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Posts Tagged ‘event displays’

I’m often asked as a high energy physicist how do we know that the elementary particles exist.  One might think such questions are absurd.  But, if the scientific method is to stand for anything, then these questions must have merit (and be taken seriously).  After all, it is our duty as scientists to take an unbiased skeptical viewpoint; and to report on what is actually observed in nature.

Trust me, I would find a world were Hogwarts Castle actually existed as a school of magic far more interesting. But alas, nature has no room for such things as wands or Horcruxes.

But I thought I’d try to discuss this week how the gluon was “discovered” decades ago.  The gluon is represented by the “g” in our “periodic table” of elementary particles:

Experimentally observed members of the Standard Model (Ref. 1)

The gluon is what’s called a “vector boson,” meaning it has spin 1 (in units of planck’s fundamental constant, ℏ).  And it is the mediator of the strong nuclear force.  The force which is responsible for binding quarks into hadrons and keeping atomic nuclei together.  When I say the gluon is a mediator, I mean that when a quark interacts with another quark or anti-quark, it does so by exchanging gluons with the other quark/anti-quark.  In fact gluons themselves interact with other gluons by exchanging gluons!!!

But how exactly do the quarks/anti-quarks and gluons interact?  Well quarks & gluons (whenever I say quarks, my statement also applies to anti-quarks) carry something called Color Charge.  Color is a type of charge (similar to electric charge) in physics.  It comes in three types labelled as red, green & blue.  Now where as electric charge has a postive and a negtive, color charge has a “color” (i.e. red charge) and an “anti-color” (i.e. anti-red charge).  It is this color charge that gives rise to the strong nuclear force, and is what is responsible for the interaction of quarks and gluons with each other.  The quantum theory associated with the interactions of quarks and gluons is known as Quantum Chromodynamics (QCD, “Chromo-“ for color!).

However, no particle with a color charge can be directly observed in the universe today.  This is due to something called “Color Confinement,”  which causes colored particles to form bind together into “white” (all colors present in equal parts), or “colorless” (net color is zero) states.  We sometimes call these states “color neutral” or “color singlet” states.  Flip Tanedo has written this nice post about Color Confinement if you’d like to know more.

So if an experimentalist cannot directly observe a gluon, how were they discovered?  One of the best answers to this question comes from electron-positron colliders, such as the LHC’s predecessor: the Large Electron-Positron Collider (LEP), and this is where our story takes us.

Jet’s in Electron-Positron Collisions

While electrons & positrons do not carry color charge, they can produce colored particles in a collision.  The Feynman Diagram for such a process is shown here:

Here an electron and a positron annihilate, emit a virtual photon, which then pair produces a quark and an anti-quark (Image courtesy of Wikipedia, Ref. 2)

Since the quark & anti-quark produced carry color; they must hadronize, or bind together, to form color neutral states.  This hadronization process then gives rise to the formation of jets.

If the momentum of the colliding electron and the positron are equal but opposite (the angle between them is 180 degrees), the two jets produced would appear to be “back-to-back.”  Meaning that the angle between them is also 180 degrees (For those of you counting, you must look in the center-of-momentum frame).

The reason for this is that momentum must be conserved.  If the electron comes in with Y momentum, and the positron comes in from the opposite direction with -Y momentum, then the total momentum of the collision is zero.  Then if I sum over all the momentum of all the particles produced in the collision (termed “outgoing” particles), this sum must also equal zero.  In this case there are only two outgoing particles, and the angle between them must be 180 degrees!

We call such a collision event a “di-jet event,” because two jets are created.  Here’s an example of a Di-Jet Event as seen by the CMS Detector, and would look identical to what is observed in an electron-positron collider.

Di-Jet Event within the CMS Detector, as seen in looking down the beam-pipe in the xy-plane.

The two protrusions of rectangles together with the solid and dotted purple lines represent the two jets in the above image.  The black lines represent each jet’s direction.  Notice how the angle between them is almost exactly 180 degrees.

