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Searching for Long-Lived BSM Particles at the LHC

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                                   Marie-Hélène Genest (2022), Scholarpedia, 17(12):54697.
 [3]doi:10.4249/scholarpedia.54697         revision #198841 [[4]link to/cite this article]
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   Curator: [7]Marie-Hélène Genest
   Contributors:


   [8]Jamie Boyd

   [9]Lesya Shchutska

   [10]Lydia Roos
     * [11]Dr. Marie-Hélène Genest, LPSC-IN2P3, CNRS/UGA, Grenoble, France


Contents

     * [12]1 Long-lived particles: what are they and where could they come from?
     * [13]2 Signatures of long-lived particles at the LHC
     * [14]3 Search strategies and unique difficulties
     * [15]4 Main LHC detectors
     * [16]5 Current and proposed dedicated detectors
     * [17]6 References

Long-lived particles: what are they and where could they come from?

   By definition, a long-lived particle (LLP) is an [18]unstable particle with a
   sizeable lifetime. The definition of sizeable, in turn, will depend on the
   experimental apparatus,  as it should be long enough to allow for the distance
   between the LLP production and decay points to be observable. In the Large Hadron
   Collider (LHC) experiments, LLPs could be produced in the proton-proton (p-p)
   collisions and manifest themselves in the detectors via their late decay, leading
   to a signature of visible particles which do not originate from the nominal
   interaction point. For a given proper decay lifetime \(\tau\) (the lifetime as
   measured in the particle rest frame) and an initial number of particles \(N_0\),
   the expected number of particles surviving after a time \(t\) measured in the
   rest frame follows an exponentially decreasing probability function\[N(t) =
   N_0e^{-t/\tau}\]

   As high-energy collisions can lead to large relativistic boosts of the produced
   particles,  if these are light compared to the collision energy, the displacement
   \(L\) corresponding to the proper lifetime can be enhanced in the laboratory
   frame\[L = \gamma\beta c\tau\]

   where \(\gamma = 1/(1-\beta^2)^{1/2}\) is the Lorentz factor and \(\beta=v/c\) is
   the ratio of the particle's velocity \(v\) to the speed of light \(c\).

   The fraction of decays in the various detector systems will thus change with the
   lifetime and boost, and multiple types of searches might be needed to cover the
   various signatures, as will be described below.

   For a particle to have a long lifetime, the decay must be suppressed, which can
   happen through two mechanisms: small couplings to decay mediators or a suppressed
   decay [19]phase space. These mechanisms are at play even in the Standard Model
   (SM), which contains multiple LLPs, such as charged pions
   (\(\tau_{\pi^{\pm}}=26\) ns), muons (\(\tau_{\mu}=2.2\) \(\mu s\)) or neutrons
   (\(\tau_{n}=880\) s). It is therefore not unexpected that they could also be at
   play in theories beyond the SM (BSM), and there are indeed many BSM models
   predicting LLPs.

   The small-coupling case can happen for example if the interaction violates an
   approximate symmetry which would otherwise forbid the decay to take place, or if
   there is a small mixing with SM particles. This can happen in dark/hidden-sector
   theories, i.e. BSM theories in which the postulated new particles are not charged
   under the usual SM interactions but communicate with the SM only through a new
   mediator which is heavy and/or only weakly coupled to the SM. An example of this
   mechanism arises in some dark-photon (\(A'\)) theories. In this scenario, the
   hypothetical massive vector boson \(A'\), which could mediate the interactions of
   dark matter particles, couples to the SM through a mixing with the SM photon with
   strength \(\epsilon\) - for small values of this mixing parameter, the dark
   photon \(A'\) becomes long lived.

