# Muon Collider: a window to the future

### Maria Enrica Biagini, Donatella Lucchesi

1 The proposed future collider projects

The last decade has seen the consolidation of some fundamental open questions in our understanding of the Universe, such as the nature of dark matter, the origin of the observed matter-antimatter imbalance, the role of gravity with the discovery of the gravitational waves and the neutrino oscillations. Particle accelerators have been and currently are one of the best tools to investigate the new phenomena confirming theoretical models, like the standard model, or disproving them. The study of the Higgs boson properties at the LHC experiments completes what is expected by the standard model and, in order to answer the aforementioned questions, a major leap forward is needed. As of today, it is not obvious what kind of accelerator will be the LHC successor. In fact, the recent document “Update of the European Strategy for Particle Physics” sees the High-Luminosity LHC (HL-LHC) with the upgraded experiments as the next step in high-energy physics. According to the current schedule, the LHC collider will be in use until 2036, and in the meanwhile the successor of LHC has to be defined. Consequently, the same document proposes to study several possible options, notably including high-intensity electron-positron colliders and an extreme energy proton-proton collider. The document also suggests investigating the possibility of having bright muon beams.

There are different proposals of electron-positron colliders:

• Circular machines: Circular Electron-Positron Collider (CepC) in China at maximum center-of-mass energy of 240 GeV and Future Circular Collider $e^+ e^–$ (FCC-ee) at CERN at 350 GeV center-of-mass energy. They have a limited reach in the center-of-mass energy due to synchrotron-radiation losses.
• Linear machines: International Linear Collider (ILC) in Japan and Compact Linear Collider at CERN where the electric bill constitutes an important expense if center-of- mass energies above few TeV have to be reached.

The proposed proton-proton colliders at present are the Future Circular Collider (FCC-hh) at CERN that aims to reach a center-of-mass energy of the order of 100 TeV, and the SppC in China, both needing a ring of the order of 100 km. This implies a huge civil engineering work and cost. In this scenario, the Muon Collider (MC) is becoming a more concrete possibility.

In order to study and solve the many technical challenges of a Muon Collider, an International Collaboration (MC Design Study) is being established, based at CERN, to propose a feasible design and define the needed R&D. The Collaboration shall provide a baseline concept for a Muon Collider, well-supported performance expectations, and assess the associated key risks as well as the cost in particular for the electricity consumption. It shall focus on the high-energy frontier and consider options with a center-of-mass energy of 3 TeV and of 10 TeV or more, identify an R&D path to demonstrate its feasibility and support its performance claims. The different R&D topics will be addressed by several dedicated working groups.

Colliding muons imply a change of paradigm: a different way of thinking about accelerator, detector and machine- detector-interface is required, as will be discussed in the following.

2 Why the Muon Collider

Muons are elementary particles, over 200 times heavier than the electrons, therefore much less subject to synchrotron radiation emission than the electron/positron beams. For this reason, Muon Colliders are the ideal accelerators to reach multi-TeV center-of-mass energies, otherwise forbidden in conventional $e^+ e^–$ linear colliders because of the cost. Moreover, muons can be accelerated to very high energy in a circular ring of the dimension of the currently available ones. The electric power necessary to operate the full complex has not been calculated yet, however a preliminary evaluation classifies the Muon Collider as a “green” machine. In fact, this is demonstrated in fig. 1 where the yearly integrated luminosity per energy consumption in Terawatt-hour is plotted as a function of the center-of-mass energy. The Muon Collider, in red, is compared to the future $e^+ e^–$ linear collider ILC and CLIC and to the circular machine, FCC-ee. The figure includes also p-p colliders, the present LHC, the possible high-energy LHC (HE-LHC) and the future FCC-hh. The Muon Collider performs just as well as the proton-proton machine in terms of power consumption. Moreover, in this collider the delivered number of useful collisions (referred to in the following as luminosity) increases as the center-of-mass energy grows. In summary, operating a Muon Collider at high energy is convenient with respect to other accelerators.

