[HN Gopher] New ATLAS result weighs in on the W boson
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New ATLAS result weighs in on the W boson
Author : JohnHammersley
Score : 52 points
Date : 2023-03-23 13:31 UTC (9 hours ago)
(HTM) web link (atlas.cern)
(TXT) w3m dump (atlas.cern)
| pierrebai wrote:
| This is ludicrous, but this kind of thing have been going on in
| science and physics in particular for a long time. Let's call the
| phenomenon maths side-blinders.
|
| One experiment finds the mass of thw W boson to be 80370 +/- 19
| MeV, another 80434 +/-9 MeV. Clearly, both results are
| incompatible, their rnage don't overlap. Of course, these are
| statistical ranges. But even with 95% certainty, their
| differences is _many times_ the uncertainty, so it 's not just
| that they're a bit off. IOW, we can be 100% (not 95%) sure that
| at least one, if not both, are incorrect.
|
| Yet they are boldly reported with those uncertainty ranges, even
| though, clearly , those ranges cannot be correct. And then ATLAS
| double down by "apply more statistical analysis" to narrow their
| uncertainty range!
|
| There should not be work on "improved stats analysis", but more
| work on finding where the systemic error between the two
| experiment lies. I truly don't see the point of retreading the
| same data set to change the value and uncertainty range when
| clearly there is something wrong with the data, the science, the
| experiment or all of the above.
|
| PS: what I'd like to see, is the labs to say something along the
| line: given result A and B being incompatible, statistically
| there is a (say) 99.99% chance that one or both experiment has a
| hidden flaw or that there is a major flaw in the standard model.
| mannykannot wrote:
| I am pretty sure that what you say in your PS is so obvious, at
| least to anyone who cares what the mass of the W boson is to a
| precision of ~+-2e-4, that it does not need saying. What does
| need saying is how bad the disagreement is, and when you are at
| the point where this difference has arisen and cannot quickly
| be resolved, publishing all the papers seems to be the best way
| to give the most complete information, available at that time,
| about the problem.
|
| Once the contradictory Fermilab results were made available,
| what should the ATLAS group do? Well, one thing would be to
| double-check their own analysis, and it seems plausible that
| this is what led to the latest result. Using improved methods
| might just as well have revealed a problem as end up as it did,
| supporting the original analysis.
| gnramires wrote:
| This experiment is way out of my depth, (and I don't know if
| that's going on here? <shrug>) but I think what you're
| describing is an interesting phenomenon, which I'll try to
| explain.
|
| I think it's important to keep in mind is those cases certain
| _models_ are being tested. In good science, you do experiments
| with certain models in mind, and of course associated models of
| your apparatus and machines themselves, and then you compare
| the results for consistency. You have to be very careful with
| any other additions in your statistical analysis that wasn 't
| generated by the model, including the models of the machines
| (say some kind of 'epistemic uncertainty' or 'procedural
| uncertainty' or something like that) to correct after the fact,
| as I believe it potentially invalidates the base models by
| itself.
|
| For example, say you measure gravity at sea level with 1
| apparatus and report gravity is 9.9 +/- 0.1. Then you get a
| second apparatus, and measure 9.2 +/- 0.1 (i.e. something went
| wrong). The difference is significant. Then you realize there
| must be some error, so you add a 'experimental error
| parameter', which you can tune, and it has implications on both
| measurements: you adjust it until the uncertainties are
| compatible (which is to be expected from consistency of
| experiments), and arrive say at 9.9 +/- 0.6 and 9.2 +/- 0.6 for
| the first and second experiment. This new parameter clearly
| doesn't belong in the model, and there's no model for _the
| parameter itself_ : there's no explanatory mechanism involved,
| only a new free parameter. Something you could say honestly, is
| that we known there's experimental error in one or both of the
| experiments, or the base models are significantly incorrect.
| But you can't say take an average of both results and say
| gravity is 9.55 +/- (..), because the existence of either
| experimental error or base model error (at least to a few sigma
| of certainty) invalidates this procedure -- that is, unless you
| just want a guess for some sort of immediate practical
| application and the "experimental error" is acceptable.
|
| Another common and well known effect in experiments is knowing
| the result you want to get, and trying different "adjustments"
| or redoing analysis until (subconsciously or not) the analysis
| yields the result that agrees with previous observations. This
| has been reported by Feynman in his books. I believe some
| modern experiments shield against this by, among other ways,
| not seeing the results of an analysis until you're sure the
| experiments/analysis is good (so you can't fine tune the
| experiments analysis to get known results).
