The paper by Thomas Gehrmann and collaborators I cited a few days ago has inspired me to have a closer look at the problem of understanding the features of extensive air showers – the phenomenon of a localized stream of high-energy cosmic rays originated by the incidence on the upper atmosphere of a very energetic proton or light nucleus.
Layman facts about cosmic rays
While the topic of cosmic rays, their sources, and their study is largely terra incognita to me -I only know the very basic facts, having learned them like most of you from popularization magazines-, I do know that a few of their features are not too well understood as of yet. Let me mention only a few issues below, with no fear of being shown how ignorant on the topic I am:
- The highest-energy cosmic rays have no clear explanation in terms of their origin. A few events with energy exceeding $10^{20} eV$ have been recorded by at least a couple of experiments, and they are the subject of an extensive investigation by the Pierre Auger observatory.
- There are a number of anomalies on their composition, their energy spectrum, the composition of the showers they develop. The data from PAMELA and ATIC are just two recent examples of things we do not understand well, and which might have an exotic explanation.
- While models of their formation suppose that only light nuclei -iron at most- are composing the flux of primary hadrons, some data (for instance this study by the Delphi collaboration) seems to imply otherwise.
The paper by Gehrmann addresses in particular the latter point. There appears to be a failure in our ability to describe the development of air showers producing very large number of muons, and this failure might be due to modeling uncertainties, heavy nuclei as primaries, or the creation of exotic particles with muonic decay, in decreasing order of likelihood. For sure, if an exotic particle like the 300 GeV one hypothesized in the interpretation paper produced by the authors of the CDF study of multi-muon events (see the tag cloud on the right column for an extensive review of that result) existed, the Tevatron would not be the only place to find it: high-energy cosmic rays would produce it in sizable amounts, and the observed multi-muon signature from its decay in the atmosphere might end up showing in those air showers as well!
Mind you, large numbers of muons are by no means a surprising phenomenon in high-energy cosmic ray showers. What happens is that a hadronic collision between the primary hadron and a nucleus of nitrogen or oxygen in the upper atmosphere creates dozens of secondary light hadrons. These in turn hit other nuclei, and the developing hadronic shower progresses until the hadrons fall below the energy required to create more secondaries. The created hadrons then decay, and in particular 
Muons have a lifetime of two microseconds, and if they are energetic enough, they can travel many kilometers, reaching the ground and whatever detector we set there. In addition, muons are very penetrating: a muon needs just 52 GeV of energy to make it 100 meters underground, through the rock lying on top of the CERN detectors. Of course, air showers include not just muons, but electrons, neutrinos, and photons, plus protons and other hadronic particles. But none of these particles, except neutrinos, can make it deep underground. And neutrinos pass through unseen…
Now, if one reads the Delphi publication, as well as information from other experiments which have studied high-multiplicity cosmic-ray showers, one learns a few interesting facts. Delphi found a large number of events with so many muon tracks that they could not even count them! In a few cases, they could just quote a lower limit on the number of muons crossing the detector volume. One such event is shown on the picture on the right: they infer that an air shower passed through the detector by observing voids in the distribution of hits!
The number of muons seen underground is an excellent estimator of the energy of the primary cosmic ray, as the Kascade collaboration result shown on the left shows (on the abscissa is the logarithm of the energy of the primary cosmic ray, and on the y axis the number of muons per square meter measured by the detector). But to compute energy and composition of cosmic rays from the characteristics we observe on the ground, we need detailed simulations of the mechanisms creating the shower -and these simulations require an understanding of the physical processes at the basis of the productions of secondaries, which are known only to a certain degree. I will get back to this point, but here I just mean to point out that a detector measuring the number of muons gets an estimate of the energy of the primary nucleus. The energy, but not the species!
As I was mentioning, the Delphi data (and that of other experiments, too) showed that there are too many high-muon-multiplicity showers. The graph on the right shows the observed excess at very high muon multiplicities (the points on the very right of the graph). This is a 3-sigma effect, and it might be caused by modeling uncertainties, but it might also mean that we do not understand the composition of the primary cosmic rays: yes, because if a heavier nucleus has a given energy, it usually produces more muons than a lighter one.
