CEDAR's system upgrade, some ideas Group Illinois-Torino Flavio Tosello – INFN-Torino April 4, 2014

Basic information (1)

Basic information (2) - h GeV/c mainz.de/jasinsk/ K π pbar

● Operation at high beam intensity ● CEDARs set on minority particles have been operated satisfactorily in beam with total intensity exceeding 5 x 10 7 part./s ● Such intensity are often achieved by sacrificing somewhat the beam parallelism so that the counting rate of the individual PM can be much larger than the rate of wanted particles. Even in these conditions the PMs do tolerate single rates of 10 7 counts/s. ● The efficiency depends strongly on the relative values of LD and beam divergence CEDAR design specs (1) (from CERN 82-13)

CEDAR design specs (2) (from CERN 82-13) ● Efficiency (section 5.2 of [1]) ● Counting efficiency depends on : status of optics and PMs, LD (Light Diaphragm) opening, gas pressure, bream momentum, beam divergence, CEDAR alignment, choice of coincidence level (6-, 7-, 8-fold) ● Each CEDAR is equipped with 8 PMTs. Typical numbers of photo-electrons (NPE) are between 2.5 and 3 for CEDAR-N → high PMT efficiency required → Past experience indicate that 6-fold coincidence has a rejection power good ● enough for practicallly all the applications so far ● The average efficiency drops when beam divergence is larger than angular acceptance, given by LD setting divided by the focal length. At very high energies, where one may have to close LD to 0.1mm, the acceptance is 0.1mm/3.88m = 26 μrad for a CEDAR-N. Keeping a beam parallel to this level requires a lot of attention. In the best case there will always be some tail in the divergence distribution for which the counting efficiency is lower → Till now, most of COMPASS data taking have been done with LD = 0.5 mm

CEDAR design specs (3) (from CERN 82-13)

● MC ● Photon propagation in CEDAR is calculated by optic matrix operations with the paraxial approach. The photon properties are represented by the distance to the optical line Y and the angle Θ to it. The propagation matrix depends on the temperature and the pressure of the helium gas as well as the wavelength of the Cherenkov photons. Temperature and pressure were assumed to be uniform and constant for a group of simulated particles. In principle one should calculate for each wavelength a propagation matrix. In order to reduce computing time, the sensitive range of PM was divided into steps of 1 nm. For each step one mean matrix was calculated taking the temperature in the detector into account. ● Only photons within the sensitive range of the PMs were simulated : [λmin, λmax] = [190 nm, 600 nm]. The probability for a wavelength follows the 1/λ^2 dependence. The quantum efficiency curve of the PMs was parametrized as a function of the wave length ● Only pions and kaons were considered in the simulation. The number of simulated pions equalled the number of kaons until results of both particle types were combined. There the correct ratio between π/K was taken into account as calculated in section 2.2. ● The track path was tilted according to a Gaussian shaped track divergence distribution. In addition each photon emission point was smeared due to multiple scattering effects in the gas. ● In addition an overall efficiency of the electronics was multiplied as a constant term with the PM quantum efficiency in order to simulate effects not taken into account separately as thresholds, resolutions and smearing. Prometeusz Jasinsky - PhD thesis – MC (1)

(diaphragm LD = 0.5mm) Including PM Q.E. Including cutoff λ>230nm (quartz windows) Including beam divergence Prometeusz Jasinsky - PhD thesis – MC (2)

Prometeusz Jasinsky - PhD thesis – MC (3)

MC performances in H2-line case and in M2-line case. In M2 case MC efficiency had to be reduced to compensate un-accounted effects (e.g. pileup, non uniform temperature field, CEDAR alignment ) Prometeusz Jasinsky - PhD thesis – MC (4)

Prometeusz Jasinsky - PhD thesis – Data analysis (1) Are our requirements (for DY) more stringent on purity or on efficiency ? Can we tag kaons with 8-fold majority ?

Prometeusz Jasinsky - PhD thesis – Data analysis (2)

Prometeusz Jasinsky - PhD thesis – Data analysis (3)

Data from the DCS database of the 2009 run Time [s] Temp.Diff. [°C] Temperature [°C] Temp.Diff. [°C] Temperature [°C] CEDAR-1T HEAD T MID T TAIL CEDAR-2T HEAD T MID T TAIL CEDAR-1 T HEAD -T MID T TAIL -T MID CEDAR-2 T HEAD -T MID T TAIL -T MID CEDAR-1 CEDAR-2 T HEAD -T MID [°C] T TAIL -T MID [°C] T HEAD -T MID [°C] T TAIL -T MID [°C]

CEDA R COINTRIN Estimation of effectiveness of existing thermal shield (somehow optimistic)