| bmitc wrote:
| What are the current implications of the experiments at CERN and
| for ATLAS? My current interpretation is that these are massive
| engineering marvels that are basically just data generators for
| some physicists to run statistics on. That is a pessimistic view,
| but I am just not aware of what's all going on there.
|
| Compare this to LIGO, where one could make a similar argument,
| but LIGO is producing very explicit data that is driving tons of
| research and discoveries and is providing tandem support for
| electromagnetic observations.
| fvold wrote:
| It's ostensibly "pure science", where we "just" gain more
| information on how the most fundamental building blocks of
| reality work.
|
| It's hard to predict what new technology can come of this. For
| example, who could have predicted the transistor?
|
| I think the information gained is valuable in itself, but way
| smarter people than me will be looking way harder at it, and
| suddenly a real-world marketable application pops out. If we
| could predict it happening, _that_ would be the pop.
|
| Also, don't underestimate the massive amounts of learning being
| done in engineering just by designing experiments and
| building/maintaining accelerators. That, alone, might be worth
| it.
| bmitc wrote:
| Those are fairly general sentiments that apply to a lot of
| things, though. However, funding is not infinite, and CERN
| has received _a lot_ of funding over the years.
| qwezxcrty wrote:
| tldr: The ATLAS Collaboration has measured the W boson mass and
| it's in agreement with the Standard Model.
|
| Unfortunately, no new physics yet and we can now forget about the
| previous excited reports.
| flangola7 wrote:
| Has anything ever pointed to the standard model being
| incorrect?
| magicalhippo wrote:
| The problem with the statement "is the standard model
| incorrect" is that "the standard model" is a bit of a moving
| target.
|
| Neutrinos for example are not fully understood since we found
| neutrino masses, and this is an active area of research[1].
| However depending on who you ask, they may say neutrino
| masses is part of the standard model or it's beyond the
| standard model (ie "new physics"). AFAIK those that say it's
| part of the standard model include some extension or
| modification[2] in their definition of the standard model
| which the other folks consider as something extra.
|
| So from what I can gather asking "is the standard model
| incorrect?" is a bit like asking "can I run this RISC V
| program on this RISC V processor?" The answer kinda depends
| on which extensions you include in the base definition.
|
| [1]: https://en.wikipedia.org/wiki/Neutrino#Research
|
| [2]: https://en.wikipedia.org/wiki/Neutrino_oscillation#Ponte
| corv...
|
| [3]: https://en.wikipedia.org/wiki/Sterile_neutrino
| qwezxcrty wrote:
| For example, the paper featured on the front page of Science
| Volume 376.
| hislaziness wrote:
| "For this new analysis, ATLAS physicists revisited its data
| collected in 2011 at a centre-of-mass energy of 7 TeV
| (corresponding to 4.6 fb-1, also used in ATLAS' previous
| measurement). Researchers employed improved statistical methods
| and refinements in the treatment of the data, enabling them to
| reduce the uncertainty of their mass measurement by more than
| 15%."
| kzrdude wrote:
| I didn't notice the scale of the X axis at first, it seems like
| the difference between measurements is not that great.
| jfengel wrote:
| It's not that great. But it's the difference between "in
| accordance with the best theory" and "suggestive of unknown
| physics".
|
| Any unknown physics is likely to be a very small effect, at
| least numerically. That small effect could require a radical
| change in the understanding of physics -- like the tiny, hard-
| to-measure difference between Newtonian gravity and general
| relativity.
|
| Unfortunately, it's yet another case where a small apparent
| effect turned out to be just noise rather than an actual
| novelty.
| buildbot wrote:
| Very cool. Unrelated, but the technology and engineering behind
| ATLAS is immense. ~98 million data channels streaming out at 2Ghz
| (at least that is what I recall). Cut down to a manageable level
| by hundreds of FPGAs in the L1 trigger.
|
| https://cds.cern.ch/record/2745767/files/ATL-INDET-PROC-2020...
| stefanpie wrote:
| Yes, the FPGAs! Atlas and other experiments at CERN require
| such complex and hard-read time requirements, making them great
| applications of FPGAs. Right now, I am working on a project to
| try and deploy graph neural networks for track reconstruction
| and filtering (on the faster L1 trigger), as well as
| reconstructing other quantifiable measurements using graph
| neural networks (I believe this is not on the L1 trigger but
| requires some element of real-time). What's cool is they have
| copies of the FPGA boards that are at CERN at Fermilab, which
| makes remote development easier. Also, a fun fact: the
| detectors that surround the core are not all perfect concentric
| circles and are a big mix of different geometries and types of
| detectors, making the problem more interesting.