Another very important parameter to consider, when comparing with a proton collider, is the physics discovery potential. Electron-positron colliders and Muon Collider at the same energy have roughly the same potential, while comparing proton-proton collider and Muon Collider is complex. Protons are particles made of quarks and gluons, they carry only a fraction of the energy carried by the accelerated protons. When protons beams collide, the interaction is between their constituents therefore at an energy lower than the protons one. In the case of muons the full energy can be exploited in the interaction. Figure 2 illustrates the center-of-mass energy at which a proton collider equals that one of a Muon Collider to produce with the same cross section two heavy new particles with mass approximately equal to half the Muon Collider energy (blue curve). The vertical dashed line indicates 100 TeV, the center-of-mass energy proposed for the FCC-hh machine, that corresponds to 14 TeV for the Muon Collider. The advantages of working with muon collisions are many more and they will be discussed in the next section.

The reason why there are no Muon Colliders as of today is that building such a machine is very challenging. Muons are unstable particles, differently from protons and electrons. They decay with a lifetime of 2.2∙10–6 s if at rest, while in a machine with a center-of-mass energy of 3 TeV each beam has an energy of 1.5 TeV and the muons have a longer lifetime, 3.1∙10–2 s. In this very short time, the produced muons have to be accelerated and transferred in the collider to make them interact, possibly several times. The recent technological developments are opening the door to the possibility of designing such a facility even though there are challenges to face before declaring victory.

3 The Muon Collider facility

A Muon Collider is indeed a complete accelerator facility. Three stages are needed: muons have to be produced, accelerated and finally brought to collision in such a way to have enough interactions useful for physics measurements. Most of the facility designs are based on muons production as tertiary particles by decay of pions created with an intense, typically several MW, proton beam interacting with a heavy material target. In order to achieve high luminosity in the collider, the muon beam, produced with low energy and hence a limited lifetime, and with very large transverse and longitudinal emittances, has to be cooled by approximately five orders of magnitude in the six-dimensional (6D) transverse and longitudinal phase space. Then the beam has to be accelerated rapidly to avoid muon decays. Production, acceleration and collision of muon beams is an amazing technical challenge, due to the muon short lifetime and to the difficulties of their production. A MC accelerator complex has been studied for several years by the MAP (Muon Accelerator Project) collaboration in the US, established in order to develop the concepts and address the feasibility of novel technologies required for MC and neutrino factories based on a proton driver source. Figure 3 shows a schematic layout of the MAP accelerator complex. In the following a short overview of the different stages will be given.

The cooling stage: since muons are not free in nature, they need to be produced by interaction of pions with a target. However, the muons are produced with large angular deviations and then large beam emittances, hence they have to be collected, cooled and focused in order to become high-quality beams suitable for a high-luminosity collider. The process studied by MAP to damp the muon 6D emittance is quite complex. The main ingredients are:

• a proton driver, an intense proton source able to create pions which will decay into muons. Protons are produced and accelerated through a superconducting (SC) Linac, accumulated in an accumulator ring, bunched in a buncher ring to build 2 ns bunches, and finally compressed in a bunch compressor, which provides a 90° rotation in the longitudinal phase space;
• a front-end, where pions are produced by interaction of the proton beam with a MW class target which must stand the proton high power, immersed in a high solenoidal field to capture and guide pions into the decay channel, where muons are captured in a bunches train, with a time-dependent acceleration due to their different energies;
• several cooling stages: muon cooling is achieved by a process called “ionization cooling” to reduce the 6D phase space by 5 orders of magnitude, so the muon beam will fit inside the first acceleration stage acceptance. This process is critical to achieve the muon beam characteristics needed for a high-luminosity MC. The challenge of the cooling process is the short muon lifetime, so cooling must take place more quickly than any of the cooling methods presently in use. For a description of the method see Box 1. A transverse ionization cooling proof-of-principle experiment, MICE (Muon Ionization Cooling Experiment), has been carried out at RAL, UK. A solenoidal cooling channel was built (without RF cavities) and the ionization cooling of muons was demonstrated by using both liquid hydrogen and lithium hydride absorbers. The cooling effect has been observed through the measurement of both an increase in the number of small-amplitude particles and an increase in the phase-space density of the beam. The results agree with the simulations.

