Looking for Exotic SUSY Signals
We do not know where the first signs of new physics will show up, so it is best to look everywhere at the same time. This includes classical channels for SUSY like those I discussed earlier, (jets and missing energy with or without isolated, high-pT leptons) as well as strange signals that were suggested long after SUSY was an established favorite theory.
One of the more exotic signals for new physics comes from a heavy charged quasi-stable particle – one that travels well below the speed of light and which leaves a trail of anomalous ionization in the detector. Such hypothetical particles have various names – “CHAMPS” at CDF and “SMPs” at ATLAS, for example. They appear as predictions from some versions of SUSY and also in other theories. From an experimenter’s viewpoint, they are simply wonderful things to go look for, since a search for SMPs draws upon the capabilities of the experiment that are not important for most other searches for new physics. In a word, such searches are “fun.”
The ATLAS Collaboration employed two subdetectors to try to find stable charged massive particles (arXiv:1103.1984 10-March). An important strength of their approach is that these two detectors provide independent information with different systematics, so a false signal in one will not be correlated with a false signal in the other. Neither detector is powerful enough to find a clean signal, but the two combined are quite powerful.
Firstly they use their pixel detector to measure the specific ionization of a particle. The ionization is high for particles that are traveling slowly, so even if the pT is high, the velocity v will be small if the mass M is high. They require pT>50 GeV, so only a particle with a mass of hundreds of GeV will give a large ionization signal in the pixel chambers. They use standard measures and calibration techniques for establishing their ionization signals. The observed distribution confirms their simulation of standard processes. There are a bit more than 5k particles.
Secondly they use their tile calorimeter to measure the time-of-flight. This device provides a time resolution of about 1 ns based on time-sampled signals from three layers of scintillating tiles which are interspersed with iron. These layers are 2.3 to 5.3 m away from the interaction point, and an SMP can produce up to six independent time measurements.
The basic performance of these two devices can be gleaned from the plots below. Notice that signals from hypothetical SMPs are very different from particles produced in standard model processes. A signal of this type would be experimentally much more dramatic than, say, an excess in the tail of the
missing energy distribution, or a few extra tri-lepton events.
The ATLAS scientists understand these two detectors well enough to be able to construct an accurate model for the resolution as a function of pT. Using a Monte Carlo technique, they convolve these resolution functions with observed particle spectra to predict the tails of the distributions, as shown in these plots:
The data conform to this prediction very well, which means there is no obvious sign for SMPs, unfortunately. Notice the dramatic difference between standard model and new physics signals. Keep in mind, also, that these two quantities are independent, so a signal in one would be confirmed, presumably, in the other. In practice the two observables are combined and a double two-sigma window defined to try to isolate a clean signal sample as a function of the hypothetical SMP mass. For a nominal mass of 100 GeV, 5.4 events are predicted and 5 are observed; for higher masses the predicted background falls rapidly and no events are observed. The systematic uncertainty on the signal yield slightly less than 20%. A plot of the upper limit on the cross section as a function of mass is given below.
The green falling curves are theoretical predictions for gluinos and squarks; the lower limit on gluinos is about 580 GeV depending on the details of their hadronization and the way the resulting hadron (called an R-hadron) interacts with the detector material. This is much, much better than the result published earlier by the CMS Collaboration (arXiv:1101.1645 9-Jan-2011). (The results on squarks are likewise much better than bounds coming from LEP and CDF.)
Why is the ATLAS limit so much better than the CMS one? The reason is very simple: The CMS result was based on only 3.1 pb-1 while this nice ATLAS result uses ten times more data. The CMS analysis used the ionization in the silicon detectors, and the presence of hits in the muon chambers; no time-of-flight information was used. Finally the R-hadron scenario was a pessimistic one. The upper limits on cross sections are about 7 pb for the CMS analysis, to be compared to about 0.8 pb for the ATLAS analysis. If these background-free limits scale as 1/Luminosity, the two analyses appear to have similar sensitivity.
In any case, the bottom line is that ATLAS has put a new stringent upper bound on quasi-stable charged massive particles. The reach should extend up toward 1 TeV later in 2011.
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