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Brian Dorney | USLHC | USA

View Blog | Read Bio

Keeping up with CMS

In the fast pace world of particle physics its sometimes hard (even for those of us actively involved in it) to keep up with current events.  However, Lucas Taylor (the Project Manager of “CMS Centers Worldwide”), has designed a system that helps students, faculty members, post-docs and other researchers to stay active with the day-to-day dealings of the CMS Collaboration without the need to always be on site at CERN.  This system, is actually more like a series of locations.  They are called CMS Centers and there are in fact 49 of them worldwide!  And when the LHC sent its first proton beam around its circumference on Sept. 10th 2008, the world’s largest press conference for a scientific event (since first moon landing) took place in the CERN’s very own CMS Center [1].  In total, 37 media organizations came to the center for the first LHC Beam [1].

The major centers are located at:

  • CERN in Geneva, Switzerland
  • Fermi National Accelerator Laboratory (FNAL) in Batavia, Illinois, USA
  • DESY (Deutsches Elektronen Synchrotron,  or the German Electron Synchrotron) in Hamberg, Germany

However these are just three of the many centers worldwide.  A complete map is shown here:

Current map of current CMS Centers worldwide, courtesy of CERN

But what kind of information is available at a CMS Center?  And more importantly, do you actually have to go to one to see this information? Certain information is available to the general public (which is what I will show here), but most of the information is only available at an actual center.  My own host institution has a CMS Center, if you look on the above map you’ll see it listed in Melbourne, Florida.

At a CMS Center one of the things that can be seen is what’s called the LHC Page 1, which the general public can view here.  I’ll show a frozen image of this page below, and walk you through what type of information can be found on this page.

LHC Page 1, Courtesy of the CERN CMS Center

  • A: Shows the target beam energy (if any) of the current experimental requirements
  • B: Shows what’s happening in with the Collider’s twin proton beams, here they are testing the injection system (hence the “Injection Probe Beam”), basically the LHC is fed beams of particles from the CERN accelerator complex, which does the job of taking particles (in this case protons) up to a small percent of the speed of light.  Then the LHC takes over accelerating particles to ~99.9999% the speed of light.
  • C: Shows an axis representing the beam’s intensity, in this still this is the black line in the central chart (and can also be viewed above at the BCT TI1 & TC2 listings).  The beam intensity is also known as the luminosity.  This is how many particles are travelling through a unit area (in cm^2) per unit time.  So the beam intensity/luminosity is 1.10e9 (cm^2 · s^-1) in this image.
  • D: Shows any comments relevant to the current state of the accelerator
  • E: Shows a plot of the current beam energy, with respect to time.  The energy unit is GeV, or giga electron volts.  An electron volt is the amount of energy it takes to move one electron, through a potential difference of one volt…and a giga electron volt is 100,000,000 eV.

Another thing that can be seen at a CMS Center is what’s called CMS Page 1, which may also be viewed by the general public at this link (sometimes this link switches between CMS Page 1 and CMS DAQ Page that I’ll talk about in a little bit).  I’ll show another freeze frame of this image, and try and walk you through some of the information that can be shown on this page.

