The Physics of the LHC

1
The Physics of the LHC

• The Compact Muon Solenoid at the Large Hadron
  Collider
• Dan Green
• Fermilab
• US CMS Project Manager

2
Outline

• Why do we go to the energy frontier?
• What is the CMS collaboration?
• What is the Standard Model? How do we detect the
  fundamental particles contained in the SM?
• The Higgs boson is the missing object in the SM
  periodic table. What is the CMS strategy to
  discover it?
• What might we find at CMS in addition to the
  Higgs?

3
What and Where is CERN, LHC, CMS?
European Center for Nuclear Research (CERN)
Large Hadron Collider (LHC)
Compact Muon Solenoid (CMS)
4
High Energy Physics-Natural Units

• Dimensions are taken to be energy in HEP.
  Momentum and mass are given the dimensions of
  energy, pc, mc2. The basic energy unit is the
  electron Volt, the energy gained when an electron
  falls through a potential of 1 Volt 1.6 x 10
  -19 Joule.
• The connection between energy and time, position
  and momentum is supplied by Planck's constant,
  , where 1 fm 10-13
  cm. Thus, inverse length and inverse time have
  the units of energy. The Heisenberg uncertainty
  relation is
• Charge and spin are "quantized" they only take
  discrete values, e or . Fermions have spin
  1/2, 3/2 ..., while bosons have spin 0,1,. The
  statistics obeyed by fermions and bosons differs
  profoundly. Bosons can occupy the same quantum
  state - e.g. superconductors, laser. Fermions
  cannot (Pauli Exclusion Principle) - e.g. the
  shell structure of atoms.

5
Size and the Energy of the Probe Particle

• In order to "see" an object of size r one must
  use "light" with a wavelength l lt r. Thus,
  visible light with l 3000 A ( 1 A 10-8 cm,
  size of an atom) can resolve bacteria. Visible
  light comes from atomic transitions with eV
  energies ( 2000 eVA).
• To resolve a virus, the electron microscope with
  keV energies was developed, leading to an
  increase of 1000 in resolving power.
• To resolve the nucleus, 105 time smaller than the
  atom one needs probes in the GeV (109 eV) range.
  The size of a proton is 1 fm 10-13 cm.
• The large Hadron Collider (LHC) at the CERN will
  explore Nature at the TeV scale or down to
  distances 0.0002 fm.

6
Progress in HEP Depends on Advancing the Energy
Frontier
7
CMS Detector Subsystems
8
CMS Plans a working detector in 2005
9
The CMS Collaboration
10
Particle Physics in the 20th Century

• The e- was discovered by Thompson 1900. The
  nucleus was discovered by Rutherford in 1920.
  The e, the first antiparticle, was found in
  1930. The m , indicating a second generation,
  was discovered in 1936.
• There was an explosion of baryons and mesons
  discovered in the 1950s and 1960s. They were
  classified in a "periodic table" using the SU(3)
  symmetry group, whose physical realization was
  point like, strongly interacting, fractionally
  charged "quarks". Direct evidence for quarks and
  gluons came in the early 1970s.
• The exposition of the 3 generations of quarks and
  leptons is only just, 1996, completed. In the mid
  1980s the unification of the weak and
  electromagnetic force was confirmed by the W and
  Z discoveries.
• The LHC, starting in 2005, will be THE tool to
  explore the origin of the breaking of the
  electroweak symmetry (Higgs field?) and the
  origin of mass itself.

11
Electro - Weak Unification

• The weak interactions are responsible for nuclear
  beta decay. The observed rates are slow,
  indicating weak effective coupling. The decays of
  the nuclei, n, and m are parametrized as an
  effective 4 fermion interaction with coupling, G
  10-5 GeV-2, Gm G2Mm5.
• The weak SU(2) gauge bosons, W Zo W- , acquire a
  mass by interacting with the "Higgs boson vacuum
  expectation value" of the field, while the U(1)
  photon, g , remains massless. MW gWltfgt
• The SU(2) and U(1) couplings are "unified" in
  that e gWsin(qW). The parameter qW can be
  measured by studying the scattering of n p,
  since this is a purely weak interaction process.
• The coupling gW can be connected to G by noting
  that the 4 fermion Feynman diagram can be related
  to the effective 4 fermion interaction by the
  Feynman "propagator", G gW2/MW2. Thus, from G
  and sin(qW) one can predict MW. The result, MW
  80 GeV was confirmed at CERN in the pp collider.
  The vacuum Higgs field has ltfgt 250 GeV.