| buildbot wrote:
| Oh that is really cool! Way back in my undergrad I worked on
| a tiny tiny part of the insertable B layer, writing some very
| bad FPGA firmware to control the T3MAPS test chip (I think
| this was for Phase 2 upgrades?). FPGAs everywhere in this
| domain!
|
| And yeah the detectors are arranged in a really complicated
| way, I can imagine that is a hide headache.
| magicalhippo wrote:
| I recall watching Marco Reps' video[1] where he built the
| (relatively) high-speed high-precision ADC they designed and
| published as an open hardware project[2], to be used to
| accurately monitor current in the magnets.
|
| A small but important piece of a giant machine puzzle.
|
| [1]: https://www.youtube.com/watch?v=D28uSzCs7-k
|
| [2]: https://ohwr.org/project/opt-adc-10k-32b-1cha/wikis/home
| buildbot wrote:
| I love how nearly everything hardware at CERN is published
| under the open hardware license. I think even the pixel chip
| designs maybe, but I could be wrong.
| qwezxcrty wrote:
| Timepix is not public but licensed to commercial vendors as
| far as I know.
|
| But anyway I appreciate they make such (a readout chip for)
| detectors available so we could benefit in one of our
| previous projects (not about particle physics).
| hobo_mark wrote:
| The Timepix is a TSMC chip, I do not see how you could
| make that opensource without breaking NDAs with the fab.
| Something like the Skywater PDK would be ideal for open
| science (provided it could perform as well under
| radiations of course).
| Pulcinella wrote:
| I know this is anthropomorphizing, but the Weak force always
| seemed like the weird hack or cludge of the fundamental forces.
| Yeah it works, but do we really need two different W bosons and
| also a Z boson? The electromagnetic force does just fine with
| just the photon.
|
| More anthropomorphizing, the fact that there are three
| generations of matter feels like a premature optimization or
| someone applied too much abstraction and extensibility to the
| universe. One or two generations makes sense, but three feels
| like someone thought the system would need to scale to an
| infinite number of generations and then that didn't happen.
| addaon wrote:
| Keep in mind that the photon is the fourth electroweak boson,
| giving a nice power of two. W+, W-, Z0, and the photon are all
| mixed from W1, W2, W3, and B, after symmetry breaking. The
| photon doesn't couple to Higgs, but still, at higher energy
| there's much more symmetry here than appears at first glance.
| sebzim4500 wrote:
| Worth noting that the electromagnetic force does not actually
| just use the photon.
|
| The photon is the force carrier for hypercharge, and what we
| experience as electromagnetism on the macro scale is really a
| combination of hypercharge and the weak nuclear force.
|
| Caveat: I haven't used this stuff in years, it's possible I am
| talking shit.
| Iwan-Zotow wrote:
| no, you don't
| RHSman2 wrote:
| [flagged]
| beezle wrote:
| Anyone with a good link on why CDF is the outlier?
| beezle wrote:
| Yeah the PR really didn't get into it. However, Tommaso helped
| quite a bit with two posts from last year
|
| https://www.science20.com/tommaso_dorigo/is_the_cdf_w_mass_m...
|
| https://www.science20.com/tommaso_dorigo/how_inconsistent_re...
| elashri wrote:
| I think Fermilab's report [1] is useful to explain that
|
| [1] https://news.fnal.gov/2022/04/cdf-collaboration-at-
| fermilab-...
| kzrdude wrote:
| I think it didn't explain it.
|
| Well, there is this
|
| > The CDF measurement was performed over the course of many
| years, with the measured value hidden from the analyzers
| until the procedures were fully scrutinized. When we
| uncovered the value, it was a surprise.
|
| Which we can interpret. One way is: We were working blindly
| on this, so our value is not biased towards hitting a
| particular number.
| filmor wrote:
| Hah, my Bachelor thesis from 2009 explored a measurement channel
| for the W and Z bosons a bit (qq->WZ->llnl, so two quarks become
| a W and a Z which then decay into in total 3 charged leptons and
| a neutrino, "Di-Boson Events with Leptonic Final States"), sadly
| with simulated data as the restart after the issues from 2008 was
| delayed. The simplest way to measure the W and Z masses was to
| "pattern match" potential events in the data (which were already
| heavily preprocessed with "pseudo particles" derived from the
| tracks in the inner detector, myon detection and calorimeters),
| in my case Z->uu or Z->ee, try ones best to filter out parallel
| channels (e.g. Z->t tbar) and then derive information from the
| histograms of the "transverse mass", which is essentially the
| effective mass with the momentum component longitudinal to the
| beam deducted (https://en.wikipedia.org/wiki/Transverse_mass).
