NA62

The NA62 experiment is an experiment at the SPS accelerator of the European nuclear research center CERN , which is currently in the development phase. The aim of the experiment is to investigate the extremely rare decay of the kaon into a pion and two neutrinos ( ). The first planned recording of data is planned for 2014. At this point in time, it will be the first experiment in the world to investigate kaon decays with this extremely small decay probability (order of magnitude 10 ⁻¹⁰ ). The spokesman for the NA62 experiment is Augusto Ceccucci . ${\ displaystyle K ^ {+} \ rightarrow \ pi ^ {+} \ nu {\ bar {\ nu}}}$

aims

The investigation of the kaon decay , because of its rarity, offers an excellent opportunity to test the Standard Model of particle physics with high precision. Within the framework of this model, the probability of this decay is predicted very precisely. If the result of the measurement deviates from the predicted result, this is a signal for physics beyond the standard model ("New Physics"). In addition, the matrix element | V _td | of the CKM matrix can be determined with high accuracy. ${\ displaystyle K ^ {+} \ rightarrow \ pi ^ {+} \ nu {\ bar {\ nu}}}$

construction

overview

NA62 is a so-called “fixed target” experiment. This means that accelerated particles hit a target at rest. At NA62, accelerated protons from the SPS accelerator hit a resting beryllium wire. The collision produces kaons, among other things, which disintegrate along the approximately 275-meter-long experiment. The NA62 experiment is located underground in the North Area (hence the abbreviation NA ) of the SPS accelerator.

The great challenge of the experiment lies in the extreme rarity of the decay to be examined. 10 ¹³ kaons have to be generated in order to measure 100 of the desired decays. This number of kaons corresponds to the number of stars in about 50 galaxies of the type of our Milky Way. The measurement assumes an efficient suppression of all other possible decays of the kaon. This so-called background is suppressed by a factor of 10 ¹² , i.e. H. filtered out. Various detector systems work together at NA62.

Detectors

The individual sub-detectors of the NA62 experiment are being developed and built by working groups across Europe. In Germany, the working group “Experimental Particle and Astroparticle Physics” (ETAP) from Johannes Gutenberg University Mainz is involved. Completed detector components are currently being installed at CERN. The following is an overview of the detectors used and their function.

Identification of the kaon and determination of the momentum

When the protons collide with the beryllium wire, the kaons represent only a small part of the particles produced. Essentially, more protons and pions are created. In order to be able to assign measured daughter particles to a kaon, the kaons must be identified after they have been generated. This is achieved with the so-called CEDAR. The CEDAR is a tube filled with hydrogen. Particles that pass through the hydrogen emit Cherenkov radiation . This radiation is reflected by mirrors at the end of the CEDAR onto a screen. The arrangement of the diaphragm and mirrors is chosen so that only Cherenkov radiation generated by kaons falls on the diaphragm.

To study the decay, it is necessary to know the momentum of the kaon. This is determined with the GigaTracker (GTK). This sub-detector consists of three units (GTK 1–3), between which there are magnets. With the help of the curvature of the orbit caused by the Lorentz force , the impulse of the kaon can be determined. The individual stations each consist of 18,000 silicon detectors with the dimensions 300 µm × 300 µm.

Identification of the pion and determination of the impulse

Only the pion from the desired decay of the kaon into a pion and two neutrinos can be seen in the following detectors. Due to their physical properties, the neutrinos cannot be measured directly in this experiment. The momentum of the pion and other charged particles are determined with the aid of a spectrometer consisting of four straw detectors and a magnet. Two of the straw detectors are located in front of the magnet and determine the position of the charged particles. The magnet deflects the charged particles and the position is determined again in the following straw detectors. As with the measurement in the GigaTracker, conclusions can be drawn from the curvature of the path about the momentum of the charged particles. A ring imaging Cherenkov detector (RICH detector) is located behind the spectrometer. The 17 meter long detector is filled with neon gas. The Cherenkov effect is again used to identify the pions. The resulting radiation cone is reflected by mirrors at the end of the detector onto 2000 photomultipliers , which read out the light. The particle can be identified from the size of the measured rings.

Veto detectors

In order to filter the desired decay out of all possible decays of the kaon, detectors must be able to recognize the subsurface. These detectors are called veto detectors because they recognize certain decays and as a result these are not recorded. Two veto systems are used in the NA62 experiment, the photo veto and muon veto system. Since the desired decay does not emit photons or muons, such decays can be discarded if one of these particles is discovered.

Photo veto detectors

The task of the photo detectors is to detect decays that produce one or more photons. Various detectors are used to ensure complete room coverage. The Large Angle Veto (LAV) consists of twelve stations that are positioned along the decay path of the kaons. Lead glass is installed in the stations , in which the photons are detected by Cherenkov radiation. The LAV is responsible for large angles of radiation of the photons from 8.5 to 50 mrad. The mean angular range from 1 to 8.5 mrad is covered by the liquid krypton calorimeter. This detector was already used in the previous NA48 experiment . The liquid krypton calorimeter is a homogeneous calorimeter that can register electromagnetic showers caused by photons completely in the cylindrical volume with an area of 5.3 m² and a depth of 127 cm. The small-angle calorimeter (SAC) is located at the end of the experiment to detect photons that are emitted in a very small solid angle (less than 1 mrad). A magnet in front of the SAC deflects all charged particles so that the uncharged photons hit the detector and can be registered there.

Muon Veto Detectors

The muon veto system consists of three detectors that are positioned one behind the other at the end of the experiment setup, but only in front of the SAC. The first two muon veto detectors (MUV1 and MUV2) are hadronic calorimeters , which are supposed to distinguish pions from muons based on the size of the particle showers generated . The calorimeters consist of iron and scintillator layers . Charged particles generate light in the scintillators, which is read out. The scintillator layers are segmented so that the size of the particle showers generated can be determined. The third muon veto detector (MUV3) is separated from the first two muon veto detectors by an 80 cm thick iron block. Muons can pass this iron block, while pions interact hadronically and get stuck in the block. The MUV3 consists of a layer of scintillators that are read out by photomultipliers. Every signal seen is identified as a muon and the decay is not saved for further analysis.

Individual evidence

↑ Joachim Brod, Martin Gorbahn: Electroweak corrections to the charm quark contribution to . ${\ displaystyle K ^ {+} \ rightarrow \ pi ^ {+} \ nu {\ bar {\ nu}}}$ In: Physical Review D. 78, No. 3, 2008, p. 034006, doi : 10.1103 / PhysRevD.78.034006 .
↑ Wikipedia article: Milky Way . Retrieved April 6, 2012, 3:15 pm

literature

Proposal to measure the rare decay K ⁺ → π ⁺ν ν at the CERN SPS (PDF; 2.0 MB) CERN . June 11, 2005. CERN-SPSC-2005-013. Retrieved September 28, 2009.
NA62 Technical Design . CERN . December 2010. CERN NA62-10-07. Retrieved April 29, 2011.
Andreas Winhart: A precision test of lepton universality in K → lν decays by NA62 at CERN (PDF; 4.4 MB) In: EMG Seminar . July 15, 2009. Retrieved September 28, 2009.

Web links

[1] Joachim Brod, Martin Gorbahn: Electroweak corrections to the charm quark contribution to . ${\ displaystyle K ^ {+} \ rightarrow \ pi ^ {+} \ nu {\ bar {\ nu}}}$ In: Physical Review D. 78, No. 3, 2008, p. 034006, doi : 10.1103 / PhysRevD.78.034006 .

[2] Wikipedia article: Milky Way . Retrieved April 6, 2012, 3:15 pm