Dan Maloney  3:00 PM
Not sure I've seen him check in yet, though. You out there Pat?
Patrick Allison  3:01 PM
I'm here now! Internet connection literally went down at 3 exactly. Fun.
Dan Maloney  3:01 PM
Good timing! Welcome aboard
Dan Maloney  3:01 PM
Cold and snow there? We're getting womped
Patrick Allison  3:02 PM
I am so not going to be able to compete with live event detector tracks, FWIW
Patrick Allison  3:02 PM
Yeah, everything's closing due to the cold. Sub-zero wind chills. Fun.
Dan Maloney  3:03 PM
No worries at all, Hack CHats are about cool people doing cool stuff, and you've got that going. Tell us a little about how you got where you are
Patrick Allison  3:04 PM
Sure! My freshman year at Penn State I apparently impressed my freshman physics prof, who recommended me to Prof. Jim Beatty, who was a new hire, starting up the particle astrophysics group. That got me started into balloon experiments, cosmic ray astrophysics, and eventually my PhD dissertation on the Pierre Auger Observatory.
Patrick Allison  3:05 PM
Might seem like a leap from cosmic rays to neutrinos, but the two fields are *very* intrinsically linked - and from there it led into working on the Antarctic Impulsive Transient Antenna, which is a balloon experiment looking for ultrahigh energy (> 10^18 eV) neutrinos in the Antarctic ice
Darryl Masson  3:06 PM
Some might argue that neutrinos *are* the most interesting cosmic rays
kjansky1  3:06 PM
Do you know if we are closer to determining the relative masses of the e,m & t neutrinos and do you think the correlate with the masses of their corresponding leptons?
Patrick Allison  3:06 PM
Absolutely! In fact the reason I say they're intrinsically linked is that at the *highest* energies - the one's we're looking for - we know that the neutrinos exist because the cosmic rays we see have to produce them.
Patrick Allison  3:07 PM
For the first part of that question - how close are we to getting the absolute masses - definitely can't answer that. There are a lot of experiments out there trying to do so, but it's very, very hard.
Patrick Allison  3:08 PM
For the second part, I think it's safe to say that no, neutrino masses are *not* linked to the masses of the e/mu/tau at all. There's just far, far too big a gap between them
Darryl Masson  3:08 PM
Has KATRIN released anything recently? I know their sensitivity is no longer competitive, but they're the only big name in town for a direct mass measurement
The # of #neutrinos observed in #Super-Kamiokande is ~30/day. In terms of prob. it may be nearly impossible to find neutrinos on this monitor. Almost all events are cosmic ray muon events." One was detected moments after I opened the visor on my phone for the first time! You can access the public real time visor at https://www-sk.icrr.u-tokyo.ac.jp/realtimemonitor/ The neutrinos collide with the matter inside the huge tank and generate ring shaped light beams that propagate in a cone. This light is detected by an array of photomultipliers. A good project would be replicating the detector array with RGB LEDs, so a small computer would access the web image and display the current state of the detector.
Patrick Allison  3:09 PM
I haven't seen anything since 2022, but I think they're still running? So maybe in the next few years. But yeah, it'd be surprising if they found something.
Thomas Shaddack  3:09 PM
There are arrays of WS2812b-like LEDs that could be easily adapted to the purpose.
kjansky1  3:10 PM
OK how about the existence of the possibly 4th neutrino and do you think neutrinos are Majorama particles?
The cool part is that the display should be the same shape of the tank, like a cylindrical display!
Darryl Masson  3:11 PM
(Full disclosure - I did my PhD and postdoc on the XENON dark matter detectors. They aren't designed for neutrino detection but solar neutrinos form a significant part of the background because a sub-keV energy threshold picks up a whole lot...)
