1 Methods of Experimental Particle Physics Alexei Safonov Lecture #15

Today Lecture Coming back to detectors: Calorimeters Electromagnetic and Hadronic Calorimeters Particle Flow Next Time: Presentations Dzero Calorimeter CMS Calorimeter More on particle Flow Summary of Particle Identification Trigger 2

QCD Production There is a reason why QCD is called a strong interaction The cross-sections for strong processes are large Strong processes are: Once perturbative QCD works: 2 jet production (order alpha_s), 3 jet production (alpha_s squared), … At lower energies QCD contribution is even larger (although “jets” are softer and it’s hard to talk of partons at some point as it’s non-perturbative) 3

Collisions at LHC Finding anything at a hadron collider requires first getting rid of enormous backgrounds due to QCD multi-jet production Can’t even write all these events on disk, need trigger - will talk later 4 7 TeVProton colliding beams Proton Collisions 1 billion (10 9 ) Hz Parton Collisions Bunch Crossing 40 million (10 6 ) Hz 7.5 m (25 ns) New Particles 1 Hz to 10 micro (10 -5 ) Hz (Higgs, SUSY,....) x mass of proton (14 TeV) = Collision Energy Protons fly at % of speed of light 2808 = Bunches/Beam 100 billion (10 11 ) = Protons/Bunch

QCD “Backgrounds” Everything is dominated by this “jet” production If we want to learn how to get rid of them, we need to understand them and be able to recognize them But you also may want to learn more about them (e.g. PDFs) Pions are the lightest mesons around m=140 MeV (just slightly heaver than a muon) Can have strange quarks too – just slightly heavier strange mesons are Kaons (m~500 MeV) Both can be charged and neutral – need to catch both We know from theory what these jets are: A shower of particles made of quarks and gluons, i.e. hadrons Predominantly light ones (mesons are hadrons consisting of 2-quarks, can’t have less) 5

Charged and Neutral Particles We know how to measure momenta of charged particles Make a tracker, put it in the magnetic field, charged particles ionize media, convert into electric signals… Need to deal with the neutrals Actually all we learnt so far was always aimed at registering charged particles Scintillators, chambers, silicon, gas – all of them How do you see neutrals? Break them and watch for charged daughters 6

Calorimeters Let’s make a list of neutral particles we need to be able to catch: Photons – we know these tend to radiate a lot when going through material – remember radiation length? Neutral pions – these decay to pairs of photons (fairly fast – via electromagentic interaction) Neutral kaons These can be fairly long living, there are two types, one won’t decay on its own while flying through the detector Neutrons similar to kaons These won’t interact as easily as photons with the material – nuclear interactions 7

Two Types of Neutrals Photons and everything that decays to photons Small amount of material will cause them to radiate, can collect the light using e.g. scintillators Neutrons and long- living kaons: May or may not interact within the amount of material you would want to put to catch all photons Have to put much more material to force them to interact Some number of nuclear interaction lengths to assure even one interaction 8

Sandwich Calorimeters Following on what we discussed, an easy solution is a sandwich detector: Absorber (steel, lead) breaks particles Scintillator collect the light from the shower And keep going Then somehow make a relationship between how much light you collect and the energy Do a test beam 9

Calorimeters Electromagnetic calorimeters turn out to be relatively easy A lot of radiations, EM showers (cascades) are easy to predict Turns out things are not so easy with hadrons 40%/Sqrt(E) or even more is not unusual 10

Neutral Hadron Shower Each nuclear interaction is likely to produce new hadrons If charged, they can be detected as they will ionize If neutral they may decay into “easily interacting particles” (like neutral pions decaying to photons) Or not – in this case you will want this new neutral hadron to interact again Need more than a couple of interaction length to get these “secondaries” in order to contain the whole hadronic shower within your calorimeter But probabilities of interaction are small, so whether they will interact and how soon – is very hard to predict Hadron showers have large fluctuations in terms of their shape, what they decay into, position of the maximum etc. The amount of light from a particular jet would depend on how “lucky” you are 11

Charged Hadrons A jet contains both neutral and charged hadrons E.g. charged pions are always there They also interact via nuclear interactions Often hard to disentangle which one is which (depends on segmentation, but making the calorimeter overly segmented makes little sense as showers can be fairly broad) 12

Calorimeter Response Two issues: As a jet is a mix of particles interacting differently (nuclear vs EM), they will look differently Response (light vs energy) to electromagnetic particles is usually different than for hadrons as those interact differently Need to work really hard to make the response (E/H) close to one – this would be a compensating calorimeter Most calorimeters are non-compensating You can’t really improve you hadronic measurement by measuring light better Fluctuations in collected light are dominated by what this particular hadron does as it goes through the calorimeter Not by how well you measure the light When you have many particles together, things sort of average out a bit, but you still end up with poor resolution Things get better with energy as resolution goes down with E, but at energies of the order of GeV resolution is usually not too good. 13

Particle Flow 14