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Lots of people who are NOT on the ANITA collaboration are talking about this topic of how “NASA scientists may have uncovered a parallel universe.” Well, as someone who is actually on the collaboration and also deeply interested in making science communication more accessible, I figured I will blog about the SCIENCE that is getting twisted around to make tabloids.
I will not be sharing links to any of the tabloids because those clickbait articles have already made enough money off of misinforming the public and creating sensation without actually educating anyone about science.
The only consolation prize is if this increases enthusiasm for particle astrophysics, I am all for the increased enthusiasm part. Yes, it is a very cool and awesome field to be in and has involved experiences of a lifetime even as a graduate student.
By all means, I hope Congress funds particle astrophysics more. we are a small but extremely hard-working community of scientists.
Some ANITA acronyms
ANITA = Antarctic Impulsive Transient Antenna
VPol = Vertically Polarized
HPol = Horizontally Polarized
UHE = Ultra High Energy
CR = Cosmic Ray
EAS = Extensive Air Shower
The audio version of this post:
An excerpt from my already public thesis is below
Although built primarily for the detection of UHE neutrinos, ANITA is able to observe radio waves from EAS or CR interactions in the air in a sideband channel.
Aside: Like a side hustle! Cosmic ray detection is the side hustle of ANITA.
Neutrino candidates in ANITA are expected to be primarily VPol, while CR candidates are primarily HPol. Interaction of the charged particles in the air with the local vertically upward-pointing geomagnetic field renders the radio signal from CR candidates its HPol signature.
This is a fortunate difference between radio signals from neutrinos and that from cosmic rays.
Although ANITA has not yet discovered UHE neutrinos, it has measured signals from several CR candidates. There were 16 CR candidates in the ANITA-1 flight, a few in the ANITA-2 flight, and about 20 in the ANITA-3 flight. Fewer CR candidates were measured in the ANITA-2 flight as the hardware was designed to not trigger on HPol events in this flight.
There are two ways in which ANITA can measure the radio signature of cosmic rays. These are 1) direct and 2) reflected as shown in a cartoon in the picture above.
- Radio waves due to CR initiated particle showers in the air interacting with the local geomagnetic field can either reach the ANITA payload directly as shown with a red line in the cartoon on the left side.
- Or, radio waves from the particle shower interaction with the geomagnetic field can reach the payload by reflection off of the ice as shown with two red lines in the cartoon on the right side.
Unusual upgoing events
We have reported on the observation of two unusual, upgoing events. At the time, July 2018, only two.
They are referred to as “mystery event” 1 and 2, respectively.
They were both EAS candidates. Mystery event 1 is from the ANITA-1 flight and mystery event 2 is from the ANITA-3 flight. They were both found to be HPol events and CR-like.
Whether ANITA observed an event directly or by reflection can be determined from the polarity of the event waveform.
Typically, ANITA measures CR events as reflected events whose polarity is opposite to that of directly observed events. The two mystery events had polarity consistent with that of being directly observed events. The polarity of mystery event 2 can be seen in the waveform picture shown above.
Here the field strength in mV per meter is in the vertical axis and time is in the horizontal axis. A dip or trough in the waveform can be seen. This trough would be a peak for an event of opposite polarity. It can be seen from the cartoon that directly observed EAS events in ANITA have shallower elevation angles than those observed by reflection off of the ice.
Directly observed CR events are relatively rare and usually observed at shallow angles of a few degrees below the horizontal.
Both mystery events 1 and 2 have steeper elevation angles of −27.4◦ and −35◦ , respectively.
These elevation angles are typical of reflected events, not direct. This is shown with a purple line on the right side of the cartoon.
To summarize, the mystery events had polarity consistent with being direct events but elevation angles similar to that of reflected events. The unusualness of the two mystery events lies in the incompatibility between their observed polarity and the elevation angle at which they were seen.
A detailed summary of Mystery Event 1
I summarize here our publication on Mystery Event 1.
In this publication, we reported on four CR or CR-like events observed with ANITA.
From ANITA’s first flight in 2007, 16 ultra-high-energy cosmic rays (UHECR) air showers were reported, 13 of which were consistent with geomagnetically-induced radio pulses seen in the reflection off of the Antarctic ice surface.
Three of these 16 events in the signal box (expected background was 1.6 events) from the initial blind analysis were deemed background: two of unknown origin and one a likely thermal noise fluctuation with no apparent signal content.
