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Things I should have blogged on last week April 13, 2009

Posted by dorigo in cosmology, news, physics, science.
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It rarely happens that four days pass without a new post on this site, but it is never because of the lack of things to report on: the world of experimental particle physics is wonderfully active and always entertaining. Usually hiatuses are due to a bout of laziness on my part. In this case, I can blame Vodafone, the provider of the wireless internet service I use when I am on vacation. From Padola (the place in the eastern italian Alps where I spent the last few days) the service is horrible, and I sometimes lack the patience to find the moment of outburst when bytes flow freely.

Things I would have wanted to blog on during these days include:

  • The document describing the DZERO search of a CDF-like anomalous muon signal is finally public, about two weeks after the talk which announced the results at Moriond. Having had in my hands a unauthorized draft, I have a chance of comparing the polished with the unpolished version… Should be fun, but unfortunately unbloggable, since I owe some respect to my colleagues in DZERO. Still, the many issues I raised after the Moriond seminar should be discussed in light of an official document.
  • DZERO also produced a very interesting search for t \bar t h production. This is the associated production of a Higgs boson and a pair of top quarks, a process whose rate is made significant by the large coupling of top quarks and Higgs bosons, by virtue of the large top quark mass. By searching for a top-antitop signature and the associated Higgs boson decay to a pair of b-quark jets, one can investigate the existence of Higgs bosons in the mass range where the b \bar b decay is most frequent -i.e., the region where all indirect evidence puts it. However, tth production is invisible at the Tevatron, and very hard at the LHC, so the DZERO search is really just a check that there is nothing sticking out which we have missed by just forgetting to look there. In any case, the signature is extremely rich and interesting to study (I had a PhD doing this for CMS a couple of years ago), thus my interest.
  • I am still sitting on my notes for Day 4 of the NEUTEL2009 conference in Venice, which included a few interesting talks on gravitational waves, CMB anisotropies, the PAMELA results, and a talk by Marco Cirelli on dark matter searches. With some effort, I should be able to organize these notes in a post in a few days.
  • And new beautiful physics results are coming out of CDF. I cannot anticipate much, but I assure you there will be much to read about in the forthcoming weeks!

DZERO refutes CDF’s multimuon signal… Or does it ? March 17, 2009

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Hot off the press: Mark Williams, a DZERO member speaking at Moriond QCD 2009 -a yearly international conference in particle physics, where HEP experimentalists regularly present their hottest results- has shown today the preliminary results of their analysis of dimuon events, based on 900 inverse picobarns of proton-antiproton collision data. And the conclusion is…

DZERO searched for an excess of muons with large impact parameter by applying a data selection very similar, and when possible totally equivalent, to the one used by CDF in its recent study. Of course, the two detectors have entirely different hardware, software algorithms, and triggers, so there are certain limits to how closely one analysis can be replicated by the other experiment. However, the main machinery is quite similar: they count how many events have two muons produced within the first layer of silicon detector, and extrapolate to determine how many they expect to see which fail to yield a hit in that first layer, comparing to the actual number. They find no excess of large impact parameter muons.

Impact parameter, for those of you who have not followed this closely in the last few months, is the smallest distance between a track and the proton-antiproton collision vertex, in the plane transverse to the beam direction. A large impact parameter indicates that a particle has been produced in the decay of a parent body which was able to travel away from the interaction point before disintegrating. More information about the whole issue can be found in this series of posts, or by just clicking the “anomalous muons” tab in the column on the right of this text.

