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    3 Questions: Marking the 10th anniversary of the Higgs boson discovery

    This July 4 marks 10 years since the discovery of the Higgs boson, the long-sought particle that imparts mass to all elementary particles. The elusive particle was the last missing piece in the Standard Model of particle physics, which is our most complete model of the universe.

    In early summer of 2012, signs of the Higgs particle were detected in the Large Hadron Collider (LHC), the world’s largest particle accelerator, which is operated by CERN, the European Organization for Nuclear Research. The LHC is engineered to smash together billions upon billions of protons for the chance at producing the Higgs boson and other particles that are predicted to have been created in the early universe.

    In analyzing the products of countless proton-on-proton collisions, scientists registered a Higgs-like signal in the accelerator’s two independent detectors, ATLAS and CMS (the Compact Muon Solenoid). Specifically, the teams observed signs that a new particle had been created and then decayed to two photons, two Z bosons or two W bosons, and that this new particle was likely the Higgs boson.

    The discovery was revealed within the CMS collaboration, including over 3,000 scientists, on June 15, and ATLAS and CMS announced their respective observations to the world on July 4. More than 50 MIT physicists and students contributed to the CMS experiment, including Christoph Paus, professor of physics, who was one of the experiment’s two lead investigators to organize the search for the Higgs boson.

    As the LHC prepares to start back up on July 5 with “Run 3,” MIT News spoke with Paus about what physicists have learned about the Higgs particle in the last 10 years, and what they hope to discover with this next deluge of particle data.

    Q: Looking back, what do you remember as the key moments leading up to the Higgs boson’s discovery?

    A: I remember that by the end of 2011, we had taken a significant amount of data, and there were some first hints that there could be something, but nothing that was conclusive enough. It was clear to everybody that we were entering the critical phase of a potential discovery. We still wanted to improve our searches, and so we decided, which I felt was one of the most important decisions we took, that we had to remove the bias — that is, remove our knowledge about where the signal could appear. Because it’s dangerous as a scientist to say, “I know the solution,” which can influence the result unconsciously. So, we made that decision together in the coordination group and said, we are going to get rid of this bias by doing what people refer to as a “blind” analysis. This allowed the analyzers to focus on the technical aspects, making sure everything was correct without having to worry about being influenced by what they saw.

    Then, of course, there had to be the moment where we unblind the data and really look to see, is the Higgs there or not. And about two weeks before the scheduled presentations on July 4 where we eventually announced the discovery, there was a meeting on June 15 to show the analysis with its results to the collaboration. The most significant analysis turned out to be the two-photon analysis. One of my students, Joshua Bendavid PhD ’13, was leading that analysis, and the night before the meeting, only he and another person on the team were allowed to unblind the data. They were working until 2 in the morning, when they finally pushed a button to see what it looks like. And they were the first in CMS to have that moment of seeing that [the Higgs boson] was there. Another student of mine who was working on this analysis, Mingming Yang PhD ’15, presented the results of that search to the Collaboration at CERN that following afternoon. It was a very exciting moment for all of us. The room was hot and filled with electricity.

    The scientific process of the discovery was very well-designed and executed, and I think it can serve as a blueprint for how people should do such searches.

    Q: What more have scientists learned of the Higgs boson since the particle’s detection?

    A: At the time of the discovery, something interesting happened I did not really expect. While we were always talking about the Higgs boson before, we became very careful once we saw that “narrow peak.” How could we be sure that it was the Higgs boson and not something else? It certainly looked like the Higgs boson, but our vision was quite blurry. It could have turned out in the following years that it was not the Higgs boson. But as we now know, with so much more data, everything is completely consistent with what the Higgs boson is predicted to look like, so we became comfortable with calling the narrow resonance not just a Higgs-like particle but rather simply the Higgs boson. And there were a few milestones that made sure this is really the Higgs as we know it.

    The initial discovery was based on Higgs bosons decaying to two photons, two Z bosons or two W bosons. That was only a small fraction of decays that the Higgs could undergo. There are many more. The amount of decays of the Higgs boson into a particular set of particles depends critically on their masses. This characteristic feature is essential to confirm that we are really dealing with the Higgs boson.

