Angular distribution as an effective probe of new physics
in
semihadronic threebody meson decays
Abstract
We analyze, in a fully modelindependent manner, the effects of new physics on a few semihadronic threebody meson decays of the type , where are well chosen pseudoscalar mesons and denote fermions out of which at least one gets detected in experiments. We find that the angular distribution of events of these decays can probe many interesting new physics, such as the nature of the intermediate particle that can cause leptonflavor violation, or presence of heavy sterile neutrino, or new intermediate particles, or new interactions. We also provide angular asymmetries which can quantify the effects of new physics in these decays. We illustrate the effectiveness of our proposed methodology with a few well chosen decay modes showing effects of certain new physics possibilities without any hadronic uncertainties.
pacs:
13.20.v, 14.60.St, 14.80.jI Introduction
New physics (NP), or physics beyond the standard model, involves various models that extend the well verified standard model (SM) of particle physics by introducing a number of new particles with novel properties and interactions. Though various aspects of many of these particles and interactions are constrained by existing experimental data, we are yet to detect any definitive signature of new physics in our experiments. Nevertheless, recent experimental studies in meson decays, such as B2KorKstLL (), Aaij:2015esa (), B2DorDstLN () and Aaij:2017tyk () (where can be or ) have reported anomalous observations raising the expectation of discovery of new physics with more statistical significance. In this context, modelindependent studies of such semileptonic threebody meson decay processes become important as they can identify generic signatures of new physics which can be probed experimentally. In this paper, we have analyzed the effects of new physics, in a modelindependent manner, on the angular distribution of a general semihadronic threebody meson decay of the type , where and are the initial and final pseudoscalar mesons respectively, and denote fermions (which may or may not be leptons but not quarks) out of which at least one gets detected experimentally. Presence of new interactions, or new particles such as fermionic dark matter (DM) particles or heavy sterile neutrinos or long lived particles (LLP) would leave their signature in the angular distribution and we show by example how new physics contribution can be quantified from angular asymmetries. Our methodology can be used for detection of new physics in experimental study of various threebody pseudoscalar meson decays at various collider experiments such as LHCb and Belle II.
The structure of our paper is as follows. In Sec. II we discuss the most general Lagrangian and amplitude which include all probable NP contributions to our process under consideration. The relevant details of kinematics is then described in Sec. III. This is followed by a discussion on the angular distribution and the various angular asymmetries in Sec. IV. In Sec. V we present a few well chosen examples illustrating the effects of new physics on the angular distribution. In Sec. VI we conclude by summarizing the important aspects of our methodology and its possible experimental realization.
Ii Most general Lagrangian and Amplitude
Following the modelindependent analysis of the decay as given in Ref. Kim:2016zbg () and generalizing it for our process where can be etc. as appropriate and can be , , , , , , , , , , , (with denoting leptons, being sterile neutrino, as fermionic dark matter and as long lived fermions)^{1}^{1}1It is clear that we can not only analyze processes allowed in the SM but also those NP contributions from fermionic dark matter in the final state as well as including flavor violation. Our analysis as presented in this paper is fully modelindependent and general in nature., we can write down the effective Lagrangian facilitating the decay under consideration as follows,
(1)  
where , , , , , are the different hadronic currents which effectively describe the quark level transitions from to meson. It should be noted that we have kept both and terms. This is because of the fact that the currents and describe two different physics aspects namely the magnetic dipole and electric dipole contributions respectively. In the SM, vector and axialvector currents (mediated by photon, and bosons) and the scalar current (mediated by Higgs boson) contribute. So every other term in Eq. (1) except the ones with , and can appear in some specific NP model. Since, in this paper, we want to concentrate on a fully modelindependent analysis to get generic signatures of new physics, we shall refrain from venturing into details of any specific NP model, which nevertheless are also useful. It is important to note that , and can also get modified due to NP contributions.
In order to get the most general amplitude for our process under consideration, we need to go from the effective quarklevel description of Eq. (1) to the meson level description by defining appropriate form factors. It is easy to write down the most general form of the amplitude for the process depicted in Fig. 1 as follows,
(2) 
where , , , , and are the relevant form factors, and are defined as follows,
(3a)  
(3b)  
(3c)  
(3d)  
(3e)  
(3f) 
with and , in which are the 4momenta of the and respectively (see Fig. 1). All the form factors appearing in the amplitude in Eq. (2) and as defined in Eq. (3) are, in general, complex and contain all NP information. It should be noted that for simplicity we have implicitly put all the relevant CabibboKobayashiMaskawa matrix elements as well as coupling constants and propagators inside the definitions of these form factors. In the SM only and are present. Presence of NP can modify these as well as introduce other form factors^{2}^{2}2It should be noted that the form factors, especially the ones describing semileptonic meson decays, can be obtained by using the heavy quark effective theory HQET (), the lattice QCD Lattice (), QCD lightcone sum rule Lightcone () or the covariant confined quark model CCQM () etc. In this paper we present a very general analysis which is applicable to a diverse set of meson decays. Hence we do not discuss any specifics of the form factors used in our analysis. Moreover, we shall show, by using certain examples and in a few specific cases, that one can also probe new physics without worrying about the details of the form factors. Nevertheless, when one concentrates on a specific decay mode, considering the form factors in detail is always useful.. These various NP contributions would leave behind their signatures in the angular distribution for which we need to specify the kinematics in a chosen frame of reference.
