Maurizio Pierini CERN here representing Prof. Spiropulu

An historical Introduction to
(modern) particle physics
Maurizio Pierini
CERN
here representing Prof. Spiropulu
Modern Particle Physics is all about…
…Boxes…
Some particle
traveling in space
can produce(*) and
absorb particles
while traveling
(*) These particles don’t exist (they are virtual particles). This is only possible because
Quantum Mechanics allows energy changes for small amount of times (as if energy was not
conserved)
ΔEΔt ≈
Modern Particle Physics is all about…
…Shaking Boxes…
+
This is the “naked”
particle
=
these are the
quantum effects
dressing it (loop)
the particles that we
physically observe. for
instance, in cosmic rays
The particles “running in the loop” change the physical property of the traveling particle.
The naked particle is NOT the same as what we see
!
By observing the differences, we can infer what’s in the loop…
!
… like shaking a box to know what is inside
Modern Particle Physics is all about…
…Opening Boxes…
We can do more than observe particles collide
We can guide them with magnetic fields, accelerate them with electric fields, and make
them collide in controlled conditions, to produce new particles
We are testing the same processes, but this time the virtual particles become real (and
observable)
A few examples (more later)
Gargamelle (CERN, 1973) observes the
“neutral currents”
UA1 (CERN,) discovers
the Z boson from
_
dilepton events in pp collisions
54
The charm quark is discovered
(1975) through the production
of a charmonium state (J/ψ)
Glashow Iliopoulos and Maiani
(1970) predict a 4th quark to
explain the absence of flavorchanging neutral currents
51
51
Particle Physics before
November 1975
’30: A simple picture
The subatomic world was very simple
- protons and neutrons at the centre in the nuclei
- electrons around them
- electromagnetic interactions (photons) keep them together
!
But not everything was understood
- what keeps protons and nucleons together?
- what is the physics behind the cosmic rays?
11
11
’30: A simple picture
The particle origins
In 1905 Victor Hess performed a
series of high-altitude balloon
experiments and found ionizing
radiation the origin of which is
beyond the Earth’s atmosphere.
Cosmic rays were the only source
of high energy particles to study
until accelerators were developed.
6
Particle Physics before November 1975
There was no particle collider at that time
!
Particles were observed from radioactive
sources and cosmic rays
!
Physicists observed these radiation
crossing gas chambers and vaporising
gas inside them (cloud chambers)
Using a magnetic field (Lorentz force)
they could observe the particles bending
11
11
taking pictures and looking at
them, one by one…
The discovery of anti-matter
cosmic rays
detection of high energy particles
Anderson, Caltech
1936 Nobel with Hess
magnetic field
what an
electron would
have looked
like
shield (to stop protons)
what
Anderson
saw
positron in cloud chamber
Particle Physics before November 1975
The picture gets more complicated
- anti-matter was somehow predicted, but still not understood
- nuclear interactions were studied, but the equivalent of the photon
for string interaction (the Yukawa meson) was not found yet
11
11
The missing piece of the puzzle was a
charged particle that could mediate the
transition from neutrons to protons
The discovery of the μ meson
In 1932, P. Kunze in Rostock
observed two kinds of tracks in
his chamber. The effect was not
understood and not expected
In 1937, Neddermeyer and Anderson
characterised the two kinds of
particles measuring the energy loss
in their device. Particle not loosing all
their energy (penetrating) were
observed and the mesotron (or mu
Figure 2: Distribution of fractional losses in 1 cm of platinum.
meson) was discovered.
observed losses for the nonpenetrating group, but they could have a v
The discovery
of the μ meson
FEBRUARYn 1, 194'I
71, NUMBER
o
t
p
e
l
If the muon
is the
meson, and
Eciitor
to tie
etters
AL REVIEW
VOLUM'E
3
~Mgnetized
A
since protons are positive,
CA negative
TION of briefmuons
reports of
importantbedi sshould
s in physics may be secured by addressing them
absorbed
(p+mn)
The cLosingfrom
date foratoms
this department
rtment.
is,
the month, muon
the preceding
the 1st of
the 8th of should
e ofwhile
positive
for the issue of the 15th, the Z3rd of the preceding
decay
will be sent to the authors. The Board of
proof
netized
0
I
5
i
Scale
y
(0
Iron
plates
l
I
I
I
=
Cm.
D
D
D
I
r
L
a
I
~d I
--&&m
-- -0
~
7lf
I-+ l
I
&r--~
ir
s not hold itself responsible for the opinions ex!
should not
the correspondents.
Communications
words in length.
Lg
I
I
Conversi, Pancini, and
Piccioni looked for this
FIG. i. Disposition of counters, absorber, and magnetized iron plates.