The modeling uncertainties are due to the fact that the very forward production of hadrons in a nucleus-nucleus collision is governed by QCD at very small energy scales, where we cannot calculate the theory to a good approximation. So, we cannot really compute with the precision we would like how likely it is that a 1,000,000-TeV proton, say, produces a forward-going 1-TeV proton in the collision with a nucleus of the atmosphere. The energy distribution of secondaries produced forwards is not so well-known, that is. And this reflects in the uncertainty on the shower composition.
Enter CMS
Now, what does CMS have to do with all the above ? Well. For one thing, last summer the detector was turned on in the underground cavern at Point 5 of LHC, and it collected 300 million cosmic-ray events. This is a huge data sample, warranted by the large extension of the detector, and the beautiful working of its muon chambers (which, by the way, have been designed by physicists of Padova University!). Such a large dataset already includes very high-multiplicity muon showers, and some of my collaborators are busy analyzing that gold mine. Measurements of the cosmic ray properties are ongoing.
One might hope that the collection of cosmic rays will continue even after the LHC is turned on. I believe it will, but only during the short periods when there is no beam circulating in the machine. The cosmic-ray data thus collected is typically used to keep the system “warm” while waiting for more proton-proton collisions, but it will not be a orders-of-magnitude increase in statistics with respect to what has been already collected last summer.
The CMS cosmic-ray data can indeed provide an estimate of several characteristics of the air showers, but it will not be capable of providing results qualitatively different from the findings of Delphi -although, of course, it might provide a confirmation of simulations, disproving the excess observed by that experiment. The problem is that very energetic events are rare -so one must actively pursue them, rather than turning on the cosmic ray data collection when not in collider mode. But there is one further important point: since only muons are detected, one cannot really understand whether the simulation is tuned correctly, and one cannot achieve a critical additional information: the amount of energy that the shower produced in the form of electrons and photons.
The electron- and photon-component of the air shower is a good discriminant of the nucleus which produced the primary interaction, as the plot on the right shows. It in fact is a crucial information to rule out the presence of nuclei heavier than iron, or the composition of primaries in terms of light nuclei. Since the number of muons in high-multiplicity showers is connected to the nuclear species as well, by determining both quantities one would really be able to understand what is going on. [In the plot, the quantity Y is plotted as a function of the primary cosmic ray energy. Y is the ratio between the logarithm of the number of detected muons and electrons. You can observe that Y is higher for iron-induced showers (the full black squares)].
Idea for a new experiment
The idea is thus already there, if you can add one plus one. CMS is underground. We need a detector at ground level to be sensitive to the “soft” component of the air shower- the one due to electrons and photons, which cannot punch through more than a meter of rock. So we may take a certain number of scintillation counters, layered alternated with lead sheets, all sitting on top of a thicker set of lead bricks, underneath which we may set some drift tubes or, even better, resistive plate chambers.
We can build a 20- to 50-square meter detector this way with a relatively small amount of money, since the technology is really simple and we can even scavenge material here and there (for instance, we can use spare chambers for the CMS experiment!). Then, we just build a simple logic of coincidences between the resistive plate chambers, imposing that several parts of our array fires together at the passage of many muons, and send the triggering signal 100 meters down, where CMS may be receiving a “auto-accept” to read out the event regardless of the presence of a collision in the detector.
The latter is the most complicated thing to do of the whole idea: to modify existing things is always harder than to create new ones. But it should not be too hard to read out CMS parasitically, and collect at very low frequency those high-multiplicity showers. Then, the readout of the ground-based electromagnetic calorimeter should provide us with an estimate of the (local) electron-to-muon ratio, which is what we know to determine the weight of the primary nucleus.
If the above sounds confusing, it is entirely my fault: I have dumped here some loose ideas, with the aim of coming back here when I need them. After all, this is a log. a Web log, but always a log of my ideas… But I wish to investigate more on the feasibility of this project. Indeed, CMS will for sure pursue cosmic-ray measurements with the 300M events it has already collected. And CMS does have spare muon chambers. And CMS does have plans of storing them at Point 5… Why not just power them up and build a poor man’s trigger ? A calorimeter might come later…