Iii Decay Kinematics
We shall consider the decay in the GottfriedJackson frame, especially the centerofmomentum frame of the system, which is shown in Fig. 2. In this frame the parent meson flies along the positive direction with 4momentum and decays to the daughter meson which also flies along the positive direction with 4momentum and to , which fly away backtoback with 4momenta and respectively, such that by conservation of 4momentum we get, , , and . The fermion (which we assume can be observed experimentally) flies out subtending an angle with respect to the direction of flight of the meson, in this GottfriedJackson frame. The three invariant masssquares involved in the decay under consideration are defined as follows,
(4a)  
(4b)  
(4c) 
It is easy to show that , where and denote the masses of particles and respectively. In the GottfriedJackson frame, the expressions for and are given by
(5a)  
(5b) 
where
(6a)  
(6b)  
(6c) 
with the Källén function defined as,
It is clear that , and are functions of only. For the special case of (say) we have and . It is important to note that we shall use the angle in our angular distribution.
Iv Most general angular distribution and angular asymmetries
Considering the amplitude as given in Eq. (2), the most general angular distribution in the GottfriedJackson frame is given by,
(7) 
where , and are functions of and are given by,
(8a)  
(8b)  
(8c) 
with
(9a)  
(9b)  
(9c)  
(9d)  
(9e)  
(9f)  
(9g)  
(9h) 
In the limit , which happens when or etc., our expressions for the angular distribution matches with the corresponding expression in Ref. Kim:2016zbg (). It is important to remember that in the SM we come across scalar, vector and axial vector currents only. Therefore, in the SM, , which implies that,
(10a)  
(10b)  
(10c) 
It is interesting to note that in the special case of , such as in , we always have . For specific meson decays of the form allowed in the SM, one can write down , and , at least in principle. The SM prediction for the angular distribution can thus be compared with corresponding experimental measurement. In order to quantitatively compare the theoretical prediction with experimental measurement, we define the following three angular asymmetries which can precisely probe , and individually,
(11a)  
(11b)  
(11c) 
The angular asymmetries of Eq. (11) are functions of and it is easy to show that . We can do the integration over in Eq. (7) and define the following normalized angular distribution,
(12) 
where
(13) 
for and with
(14) 
From Eq. (13) it is easy to show that which also ensures that integration over on Eq. (12) is equal to . It is interesting to note that the angular distribution of Eq. (12) can be written in terms of the orthogonal Legendre polynomials of as well,
(15) 
Here we have followed the notation of Ref. Gratrex:2015hna () which also analyzes decays of the type , with only leptons for , in a modelindependent manner but using a generalized helicity amplitude method. The observables of Eq. (15) are related to , and of Eq. (12) as follows,
(16a)  
(16b)  
(16c) 
These angular observables ’s can be obtained by using the method of moments Gratrex:2015hna (); Beaujean:2015xea (). Another important way to describe the normalized angular distribution is by using a flat term and the forwardbackward asymmetry AngDist:Hiller () as follows,
(17) 
This form of the angular distribution has also been used in the experimental community AngDist:Expt () in the study of . The parameters and are related to , and as follows,
(18a)  
(18b) 
Thus we have shown that Eqs. (12), (15) and (17) are equivalent to one another. In this paper, we choose to work using the normalized angular distribution in terms of , and as shown in Eq. (12). This is because the terms , and can be easily determined experimentally by using the vs Dalitz plot which does not depend on any specific frame of reference. This Dalitz plot can be easily divided into four segments , , and as shown in Fig. 3. The segments are decided as follows,
Segment  :  , 
Segment  :  , 
Segment  :  , 
Segment  :  . 
The terms , and can thus be expressed in terms of the following asymmetries,
(19a)  
(19b)  
(19c) 
where denotes the number of events contained in the segment . Since the vs Dalitz plot does not depend on the frame of reference, we need not constraint ourselves to the GottfriedJackson frame of Fig. 2 and can work in the laboratory frame as well. Furthermore, we can use the expressions in Eq. (19) to search for NP.
V Illustrating the effects of new physics on the angular distribution
v.1 Classification of the decays
It should be emphasized that for our methodology to work, we need to know the angle in the GottfriedJackson frame, or equivalently the vs Dalitz plot, which demand that 4momenta of the final particles be fully known. Usually, the 4momenta of the initial and final pseudoscalar mesons are directly measured experimentally. However, depending on the detection possibilities of and we can identify three distinct scenarios for our process . We introduce the notations and to denote whether the fermion gets detected (✓) or not (✗) by the detector. Using this notation the three scenarios are described as follows.

. Here both and are detected, e.g. when or .

. Here either or gets detected, e.g. when , , , .

. Here neither nor gets detected, e.g. when , , , , , , etc.
It should be noted that the above classification is based on our existing experimental explorations. What is undetected today might get detected in future with advanced detectors. In such a case we can imagine that, in future, the modes grouped in S2 might migrate to S1 and those in S3 might be grouped under S2. Below we explore each of the above scenarios in more details.
v.2 Exploration of new physics effects in each scenario
The first scenario (S1) is an experimenter’s delight as in this case all final 4momenta can be easily measured and the vs Dalitz plot can be obtained. Here, our methodology can be used to look for the possible signature of new physics in rare decays such as