Wednesday, November 30, 2016

Academics Aren't Paid (Directly) For Their Publications

As 4Gravitons (who is a graduate student or post-grad at the Perimeter Institute in Waterloo, Canada, one of the premier theoretical physics shops in the world) recently pointed out in his blog:
In fact, academics don’t get paid by databases, journals, or anyone else that publishes or hosts our work. In the case of journals, we’re often the ones who pay publication fees. Those who write textbooks get royalties, but that’s about it on that front.
I grew up with a father who was a professor and a mother who was a university administrator who helped professors get grants for research and comply with human subjects requirements, so I've know this for as long as I knew it was something to know about. But, lots of people don't realize this fact.

Now, this doesn't mean that professors don't receive economic benefit from publishing. The business model of academia works like this:

1. You need to do research, ideally publishable in some form, to earn a PhD, and a PhD or good progress towards earning one in the very near future, is the basic prerequisite for being hired as a professor.

2.  Professors are initially hired for one to three year fixed terms as lecturers or as tenure track "Assistant Professors".

3.  An Assistant Professor is evaluated for tenure (usually after three years, but practice varies and sometimes there are multiple stints as an Assistant Professor at the same institution or successive ones). If you get tenure, you are usually simultaneously promoted to "Associate Professor" and get a raise. If you don't get tenure, you may be given another shot, but usually, you are terminated.

4.  An Associate Professor with tenure can then be evaluated for promotion to full "Professor" with greater prestige and higher pay.

In all of the main career steps in an academic's life: getting a PhD, getting hired as a tenure track professor, earning tenure, getting promoted to Associate Professor, and getting promoted to full Professor, the dominant consideration is what research you have published in peer reviewed scholarly journals and how significant that research is (e.g. measured by citations in other scholarly work). There are other factors, but that is the dominant one. Hence the phrase, "publish or perish".

Your publications are also the primary consideration in how prestigious and high paying a post you will be hired at (often after landing a first post elsewhere) and of your prestige in your field and the academic profession in general.

A professor at a research university has a teaching load expected to use about 25%-67% of his or her time, with the most esteemed professors having the lightest teaching loads. In the balance of your time you are expected, but not required (once you have tenure) to do research, most of which should be potentially publishable in peer reviewed journals.

Subsidies for universities and colleges from state governments that make this possible is the main way that state government finances basic research.

So professors have strong incentives to publish, but are not directly rewarded for the publications themselves (many of which have arguably weak claims to intellectual property protection due to exceptions for factual compilations and scientific principles).

This is a good thing, because, in the end, their incentive is to produce more papers, not necessarily for those papers to have lots of readers, and even very respectably cited papers are often read by a very small number of readers and purchased by few customers other than academic libraries.

Emergent Gravity

Sabine Hossenfelder's latest post at Backreaction on "Emergent Gravity" is one of her best educated layman oriented posts explaining concepts in fundamental physics yet.

Notably, she emphasizes the connection, lacking in many other explanations of this approach, to the similarities between thermodynamics (which is know is emergent via statistical mechanics from the mechanics of atoms and molecules) and gravity.
Emergent gravity has been in the news lately because of a new paper by Erik Verlinde
. . . 
Almost all such attempts to have gravity emerge from some underlying “stuff” run into trouble because the “stuff” defines a preferred frame which shouldn’t exist in general relativity. They violate Lorentz-invariance, which we know observationally is fulfilled to very high precision. 
An exception to this is entropic gravity, an idea pioneered by Ted Jacobson 20 years ago. Jacobson pointed out that there are very close relations between gravity and thermodynamics, and this research direction has since gained a lot of momentum. 
The relation between general relativity and thermodynamics in itself doesn’t make gravity emergent, it’s merely a reformulation of gravity. But thermodynamics itself is an emergent theory – it describes the behavior of very large numbers of some kind of small things. Hence, that gravity looks a lot like thermodynamics makes one think that maybe it’s emergent from the interaction of a lot of small things. . . . as long as you’re not looking at very short distances, it might not matter much exactly what gravity emerges from. Like thermodynamics was developed before it could be derived from statistical mechanics, we might be able to develop emergent gravity before we know what to derive it from.
This is only interesting, however, if the gravity that “emerges” is only approximately identical to general relativity, and differs from it in specific ways. For example, if gravity is emergent, then the cosmological constant and/or dark matter might emerge with it, whereas in our current formulation, these have to be added as sources for general relativity. 
So, in summary “emergent gravity” is a rather vague umbrella term that encompasses a large number of models in which gravity isn’t a fundamental interaction. The specific theory of emergent gravity which has recently made headlines is better known as “entropic gravity” and is, I would say, the currently most promising candidate for emergent gravity. It’s believed to be related to, or maybe even be part of string theory, but if there are such links they aren’t presently well understood.
She references the following article for a more technical description of many of the leading theories.
We give a critical overview of various attempts to describe gravity as an emergent phenomenon, starting from examples of condensed matter physics, to arrive to more sophisticated pregeometric models. The common line of thought is to view the graviton as a composite particle/collective mode. However, we will describe many different ways in which this idea is realized in practice.

