The second plenary session of the day looks at some deeper questions of theory.
11:00 am: Naturalness, JoAnne Hewett
Naturalness told us to expect NP as soon as we turned the LHC on. We didn't see it, at run I or (so far) run II. What should we do? Panic? Hope the 750 GeV excess persists? Give in to anthropic arguments?
Let's panic (it's more fun).
How is naturalness defined? For concreteness, consider SUSY. There are several fine tuning measures that have been advanced: the traditional Guidice value based on variation of the Higgs VEV to input parameters; one based on large-logarithms; and one based on EW-scale inputs. Different choices lead to different answers, so there is a definite subjective element in what an individual scientist will consider the true fine-tuning.
Model building can give us further ideas. Consider the CMSSM. Pre-LHC, the best-fit values for the SUSY parameters were just beyond the limits: turn on the machine and see it. But now m0 and M1/2 are up at the TeV-scale, which most people will agree is in some tension with naturalness.
But of course, SUSY is not a single model. The pMSSM is in many respects just as valid a model, but the extra parameters will offer different phenomenology. A 105-point scan of the parameter space finds points with gluinos and squarks down to a few hundred GeV still allowed by LHC data. This analysis was repeated and confirmed by ATLAS. We see several features in these lighter sparticle spectra: Compressed spectra (with little MET), Stealth SUSY (with complicated decays) and kinematic suppression. The 13 TeV results extend the exclusions but there remain many points with sub-TeV gluinos. Even with 3 ab-1 of data, there will be a handful of points that survive.
However, focusing on a specific low-fine-tuning subset of pMSSM data, they will be excluded by the LHC with the full data set. Common feature of these models: suite of light and compressed EW gauginos, leading to a complex stop decay pattern that weakens constraints. This provides us with a guide for future developments in search techniques. A recent example is to look for charginos boosted by QCD ISR together with photon FSR (that tends to align with the MET).
Move on to discussion of RS models. I saw several of these points in Tom Rizzo's talk on Tuesday about explaining the 750 GeV excess using KK gravitons. Need to use large BLKTs to suppress couplings of KK gluon. But the general conclusion is that BLKTs are naturally expected, and extremely important for the KK phenomenology.
Question
Naturalness can be made rigourous in Bayesian statistics (up to priors).
pMSSM requires throwing away (some of) the motivations for SUSY. Additionally, 105 points sounds too few (108 done by questioner John Ellis). How do fraction of points that survive compare to e.g. Bayesian analyses? Do not; there is no statistical power associated with these statements.
750 GeV excess can be explained with radion, as Jack Gunion (asking the question) and I talked about at this conference.
11:30 am: Supersymmetry: to be or not to be? Hermann Nicolai
Oh look, another speaker who didn't feel the need to upload their talk in advance.
"Take a more fundamentalist approach". Word choice; the SUSY = religion jokes are easy enough to make as it is.
Why SUSY? Start with Coleman-Mandula, which is where I started back as a grad student, so I'm feeling more sympathetic already.
Interesting point: positive cosmological constant (as appears to be realised in nature) is not consistent with SUSY!
Maximal rigid SUSY is N = 4; maximal supergravity is N = 8. These theories are essentially unique due to the restrictions of the SUSY algebra. However, people immediately realised that N = 1 SUSY is the unique theory with chiral fermions (and elementary gauge bosons). It is also motivated by string theory, and is the only theory we can hope to see at the LHC (check this).
A problem with SUSY since the beginning is how to break it. Ultimately, no completely compelling mechanism. Spontaneous breaking insufficient, hence the soft breaking usually adopted. Problem is worse in string theory. (Time-dependent cosmological backgrounds always break SUSY.)
What if the LHC does not find SUSY? Original SUSY model expected a 3 GeV photino! Opinion: should have seen (traces of) N = 1 SUSY at LEP. But then what? Other ideas for naturalness & DM that would not need SUSY. But more speculative idea: what is the fate of space-time SUSY in quantum gravity theories with emergent space and time?
Move on to maximal theories. As essentially unique, no freedom to fiddle with things which would apparently make them maximally testable. But connection to "more important" exceptional symmetries. For D < 3, these exceptional symmetries are infinite-dimensional. The theme seems to be that this makes things impossible to analyse...
I admit, I got a bit lost there.
Going back to N = 8 supergravity, could perhaps be finite (which would be bad for string theory). Odd coincidence: 56 fermions equals 3 x 16 + 8, which means ... what, exactly? Something about how we wouldn't expect any new fermions, but I don't follow why.
That said, I think this was probably the best this talk could have been; I'm not familiar with the technical details, but explaining them would have taken too long and been pointless for many people.
12:00 pm: Non-linear Supersymmetry, Renata Kallosh
Another speaker who has not uploaded their talk.
Inflaton model where non-canonical kinetic term is motivated from SUSY Kahler potential.
Eventually got to some point, about supergravity with a positive cosmological constant. SUSY is realised non-linearly, such that a fermion transforms into a function of fermions (only). SUSY is spontaneously broken/non-linearly realised; equivalent to taking scalar particle to infinity. But I kind of lost interest with the first half of this talk and missed how it all fit together.