Now suppose either the quark or the anti-quark in the above Feynman Diagram was very energetic, and radiated off another particle.  QCD tells us that this particle that is radiated is a gluon.  The Feynman Diagram for this “gluon radiation” would look like the above diagram, but with one additional “line,” as shown here:


Gluon radiation from an anti-quark in an electron-positron collision (Image courtesy of Wikiepdia, Ref. 2)

 

We say this Feynman Diagram describes the process e+e →qqg.  Here the anti-quark is shown as radiating a gluon, but the quark could have just as easily radiated a gluon.  If the radiated gluon is very energetic, the theory tells us it would have a different direction from the quark and the anti-quark.  Thus the gluon would make its own jet!

Now an experimentalist has something to look for! If gluons exist, we should see events in which we have not two, but three jets created in electron-positron collisions.  Due to momentum conservation, these three jets should also all lie in the same plane (called “the event plane”); and if the gluon has enough energy, the three jets should be “well separated,” or the angles between the jets are large.

Such electron-positron collision events were observed in the late 1970s/early 1980s at the Positron Electron Tandem Ring Accelerator (PETRA) at the Deutsches Elektronen Synchrotron (DESY).  Here are two examples of three jet events observed by the JADE detector (one of the four detectors on PETRA):

A Tri-Jet event observed in the JADE Detector, again looking down the beampipe (Ref. 3)

 

Another Tri-Jet event observed in the JADE detector (Ref. 4)

From these event displays you can see the grouping of charged & neutral tracks (the solid & dotted lines in the images) in three regions of the JADE detector.  Notice how the tracks are clustered, we say they are “collinear.”  The reason they are appear collinear is because when a quark/gluon hadronizes, the hardonization process must conserve momentum.  The particles produced from hadronization must travel in the same direction as the original quark/gluon.  Then because of this collinear behavior the tracks group together to form jets.  Notice also how the jets are no longer back-to-back, but are well separated from each other (as expected).

While these images were first reported decades ago, we still observe three jet events today at the LHC and other colliders.  Here is an example of a three jet event as recorded by the CMS Detector:

 

A Tri-Jet event in CMS

 

But now let’s actually compare some theoretical predictions of QCD to the experimental data seen at PETRA and see if we can come up with a reason to believe in the gluon.

 

QCD Wins the Day

The MARK-J Collaboration (also one of the detectors at PETRA) decided to investigate three jet events based on two models of the day, the first of which was QCD [4], now a fully formalized theory, which interpreted three jet events as:

e+e →qqg

In which a gluon is produced in the collision, in addition to the quark and anti-quark.  The second model they used was what was called the quark/anti-quark model, or phase-space model [4].  Which interpreted three jet events as simply:

e+e →qq

In which only a quark and an anti-quark are produced.

To compare their theoretical predictions to the experimental data they looked at how energy was distributed in the detector.  They looked to see how well the two predictions matched what was observed by using something called a “normalized chi-squared test”  (a test which is still widely used today across all areas of research).

In a normalized chi-squared test, you perform a statistical test between the two “data sets” (in this case one set is the experimental data, the other is the theoretical prediction), from this test you get a “chi-squared” value.  If the “chi-squared” value divided by the “number of degrees of freedom” (usually the number of data points available) is equal to one, then we say that the two data sets are well matched.   Or, the theoretical prediction has matched the experimental observation.  So if one of the two above models (QCD, and the “Phase-Space” model) has a normalized chi-squared value of one or near one when compared with the data, then that is the model that matches nature!

So to make their energy distributions, the MARK-J Collaboration decided to work in a coordinate system defined by three axes [4,5].  The first of which was called the “Thrust” axis, defined as the direction for which the “energy flow” is maximum [4,5].  This basically means the Thrust axis is taken as the direction of the most energetic jet in the event.

The second axis, the “Major” axis, is taken to be perpendicular to the Thrust axis; but with the requirement that the projected energy of the most energetic jet onto the Major axis in is maximized [4,5].  Meaning if I took the dot product between the Major axis and the direction of the most energetic jet, this dot product would always be maximum (but still keep the Major axis and the Thrust axis perpendicular).  This additional requirement needs to be specified so that the Major axis is unique (there are an infinite number of perpendicular directions to a given direction).

