Heavy QCD Axion at Belle II: Displaced and Prompt Signals

Abstract

The QCD axion is a well-motivated addition to the standard model to solve the strong CP problem. If the axion acquires mass dominantly from a hidden sector, it can be as heavy as O(1) GeV, and the decay constant can be as low as O(100) GeV without running into the axion quality problem. We propose new search strategies for such heavy QCD axions at the Belle II experiment, where the axions are expected to be produced via B → Ka. We find that a subsequent decay a → 3π with a displaced vertex leads to a unique signal with essentially no background, and that a dedicated search can explore the range O(1-10) TeV of decay-constant values. We also show that a → γγ can cover a significant portion of currently unexplored region of 150 ma 500 MeV.

Axion-like particle searches at DarkQuest

“…Longer-term prospects for discovering gluon-coupled ALPs include phase 2 of FASER [81], MATHUSLA [32,91] and DUNE [66]. Heavier gluon-coupled ALPs can be produced and detected at Belle II [92], LHCb [93], ATLAS [84,94] and CMS [94][95][96][97], especially using "triggerless" techniques [98]. In the left panel we compare the reach of phase 1 and 2 of DarkQuest (black solid and dashed lines, respectively) to existing constraints in gray (these are described in section 6.2.1).…”

New Physics Searches at Kaon and Hyperon Factories

“…For example, in K + → π + γγ search, the diphoton invariant mass shifts to a lower value than the ALP mass as m γγ m a (1 − d/L) where L is the distance from the K + decay point to the ECAL, and d is the displacement of ALP decay. Similar issue was discussed in the context of B ± → K ± a(→ γγ) [141,142].…”

“…For m a 230 MeV, an ALP coupling to gluons is excluded by the K + → π + γγ measurement at NA48/NA62 [147,148], while a 10 −10 sensitivity in the branching ratio is necessary for a definitive test of the electroweak scenario. Note that the recent Babar analysis excludes the large paramater space in a mass range 175 m a 260 MeV for the gluon and electroweak scenarios [141,142]. Similarly, reducing the bounds on B(K L → π 0 γγ) to ∼ 10 −10 at KOTO or KLEVER would test the scenarios (a) and (b) for m a 170 MeV.…”

“…(26). Complementary bounds from other collider probes such as LEP [135,136], CDF [137], GlueX [138,139] and Bfactories [100,106,[140][141][142] 4 can also be relevant. For the large decay constant, the ALPs are long-lived,…”

“…The bound will be stronger if Λ UV = 4πf a because the RGE effect is enhanced. The projection of this channel at Belle II is discussed in Ref [142]…”