Resonant di-Higgs boson production in the bb¯WW channel: Probing the electroweak phase transition at the LHC

Abstract

We analyze the prospects for resonant di-Higgs production searches at the LHC in the bbW þ W − (W þ → l þ ν l , W − → l −ν l ) channel, as a probe of the nature of the electroweak phase transition in Higgs portal extensions of the Standard Model. In order to maximize the sensitivity in this final state, we develop a new algorithm for the reconstruction of the bbW þ W − invariant mass in the presence of neutrinos from the W decays, building from a technique developed for the reconstruction of resonances decaying to τ þ τ − pairs. We show that resonant di-Higgs production in the bbW þ W − channel could be a competitive probe of the electroweak phase transition already with the data sets to be collected by the CMS and ATLAS experiments in run 2 of the LHC. The increase in sensitivity with larger amounts of data accumulated during the high-luminosity LHC phase can be sufficient to enable a potential discovery of the resonant di-Higgs production in this channel. I. MOTIVATIONWith the discovery of the Higgs boson at the Large Hadron Collider (LHC) [1,2], exploring the thermal history associated with electroweak symmetry breaking (EWSB) has taken on heightened interest. In the Standard Model (SM), EWSB in the early Universe occurs through a crossover transition. In contrast, beyond-the-StandardModel (BSM) scenarios may lead to a bona fide electroweak phase transition (EWPT). If such a transition occurred and was both first order and sufficiently strong, it could have provided the conditions needed for generating the observed cosmic matter-antimatter asymmetry.Electroweak baryogenesis (EWBG) (for recent reviews, see Refs. [3,4]) is one of the most widely studied and experimentally testable scenarios for explaining the origin of the cosmic matter-antimatter asymmetry, characterized by the baryon-to-entropy density ratio Y B ¼ n B =s as (most precisely) measured by Planck [5]:Successful baryogenesis requires three ingredients in the particle physics of the early Universe, the so-called "Sakharov criteria" [6]: (i) baryon-number (B) violation,(ii) C and CP violation, and (iii) departure from thermal equilibrium or a breakdown of CPT invariance. The SM contains the requisite B violation in the guise of electroweak sphalerons, but it fails with regard to the last two criteria. CP violation in the SM, via the CKM mixing matrix, is too feeble. In the minimal SM, the maximum Higgs mass for a first-order EWPT is m h ∼ 70-80 GeV, as confirmed by a variety of theoretical Monte Carlo simulations [7][8][9][10][11]; while for the observed m h ∼ 125 GeV, EWSB occurred through a crossover phase transition in the early Universe, which would not provide for the necessary out-of-equilibrium conditions. In contrast, if the observed Higgs boson resides within an extended scalar sector, the nature and properties of the EWPT could differ significantly from those of the SM. In that case, the Universe could have undergone a strong firstorder EWPT even for a SM-like Higgs boson of mass m h ∼ 125 GeV. The additional scalar degrees of freedom can...

Non-resonant collider signatures of a singlet-driven electroweak phase transition

“…• As mentioned in the Introduction, resonant di-Higgs production at the LHC [11,14,29,38] and a future 100 TeV hadron collider [12,13,15,39] would be expected to probe portions of the parameter space with m 2 > 2m 1 . Ref.…”

“…A particularly powerful probe will be measurements of the Zh production cross-section at lepton colliders, which can deviate from its SM predicted value due to mixing effects and the wavefunction renormalization of the Higgs-like scalar h [18,19]. A combination of these approaches, in addition to gravitational wave experiments such as LISA [20], show promise in probing the EWPT in singlet models [10][11][12][13][14][15][16][17][20][21][22][23][24][25][26].…”

“…refs. [11][12][13][14][15] and references therein). This is an attractive channel because of its potentially large cross-section, especially at a 100 TeV collider.…”

Standard model with a complex scalar singlet: Cosmological implications and theoretical considerations

“…The possibilities for a SFOEWPT with an additional real singlet scalar (dubbed the "xSM" [17]), along with its phenomenological consequences for collider studies, have been studied extensively [18][19][20][21][22][23][24][25][26][27][28][29]. The xSM implies the existence of two neutral mass eigenstates, H 1;2 , that are mixtures of the neutral doublet and real singlet.…”

Pair production of the Elementary Goldstone Higgs boson at the LHC

“…3(c). If a new scalar is sufficiently heavy and can decay on-shell into two SM-like Higgs bosons, it has been shown that the new scalar might significantly enhance the di-Higgs production rate over the SM prediction via its resonant production by gluon-gluon fusion [21][22][23][24][25][26][29][30][31][32][33]. Using the narrow width approximation, the resonant production cross section can be approximately written as…”