Transcript Thr Search for the Higgs Boson

The search for
the Higgs Boson
Bill Murray,
RAL,
March 2002
What is the Higgs?
Why do we think it exists?
Have we found it?
1
Outline:





An quick review of particle physics
An introduction to Higgs’ theory
What do we know about it already?
What did we see in CERN 2000?
Where next?
2
A question to start you off:
50% chance the ‘Theory of Everything’
will be known within 20 years.
 The future of physics is to be found in
the 6th place of decimals.
 Physics as we know it will be over in 6
months.

When were these said?
3
The Matter Particles
u
Proton
u
Mass: 1.7 10-27 kg
charge: +1
d
Neutron
Electron
Neutrino
Charge: 0
e
Mass: 0.0005 proton mass
charge: -1

Mass: 10-11 proton mass?
Charge: 0
4
The particles of Matter
u
‘up’ quark
d
‘down’ quark

neutrino
e
Electron
e
5
The particles of Matter

u
Group together as a family
e
d
All ordinary matter is composed of these

(Each also has an antiparticle)
e
6
The Matter particles

u

c

t
e
d
m
s
t
b
1st Generation
2nd Generation
3rd Generation
Ordinary
matter
Cosmic rays
Accelerators
Why 3
??
7
The Matter particles
1.7GeV
1.7GeV
175GeV

u

c

t
e
d
m
s
t
b
1st Generation
2nd Generation
0.1GeV
3rd Generation
4.5GeV

E=mc2
Others are lighter
e
1GeV~Proton Mass
8
The Matter particles
1974
1995
2001

u

c

t
e
d
m
s
t
b
1st Generation
1897
2nd Generation
3rd Generation
1977
1975
9
Forces in Ordinary Physics
Classically, forces are described by charges and fields
+
+
Field
+
10
Forces in Particle Physics
High energies and small distances  quantum mechanics
Continuous field  exchange of quanta
For Electromagnetism
+
+
The quanta are photons, 
11
The Forces of Nature
Force
Realm
Particle
photon
ElectroMagnets,
magnetism DVD players
Strong
Fusion
Gluon
+
-
W ,W ,
-decay,
0
(sunshine) Z
Gravitation Not in the same
framework
Weak
Higgs may give
a link?
12
The Forces of Nature
Force
Mass, GeV
Electro0
magnetism
Strong
0
Particle
photon
Gluon
W’s

Z

Gravitation Not in the same
framework
Weak
13
Operation of the Forces
Electron
photon
Positron
Feynman
Diagram
14
Why do we need the Higgs?

If we require that the laws of Physics
follow a certain symmetry, we are forced
to introduce electromagnetism

A similar symmetry predicts the existence
of the Weak force.

That symmetry also insists that the
photon, W & Z bosons have no mass!
15
Why do we need the Higgs?
The W and Z have a mass
 In contradiction to the theory which
generated them.
 If the world is symmetric they exist,
but if the world is symmetric they are
massless.


In 1964 Peter Higgs saw a way out:
`Spontaneous symmetry breaking’
16
??
Spontaneous Symmetry Breaking?
??
??
??
??
Propose a symmetric potential
giving a non-symmetric result.
A very neat solution!
Introduce a new field with this
property
Matter Particles:
Force Particles:
Interact with Higgs field
slows them down 
generates mass
W,Z particles gain mass
Photons/gluons have
different symmetries; stay
17
massless
Origin of Mass ?
1. Start with a mass-less particle
m=0, v = speed of light
2. Introduce a new field H that interacts with the particle
3. Let H be non-zero in the vacuum
H
0
H
0
m=0, v = speed of light V < c, m > 0
H
0
H
0
18
The Waldegrave Higgs challenge
In 1993, the then UK Science Minister,
William Waldegrave, issued a challenge to
physicists to answer the questions:
'What is the Higgs boson, and why do we
want to find it?’
on one side of a sheet of paper.
David Miller of UCL won a bottle of champagne for
the following:
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The Waldegrave Higgs challenge
Imagine a room full of
political activists
20
The Waldegrave Higgs challenge
The Prime Minister
walks in
21
The Waldegrave Higgs challenge
He is surrounded by a
cluster of people
Analogous to
generation of Mass
22
The Waldegrave Higgs challenge
The same room again
23
The Waldegrave Higgs challenge
A interesting rumour is
introduced
24
The Waldegrave Higgs challenge
Thanks to
D. Miller
and CERN
©
Photo
CERN
Soon we have a cluster
of people discussing it
Analogous to Higgs
boson
25
What does Higgs theory imply?
Higgs’ mechanism gives
mass to W and Z bosons,
and to the matter particles.
Mass of the W predicted
It also predicts one
extra particle:
The Higgs boson
(Nb. The Higgs has no spin a boson)
We can test it
The Higgs
Boson mass is
not predicted
26
The W, the top quark and Higgs

