Exploring the lifetime and cosmic frontier with the MATHUSLA detector
PPrepared for submission to JINST
XV Workshop on Resistive Plate Chambers and Related Detectors RPC202010-14 February 2020Rome, Italy
Exploring the lifetime and cosmic frontier with theMATHUSLA detector on behalf of MATHUSLA collaboration
Cristiano Alpigiani a a University of Washington, Seattle
E-mail:
Abstract: The MATHUSLA detector to be installed on the surface above and somewhat displacedfrom the CMS interaction point (IP) will cover an area of 100 ×
100 m containing many layers ofscintillators planes to establish the space and time coordinates of charged particle tracks. This isan unprecedented detector in terms of size and continuous sensitivity over an area of 10 m . Thisdocument describes the present MATHUSLA detector concept that is sensitive to both long-livedparticles produced in the LHC collisions in CMS and cosmic ray extended air showers (EAS). Theability to improve significantly cosmic ray studies by adding a 10 m layer of RPCs that have bothdigital and analogue readout similar to the ARGO-YBJ experiment will be discussed with focus onlarge zenith angle EAS.Keywords: Large detector systems for particle and astroparticle physics; Particle tracking detectors a r X i v : . [ phy s i c s . i n s - d e t ] J un ontents Long-lived particles (LLPs) occur in many extensions to the Standard Model (SM) with lifetimesthat can be as long as the Big Bang Nucleosynthesis (BBN) bound of about c τ (cid:46) –10 m [1].Examples of models where such particles are predicted which can be produced at the Large HadronCollider (LHC) include: Supersymmetric (SUSY) models such as RPV SUSY [2] and StealthSUSY [3], models addressing the hierarchy problem such as Hidden Valleys [4], and modelsaddressing dark matter [5].The main experiments at the LHC have extensive programs to search for such particles, coveringlifetimes from a few centimeters to tens of meters. Searches for LLPs decaying into final statescontaining jets were carried out at the Tevatron ( √ s = .
96 TeV) by both the CDF [6] and D0 [7]Collaborations, at the LHC by the ATLAS and LHCb Collaborations in proton–proton collisions at √ s = √ s = √ s =
13 TeV [15–17]. To date, no search hasobserved evidence of beyond the Standard Model, neutral LLPs. However, their reach is limited bydifferent factors such as the trigger, the presence of backgrounds from the collision or beam effects,and ultimately by the size of the detectors. For example, searches for LLPs decaying to hadrons(leptons) with less than a few 100 GeV ( ∼
10 GeV) of visible energy in the event have particularlylow trigger efficiency and are highly constrained by QCD and other backgrounds. These limitationscould risk missing a discovery should LLP with a lifetime close to the BBN be created at the LHCcollisions.MATHUSLA (MAssive Timing Hodoscope for Ultra-Stable neutraL pArticles) [18] is a pro-posed large-scale surface detector to be located above ATLAS [19] or CMS [20] to study LLPproduced by the High-Luminosity LHC (HL-LHC) [21]. The ∼
90 m of rock between the in-teraction point (IP) and the detector’s decay volume on the surface gives enough shielding forMATHUSLA to work in a clean environment. Being a background-free experiment increasesthe sensitivity to LLP lifetimes up to lifetimes of 10 m and extends the sensitivity of the main– 1 –etectors by orders of magnitude. LLP decays would be reconstructed as displaced vertices ofupwards traveling charged particles. As a secondary physics objective, MATHUSLA would also beable to perform cosmic-ray physics measurements and help solve important puzzles in astroparticlephysics.A white paper describing the need for a detector like MATHUSLA was published by a largenumber of experimentalists and theorists in 2018 [22]. The MATHUSLA collaboration has madesignificant progress on the detailed background and design studies and presented a Letter of In-tent [23] to the LHCC. A test stand with a detector layout similar to the one envisioned for theMATHUSLA detector was assembled in the surface above ATLAS and took data during 2018.It was composed of one external layer of scintillators in the upper part and one in the lower partwith six layers of RPCs (from the ARGO-YBJ experiment) between them. The overall structurewas ∼ . ∼ . × . . The analysis of the test stand data has just beenpublished [24]. The results provided empirical information on backgrounds coming from the LHCas well as from cosmic rays (CR). The collaboration is now seeking to construct a MATHUSLAdemonstrator detector unit by 2021. The full-scale detector could become operational by 2025–26. LLP near the BBN lifetime bound arising from exotic Higgs decays could be discovered if thedetector had a linear size of ∼
20 m in the direction of travel with good geometric coverage ( ∼ O (100 m). The ∼
