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New Particle Discovered At CERN
New submitter ph4cr writes with news that a new particle has been discovered at CERN that confirms theoretical predictions. A pre-print of the academic paper is available at the arXiv (PDF). From the article:
"Physicists from the University of Zurich have discovered a previously unknown particle composed of three quarks in the Large Hadron Collider (LHC) particle accelerator. A new baryon could thus be detected for the first time at the LHC. The baryon known as Xi_b^* confirms fundamental assumptions of physics regarding the binding of quarks. ... In the course of proton collisions in the LHC at CERN, physicists Claude Amsler, Vincenzo Chiochia and Ernest Aguiló from the University of Zurich's Physics Institute managed to detect a baryon with one light and two heavy quarks. The particle Xi_b^* comprises one 'up,' one 'strange' and one 'bottom' quark (usb), is electrically neutral and has a spin of 3/2 (1.5). Its mass is comparable to that of a lithium atom. The new discovery means that two of the three baryons predicted in the usb composition by theory have now been observed."
I don't understand much about particle physics, but perhaps someone could give a quick explanation of how a particle made of three quarks has a mass equivalent to an entire atom of atomic number 3 and atomic weight almost 7? Is it because a bottom quark is one of its constituents?
Protons and neutrons are composed of strictly up and down quarks, in (uud) and (udd) combinations for protons and neutrons respectively. Up quarks weigh about 2.5 MeV and down quarks weigh about 5.0 MeV. A strange quark weighs about 100 MeV, and a bottom Quark weighs (very) roughly 4.2 GeV. It's because of the bottom quark that Xi_b^* weighs so much.
Source: http://pdglive.lbl.gov/Rsummary.brl?nodein=Q123
http://pdglive.lbl.gov/Rsummary.brl?nodein=Q005
It's pronounced "chi b star" and the discovery was by the CMS collaboration. The analysis was done by physicists from Zurich (apparently including one of my former postdocs) but they require data generated by the experiment so typically we credit the experiment. The discovery is of a new bound state of 3-quarks - not a new fundamental particle - so while interesting and definitely worthwhile it is not particularly exciting.
I find it funny that TFS talks about a Xi_b^* baryon with usb quarks, and goes on about its spin, as if it was common knowlegde, but has to precise that 3/2 is 1.5.
... and has a spin of 3/2 (1.5). ...
I don't know about the rest of the summary, but I can confirm that 3/2 is in fact 1.5.
USB quark (1.0)
Xi_b^* is actually the LaTeX code needed to generate the name of the particle. The particle's name is actually this (png image)
Indeed, without having that massive boondoggle at CERN we could have funded at least 1 more month of the Iraq war!
It gets even more amusing when you consider that a proton has a mass of about 938MeV/c , whereas the three quarks it is made up doesn't even add up to 10MeV/c. The binding energy of protons and neutrons is immense compared to the particles they are composed of.
I know this is slightly in jest, but this paper is not the sum-total of all of the work at the LHC.
There are 6 projects, each with hundreds of scientists, all of whom are juggling many papers at once. This Xi stuff is completely independent from Higgs searches, and it is one of many particles already discovered or confirmed at the LHC. So this isn't a Higgs-worthy discovery, although I think it is pumped-up a bit because CERN has really good press, and it looks good that the LHC is finding new physics.
Otherwise, this would just be a normal story. New Baryons or Mesons (like this one) are found a few times a year.
Someone's been looking at my coworker's variable names.
I just want to provide a little context to this announcement. As shown in the article, this is a Baryon, made up of 3 quarks. With 6 possible types of quarks, and 3 spots, this makes for many possible combinations of Baryons, a lot that have been found. Here is a current list of baryons:
PDG Baryon List
The proton and neutron are the p and n in the top left. The new Cascade (Xi_b) will be in the bottom right, in the "Bottom quark" section.
So this is neat and all, but hyped up a bit because its the LHC. A couple of these new Baryon (and also Mesons) are confirmed every year.
It's worth noting that the composite particle's masses are generally due primarily to the massless gluons who's immense energy contributes to the bound particle
Proton (uud): ~10MeV/c^2 in quarks , 938MeV total
Neutron (udd): ~12.5MeV in quarks, 940MeV total
Xi_b^* (usb): ~4293MeV in quarks, ~6517MeV total (7amu * 931 MeV/amu)
So not only is Xi_b^* composed of much higher mass quarks, but it would appear to have roughly twice the binding energy as well.