Lorenzo Sindoni, Emergent Models for Gravity: an Overview of Microscopic Models (May 12, 2012).

The notion that you might be able to derive gravity from first principles through a clever macro-level understanding of particle physics is very exciting indeed. It would be miraculous enough for dark matter and dark energy to emerge naturally from a quantum gravity theory. But, it would be even more amazing if quantum gravity itself could be derived from and emerged naturally from fundamental particle physics. 

Certainly, that hasn't been established yet, but it does seem like a very plausible possibility.

Friday, November 25, 2016

Glueballs Still Elusive

Despite combing through 260 million events that should be able to produce a type of glueball resonance that can't be confused with quarkonium, the researchers once again after about four decades of fruitless searching, have come up empty. The quality of this particular search at Belle, and the relentless failure of searches over the decades to find any trace of glueballs despite increasingly sophisticated efforts to find them has me wondering: 

Does some, as yet unarticulated missing principle of quantum chromodynamics (QCD) forbids glueballs?

The particularly confounding aspect of this is that glueballs are relatively easy to describe mathematically, since they implicate just one of the Standard Model physical constants, the strong force coupling constant. Unlike other hadrons, no physical constants related to quark masses, the weak force coupling constant, or the CKM matrix needs to be known to describe them. 

The masses they are predicted to have are not very different from all sorts of other known hadrons (this search focused on glueballs predicted to have masses from 2.8 to 4.59 GeV (at two sigma extremes from the predicted masses), around the predicted mass of D mesons and B mesons, and well defined, distinctive quantum numbers. These are essentially completely defined from theory.

A search with 260 million events shouldn't be able to miss them if they are produced at all in the studied process at any meaningful rate, but the collaborators found nothing. Branching fractions of as much as 1 per 5,000 decays to glueballs from the Upsilon mesons whose decays were studied, which were promising candidates for giving rise to decay to glueballs at a detectable branching fraction, have been ruled out.

What do we not know about QCD that causes this? If they do exist, why can't we find traces of any of them?

The fact that these are "oddballs" can mix with quarkonium states is particularly notable, because the usual excuse for not being able to see glueball resonances doesn't apply here.

In QCD, the usual rule is that everything that is permitted is mandatory, so finding a decay that isn't prohibited by any of the rules of QCD that can't be detected is a big signal that we're missing something, although it is notable that no QCD theoretical estimate of the branching fraction was provided in this study, as some resonances are simply very rare.
The existence of bound states of gluons (so-called “glueballs”), with a rich spectroscopy and a complex phenomenology, is one of the early predictions of the non-abelian nature of strong interactions described by quantum chromodynamics (QCD). However, despite many years of experimental efforts, none of these gluonic states have been established unambiguously. Possible reasons for this include the mixing between glueballs and conventional mesons, the lack of solid information on the glueball production mechanism, and the lack of knowledge about glueball decay properties. Of these difficulties, from the experimental point of view, the most outstanding obstacle is the isolation of glueballs from various quarkonium states.
Fortunately, there is a class of glueballs with three gluons and quantum numbers incompatible with quark-antiquark bound states, called oddballs, that are free of this conundrum. The quantum numbers of such glueballs include J P C = 0 −−, 0 +−, 1 −+, 2 +−, 3 −+, and so on. Among oddballs, special attention should be paid to the 0 −− state (G0−− ), since it is relatively light and can be produced in the decays of vector quarkonium or quarkoniumlike states.

