Session Y07: Muon Collider Symposium IV

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\section{When the 5 MW, 2.5 GeV, 1.3 $\backslash $textmu s proton pulses hit the ESSnuSB neutrino target there will be a copious production of not only neutrinos but also of muons. These muons can be used to realize low energy versions of nuSTORM for neutrino cross-section measurements and sterile neutrino searches and of a Neutrino Factory for high precision PMNS parameter measurements. An overview will be given of the implementations of nuSTORM and Neutrino factory projects on the ESS site and the design work that will be required to evaluate their technical challenges and physics performances.}   

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Collimated beams for a Muon Collider can be produced with an intense positron beam stored in a large energy acceptance ring and directed to a multi-target system, producing muon pairs at threshold. To achieve significantly high muon beam intensities, each target of this system must be subject to a high power load and a large integrated Peak Energy Density Deposition (PEDD). Feasibility studies have been carried out investigating the properties of suitable targets: different low-Z materials, varying target thickness and configuration have been considered. A theoretical model has been developed to simulate the time evolution of the target heating, from energy deposition, to space-time temperature field calculation, and to thermal stress field derivation from temperature gradients. A targetry system with multiple elements has been simulated to evaluate the interference effect in each element’s heat radiation. Results of the simulations for Be- and C-composites targets will be presented for a specific positron beam bunch pattern.   

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Low energy probes of lepton flavor violation (LFV) are indirectly probing new physics beyond the TeV scale, with order of magnitude advances expected in the future. A high energy muon collider would have the reach to probe similar processes at higher energies, e.g., via $\mu^+ \mu^- \to \tau^{\pm} \mu^{\mp}$, which can be compared to the low-energy flavor-violating decay bounds. Alternatively, in particular models of new physics, new particles with flavor-violating interactions can be produced directly, such as mixed slepton pair production in the MSSM. I’ll present some first estimates of the physics reach of a muon collider for both of these scenarios, with an emphasis on the complementarity between low-energy precision experiments and high-energy muon collider searches.   

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Certain dimension-eight operator coefficients of the Standard Model Effective Field Theory (SMEFT) are subject to positivity bounds, derived from fundamental principles of Quantum Field Theory, such as unitarity, locality, analyticity, and Lorentz invariance. We discuss the unique advantage of a multi-TeV muon collider in probing these positivity bounds. We point out a special channel, mu+mu- (e+ e-) -> gamma gamma, for which the leading new physics contribution comes only from dimension-8 operators. The positivity bounds are thus applicable to the most direct observable --- the diphoton cross-sections. This unique feature provides a clear, robust, and unambiguous test of the principles of Quantum Field Theory. We estimate the capability of various future lepton colliders and point out the importance of having a high center-of-mass energy in probing this channel. We also perform a combined analysis of the gamma gamma, Z gamma, and ZZ processes in the high energy limit and point out the important interplay among them.   

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Weakly Interacting Massive Particles (WIMPs) are theoretically appealing Dark Matter (DM) candidates. Generalizing the Minimal Dark Matter paradigm, we consider stable DM candidates within a single $SU(2)_L$ $n$-plet, with odd $n$ in order to avoid strong constraints from direct detection and $n \leq 7$ to ensure calculability. WIMPs are perfect candidates for a high-energy lepton collider, given the electroweak nature of the signal, and since their thermal masses lie in the range $1-45$ TeV \footnote{For previous work, see M. Cirelli et al, JHEP, 10 (2014) [Erratum: JHEP 01, 041 (2015)], M. Low et al, JHEP, 08 (2014), T. Han et al, arXiv:2009.11287}. \\ We consider the reach of the mono-$\gamma$, the mono-W and the di-$\gamma$ channels, for both scalar and fermion WIMPs. We compute the signal-to-noise ratio and the reaches for each $n$-plet and compare them with their respective thermal targets. Our key result is that a 30 TeV muon collider can probe the DM 5-plets at 95\% CL, while to produce the 7-plets a higher energy machine is required.   

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Multi Higgs production at a future high-energy muon collider provides a unique opportunity to gather information on the Higgs potential, ie. the determination of the triple and quartic self-couplings. We explore the potential of combining information from H,HH,HHH production to constrain possible coupling deformations induced by new physics at high scales, including the first matrix-element based evaluation of the irreducible backgrounds.   

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After the triumph of discovering the Higgs boson at the CERN Large Hadron Collider, people are getting increasingly interested in studying the Higgs properties in detail and searching for the physics beyond the Standard Model (BSM). A multi-TeV muon collider provides a clean experimental environment for the Higgs precision measurements and for the discovery of new particles. However, in high-energy leptonic collisions well above the electroweak scale $M_Z$, the collinear factorization of the photon parton distribution function (PDF) based on the ``equivalent photon approximation (EPA)'' is not well defined anymore. All the SM particles should be treated as partons that radiated off the beam particles, and the electroweak parton distribution functions (EW PDFs) should be adopted as a proper description for partonic collisions of the initial states. In our work, the Dokshitzer-Gribov-Lipatov-Altarelli-Parisi (DGLAP) formalism is employed to perturbatively resum the potential large logarithms emerging from the initial-state radiation (ISR). We present EW parton luminosities and semi-inclusive cross sections for several important SM processes at a future multi-TeV muon collider and show it is appropriate to adopt the EW PDFs for future high-energy leptonic colliders.   

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Muon colliders offer a great opportunity to discover and prove new physics beyond the Standard Model. Dark SUSY models, for instance, couple the supersymmetric particles with the dark sector, and long-lived dark matter particles are expected to decay with a clear signature, i.e. very collimated muon pairs. The study of decay channels with dark matter particles coming from neutralinos produced in muon collisions at 3 TeV centre-of-mass energy is presented for the time being without the effects of the machine Beam-Induced Background. Preliminary results of the muon reconstruction performances, obtained by analyzing the final state, characterized by muon pairs, are shown for a possible range of neutralino mass.   

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Charged particle trajectory reconstruction at a Muon Collider detector is more similar to the hadron collider environment than an electron collider. The presence of the beam-induced background leaves a large hit multiplicity in the tracking detector that complicates the pattern recognition stage of track reconstruction. The BIB hits increase the possible hit combinations that need to be filtered to create valid track candidates. This is analogous to the problem from pile-up hits in an hadron collider detector. The A Common Tracking Software (ACTS) is a library that implements the tracking algorithms developed by the collider tracking community, with a particular focus on hadronic environments. In addition to clever algorithms, it further tackles the tracking performance issue by heavily optimizing the code and exploring novel computing architectures. Due to the experiment-independent design, ACTS allows other experiments to leverage their complex tracking algorithms in different settings. This contribution will explore the usage of ACTS to perform the track reconstruction for simulated muon collider events with full beam-induced background.