The acceleration stage: after muons are produced, due to their short lifetime at rest, a very fast acceleration chain is needed to bring the beams to the required collider energy. The acceleration can be achieved in stages. The key issues driving the design are: to limit the number of decays and avoid longitudinal emittance growth; to have good power efficiency; to limit the cost (RF cavities being the driving cost factor). Since RF cavities are expensive, as many passes as possible should be done through them: small circumference of acceleration stages are preferred, with high dipole fields and a large dipole packing factor. The acceleration gradient should be large due to the short lifetime. To reduce the emittance growth, an increase in circumference, with a small momentum compaction factor, could help. Several kinds of accelerators have been considered for the fast acceleration process: Linac, Recirculating Linear Accelerator (RLA), Fixed Field Alternating Gradient (FFAG), or Rapid Cycling Synchrotron (RCS).

Linac are single-pass, so they are quite inefficient and also expensive.

In the multi-pass Recirculating Linear Accelerator the beam goes through separate passes (arcs) depending on its energy. This solution is preferred at low energy, its main limitation being the geometry of the in/out of the different arcs. A “dogbone” layout, with one Linac and several arcs at both ends, would help. To preserve the longitudinal emittance, relatively long arcs are needed and focusing of the beam and matching between Linac and arcs will be necessary.

Fixed Field Alternating Gradient are rings where magnetic fields do not vary with time, so there is a single beamline for many energies. This means that the dipole magnets need to have a very large aperture. The tolerated decay and emittance growth determine the circumference overturns ratio.

Finally, Rapid Cycling Synchrotrons are pulsed synchrotrons where magnet fields are proportional to the beam momentum. They are preferred to Fixed Field Alternating Gradient accelerators, since higher RF frequencies and/or more turns can be chosen. The RF cavities, as many as possible, should be distributed uniformly. The magnet field increases rapidly (less than 1 ms), however the average field available is low (around 1.5 T), so a hybrid solution was proposed. In a hybrid pulsed synchrotron, the average bend field is increased by interleaving fixed-field superconducting dipoles and bipolar pulsed warm dipoles, so that more RF passes, and a shorter circumference is possible, however a larger magnet aperture is needed. At higher energies this solution may be preferred because of the longer beam lifetime. At the moment, a choice on which method is best has not been made and will be one of the crucial subjects of the International Muon Collaboration studies.

The collider stage: the final stage of the MC facility is the collider ring, where the physics data are collected. In a collider, a high collision frequency, a small beam emittance and a high number of particles per bunch are needed to reach high luminosity. These requirements are quite challenging in the case of a MC, where a luminosity of the order of 1035 cm–2s–1 is desired on a wide range of energies. The design of the rings needs to cope with several requirements in order to maximize luminosity: low beta-functions (see Box 2) at the interaction point (few mm in the range of 3–6 TeV center-of-mass energy), small emittances, short bunch lengths, small ring circumference to increase the collision rate in the short muon lifetime. At present there is a limitation on the maximum magnetic field achievable in the superconducting dipoles, which also need to be protected from the backgrounds produced by muon decay particles. The short lifetime has consequences in the high rate of decay backgrounds, therefore a careful design of the interaction region is needed, together with the detector protection design: machine-detector-interface is a key issue in this case. Since beam emittance and energy spread in the MAP production scheme are quite large, the rings need to be designed with large physical, dynamic and momentum apertures and also a small momentum compaction factor to obtain a short bunch length with a reasonable RF voltage. Moreover, since the energy range of operation should be very large, spanning from 3 to 14 TeV center-of-mass energy, a lattice design optimized for all energies is quite difficult to achieve. The design of the rings has been done in the past for several different energies, with different ring circumferences. In the framework of the International Muon Collider Collaboration this topic will be the subject of a dedicated study group.

The muon decay $\mu^{-} \rightarrow e^{-}\bar{\nu}_{e}\nu_{\mu}$ and its charge conjugate are the major source of background in a Muon Collider. It can be so high that it could make it impossible to perform any physics measurement. Electrons, positrons and synchrotron photons, successively radiated, interact with the machine components and the surrounding environment producing secondary particles, mainly charged and neutral hadrons, Bethe-Heitler muons, electrons and photons, that eventually may reach the detector. Studies performed by the MAP Collaboration demonstrated that two tungsten cone- shaped shields (nozzles) in proximity of the interaction point, accurately designed and optimized for each specific beam energy, can mitigate the background arriving at the detector. Figure 7 shows a simulation obtained with FLUKA of a muon beam of 1.5 TeV circulating in a realistic ring with a realistic nozzle and the support structures of the experimental hall. The detector is represented by the black box and the decay products are not traced into the detector. The figure clearly shows the amount of background that reaches the detector. The optimization of the machine-detector-interface is an activity that sees accelerator and detector physicists working together to balance the level of the background in the detector region and the luminosity for each center-of-mass option.