Patrick Allison  3:12 PM
2 points regarding any additional neutrinos: more than 3 *flavors* (probably not) and more than just the e/mu/tau types (... maaaybe). The problem that people run into with the 2nd question is that some theorists love to just "handwave" away neutrino masses saying "oh we can explain that easy" with something called the seesaw mechanism. But that *requires* there to be another neutrino... just with an absurd energy.
Patrick Allison  3:13 PM
Neutrino masses are physics outside the Standard Model. They just are. That's why neutrinos (and all weak interactions in general) are cool to study.
Patrick Allison  3:14 PM
I gave a talk at a high school once and it led with "the weak force has more Nobel Prizes than all the other forces". Totally true
Darryl Masson  3:15 PM
There's been some 3-ish sigma hints from some neutrino experiments at possible sterile neutrinos. Do you think DUNE will pick up something interesting when it (finally) starts running, or do we have enough experiments at this point that we should expect to see 3-sigma non-effects?
kjansky1  3:15 PM
Could they be their own antiparticles?
Patrick Allison  3:15 PM
Man, everyone's making me make bold predictions! Yeah, I think it's the latter - I think you're just starting to see noise.
Patrick Allison  3:16 PM
Neutrinos being their own antiparticles is the idea behind them being a Majorana particle. A lot of theorists really love that, but man, it'd be weird to have everything being Dirac and then *one* Majorana
Patrick Allison  3:17 PM
But then again Rocky Kolb used to say the same thing about the Higgs particle ("it'd be nice if experimentalists could just find *one* scalar? Just one? Please?") sooo
kjansky1  3:17 PM
And they did!
Darryl Masson  3:18 PM
but did we find _the_ Higgs or _a_ Higgs?
kjansky1  3:18 PM
Here again more Higgs?
Patrick Allison  3:18 PM
Hey I found the link to the old ANITA online monitor: https://www.hep.ucl.ac.uk/uhen/anita/aware/awareHk.php
Dan Maloney  3:18 PM
So for the particle physics noobs like me, where are all these neutrinos we're looking for come from? I'd guess mostly from the sun, but maybe some come from other stars? Other galaxies?
Thomas Shaddack  3:19 PM
Thought. Can neutrinos be used for diagnostics/monitoring of nuclear reactors, or nuclear tests?
Darryl Masson  3:19 PM
There are a few "main" sources of neutrinos - the sun, us/the earth, and the universe
Patrick Allison  3:20 PM
Oh, that's a good question, and it's got a lot of answers. The vast majority that we see come from the Sun. We've seen a *handful* from a single supernova.
Patrick Allison  3:20 PM
At higher energies - a little below ~1 PeV - we know that there are astrophysical sources of neutrinos. We have *no idea* what they are.
Darryl Masson  3:20 PM
I've seen some discussion on trying to identify geo-neutrinos out of all the others as a way of probing internal structure of the inner core (which is where most of the radioactive heavy elements settled during planetary formation)
Patrick Allison  3:21 PM
There's an interesting problem with trying to figure out where those neutrinos come from. There's a paradox called Olber's paradox which says "if the universe is infinite, why isn't the sky white?" because you should be able to just point somewhere, and there's a star. The answer to that is twofold: 1, the Universe is too young, and 2, dust. There is no "neutrino dust" and so Olber's paradox is a bigger problem for neutrino sources.
Darryl Masson  3:22 PM
does the cosmic neutrino background count as cosmic rays? I've so far only see one proposal on ways to measure it
Patrick Allison  3:22 PM
But at the *highest* energies - around 10^18 eV - we've never seen those neutrinos, but we *know* they are there. Because we know high energy cosmic rays exist, and those *must* produce neutrinos when they hit the CMB.
Dan Maloney  3:22 PM
Do detectors provide some kind of directional data? If so, I'd imagine Sun-generated neutrinos would mostly hit Earth along the plane of the ecliptic. And thanks for blowing my mind with the white universe thing.
