The Largest Liquid Argon Neutrino Detector in the World: ProtoDUNE

Title: “ProtoDUNE and a Dual-Phase LArTPC

Author: Andrea Scarpelli on behalf of the DUNE Collaboration

Reference: arXiv:1902.04780

ProtoDUNE is helping lead the way for the next generation of long-baseline neutrino experiments, and is exceeding expectations.

DUNE, the Deep Underground Neutrino Experiment, will deploy four massive Liquid Argon Time Projection Chambers (LArTPCs) totaling 40 kilotons of liquid argon to study neutrinos, one of the universe’s most elusive particles. Time Projection Chambers use sensitive volumes of gas or liquid exposed to electric and magnetic fields to reconstruct 3D particle trajectories and interactions. Liquid argon is advantageous for several reasons. It’s relatively inexpensive, produces light when energetic charged particles pass by (scintillates), and won’t absorb electrons produced by ionizing radiation before they reach the detector arrays (since it’s a noble element). It is also very dense, increasing the chances of a particle interacting in the detectors compared to other materials.

The 1100 scientists involved with DUNE plan to use two detectors and the world’s most intense neutrino beam. A smaller detector, located at the Fermi National Accelerator Laboratory in Batavia, Illinois, will record particle interactions near the neutrino beam source. The largest detector, buried more than a kilometer underground, will be located at the Sanford Underground Research Laboratory in Lead, South Dakota. This is 1,300 kilometers away from the neutrino source, known as the “baseline” of the long-baseline neutrino experiments. This distance allows the neutrinos time to oscillate, or change their flavor as they travel, before being detected in the large LArTPC. Scientists can then learn more about neutrinos by using the physics of neutrino oscillation. DUNE aims to measure the neutrino oscillation parameters, determine the neutrino mass hierarchy and search for CP violation in the leptonic sector of the Standard Model.

LBNE_Graphic_061615_2016.jpg
DUNE will consist of two detectors: one located at Fermilab in Batavia, IL, and one 1,300 km downstream of the neutrino beam source, at the Sanford Underground Research Laboratory in Lead, South Dakota. Source: http://www.dunescience.org/

Before DUNE can get started, characterizing the detector response and proving the feasibility of the DUNE technology at the kiloton scale is crucial. That’s where ProtoDUNE comes in.

In LArTPCs, 3D images of particle events are formed by collecting electrons produced during Argon ionization and drifting them onto detectors using an applied field. The most common setup for these detectors is a Single-Phase (SP) LArTPC, where electrons are drifted horizontally onto a readout. However, because in this setup the drift length cannot exceed 3-4 meters, using this technology for DUNE requires a complicated design with several TPCs within the volume of argon. An alternative approach is a Dual-Phase (DP) setup, where the electrons are drifted vertically and amplified by a strong field applied inside a thin layer of argon gas just before the readout. This amplification is used to compensate for electron losses or noise caused by the larger drift length differences.

protodune
Left: Model of ProtoDUNE-SP (Single-Phase). Right: Model of ProtoDUNE-DP (Dual-Phase). Source: arXiv:1902.04780

The ProtoDUNE experiment, located at CERN, is designed to test both the SP and DP setups for the DUNE far detector. Using a total Ar mass of 0.7 kilotons, ProtoDUNE will also test the detector response and calibration with both a charged particle beam and using cosmic rays. Testing the designs of the DUNE components at a 1:1 scale wherever possible will demonstrate the technology and allow DUNE to move forward.

201803-89_02
Scientists inside of ProtoDUNE-DP’s field cage, which use high voltages to drift particles towards the readouts using an electric field. Source: Symmetry magazine

In the fall of 2018, ProtoDUNE came alive, and measured neater tracks with less noise from electronics than had been expected. While DUNE will be much larger, this first prototype detector, ProtoDUNE-SP, is now the largest LArTPC ever to be constructed, and to have it functioning well is a key step towards realizing DUNE. However, there is certainly a lot more work to do before DUNE’s two detectors come alive in 2026. Stay tuned!