    What we found since then is that the Higgs boson does not only decay to bosons, but also to fermions, which is not obvious because bosons are force carrier particles while fermions are matter particles. The first new decay was the decay to tau leptons, the heavier sibling of the electron. The next step was the observation of the Higgs boson decaying to b quarks, the heaviest quark that the Higgs can decay to. The b quark is the heaviest sibling of the down quark, which is a building block of protons and neutrons and thus all atomic nuclei around us. These two fermions are part of the heaviest generation of fermions in the standard model. Only recently the Higgs boson was observed to decay to muons, the charge lepton of the second and thus lighter generation, at the expected rate. Also, the direct coupling to the heaviest  top quark was established, which spans together with the muons four orders of magnitudes in terms of their masses, and the Higgs coupling behaves as expected over this wide range.

    Q: As the Large Hadron Collider gears up for its new “Run 3,” what do you hope to discover next?

    One very interesting question that Run 3 might give us some first hints on is the self-coupling of the Higgs boson. As the Higgs couples to any massive particle, it can also couple to itself. It is unlikely that there is enough data to make a discovery, but first hints of this coupling would be very exciting to see, and this constitutes a fundamentally different test than what has been done so far.

    Another interesting aspect that more data will help to elucidate is the question of whether the Higgs boson might be a portal and decay to invisible particles that could be candidates for explaining the mystery of dark matter in the universe. This is not predicted in our standard model and thus would unveil the Higgs boson as an imposter.

    Of course, we want to double down on all the measurements we have made so far and see whether they continue to line up with our expectations.

    This is true also for the upcoming major upgrade of the LHC (runs starting in 2029) for what we refer to as the High Luminosity LHC (HL-LHC). Another factor of 10 more events will be accumulated during this program, which for the Higgs boson means we will be able to observe its self-coupling. For the far future, there are plans for a Future Circular Collider, which could ultimately measure the total decay width of the Higgs boson independent of its decay mode, which would be another important and very precise test whether the Higgs boson is an imposter.

    As any other good physicist, I hope though that we can find a crack in the armor of the Standard Model, which is so far holding up all too well. There are a number of very important observations, for example the nature of dark matter, that cannot be explained using the Standard Model. All of our future studies, from Run 3 starting on July 5 to the very in the future FCC, will give us access to entirely uncharted territory. New phenomena can pop up, and I like to be optimistic. More

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    The promise and pitfalls of artificial intelligence explored at TEDxMIT event

    Scientists, students, and community members came together last month to discuss the promise and pitfalls of artificial intelligence at MIT’s Computer Science and Artificial Intelligence Laboratory (CSAIL) for the fourth TEDxMIT event held at MIT. 

    Attendees were entertained and challenged as they explored “the good and bad of computing,” explained CSAIL Director Professor Daniela Rus, who organized the event with John Werner, an MIT fellow and managing director of Link Ventures; MIT sophomore Lucy Zhao; and grad student Jessica Karaguesian. “As you listen to the talks today,” Rus told the audience, “consider how our world is made better by AI, and also our intrinsic responsibilities for ensuring that the technology is deployed for the greater good.”

    Rus mentioned some new capabilities that could be enabled by AI: an automated personal assistant that could monitor your sleep phases and wake you at the optimal time, as well as on-body sensors that monitor everything from your posture to your digestive system. “Intelligent assistance can help empower and augment our lives. But these intriguing possibilities should only be pursued if we can simultaneously resolve the challenges that these technologies bring,” said Rus. 

    The next speaker, CSAIL principal investigator and professor of electrical engineering and computer science Manolis Kellis, started off by suggesting what sounded like an unattainable goal — using AI to “put an end to evolution as we know it.” Looking at it from a computer science perspective, he said, what we call evolution is basically a brute force search. “You’re just exploring all of the search space, creating billions of copies of every one of your programs, and just letting them fight against each other. This is just brutal. And it’s also completely slow. It took us billions of years to get here.” Might it be possible, he asked, to speed up evolution and make it less messy?

    The answer, Kellis said, is that we can do better, and that we’re already doing better: “We’re not killing people like Sparta used to, throwing the weaklings off the mountain. We are truly saving diversity.”

