LHC says quarks still elementary

[Vanadium 50]

The ATLAS experiment has recently sent two papers for publication. Both look for evidence of quark substructure, in two different ways. http://arxiv.org/abs/1008.2461" , submitted to Physical Review Letters, looks for an excited quark, and does not see any evidence of one. Based on that, they conclude that such an excited quark, if it exists, must have a mass above 1.26 TeV. The previous best limit was 0.87 TeV.

http://arxiv.org/abs/1009.5069" , submitted to Physics Letters B, essentially repeats the Rutherford experiment with quarks, and sees no evidence of substructure below a scale of 3.4 TeV (corresponding to about 6 x 10^-20 m). The previous best limit was 2.8 TeV.

 

[xepma]

Standard Model is still standing strong ;)

 

[jal]

...no evidence of substructure below a scale of 3.4 TeV (corresponding to about 6 x 10-20 m)

WHAT!? NO particles?

What about all those pretty theories that go to GUT scale or to PLANK scale?

Can't have that. It will cause too many layoffs!

jal

 

[daschaich]

WHAT!? NO particles?

No quark substructure. Not the same thing.

Thanks for posting those papers, Vanadium 50. I really should keep a closer eye on hep-ex.

 

[jal]

Cough, cough!

http://cdsweb.cern.ch/record/1294718/files/CERN-PH-EP-2010-028.pdf

Search for New Particles in Two-Jet Final States in 7 TeV Proton-Proton Collisions
with the ATLAS Detector at the LHC

This result extends the reach of previous experiments and constitutes the first exclusion of physics beyond the Standard Model by the ATLAS experiment. In the future, such searches will be extended to exclude or discover additional hypothetical particles over greater mass ranges.

 

[Parlyne]

Cough, cough!

http://cdsweb.cern.ch/record/1294718/files/CERN-PH-EP-2010-028.pdf

Search for New Particles in Two-Jet Final States in 7 TeV Proton-Proton Collisions
with the ATLAS Detector at the LHC

These results put new bounds on possible quark substructure. Quark substructure is one possible type of new physics. There have previously been no new bounds on any type of new physics from the LHC. Thus, these are the first LHC results that put new bounds on new physics. You should not, however, take that to mean that these new bounds apply to every possible type of new physics. They don't.

 

[tom.stoer]

@jal: GUT, SUSY, SUGRA and superstrings do not predict any quark substructure.

 

[daschaich]

@jal: GUT, SUSY, SUGRA and superstrings do not predict any quark substructure.

At least not at the TeV scale.

 

[kuon]

I think though that there are some models where RS is used where the quarks are composite...

Maybe somebody with more knowledge on the subject can say something about it...

 

[jal]

@jal: GUT, SUSY, SUGRA and superstrings do not predict any quark substructure.

Are we playing with the definitions of quarks and substructures?

What I understood from those papers is that they have probed to 3.4 TeV (corresponding to about 6 x 10-20 m) and have found only quarks and nothing else, (no smaller particles, no other particles) that would indicate that there is anything else that makes up quarks.

I guess that I'll leave open the possibility that if there is anything else then it must be hiding in another dimension which we have no means of probing. (Lisa Randall can breath for a while longer)

jal

 

[tom.stoer]

Are we playing with the definitions of quarks and substructures?

What I understood from those papers is that they have probed to 3.4 TeV (corresponding to about 6 x 10-20 m) and have found only quarks and nothing else, (no smaller particles, no other particles) that would indicate that there is anything else that makes up quarks.

I guess that I'll leave open the possibility that if there is anything else then it must be hiding in another dimension which we have no means of probing. (Lisa Randall can breath for a while longer)

We are not playing with definitions.

I can only repeat what I said: GUT, SUSY, SUGRA and superstrings do not predict any quark substructure. All these theories predict additional, new particles, but these particles are not substructures of quarks (superstring theory predicts a different high energy behavior of the amplitudes). So it is important to distinguish whether an experiment rules out a substructure or whether it rules out new particles.