Two 0 −− oddballs are predicted using QCD sum rules with masses of (3.81 ± 0.12) GeV/c 2 and (4.33 ± 0.13) GeV/c 2 , while the lowest-lying state calculated using distinct bottom-up holographic models of QCD [3] has a mass of 2.80 GeV/c 2 . Although the masses have been calculated, the width and hadronic couplings to any final states remain unknown.

Possible G0−− production modes from bottomonium decays are suggested in Ref. [2] including Υ(1S, 2S) → χc1+G0−− , Υ(1S, 2S) → f1(1285)+G0−−, χb1 → J/ψ+G0−− , and χb1 → ω + G0−− . In this paper, we search for 0 −− glueballs in the production modes proposed above and define G(2800), G(3810), and G(4330) as the glueballs with masses fixed at 2.800, 3.810, and 4.330 GeV/c 2 , respectively. All the parent particles in the above processes are copiously produced in the Belle experiment, and may decay to the oddballs with modest rates.
Full pdf here.

The abstract and paper are as follows:
We report the first search for the J P C = 0−− glueball in Υ(1S) and Υ(2S) decays with data samples of (102 ± 2) million and (158 ± 4) million events, respectively, collected with the Belle detector. No significant signals are observed in any of the proposed production modes, and the 90% credibility level upper limits on their branching fractions in Υ(1S) and Υ(2S) decays are obtained. The inclusive branching fractions of the Υ(1S) and Υ(2S) decays into final states with a χc1 are measured to be B(Υ(1S) → χc1 + anything) = (1.90 ± 0.43(stat.) ± 0.14(syst.)) × 10−4 with an improved precision over prior measurements and B(Υ(2S) → χc1 + anything) = (2.24 ± 0.44(stat.) ± 0.20(syst.)) × 10−4 for the first time.
Belle Collaboration, "Search for the 0−− Glueball in Υ(1S) and Υ(2S) decays" (November 22, 2016).

UPDATE November 29, 2016:

A preprint of a back to the drawing board paper has been posted and notes these results while estimating a best fit for an oddball mass two GeV heavier than the mass used by the Belle Collaboration that is still consistent to within the large two sigma error bars with the heavier Belle Collaboration values.
We present the new results for the exotic glueball state 0−− in the framework of the QCD sum rules. It is shown that previously used three-gluon current does not couple to any glueball bound state. We suggest considering a new current which couples to this exotic state. The resulting values for mass and decay constant of the 0−− glueball state are MG = 6.3 +0.8 −1.1 GeV and FG = 67 ± 6 keV, respectively.
Alexandr Pimikov, Hee-Jung Lee, Nikolai Kochelev, Pengming Zhang, "Revision of exotic 0−− glueball" (November 26, 2016).

Their predicted mass at two sigma error bars is 4.1 GeV to 7.9 GeV which isn't too impressive for a pure theoretical calculation that is basically a function of just one experimentally measured Standard Model constant (the strong force coupling constant) that is known to a precision of about 1%. The Belle estimate has a mere 3% uncertainty.

The pseudo-scalar bottom eta meson which is a form of bottomonium has a mass of 9.398 +/- 0.0032 GeV. The measured mass of the parent mesons Y(1S) which is called an upsilon meson, is also a form of bottomonium and is 9.46030 +/- 0.00026 GeV. The measured mass of the Y(2S) which is an excited upsilon meson, which is another form of bottomonium isn't well established in sources I've found off the bat, but would be expected to be heavier than 9 .46 GeV. Measured masses of two kinds of Y bosons with the right quantum numbers whose exact excitation has not been determined are 10.81 GeV and 11.02 GeV. So, the parent would not be barred from decaying into an oddball of this type even if it is on the heavy side of the estimated range, by mass-energy conservation.

