Beautiful beam-halo tracks in CMS!


In posting LHC updates, I want to be careful not to say anything or present anything that is still internal to the collaboration, which usually precludes plots of hours-old data.  However, our first event displays are already available on a publicly-accessible site, so enjoy!

Here’s an annotation of the what I think is the clearest one:

Upstream of our detector, protons from the first LHC beam collided with atomic nuclei of gas atoms and metallic beam collimators (which clip the beam to protect the magnets), producing a “halo” of muons, roughly parallel to the beam in the beampipe.  In the picture above, you’re looking at a reconstruction of one of these muons from detector signals alone.  For clarity, only the subdetectors which saw anything are drawn (they’re the trapezoids hovering in space), and their measurements are presented as intersecting yellow and purple lines.  The yellow and purple are nearly orthogonal measurements, each sensitive to the passing particle’s position in one dimension: where they cross is where the muon passed through.  If you look closely, you’ll see that these points line up.  The software recognized this as a track and fitted a blue line to them, which looks as though it points back to a beam-gas collision in the beamline somewhere to the right of this picture.

The computer-generated image doesn’t give a sense of scale, so I’ve added Big Bird.  It’s also unclear at this level of zooming that the position of the muon as it passes through them is measured with an accuracy of about 300 microns.

Cute event displays are absolutely pouring in on CMS mailing lists.  Meanwhile, it sounds like the LHC is getting close to running protons in the other direction as well (it was delayed due to a cryogenics problem).  As a reminder, they’re not planning to collide them or ramp them up to high energies or high intensities today: that will be a slow, careful process over the next one or two months.  I wonder if they’re going to have festive event like this when the beams actually do collide, or if the media will lose interest by then.


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5 Responses to “Beautiful beam-halo tracks in CMS!”

  1. Greg Muller Says:

    Is the relative time precision of these detectors precise enough to be useful? I mean, it would have to be really, really precise to say anything about direction of these presumably relativistic muons, but you guys have done crazier awesome stuff than that over there.

  2. Jim Pivarski Says:

    Yes! (We wouldn’t engineer precision if we couldn’t use it.) Determining particles’ directions with high precision is easy, the hard part is measuring the curvature of tracks in a magnetic field (errors grow quadratically with lever arm, rather than linearly).

    Both of these primarily use the spatial information, but timing is also important. One prospective signal would be new massive charged particles, which look just like muons except that they’re slow: less relativistic than you’d expect (0.9c instead of 0.99999c). This is not such a hard measurement— after all, Big Bird is 7 nanoseconds tall and 1-2 nanosecond timing is not hard to find in off-the-shelf electronics.

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