 

Further Reading:

  1. DUNE website: http://www.dunescience.org/
  2. “Long-Baseline Neutrino Facility (LBNF) and Deep Underground Neutrino Experiment (DUNE) Conceptual Design Report Volume 1: The LBNF and DUNE Projects”: https://arxiv.org/abs/1601.05471
  3. “Understanding the energy resolution of liquid argon neutrino detectors”: https://arxiv.org/pdf/1811.06159.pdf

Are you smarter than an ATLAS algorithm?

Article: ‘That looks weird’ — evaluating citizen scientists’ ability to detect
unusual features in ATLAS images of LHC collisions
Authors:  A.J. Barra, A. Haasb, C.W. Kalderon
Reference: arXiv:1610.02214v1

As it turns out, non-expert citizen scientists can match or beat out ATLAS algorithms in identifying features in images of LHC collisions.

The ability of the general public to identify long-lived particles and other unusual features in images of LHC collisions recorded by the ATLAS experiment was studied using data from the Higgs Hunters project. The Higgs Hunters project was launched by NYU scientists and colleagues in 2014, and allows members of the general public to study LHC images to help search for previously unobserved particles. Writing computer algorithms to identify “weird looking things” in these images can be difficult, and human eyes, be they expert or non-expert, can help with the hunt. The Higgs Hunters project scientists are specifically searching for previously unobserved particles that could be created via decay of the Higgs. In some cases, the tracks left in the ATLAS experiment could be picked out better by human eyes than computer programs.

This isn’t the first time the scientific community has reached out to non-experts to classify images or aid in scientific pursuits. Through the Galaxy Zoo project, for example, citizen scientists contributed to the results of 48 scientific papers by classifying galaxy shapes and spotting unusual objects in images from the Sloan Digital Sky Survey and other image datasets. In a more indirect way, the public has also been previously invited to contribute to CERN’s science by donating their personal computer’s idle time to help simulate proton-proton collisions. The Higgs Hunters project builds on this history by being the first to allow the general public to take an active role in searching for new particles at the LHC, which not only has the practical benefit of helping scientists out in their search, but also inspires the non-scientist public to take an interest in the field of particle physics.

The task selected for the Higgs Hunters project was that of identifying new particles, \phi, dubbed “baby bosons,” as they decay within the ATLAS detector. Such particles are predicted in theories in which an additional scalar mixes weakly with the Higgs boson. These processes were chosen because they had not been previously unobserved and would generate a signature that is fairly easily identifiable by eye, rendering the citizen scientists competitive with standard reconstruction algorithms. The discovery of these particles would be a high impact scientific discovery, a key motivating feature when employing citizen scientists.

Before being presented to volunteers, images were pre-selected to include those containing a muon and an antimuon with invariant mass consistent with the mass of the Z boson. Such events are consistent with the Z boson decay processes Z \rightarrow \mu^+ + \mu^- and have an increased probability of also containing a Higgs boson, since virtual Z bosons may emit Higgs bosons through the ‘Higgs-strahlung’ process, Z^* \rightarrow Z+H. Data were selected from the 2012 data-taking period between April and December. The ability of the volunteers to identify the desired events was calibrated using test images which showed Monte Carlo simulations of the process of interest, H \rightarrow \phi + \phi. All of the images presented to volunteers, be they simulations or real data, were processed using the ATLAS reconstruction software.

An example ATLAS detector image presented to citizen scientists, generated from a computer simulation of the process H → φ+φ. The green lines emanating from the center indicate the reconstructed muon and antimuon used to select the event. The red dotted line indicates the direction of missing momentum transverse to the beam. Source: arXiv:1610.02214v1

As of October 2016, classifications had been performed by ~32,000 citizen scientists of a wide arrange of ages and backgrounds from 179 countries. Peaks in volunteer activity occurred soon after the project launch and when CERN news stories were published about the project. Each image was classified by ~60 people. The number of classifications completed by each citizen scientist follows an approximate power-law behavior, with most volunteers classifying just a handful of images, but ~1,000 people providing 100+ classifications each. The most dedicated enthusiast provided nearly 20,000 classifications. In total, 1,200,000 features of interest were classified on ~39,000 distinct images.