There are many things to say, but I will not say them all here now, because I am still digesting the presentation, the accompanying document produced by DZERO (not ready for public consumption yet), and the implications and subtleties involved. However, let me flash a few of the questions I am going to try and give an answer to with my readings:

  • The paper does not address the most important question – what is DZERO’s track reconstruction efficiency as a function of track impact parameter ? They do discuss with some detail the complicated mixture of their data, which results from triggers which enforce that tracks have very small impact parameter -effectively cutting away all tracks with an impact parameter larger than 0.5cm- and a dedicated trigger which does not enforce an IP requirement; they also discuss their offline track reconstruction algorithms. But at a first sight it did not seem clear to me that they can actually reconstruct effectively tracks with impact parameters up to 2.5 cm as they claim. I would have inserted in the documents an efficiency graph for the reconstruction efficiency as a function of impact parameter, had I authored it.
  • The paper shows a distribution of the decay radius of neutral K mesons, reconstructed from their decay into pair of charged pions. From the plot, the efficiency of reconstructing those pions is excessively small -some three times smaller than what it is in CMS, for instance. I need to read another paper by DZERO to figure out what drives their K-zero reconstruction efficiency to be so small, and whether this is in fact due to the decrease of effectiveness with track displacement.
  • What really puzzles me, however, is the fact that they do not see *any* excess, while we know there must be in any case a significant one: decays in flight of charged kaons and pions. Why is it that CDF is riddled with those, while DZERO appears free of them ? To explain this point: charged kaons and pions yield muons, which get reconstructed as real muons with large impact parameter. If the decay occurs within the tracking volume, the track is partly reconstructed with the muon hits and partly with the kaon or pion hits. Now, while pions have a mass similar to that of muons, and thus the muon practically follows the pion trajectory faithfully, for kaons there must be a significant kink in the track trajectory. One expects that the track reconstruction algorithm will fail to associate inner hits to a good fraction of those tracks, and the resulting muons will belong to the “loose” category, without a correspondence in the “tight” muon category which has muons containing a silicon hit in the innermost layer of the silicon detector. This creates an excess of muons with large impact parameter. CDF does estimate that contribution, and it is quite large, of the order of tens of thousands of events in 743 inverse picobarns of data! Now where are those events in the DZERO dataset, then ?

Of course, you should not expect that my limited intellectual capabilities and my slow reading of a paper I have had in my hands for no longer than two hours can produce foulproof arguments. So the above is just a first pass, sort of a quick and dirty evaluation. I imagine I will be able to give an answer to those puzzles myself, at least in part, with a deeper look at the documentation. But, for the time being, this is what I have to say about the DZERO analysis.

Or rather, I should add something. By reading the above, you might get the impression that I am only criticizing DZERO out of bitterness for the failed discovery of the century by CDF… No, it is not the case: I have always thought, and I continue to think, that the multi-muon signal by CDF is some unaccounted-for background. And I do salute with relief and interest the new effort by DZERO on this issue. I actually thank them for providing their input on this mystery. However, I still retain some scepticism with respect to the findings of their study. I hope that scepticism can be wiped off by some input – maybe some reader belonging to DZERO wants to shed some light on the issues I mentioned above ? You are most welcome to do so!

UPDATE: Lubos pitches in, and guess what, he blames CDF… But Lubos the experimentalist is not better than Lubos the diplomat, if you know what I mean…

Other reactions will be collected below – if you have any to point to, please do so.

CMS and extensive air showers: ideas for an experiment February 6, 2009

Posted by dorigo in astronomy, cosmology, physics, science.
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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 K^+ \to \mu^+ \nu_{\mu}, \pi^+ \to \mu^+ \nu_{\mu} decays will create a lot of muons.

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…

Some notes on the multi-muon analysis – part IV February 2, 2009

Posted by dorigo in news, physics, science.
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In this post -the fourth of a series (previous parts: part I, part II, and part III)- I wish to discuss a couple of attributes possessed by the “ghost” events unearthed by the CDF multi-muon analysis. A few months have passed since the publication of the CDF preprint describing that result, so I think it is useful for me to make a short summary below, repeating in a nutshell what is the signal we are discussing and how it came about.

Let me first of all remind you that “ghost events” are a unknown background component of the sample dimuon events collected by CDF. This background can be defined as an excess of events where one or both muons fail a standard selection criterion based on the pattern of hits left by the muons in the innermost layers of the silicon tracker, SVX. I feel I need to open a parenthesis here, in order to allow those of you who are unfamiliar with the detection of charged tracks to follow the discussion.