But why mention mass != weight? In a uniform gravitational field mass and weight are directly proportional to each other and can be used interchangeably using the gravitational acceleration as the conversion factor. The distinction is only relevant if you're either
1) operating within a non-constant gravitational field (i.e. in space) or comparing weights of different planets
or
2) You've discovered the first matter ever detected with different gravitational mass and inertial mass
Since (1) doesn't apply, and (2) almost certainly doesn't the distinction seems irrelevant
--- Most topics have many sides worth arguing, allow me to take one opposite you.
I didn't mod it down, but I can answer: probably because there's absolutely no call for calling someone a moron over a simple enough question, especially where something like QM is concerned, but even more because it assumes the OP should have knowledge of LaTex, as though everyone in the world should and anyone who doesn't is stupid.
Funny how the geeks never liked being called "nerd" and "dork" by the jocks in school, but in their own climate, quite a few of them dish it out just as bad. Now who's the bully? Honestly, unprovoked name-calling is just flat-out childish and mean spirited... thus, trollish.
Look back up at my post, now look back down, you're on the Internet. Now look back up. I'm a signature.
You're asking a couple distinct, and reasonable, questions. About "blind testing" -- I don't know the details for this particular result, but particle physicists put quite a bit of effort into making sure that they aren't fooling themselves. One of the best ways of doing it is so-called "blind analysis". The idea there is to define your entire data analysis strategy based solely on simulated data. There are pretty good simulations available of both the expected backgrounds, and of the process you are trying to actually find (the signal). So you define all of the methods you are going to use using these simulations before you look at the data. This ensures that you don't bias yourself into "finding something" in the data that isn't really there. (I don't know if a strict blinding procedure was used for this analysis, but likely something similar was done.)
The formal peer review system will come into effect now that the result is submitted to a journal. The paper will be distributed to some anonymous referees who will try to judge the merits of the physics and decide whether it merits publication. But I should note that the peer review process in modern particle physics actually starts long before the result is made public. Although there are only 3 or 4 main analysts, the paper is signed by the entire 3000 person CMS Collaboration (of which I am a member). So we have a very stringent internal review process to ensure that the result is sound before we release it with 3000 names taking responsibility. That doesn't mean that particle physics collaborations never make mistakes, but it does mean that results are scrutinized by a number of more or less unbiased eyes before they are made public.
Additionally, if the results are real, they can be replicated. LHC collides particles not only in the heart of the CMS detector, but there is also (among others) the ATLAS detector. This detector has more or less the same goals as CMS, but is built and operated by different people using a different detector design (both on the level of individual electronic chips and sensors, and on overall design choices), as well as different and mostly independently written software.
So I guess someone with access to ATLAS data should now write up the analysis and see if they can find it too.
--- Physicist who did his master thesis with sensors for ATLAS tracker, now doing a PhD on accelerator cavities for the CLIC future high-energy electron-positron collider.
The Baryon multiplets are.
Spin 1/2 (you can draw this as a hexagon)
Xi^0 Xi^-
Sigma^- Sigma^0 Sigma^+
Lambda
Neutron Proton
Spin 3/2 (draw this as a triangle)
Omega^-
Xi^0 Xi^-
Sigma^- Sigma^0 Sigma^+
Delta^- Delta^0 Delta^+ Delta^++
There are plenty of Baryons yet to be found, including most massively of all, the Omega Triple Bottom, which is (bbb) instead of (sss)
You are not everyone. I was beaten up about once a week on average for it from grade 2 until grade 8. I wish I was exxagerating, but I'm not. Not everyone is given the lavish life you enjoyed.
So, shut the fuck up, Bully. You didn't live the hardest life evar (and neither did I. I know because I'm still alive).
Physicists from the University of Zurich have discovered a previously unknown particle
I'm going to go with "The particle formerly known as unknown", or just "The particle".
It's LaTeX, you moron. It's the Greek letter Xi with a "b" in the subscript and a "*" in the superscript. So just call it Xi-b-star or Xi-b-asterix. Simple.
And how would you write Xi-b-obelix? ;-)
The Tao of math: The numbers you can count are not the real numbers.
In fact this particle is pretty massive already, as TFA notes (around the mass of a lithium atom). So presumably a Xi_b^obelix would mass as much as a menhir.
From scarped cliff or quarried stone she cries "A thousand types are gone, I care for nothing, no not one."