Supersymmetry Theory Dealt a Blow 143
Dupple writes in with some news from the team at the Large Hadron Collider. "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. Many researchers had hoped the LHC would have confirmed this by now. Supersymmetry, or SUSY, has gained popularity as a way to explain some of the inconsistencies in the traditional theory of subatomic physics known as the Standard Model. The new observation, reported at the Hadron Collider Physics conference in Kyoto, is not consistent with many of the most likely models of SUSY. Prof Chris Parke, who is the spokesperson for the UK Participation in the LHCb experiment, told BBC News: 'Supersymmetry may not be dead but these latest results have certainly put it into hospital.'"
Bad summary (Score:5, Informative)
The summary, like the article, jumps straight into "OMG CONFLICT" without bothering to tell us what's going on. From later in the article:
Researchers at the LHCb detector have dealt a serious blow to [supersymmetry]. They have measured the decay between a particle known as a Bs Meson into two particles known as muons. It is the first time that this decay has been observed and the team has calculated that for every billion times that the Bs Meson decays it only decays in this way three times. If superparticles were to exist the decay would happen far more often. This test is one of the "golden" tests for supersymmetry and it is one that on the face of it this hugely popular theory among physicists has failed. ...
The results are in fact completely in line with what one would expect from the Standard Model. There is already concern that the LHCb's sister detectors might have expected to have detected superparticles by now, yet none have been found so far.
But it sounds like this is only a problem for some variants of supersymmetry:
"If new physics exists, then it is hiding very well behind the Standard Model," commented Cambridge physicist Dr Marc-Olivier Bettler, a member of the analysis team. The result does not rule out the possibility that super particles exist. But according to Prof Parkes, "they are running out of places to hide". 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."
Re:And? (Score:5, Informative)
In a nutshell: the Standard Model of particle physics, developed in the 60s and 70s, has once again been shown to be a remarkably robust and effective description of reality. Thus far, no proposed extension to the SM has been corroborated by any convincing evidence. However, there *are* problems with the SM - it's just the resolution of these problems is at present beyond us.
Re:B Rays? (Score:5, Informative)
In a fairly hand-wavy way, supersymmetry predicts we should see this quite a lot, but the experiment shows it happens far less frequently, implying the current version of SUSY is either incorrect or completely wrong.
Lets get started... (Score:5, Informative)
Re:do not confuse this (Score:4, Informative)
Indeed that was a problem from the past where the existence of the PUSY was the main question, the today stereotype is, after confirming the PUSY, the problem now in that there is a sole PUSY.
Re:Bad summary (Score:4, Informative)
SUSY is not a theory which is altered every time a new relevant discovery is made. It's a (quite large) family of theories, some of which are ruled out every time a new relevant discovery is made.
Re:Bad summary (Score:5, Informative)
But it sounds like this is only a problem for some variants of supersymmetry:
Yes and no.
Actually just 'yes'. SUSY is essentially a mirror image of the Standard Model about which we know very little indeed (only limitations on it). Hence the best models assume nothing which is not expressly forbidden and so we end up with ~120 free parameters vs the 25 free parameters for the Standard Model which we have measured and so excluded many of the possibilities. For example the we set the mass of a photon and a gluon to zero in the Standard Model because we have no evidence that they have a mass and the Lagrangian requires zero mass for it to have the correct symmetries. However in fact all we can do is put an upper limit on the mass from experiment: this is a better example of the illustration you are trying to make.
The Standard Model already heavily suppresses Bs->mumu decay all this has shown is that SUSY, if it exists, likewise heavily suppresses it. This is a very interesting result but, far from falsifying SUSY, it just means that SUSY is perhaps more like the Standard Model than we think it needs to be. Since we have no clue about how Supersymmetry is broken this is not too surprising...so I'd say it's very interesting and certainly constrains SUSY but it is by no means its death knell. Indeed arguments about excluding phase space and so therefore making a theory less probably are somewhat akin to arguing that choosing the numbers 1,2,3,4,5,6 in a lottery is stupid because they will never come up. If SUSY is there nature has chosen one set of parameters for it and, if that happens to be the last place we look it is the last place we'll find it. However if we find no hints of SUSY particles at the LHC once we run with a higher energy (March 2015) then it will start to be in trouble because at that point it becomes a less likely solution to the problem it was actually invented to explain: why is the Higgs mass so much less than the energy scale of gravity?
Re:Great! (Score:4, Informative)
Well, assuming that something is found that is not consistent with the standard model. There is actually something from the LHC that is not consistent with the standard model, the LHCb discovery of CP violation in charm decays [blogspot.com]. This is "only" 3.5 sigma, and needs some serious theoretical work to be sure the SM prediction is even right, but as things stand it is evidence for new physics [arxiv.org].
