How do you see a subatomic neutrino in a “haystack” of particles streaming from space? That’s the daunting prospect going through physicists learning neutrinos with detectors close to Earth’s floor. With little to no shielding in such non-subterranean places, surface-based neutrino detectors, normally trying to find neutrinos produced by particle accelerators, are bombarded by cosmic rays—relentless showers of subatomic and nuclear particles produced in Earth’s ambiance by interactions with particles streaming from more-distant cosmic places. These ample vacationers, largely muons, create an online of crisscrossing particle tracks that may simply obscure a uncommon neutrino occasion.
Fortunately, physicists have developed instruments to tone down the cosmic “noise.”
A workforce together with physicists from the U.S. Department of Energy’s Brookhaven National Laboratory describes the method in two papers lately accepted to be printed in Physical Review Applied and the Journal of Instrumentation (JINST). These papers show the scientists’ potential to extract clear neutrino alerts from the MicroBooNE detector at DOE’s Fermi National Accelerator Laboratory (Fermilab). The methodology combines CT-scanner-like picture reconstruction with data-sifting methods that make accelerator-produced neutrino alerts stand out 5 to 1 in opposition to the cosmic ray background.
“We developed a set of algorithms that reduce the cosmic ray background by a factor of 100,000,” stated Chao Zhang, one of many Brookhaven Lab physicists who helped to develop the data-filtering methods. Without the filtering, MicroBooNE would see 20,000 cosmic rays for each neutrino interplay, he stated. “This paper demonstrates the crucial ability to eliminate the cosmic ray backgrounds.”
Bonnie Fleming, a professor at Yale University who’s a co-spokesperson for MicroBooNE, stated, “This work is critical both for MicroBooNE and for the future U.S. neutrino research program. Its impact will extend notably beyond the use of this “Wire-Cell’ evaluation method, even on MicroBooNE, the place different reconstruction paradigms have adopted these data-sorting strategies to dramatically cut back cosmic ray backgrounds.”
MicroBooNE is one among three detectors that type the worldwide Short-Baseline Neutrino program at Fermilab, every situated a special distance from a particle accelerator that generates a fastidiously managed neutrino beam. The three detectors are designed to depend up various kinds of neutrinos at rising distances to search for discrepancies from what’s anticipated based mostly on the combo of neutrinos within the beam and what’s recognized about neutrino “oscillation.” Oscillation is a course of by which neutrinos swap identities amongst three recognized sorts, or “flavors.” Spotting discrepancies in neutrino counts might level to a brand new unknown oscillation mechanism—and presumably a fourth neutrino selection.
Brookhaven Lab scientists performed a significant position in designing the MicroBooNE detector, significantly the delicate electronics that function inside the detector’s super-cold liquid-argon-filled time projection chamber. As neutrinos from Fermilab’s accelerator enter the chamber, on occasion a neutrino will work together with an argon atom, kicking some particles out of its nucleus—a proton or a neutron—and producing different particles (muons, pions) and a flash of sunshine. The charged particles that get kicked out ionize argon atoms within the detector, knocking a few of their electrons out of orbit. The electrons that type alongside these ionization tracks get picked up by the detector’s delicate electronics.
“The whole trail of electrons drifts along an electric field and passes through three consecutive planes of wires with different orientations at one end of the detector,” Zhang stated. “As the electrons approach the wires, they induce a signal, so that each set of wires creates a 2D image of the track from a different angle.”
Meanwhile, the flashes of sunshine created on the time of the neutrino interplay get picked up by photomultiplier tubes that lie past the wire arrays. Those mild alerts inform scientists when the neutrino interplay came about, and the way lengthy it took the tracks to reach on the wire planes.
Computers translate that timing into distance and piece collectively the 2D monitor photos to reconstruct a 3D picture of the neutrino interplay within the detector. The form of the monitor tells scientists which taste of neutrino triggered the interplay.
“This 3D “Wire-Cell’ picture reconstruction is much like medical imaging with a computed tomography (CT) scanner,” Zhang explained. In a CT scanner, sensors capture snapshots of the body’s internal structures from different angles and computers piece the images together. “Imagine the particle tracks going by way of the three wire planes as an individual going into the scanner,” he stated.
