Modified image from ATLAS Supersymmetry searches page

How will supersymmetric particles be searched for?

Almost all theories involving supersymmetry agree on one thing: the lightest of the supersymmetric particles will be either completely stable or very long-lived, and, moreover, it interacts very weakly with ordinary particles. Since particle detectors are built from ordinary particles, they will not be able to capture the lightest supersymmetric particle.

The presence of such a particle, however, can be easily noticed indirectly – from the imbalance of the transverse momentum of the registered particles. High-energy protons collide along the axis, their total transverse momentum is practically zero, which means that the total momentum of all generated particles – both those that the detectors see and those for which the detectors are transparent – will also be close to zero. Therefore, if the transverse momentum of all measured particles differs markedly from zero, it means that one or several particles were born in the collision, which carried away the missing transverse momentum.

The analysis of events with a “missing” transverse momentum will be one of the key points of the supersymmetry search strategy. At first, such events will simply be counted, then it will be checked whether such events can be described by the Standard Model (after all, the neutrino is not registered by the detector and also carries away the transverse momentum). Finally, if it turns out that such events are not described by the Standard Model, it will be tested which predictions of which variants of supersymmetric theories will best describe the data.

Fig. 1. Possible scheme for the decay of a heavy squark (quark superpartner). As a result of a chain of decays, five quarks and a stable lightest supersymmetric particle are obtained, which does not leave a trace in the detector. Image from www-d0.fnal.gov

Since squarks and gluinos – superpartners of quarks and gluons – also feel strong interactions, they will be born among superparticles most often. Therefore, a process of the form “several hadronic jets plus a missing transverse momentum” will be one of the clearest manifestations of supersymmetry (see Fig. 1).

Fig. 2. An example of the process of production of a pair of supersymmetric particles that decay into two hadronic jets, two photons, and two gravitinos (superpartner of the graviton), which carry away the transverse momentum. Image from www-d0.fnal.gov

In addition to looking for events with missing transverse momentum, physicists will pay attention to any combination of particles that are unlikely to be produced in the Standard Model. For example, in some versions of supersymmetric theories, the characteristic production of several leptons or several photons is predicted (an example of such an event is shown in Fig. 2). All such deviations from the Standard Model will be carefully checked for “involvement” in supersymmetry.

Studying the Higgs bosons of supersymmetric models

Even if the superpartner particles of ordinary particles turn out to be too heavy and cannot be directly produced in the collider, physicists will still have the opportunity to test the supersymmetry predictions for the Higgs bosons. Unlike the Standard Model, supersymmetric theories have several Higgs bosons with different properties, including electrically charged ones. These particles are unstable, so they will not be looked for directly in the detector, but through the traces of their decay into ordinary particles. One way or another, the discovery of the Higgs boson (or bosons) and a careful study of their properties (production cross section, preferred decay options) will be an important stage in the search for supersymmetry.

How the results will be presented

There are many different variants of supersymmetric theories, and each of them has many free parameters. When experimenters look for supersymmetry, they certainly won’t try to test the whole infinite set of possible theories. Instead, a few simple reference cases of supersymmetric models are selected and (inconsistent) data validated against them.

One such model is mSUGRA, a minimal supersymmetric model that includes gravity. For the experiment, its most important parameters are the masses of superparticles, specifically, two quantities: the mass of superpartners of fermions (m0) and the mass of superpartners of gauge bosons (m1 / 2) at the moment of supersymmetry breaking (the masses of real superparticles do not differ too much from these parameters). Thus, different models will correspond to different points on the plane (m0, m1 / 2).