12
The Standard Model of Elementary Particle Physics

• Matter consists of half integral spin fermions.
  The strongly interacting fermions are called
  quarks. The fermions with electroweak
  interactions are called leptons. The uncharged
  leptons are called neutrinos.
• The forces are carried by integral spin bosons.
  The strong force is carried by 8 gluons (g), the
  electromagnetic force by the photon (?), and the
  weak interaction by the W Zo and W-. The g and ?
  are massless, while the W and Z have 80, 91 GeV
  mass.

J 1
g,?, W,Zo,W-
Force Carriers
u d
c s
t b
2/3 -1/3
Quarks
J 1/2
Q/e
e ?e
? ???
? ??
1 0
Leptons
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CMS in the Collision Hall
Tracker ECAL HCAL Magnet Muon
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Detection of Fundamental Particles
SM Fundamental Particle Appears As ?
? (ECAL shower, no track) e
e (ECAL shower, with track) ?
? (ionization only) g
Jet in ECAL HCAL q u, d, s
Jet (narrow) in ECALHCAL q c, b
Jet (narrow) Decay Vertex t --gt W b
W b ?e???? Et
missing in ECALHCAL ?--gtl ?? ?l Et
missing charged lepton W --gt l ?l
Et missing charged lepton,
EtM/2 Z --gt l l-
charged lepton pair --gt ?l ?l Et
missing in ECALHCAL
15
Dijet Events at the Tevatron

• The scattering of quarks inside the proton leads
  to a "jet" of particles traveling in the
  direction of, and taking the momentum of, the
  parent quark. Since there is no initial state Pt,
  the 2 quarks in the final state are "back to
  back" in azimuth.

16
A FNAL Collider (D0) Event

• The D0 detector has 3 main detector systems
  ionization tracking,liquid argon calorimetry ( EM
  , e , and HAD , jets ,), and magnetized steel
  ionization tracker muon , m , detection/identifica
  tion. This event has jets, a muon, an electron
  and missing energy , n.

17
A FNAL Collider (CDF) Event

• The CDF detector has 3 main detector systems
  tracking - Si ionization in a magnetic field,
  scintillator sampling calorimetry, (EM - e, g and
  HAD - h), and ionization tracking for muon
  measurements. Missing energy indicates n in the
  final state.Si vertex detectors allow one to
  identify b and c quarks in the event.

18
W --gt e ? at the Tevatron

• The W gauge bosons can decay into
  quark-antiquarks, e.g. u d, or into lepton
  pairs, e ne, m nm, t nt. There can also be
  radiation associated with the W, gluons which
  evolve into jets.

19
Z --gt e e and ? ? Events at the Tevatron

• The e appear in the EM and not the HAD
  compartment of the calorimetry, while the m
  penetrate thick material.

20
The Generation of Mass by the Higgs Mechanism

• The vacuum expectation value of the Higgs field,
  ltfgt, gives mass to the W and Z gauge bosons, MW
  gWltfgt. Thus the Higgs field acts somewhat like
  the "ether". Similarly the fermions gain a mass
  by Yukawa interactions with the Higgs field, mf
  gfltfgt. Although the couplings are not predicted,
  the Higgs field gives us a compact mechanism to
  generate all the masses in the Universe.
• G(H-gtff) gf2MH
  g2(Mf/MW)2MH , g gW
• G(H-gtWW)
  g2MH3/MW2 g2(MH/MW)2MH
• G MH3 or G/MH
  MH2 gt G/MH 1 _at_ MH 1 TeV

f, W, Z f, W, Z
g
H
21
Higgs Cross section
CDF and D0 successfully found the top quark,
which has a cross section 10-10 the total cross
section. A 500 GeV Higgs has a cross section
ratio of 10-11, which requires great
rejection power against backgrounds and a high
luminosity.
22
CMS Tracking System

• The Higgs is weakly coupled to ordinary matter.
  Thus, high interaction rates are required. The
  CMS pixel Si system has 100 million elements so
  as to accommodate the resulting track densities..