   A long lifetime through this mechanism can also happen if the mediator of the
   decay is very massive, thus reducing the effective coupling. This happens for
   example in some supersymmetric (SUSY) theories. In these theories, every fermion
   of the SM has a new scalar particle counterpart, and every boson - a new fermion;
   the Higgs sector is also enlarged and the supersymmetric partners of the gauge
   and Higgs bosons can mix, creating charged and neutral states called charginos
   and neutralinos, the lightest neutralino often being stable and a suitable
   dark-matter candidate. As the superpartners do not have the same masses as their
   SM counterparts, SUSY is not an exact symmetry and must be broken. For split SUSY
   models, the scalar partners of the SM fermions are very heavy, \(O(10^{2-3})\)
   TeV, while the fermionic partners of the SM bosons are at a mass scale accessible
   at the LHC. The superpartner of the [20]gluon, the gluino (\(\tilde{g}\)), could
   then be pair-produced in the p-p collisions, but its decay, into a pair of quarks
   and the lightest neutralino (\(\tilde{\chi}\)) through a virtual squark
   production (\(\tilde{q}^*\)) , would be suppressed
   (\(\tilde{g}\rightarrow\tilde{q}^*\bar{q}\rightarrow\tilde{\chi}q\bar{q}\)). This
   long-lived gluino, as it carries colour charges, would hadronise with SM
   particles into a so-called R-hadron before decaying.

   The suppressed decay phase space can happen if the mass difference between the
   decaying particle and its decay products is small, in a so-called compressed mass
   spectrum. This can happen in the anomaly-mediated SUSY breaking models, for which
   the masses of the lightest chargino and neutralino differ only by loop-level
   corrections of \(O(160)\) MeV: the lightest chargino, which decays into the
   lightest neutralino and a charged pion in this case, hence becomes long-lived as
   the decay phase space is suppressed.

Signatures of long-lived particles at the LHC

   Figure 1: Overview of some LLP signatures in the various sub-dectors of a generic
   detector like ATLAS, shown in the transverse plane with respect to the beam line:
   the collision takes place in the center and the particles will cross in turn the
   inner tracking detector (in green), the electromagnetic and hadronic calorimeters
   (in turquoise and blue) and the muon spectrometer (in grey). The dotted lines
   show invisible particles, the full lines represent tracks, and the shaded areas
   are calorimeter energy deposits. The represented signatures (see the text) are
   from the top and in a clockwise fashion: a disappearing track, a kinked track, a
   non-pointing photon, an emerging jet, a heavy charged LLP going through the
   detector, a displaced hadronic jet in the calorimeter, the same in the muon
   spectrometer, a displaced electron, a displaced muon pair, a displaced electron
   pair and finally, a displaced hadronic jet in the inner detector. (Credit:
   Heather Russell, under the Creative Commons Attribution-NonCommercial 4.0
   International License).

   In a general-purpose detector like ATLAS or CMS, the particles produced in the
   collisions at the LHC will move away from the interaction point, first going
   through the inner tracking detector and, energy and lifetime permitting, the
   electromagnetic and hadronic calorimeters and finally, the muon spectrometer. The
   signature will thus not only depend on the lifetime of the particle, but also on
   its nature and hence on its possible interactions with the various sub-detectors.
   Some examples are described below and graphically shown in Figure 1.

