless, one had to be able to attribute a given pp decay to either the KS or the KL beam. The beam rates had to be higher to reach 5σ and the blue tube therefore had to move from H4 to ECN3. The beam design involved one of the most exciting studies we did over all these years: the use of a bent crystal. This was put just downstream of the KL target and its sweeping magnet and recuperated the required small fraction (5 10-5) of the non-interacting protons. Those were subsequently transported along the KL beam axis and finally deflected onto a KS production target 120 metres downstream and laterally only 72 mm away from the KL beam axis. From there the two neutral beam lines converged gently with a very small angle, 0.6 mrad, to the NA48 detectors, that now included a magnetic spectrometer, MNP33.
A bit downstream of the bent crystal, the protons were tagged by a small scintillating hodoscope and associated with the event in the main detectors via time-of-flight. The different decay vertex distributions were corrected for by lifetime-weighting. The experiment worked very well and the final result reached a precision of almost 7σ: (14.7 ± 2.2) 10-4, in nice agreement with the NA31 result and excluding the E731 result. In parallel the new Fermilab experiment, KTEV, got now a result nicely compatible with NA48 and with similar precision. These are still the best measurements of ε’/ε up to now. Often people thought the experiment would only publish this one number. This turned out to be completely wrong: NA48 published tens of additional papers on many rare decay channels. The experiment also developed very innovative detector technologies, the most impressive one being the liquid krypton calorimeter. The KL beam design has served as basis for the HIKE phase 2 beam design, proposed last autumn.
When NA48 came to an end, the collaboration wanted to study possible CP violation in charged kaon decays. This was done by comparing the decay distributions (the so-called Dalitz plots) in K+ and K– decays into three charged pions.
Encouraged by the success of the simultaneous beams concept, simultaneous K+ and K– beams were designed and quickly installed in the 2002-2003 YETS. The somewhat innovative idea was to put an achromat of 4 dipoles following the T10 production target, the first two of which provided opposite vertical offsets for K+ and K– at the TAX which would be compensated by the last two dipoles of the achromat. The TAX had holes that allowed to select either one or both beams. There was only very weak focusing with four weak quadrupoles from the T10 target to experiment that introduced no difference between positive and negative beams. Note that a similar concept is now under consideration for the NUTAG experiment. The detector was the same as for the original NA48 experiment, apart from the addition of innovative small TPC-like trackers, KABES, with minimal material budget. The experiment provided a measurement of unprecedented precision, but no evidence for CP violation within the precision achieved. The last paper of NA48/2 is actually in preparation just now!
During the running of NA48/2, the proposal for NA62 was being prepared and submitted in June 2005. The experiment was approved by the Research Board only at the end of 2008. While waiting, the collaboration used the existing beam (but with K+ only) to measure precisely the ratio between the K+ → e+ν and K+ → μ+n branching ratios. NA48/3 provided again the most precise measurement so far, in agreement with the Standard Model. At the same time, it was an excellent preparation for NA62, with only one charged track in the final state.
Following the approval of NA62, the construction of the new higher-intensity beam line and of the new experiment started. NA62 aimed to measure the branching ration of the very rare decay K+ → π+νν to 10% precision. The measurement is excessively difficult, as the neutrinos cannot be detected. The SM branching ratio is 8 10-11 and this implies that the experiment must be very certain that they have not overlooked any other particles than neutrinos at this level. Indeed, they had to slightly adapt their goal and they now expect to reach about 15% by LS3. Probably you are already quite aware of NA62, as it has been very actively running in ECN3 for the last decade. However, and maybe less well known, similarly to NA48, the experiment has published numerous other important and often cited papers on topics, such as lepton number violation, lepton flavour violation and even dark matter searches.
In parallel to running the beam line for NA62 physics data-taking, the P42 beam serves as a test bed in preparation for the much higher intensity beams expected after LS3. This very important kaon research line has made great contributions to kaon physics and even to particle physics as a whole. It is complementary to CP-violation physics in the B-sector with LHCb and Belle-II and in synergy the combination with their results allows to better refine the knowledge of the Standard Model.
All kaon experiments rely on a close interplay between the detectors and the beam line. Therefore, they always have been a fantastic playground for our beam physicists with lots of difficult challenges and innovative approaches.
But also the technical teams in the group could regularly demonstrate their expertise. A few telling examples are the XTGV train in vacuum in NA31, as well as the thin 2 m diameter Kevlar windows separating the decay volume from a Helium tank at 1 bar for NA31 and NA48, the bent crystal technology and its precise goniometer and the delicate KS collimator for NA48, a new beam dump 24 m underground behind ECN3 and recently the hydrogen Cedar for NA62. But there are too many to all be listed here.