Leptoquarks born at DESY?

17 Jun 2002

At the HERA collider at the DESY lab in Hamburg, Germany, 820-GeV protons smash into 27.5-GeV electrons or positrons. At these very high energies, a positronessentially scatters not from the proton as a whole but from individual quarks inside the proton. Two important parameters help to characterize such interactions: x,the fractionof the proton's momentum carried by the struck quark, and Q^2, the square of the momentum transferred between proton and positron.

The two large collaborations at HERA, the H1 and Zeus groups, have searched their three-year data inventory for a class of events at high x and high Q^2. Bothgroups report an excess of very-high-Q^2 events compared to predictions based on the standard model of particle physics. For Q^2 events above a value of 15,000GeV^2, the H1 group found 12 events, against an expected background of 4.7 events. Meanwhile, the Zeus group observed that their data agreed with theory forQ^2 up to15,000. Above that, however, agreement worsens; above a Q^2of 35,000, where only one-tenth of an event would be expected,2 events were recorded;one of them represents the highest Q^2(46,000) value ever observed for a lepton-proton interaction.

If confirmed by more data, these surplus events at high Q^2 could be a sign of some new phenomenon beyond the standard model. In one scenario, the electron andquark fuse into a "leptoquark," a particle (with a mass around 200 GeV/c^2) that would, among other things, help to facilitate proton decay.