Darryl Masson  3:23 PM
most detectors have *some* directionality
Darryl Masson  3:23 PM
Borexino released a neutrino image of the solar core, but the resolution isn't particularly good
Patrick Allison  3:24 PM
Oh, I left off the main source of neutrinos for a lot of these detectors, actually: the Earth's atmosphere. From cosmic rays. Again. (See how these things are linked?)
Patrick Allison  3:24 PM
Basically, cosmic rays hit the atmosphere, produce buckets of particles, many of which are muons, and those muons decay, producing neutrinos which zip through the Earth and interact.
Thomas Shaddack  3:25 PM
that's not background noise. that's background scream.
Patrick Allison  3:25 PM
In those cases you know they're neutrinos because you see a muon coming "up," and, uh, muons can't get through the Earth. Using the entire Earth as a "filter" or "shield" is very common
Darryl Masson  3:26 PM
even just part of the Earth as a shield is super common
kjansky1  3:27 PM
Do you think that the matter antimatter disparity could be partially due to reverse Compton scattering of high energy photons by neutrinos and leading to the formation of neutrons, with the synthesis of baryonic matter in the Universe?
Patrick Allison  3:27 PM
What's funny is at the energies ANITA and the experiments I work on are at - the Earth is opaque! The neutrinos can't get through it at all. So you primarily look for "earth skimming" neutrinos - one that just *barely* go through.
Patrick Allison  3:28 PM
Cosmic ray physics/neutrino physics often think about the amount of matter you pass through in terms of "column density" - density integrated along a path. At 10^18 eV neutrinos will interact with about O(1000) g/cm^2.
kjansky1  3:28 PM
Muon tomography of the Giza pyramids is a great application used for detecting potential hidden chambers.
Patrick Allison  3:30 PM
And because I'm still peeved about not having a live event display or something, here's a map of a bunch of the observed events that ANITA saw near the end of its last flight. None of which are neutrinos, sadly.
Patrick Allison  3:30 PM
Patrick Allison  3:31 PM
The hardest part about detecting neutrinos is a lot like for dark matter - just killing all the background.
Patrick Allison  3:31 PM
You'd like to think that Antarctica is radio quiet, but I can tell you it *definitely* is not.
Patrick Allison  3:32 PM
The "purple crosses" there are the events - the red/orange are the balloon's flight path around Antarctica.
Darryl Masson  3:32 PM
is there anywhere left on the planet that's actually "radio quiet"?
Darryl Masson  3:32 PM
(I guess pick your favorite frequency band)
Dan Maloney  3:32 PM
How much does the fact that Antarctica isn't a homogeneous block of ice factor into ANITA experiments?
Patrick Allison  3:33 PM
Definitely not for a broadband balloon experiment! One of the biggest problems we have is that the US military replaced their comm satellites with ones that are square in ANITA's band, so it's a royal pain in the neck
kjansky1  3:33 PM
Everywhere will be very radio quiet just after the next Carrington event !
Patrick Allison  3:34 PM
@Dan Maloney Great question. It mainly hurts us in that some parts of the continent are very shallow, and others are deep, and... we can't stay in one spot.
Patrick Allison  3:34 PM
The ice itself is radio transparent across the entire continent, which is really nice. They do lots of radar bounce experiments to measure it.
Patrick Allison  3:35 PM
Thankfully the best part of the continent for us is the worst for humans, too - the East Antarctic (no jokes about "everything's north of the south pole")
Patrick Allison  3:36 PM
In that image you see above the colors are ice depth, so you can see our last flight wasn't... great, but it wasn't terrible. It sucked because we ended up spiralling into the South Pole and just hanging there.
Patrick Allison  3:36 PM
But the electronics went belly-up shortly after we got stuck there, so it wasn't *that* big a loss.
Patrick Allison  3:37 PM
and yes since I'm the one who's the electronics guru that one ticked me off
Dan Maloney  3:37 PM
Does the balloon do active steering? Or is it just riding the circumpolar winds?
