Localized $4\sigma$ and $5\sigma$ Dijet Mass Excesses in ALEPH LEP2 Four-Jet Events

We investigate an excess observed in hadronic events in the archived LEP2 ALEPH data. This excess was observed at preselection level during data-MC comparisons of four-jet events when no search was being performed. The events are clustered into four jets and paired such that the mass difference between the two dijet systems is minimized. The excess occurs in the region $M_1+M_2\sim 110\mbox{ GeV}$; about half of the excess is concentrated in the region $M_1\sim 80\mbox{ GeV}$, $M_2\sim 25\mbox{ GeV}$, with a local significance between $4.7\sigma$ and $5.5\sigma$, depending on assumptions about hadronization uncertainties. The other half of the events are in a broad excess near $M_1\sim M_2\sim 55\mbox{ GeV}$; these display a local significance of $4.1-4.5\sigma$. We investigate the effects of changing the SM QCD Monte Carlo sample, the jet-clustering algorithm, and the jet rescaling method. We find that the excess is remarkably robust under these changes, and we find no source of systematic uncertainty that can explain the excess. No analogue of the excess is seen at LEP1.


How this situation arose
And we saw a ∼ (3 − 4)σ bump around Σ ∼ 55 GeV.  (This plot after refinements in analysis.) Not the first time a bump had been seen in this location: Aleph had seen a 55-GeV bump in Σ in 4-jet search for hA at √ s = 130 − 136 GeV (5.7 pb −1 ) (Buskulic, Z. Phys. C 71, 179). Not confirmed by other experiments, not seen in higher energy data.
We decided this merited further study, started developing analysis tools.

MC Simulation of SM QCD
Almost all of the SM production at preselection level is e + e − → Z /γ → qq → hadrons (QCD), & electroweak processes, e.g. e + e − → W + W − , ZZ → qqqq. The electroweak processes are pretty easy to simulate well; tend to have well-separated jets. QCD not so easy: qq radiates gluons at low energies, angles. Excess occurs in region dominated by e + e − → Z /γ → hadrons. LEP era: QCD simulated by interfacing e + e − → Z /γ → qq ME to parton shower. (Also some matching to qqg ME). No 4-parton ME. MC generators have advanced a lot since. Forcing events into 4 jets; very desirable to have 4-parton ME. Sherpa lets us do this; can generate MEs for final states with different numbers of partons, interface them to parton shower, correctly merge them into one inclusive sample. Some MEs can be generated @NLO. We simulated QCD w/Sherpa using MEs for final states of up to 6 partons.

MC tuning
We did 2 tunes of Sherpa using MEs for up to 6 final-state partons.
For first (LO) tune, all MEs were at LO.
For other (NLO), MEs for final states of up to 4 partons were at NLO using BlackHat.
PYTHIA used for hadronization.
We placed an emphasis on being able to reproduce event-shape variables and multi-jet distributions.
Compare MC generated with our 2 tunes to data, to each other, and to QCD generated with KK2f, a LEP-era qq ME generator, interfaced to PYTHIA for parton shower.
Compare at both Rivet level and at full simulation, at both LEP1 and LEP2. Inter-jet angles particularly important. Closely related to reconstructed dijet masses. Jets energy-ordered, Jet 1 most energetic.
Overall, LO tune seemed to give the best agreement with data.  Similar behavior at LEP1, LEP2, up to overall energy scale.
Wanted to see if could use this to get even better MC.  Disagreement between KK2f and other samples much reduced, take as indication reweighting reduces systematics. Take reweighted LO Sherpa as most trusted SM QCD estimation, retain others for systematic studies.
Preselection, jet clustering, and jet rescaling Preselection: Want to retain hadronic events while removing two-photon events and events with hard ISR.
Most cuts identical/similar to standard cuts in ALEPH 4-jet analyses.
Jet clustering: We use the jet-clustering algorithm Luclus, as it has been noted to have better resolution for jet angles and energies.
Most common algorithm at LEP was Durham; will compare several.
For this reason, dijet masses determined mostly from inter-jet angles.
Rescaled momenta such that jet masses were kept constant.

Jen Kile
Localized 4, 5σ Aleph Dijet Mass Excesses July 15, 2019 QCD@LHC 13 / 35 Using above choices for QCD MC sample, preselection cuts, and jet clustering and rescaling, we see this in the M 1 -M 2 plane at LEP2: M 1 = mass of dijet containing most energetic jet.
Systematics not included.   All show highly significant excesses. We also tried 6 different jet-clustering algorithms. Seems to reduce significance in both regions.
Perhaps gives some clue about jet structure of these events.
What happens if you ask that Luclus and Durham agree on Σ? May indicate excess more "four-jetty" than expected from background.
Corrections and uncertainties to MC samples: Topology of Region A events is very 1 − 3, with one jet of E ∼ √ s/2 in one hemisphere, and three jets in the other.
Jets paired to minimize dijet mass difference. Sensible for 55-GeV pair-production; less so for production of 80-GeV, 25-GeV resonances.
Also, decay angle θ dec of 80-GeV dijet is strongly peaked near 0: Both features consistent w/less-energetic jet in the 80-GeV system being softer than would be expected from a genuine 80-GeV particle.
Does not look very resonance-like. Looks a lot like QCD background.  Excess dependence on √ s? → Compatible w/being proportional to QCD bkg, but error bars large.