Untangling the cosmic net
It sounds virtually easy—for those who overlook in regards to the 1000’s of cosmic rays that stream by way of the detector on the identical time. Their ionization trails additionally drift by way of the scanning wires, creating photos that seem like a tangled net. That’s why MicroBooNE scientists have been engaged on subtle “triggers” and algorithms to sift by way of the information to allow them to extract the neutrino alerts.
By 2017, they’d made substantial progress lowering the cosmic ray noise. But even then, cosmic rays outnumbered neutrino tracks by about 200 to 1. The new papers describe additional methods to scale back this ratio, and flip it to the purpose the place neutrino alerts in MicroBooNE now stand out 5 to 1 in opposition to the cosmic ray background.
The first step includes matching the alerts revealed by particles generated in neutrino interactions with the precise flashes of sunshine picked up by the photomultiplier tubes from that interplay.
“This is not easy!” stated Brookhaven Lab physicist Xin Qian. “Because the time projection chamber and the photomultiplier tubes are two different systems, we don’t know which flash corresponds to which event in the detector. We have to compare the light patterns for each photomultiplier tube with all the locations of these particles. If you’ve done all the matching correctly, you will find a single 3D object that corresponds to a single flash of light measured by the photomultiplier tubes.”
Brooke Russell, who labored on the evaluation as a Yale graduate scholar and is now a postdoctoral fellow at DOE’s Lawrence Berkeley National Laboratory, echoed these feedback on the problem of light-matching. “Given that the charge information is in some cases not fully complementary to the light information, there can be ambiguities in charge-light pairings on a single-readout basis. The algorithms developed by the team help to account for these nuances,” she stated.
Still, the scientists should then evaluate the timing of every monitor with the time accelerator neutrinos had been emitted (an element they know as a result of they management the accelerator beam). “If the timing is consistent, then it is a possible neutrino interaction,” Qian stated.
The algorithm developed by the Brookhaven workforce brings the ratio down to at least one neutrino for each six cosmic ray occasions.
Rejecting further cosmic rays will get a bit simpler with an algorithm that eliminates tracks that utterly traverse the detector.
“Most cosmic rays go through the detector from top to bottom or from one side to the other,” stated Xiangpan Ji, a Brookhaven Lab postdoc engaged on this algorithm. “If you can identify the point of entry and exit of the track, you know it’s a cosmic ray. Particles formed by neutrino interactions have to start in the middle of the detector where that interaction takes place.”
That brings the ratio of neutrino interactions to cosmic rays to 1:1.
An further algorithm screens out occasions that begin outdoors the detector and get stopped someplace within the center—which look much like neutrino occasions however transfer in the other way. And one last fine-tuning step guidelines out occasions the place the sunshine flashes do not match effectively with occasions, to convey the detection of neutrino occasions to the exceptional degree of 5 to 1 in contrast with cosmic rays.
“This is one of the most challenging analyses I have worked on,” stated Hanyu Wei, the Brookhaven Lab postdoctoral fellow main the evaluation effort. “The liquid-argon time projection chamber is a new detector technology with lots of surprising features. We had to invent many original methods. It was truly a team effort.”
Zhang echoed that sentiment and stated, “We expect this work to significantly boost the potential for the MicroBooNE experiment to explore the intriguing physics at short baselines. Indeed, we’re looking forward to implementing these techniques in experiments at all three short-baseline neutrino detectors to see what we learn about neutrino oscillations and the possible existence of a fourth neutrino type.”
Neutrino Event Selection within the MicroBooNE Liquid Argon Time Projection Chamber utilizing Wire-Cell 3-D Imaging, Clustering, and Charge-Light Matching. arxiv.org/abs/2011.01375
Cosmic Ray Background Rejection with Wire-Cell LArTPC Event Reconstruction within the MicroBooNE Detector. arxiv.org/abs/2101.05076
Brookhaven National Laboratory
Physicists obtain vital enchancment in recognizing accelerator-produced neutrinos in a cosmic haystack (2021, June 9)
retrieved 9 June 2021
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