Z'向量玻色子对μ子磁矩异常的轻子风味解释

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本文主要探讨了在标准模型的基础上引入额外的一个Z'（Z-prime）向量玻色子，以解释长期存在的μ子磁矩（g-2）的异常。Z'玻色子在这个扩展模型中具有特殊的特性，它只对第二代（μ子和τ子）以及对应的中微子（νμ和ντ）产生非对角线的风味作用。这一模型的动机是针对μ子g-2测量结果与其理论预测之间的显著偏差。作者分析了简化模型的设计和实现，特别关注了Z'的质量范围，以寻找能够缓解μ子磁矩问题的最佳参数空间。研究发现，相比于τ子，一个质量较轻的Z'并不能提供有效的（g-2）μ解，这与实验观测相冲突。因此，模型中的Z'玻色子需要较重，从而排除了轻型Z'的可能性。论文还提出了未来实验可以验证这一模型的一些途径。在佳丽2实验中，可以观察到违反轻子风味统一性的τ子衰变，显示出Z'对特定轻子的偏好。在高能物理实验如HL-LHC（High Luminosity Large Hadron Collider，高亮度大型强子对撞机）和FCC-ee（Future Circular Collider - electron-positron，未来环形对撞机 - 电子-正电子）中，可能会出现新的四轻子信号，这些信号包括同号介子和二重子的组合，这些现象与Z'的存在密切相关。此外，中微子望远镜如IceCube和KM3NeT有望在高能中微子光谱中检测到由于Z'玻色子引起的共振吸收效应，这提供了另一种独立的探测手段。这篇文章通过严格的理论分析和对现有实验数据的考虑，提出了一种可能的Z'解释μ子磁矩异常的方案，并指出了未来实验验证的关键路径。这一工作对于理解基本粒子物理学的精细结构以及可能的新物理现象具有重要意义。

Physics Letters B 762 (2016) 389–398

Contents lists available at ScienceDirect

Physics Letters B

www.elsevier.com/locate/physletb

Lepton ﬂavor violating Z



explanation of the muon anomalous

magnetic moment

Wolfgang Altmannshofer

, Chien-Yi Chen

b,c

, P.S. Bhupal Dev

d,∗

, Amarjit Soni

Department of Physics, University of Cincinnati, Cincinnati, OH 45221, USA

Department of Physics and Astronomy, University of Victoria, Victoria, BC V8P 5C2, Canada

Perimeter Institute for Theoretical Physics, Waterloo, ON N2J 2W9, Canada

Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, D-69117 Heidelberg, Germany

Physics Department, Brookhaven National Laboratory, Upton, NY 11973, USA

a r t i c l e i n f o a b s t r a c t

Article history:

Received

12 August 2016

Accepted

22 September 2016

Available

online 28 September 2016

Editor:

B. Grinstein

Keywords:

Muon

anomalous magnetic moment

Light

gauge boson

Lepton

ﬂavor violation

We discuss a minimal solution to the long-standing (g − 2)

anomaly in a simple extension of the

Standard Model with an extra Z



vector boson that has only ﬂavor off-diagonal couplings to the second

and third generation of leptons, i.e. μ, τ , ν

, ν

and their antiparticles. A simpliﬁed model realization,

as well as various collider and low-energy constraints on this model, are discussed. We ﬁnd that the

(g − 2)

-favored region for a Z



lighter than the tau lepton is totally excluded, while a heavier Z



solution is still allowed. Some testable implications of this scenario in future experiments, such as lepton-

ﬂavor

universality-violating tau decays at Belle 2, and a new four-lepton signature involving same-sign

di-muons and di-taus at HL-LHC and FCC-ee, are pointed out. Acharacteristic resonant absorption feature

in the high-energy neutrino spectrum might also be observed by neutrino telescopes like IceCube and

KM3NeT.

(http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP

1. Introduction

The anomalous magnetic moment of the muon a

≡ (g −2)

among the most precisely known quantities in the Standard

Model (SM), and therefore, provides us with a sensitive probe of

new physics beyond the SM (BSM) [1,2]. There is a long-standing

3.6 σ discrepancy between the SM prediction [3–5] and the mea-

sured

value of a

[6]:

a

≡a

exp

−a

 (288 ±80) ×10

−11

. (1)

The uncertainties in the experimental measurement, which come

from the E821 experiment at BNL [7], can be reduced by about a

factor of four in the upcoming Muon g − 2 experiment at Fermi-

lab [8].

If comparable progress can be made in reducing the uncer-

tainties

of the SM prediction [9–12], we will have a deﬁnite answer

to the question whether or not a

is evidence for BSM physics.

Thus from a theoretical point of view, it is worthwhile investi-

gating

simple BSM scenarios which can account for the (g − 2)

anomaly, should this endure, and at the same time, have comple-

mentary

tests in other ongoing and near future experiments. With

Corresponding author.

E-mail

address: bhupal.dev@mpi-hd.mpg.de (P.S.B. Dev).

this motivation, we discuss here a simple Z



interpretation of the

(g −2)

anomaly.

suﬃciently muonphilic Z



can address the (g − 2)

dis-

crepancy [13–30];

however, in order to avoid stringent bounds

from the charged lepton sector, while being consistent with a

sizable contribution to (g − 2)

, the Z



coupling must violate

lepton universality.

For instance, a sizable Z



coupling to elec-

trons

is strongly constrained over a large range of Z



masses

from e

−

→e

−

measurements at LEP [48], electroweak pre-

cision

tests [49,50], e

−

→ γ 



−

(with  = e, μ) at BaBar [51],

→ γ 



−

at NA48/2 [52], the g − 2of the electron [18], and

neutrino–neutrino scattering in supernova cores [53,54]. Similarly,

a sizable ﬂavor-diagonal Z



coupling to muons is strongly con-

strained

from neutrino trident production ν

N → ν

Nμ

−

[25]

using

the CCFR data [55]. In addition, charged lepton ﬂavor-

There are other experimental hints of lepton ﬂavor violation or the breakdown

of lepton ﬂavor universality in processes involving muons and taus, e.g. in B

→



−

decays at the LHCb [31], in B → D

(∗)

τν decays at BaBar [32], Belle [33,34]

and

LHCb [35], and in the h →μτ decay at both CMS [36] and ATLAS [37] (which

however seems to have disappeared in the early run-II LHC data [38,39]). See e.g.

Refs. [40–47] for the most recent attempts to explain some of these anomalies. In

this work we concentrate on (g − 2)

and only comment on h → μτ .

http://dx.doi.org/10.1016/j.physletb.2016.09.046

0370-2693/

SCOAP

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