We can calculate the mass of the W boson
 Need the mass of the Z and the strength of
the forces; these are well known
 It is also affected by:
– Top quark mass:
Weak effect
– Higgs mass:
Tiny effect
Checking this tests Higgs’ mechanism
27
W mass
The W, the top quark and Higgs
The W mass
almost constant
28
The W, the top quark and Higgs
Expanded
scale
The W mass
depends upon the
top and Higgs
masses
29
CERN’s Collider ring
4
2 LEP
LHC experiments:
experiments
-, E ~ ~
LHC
LEP: :pp,
e+eE
210 GeV
GeV
cms
cms 14000
DELPHI
CERN
©
30 Photo
CERN
What are LEP and LHC?
LEP
LHC
Beams of
Electrons
Protons
Energy, GeV
208
14,000
Max. Higgs
Mass
Detailed?
115
~1000
Yes
No
Work
Operation 1989-2000
2006started
1980’s
Complementary machines – but needing the same tunnel
31
In the LEP tunnel
27km of
vacuum
pipe and
bending
magnets
©
Photo
CERN
32
Now to be the LHC tunnel
27km of
vacuum
pipe
8.3Tesla
bending
magnets,
3o above
absolute
zero
©
Photo
CERN
33
A LEP Experiment: ‘DELPHI’
One of
four
rather
similar
detectors
Note the
person
Onion-like layered
structure, about
10m in each
direction
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Another experiment: ‘OPAL’
One of
four
rather
similar
detectors
©
Photo
CERN
Note the
people
Assembly in 1989
35
An LHC experiment: ATLAS
©
Photo
CERN
Note the
people
In construction
36
Previous work at LEP
From 1989 to 1995
LEP created
20,000,000 Z bosons

Z mass
91GeV
These were used for
detailed studies of its
properties
For example, here you
see the analysis which
established the number
of neutrinos as 3
They can say
something about the
Higgs too.
37
Feynman Diagram for Z production
Electron
Z
Cannot see the Z
Only its decay
products
Positron (anti-electron)
38
Indirect Search for the Higgs Boson
top
Higgs
quark
Basic Feynman
More complicated
diagramsdiagram
have ‘loop’
effects.
The measured particles
are the same.
This is allowed by
Heisenberg’s
‘uncertainty
principle’
39
Indirect Search for the Higgs Boson
What is affected?
Z decay rate to b’s sensitive to top mass
Angular distributions sensitive to W & H mass
top
 1%
0.1%
Precision
needed!
 1%
Higgs
By carefully studying Z’s we:
• Predict mtop and mW;
Compare with measurements;
• Predict mH;
Compare with measurements.
40
How to recognize Z decays:
e) Z->
- Two jets, many particles
Z  qq:
Z  e+e-, m+m-: Two charged
particles (e or m.)
Z  t+t-: Each t gives 1 or 3 tracks
42
How to recognize Z decays:
e) Z->
Z  :
Not detectable.
b quarks: These
quarks have detached vertices,
measured in accurate vertex detectors
(weak and slow decays
to lighter quarks)
Interactive Z decays
43
One example Z distribution:
Angular distributions
•This distribution depends
on the W mass
Zm+m-
DELPHI
1993-1995
46GeV
Many things are used:
Z mass,
Several angular
distributions,
total Z decay rate,
Z decay fraction to bb
47GeV
45GeV
Cos(q) of outgoing m44
The W, the top quark and Higgs
Result from Z studies
The W and top
masses from Z
studies agree
with theory
i.e. they lie on the
curves
They can be
checked by
direct
measurement
45
The W, the top quark and Higgs
W mass from
LEP and
Tevatron
Completely
consistent!
This suggested the
top quark had a
mass of 175GeV
before it had been
discovered
46
The W, the top quark and Higgs
Top mass
from
Tevatron
Again,
incredible
consistency
The top mass
directly measured
agrees completely
with the predicted
one
47
The W, the top quark and Higgs
10GeV
Scale has
been
expanded
further
100GeV
1000GeV
The data
(especially if they
are averaged)
suggest a Higgs
mass around
100GeV
This procedure
worked for the top
quark. Will it work
again?
48
Summary of model

