JLAB12

Responsabili dell'eperimento JLAB12

Marzio De Napoli
Stanza: 345
Telefono: 095/3785331
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Concetta Sutera
Stanza:
Telefono: 095/378
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Attività nelle quali è coinvolta la Sezione INFN di Catania:

Search for Light Dark Matter at Jefferson Lab

According to the standard model of cosmology, Dark Matter constitutes about 84.5% of the total matter in the universe. Unlike normal matter, dark matter does not interact with the electromagnetic force and its existence is only inferred from its gravitational effects on visible matter, radiation, and the large-scale structure of the universe. The most common view is that dark matter is not baryonic, but that it is made up of other, more exotic particles created in the big bang, stable enough to still be around today.

HPS (Heavy Photon Search)

The search for low-mass hidden sectors weakly coupled to the standard model (SM) has received increased attention over the last decade. Hidden sectors are motivated by the existence of dark matter, appear in myriad extensions of the SM, and have been invoked to explain a wide variety of experimental anomalies. A prototypical hidden sector consists of a spontaneously broken “hidden” U(1) gauge symmetry, whose mediator is the “heavy photon” or “dark photon”, A0. The heavy photon interacts with SM particles through kinetic mixing with the U(1)Y (hypercharge) gauge boson. This mixing generates an interaction between the A0 and the SM photon at low energies, allowing dark photons to be produced in charged particle interactions and, if sufficiently massive, to decay into pairs of charged particles like eþe− or hidden-sector states.

The Heavy Photon Search (HPS) is a new experiment at Thomas Jefferson National Accelerator Facility in Newport News, Virginia, searching for the Dark Photon. The HPS experiment measures forward going electrons and positrons produced by electron bremsstrahlung in a thin tungsten target by an intense electron beam of energies between 1.1 and 6.6 GeV. The e⁺ - e^-pairs are identified by a lead tungstate crystal calorimeter, providing the fast trigger, and their momenta and decay vertexes are measured with a very high rate silicon tracker/vertexer situated in a dipole magnet. Heavy photons are thus identified as bumps in the invariant mass spectrum of the electron-positron pairs, and by observing that their decay vertex is separated from the target.

https://journals.aps.org/prd/abstract/10.1103/PhysRevD.98.091101

https://confluence.slac.stanford.edu/display/hpsg/Heavy+Photon+Search+Experiment

BDX (Beam Dump eXperiment)

The best constraints on multi-GeV dark matter interactions are from underground searches for nuclei recoiling off non-relativistic dark matter particles in the Galactic halo. However, these searches are essentially blind to few-GeV or lighter dark matter, whose nuclear scattering transfers invisibly small kinetic energy to a recoiling nucleus. The Beam Dump eXperiment (BDX) at Jefferson Lab proposes an approach to search for dark matter in this lower mass range by producing it in an electron beam-dump and then detecting its scattering in a small ( ̴m³) downstream detector.

A dark photon produced by electron bremsstrahlung in a beam dump could decay into a pair of light dark matter particles (χ). This so called “invisible decay” represents a possible complementary decay channel to the “visible” decay into e⁺-e^-pairs searched by HPS. The kinematics of the decay process is strongly forward peaked with the dark matter particles produced with almost the same electron beam energy. Due to the very low interaction probability with ordinary matter, the dark matter beam will cross undisturbed the beam-dump and the dirt before reaching the detector. On the contrary, most of the produced Standard Model particles will be stopped in the dump itself and in the surrounding shielding.

A fraction of these relativistic dark matter particles then would scatter off the electrons of the detector volume via a dark photon exchange. The χ-proton scattering will produce a slow recoil proton with ̴MeV energy whereas the χ-electron scattering will give rise to an electromagnetic shower generated by a ̴GeV recoil electron. BDX aims to measure both signals by using an electromagnetic calorimeter made by scintillating crystals. The calorimeter will be completely surrounded by layers of active veto detectors and passive shielding to reduce as much as possible the beam-unrelated cosmogenic background that, interacting with the detector nuclei, could produce signals that mimicking the signals expected from the χ-proton scattering. To quantify the background rejection capability of BDX and finalize the experimental setup design, a campaign of cosmogenic background measurements will be performed in 2015-2016 at the Laboratory Nazionali del Sud in Catania. For this purpose the different components of a complete BDX prototype are presently under construction in Catania and Genova.
A future BDX experiment performed downstream of the beam-dump at one of the high intensity JLab experimental Halls, receiving up to 10²² electrons-on-target at 12 GeV of incident energy in a one-year period, will be sensitive to large regions of dark matter parameter space, exceeding the discovery potential of existing and planned experiments by two orders of magnitude in the MeV-GeV dark matter mass range.

https://arxiv.org/abs/1607.01390