CMS Page 1, Courtesy of the CERN CMS Center

  • A: Shows the intensity (or luminosity) of both beams that are colliding within the CMS Detector (currently this is very low, the highest we achieved in 2010 was of the order of 10^32, twenty orders of magnitude higher than what is currently shown here!)
  • B: Shows the beam energy
  • C: Shows the current status of the CMS Detector, Running means CMS is taking data…this will sometimes read “Offline” when we are not taking data…but this lets you know whether or not the detector is operating
  • D: Shows  a plot of integrated luminosity in units of 1/nb (nb read, “nano-barn”).  A barn, is a unit of cross-sectional area.  One barn corresponds to an area of 10^-24 cm^2.  (You may make “you couldn’t hit the broad side of a barn” reference now!).  When we are colliding proton beams for experimental studies relating to physics analysis the plot will show “Delivered” and “Recorded” integrated luminosities.  “Delivered” corresponds to what the Large Hadron Collider is giving the detector, and “Recorded” corresponds to how much of what was delivered was written to our tape drives as useful data.
  • E: Shows comments made by the shift leader at point 5 (the CMS Control Room) that are relevant to the current experimental study.
  • F: Shows a readout of all of the CMS Detector’s sub systems, the Detector is like an onion (and also an Ogre), it has layers.  These layers are (starting from the top):
    1. CSC: Cathode Strip Chambers, these are part of the Detector’s (very impressive) Muon Detection system.  The staple of our name, Compact Muon Solenoid.
    2. DT: Drift Tubes, these are also part of the Muon Detection system.
    3. ECAL: Electromagnetic Calorimeter, these are scintillating lead tungstate (PbWO4) crystals.  They have a short characteristic radiation length, and are responsible for photon & electron detection.
    4. ES: Electromagnetic PreShower, this system causes cascades of of particles to form in the detector (also known as a shower).  This is specifically designed to shower high energy electrons and photons into the ECAL.
    5. HCAL: Hadronic Calorimeter, these are “towers” (or stacks) of brass scintillating plates.  They are responsible for picking up heavy particles (bayrons and mesons) and neutral particles that do not leave a signal anywhere else in the detector.  They are very dense material and have a sufficient nuclear interaction length to do the job.
    6. PIXEL: Silicon Pixel Detector, this is a very advanced detector made up of silicon pixel strips, located only centimeters from the proton-proton collision.  This is the closest detector element to the interaction point (aka collision point), and gives us very good momentum resolution, and track impact parameter measurements.
    7. RPC: Resistive Plate Chambers, are the last element of the muon detection system.
    8. TRACKER: Silicon Strip Tracking Detector, this is also a very advanced detector, it encompasses the Pixel Detector, and is the second closest detection element to the beam pipe.  We use this to make precision momentum measurements, secondary decay vertices, and track impact parameter measurements.  If you combine the entire tracker (both Pixel and Silicon Strip) has over 10^7 channels that are readout by the Detector’s electronics!!!!
    9. CASTOR: Castor is a forward element of the Hadronic detection system.  It is a little ways away from the rest of the detector down the beam pipe in both directions.  CASTOR is responsible for picking up neutral particles that come out at very small scattering angles, almost co-linear with the beam pipe.
    10. TRG: Trigger System, responsible for selecting events to record.
    11. DAQ: Data Acquisition System
    12. DQM: Data Quality Monitoring System (this ensures that the data we are recording is good data!)
    13. SCAL: I’m actually not sure what this is unfortunately 🙁
    14. HFLUMI: This is part of the Forward Hadron Calorimeter, this sits at both ends of the detector, and trys to capture heavy  hadrons and neutral particles at low scattering angles to the beam pipe.
    15. Fill/Run  Number & Lumi Section: This is how we label events so that we can investigate them individually later.
    16. Physics Bit Set: This is a list of technical triggers that give information about the detector, and records this to the data when it is taken.
    17. Magnet: it usually operates near 4T (largest of its kind in the world!)

An interactive cartoon of many of these systems and how particles interact with them can be found here.

Another bit of information most CMS Centers will show is the CMS DAQ.  And this too is available to the public at this link.  Here’s another free-frame so we can walk through what information is available on this page as well.

CMS DAQ Page, Courtesy of CERN CMS Center

  • A: Shows a basic summary of the DAQ System.  Here we have; the beam setup, the run number, the rate of level 1 triggers accepting events (basically how many events our level1 trigger accepts in a second), the size of the event (in kilobytes), the acceptance rate of final events (events that have been accepted by both the level 1 trigger and the high level trigger), and the percent of the high level trigger (HLT) computing power we are using.
  • B: This image will vary from time to time, sometimes it shows a tiny version of CMS Page 1, LHC Page 1, or a live event display! (See below for more details on that).
  • C: Shows the status of all the detector elements while the data is being taken.
  • D: Shows the status of the current data streams.
  • E: Shows the a plot of; our trigger rate (in green), or CPU performance (in pink), and the number of events accepted and stored (the white shaded region, this region gets larger and larger while we take data, hence the increasing trend).
  • F: And my favorite, a tiny little statement that says “Physics On,” as if we could turn it Off!

These aren’t the only things that a CMS Center will show.  The CMS Center at my host institution has 5 displays in total, so we run 5 displays at a time.  However, the CMS Center at CERN has 25 consoles in total, with six monitors per console [2], so that’s 150 screens in total!!!!  A CMS Center may also show the status of each individual channel in the detector’s subsystems, the status of the calorimeters; and possibly even the status of the supercomputing farms available.   This and much more…but for this, you must go to the Center to observe this information!  So if you ever have the opportunity, I highly recommend it.  I myself have seen the Center at FNAL on the night of September 10th, 2008, and it was awe-inspiring to say the least.  FNAL’s CMS Center is also a remote operations ceneter (ROC).  Scientists at Fermi may remotely monitor (and control) various aspects of the CMS Detector at the ROC.  This allows them to be part of the action without needing to be present at CERN.