90 meters ofrock between the collision point and the surface eliminates most backgrounds associated with pp collisions. Still, a large background of cosmic muons and backgrounds from high energy muonsand neutrinos coming from the IP must be rejected which requires good tracking and vertexingcapabilities. The proposed detector, illustrated in the left panel of figure 1 along with two possibledisplaced vertices from LLP decays, is a large box of 100 × ×
25 m volume, with a robusttracking system on its upper part, 25 m air decay volume and a tracking veto on the floor. Anadditional double-tracking layer 5 m below the main tracking system allows to enhance the particleposition measurement precision close to the floor.The expected sensitivity for a SM-like Higgs boson producing two long-lived scalars decayinginto hadronic jets for the current benchmark geometry in the expected luminosity for the HL-LHCis shown in the right plot of figure 1. The MATHUSLA limit is obtained assuming 4 LLPs decayingin the detector volume in a zero-background regime. It is compared to the exclusion projection fora single displaced vertex search in the ATLAS Muon System considering the background expectedat the HL-LHC [25].The dominant background comes from CRs, with a rate in the MHz range. Their rejectiondepends on the robust ceiling tracking system, with spatial and temporal resolutions in cm andnanosecond range, respectively. If the tracking layers span a vertical distance of a few meters, full4-dimensional track and displaced vertex reconstruction is possible, which significantly reduces thecombinatorial backgrounds as tracks must intersect in both space and time to form a vertex. BothResistive Plate Chambers (RPCs) and plastic scintillators are time-tested technologies that meet the– 2 – .001 0.100 10 1000 10 - - - - c τ X ( m ) B r ( h -> XX ) σ pp → h → XX ( f b ) s =
14 TeV, 3ab - h → XX, X → jj m X =
20 GeV
BBN L i m i t HL - LHC h → invisiblesensitivity A TL A S M S D V M A T H U S L A a t C M S D ecays do m i n a n t l y i n M a i n D e t ec t o r Figure 1 . Left: simplified MATHUSLA layout. Right: sensitivity comparison for a SM-like Higgs bosonproducing two long-lived scalars with a mass of 20 GeV decaying into hadronic jets, for MATHUSLA inred and the ATLAS exclusion projection using a search in the Muon System [25]. The MATHUSLA curvecorresponds to 4 LLPs decaying in the detector volume in a zero-background regime for a total integratedluminosity of 3 ab − expected over the entire HL-LHC data taking period. needed specifications. Since CRs travel downwards and do not inherently form displaced vertices,this signal requirement is expected to allow MATHUSLA to reach the near-zero-background regime.The expected rate of muons from the HL-LHC collisions is O (1 Hz). They are upwardstraveling muons that do not generally produce a displaced vertex and that can be vetoed by the floortracker. Upward going atmospheric neutrinos are estimated to be of order 10 to 100 per year, mostcan be rejected using the time of flight information of the LLP decay products. Moreover, they canbe measured when there are no LHC beams. Neutrinos from LHC collisions are a subdominantbackground, estimated to be a few events during the entire HL-LHC data taking, and can berejected with geometrical cuts and timing vetoes on non-relativistic charged tracks associated withthe scattering event. A more detailed description of the possible backgrounds can be found inref. [23], while more precise rate estimates from the analysis of the data collected by the test standare described in ref. [24]. Figure 2 . Schematics of the modular design of MATHUSLA (left) and structure of the modules (right).
MATHUSLA is designed to be a large area detector, requiring to cover a wide surface with detectormaterial. Building MATHUSLA as an array of independent modules makes it a flexible and scalabledetector, easy to adapt to the available land and the specific site conditions. It also allows for a– 3 –taged integration with an incremental ramp-up. One of the advantages of MATHUSLA is that itis entirely parasitic: its construction and operation are not expected to interfere with the operationof ATLAS or CMS and its staged construction can happen as a completely independent plan fromthat of the HL-LHC and the experiments upgrade work.The current design considers individual modules with a volume of 9 m × ×
25 m and aseparation of ∼ × ×
100 mdetector. The strategy is to collect all detector hits with no trigger selection and separately recordthe trigger information. The data rate is dominated by CRs, 1/(cm minute), which gives ∼ , two hits per module with 4 bytes perreadout, the readout of 9 layers gives ∼ β = .
7, an optical fibretransmission to CMS with v fibre = . µ s /
100 m will guarantee the detector around 3 . µ s or moreto form trigger and get information to CMS Level-1 trigger (at the HL-LHC the CMS trigger willhave a latency time of around 12 µ s). CERN owns an available piece of land near CMS that would be a suitable site for the detector [23].The MATHUSLA collaboration is working with Civil Engineers from CERN to define the buildingand the layout of the detector. Figure 3 shows the details on the planned position (left) and the sizeof the detector (right).
