Particle Detectors : an introduction. Erik Adli/Are Strandlie, University of Oslo, August 2017, v2.3

Transcription

1 Particle Detectors : an introduction Erik Adli/Are Strandlie, University of Oslo, August 2017, v2.3

2 Experimental High-Energy Particle Physics Event rate in ATLAS : N = L x (pp) 10 9 interactions/s Mostly soft ( low p T ) events Interesting hard (high-p T ) events are rare 2

3 Decay products Most unstable particles decay very close to the interaction point. Higgs lifetime ~ s. Vector boson lifetime ~ s. The detector detect the decay products (in this case, the electrons and positron) Due to the low cross section of interesting physics, we maximize the numbers of collisions per second 3

4 Z 0 production at the UA1 experiment at the CERN SppS collider (1983)

5 Higgs production at the ATLAS experiment at the CERN LHC collider (2012)

6 How to identify single particle tracks from the large number of signals in the detector? Lectures by Are. In this introduction we discuss the overall anatomy of a HEP-detector and how particles interact the detector, as a prerequisite for Are's part. Material will be repeated in the more detailed detector lectures by Steinar (November)

7 Cross section of a HEP-detector 7

8 What is measured? We want to measure which type of particle, as well as their momentum. Particles : e -, e + p, n, m, p etc. Detector principle : a charged particle interacts with (atomic) electrons in the detector material, and deposits energy Three main detector technologies (to be discussed in detail by Steinar, Leo) Gas detectors Semiconductors Scintillators Ionization of gas atoms, amplification of signal due to high voltage Production of electron-hole pairs 8 Material state excited by ionizing radiation. Any technology may in principle be used for the different detector layers

9 Energy loss: heavy charged particles Heavy charged particles transfer energy mostly to the atomic electrons, ionising them. We will later come back to not so heavy particles, in particular electrons/positrons. Usually the Bethe Bloch formula is used to describe this - and most of features of the Bethe Bloch formula can be understood from a very simple model : 1) Let us look at energy transfer to a single electron from heavy charged particle passing at a distance b 2) Let us multiply with the number of electrons passed 3) Let us integrate over all reasonable distances b electron,m e b ze,v FYS4550, 2005 Steinar Stapnes 9

10 Heavy charged particles Bethe-Bloch FYS4550, 2005 Steinar Stapnes 10

11 Electrons and Positrons Electrons/positrons; modify Bethe Bloch to take into account that incoming particle has same mass as the atomic electrons Bremsstrahlung in the electrical field of a charge Z comes in addition : goes as 1/m 2 e e The critical energy, E c, is defined as the point where the ionisation loss is equal the bremsstrahlung loss. FYS4550, 2005 Steinar Stapnes 11

12 Electromagnetic showers electron and photons: electromagnetic showers, caused by bremsstrahlung and photon pair production while going through a material. Radiation length, X 0 : distance when 1/e of the total energy remains Critical energy, E c : limit for bremsstrahlung Electron, positrons and photons: the same energy loss mechanism

13 Magnetic fields: momentum measurements A strong magnetic field, B, is usually set up in a detector region. In this region, particles will follow a curved motion. This allows to estimate the particle momentum and charge, based on the particle tracks. Tracker layer: low energy loss (de/dx) Measureable quantity: the sagitta, s From geometry : 2 v ebl F B m p qbr r 8s 2 13

14 Momentum resolution scaling : increases with p decreases with B and L decreases with # of measurements Scattering effects give a lower limit and bounds on how precise your measurements should be,

15 Calorimeters: energy measurements Calorimeter: a layer of material with sufficient stopping power (large de/dx) to decelerated a particle and measure its energy. Ideally, all the particle energy should be deposited in the calorimeter (most precise). Electromagnetic calorimeter (CMS lead crystal) 15

16 Electromagnetic calorimeters The total track length : Intrinsic resolution : T N ( E) E tracks E 0 E ( T ) T 0 C 0 1 T 1 E E c : limit for secondary production (T) ~ T, from stochastic nature of shower production. Text from C.Joram Note the complementarity to the momentum resolution in a tracker ( ~ p vs. ~ 1/ E ) Steinar Stapnes 16

17 Vertexing and secondary vertices Several important measurements depend on the ability to tag and reconstruct particles coming from secondary vertices hundreds of microns from the primary (giving track impact parameters in the tens of micron range), to identify systems containing b,c, s; i.e generally systems with these types of decay lengths. This is naturally done with precise vertex detectors where three features are important : Robust tracking close to vertex area The innermost layer as close as possible Minimum material before first measurement in particular to minimize the multiple scattering (beam pipe most critical). The vertex resolution of is therefore usually parameterized with a constant term (geometrical) and a term depending on 1/p (multiple scattering) and also (the angle to the beam-axis). Secondary x Primary collision x Steinar Stapnes 17

18 Arrangement of detectors We see that various detectors and combination of information can provide particle identification; for example p versus EM energy for electrons; EM/HAD provide additional information, so does muon detectors, EM response without tracks indicate a photon; secondary vertices identify b,c, s; isolation cuts help to identify leptons FYS4550, 2005 From C.Joram Steinar Stapnes 18

19 Detector systems From C.Joram Steinar Stapnes 19

20 LHC and ATLAS

21 LHC-eksperimentene 21

22 LHC og dens detektorer LHC detectors 22

23 The LHC ATLAS-detector ATLAS compared with CERN building 40 (5 floors) Diameter Lengde toroidemagnet Ende-til-ende lengde Vekt 25 m 26 m 46 m 7000 tonn 23

24 ATLAS: Inner detector Semiconductor tracker 24

25 Development of the inner detetor Steinar Stapnes 25

26 ATLAS: calorimeters Left: electromagnetic calorimeter (liquid Argon). Decelerates e,p, by electromagnetic showers. Right: hadronic calorimeter. Compact material (iron, steel that stops p, n (hadrons) 26

27 Calorimeter system FYS4550, 2005 Steinar Stapnes 27

28 ATLAS: muon chambers Muons are the only charged particles that traverses all layers of ATLAS (point-like, little bremsstahlung) The muon chambers are therefore placed in the outer layers of the detectors, outside the calorimeters. 28

29 ATLAS: the magnet systems ATLAS: Separate magnets 1) Inner detector (Solenoid) 2) Muon spectrometer (Toroids) 29

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31 Construction of ATLAS FYS4550, 2005 Steinar Stapnes 31

32 Installasjon av en toroidemagnet i ATLAS-hulen 32

33 ATLAS toroidemagneter 33