The third axis, called the “Minor” axis, is then perpendicular to these two.  However, it turns out that energy flow along this direction is very close to the minimum energy flow along any axis [4,5].

But let’s not get bogged down in these definitions.  All this is doing is setting up a way for us to compare different events all at once; since no two events will have jets oriented in exactly the same way.  In addition, these definitions also identify the event plane for each collision event.

So here’s what the energy distributions turned out looking like for all events considered:

 

Energy distributions in selected three jet events recorded by the MARK-J Collaboration. The black dots are the data points, the dotted line is the theoretical prediction, more details below (Ref. 5).

 

The angles in the above plots correspond to the where in the energy was deposited within the MARK-J Detector.  The distance from the black dots to the center of each graph is proportional to the amount of energy deposited in the detector in this direction [4,5].

Forty events in total were used to make the above distributions [4].  Each event’s jet topologies where re-oriented so they matched the definitions of the Thrust, Major & Minor axes outlined above.  From the top diagram labeled as “Thrust-Major” plane we see three “lobes” or clustering of data points.  This indicates that the three jet structure, or topology, of these forty events.

By rotating the entire picture along the thrust axis by 90 degrees we end up looking at the “Thrust-Minor” plane, the bottom diagram.  Notice how we now only have two clusterings of data points or lobes.  This is because we are looking at the event plane edge on.  Imagine looking at the Mercedes-Benz symbol.  The plane that the three spokes in it is the “Thrust-Major” Plane.  Then if I turn it so I can see only the metal rim of the Mercedes symbol, I’m looking in the “Thrust-Minor” plane.  So the bottom diagram then illustrates that these events have the jets all lying in a plane, as expected due to momentum conservation.

Now how well did the two theoretical predictions mentioned above match up to the experimental observations?

The “phase space” model (no gluons) was not plotted in the above diagrams.  However, the normalized chi-squared value between the experimental data and the “phase space” model was reported as 222/70 [4]; which is nowhere near one! Researchers took this to mean that this theoretical model does not do a good job at describing the observed behavior in nature (and is thus wrong, or missing something).

Now the QCD prediction (with gluons!) is shown as the dotted line in the above diagrams.  See how well it matches the experimental data?  In fact the normalized chi-squared value between the data and the predictions of QCD was 67/70 [4,5]; now this is close to one! So the predictions of QCD with three jet events being caused by the radiation of an energetic gluon has matched the experimental observation, and given us the proof we needed to believe in gluons!

However, the story of the gluon did not end there.  Much more was needed to be done, for example QCD predicts the gluon to have spin 1.  These measurements which I have outlined in this post did not measure the spin of the gluon.  More work was needed for that; but safe to say by the mid 1980s the gluon was well established as an elementary particle, and we have lived with this knowledge ever since.

Until next time,

-Brian

 

References

[1] Wikipedia, The Free Encyclopedia, “The Standard Model of Elementary Particles,” http://en.wikipedia.org/wiki/File:Standard_Model_of_Elementary_Particles.svg, July 5th, 2011.

[2] Wikipedia, The Free Encyclopedia, “Feynmann Diagram Gluon Radiation,” http://en.wikipedia.org/wiki/File:Feynmann_Diagram_Gluon_Radiation.svg, July 5th, 2011.

[3] P. Söding, “On the discovery of the gluon,” The European Physical Journal H, 35 (1), 3-28 (2010).

[4] P. Duinker, “Review of e+e- physics at PETRA,” Rev. Mod. Phys. 54 (2), 325-387 (1982).

[5] D.P. Barber, et. al., “Discovery of Three-Jet Events and a Test of Quantum Chromodynamics at PETRA,” Phys. Rev. Letters, 43 (12), 830-833 (1979).

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Anatomy of a Jet in CMS

Wednesday, June 1st, 2011

We talk often about Jets here at US LHC.  We talk about ways to identify them, their structure, and we even mention some crazy phenomenon involving them.  But one thing we don’t always talk about is what a jet looks like.  And this is what I would like to show today, in gory detail.  So this post is about pictures, lots and lots of pictures.

