Transcript Particle physics tomorrow LHC

Particle physics
for
tomorrow :
LHC
Pierre Darriulat, Ha Noi, February 2007
Content
• The present state: major questions
• Electron vs proton colliders
• The Large Hadron Collider at CERN
• Perspectives for Viêt Nam
The present state: major questions
Standard model of massless particles
- Space-time symmetry
- Exchange symmetries
- Gauge invariance
Standard model of massive particles
- Spontaneous symmetry breaking, Higgs mechanism
- Supersymmetry
Major questions
- Three families, lepton-quark symmetry
- Where is (are) the Higgs(es)?
- Is the world supersymmetric?
- Astrophysics/cosmology: inflation, dark matter, dark
energy and quantization
A new domain to explore
Standard model of massless particles :
space-time symmetry
• Poincaré group of Lorentz transformations:
- translations (energy-momentum is a 4-vector),
- space rotations (spin)
- Lorentz boosts (space-time rotations, left and right
representations, Dirac spinors, antiparticle-particle relation)
• Particles defined by covariant spin and mass (Casimir)
• The Standard Model starts by assuming the existence of
a single spin ½ fermion species, f
Standard model of massless particles :
exchange symmetry
• The single fermion species (out of some 1080 fermions in the
universe!) may exist in different forms, specified by indices:
fi,j,k…
• Group symmetries define the exchange from one index to
another: Ui1,i2
• SU(3)×SU(2)×U(1) describes color×weak-isospin×charge
(hypercharge) exchanges associated with strong, weak and
electromagnetic forces respectively
• Quark-lepton symmetry and three families are not
understood. Unification is believed to take place at GUT
scale, ~1016 GeV, close to the Planck scale (1019GeV)
Standard model of massless particles :
gauge invariance
• Gauge invariance (local) requires that we may choose the
phases of the fields as we like at any point of space-time and
ascertain that the exchanged states still satisfy Dirac
equation. It is not possible.
• The way out is to introduce massless gauge vector bosons
that compensate exactly for the effect. We need as many as
there are generators in the exchange group. They couple
directly to the fermion field
• U(1) gives the photon; SU(2) gives three weak bosons, W+,
W- and Z; SU(3) gives 8 gluons.
In fact the photon and weak bosons mix with weak (Weinberg)
angle θW , sin2 θW = 0.23
Standard model of massive particles :
spontaneous symmetry breaking,
Higgs mechanism
• The favored way to generate masses uses the fact that
SU(2)×U(1) symmetry breaking relates to non-zero masses
(mass terms are of the form fLfR)
• Introducing a pair of complex scalar fields with a locus of
degenerate minima generates 3 Goldstone bosons that give
masses to the weak bosons and a 4th scalar: the Higgs boson
• More complex schemes are possible with several Higgs
bosons, but the mechanism remains the same
• However no Higgs boson has yet been observed, the current
mass limit is 114 GeV.
Current limit on Higgs mass (from LEP)
Standard model of massive particles :
Supersymmetry
• The Higgs mechanism will generate weak boson masses
commensurate with the only available scale, MPlanck=1019GeV,
rather than 100 or so GeV as required.
• The favored way to prevent this to happen is to introduce
supersymmetry (SUSY), a symmetry between bosons and
fermions.
• While fermions are prevented to acquire large masses by
SU(2)×U(1) symmetry, SUSY will do it for bosons.
• SUSY is in fact a fundamental symmetry of space-time. Its
commutators are proportional to momentum and gauging it
generates gravity (SUGRA)
• However, no SUSY partner of any known particle has yet been
observed. They are expected in the 100 to 1000 GeV range.
Major questions: three families, lepton-quark symmetry
Quarks: (u,d) (c,s) (t,b)
Leptons: (e, νe) (μ, νμ) (τ, ντ)
Major questions: Where is (are) the Higgs(es)?
