1. John Womersley Introduction Fermilab DØ Run 2B Silicon Workshop January 2001

2. John Womersley 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. John Womersley 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. John Womersley Mass of Hadrons Mass of a proton = 938 MeV Mass 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. John Womersley 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. John Womersley 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. John Womersley Fundamental particles and forces leptons q = 1e   q = 0 e   quarks q = 2 / 3 uct q = – 1 / 3 dsb 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 d u s c b t e   1 TeV 1 GeV 1 MeV 1 keV 1 eV d u s c b t d u s c b t ee   W photon mass = 0 mass = 80.4 GeV masses

8. John Womersley What does mass mean? For an elementary pointlike particle –propagates through the vacuum at v < c –Lorentz transform mixes LH and RH helicity states: symmetry 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? Massless field Something else mix Massive field

9. John Womersley 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. John Womersley Physics  Integrated Luminosity Radiation dose Instantaneous Luminosity Tracking confusion Detector Upgrade 15 fb -1 110-190 GeV m H probability density, J. Ellis (hep-ph/0011086) Schedule 

11. John Womersley Run 2B silicon strategy At the start of this process, a minimal solution seemed attractive (to me at least) –lab’s 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. John Womersley 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 This meeting

13. John Womersley 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 Fermilab’s 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 Let’s assume success and plan for it!

14. John Womersley 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.