We can’t see jets with our eyes.  The particles that make up a jet are just to small.  But using a device as large as the CMS detector, we can take a “snap-shot” of a jet created in a proton-proton collision at the LHC.  So it behooves us to start with a brief illustration of CMS.

(Clicking on any of the images below allows you to blow them up in another window, just in case you need a bigger picture)

The CMS Detector

It’s a gigantic cylinder, twenty-one meters long and fifteen meters in diameter!  For comparison, the average height of all American women & men over 20 years old is 1.62 & 1.76 meters (5′ 4″, 5′ 9″), respectively.  But being gigantic isn’t the only thing CMS has in common with an ogre, CMS also has layers; and each layer is a different sub-detector responsible for identifying a different class of particles.

But Here’s computer generated image of CMS, with a cut-away section showing some of these layers:

 

Cut away view of CMS, Hadronic & Electromagnetic Calorimeters not shown.  The camera guide in the bottom right shows CMS’s coordinate axis, (x,y,z) for (red,green,blue).

 

The blue disks and red rectangles on the outside of the detector are part of CMS’s muon chambers (the sub-detector responsible for picking up muons).

The inner cylinder represents CMS’s silicon tracker, this sub-detector is a rather complex instrument.  Its made of silicon strips, and is essentially a giant CCD camera (with over ten million pixels).  The silicon tracker is responsible for reconstructing the trajectories of charged particles as they pass through CMS; this is done by basically playing a giant game of connect the dots.  A close-up of the silicon tracker is shown here:

 

Close-up of the silicon tracker

 

The green and yellow portions are the silicon tracker.  The grey/silver part is what’s called the silicon pixel detector.  It is less then an inch away from where the proton-proton collisions occur in CMS, and thus the closest detection element.

While not shown in the image above, CMS’s calorimeters and superconducting magnet are located between the silicon tracker and the muon system.  They can be seen in this interactive applet on particle detection, of which I’ve taken a screenshot of and shown below:

 

 

The calorimeters are responsible for measuring the energy and momentum of charged & neutral particles (the tracker plans a role in this as well).  They are critical to jet identification & reconstruction…without them we would not be able to do any jet physics in CMS.

Basically what happens is calorimeters are designed so that a particle loses all of its energy as it travels through the calorimeter.  From the energy deposited, and the location of where the deposit occurs we can determine the direction and momentum of charged particles (again, the tracker also plays a role in this).

CMS has two types of calorimeters: an electromagnetic calorimeter (ECAL) for detecting electrons and photons; and a hadronic calorimeter (HCAL) for detecting heavy particles that can pass through the ECAL.  The HCAL is also the only place in CMS where we can detect neutral particles with non-zero mass (such as a neutron).

I should mention that neutrino’s escape detection, and we have to infer their presence by looking for “missing energy.”

But now that I’ve introduced you to CMS, let’s get down to business and talk about Jets.

 

Jets in CMS

I use jets extensively in my own research and its sometimes hard to get a handle on what a jet really is.  I like to think of it like a shotgun blast of particles slamming into the detector.  Jets arise from the hadronization of colored particles, and because of this they are made up of many particles.  Jets can be made of leptons, hadrons and even bosons (specifically the photon).  These particles are usually collimated in a given direction, and you can kinda draw a cone around them (like a shotgun blast!).

For this reason in CMS we like to think of jets as cones, like in this image:

 

A jet cone in CMS

 

This is a jet cone created in a single proton-proton collision recorded by CMS detector in 2010.  In this image I’ve turned the silicon tracker’s graphics off, along with everything else that happened in this event (it can get really messy anyway).  A zoom in of this jet cone can be seen here:

 

Zoom in of the jet cone

 

Now this jet cone may look small in comparison to the entire CMS Detector; but don’t be fooled, I choose a very energetic jet for this post.  This jet’s component of momentum in the xy-plane (green & red axis above) is 115 GeV/c.  Most jets created in proton-proton collisions have xy-momentum components of less then 30 GeV/c.  In fact, if you plot the number of jets detected against their xy-momentum components, you get a distribution that looks similar to an decaying exponential.  So, 115 GeV/c jet is rather energetic.