Si pixels Si Strips - an all Si detector is
demanded by the high luminosity required to do
the Physics of the LHC
23
If MH lt 160 GeV use H --gt ZZ --gt 4e or 4?
Fully active crystals are the best resolution
possible needed for 2 photon decays of the Higgs.
24
The Hadron Calorimeter

• HCAL detects jets from quarks and gluons.
  Neutrinos are inferred from missing Et.

Scintillator WLS gives hermetic readout for
neutrinos
25
The CMS Muon System

• The Higgs decay into ZZ to 4? is preferred for
  Higgs masses gt 160 GeV. Coverage to ? lt 2.5 is
  required (? gt 6 degrees)

26
CMS Trigger and DAQ System
1 GHz interactions 40 MHz crossing rate lt 100 kHz
L1 rate lt10 kHz L2 rate lt 100 Hz L3 rate to
storage medium
The telecomm technology is moving very rapidly. A
L2 and L3 in software using the full event is
possible
27
Higgs Discovery Limits
The main final state is ZZ --gt 4l. At high masses
larger branching ratios are needed. At lower
masses the ZZ and ??? final states are
used. LEP II will set a limit 110 GeV. CMS will
cover the full range from LEPII to 1 TeV.
28
LEP,CDF D0 Data Indicate Light Higgs
29
Higgs Mass - Upper Limit
In quantum field theories the constants are
altered in high order processed (e.g. loops).
Asking that the Higgs mass be well behaved up to
a high mass scale (no new Physics) implies a low
mass Higgs.
30
12 Unresolved Fundamental Questions in HEP

• How do the Z and W acquire mass and not the
  photon?
• What is MH and how do we measure it?
• Why are there 3 and only 3 light generations?
• What explains the pattern of quark and lepton
  masses and mixing?
• Why are the known mass scales so different? ?QCD
  0.2 GeV ltlt EW vev 246 GeV ltlt MGUT 1016 GeV
  ltlt MPL 1019 GeV
• Why is charge quantized?
• Why do neutrinos have such small masses
• Why is matter (protons) stable?
• Why is the Universe made of matter?
• What is dark matter made of?
• Why is the cosmological constant small?
• How does gravity fit in with the strong,
  electromagnetic and weak forces?

31
Grand Unified Theories

• Perhaps the strong and electroweak forces are
  related. In that case leptons and quarks would
  make transitions and p would be unstable. The
  unification mass scale of a GUT must be large
  enough so that the decay rate for p is lt the rate
  limit set by experiment.
• The coupling constants "run" in quantum field
  theories due to vacuum fluctuations. For example,
  in EM the e charge is shielded by virtual ?
  fluctuations into ee- pairs on a distance scale
  set by, le 1/me. Thus a increases as M
  decreases, a(0) 1/137, a(MZ) 1/128.

32
Why is charge quantized?

• There appears to be approximate unification of
  the couplings at a mass scale MGUT 1014 GeV.
• Then we combine quarks and leptons into GUT
  multiplets - the simplest possibility being
  SU(5).
• d1 d2 d3 e ? 3(-1/3 ) 1 0 0
• Since the sum of the projections of a group
  generator in a group multiplet is 0 (e.g. the
  angular momentum sum of m), then charge must be
  quantized in units of the electron charge.
• In addition, we see that quarks must have 1/3
  fractional charge because there are 3 colors of
  quarks - SU(3).

33
GUT Predicts ??W

• A GUT has a single gauge coupling constant. Thus,
  ? and ?W must be related. The SU(5) prediction is
  that sin(?W) e/g ??3/8.
• This prediction applies at MGUT
• Running back down to the Z mass, the prediction
  becomes ?3/81 - 109 ?/18?(ln(MGUT/MZ))1/2
• This prediction is in agreement with the
  measurement of ?W from the W and Z masses.

