arXiv:1107.0630v2 [hep-ph] 6 Nov 2011
B → K1ℓ+ℓ−Decays in a Family Non-universal Z′Model
Ying Li∗, Juan Hua
Department of Physics, Yantai University, Yantai 264-005, China
Department of Physics, Chung Yuan Christian University, Chung-Li, Taiwan 320, Republic of China
(Dated: November 8, 2011)
The implications of the family non-universal Z′model in the B → K1(1270,1400)ℓ+ℓ−(ℓ = e,µ ,τ) decays
are explored, where the mass eigenstates K1(1270,1400) are the mixtures of1P1and3P1states with the mixing
angle θ. In this work, considering the Z′boson and setting the mixing angle θ = (−34±13)◦, we analyze the
branching ratio, the dilepton invariant mass spectrum, the normalized forward-backward asymmetry and lepton
polarization asymmetries of each decay mode. We find that all observables of B→K1(1270)µ+µ−are sensitive
to the Z′contribution. Moreover, the observables of B → K1(1400)µ+µ−are relatively strong θ-dependence;
thus, the Z′contribution will be buried by the uncertainty of the mixing angle θ. Furthermore, the zero crossing
position in the FBA spectrum of B → K1(1270)µ+µ−at low dilepton mass will move to the positive direction
with Z′contribution. For the tau modes, the effects of Z′are not remarkable due to the small phase space. These
results could be tested in the running LHC-b experiment and Super-B factory.
The flavor changing neutral currents (FCNC) b → sℓ+ℓ−(ℓ = e,µ,τ), forbidden in the standard model (SM) at the tree level,
are very sensitive to the flavor structure of the SM and to the new physics (NP) beyond the SM. The rare decays B → K1ℓ+ℓ−
involving axial-vector strange mesons, also induced by b → sℓ+ℓ−, have been the subjects of many theoretical studies in the
frame work of the SM [1–4] and some NP models, such as universal extra dimension , models involving supersymmetry 
and the fourth-generation fermions . Generally, these semileptonic decays provide us with a wealth of information with a
number of physical observables, such as branching ratio, dilepton invariant mass spectrum, the forward backward asymmetry,
lepton polarization asymmetry and other distributions of interest, which play important roles in testing SM and are regarded as
probes of possible NP models.
In the quark model, two lowest nonets of JP= 1+axial-vector mesons are usually expected to be the orbitally excited q¯ q′
states. In the context of the spectroscopic notation n2S+1LJ, where the radial excitation is denoted by the principal number n,
there are two types of lowest p-wave meson, namely,13P1and11P1. The two nonets have distinctiveC quantumnumbers,C =+
orC = −, respectively. Experimentally, the JPC= 1++nonet consists of a1(1260), f1(1285), f1(1420), and K1A, while the 1+−
nonet contains b1(1235), h1(1170), h1(1380) and K1B. The physical mass eigenstates K1(1270) and K1(1400) are mixtures of
K1Aand K1Bstates owing to the mass differenceof the strange and non-strangelight quarks, and the relation could be written as:
, with M =
In the past few years, many attempts have been made to constrain the mixing angle θ [8–11]. In this study, we will use
θ = −(34±13)◦for numerical calculations, which has been extracted from B → K1(1270)γ and τ → K1(1270)ντby one of us
in  , and the minus sign is related to the chosen phase of |K1A? and |K1B?.
To make predictions of these exclusive decays, one requires the additional knowledge about form factors, i.e., the matrix
elements of the effective Hamiltonian between initial and final states. This problem, being a part of the nonperturbative sector
of QCD, lacks a precise solution. To the best of our knowledge, a number of different approaches had been used to calculate
the decay form factors of B → K1decays, such as QCD sum rules , light cone sum rules (LCSRs) , perturbative QCD
approach  and light front quark model . Among them, the results obtained by LCSRs which deal with form factors at
small momentum region, are complementary to the lattice approach and have consistence with perturbative QCD and the heavy
quark limit. On this point, we will use the results of LCSRs  in this work.
In some new physics models, Z′gauge boson could be naturally derived in certain string constructions  and E6models
 by adding additional U(1)′gauge symmetry . Among many Z′models, the simplest one is the family non-universal
Z′model. It is of interest to note that in such a model the non-universal Z′couplings could lead to FCNCs at tree level as well
as introduce new weak phases, which could explain the CP asymmetries in the current high energy experiments. The effects
of Z′in the B sector have been investigated in the literature, for example see Ref. [19–21]. The recent detailed review is Ref.
