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Old 29 Apr 2012, 06:43 PM   #359876 / #1
phands
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Default New Particle Discovered at CERN

http://www.sciencedaily.com/releases...0427095621.htm

Cool stuff......

Quote:
ScienceDaily (Apr. 27, 2012) — 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 particle physics, the baryon family refers to particles that are made up of three quarks. Quarks form a group of six particles that differ in their masses and charges. The two lightest quarks, the so-called "up" and "down" quarks, form the two atomic components, protons and neutrons. All baryons that are composed of the three lightest quarks ("up," "down" and "strange" quarks) are known. Only very few baryons with heavy quarks have been observed to date. They can only be generated artificially in particle accelerators as they are heavy and very unstable.

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.

The discovery was based on data gathered in the CMS detector, which the University of Zurich was involved in developing. The new particle cannot be detected directly as it is too unstable to be registered by the detector. However, Xi_b^* breaks up in a known cascade of decay products. Ernest Aguiló, a postdoctoral student from Professor Amsler's group, identified traces of the respective decay products in the measurement data and was able to reconstruct the decay cascades starting from Xi_b^* decays.

The calculations are based on data from proton-proton collisions at an energy of seven Tera electron volts (TeV) collected by the CMS detector between April and November 2011. A total of 21 Xi_b^* baryon decays were discovered -- statistically sufficient to rule out a statistical fluctuation.

The discovery of the new particle confirms the theory of how quarks bind and therefore helps to understand the strong interaction, one of the four basic forces of physics which determines the structure of matter.

The University of Zurich is involved in the LHC at CERN with three research groups. Professor Amsler's and Professor Chiochia's groups are working on the CMS experiment; Professor Straumann's group is involved in the LHCb experiment.

CMS detector

The CMS detector is designed to measure the energy and momentum of photons, electrons, muons and other charged particles with a high degree of accuracy. Various measuring instruments are arranged in layers in the 12,500-ton detector, with which traces of the particles resulting from the collisions can be recorded. 179 institutions worldwide were involved in developing CMS. In Switzerland, these are the University of Zurich, ETH Zurich and the Paul Scherrer Institute.
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Old 29 Apr 2012, 11:39 PM   #359963 / #2
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Woohoo!
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Old 30 Apr 2012, 05:43 AM   #360020 / #3
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Here's how this particle fits in.

Every known hadron state is one of these:
  • Meson: q + q*
  • Ordinary baryon: q + q + q
  • Antibaryon: q* + q* + q*
q = ordinary quark, q* = antiquark
In general, (# ordinary quarks) - (# antiquarks) must be a multiple of 3, though it can be a negative one.

Mesons can have total spin either 0 or 1.
Baryons can have total spin either 1/2 or 3/2.
Orbital angular momentum can combine with both of these; it is always an integer. The lowest states have zero OAM.

For each set of quark flavors, there's 1 baryon with total spin 3/2, but for 1/2, it's more complicated:
All the same: 0
Two the same, one different: 1
All different: 2

There are 5 quark flavors that can form hadrons: up, down, strange, charm, and bottom. The 6th one, top, is too short-lived for that.

I went over to Particle Data Group and checked on what has been observed.

For mesons, every combination of hadron-making flavors has been observed:
uu*, ud*, du*, us*, su*, ds*, sd*, ss*, uc*, cu*, dc*, cd*, cc*, ub*, bu*, db*, bd*, sb*, bs*, cb*, bc*, bb*

For baryons, every combination of the light flavors has been observed:
uuu, uud, udd, ddd, uus, uds, dds, uss, dss, sss

For charm:
uuc, udc, ddc, usc, dsc, ssc

For bottom:
udb, usb, dsb, ssb

This most recent particle is a spin-3/2 usb particle; the previously observed usb one is a spin-1/2 one.

From this discovery, it looks like the LHC may be able to make more bottom-containing baryons.
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Old 30 Apr 2012, 10:14 AM   #360043 / #4
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I don't understand all that mathy stuff, but I guess my confusion/objection is that they didn't DISCOVER a new particle. This particle was part of the current model I presume, but what they did was CONFIRM / OBSERVE the particle.

Right?
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Old 30 Apr 2012, 10:29 AM   #360049 / #5
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Why doesn't the initial observation of something never seen before count as a discovery?
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Old 30 Apr 2012, 10:34 AM   #360050 / #6
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Quote:
Originally Posted by kennyc View Post
I don't understand all that mathy stuff, but I guess my confusion/objection is that they didn't DISCOVER a new particle. This particle was part of the current model I presume, but what they did was CONFIRM / OBSERVE the particle.

Right?
It counts as a discovery, even if it is a discovery of a predicted particle.
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Old 30 Apr 2012, 10:38 AM   #360051 / #7
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Quote:
Originally Posted by lpetrich View Post
Quote:
Originally Posted by kennyc View Post
I don't understand all that mathy stuff, but I guess my confusion/objection is that they didn't DISCOVER a new particle. This particle was part of the current model I presume, but what they did was CONFIRM / OBSERVE the particle.

Right?
It counts as a discovery, even if it is a discovery of a predicted particle.
I understand, but think it's a funny use of the word.

It's a bit like saying I discovered catfish. It had been predicted there were catfish in the local pond.

At least in Canada they got the headline right:

Hadron Collider scientists detect baryon Xi-b subatomic particle


http://www.ottawacitizen.com/technol...645/story.html
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Old 30 Apr 2012, 04:33 PM   #360115 / #8
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Quote:
Originally Posted by kennyc View Post
It's a bit like saying I discovered catfish. It had been predicted there were catfish in the local pond.
A better analogy would be if there were a missing link between catfish and an evolutionary ancestor. The missing link is predicted to have existed but has no experimental evidence. Then, if you find a fossil of the missing link, it would certainly be a discovery.
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Old 02 May 2012, 01:23 AM   #360628 / #9
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meeh.
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