   In the inner tracking detector A displaced [21]vertex (DV) signature can arise
   when an LLP decays to several charged particles in the inner tracking detector
   (ID) volume, leading to tracks whose intersection is not compatible with the
   primary vertex of the event, where the p-p collision took place, but instead form
   a DV at the position of the LLP decay. Another, more exotic, signature from LLPs
   in the ID is the so-called disappearing track signature, which can be produced if
   a charged LLP decays into an invisible state and a charged particle which has too
   little energy to be seen in the detector. Such is the case in the compressed
   supersymmetric model discussed above in which an LLP chargino, produced in the
   p-p collision and recorded as a track in the first layers of the inner detector,
   eventually disappears by decaying into an almost mass-degenerate invisible
   neutralino and a very low-energy charged pion which is not seen. A charged LLP
   could also decay into an energetic charged particle, which does not follow the
   direction of its LLP parent, and an invisible state; in this case, the track
   initiated by the LLP does not disappear but appears to abruptly change direction,
   thus giving a kinked track signature. Another LLP signature in the ID would be
   that of a track that is associated to an otherwise well-reconstructed lepton
   (electron or muon) but which would be displaced, i.e. with a larger than usual
   impact parameter with respect to the primary vertex, if this lepton comes from an
   LLP decay. As a final example, the LLP could be a high-mass charged particle (for
   example an R-hadron as discussed before) which would have, even at high energy, a
   low velocity compared to the speed of light. Even in the case in which the
   lifetime of such a particle would allow it to cross the full detector before
   decaying, it could leave a distinctive signature in the ID in the form of an
   abnormally large ionisation loss, dE/dx. Its low velocity could also be measured
   through time-of-flight techniques, which could combine the ID information with
   information from the muon spectrometer if the particle goes through all the
   detector volume, or with timing information from the calorimeters.

   In the calorimeters The decay of LLPs can involve the production of hadronic
   jets, if their decay products involve SM quarks and/or gluons. Such jets could be
   identified in the detectors in various ways. If the decay happens in the
   calorimeter, the formed jets will be displaced, possibly exhibiting an atypically
   large ratio of energy deposited in the hadronic calorimeter with respect to that
   deposited in the electromagnetic calorimeter, and a lower number of associated
   tracks in the ID. In some dark QCD models, dark quarks, from a hidden sector, are
   produced in the collisions and then hadronise in the dark sector to form dark
   hadrons; these dark hadrons are invisible to the detector until some of them
   decay back to SM particles, such as SM quarks. In such a case, the initially
   formed dark jet would gradually become visible in the detector, as the LLPs
   inside it start to decay, leading to a possible emerging jet signature: multiple
   DVs in the ID associated with a jet in the calorimeters. If on the other hand the
   jet is coming from a massive neutral LLP decay in the calorimeter, it can lead to
   a delayed jet signature: the timing of the calorimeter is then used to measure
   the time of the jet with respect to the expected arrival time for a massless
   particle coming from the interaction point. Besides timing information, the
   geometrical segmentation of some calorimeters can also be used to recover some
   information: an example is to look for non-pointing photons, that is photons
   identified in the electromagnetic calorimeter whose flight direction does not
   coincide with the direction from the primary vertex. Such a signature could
   happen for example in some supersymmetric models in which a heavy neutralino LLP
   decays into a gravitino, which is invisible to the detector, and a Higgs boson
   which subsequently decays into two non-pointing photons.

   In the muon spectrometer LLPs with large enough lifetimes could lead to a DV
   signature in the muon spectrometer, via the same mechanism as the DV signature in
   the ID. Very displaced hadronic jets can also appear in the muon spectrometer,
   leading to a possibly spectacular, multiple-track signature which is completely
   isolated from any activity in the ID or the calorimeters.

   Some even more exotic signatures can happen for very long lifetimes, in models in
   which the LLPs would interact and come to rest inside the detector and decay at a
   later time, giving energetic events which are decorrelated in time from the
   collision activity in the detector, even possibly producing signal when the LHC
   is not running.

Search strategies and unique difficulties

   Figure 2: Reconstruction efficiency of tracks from displaced charged particles
   which come from the decay of an LLP at a radius \( r_{prod} \) in the inner
   detector of ATLAS. The standard tracking [22]algorithm, requiring some
   association of the tracks to the primary vertex, is quite inefficient for decays
   happening at large radii: the large-radius tracking algorithm is necessary to
   recover some efficiency for these LLP decays.(Credit: ATLAS Collaboration,
   ATL-PHYS-PUB-2017-014)

