Online volunteers are being asked to spot tiny explosions that could be evidence for new particles that will require new models of physics.
Higgs Hunters [www.higgshunters.org], a project launched today by UK and US scientists working on the ATLAS experiment, enables members of the public to view 25,000 images recorded at CERN’s Large Hadron Collider. By tagging the origins of tracks on these images volunteers could spot sub-atomic explosions caused when a Higgs boson ‘dies’, which some scientists think could generate a kind of particle new to physics.
‘If anything discovering what happens when a Higgs boson ‘dies’ could be even more exciting that the original discovery that the Higgs boson exists made at CERN back in 2012,’ said Professor Alan Barr of Oxford University’s Department of Physics, lead scientist of the Higgs Hunters project. ‘We want volunteers to help us go beyond the Higgs boson ‘barrier’ by examining pictures of these collisions and telling us what they see.’
In the ATLAS experiment at CERN’s Large Hadron Collider protons are smashed together at up to one billion kilometres per hour. Such collisions can generate Higgs bosons: these are known to rapidly decay into other particles and some scientists believe these could include a new type of previously unobserved particle. Simulations predict that these new particles should leave tell-tale tracks inside the ATLAS experiment, which computer programs find difficult to identify, but which human eyes can often pick out.
Professor Andy Haas of New York University said: ‘Writing computer algorithms to identify these particles is tough, so we’re excited to see how much better we can do when people help us with the hunt.’
Professor Chris Lintott of Oxford University’s Department of Physics, Zooniverse Principal Investigator, said: ‘The most exciting citizen science comes when you find the unexpected lurking amongst the data, and who knows what could be out there in the data from the ATLAS experiment?’
Professor Dave Charlton, spokesperson of the ATLAS Collaboration, said: ‘With the Higgs Hunters project, people can look directly at ATLAS data to help us find unexpected phenomena – perhaps volunteers will be able to spot new physics with their own eyes!’
A successful detection of new particles would be a huge leap forward for particle physics, as they would lie beyond the Standard Model – the current best theory of the fundamental constituents of the Universe.
By Frank Wuerthwein, Keith Ulmer and Guillelmo Gomez Ceballos.
Among the leading candidates to describe physics beyond the standard model of particle physics is Supersymmetry, a new symmetry that posits the existence of a partner particle for each known particle in the standard model. Supersymmetry, or “SUSY” as it has come to be known, may help explain the nature of dark matter and the large difference in strength between the fundamental forces of nature. Each year, new experimental results and theoretical developments are reported in the “SUSY” conference series, with the 2014 edition (SUSY2014) happening this week in Manchester, England.
Experimental evidence for SUSY has been sought for many years at multiple colliders, including a vast array of search results from the CMS experiment at the Large Hadron Collider at CERN. With data from Run 1 of the LHC collected through the end of 2012, the full set of results thus far has not revealed any striking signs of physics beyond the standard model . New searches presented at SUSY2014 have begun to probe increasingly complicated potential decay chains and to combine multiple searches to access more challenging new physics scenarios. Below we highlight some of the most recent results first presented this summer at SUSY14 and ICHEP 2014 .
Search for new physics in the final states hh, Zh, and ZZ plus MET
After its discovery only two years ago, the Higgs boson is already a powerful tool in the search for new physics. Earlier this year, CMS submitted for publication  a set of searches for associate production of W, Higgs, and missing transverse energy (“MET”, indicative of particles escaping the detector). At ICHEP this summer, CMS presented the first combined searches for hh, Zh, and ZZ plus MET. No excess above standard model backgrounds is observed. Figure 1 shows the interpretation of the results in terms of limits on higgsino pair production as a function of the higgsino mass and decay branching fraction. Within the framework of Gauge Mediated Supersymmetry Breaking (GMSB), the neutral higgsino decays to a gravitino and either a higgs or Z boson. The left plot in Figure 1 shows that CMS excludes higgsino production up to ~ 300GeV when the higgsino decays at equal rate to either of these two decays. The right plot in Figure 1 indicates that four different final states dominate the sensitivity in different parts of the 2D parameter space, clearly demonstrating that searches for new physics with one or two higgs bosons in the final state benefit greatly from combining many different decay channels.
