New CMS Search for Exotic Particles

CMS posted a new preprint on a search for several types of exotic particles on Monday using 19.7 fb-1 of integrated luminosity at 8 TeV center-of-mass energy. The paper focuses on the theoretical models of WIMP dark matter, extra dimensions, and unparticles.

WIMP, or weakly interacting massive particle, dark matter is the most popular model of particle dark matter. It postulates that dark matter is composed of heavy particles that don’t interact much with regular matter, similar to what one might term a superheavy neutrino. Theories of extra dimensions postulate that there are additional spatial dimensions to the familiar three. If these dimensions are finite in size (“compactified:” like a tiny loop or torus, to give examples in 1D and 2D) and are small enough, we could see particles that seem equivalent to Standard Model particles but are much heavier. Unparticle physics is something that I don’t know very about, but it apparently proposes the existence of scale-invariant fields: where things like the mass and momentum all scale in the same way. This is very different from particles, where the mass is always constant regardless of the momentum.

The CMS paper looks for a particular event topology and uses this to set exclusion limits on models of these exotic particles. The channel that they look at is monojets with large missing transverse energy. The idea is that when quarks in the protons in the beams collide to create WIMPs, Kaluza-Klein particles (related to extra dimensions), or unparticles, they can emit a gluon prior to the interaction or can emit one as part of the interaction (if there is an interaction between gluons, the new particles and another particle mediating the interaction). A high energy gluon will fragment into a number of quarks, which themselves organize into hadrons that are measured by the detector. Thus, high energy gluons lead to a cone-shaped spray of hadrons called a “jet.” The interactions CMS searches for are “monojets” because they have a high energy jet and nothing else. If the jet points away from the beam direction, there will be a large imbalance if the momenta of all the particles in the transverse directions are added. Conservation of momentum requires that the total transverse momentum be basically zero, so this is a sign that an effectively noninteracting particle (or unparticle) carried away the excess momentum. Some models will predict cross sections for this process, so the experimental result can be compared to theoretical models.

Backgrounds for these interactions, according to the paper, mostly involve intermediate vector boson (W and Z) decays with associated jet production in the same interaction. The Z in particular can decay to neutrinos, which are invisible, so any jets in the event could appear like a monojet with large missing transverse energy. The results in all cases of the analysis are consistent with the Standard Model. So, CMS can set exclusion limits on the properties of these theoretical models. As in many such cases, the analysis contains a number of caveats, so the limits are for the specific models tested not for the most generic versions of these theories.


On Michelle Malkin, Or, How to Lie With Statistics

As I mentioned in a previous post, Michelle Malkin recently wrote an article tangentially related to the Ferguson, MO shooting that is an excellent example of how to mislead readers with statistics.

For some other background, Malkin is one of the most odious figures in the American political landscape. She literally wrote a book defending  the use of concentration camps and racial profiling against disfavored minorities, using the examples of Japanese Americans in the Second World War and Arab and Muslim Americans in the aftermath of the September 11th  attacks. Reason – another conservative-ish magazine which (along with many others) eviscerated this book, asking “Could it be that she actually supports the idea of detaining American Arabs and Muslims?” – has also responded to the article. I’ll reference that response later. I’m choosing this response because the two publications often find themselves on the same side in political arguments. Reason tends to lean toward anti-government activism, while the National Review still seems to be stuck in a Cold War anti-Communist mindset (with occasional, probably disingenuous, forays into libertarianism), seeing Stalinists hiding under every rock and behind every bush (and Obama too).

The gist of Malkin’s article is that we shouldn’t be concerned about overzealous and authoritarian police actions because being a police officer is a dangerous job. This central point is a fallacy. Being a police officer may be a dangerous job, and the vast majority of police officers may be doing their best to protect the average citizen, but this does not mean that the public should not concern itself with police misconduct or even the appearance of police misconduct. Public oversight of public institutions is a key part of democratic governance. If a significant part of the population does not feel that such an important institution as law enforcement is supporting their interests (regardless of whether this feeling is factually correct), that is a problem for everyone.  Institutions depend on the cooperation and support of the people.