Patrick Allison  3:38 PM
Just riding the vortex. Steering is something they've thought about for the ultra-long duration balloons (ULDB). But those require pressurized balloons (sometimes called super-pressure balloons) and the engineering there is VERY hard
kjansky1  3:38 PM
Hey China seems to have the balloon steering part figured out.
Patrick Allison  3:39 PM
I actually worked with one of the math profs working on that. Yes, math - because the main problem is getting the gores to deploy properly and it turns out that's a math problem. A "gore" in a balloon is one of those pumpkin-like sections.
Patrick Allison  3:40 PM
I should actually put up an image of the ANITA experiment, which is a radio array on a balloon, so here you go:
Patrick Allison  3:40 PM
kjansky1  3:41 PM
Wow all those horns, what frequency?
Patrick Allison  3:41 PM
200-1200 MHz. 48 total. Its successor, PUEO, says "aren't you cute" and goes up to 112 total horns at 2/3rd scale (so 300-1500 MHz)
Dan Maloney  3:42 PM
Not gonna lie, looks like that came off a "Mad Max: Fury Road" truck. Just needs a Doof Warrior and a fire-breathing guitar
Patrick Allison  3:42 PM
We abandoned the low frequency part because the military satellites just make it useless. Even 300-400 MHz is pretty polluted thanks to the MUOS satellites.
kjansky1  3:42 PM
Oh boy Inmarsat and a whole slew of other RFI , good luck.
Patrick Allison  3:44 PM
To be honest I'm surprised more satellites haven't been a problem, but it's really very much just the milsats. They're not even *that* loud, actually - only about maybe ~7-10 dB above thermal noise. But ANITA/PUEO *work* at thermal noise levels.
salec  3:44 PM
A question more fitting to the start of this chat (sorry): could gravitational lensing help estimate the (average) mass of neutrino?
Patrick Allison  3:45 PM
Probably not. One of the best limits we had for a while came from the SN1987A neutrinos, so a supernova would help... but lensing wouldn't there.
Patrick Allison  3:46 PM
I think the direct-detection experiments (like KATRIN) are probably still the best bet. It's just so hard.
Patrick Allison  3:47 PM
Thomas Shaddack  3:47 PM
can the antennas be partly shielded so they don't listen to the birds up but more the ice down? can the Askaryan radiation (sounds positively startrekish) pulses be discriminated from the anthropogenic signals?
Patrick Allison  3:47 PM
Managed to grab one of the events from ANITA-4's event display.
Patrick Allison  3:48 PM
@Thomas Shaddack Great question. The neutrinos we're mostly looking at are going to be *very* close to the limb of the Earth, so the problem is that if you point the antennas down any more, you lose sensitivity neutrinos.
Patrick Allison  3:49 PM
So it's a bit of a balancing act. We managed the satellites last time by having el-cheapo tunable notch filters (just a tunable trap for any ham geeks). In PUEO there will be a tunable digital notch filter.
kjansky1  3:49 PM
Are the pulses circularly polarized?
Patrick Allison  3:50 PM
Linearly, which is a big help! Because as you're probably guessing, the satellites are (pure) circular. This lets us *distinguish* the two...
Patrick Allison  3:50 PM
But the problem of trying to *reject* circular pol at a trigger level is really hard.
Patrick Allison  3:51 PM
Trying to do *anything* in an FPGA at 3 GSa/s is hard.
Darryl Masson  3:51 PM
what's the deadtime like?
Darryl Masson  3:51 PM
I would guess the options for ring-buffering at 3 GHz are more limited than at 1 GHz which is the highest I ever dealt with
Patrick Allison  3:52 PM
If we don't have a notch, it's terrible, you lose about 25-50% of your livetime to the milsats. With a notch it's only around 8% or so.
Thomas Shaddack  3:53 PM
can we just shoot the sats down? for science?