The decay neutrinos coming from muon decays do not affect the detector design due to the low interaction cross section, but the secondary radiation, hadrons, muons and electrons produced by the neutrino interaction with the earth could constitute a radiological hazard. Preliminary studies, conducted with simulations in the MAP facility configuration, show that the neutrino radiation from a muon beam with an energy in the order of TeV is concentrated in the machine plane. Therefore, if the collider is installed underground, the neutrinos interact with a certain length of earth depending on the collider depth, before escaping on the surface. Assuming the MAP muon beam parameters, the dose-equivalent rate for different hypotheses of the machine depth, and muon beam energy, has been roughly estimated. Along the collider arcs this dose-equivalent does not represent a hazard if the beam energy is below about 1.5 TeV, while in the straight sections, where the neutrinos from muon decay are much more collimated, the radiation induced by the interaction with the material can create hazards if the beam energy is above 1 TeV. Methods to mitigate the effects, like introducing small oscillations to the beam orbit (beam wobbling) to name one, have been proposed and seem very effective. They have to be studied in detail taking in consideration the site configuration. The International Muon Collider Collaboration proposes to evaluate the possibility of locating the accelerator at CERN and therefore the Radiation and Safety Office is conducting the study.

4 A new idea for a muon source

To avoid the rather complicated and technically difficult cooling process needed with the proton-based MAP muon source, the idea to create muon pairs from the interaction of a high-energy positron beam with a high-density target was conceived at Snowmass in 2013. In the following years a Low Emittance Muon Accelerator (LEMMA) concept was developed at the INFN Frascati National Laboratories (Italy). The scheme is based on the muon production from a 45 GeV $e^+$ beam annihilating with the electrons of a target close to threshold for $\mu^{+}\mu^{-}$ pair creation, thus generating muon beams with low enough transverse emittance for a high-energy collider. The advantage of this scheme is that small emittance muon bunches can be created, without the need for a complicated cooling process. Since the bunches have lower emittance, a relatively lower number of muons is required to achieve high luminosity, this lowers also the background rates due to the muons decays, and in particular the boundary radiation limitations due to the neutrino-induced hazard.

The initial design foresaw an $e^{+}$ storage ring with an internal target, in order to allow for multiple interactions of the $e^+$ with the electrons at rest in the target, and the subsequent formation of a muon bunch. However, this layout has encountered several limiting difficulties. An alternative design was conceived, where $e^{+}$ bunches are extracted to impinge on multiple targets in one or more dedicated straight sections. This scheme could release the impact of the average power on the target and also reduce the number of $e^{+}$ required from the positron source. The facility layout is schematically shown in fig. 8.

The complex consists of a chain of several components. A Positron Source (PS) at 300 MeV produces positrons which are accelerated in a 5 GeV Linac and then stored in a 5 GeV Damping Ring (DR) to reduce the beam emittances. A SC Linac (or an Energy Recovery Linac (ERL)) is used to accelerate positrons to 45 GeV and inject them into a 45 GeV Positron Ring (PR) to accumulate the 1000 bunches needed for muon production. Some delay loops will synchronize the positron bunches extracted before passing through one or more Target Lines (TL) for the muon production. After the production muons are collected in two Accumulation Rings (AR), where they are stored until the bunch has a suitable number of particles, the same bunch will pass the same TL (and the AR) several times until it reaches the required number of muons. To restore the initial positron-beam current and to release the stress on the PS, which has to produce a number of positrons above the current state-of-the-art sources, an “embedded” positron source, where positrons are produced by the photons coming from the interaction $e^+$ /target, can be envisaged. Another option under study is to recuperate the “spent” positron beam, with a large energy spread, at the exit of the TL after the muon production and inject it back into the 45 GeV ring.

The LEMMA scheme poses several technical challenges and has some issues. First of all, a high-rate positron source is needed, one order of magnitude larger than the ILC and CLIC ones. The muon production rate at the moment is quite lower than in the MAP scheme, solutions to recombine muons into one bunch should be studied. Moreover, the target material choice is critical, since the target needs to have a high density for a high production rate, to stand the heat load and mechanical stress from the interaction of the intense positron beam for many pulses, and at the same time to have a low density to avoid multiple scattering which would increase the muon bunch emittance. Some R&D is ongoing worldwide, also as a number of proposals for the next US Snowmass process due by summer 2022. Examples of future R&D topics are the target studies, as well as the design of a powerful positron source, the latest in synergy with the future $e^{+}e^{–}$ colliders FCC-ee and CepC.

