Physics

The pion (π0) decays into exactly two photons, whose positions and energies will be detected by CsI crystals, which are connected via optical fiber cables to a computer system. The small branching ratio for this particular decay makes it a significant challenge to create a trigger and data acquisition system that will recognize and store only relevant data. Creating such a system has been the primary focus of the K0TO group at the University of Michigan Physics Department, although some members of the K0TO group are also working on creating Monte-Carlo simulations of the decay.

Kaon decays are known examples of charge parity (CP) violation. CP symmetry refers to the idea that there should be an equal amount of matter and antimatter (particles and antiparticles) in the universe, yet observations indicate that matter dominates. CP violation occurs when the probability of a particle's decay into other particles is different from the probability that its antiparticle will decay into the mirror antiparticles, so the CP violation expected in this experiment should provide some insight into the imbalance between matter and antimatter.

The neutral kaon, K0, is a meson composed of a down quark and an antistrange quark. The K0 particle can have different lifetimes, so KL refers to a neutral kaon with the longer lifetime of 5.11*10-8 seconds. While kaons usually decay into one pion and an electron, two pions, or three pions, where each pion decays into two photons, we are only interested in the rare events where it decays into one pion (and therefore two photons) and two neutrinos, like the event in the diagram above.

We cannot actually see the pion, though, or detect the neutrinos. This is why we use a detector made of an array of CsI crystals, since they can keep track of where each photon goes and how much energy they have during an event. By using this data, we can reconstruct the trajectory and energy of the pion, which will give us more information about what exactly was happening in the event.

In order to save data from only the events we are interested, we must first make a clean KL beam. Techniques for this include shooting protons on the target (see above), using a long beam line to kill off particles with a shorter lifetime (like those from KS particles that decay after a mere 8.95*10-11 s), absorbing core photons and sweeping away charged particles, and shaping collimators to minimize halo particles generated by scattering off the collimator surface. We also need a high acceptance detector, a hermetic veto for photons and charged particles with photon detection inefficiency of about 10-4, and a pencil size beam for maximum calorimeter acceptance and also to constrain the KL decay vertex.

After taking data, we will need to reconstruct the KL → π0νν kinematics, using the information stored about the energy and position of each photon, as detected by the CsI crystals. By storing information from events where only two photons were detected and then calculating the angle between the photons, we can use this knowledge, along with the assumption that the KL was on the beam line, to determine the Zvtx of the pion. We then use this to identify kaon backgrounds, which are other possible decays that looked like what we wanted but were actually different; we will know this by figuring whether the energy of a photon and its trajectory angle make logical sense, or if there is a larger two photon energy ratio than we expect, among other things. Finally, we will reduce the halo neutron background, and will then have a data set that is ready for further analysis.

 

 

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