cartoon of a disappearing track
New, undiscovered charged particles could be produced by the LHC, leaving tracks in CMS that look similar to tracks from Standard Model particles. If the new particle decays to other, unseen new particles inside the CMS tracker, a very striking signature would be observed: a 'disappearing track'. CMS has carried out a new search for this distinct signature that exploits its upgraded inner tracker to add sensitivity to shorter lifetimes.
One of the most exciting possibilities for physics beyond the Standard Model is the existence of a new undiscovered long-lived particle (LLP). Most particles in the Standard Model decay so quickly that the resulting products seem to come directly from the collision region. LLPs, on the other hand, travel a measurable distance and then decay in the detector. If the decay products can't be seen by the detector, this signature would be completely different from anything produced by the Standard Model: a charged particle that seems to 'disappear' in the middle of CMS!
Many new physics scenarios predict particles that could produce this signature. One example is anomaly-mediated supersymmetry breaking (AMSB), which hypothesizes two new particles (along with many others): the charged ‘chargino’ and neutral ‘neutralino’. In AMSB, these two have masses very close to one another's, and the chargino decays into a neutralino and charged pion. This small mass difference has two important consequences: the phase space for chargino decay is limited, giving it a long lifetime, and when it does decay there is very little energy left for the pion. The charged pion is bent into a tight helix by the 4T superconducting solenoidal magnet in CMS, and its interactions leave too small of an impact to be seen by the detector. The neutralino travels through the detector unseen, and the disappearing act is complete for the chargino! Figure 1 below shows how this might look in the recorded data.
Figure 1: Illustration of the decay of a long-lived chargino in the CMS tracker. The chargino (red) travels through four layers of the tracker (concentric grey circles) before decaying to a neutralino (cyan) and pion (gold). The neutralino cannot be seen with the CMS detector, and the pion is too low-energy to be observed. What is left is a short track that disappears in the middle of the tracker. Thanks to the upgraded tracker, CMS is now sensitive to tracks as short as four layers.
So how does CMS search for this signature? First and foremost by making sure a track isn't due to a known Standard Model particle, like a lepton or a hadron jet. Any track too close to one of these is rejected from consideration. To be as careful as possible, the data is used to painstakingly map out parts of the detector that might be inefficient at finding leptons, and tracks in these regions are also rejected. After this any tracks are highly unlikely to be a lepton, and we can ask that it disappears: the track has to stop interacting with the tracker at least three layers before its outermost layer, it can’t leave any hits at all in the muon detectors, and there has to be very little energy deposited in the calorimeters close to the track. Figure 2 below shows how a simulated chargino track looks in CMS.
Figure 2: A display of a simulated AMSB chargino event in the CMS detector. The disappearing track from the chargino can be seen in the middle of the display. In this event, the chargino passes through four layers of the tracker before disappearing.
After making these stringent requirements on candidate tracks, only an estimated 48 background events are expected in an entire two years of LHC data. These events are from either the rare time a lepton could still seem to disappear or from accidentally combining unrelated tracker hits into a ‘spurious’ track. With only 48 expected events, these backgrounds are incredibly rare: in some cases, their estimated rate is only one in a hundred million events! Having calculated the number of expected background events and every uncertainty in the search, we look in the data and see how many candidate signal events we observe: exactly 48. While this doesn’t suggest a large number of AMSB events, we can use these results to calculate the maximum rate of AMSB chargino production that would still be consistent with the observation. Figure 3 below shows the exclusion curve in chargino mass and lifetime, where everything to the left of the curve is ruled out to a high level of confidence.
Figure 3: Results of the search for disappearing tracks. The black curve represents upper limits on the mass of the chargino (x-axis) as a function of the chargino lifetime (y-axis). Chargino masses to the left of the curve are excluded by this search. The upgraded tracker is much more sensitive to short tracks, and the exclusion extends to much shorter lifetimes than was possible previously.
Charginos with a lower mass would be produced more frequently than charginos with a higher mass, which is why the left side of the curve is excluded. A lifetime of about 3 nanoseconds is the most likely to leave a candidate disappearing track for the search and the size of the CMS detector, so the exclusion is highest in mass for that hypothesis, excluding particles that create disappearing tracks with masses below 884 GeV. These are the strongest limits in the world for this type of new physics, and show the powerful improvement of the CMS tracker upgrade in searching for shorter lifetimes.