Detectors for High Energy Experiments 1905…..

Ernest Rutherford in his lab …2009

4.6Km

Atlas Detector General consideration on HEP detector design From a detector in HEP we want to have info of

• number of particles • event topology (picture) Can’t be achieved with a • particle identity single detector ! • energy of a particle • momentum of a particle • charge of a particle

⇒ integrate detectors into detector systems, Create a general purpose detector

(for a very brief review, see Dan Green pages 291-301. Else, look at the slides) First let’s recall momentum scale of particles in HEP experiments

LEP, vs positrons collisions, center of mass energy 91 GeV Final state : mostly two partons each carrying 45 GeV energy leptons : 1-3 charged particles, with O(10 GeV), quark (I.e. Jet): roughly 10 charged particles, with O(1 GeV) energy each

LHC, protons vs protons collisions, center of mass energy 16 TeV (16 000 GeV) Final state: several partons, hence few jets with few 10s particles each and few isolated leptons from non-strong inter. processes, e.g. Z decay

Leptons, jets : 0(10-100 GeV) energy

Different momentum scale reflects in different size of the detectors ALEPH 10 meters, 1.5 Tesla

Weight 3500 Tons or ATLAS : 22 meters in diameter and 2 Tesla field , and weighs 7000 tons! Number of particles, event topology

For this we need a “tracking” detector, which allows us to “see” the path of each charged particle in the measuring device. A “tracking” detector is also needed - together with calorimeters eg.- to measure the number of particles involved in the event observed

ALEPH Time Projection Chamber It is very common to use in addition a very small tracking detector, called “vertex” detector. It is very usefull to take a good “close-up” picture of the most long-lived particles In a high energy collision DELPHI

500 micrometers

Vertex detector are most often done of silicon. They are usually very compact, sit as close as possible to the interaction point (where particles come out), and need to be radiation hard, because they sit very close to the beam where there is lots of radiation Energy measurement

For electrons and : fully destructive measurement in the electromagnetic calorimeter. Photons and electrons/positrons get completely stopped due to pair production, bremstralhung processes (EM processes)

For : Fully destructive process in the hadronic calorimeter. Hadrons stopped by strong interactions with the material of the calorimeter (needs to be high density, high Z,A)

Best measurement of energy obtained when shower is fully contained in the calorimeter Particle Identity

TR e± identification TOF π/K separation dE/dx RICH

10-1 100 101 102 103 104 p [GeV/c]

p = m0 βγc Simultaneous measurement of p and dE/dx defines dE 1 ln 2 2 ∝ 2 (β γ ) mass m, hence the particle identity dx β

e µ π Pion/kaon separation (2 sigma) requires DE/DX K resolution of < 5% Average energy loss for e,µ,π,K,p in 80/20 Ar/CH4 p (NTP) (J.N. Marx, Physics today, Oct.78) DELPHI RICH : RIng CHerenkov Imaging Detector

2 radiators + 1 photodetector spherical mirror

C5F12 (40 cm, gas) C4F10 (50 cm, gas)

Photodetector TMAE-based

C6F14(W. (1 Adam cm, etliquid) al. NIM A 371 (1996) 240) Two particles from a hadronic jet (Z-decay) in the DELPHI gas and liquid radiator + hypothesis for π and K

 2 2   1   1 E   1 p + m  θC = arccos  = arccos ⋅  = arccos ⋅  nβ   n p   n p        n = 1.28

C6F14 liquid π/K π/K/p K/p DELPHI

n = 1.0018

C5F12 gas π/h π/K/p K/p

When particles become relativistic , then cherenkov radiation cannot help distinguish identity -> Transition radiation used Transition Radiation Detectors (eg ATLAS):

W 1 N ph ≈ ∝ α ≈ number of photons small hω 137 Need many transitions → build a stack of many thin foils with gas gaps

 stacks of CH2 foils are used  hydrocarbon foam and fiber materials Low Z material preferred to keep re-absorption small (∝Z5)

R D R D R D R D sandwich of radiator stacks and detectors → minimize re-absorption

TR X-ray detectors:

• Detector should be sensitive for 3 ≤ Eγ ≤ 30 keV. Intrinsic problem: detector “sees” TR and dE/dx θ ∝1 γ ) dE/dx TR (10 keV) Xe

height ≈200 e- ≈500 e- cm (1

Pulse t Discrimination Energy radiated ~γ , basically only electrons with by threshold energy O(10 GeV) emit TR Let’s find some tools …

and put everything together ! We have seen how we can get information about • number of particles • event topology • momentum / energy • particle identity

