Tag: Fermilab

• Challenging the neutrino signal anomaly

  A gentle reminder before we begin: you’re allowed to be interested in particle physics. 😉

  Neutrinos are among the most mysterious particles in physics. They are extremely light, electrically neutral, and interact so weakly with matter that trillions of them pass through your body each second without leaving a trace. They are produced in the Sun, nuclear reactors, the atmosphere, and by cosmic explosions. In fact neutrinos are everywhere — yet they’re almost invisible.

  Despite their elusiveness, they have already upended physics. In the late 20th century, scientists discovered that neutrinos can oscillate, changing from one type to another as they travel, which is something that the simplest version of the Standard Model of particle physics — the prevailing theory of elementary particles — doesn’t predict. Because oscillations require neutrinos to have mass, this discovery revealed new physics. Today, scientists study neutrinos for what they might tell us about the universe’s structure and for possible hints of particles or forces yet unknown.

  

  However, detecting neutrinos is very hard. Because they rarely interact with matter, experiments must build massive detectors filled with dense material in the hopes that a small fraction of neutrinos will collide inside with atoms. One way to detect such collisions uses Cherenkov radiation, a bluish glow emitted when a charged particle moves through a medium like water or mineral oil faster than light does in that medium.

  (This is allowed. The only speed limit is that of light in vacuum: 299,792,458 m/s.)

  The MiniBooNE experiment at Fermilab used a large mineral-oil Cherenkov detector. When neutrinos from the Booster Neutrino Beamline struck the atomic nuclei in the mineral oil, the interaction released charged particles, which sometimes produced rings of Cherenkov radiation (like ripples) that the detector recorded. In MiniBooNE’s data, the detection events were classified by the type of light ring produced. An “electron-like” event was one that looked like it had been caused by an electron. But because photons can also produce nearly identical rings when they strike the nuclei, the detector couldn’t always tell the difference. A “muon-like” event, on the other hand, had the distinctive ring pattern of a muon, which is a subatomic particle like the electron but 200-times heavier, and which travels in a straighter, longer track. To be clear, these labels described the detector’s view; they didn’t  guarantee which particle was actually present.

  MiniBooNE began operating in 2002 to test an anomaly that had been reported at the LSND experiment at Los Alamos. LSND had recorded more electron-like” events than predicted, especially at low energies below about 600 MeV. This came to be called the “low-energy excess” and has become one of the most puzzling results in particle physics. It raised the possibility that neutrinos might be oscillating into a hitherto unknown neutrino type, sometimes called the sterile neutrino — or it might have been a hint of unexpected processes that produced extra photons. Since MiniBooNE couldn’t reliably distinguish electrons from photons, the mystery remained unresolved.

  To address this, scientists built the MicroBooNE experiment at Fermilab. It uses a very different technology: the liquid argon time-projection chamber (LArTPC). In a LArTPC, charged particles streak through an ultra-pure mass of liquid argon, leaving a trail of ionised atoms in their wake. An applied electric field causes these trails to drift towards fine wires, where they are recorded. At the same time, the argon emits light that provides the timing of the interaction. This allows the detector to reconstruct interactions in three dimensions with millimetre precision. Crucially, it lets physicists see where the particle shower begins, so they can tell whether it started at the interaction point or some distance away. This capability prepared MicroBooNE to revisit the “low-energy excess” anomaly.

  MicroBooNE also had broader motivations. With an active mass of about 90 tonnes of liquid argon inside a 170-tonne cryostat, and 8,256 wires in its readout planes, it was the largest LArTPC in the US when it began operating. It served as a testbed for the much larger detectors that scientists are developing for the Deep Underground Neutrino Experiment (DUNE). And it was also designed to measure the rate at which neutrinos interacted with argon atoms, to study nuclear effects in neutrino scattering, and to contribute to searches for rare processes such as proton decay and supernova neutrino bursts.

  (When a star goes supernova, it releases waves upon waves of neutrinos before it releases photons. Scientists were able to confirm this when the star Sanduleak -69 202 exploded in 1987.)

  

  Initial MicroBooNE analyses using partial data already challenged the idea that MiniBooNE’s excess was due to the anomaly. However, the collaboration didn’t cover the full range of parameters until recently. On August 21, MicroBooNE published results from five years of operations, corresponding to 1.11 x 10²¹ protons on target, which was about a 70% increase over previous analyses. This complete dataset together with higher sensitivity and better modelling has provided the most decisive test so far of the anomaly.

  The MicroBooNE detector recorded neutrino interactions from the Booster Neutrino Beamline, a setup that produces neutrinos, using its LArTPC detector, which operated at about 87 K inside a cryostat. Charged particles from neutrino interactions produced ionisation electrons that drifted across the detector and were recorded by the wire. Simultaneous flashes of argon scintillation light, seen by photomultiplier tubes, gave the precise time of each interaction.