The final thing that may usually be seen at a CMS Center (and is also available to the general public) is the live event  feed coming right off of the detector (basically the proton-proton collisions as they happen!!!!!).   I personally think this is one of the most inspiring views available from CMS.  It let’s you see the “mini-big-bang’s” in action.  When two protons collide in CMS, they literally explode into hundreds of pieces, and these pieces are picked up by our detector (if they interact with matter in an ordinary way).

We use a program called “Fireworks Event Display” to visualize the signal the CMS Detector picks up, and to see what’s actually happening “on-line” in the collisions.  Its best to view this page when the collider is performing 7 TeV Collisions (we aren’t just yet, but will be soon), so check on this link in the near future to see some very interesting events (that could possibly change the face of modern physics as we know it!!!).  The link for the live feed is here.  It updates every few minutes with a new event (or if its a particularly interesting event, it will stay on the screen for some time to give it the needed “publicity”).

You can find some previous event displays on the public page of the CMS Collaboration.  Here is a sample, I’ll briefly explain the various elements we are looking at:

A Proton Proton Collision Event at 7 TeV, Courtesy of CERN and the CMS Collaboration

Here we have several things going on:

  • The yellow curved lines are what are called tracks, they are caused by charged particles hitting the Pixel Tracker and the Silicon Strip Tracker.  From each hit on pixel or a strip, we can reconstruct the charged particles path as shown above.
  • The red “rectangles” are hits in the Electromagnetic Calorimeter.  These are predominately made by photons and electrons.
  • The blue “rectangles” are hits in the Hadronic Calorimeter.  These are caused by “heavy” particles (like baryons and mesons) that interact strongly with the nuclei of the brass scintillating plates of HCAL.  These particles may also be neutral, and HCAL is how we detect these neutral particles.
  • In the lower right plot, the outermost rectangles that form a ring are the muon detection system of CMS, these are the farthest from the proton-proton collision in the entire detector.
  • A 3D View of the detector (Left)
  • A view of the Rho-Z plane, in cylindrical coordinates (Top Right)
  • A view of the Rho-Phi plane, in cylindrical coordinates (Bottom Right)

Sometimes more “Physics Objects” are shown in event displays; like jets, muons, and MET’s.  Jets are conic depositions of energy in the calorimeters in a collimated line.  Jets are due to hadronic activity, when quarks are formed in the collision (or release from the exploding protons), they must hadronize immediately into bound states due to color confinement.  This process is observed in Jets in the calorimeter.  Muons are the heavier brother of the electron, and they will transverse the entire detector (and sometimes the entire atmosphere) before decaying into lighter particles.  MET’s are what’s called Missing Transverse Energy.  In an event, we sum up all the momentum vectors that we observe.  We orient our coordinate axis so that the z-axis is along the beam pipe.  The colliding protons have equal and opposite momentum, only in the z-direction.  So all the momentum vectors in the xy-plane (perpendicular to the detector) must sum to zero.  If it doesn’t its an indication that a particle (like a neutrino) has escaped detection, and went in the direction necessary to balance the momentum vectors in the transverse plane to zero.

You can find more images of real proton-proton collision events here.

And finally some pictures of the CERN CMS Center, the FNAL CMS Center, and my host institution’s (Florida Institute of Technology) CMS Center are shown below.

The CMS Center at CERN [1]:

The CMS Center At FNAL [2]:

And at the Florida Institute of Technology:

CMS Center at the Florida Institute of Technology, showing myself and fellow graduate student, Rob Lucia, hard at work. (Photo taken by Dr. Igor Vodopiyanov)

Well that’s all for now, but hopefully you’ve found this informative.  Now you too can be part of the action by checking the links above to see in real-time what’s happening at Point 5 and with CMS and the Large Hadron Collider.



[1] Lucas Taylor, “How to create a CMS Centre @ My Institute,” April 8th 2011, https://cms-docdb.cern.ch/cgi-bin/DocDB/RetrieveFile?docid=2527&filename=CMS-Centres-Worldwide-1-5-A5.pdf

[2]  Lucas Taylor et al., “CMS centres for control, monitoring, offline operations and prompt analysis,” J. Phys.: Conf. Ser. 119 072029 doi: 10.1088/1742-6596/119/7/072029.

For more information on the FNAL CMS Center please see: http://cms.fnal.gov/index.shtml


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