   Given the atypical signatures expected, some of which were mentioned above,
   dedicated algorithms are often necessary to search for these objects in the
   detector. Large radius tracking is one such algorithm: after reconstructing the
   standard tracks which are pointing towards the primary interaction point, an
   additional algorithm running on the remaining unassociated hits in the ID can
   build a collection of tracks which originate at larger radius in the ID, and
   which are then used to search for DVs. Indeed, as can be seen in Figure 2, the
   standard tracking algorithm, which has constraints on the association of the
   tracks to the primary vertex, is not efficient for LLPs decaying at large radii
   in the ID, corresponding to longer lifetimes: the large radius tracking algorithm
   is necessary to have a non-negligible efficiency for these scenarios.
   Figure 3: Example of a material map from CMS. (Credit: CMS Collaboration as
   published in JINST 13 (2018) P10034)

   Great care must also be taken when using existing algorithms and analysis
   techniques, as standard procedures might be detrimental to some LLP searches.
   This can be the case for some event cleaning procedures for example, which are
   often undertaken in more standard analyses in order to avoid detector-related
   issues. In some recommended cleaning selections, events with atypical energy
   deposits in some detector parts, that could be due to electronic noise for
   example, are removed from the dataset. While these procedures usually have a
   negligible impact on the events targeted by standard analyses, i.e. those
   searching for the prompt decay of new particles, it may not be the case for some
   LLP searches for which they could remove a significant fraction of the signal
   candidates.

   Standard Model processes can sometimes mimic the LLP signatures and might hence
   produce a large background to these searches. This is the case in the production
   of displaced vertices, which can frequently happen from the interaction of SM
   particles emitted from the collision point with the dense material of some
   detector parts (sensitive material, detector supports or services), such as
   photons converting into an electron-positron pair. These can however be removed
   by fiducial selections, that is by vetoing any DV that falls within a region
   compatible with the presence of such material, using the known material map of
   the detector, an example of which is shown in Figure 3. Applying selections on
   the mass of the DV can also help in that respect, and will also remove background
   from known low-mass SM [23]resonances.

   Other LLP signatures are so unique that a very limited number of Standard Model
   processes, if any, can produce them. The fact that some LLP signatures are free
   from collision SM backgrounds can make them particularly interesting as the
   collected dataset increases in size. The sensitivity of standard searches, when
   they are not limited by systematic uncertainties, is roughly given by the ratio
   \(S/\sqrt{B}\) where \(S\) is the expected number of signal events and \(B\) is
   the number of background events. As the collected dataset increases, so do S and
   B, at the same rate: the sensitivity thus increases proportionally to the square
   root of the dataset size. For searches in which the background is non existent,
   or unrelated to the collision rate, the sensitivity can grow more quickly, being
   directly proportional to the expected number of signal events. Even for these
   searches though, there can be sources of background, which are often more diverse
   in origin, including rare mis-reconstruction of objects or non-collision
   background which can be linked to noise in some parts of the detector, cosmic
   rays, or be induced by interactions of protons in the beam with collimators or
   with residual gas molecules in the beam pipes. Estimating the level of these
   backgrounds, which are difficult if not impossible to simulate reliably, requires
   designing background estimation methods which rely on samples in data selected to
   have little or no expected signal while being enriched in the background of
   interest (so-called data-driven techniques). For example, in order to estimate
   the number of cosmic-ray or beam induced events, one can measure the probability
   of mimicking the signal signature in an independent dataset which would be empty
   of signal contamination. Such a dataset could be a cosmic-ray dataset taken when
   there is no collisions in the first case, or a dataset corresponding to times in
   which only one proton bunch crosses the detector (called unpaired bunch) in the
   second case. The probability measured would then need to be applied appropriately
   to the dataset of the search in order to compute how many such background events
   are expected to pass the signal selection in the final analysis.

