2 new Baryon particles discovered

[dragon]

Geneva 19 November 2014. Today the collaboration for the LHCb experiment at CERN1’s Large Hadron Collider announced the discovery of two new particles in the baryon family. The particles, known as the Xi_b'- and Xi_b*-, were predicted to exist by the quark model but had never been seen before. A related particle, the Xi_b*0, was found by the CMS experiment at CERN in 2012. The LHCb collaboration submitted a paper reporting the finding to Physical Review Letters.

Like the well-known protons that the LHC accelerates, the new particles are baryons made from three quarks bound together by the strong force. The types of quarks are different, though: the new X_ib particles both contain one beauty (b), one strange (s), and one down (d) quark. Thanks to the heavyweight b quarks, they are more than six times as massive as the proton. But the particles are more than just the sum of their parts: their mass also depends on how they are configured. Each of the quarks has an attribute called "spin". In the Xi_b'- state, the spins of the two lighter quarks point in opposite directions, whereas in the Xi_b*- state they are aligned. This difference makes the Xi_b*- a little heavier.

“Nature was kind and gave us two particles for the price of one," said Matthew Charles of the CNRS's LPNHE laboratory at Paris VI University. "The Xi_b'- is very close in mass to the sum of its decay products: if it had been just a little lighter, we wouldn't have seen it at all using the decay signature that we were looking for.”

"This is a very exciting result. Thanks to LHCb's excellent hadron identification, which is unique among the LHC experiments, we were able to separate a very clean and strong signal from the background," said Steven Blusk from Syracuse University in New York. “It demonstrates once again the sensitivity and how precise the LHCb detector is.”

As well as the masses of these particles, the research team studied their relative production rates, their widths - a measure of how unstable they are - and other details of their decays. The results match up with predictions based on the theory of Quantum Chromodynamics (QCD).

QCD is part of the Standard Model of particle physics, the theory that describes the fundamental particles of matter, how they interact and the forces between them. Testing QCD at high precision is a key to refine our understanding of quark dynamics, models of which are tremendously difficult to calculate.

“If we want to find new physics beyond the Standard Model, we need first to have a sharp picture,” said LHCb’s physics coordinator Patrick Koppenburg from Nikhef Institute in Amsterdam. “Such high precision studies will help us to differentiate between Standard Model effects and anything new or unexpected in the future.”

The measurements were made with the data taken at the LHC during 2011-2012. The LHC is currently being prepared - after its first long shutdown - to operate at higher energies and with more intense beams. It is scheduled to restart by spring 2015.



Further information

Caption diagram : The mass difference spectrum: the LHCb result shows strong evidence of the existence of two new particles the Xi_b'- (first peak) and Xi_b*- (second peak), with the very high-level confidence of 10 sigma. The black points are the signal sample and the hatched red histogram is a control sample. The blue curve represents a model including the two new particles, fitted to the data. Delta_m is the difference between the mass of the Xi_b0 pi- pair and the sum of the individual masses of the Xi_b0 and pi-.. INSET: Detail of the Xi_b'- region plotted with a finer binning.

Link to the paper on Arxiv: http://arxiv.org/abs/1411.4849 (link is external)
More about the result on LHCb’s collaboration website: http://lhcb-public.web.cern.ch/lhcb-pub ... l#StrBeaBa
Observation of a new Xi_b*0 beauty particle, on CMS’ collaboration website: http://cms.web.cern.ch/news/observation ... y-particle
Footnote(s)

1. CERN, the European Organization for Nuclear Research, is the world's leading laboratory for particle physics. It has its headquarters in Geneva. At present, its Member States are Austria, Belgium, Bulgaria, the Czech Republic, Denmark, Finland, France, Germany, Greece, Hungary, Israel, Italy, the Netherlands, Norway, Poland, Portugal, Slovakia, Spain, Sweden, Switzerland and the United Kingdom. Romania is a Candidate for Accession. Serbia is an Associate Member in the pre-stage to Membership. India, Japan, the Russian Federation, the United States of America, Turkey, the European Commission and UNESCO have Observer Status.
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[LaCroix]

These guys should take a break. That's how many new particles they have found by now?

[Simon_Jester]

They are taking a break. This is from the data they were gathering two years ago.

[Borgholio]

We sure are getting our money's worth out of LHC. I wonder how many of these particles were predicted beforehand, vs discovered on the spot without being expected?