Left: Start date of new citizen scientists joining the project for the first time. Right: Number of classifications per citizen scientist, displaying an approximate power-law behavior. Source: arXiv:1610.02214v1

It was found that the citizen scientists’ performance competed very well with that of the computer algorithm, even beating it for events with low mass (8 GeV) baby bosons regardless of image view and boson lifetime value. As the mass of the boson increased, the algorithm began beating out the human volunteers. A “weird thing” was also spotted: an image showing a collision apparently containing a jet of multiple collimated muons. In the Standard Model, jets are always of hadrons, not muons. After further investigation by the science team, it was found that the event was due to an unusual interaction of a known particle with the detector, rather than an unusual or new particle. While this didn’t represent a discovery of new physics, the potential of citizen scientists to pick out “weird stuff” could lead to the future identification of interesting features in LHC collision data.

The volunteers responded positively to the project, with the overwhelming majority (>97%) interested in continuing their participation in a future CERN physics project. 47% of respondents said they were more likely to go on to study physics as a result of participating in the project, and 80% felt that their knowledge of particle physics had been improved.

In the future, relying on non-expert citizen scientists could help scientific collaborations classify data as well or better than computer algorithms could, while engaging the public in physics in a valuable and influential way. Just last month, LIGO launched its “Gravity Spy” citizen science program, in which participants search LIGO data for “glitches” that can help LIGO scientists distinguish between the signals they observe.

Background reading:

Horton Hears a Sterile Neutrino?

Article: Limits on Active to Sterile Neutrino Oscillations from Disappearance Searches in the MINOS, Daya Bay, and Bugey-3 Experiments
Authors:  Daya Bay and MINOS collaborations
Reference: arXiv:1607.01177v4

So far, the hunt for sterile neutrinos has come up empty. Could a joint analysis between MINOS, Daya Bay and Bugey-3 data hint at their existence?

Neutrinos, like the beloved Whos in Dr. Seuss’ “Horton Hears a Who!,” are light and elusive, yet have a large impact on the universe we live in. While neutrinos only interact with matter through the weak nuclear force and gravity, they played a critical role in the formation of the early universe. Neutrino physics is now an exciting line of research pursued by the Hortons of particle physics, cosmology, and astrophysics alike. While most of what we currently know about neutrinos is well described by a three-flavor neutrino model, a few inconsistent experimental results such as those from the Liquid Scintillator Neutrino Detector (LSND) and the Mini Booster Neutrino Experiment (MiniBooNE) hint at the presence of a new kind of neutrino that only interacts with matter through gravity. If this “sterile” kind of neutrino does in fact exist, it might also have played an important role in the evolution of our universe.

Horton hears a sterile neutrino? Source: imdb.com

The three known neutrinos come in three flavors: electron, muon, or tau. The discovery of neutrino oscillation by the Sudbury Neutrino Observatory and the Super-Kamiokande Observatory, which won the 2015 Nobel Prize, proved that one flavor of neutrino can transform into another. This led to the realization that each neutrino mass state is a superposition of the three different neutrino flavor states. From neutrino oscillation measurements, most of the parameters that define the mixing between neutrino states are well known for the three standard neutrinos.

The relationship between the three known neutrino flavor states and mass states is usually expressed as a 3×3 matrix known as the PMNS matrix, for Bruno Pontecorvo, Ziro Maki, Masami Nakagawa and Shoichi Sakata. The PMNS matrix includes three mixing angles, the values of which determine “how much” of each neutrino flavor state is in each mass state. The distance required for one neutrino flavor to become another, the neutrino oscillation wavelength, is determined by the difference between the squared masses of the two mass states. The values of mass splittings m_2^2-m_1^2 and m_3^2-m_2^2 are known to good precision.

