The newest on the Large Hadron Collider’s hunt for the theoretical particle that might comprise dark matter.
The Large Hadron Collider (LHC) is renowned for the hunt for and discovery of the Higgs boson, however in the 10 years because the maker clashed protons at an energy greater than formerly attained at a particle accelerator, scientists have actually been utilizing it to attempt to hound a similarly amazing particle: the theoretical particle that might comprise an unnoticeable kind of matter called dark matter, which is 5 times more widespread than common matter and without which there would be no universe as we understand it. The LHC dark-matter searches have up until now turn up empty-handed, as have non-collider searches, however the amazing work and ability put by the LHC scientists into discovering it has actually led them to limit a lot of the areas where the particle might lie concealed – essential turning points on the course to a discovery.
“Before the LHC, the space of possibilities for dark matter was much wider than it is today,” states dark-matter theorist Tim Tait of UC Irvine and theory co-convener of the LHC Dark Matter Working Group.
“The LHC has really broken new ground in the search for dark matter in the form of weakly interacting massive particles, by covering a wide array of potential signals predicted by either production of dark matter, or production of the particles mediating its interactions with ordinary matter. All of the observed results have been consistent with models that don’t include dark matter, and give us important information as to what kinds of particles can no longer explain it. The results have both pointed experimentalists in new directions for how to search for dark matter, and prompted theorists to rethink existing ideas for what dark matter could be – and in some cases to come up with new ones.”
Make it, break it and shake it
To search for dark matter, experiments basically “make it, break it or shake it”. The LHC has actually been attempting to make it by clashing beams of protons. Some experiments are utilizing telescopes in area and on the ground to search for indirect signals of dark-matter particles as they clash and break themselves out in area. Others still are chasing after these evasive particles straight by looking for the kicks, or “shakes”, they offer to atomic nuclei in underground detectors.
The make-it method is complementary to the break-it and shake-it experiments, and if the LHC identifies a prospective dark-matter particle, it will need verification from the other experiments to show that it is certainly a dark-matter particle. By contrast, if the direct and indirect experiments identify a signal from a dark-matter particle interaction, experiments at the LHC might be developed to study the information of such an interaction.
Missing-momentum signal and bump searching
So how has the LHC been trying to find indications of dark-matter production in proton accidents? The primary signature of the existence of a dark-matter particle in such accidents is the so-called missing out on transverse momentum. To search for this signature, scientists accumulate the momenta of the particles that the LHC detectors can see – more specifically the momenta at ideal angles to the clashing beams of protons – and determine any missing out on momentum required to reach the overall momentum prior to the accident. The overall momentum must be absolutely no due to the fact that the protons take a trip along the instructions of the beams prior to they clash. But if the overall momentum after the accident is not absolutely no, the missing out on momentum required to make it absolutely no might have been brought away by an unnoticed dark-matter particle.
Missing momentum is the basis for 2 primary kinds of search at the LHC. One type is directed by so-called total brand-new physics designs, such as supersymmetry (SUSY) designs. In SUSY designs, the recognized particles explained by the Standard Model of particle physics have a supersymmetric partner particle with a quantum residential or commercial property called spin that varies from that of its equivalent by half of a unit. In addition, in lots of SUSY designs, the lightest supersymmetric particle is a weakly connecting enormous particle (SISSY). Pansies are among the most fascinating prospects for a dark-matter particle due to the fact that they might create the existing abundance of dark matter in the universes. Searches targeting SUSY Pansies search for missing out on momentum from a set of dark-matter particles plus a spray, or “jet”, of particles and/or particles called leptons.
Another kind of search including the missing-momentum signature is directed by streamlined designs that consist of a WIMP-like dark-matter particle and a conciliator particle that would engage with the recognized common particles. The conciliator can be either a recognized particle, such as the Z boson or the Higgs boson, or an unidentified particle. These designs have actually acquired substantial traction in the last few years due to the fact that they are extremely easy yet basic in nature (total designs specify and therefore narrower in scope) and they can be utilized as criteria for contrasts in between arise from the LHC and from non-collider dark-matter experiments. In addition to missing out on momentum from a set of dark-matter particles, this 2nd kind of search searches for a minimum of one extremely energetic things such as a jet of particles or a photon.
In the context of streamlined designs, there’s an alternative to missing-momentum searches, which is to look not for the dark-matter particle however for the conciliator particle through its change, or “decay”, into common particles. This method searches for a bump over a smooth background of occasions in the accident information, such as a bump in the mass circulation of occasions with 2 jets or more leptons.
