Transcript Physics 7802.01 Introduction

P780.02 Spring 2002 L19
Richard Kass
Intro to HEP/Nuclear Experiments
What are the ingredients of a high energy or nuclear physics experiment?
Consider three examples of different types of experiments:
FIXED TARGET (FOCUS, SELEX, E791)
COLLIDING BEAM (CLEO, CDF, STAR)
ACTIVE EXPERIMENT (Super K, SNO)
Some Common features:
energy/momentum measurement
particle identification
trigger system
data acquisition and storage system
software
hardworking, smart people…
Some Differences:
experiment geometry
data rate
single purpose Vs multi-purpose
P780.02 Spring 2002 L19
Particle Detection
Richard Kass
In order to detect a particle it must interact with matter!
The most important “detection” processes are electromagnetic.
Energy loss due to ionization
electrons
particles heavier than electrons (e.g. m, p, k, p)
Energy loss due to photon emission
Hadrons (p,k,p) interact with matter
bremsstrahlung (mainly electrons)
via the strong interaction and create
Interaction of photons with matter
particles through inelastic collisions.
photoelectric effect
These particles lose their energy via
Compton effect
EM processes:
pair production (g e+e-)
p0ggor p+m+n,m+e+nn
Coulomb scattering (multiple scattering)
Other/combination of electromagnetic processes
cerenkov light
scintillation light
electromagnetic shower
transition radiation
Calculation of above processes involve classical EM and QED
P780.02 Spring 2002 L19
Fixed Target Experiment
Richard Kass
Imagine an experiment designed to search for baryons with Strangeness=+1
These particles would violate the quark model since baryons always have
negative strangeness in the quark model.
A candidate reaction is: p-pk-X+
Since this is a strong reaction we need to conserve:
baryon number: X has B=+1
strangeness:
X has to have +1
electric charge: X has to have Q=+1
General requirements of experiment:
we need to know that only k- and one other particle produced in final state
To achieve this we will have to:
get a beam of p-’s with well defined momentum (we need an accelerator)
get a target with lots of protons (e.g. liquid hydrogen)
identify p-’s and k-’s
eliminate background reaction: p-p p-p
measure the momentum of the p-’s and k-’s
eliminate background reactions: p-pk-k+n or k-kop
a way to record the data
P780.02 Spring 2002 L19
Richard Kass
Example of fixed target experiment: FOCUS
Momentum: silicon+drift chambers+PWC’s+magnet
Energy: EM+hadronic calorimeters
Particle ID: Cerenkov Counters, muon filter calorimeter
Real life view
P780.02 Spring 2002 L19
Richard Kass
Colliding Beam: CLEO III Experiment
General purpose detector to study lots of different final states
produced by e+e- annihilations at 10 GeV cm energy
e+e-B+B-
B+*+
B-D*0pD*0 D0g
m+mD0 K- p+
*+s0p+
s0 p+p-
Must have cylindrical geometry since beams pass
through the detector
Must measure:
momentum of charged particles
energy of g’s and po’s
Must identify particles:
charged: e, m, p, k, p
neutral: g, p0, k0, L
P780.02 Spring 2002 L19
Richard Kass
CLEO III Charged Particle Tracking Detectors
Installation of
CLEO III silicon
detector
Silicon detector has:
1.25x105 strips
Each strip has its own:
RC, preamp, ADC
Drift Chamber
has about 104
sense wires
Readout
cables
hybrids:
holds amps
Silicon wafers
(layer 4)
Drift chamber End Plates
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CLEO III Drift Chamber
A drift chamber measures
position by measuring the
time it takes for ions to drift to
a wire: x=vt. (assume we know v)
47 Layers of sense wires
9796 sense wires
measures r-f coordinate
100mm
Gas is He:C3H8 (60:40)
drift chamber cell
X=field wire
O=sense wire
x x x
x o x
x x x
Richard Kass
P780.02 Spring 2002 L19
Richard Kass
Charged particle tracking and momentum resolution
Why do we need charged particle tracking in an experiment?
FDetermine the number of charged particle produced in a reaction.
FDetermine the identity of a charged particle (e.g. p, K, p ID using dE/dx).
FDetermine the momentum of a charged particle.
We measure the momentum of a charged particle by determining its trajectory in a
known magnetic field.
Simplest case: constant magnetic field and p^B trajectory is a circle with p=0.3Br
We measure the trajectory of the charged particle by measuring its coordinates
(x, y, z or r, z, f, or r, q, f) at several points in space.
We measure coordinates in space using one or more of the following devices:
Multiwire Proportional Chamber
low spatial resolution (1-2 mm)
Drift Chamber
moderate spatial resolution (50-250mm)
Silicon detector
high spatial resolution (5-20 mm)
Most common
Simplest case: determine radius of circle with 3 points
Better momentum resolution better mass resolution
Many particles of interest are observed through their decay products:
D0K-p+,D+K-p+ p+,Lpp-, K0p+ p+
By measuring the momentum of the decay products we measure the mass of the parent.
1
m  m1+m2  m2=(E1+E2)2-(p1+p2)2 = m12+ m22 +2[(m12+ p12 )1/2 (m22+ p22 )1/2 - p1p2 cosa]
a
For fixed a: m2 / m2   p / p
2
P780.02 Spring 2002 L19
Richard Kass
Momentum and Position Measurement
(L/2, y2)
y

