Neutral Xi_b^star, Xi(b)^{*0}, Ξb*0, whatever
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Carl Franzen, "Big Bang Machine Discovers Brand New Particle", TPM IdeaLab 4/27/2012:
An entirely new type of particle has been discovered by scientists using the world’s largest and most powerful particle accelerator, the Large Hadron Collider (LHC), near Geneva, Switzerland.
The discovery of the new particle, called “neutral Xi_b^star baryon,” was made by the CMS experiment, one of six separate particle physics experiments running at the LHC. It was announced Friday by Symmetry Magazine.
Here's the announcement from Symmetry ("CMS collaboration discovers its first new particle", Symmetry Breaking 4/27/2012):
Members of the CMS collaboration announced the experiment’s first discovery of a new particle today. In a paper submitted to Physical Review Letters, the CMS collaboration described the first observation of an excited, neutral Xi_b baryon, a particle made up of three quarks, including one beauty quark.
Tommaso Dorigo offers another take, along with an extended explanation ("CMS Discovers new Xi Baryon", Science 2.0 4/27/2012):
No, unfortunately not yet the discovery of the century. Still, the new particle found by CMS in its 2011 dataset is a very important piece of the puzzle of low-energy spectroscopy. Here "low" should be taken with a pinch of salt: the new particle, an excited state of the Ξ_b series, has a mass only slightly lower than six GeV, and is thus "heavy" if compared to most other hadrons.
Here's the abstract of the .pdf version of the paper (CMS Collaboration, "Observation of an excited Xi(b) baryon", arXiv 4/26/2012):
And the corresponding plain-text version, from the arXiv page:
The observation of an excited b baryon via its strong decay into Xi(b)^- pi^+ (plus charge conjugates) is reported. The measurement uses a data sample of pp collisions at sqrt(s) = 7 TeV collected by the CMS experiment at the LHC, corresponding to an integrated luminosity of 5.3 inverse femtobarns. The known Xi(b)^- baryon is reconstructed via the decay chain Xi(b)^- to J/psi Xi^- to mu^+ mu^- Lambda^0 pi^-, with Lambda^0 to p pi^-. A peak is observed in the distribution of the difference between the mass of the Xi(b)^- pi^+ system and the sum of the masses of the Xi(b)^- and pi^+, with a significance exceeding five standard deviations. The mass difference of the peak is 14.84 +/- 0.74 (stat.) +/- 0.28 (syst.) MeV. The new state most likely corresponds to the Xi(b)^{*0} baryon, the J^P=3/2^+ excitation of the Xi(b)^0.
The major newswires and news outlets haven't covered this discovery, perhaps because they're trying to decide how to spell, typeset, and explain the particle's name.
From one point of view, it seems to be pretty simple. This is a new kind of Xi baryon, of which Wikipedia already lists 16 types, from the plain "Xi" (Ξ0) to the "charmed bottom Xi" (Ξ0
cb), explaining that
The Xi baryons or cascade particles are a family of subatomic hadron particles which have the symbol Ξ and have a +2, +1 or -1 elementary charge or are neutral. They are baryons containing three quarks: one up or down quark, and two heavier quarks. They are sometimes called the cascade particles because of their unstable state; they decay rapidly into lighter particles through a chain of decays.
But the physicists themselves exhibit quite a bit of variation in writing the new particle's name. There's romanized Xi vs. greek Ξ, there's subscripted b vs. "_b" vs. "(b)", there's superscripted "*0" versus "^{*0}" versus the initial modifier sequence "excited, neutral", and probably some other bound states of typographical matter not yet observed.
Why should we care about this discovery, however it's typeset? An explanation from the horse's mouth:
“Besides helping to understand how quarks bind and therefore further validate the theory of strong interactions, one of the four basic forces of physics, this measurement represents a tour-de-force that opens up good perspectives for future discoveries,” wrote Carlos Lourenco, a senior researcher with the European Organization for Nuclear Research (CERN), the organization that oversees the experiments at the giant accelerator, in an email to TPM.
The particle’s rarity is due in part to its composition: It is made up of three quarks which normally aren’t found locked together.
“When these three quarks get together, they immediately divorce, instantaneously,” Lourenco explained.