Two words on tracks and their quality

The silicon tracker of CDF, SVX, is made up by seven concentrical cylinders of solid-state sensors (see figure on the right: SVX in Run II is made by the innermost L00 layer in red, plus four blue SVX II layers, plus two ISL layers; also shown are the two innermost Run I SVX’ layers, in hatched green), surrounding the beam line. When electrically charged particles created in a proton-antiproton collision travel out of the interaction region lying at the center, they cross those sensors in succession, leaving in each a localized ionization signal -a “hit”.

CDF does not strictly need silicon hits to track charged particles, since outside of the silicon detector lies a gas tracker called COT (for Central Outer Tracker), capable of acquiring up to 96 additional independent position measurements of the ionization trail; however, silicon hits are a hundred times more precise than COT ones, so that one can define two different categories of tracks: COT-only, and SVX tracks. Only the latter are used for lifetime measurements of long-lived particles such as B hadrons, since those particles travel at most a few millimeters away from the primary interaction point before disintegrating: their decay products, if tracked with the silicon, allow the decay point to be determined.

Typically, CDF loosely requires an SVX track to have three or more hits; however, a tighter selection can be made which requires four or more hits, additionally enforcing that two of those belong to the two innermost silicon layers. These tight SVX tracks have considerably better spatial resolution on the point of origin of the track, since the two innermost hits “zoom in” on it very effectively.

Back to ghosts: a reminder of their definition

Getting back to ghost events, the whole evidence of their presence is that one finds considerably more muon pairs failing the tight-SVX tracking selection than geometry and kinematics would normally imply in a homogeneous sample of data. Muons in ghost events systematically fail hitting the innermost silicon layers, just as if they were produced outside of it by the decay of a long-lived, neutral particle.

Because of its very nature -an excess of muon pairs failing the tight-SVX criteria- the “ghost sample” is obtained by a subtraction procedure: one takes the number T of events with a pair of tight-SVX muons, divides their number by the geometrical and kinematical efficiency \epsilon that muons from the various known sources pass tight-SVX cuts, and obtains a number E, which subtracted from the number O of observed dimuon pairs allows to spot the excess G, as follows: G = O-E = O-T/\epsilon.

Mind you, we are not talking of a small excess here: if you have been around this blog for long enough, you are probably accustomed to the frequent phenomenon of particle physicists getting hyped up for 10-event excesses. Not this time: the number of ghost muon events exceeds 70,000, and the nature of this contribution is clearly of systematic origin. It may be a background unaccounted by the subtraction procedure, or a signal involving muons that are created outside of the innermost silicon layers.

In the former three installments of this multi-threaded post I have discussed with some detail the significant sources of reconstructed muons which may contribute to the ghost sample, and be unaccounted by the subtraction procedure: muons from decays in flight of kaons and pions, fake muon tracks due to hadrons punching through the calorimeter, and secondary nuclear interactions. Today, I will rather assume that the excess of dimuon events constitutes a class of its own, different from those mundane sources, and proceed to discuss a couple of additional characteristics that make these events really peculiar.

The number of muons

In the first part of this series I have discussed in detail how the excess of ghost events contains muons which have abnormally large impact parameters. Impact parameter -the distance of the track from the proton-antiproton collision point, as shown by the graph on the right- is a measure of the lifetime of the body which decays into the muons, and the observation of large impact parameters in ghost events is the real alarm bell, demanding that one needs to really try and figure out what is going on in the data. However, once that anomaly is acknowledged, surprises are not over.

The second observation that makes one jump on the chair occurs when one simply counts the number of additional muon candidates found accompanying the duo which triggered the event collection in the first place. In the sample of 743,000 events with no SVX hit requirements on the two triggering muons, 72,000 events are found to contain at least a third muon track. 10% is a large number! By comparison, only 0.9% of the well-identified \Upsilon(1S) \to \mu \mu decays contained in the sample is found to contain additional muons besides the decay pair. However, since the production of \Upsilon particles is a quite peculiar process, this observation need not worry us yet: those events are typically very clean, with the b\bar b meson accompanied by a relatively small energy release. In particle physics jargon, we say that \Upsilon mesons have a soft P_T spectrum: they are produced almost at rest in most cases. There are thus few particles recoiling against it -and so, few muons too.