All counters "D" are connected in parallel.
difference in ’45-’46 (under
of Negative
Mesons
Disintegration
WWII bombs)
in Rome,
using ments using, successively, iron and carbon as absorbers.
M. CONVERSI, E. PANCINI,
O. PICCIONIC
Norecording
sign ofequipment
a difference
observed.
was one which
two of us had
The
an
array
of
gas
counters
and
entro di Fisica Nucleare del C. N. R. Istituto di
of
the meson's mean
used inwas
a measurement
previously
Fisica dell'Unieersitit di Roma. Italia
The
muon
not
the
meson.
absorbers
in
a
magnetic
field
life. It gave threefold (III) and fourfold (IV) delayed
December 21, 1946
First questions
break
simple
The difference
(III) —
(IV) (afterpicture
applying a
(what a muon 'detector looks coincidences.
correction for the lack of e%ciency of the fourfold
ous Letter to the Editor, we gave a first account
slight
What was the muon, then?
like today)
vestigation
of the difference in behavior between
coincidences) was owing to mesons stopped in the absorber
- Where
d negative mesons stopped in dense materials.
electron which produced a
and
disintegration
ejecting a was
the meson?
!
AND
and Araki' showed that, becuase of the Coulomb
delayed
coincidence.
The minimum
detected delay was
The meson was eventually found
the pion
electron (e)
(π)
(µ)
particle tracks left in a photographic emulsion during the
decay of a pion.
The pion was discovered by Cecil Powel and Giuseppe Occhialini in 1947 using
photographic emulsions at the Pic du Midi, high in the French Pyrenees.
p.s. e, µ are leptons 9
Particle Physics before November 1975
Where does this proton come from?
All the cosmic rays ingredients
were know
- Still, no coherent picture
- In particular, the muon was like
an unnecessary ingredient
-
“Who ordered that?”
I. I. Rabi
11
11
Particle Accelerators
Cosmotron
3 GeV protons Brookhaven National Laboratory(1952)
E. O. Lowrence
(Berkeley, 1929)
12
Particles accelerated in single beams
and collided against fixed target
ADA:First e+e- collider
!
It was then realised that a beam vs
beam collision allowed to reach higher
energies
!
Particle accelerators became particles
colliders
!
B. Touschek!
(Frascati, Italy)!
Particle Physics before November 1975
11
11
New particles appearing every year
New phenomena discovered studying them (e.g., P, C, and CP violation)
We had theories who could describe the phenomenology
But there were too many particles for this to be the ultimate theory
Something more fundamental was missing
The discovery era: 1970-1980
More powerful facilities built, to look for some predicted new physics
Often, something else was found
More and more puzzles in observations
A generation of physicists put the pieces together
41
The Quarks
•
The concept of charged currents was
known, and people realised that there
were several kind of objects (different
flavors of currents)
•
Quarks were proposed independently
by Gell-Man (Caltech) and Zweig
(CERN, a student of Feynman at
Caltech) to describe the confused
pattern of particle zoo
•
Deep inelastic scattering (Feynman
parton model) proved the existence of a
structure inside the protons
•
GIM used this concept to predict the
existence of the fourth quark, and its
mass (~1-1.5 GeV)
•
The new theory of nuclear interaction
was now testable at accelerators/
colliders
picture iPad
51
New Bosons
The glue of weak and
strong interactions were
discovered
The bricks were there
(leptons and quarks)
A deeper structure has
emerged
53
52
Our understanding
was now
fitting
a model
What is a model?
23
Add boxes and shakes
fermions
quarks
leptons
bosons
gauge
bosons
graviton
20
The electron and quark interact electromagnetically by
the exchange of a photon. The lines, wiggles and
vertices represent a mathematical term in the
21
calculation of the interaction.
  a list of particles with their “quantum numbers”,
  about 20 numbers that specify the strength of the
various particle interactions,
  a mathematical formula that you could write on a
napkin.
24
1940-70
Particle zoo experiments
40
39
g
25
g
26
g
27
g
28
46
Discovery vs Measurement
•
The history of particle physics is made of
discovery eras, followed by a consolidation of
the understanding, with precisions
•
So far, the understanding came with new insight,
which paved the way to new discoveries
•
To a large extent, this was the history of proton
colliders (many things produced in chaotic
environment) vs ee colliders (clean
environment, typically operated as a factory of
Z→μμ at LHC
Z→μμ at LEP
ee vsofhadron
colliders
history
colliders
The next one?
article physics
0 years.
