In this sorry age for Supersymmetry (SUSY) phenomenologists, it is quite easy to step on an aching toe while discussing the results of the Large Hadron Collider experiments, whose results have let these physicists down by excluding the presence of SUSY where most of them used to put their moneys until yesterday.
The knee-jerk reaction I frequently observe (e.g. see the thread of my recent post on the cMSSM) in the most stubborn SUSY enthusiasts is to seek refuge in a concept which could be summarized as follows: "There is only one true realization of SUSY in Nature, and one point of the parameter space which corresponds to it; the fact that the LHC has kept excluding parameter space points that do not correspond to the true realization of SUSY says nothing on the correctness of SUSY anyway".
Now, let us step back for a second and make a few simple-minded examples of the implications of this kind of reasoning, to put it in the correct context.
1) In some cultures a popular game played in special events is to hide a small coin or jewel in a big cake; everybody then gets a slice, and the person who finds the precious treasure can keep it. Now imagine you play such a game, and you start eating your slice bit by bit, to be sure you are not gulping down the treat with the cake. You keep finding nothing, and your dish is soon close to empty; only the tip of your slice remains to be checked. You therefore now grow extremely excited: surely you're going to find it in the next bit !
2) You arrive at a deserted train station in the evening. You know that there's one train exactly every hour to your destination; however, you do not know the minutes at which trains pass. You also seem to remember that at some time late in the evening trains stop circulating. You sit and wait, and after 58 minutes have passed your train has not come yet. You then rise from the bench and pick up your suitcase, certain that the train is about to arrive.
3) In a science-fiction story (I believe it is Ray Bradbury's "The Martian Chronicles", but I could be mistaken) a man decides to seek the help of a private detective. The detective explains that his client can be confident the case will be solved: he failed to solve the previous 150 ones, so it's extremely improbable that he'll fail on this one, too.
In the three above examples, there is an obvious flaw in the reasoning of the protagonist: a wrong a priori assumption. The failure to account for the unknown probability that reality is according to one's wish is a childish mistake that we sometimes fall in even as adults.
What SUSY enthusiasts in fact do when they resort to the "Nature chose elsewhere to hide" argument is to manifest that their prior belief in SUSY being the correct theory of nature is 100%. This, in a Bayesian formalism, can be mathematically described as a "point mass" prior probability density function (PDF): a Dirac delta function, containing all the probability at one value (whatever value we choose to describe "SUSY is true" on a real axis). Now, the problem with a point mass prior PDF is that no experimental observation -none at all- inserted in the Bayesian equation can produce a posterior which is different from the prior: a granitic belief cannot be shaken, regardless of the evidence against it !
I believe Science progresses more rapidly if scientists keep their minds open to the widest range of possibilities. Well, let me restate that: I believe Science does not progress at all if scientists fail to do so ! I am therefore inclined to believe that choosing a point mass PDF for one's beliefs on the correctness of a unconfirmed theory is a wrong, anti-scientific attitude. I certainly acknowledge that SUSY is a beautiful idea, and I indeed would be happy if it were found some day (even better, if I myself found it ! I am indeed searching for SUSY particles in my research time with the CMS experiment!); yet the failure to observe SUSY as we raise the energy of proton-proton collisions and the accumulated size of our datasets in ATLAS and CMS cannot be dismissed as "no information". It is important information !
Keeping oneselves anchored to a point-mass PDF that "SUSY is correct" equates to dismissing as garbage all the negative results of the LHC searches. I will say more: it equates to saying that it is useless to do experimental research, because SUSY might be hiding where we have no access with particle collisions or other experiments. Given that, and given that we must already be sure that SUSY is correct, why searching for it ?
I am confident, though, that the attitude of those SUSY enthusiasts who choose the point-mass PDF is going to change if we continue excluding parameter space points at the LHC. Phenomenologists are pragmatic and smart people (someone funnily used the word "street-smart" in connection to one of them in the comments thread I mentioned above), so even the stubborn among them will soon choose some other point mass to anchor themselves and their careers to.
SUSY: A Matter Of Prior Beliefs