Now, the 10% number quoted above is not an accurate estimate of the fraction of ghost events containing additional muons, since it is extracted from the total sample -the 743,000 events. The subtraction procedure described above allows to estimate the fraction in the ghost sample alone: this is actually larger, 15.8%, because all other sources contribute fewer multi-muon events: only 8.3%. These fractions include of course both real and fake muons: in the following I try to describe how one can size up better those contributions.

Fake muons

A detailed account of the number of additional muons in the data and the relative sources that may be originating them can be tried by using a complete Monte Carlo simulation of all processes contributing to the sample, applying some corrections where needed. As a matter of fact, a detailed accounting of all the physical processes produced in proton-antiproton collisions is rather an overkill, because events with three or more muon candidates are a rare merchandise, and they can be produced by few processes: basically the only sizable contributions come from sequential heavy flavor decays and fake muon sources. Let us discuss these two possibilities in turn.

Real muon pairs of small invariant mass, recoiling against a third muon, are usually the result of sequential decays of B-hadrons, like in the process B \to \mu \nu D \to \mu \nu X (see picture on the left, where the line of the decaying quark is shown emitting sequentially two lepton pairs in the weak decays). The two muons from such a chain decay cannot have a combined mass larger than 5 GeV, which is (roughly speaking) the mass of the originating B hadron. In fact, by enforcing that very requirement (M_{\mu \mu} >5 GeV) on the two muons at trigger level, CDF enriches the collected dataset of events where two independent heavy-flavor hadrons (B or D mesons, for instance) are produced at a sizable angle from each other. A sample event picture is shown below in a transverse section of the CDF detector. Muon detection systems are shown in green, and in red are shown the track segments of two muons firing the high-mass dimuon trigger.

(You might well ask: Why does CDF requires a high mass for muon pairs ? Because the measurements that can be extracted from such a “high-mass” sample are more interesting than those granted by events with two muons produced close in angle, events which are in any case likely to be collected into different datasets, such as the one triggered by a single muon with a larger transverse momentum threshold. But that is a detail, so let’s go back to ghost muons now.)

When there are three real muons, one thus has most likely a $b \bar b$ pair, with one of the quarks producing a double semileptonic decay (two muons of small mass and angle), and the other producing a single semileptonic decay (with this third muon making a large mass with one of the other two): for instance, B \bar B \to (\mu^- \bar \nu X) (\mu^+ \nu D) \to (\mu^- \bar \nu X)(\mu^+ \nu \mu^- \bar \nu Y), in the case of two B mesons; in the decay chain above, X and Y denote a generic hadronic state, while D is a hadron containing a anti-charm quark. B hadron decays can produce three muons also when one of them decays to a J/\Psi meson, which in turn decays to a muon pair. Other heavy flavor decays, like those involving a c \bar c pair, can at most produce a pair of muons, and the third one must then be a fake one.

The HERWIG Monte Carlo program, which simulates all QCD processes, does make a good guess of the production cross-section of b-quark pairs and c-quark pairs produced in proton-antiproton collisions, in order to simulate all processes with equanimity; but those numbers are not accurate. One improves things by taking simulated events that contain those production processes such that they match the b \bar b and c \bar c cross-sections which are measured with the tight-SVX sample, the subset devoid of the ghost contribution.

The CDF analysis then proceeds by estimating the number of events where at least one muon track is in reality a hadron which punched through the detector. The simulation can be trusted to reproduce the number of hadrons and their momentum spectrum, but the phenomenon of punch-through is unknown to it! To include it, a parametrization of the punch-through probability is obtained from a large sample of D \to K \pi decays, collected by the Silicon Vertex Tracker, a wonderful device capable of triggering on the impact parameter of tracks. The D meson lives long enough that the kaon and pion tracks it produces have sizable impact parameter, and millions of such events have been collected by CDF in Run II.

