Search for long-lived neutral particles produced in pp collisions at s=13  TeV decaying into displaced hadronic jets in the ATLAS inner detector and muon spectrometer

Abstract

A search is presented for pair production of long-lived neutral particles using 33 fb −1 of ffiffi s p ¼ 13 TeV proton-proton collision data, collected during 2016 by the ATLAS detector at the LHC. This search focuses on a topology in which one long-lived particle decays in the ATLAS inner detector and the other decays in the muon spectrometer. Special techniques are employed to reconstruct the displaced tracks and vertices in the inner detector and in the muon spectrometer. One event is observed that passes the full event selection, which is consistent with the estimated background. Limits are placed on scalar boson propagators with masses from 125 GeV to 1000 GeV decaying into pairs of long-lived hidden-sector scalars with masses from 8 GeV to 400 GeV. The limits placed on several low-mass scalars extend previous exclusion limits in the range of proper lifetimes cτ from 5 cm to 1 m.

Exclusive displaced hadronic signatures in the LHC forward region

“…The existing LHCb search on B + → K + S(µ + µ − ) [41] has excluded the parameter space for the best sensitivity region of our kaon search, unless the muon coupling to S is < ∼ 20% of the size in the Higgs mixing model. As we will discuss, the displaced muon searches from both ATLAS and CMS [12,42] can also set useful bounds comparing to our projected LHCb S → KK sensitivity.…”

“…5 shows that for M ∼ 10 2 TeV, the displaced kaon search can probe BR(h → SS) down to the 10 −3 (10 −4 ) level with 15 (300) fb −1 of data. Hadronic decays of GeV scale LLPs with p T < ∼ 60 GeV are a challenging signature to look for at ATLAS/CMS (e.g., see the recent ATLAS search [12]), and the displaced kaon search can be the discovery machine for such signatures. that allows S to decay both into hadrons and leptons.…”

“…However, differently from the hadrophilic scenario, we now have BR(S → µµ) ≈ 8% in the same m S range. Since ATLAS/CMS are excellent at identifying muons, they can provide important bounds on the same model by looking at displaced muon signatures [12,42]. In Fig.…”

“…Due to the large hadronic background and a limited ability to identify the SM hadrons, the ATLAS/CMS studies rely on various inclusive searches of displaced jet signatures, which are mainly sensitive to signals with LLP mass much larger than O(10) GeV scale or transverse energy H T > O(100) GeV (e.g., [8,9]). Some ATLAS/CMS studies can lower the mass and energy requirements by focusing on decays in the hadronic calorimeter (HCAL) or muon spectrometer (MS) (e.g., [10][11][12]), but the LLP mass still needs to be > ∼ 5 GeV. The LHCb study of LLP decaying to jets [13] also performs a similar inclusive search, and the current bounds only apply when m S > ∼ 10 GeV and track multiplicity > ∼ 15.…”

Probing long-lived particles at Higgs factories

“…Some searches for LLPs have also been performed at ATLAS and CMS: see for examples Refs. [4][5][6][7][8]. Nevertheless, the current triggers are not optimized for detecting LLPs so that perhaps many signals might have been missed.…”