  In neutrino physics, a category of events grouped by what the detector sees in the final state is called a channel. Researchers call it a signal channel when it matches the kind of event they are specifically looking for, as opposed to background signals from other processes. With MicroBooNE, the team stayed on the lookout for two signal channels: (i) one electron and no visible protons or pions (abbreviated as 1e0p0π) and (ii) one electron and at least one proton above 40 MeV (1eNp0π). These categories reflect what MiniBooNE would’ve seen as electron-like events while exploiting MicroBooNE’s ability to identify protons.

  One important source of background noise the team had to cut from the data was cosmic rays — high-energy particles from outer space that strike Earth’s atmosphere, creating particle showers that can mimic neutrino signals. In 2017, MicroBooNE added a suite of panels around the detector. For the full dataset, the panels cut an additional 25.4% of background noise in the 1e0p0π channel while preserving 98.9% of signal events.

  

  In the final analysis, the MicroBooNE data showed no evidence of an anomalous excess of electron-like events. When both channels were combined, the observed events matched the expectations of the Standard Model of particle physics well. The agreement was especially strong in the 1e0p0π channel.

  In the 1eNp0π channel, MicroBooNE actually detected slightly fewer events than the Model predicted: 102 events v. 134. This shortfall, of about 24%, is however not enough to claim a new effect but enough to draw attention. But rather than confirming MiniBooNE’s excess, this result suggests there’s some tension in the models the scientists use to simulate how the neutrinos and argon atoms will interact. Argon has a large and complex nucleus, which makes accurate predictions challenging. The scientists have in fact stated in their paper that the deficit may reflect these uncertainties rather than new physics.

  The new MicroBooNE results have far-reaching consequences. Foremost, the results reshape the sterile-neutrino debate. For two decades, the LSND and MiniBooNE anomalies had been cited together as signs that the neutrino was oscillating into a previously undetected state. By showing that MiniBooNE’s excess was not due to extra electron-like interactions, MicroBooNE shows that the ‘extra’ events were not caused by excess electron neutrinos. This in turn casts doubt on the simplest explanation, of sterile neutrinos.

  As a result, theoretical models that once seemed straightforward now face strong tension. While more complex scenarios remain possible, the easy explanation is no longer viable.

  

  Second, they demonstrate the maturity of the LArTPC technology. The MicroBooNE team successfully operated a large detector for years, maintaining the argon’s purity and low-noise electronics required for high-resolution imaging. Its performance validates the design choices for larger detectors like DUNE, which use similar technology but at kilotonne scales. The experiment also showcases innovations such as cryogenic electronics, sophisticated purification systems, protection against cosmic rays, and calibration with ultraviolet lasers, proving that such systems can deliver reliable data over long periods of operation.

  Third, the modest deficit in the 1eNp0π channel points to the importance of better understanding neutrino-argon interactions. Argon’s heavy nucleus produces complicated final states where protons and neutrons may scatter or be absorbed, altering the visible event. These nuclear effects can lead to mismatches between simulation and data (possibly including the 24% deficit in the 1eNp0π signal channel). For DUNE, which will also use argon as its target, improving these models is critical. MicroBooNE’s detailed datasets and sideband constraints will continue to inform these refinements.

  Fourth, the story highlights the value of complementary detector technologies. MiniBooNE’s Cherenkov detector recorded more events but couldn’t tell electrons from photons; MicroBooNE’s LArTPC recorded fewer events but with much greater clarity. Together, they show how one experiment can identify a puzzle and another can test it with a different method. This multi-technology approach is likely to continue as experiments worldwide cross-check anomalies and precision measurements.

  Finally, the MicroBooNE results show how science advances. A puzzling anomaly inspired new theories, new technology, and a new experiment. After five years of data-taking and with the most complete analysis yet, MicroBooNE has said that the MiniBooNE anomaly was not due to electron-neutrino interactions. The anomaly itself remains unexplained, but the field now has a sharper focus. Whether the cause lies in photon production, detector effects or actually new physics, the next generation of experiments can start on firmer footing.

  2025.09.06

• Hype from Fermilab

  Where do you think the following bit of text is from?

  A wormhole, also known as an Einstein-Rosen bridge, is a hypothetical tunnel connecting remote points in spacetime. While wormholes are allowed by Albert Einstein’s theory of relativity, wormholes have never been found in the universe. In late 2022, the journal Nature featured a paper co-written by Joe Lykken, leader of the Fermilab Quantum Institute, that describes observable phenomenon produced by a quantum processor that “are consistent with the dynamics of a transversable wormhole.” Working with a Sycamore quantum computer at Google, a team of physicists was able to transfer information from one area of the computer to another through a quantum system utilizing artificial intelligence hardware.

