Alex Wright came to Princeton in September 2009 after completing his PhD at Queen's University, in Ontario, Canada, where he worked on the Sudbury Neutrino Observatory and SNO+ solar neutrino experiments.

At Princeton, Alex has continued to study neutrinos as part of the Borexino collaboration. Borexino is a liquid scintillator based solar neutrino experiment located in the Gran Sasso underground laboratory in Italy. The experiment, which was designed to study the lower energy solar neutrinos, has already produced the first direct measurement of the flux of ⁷Be solar neutrinos and the lowest energy measurement to date of the ⁸B solar neutrinos. The collaboration has also recently published a measurement of the flux of geo-neutrinos, which are anti-neutrinos emitted during the decay of radioactive isotopes within the earth. Borexino continues to take data, with the goal of obtaining more precise measurements of the ⁷Be and ⁸B fluxes and perhaps detecting other low energy neutrino fluxes as well. These measurements will help us to better understand both the mechanism behind solar neutrino oscillations and the nuclear fusion reactions that power the sun.

Alex is also interested in the search for dark matter, an as yet undetected non-luminous form of matter that is thought to make up almost one quarter of the universe. One well motivated group of theories predict that the dark matter should be comprised of relatively massive elementary particles which interact with normal matter only through gravitation and the weak force (which is also responsible for phenomena like nuclear decay). If this theory is correct, these Weakly Interacting Massive Particles, or WIMPs, should be detectable via the nuclear recoils produced by their occasional interactions with matter.

Here at Princeton Alex is working, in conjunction with the other members of the Particle Astrophysics group, to develop a new experiment to search for signs of these WIMP interactions. The new detector, called DarkSide (the proposal for the DarkSide experiment can be found here), will search for WIMP interactions in a target of liquid argon. DarkSide will be a "two-phase" detector, meaning that it will contain both gaseous and liquid argon. This will allow the experiment to measure each nuclear recoil event in two ways, by separately detecting the scintillation light and ionization electrons produced by the recoiling nuclei. The sensitivity of the experiment will be enhanced through the use of argon that has been naturally depleted in ³⁹Ar, a radioactive isotope that provides a significant background in experiments which use atmospheric argon, and by the development of an active neutron shield based on boron-doped liquid scintillator. The background rejection ability afforded by the use of the parallel detection channels in the two-phase detector, coupled with the use of depleted argon and the highly efficient neutron veto should give DarkSide the lowest background rate yet recorded in a dark matter detector. This, in turn, will set the stage for larger, more sensitive experiments and, hopefully, for the detection of WIMP dark matter.

Selected Publications

• G. Bellini et al. (The Borexino Collaboration).
  First Evidence of pep Solar Neutrinos by Direct Detection in Borexino.
  Phys. Rev. Lett. 108:051302 (2012). Preprint. 
   
• B. Aharmim et al. (The SNO Collaboration).
  Combined Analysis of all Three Phases of the Solar Neutrino Data from the Sudbury Neutino Observatory.
  Preprint. Submitted to Phys. Rev. C.
   
• B. Aharmim et al. (The SNO Collaboration).
  Measurement of the v_e and Total ⁸B Solar Neutrino Fluxes with the Sudbury Neutrino Observatory Phase-III Data Set.
  Preprint. Submitted to Phys. Rev. C.
   
• B. Beltran et al.
  A Monte Carlo Simulation of the Sudbury Neutrino Observatory Proportional Counters.
  New J. Phys. 13:073006 (2011). Preprint.
   
• G. Bellini et al. (The Borexino Collaboration).
  Precision Measurement of the ⁷Be Solar Neutrino Interaction Rate in Borexino.
  Phys. Rev. Lett. 107:141302 (2011). Preprint.
   
• A.Wright, P. Mostiero, B. Loer, and F. Calaprice.
  A Highly Efficient Neutron Veto for Dark Matter Experiments.
  Nucl. Inst. Meth. A 644:18-26 (2011). Preprint.
   
• B. Aharmim et al. (The SNO Collaboration).
  Low-Energy-Threshold Analysis of the Phase I and Phase II Data Sets of the Sudbury Neutrino Observatory.
  Phys. Rev. C 81:055504 (2010). Preprint.
   
• B. Aharmim et al. (The SNO Collaboration).
  An Independent Measurement of the Total Active ⁸B Solar Neutrino Flux Using an Array of ³He Proportional Counters at the Sudbury Neutrino Observatory
  Phys. Rev. Lett. 101:111301 (2008). Preprint.