Figure 3 . Left: CERN-owned land near CMS and location of MATHUSLA. The IP in CMS is marked as ared star. Right: Schematics showing the currently proposed dimensions of the MATHUSLA building. – 4 –he current proposal contemplates a 100 m ×
102 m experimental area located on the surfaceof CMS together with a 30 m ×
100 m adjacent area for the detector assembly. The total height of ∼
40 m includes a ∼
25 m decay volume, 21 m of which would be excavated, and 12 m in the upperpart to host the tracker and the cranes’ system for assembly and maintenance. Having a large partof the decay volume underground brings it closer to the IP, which increases the solid angle in theacceptance for LLPs generated in the collisions. To adjust to the available land, this proposal has a7.5 m offset to the centre of the beams. The site allows for the detector to be as close as 68 m awayfrom the IP.The original MATHUSLA proposal [18] assumed a distance of 100 m from the IP bothhorizontally and vertically. Reducing these distances as explained above, the current proposal canreach a similar LLP sensitivity as the original detector with a final detector design more optimised,with smaller geometry that is cost-efficient and tailored to the available experimental site.
MATHUSLA has all the qualities needed to act as an excellent CR telescope. MATHUSLA’s largearea provides good efficiency for extended air showers from primary CRs. Its combination withhigh-resolution directional tracking and proximity to ATLAS or CMS for correlated shower coremeasurements could allow more detailed studies of the core structure, crucial to determine theatomic number of the primary cosmic particles. These measurements, which do not interfere withthe primary goal of LLP discovery, represent a “guaranteed physics return” on the investment of thedetector, as well as an opportunity to establish a CR physics program. The qualitative CR physicscase was discussed in [22].CRs up to the knee (3–4 10 eV) originate in supernova remnants and are accelerated bythe 1st order Fermi mechanism in shock waves. The evolution of light nuclei spectra (p+He)could be an indication of the contribution of different populations of CRs (coming from differentchemical compositions). Around the knee, CR measurements are performed through EAS arraysexperiments. It is still not clear whether the mass of the knee is due to p and He spectra or highernuclei, and the results of the current experiments show some disagreement in this region. Therefore,analyse the primary proton spectrum is crucial to understand CR acceleration and propagation inthe Galaxy, and precise measurements of the flux could allow to calculate the rate of secondary CRand atmospheric neutrinos.Figure 4 shows an example of a vertical shower reconstructed by the MATHUSLA detectorusing 7 layers of scintillator detectors (4 cm × particles/m ,and they are subdivided into Big Pads (BP). The position and arrival times of the particles can bemeasured with individual strips in each BP. For the current simulations, the BP provides space-timedata of the shower with a time resolution of 1 ns and can have different sizes (2 × , 2 × ,1 × ). The detector is considered 100% efficient and the signal is proportional to the particledensity (linear response) assuming identical sensitivity to all charged particles.The simulation is performed using CORSIKA 7.6400. For the high-energy ( E h >
200 GeV)and low-energy hadronic models, QGSJET-II-04 and Geisha are used, respectively. Only chargedparticles ( e ± , µ ± , π ± , K ± , p ± ) are registered in each layer. Unstable particles are allowed to decay,– 5 –ut the decay products are removed from the simulation. The core of the air shower is scatteredrandomly in MATHUSLA, and for each shower event, the coordinates of the hit bars/BP and thearrival times of 1st particle to each hit bar/BP are registered. Figure 4 . Example of a vertical air shower in MATHUSLA.