  If you’ve been following the hoopla surrounding this paper, esp. over the way it was reported by Quanta and many other outlets, your first guess might be that this is yet another news outlet that ignored the difference between an actual, physical wormhole and a simulation of a mathematical version of an actual, physical wormhole (the paper’s authors, a group to which Lykken belongs, accomplished the latter). But no: this text is from Fermilab itself! It appears on a page announcing a forthcoming lecture by Lykken on February 17. (Hat-tip to Peter Woit for discovering and flagging this on his blog.)

  What I’d like to point out here, for the hundredth time I’m sure, is that hype originates more often than you think from university and institute press offices rather than in the minds and hearts of science journalists. Insufficiently critical reportage (awareness of which is sometimes only possible in hindsight) often fails to stop hype from reaching a larger audience but it seldom creates hype in the first place. This may seem like a fine point but it matters when there is a tendency to overlook the role of press officers, and some scientists themselves (including Lykken), in building the narratives around their and their colleagues’ work.

  2023.02.09

• Why are the Nobel Prizes still relevant?

  Note: A condensed version of this post has been published in The Wire.

  Around this time last week, the world had nine new Nobel Prize winners in the sciences (physics, chemistry and medicine), all but one of whom were white and none were women. Before the announcements began, Göran Hansson, the Swede-in-chief of these prizes, had said the selection committee has been taking steps to make the group of laureates more racially and gender-wise inclusive, but it would seem they’re incremental measures, as one editorial in the journal Nature pointed out.

  Hansson and co. seems to find the argument that the Nobel Prizes award achievements at a time where there weren’t many women in science tenable when in fact it distracts from the selection committee’s bizarre oversight of such worthy names as Lise Meitner, Vera Rubin, Chien-Shiung Wu, etc. But Hansson needs to understand that the only meaningful change is change that happens right away because, even for this significant flaw that should by all means have diminished the prizes to a contest of, for and by men, the Nobel Prizes have only marginally declined in reputation.

  Why do they matter when they clearly shouldn’t?

  For example, according to the most common comments received in response to articles by The Wire shared on Twitter and Facebook, and always from men, the prizes reward excellence, and excellence should brook no reservation, whether by caste or gender. As is likely obvious to many readers, this view of scholastic achievement resembles a blade of grass: long, sprouting from the ground (the product of strong roots but out of sight, out of mind), rising straight up and culminating in a sharp tip.

  However, achievement is more like a jungle: the scientific enterprise – encompassing research institutions, laboratories, the scientific publishing industry, administration and research funding, social security, availability of social capital, PR, discoverability and visibility, etc. – incorporates many vectors of bias, discrimination and even harassment towards its more marginalised constituents. Your success is not your success alone; and if you’re an upper-caste, upper-class, English-speaking man, you should ask yourself, as many such men have been prompted to in various walks of life, who you might have displaced.

  This isn’t a witch-hunt as much as an opportunity to acknowledge how privilege works and what we can do to make scientific work more equal, equitable and just in future. But the idea that research is a jungle and research excellence is a product of the complex interactions happening among its thickets hasn’t found meaningful purchase, and many people still labour with a comically straightforward impression that science is immune to social forces. Hansson might be one of them if his interview to Nature is anything to go by, where he says:

  … we have to identify the most important discoveries and award the individuals who have made them. If we go away from that, then we’ve devalued the Nobel prize, and I think that would harm everyone in the end.

  In other words, the Nobel Prizes are just going to look at the world from the top, and probably from a great distance too, so the jungle has been condensed to a cluster of pin-pricks.

  Another reason why the Nobel Prizes haven’t been easy to sideline is that the sciences’ ‘blade of grass’ impression is strongly historically grounded, with help from notions like scientific knowledge spreads from the Occident to the Orient.

  Who’s the first person that comes to mind when I say “Nobel Prize for physics”? I bet it’s Albert Einstein. He was so great that his stature as a physicist has over the decades transcended his human identity and stamped the Nobel Prize he won in 1921 with an indelible mark of credibility. Now, to win a Nobel Prize in physics is to stand alongside Einstein himself.

  This union between a prize and its laureate isn’t unique to the Nobel Prize or to Einstein. As I’ve said before, prizes are elevated by their winners. When Margaret Atwood wins the Booker Prize, it’s better for the prize than it is for her; when Isaac Asimov won a Hugo Award in 1963, near the start of his career, it was good for him, but it was good for the prize when he won it for the sixth time in 1992 (the year he died). The Nobel Prizes also accrued a substantial amount of prestige this way at a time when it wasn’t much of a problem, apart from the occasional flareup over ignoring deserving female candidates.