"This was hard to do and it's the latest in a series of results showing that we were right all along" is not an exceptionally compelling hook, aside from the problem of how to write the name. Maybe it's time for the Les Horribles Cernettes to add a new verse to their 1994 hit Strong Interaction, featuring the three-way (bottom-strange-up) and "instantaneous divorce" themes.
The original lyrics:
You quark me up (yeah yeah, I feel your charme)
You quark me down (tau tau, I feel so strange)
You quark me top (go go on hypercharge)
You quark me bottom (shoot shoot on isospin)
You spin me 'round 'round 'round 'round yeah
You spin me 'round 'round 'round 'round yeah
You spin me 'round 'round 'round 'round yeah
You spin me 'round 'round 'round 'round yeah
I feel your attraction
It's a strong interaction
Update — AFP has a story out ("Hadron Collider scientists detect baryon Xi-b subatomic particle", reprinted in the Ottawa Citizen), which goes with the typographical decay product "Xi-b".
D.O. said,
April 28, 2012 @ 11:12 am
The whole naming system for elementary particles is slightly nutty. It should be an interesting topic for linguists to figure out at which point the legacy system becomes so cumbersome that people abandon it.
Coby Lubliner said,
April 28, 2012 @ 11:23 am
As long as there's funny typographic stuff going on, I wonder what happened to the cedilla in Carlos Lourenço's name.
Peter Taylor said,
April 28, 2012 @ 11:31 am
_bis just LaTeX notation for subscripted b, and^{*0}is just LaTeX notation for superscripted *0. As variation between ways of writing the name go, it's about as significant as using block capitals vs cursive. The really interesting one is (b) in the plaintext version of a document which was almost certainly written in LaTeX.[(myl) Yes, I wondered about that — could there be a set of LaTeX macros that render X(y) as X with subscript y?]
Blaise Pascal said,
April 28, 2012 @ 11:50 am
I would say the "proper" typography for this particle is the Greek Ξ, the superscripted *0, and the subscripted b, as it's listed in the type-set version of the paper on the arXiv.
All the other forms are attempts to deal with typography which doesn't have a Ξ, subscripts, or superscripts. If you don't have a Ξ, romanize it to Xi. If you don't have subscripts, use (b) to replace.
The scientific typesetting system TeX uses _x to indicate a subscripted x and an ^x to indicate a superscripted x, and uses {} for grouping. Thus _b is TeXese for subscripted b and ^{*0} is TeXese for superscripted *0. I'm somewhat surprised that you didn't run into a \Xi, as that is TeXese for a Ξ.
The * means "excited", and the 0 means "neutral", so "excited Ξ^{*0}", "neutral Ξ^{*0)", and "excited, neutral Ξ^{*0}" all have varying degrees of redundancy. Saying "excited, neutral Xi(b)" unrolls all the various super- and sub- scripts neatly.
According to the Wikipedia page, this could also be called an excited bottom Xi.
[(myl) But Tommaso Dirigo, who surely knows the correct nomenclature and has relevant HTML hacking skills, goes with Ξ_b, Ξ*_b, etc., rather than Ξb etc. And the CMS website itself uses the headline "Observation of a new Xi_b^*0 beauty particle" — though the body of the article does shift to Ξ*b0, spelled
Ξ<sup>*</sup><sub>b</sub><sup>0</sup>
A further source of variation is the fact that the CMS team apparently hasn't yet given up the fight on "bottom" vs. "beauty"…]
Jimbino said,
April 28, 2012 @ 12:02 pm
Is this guy Carlos Laurenco or really Carlos Laurenço referred to by some reporter who hasn't learned to use the international keyboard that has the c-cedilla (ç)? And are there any other words that use 'ç' besides façade in standard American English?
[(myl) Recordings from the instrumentation at TPM suggest that the cedilla decayed via the emission of a gamma-ray photon.]
Rod Johnson said,
April 28, 2012 @ 12:05 pm
Aperçu.
[(myl) Curaçao. Also garçon, soupçon, Provençal.]
Dan Hemmens said,
April 28, 2012 @ 12:42 pm
The whole naming system for elementary particles is slightly nutty. It should be an interesting topic for linguists to figure out at which point the legacy system becomes so cumbersome that people abandon it.
