Methods of Experimental Particle Physics

1. Methods of Experimental Particle Physics Alexei Safonov Lecture #15

2. 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

3. 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)

4. Collisions at LHC 7.5 m (25 ns) Bunch Crossing 40 million (106) Hz Proton Collisions 1 billion (109) Hz Parton Collisions New Particles 1 Hz to 10 micro (10-5) Hz (Higgs, SUSY, ....) 14 000 x mass of proton (14 TeV) = Collision Energy Protons fly at 99.999999% of speed of light 2808 = Bunches/Beam 100 billion (1011) = Protons/Bunch • 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 7 TeV Proton Proton colliding beams

5. 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)

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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)

13. 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 20-100 GeVresolution is usually not too good.

14. Particle Flow