The extraction of the probability is quite simple: take the kaon tracks from D decays, and find the fraction of these tracks that are considered muon candidates, thanks to muon chamber hits consistent with their trajectory. Then, repeat the same with the pion candidates. The result is shown in the graphs below separately for kaon and pion tracks. In them, the probability has been computed as a function of the track transverse momentum.

Besides the above probabilities and the tuning of the b \bar b cross section, a number of other details are needed to produce a best-guess prediction of the number of multi-mion events with the HERWIG Monte Carlo simulation. However, once all is said and done, one can verify that there indeed is an excess in the data. This excess appears entirely in the ghost muon sample, while the tight-SVX sample is completely free from it. Its size is again very large, and its source is thus systematical -no fluctuation can be hypothesized to have originated it.

The mass of muon pairs in multi-muon events

To summarize, what happens with ghost events is that if one searches for additional muon tracks around each of the triggering muons, one finds them with a rate much higher than what one observes in the tight-SVX dimuon sample. It is as if a congregation of muons is occurring! The standard model is unable to even getting close to explain how events with so many muons can be produced. The source of ghost events is thus really mysterious.

Now, if you give to a particle physicist the momenta and energies P_x. P_y, P_z, E of two particles produced together in a mysterious process, there is no question on what is going to happen: next thing you know, he will produce a number, m^2=(\Sigma E)^2-(\Sigma P_x)^2 -(\Sigma P_y)^2 - (\Sigma P_z)^2. m is the invariant mass of the two-particle system: if they are the sole products of a decay process, m is a unbiased measurement of the mass M_x of the parent body. If, instead, the two particles are only part of the final state, m will be smaller than M_x; still, a distribution of the quantity m for several decays will say a lot about the parent particle X.

Given the above, it is not a surprise that the next step in the analysis, once triggering muons in ghost events are found to be accompanied by additional muons at an abnormal rate, is to plot the invariant mass of those two-muon combinations.

There is, however, an even stronger motivation from doing that: an anomalous mass distribution of lepton pairs (then electron-muon pairs, not dimuons -I will come back to this detail later) had been observed by the same authors in Run I. That excess of dilepton pairs was smaller numerically -the dataset from which it had been extracted corresponded to an integrated luminosity 20 times smaller- but had been extracted with quite different means, from a different trigger, and with a considerably different detector (the tracking of CDF has been entirely changed in Run II). The low-mass excess of dilepton pairs remained a unexplained feature, calling for more investigation which had to wait a few years to be performed. The mass distribution of electron-muon combinations found by CDF in Run I is shown in the graph on the right: the excess of data (the blue points) over known background sources (the yellow histogram) appears at very low mass.

In Run II, not only does CDF have 20 times more data (well, sixty times so by now, but the dataset on which this analysis was performed was frozen one and a half years ago, thus missing the data collected and processed after that date): we also have more tools at our disposal. The mass distribution of muon pairs close in angle, belonging to ghost events with three or more muon candidates, can be compared with the tuned HERWIG simulation both for ghost event sample and for the tight SVX sample: this makes for a wonderful cross-check that the simulation can be trusted on producing a sound estimate of that distribution!

The invariant mass distribution of muon pairs close in angle in tight-SVX events with three or more muon tracks is shown on the left. The experimental data is shown with full black dots, while the Monte Carlo simulation prediction is shown with empty ones. The shape and size of the two distributions match well, implying that the Monte Carlo is properly normalized. Indeed, the tight-SVX sample is the one used for the measurements of b \bar b and c \bar c cross sections: once the Monte Carlo is tuned to the values extracted from the data, its overall normalization could mismatch the data only if fake-muon sources were grossly mistaken. That is not the case, and further, one observes that the number of J/\Psi \to \mu \mu decays -which end up all in one bin in the histogram, at 3.1 GeV of mass- are perfectly well predicted by the simulation: again, not a surprise, since those mesons can make it to a three-muon dataset virtually only if they are originated from B hadron decays. So, the check in tight-SVX events fortifies our trust on our tuned Monte Carlo tool.

