Prospects in the search for a new light $Z'$ boson with the NA64$μ$ experiment at the CERN SPS

Abstract

A light Z vector boson coupled to the second and third lepton generations through Lµ − Lτ current with mass below 200 MeV provides a very viable explanation in terms of new physics to the recently confirmed (g − 2)µ anomaly. This boson can be produced in the bremsstrahlung reaction µN → µN Z after a high energy muon beam collides with a target. NA64µ is a fixed-target experiment using a 160 GeV muon beam from the CERN Super Proton Synchrotron accelerator looking for the Z production and its subsequent decays, Z → invisible. In this paper, we present the study of the NA64µ sensitivity to search for such a Z . This includes a realistic beam simulation, the detailed detectors description and a discussion about the main potential background sources. A pilot run is scheduled in order to validate the simulation results. If those are confirmed, NA64µ will be able to explore all the remaining phase space which could provide an explanation for the g − 2 muon anomaly.

Probing New Gauge Forces with a High-Energy Muon Beam Dump

“…5. We also show the projected limits from other muon beam experiments, M 3 [74] and NA64µ [24]. Note that other proposed experiments such as Ref.…”

“…The sensitivity improvements beyond other similar proposed experiments, such as Refs. [18][19][20][21][22][23][24], stem from two features of such an experimental setup: the increased beam energy and direct coupling to muons.…”

“…In a beam-dump experiment, the high energy muon beam is 'dumped' into a dense material to greatly increase the total rate of interaction at the price of center-of-mass energy. This experimental setup can therefore test couplings too small to be probed at the main collider by several orders of magnitude in a slightly lower mass range [18][19][20][21][22][23][24].…”

On (g − 2)μ from gauged U(1)X

“…The NA64 search [118] constrains the X couplings to electrons, which enter the prediction for the X production rates. Future NA64µ [119,120] and M 3 [121] experiments will feature muons in the incoming beam and will have the potential to cover the entire parameter space of the L µ −L τ model, see figure 2 in ref. [65].…”

Axial vectors in DarkCast

“…In addition, NA64 µ [76,100] is a planned fixed-target experiment in which a dark photon can be produced via muon bremsstrahlung and subsequently decays invisibly. Future bounds that will be set by NA64 µ can be recast by adapting eq.…”