But what makes up this jet!?  Simple answer A LOT!  This one jet shown above is made up of over 20 different particles, all of which are conveniently hidden at the moment.  So let’s go about fleshing this jet out, piece by piece.

 

Jet Anatomy

So what did CMS see as this jet hit the detector?  Let’s start with the silicon tracker:

 

A jet & its tracks in CMS

 

So here I’ve turned off the view of the muon chambers, and just shown the jet cone and the tracks (the green lines) reconstructed in the silicon tracker.

A few things to note, the green lines appear to be coming from the same point in space (for the most part).  This point is called the primary vertex, its the point at which the proton-proton collision actually occurred.  Another interesting feature is that these tracks go outside of the jet’s cone!  What’s up with that!?

Well the answer is two-fold.  First, these tracks where made by charged particles; and thus their trajectories are bent in the presebce of CMS’s magnetic field.  In other words, we forced the tracks to go outside the jet cone by having a superconducting magnet in our detector (this allows us to make more precise momentum measurements).  Second, treating jets as cones is just a model (which works well).  It often happens that these tracks are indeed all inside the cone, but I purposefully chose a jet with tracks that had large curvature for this demonstration.

In fact, jets are created by using different algorithms; and not all algorithms use a cone geometry!  There are many different algorithms that you can use, they all have subtle differences…but that is really a story for another day.  I just want to show  you what makes up a jet, and what it looks like to the detector.

So this is all the information that the silicon tracker gave us about the jet.  It’s time to ask the calorimeters what they saw, starting with ECAL:

A Jet, its tracks, and its energy deposits in CMS’s electromagnetic calorimeter

 

These yellow-ish squares represent the energy deposited in ECAL.  How far these squares protrude from that wire-frame represents how much energy has been deposited (for those of you who are keeping track, the scale is 10 GeV per meter).  I’ll show some images illustrating this protrusion later on; right now I want to talk about the relation between the tracks, the jet cone and these ECAL energy deposits.

It looks like the jet cone is centered on the bulk of the energy deposits in ECAL.  We actually intended this to happen because of something called clustering.  We group pieces of the calorimeters (both ECAL & HCAL) into clusters, and then clusters into superclusters.  And it is these clusters/superclusters that we go to when beginning to construct a jet.

It also looks like some of the tracks in our silicon tracker match up with these ECAL energy deposits.  But, there are clearly some energy deposits that don’t match up to any track!  What has happened here!?

The answer, is photons and other neutral particles!

The silicon tracker is incapable of detecting particles without an electric charge (like the photon).  But the ECAL was designed specifically to capture electrons & photons (which is why it is called the electromagnetic calorimeter).  Adding “photon candidates” to the picture gives us this result:

 

A Jet, its tracks, its “photon candidates”, and its energy deposits in CMS’s electromagnetic calorimeter

 

These dotted purple-ish lines are the trajectories of the “photon candidates” in this jet.  I say candidates because they might not actually be photons.  To be an actual photon the candidate must past very stringent quality requirements which only a real photon will satisfy.  I haven’t enforced any quality requirements here, so all bets are off (remember I’m just trying to show what made up this jet!).

Again all these photon candidates appear to be coming from the primary vertex, most of them are within the jet’s cone (not all); and almost every photon candidate is linked to an energy deposit in the ECAL.  These photon candidates are also linked to ECAL energy deposits that are not linked to tracks identified by the silicon tracker.

But we still have some ECAL deposits that are clearly not linked to either tracks identified by the silicon tracker or photon candidates!  We need to bring up the rest of the neutral particles:

 

A Jet, its tracks, its “photon candidates”, its neutral hadrons, and its energy deposits in CMS’s electromagnetic calorimeter

 

The dotted blue lines are the “neutral hadron candidates” within this jet.  Similar observations as before can be made.  But since neither a photon or a neutral hadron leaves a track in the silicon tracker, how do I distinguish between them?   This is were HCAL comes into play.