Re:Lets get started... (Score:5, Informative)
The binding energy is negative and lowers the mass of nuclei.
Yes ... and no.
Yes, the binding energy of protons and neutrons to each other lowers the mass of a nucleus, such that a carbon-12 atom has less mass than 6 separate protons and 6 separate neutrons, but there is also the binding energy of the three quarks within each proton and each neutron. That is a honking big positive number, such that most of the mass (somewhere close to 99% [wikipedia.org]) is actually from the interaction (virtual gluons and such) between the quarks, rather than the rest mass of the quarks themselves. Since 99.9% of the mass of an atom comes from the protons and neutrons, about 99% of the mass of any object you interact with daily comes not from fundamental particles, but rather the energy of interaction between quarks.
So when the GP says "between and within protons and neutrons", he's correct, although dropping the "between" would make him slightly more accurate. I don't know enough about QCD to make any assessment of whether the Higgs field contributes significantly to the magnitude of that binding energy. (That is, if we had a zero-valued (or near-zero valued) Higgs field, would the magnitude of the quark binding energy (and thus the mass of everyday objects) be significantly different. )
the paper (Score:5, Informative)
Here is the paper: https://cdsweb.cern.ch/record/1493302/files/PAPER-2012-043.pdf [cdsweb.cern.ch]
Some blogs discussing the significance of the result:
http://www.science20.com/quantum_diaries_survivor/lhcb_evidence_rare_decay_bs_dimuons-96311 [science20.com]
http://motls.blogspot.com/2012/11/superstringy-compactifications.html#more [blogspot.com]
http://profmattstrassler.com/ [profmattstrassler.com]
Particle physics isn't my field, but neither the paper nor the blog posts seem to be interpreting it, as the BBC does, as evidence against supersymmetry.
Re:And? (Score:5, Informative)
It always baffles me why everybody is so focused on developing completely new and revolutionary physics.
The fundamental problem with the standard model is gravity. In terms of particle interactions, they have it covered via the Higgs particle and gravitinos. But the standard model doesn't have curvature of space.
Re:And? (Score:5, Informative)
The greatest progress has been made in refining the Standard Model, rather than replacing it.
Which is why a lot of folks were gunning for SUSY, because that's more or less exactly what is -- an extension, rather than a replacement, for the Standard Model.
In SUSY we keep everything we already know and love about the Standard model, but there is also a symetry where each existing particle has a partner with 1/2 spin difference.
Which as a consequence would apparently solve a number of known issues with the Standard Model -- which is attractive because we know the SM is good, but flawed -- and also provide possible solutions for other mysteries like Dark Matter.
So, basically, rulling out SUSY would be a setback for the (very reasonable and desireable) "refinement" model of advancing physics.
Maybe you're going off the fact that String Theory, a revolutionary new model of physics, also predicts SUSY?
Re:And? (Score:5, Informative)
It can be thought of as an attempt to do probabilistic type arguments, when you don't have any data to do probability with.
Suppose that astronauts find a long abandoned alien base on the Moon. All equipment was carefully removed, but we know that the doors and corridors are all 4 meters wide and 3 meters high. It would be "natural" to assume that the aliens (or their machinery) were typically less, but not much less, than 3 meters tall. That could be wrong - maybe they are 1 meter birds who like room to fly in. Or maybe they are 4 meter giants who don't mind stooping. But, in the absence of any other evidence, it is a "natural" assumption. Such assumptions are very common in places like cosmology and quantum gravity.
One argument from naturalness is that dimensionless constants should "naturally" be near one, without a good reason to have some specific value. (The other "natural" is of course zero.).
Take the axion and CP violation. You can add a term to the QCD Lagrangian which violates Charge+Parity (or CP), which means that this term allows for particles and their antiparticles to behave differently. This term is multiplied by a constant denoted by theta, with theta = 0 meaning no CP violation. It turns out you can restrict theta to be 10^-10 experimentally. So, presumably, theta IS zero (as zero is a much more "natural" number than the really tiny 10^-10). The axion came from assuming that theta really described a new field (with a new particle, the axion), and was driven towards zero in the evolution of the universe. It seemed much more "natural" to say that "after about the first microsecond of the big bang theta is driven to be zero" than just saying "this constant is really tiny."
The reason I said that about the cosmological constant (lambda) is that it is about 0.7 and (in the same units) the standard model value for it is about 10^122. (Or,
in natural units, its current value is about 10^-122.) That is an extraordinary result. Many people were sure that lambda was exactly zero (as that could also be "natural,") but it isn't. Note that the value for the axion's theta is by contrast almost routine. If theta is like winning the lottery, lambda is like having every atom in the universe winning the lottery simultaneously for every nanosecond that the universe has existed. So, I regard these arguments as less persuasive than I did 20 years ago,