A fourth flavor? Adding a sterile neutrino to the mix

A “sterile” neutrino is referred to as such because it would not interact weakly: it would only interact through the gravitational force. Neutrino oscillations involving the hypothetical sterile neutrino can be understood using a “four-flavor model,” which introduces a fourth neutrino mass state, m_4, heavier than the three known “active” mass states. This fourth neutrino state would be mostly sterile, with only a small contribution from a mixture of the three known neutrino flavors. If the sterile neutrino exists, it should be possible to experimentally observe neutrino oscillations with a wavelength set by the difference between m_4^2 and the square of the mass of another known neutrino mass state. Current observations suggest a squared mass difference in the range of 0.1-10 eV^2.

Oscillations between active and sterile states would result in the disappearance of muon (anti)neutrinos and electron (anti)neutrinos. In a disappearance experiment, you know how many neutrinos of a specific type you produce, and you count the number of that type of neutrino a distance away, and find that some of the neutrinos have “disappeared,” or in other words, oscillated into a different type of neutrino that you are not detecting.

A joint analysis by the MINOS and Daya Bay collaborations

The MINOS and Daya Bay collaborations have conducted a joint analysis to combine independent measurements of muon (anti)neutrino disappearance by MINOS and electron antineutrino disappearance by Daya Bay and Bugey-3. Here’s a breakdown of the involved experiments:

  • MINOS, the Main Injector Neutrino Oscillation Search: A long-baseline neutrino experiment with detectors at Fermilab and northern Minnesota that use an accelerator at Fermilab as the neutrino source
  • The Daya Bay Reactor Neutrino Experiment: Uses antineutrinos produced by the reactors of China’s Daya Bay Nuclear Power Plant and the Ling Ao Nuclear Power Plant
  • The Bugey-3 experiment: Performed in the early 1990s, used antineutrinos from the Bugey Nuclear Power Plant in France for its neutrino oscillation observations
Screen Shot 2016-09-12 at 10.22.49 AM
MINOS and Daya Bay/Bugey-3 combined 90% confidence level limits (in red) compared to the LSND and MiniBooNE 90% confidence level allowed regions (in green/purple). Plots the mass splitting between mass states 1 and 4 (corresponding to the sterile neutrino) against a function of the \mu-e mixing angle, which is equivalent to a function involving the 1-4 and 2-4 mixing angles. Regions of parameter space to the right of the red contour are excluded, counting out the majority of the LSND/MiniBooNE allowed regions. Source: arXiv:1607.01177v4.

Assuming a four-flavor model, the MINOS and Daya Bay collaborations put new constraints on the value of the mixing angle \theta_{\mu e}, the parameter controlling electron (anti)neutrino appearance in experiments with short neutrino travel distances. As for the hypothetical sterile neutrino? The analysis excluded the parameter space allowed by the LSND and MiniBooNE appearance-based indications for the existence of light sterile neutrinos for \Delta m_{41}^2 < 0.8 eV^2 at a 95% confidence level. In other words, the MINOS and Daya Bay analysis essentially rules out the LSND and MiniBooNE inconsistencies that allowed for the presence of a sterile neutrino in the first place. These results illustrate just how at odds disappearance searches and appearance searches are when it comes to providing insight into the existence of light sterile neutrinos. If the Whos exist, they will need to be a little louder in order for the world to hear them.

 

Background reading:

The CMB sheds light on galaxy clusters: Observing the kSZ signal with ACT and BOSS

Article: Detection of the pairwise kinematic Sunyaev-Zel’dovich effect with BOSS DR11 and the Atacama Cosmology Telescope
Authors: F. De Bernardis, S. Aiola, E. M. Vavagiakis, M. D. Niemack, N. Battaglia, and the ACT Collaboration
Reference: arXiv:1607.02139

Editor’s note: this post is written by one of the students involved in the published result.

Like X-rays shining through your body can inform you about your health, the cosmic microwave background (CMB) shining through galaxy clusters can tell us about the universe we live in. When light from the CMB is distorted by the high energy electrons present in galaxy clusters, it’s called the Sunyaev-Zel’dovich effect. A new 4.1σ measurement of the kinematic Sunyaev-Zel’dovich (kSZ) signal has been made from the most recent Atacama Cosmology Telescope (ACT) cosmic microwave background (CMB) maps and galaxy data from the Baryon Oscillation Spectroscopic Survey (BOSS). With steps forward like this one, the kinematic Sunyaev-Zel’dovich signal could become a probe of cosmology, astrophysics and particle physics alike.