W mass agrees with Higgs theory
– to 1 part in 1000

Loop effects verified:
– Top mass agrees with prediction

Higgs mass should be:
+53
-35
88 GeV
49
The direct search

People have looked for a Higgs at LEP
 By 2000 we knew that Higgs could not
weigh less than 108GeV
 The ‘best fit’ on the previous slide is below
that…
– Not a serious conflict, but
The Higgs should not be far away
50
Allowed mass with time
EW fits assume a Higgs
Search looks for one
01
Excluded by
Direct Search
20
00
20
99
19
98
19
97
19
96
Excluded by EW
fits
Allowed
19
19
95
1000
900
800
700
600
500
400
300
200
100
0
51
LEP Higgs production: HZ
Predicted LEP events,
4 experiments
Make a Higgs and
a Z together
So need Energy
greater than Higgs
mass plus Z mass
52
Higgs and Z decay channels
Recall: Both Z & H are made
Z decay modes
Higgs decay modes
WW
tau
b quarks
B quarks

quarks
gluons
neutrinos
WW
electrons
other
e.g. ZHbb
taus
Any quark
muons
All Z decay modes used
Higgs only into b-quarks
53
How is Higgs search done?
•Look at events one-by-one (with software!)
•Identify leptons and quark jets
– implied if energy missing
•Require two jets from b quarks
 these might be from a Higgs
•Check that the rest of the event looks like a Z boson
•Calculate best-guess Higgs mass
•Make an estimate of how likely it is to be the signal
54
Example of a background:
- W+W-  q1q2q3q4:
Four well separated jets.
W+W-
-  q1q2l:
Two hadronic jets,
One lepton, missing energy.
+
WW
W+W-  l11l2-2:
Two leptons, missing energy
55
A DELPHI candidate
Need to decide which jets are
from Higgs, which from Z.
Decision not easy in many cases.
In this case, two possible :
Mass 97.4 or 113.4
But we take the overall best:
MH=97.4GeV/c2
The one that got away?
56
LEP machine in 2000
The accelerator was driven to its limit in energy
Despite all that, the amount of data was large
‘Higgs discovery mode’
57
Beam energy 99/00: From 192 to 209 GeV
1) Increase gradient & Cryogenics upgrade
3) Re-install 8 Cu cavities
30
Number of cavities
E: 207  207.4 GeV; mH: 114  114.25 GeV
200 GeV
7.0 MV/m
96 GeV
100 GeV
104 GeV
25
4) Use orbit correctors as magnetic dipoles
20
192 GeV
6.0 MV/m
15
204 GeV
7.5 MV/m
10
E: 207.4  207.8 GeV; mH: 114.25  114.5 GeV
5) Decrease the RF frequency
5
E: 207.8  209.2 GeV; mH: 114.5 
115.1 GeV
9.
4
9
9.
2
8.
8
8.
6
8.
4
8
8.
2
7.
8
7.
6
7.
4
7
7.
2
6.
8
6.
6
6.
4
6
6.
2
5.
8
5.
6
5.
4
5
5.
2
4.
8
4.
6
4.
4
4.
2
4
0
Accelerating field [MV/m]
Accelerating field (MV/m)
f = 0 Hz
E: 192  204 GeV; mH: 100  112 GeV
2) Improve stability & Decrease security margin
• Two- to one-klystron margin (1h30):
E: 204  205.5 GeV; mH: 112  113 GeV
LEP:
f=350 MHz
More dipolar
magnetic field
seen in the
quadrupoles!
• Raise energy so no margin at all (15 mins):
E: 205.5  207 GeV; mH: 113  114 GeV
f = -50 Hz
58
First data above 206 GeV:
_ _
e+e-  bbqq
Missing
Momentum
First Serious Candidate
(14-Jun-2000, 206.7 GeV)
• Mass 114.3 GeV/c2;
• Good HZ fit;
• Poor WW and ZZ fits;
• P(Background) : 2%
• s/b(115) = 4.6
The purest candidate event ever!
High pT muon
b-tagging
(0 = light quarks, 1 = b quarks)
• Higgs jets: 0.99 and 0.99;
• Z jets: 0.14 and 0.01.
59
Some candidate events at 115
2
GeV/c
27-Jun-2000
Mass: 113 GeV
s/b115 = 0.7
31-Jul-2000
Mass: 112 GeV
s/b115 = 2.4
ALEPH
21-Aug-2000
Mass: 110 GeV
s/b115 = 0.9
DELPHI
21-Jul-2000
Mass: 114 GeV
s/b115 = 0.4
e+e- 
14-Oct-2000
Mass: 114 GeV
s/b115 = 0.7
_ _
bb
L3
60
Significant high mass events
EXP
1
ALEPH
Energ Z decay
y
206.7
quarks
M
s/b
weight
114.3
4.6
1.73
2
ALEPH
206.7
quarks
112.9
2.4
1.21
3
ALEPH
206.5
quarks
110.0
0.9
0.64
4
L3
206.4