Now, let us look at how things are going in the ghost muon sample (see graph on the right). Here, we observe more data at low invariant mass than what the Monte Carlo predicts: there is a clear excess for masses below 2.5 GeV. This excess has the same shape as the one observed in Run I in electron-muon combinations!

Please take a moment to record this: in CDF, some of the collaborators who objected to the publication of the multi-muon analysis did so because they insisted that more studies should be made to confirm or disprove the effect. One of the objections was that the electron-muon sample had not been studied yet. The rationale is that if the ghost events are due to a real physical process, then the same process should show up in electron-muon combinations; otherwise, one is hard-pressed to avoid having to put into question a thing called lepton universality, which -at least for Standard Model processes- is a really hard thing to do. However, the electron signature in CDF is very difficult to handle, particularly at low energy: backgrounds are much harder to pinpoint than for muons. Such a study is ongoing, but it might take a long time to complete. Run I, instead, is there for us: and there, the same excess was indeed present in electron events too!

Finally, there is one additional point to mention: a small, but crucial one. The J/\Psi signal is in perfect match with the simulation prediction! This observation confirms that the tuned cross section of b \bar b production is right dead-on. Whatever these ghost events are, they sure cannot be coming from B production. Also, note that the agreement of the J/\Psi signal with Monte Carlo expectations constitutes proof that the efficiency of the tight-SVX requirements -the 24% number which is used to extract the numerical excess of ghost events- is correct. Everything points to a mysterious contribution which is absent in the Monte Carlo.

The above observations conclude this part of the discussion. In the next installment, I will try to discuss the additional oddities of ghost events -in particular, the rate of muons exceeding the triggering pair is actually four times higher than in QCD events. I will then examine some tentative interpretations that have been put forth in the course of the three months that have passed since the publication.

Babysitting this week February 1, 2009

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Blogging is one of the activities that will get slightly reduced this week, along with others that are not strictly necessary for my survival. Mariarosa has left for Athens this morning with three high-school classes of her school, Liceo Foscarini. They will visit Greece for a whole week, and be back to Venice on Saturday.

I am not scared by the obligation of having to care for my two kids, and I do like such challenges -I maintain that my wife should not complain too much when it is me who leaves for a week, much more frequently- but of course the management of our family life will take all of my spare time, plus some.

Blogging material, in the meantime, is piling up. There are beautiful results coming out of CDF these days (isn’t that becoming a rule?). Furthermore, recently the Tevatron has been running excellently, and the LHC seems in the middle of a crisis over whether to risk a second, colossal failure by pushing the energy up to 10 TeV to put the Tevatron off the table in the shortest time possible, or to play it safe and keep the collision energy at 6 TeV, accepting the risk of being scooped of the most juicy bits of physics left over to party with.

And multi-muons keep me busy these days. Besides the starting analysis in the CMS-Padova group, there are papers worth discussing in the arxiv. This one was published a few days ago, and we had in Padova last Thursday one of the authors, Thomas Gehrmann, discussing QCD calculations of event shapes observables in a seminar- which of course allowed me to chat with him about his hunch on the hidden valley scenarios he discusses in his paper. More on these things next week, after I set my kids to sleep!

Multi-muon news January 26, 2009

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This post is not it but no, I have not given up on my promise to complete my series on the anomalous multi-muon signal found by CDF in its Run II data. In fact, I expect to be able to post once more on the topic this week. There, I hope I will be able to discuss the kinematic characteristics of multi-lepton jets. [I am lazy today, so I will refrain from adding links to past discussions of the topic here: if you need references on the topic, just click on the tag cloud on the right column, where it says "anomalous muons"!]

