Wednesday, December 12, 2018

From The Lab The Sterile Neutrino Controversy Continues, Still No Sign Of SUSY And CKM Fits

* A recent review paper below suggests that there is strong evidence for a 1 eV sterile neutrino.

I am far more skeptical based upon the fact that this estimate omits experimental searches that have had negative results like the MINOS, MINOS+, Daya Bay and JUNO experiments. The negative results exist at the level of reactor experiments that haven't found the 1 eV sterile neutrino anomaly or can explain it somehow (unlike the cherry picked examples of those that have), and aggregating positive results of an anomaly without including negative results is a gross example of a look elsewhere effect that can't be ignored and would greatly reduce the true statistical significance of these observations.

Cosmology data also disfavor from cosmology measurements that show the effective number of neutrino types under 10eV (Neff) to be 3 rather than 4 (after considering a radiation adjustment of about 0.046 to each number) at six sigmas of confidence, when it should be about 4.046 with a 1 eV sterile neutrino. A 1 eV sterile neutrino that oscillates with the active neutrinos would also drive up the sum of the neutrino masses from cosmology measurements to well above the measured constraints on that sum of masses (even if they are divided by four and the multiplied by three to adjust for the additional number of neutrino types and the active neutrinos were given a minimum combined mass it would imply sum of neutrino masses renormalized to three types of about 0.8 eV, about six times current limits). Specifically Neff=2.99±0.17, and the neutrino mass is tightly constrained to ∑mν<0.12eV. You need not just sterile but "secret" neutrinos to overcome these cosmology data problems.

Also, a sterile neutrino would be "hot" rather than "cold" (ca. GeV mass scale) or "warm" (keV mass scale), so it couldn't provide a dark matter candidate either. Yet, there is no astronomy evidence for "hot dark matter" which would reduce the amount of large scale structure at the galactic level in the universe.

The hypothetical sterile neutrino in the phenomenological model that is being fit to in order to evaluate anomalies in the observed results is also not a good fit in the overall scheme of Standard Model fundamental particles. An additional active neutrino type, for example, would be contrary to W and Z boson decay data and would require a fourth generation of fundamental fermions, so that can be ruled out. But, oscillations with fundamental particles to a sterile neutrino that don't have the same quantum number with regard to weak isospin as the other neutrino types would also seem problematic, and should also lead to non-tree level process effects that aren't observed.

In short, despite the high claimed significance of this result, I am quite comfortable that these results are due to systemic measurement errors not accounted for in a certain type of reactor experiment, rather than being true indications of a new fundamental particle that is a sterile neutrino that oscillates with low frequencies to the three active neutrino types and has a mass on the order of 1 eV.
For a long time there were 3 main experimental indications in favor of the existence of sterile neutrinos: νe¯ appearance in the νμ¯ beam in the LSND experiment, νe¯ flux deficit in comparison with theoretical expectations in reactor experiments, and νe deficit in calibration runs with radioactive sources in the Ga solar neutrino experiments SAGE and GALEX. All three problems can be explained by the existence of sterile neutrinos with the mass square difference in the ballpark of 1 eV2. Recently the MiniBooNE collaboration observed electron (anti)neutrino appearance in the muon (anti)neutrino beams. The significance of the effect reaches 6.0σ level when combined with the LSND result. Even more recently the NEUTRINO-4 collaboration claimed the observation of νe¯ oscillations to sterile neutrinos with a significance slightly higher than 3σ. If these results are confirmed, New Physics beyond the Standard Model would be required. More than 10 experiments are devoted to searches of sterile neutrinos. Six very short baseline reactor experiments are taking data just now. We review the present results and perspectives of these experiments.