11:00 am: Naturalness, JoAnne Hewett
Naturalness told us to expect NP as soon as we turned the LHC on. We didn't see it, at run I or (so far) run II. What should we do? Panic? Hope the 750 GeV excess persists? Give in to anthropic arguments?
Let's panic (it's more fun).
How is naturalness defined? For concreteness, consider SUSY. There are several fine tuning measures that have been advanced: the traditional Guidice value based on variation of the Higgs VEV to input parameters; one based on large-logarithms; and one based on EW-scale inputs. Different choices lead to different answers, so there is a definite subjective element in what an individual scientist will consider the true fine-tuning.
Model building can give us further ideas. Consider the CMSSM. Pre-LHC, the best-fit values for the SUSY parameters were just beyond the limits: turn on the machine and see it. But now m0 and M1/2 are up at the TeV-scale, which most people will agree is in some tension with naturalness.
But of course, SUSY is not a single model. The pMSSM is in many respects just as valid a model, but the extra parameters will offer different phenomenology. A 105-point scan of the parameter space finds points with gluinos and squarks down to a few hundred GeV still allowed by LHC data. This analysis was repeated and confirmed by ATLAS. We see several features in these lighter sparticle spectra: Compressed spectra (with little MET), Stealth SUSY (with complicated decays) and kinematic suppression. The 13 TeV results extend the exclusions but there remain many points with sub-TeV gluinos. Even with 3 ab-1 of data, there will be a handful of points that survive.
However, focusing on a specific low-fine-tuning subset of pMSSM data, they will be excluded by the LHC with the full data set. Common feature of these models: suite of light and compressed EW gauginos, leading to a complex stop decay pattern that weakens constraints. This provides us with a guide for future developments in search techniques. A recent example is to look for charginos boosted by QCD ISR together with photon FSR (that tends to align with the MET).
Move on to discussion of RS models. I saw several of these points in Tom Rizzo's talk on Tuesday about explaining the 750 GeV excess using KK gravitons. Need to use large BLKTs to suppress couplings of KK gluon. But the general conclusion is that BLKTs are naturally expected, and extremely important for the KK phenomenology.
Question
Naturalness can be made rigourous in Bayesian statistics (up to priors).
pMSSM requires throwing away (some of) the motivations for SUSY. Additionally, 105 points sounds too few (108 done by questioner John Ellis). How do fraction of points that survive compare to e.g. Bayesian analyses? Do not; there is no statistical power associated with these statements.
750 GeV excess can be explained with radion, as Jack Gunion (asking the question) and I talked about at this conference.
11:30 am: Supersymmetry: to be or not to be? Hermann Nicolai
Oh look, another speaker who didn't feel the need to upload their talk in advance.
"Take a more fundamentalist approach". Word choice; the SUSY = religion jokes are easy enough to make as it is.
Why SUSY? Start with Coleman-Mandula, which is where I started back as a grad student, so I'm feeling more sympathetic already.
Interesting point: positive cosmological constant (as appears to be realised in nature) is not consistent with SUSY!
Maximal rigid SUSY is N = 4; maximal supergravity is N = 8. These theories are essentially unique due to the restrictions of the SUSY algebra. However, people immediately realised that N = 1 SUSY is the unique theory with chiral fermions (and elementary gauge bosons). It is also motivated by string theory, and is the only theory we can hope to see at the LHC (check this).
A problem with SUSY since the beginning is how to break it. Ultimately, no completely compelling mechanism. Spontaneous breaking insufficient, hence the soft breaking usually adopted. Problem is worse in string theory. (Time-dependent cosmological backgrounds always break SUSY.)
What if the LHC does not find SUSY? Original SUSY model expected a 3 GeV photino! Opinion: should have seen (traces of) N = 1 SUSY at LEP. But then what? Other ideas for naturalness & DM that would not need SUSY. But more speculative idea: what is the fate of space-time SUSY in quantum gravity theories with emergent space and time?
Move on to maximal theories. As essentially unique, no freedom to fiddle with things which would apparently make them maximally testable. But connection to "more important" exceptional symmetries. For D < 3, these exceptional symmetries are infinite-dimensional. The theme seems to be that this makes things impossible to analyse...
I admit, I got a bit lost there.
Going back to N = 8 supergravity, could perhaps be finite (which would be bad for string theory). Odd coincidence: 56 fermions equals 3 x 16 + 8, which means ... what, exactly? Something about how we wouldn't expect any new fermions, but I don't follow why.
That said, I think this was probably the best this talk could have been; I'm not familiar with the technical details, but explaining them would have taken too long and been pointless for many people.
12:00 pm: Non-linear Supersymmetry, Renata Kallosh
Another speaker who has not uploaded their talk.
Inflaton model where non-canonical kinetic term is motivated from SUSY Kahler potential.
Eventually got to some point, about supergravity with a positive cosmological constant. SUSY is realised non-linearly, such that a fermion transforms into a function of fermions (only). SUSY is spontaneously broken/non-linearly realised; equivalent to taking scalar particle to infinity. But I kind of lost interest with the first half of this talk and missed how it all fit together.
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