I'm not sure the cumbersomeness is so much a consequence of a nutty legacy system as a consequence of a system that seeks to convey technical information in a systematic way. It's like the names for organic chemicals – names like "1-Chloro-1, 1-diflouroethane" seem clunky to non-chemists but they're a fairly sensible name for technical use. Certainly it tells you more about the structure of the chemical than just calling it "Freon".
Sili said,
April 28, 2012 @ 12:44 pm
Could be worse. Could be chemistry.
Always amusing to hear reporters try to pronounce systematic names – or even better, explain them. Did you know that "γ-hydroxybutanoic acid" comprises the three compounds "gamma", "hydroxy" and "butanoic acid"?
Sili said,
April 28, 2012 @ 1:04 pm
I'm not sure "ethane" maps perfectly to "Ξ".
To me as a non-particle physicist, the analogy seems more like "Freon 142b". Certainly, if you know the special naming systems for freons, you can decipher that into a structure, but if you're just an ordinary (organic) chemist, it's no less opaque than "furfural".
As systematic name would, I think, name the quarks involved, perhaps include the charge for redundancy, and then give the excited state explicitly. So this new particle could be (usb)-(I=3/2).
James Wimberley said,
April 28, 2012 @ 1:08 pm
It's a type of beauty particle, then? The Harlowe, the Russell, the Monroe.. Also the Valentino, the Flynn and so on. Further subcategories can be devised using hair colour and sexual orientation. Do you run out of particles or starlets first?
D.O. said,
April 28, 2012 @ 1:18 pm
I don't think that current system is really a heavy burden. After all, journalists do not write about details of particle physics too often and all people who do study those particles will adapt to capital Greek letters all surrounded by letters and numbers or whatever other system pretty easily. It's just interesting how naming conventions do not adapt quickly to more recent understanding of what's really going on.
D.O. said,
April 28, 2012 @ 1:22 pm
Didn't know it was excited. We definitely need a new symbol for a despirited particle. Should we stick asterisk in a new place or search for something different? How about bringing back the cedilla, woudn't it be ironic?
MattF said,
April 28, 2012 @ 1:28 pm
Well, IMHO, it's something of an exaggeration to call this a 'discovery.' We're talking about a kind of spectroscopy– at a high energy (and at a high cost), but spectroscopy none the less. The existence and rarity of the particle in question is entirely consistent with, and a consequence of, the Standard Model and the laws of quantum mechanics. If the researchers did not find it where they expected it to be, that would have been a much bigger surprise.
Sili said,
April 28, 2012 @ 1:29 pm
Of course. But how could one fail to find it, if one did not look for it?
Garrett Wollman said,
April 28, 2012 @ 1:33 pm
For a lot of people who write on scientific subjects, or even just work with scientists, the use of the TeX math-mode notation is so completely internalized as to be unremarkable, even when it leaks into completely un-TeXnical contexts. One of my duties is running my lab's technical-report publication system, and I frequently see theory people pasting the (La)TeX source of the abstract section of their paper directly into the abstract metadata field, even though we tell them explicitly not to do this (there is no, repeat no, formatting of any kind in abstracts, just plain-Jane ASCII text). Seminar announcements (published on our Web site, i.e., in HTML) often contain similar leakage, usually from the same people (theoretical computer scientists and their administrative assistants).
On the other hand, if I needed to represent sub-/superscripts in plain text, such as an email message, I'd probably do the same thing.
Garrett Wollman said,
April 28, 2012 @ 1:34 pm
And now it occurs to me that for extra verisimilitude I should have written "non-\TeX{}nical contexts" above before hitting the "Reveal Errors" button.
Dick Margulis said,
April 28, 2012 @ 1:57 pm
Regarding (b) to represent subscript b, I don't know where that arose in the immediate context, but this was standard notation back in Fortran days (and is presumably still used in referencing arrays in modern programming languages). Semantically, this may be an entirely different use of the subscript position, but it's easy to see how someone accustomed to representing "x sub b" as x(b) might do the same (erroneously, I take it) in this context.