. In Ref. , Chang et.al obtained the explicit picture of Z′couplings with the data of¯Bs−Bsmixing, B → K(∗)ℓ+ℓ−,
B → µ+µ−, B → Kπ and inclusive decays B → Xsℓ+ℓ−. So, it should be interesting to explore the discrepancy of observables
between predictions of SM and those of the family non-universalZ′model. Motivated by this, we shall address the effects of the
Z′boson in the rare decays B → K1ℓ+ℓ−.
In experiments, B → K1ℓ+ℓ−have not yet been measured, but are expected to be observed at LHC-b  and Super-B factory
. In particular, it is estimated that there will be almost 8000 B → K∗µ+µ−events with an integrated luminosity of 2fb−1in
the LHC-b experiment[23, 25]. Althoughthe branchingratio of B→K1(1270)µ+µ−calculated in  is one orderof magnitude
smaller than the experimentally measured value of B → K∗µ+µ−, we still expect the significant number of events for this
The remainder of this paper is organized as follows: in section 2, we introduce the effective Hamiltonian responsible for
the b → sℓ+ℓ−transition in both SM and Z′model. Using the effective Hamiltonian and B → K1form factors, we obtain the
branchingratios as well as variousrelated physical observables. In section 3, we numericallyanalyze the consideredobservables
of B → K1ℓ+ℓ−. This section also includes a comparisonof the results obtained in Z′model with those predicted by the SM. We
will summarize this work in the last section.
A.The Effective Hamiltonian for b → s transition in SM
By integrating out the heavy degrees of freedom including top quark,W±and Z bosons above scale µ = O(mb), the effective
Hamiltonian responsible for the b → sℓ+ℓ−transitions is given as [27, 28]:
Hef f(b → sℓ+ℓ−) = −GF
where we have neglected the terms proportionaltoVubV∗
in . Specifically, the operators O9and O10are given as
uson account of |VubV∗
ts| <0.02. The local operators can be found
In SM, the Wilson coefficients Ciat scale µ = mbcalculated in the naive dimensional regularization (NDR) scheme  are
collected in Table I. It should be stressed that for b → sℓ+ℓ−processes, the quark decay amplitude can also receive additional
TABLE I: The SM Wilson coefficients at the scale µ = mb.
contributions from the matrix element of four-quark operators,
i=1?ℓ+ℓ−s|Oi|b?, which are usually absorbed into the effective
7,9in Table I are defined respectively as Wilson coefficients. The effective coefficientsCeff
αsC9+YSD(z, ˆ s)+YLD(z, ˆ s),
with definitions z = mc/mb, ˆ s = q2/m2
from the c¯ c resonances regions, which can be calculated reliably in the perturbative theory. On the contrary, the long-distance
b. YSD(z, ˆ s) represents the short-distance contributions from four-quarkoperators far away
contributions YLD(z, ˆ s) from four-quark operators near the c¯ c resonances cannot be calculated and are usually parameterized
in the form of a phenomenological Breit-Wigner formula. Currently, the light-cone distribution amplitudes of the axial-vector
mesonsactuallyhavenotyet beenwell studied,since contributionsoftwo axial-vectormesonsin thehadronicdispersionrelation
cannot be separated in all cases. Moreover, the width effect of axial-vector meson is so large that the traditional approach like
the sum rules cannot deal with it effectively. The manifest expressions and discussions for YSD(z, ˆ s) and YLD(z, ˆ s), are refereed
to Ref. . Since the contribution of long distance can be vetoed effectively in the experimental side, we will not discuss it in
the current work. Furthermore, for the Ceff
7, we here also ignore the long-distance contribution of the charm quark loop, which
is suppressed heavily by the Breit-Wigner factor.
B.Family Non-universal Z′Model and Parameter Constraint
As stated before, in the family non-universal Z′model, there exists the flavor changing neutral current even at the tree level
due to the non-diagonal chiral coupling matrix. Assuming that the couplings of right-handed quark flavors with Z′boson are
diagonal and ignoring Z−Z′mixing, the Z′part of the effective Hamiltonian for b → sl+l−can be written as [19–21]
ef f(b → sℓ+ℓ−) = −2GF
To match the effective Hamiltonian in SM, as shown in Eq.(2), the above equation is reformulated as
ef f(b → sℓ+ℓ−) = −4GF
DK1(ˆ s) =
(1+ ˆ mK1)VK1
1(ˆ s)−(1− ˆ mK1)VK1
2(ˆ s)−2 ˆ mK1VK1
−2 ˆ mbCeff
EK1(ˆ s) =
1+ ˆ mK1
FK1(ˆ s) = (1+ ˆ mK1)C10VK1
GK1(ˆ s) =
1+ ˆ mK1
HK1(ˆ s) =
(1+ ˆ mK1)VK1
1(ˆ s)−(1− ˆ mK1)VK1
2(ˆ s)−2 ˆ mK1VK1
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