  That their laureates have almost always been from Europe and North America further cemented the prizes’ impression that they’re the ultimate signifier of ‘having made it’, paralleling the popular undercurrent among postcolonial peoples that science is a product of the West and that they’re simply its receivers.

  That said, the prize-as-proxy issue has contributed considerably as well to preserving systemic bias at the national and international levels. Winning a prize (especially a legitimate one) accords the winner’s work with a modicum of credibility and the winner, of prestige. Depending on how the winners of a prize to be awarded suitably in the future are to be selected, such credibility and prestige could be potentiated to skew the prize in favour of people who have already won other prizes.

  For example, a scientist-friend ranted to me about how, at a conference he had recently attended, another scientist on stage had introduced himself to his audience by mentioning the impact factors of the journals he’d had his papers published in. The impact factor deserves to die because, among other reasons, it attempts to condense multi-dimensional research efforts and the vagaries of scientific publishing into a single number that stands for some kind of prestige. But its users should be honest about its actual purpose: it was designed so evaluators could take one look at it and decide what to do about a candidate to whom it corresponded. This isn’t fair – but expeditiousness isn’t cheap.

  And when evaluators at different rungs of the career advancement privilege the impact factor, scientists with more papers published earlier in their careers in journals with higher impact factors become exponentially likelier to be recognised for their efforts (probably even irrespective of their quality given the unique failings of high-IF journals, discussed here and here) over time than others.

  Brian Skinner, a physicist at Ohio State University, recently presented a mathematical model of this ‘prestige bias’ and whose amplification depended in a unique way, according him, on a factor he called the ‘examination precision’. He found that the more ambiguously defined the barrier to advancement is, the more pronounced the prestige bias could get. Put another way, people who have the opportunity to maintain systemic discrimination simultaneously have an incentive to make the points of entry into their club as vague as possible. Sound familiar?

  One might argue that the Nobel Prizes are awarded to people at the end of their careers – the average age of a physics laureate is in the late 50s; John Goodenough won the chemistry prize this year at 97 – so the prizes couldn’t possibly increase the likelihood of a future recognition. But the sword cuts both ways: the Nobel Prizes are likelier than not to be the products a prestige bias amplification themselves, and are therefore not the morally neutral symbols of excellence Hansson and his peers seem to think they are.

  Fourth, the Nobel Prizes are an occasion to speak of science. This implies that those who would deride the prizes but at the same time hold them up are equally to blame, but I would agree only in part. This exhortation to try harder is voiced more often than not by those working in the West, with publications with better resources and typically higher purchasing power. On principle I can’t deride the decisions reporters and editors make in the process of building an audience for science journalism, with the hope that it will be profitable someday, all in a resource-constrained environment, even if some of those choices might seem irrational.

  (The story of Brian Keating, an astrophysicist, could be illuminating at this juncture.)

  More than anything else, what science journalism needs to succeed is a commonplace acknowledgement that science news is important – whether it’s for the better or the worse is secondary – and the Nobel Prizes do a fantastic job of getting the people’s attention towards scientific ideas and endeavours. If anything, journalists should seize the opportunity in October every year to also speak about how the prizes are flawed and present their readers with a fuller picture.

  Finally, and of course, we have capitalism itself – implicated in the quantum of prize money accompanying each Nobel Prize (9 million Swedish kronor, Rs 6.56 crore or $0.9 million).

  Then again, this figure pales in comparison to the amounts that academic institutions know they can rake in by instrumentalising the prestige in the form of donations from billionaires, grants and fellowships from the government, fees from students presented with the tantalising proximity to a Nobel laureate, and in the form of press coverage. L’affaire Epstein even demonstrated how it’s possible to launder a soiled reputation by investing in scientific research because institutions won’t ask too many questions about who’s funding them.

  The Nobel Prizes are money magnets, and this is also why winning a Nobel Prize is like winning an Academy Award: you don’t get on stage without some lobbying. Each blade of grass has to mobilise its own PR machine, supported in all likelihood by the same institute that submitted their candidature to the laureates selection committee. The Nature editorial called this out thus:

  As a small test case, Nature approached three of the world’s largest international scientific networks that include academies of science in developing countries. They are the International Science Council, the World Academy of Sciences and the InterAcademy Partnership. Each was asked if they had been approached by the Nobel awarding bodies to recommend nominees for science Nobels. All three said no.

  I believe those arguments that serve to uphold the Nobel Prizes’ relevance must take recourse through at least one of these reasons, if not all of them. It’s also abundantly clear that the Nobel Prizes are important not because they present a fair or useful picture of scientific excellence but in spite of it.

  2019.10.16