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Methods of Experimental Particle Physics

Alexei Safonov

Lecture #15

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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

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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)

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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

7 TeV Proton Proton colliding beams

Proton Collisions 1 billion (109) Hz

Parton Collisions

Bunch Crossing 40 million (106) Hz

7.5 m (25 ns)

New Particles 1 Hz to 10 micro (10-5) Hz (Higgs, SUSY, ....)

14 000 x mass of proton (14 TeV) = Collision EnergyProtons fly at 99.999999% of speed of light

2808 = Bunches/Beam100 billion (1011) = Protons/Bunch

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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)

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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

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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

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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

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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

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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

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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

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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)

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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 GeV resolution is usually not too good.

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Particle Flow