Fermilab

1
Introduction

• John Womersley
• Fermilab
• DØ Run 2B Silicon Workshop
• January 2001

2
Mass shapes the Universe

• through gravitation, the only force that is
  important over astronomical distances
• Despite the successes of general relativity, we
  still do not understand gravity in a quantum
  framework
• but we believe we are getting closer to
  understanding the origin of mass

3
Mass in the cosmos

• Masses of Atoms
• rest masses of the fermions
• binding energies
• Dark Matter
• dynamical mass much greater than visible luminous
  material
• primordial nucleosynthesis, D/He abundance ? all
  this mass cannot be baryonic
• new particles?

4
Mass of Hadrons

• Mass of a proton 938 MeVMass of two u quarks
  plus a d quark 10 ? 5 MeV
• 99 of the mass of a proton (and therefore of the
  mass of a hydrogen atom) is due to the binding
  energy
• Quantum Chromodynamics (QCD)
• the strong force that acts on quarks
• a gauge theory (like electromagnetism)
• unlike electromagnetism, the vector bosons of the
  theory (gluons) themselves carry the charge
  (color)
• gluons are self-interacting
• coupling constant runs rapidly force becomes
  strong for small momentum transfers
• confinement

Compilation of experiments
5
Understanding QCD

• Precisely testable QCD calculations are available
  for high momentum transfer processes at particle
  accelerators
• e.g. DØ measurements of production of jets in?pp
  collisions
• Soft QCD is calculable only numerically lattice
  gauge theory
• initially somewhat disappointing
• recent advances in computing, and in the
  techniques used, lead to reasonably credible
  results
• predicted and measured hadron masses

6
Does this mean we understand mass?

• There is not much doubt that QCD is the theory of
  the strong interaction, and we are making
  progress in understanding how to calculate
  reliably in this framework
• and recall that 99 of the mass of the (visible)
  universe is QCD
• But
• we still need to understand fermion masses
• second and third generations of quarks and
  leptons are much more massive
• the masses exhibit patterns
• we still need to understand vector boson masses
• mass of the W and Z bosons is what makes the weak
  force weak

7
Fundamental particles and forces
masses

• leptons q 1 e ? ? q 0 ?e ??
  ??
• quarks q 2/3 u c t q 1/3 d s b
• Forces
• QCD
• Electroweak force
• interaction between quarks and leptons, mediated
  by photons (electromagnetism) and W and Z bosons
  (weak force)
• same couplings to matter(except angles)
• very different masses

mass 80.4 GeV
W
photon mass 0
8
What does mass mean?

• For an elementary pointlike particle
• propagates through the vacuum at v lt c
• Lorentz transform mixes LH and RH helicity
  statessymmetry is broken
• mass is equivalent to an interaction with the
  (Quantum Mechanical) vacuum
• coupling strength mass
• For a spin-1 state like a photon, there is an
  extra effect
• massless ? two polarization states
• massive ? three polarization states
• where does this additional degree of freedom come
  from?

9
The Higgs Mechanism

• Hence, in the Standard Model (Glashow, Weinberg,
  Salam, t Hooft, Veltmann)
• electroweak symmetry breaking through
  introduction of a scalar field ? ? masses of W
  and Z
• Higgs field permeates space with a finite vacuum
  expectation value
• cosmological implications! (inflation)
• If ? also couples to fermions ? generates fermion
  masses
• An appealing picture is it correct?
• One clear and testable prediction there exists a
  neutral scalar particle which is an excitation of
  the Higgs field
• All its properties (production and decay rates,
  couplings) are fixed except its own mass
• Highest priority of worldwide high energy physics
  program find it!

10
mH probability density, J. Ellis
(hep-ph/0011086)
15 fb-1
110-190 GeV

• Physics ? Integrated Luminosity
• Radiation dose
• Instantaneous Luminosity Tracking
  confusion
• Detector Upgrade

Schedule ?
11
Run 2B silicon strategy

• At the start of this process, a minimal solution
  seemed attractive (to me at least)
• labs guidance was clear (2.5M cost, etc.)
• one might think that smaller quicker, easier
• But we now have a better understanding
• review committee report
• there are too few SVX2e chips
• there are likely to be problems with pattern
  recognition
• difficult to insert a L00 inside the existing
  SMT
• Hence we will define a more ambitious solution as
  our baseline plan

12
This meeting

• Move towards this baseline design
• my summary of the objective
• a design whose cost can be estimated
• a design whose performance can be simulated
• permit us to order prototype sensors
• Components
• SVX4 chip (December 2000 decision)
• a limited number of species of detectors
• all detectors single sided
• a layer 0 closer to the beam and a new beam pipe
• improves impact parameter resolution despite
  increased material
• more layers
• resolve pattern recognition issues
• but reduced coverage in rapidity and no F-disks

13
How to pay for this?

• We are submitting a MRI proposal to NSF for 2M
• the physics is sexy and easy to motivate
• if funded, would enable us to build roughly the
  scale of detector we want with Fermilabs
  contribution remaining of order 2.5M
• Our previous MRI record is successful, but no
  guarantees. . .
• review technical and funding progress summer
  2001, may have to fall back at that time
• Lets assume success and plan for it!

14
One note of caution

• Installation and commissioning of the Run 2a
  detector is at a critical point
• we must balance our enthusiasm for Run 2b with
  the need to ensure the success of the current run
• It is appropriate for a few people in D0 to work
  at the ? 50 level on Run2b but for most of the
  rest of us it should remain a side activity.
  Please do not let your Run 2a responsibilities
  suffer.