Fig. 3. Plane of parameters of the simplest versions of supersymmetric models. Each point on this plane corresponds to some variant of the model. An asterisk marks one of the “reference” parameter sets. Dashed lines show models with various masses of squarks and gluinos. The filled areas correspond to those areas of parameters that have already been checked and closed in previous colliders. Modified image from ATLAS Supersymmetry searches page

In fig. 3 shows this plane in the most interesting mass range. Searches for supersymmetry at previous colliders (LEP and Tevatron) have already covered small regions of mass, but only the LHC will be able to scan the entire imaged area. Similar planes of parameters are drawn for other variants of supersymmetric models.

additional literature

ATLAS Supersymmetry searches and CMS Supersymmetry Physics Results – pages with all public results of ATLAS and CMS collaborations on supersymmetry search. J. L. Feng, J.-F. Grivaz, J. Nachtman. Searches for supersymmetry at high-energy colliders // Rev. Mod. Phys. 82, 699-727 (2010); the text is available in the e-print archive (arXiv: 0903.0046). L.N.Smirnova. Investigations of supersymmetry in the ATLAS detector // chapter from the manual devoted to the ATLAS detector.”

The detector is the most important part of the accelerator experiment; this is the “eyepiece of the microscope” with which physicists can see the structure of the nucleus and elementary particles.

Inside the detector, particles from colliding beams collide and generate new unstable particles. They immediately disintegrate into more stable particles, which scatter in all directions. These decay products fly through the detector and leave their traces in it – for example, they ionize matter in their path and make special scintillation crystals glow. From these traces, physicists find out what kind of particles they were, at what angle and with what energy they flew, what charge and mass they had. Various components of the detector, located in layers around each other, help to collect all this information.

The detector is the largest and most complex setup on an accelerator. Colliding beams collide inside the detector, and in these collisions a lot of unstable particles are born, which scatter in all directions. The task of the detector is to track these particles and measure their charges, impulses and energies. Having learned all these parameters, experimenters will be able to determine what kind of subnuclear process gave rise to them, which means they will be able to check the calculations of theoretical physicists. We can say that the detector is the very “eyepiece of the microscope” with which a person can see the deepest structure of our world.

Vertex detector

The vertex detector is a very compact detector that is located close to the vacuum tube, very close to the collision of the particles. Its goal is to reconstruct as accurately as possible the first centimeters of the trajectories of the emitted particles and find their “tops”, that is, the points in space where these particles were born. This information is especially useful when a large number of particles are produced – it can be used to find out which of them are decay products of unstable intermediate particles, and which were immediately born in a collision.

The vertex detector looks like a thin layer of semiconductor wafers with many tracks to drain the charge. When a charged particle pierces it through and through, in each layer, in the place where the particle passed, a cloud of electrons, knocked out of the semiconductor, appears and begins to move. Microelectronics collects the generated charge and makes it possible to determine the points of passage of a particle with high accuracy and very quickly. The spatial trajectory of the particle is then reconstructed from several such points.

Track detector

The next is a track detector, about a meter in size. It measures how the trajectories of the emitted particles (“tracks”) bend in the magnetic field that penetrates the detector. Knowing the radius of curvature of the trajectory, you can calculate the momentum of the particle. Drift cameras are often used as track detectors. Thin wires under tension are stretched in them with a fine pitch. The charges generated by the passing particle are deposited on the nearest wire, informing the recording equipment where the particle has flown. From signals from many wires, the trajectory of the particle is formed.

If several particles were born in a collision, their trajectories are usually easily reconstructed. But when hundreds of particles scatter from the vertex (this happens, for example, in a collision of heavy nuclei), then a real jumble of hundreds of arcs appears in the track detector. To understand what happened at the moment of collision, it is necessary to restore all trajectories to a single one and find out which arc belongs to which particle. This can be done thanks to specially developed complex algorithms for processing “raw” data.

Calorimeter

Next are multilayer calorimeters – detectors that measure the energy of particles. Knowing the energy of a particle and its momentum, one can use the formulas of relativistic dynamics to calculate its mass – and therefore, find out what type of this particle.

The energy of a particle can be measured with good accuracy if it is completely absorbed in the substance. Part of this energy will be spent on the creation of light quanta, which can be captured with the help of very sensitive photodetectors – photomultipliers – and with this help restore the energy of the original particle. Unlike the vertex and track detectors, which have a very weak effect on the particle, the calorimeter completely absorbs it. Therefore, the calorimeters must be located in the outer layers of the detector.