Glen Gordon said,
April 28, 2012 @ 4:03 pm
Isn't this all just a ridiculously opaque way of saying that they discovered "quark soup"? The article says the three quarks that the particle is comprised of are normally not found together and it rips apart very quickly. Now, does this really constitute a particle? Couldn't we then call *any* combination of charged quarks, no matter how absurd and unstable, a "particle"?
robert said,
April 28, 2012 @ 4:16 pm
Organic chemistry nomenclature is reasonably transparent because most of the chemicals have a tree structure, broadly like human language. There are substantial differences, but the similarity is enough to allow a naming system that works roughly like normal word formation or syntax, making it relatively clear – though it doesn't handle too well polycyclic molecules, which definitely aren't trees.
The subatomic particle zoo does have structure, but it's not a tree structure, so naming it in a systematic and transparent way is a lot harder.
Sili said,
April 28, 2012 @ 4:33 pm
How do you define "absurd"? It's not like we have found penta-quarks yet, but it'd still be fascinating if they existed.
And a short lifetime isn't the same thing as not being bound. A marriage is still a marriage even if it's dissolved after a weekend.
Sili said,
April 28, 2012 @ 4:35 pm
Yet we've managed it for inorganic chemistry.
We just say what the compounds consist of – u, s, b here – and if necessary what type of compound it is – I=3/2 here.
Dan Hemmens said,
April 28, 2012 @ 4:39 pm
Couldn't we then call *any* combination of charged quarks, no matter how absurd and unstable, a "particle"?
Not quite, as MattF points out above this particle hasn't so much been *discovered* as *observed*. And in the wacky world of particle physics there's a massive difference between something that is possible but extremely rare and something that is flat out impossible. To draw a linguistic analogy, it's like the difference between a very, very uncommon word (like "claustrophilia" and "koumpounophobia" to use examples from recent LL posts) and completely random strings of letters.
As the OP observes, this "discovery" basically boils down to being "the latest in a series of results that show we were right all along."
Rod Johnson said,
April 28, 2012 @ 4:43 pm
Sili: maybe we should call these short-lived particles "Kardashian particles" then. :)
Back on the cedilla beat for a sec: actually, "facade" is the one member of that set that *doesn't* have a cedilla in English, as far as I know.
Sili said,
April 28, 2012 @ 4:53 pm
So much for the "Gellmann Was Wrong" headlines.
Sounds better than "Spears' particle", I guess.
Circe said,
April 28, 2012 @ 5:45 pm
That, I think, would be TeX sacrilege. I believe the notation for super and subscripts are built in at the much lower level of TeX, and cannot be modified easily by macro systems like LaTeX built on top of TeX.
Jerry Friedman said,
April 28, 2012 @ 6:28 pm
@Glen Gordon: "Quark soup" would be this.
@Rod Johnson: I was surprised to learn recently that the on-line OED lists "façade" but not "facade". It doesn't give diacritical variations for any of the words I looked for that might be spelled with a cedilla. For instance, it doesn't admit "jacana" can be spelled with one.
@Circe: I'll bet someone could make TeX render X(y) as Xy using active characters. However, they'd be crazy. How would they write X(y)?
Jerry Friedman said,
April 28, 2012 @ 6:29 pm
(Forgot the "sub" tag only works in the preview.)
Andy Averill said,
April 28, 2012 @ 10:37 pm
Also niçoise, as in the salad.
Glen Gordon said,
April 28, 2012 @ 10:46 pm
@Glen Gordon: "Quark soup" would be this."
Maybe the universe then is a really really really large particle comprised of a ridiculously large number of quarks then. Somebody should really find a good name for _that_ exotic combination too. ;o)
Could someone be kind enough to accurately describe the scientific difference between a ridiculously short-lived "exotic baryon" and "a local puddle of quark plasma"?
Rolig said,
April 29, 2012 @ 4:18 am
But what I am curious about is, how do you call this particle? How does one pronounce it verbally? "Xi [zye] subscript b superscript star zero"? "An excited neutral Xi-b [zye-bee] particle"? What is its name? Science journalists writing for the the public should provide this information, not simply present a string of enigmatic symbols.
[(myl) "Excited neutral beauty cascade" gets my vote.]
Eugene van der Pijll said,
April 29, 2012 @ 6:19 am
Could someone be kind enough to accurately describe the scientific difference between a ridiculously short-lived "exotic baryon" and "a local puddle of quark plasma"?
An accurate description is difficult, but here's the idea.
