August 26, 1997
One symmetry on the scale of quarks and leptons is that between generations: all generations look the same apart from their masses. Another symmetry exists within each generation: there are two quarks and two leptons, and a pair of quarks behaves in some ways like a pair of leptons. There could be a direct interaction between quarks and leptons so that leptons can change into quarks and vice versa. If this is so, there must be new particles which carry information between them --- leptoquarks.
Since people have already searched for leptoquarks, we know that unless they are extremely massive, each generation must have its own leptoquark --- a given leptoquark can only interact with the quarks and leptons of a single generation. First generation leptoquarks only interact with up and down quarks, electrons and electron-neutrinos. Second generation leptoquarks only interact with charm and strange quarks, muons and muon-neutrinos and third generation leptoquarks only with top and bottom quarks, taus and tau-neutrinos.
Two experiments at DESY (a laboratory in Germany), H1 and ZEUS, announced in February, 1997 that they saw too many events with unusual properties (there was less than a 1% chance that ``too many'' was actually the right number). One possible explanation for these events is the production of first generation leptoquarks (others include another new type of particle or simple statistical fluctuation!) with a mass of about 200 GeV/c2 (about 200 times the mass of a proton). Although we were already searching for leptoquarks, the announcement by H1 and ZEUS caused us to devote more resources (especially people) to the analysis.
Since the current, exceptional, interest is in first generation leptoquarks, the following discussion will concentrate on them.
Leptoquarks, if they exist, would be produced in collisions between high energy protons and anti-protons (actually between one of the quarks in the proton and one in the anti-proton) at Fermilab as long as the energy released during the collision is sufficient to create a leptoquark pair (remember E = mc2; you need enough E to make two particles of mass m). They could also be produced in collisions between protons and electrons (direct interaction between a quark and an electron) at DESY.
After creation, a leptoquark would split almost immediately into a quark and a lepton and could be identified by looking for these decay products. The quark, which cannot exist alone, immediately ``dresses'' itself by the creation of many quark--anti-quark pairs to form a ``jet'' of particles. Jets are identified by their large energy deposition in a calorimeter (an energy-measuring device). The lepton can be an electron or a neutral, almost non-interacting particle called a neutrino. An electron is identified by the presence of an isolated track in a tracking chamber and energy deposition in the electromagnetic portion of a calorimeter. Neutrinos are identified by ``missing'' energy. Since neutrinos interact very rarely, they escape the detector, carrying energy away. We know how much energy was involved in the collision, so the difference between the energy of the collision and the energy we measure in the calorimeter is the energy carried away by the neutrino.
We searched for the production of pairs of leptoquarks (actually, the production of a leptoquark and an anti-leptoquark) in two ways. For the first way, we assumed that both of the leptoquarks decay into an electron and a quark leading to a final state with two electrons and two jets (and no missing energy). For the second way, we assumed that one of the leptoquarks decays into an electron and a quark and the other into a neutrino and a quark, leading to a final state with one electron, two jets, and missing energy.
Since other interactions can produce events with the same final states, we had to find a way to select events which looked like they came from leptoquarks rather than from other particles we already know about. We did this by making a series of ``cuts.'' We required that the electrons and the jets have a minimum amount of energy and that there be a lot of missing energy (for the final state which should have missing energy). We picked cuts which would give us a small number of expected events (about 0.4 events --- of course we couldn't see 0.4 events, only none or one or two) and as many leptoquark events as possible.
We didn't see any events which could be attributed to leptoquarks. In fact, we didn't see any events at all after making the cuts! The next step is to set a lower limit on the leptoquark mass. To do this, we need to know how often leptoquarks are produced since all we know is that we didn't see any in our data set. The likelihood of producing a pair of leptoquarks depends on their mass --- the more massive a leptoquark is, the less likely it is that a pair of them will be produced in a collision. Theorists have done the necessary calculations and, based on their results, we can set a mass limit which depends on how the leptoquark decays (since we've never seen a leptoquark, we don't know which way it prefers to decay).
If leptoquarks always decay into an electron and a quark, they are more massive than 225 GeV/c2, well above the result from HERA. If they had a mass of 225 GeV/c2, we would have seen about three events. If they decay half the time into an electron and a quark and the other half the time into a neutrino and a quark, they must be more massive than 204 GeV/c2. Again, we would have seen about three events if their mass was 204 GeV/c2.
The plot shows beta (the fraction of the decays which are to electron and quark) vs. leptoquark mass. The curves are the results of the individual analyses (two electrons and two jets or one electron, two jets, and missing energy from a neutrino), and the combination of the analyses. The area to the left of the red curve (the combination) is excluded. Also shown on the plot is an analysis done using only a small part of the data which searched for the case when both leptoquarks decayed to neutrino and quark (two jets and lots of missing energy).
The most reasonable explanation of the HERA events as leptoquarks requires that they decay nearly 100% of the time to electron and quark. Our results show that these ``extra'' events are almost certainly not leptoquarks!
A copy of our paper ``Search for Scalar Leptoquark Pairs Decaying to Electrons and Jets in pbarp Collisions,'' submitted to Physical Review Letters, is available as a preprint from Fermilab-Pub-97-252-E. The paper describing the search for leptoquarks in the electron, jets and missing energy channel is in preparation.
We have also searched for second generation leptoquarks which decay into a muon and a quark or a muon-neutrino and a quark. And for third generation leptoquarks which decay into a tau-neutrino and a b quark. No evidence for any of them yet!
For further information contact Prof. Susan Blessing, Florida State University at blessing@hep.fsu.edu
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Last modified: 15 Aug 97