| heywhatupboys wrote:
| Ever noticed how physicists were all the rage in late 2000's, but
| now not so much? seems like the world has stopped caring about
| elementary particles.
| dkural wrote:
| It's not so much the world that's stopped caring, but there's
| been no real advance. The Higgs was expected, and found. All
| the other theories were refuted by the LHC. No new particles,
| no super-symmetry, nothing. The field of particle physics is at
| a dead-end currently.
| mrguyorama wrote:
| And even if it isn't a dead end, moving forward requires EVEN
| LARGER accelerators for higher energies, which are expensive,
| which requires buy in from the public, and the public DGAF
| about "understanding particle physics". Also, very little of
| this possible future advancement could be meaningful to
| everyday people, as it's astoundingly unlikely for something
| that "breaks physics" to actually have significant effects,
| otherwise we should have found it ages ago. Most likely, any
| new physics from particle physics would likely just add more
| decimal places of accuracy to existing models.
| magicalhippo wrote:
| > moving forward requires EVEN LARGER accelerators for
| higher energies
|
| Right, I recently saw a presentation my an experimentalist
| arguing that after HL-LHC[1] (a major upgrade to LHC), the
| next sensible size for a LHC-style collider would be one
| that occupied the Gulf of Mexico. And they meant
| literally[2], the proposal was to have submerged segments,
| using existing technology from the offshore oil industry,
| spanning the gulf.
|
| [1]: https://en.wikipedia.org/wiki/High_Luminosity_Large_Ha
| dron_C...
|
| [2]: https://s3.cern.ch/inspire-prod-
| files-3/3e7c3bd810a4730714d3...
| mrguyorama wrote:
| It's incredibly depressing. There's no path to fully
| exploring particle physics without basically a worldwide
| utopia to fund ludicrously expensive, probably
| unsuccessful projects. Sure, there's not likely to be
| anything incredible to come out of further research in
| the that field, but the idea of being unable to learn
| about our universe because nobody cares and politicians
| don't want to pay for it sucks.
| magicalhippo wrote:
| * * *
| Idiot_in_Vain wrote:
| If you want a short introduction on why physicists care so much
| about the weight of the W boson:
| https://www.youtube.com/watch?v=G0Q4UAiKacw
| ttpphd wrote:
| Would love to have an explanation of statistical vs total
| uncertainty, how they are quantified and how they are different.
| SaberTail wrote:
| I haven't read the paper yet, but I can give a general idea
| based on my experience working on similar experiments.
|
| The W boson doesn't live long enough to make it to the
| detector. All they can see are the decay products. The detector
| is like an onion, with different layers measuring the energies
| and/or momentum of different decay products.
|
| So what a search like this is trying to do is look at these
| decay products, figure out which ones came from a W boson,
| combine the various energy and momentum measurements, and use
| that and relativity to determine the rest mass.
|
| The measurement process always has some error, so you want to
| combine multiple measurements. This is how you get the
| statistical uncertainty. As an oversimplified example, imagine
| averaging multiple measurements. The more measurements you get,
| the smaller this error gets as your average gets closer to the
| true value. In actuality they're fitting some distribution to
| the mass measurements, but the same idea applies. The key thing
| is that the more data they collect, the smaller the statistical
| uncertainty gets.
|
| But there's also systematic uncertainty. Lots of things
| contribute to this, and it's effectively an indicator of how
| well they understand the detector and how well they understand
| the decay process. For example, the various systems need to be
| calibrated to convert their measurements to actual energies and
| momentums. These calibrations aren't perfect, and so the
| measurements aren't perfect. Or, when they determine which
| events look like W boson decays, they might be selecting some
| small fraction of other processes, or rejecting events in a way
| that biases for higher or lower mass. These will throw off the
| final measurement. They try to correct for these, but there's
| never enough data to do so perfectly.
|
| The key thing about systematic uncertainties are that no amount
| of measuring more decays will overcome them. There are other
| things you can do to bring them down, such as improving your
| detector simulations and doing additional calibrations. But
| there's a limit to how much they can bring this down.
|
| As an example, imagine trying to measure the height of a
| building with a long measuring tape. You might measure it many
| times and then average it, and that would give you some very
| small statistical error. But you don't know that the measuring
| tape was printed completely accurately, you don't know that it
| isn't stretching due to gravity or temperatures, you don't know
| that you got the tape perfectly lined up with the building.
| Those are all systematic uncertanties, and so when you report
| your measurement, those will be included in your total
| uncertainty.
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