WHAT ARE THE OBSERVABLE PARTICLES--We measure the particles that are produced at an angle that will make it pass through our detector (see COVERAGE below). Only those particles that are charged, or undergoes strong or electromagnetic interactions will be observed. Some particles that are neutral and do no interact in our detector will show up as MET (Missing Energy transverse), by carrying off large Transverse Momentum which is not balanced by the other particles in the event which we observe (these non-interacting neutral particles includes neutrinos, as well as possibly some new particles of similar persuasion).
Conservation of Momentum says that if we have a 40 GeV transverse momentum particle that goes to the left at 90 degrees, there must be a total of 40 GeV transverse momentum particle(s) going off to the right. If we do not see any energetic particles to the right, (and we have no hole in our coverage), then whatever that carries 40 GeV transverse momentum off to the right must be one or more particles that we do not detect !!! such as a neutrino. In fact, if the 40 GeV particle to our left is an electron, or a muon, this is a perfect candidate for a W boson, an 80 GeV object that would decay (roughly 11 % of the time each) into a e + nu or mu + nu.

WHAT ARE THE CHARACTERISTICS OF THESE PARTICLES THAT WE OBSERVE--Particles we are able to observe can be catagorized into

1. Charge or Neutral --Positions of charge particles can be measured accurately in our SVX or COT (central Tracker) through the ionization left by the particle as it pass through (these ionizations are amplified and turned into electronic signals which we record--the properties of which allow us to accurately measure the position of the track) --and, since we have a magnetic field, the degree of bending of the track in the end-on view tell us the momentum of the track--the straighter the track, the higher the momentum. [insert here 2 figures of isolated tracks--one 40 GeV and one 1 GeV]
2. Whether they undergoes strong interaction or not --e.g., Muons do not undergo either strong interaction or em shower; photons (whether direct or coming from decays of other particles such as neutral pions) and electrons only undergoes em interaction, not strong; Neutrinos do no undergo either em or strong interaction. Almost all other particles (protons, neutrons, charged pions, Kaons, etc.) undergoes both strong (and, if charged) em interaction.
3. Whether they undergoes EM (Electro-magnetic) shower or not--only electrons and photons (though many photons come from the decays of other particle--such as pi-zero, the neutral pion) have a shower that leaves all its energy in the front EM calorimetry.