• MH>114 GeV from LEP (radiated from Z)
• MH<900 GeV from unitarity ΓH ~ MH3
• LEP hints that MH is less than 200 or so GeV
• When several Higgses are expected (5 in the MSSM), the lower mass one
obeys the same (or in fact lower) mass limit as in the single Higgs case
• H couples to other particles in proportion to their mass. Couplings to W,
Z and t dominate
• Dominating cross section is from fusion of bremsstrahled weak bosons
(e+e- and pp colliders), gluons (pp colliders) or photons (e+e- colliders),
the latter two via t,b or W,Z loops. They are at the pb level, implying
luminosities of at least some 1033cm-2s-1 (3 to 4 Higgses produced per
hour).
• Identification and background rejection are difficult: only a fraction of
the produced Higgses are detected and identified
Major questions: Is the world supersymmetric?
• Supersymmetric partners of known particles include scalar
bosons (sleptons and squarks) and spin ½ fermions (gauginos
and higgsinos).
• Always produced in pairs: g→g+g becomes g→gsusy+gsusy
Z→W+W becomes Z→Wsusy+Wsusy f→f+(W,Z) becomes
f→fsusy+(W,Z)susy , e→e+γ becomes e→esusy+γsusy etc…
• The lowest mass SUSY partner (LSP) should be stable and
not interacting: detection relies on presence of missing
mass/transverse momentum (strong neutrino background).
• Here again cross-sections are at the pb scale requiring
luminosities at the 1033 cm-2s-1 level
Major questions in astrophysics/cosmology:
inflation, dark matter, dark energy and quantization
• Inflation occurs just after the big bang at GUT times and requires a
better understanding of Grand Unification, quantization of gravity
and large distance behavior of gravity.
• The cosmic microwave background data imply that the universe is flat;
but then, we are only able to identify 27% of its energy density. The
missing 73% are well described by a cosmological constant, namely a
force dominating at large distances and causing, recently, an
acceleration of the expansion of the universe. It gives evidence for our
poor understanding of gravity at large distances.
• Normal matter accounts for only 4% of the energy density in the
universe, 1% in the form of stars and 3% in the form of hot gases:
strong evidence in favor of 23% of the energy density of the universe
being in the form of weakly interacting (therefore neutral), massive
particles from cluster binding energy and velocity curves of stars away
from the galactic plane. The LSP is the favorite candidate.
• Above Planck mass, MPlanck=1/√GNewton~1019GeV , general relativity
and quantum theory become incompatible: need for a new theory,
superstrings being the currently favored candidate
A new domain to explore
• Of the six major questions that have been identified:
- quantization of gravity
- lepton-quark symmetry and three families (GUT scale, inflation)
- dark energy
- dark matter
- where is (are) the Higgs(es)?
- where are the SUSY partners?
the last three are likely to find their answer in a new domain that
can be explored with particle accelerators (colliders), a mass range
between 100 and 1000 GeV.
• Such accelerators must have a luminosity above the 1033cm-2s-1 range.
• Technically, two options are possible:
- a proton-proton circular collider with at least 10TeV centre of mass
energy. This is the LHC (Large Hadron Collider) under completion at CERN
- an electron-positron linear collider with at least 1TeV centre of mass
energy, currently in its design stage
Electron vs proton collider
• Collider vs fixed target
• Synchrotron radiation
• Proton-proton circular colliders
- Limiting luminosity factors
- Protons as composite particles
- Energy vs luminosity
- Questions of background
• Electron-positron linear colliders
- Limiting luminosity factors
- Annihilations, resonant or non-resonant,
bremsstrahlung
• Electron vs proton collider: a summary
Collider vs fixed target
• Collider is compulsory. To reach 1TeV centre of mass
energy in a p-p fixed target collision we need a beam
energy E such that
(E+.001)2-E2=.002E=1, namely E = 500 TeV
Compared to E = 0.5 TeV in the collider mode!
• Collider luminosity (i.e. ratio of event rate to cross-section)
is L=N2/(A×Δt), N=particles per bunch, A = bunch crossarea, Δt = time between bunches
• Circular colliders benefit of reusing the same bunches but
they must preserve their quality. Linear colliders may
damage them: they are used only once!