The introduction to this paper notes that:
The glueballs are composite particles that contain gluons and no valence quarks. The glueballs carry very important information about the gluonic sector of QCD and their study is one of the fundamental tasks for the strong interaction. While the glueballs are expected to exist in QCD theoretically, there was no clear experimental evidence and so the glueballs remain yet undiscovered (see reviews [1, 2]). This is the reason why the investigation of the possible glueball’s candidates are included in the programs of the running and projected experiments such as Belle (Japan), BaBar (SCAC, USA), BESIII (Beijing, China), RHIC (Brookhaven, USA), LHC (CERN), GlueX (JLAB, USA), NICA (Dubna, Russia), HIAF (China) and FAIR (GSI, Germany). 
One of the main problems of the glueball spectroscopy is the possible large mixing of the glueballs with ordinary meson states, which leads to the difficulties in disentangling the glueballs in the experiment. In this connection, the discovery of the exotic 0−− glueball, which can not be mixed with the qq¯ states, is one of the fundamental tasks of the glueball spectroscopy. Therefore, it is very important to investigate the properties of this glueball within the QCD’s based approach. One of such approaches is the QCD Sum Rules (SR). The first study of the 0−− glueball by the QCD SR method has been performed recently in [3] where the authors introduced a very specific interpolating current for this three-gluon state. Unfortunately, they only considered SR for the mass of the glueball and did not check the SR for the decay constant. Below we show that their current has pathology, which leads to the negative sign of the imaginary part of the corresponding correlator and, as the result, SR become inconsistent. Considering the fact the study of the glueball is a very hot topic nowadays and the prediction of the value of the exotic glueball mass is crucial for the experimental observation, the revision of the exotic glueball properties within QCD SR is required. 
In this Letter, a new interpolating current, which couples to the 0−− exotic glueball state, is suggested. We calculate the Operator Product Expansion (OPE) for the correlator with this current up to dimension-8 and show that there is a good stability of SR for both mass and decay constant of this state.
The paper then reaches the result stated in the abstract and goes on to conclude that:
Our final result is: MG = 6.3 +0.8 −1.1 GeV,  FG = 67 ± 6 keV. (11)
The SR analysis in full QCD (Nf = 3) and nonzero quark condensate hJ 2 i) leads to a reduction of the glueball mass by 0.2 GeV. The mass of the exotic glueball in Eq.(11) is not far away from the recent unquenched lattice result MG = 5.166 ± 1.0 GeV [12] obtained with a rather large pion mass mπ = 360 MeV. 
Here we would like to note that there are three sources of uncertainties in the above analysis for the mass and decay constant: the variation of gluon condensate, stability of SR triggering Borel parameter M2 dependence in terms of criteria δ min k , and roughly estimated SR uncertainty coming from the OPE truncation. The latter uncertainty for the decay constant comes from the definition of the fiducial interval, Eq. (9), in the standard assumption that the contribution from missing terms is of the order of the last included nonperturbative term squared: (1/3)2 ∼ 10%. The same error for mass can be expected to be suppressed since the related errors for R (SR) k+1 and R (SR) k are correlated. The best threshold value is s bf 0 = 52.4 +12.6% −16.2% GeV2 when only uncertainty of the gluon condensate is included. Note that the fiducial interval for the central value of the gluon condensate is M2 ∈ [3.7, 7.3] GeV2 . We also mention that here we present the results from the k = 0 case for SR (see Eqs.(8,10)). The mass estimation for higher values of k = 1, 2, 3 are in agreement with the considered k = 0 case within the error bars. 
Summarizing, we present the revision of the QCD SR result for the exotic three-gluon glueball state with quantum numbers J P C = 0−−. A new interpolating current for this glueball has been constructed. By using this current, we have analyzed the QCD SR consisting of contributions up to the operators of dimension-8 and obtained the estimation of the mass and decay constant of the exotic glueball. 
After the paper was completed we were informed about the negative result of the searching of the low mass exotic 0 −− glueball by the Belle Collaboration [13].