The Kinematic Sunyaev-Zel’dovich Effect

It rolls right off the tongue, but what exactly is the kinematic Sunyaev-Zel’dovich signal? Galaxy clusters distort the cosmic microwave background before it reaches Earth, so we can learn about these clusters by looking at these CMB distortions. In our X-ray metaphor, the map of the CMB is the image of the X-ray of your arm, and the galaxy clusters are the bones. Galaxy clusters are the largest gravitationally bound structures we can observe, so they serve as important tools to learn more about our universe. In its essence, the Sunyaev-Zel’dovich effect is inverse-Compton scattering of cosmic microwave background photons off of the gas in these galaxy clusters, whereby the photons gain a “kick” in energy by interacting with the high energy electrons present in the clusters.

The Sunyaev-Zel’dovich effect can be divided up into two categories: thermal and kinematic. The thermal Sunyaev-Zel’dovich (tSZ) effect is the spectral distortion of the cosmic microwave background in a characteristic manner due to the photons gaining, on average, energy from the hot (~107 – 108 K) gas of the galaxy clusters. The kinematic (or kinetic) Sunyaev-Zel’dovich (kSZ) effect is a second-order effect—about a factor of 10 smaller than the tSZ effect—that is caused by the motion of galaxy clusters with respect to the cosmic microwave background rest frame. If the CMB photons pass through galaxy clusters that are moving, they are Doppler shifted due to the cluster’s peculiar velocity (the velocity that cannot be explained by Hubble’s law, which states that objects recede from us at a speed proportional to their distance). The kinematic Sunyaev-Zel’dovich effect is the only known way to directly measure the peculiar velocities of objects at cosmological distances, and is thus a valuable source of information for cosmology. It allows us to probe megaparsec and gigaparsec scales – that’s around 30,000 times the diameter of the Milky Way!

A schematic of the Sunyaev-Zel’dovich effect resulting in higher energy (or blue shifted) photons of the cosmic microwave background (CMB) when viewed through the hot gas present in galaxy clusters. Source: UChicago Astronomy.

 

Measuring the kSZ Effect

To make the measurement of the kinematic Sunyaev-Zel’dovich signal, the Atacama Cosmology Telescope (ACT) collaboration used a combination of cosmic microwave background maps from two years of observations by ACT. The CMB map used for the analysis overlapped with ~68000 galaxy sources from the Large Scale Structure (LSS) DR11 catalog of the Baryon Oscillation Spectroscopic Survey (BOSS). The catalog lists the coordinate positions of galaxies along with some of their properties. The most luminous of these galaxies were assumed to be located at the centers of galaxy clusters, so temperature signals from the CMB map were taken at the coordinates of these galaxy sources in order to extract the Sunyaev-Zel’dovich signal.

While the smallness of the kSZ signal with respect to the tSZ signal and the noise level in current CMB maps poses an analysis challenge, there exist several approaches to extracting the kSZ signal. To make their measurement, the ACT collaboration employed a pairwise statistic. “Pairwise” refers to the momentum between pairs of galaxy clusters, and “statistic” indicates that a large sample is used to rule out the influence of unwanted effects.

Here’s the approach: nearby galaxy clusters move towards each other on average, due to gravity. We can’t easily measure the three-dimensional momentum of clusters, but the average pairwise momentum can be estimated by using the line of sight component of the momentum, along with other information such as redshift and angular separations between clusters. The line of sight momentum is directly proportional to the measured kSZ signal: the microwave temperature fluctuation which is measured from the CMB map. We want to know if we’re measuring the kSZ signal when we look in the direction of galaxy clusters in the CMB map. Using the observed CMB temperature to find the line of sight momenta of galaxy clusters, we can estimate the mean pairwise momentum as a function of cluster separation distance, and check to see if we find that nearby galaxies are indeed falling towards each other. If so, we know that we’re observing the kSZ effect in action in the CMB map.