   As with any other LHC analysis, a further hurdle in the search for LLPs is the
   trigger system which must pick interesting collision events in a short time,
   based on limited detector information. Indeed, of the 40 MHz of LHC p-p
   collisions, ATLAS and CMS store on disk for further offline analysis only about a
   kHz of p-p collisions due to bandwidth and storage restrictions; one must thus
   make sure that eventual signal events are picked up for further processing
   amongst the myriad of collisions. Depending on the nature of the LLP and on its
   production mechanism, the LLP can, in some models, be expected to be accompanied
   by other more standard, energetic objects in the detector, such as leptons, jets
   or missing transverse momentum. This can be especially helpful at the trigger
   level, as one can then rely on the presence of these objects in order to select
   the interesting events. In some other situations, a dedicated trigger is needed.
   The goal is then to devise it in such a way as to collect these peculiar events,
   by making a decision based on the available trigger information within the
   allocated trigger timescale and without affecting the recording bandwidth
   significantly. Given the relative rarity of the LLP signature, this can often be
   achieved but may require dedicated triggering algorithms without which the events
   would be lost. A particularly challenging scenario is one in which LLPs are very
   heavy: produced without a large boost, they could arrive too late in the detector
   to be considered by the triggering algorithms.

Main LHC detectors

   The scenarios described above and in Figure 1 focus mainly on a multipurpose
   general detector that is large, has an almost full coverage around the
   interaction point and is made of multiple subdetectors which can, together, cover
   a wide range of signatures, such as ATLAS or CMS. Indeed, CMS and ATLAS are
   sensitive to a large range of models and parameters, as shown in Figure 4 which
   gives an overview of excluded lifetimes for a selection of ATLAS analyses.
   Multiple models are given as examples in this Figure, such as supersymmetric
   scenarios, Higgs decays to LLPs such as dark photons, new long-lived scalar
   particles or long-lived heavy neutral leptons, and the exclusions span multiple
   orders of magnitude in lifetimes and masses, depending on the models.
   Figure 4: Ranges of new particle lifetimes excluded at the 95% confidence level
   for a selection of ATLAS analyses. (Credit: ATLAS Collaboration,
   ATL-PHYS-PUB-2022-034)

   The LHCb detector is also an excellent tool to search for LLPs: it was optimised
   to study b-hadrons, which are, in fact, SM LLPs whose identification often relies
   on the presence of a displaced vertex. Running at a lower instantaneous
   luminosity to avoid large pileup and only instrumented in the forward direction
   (i.e. in a rather small angle around the beam pipe), its coverage is unique and
   thus complementary to that of ATLAS and CMS, being able to probe lower masses
   with its forward acceptance and low-energy triggers, and lower lifetimes due to
   its excellent vertexing  capabilities and detector instrumented for decays with a
   larger forward boost. An example of unique coverage brought by the LHCb detector
   is shown in Figure 5, which shows the limits in the dark photon (\(A'\))
   parameter space. The LHCb search looks for a dimuon displaced vertex which would
   come from the decay \( A'\rightarrow \mu\mu \). As shown in the Figure, the LHCb
   coverage is unique amongst all experiments, probing the 200-350 MeV range for
   \(10^{-5} < O(\epsilon) < 10^{-4}\).
   Figure 5: Limits in the dark photon (\(A'\)) parameter space of the \(\gamma-A'\)
   mixing parameter \(\epsilon\) versus the dark photon mass. The LHCb LLP coverage,
   based on a displaced signature of \( A'\rightarrow \mu\mu \), is displayed by the
   two excluded islands in the middle of the plot. LHCb offers a unique coverage
   amongst all experiments, probing the 200-350 MeV range for \(10^{-5} <
   O(\epsilon) < 10^{-4}\). (Credit: LHCb Collaboration, Phys. Rev. Lett. 124 (2020)
   041801).