In physics, if you have two objects that are bound together, you can describe the combination of them as a single object. For example, the earth and the moon can be described as a single body. The mass of this body is equal to the mass of the earth, plus the mass of the moon, plus the energy contained in the movement of the moon around the earth. (Mass ans energy are equivalent, as stated by Einstein: E=mc2.) In celestial mechanics, the energy term can be ignored, because it's really small compared to the masses of the individual objects.
In the same way, you can calculate the "mass" of any combination of particles in a Sub-atomic reaction. I put the word "mass" in quotes, because in this case, the calculated mass is dominated by the energy of the movement of the particles with respect to each other. For example, a proton has a mass of about 900 MeV, but it consists of 3 quarks of about 10 MeV each.
Now, in a "local puddle of quark plasma", when you calculate the "combined mass" of any three quarks, the result will be a random number, but if there's an "exotic baryon", the "combined mass" will always be a fixed number, which is the mass of the baryon. In the case of the most exotic of these baryons, they do not exist long enough to have any effect as "distinct particle", but the trace of them still exist as a correlation between the energies of the decay products.
This is also how you discover these baryons: you observe a large number of particles produced in a nuclear collision, and then you calculated the "mass" of each combination of particles. If some of these particles were formed in the decay of an unknown particle, you will find that an exceptionally large number of combinations have the same combined mass. (See the Wikipedia article on Resonance for an example.)
(This is a simplified description, of course. For example, the decay of a Xi is a multi-step process, as shown in the diagram at the top of this post, and if you want to find this resonance, you'd first try to reconstruct Lambda particles from the observed protons and pions, and J/psi's from the observed muons, and then combine those reconstructed particles, etc.)
Matt McIrvin said,
April 29, 2012 @ 7:41 am
@Glen Gordon: This kind of unstable composite particle is often referred to as a "resonance"; specifically, a short-lived excited resonance of this particular three-quark system. The shorter-lived a resonance is, the fuzzier its "spectral line" will be; that is, the less it behaves like a particle with a well-defined mass. But it's a matter of degree.
There's another linguistic angle I've noticed concerning this particular type of particle, which is that English-speaking physicists often avoid saying "Xi" when talking about the various sorts of Xi resonance, probably because it's slightly hard for them to pronounce, and also maybe to avoid confusion between the letters xi and chi, which looks more like an X.
So they often just pronounce it "cascade" instead, referring to the distinctive cascading decay pattern of Xi particles. This one would be pronounced "neutral cascade-b-star", I think.
Matt McIrvin said,
April 29, 2012 @ 7:47 am
…though often the word "resonance" is colloquially reserved just for the even shorter-lived ones that are mostly distinguished from their stabler counterparts by having more excited orbital wavefunctions for the components. They'll be denoted by having a number in parentheses giving their mass-energy in mega-electronvolts.
Rich Magahiz said,
April 29, 2012 @ 1:29 pm
Actually, Sili, my former research collaboration came out with evidence interpreted as evidence for a pentaquark. The interpretation was that it consists not of five quarks but of four quarks and an antiquark. There was controversy later when other experiments failed to confirm that first report, however.
Brett said,
April 29, 2012 @ 1:29 pm
I was expecting the whole post would be about the fact that a Xi particle is called a "cascade" in English, since that is a genuine linguistic oddity of these kinds of baryons. The various subscripts and superscripts are really quite transparent if one is used to working with them, but the "cascade" thing is really unique. Nor is the "cascade" usage limited to speech; it can be found in many books, especially older ones.
Mark F. said,
April 29, 2012 @ 4:26 pm
The issue of how to say it out loud has an interesting angle too. There seems to be a difference in pronunciation cultures (perhaps just subsets of them) between math and physics on phi, psi, and xi. Physicists seem to use the anglicized pronunciations of /fai/, /sai/ and /zai/, while a lot of my math professors in grad school used what I'm told is something like the classicist's pronunciation of /fi/, /psi/ and /ksi/. Of course, neither group pronounces pi as /pi/.
Matt McIrvin said,
April 29, 2012 @ 4:43 pm
For xi, in physics, I've usually heard either /zai/ or /ksai/. Or "cascade" for the particle.
Layra said,
April 29, 2012 @ 11:27 pm
As a mathematician, I have heard precisely one of my professors ever pronounce pi as /pi/, but had dismissed it as him being Russian.
I actually hear a lot of variance in the pronunciation of phi and psi even within individuals, but xi is always pronounced /xi/.