[SCRawl]

We sure are getting our money's worth out of LHC. I wonder how many of these particles were predicted beforehand, vs discovered on the spot without being expected?

These ones were predicted, as stated in the OP. I strongly suspect that everything they've found has been predicted.

[Borgholio]

Yeah...I guess I'm just curious if the LHC has found anything totally unexpected so far.

[Elheru Aran]

Yeah...I guess I'm just curious if the LHC has found anything totally unexpected so far.

Part of the problem is just the amount of time it takes to analyze everything that comes out of it. They can mark off the usual stuff-- protons, electrons, what have you-- fairly quickly, but the funky stuff takes a while to figure out. Combine that with the sheer number of tests being run on that thing-- it's probably booked up for years with multiple runs every few days-- and it's no wonder things take time to work out and come out with new results. Plus, more often than not, likely the results are going to duplicate stuff that's happened before-- really new stuff that they can actually publish aren't going to be discovered all that often. And then you have the whole gap between research and publishing and the time for the mainstream media to actually notice the boring new scientific discovery...

That's my understanding of the situation, anyway.

[Simon_Jester]

We sure are getting our money's worth out of LHC. I wonder how many of these particles were predicted beforehand, vs discovered on the spot without being expected?

All of them.

So far, the main contribution of the LHC has been that it provides additional confirmation that our basic theories are sound to describe the known universe... and to provide a baseline of data that tells us exactly what to expect if we do start poking around looking for exotic and unknown physics. Much as, in order to understand atomic physics, we had to thoroughly understand classical electromagnetism a la Maxwell.

Now, there may well be unknown and unidentified particles and forces suggested by further analysis of the LHC data. But, yes, it's going to take a lot of time and number-crunching to find them in the background clutter.

[fnord]

Simon, sort of like peeling away the layers of stuff we already know about and seeing what's left at each layer?

[Simon_Jester]

Well, there's the distinct possibility that we actually found the last layer of onion. There is, as yet, no firm consensus about what might exist that would need new physics beyond the Standard Model to explain.

That said, basically yes- if there is anything to be seen other than what we could already predict, we need a very thorough grasp of what "what we predict" is. Basically, the desire to be able to say "no, that's an exotic mutant blob of quarks of a type we already understand, and that's interesting, the Higgs is a little more massive than we thought, which implies..."

Hopefully this will give rise to a question analogous to the one that gave rise to quantum mechanics, which was:

"According to our very thorough and rigorous understanding of electromagnetism and statistical mechanics, the inside of a heated cavity should dump all its energy into ultra-high frequency energy states and be full of ricocheting X-rays. Why doesn't that happen?"

The answer turned out to be "despite all our earlier intuition, the energy of electron orbitals and of electromagnetic radiation is quantized; it comes in fixed amounts that cannot be 'tweaked' up or down willy-nilly." Which gave rise to, well, much of the physics of the 20th century, courtesy of our command of the physics of the 19th century.

[jwl]

The nearest thing to an unknown particle is the possibility that they may have found a particle with four quarks, but even that isn't definitely against quantum chromodynamics, it's just too difficult to calculate whether it is possible ort not. Of course, there is also the higgs, which is currently looks like it follows the standard model but it may not do.

Well, there's the distinct possibility that we actually found the last layer of onion. There is, as yet, no firm consensus about what might exist that would need new physics beyond the Standard Model to explain.

That said, basically yes- if there is anything to be seen other than what we could already predict, we need a very thorough grasp of what "what we predict" is. Basically, the desire to be able to say "no, that's an exotic mutant blob of quarks of a type we already understand, and that's interesting, the Higgs is a little more massive than we thought, which implies..."

Hopefully this will give rise to a question analogous to the one that gave rise to quantum mechanics, which was:

"According to our very thorough and rigorous understanding of electromagnetism and statistical mechanics, the inside of a heated cavity should dump all its energy into ultra-high frequency energy states and be full of ricocheting X-rays. Why doesn't that happen?"

The answer turned out to be "despite all our earlier intuition, the energy of electron orbitals and of electromagnetic radiation is quantized; it comes in fixed amounts that cannot be 'tweaked' up or down willy-nilly." Which gave rise to, well, much of the physics of the 20th century, courtesy of our command of the physics of the 19th century.

There is something that needs beyond the standard model physics to explain: neutrino oscillations.

[Simon_Jester]

[blinks]

I'd entirely forgotten about neutrino oscillation.