Current and proposed dedicated detectors

   Besides these large, general detectors, other detectors specialising in LLP
   detection have been installed or proposed at the LHC, usually instrumenting
   service areas which may not be used otherwise. These detectors, further away from
   the interaction points where the collisions take place, can often benefit from
   existing passive or active shielding, possible complementary collision
   information from their associated general-purpose detector, a dedicated design
   and a good positioning to reconstruct LLPs in a lower background environment.
   Figure 6: FASER projected sensitivity to dark photons, compared to other
   experiments, such as MATHUSLA.(Credit: FASER Collaboration,
   [24]https://faser.web.cern.ch/)

   Such is the case for FASER, which has been installed in an unused tunnel 480
   meters away from the ATLAS interaction point and which has started taking data in
   LHC Run-3. The idea is to look for long-lived light particles produced in meson
   decays in the very forward region of LHC collisions which is not instrumented in
   ATLAS. While the proton beam will follow the curved LHC path and most of the
   other SM particles emitted in the forward region will be absorbed by the 100
   meters of rock and concrete between ATLAS and FASER, the neutral,
   weakly-interacting particles produced in the collisions will travel straight
   towards FASER and could decay inside its cylindrical decay volume (radius of 10
   cm, length of 1.5 meters) into an electron-positron or a photon pair. These
   particles would then pass through a strong magnetic field, a spectrometer and a
   calorimeter, allowing their identification. One scenario FASER can probe is the
   production of dark photons, see Figure 6. In front of the main FASER detector an
   emulsion/tungsten neutrino detector is placed, called FASER\(\nu\).

   Another recently installed detector is SND@LHC: similarly to FASER, it probes the
   forward region of the ATLAS detector, being located at the same distance of 480
   meters away from the interaction point, on the other side. Differently from FASER
   though, the detector is slightly off-axis with respect to the beam inside ATLAS,
   thus providing a complementary coverage. The detector, 1-meter wide and 2.6-meter
   long, is shielded by about 100 meters of rock, has an active scintillator veto, a
   vertex detector and electromagnetic calorimeter made of emulsion/tungsten and
   scintillating fibers and a muon spectrometer and hadronic calorimeter made of
   scintillator and iron. While being primarily intended to study neutrinos coming
   from the collisions, like FASER\(\nu\), it can also be used to look for
   long-lived, feebly-interacting particles scattering or decaying in the detector
   volume.

   Larger versions of both FASER and SND@LHC are proposed as part of the Forward
   Physics facility (FPF). The proposed FPF would be a new underground cavern
   hosting forward experiments which would be shielded by concrete and rock and
   located a few hundreds of meters away from the ATLAS interaction point. Another
   recently proposed forward detector is FACET, which would complement the CMS
   detector in the very forward region in view of the high-luminosity phase of the
   LHC (HL-LHC).

   Instead of looking for LLPs produced in the decay of light SM particles such as
   mesons in the very forward region, detectors can also be placed in a transverse
   way, close to the surface, enhancing the lifetime sensitivity of searches for
   LLPs produced in the decay of heavy particles such as the Higgs boson. This is
   the case for the MATHUSLA project, which proposes to place a large (100 m large x
   100 m long x 25 m high) detector close to the surface, above the CMS interaction
   point, looking for decays of LLPs produced in the p-p collisions but escaping CMS
   as they have very long lifetimes. It would consist in multiple layers of tracking
   detector to search for decays inside a 20-meter-high air-filled decay volume.
   Most of the background from SM particles would be absorbed by the O(100-m)
   overhead rock on the way to MATHUSLA, except high-energy muons coming from the
   interaction point (which can be vetoed through a scintillator layer at the
   entrance of the detector) or from cosmic rays (which can be identified and
   removed based on their direction of propagation inside the multiple tracking
   layers). A test stand installed above the interaction point of ATLAS as a proof
   of concept was operated in 2018. If approved, this experiment could be installed
   to take data during the HL-LHC. The projected sensitivity to a Higgs decaying to
   two long-lived scalars X is shown in Figure 7, showing its complementarity to the
   ATLAS dectector at longer lifetimes.
   Figure 7: MATHUSLA projected sensitivity to a Higgs boson decaying to two
   long-lived scalars X, shown in the plane of the decay branching ratio versus the
   lifetime of X, showing its complementarity to the ATLAS detector at longer
   lifetimes.(Credit: MATHUSLA Collaboration,
   [25]https://mathusla-experiment.web.cern.ch/node/9)