The main disadvantages of using only scintillators are the impossibility of measuring the arrivaltimes of the particles at the front of the EAS and the number of particles for events with zenithangles θ < ◦ . This will not allow to measure the arrival direction of CRs with a high precisionand it will make difficult to perform CR composition studies. The scintillator bar saturates at onehit making possible to find the impact point and arrival direction for one particle. For the currentstudies, we assign the coordinates of the centre of the bar to the hit, and register only the arrivaltime of the 1st particle that arrives to the scintillator bar.The RPC digital readout might allow to improve the measurement of the spatial and temporalstructure of an Extensive Air Shower (EAS), and perform low-density measurements. The analoguesystem has the advantage of allowing to measure the high density of particles up to 10 /m in thestreamer working mode, as shown by Argo-YBJ. In MATHUSLA, the RPCs will work in avalanchemode. This will extend of at least one order of magnitude the linearity range in the hit densitymeasurement. This will allow to study the shower core profile with an unprecedented detail, thusexpanding the measurements of CRs beyond the knee. The additional RPC layer could allow toprecisely measure the shower front by having a good time-spatial determination of it.This could improve the determination of the core and the arrival direction of the shower,important for vertical EAS, where the saturation effects in the scintillation planes can lower the coreand arrival direction precision. On the other hand, with measurements of the density of chargedparticles, the lateral distribution function (LDF) of charged particles can be obtained event-by-event, which can help to determine the energy scale of the primary CR and the composition of theCR nuclei. The energy scale can be estimated from the amplitude of the lateral distribution, andthe primary composition could be studied by using the steepness of the LDF (the lighter and moreenergetic the air shower, the bigger the steepness of the LDF). Moreover, one RPC layer can improvethe measurements of the vertical and inclined events on the energy and the deposited charge. Allthese additional information could allow the reconstruction of the all particle energy spectrum from– 6 –ertical and inclined events up to 100 PeV, obtain large scale anisotropy maps in arrival directionsof the CRs, measure small scale anisotropies in arrival directions, and search for point sources.Moreover, they will also allow testing, more precisely, the hadronic interaction models. Figure 5 . Preliminary performance (still to be optimised) for measurements of the arrival direction andcore position for vertical events (number of hits > 50 and θ < ◦ ) using information from scintillator andRPC detectors. Figure 5 shows the preliminary performance (no optimisation is performed yet) for measure-ments of the arrival direction and core position for vertical events (number of hits > 50 and θ < ◦ )using information from scintillator and RPC detectors. For comparison, the Argo-YBJ angularaccuracy was lower than 6 ◦ in the TeV range. LLPs occur in a wide variety of beyond the Standard Model scenarios addressing the most fun-damental mysteries of particle physics. In this document, I presented the MATHUSLA detectorthat could extend the sensitivity to long decay lifetimes by orders of magnitude compared to LHCdetector searches. Moreover, MATHUSLA could act as a CR telescope, and it could performvery precise CR measurements up to the PeV scale. By integrating a device with the possibilityto measure arrival times and particle densities of extensive air showers, as an RPC, MATHUSLAcan be employed as a CR detector and monitor a big portion of the sky above ( θ < ◦ ), withoutlimitation to inclined events. – 7 – eferences [1] A. Fradette and M. Pospelov, BBN for the LHC: constraints on lifetimes of the Higgs portal scalars ,Phys. Rev. D96 (2017), no. 7 075033, [arXiv:1706.01920].[2] R. Barbier et al.,
R-parity violating supersymmetry , Phys. Rept. (2005) 1-202,10.1016/j.physrep.2005.08.006, [ arXiv:hep-ph/0406039 ].[3] J. Fan, M. Reece, J. T. Ruderman,
Stealth Supersymmetry , JHEP (2011) 12,10.1007/JHEP11(2011)012, [ arXiv:1105.5135 [hep-ph] ].[4] M. J. Strassler, K. M. Zurek, Echoes of a Hidden Valley at Hadron Colliders , Phys. Lett. B (2007)374-379, 10.1016/j.physletb.2007.06.055, [ arXiv:hep-ph/0604261 ].[5] D. Tucker-Smith, N. Weiner,
Inelastic dark matter , Phys. Rev. D (2001) 043502,10.1103/PhysRevD.64.043502, [ arXiv:hep-ph/0101138 ].[6] CDF Collaboration, Search for heavy metastable particles decaying to jet pairs in p ¯ p collisions at √ s = . TeV , Phys. Rev. D (2012) 012007, 10.1103/PhysRevD.85.012007, [ arXiv:1109.3136[hep-ex] ].[7] D0 Collaboration, Search for Resonant Pair Production of Neutral Long-Lived Particles Decaying to bb in pp Collisions at √ s = .