Trajectory of
charged particle
s=sagitta
x


(0, y1)
(L, y3)
z
Assume: we measure y at 3 equi-spaced measurements in (x, y) plane (z=0)
each y measurement has precision y
have a constant B field in z direction so p^=0.3Br
Note: The exact
The sagitta is given by:
expression for s is:
y1 + y3 L2
L2
0.3BL2
s  y2 


2
8r 8 p^ /( 0.3B)
8 p^
L2
sr- r 4
2
The error on the sagitta, s, due to measurement error is found using propagation of
errors to be:
 s  3 / 2 y
Thus the momentum (^to B) resolution due to position measurement error is:
 p^
p^

s
s

3 / 2 y
2
(0.3L B) /(8 p^ )

8 p^ 3 / 2 y
2
0.3L B
 32.6
p^ y
2
LB
(m, GeV/c, T)
P780.02 Spring 2002 L19
Mass Resolution and Physics
Richard Kass
Discovery of the b-quark at Fermilab (1977).
Used a double arm spectrometer to measure
invariant mass of m-pairs. Had to do an
elaborate fit to find 3 bb resonances:
U(1S), U(2S), U(3S)
pBem+m-X
1977
PRL 39, 252 (1977)
PRL 39, 1240 (1977)
m+m-
Double arm spectrometer (E288)
1986
Upgraded double
arm spectrometer
(E605) clearly
separates the 3 states:
improved mass resolution
and particle ID (RICH)
Better fit
P780.02 Spring 2002 L19
Richard Kass
Energy Measurement (Calorimetery)
Why measure energy ?
I) Not always practical to measure momentum.
An important contribution to momentum resolution is proportional to the momentum.
Example: suppose we want to measure the momentum of a charged particle
such that we can tell whether it is positively or negatively charged (to within 3).
We demand: p/p < 0.33
A more detailed analysis of momentum resolution gives:
 p^
p^

720  p^
(m, GeV/c, T)
n + 4 (0.3BL2 )
Use CLEO or CDF-like parameters: B=1T, L=1m, n=100, =150mm and find p^:
n + 4 (0.3BL2 )
104 (0.3)(1)(12 )
p^  (0.33)
 (0.33)
 2.5  102 GeV/c
-4
720

720 1.5  10
Thus above  250 GeV/c we can’t reliably measure the charge of the particle!
L There are practical limits on the values of B, L, , n, etc.
II) Some interesting particles do not have electrical charge.
Momentum measurement using B-field only works for charged particles.
What about photons, p0’s and h’s (both decay to gg), KL’s, neutrons, etc ?
P780.02 Spring 2002 L19
Calorimetry
Richard Kass
In addition to measuring energy calorimeter information can also be used to:
identify particles (e.g. g’s, e’s)
measure space coordinates of particles (no B-field necessary)
form a “trigger” to signal an interesting event
eliminate background events (e.g. cosmic rays, beam spill)
can be optimized to measure electromagnetic or hadronic energy
Calorimeter usually divided into active and passive parts:
Active: responsible for generation of signal (e.g. ionization, light)
Passive: responsible for creating the “shower”
Many choices for the “active” material in a calorimeter:
inorganic crystals (CsI used by CLEO, BELLE, BABAR)
organic crystals (ancthracene) {mainly used a reference for light output}
plastic scintillator
liquid scintillator (used by miniBoone)
Noble liquids (argon used by D0)
gas (similar gases as used by wire proportional chambers)
glass (leaded or doped with scintillator)
Many choices for the “passive” material in a calorimeter:
high density stuff: marble, iron, steel, lead, depleted uranium
lower(er) density stuff: sand, ice, water
P780.02 Spring 2002 L19
Particle ID with Calorimeters
Richard Kass
electron/positron: Charged particle undergoes EM shower in calorimeter,
compare momentum (measured in drift chamber) with energy, require E/p1.
Not efficient when electron has same energy as a minimum ionizing particle
(both have E/p  1), also background from reactions: pp0X.
photon: EM shower in calorimeter not matched to charged track in drift chamber.
muon: Charged track in drift chamber that does not shower in EM calorimeter or
interact in hadron calorimeter. Background from pions (and kaons) that decay in
flight (pmn) and/or non-interacting p/K.
neutrino: Compare visible energy (calorimeter) with measured momentum (drift
chamber) and look for imbalance in event. Could be more than one neutrino missing!
neutron or KL: Hadronic shower in calorimeter that does not match to charged track
in drift chamber. Need hadronic calorimeter.
p0, h: measure invariant mass of gg combinations.
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A CLEO Event
Richard Kass
A fully reconstructed event.
Lots of g’s in event.
P780.02 Spring 2002 L19
A CLEO Event
Richard Kass
CLEO event with
muons and electrons
muon
P780.02 Spring 2002 L19
Richard Kass
Ring Imaging Cerenkov Counters (RICH)
RICH counters use the cone of the Cerenkov light.
The ½ angle (q) of the cone is given by:
2
2