Synchrotron radiation
• Bending a charged particle in a magnetic field (circular colliders)
implies radiating a photon forward (synchrotron radiation) and
therefore loosing energy.
• The energy loss per turn is 10-4 E4/R MeV per turn for electrons with
E in GeV and R in km.
For protons it is (mp/me)4 times smaller, namely
10-3 E4/R keV per turn with E in TeV and R in km.
• A xTeV beam in a xT magnetic field means a circumference of 20km.
The energy loss per turn is for electrons (x=1) 30TeV ! And for
protons (x=10) 3keV! An accelerating gradient of 10 MeV/m means
0.2TeV per turn.
• Only two choices in practice: a circular proton collider or a linear
electron collider. The latter is more difficult.
Proton-proton circular colliders:
limiting luminosity factors
• To first order one beam acts on the other as a lens of
convergence C=1/f~N/(γA) and A=β*ε=β*εn/γ ,
where ε = emittance and εn= invariant (Liouville). β* is
defined by the optics, dominated by low β intersections. It
can be corrected on average but not for its fluctuations ΔC~C
• The tune shift ΔQ=C β*/4π ~N/(4π εn) is a measure of the
perturbation that cannot be corrected. In a circular collider it
must be kept down to the percent level. Circular colliders are
operated near the beam-beam limit.
Proton-proton circular colliders:
protons as composite particles
• Contrary to electrons,
protons are composite
particles made of partons:
three valence quarks and a
gluon sea (including quarkantiquark pairs).
• Proton colliders are in fact
used as parton colliders, only
two partons, one from each
proton, take part in the
interesting physics
Include here a
copy of the
longitudinal
density from
page 3 of my
talk to machine
physicists
Proton-proton circular colliders:
energy vs luminosity
• The available centre of mass energy is √s*=√(x1x2s), but the available
luminosity is L*=F(x1)F(x2)L, the F’s being parton densities. A large
energy requires large x values, a large luminosity requires low x
values. The effective mass reach of a proton collider depends upon
both its energy and luminosity
Proton-proton circular colliders:
questions of background
• Most collisions (1011pb!) do not resolve partons and are
governed by the proton size (σ~πRp2). They produce low
transverse momentum (~ħ/Rp) particles peaked along the
beams (rapidity plateau) and must be filtered out
• Large transverse momentum hadron jets resulting from
gluon-gluon collisions are a major source of background
• W,Z bosons and t quarks are copiously produced at LHC
energies and provide a disturbing background of leptons
(charged and neutrinos), usually a good signature for
interesting physics in hadron colliders
Electron-positron linear colliders:
limiting luminosity factors
• Linear colliders can operate much beyond the beam limit
but new limitations come into play:
• For a bunch length l, the disruption factor D=lC should not
exceed unity. Otherwise the bunches are so distorted that
one looses luminosity significantly
• Field depth effects ~ l/β*
• Collision angle (crab crossing) ~ l/√A
• Wake fields (head-tail forces) ~ l/√λRF
Electron-positron linear colliders:
annihilations, resonant or non-resonant, a comment
• Lower energy electron colliders have benefited from
resonant situations (all quarkonia, Z at LEP) or at least
from the fact that the interesting final state resulted from
electron positron annihilation (no underlying event, like
in W pair production at LEP). Such favorable situations
are nor expected to often repeat at TeV energies. As an
example the standard Higgs should be produced from the
fusion of two weak bosons bremsstrahled from the beams
Electron vs proton colliders: a summary
• In proton colliders the available parton-parton centre of mass
energy and luminosity are significantly lowered. The overwhelming
low transverse momentum products are a nuisance and must be
filtered out.
• Proton colliders can reach high luminosities because they can be
circular. They are magnet-limited.
• Electron colliders are “cleaner” but the favorable situation of
resonant annihilations is not expected to repeat here.
• They must be linear because of synchrotron losses. They are RFlimited and in order to achieve sufficient luminosities they need to
stretch parameters beyond reasonable limits.