   There is also a proposal to install a 10 x 10 x 10 m\(^3\) detector, CODEX-b,
   about 25 meters away from the LHCb interaction point, in the old DELPHI
   experiment cavern, in view of HL-LHC. In this case as well, backgrounds would be
   reduced by an existing concrete wall, complemented by passive shielding and
   active vetos to be installed. In the baseline configuration, resistive-plate
   chambers (RPCs) would be used on each face of the cubic detector, which would be
   complemented by further RPCs inside the volume. The broad LLP search program of
   this experiment would benefit from the ability to combine information with the
   existing data stream of the LHCb detector. A demonstrator of smaller size (2 x 2
   x 2 m\(^3\)), CODEX-b, should be installed in 2022-2023 and be operated during a
   part of Run-3, as a proof of concept.

   An 18-meter diameter, 56-meter long shaft above [26]the ATLAS experiment, which
   has been used for the ATLAS detector installation in the cavern but which is not
   used during normal LHC operation, could also be instrumented with RPCs to search
   for LLPs in view of the HL-LHC, as proposed in the ANUBIS project. This
   experiment could be combined with ATLAS information to veto and estimate the SM
   particle backgrounds, using timing information to reject cosmic rays which would
   travel in the opposite direction. A demonstrator, proANUBIS, should be installed
   during Run-3.

   On a longer timescale, the AL3X project would aim to exploit the cavern and part
   of the current ALICE experiment, should its current heavy-ion program finish
   after Run-4. In this project, the current interaction point would need to be
   upgraded to run at the nominal HL-LHC luminosity and also to be moved, in order
   to allow LLPs to travel some distance before decaying in the detector. AL3X would
   be re-using the existing L3 magnet and ALICE time projection chamber, adding some
   shielding material in front.

   While the dedicated detectors above are able to extend the coverage for neutral
   LLPs, MoEDAL+MAPP and milliQan are specifically designed to cover charged LLPs.

   The MoEDAL experiment, which is installed in the LHCb cavern since Run-1, targets
   long-lived highly-ionising particles which would be produced in the collisions,
   such as hypothetical magnetic monopoles, or singly-charged massive LLPs, such as
   long-lived supersymmetric partners of the leptons. The detector consists of two
   parts which can passively record events during the runs and are then read out to
   look for signs of new physics. The first part is a plastic nuclear track
   detector, in which the passage of charged particles would be recorded as visible
   track damage. The second part is a magnetic monopole trapper made of nearly one
   ton of aluminium, which would, as its name indicates, capture magnetic monopoles.
   The scanning of the volume with a SQUID magnetometer would then reveal their
   presence. The experiment has placed stringent limits on these models, as shown in
   Figure 8. For Run-3, the detector is upgraded to MoEDAL-MAPP: the MAPP addition
   comprises two new detectors, one (MAPP-LLP) looking for LLPs decaying to charged
   particles, and the other one (MAPP-mQP) looking for millicharged LLPs, that is
   particles carrying a fractional electric charge as low as \(O(0.001e)\).
   Figure 8: Limits set by the MoEDAL experiment on the mass of the magnetic
   monopoles as a function of their magnetic charge, compared to some
   general-purpose detector results. (Credit: J. Pinfold @ Phil.Trans.Roy.Soc.Lond.A
   377 (2019) 2161, 20190382 )

   Millicharged LLPs are also the target of the milliQan experiment. After running a
   scintillation-bar prototype in 2018, two complementary, scintillation-based
   detectors will be used in Run-3, with a possible extension proposed for the
   HL-LHC. Installed in a drainage and observation gallery 70 meters underground and
   33 meters away from the CMS interaction point, the detectors benefit from rock
   shielding. The projected sensitivity is shown in Figure 9. An upgrade, called
   FORMOSA, is also proposed as part of the FPF.
   Figure 9: Existing sensitivity using the 2018 demonstrator and projected
   sensitivity to millicharged particles by the milliQan collaboration, compared to
   other experiments, shown in the plane of the charge versus the mass of the
   particles. (Credit: milliQan Collaboration @ Phys. Rev. D 104 (2021) 3, 032002 )