Now: Geometrical concepts for detector design

Fix target geometry Geometry

“Magnet spectrometer” “4π Multi purpose detector” Target tracking filter N

S beam magnet calorimeter barrel (dipole) endcap endcap • Limited solid angle dΩ • “full” dΩ coverage: coverage Hermetic detector • rel. easy access (cables, • very restricted access maintenance) LHC-B Low density  high density high precision  low precision high granularity  low granularity track density ∝ 1/r2

Typical arrangement µ+ of subdetectors e- γ

vertex location p (Si detectors)  main tracking (gas or Si detectors)   e.m. calorimetry  magnet coil  calorimetry / return yoke  muon identification / tracking  In collider geometry Magnetic field configurations:

solenoid B toroid B

Imagnet coil

Imagnet

+ Large homogenous field + Rel. large fields over large volume inside coil + Rel. low material budget - weak opposite field in return yoke - non-uniform field - Size limited (cost) - complex structure - rel. high material budget Example: • ATLAS (Barrel air Examples: toroid, SuperConducting, 3 K , 4 T) • DELPHI (SC, 1.2T) • L3 (NC, 0.5T) • CMS (SC, 4T) ATLAS TOROID ATLAS SOLENOID

Some other practical considerations before building a detector

Find compromises and clever solutions between :

• Mechanical stability, precision ⇔ distortion of resolution (eg. due multiple scattering) or loss of information (e.g. conversion of gammas)

• Hermeticity ⇔ routing of cables and pipes • Hermeticity ⇔ thermal stability • Hermeticity ⇔ accessibility, maintainability

… and always keep an eye on cost Now let’s see some concrete examples

Before the tracks from one event have crossed the ATLAS detector, the tracks from the next one are getting into the detector ! So detector readout system should be fast .

Need high granularity tracking and calorimetry

H → bb event @ high luminosity (10^34) 1000 charged tracks per event Radius: 2cm 10cm 25cm 60cm 2 NTracks/(cm *25ns) 10.0 1.0 0.10 0.01

Experimenter ATLAS CMS

- General purpose - General purpose - Origin of mass - Origin of mass - Supersymmetry - Supersymmetry - 2,000 scientists - 1,800 scientists from from 34 countries over 150 institutes

LHCb ALICE

- to study the - heavy ion collisions, differences to create quark- between matter gluon plasmas and antimatter - 50,000 particles in - detect over 100 each collision million b and b-bar mesons each year

Niels Bohr Institutet is in Alice and Atlas experiments ATLAS width 44 meters, diameter 22 meters, Weight 7000 t Magnetic field in center: superconducting solenoid 2T Magnetic field at Muon det.: superconducting toroid 4 T ATLAS MUON SPECTROMETER

Four different types of detectors: Trigger Resistive Plate Chambers (RPC) chambers and Thin Gap Chambers (TGC). Fast, good for trigger

Monitored Drift Tube (MDT) and Cathode Strip Chambers (CSC) for tracking with precision Precision chambers Monitored Drift Tube chambers

 Precision measurements in the muon spectrometer are performed by chambers of Monitored Drift Tubes (MDT)  The basic elements are aluminum tubes with a 3 cm diameter and a wire at HV in the middle  The basic measurement is the drift time of ionized electrons to the wire  The measurement resolution is ~80 µm  Each chamber has 2 superlayers, each with 3 or 4 layers of tubes L1 Trigger Muon Muon reconstruction in ATLAS detector

MDT RPC/TGC

µ

+ ++

Barrel Toroid MDT RPC/TGC + + ++

µ ++ ++ + + + +++ + End Cap Calorimeter ++ Toroid

Inner Detector Muon reconstruction in ATLAS detector

 In ATLAS, muon tracks can be reconstructed independently in the muon spectrometer. A search for all µ is performed

Track reconstruction in the + ++ Muon Spectrometer is done with MOORE or MuonBoy

+ ++ ++

++ ++

+ ++ + +++ +

 Large volume toroidal field – bending in _ direction  Low detector occupancy  Accurate high momentum measurements Muon reconstruction in ATLAS detector

 Following this, muon tracks or segments are combined with inner detector tracks to obtain the muon momentum at the interaction point

+ ++ MuId/Staco  Extrapolate muon tracks back to the primary vertex region + + ++  Combines them with Inner detector tracks µ ++ ++ + + + +++ + ATLAS MUON DETECTOR Higgs to 4

Higgs to 4 muons and Higgs to 4 electrons, 30 fb-1

Higgs to 4 electrons Inner Detector

 The ATLAS Inner Detector (ID) is inside a 2T solenoid magnet  There are 3 detector types:  semi-conductor pixel  semi-conductor strips  transition radiation tracker  The pixel and SCT will provide a few very accurate points  The TRT will provide continuous tracking – 36 points  Each contributes similarly to the resolution Particle goes through Pixel detector

 3 barrel and 8 disk layers of 140 MILLION pixels on 2228 Silicon semiconductor modules  The 140 MILLION channels are read out providing a resolution of 10 µ in r-φ and 50 µ in z SCT  SCT is designed to provide eight precision measurements per track in the intermediate radial range  contributing to measurement of  Momentum  Impact parameter  Vertex position  In the barrel SCT eight layers of silicon microstrip detectors  The end-cap modules use tapered strips with one set aligned radially.