In the meantime, I am happy to report that I have just started working at the same analysis for the CMS experiment! In Padova we have recently put together a group of six -one professor, three researchers, a PhD student, and a undergrad- and we will pursue the investigation of the same signature seen by CDF.  And today, together with Luca, our new brilliant PhD student, I started looking at the reconstruction of neutral kaon decays K^\circ \to \pi^+ \pi^-, a clean source of well-identified pion tracks with which we hope to be able to study muon mis-identification in CMS.

Meanwhile, the six-strong group in Padova is already expanding. Last Wednesday professor Fotios Ptochos, a longtime colleague in CDF, a good friend, and crucially one of the authors of the multi-muon analysis, came to Padova and presented a two-hour-long seminar on the CDF signal in front of a very interested group of forty physicists spanning four generations -from Milla Baldo Ceolin to our youngest undergraduates. The seminar was enlightening and I was very happy with the result of a week spent organizing the whole thing! (I will have to ask Fotios if I can make the slides of his talk available here….)

Fotios, a professor at the University of Cyprus, is a member of CMS, and a true expert of measurements in the B-physics sector at hadron machines. We plan to work together to repeat the controversial CDF analysis with the first data that CMS will collect -hopefully later this year.

The idea of repeating the CDF analysis in CMS is obvious. Both CDF and D0 can say something on the signal in a reasonable time scale, but whatever the outcome, the matter will only be settled by the LHC experiments. Imagine, for instance, that in a few months D0 publishes an analysis which disproves the CDF signal. Will we then conclude that CDF has completely screwed up its measurement ? We will probably have quite a clue in that case, but we will need to remain possibilistic until at least a third, possibly more precise, measurement is performed by an independent experiment.That measurement is surely going to be worth a useful publication.

And now imagine, on the contrary, that the CDF signal is real…

Some posts you might have missed in 2008 – part II January 6, 2009

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Here is the second part of the list of useful physics posts I published on this site in 2008. As noted yesterday when I published the list for the first six months of 2008, this list does not include guest posts nor conference reports, which may be valuable but belong to a different place (and are linked from permanent pages above). In reverse chronological order:

December 29: a report on the first measurement of exclusive production of charmonium states in hadron-hadron collisions, by CDF.

December 19: a detailed description of the effects of parton distribution functions on the production of Z bosons at the LHC, and how these effects determine the observed mass of the produced Z bosons. On the same topic, there is a maybe simpler post from November 25th.

December 8: description of a new technique to measure the top quark mass in dileptonic decays by CDF.

November 28: a report on the measurement of extremely rare decays of B hadrons, and their implications.

November 19, November 20, November 20 again , November 21, and November 21 again: a five-post saga on the disagreement between Lubos Motl and yours truly on a detail on the multi-muon analysis by CDF, which becomes a endless diatriba since Lubos won’t listen to my attempts at making his brain work, and insists on his mistake. This leads to a back-and-forth between our blogs and a surprising happy ending when Motl finally apologizes for his mistake. Stuff for expert lubologists, but I could not help adding the above links to this summary. Beware, most of the fun is in the comments threads!

November 8, November 8 again, and November 12: a three-part discussion of the details in the surprising new measurement of anomalous multi-muon production published by CDF (whose summary is here). Warning: I intend to continue this series as I find the time, to complete the detailed description of this potentially groundbreaking study.

October 24: the analysis by which D0 extracts evidence for diboson production using the dilepton plus dijet final state, a difficult, background-ridden signature. The same search, performed by CDF, is reported in detail in a post published on October 13.

September 23: a description of an automated global search for new physics in CDF data, and its intriguing results.

September 19: the discovery of the \Omega_b baryon, an important find by the D0 experiment.

August 27: a report on the D0 measurement of the polarization of Upsilon mesons -states made up by a b \bar b pair- and its relevance for our understanding of QCD.

August 21: a detailed discussion of the ingredients necessary to measure with the utmost precision the mass of the W boson at the Tevatron.

August 8: the new CDF measurement of the lifetime of the \Lambda_b baryon, which had previously been in disagreement with theory.

August 7: a discussion of the new cross-section limits on Higgs boson production, and the first exclusion of the 170 GeV mass, by the two Tevatron experiments.