For the measurement quoted in their paper, the ACT collaboration finds the average pairwise momentum as a function of galaxy cluster separation, and explores a variety of error determinations and sources of systematic error. The most conservative errors based on simulations give signal-to-noise estimates that vary between 3.6 and 4.1.

The mean pairwise momentum estimator and best fit model for a selection of 20000 objects from the DR11 Large Scale Structure catalog, plotted as a function of comoving separation. The dashed line is the linear model, and the solid line is the model prediction including nonlinear redshift space corrections. The best fit provides a 4.1σ evidence of the kSZ signal in the ACTPol-ACT CMB map. Source: arXiv:1607.02139.
The mean pairwise momentum estimator and best fit model for a selection of 20000 objects from the DR11 Large Scale Structure catalog, plotted as a function of comoving separation. The dashed line is the linear model, and the solid line is the model prediction including nonlinear redshift space corrections. The best fit provides a 4.1σ evidence of the kSZ signal in the ACTPol-ACT CMB map. Source: arXiv:1607.02139.

The ACT and BOSS results are an improvement on the 2012 ACT detection, and are comparable with results from the South Pole Telescope (SPT) collaboration that use galaxies from the Dark Energy Survey. The ACT and BOSS measurement represents a step forward towards improved extraction of kSZ signals from CMB maps. Future surveys such as Advanced ACTPol, SPT-3G, the Simons Observatory, and next-generation CMB experiments will be able to apply the methods discussed here to improved CMB maps in order to achieve strong detections of the kSZ effect. With new data that will enable better measurements of galaxy cluster peculiar velocities, the pairwise kSZ signal will become a powerful probe of our universe in the years to come.

Implications and Future Experiments

One interesting consequence for particle physics will be more stringent constraints on the sum of the neutrino masses from the pairwise kinematic Sunyaev-Zel’dovich effect. Upper bounds on the neutrino mass sum from cosmological measurements of large scale structure and the CMB have the potential to determine the neutrino mass hierarchy, one of the next major unknowns of the Standard Model to be resolved, if the mass hierarchy is indeed a “normal hierarchy” with ν3 being the heaviest mass state. If the upper bound of the neutrino mass sum is measured to be less than 0.1 eV, the inverted hierarchy scenario would be ruled out, due to there being a lower limit on the mass sum of ~0.095 eV for an inverted hierarchy and ~0.056 eV for a normal hierarchy.

Forecasts for kSZ measurements in combination with input from Planck predict possible constraints on the neutrino mass sum with a precision of 0.29 eV, 0.22 eV and 0.096 eV for Stage II (ACTPol + BOSS), Stage III (Advanced ACTPol + BOSS) and Stage IV (next generation CMB experiment + DESI) surveys respectively, with the possibility of much improved constraints with optimal conditions. As cosmic microwave background maps are improved and Sunyaev-Zel’dovich analysis methods are developed, we have a lot to look forward to.

 

Background reading:

The dawn of multi-messenger astronomy: using KamLAND to study gravitational wave events GW150914 and GW151226

Article: Search for electron antineutrinos associated with gravitational wave events GW150914 and GW151226 using KamLAND
Authors: KamLAND Collaboration
Reference: arXiv:1606.07155

 

After the chirp heard ‘round the world, the search is on for coincident astrophysical particle events to provide insight into the source and nature of the era-defining gravitational wave events detected by the LIGO Scientific Collaboration in late 2015.

By combining information from gravitational wave (GW) events with the detection of astrophysical neutrinos and electromagnetic signatures such as gamma-ray bursts, physicists and astronomers are poised to draw back the curtain on the dynamics of astrophysical phenomena, and we’re surely in for some surprises.

The first recorded gravitational wave event, GW150914, was likely a merger of two black holes which took place more than one billion light years from the Earth. The event’s name marks the day it was observed by the Advanced Laser Interferometer Gravitational-wave Observatory (LIGO), September 14th, 2015.  LIGO detections are named “GW” for “gravitational wave,” followed by the observation date in YYMMDD format. The second event, GW151226 (December 26th, 2015) was likely another merger of two black holes, having 8 and 14 times the mass of the sun, taking place 1.4 billion light years away from Earth. A third gravitational wave event candidate, LVT151012, a possible black hole merger which occurred on October 12th, 2015, did not reach the same detection significance a the aforementioned events, but still has a >50% chance of astrophysical origin. LIGO candidates are named differently than detections. The names start with “LVT” for “LIGO-Virgo Trigger,” but are followed by the observation date in the same YYMMDD format. The different name indicates that the event was not significant enough to be called a gravitational wave.

