Probing baryogenesis with displaced vertices at the LHC

Abstract

The generation of the asymmetric cosmic baryon abundance requires a departure from thermal equilibrium in the early universe. In a large class of baryogenesis models, the baryon asymmetry results from the out-of-equilibrium decay of a new, massive particle. We highlight that in the interesting scenario where this particle has a weak scale mass, this out-of-equilibrium condition requires a proper decay length larger than O(1) mm. Such new fields are within reach of the LHC, at which they can be pair produced leaving a distinctive, displaced-vertex signature. This scenario is realized in the recently proposed mechanism of baryogenesis where the baryon asymmetry is produced through the freeze-out and subsequent decay of a meta-stable weakly interacting massive particle ("WIMP baryogenesis"). In analogy to missing energy searches for WIMP dark matter, the LHC is an excellent probe of these new long-lived particles responsible for baryogenesis via the low-background displaced vertex channel. In our paper, we estimate the limits on simplified models inspired by WIMP baryogenesis from two of the most sensitive collider searches by CMS and ATLAS with 8 TeV LHC data. We also estimate the LHC reach at 13 TeV using current strategies, and demonstrate that up to a factor of 100 improvement in cross-section limits can be achieved by requiring two displaced vertices while lowering kinematic thresholds. For meta-stable WIMPs produced through electroweak interactions, the high luminosity LHC is sensitive to masses up to 2.5 TeV for lifetimes around 1 cm, while for singlets pair-produced through the off-shell-Higgs portal, the LHC is sensitive to production cross sections of O(10) ab for benchmark masses around 150 GeV. Our analysis and proposals also generally apply to displaced vertex signatures from other new physics such as hidden valley models, twin Higgs models and displaced supersymmetry.

Long-lived, colour-triplet scalars from unnaturalness

“…However recasting limits from these searches is difficult without access to the complete detector simulations used by the collaborations. Nevertheless several recent papers [7][8][9][10]12] have demonstrated that, with some reasonable assumptions, good agreement with the full experimental analyses can be achieved. The most relevant search for our model is the ATLAS displaced-vertex search [29] and we shall take a similar approach to that of ref.…”

“…Previous studies of long-lived particles have primarily focused on supersymmetric models, motivated by the idea of split supersymmetry [3][4][5][6] or simplified toy models with Rparity violation [7][8][9][10][11][12]. Our work is the first analysis of models based on the composite Higgs idea.…”

Baryogenesis via particle-antiparticle oscillations

“…(See Ref. [29].) Furthermore, if there are lepton-number-violating decays (as well as baryon-number-violating decays), the decays can produce a same-sign lepton asymmetry [28].…”

“…(A detailed study of using displaced vertices to probe baryogenesis was given in Ref. [29].) After setting this width, a mass difference of 2 × 10 −6 eV is chosen to make the oscillations more visible.…”

Long-lived colored scalars at the LHC

“…Displaced vertices are well motivated scenarios of models beyond the Standard Model. They appear within the context of SUSY breaking [12][13][14], models of weak-scale R-parity violation [15][16][17], Hidden valleys [18][19][20][21], baryogengesis [22,23], dark QCD [24] and late decays of righthanded neutrinos within SUSY extensions of the Standard Model [25]. In addition, there is an increasing activity in the recasting front, to better constrain a wide range of these models using the existing LHC displaced vertex analyses [23,24,26].…”