by Mike Albrow, January 29, 2007

[Search for New Physics Using High Mass Tau Pairs from 1.96-TeV $ p\bar{p}$ Collisions]

Search for High Mass Tau Pairs

Electrons and their antiparticles, positrons, are fundamental particles with heavier partners, muons ($ \mu$) (about 206.8 times heavier) and taus ($ \tau$) (about 3477 times heavier). These are all called ``leptons". Nobody understands this replication, but it happens with quarks too ... the light quarks (called ``up" and ``down") that combine to make protons and neutrons have heavier partners called strange, charm, bottom and top. We hope that one day we will understand the pattern of particles and their masses. We hope also to understand the pattern of forces and their strengths. The fundamental forces we know are electromagnetism, the strong and weak nuclear forces, and gravity. We normally consider that gravity is too weak a force to affect in any way experiments we can do with fundamental particles, but recent ideas that space may have more than 3 dimensions if you look on very small scales could change that. Quantum theory tells us that the forces are all carried by particles: photons (particle of light) for electromagnetism, ``gluons" for the strong force between quarks, and a relatively heavy triplet of particles called ``W- and Z-bosons" for the weak force. ``Relatively heavy" means about 85 and 97 $ \times$ the mass of a proton, respectively.

Perhaps there are more yet-undiscovered forces? If so they will have yet-undiscovered carrier particles. Finding them would make great progress towards a more complete theory. For example there could be a heavier version of the Z-boson, which would imply a new ``superweak force" acting between quarks and leptons. All the force carriers mentioned so far have one unit of ``spin" (angular momentum), except the graviton (if it exists) which would have 2 units. But it is expected, in our best theories of particle interactions, that there should also exist at least one fundamental force carrier with no spin (spin = 0) called a Higgs boson. In some theories there are more than one such particle, and they should couple more strongly to heavier particles. They could be produced in proton-antiproton collisions at the Fermilab Tevatron in a quark-antiquark annihilation, and then some would decay to a pair of leptons, mostly to a pair of taus ( $ \tau^+\tau^-$) because they are heaviest. The taus disintegrate after travelling a short distance (typically 1 mm) usually into an electron or a muon or one or three pions (one of the many strongly interacting particles), always together with an uncharged partner to the tau, a ``tau neutrino" ( $ \nu_{\tau}$) which we cannot detect.

In this study we selected very rare collisions, roughly one in $ 10^{12}$, that had a pair of taus with very large momentum at large angles to the collision axis. We then said: ``Supposing this pair came from the decay of a massive particle, what mass would that particle have had"? We plotted the result and saw a peak near the mass of the Z-boson, from the deacay $ Z\rightarrow\tau^+\tau^-$. A new more massive particle might have shown up as another peak, but we did not see one. We did find 4 events with masses above 120 GeV (the Z mass is 91.2 GeV) but we expected, without any new physics, $ 2.8 \pm 0.5$ which is in good enough agreement. As we did not see any evidence for a new peak, we are able to rule out (with a certain degree of confidence) some theoretical possibilities. For example a partner to the Z-boson, that coupled in the same way to taus, is very probably (our confidence level can be quantified and is 95%) heavier than 400 GeV. Even though this study did not discover a new particle and/or a new force, the result is still important as it rules out certain theoretical possibilities. If theorists did not have experimental constraints like these they could waste much time on unlikely (or impossible) scenarios.

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Plain English Summary: High Mass Tau Pairs

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The translation was initiated by Ben Kilminster on 2007-01-29

Ben Kilminster 2007-01-29