• Both colliders offer very difficult experimental conditions and
require detectors of unprecedented sophistication and complexity.
• A proton collider (LHC) is under completion, an electron collider is
in its design phase.
The Large Hadron Collider at CERN
OTHER RELEVANT
PARAMETERS
LHC
NLC
N
1011
1.5 1010
Δt
25 ns
556 μs
I
0.5 A
4.5 μA
A
(16 μm)2
2.5nm×220nm
L
27 km
2×14 km
L
1034 cm-2s-1
1034cm-2s-1
√s
14 TeV
1 TeV
Nex
2+2
1
LHC:
Beam power 7000 GW
Stored energy 700 MJ
Collision lifetime 10 h
Ramping time 20 mn
Synch. losses 0.44 W/m
Magnets Nb/Ti,
Cu/SC=1.7
LH 2K 0.81 TeV/T
NLC:
Beam power 4.5 MW
P(line)
280 MW
Bunch length 100 μm
Gradient
100 MV/m
Detectors
• Two general purpose 4π coverage: ATLAS and CMS; one for ion-ion
collisions: ALICE; and one for CP-violation studies in the b sector,
LHCb.
• ATLAS and CMS will see 20 to 40 events per bunch crossing (25ns),
ie 1GHz and 1011 to 12 tracks/s! They include magnetic momentum
analysis, calorimetry, muon spectrometry and hermeticity (missing
mass and/or momentum)
• ATLAS is 46m long, 25 m in diameter, 0.3g/cm3. It has an air core
toroidal magnet. CMS has a large, 6 m bore, solenoid with 4 T field
and is 22 m long and 15 m in diameter, 3g/cm3. Both use silicon
trackers.
• Both have radiation hard components. For calorimetry, ATLAS uses
liquid argon and CMS PbWO4 crystals.
• Major computing effort (GRID)
• ATLAS and CMS each have some 2000 members from 170 institutions,
nearly 40 countries!
Installation of the 1000th LHC dipole (out of 1232)
September 5th 2005
In brief…
• Before the end of the decade (officially already this year)
LHC will open a new window on particle physics and
astrophysics in the TeV range and address the questions of
the mass generation mechanism and of supersymmetry, both
having major impacts on our understanding of the world
• It will also study ion-ion collisions in the quark-gluon plasma
regime and CP-violation in the b-sector
• Many other questions will be addressed and, most importantly,
the ATLAS and CMS detectors are prepared… to detect the
unexpected
• This might offer an exciting and challenging opportunity for
the young most talented Vietnamese physicists
Perspectives for Viêt Nam
• The opportunity for Viêt Nam to join LHC
experiments shows that in particle physics as in
astrophysics developing countries may take part in the
most prestigious frontier experiments.
• CERN, a European laboratory, is de facto becoming a
world laboratory. Vietnamese young physicists
working on LHC experiments will be in contact with –
and competing with – particularly brilliant colleagues
from all over the world.
• To take up this challenge, a number of conditions
should be fulfilled
• Vietnamese physicists wishing to
contribute to LHC experiments must join
efforts in order to make a single team,
independently from whichever university
or institute they are belonging to. This is
an essential condition to reach a critical
size below which no effective Vietnamese
contribution would be possible.
• Reasonable wages, responsibilities and
working conditions should be offered in
order to attract talented physicists and
avoiding undue demotivation.
• Impartial selection panels including
members from various institutions and
possibly various countries should be used
to select the best possible physicists.
• The myth according to which theory is
worth more than experiment and theorists
more clever than experimenters, a myth
present in many developing countries,
should be abolished : students should be
taught that physics is made from an
incessant exchange between the two,
without hierarchy, and that a good physicist
must be conversant in both disciplines.
• While there is still a long way to go to
fulfill these conditions, it could be done
very fast if there were enough motivation
and determination to take up the challenge.
• Failing to do so would simply result in
providing western science with more cheap
labor and aggravating the already
catastrophic brain drain.