Compact Muon Solenoid on the lookout for new physics
With Run 3 of the Large Hadron Collider (LHC) just around the corner, the LHC experiments are still publishing new results based on the data from the previous runs. Despite no new discoveries being announced, a small number of analyzes reveal minor deviations from expectations. At the current level, these anomalies can still be attributed to random fluctuations in data, but they indicate regions that should be scrutinized once the new flow of collisions arrives. Below are a few examples recently published by the compact muon solenoid (CMS) collaboration.
In 2017, CMS recorded a spectacular collision with four particle jets in the final state. The invariant mass of all four jets was 8 TeV and the jets could be divided into two pairs each with an invariant mass of 1.9 TeV. Such a configuration could be produced if a new particle with a mass of 8 TeV is created by the collision of proton beams and then decays into a pair – again, new – particles with a mass of 1.9 TeV. In a new analysis recently published by CMS, a search for such twin pairs of jets with matching invariant masses is performed for data collected up to the end of LHC Run 2. Surprisingly, a second event with similar salient properties was found, with a 4 jet masses of 8.6 TeV and 2 jet masses of 2.15 TeV. These two events can be clearly seen in the graph below, where the 4-ray events are plotted as a function of the 2-ray and 4-ray masses.
While almost all observed events involving two pairs of jets are produced by strong interactions between the colliding photons, events with such high invariant masses are extremely unlikely. The probability of seeing two events at these masses with no new phenomena present is on the order of 1 in 20,000, which corresponds to a local significance of 3.9σ. While at first glance this may seem like a very strong signal, given that the area of masses being analyzed is large, it’s important to also look at global significance, which indicates the likelihood of detecting an excess somewhere. in the analyzed region. For the two events, the global significance is only 1.6σ.
Two other searches for new heavy particles report small excesses in data. In a search for high-mass resonances that decay into a pair of W bosons (which subsequently decay into leptons), the highest deviation corresponds to a signal hypothesis with a mass of 650 GeV, with local significance at 3.8σ and global significance at 2 ,6σ. In a search for heavy particles that decay into pairs of bosons (WW, WZ or other combinations, including Higgs bosons) that then decay into pairs of jets, the data deviates from expectations in two places. The signal hypothesis is a W’ boson with a mass of 2.1 or 2.9 TeV, decaying to a WZ pair and the highest local significance is 3.6σ, with a global significance of 2.3σ.
Another new result comes from searches looking for additional Higgs boson particles that decay into tau pairs. For a new particle with a mass of 100 GeV, a small excess can be seen in the data with 3.1σ local and 2.7σ global significance. Interestingly, this coincides with a similar excess seen by CMS in a previous search for low-mass resonances in the two-photon end state. Another excess is visible in the high mass range, with the largest deviation from expectation observed for a mass of 1.2 TeV with a local (global) significance of 2.8σ (2.4σ).
The final state of the tau pair was also used to search for hypothetical new particles called leptoquarks. This is of particular interest because leptoquarks could potentially explain the taste anomalies observed by the LHCb experiment, so if the anomalies are indeed a manifestation of some new phenomena, this would be a way to look at these phenomena independently from a different angle. . CMS has not found an excess so far, but the analysis is only just beginning to become sensitive to the range of leptoquark parameters that could fit the taste anomalies, so more data is needed to fully explore the leptoquark hypothesis.
The new LHC data collection period starts in July, with higher energy and significantly improved detectors, promising a new stream of data for the search for new phenomena.
ATLAS experiment looks for rare Higgs boson decays into a photon and a Z boson
More details at CERN Courier: cerncourier.com/a/dijet-excess-intrigue-at-cms/
Quote: Compact Muon Solenoid in Search of New Physics (2022, June 20) retrieved June 21, 2022 from https://phys.org/news/2022-06-compact-muon-solenoid-lookout-physics.html
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