1
-1
-1  m + p 
cosq 
 q  cos


n
np


2q
r
L
The radius of the cone is: r=Ltanq, with L the distance to the where the ring is imaged.
For a particle with p=1GeV/c, L=1 m, and LiF as the medium (n=1.392) we find:
p
K
P
q(deg)
43.5
36.7
9.95
r(m)
0.95
0.75
0.18
Great p/K/p separation!
Thus by measuring p and r we can identify what type of particle we have.
Problems with RICH:
optics very complicated (projections are not usually circles)
readout system very complicated (e.g. wire chamber readout, 105-106 channels)
elaborate gas system
photon yield usually small (10-20), only a few points on “circle”
P780.02 Spring 2002 L19
Richard Kass
CLEO’s Ring imaging Cerenkov Counter
The figures below show the CLEO III RICH structure. The radiator is LiF, 1 cm thick,
followed by a 15.7 cm expansion volume and photon detector consisting of a wire chamber
filled with a mixture of TEA and CH4 gas. TEA is photosensitive. The resulting photoelectrons
are multiplied by the HV on the wires and the resulting signals are sensed by a rectangular
array of pads coupled with highly sensitive electronics.
P780.02 Spring 2002 L19
Performance of CLEO’s RICH
Number of detected
photons on 5 GeV
electrons
Richard Kass
D*’s without/with
RICH information
Preliminary data
on p/K separation
A track in the
RICH
P780.02 Spring 2002 L19
Richard Kass
CLEO’s Ring imaging Cerenkov Counter
Lithium Floride (LiF) radiator
Assembled radiators.
They are guarded by
Ray Mountain. Without
Ray “living”at the factory
that produced the LiF
radiators we would still
be waiting for the order
to be completed.
Assembled
photodetectors
A photodetector:
CaF2 window+cathode pads
P780.02 Spring 2002 L19
Richard Kass
Example of active experiment: SuperKamiokande
Original purpose of experiment was to search for proton decay: pe+p0
Baryon and lepton number violation predicted by many grand unified models (e.g. SU(5))
General Requirements for experiment
Need lots of protons (decay rate of 1032 years7x103 tons of H2O)
Size: Cylinder of 41.4m (Height) x 39.3m (Diameter)
Weight: 50,000 tons of pure water
Need to identify e-’s and p0’s
Reject unwanted backgrounds (cosmic rays, natural radiation)
103m underground at the Mozumi mine
of the Kamioka Mining&Smelting Co Kamioka-cho, Japan
Inside SuperK
P780.02 Spring 2002 L19
SuperKamiokande
Richard Kass
Closer look at experimental requirements:
Identifying p’0s tricky since p0gg thus must identify g’s
Need to measure energy or momentum of e and p0
impractical to use magnetic field  measure energy using amount of Cerenkov light
detect cerenkov light using photomultiplier tubes
11,200 photomultiplier tubes, each 50cm in diameter , the biggest size in the world
Energy Resolution: 2.5% @ 1 GeV and 16% (at 10 MeV)
Energy Threshold: 5 MeV
Need to measure direction of e and po to see if they come from common point
cerenkov light is directional
Need to measure timing of e and po to see if they were produced at common time
cerenkov light is “quick”, can to timing to few nanoseconds
BUT DON’T FORGET
CIVIL ENGINEERING!
Nov 12: accident destroys 1/3 of phototubes
Nov. 13: Bottom of the SK detector
covered with shattered PMT glass pieces
and dynodes.
P780.02 Spring 2002 L19
The SNO Detector
Located in a mine in Sudbury Canada
Uses “Heavy” water (D2O)
Detects Cerenkov light like SuperK
Richard Kass
SNO=Sudbury Neutrino Observatory
Nucl. Inst. and Meth. A449, p172 (2000)