SCT spatial resolution ~ 100 micrometers ATLAS tracking

Disadvantage of using silicon or non-gaseous material Is loss of energy and multiple scattering in inner part of the detector ATLAS Transition Radiation Tracker SCT and TRT Barrel inserted Feb 2006 Calorimeter

 The EM calorimeter, and part of the Hadron calorimeter are made of an accordion like arrangement of lead radiator and liquid argon measurement medium  There are over 100000 channels in the barrel and 70000 in the endcap  The calorimeter takes part in the level 1 trigger Calorimeter is made of metal plates (absorbers) and sensing elements. Sensing elements are : Liquid Argon in inner part (shower electrons create ionization In the narrow gaps of liquid argon) …

… and tiles of scintillating plastic on the outside ATLAS electromagnetic calorimeter . Very high granularity ATLAS Tile Hadronic calorimeter

H→γγ

 Why is this channel so difficult?  The final state is 2 neutral particles  No momentum and direction measurements in the tracking detector are available σE 10%  Photons shower in the EM calo, with energy resolution ≈ E E  The invariant mass of a pair of photons has to be calculated – mass resolution is related to the single particle momentum resolution  We expect a wide distribution  Almost every π0 decays into 2 photons ν There are many π0 produced in each collision ν Highly boosted π0 produce γ very close to each other ν The calorimeter has to be highly segmented to tell one γ from 2 γ ν π0 →γγ is a big combinatorial background under the H→γγ peak. υ This channel dictated the design of the EM calo H→γγ

Signal and background After background subtraction SM Higgs

Tau leptons are quite important for discovery

MSSM SUSY Higgs pions

Had. 0 π ± inner detector calo π ±

(%) EM π hadronic calorimeters calo π± resolution

pT (GeV)

two algorithms (approaches) τ decay in ATLAS for tau identification , for the two different discrimination of a tau jet from a quark/gluon initiated PT regimes, in order to get the best momentum determination jet is mainly given by the shower shape in the calorimeter and by the number of tracks in the jet Powerfull combination of calorimeter and tracking information, to observe tau jets

12 REM DET Nstrip

st Strip Width Charge ET/pT(1 track) Some variables used for tau vs quark/gluon jet separation

Jet energy scale

 The signal in the calorimeter requires translation into the energy of the particle  This translation is particle type and detector region dependent  Pions leave a different signal than electrons for the same energy loss  Different sampling depths result in different calibration  Jets are more complicated still ν π0 and charged π, but also muons/electrons/ ν Tau leptons (hadronic decay) are a special jet, require sspecial calibration  These calibrations are started at test-beams  Continue using simulation  Will continue using well understood samples  Z+jets “Missing energy”

 The total cm energy will be 14 TeV  Most final state energy will go down the beam-pipe unmeasured  Hard interaction energy unknown and differs by event

 Products characterized by momentum transverse to the beam-line pT  No way to measure “missing energy” out of unknown total

 What we measure is the pT imbalance in the final state

 Measured as vector No missing ET Missing ET sum of energy deposition in calo cells  Characterizes events with particles that leave the ~ ~0 uR → χ1 + u detector 0 + − ~ ~0 ~0 Z → e e s → χ1 + Z + s → χ1 + d + d + s unobserved Missing ET continued

 What particles result in missing ET?   The SUSY LSP or neutral stable NLSP  Muons?  They leave little energy in the calorimeter, so if not accounted for, will produce fake missing ET

 They are not accounted for in the calorimeter trigger so high pT muons can produce a missing ET trigger  This should be corrected at Event Filter or offline  Charged stable NLSP?  Like muons Fake missing ET  No other source of missing ET in event

 Detector malfunction can fake missing ET  A “hot” or “dead” area in the calorimeter will change the ET balance artificially  Particles going through cracks also create fake missing ET

H → µ +µ −e+e− Missing ET resolution

 A lot of work on understanding missing ET and its dependence on  topologies  jet energy calibration  e/π/γ energy corrections  crack and dead areas  Jet punch through seen as muon

Fast scintillator Radiation hard High Z, so shower development possible