21 ms·
ATLAS physicist here -- you're mostly right! The existence of a Higgs boson has been confirmed with extremely high confidence. And this news... it's exciting in
by cshimmin 8y ago
ATLAS physicist here -- you're mostly right! The existence of a Higgs boson has been confirmed with extremely high confidence. And this news... it's exciting in the sense that it's an important result, but it's a bit of a damper on those of us who were hoping to discover new secrets about the universe.
Interestingly, the primary production mechanism for the Higgs boson at the LHC proceeds via the fusion of gluon particles from the colliding protons. This seems strange at first glance, since the Higgs interacts with particles proportionally to their mass, while the gluons are massless. In the standard model, there is a higher-order process wherein the gluons annihilate into a virtual "loop" of top quarks (which are very massive), which then "lend" their mass to produce the Higgs.
Because previous measurements have been consistent with the Standard Model prediction, we already had some evidence that the coupling to top quarks was nominal. However, there are other possible scenarios involving exotic physics that could mimic this production mechanism, which could imply the existence of new forces/particles, and that the Higgs boson does not couple to "normal" particles in the predicted fashion. Because of the nature of these heavy virtual loop processes, it turns out that it's pretty hard to distinguish between different such scenarios by examining the kinematic properties of the events. Hence, the main observable is the rate, and even that can be made to match a wide range of values with or without standard couplings, given a clever enough theory.
This recent result demonstrates directly that the Higgs boson does indeed couple to the top quark in a matter roughly consistent with the Standard Model expectation (the CMS measurement shows an upward fluctuation, indicating even stronger coupling than expected, although this is probably just a statistical fluctuation). Therefore, no fancy/exotic physics (such as string theory, strong gravity, etc) are required to explain the observed production of Higgs bosons so far at the LHC.
- dspillett 8y ago> The existence of a Higgs boson has been confirmed with extremely high confidence. And this news... it's exciting in the sense that it's an important result, but it's a bit of a damper on those of us who were hoping to discover new secrets about the universe. There is a certain beauty in not only accepting that you might be wrong, but actively wanting to be wrong because that could be so much more interesting! And a certain sadness in the disappointment some obviously have when they accept that the less interesting outcome is the one that they receive.
- cshimmin 8y agoWell, it's a bit selfish really. It's a terribly interesting outcome as it is, but the particle physicists in the 70's and 80's got to have all the fun.
- deleted 8y ago[deleted]
- da_chicken 8y agoSort of how the chemists in the 18th and 19th centuries got to have all the fun. "I'll call it 'Aluminum.' No, wait! 'Aluminium!' Uh... ship that first print run of my book with the wrong spelling to North America. Nobody will notice!" "I'll call it Potassium, because it's found in potash." "That's not very creative." "Fine, I'll call it Kalium and give it a symbol of K." "Where'd you get Kalium from?" "Well, it's very alkaline, you see. It's from potash, after all." "That's still not very creative!" "Fine. We'll call it Potassium, but give it a symbol of K. Happy now?"
- vidarh 8y agoIt's still called Kalium in a bunch of languages.
- staticautomatic 8y agoIs that also the case for "natrium?"
- drsopp 8y agoI'm a high school science teacher. Could you point me to the typical methods you use to calculate the level of confidence of results at CERN?
- cshimmin 8y agoSure thing! In broad strokes, the most formal way (in our field's typically frequentist paradigm) to calculate confidence/significance is to build a robust statistical model for your entire experiment (including the random effect that various uncertainties, etc will have on the outcome), and then to randomly synthesize many many outcomes of your experiment with respect to a given hypothesis. The outcome of each "pseudoexperiment" is boiled down to a single number, known as a test-statistic, and this way we can generate a distribution of that test statistic that gives you an idea of the probability of any given outcome. Then, when we do the experiment for real and get a single outcome, we can evaluate the compatibility of that outcome with the distribution of possible outcomes sampled from the statistical model (e.g. by calculating a p-value). Generally the Profile Likelihood Ratio is chosen as the test statistic at LHC experiments. This is possibly the most highly-cited statistics paper in our field: https://arxiv.org/abs/1007.1727 https://arxiv.org/abs/1007.1727 It provides asymptotic formulae that allow us to estimate these p-values without having to simulate our experiment tens of thousands of times. But it also gives a nice overview of the different test statistics one might use. "LHC Statistics for Pedestrians" is short and informative, but a bit haphazard and probably assumes you're familiar with a lot of jargon: https://cds.cern.ch/record/1099994/files/p205.pdf https://cds.cern.ch/record/1099994/files/p205.pdf "Practical Statistics for the LHC" is a much more complete and thorough introduction, but it's quite long and technical: https://arxiv.org/abs/1503.07622 https://arxiv.org/abs/1503.07622 "Statistics for searches at the LHC": https://arxiv.org/abs/1307.2487 https://arxiv.org/abs/1307.2487 Similar to the previous one, but maybe more words and less math. Perhaps one of these will resonate with you more than the others! Lastly, here is a public note that was created jointly by the ATLAS and CMS collaborations prior to the Higgs boson discovery. Basically, we got together and came up with a consistent way to present new results from the LHC, and this note documents those protocols: https://cds.cern.ch/record/1379837 https://cds.cern.ch/record/1379837
- 8y ago
- chaotic_clanger 8y agohaving you here i'd like to take the opportunity and to ask about tetraquarks. if i recall correctly there have been several identified tetraquark particles over past years (many of detections coming from LHC) with hint of possibility of pentaquark. i always wondered how does standard model includes/describes these?
- cshimmin 8y agoGood question! That kind of stuff is a bit out of my area of expertise, but I have asked around my colleagues at CERN and various Unis with more or less the same question. As far as I know, there's nothing too exotic (from a theoretical perspective), about tetra/pentaquarks. They are just QCD bound states of more than 3 quarks. The problem with QCD is that it's a non-perturbative theory, so we generally can't even do a good job computing the hadronic bound states that we already know exist! So, I get the impression that these observations are mostly of interest to people doing QCD on the lattice (or other calculational methods), as inputs to their modeling. I.e. perhaps having an "answer key" for some tetraquark masses will allow them to better determine whether they should be modeling these states as bound states of two mesons (which each have 2 quarks), or as a totally different 4-quark structure.
- letsgetphysITal 8y ago> the gluons annihilate into a virtual "loop" of top quarks (which are very massive), which then "lend" their mass to produce the Higgs. Why are the words "loop" and "lend" in quotation marks? Are they just a handy visual a lay person would understand to describe a more complicated phenomenon? Is there no better word to describe what is happening, including within your field, but it isn't a technically accurate description? I'm envisioning a room full of highly educated folk excitedly talking over each other... "Well it looks like a ring, but it doesn't quite meet up..." "It looks like a disc, but with more 'weight' to it..." "No, it looks like a loop-the-loop on a roller coaster!"
- cshimmin 8y agoLoop is definitely a technical term. I guess I shouldn't have quoted it? It refers to a Feynman diagram that looks like this: https://en.wikipedia.org/wiki/One-loop_Feynman_diagram https://en.wikipedia.org/wiki/One-loop_Feynman_diagram In quantum field theory, perturbative physical processes can be expressed as the sum of all possible diagrams compatible with the initial and final state of interest. Each diagram corresponds to an integral, and diagrams that have loops are special because they are underconstrained by the initial/final conditions. Hence, you are left with an additional integral over every possible combination of momenta that can be exchanged within the loop. These integrals are usually hard or impossible to compute and may require weird mathematical techniques (like dimensional regularization). So, yes, (theoretical) particle physicists talk about loops a lot! "Lend" here is total b.s. that I made up. The point is that the gluons can't directly interact with the Higgs boson, so they use their collisional energy to create some massive top quarks that can.
- letsgetphysITal 8y agoSo a little from column A, a little from column B. Got it! In all seriousness, I'm just interested in how much is being obscured when folk like Cox do a TV show, obviously written for those interested but not involved like myself. I've no idea what anything in your paragraph meant past the word "In" (yes, the first word), but I can definitely picture a loop, and get that there is a something which is transmitted / moves / traded / otherwise-not-at-its-origin within that loop, and trying to understand what that something is will leave me drooling into the carpet. Still, I'm glad I asked. Thanks!
- jcims 8y agoIs there any way to describe the scale of these particles relative to those I learned about in high school chemistry (proton/neutron/etc)? Is that even a sensible question?
- cshimmin 8y agoTotally sensible, and it's actually probably the easiest way for me to think about it as well :) A proton or neutron has a mass of approximately 1 GeV/c^2. The Higgs boson has a mass of about 125 GeV/c^2 -- so it's totally appropriate to think about it as being 125x heavier than a proton, or about as heavy as an Iodine nucleus. The top quark has a mass of 170 GeV/c^2 -- it's the most massive fundamental particle we've discovered so far!
- jcims 8y agoWow! I'm glad I asked because this just flipped my mental model completely. Now I have to do some investigating, thank you!!!
- ff317 8y agoSo I have another silly non-physicist question at this point: if Higgs is what gives particles mass, and it has a mass of 125 GeV/c^2, why doesn't this imply mass can only be conferred in units of this amount? We obviously have smaller particles with "real" masses (e.g. nuclei lighter than iodine). Is it that the Higgs is coupled by a varying degree which confers only part of its mass to the coupled particle? In that case, can a single Higgs be lightly-coupled to several distinct particles? Does this imply some other spooky connections/behaviors between multiple particles sharing partial couplings to a single Higgs?
- raverbashing 8y agoSo is the Higgs what "unifies" the electroweak and strong force?
- bardworx 8y agoFrom article, and what I’ve read previously, Higgs provides weight to particles and does not have anything to do with electroweak and strong force. I could be wrong as I’m a layman in these things.
- filmor 8y agoIt does have something to do with the electroweak force, the Higgs mechanism allows gauge bosons (like the Z_0, W_+ and W_-) to have mass.
- raverbashing 8y ago> I could be wrong Yes, you are ;) Start here: https://home.cern/about/physics/z-boson https://home.cern/about/physics/z-boson
- cm2187 8y agoAs someone who doesn't know what I am talking about, can I ask a stupid and naive question? By converting energy into mass and mass into energy, would it be conceptually feasible to generate thrust in a closed circuit? My thinking is that you could use that for propulsion in space. We are currently relying on throwing stuff out to generate a reaction force and of course we quickly run out of things to throw. But if you could change the mass of particules in flight, you could increase the mass on the way forth and decrease the mass on the way back, generating continuous thrust?
- EliRivers 8y agoYou'd still have conservation of momentum; you don't have to have mass (or at least, the kind of mass I suspect you're thinking of) to have momentum.
- cm2187 8y agoThanks. I must be missing something. If I have a particle emitted from point A and stopped in point B, travelling between the two at velocity v, and that I can magically change its mass in flight from M (when it leaves A) to m (when it reaches B), with M > m, wouldn't I create kinetic energy for the overall system of 0.5 * (M-m) * v^2 ?
- andrepd 8y agoNo, because its velocity changes too. In relativistic mechanics, the energy momentum relation holds: E^2 = p^2 + m^2, and energy is conserved in a closed system. If on a system with 1 particle the mass changes, the momentum must change to compensate as well.
- Koshkin 8y ago> hoping to discover new secrets about the universe I am curious as to what kind of new secrets. Because the recent examples - the discovery of gravitational waves and the Higgs only confirmed well-established theoretical predictions.
- neolefty 8y agoMy guess: Those "new secrets" would be in the category of "unknown unknowns"[1]. We don't even know where to look for them, so we looked somewhere we hadn't before—in higher energies. And we used a "known unknown" as a stepping stone—the Higgs, which we were pretty sure existed, and we had an idea of what its properties would be. [1] https://en.wikipedia.org/wiki/There_are_known_knowns https://en.wikipedia.org/wiki/There_are_known_knowns
- cshimmin 8y agoWell said. Some particular examples of things we're looking for in this case, however, include supersymmetry, strong gravity / extra dimensions, and dark matter. All of these theories generally can be contrived one way or another to alter the predictions of the standard Higgs theory. But we are also looking everywhere we can and trying to do precise measurements because we may find something surprising as well!
- fusiongyro 8y agoDo we have any experimental evidence of shortcomings of the standard model? It seems to me as a lay person that it keeps getting confirmed at higher and higher energies, and hints of science beyond it remain elusive. Is that true?
- csomar 8y agoI have an unrelated question as you seem knowledgeable: Does the Quantum version of mass/gravity conflict with Einstein theory of general relativity/spacetime?