Sandy Nicholson said,
April 30, 2012 @ 3:53 am
As a mathematician-turned-linguist, I agree with Layra that π/Π is usually pronounced /pai/ by mathematicians (who could hardly say /pi/, given that there is another common letter with that pronunciation!). But ξ/Ξ is definitely pronounced /ksi/ (not /xi/!). (I pronounce it that way anyway, but I couldn’t swear that I’d never heard a mathematician say /ksai/.) I also say /fi/ and /psi/, but I have heard plenty of mathematicians say /fai/ and /psai/ (and possibly /sai/).
For what it’s worth, I’m also a person who doesn’t think twice about reading plain text with LaTeX-source-inspired notation for subscripts etc.
Army1987 said,
April 30, 2012 @ 4:08 am
I'd pronounce it either "xi-bee-star-zero" or "excited neutral bottom cascade", depending on the same things depending on which I'd say "del cross bee" or "the curl of the magnetic field".
[(myl) "Excited neutral beauty cascade" is more upbeat, and sort of follows the lead of the CMS group itself. But "Excited neutral bottom cascade" is certainly a more memorable image.]
Rod Johnson said,
April 30, 2012 @ 7:12 am
Layra: /xi/? With a velar fricative?
Jerry Friedman said,
April 30, 2012 @ 9:06 am
I've heard physicists use both. In teaching freshman physics, I say /fi/—we usually don't mention ψ or ξ, and at this point I'm not sure how I pronounce ψ. (ξ is /ksi/ for me.)
A friend and fellow physics major in college pronounced Φ /faɪ/ and φ /fi/, just to keep things clear. He might still, actually. He also said capital Latin letters louder than small letters when there was a risk of confusion. I do that occasionally to entertain my students.
Yet another John said,
April 30, 2012 @ 11:08 am
Another data point from a mathematician. In my idiolect:
φ is /fi/, almost always, I think. /fai/ is understandable but sounds odd to me, unless we are talking about the name of a fraternal organization (in which case it is always /fai/).
ψ is usually /sai/, though I think in free variation with /si/ and /psi/ (depending on the phase of the moon).
Ξ is /ksi/, I think. This comes up rarely, so I'll be sure to articulate it carefully to avoid confusion with the other two.
One time in grad school we had a professor who used an ambiguously-shaped squiggle on the chalkboard which he called /zai/. We never did figure out which letter it was supposed to be, or if he was simply improvising a new Greek letter on the spot.
Rodger C said,
April 30, 2012 @ 11:34 am
@Yet: I think he was probably saying "xi" and making a lower-case one, which could easily degenerate into three vertical wiggles.
Glen Gordon said,
April 30, 2012 @ 11:04 pm
@Matt McIrvin, Eugene van der Pijll and others for a very precise answer. My head just exploded. I'll have to think a great deal more about all this obviously. And pentaquarks, that's a neat hypothesis too.
As for how "xi" is pronounced, I find that because English speakers find word-initial /ks/ unnatural, we will tend to avoid it if possible. So while /ksaj/ is more true to its Greek origin, /zaj/ is an attempt at naturalizing the foreign sequence of sounds. (And /xaj/ in English is just plain wrong on so many levels.) I speak French too and it always fascinates me how what sounds "natural" in one language just doesn't in another, and further, how we can hold all these different languages and their rules in our head without having a seizure. The brain is an interesting organ. So "xi" is /ksi/ in French and I've never heard */zi/ in that language.
Sili said,
May 1, 2012 @ 8:23 am
Another datapoint:
Rucca said,
May 2, 2012 @ 11:42 am
Maybe it is because the name is so complicated, “Xi_b^{0star} baryon", which should be read as "neutral excited strange and beautiful baryon", that the people who made the discovery, in CMS, call the particle simply as "the Ernestion", because it was first found by a postdoc called Ernest.
Sandy Nicholson said,
May 4, 2012 @ 8:58 am
Glen Gordon wrote: /xaj/ in English is just plain wrong on so many levels.
As a Scot (and perhaps more significantly, as a Gaelic-speaker), I tend to pronounce χ/Χ as /xi/ or occasionally /xai/, never as /kai/. (Admittedly, Scottish English uses /x/ rarely and never word-initially (as far as I know).