34
Why is matter (protons) stable?

• There is no gauge motivated conservation law
  making protons stable.
• Indeed, SU(5) relates quarks and leptons and
  possesses leptoquarks with masses the GUT
  mass scale.
• Thus we expect protons (uud) to decay via uu --gt
  ed , ud --gt d?. Thus p --gt e??o or ??
• Looking at the GUT extrapolation, we find 1/?
  40 at a GUT mass of 1014 GeV.
• One dimensional grounds, the proton lifetime
  should be
• ?p 1/?p ?GUT2(Mp/MGUT)4Mp or ?p 4 x 1031
  yr.
• The current experimental limit is 1032 yr. The
  limit is in disagreement with a careful estimate
  of the p decay lifetime in simple SU(5) GUT
  models. Thus we need to look a bit harder at the
  grand unification scheme.

35
9 - Why is the Universe made of matter?

• The present state of the Universe is very
  matter-antimatter asymmetric.
• The necessary conditions for such an asymmetry
  are the CP is violated, that Baryon number is not
  conserved, and that the Universe went through a
  phase out of thermal equilibrium.
• The existence of 3 generations allows for CP
  violation.
• The GUT has, of necessity, baryon non-conserving
  reactions due to lepto-quarks.
• Thus the possibility to explain the asymmetry
  exists in GUTs, although agreement with the data,
  NB/N? 10-9, and calculation may not be
  plausible.

36
SUSY and Evolution of ??
It is impossible to maintain the big gap between
the Higgs mass scale and the GUT mass scale in
the presence of quantum radiative corrections.
One way to restore the gap is to postulate a
relationship between fermions and bosons. Each SM
particle has a supersymmetric (SUSY) partner with
spin 1/2 difference. If the mass of the SUSY
partners is 1 TeV, then the GUT unification is
good - at 1016 GeV
37
Galactic Rotation Curves
The rise of v as r (Keplers law) is observed, but
no falloff is observed out to 60 kpc, well beyond
the luminous region of typical galaxies. There
must be a new dark matter.
38
Summary for CMS Physics

• CMS will explore the full (100 - 1000 GeV)
  allowed region of Higgs masses. Precision data
  indicates that the Higgs is light.
• The generational regularities in mass and CKM
  matrix elements will probably not be informed by
  data taken at CMS.
• There appears to be a GUT scale which indicates
  new dynamics. The GUT explains charge
  quantization, the value of ?W and perhaps the
  matter dominance of the Universe and the small
  values of the neutrino masses. However it fails
  in p decay and quadratic radiative corrections to
  Higgs mass scales..
• Preserving the scales, (hierarchy problem) can
  be accomplished in SUSY. SUSY raises the GUT
  scale, making the p quasi-stable. The SUSY LSP
  provides a candidate to explain the observation
  of galactic dark matter. A local SUSY GUT
  naturally incorporates gravity. It can also
  possibly provide a small cosmological constant. A
  common GUT coupling and preservation of loop
  cancellations requires SUSY mass lt 1 TeV. CMS
  will fully explore this SUSY mass range either
  proving or disproving this attractive hypothesis.

39
What will we find at the LHC?

• There is a single fundamental Higgs scalar field.
  This appears to be incomplete and unsatisfying.
• Another layer of the cosmic onion is uncovered.
  Quarks and/or leptons are composites of some new
  point like entity. This is historically plausible
  atoms ? nuclei ? nucleons ? quarks.
• There is a deep connection between Lorentz
  generators and spin generators. Each known SM
  particle has a super partner differing by ½
  unit in spin. An extended set of Higgs particles
  exists and a whole new SUSY spectroscopy exists
  for us to explore.
• The weak interactions become strong. Resonances
  appear in WW and WZ scattering as in ? ? ? ?. A
  new force manifests itself, leading to a new
  spectroscopy.
• There are more things in heaven and earth than
  are dreamt of