Tuesday, July 23, 2013

SUSY Increasingly Irrelevant

New large hadron collider data on B_s meson decays into muon pairs has confirmed the Standard Model expectation and once again has constrained the parameter space of any supersymmetry (SUSY) theory.  The process is so rare that it hasn't been possible in experiments until now to get enough events to provide a statistically significant confirmation or refutation of the Standard Model prediction of the frequency with which B_s mesons decay into muon pairs (this happens about three times per B_s meson decays and producing a B_s meson in the first place isn't an easy thing).

The frequency with which these decay products of B_s mesons is produce a muon pair is very sensitive to the existence of heavy fundamental particles.

In part, this is because the signal associated with any indirect influence of heavy fundamental particles in other kinds of indirect hadron decays are swamped by a background of direct decay processes. But, a decay of a B_s meson into two muons can't happen directly.  This can only happen through intermediate steps that are rare to start with and so wouldn't statistically swamp other possible rare decay paths of B_s mesons into two muons involving currently unknown heavy particles.

The decays of B_s mesons into muon pairs are also sensitive to high energy processes, because in many other decays the source particle is too light have enough mass-energy to include loops involving heavy particles at any measurable frequency.  Virtual processes can "tunnel" to particles in intermediate steps that seem to violate mass-energy conservation so long as the end products do not, but the less mass-energy you start with, the less likely a virtual process of a given mass-energy is to take place.  The B_s meson is the third heaviest pair of quarks that can form a meson (after a pair of bottom quarks and a bottom quark-charm quark pair), so it can more easily have decay chains involving heavy virtual particles, but only if such particles exist.

The fact that a B_s meson is a pseudo-scalar spin-zero particle also rules out a lot of decays that would be possible in other systems, further reducing the background to signal ratio in the measured decay products.

As Jester explains, the closeness of the number of muon pairs observed to the Standard Model expectation is inconsistent with certain kinds of particles of less than 100 TeV mass (aka 10^5 GeV) in beyond the Standard Model theories that don't have special features that suppress decays via the new fundamental particle (although still far below the 10^16 GeV of the "GUT scale").  More recent prognostications have suggested that SUSY particle masses might mostly be on the order of 30 TeV.  The 100 TeV naive exclusion range is about a hundred times heavier than theorists had expected supersymmetric particles mass to be in the early days of SUSY theory.  The heaviest known fundamental particle, top quark, is about 0.17 TeV.  The data also disfavor low values for the supersymmetry parameter tan beta, which is basically the ratio of the two main mass constants of minimal or near minimal supersymmetry models.  Jester calls this result "just another handful of earth upon the coffin" of supersymmetry.

I'd put it differently.  If supersymmetry exists, there is just one set of supersymmetry parameters and they could be anywhere in the parameter space for reasons that may defy any human explanation.  We still have no good story to explain the host of parameters in the Standard Model, even though we have measured them to some degree of precision and have seen some trends.  So, even the profound shrinking of the supersymmetry parameter space and ruling out of some subtypes of supersymmetry models entirely, doesn't necessarily rule it out (although the many decades in which not a scintilla of evidence strongly hinting that SUSY exists has been found).

While SUSY can't really be definitively ruled out until a vice of parameter constraints reduce SUSY parameter space to zero, however, as more data come in SUSY starts looking more and more like the Standard Model at higher and higher energies. Results like this make any supersymmetry theory that could exist increasingly irrelevant with no meaningful phenomenological impact outside processes not seen in nature since the early parts of the Big Bang.  When you reach a point where you are talking about particles that wouldn't even have measurable effects on neutron stars, black holes, black holes, quasars, and cosmology in a young universe, you are talking about particles and a theory that may be largely irrelevant, even if it is correct.

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