96 TeV, Phys. Rev. Lett. (2009) 071801,10.1103/PhysRevLett.103.071801, [ arXiv:0906.1787 [hep-ex] ].[8] ATLAS Collaboration,
Search for a Light Higgs Boson Decaying to Long-Lived Weakly InteractingParticles in Proton-Proton Collisions at √ s = TeV with the ATLAS Detector , Phys. Rev. Lett. (2012) 251801, 10.1103/PhysRevLett.108.251801, [ arXiv:1203.1303 [hep-ex] ].[9] LHCb Collaboration,
Search for long-lived particles decaying to jet pairs , Eur. Phys. J. C (2015)152, 10.1140/epjc/s10052-015-3344-6, [ arXiv:1412.3021 [hep-ex] ].[10] ATLAS Collaboration, Search for long-lived, weakly interacting particles that decay to displacedhadronic jets in proton-proton collisions at √ s = TeV with the ATLAS detector , Phys. Rev. D. (2015) 012010, 10.1103/PhysRevD.92.012010, [ arXiv:1504.03634 [hep-ex] ].[11] ATLAS Collaboration, Search for massive, long-lived particles using multitrack displaced vertices ordisplaced lepton pairs in pp collisions at √ s = TeV with the ATLAS detector , Phys. Rev. D (2015) 072004, 10.1103/PhysRevD.92.072004, [ arXiv:1504.05162 [hep-ex] ].[12] CMS Collaboration, Search for long-lived neutral particles decaying to quark-antiquark pairs inproton-proton collisions at √ s = , Phys. Rev. D (2015) 012007,10.1103/PhysRevD.91.012007, [ arXiv:1411.6530 [hep-ex] ].[13] LHCb Collaboration, Updated search for long-lived particles decaying to jet pairs , Eur. Phys. J. C (2017) 812, 10.1140/epjc/s10052-017-5178-x, [ arXiv:1705.07332 [hep-ex] ].[14] LHCb Collaboration, Search for massive long-lived particles decaying semileptonically in the LHCbdetector , Eur. Phys. J. C (2017) 224, 10.1140/epjc/s10052-017-4744-6, [ arXiv:1612.00945[hep-ex] ].[15] CMS Collaboration, Search for new long-lived particles at √ s = TeV , Phys. Lett. B (2018)432, 10.1016/j.physletb.2018.03.019, [ arXiv:1711.09120 [hep-ex] ].[16] ATLAS Collaboration,
Search for long-lived particles produced in pp collisions at √ s = TeV thatdecay into displaced hadronic jets in the ATLAS muon spectrometer
Phys. Rev. D (2019) 052005,10.1103/PhysRevD.99.052005, [ arXiv:1811.07370 [hep-ex] ]. – 8 –
17] ATLAS Collaboration,
Search for long-lived neutral particles in pp collisions at √ s = TeV thatdecay into displaced hadronic jets in the ATLAS calorimeter , Eur. Phys. J. C , 481 (2019),10.1140/epjc/s10052-019-6962-6, [ arXiv:1902.03094 [hep-ex] ].[18] J. P. Chou, D. Curtin, and H. J. Lubatti, New Detectors to Explore the Lifetime Frontier , Phys. Lett.
B767 (2017) 29–36, [ arXiv:1606.06298 ].[19] ATLAS Collaboration,
The ATLAS Experiment at the CERN Large Hadron Collider , 2008 JINST 3S08003.[20] CMS Collaboration,
The CMS experiment at the CERN LHC , JINST (2008) S08004,10.1088/1748-0221/3/08/S08004[21] G. Apollinari, O. Bruening, T. Nakamoto, L. Rossi, High Luminosity Large Hadron ColliderHL-LHC , [ arXiv:1705.08830 [physics.acc-ph] ].[22] D. Curtin et al.,
Long-Lived Particles at the Energy Frontier: The MATHUSLA Physics Case , arXiv:1806.07396 .[23] C. Alpigiani et al., A Letter of Intent for MATHUSLA: a dedicated displaced vertex detector aboveATLAS or CMS , CERN-LHCC-2018-025, LHCC-I-031 (2018) [ arXiv:1811.00927 ].[24] M. Alidra, C. Alpigiani, A. Ball, P. Camarri, R. Cardarelli et al.,
The MATHUSLA Test Stand ,[ arXiv:2005.02018 ], submitted to Nuclear Instruments and Methods in Physics Research.[25] A. Coccaro, D. Curtin, H. J. Lubatti, H. Russell, J. Shelton, Data-driven Model-independent Searchesfor Long-lived Particles at the LHC , Phys. Rev. D arXiv:1605.02742] .[26] D. Curtin and M. E. Peskin, Analysis of Long Lived Particle Decays with the MATHUSLA Detector , Phys. Rev.
D97 (2018), no. 1 015006, [ arXiv:1705.06327 ].[27] GEANT4 Collaboration,
GEANT4: A Simulation toolkit , Nucl. Instrum. Meth. 407
A506 (2003)250-303.[28] T. Sato,
Analytical model for estimating the zenith angle dependence of terrestrial cosmic ray fluxes ,409 PLOS ONE (08, 2016) 1-22.[29] T. Sjostrand, S. Mrenna, and P. Z. Skands, A Brief Introduction to PYTHIA 8.1 , Comput. Phys.Commun. 411 (2008) 852-867, [ arXiv:0710.3820 ].].