1. Electron (includes positrons)--A Lepton of the first family (see additional intro for more details of the Standard Model and the 3 families of leptons)-- charged particle that undergoes EM shower, and thus would leave all its energy in the front EM calorimeter
2. Muons-- A lepton of the second family-- --charged particle, does not interact in the calorimetry--so will only leave a small amount of energy there (.25 GeV in teh EM and about 1.5 GeV in the HAD calorimetry). It will penetrate the IRON and will leave a track in the Muon chambers. While the muon is unstable, its lifetime (microseconds) is long enough so that almost all of then will traverse our detector before decaying.
3. Taus --A lepton of the third family --This short-live particle would decay into many different states--of those without either an electron of a muon, 2/3 of the times, with one charge pion ( along with neutral pi-zeros, and neutrino); 1/3 of the time into 3 charge pions (+ neutral pi-zeros and neutrino). Thus, the evidence for these are narrow jets with a 2 to 1 ration of 1-prong to 3 prongs. We can not tell that a particular event is a Tau jet--only that in a sample of dozens (or hundreds) or events, that there is a certain proportion of Taus.
4. Photons --neutral particle that leaves all its energy in the EM calorimetry --Note that some photons come directly from the collision, while others come from the decays of such particles as neutral pions--it is a difficult task to determine what fraction of observed photons come from which source !!! This detective work is really difficult.
5. Jets (Quark or gluon) --a collimated bunch of charged and neutral particles usually with many charged particles--like streamers from a firework explosion.
6. b-jets --Jets containing a b quark --this special catagory of a jet come from a b quark --and thus contain a B meson or B hadron among its decays. The B meson/hadron is special because it lives for a few hundreds of a nanoseconds, and thus will travel perhaps a few mm before decaying. We specially made very accurate detectors (see SVX below) very close to the beam and collision in order to measure the decaying tracks from such a B meson, and determine that such a "long-live" particles exist in such an event. Identifying b jets are crucial for many physics (not just measurement involving b's)-- since many heavy objects prefer to decay into b-jets (e.g., each Top quark decays into at least on b jet; in many models of the Higgs meson, it preferentially decays into 2 b's).
7. c-jets --jets containing a c (charm) quark--similar to b jets, except that the D (charmed) meson has a lower mass (less than 2 Gev) than the B meson (5.2 GeV mass) and a slightly shorter lifetime. Thus, we can statistically differentiate between them (i.e., determine from a large sample--perhaps hundreds or more--what fractions are b-jets, c-jets, and light quark jets.
8. Neutrinos or similar--as mentioned about, only inferred from the lack of momentum conservation in the event. We can only inferr the transverse comoponent of this particle, not the component along the beam. Also, if there is more than one neutrino, we only know the combined transverse momentum of all the neutrinos (or other neutral non-interacting particles) combined
9. Soft particles --In addition to these energetic particles, each event typically have dozens of soft particles, often no more than 1 GeV, which, unfortunatly, make it sometimes difficult to reconstruct the event correctly (especially the tracking) by obscuring the high mementum tracks we are particularly interested in.
10. Note that even if all these particles go through over coverage area, a small fraction of the particles will not be recognized (we say-- reconstructed) properly. This is a bias that we have to understand before we can definitely say what we observe, and constitutes what we call "systematic uncertainty" in our final results (where any measure quantity always have a systematic as well as statistical errors (or uncertainty). So, we can never say that "We measure the parameter xyz to be exactly 1.024"--we will always qualify the results by saying something like this -- " We measure the parameter xyz to be 1.024 +- .003 systematic and +- .004 statistical errors".
    The statistical error comes from the fact that we have a finite number of observed events. For example, if you flip a coin (not a loaded on, please) 100 times, you may find that there are 48 heads and 52 tails--that's because 100 times give you a statistical error of 1/2 divided by square root of 100--so 5% statistical error-- so, the result which is 2% away from the correct one is entirely reasonable (you may also been hearing about statistical uncertaintly in recent political polling results--same idea).

HOW WE MEASURE THESE PARTICLES

DETECTOR COMPONENTS --In order to measure the observable particles, we have several detector systems. Lets consider a particle coming off the collision at really large angle--say, 40-90 degrees from the beam. it will meet in order the following detector systems [need to include figures]--

• SVX--SILICON VERTEX TRACKER--(Charged particle only) --These are among the most accurate devices in measuring the position of charged particles-- accuracy of about few tens of microns !!! 6 layers of SVX provide an accurate determination of all tracks, and can tell tracks that does not point to the collision vertex--and thus candidates for being from the decay of a B meson (or a D (charm) meson).
• COT --CENTRAL TRACKER --(Charged particle only) --these measure the position of a charged track accurately, and, based on the bending, also the momentum of the track
• EM CALORIMETRY --these front calorimetry consists of Lead sheets sandwiched with scintillator (which measures the ionization from tracks in the EM shower). The total amount of light observed is linearly correlated with the energy of the electron or photon--thus allowing us to measure the energy of the electron or photon. An energetic shower with no track pointing to it is a photon candidate (though it could be a neutral pion also), while one that have a track with the same momentum would be a straight-forward electron candidate.
• HAD CALORIMETRY --(The Back calorimetry) --iron plates with scintillator sandwiched in-between. Almost all EM shower would be over by the time the HAD calorimetry is met. For hadrons (charged pions, kaons, and protons), most of the energy would be left in the HAD calor, with perhaps 10-30% in the EM calorimetry.
• IRON ABSORBER --Behind the calorimetry, used to absorb all Hadronic shower
• MUON CHAMBERS --If the charged particle makes it to the muon chamber, it is very unlikely to be a hadron (though there is one chance in perhaps 10,000)--and thus the particle is a good muon candidate--especially if the slope of the track is consistant with the momentum measured in our COT tracker.