Patrick Allison  3:53 PM
Hey, for the 280 MHz one, it'd be for more than that, those are the ones used by Brazilian pirates
Dan Maloney  3:54 PM
SpaceX is planning 144 launches this year, mostly to put up more Starlink birds. You might get your wish accidentally
Patrick Allison  3:54 PM
But it's the US military, they don't move quickly. I haven't seen any info on de-orbiting/retiring those guys. Which would be *really* nice.
Patrick Allison  3:55 PM
@Dan Maloney Funny thing is that we're planning on *using* Starlink on PUEO. We used to use the fancy fast Iridium, but it never worked
kjansky1  3:55 PM
Those pirate milsats are at geo not likely they can bring them down unless somebody goes up and gets them.
Patrick Allison  3:56 PM
We did also use the super-slow iridum (plus good old fashioned TDRSS) as well. Ballooning is NASA, so interfacing with those guys is fun.
Patrick Allison  3:57 PM
OpenPort, that's what the fancy Iridium was called. All the ballooning projects had problems with it.
kjansky1  3:57 PM
Any follow on flights planned?
Patrick Allison  3:59 PM
For ANITA, we did a large-scale redesign and revamp thanks to the new technology available now with high-speed digitizers, so we renamed it to PUEO. We now do digital beamforming at the trigger level to provide a big sensitivity boost. That *should* be flying in the next few years or so, hopefully?
kjansky1  4:00 PM
Best of luck on it.
Patrick Allison  4:00 PM
Antarctic programs and ballooning in general have had tough times recently due to COVID restrictions at Pole plus poorly-timed US budget fights. September is the *wrong* damn time to fight on the budget.
Patrick Allison  4:00 PM
(The government shutdown a number of years ago cost an entire Antarctic season because if you shutdown the government during supply run to Antarctica... no pole season)
Thomas Shaddack  4:01 PM
the most hostile environment in the universe, for space probes, is not venus, or sun vicinity, or deep space - it's the congress floor.
Patrick Allison  4:01 PM
But PUEO is a bit of a "flagship" program (we're "big budget" for balloons) so we kinda jump the line a bit.
Patrick Allison  4:01 PM
Patrick Allison  4:01 PM
Here's an example of the frequency spectrum ANITA sees, so you can see the two clear notches:
Patrick Allison  4:02 PM
and if that frequency spectrum looks horribly non-flat, yes, it is - that's mostly due to the custom digitizer we used.
Patrick Allison  4:03 PM
I guess I should also include a picture of PUEO too:
Patrick Allison  4:03 PM
Patrick Allison  4:04 PM
I probably do need to get going, but if there are any last questions?
Thomas Shaddack  4:04 PM
looks like an electronic warfare passive surveillance system.
kjansky1  4:04 PM
So all those horns are in a phased array/
Thomas Shaddack  4:05 PM
obviously. hence the electronic beam steering.
Patrick Allison  4:05 PM
16 of them maximum.
Patrick Allison  4:06 PM
4 of them point in the same direction, and then they're spaced at 15 degree intervals, so even when you're 2 of those "columns" off the signal isn't *that* far down
Thomas Shaddack  4:06 PM
with coherent clock on the SDR units, the beam steering could be done ex-post if the raw iq files are saved.
Patrick Allison  4:06 PM
Yeah, for reconstructing the incoming direction, the beamforming is done much more finely afterwards. We call it making an "interferometric map"
Patrick Allison  4:07 PM
But we have to do at least the first stage of beamforming live to get the trigger sensitive enough.
Dan Maloney  4:07 PM
I was just about to call time myself -- time to brave the storm to fetch my wife and kid from campus. Thanks so much for joining us today, Patrick, this was really interesting stuff. And thanks to everyone for such good questions -- way more interest in neutrinos out there than I thought there would be, great to see!
Thomas Shaddack  4:08 PM
can you be trigger-happy and then throw out the false alarms?

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