There is strong astrophysical evidence that most of the matter making up the Universe is not “ordinary matter” (i.e. it is not constituted by particles described by the Standard Model). The most popular hypothesis among physicists is that it consists of a new kind of matter, electrically neutral and stable, which does not interact (at all or very weakly) with ordinary matter and therefore does not produce electromagnetic radiation, i.e. it is “dark”.Many theoretical models envisage various types of dark matter, among which of particular interest is the possibility that there is a particular type of particle, similar to the carriers of interaction in known forces (such as the photon for electromagnetic force), which acts as a “bridge” or “portal” between the “dark” particles of the new type of matter and the ordinary matter of the Standard Model, through a new, very weak force. One way to study this type of “dark” photon is searching its production in the annihilations of electrons and anti-electrons (positrons) in photon pairs, one of which ordinary and one dark. PADME is a new experiment aiming to seek this type of event through the accurate reconstruction of the missing mass in the balance between the initial state, constituted by the electron-positron pair (using the positrons of the Beam Test Facility) and the final state in which only the ordinary photon is detected.
For this purpose, the main detector is a so-called calorimeter, i.e. a device able to measure the electromagnetic energy deposited with great precision, at the same time ensuring determination of the direction of the particles hitting it. In order to produce an optimal number of interactions that produce dark photons compared to the background constituted by events due to “normal” electromagnetic interaction, a positron beam must be directed, with great accuracy (as regards the direction of incidence and the point of impact) towards a target consisting of the electrons of the atoms of a very thin and light material: in our case the carbon of a synthetic diamond which in turn can be used as a detector. All the experimental equipment must be in a vacuum in order to minimize interactions with air molecules and a magnetic field must at the same time sweep the beam of positrons that have not produced annihilations away from the calorimeter and allow the events in which the positrons interact with the target producing only one photon by radiation (so-called “breaking” or Bremsstrahlung radiation) to be excluded. The magnetic field of the right intensity and with the large volume required for PADME is produced by a spare magnet made available by CERN in Geneva.
Research material “PADME: how it works”
Article from the magazine Asimmetrie “The dark side – Hunting the dark matter” by Mauro Raggi
Translation by Camilla Paola Maglione, Communications Office INFN-LNF