Three additional CRs were also found in cross-correlation analysis after unblinding including two events that were identified as Earth-skimming CRs, a previously unobserved class of events. The Earthskimming events were directly observed and thus had opposite polarity as the reflected events.
In addition to these two Earth-skimming events observed in ANITA-1, another event of the same type was observed in ANITA-2.
On reviewing the events in the signal box of ANITA-1, it was found that one of the events, previously deemed background, was dominated by HPol content and consistent with geomagnetic parameters of a CR.
It arrived at the payload from a direction of 27.4 ◦ below the horizontal, a fairly typical angle for a reflected CR event. Interestingly, however, it did not correlate well with a reflected CR signal shape.
A re-evaluation of this event led to the finding that the polarity of this event is consistent with an air shower seen directly, without reflection. Naturally, then, this event was compared to the other class of CRs, the Earth-skimming events. But the three Earth-skimming CR events showed to have much shallower angles associated with them, namely, 4.3, 3.3 and 3.0 degrees below horizontal.
So, the steep angle of 27.4◦ below horizontal of this CR-like event poses interesting problems for interpretation.
The publication referenced here investigates these four unusual upward-directed events seen by ANITA with the goal of explaining what relation if any, these may have with tau lepton initiated air showers.
In case any of these events are associated with tau leptons, then an exciting interpretation would be (but not confirmed) that the tau lepton originated from the charged current interaction of a UHE tau neutrino in the ice or, more likely, in the rock below the ice, and then survived long enough to produce a directly (as opposed to reflected) observed air shower.
A radio signal detected by ANITA can have a mix of polarization states, that is, it can be part unpolarized, part linearly polarized and part circularly polarized.
Stokes parameters I, Q, U, and V can be estimated for signals to figure out the different polarization components of a signal. The I parameter is for unpolarized, Q is for linearly polarized (+Q for HPol and Q for VPol), U is for linearly polarized along 45 degrees (+U for positive slope, -U for negative slope) and V is for circularly polarized (+V for left circularly polarized and V for right circularly polarized).
In the case of CRs, at least a few percent of circular polarization may be expected in the signal due to possible interference between the primary geomagnetic radiation and secondary Cherenkov radiation. Upon estimating the Stokes parameters for the four events, it was found that in addition to linear polarization, the two stronger Earth-skimming events have circular polarization components.
The CR-like mystery event, too, has both linear and circular polarization ( 10%) components. The presence of circular polarization, the good correlation with the other three upward-directed CR events, and with the inverted reflected CR events, excess of noise in trailing part of the signal similar to reflected CRs, all support that the CR-like mystery event originates primarily from geomagnetically-induced radiation.
The two competing hypotheses about the CR-like mystery event are as follows:
- it could be a reflected CR event whose polarity got distorted by some unknown process
- OR it could be a direct air shower event caused by interactions or decay of a secondary lepton from a neutrino interaction in the Earth.
The most probable candidate for a secondary lepton in the latter hypothesis is a tau lepton. Not confirmed though.
After accounting for uncertainties, the minimum possible emergence angle for the hypothesized tau event is 25.4 degrees below horizontal which corresponds to an Earth chord distance of 5460 km. This chord distance is about 17,000 km water equivalent which is far greater than the Standard Model (SM) predicted neutrino interaction length of about 2000 km water equivalent.
A neutrino with an energy of order EeV or more with Standard Model cross-section would not make it far enough through this Earth chord to produce a tau lepton late enough for the tau to then survive long enough to produce an air shower.
Since the energies of the events are known to be of order EeV or more from the observed radio pulses, the only other parameter that can be tweaked to consider the tau hypothesis is the cross-section.
It is found that for SM cross-sections of the neutrino, more events closer to the horizon should have been seen. When the cross-section is suppressed, however, ANITA’s acceptance (prediction for the number of neutrinos expected to be seen by ANITA, call it apples) for such events agree with current limits (call it oranges) with the caveat that the limits are all derived using SM cross-sections (cannot compare apples to oranges).
However, it is interesting to see that suppressing the cross-section by a factor of about 3-5, as initially estimated, does make the CR-like mystery event present itself reasonably as a tau neutrino candidate.
Our conclusion is that we need to further study these mystery events, not that there are parallel universes. The concept of a parallel universe in itself is not shocking, it is simply that we have not published to say “parallel universe” or connected our results with a parallel universe. So, scientifically, there is no result stating that right now.
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