Unlike heavy Standard Model particles– which decay within a few millimeters of the Large Hadron Collider (LHC) collision point– new, long-lived particles (LLPs) might travel significant ranges through the ATLAS detector prior to rotting.
Studying the decay of any particle is a complex task, however it is typically made much easier by assuming that it rotted near the LHC accident point. If the particle is heavy, these energy deposits would be unusually big and could be used to infer the mass of the particle that produced them. When particles decay to quarks, they go through a process called hadronization, which leads to sprays of collimated particles in the ATLAS detector called jets. These would leave to a very unusual signature in the experiment: the jets would have no associated particle trajectories in the tracking detector; would be extremely narrow compared to their Standard Model equivalents, given that the spray of particles wouldnt have time to end up being spatially separated; and would leave a high portion of their overall energy in the hadronic part of the calorimeter.
To ensure no stone is left unturned, ATLAS physicists have actually developed a variety of new techniques to look for LLPs with different possible characteristics. 4 brand-new outcomes from this effort have actually been presented at the current Lepton-Photon and La Thuile conferences.
The hunt for right-handed neutrinos
Neutrinos are a few of the most strange particles in the Standard Model. Physicists have long puzzled over why neutrinos are just ever observed to be “left-handed” (i.e. their spin and momentum are opposed), while all other known particles can likewise be observed in “right-handed” states.
Figure 1: Display of a simulated occasion where a neutral long-lived particle is produced at the centre of the detector (left red dot) and decays after passing through some distance (best red dot). This signature is referred to as a “displaced vertex.” Credit: ATLAS Collaboration/CERN
One possibility is that right-handed neutrinos exist however are really heavy, and for that reason harder to produce in nature. These new particles– called “heavy neutral leptons” (HNLs)– would all at once offer right-handed partners to Standard Model neutrinos and discuss why neutrinos are so light. HNLs would display long-lived speculative signatures if the interaction strength in between HNLs and Standard Model neutrinos is little.
In a new search for these heavy neutrinos, ATLAS physicists looked for leptons originating from a typical displaced vertex (see Figure 1) in the ATLAS charged-particle tracking detector, where a HNL could have decomposed to a mix of electrons, muons and missing energy. Utilizing the decay items, they rebuilded the possible HNL mass which would be various for signal events than for background occasions, as displayed in Figure 2. As an outcome, physicists had the ability to set limitations on HNL masses between 3 and ~ 15 GeV, and had the ability to report on HNL decomposes to electron-muon pairs for the really very first time!
Figure 2: The reconstructed HNL mass circulation for the observed information (black dots), the anticipated background with its uncertainty (lilac/blue), and simulated signal for three various mass hypotheses (colored lines). Credit: ATLAS Collaboration/CERN
The ATLAS Collaboration has devised a variety of new methods to search for long-lived particles that decay away from the central LHC accident point.
Following the steps of charged LLPs
When browsing for brand-new particles, physicists have to look for their decay products– or do they? If the particle is heavy, these energy deposits would be unusually large and might be utilized to presume the mass of the particle that produced them.
Figure 3: The reconstructed mass spectrum for the Standard Model expectation (blue line) and observed information (black dots). Different possible signals that are checked by the search are shown in rushed colours. A little excess of events is observed above 1000 GeV in mass, which is nevertheless not compatible with a slow charged particle hypothesis when looking into time measurements. Credit: ATLAS Collaboration/CERN
Nevertheless, predicting the Standard Model background procedures in this search is really tough. ATLAS physicists employed an advanced “data-driven” technique to deal with the problem, using tracks with routine energy deposits to forecast the background processes that could mimic a signal.
The observed data concur with the expected Standard Model background, other than for the existence of numerous occasions forming an excess in a high-energy and high-mass area, as displayed in Figure 3. Appealing, the time-of-flight measurements in external detector subsystems suggest that none of the prospect tracks match the slow-moving and heavy charged particle hypothesis.
Harnessing the power of maker learning
When particles decay to quarks, they undergo a process called hadronization, which results in sprays of collimated particles in the ATLAS detector called jets. If a new, neutral LLP were to decay to quarks in the outer layer of the calorimeter, it would leave behind “displaced” jets. These would delegate a really unusual signature in the experiment: the jets would have no involved particle trajectories in the tracking detector; would be really narrow compared to their Standard Model counterparts, considering that the spray of particles would not have time to end up being spatially separated; and would leave a high portion of their total energy in the hadronic part of the calorimeter. An example of among these uncommon jets can be seen in case display screen above.
In a brand-new analysis, ATLAS researchers made use of the uncommon characteristics of displaced jets to browse for sets of neutral LLPs rotting in the calorimeter. The Deep Neural Network they developed for the analysis is particularly unique: it is able to differentiate displaced jets from background interactions of the LHC beam with product from the accelerator itself, as well as from jets from Standard Model quark decays. Even more, scientists utilized an adversarial training scheme to guarantee the algorithm did not exploit recognized differences between information and simulation. The result was utilized to estimate the background for the search, and no significant excess of events has been found so far.
What if the neutral LLP rots to leptons instead of quarks? Dark photons are a class of LLPs speculated to act in this method. If such decays were to happen in the ATLAS calorimeter or muon system, they would result in collimated groups of leptons called lepton-jets.
Scientists likewise browsed for lepton-jets in another brand-new analysis, using state-of-the-art machine-learning techniques to distinguish LLP-candidate lepton-jets from background processes, such as cosmic rays and LHC beam-gas interactions. For the very first time, ATLAS physicists searched for brand-new particles using maker learning strategies exploiting patterns of raw energy deposits in each layer of the detector. No excess of events was seen, they set rigid new limits on the existence of dark photons and were able to probe dark-photon decays to electrons for the very first time!
Into Run 3
At the core of these brand-new analyses is one crucial question: what if brand-new particles are concealing from standard searches? ATLAS scientists have developed unique, innovative methods to check out the abundant diversity of possible LLP rots.
ATLAS event screen for an information event with a set of “displaced jets”, which are narrow, mostly trackless and have the majority of their energy in the hadronic part of the calorimeter. Credit: CERN
Physicists at the ATLAS experiment are on the hunt for new, long-lived particles to assist discuss numerous impressive secrets of our Universe.
Physicists at the ATLAS experiment are on the hunt for brand-new, long-lived particles to help discuss several exceptional secrets of our Universe. High-energy crashes permit researchers to study heavy particles that decay very rapidly, like the Higgs boson. Unlike heavy Standard Model particles– which decay within a couple of millimeters of the Large Hadron Collider (LHC) collision point– brand-new, long-lived particles (LLPs) might travel considerable ranges through the ATLAS detector prior to decaying.
Studying the decay of any particle is a complex job, but it is typically made much easier by assuming that it decayed near the LHC crash point. This leaves LLPs in a blind spot, as they might decay throughout the detector. Further, as the layers of the ATLAS experiment are instrumented differently, proof of LLPs would look different depending upon which layer the particle decays in.