The introduction to this paper notes that:
Oscillations of the three neutrino flavors are well established. Two mass differences and three angles describing such oscillations have been measured [1]. Additional light active neutrinos are excluded by the measurements of the Z boson decay width [2]. 
Nevertheless, existence of additional sterile neutrinos is not excluded. Moreover, several effects observed with about 3σ significance level can be explained by active-sterile neutrino oscillations. 
The GALEX and SAGE Gallium experiments performed calibrations with radioactive sources and reported the ratio of numbers of observed to predicted events of 0.88 ± 0.05 [3]. This deficit is the so called “Gallium anomaly” (GA). 
Mueller et al. [4] made new estimates of the reactor ˜νe flux which is about 6% higher than experimental measurements at small distances. This deficit is the so called “Reactor antineutrino anomaly” (RAA). 
Both anomalies can be explained by active-sterile neutrino oscillations at Very Short Baselines (VSBL) requiring a mass-squared difference of the order of 1 eV2 [5]. 
The LSND collaboration reported observation of ˜νµ → ν˜e mixing with the mass-squared difference bigger than ∼ 0.1 eV2 [6]. The initial results of the MiniBooNE tests of this signal were inconclusive and probably indicated additional effects [7]. However, in May, 2018 the MiniBooNE collaboration presented the 4.7σ evidence for electron (anti)neutrino appearance in the muon (anti)neutrino beams [8]. The effect significance reaches 6.0σ when the MiniBooNE and LSND results are combined. The MINIBooNE and LSND data are consistent, however the energy spectrum of the excess does not agree too well with the sterile neutrino explanation. 
The best point in the sterile neutrino parameter space corresponds to a very large mixing (sin2 2θ = 0.92) and a small mass square difference of ∆m2 14 = 0.041eV2 (see Figure 1). However, this region in the sterile neutrino parameter space is disfavored by other experiments and only a small area with larger mass square differences up to 2 eV2 and smaller mixing is still allowed by the global fits [9, 10]. 
Very recently the NEUTRINO-4 collaboration claimed the observation of the of ˜νe oscillations to sterile neutrinos with a significance slightly larger than 3σ [11]. The measured sterile neutrino parameters are surprisingly large: ∆m2 14 = 7.22eV2 and sin2 (2θ14) = 0.35. These values are in contradiction with the limits obtained by the reactor ˜νe flux measurements at larger distances (see, for example [12]). However, these limits depend on the phenomenological predictions of the reactor ˜νe flux which are model dependent. 
There are also cosmological constraints on the effective number of neutrinos [2, 13]. However, in several theoretical models sterile neutrinos (at least with not too large masses) are still compatible with these constraints. Details can be found in a review of sterile neutrinos [14]. 
At the DANSS detector in Russia:
The optimum point of the RAA and GA fit is clearly excluded. Figure 3 shows the 90% and 95% Confidence Level (CL) area excluded by DANSS in the ∆m2 14, sin2 2θ14 plane. The excluded area covers a large fraction of regions indicated by the GA and RAA. In particular, the most preferred point ∆m2 14 = 2.3 eV2 , sin2 2θ14 = 0.14 [5] is excluded at more than 5σ CL.
As this summary by the author illustrates, four different kinds of anomalies seen at several different experiments are not consistent with each other. The GA and RAA effects are in the opposite directions. The lack of replication of the anomalies in different kinds of neutrino experiments also casts doubt on the hypothesis that these anomalies can really be explained by sterile neutrinos.

* Meanwhile, another paper, in a seemingly endless stream of them at the LHC, looks for signs of supersymmetry (a.k.a. SUSY) and sees nothing. The latest paper include Run-2 data at higher energies than Run-1. Supersymmetry has pretty much been ruled out at scales on the order of 1 TeV and are hard to square with the data up to something like 10 TeV. 

Moreover, since supersymmetry was conceived to deal with naturalness and hierarchy issues at the electroweak energy  scale (ca. 0.246 TeV, which is the vacuum expectation value of the Higgs field), it has pretty much been ruled out as a solution to the issue it was originally devised to address.
Results of a search for supersymmetry are presented using events with a photon, an electron or muon, and large missing transverse momentum. The analysis is based on a data sample corresponding to an integrated luminosity of 35.9 fb−1 of proton-proton collisions at √s= 13 TeV, produced by the LHC and collected with the CMS detector in 2016. Theoretical models with gauge-mediated supersymmetry breaking predict events with photons in the final state, as well as electroweak gauge bosons decaying to leptons. Searches for events with a photon, a lepton, and missing transverse momentum are sensitive probes of these models. No excess of events is observed beyond expectations from standard model processes. The results of the search are interpreted in the context of simplified models inspired by gauge-mediated supersymmetry breaking. These models are used to derive upper limits on the production cross sections and set lower bounds on masses of supersymmetric particles. Gaugino masses below 930 GeV are excluded at the 95% confidence level in a simplified model with electroweak production of a neutralino and chargino. For simplified models of gluino and squark pair production, gluino masses up to 1.75 TeV and squark masses up to 1.43 TeV are excluded at the 95% confidence level.

* Finally, there is an updated summary of and global fit of measurements of the four Standard Model parameters of the CKM matrix.
[T]he results of the global fit under the SM hypothesis remain excellent: the p-value is 51%, which corresponds to 0.7σ, if all uncertainties are treated as Gaussian. . . . 
The consistent overall picture allows for a meaningful extraction of the CKM matrix elements, the extracted Wolfenstein parameters being (68% C.L. intervals) 
A = 0.8403 +0.0056 −0.0201 (2% unc.), 
λ = 0.224747 +0.000254 −0.000059 (0.07% unc.), 
ρ¯ = 0.1577 +0.0096 −0.0074 (5% unc.), and 
η¯ = 0.3493 +0.0095 −0.0071 (2% unc.).
Also, to be clear the comparison to the Standard Model hypothesis above is merely testing the relationships of the parameters to each other for consistency purposes. There is no established theory in the Standard Model to determine what the specific absolute value of the Standard Model parameters should be.

The most powerful confirmation of all of the CKM matrix construct, however, is that the data from myriad different processes can produce consistent measurements of each of the parameters at all, which suggests that the theoretical construct of the CKM matrix is sound.

The CKM matrix element measurements, while not measured supremely precisely, still place bounds on beyond the Standard Model physics indirectly that limit the scales at which it would manifest to not lower than about 114 TeV, far beyond what any near term collider could seem, and arguably much higher than that.

1 comment:

neo said...

"Moreover, since supersymmetry was conceived to deal with naturalness and hierarchy issues at the electroweak energy scale (ca. 0.246 TeV, which is the vacuum expectation value of the Higgs field), it has pretty much been ruled out as a solution to the issue it was originally devised to address."


i said exactly this on PF, and a moderator warned me of making incorrect statements.