Dark Matter Searches at Colliders – part III May 6, 2008Posted by dorigo in cosmology, physics, science.
Long overdue, here is the final part of a long post on the searches for new particles that may be the solution of a long-standing problem in astrophysics today: the missing mass in our Universe.
The large majority of cosmologists have become convinced, through the analysis of masses of data collected in the last two decades, that four-fifths of the matter in the Universe is non-baryonic. If we neglect particles which can only be created in high-energy collisions and decay in ridiculously small amounts of time, Baryons exists in just two forms: protons and neutrons. These make up the nuclei of atoms, and provide the fuel for stars to shine as they fuse into helium nuclei.
Non-baryonic matter does exist, and we know it well: we have electrons and neutrinos; but these are irrelevant. Electrons weigh less than a thousandth of a proton -and there are just as many electrons as protons around, to a very good approximation. As for neutrinos, despite our ignorance on their mass, they cannot make up the deficit of mass observed in the rotation speed of galaxies (exhibit one in support to Dark Matter: the speed of rotation does not decrease as much as it should if their mass was concentrated in stars) or in clusters of galaxies (exhibit two: gravitational effects we may detect visually do not match the observed distribution of galaxies in these agglomerates).
One intriguing solution to the problem lies in hypothesizing that a massive particle called neutralino wanders around in huge amounts, slow and unbothered by its close encounters with ordinary matter. Neutralinos would be electrically neutral, they would not interact strongly with matter, and they would be perfectly stable, lest they violate a very convenient quantum-mechanical conservation law. For more details on these hypotheses, see part II of this post.
So how can collider experiments detect this evanescent particle ? By producing pairs of higher-mass supersymmetric particles, which would chain-decay into non-supersymmetric ones plus a pair of those lightest supersymmetric particles, LSP. On the right you can see a decay chain whereby a gluino – a SUSY particle produced in large amounts in hadron collisions, due to its strongly interacting nature – emits a squark, the squark in turn emits another quark and decays into an excited neutralino, this emits a slepton, and the slepton ends up producing the lightest neutralino. All in all, from each of these chains (one per decay of each of the produced gluinos) one should observe two jets of hadrons from the quark hadronization, two leptons, and some missing energy. The missing transverse energy stolen by each neutralino would add as two vectors add in a plane: only rarely they would cancel each other out. In the graph below, for instance, two neutralinos leaving in different directions (the two dashed lines pointing towards the upper and lower left, in the transverse cut-away view of the ATLAS detector) would create a missing transverse energy vector pointing roughly mid-way between their exit directions.
The Tevatron experiments have searched for these events in their Run II data. The search in CDF considered the signature of two, three, or four hadronic jets plus a significant amount of missing energy from the neutralinos. This signature can be mimicked very effectively by the frequent, generic production of many jets by quantum chromodynamics interactions between quarks and gluons; the missing energy is thus required to be large and significant to suppress these processes.
The CDF experiment applied three different sets of selection cuts on their data to seek sensitivity to different regions of the parameter space of Supersymmetry. Indeed, as the mass of gluinos, squarks, and sleptons varies, so does the visible final state. For instance, if squarks and gluinos have a similar mass one is unlikely to detect a hadronic jet from the quark that is emitted in the transformation of the former into the latter. The signature pf pair-produced gluinos then more closely resembles one with only two jets and missing energy.
The figure on the right shows the final selection of the data in one of the three search regions. It is clear that known Standard Model processes provide a good modeling of the observed distribution of missing transverse energy in the data (black points with error bars), whereas a supersymmetric signal (the empty histogram in green, overlaid to SM contributions) would have instead stood out and created a disagreement.
From the distributions an upper limit can be extracted on the amount of signal contained in the data, and from the latter a limit is obtained in the cross section of gluino pair production: this translates into a mass exclusion range for squarks and gluinos. The final summarizing plot is shown below.
The plane is spanned by the mass of the two hypothetical particles. Colored areas have been excluded by different experiments; the CDF search extends the excluded region by the size of the red-painted area. We thus learn that gluinos cannot be lighter than 300 GeV, whatever the squark mass, otherwise CDF would have seen a bunch of anomalous events with large missing energy and jets.