July 18: a search for narrow resonances decaying to muon pairs in CDF data excludes the tentative signal seen by CDF in Run I.

July 10: An important measurement by CDF on the correlated production of pairs of b-quark jets. This measurement is a cornerstone of the observation of anomalous multi-muon events that CDF published at the end of October 2008 (see above).

July 8: a report of a new technique to measure the top quark mass which is very important for the LHC, and the results obtained on CDF data. For a similar technique of relevance to LHC, also check this other CDF measurement.

Scientific wishes for 2009 December 31, 2008

Posted by dorigo in astronomy, Blogroll, cosmology, personal, physics, science.
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I wish 2009 will bring an answer to a few important questions:

  • Can LHC run ?
  • Can LHC run at 14 TeV ?
  • Will I get tenure ?
  • Are multi-muons a background ?
  • Are the Pamela/ATIC signals a prologue of a new scientific revolution ?
  • Will England allow a NZ scientist to work on Category Theory on its soil ?
  • Is the Standard Model still alive and kicking in the face of several recent attempts at its demise ?

I believe the answer to all the above questions is yes. However, I am by no means sure all of them will be answered next year.

Hectic week December 4, 2008

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The regulars here will have already noticed by now that my posting rate has fallen this week. I have been busy with three different physics analyses, trying to make some progress in each.

The first project is the calibration of the momentum scale in CMS. I have discussed the issue elsewhere a couple of times; I am slowly converging to an understanding of how to treat the Z boson lineshape -which receives contributions from a number of different sources and effects: parton distribution functions in the projectiles, electromagnetic and weak radiation effects, interaction of the final state products of Z decay with the material of the tracker. All this must be dealt with in a coherent fashion to extract the most information possible from the Z decays we will reconstruct in CMS. We have a small but focused group working at the momentum scale calibration, including worthy physicists from Torino University, plus Marco and me. This week, I have tried to determine the effect of parton distribution functions alone, to insert it in our algorithm, but something still escapes me, and I want to do things as well as I can -which sometimes take little extra effort from a mediocre result, but in this case seems to be requiring a lot more care.

The second is the search for Higgs boson decays in the final state arising when H decays to two Z bosons, and one of the Z decays to a lepton pair, while the other decays to a pair of jets. Usually this final state, which is very hard to exploit at low Higgs masses due to the large backgrounds, is used for high-mass searches only (above 200 GeV). We want to extend it to lower masses, where the Higgs is more likely to be, using the Z \to b \bar b decay, which Mia and I have a lot of experience in detecting in hadronic environments. Mia will present some results of this study tomorrow at CERN, so we have been working at this heavily this week.

The third topic is the evaluation of the chances of CMS to detect a similar signature of multi-muon events that CDF has seen in its data. The CDF signal is probably just a not well understood background, but it makes sense to size up the capability of CMS to detect a similar signature with early data. This requires understanding muon sources without using real data, and it is a bit far-fetched, but it is perfectly sound as a masters’ thesis topic, one on which Franco and I in fact have a student working. I have not worked much on this topic this week, but it still has absorbed a little of CPU.

I have a thick agenda of pending things to do, which has grown longer in the last few days. One thing is to post more commentaries on the multi-muon analysis by CDF here. Another is to progress with a document I am writing. A third is to review a 40-pages long CDF paper draft for the Spokespersons Reading Group, to which I proudly belong. A fourth is to organize the upcoming meeting of the CMS-Padova software-analysis group, which will convene in ten days. A fifth is to prepare my next trip to CERN, which will be from next Monday to next Friday. I do hope that I will be able to post more in the next few days… if I survive.

Live streaming for the radio interview November 22, 2008

Posted by dorigo in internet, news, personal, physics, science.
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In less than an hour I will participate in the program “Caccia al Fotone”, aired by Radio Città Fujiko, on 103.1FM (if you are in Italy). If you want to follow it on the internet, there is a streaming available:

You can send an SMS or an email to ask questions. See the site of the program for directions.


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