References

   J. Alimena et al, Searching for long-lived particles beyond the Standard Model at
   the Large Hadron Collider, Phys. G: Nucl. Part. Phys. 47 090501 (2020),
   [27]https://arxiv.org/abs/1903.04497

   L. Lee, C. Ohm, A. Soffer and T. T. Yu, Collider Searches for Long-Lived
   Particles Beyond the Standard Model, Prog. Part. Nucl. Phys. 106, 210-255 (2019),
   [28]https://arxiv.org/abs/1810.12602

   ATLAS Collaboration, Performance of the reconstruction of large impact parameter
   tracks in the inner detector of ATLAS, ATL-PHYS-PUB-2017-014,
   [29]http://cds.cern.ch/record/2275635

   ATLAS Collaboration, Summary Plots for Heavy Particle Searches and Long-lived
   Particle Searches - July 2022, ATL-PHYS-PUB-2022-034,
   [30]http://cds.cern.ch/record/2815305

   CMS Collaboration, Precision measurement of the structure of the CMS inner
   tracking system using nuclear interactions, JINST 13 (2018) P10034

   LHCb Collaboration, Search for \( A'\rightarrow \mu^+\mu^- \) decays, Phys. Rev.
   Lett. 124 (2020) 041801, [31]https://arxiv.org/abs/1910.06926

   [32]https://faser.web.cern.ch/

   [33]https://snd-lhc.web.cern.ch/

   J. L. Feng et al, The Forward Physics Facility at the High-Luminosity LHC,
   [34]https://arxiv.org/abs/2203.05090

   S. Cerci et al, FACET: A new long-lived particle detector in the very forward
   region of [35]the CMS experiment, to appear in JHEP,
   [36]https://arxiv.org/abs/2201.00019

   [37]https://mathusla-experiment.web.cern.ch/

   M. Bauer, O. Brandt, L. Lee and C. Ohm, ANUBIS: Proposal to search for long-lived
   neutral particles in CERN service shafts, [38]https://arxiv.org/abs/1909.13022.

   G. Aielli et al, The Road Ahead for CODEX-b, [39]https://arxiv.org/abs/2203.07316

   V. V. Gligorov, S. Knapen, B. Nachman, M. Papucci, and D. J. Robinson, Leveraging
   the ALICE/L3 cavern for long-lived particle searches, Phys. Rev. D 99, 015023
   (2019)

   [40]https://moedal.web.cern.ch/

   J. L. Pinford, The MoEDAL experiment: a new light on the high-energy frontier,
   Phil.Trans.Roy.Soc.Lond.A 377 (2019) 2161, 20190382

   milliQan Collaboration, Sensitivity to millicharged particles in future
   proton-proton collisions at the LHC with the milliQan detector, Phys. Rev. D 104
   (2021) 3, 032002, [41]https://arxiv.org/abs/2104.07151

   S. Foroughi-Abari, F. Kling and Y-D. Tsai, FORMOSA: Looking Forward to
   Millicharged Dark Sectors, Phys. Rev. D 104, 035014 (2021),
   [42]https://arxiv.org/abs/2010.07941
   Sponsored by: [43]Dr. Lydia Roos, CNRS/IN2P3/LPNHE, Paris, France
   [44]Reviewed by: [45]Prof. Lesya Shchutska, EPFL, Lausanne, Switzerland
   [46]Reviewed by: [47]Dr. Jamie Boyd, CERN, Geneva, Switzerland
   Accepted on: [48]2022-12-08 14:15:41 GMT
   Retrieved from
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