The Tevatron protons and antiprotons do not have enough energy to investigate supersymmetric particles of mass much larger than the limit discussed above: so if Supersymmetry is the right theory of Nature, it may turn out to be the job of the Large Hadron Collider to discover it. With its 7-fold increase in energy and hundred-fold increase in interaction rates, the LHC is expected to provide a clear-cut answer: discover supersymmetry, or rule it out for good. As you can see in the plot below (where the plane is spanned by two convenient parameters among the multitude of choices: and ), the discovery reach of the CMS experiment extends to mass values in excess of a TeV – where supersymmetric particles would be close to useless, because they would not have a chance to solve the problems of electroweak symmetry breaking for which they were once invented.
The graph is complicated and it requires some more explanation: the blue areas are excluded by theoretical constraints and experimental searches, and the green area is also excluded. The colored wavy lines show instead the limits that CMS will be able to set in the plane -intending it will exclude anything to the left of the curves – with different searches, labeled by their respective “smoking guns”. The red curve is labeled for missing transverse energy, and it is one of the most performant in excluding the parameter space.
So, indeed, CMS and ATLAS will have an easy way to find signals of supersymmetry across the table -the wide space of parameters: they just need to study their distribution of missing transverse energy, just as we saw CDF do in the analysis mentioned above. The fanthom signal of a neutralino, which cannot interact with the detector and leaves unseen, turns out to be more striking at the end of the day than the multitude of jets and charged leptons the pyroclastic Supersymmetric production events would give rise to. Seeing events with a large amount of missing transverse energy would not allow us to determine which form of supersymmetry we are dealing with – whether a minimal supersymmetric extension of the Standard Model with two higgs boson doublets, or more complicated schemes. However, it would still allow us to claim that we have evidence for THE candidate particle which constitutes 80% of the stuff the Universe is made of.
I need to warn the reader here: of course, ATLAS and CMS have already studied dozens of methods, some of which are quite complicated, to extract very detailed information on Supersymmetry and very clean signatures of its presence from LHC data. These analyses focus on kinematical properties of the supersymmetric decays which are very model-dependent, and very complicated to explain. Although I reported about these methods in my seminar, I take the liberty here of jumping ahead a little…
So what instead if SUSY is not, after all, the right idea ?
Despite the general enthusiasm of theorists, phenomenologists, and other assorted believers, in fact, we have to keep a cool mind. Let’s review the cost of the purchase we have to make if we are to marry Supersymmetry:
- twenty brand-new particles, never before seen
- at least 104 new parameters, whose value is unknown and to be determined by improbable experiments
- a strict conservation of R-parity, the number you get by adding together spin, baryon, and lepton number in a suitable combination – the combination allows the proton and the lightest neutralino to remain stable
- We also have to agree that despite the fact that in principle the Tevatron and LEP colliders could have well stumbled into Supersymmetry, they haven’t – new physics chose to hide in the far away corner, just like the small coin that you dropped from your pocket.
Some of us think the above is too much to buy, for a theory which “solves” the mystery of a unnaturally small mass of the Higgs boson (provided the Higgs exists and is light as every evidence still suggests) and which collapses two crossings between running coupling constants into one single point. Ockham’s razor comes a-slashing: “entia non sunt multiplicanda praeter necessitatem“, one must not multiply entities. The most economical explanation is the best one… The razor cuts unnecessary entities.
One should mention, at the end of this long post which focused on the searches for just one candidate for dark matter – the one which hadron colliders may have a chance to find, the neutralino – that there is a long list of alternatives, of many flavors: kaluza-klein gravitons, sneutrinos, gravitinos, little higgses, axions, primordial black holes, charged massive particles, heavy neutrinos, sterile neutrinos, you name them.
It is for this very reason that in the end, LHC searches will require to follow the very important two-step procedure outlined by M.Mangano in a recent paper: first establish that an anomaly exists in the data, and only after it has been demonstrated to be utterly unexplainable by known phenomena, proceed with an exotic explanation.
To conclude, dark matter candidates have been searched at past and present collider experiments with no success. LHC appears to have the right energy and the potential to finally discover the source of this astounding enigma. In any case, we will know in a few years whether Supersymmetry is real or just a crazy concoction. If SUSY exists, new accelerators will be needed to investigate it in detail, but if it doesn’t, particle physics may be at a dead end. Despite this threatening possibility, we have extremely exciting years ahead of us!