Next: How the properties of particles are studied at an accelerator

Pre-accelerator

First of all, the particles must be created and then accelerated to a low energy. All this is done in a small pre-accelerator. Electrons and protons are extracted from ordinary matter, for example, using an electric field or ionization. The particles are “pulled” by the electric field, accelerated under its action, and then fall into a small synchrotron called a “storage”. Particles accumulate in it, and when there are enough of them, they are “injected” into the main accelerator. There, experiments begin with them, and in the preliminary accelerator the particles are again accumulated from scratch. Each such cycle takes several hours.

If it is necessary to carry out experiments with particles that are absent in ordinary matter (for example, antiprotons), then the scheme becomes more complicated. First, as before, protons are received, then the proton beam is directed to a special target (converter). When protons collide with target nuclei, a jumble of particles is born, among which there are antiprotons. With the help of magnetic fields, these antiprotons are extracted and then sent to the storage ring.

Beam “taxiing” system. Swivel magnets

When physicists talk about the movement of particles inside an accelerator, they call it collectively: a particle beam. This beam is not smeared along the entire length of the tube, but is collected in separate bunches of particles. Usually a clot is a long (several centimeters or tens of centimeters) and thin (tens of microns) “needle”, consisting of particles flying alongside.

According to Newton’s first law, particles in a free state tend to move in a straight line. Therefore, in order to keep them inside the ring accelerator, their trajectory has to be wrapped using a magnetic field. For this, special rotary magnets are installed along the accelerating ring at some distance from each other. As a result, the trajectory of the beam becomes similar to a rounded polygon: at its vertices, the beam rotates by a small angle, and then flies to the next magnet in a straight line. It is on the straight sections that all the rest of the equipment is installed.

The higher the energy of the particles, the more difficult it is to wrap them in an arc of the desired radius and the more powerful bending magnets have to be used. The LHC uses bending magnets with an induction of 8 Tesla (about 100,000 times stronger than the Earth’s magnetic field). Such a strong field can be obtained only in superconducting electromagnets and only at very low temperatures. As a result, the entire installation (and this ring with a perimeter of 27 km!) Has to be cooled to very low temperatures (below 2 K). This once again emphasizes that the accelerating ring is not just a “pipe with a magnetic field”, but a very complex technical structure.

The magnetic field in bending magnets is not uniform; https://123helpme.me/a-tree-grows-in-brooklyn/ it is slightly weaker in the inner part and slightly stronger in the outer part of the arch. This is done in order to bring back a beam that has slightly lost its optimal orbit.

Monitoring system and “emergency exit” for the beam

Despite the fact that the particle beam contains not so many particles (the total mass of all particles in the beam is usually nanograms or less!), It can store enormous kinetic energy. For example, the proton beam at the LHC has an energy comparable to the kinetic energy of a flying jet plane. If control over the beam is lost, it breaks free and burns through the wall of the vacuum tube, accelerator equipment, and even multi-meter concrete walls. Therefore, a tracking system for the beam position is absolutely essential for the safe operation of the accelerator.

The tracking system in real time controls where exactly inside the vacuum tube the beam trajectory passes at a given moment. If it deviates slightly from the axis of the pipe, the magnetic fields try to align its position. If the deviation becomes critical, then a “beam drop” occurs – a special very fast magnet turns on abruptly and takes the beam out of the accelerator ring through a special “emergency exit” into the distance, where a huge concrete target absorbs all its energy. Usually, it is enough to make one emergency exit for each of the two colliding beams: the beam instability does not develop so quickly, and the beam will have time to reach its exit during this time.

The regular discharge of the beam also occurs in the normal mode. Flying in the accelerator, the beam gradually loses particles – some are eliminated in collisions in the detector, some are simply scattered by the residual gas molecules in the vacuum chamber. Every few hours, when the beam weakens several times, it is “dropped” onto the same target standing at a distance, and a new portion of particles is injected into the accelerator.

Accelerating section

When the particles have just been “injected” from the preliminary into the main accelerator, they still have too little energy, and they need to be accelerated. This is done in a special accelerating section – the klystron. The klystron is a special vacuum chamber of a bizarre shape, vaguely resembling an empty microwave.