Bottom-quark forward-backward asymmetry, dark matter, and the LHC

Abstract

The LEP experiment at CERN provided accurate measurements of the Z neutral gauge boson properties. Although all measurements agree well with the standard model (SM) predictions, the forward backward asymmetry of the bottom-quark remains almost 3σ away from the SM value. We proposed that this anomaly may be explained by the existence of a new Uð1Þ D gauge boson, which couples with opposite charges to the right-handed components of the bottom and charm quarks. Cancellation of gauge anomalies demands the presence of a vector-like singlet charged lepton as well as a neutral Dirac (or Majorana) particle that provides a dark matter candidate. Constraints from precision measurements imply that the mass of the new gauge boson should be around 115 GeV. We discuss the experimental constraints on this scenario, including the existence of a di-jet resonance excess at an invariant mass similar to the mass of this new gauge boson, observed in boosted topologies at the CMS experiment.

Asymmetric heavy-quark hadroproduction at LHCb: predictions and applications

“…For the benchmark parameters (5.5) the combination of the constraints [66][67][68] can however be shown to be fulfilled for most of the Z -boson masses in (5.7). In fact, the search [66] features an 2.9σ local excess for dijet invariant masses around 115 GeV, which has been interpreted in [63] as a Z boson in the U (1) D model with M Z 115 GeV and (5.6). In the following, we point out that Z boson with the properties (5.6) and (5.7), can also be probed by the LHCb measurement of Z → bb production in the forward direction [17].…”

“…We will compare the results of our sensitivity studies to the constraints on the Zbb and Zcc couplings that arise from the Z-pole measurements performed at LEP [1]. Our second application consists in using the recent LHCb measurement of Z → bb production [17] to put constraints on the new-physics model proposed in [63] which aims at explaining the long-standing LEP anomaly of the forward-backward asymmetry of the bottom quark.…”

“…While this deviation could be a result of statistical fluctuations, it is intriguing since it also could be associated with a large modification of the right-handed Zbb coupling cf. have been used in the article [63] as a benchmark. In fact, for these choices the Zbb and Zcc couplings in the U (1) D model take the values…”

“…Constraints on the Peskin-Takeuchi parameter T now put a bound on the size of the allowed mass splitting M Z − M Z [63]. For the s α value given in (5.4), one finds that the constraint T = 0.07 ± 0.12 [65] translates into the following 95% CL limit on the mass of the new gauge boson…”

“…As pointed out in [63], the presence of a Z boson in this mass range is subject to several constraints. The first constraint comes from the CMS search [66] for narrow spin-1 resonances decaying to a qq pair in association with a high-transverse momentum jet from ISR.…”