5 The guaranteed physics discovery

The possibility to reach very high energy with muon collisions has motivated several theoretical studies that are demonstrating that a Muon Collider would perform much better than other proposed machines, extending the reach of the searches for new particles and new phenomena. But even if the answers to the main open questions through the direct searches sat at energies much higher than the ones reachable by any future machine, the Muon Collider would nonetheless offer a unique possibility: the precise determination of the Higgs field potential.

The Higgs boson, discovered at LHC in 2012, was introduced in the standard model as a consequence of the mechanism by which the elementary particles were assumed to acquire mass. Its discovery was a tremendous success of both the model for its predictive power and the experiments for being able to catch such an elusive particle. Elementary fermions and bosons interact with the Higgs field, a process by which they acquire mass, and the strength of these couplings predicted by the theory will be evaluated by the LHC experiments and could be precisely measured at any of the future colliders, while the determination of the shape of the Higgs field will not be possible.

Why is it so important to determine it? In the current understanding of the Universe and its evolution, before the electroweak symmetry breaking, all the elementary bosons and fermions were massless, the potential of the Higgs field was symmetric with respect to the vacuum. At a certain moment, or better at a given temperature, the Higgs field potential assumed the so-called “Mexican” hat form with infinite possible minimum configurations, the bottom of the hat. The choice of one configuration is referred to as the spontaneously breaking of the electroweak symmetry. In the standard model, this potential configuration is parametrized in terms of the expectation value on the vacuum, $\nu = 1 / (\sqrt{2}G_{F})^{1/2} \sim 246$ GeV, with $G_F$ the Fermi constant, $\lambda$ the strength of the coupling of the Higgs to itself and the Higgs mass $m_{H} = \sqrt{2\lambda} \nu$.

Processes where a virtual Higgs boson produces two or three Higgs bosons, governed by the so-called trilinear and quadrilinear Higgs self-couplings, are of paramount importance to verify if the Higgs potential corresponds to the one predicted by the standard model. In order to be as general as possible and to include deviations from the standard model, the Higgs potential is written assuming two different values for trilinear ($\lambda_{3}$) and quadrilinear ($\lambda_{3}$) self-couplings:

$V ( h ) = \frac{1}{2} m^{2}_{H} h^{2} + \lambda_{3}\nu h^{3} \frac{1}{2}\lambda_{4}h^{4}$,

where $\lambda_{3} = \lambda_{4} = \lambda ( m^{2}_{H} ) / (2\nu^{2})$ in the standard model and the scalar field, $h$ an expansion around $\nu / \sqrt{2}$, describes a physical Higgs boson.

The direct measurement of $\lambda_{3}$ and $\lambda_{4}$ will verify the actual nature of the observed Higgs particle and the determination of the shape of the Higgs potential could dramatically affect the knowledge of cosmology being related to the spontaneous symmetry breaking.

Experimentally $\lambda_{3} ( \lambda_{4} )$ can be measured by counting events where a virtual Higgs boson produces two (three) Higgs bosons. These processes are expected to have extremely small cross sections and to require collisions at very high energy, reachable only by the muon collisions.

The next question is if an experiment at a Muon Collider will be able to detect and reconstruct events where two (three) Higgs bosons decay in two b-jets, the most probable process, given the high level of beam-induced background.

A dedicated machine-detector-interface has been designed for 1.5 TeV center-of-mass energy and used with the full simulation of the detector to find new strategies that exploit new technologies in order to mitigate the effect of the background. Recent studies have demonstrated that high granularity silicon-based tracking detectors and calorimeters combined with the arrival time of each particle give very good performance in physics object reconstruction including b-jets. The expectations for 3 TeV center-of-mass energy are similar even though we should have a lower level of background since at higher energies muons live longer. Figure 9 shows the decay of two Higgs bosons into b-quark jets, produced by muon collision at 3 TeV center-of-mass energy as they appear in the detector. The reconstructed four b-jets are clearly visible, here the beam-induced background is not shown. In four years of data taking at an instantaneous luminosity of 4.4·1034 cm-2s-1 around 70 events fully reconstructed are expected with a ratio of signal-to-physics background of about 1 to 10. Already at 3 TeV, considered as low energy for a Muon Collider, it would be possible to extract information on the Higgs boson potential, unique to the Muon Collider.

