Unveiling the High-Rate Frontier: ATLAS' Quest for New Particles
Nature's penchant for symmetry is a guiding principle at the Large Hadron Collider (LHC), where the majority of proton-proton collisions result in a remarkably symmetrical pattern: two concentrated sprays of hadrons, or jets, emerging back-to-back with nearly equal momentum. These 'dijet' events are like breadcrumbs leading physicists beyond the Standard Model.
The challenge lies in the abundance of these events. The ATLAS experiment, with its 'trigger' system, must select the most intriguing collisions from a billion occurring every second. The first-level trigger reduces the rate to a manageable 100,000 events per second, and then the high-level trigger further narrows it down to just 1,200 events per second. This means most dijet events are discarded due to bandwidth constraints.
But here's where it gets controversial: the ATLAS Collaboration has developed an innovative approach called trigger-level analysis (TLA). Instead of recording all collision data, TLA saves only the essential information, reducing the average event size from a hefty 1 megabyte to a mere 6.5 kilobytes. This allows ATLAS to record far more events, especially those with low dijet masses. Similar strategies have been employed by other collaborations, but ATLAS has taken it a step further.
With this approach, ATLAS researchers are exploring an extraordinary dataset - 60 billion trigger-level events collected during LHC Run 2 - in search of new particles. In a recent paper published in Physical Review D, the ATLAS Collaboration presented a search for new particles decaying into dijets, based on this vast dataset. Using TLA, physicists were able to record data at an impressive rate, over 20 times faster than the standard high-level trigger.
Dijet searches are like looking for a needle in a haystack - they search for a small excess in the reconstructed dijet mass spectrum, which could indicate a new particle beyond the Standard Model. While traditional dijet searches focus on masses above 1 TeV, this trigger-level analysis pushes the boundaries, extending sensitivity down to 375 GeV. This is the lowest inclusive dijet mass ever studied at the LHC, without relying on additional constraints.
The data analysis revealed a compatibility with the smoothly falling Standard Model background. The most significant excess observed was for a Z' signal with a mass of 650 GeV, with a global significance of 2.2σ. Through meticulous efforts to improve background estimation and jet calibration, physicists set world-leading limits on the mass and interaction strength of potential new particles in various simplified models.
And this is the part most people miss: trigger-level analyses of Run 3 data hold even greater promise. With an upgraded TLA readout stream, ATLAS has recorded more complex collision event signatures in an even larger dataset, expanding the experiment's sensitivity to new physics at the high-rate frontier.
So, what do you think? Is this innovative approach a game-changer for particle physics research? Or are there potential pitfalls we should consider? Let's discuss in the comments!
