Cern: First evidence for the decay Bs → μ+μ−

  • #1
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BBC News reported this

http://www.bbc.co.uk/news/science-environment-20300100
Researchers at the Large Hadron Collider have detected one of the rarest particle decays seen in Nature. The finding deals a significant blow to the theory of physics known as supersymmetry.
Here's the actual paper

https://cdsweb.cern.ch/record/1493302/files/PAPER-2012-043.pdf [Broken]
First evidence for the decay Bs → μ+μ−
The LHCb collaboration
What does this actually mean for Susy etc?
 
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Answers and Replies

  • #3
PAllen
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I notice the following:

- Dorrigo makes no claim re. supersymmetry
- Matt Strassler , in his blog, also makes no such claim; only that SM survives another test.
- The actual paper does not mention Susy

Gordon Kane, comments below that that the result is not unexpected for SUSY/string.


http://motls.blogspot.com/2012/11/superstringy-compactifications.html#more


Is there any source for the 'contradictions' besides BBC?
 
  • #4
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As always, SUSY can evade these things just by having the superpartners be heavy enough to not to contribute much to these processes. These flavour constraints eat up various chunks of parameter space that are different to direct searches and dark matter constraints etc. though, so they are still important.
 
  • #5
MathematicalPhysicist
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@kurros, then how can you test supersymmetry if there's no strict prediction on the sizes of the masses?
(even a range of the predicted masses).
 
  • #6
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@kurros, then how can you test supersymmetry if there's no strict prediction on the sizes of the masses?
(even a range of the predicted masses).
Well the most awesome way would be if the LHC sees the superpartners directly one of these days. Aside from that, you can only get an idea of what sorts of masses they should have through these indirect measurements. With Bs->mu+mu- for instance, if it had been observed to occur at a rate somewhat higher than the SM predicts, then you would be able to compute what superparter masses can give you this correct value (there would be a lot of possible combinations but it would be narrowed down). Presumably there would follow the observation of other processes also deviating from the SM predictions and together this would let you narrow it down further. Maybe you would get some information from dark matter searches at some point. If the Higgs decay to two photons persists as being too frequent then that too gives you information. There is also the muon anomalous magnetic moment, which seems to deviate from the SM prediction, but which people still argue somewhat over what the SM prediction actually is (they have trouble computing it due to QCD effects).

But none of these things tell you as much as actually observing superpartners and measuring their masses directly.
 
  • #7
MathematicalPhysicist
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And what will make people abandon supersymmetry? Or nobody thinks of this option?
 
  • #8
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And what will make people abandon supersymmetry? Or nobody thinks of this option?
They will only abandon it when some new phenomenon that is wildly incompatible with it is observed. Say we get good evidence for dark matter scattering in these underground detectors, and it is totally irreconcilable with SUSY, or likewise if some new particles start appearing at the LHC which in no way could possibly be SUSY particles. Or perhaps the most likely way is if upon continued investigation of the Higgs sector we discover that there is some complicated stuff going on there that cannot be explained by SUSY.

As long as we continue to see everything compatible with the standard model, SUSY cannot be killed. SUSY predictions can always be made to be exactly the same as the SM predictions for sufficiently decoupled superpartners.

It is basically an Occams razor thing. In general, the community will assume everything is going according to the Standard Model plan until forced to concede otherwise. Next SUSY is the simplest (arguably) choice, so they will try to cram any observations into that framework. Only once SUSY really definitely cannot explain something will they turn to other ideas (although of course there are people working on other ideas all the time, I just mean those will not become mainstream until the previous mainstream ideas are really unworkable).
 
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  • #9
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I notice the following:
- Dorrigo makes no claim re. supersymmetry
Is there any source for the 'contradictions' besides BBC?
Dorigo says
The decay of the B mesons to muon pairs are ... important, because they proceed via loop quantum diagrams within which existing particles may circulate. And since these loops are virtual, even very massive particles, even ones we do not know yet about, would produce a significant contribution. So measuring the rate of the rare decays allow us to gauge whether there is new physics in store for us, or whether there is just a desert of Standard Model physics awaiting us at the high-energy frontier.
The BBC's quote was
Prof Val Gibson, leader of the Cambridge University LHCb team, said that the new result was "putting our supersymmetry theory colleagues in a spin". ... Supporters of supersymmetry, however, such as Prof John Ellis of King's College London, said that the observation is "quite consistent with supersymmetry". "In fact," he said, "(it) was actually expected in (some) supersymmetric models. I certainly won't lose any sleep over the result."
 
  • #10
bcrowell
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As long as we continue to see everything compatible with the standard model, SUSY cannot be killed. SUSY predictions can always be made to be exactly the same as the SM predictions for sufficiently decoupled superpartners.
But if SUSY doesn't exist at the electroweak unification scale, then it loses its theoretical motivation. Nobody would have proposed it if they had already known that it wasn't going to operate at the electroweak scale.
 
  • #11
Haelfix
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That's not true at all. Supersymmetry is widely used in cosmological model building, for applications in Baryogenesis and Leptogenesis, as well as the theory of inflation. It serves many purposes as well in GUT model building.

It also seems vital for quantum gravity for any number of theoretical reasons, many of which are general arguments involving black holes etc

The reason it is still so popular in electroweak model building is precisely because there is a distinct lack of credible alternatives for so many pressing questions that really must be answered.

So I agree with the poster above. It won't disappear as a credible idea unless some other new physics is observed or invented, that explains away all those problems in a simpler more elegant and natural fashion. Until that time, all that the LHC is doing is eating up parameter space.
 
  • #12
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SUSY is not going anywhere as a theory in general. But if it is ruled out as a solution to the hierarchy problem then it loses the motivation for it to be at the EW scale.

SUSY is very popular for many reasons. One of the major reasons is that it is within many physicists confort zones. I think it would be better for physics if the LHC finds something non-perturbative to push the community to understand strongly coupled theories.
 
  • #13
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So it has no agreed on effect on supersymmetry, but it does strengthen the standard model?
 
  • #14
PAllen
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So it has no agreed on effect on supersymmetry, but it does strengthen the standard model?
That is how I would describe it.
 

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