COVERAGE--You may have notice that we have little or no detector in the direction of the beam--for very good reason. Since most glancing collisions produced lots of energetic particles along the beam direction, any detector where would be hit with so many particles that the detector would die-- and since each bunch-crossing would have millions of particles, we would not know which one come from the collision we are interested in. Furthermore, there is a vacuum beam pipe for the beam to go through (so that the beam does not get wasted by collision with air)--and it is very difficult instrument next to vacuum (well, we do have some small detector--call Roman Pots, which stick into the beam to make some measurement, but not for hard collisions.

So, our Coverage starts at 1-2 degree from the beam, and covers all the "large" angle region, with progressively more sophisticated detector. Coverage from about 30 dgree to 90 degree (Central region) is the most thorough --described above. Between 10 and 30 degree, there is less tracking coverage (we see the track, but do not measure the momentum well), adequate calorimetry coverage (PLUG region), and muon coverage only down to (20 degrees).

WHAT WE RECORD --Out of the millions of collision per second, only a few gives out energetic particles at large angles--into our detector. All our detectors --tracking chambers, calorimeters, etc. --are instrumented to record electronic information--we have more than (?500,000) such channels of information !!! A small selected sample of these channels provide very fast (within microseconds) preliminary information on what happen in the collision and allow us to determine whether the particular collision is of sufficient interest (the "Trigger") so that we should record all the information from these "accepted" event (we only record about 50 events per second out of the millions of collisions)
All the event information from the (?500,000) channels are digitized-- and those that have null information (such as a scintillator that do not have a signal) are discarded. So, roughly 100,000 pieces of information are recorded for every selected event (give a take a factor of 2--depending on what's happening in the event--some events have 10-20 particles in our detector, others have 50 or even 100 or more particles).

RECONSTRUCTION --In order to determine what the event is, we need to "reconstruct" the event--that is, measure all the particles that we observe, determine their characteristics, and then decide what type of event this is. For example, if we see an event with a well-measured 40 GeV electron candidate on one side, along MET on the other side--plus other soft particles, this would be a candidate for a W decaying into an electron + neutrino.

PHYSICS ANALYSIS --Once we have collected a sample of a particular type of events (say, W-->e + nu candidates), we need to answer several questions-- (I give below a very brief overview--the whole items, even for one particular process, is extremely complicated and usually occupy one or more graduate student or post-doctoral fellow many months or even years of burning the midnight oil --many of the work from morning to midnight for 6-7 days per week !!!)

1. What is the efficiency of our detector and trigger
2. What are the Physics and detector background to the process we are studying
3. What is the results we derive from the events we obtain

Now, for some comments on these issues--for a particular process we are studying

1. What is the efficiency of our detector and trigger
   Events could be lost due to inefficiency in our trigger, reconstruction, selection criteria, or confusion.
   The trigger, which enable us to record only 50 or so events out of the millions of collision per second, could lose a small fraction of events, as well as having biases which will make us lose some events--especially with some special properties.
   Some events may not be reconstructed properly.
   In order to best study a process, we make selection criteria (cuts) so that only the "best" (i.e., the sample having the most advantageous signal to background) events are used for final analysis.
   Confusion --Also, some signal events would be lost due to tracks overlapping with other tracks in the event--and thus might be lost.
   We need to determine these and efficiencies and biases in order to obtain any physics results--this involves many studies using background and other samples. Monte carlo simulation (what-if scenarions in which we generate supposed signal and study them through a computer-simulated detector--to see what would have shown up), etc.
2. What are the Physics and detector background to the process we are studying
   For each process we are studying, there are other process which could either give us events close to or even identical with the events from the desired process.
   Some of these could be "artifacts"--events which we mis-measure some of the reconstructed particles due to electronics or other noises
   Others are correctly reconstructed events which come from other processes which mimic the process we are studying--such as an electron candidate which come from a overlap of a track with a photon energy deposit.
   Still others are events with the correct signature, but come from other physics processes
   All these must be studied and the quantities determined. If we are looking for a new physics process, we must show that the number as well as properties of events observed can not be accounted for by the "backgrounds" from old physics.
3. What is the results we derive from the events we obtain
   Once we have made a measurement, we need to provide the values of the measurement, as well as the systematic and statistical uncertainties of the measurement.
4. See Signal vs. Background for a more detailed discussion.