115.0
0.7
0.53
5
OPAL
206.6
quarks
110.7
0.7
0.53
6
206.7
quarks
114.3
0.6
0.49
7
DELPH
I
ALEPH
205.0
ee
118.1
0.6
0.47
8
ALEPH
208.1
tt
115.4
0.5
0.41
9
ALEPH
206.5
quarks
114.0
0.5
0.40
10
OPAL
205.4
quarks
112.6
0.5
0.40
61
L3 H candidate:
Two clear b-quarks
Lots of energy
missing
Could be HZ, with Z
decaying to
invisible neutrinos
Mass is 114.5GeV
Or maybe it is just a
pair of quarks.
Importance initially
overestimated
62
Combined Mass Plot
Distribution of the
reconstructed Higgs
boson mass with
increasing purity
for a signal with
mass 115 GeV/c2
Yellow is
background
Red is Higgs, if it
weighs 115GeV
Essential to
understand the
background if we
want to claim
something new
63
The roadmap of 2000
At Sept 5th seminar, we
predicted 70pb-1 more
data would give a 3s
evidence if MH is 115
On November 3rd, with
55pb-1 we estimated 2.9s
or 4 in 1000
After some corrections,
`only’ ~2s
or 4 in 100
64
What should be done?
The data looked just the way a 115GeV Higgs would look
But the statistical significance was not that strong.
Extending by one year would be expensive and delay LHC
It would also have a decent shot at answering the biggest
question in particle Physics What we are trying to find out
Friday
3rd5November:
`LEPC’
met
Monday
`SPC’
Wednesday
8rdthNovember:
: Committee
of council
Norecommendation
recommendation
No
Recommended
closure
Only the UK spoke for continuation – thank you.
65
What might 2001 have brought?
•Six months in 2001;
Expected mass spectrum:
• with similar amount of data to 2000;
• and an energy of 208.5  210 GeV;
(made possible with add’l cavities and a few tricks)
The evidence might have become
A clear discovery
Background subtracted:
~28 signal events
66
What did 2001 REALLY bring?
DELPHI cables in 2001
LEP had run its course
67
What happens next?
s = 2000 GeV
The Tevatron:
• A pp collider, near Chicago;
• Has run before – rate now
much higher
•Can find a 115GeV Higgs by
2007, but not be sure much
beyond
68
What happens next?
The LHC:
• CERN’s new pp collider;
• Designed to find the Higgs, it
can do it!
•First chance in 2007
s = 14000 GeV
69
What happens next?
TESLA:
• A Proposed e+e- collider
- CLEAN
• 500-800GeV, very high
rate
•Find the Higgs in ½ a
s = 800 GeV
day: study it carefully
70
th
19

or
th
20
Century?
50% chance the ‘Theory of
Everything’ will be known
within 20 years:
Stephen Hawking, 1997 et al.

Physics as we know it will be over in 6
months:
Max Born,1933
 The future of physics is to be found in the
6th place of decimals:
Lord Kelvin, 1890’s
71
Implications for society?

The early 20th century brought quantum
mechanics and relativity
– Lasers
– Computer chips
– Synchrotron light sources ‘Diamond’

These were 50 years or more in coming,
but fuel Microsoft's $200Bn business
 I don’t see any applications for Higgs
research, but I am not so foolhardy as to
say that understanding the origin of
mass cannot have any.
72
The Higgs discovered at LEP
•Precision studies of Z,W and top DEMAND a Higgs:
•MW agrees with Higgs’ predictions to 1 per mille,
mH = 88
+ 53
- 35
GeV/c2
• Direct Searches (4 in 100 effect)
mH =
+ 0.8
115.6- 0.8
GeV/c2
Is this a statistical fluke?
Or was there something there?
In about 5 years, Tevatron/LHC
should decide
More Precision Measurements with
Lepton Colliders will follow after 2010
73