Some comments from Prometeusz Jasinski (1) ● For a closed lid in the CEDARs (normal operation mode) flux must not exceed 2.6x10 6 TAGGED particles per SECOND. ● But with the update of M2 beam line electronics (2007) Jens Spanggaard is certainly able to reduce the gain at the PMs and thresholds by at least a factor of 10. So we could go for 2.6x10 7 TAGGED particles. ● Do not open the lid fully at a rate of 10 8 particles/s or you will burn the anodes! ● Antiprotons are 0.5% of the total flux so 5x10 5 taggs per second are expected. ● Efficiency and purity of nearly 100% can be expected (see my PhD thesis, p.58) under the conditions that beam will have same properties as for a low intensity (and this I cannot guarantee). ● Kaons (2.5% of the total flux) reach a rate of (only) 2.5x10 6, so below the specs. ● But separation is very difficult in the M2 beam line. The main problem is not the CEDAR : apart from helium leak that has to be compensated, CEDARs are mostly operating as specified. ● Rest (electronics, lid, quantum efficiency) was 80% efficient. ● The biggest issue is the beam divergence (see my PhD thesis, p.53). At best we obtained an efficiency of 40% (majority 6). We lost 50% of Ks by the beam divergence.

Some comments from Prometeusz Jasinski (2) TO DO list ● 1. Most important: improve the beam divergence, but I'm afraid Lau Gatingon was pushing the very long beam line to the limits. Then think of the rest: ● 2. Improve long term stability like temperature (tents around the CEDARs helped but not enough) and pressure (smaller gas injection portions for a better control). ● 3. Use more clever analysis methods (Bayes (Tobias Weisrock) or Likelihood (Jan Friedrich)), but those work satisfying only if long term tability is improved!!!. ● 4. At the very end and if you have solved above's questions you may take care of PMs in the CEDARs if you like. E.g. : a) reconsider the use of a lower threshold (<25-30mV, possible with the new electronics) which allows for a lower PM gain, b) recheck PM signal shapes (there is some “ringing” possibly do to impedance mismatches or to the high gain). 5. One more note: I observed a decrease in the purity when increasing the beam intensity (my PhD thesis, p.56 Fig. 3.12). Suspects are pileup specially due to non updating mode in the electronics. Proof was never delivered although we had the chance in 2009 to do so (other more urgent issues, then the year was over). ● 6. Moreover: beam halo was never studied in detail although effects were observed. P.J.'s preliminary conclusions: Antiprotons you should be easily feasible. High efficiency Kaons seem quite difficult.

Some comments from Lau Gatignon (1) Forewards 1) The Liouville theorem (phase space conservation) states that the surface of the phase space ellipse cannot be changed (except if you collimate away a part of the ellipse). For an upright ellipse (which we have in the case of a parallel beam, as well as i a focus) this surface is proportional to the product of beam size and width of the angular distribution. Therefore a very parallel beam at the CEDAR requires a very large spot at the CEDAR. 2) We have tested (with Jens Bisplinghoff) an optics that provides such a highly parallel, but large, beam at the CEDAR during the Primakoff running. The problem with this optics is that if you focus this beam at the end of COMPASS, the beam will be too large all over the full length of COMPASS: To cure this you have to collimate in the downstream part of the beam (if you collimate more than a kilometre upstream, there is far too much smearing - it was shown not to work). Those collimators would dump a large fraction of the beam and produce junk which reaches the COMPASS detectors. The conditions were found to be unbearable for COMPASS.

Some comments from Lau Gatignon (2) Proposal For Drell Yan, the background considerations are not at all the same, as the beam is dumped before COMPASS and the focus is much further upstream. So we may try to revive this optics and see whether an overall improvement can be achieved by making a different optimization including backgrounds and CEDAR efficiency. It is possible to focus the beam at the front plane of the hadron absorber. I have confirmed at the time that 1 cm sigma is possible for the 'standard' beam. It is also possible with the argued and more parallel optics at the Cedar. This would in fact lead to a slightly smaller spot at the focus. But we can always make the spot slightly larger (by focusing further way). We have in fact three sets of quadrupoles between the Cedar and the target/absorber region. We have some freedom in tuning the magnification. But : there must be a compromise between spot size and divergence. We can focus at any point downstream of the COMPASS target (but not much upstream) and even at different positions in the two planes. But under all conditions Mr.Liouville will get his way!

Preliminary conclusions and next steps ● Conclusions ● A new test with 2008 beam optics can be designed, taking into account the DY setup, to try to reduce the beam divergence in the CEDAR region ● The CEDAR PMTs can operate at 2.6x10 7 TAGGED particle/s if the PMT gain is reduced by a factor ~10 ● It seems necessary to improve the temperature stability, particularly for CEDAR 1 ● It is not clear to me if and improvement in pressure control is needed ● Better understanding is needed to find the trade-off between high efficiency and high purity and to define how to measure the particle flux ● A meeting has being organized with CERN CEDAR group (Lau Gatignon, Jens Spangaard, …), during the week 7-11 April, in order to : ● Better understand how to reduce the beam divergence ● Assess the best option to operate the PMTs at higher particle rate and in in order to maintain the responsibility on existing CEDAR electronics – Lower gain of PMTs + post-amplifier – Use of lower threshold exploiting the 2007 electronics characteristics ● Assess the best option to improve temperature and pressure control – Better insulation of the hall area around the CEDARs – Construction of an additional (external, possibly with heaters) thermal shield for each CEDAR ● Better understand if some improvement in pressure control and gas refilling is needed