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Enhanced NeUtrino BEams from kaon Tagging

Periodic Reporting for period 4 - ENUBET (Enhanced NeUtrino BEams from kaon Tagging)

Reporting period: 2019-09-01 to 2021-02-28

"The fact that our “world” is composed of matter and not of anti-matter can be thought as the basis for our own existence. In spite of its importance, the profound reason of why this happened in the Universe and especially in the quantitative terms that we observe, remains one of the big open questions of modern science. The international community of physics is currently engaged in the construction of large experiments in Japan and in the US to study with high precision subtle differences between the transformations of muon neutrinos to electron neutrinos and those involving the corresponding anti-particles (anti-νμ → anti-νe). This is technically know as the search for ""leptonic CP violation"". The observation of differences in these two processes would mark a breakthrough in particle physics: the dominance of matter over anti-matter that we observe today could actually come from the behavior of primordial neutrinos produced just after the Big Bang. ENUBET is contributing to this world-wide effort by redefining the quality of current artificial neutrino sources (beams). It offers a tool that is capable of measuring all the details of the interactions of electron neutrinos with ordinary matter with an unprecedented level of precision. This project opens a new frontier in neutrino physics providing physicists with a new investigation tool: an intense source of electron neutrinos (νe) endowed with a ten-fold increase in precision. Succeeding in measuring positrons associated with the production of electron neutrinos in decay tunnels: this is the challenge that ENUBET is tackling with a strong team of experts composed of physicists from Padova University, INFN and other European institutions. Decay tunnels are tough environments for those aiming at installing detectors there: particle fluxes are extraordinarily high reaching up to a million particles per second per squared centimeter. The “intelligent” decay tunnel that ENUBET is proposing is a large detector based on innovative Silicon photo-sensors, potentially able to overcome the difficulties (radiation resistance, response speed, costs) that were making such a program wishful thinking just a few years ago."
During the first half of the project the team working on the ENUBET project has developed particle detectors suitable for the measurement of positron particles (the anti-particle of the more familiar electron). These particles are emitted at large angles when electron neutrinos are produced from the decay of unstable particles named Kaons. Measuring positrons allows “counting” the number of the elusive neutrinos. This is an enormous advantage because in this way the intensity of the neutrino beam can be known with an unprecedented precision. The detectors developed by the ENUBET team are characterized by a high degree of compactness which allows to monitor the production of positrons in a very uniform way all along the production region. This region extends for several tens of meters: it becomes compulsory to choose a technology with a good performance/cost ratio. The solution envisaged within the project is to use the so-called “shashlik” calorimeters. These are stacks of iron and plastic slabs where organic molecules are dissolved having the property of emitting light when traversed by charged particles. This light allows the detection of positrons and other particles (pions) that are also abundantly present in the decay region. Positrons are absorbed quite effectively by iron while other particles are, in general, more penetrating. Thanks to this characteristics the detectors being developed will enable the monitoring of positrons even in presence of large amounts of disturbing spurious signals from other particles. In the first half of the project we have developed several prototypes of these detectors that have been tested with particles at the CERN PS accelerator in Geneva. We have developed two options which are distinguished by the way in which the light produced by the passage of particles is brought to sensors (called Silicon-PhotoMultipliers) that convert it into electrical signals. One of these schemes has the advantage of being particularly compact and elegant but exposing the sensors to a higher risk of premature aging. The second is safer and is based on a more traditional layout and allows larger safety margins.

In addition to this intense campaign of detector development and tests we have studied a magnetic layout capable of guiding the unstable particles that originate the neutrinos inside the region where the monitoring occurs. This “magnetic” path is currently about 27 m in length and consists of elements for focusing (called quadrupoles) and bending (dipoles). Among all the particles produced when protons are impinged of a graphite target only a subset of “useful” particles are selected having the right energy to produce the neutrinos we are interested in. A narrow and collimated beam of these particles is then guided and sent into the decay tunnel. Having such a sharp beam is mandatory since the instrumentation would not tolerate the particle intensities that would be created if the particle beam would not be properly “contained”. After having worked out and studied the most promising layout we are optimizing it to reduce the contributions from undesired particles disturbing the measurement of positrons (“backgrounds”). This is achieved through a careful study of absorbing materials along the system and the parameters of the magnets (geometry, currents).

Another interesting output of our studies has been the development of a new procedure for extracting protons from particle accelerators. This procedure has been tested at the CERN-SPS accelerator with protons having an energy of 400 GeV. By acting on the currents of focusing magnets in the accelerator ring according to a particular scheme it was possible to extract the protons in a sequence of short pulses of a few thousands of a second with a repetition of about ten bursts per second. This peculiar time structure would be useful for the ENUBET neutrino beam but we have realized that the method that has been worked out could be useful for more experiments.
The detectors developed in the context of ENUBET are very innovative. They implement the concept of converting the light produced by the passage of particle right “in situ” i.e. basically where the positrons are absorbed in the iron. The silicon-based devices converting light into electric signals are directly located in front of optical fibers trapping the light emitted by the plastic scintillators and mounted on a printed circuit board. This is an elegant solution allowing to have a very compact layout without dead regions. This layout proved to be successful. At the end of the project we are planning to build a detector (demonstrator) that could be the first part of a larger setup allowing to create a high precision neutrino beam of new generation. The idea of “monitored” neutrino beams on the other hand is challenging and the team is studying it in depth to ensure that all possible difficulties (high particle rates, unwanted backgrounds) can be dealt with. In particular a detailed analysis is being done to verify that all factors of uncertainties can be controlled at a level allowing an overall error on the intensity of the produced neutrinos at the 1 % level. This is done thanks to the measurements that we have collected already and to the simulations and reconstruction software that has been developed within our team. In parallel we are engaged in an effort to communicate this new scientific approach by participating in a large number of international conferences and events involving the community of involved scientists. At the end of the project we will produce a technical design report of a facility capable of overcoming the serious limitations of present neutrino beams including the physics reach and a cost estimate with the goal of involving a larger community of scientists and build a full scale experiment. This experiment could provide an accurate measurement of the probability with which neutrinos interact with matter and boost the opportunity for understanding the nature of matter-antimatter using neutrino beams in the near future.