Search for gluino pair production via the decays to top pairs, bottom pairs, or top and bottom plus MET
Up to now, CMS searches for gluino pair production inspired by “natural SUSY” (i.e. SUSY in which the masses of the SUSY partners are not much higher than those of the Higgs boson) have focused on final states with either four top or four b-quarks plus MET. In contrast, theoretically any combination of MET plus 4 quarks, top or bottom, is well justified. At ICHEP, CMS presented the first complete exploration of sensitivity across the full set of possible final states and branching fractions. Figure 2 shows the corresponding exclusion curves in the gluino vs neutralino mass plane. This search employs the so-called “razor” variables, and its sensitivity is dominated by all-hadronic final states. The more top quarks there are in the final state for a given gluino mass, the less momentum is left for all the decay products, and the harder it is thus to distinguish signal from background. Accordingly, the sensitivity decreases as the number of top quarks per event increases.
Searching for SUSY with an “Edge”
The dilepton invariant mass distribution for leptons from the decays χ20 to l+l- χ10, or similar decays via a slepton as an intermediate state, display the striking feature of a kinematic “edge” [5, 6]. As these decays conserve lepton flavor, this edge is present only in same-flavor events, i.e. ee and μμ, and is completely absent in the “opposite flavor” lepton sample, i.e. eμ events. In contrast, backgrounds for which each of the two leptons come from a different W decay, e.g. top pairs, WW, etc., will have identical dilepton distributions for same and opposite flavor. Thus, the eμ sample in data provides a perfect model of the background dilepton mass distribution – modulo effects from the trigger and lepton reconstruction. The kinematic edge is a sufficiently striking signature to reveal new physics even at relatively modest hadronic activity, HT and MET, i.e. in the presence of sizeable top and Z backgrounds.
CMS presented a search for such an “edge” in dilepton events with jets and MET at SUSY2014 using the full 8TeV data sample . Figure 3 overlays the dilepton mass distribution in ee plus μμ (data points), with the corresponding one from eμ (pink histogram). The blue shaded region depicts the systematic error envelope for the background prediction. A small excess is visible below the Z peak. A signal region of 20GeV < mll < 70GeV was chosen before data taking. Within this region, 860 events are observed with an expected standard model background yield of 730 ± 40. The small excess is consistent with a 2.6 sigma fluctuation of the standard model background. For more details see .
Search for additional neutral MSSM Higgs bosons in the H→ττ decay channel
Another highlight among the CMS results presented at the SUSY2014 conference is the search for additional neutral Higgs bosons decaying to τ leptons, which is the most promising channel to search for such Higgs bosons in the context of the minimal SUSY extension of the standard model, the MSSM. Following the release of a preliminary result based on the full data set of the 2011/2012 data taking period , additional results based on a new interpretation of the data have been presented at this conference for the first time . While the data selection has not changed, extensive work has set the ground for a new interpretation of the data in the context of modern benchmark models. In particular, the new models take into account the presence of the recently discovered Higgs boson with a mass of 125 GeV, as proposed in . Also for the first time the model-dependent exclusion contours as a function of the mass of the CP-odd Higgs boson, A, and the ratio of the vacuum expectation values of the two SUSY Higgs doublets, tanβ, have been derived, taking the presence of the newly discovered Higgs boson properly into account in the test statistic. As recently demonstrated by CMS , all observations of the new boson are so far compatible with the SM expectation within ~10% accuracy, which justifies the standard model hypothesis to be the better choice for the test statistic. The hypothesis test now becomes a search based on a model with three Higgs bosons compared against the standard model with only one Higgs boson. Traditional limits, based on the test statistic excluding the Higgs boson from the standard model hypothesis have also been made public on the CMS web-pages . Also made available to the public is an extended database of results based on a model-independent single-resonance search model, which will be extremely valuable to theorists engaged in model building. Figure 1 shows the exclusion contour in a modified mh,max scenario, also referred to as mh,mod+ exploiting the new statistical treatment for the statistical inference.
By Frank Wuerthwein, Keith Ulmer and Guillelmo Gomez Ceballos.