 

Two black holes spiral in towards one another and merge to emit a burst of gravitational waves that Advanced LIGO can detect. Source: APS Physics.

The following  computer simulation created by the multi-university SXS (Simulating eXtreme Spacetimes) project depicts what the collision of two black holes would look like  if we could get close enough to the merger. It was created by solving equations from Albert Einstein’s general theory of relativity using the LIGO data. (Source: LIGO Lab Caltech : MIT).

Observations from other scientific collaborations can search for particles associated with these gravitational waves. The combined information from the gravitational wave and particle detections could identify the origin of these gravitational wave events. For example, some violent astrophysical phenomena emit not only gravitational waves, but also high-energy neutrinos. Conversely, there is currently no known mechanism for the production of either neutrinos or electromagnetic waves in a black hole merger.

Black holes with rapidly accreting disks can be the origin of gamma-ray bursts and neutrino signals, but these disks are not expected to be present during mergers like the ones detected by LIGO. For this reason, it was surprising when the Fermi Gamma-ray Space Telescope reported a coincident gamma-ray burst occurring 0.4 seconds after the September GW event with a false alarm probability of 1 in 455. Although there is some debate in the community about whether or not this observation is to be believed, the observation motivates a multi-messenger analysis including the hunt for associated astrophysical neutrinos at all energies.

Could a neutrino experiment like KamLAND find low energy antineutrino events coincident with the GW events, even when higher energy searches by IceCube and ANTARES did not?

 

Schematic diagram of the KamLAND detector. Source:  hep-ex/0212021v1

KamLAND, the Kamioka Liquid scintillator Anti-Neutrino Detector, is located under Mt. Ikenoyama, Japan, buried beneath the equivalent of 2,700 meters of water. It consists of an 18 meter diameter stainless steel sphere, the inside of which is covered with photomultiplier tubes, surrounding an EVOH/nylon balloon enclosed by pure mineral oil. Inside the balloon resides 1 kton of highly purified liquid scintillator. Outside the stainless steel sphere is a cylindrical 3.2 kton water-Cherenkov detector that provides shielding and enables cosmic ray muon identification.

KamLAND is optimized to search for ~MeV neutrinos and antineutrinos. The detection of the gamma ray burst by the Fermi telescope suggests that the detected black hole merger might have retained its accretion disk, and the spectrum of accretion disk neutrinos around a single black hole is expected to peak around 10 MeV, so KamLAND searched for correlations between the LIGO GW events and ~10 MeV electron antineutrino events occurring within a 500 second window of the merger events. Researchers focused on the detection of electron antineutrinos through the inverse beta decay reaction.

No events were found within the target window of any gravitational wave event, and any adjacent event was consistent with background. KamLAND researchers used this information to determine a monochromatic fluence (time integrated flux) upper limit, as well as an upper limit on source luminosity for each gravitational wave event, which places a bound on the total energy released as low energy neutrinos during the merger events and candidate event. The lack of detected concurrent inverse beta decay events supports the conclusion that GW150914 was a black hole merger, and not another astrophysical event such as a core-collapse supernova.

More information would need to be obtained to explain the gamma ray burst observed by the Fermi telescope, and work to improve future measurements is ongoing. Large uncertainties in the origin region of gamma ray bursts observed by the Fermi telescope will be reduced, and the localization of GW events will be improved, most drastically so by the addition of a third LIGO detector (LIGO India).

As Advanced LIGO continues its operation, there will likely be many more chances for KamLAND and other neutrino experiments to search for coincidence neutrinos. Multi-messenger astronomy has only just begun to shed light on the nature of black holes, supernovae, mergers, and other exciting astrophysical phenomena — and the future looks bright.

Background reading: