13 ms·
Measurement of the W boson mass reveals 7σ deviation from calculations
- chris_overseas 5y agoDiscussed here: https://news.ycombinator.com/item?id=30948260 https://news.ycombinator.com/item?id=30948260
- andyjohnson0 5y agoThanks. As a general reader, I found that article much more accessible than the Science article.
- bejelentkezni 5y agoThe duality of man[0]. https://news.ycombinator.com/item?id=30952583 https://news.ycombinator.com/item?id=30952583
- deleted 5y ago[deleted]
- nvalis 5y agoSome comments from an ATLAS physicist doing W mass measurements at the LHC: https://non-trivial-solution.blogspot.com/2022/04/do-we-have-finally-found-new-physics.html https://non-trivial-solution.blogspot.com/2022/04/do-we-have...
- cshimmin 5y agoAs another ATLAS physicist, I can say that this is an excellent article from Prof. Schott. He is very politely arguing that "someone messed up". I'm not sure I agree so much with the point of combining the LEP experiments, which do have some tension with each other. Unless the combination is specifically taking into account correlations between uncertainties at the different experiments on the same collider (which exist, but it's really hard to handle). Another take many people in the field are expressing is that it's simply infeasible to reliably interpret statistical models at that level (especially one that is dominated by systematic uncertainty), since they are based on approximations and assumptions e.g. that certain nuisance parameters are "nicely" distributed and uncorrelated. See e.g. comments from Prof. Cranmer [1] who is one of the folks who developed the standard statistical formalism and methods used in modern particle physics experiments. [1] https://twitter.com/kylecranmer/status/1512222463094140937?s=21&t=HxQAdoaHwNMCL-HIhrA07w https://twitter.com/kylecranmer/status/1512222463094140937?s...
- spekcular 5y agoWhy don't people use nonparametric methods to get around the problem of assuming certain parameters are "nicely" distributed? (Not a physicist, but curious – this seems like the "obvious" solution.)
- nabla9 5y agoNonparametric methods are often used when the assumptions of parametric tests don't hold. In physics experiments they want to fix the structure of the model and know the assumptions. They want to know the distribution and parameters to hold. If assumptions don't hold, they must find out why, find better assumptions and fix the model. To say it differently: physicists are not trying to discover statistical laws. They are trying to discover physical laws trough statistics.
- spekcular 5y agoIt sounds like they know certain assumptions regarding parameters that are not of interest are wrong. So why explicitly model those, if we don't care about their distribution? We (apparently) only care about an accurate estimate of W boson mass.
- lazide 5y agoBecause it’s all interrelated and too many variables make it impossible to nail down anything with certainty if you don’t assume some invariants somewhere?
- nabla9 5y agoThat works if the thing is something you can remove from the experiment and model separately, then put it back. In CERN many variables are tied to this one huge machine that is one of its kind.
- spekcular 5y agoI admit I'm not familiar with the model used to aggregate the boson data. But there's an entire community of nonparametric/semiparametric statisticians that works on problems just like this. It seems crazy to me that that millions of dollars are spent to build the machines to collect this data, yet the papers are written using statistical models with distributional/independence assumptions known to be false. (The tweet linked above seems to be saying something similar.) Is there a concrete reason we can't be naive and just bootstrap confidence intervals for example? Of course I defer to the physicists here – but I'm curious whether there's some simple high-level reason the usual tricks don't work.
- slibhb 5y ago> We observed for quite some time some features in the our data, which we could not explain. Once one of my PhD students came into my office and told that he finally figured out this feature: the protons in the ATLAS detector do not collide heads-on but under a very small angle, allowing the not interacting protons to continue their travel through the LHC on the other side of the experiment. Indeed he was right - we have not been considering this effect in our simulations, however - after some calculations and speaking to the machine experts - it turned out that this effect induces a feature in our data, which is opposite in sign that we observe; so we have been left with an effect that was twice as large and unexplained. In the end it turned out to be caused by the deformation of the ATLAS detector by its own weight of more than 7000 tons over time. I know these people are incredibly smart and conscientious. And the standard model is extremely successful and well confirmed. But that's a lot of degrees of freedom.
- beezle 5y agoWaiting to see if Tammaso puts something up about it, IIRC he was a CDF member https://www.science20.com/quantum_diaries_survivor https://www.science20.com/quantum_diaries_survivor
- Certhas 5y ago"I do not think, we have to discuss which new physics could explain the discrepancy between CDF and the Standard Model - we first have to understand, why the CDF measurement is in strong tension with all others." That's... cute. I doubt it will stop the theorists from flooding the arxiv with explanaitions in the coming days/weeks. Recall what happened when there was a barely 3 sigma (local) statistical fluctuation in LHC data: https://resonaances.blogspot.com/2016/06/game-of-thrones-750-gev-edition.html https://resonaances.blogspot.com/2016/06/game-of-thrones-750... Edit: Thank you for posting the excellent article!
- neals 5y agoWhat is a sigma?
- jstx1 5y ago1 sigma = 1 standard deviation
- throw0101a 5y agoA unit used in statistics: > In statistics, the standard deviation is a measure of the amount of variation or dispersion of a set of values.[1] * https://en.wikipedia.org/wiki/Standard_deviation https://en.wikipedia.org/wiki/Standard_deviation
- tempay 5y agoThis page is likely more approachable: https://en.wikipedia.org/wiki/68%E2%80%9395%E2%80%9399.7_rule#Table_of_numerical_values https://en.wikipedia.org/wiki/68%E2%80%9395%E2%80%9399.7_rul...
- sundarurfriend 5y agoI thought you were going to link to https://simple.wikipedia.org/wiki/Standard_deviation https://simple.wikipedia.org/wiki/Standard_deviation The "Simple English Wikipedia" is a really underrated resource for understanding jargon outside your field.
- cshimmin 5y agoI work in this field (different experiment); despite the downvotes this is a reasonable question. Reposting my comment from above, since there is confusion here (the other sibling comments are incorrect). In particle physics, sigma denotes "significance", not standard deviation. Technically what we're quoting as "sigmas" are "z-values", where z=Phi^{-1}(1 - p), where Phi^{-1} is the inverse CDF of the Normal distribution and p is the p-value of the experimental result. So, 7 sigma is defined to be the level of significance (for an arbitrary distribution) corresponding to the same quantile as 7 standard deviations out in a Normal distribution.
- londons_explore 5y agoDoes it make sense to even discuss the sigma of any deviation? When you add in the "10% chance that some scientist messed up the maths or something in the experiment", then it's impossible to ever reach 7 sigma...
- morelandjs 5y agoKnown unknowns, and unknown unknowns. Still useful to quantify the known unknowns and compare significance of various events according to them.
- amelius 5y agoIf a quantity cannot be negative (such as a mass), then standard deviation isn't the best choice. EDIT: Yes, because the Gaussian distribution extends to +/- infinity; davrosthedalek explains it best, below.
- cedilla 5y agoWhat would be a better choice?
- FabHK 5y agoGP is probably referring to the coefficient of variation, sigma/mu (standard deviation divided by mean), which normalises out for example the unit of measurement. However, the 7 here is basically (x - mu)/sigma, so it is normalised (in that sense), anyway.
- davrosthedalek 5y agoNo, I think the problem (in principle) is that "standard deviation" has a special meaning for Gaussian distributions, which extend to infinity in both directions. A quantity that has a fixed range has most likely an asymmetric distribution, so one would expect an asymmetric error bar as well. But for a sigma<<the value, it's often not a big concern. A good example is efficiency measurements. I can't count how often I have seen students say something like: Our detector is 99%+-3% efficient. Obviously a detector can't be 102% efficient.
- freemint 5y ago7 sigma is actually less than one thinks because these distributions are not normal distributions.
- sdfgdf 5y ago
- bawolff 5y agoWhy aren't they normal? I know very little about this topic, but i would generally assume that measuring most natural phenomenon would be normal.
- rich_sasha 5y agoEspecially since, isn't this an average / error of a mean estimate? So even if individual observations are non-normal, this would be a perfect place for Central Limit Theorem. I know nothing about Quantum though, only maths.
- freemint 5y agoBecause it's a non-linear world? And the graphs seem very obviously skewed? And it's kurtosis also seems to differ from three?
- rich_sasha 5y agoNone of those things matter to the central limit theorem. If I have IID observations with finite 2nd moment (variance), then their average will pretty quickly converge to a Gaussian distribution. And I can relax a lot of this and still recover a variant of CLT. Of course maybe the calculation is different, eg it’s not like there are N independent observations, but rather some other complex condition solved for the mean estimate.
- krona 5y agoAlso not knowing anything about this topic, I'd assume it wasn't normal because we're talking about mass close to zero, and mass must be greater than zero.
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- codezero 5y agoHow is it that several other measurements have error bars that don’t even overlap with this one?
- CrazyStat 5y agoError bars account for the known and quantifiable sources of uncertainty. They don't (can't) account for unknown or unquantifiable sources of uncertainty, such as aspects of the experimental design that were not properly accounted for or unpredicted/unmodeled interactions with other particles or forces. Known unknowns and unknown unknowns, as Rumsfeld would put it. About a decade ago I saw a very nice figure of estimates of the speed of light over time showing this effect. Unfortunately I haven't been able to find it since.
- codezero 5y agoThanks for the reminder, this makes sense.
- aspenmayer 5y agoI found something similar on page 19 of this presentation. No errors bars, but they do provide some info about the errors of various experiments. https://www.nhn.ou.edu/~johnson/Education/Juniorlab/C_Speed/Historyof_c_F2002.PDF https://www.nhn.ou.edu/~johnson/Education/Juniorlab/C_Speed/... Edit: here’s some error bars! https://www.researchgate.net/figure/Uncertainties-in-Reported-Measurements-of-the-Speed-of-Light-1870-1960-This-figure_fig3_237392909 https://www.researchgate.net/figure/Uncertainties-in-Reporte...
- derbOac 5y agoI love that second paper!
- aspenmayer 5y agoWhat were your favorite parts?
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- aaaaaaaaaaab 5y agoOh no! Time to tweak the parameters of the standard model again!
- imtemplain 5y ago
- was_a_dev 5y agoCan anyone explain the difference between light and heavy supersymmetry? Particle physics isn't quite my field
- rybosworld 5y agoThis could be an alien race interfering with our measurements.
- sbelskie 5y agoI feel like I’ve read that book before but can’t recall what it was.
- sylens 5y agoThree Body Problem
- jkhloiujlknmk 5y agoor The Gods Themselves. Neither is great, actually. With all due respect for Asimov, who I love.
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- fallingfrog 5y agoI would guess that unless some other team replicates this result, it's probably a measurement error somewhere. Physics can be very delicate and tricky, and it's easy to make mistakes. But, even the mistakes are opportunities for learning, so it's not a waste.
- junon 5y agoCan someone explain this in laymen's terms?
- SpeakMouthWords 5y agoSomeone has run an experiment, and in this experiment they created a large amount of evidence that seems to say that quite an important particle in particle physics weighs something slightly different from what we thought it should. This is important because the weight of that particle was predicted by our generally-accepted theory of how the universe works. If the weight is different, it means the theory hasn't taken into account everything that it should.
- junon 5y agoThank you :)
- Pet_Ant 5y agoHow do ±6.4 and ±6.9 combine to ±9.4 and not ±13.3 ?
- pif 5y agohttps://physics.nist.gov/cuu/Uncertainty/combination.html https://physics.nist.gov/cuu/Uncertainty/combination.html
- fulvioterzapi 5y agoErrors do not sum like regular numbers. You want to take the square root of the sum of the squares of the errors. sqrt(6.4^2 + 6.9^2) ≈ 9.4 You can have a look here: http://ipl.physics.harvard.edu/wp-uploads/2013/03/PS3_Error_Propagation_sp13.pdf http://ipl.physics.harvard.edu/wp-uploads/2013/03/PS3_Error_...
- stocknoob 5y agoThat PDF is great, thanks for sharing.
- alephxyz 5y agoThe variances are additive but not the std dev. Sqrt(6.4^2 + 6.9^2) = 9.4
- deleted 5y ago[deleted]
- imtemplain 5y ago
- beefield 5y agoTalking about particle/quantum physics, is there a book/youtube channel/whatnot that would describe (some/main) experiments and results that have convinced physicists that classical physics does not work when you go small. I mean, I know about double slit experiment, but I guess it is a long journey from that to the Standard Model. So instead of the heavy theory, I'd like to see the stuff that made people scratch their heads in the first place.
- simonh 5y agoThis is a bit old, but still excellent. https://www.youtube.com/watch?v=XYcw8nV_GTs https://www.youtube.com/watch?v=XYcw8nV_GTs This is more up to date and specifically on challenges to the SM. Where is physics going? | Sabine Hossenfelder, Bjørn Ekeberg and Sam Henry https://www.youtube.com/watch?v=b8npmtsfsTU&t=2306s https://www.youtube.com/watch?v=b8npmtsfsTU&t=2306s
- mr_mitm 5y agoThe first real head scratchers were the black body spectrum [0] and the fact that atoms are stable. Rutherford [1] showed that atoms consist of a tiny, positively charged nucleus and rather large negatively charged shell. It was hypothesized that electrons are flying around the nucleus like planets around the sun. But we already knew at that point that moving charges emit radiation, which causes the electron to lose energy and move closer to the nucleus. So it should pretty much immediately collapse into a point. Bohr then showed that if you assume that only certain orbits were allowed, it works out pretty nicely. Nowadays we now that there is such a thing as a ground state, meaning the lowest amount of energy the electron can possibly have around a nucleus is enough to keep it moving. The idea for quantizing things came from observing the black body spectrum. If you sum up all contributions classically, you get infinity. Planck tried to see what happens if you assume that energy comes in little packets instead of a continuous spectrum. He didn't have any justification for it, but it matched the observations pretty well. [0] https://en.wikipedia.org/wiki/Black-body_radiation https://en.wikipedia.org/wiki/Black-body_radiation [1] https://en.wikipedia.org/wiki/Rutherford_model https://en.wikipedia.org/wiki/Rutherford_model
- jrpt 5y agoQuantum physics is a separate (but related) branch from particle physics so using the slash "quantum/particle" is mixing up two different things - which one do you really want? Theoretical Concepts in Physics by Malcolm Longair is a mix of history and physics, by explaining how physicists came to discover their theories. I actually don't think it says much about modern particle physics though. It includes quantum mechanics. Introduction to Elementary Particles by David Griffiths if you just want particle physics. Griffiths also has an intro book on quantum mechanics.
- a-dub 5y agohow much tearing apart of everything and quintuple checking goes on before publishing a result like this? do they stand by the result or is it more of a call for "hey, come have a look at this. we can't explain it." it's got to be anxiety inducing! (and exciting, of course)
- graderjs 5y agoScccoooopped! :P :) xx ;p https://news.ycombinator.com/item?id=30952630 https://news.ycombinator.com/item?id=30952630
- nyc111 5y agoDo we actually know how physicists define "mass" in this context? Because, in physics many words have technical meanings that only physicists can know. For instance, as a layman when I see the word "particle" I imagine a spherical thing with an extension in space. But a physicist would laugh at me because in physics a particle is not a particle, it can be a statistical bump in data, it can be a field, it can be a wave, anything but a spherical particle. But a physicist would call a wave a particle and see nothing wrong with it. The same goes for mass, what physicists call mass can be voltage for instance. So does anyone know what "mass" means in this context?
- cowboysauce 5y ago> The same goes for mass, what physicists call mass can be voltage for instance. You’re probably thinking of how a proton has a mass of 938 MeV/c^2. This is still a mass and not a voltage. 1 eV (electronvolt) is the amount of kinetic energy that an electron would have after being accelerated though an electric potential of one volt. By the mass-energy equivalence 1 eV is equivalent to a mass of ~1.783x10^-36 kg and a proton has a mass of ~1.673x10^−27 kg.
- nyc111 5y ago> You’re probably thinking of how a proton has a mass of 938 MeV/c^2. Yes, that’s what I was thinking. But it seems that there is a problem with the definition of the word “mass”. Clearly there are at least two definitions. First, the weight of an object. Here weight is measured and weight is called “mass”. There is no equivalence, same thing is called weight and mass. Weight and mass are synonyms. This mass has nothing to do with electricity and has nothing to do with motion. The second definiton of mass is related to electricity and motion. It has no meaning outside electricity. In this case, they accelerate an electric current and measure its kinetic energy and call this kinetic energy “mass”. Again these words are synonyms. Why do physicists like these silly word plays so much, I have no idea.
- patrickkrusiec 5y agoThe weight of the object is not its mass. The weight is the force an object experiences due to a gravity. You have the same mass on the Earth and the Moon, but you weigh 1/6 of your weight on Earth on the Moon because of the gravitational force is 1/6th the strength of Earth's gravitational force. The definition of mass is subtler, but you seem to confusing units with the definition of the concept. Units are necessary because you need a scale to measure physical properties. You can't measure a length and say that it's "ten". You needs units attached like feet or meters. An eV (electronvolt) is a unit of energy. Just like a kilogram (unit of mass) originally was defined as the mass of a 10cm x 10cm x 10cm cube of water at room temperature, the eV (unit of energy) is defined as the increase in kinetic energy of an electron (which has a fixed and known charge) accelerated across 1 volt. But neither the definition of the kilogram or the eV define the concepts of mass or energy, they just merely define units, which humans chose, used to measure mass or energy. Now how does mass and energy relate to each other? Simply put, the Special Theory of Relativity, developed by Einstein in 1905, states that mass and energy are equivalent to each other. Now the word "equivalent" has a precise but complicated meaning that I will not explain here (if you do want to understand it, take a course in special relativity). This relation is defined quantitatively by E=mc^2 (Energy E equals mass m times the speed of light c squared). Let's first look at another relationship, distance = velocity * time. This equation can be be rewritten as distance/time = velocity. If we use meters to measure distance and seconds to measure time, we can "divide" the units and define the units of velocity as a meters per second or m/s. Same thing with E=mc^2. We can rewrite the mass m as m = E/c^2, and let the units of mass be eV/c^2. Using eV/c^2 or kilograms or whatever to measure mass has no effect on the definition of the concept mass itself (which you can think of as an intrinsic property of objects independent of units which affects their behavior in known ways). Why do physicists make all of this so complicated? They don't. It is reality that is subtle and complex and hard to understand. Because the purpose of physics is to describe reality, it has to be subtle and complicated.
- mc4ndr3 5y agoIf my calculations were off by seven standard devs I'd quit science and go skip rocks. wth
- rpz 5y ago:) If only they spent a few billion more on the equipment maybe they’d get to six sigmas
- beezle 5y agoUnfortunately I missed the webinar this afternoon, but here is the orginal press release from Fermilab, it is fairly long: https://news.fnal.gov/2022/04/cdf-collaboration-at-fermilab-announces-most-precise-ever-measurement-of-w-boson-mass/ https://news.fnal.gov/2022/04/cdf-collaboration-at-fermilab-...