    Knowledge, moreover, is now being widely shared, passed on “horizontally” through accessible information sources, he noted, rather than “vertically,” from parent to offspring. “I would like to argue that competition in the human species has been replaced by collaboration. Despite having a fixed cognitive hardware, we have software upgrades that are enabled by culture, by the 20 years that our children spend in school to fill their brains with everything that humanity has learned, regardless of which family came up with it. This is the secret of our great acceleration” — the fact that human advancement in recent centuries has vastly out-clipped evolution’s sluggish pace.

    The next step, Kellis said, is to harness insights about evolution in order to combat an individual’s genetic susceptibility to disease. “Our current approach is simply insufficient,” he added. “We’re treating manifestations of disease, not the causes of disease.” A key element in his lab’s ambitious strategy to transform medicine is to identify “the causal pathways through which genetic predisposition manifests. It’s only by understanding these pathways that we can truly manipulate disease causation and reverse the disease circuitry.” 

    Kellis was followed by Aleksander Madry, MIT professor of electrical engineering and computer science and CSAIL principal investigator, who told the crowd, “progress in AI is happening, and it’s happening fast.” Computer programs can routinely beat humans in games like chess, poker, and Go. So should we be worried about AI surpassing humans? 

    Madry, for one, is not afraid — or at least not yet. And some of that reassurance stems from research that has led him to the following conclusion: Despite its considerable success, AI, especially in the form of machine learning, is lazy. “Think about being lazy as this kind of smart student who doesn’t really want to study for an exam. Instead, what he does is just study all the past years’ exams and just look for patterns. Instead of trying to actually learn, he just tries to pass the test. And this is exactly the same way in which current AI is lazy.”

    A machine-learning model might recognize grazing sheep, for instance, simply by picking out pictures that have green grass in them. If a model is trained to identify fish from photos of anglers proudly displaying their catches, Madry explained, “the model figures out that if there’s a human holding something in the picture, I will just classify it as a fish.” The consequences can be more serious for an AI model intended to pick out malignant tumors. If the model is trained on images containing rulers that indicate the size of tumors, the model may end up selecting only those photos that have rulers in them.

    This leads to Madry’s biggest concerns about AI in its present form. “AI is beating us now,” he noted. “But the way it does it [involves] a little bit of cheating.” He fears that we will apply AI “in some way in which this mismatch between what the model actually does versus what we think it does will have some catastrophic consequences.” People relying on AI, especially in potentially life-or-death situations, need to be much more mindful of its current limitations, Madry cautioned.

    There were 10 speakers altogether, and the last to take the stage was MIT associate professor of electrical engineering and computer science and CSAIL principal investigator Marzyeh Ghassemi, who laid out her vision for how AI could best contribute to general health and well-being. But in order for that to happen, its models must be trained on accurate, diverse, and unbiased medical data.

    It’s important to focus on the data, Ghassemi stressed, because these models are learning from us. “Since our data is human-generated … a neural network is learning how to practice from a doctor. But doctors are human, and humans make mistakes. And if a human makes a mistake, and we train an AI from that, the AI will, too. Garbage in, garbage out. But it’s not like the garbage is distributed equally.”

    She pointed out that many subgroups receive worse care from medical practitioners, and members of these subgroups die from certain conditions at disproportionately high rates. This is an area, Ghassemi said, “where AI can actually help. This is something we can fix.” Her group is developing machine-learning models that are robust, private, and fair. What’s holding them back is neither algorithms nor GPUs. It’s data. Once we collect reliable data from diverse sources, Ghassemi added, we might start reaping the benefits that AI can bring to the realm of health care.

    In addition to CSAIL speakers, there were talks from members across MIT’s Institute for Data, Systems, and Society; the MIT Mobility Initiative; the MIT Media Lab; and the SENSEable City Lab.

    The proceedings concluded on that hopeful note. Rus and Werner then thanked everyone for coming. “Please continue to reflect about the good and bad of computing,” Rus urged. “And we look forward to seeing you back here in May for the next TEDxMIT event.”

    The exact theme of the spring 2022 gathering will have something to do with “superpowers.” But — if December’s mind-bending presentations were any indication — the May offering is almost certain to give its attendees plenty to think about. And maybe provide the inspiration for a startup or two. More