Malkin’s logic doesn’t hold up to scrutiny, but let’s now look at her statistics. First, she mentions that over a recent 10 year period, there were 1501 deaths of police officers. The title interprets this as saying that an officer is “killed” every “58 hours.” Reason has tracked down the source of this statistic. The article correctly summarizes the death statistic in the source. The title, however, is deeply misleading. “Killed” implies that the officers were murdered. The source splits up the deaths into a number of different classes and shows that many of these were due to things like illness and car crashes. Even the figure for shootings can be misleading: it may include accidents and suicides in addition to murders. The number of police officers murdered is likely to be much smaller than this figure. Any number of murders is bad, but Malkin is grossly exaggerating the danger of being an officer compared to the average job.

In her first “fact:” Again, the figure of 100 officers “killed” is suspect for the reasons above. The numbers of assaults and injuries are not sourced, so we have no idea what the context is. Are these police reports, criminal complaints, indictments, convictions, or something else like extrapolated polling data? Overuse of minor charges such as disorderly conduct, resisting arrest, etc. (i.e. “contempt of cop” in unjustified cases – see people arrested for assaulting an officer’s fist with their faces) is a widely known phenomenon so we don’t know how many of these are legitimate.

In the second “fact:” Again, these figures have no context. It is obvious that these numbers are from a different dataset than the earlier 10 year figures. I would guess that these are all-time figures. A few things: New York has been the largest city in the US nearly since the beginning of the country. It would not be surprising if the NYPD has seen the most deaths of any municipal police department when it is both larger and older than nearly every other major department. Likewise, Texas is now the second largest state and has been one of the largest states for decades. It, too, will be expected to have more deaths than most states simply because of size.

Total deaths is just not a very useful statistic. Number per capita per year is more better for comparing different numbers, whether the number of deaths per police population per year or number of deaths per total population per year. This removes many size and age effects, though there are many other reasons why a direct comparison may be difficult. This has no bearing on the article because the article does not compare the numbers to anything. We have no idea how the death rate compares to the general population, or to other traditionally dangerous jobs. Additionally, the article is talking about policing today but provides historical figures. Ten year figures are reasonable because it’s all in the recent past, but historical figures like the NY and TX ones are not. Crime rates have been dropping precipitously since the early 90s, so a historical average over a period of time greater than 10 years will not accurately represent the current environment. Trends over time are critical in understanding these statistics.

Malkin also provides a comparison of the number of police deaths during part of August 2014 to the same part of August in 2013. The rate for this year is 14% higher than last year, which sounds bad. But, the numbers are 72 in 2014 and 63 in 2013.  Now let’s assume that the rate of deaths was constant in 2013 and 2014. It’s not exactly true, but I’d like to see what happens if we make this assumption. Then, we can estimate that the average number of deaths in this part of August is (72+63)/2 = 67.5. We can probably safely assume that these are primarily single deaths (again, little context is provided, but multiple death incidents are almost certainly a small fraction of deaths). In this case, the number should be Poisson distributed. The standard deviation is then (67.5)1/2=8.2. You can see that both the 2013 and 2014 numbers are well within a single standard deviation of this estimated mean. The difference is simply not even close to statistically significant. This is because integrating the number over too short a period leads to numbers that are sensitive to statistical fluctuations. While this 14% increase may be true (I have no reason to doubt it), there is no way for us to tell that it represents a real increase in the murder rate and not statistical noise. The year-to-date totals (or even seasonal totals) will have much larger numbers allowing for a more precise comparison.

Malkin ends the piece with descriptions of several recent murders of police officers and then a gratuitous jab at Al Sharpton, a favorite bogeyman of right wing commentators who hasn’t been relevant in a long time. There aren’t any stats here but it’s clearly designed to elicit an emotional response against more liberal-minded people (represented by Sharpton) in the reader.

An important question, then, is why did Malkin include such suspect statistics and poor logic? In a Twitter response to the Reason author (linked in the Reason article), Malkin insinuates that it’s not due to ignorance. Rather, she wrote the piece to push an agenda – support police action no matter what, particularly when her political opponents oppose that action. She disclaims any responsibility for the statistics because it’s her source that’s biased, not her. Evaluating sources is an important part of persuasive writing, particularly if you have such a wide audience as Michelle Malkin. Knowingly using unreliable sources without comment is unethical while inadvertently doing so when one should know better is irresponsible. Malkin places her political agenda above her commitment to the truth. By acting as a purveyor of ignorance, she reveals herself to be a demagogue who has no place in debates on serious issues.