Methods of Experimental Particle Physics

1. Methods of Experimental Particle Physics Alexei Safonov Lecture #12

2. On transition radiation IsaaCsarver

3. 3 Transition Radiation • Take the electric field solutions for a charged particle in vacuum and in medium. • Subtract the differences and you have the transition radiation. • To work, the foil must be sufficiently thick for the material to react. Jackson says the thickness is on the order of 10 microns. • As we have transition radiation from both surfaces, a well selected foil thickness and separation distance can result in coherence effects that improve detection. • Jackson 13.7, Wikipedia.org “Transition Radiation”

4. Today and Next Time • Detectors and Technologies used in modern HEP experiments • Tracking devices: • Gaseous detectors • Silicon detectors • Muon detectors • Gaseous detectors again • Calorimeters: • Electromagnetic and Hadron Calorimeters • Compensation • Trigger, DAQ etc

5. Gaseous Detectors • Gaseous tracking devices • Measure positions where charged particle left ionization to build a track • Guide electrons and ions to electrodes using electric field to collect charge • Typically use charge multiplication • E.g. an ionization electron, if put in strong electric field, will accelerate and ionize media on its path liberating more electrons and creating “avalanches” • Advantages: • They can be very “light” (gas is light!) • You only want to see where the particle went, you don’t want it to seriously interact with your tracker • Good precision • Can identify particle types by measuring how much they ionize the media at given momentum

6. Drift Chambers • Implementations vary, but same principle: • Guide electrons and ions from ionization to detector sensitive elements • Measure charge, time difference between electron and ion arrival times • Often stick a lot of sense wires, layers etc. • Figure out where the particle went in terms of its position • Use whatever you can: • Measure when the signal arrived (time gives you how far it traveled)

7. What Matters • Want charges to be large and come fast • Pick gas mixtures with low ionization energy • Easy to ionize • And with large drift velocities • To get signal fast • And with small transverse diffusion • To better measure position

8. What Else? • And you want large E field as v ~E: • But not too large or you will ionize gas by the electric field - a lot of noise (or turn it into a spark chamber) • Many of these desires contradict each other • Building these is a complex optimization problem • Resolution is usually limited by ~ 100 microns

9. Limitations • Technically difficult • Small mistakes in wire positions can cause large field distortions and make the whole chamber not working • “Slow” signals as they have to drift over not negligible distances • You still want to get sufficient multiplication to make it detectable • This is bad if you have a lot of particles and collisions happen often • You don’t want showers to start overlapping, do you? • Tevatron: 396 ns between crossings, LHC: 25 ns • Don’t take rate too well • Charge accumulation (ions) at very high rate, can cause gain losses, field distortions etc.

10. Ionization in Semiconductor • As charged particle traverses a semiconductor, you want to create an electron-hole pair • Need to give the electron enough energy to cross from valence into the conduction bend • In reality need a little more energy as you also need to spend some on creating a phonon to preserve momentum conservation • Would want a small bandgap as you want to create many electron-hole pairs without putting too much material • Roughly 4 eV per pair in a silicon diode at room temperature • Temperature dependent • Alpha, beta – determined by the material

11. Setting Things Up • A p-n junction (a diode essentially) • Doping to increase the number of charge carriers • The interface region is depleted of charge carriers • Forward bias: • Push electrons to the right, holes to the left, depleted region small, E can’t hold electrons from moving to the left, holes to the right • Equilibrium • Zero bias (no voltage)

12. Building a Detector • Apply reverse bias: • The depleted region broadened • That’s where you want ionization to happen • No current except thermal • Thermal excitations grow fast with temperature and reduction in bandgap • Want it cold or have larger bandgap to avoid noise current • A typical MIP leaves tens of keVs in a 300mm of silicon • Tens of thousands of electron-hole pairs • Move in electric field creating current • Enough to detect with low noise electronics & low noise current • Equilibrium • Zero bias (no voltage)

13. Why Silicon Detectors? • Advantages: • In special conditions can get a few micron precision, 20-50 microns would be more typical • Disadvantages: • “Heavy”: Particles interact more than you want them • Complex infrastructure: • Cooling to keep noise low, Tilting to offset drift of carriers in magnetic field • Detectors deteriorate with the radiation doze • Fast signals ~ 10’s of ns (small distance to travel) • High spatial resolution • Can make small strips or pixels of silicon (tens to 100 microns)

14. LHC Trackers • From top left clockwise: • CMS Tracker layout: pixel & strip detectors • CMS Strip Detector • ATLAS pixel detector

15. Silicon Versus Gas • Cost versus performance is important: • Silicon detectors are incredibly expensive • Gaseous detectors are much less expensive • But don’t take high rate well • One area where it can still work is muon chambers • Muons get through a lot of material without much energy loss • Only ionization, but it’s heavy enough to make those small, it doesn’t radiate and weak interactions don’t happen often • Muon chambers are usually positioned on the far periphery of the detector beyond a lot of material • Not much gets there except muons, so rates are pretty lowmaking them a good use case for gaseous detectors

16. Strip Cathode Chambers • CMS endcap muon system uses CSCs • Small chambers so easier to operate • Position resolution of ~100 microns • Using center of gravity of the avalanche

17. Drift Tubes • A single unit is a wire in enclosure • Another way to avoid difficulties with one wire goes wrong, the whole chamber is gone • Used in the central part of the CMS muon system • Good choice for the same reasons as CSC • Rates are low enough, spatial precision is sufficiently good

18. CMS DT Muon System

19. Put Everything Together • Have we missed anything? • Calorimeters! – next time (and more)