The goal of the Muon Collider project is to reach energies higher than 3 TeV, 10 TeV are currently explored. At this energy a sizable number of double and triple Higgs bosons will be produced allowing the full determination of the Higgs potential.

6 Synergies with other physics fields and social impact

The studies for a Muon Collider will have a huge impact on the state-of-the-art technologies for new accelerators. Most of the future programs to explore nature are based on the extrapolation of the conventional approach used at LEP and at LHC. Powerful lepton colliders are proposed to perform precision measurements of the Higgs properties and of other key parameters of the standard model. ILC, CepC, FCC-ee are gigantic infrastructures with dimensions in the range of 30–100 km, investments of several billion euros and construction times of about 10 years. New hadron machines are proposed to push forward the energy frontier deep into the multi-TeV region. Colliders like SppC and FCC-hh imply rings of 100 km circumference, a new generation of superconducting magnets to be developed, costs exceeding 20 billion euros and very long construction time scales. A Muon Collider would be a serious alternative to some of these programs, providing access to the most important measurements with a smaller and substantially cheaper machine, a new generation of particle colliders capable of performing precision studies, with the flexibility to be scaled to the multi-TeV regime. Furthermore, the complex of accelerators necessary to implement a Muon Collider would allow for a rich program of physics measurements, as, for example, neutrino physics. The huge amount of R&D involved in this project would be extremely useful for the accelerator and high-energy physics communities. For example, if the studies could prove it feasible, the LEMMA scheme at low energy could be used for intense photon beam production empowering many applied fields, including material studies, medical science, and lithography.

Italy, in particular, has a glorious history of design, construction and operation of accelerators for the study of the fundamental constituents of matter and their interactions, since the construction of the first machine in the '60s at the INFN Frascati National Laboratories. In recent years, due to the increasing size of the projects and the larger costs, it became more difficult to build national accelerators, therefore the training and the interest of young researchers on colliders design has diminished. The Muon Collider study, being a completely new and challenging field of research, can renew the interest of many students and young researchers in this field, for a new generation of accelerator physicists and engineers.

Looking at the applications aspect, the Muon Collider can be dreamed of as the door to a new era. For example, muon tomography is a technique that uses cosmic-ray muons to reconstruct the three-dimensional image of a given volume that can be hidden under the earth or a heavy material given the highly penetrating power of these particles. In Italy the project Mu-Ray aims at mapping the inside of Vesuvius volcano. Muon tomography is also used to monitor nuclear reactors. The earthquake of 2011 in Japan caused serious damage to the Fukushima Daiichi power plant. The direct evaluation of the real damage to the cores of the reactor was very dangerous given the high level of radiation in the site. The Toshiba Corporation has for the very first time used a muon tomography detector to analyze the cosmic- ray muons scattered by the reactor and evaluate the conditions of the cores. Data was collected for about four weeks in order to have enough muons impinging on the reactor, due to the low rate of high-energy cosmic muons.

While it is clear that the present Muon Collider proposals are for a facility of large dimensions, in the future a dream to have Muon Collider technology so advanced to build tools based on muon beams and perform much faster analyses as the ones mentioned, could be envisaged. The present Muon Collider status is very similar to the beginning of the era of electron and hadron accelerators, when it was difficult to imagine that nowadays compact accelerators are used in hospitals for cancer treatment, or table-top accelerators could have been built for industrial applications. If a compact muon beam were available, it could be used for example in the harbors or at the borders, to monitor the nuclear waste and the nuclear weapons illegal trade, methods which are already in use but that take time given the cosmic-ray muon rate.

Concluding in one sentence, the Muon Collider is a chance not to be missed.

Acknowledgments

The authors thank Susanna Guiducci, Nadia Pastrone, Lorenzo Sestini, Laura Buonincontri and Massimo Casarsa for the useful and constructive discussions. Thanks to Paola Sala, Camilla Curatolo, Francesco Collamati and Alessio Mereghetti for preparing the material on the machine-detector-interface.