 

A Jet, its tracks, its “photon candidates”, its neutral hadrons, and its energy deposits in CMS’s hadronic calorimeter

 

Now I’ve turned off the energy deposits in ECAL and turned on the deposits in HCAL (teal rectangles).  I’ve also drawn a crude circle around one of the jet’s photon candidates.  It clearly has no HCAL energy deposit around it, but all the dotted blue lines are linked to HCAL deposits.  Now we can clearly see the difference between neutral hadrons and photon candidates; one has energy deposited in HCAL, the other doesn’t.

It might be interesting to note that some photon candidates appear to have HCAL deposits.  There are two reasons for this: 1) the photon candidate isn’t a real photon (it would fail the quality requirements I mentioned above), or 2) there is a nearby hadronic particle that is actually responsible for the HCAL energy deposit.

Now Let’s add the charged hadrons into the picture as well:

 

A Jet, its tracks, its “photon candidates”, its neutral hadrons, its charged hadrons, and its energy deposits in CMS’s hadronic calorimeter

 

These bright blue lines represent the trajectories of charged hadrons.  Notice that they are also coincident with the tracks in the silicon tracker (as they should be!).  These charged hadrons also link to energy deposits in HCAL.  In addition, they also have a chance of depositing energy in ECAL as well:

 

A Jet and its components in CMS

 

I’ve turned the ECAL deposits back on in the image above.

Notice the ECAL and HCAL deposits are stacked on top of each other (with ECAL appearing first).  We like to do this because this gives us the full idea of the direction of energy deposition in the CMS detector.  Let’s turn our view around so we can see the differences in this jet’s energy deposition:

 

A jet’s calorimeter deposits in CMS (also note the curvature of the jet’s tracks!!)

 

Rotated view of a jet’s calorimeter deposits in CMS

 

Final view of a jet’s calorimeter deposits in CMS

 

So from these views we can see the amount of energy deposited around the jet’s cone.

Again, height above the wire frame corresponds to the amount of total energy deposited in a region.  In some cases this energy was deposited in both the ECAL (yellow-ish rectangles) and the HCAL (teal rectangles).  The height of these different rectangles corresponds to the amount of energy deposited in their respective calorimeters.

I’ve also now colored all of the jet’s constituents blue, this now is the complete jet, its a spray of particles that goes along a specific direction (shown by the black shaded cone).

For those of you wondering, this is a specific type of jet called an anti-kT particle flow jet.  The algorithm, anti-kT particle flow, used to “reconstruct” this jet made use of the energy deposited in the calorimeters, the tracks in the silicon tracker, and the primary vertex (for determining tracks of neutral particles).

Some algorithms make use of only the calorimeters and the primary vertex (these are called calo-jets).  But discussing the different jet algorithms is a story for another day.

Remember how I said that the scale of those calorimeter deposits was 10 GeV per meter.  Let’s put that into perspective now:

 

The complete jet recorded by CMS, note the intense curvature of some of the jet’s tracks!

 

Remember the diameter of CMS is 15 meters, so from the primary vertex to the edge of the red muon system (near my coordinate axis guide at the bottom right) is 7.5 meters.  Hopefully, this gives you an idea of the amount of energy deposited in each of the calorimeter clusters.

So this is what a jet looks like!  All in all this jet had 29 different particles that were used in its construction.

So when we talk about Jets here at US LHC (and the rest of Quantum Diaries) I hope you will have a much better idea of what a jet really is.

 

Until next time,

-Brian

For some further reading on Jets, I suggest taking a look at these older posts:

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Keeping up with CMS

Friday, April 8th, 2011

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.

-Brian

References:

[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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Live event displays

Wednesday, November 10th, 2010

You can see live event displays from ALICE here.  Perhaps you can identify the different detectors in the event display from the description of the ALICE detector.  And here’s an event display with our Time Projection Chamber in:

What you’re looking at is the charged particles we’ve seen in ALICE.  I’d tell you roughly how many we see except that I’m afraid that some theorists in our field would mistake that for a measurement.  (You know who you are!)

There’s more information about ALICE and more event displays here.

(ATLAS and CMS also have event displays.)

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