f
ders
ers &
ers
s discovered Wtau-lepton, charm,
erification of the
SM confirmation: LEP
Figure 1.3: The LEP storage ring, showing the locations of the four ex
and SPS accelerators used to pre-accelerate the electron and positron b
Year
1989
1990
1991
1992
1993
1994
1995
Centre-of-mass Integrated
energy range luminosity
[GeV]
[pb−1 ]
88.2 – 94.2
1.7
88.2 – 94.2
8.6
88.5 – 93.7
18.9
91.3
28.6
89.4, 91.2, 93.0
40.0
91.2
64.5
89.4, 91.3, 93.0
39.8
Table 1.1: Approximate
centre-of-mass energies and integrated luminos
55
experiment. In 1990 and 1991, a total of about 7 pb−1 was taken at
20 pb−1 per year in 1993 and in 1995. The total luminosity used by
analyses was smaller by 10–15% due to data taking inefficiencies and d
17
56
57
59
60
∆χ2
Measurement
6
Theory uncertainty
∆α(5)
had
5
=
0.02758±0.00035
0.02749±0.00012
2
4
incl. low Q data
mZ [GeV]
91.1875 ± 0.0021
91.1874
ΓZ [GeV]
2.4952 ± 0.0023
2.4965
R0l
σhad [nb]
0
41.540 ± 0.037
41.481
Afb
Rl
20.767 ± 0.025
20.739
Al(Pτ)
Al(Pτ)
Rb
D∅
A0,b
fb
0,b
0.0992 ± 0.0016
0.1037
Ab
Afb
0,c
0.0707 ± 0.0035
0.0742
Ac
Ab
0.923 ± 0.020
0.935
Al(SLD)
Ac
0.670 ± 0.027
0.668
sin θeff (Qfb)
0.1513 ± 0.0021
0.1480
mW*
0.2314
ΓW*
m
179.0
± 5.1
t [GeV]
178.0 ± 4.3
2
R0c
0.1480
Afb
± 6.6
500
Γ176.1
W [GeV]
Average
R0b
A0,c
fb
mW [GeV]
mH [GeV]
0,l
0.21629 ± 0.00066 0.21562
Top-Quark Mass [GeV]sin2θlept(Q ) 0.2324 ± 0.0012
eff
fb
100
0
0.1723
Al(SLD)
Excluded
0
30 CDF
3
σhad
0.01714 ± 0.00095 0.01642
0.1465 ± 0.0032
meas
0.1721 ± 0.0030
Rc
1
fit
−O |/σ
1
2
ΓZ
0.02758 ± 0.00035 0.02767
0,l
2
meas
|O
0
(5)
∆αhad(mZ)
Afb
3
Fit
χ /DoF: 2.6 / 4
80.425 ± 0.034
80.389
2.133 ± 0.069
2.093
178.0 ± 4.3
178.5
2 lept
QW(Cs)
sin2θ−−(e
θMS −e−)
2
sin θW(νN)
0
1 g2(νN)
2
L
g2R(νN)
3
*preliminary
.13: ∆χ2 (mH ) = χ2min (mH ) − χ2min as a function of mH . The line is the
result of
+ 13.2
LEP1/SLD
172.6
sing all 18 results. The associated band represents the estimate of the− theoretical
10.2
2
3
nty due to missing higher-order corrections as discussed in Section 8.4. The vertical
10
10
10
ows the 95% confidence level exclusion limit on mH of 114.4 GeV derived
from the
+ 12.3
LEP1/SLD/m
/Γ
181.1
arch at LEP-II [39]. The dashed W
curve
W is the result obtained using the theory-driven
− 9.5
H
2
Z ) determination of Equation 8.4. The direct measurements of mW and ΓW used here
Figure 8.14: Comparison of the measurements with the expectation of the SM, calculated for
minary.
125
150
175the five SM
200input parameter values in the minimum of the
Figure
on shown
the mass of the Higgs boson from each pseudo-observable
global8.15:
χ2 ofConstraints
the fit. Also
Higgs-boson
mass
and
its
68%
CL uncertainty is obtained from a five-parameter SM
is the pull of each measurement, where pull is defined as the difference of measurement and
mt [GeV]
(5)
the observable,
constraining
∆αΓhad (m2Z ) = 0.02758 ± 0.00035, αS (m2Z ) = 0.118 ± 0.003,
expectation in units of the measurement uncertainty. The direct
measurements
of m and
M
[GeV]
SM confirmation: Tevatron
Tevatron @Fermilab
63
63
Discovery of the Top
65
65
The Top Mass
66
67
decades of
Higgs hunting
2009
2011
1940-70
Particle zoo experiments
40
2009: the LHC started…
Some Reading
S. Glashow!
The Charm of Physics: Collected Essays!
the SM early days from the voice of one of it fathers!
!
R. N. Cahn, G. Goldhaber!
The Experimental Foundations of Particle Physics!
Nice historical overview + collection of original papers!