Minutes of the meeting
[Paul's talk]
(kentaro) The event rate is one per a year but it would be a bit different from the regular GW detection as we already know the signal is there. We will accumulate the signal and reach 5 sigma (or 1 sigma) to be able to discuss the science; it will be more like the event search in an accelerator.
(paul) Yes. The event rate is calculated with SNR of 5, as a standard way, and it is conservative. (The event rate of the BNS inspiral in A+ is 100.)
(matteo) How about the sky coverage?
(paul) It will be better with 2 detectors but only by a factor. We need an EM counterpart rather than a second GW detector.
(eric) SK can detect a neutrino only from a nearby gallaxies. How about HK? (SK can detect 2 and HK can detect 10 neutrino events for supernova in Andromeda.)
[Johannes's talk]
(kentaro) Why can the penultimate mass be warm?
(johannes) We have chosen to increase the temperature to 150K and extract the heat only through the radiation.
(kentaro) It seems we still see violin modes.
(johannes) Because the loss is high for the piano wire. The diameter is 550um, which comes from the breaking strength.
(kentaro) Can you use the 20cm float-zone silicon if the baseline is 2km? KAGRA mirror is 22cm in diameter.
(johannes) It is a bit small as our wavelength is 2um.
[Matteo's talk]
(dave) What kind of coating do you use?
(matteo) SiO2 and Ta2O5 without the Ti doping.
(johannes) Is there a gravity-induced birefringence?
(matteo/yoichi) Hirose-san measured the birefringence with rotating the mirror every 45 degrees and we did not see the influence of gravity. Sapphire is hard and our mirror is relatively small.
[Yuta's talk]
(dave) What is the pathway to get a 100kg sapphire?
(yuta) We are collaborating with a French group and they are trying to make a new crystal.
(dave) Have you considered Silicon for KAGRA+?
(yuta) I did not show the curve in the slides but yes. Actually the result was very good.
[Kentaro's talk]
(david) Do you have a room for the 300m SRC?
(kentaro) We need to fold the recycling cavity 10 times, extend the facility down to the parking lot, or to somehow go across the arm cavity duct.
(yoichi) Another way would be to dig a tunnel upward or downward.
(dave) How much power do you have in the PRC?
(kentaro) We will have 800W in the PRC. The limitation comes from the cooling capability of the cryo-cooler.
(dave) 10^-24 is crucial to observe the NS merger. We need 30kW with 2um or 20kW with 1.5um. What if you go to 120K?
(matteo) The temperature needs to be low with sapphire. It is not Silicon.
[General discussion]
(dave) Shall we form small working groups?
(Chunnong) There will be other options like white light cavity or intra-cavity squeezing. (Showed some plots.) Michael Page is moving to KAGRA next week.
(johannes) What if we just increase the power in KAGRA?
(kentaro) Substrate thermoelastic noise increases and it ruins the sensitivity.
(dave) It will be the key to give up the low frequency sensitivity.
(kentaro) Substrate TE is proportional to freq^-0.5 so it limits the sensitivity at HF. (Yuta showed some plots.)
(kentaro) The use of the Silicon at 123K is essential to go to HF.
(dave) Yes. How serious are KAGRA people thinking to go HF?
(kentaro) We mostly agreed to go HF in 2018 but then we encountered the birefrigence problem and we got stuck at the point.
(matteo) Silicon might also have the birefringence problem.
(yoichi) I agree.
(kentaro) As a matter of fact, KAGRA+'s goal is not as ambitious as OzGrav-HF. The sensitivity does not reach 10^-24 but 4-5 times worse.
(paul) I realized that. The factor is crucial to see one event per year.
(dave) 10^-24 is the important number.
(kentaro) Do you need to build it by 2030?
(dave) Yes, That is the time when the 2.5Gs will be working and the 3Gs are still under construction. That is why I would like to consider making KAGRA+ a HF detector. I really like to have a detector in Australia, though.
(eric) It is also important to build it by 2030 so that the detector can be a 3G enabler.
(mathew) It is important for KAGRA to reach 70Mpc in the next few years. Can people work in parallel to achieve 70Mpc, solving the birefringence problem, and also NEMO?
(dave) Email both kentaro and me if you like to join the working group. Thank you for joinning.