A general method for model-independent measurements of particle spins, couplings and mixing angles in cascade decays with missing energy at hadron colliders

Abstract

We outline a general strategy for measuring spins, couplings and mixing angles in the case of a heavy partner decay chain terminating in an invisible particle. We consider the common example of a heavy scalar or fermion D decaying sequentially to other heavy particles C, B and A by emitting a quark jet j and two leptons ℓ ± n and ℓ ∓ f . We derive analytic formulas for the dilepton ({ℓ + ℓ − }) and the two jet-lepton ({jℓ n } and {jℓ f }) invariant mass distributions for the case of most general couplings and mixing angles of the heavy partners. We then consider various spin assignments for the heavy particles A, B, C and D, and for each case, derive the relevant functional basis for the invariant mass distributions which contains the intrinsic spin information and does not depend on the couplings and mixing angles. We propose a new method for determining the spins of the heavy partners, using the three experimentally observable distributions {ℓ + ℓ − }, {jℓ + } + {jℓ − } and {jℓ + } − {jℓ − }. We show that the former two only depend on a single model-dependent parameter α, while the latter may depend on two other parameters β and γ. By fitting these distributions to our set of basis functions, we are able to do a pure measurement of the spins per se. Our method is also applicable at a pp collider such as the Tevatron, for which the previously proposed lepton charge asymmetry is identically zero and does not contain any spin information. In the process of determining the spins, we also end up with an independent measurement of the parameters α, β and γ, which represent certain combinations of the couplings and the mixing angles of the heavy partners A, B, C and D.

Spin effects in the antler event topology at hadron colliders

“…in the pure "phase space approximation". Then, any deviation from the expected "phase space" distribution will indicate spin effects, although a priori it will not be immediately clear whether those are due to non-trivial intrinsic spins of the particles {A, B, C}, non-trivial coupling chiralities, or some combination of the two [20]. In section 5 we therefore provide the expected "phase space" distributions for several kinematic variables, including the angular variable cos θ * ℓ − ℓ + proposed in [8] (see section 3.1 below for its exact definition).…”

Heavy color-octet particles at the LHC

“…This strategy has been studied extensively for similar decay chains involving leptons instead of top quarks [51][52][53][54]. In contrast to these studies, one must account for the non-negligible mass of the top quark.…”

Dark matter particle spectroscopy at the LHC: generalizing M T2 to asymmetric event topologies

“…Alternatively, one can try to measure the spin of the missing particle and show that it is different from 1/2 (the spin of the neutrino). While there is a large body of recent work on spin measurements in missing energy events , once again very few of those methods are modelindependent 2 [45,47]. Furthermore, in all considered examples in the literature the spin measurement appears to be very difficult.…”