Narrowing down the SISSY area
What outcomes have the LHC experiments attained from these SISSY searches? The brief response is that they haven’t yet discovered indications of SISSY dark matter. The longer response is that they have actually dismissed big pieces of the theoretical SISSY area and put strong limitations on the enabled worths of the residential or commercial properties of both the dark-matter particle and the conciliator particle, such as their masses and interaction strengths with other particles. Summarising the arise from the LHC experiments, ATLAS experiment partnership member Caterina Doglioni states “We have completed a large number of dedicated searches for invisible particles and visible particles that would occur in processes involving dark matter, and we have interpreted the results of these searches in terms of many different WIMP dark-matter scenarios, from simplified models to SUSY models. This work benefitted from the collaboration between experimentalists and theorists, for example on discussion platforms such as the LHC Dark Matter Working Group (LHC DM WG), which includes theorists and representatives from the ATLAS, CMS and LHCb collaborations. Placing the LHC results in the context of the global WIMP search that includes direct- and indirect-detection experiments has also been a focus of discussion in the dark-matter community, and the discussion continues to date on how to best exploit synergies between different experiments that have the same scientific goal of finding dark matter.”
Giving a particular example of an outcome gotten with information from the ATLAS experiment, Priscilla Pani, ATLAS experiment co-convener of the LHC Dark Matter WG, highlights how the partnership has actually just recently browsed the complete LHC dataset from the maker’s 2nd run (Run 2), gathered in between 2015 and 2018, to search for circumstances in which the Higgs boson may decay into dark-matter particles. “We found no instances of this decay but we were able to set the strongest limits to date on the likelihood that it occurs,” states Pani.
Phil Harris, CMS experiment co-convener of the LHC Dark Matter Working Group, highlights look for a dark-matter conciliator rotting into 2 jets, such as a current CMS search based upon Run 2 information.
“These so-called dijet searches are very powerful because they can probe a large range of mediator masses and interaction strengths,” states Harris.
Xabier Cid Vidal, LHCb experiment co-convener of the LHC Dark Matter WG, in turn notes how information from Run 1 and Run 2 on the decay of a particle referred to as the Bs meson has actually enabled the LHCb partnership to position strong limitations on SUSY designs that consist of Pansies. “The decay of the Bs meson into 2 muons is extremely conscious SUSY particles, such as SUSY Pansies, due to the fact that the frequency with which the decay happens can be extremely various from that anticipated by the Standard Model if SUSY particles, even if their masses are too expensive to be straight discovered at the LHC, hinder the decay,” states Cid Vidal.
Casting a larger web
“10 years ago, experiments (at the LHC and beyond) were searching for dark-matter particles with masses above the proton mass (1 GeV) and below a few TeV. That is, they were targeting classical WIMPs such as those predicted by SUSY. Fast forward 10 years and dark-matter experiments are now searching for WIMP-like particles with masses as low as around 1 MeV and as high as 100 TeV,” states Tait. “And the null arise from searches, such as at the LHC, have actually motivated lots of other possible descriptions for the nature of dark matter, from fuzzy dark matter made from particles with masses as low as 10−22 eV to prehistoric great voids with masses comparable to numerous suns. In light of this, the dark-matter neighborhood has actually started to cast a larger web to check out a bigger landscape of possibilities.”
On the collider front, the LHC scientists have actually started to examine a few of these brand-new possibilities. For example, they have actually begun taking a look at the hypothesis that dark matter becomes part of a bigger dark sector with numerous brand-new kinds of dark particles. These dark-sector particles might consist of a dark-matter equivalent of the photon, the dark photon, which would engage with the other dark-sector particles along with the recognized particles, and long-lived particles, which are likewise anticipated by SUSY designs.
“Dark-sector scenarios provide a new set of experimental signatures, and this is a new playground for LHC physicists,” states Doglioni.
“We are now broadening upon the speculative approaches that we recognize with, so we can attempt to capture uncommon and uncommon signals buried in big backgrounds. Moreover, lots of other existing and organized experiments are likewise targeting dark sectors and particles connecting more feebly than Pansies. Some of these experiments, such as the recently authorized FASER experiment, are sharing understanding, innovation and even accelerator complex with the primary LHC experiments, and they will match the reach of LHC look for non-WIMP dark matter, as revealed by the CERN Physics Beyond Colliders effort.”
Finally, the LHC scientists are still dealing with information from Run 2, and the information collected up until now, from Run 1 and Run 2, is just about 5% of the overall that the experiments will tape. Given this, along with the enormous understanding acquired from the lots of LHC analyses so far carried out, there’s possibly a combating possibility that the LHC will find a dark-matter particle in the next 10 years. “It’s the fact we haven’t found it yet and the possibility that we may find it in the not-so-distant future that keeps me excited about my job,” states Harris. “The last 10 years have shown us that dark matter might be different from what we had initially thought, but that doesn’t mean it is not there for us to find,” states Cid Vidal.
“We will leave no stone unturned, no matter how big or small and how long it will take us,” states Pani.