广义手征$U(1)_{B-L}$：朝向受约束更少的$Z'$与暗物质
Generalized Chiral $U(1)_{B-L}$: Towards a Less Constrained $Z'$ and Dark Matter
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中文总结 AI 辅助
该研究通过广义手征$(B-L)$荷分配放宽了LHC对新$Z'$玻色子的双轻子约束，解决了DM仅通过$Z'$门户湮灭时的 relic丰度与直接探测张力，实现了$Z'$、中微子质量与暗物质的兼容。
中文摘要 AI 辅助
通过$U(1)_X$规范对称性扩展标准模型会预言一种新的$Z'$玻色子，其质量受到大型强子对撞机（LHC）上高质量双轻子共振搜寻的严格约束。我们表明，通过在保持规范反常抵消的同时广义化手征$(B-L)$荷分配，这些约束可以被放宽。所得$U(1)_X$荷由两个独立参数参数化，对这些参数的适当选择会压低$Z'$到带电轻子的分支比，大幅削弱双轻子约束。该框架通过狄拉克I型 seesaw机制容纳狄拉克中微子质量，且无需额外离散对称性即可容纳稳定的单态标量暗物质（DM）候选者。我们发现，修改后的荷分配虽对对撞机现象学有益，但当DM仅通过$Z'$门户湮灭时，会在获得观测到的 relic丰度与满足直接探测限制之间引入张力。包含标量介导的湮灭通道可解决该张力，并开辟出广阔的可行参数空间，DM质量范围为$M_{\rm DM}\simeq60~{\rm GeV}$至$10~{\rm TeV}$。所得参数空间与观测到的 relic密度、直接探测、对撞机及电弱精密约束一致，表明广义手征荷结构可同时容纳受约束更少的$Z'$区、中微子质量及可行的暗物质现象学。
英文摘要
Extensions of the Standard Model by a $U(1)_{X}$ gauge symmetry predict a new $Z'$ boson, whose mass is severely constrained by high-mass dilepton resonance searches at the LHC. We show that these bounds can be relaxed by generalizing the chiral $(B-L)$ charge assignment while preserving gauge anomaly cancellation. The resulting $U(1)_X$ charges are parameterized by two independent parameters, and an appropriate choice of these parameters suppresses the $Z'$ branching fraction into charged leptons, substantially weakening the dilepton constraints. The framework accommodates Dirac neutrino masses through a Dirac type-I seesaw mechanism and a stable singlet scalar DM candidate without requiring an additional discrete symmetry. We find that the modified charge assignment, although beneficial for collider phenomenology, introduces a tension between obtaining the observed relic abundance and satisfying direct detection limits when DM annihilation proceeds solely through the $Z'$ portal. The inclusion of scalar-mediated annihilation channels resolves this tension and opens up a broad viable parameter space, with DM masses ranging from $M_{\rm DM}\simeq 60~{\rm GeV}$ to $10~{\rm TeV}$. The resulting parameter space is consistent with the observed relic density, direct detection, collider, and electroweak precision constraints, demonstrating that a generalized chiral charge structure can simultaneously accommodate a less constrained $Z'$ sector, neutrino masses, and viable dark matter phenomenology.
