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 ExclusionPrinciple) - 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 < 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 eV*A). • 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 ~ gW<f> • 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 <f> ~ 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

13. CMS in the Collision Hall Tracker ECAL HCAL Magnet Muon

14. 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 ECAL+HCAL q = c, b Jet (narrow) + Decay Vertex t --> W +b W + b e Et missing in ECAL+HCAL -->l +  +l Et missing + charged lepton W --> l + l Et missing + charged lepton, Et~M/2 Z --> l+ + l- charged lepton pair --> l + l Et missing in ECAL+HCAL

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/identification. 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 --> 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 --> 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, <f>, gives mass to the W and Z gauge bosons, MW ~ gW<f>. Thus the Higgs field acts somewhat like the "ether". Similarly the fermions gain a mass by Yukawa interactions with the Higgs field, mf = gf<f>. Although the couplings are not predicted, the Higgs field gives us a compact mechanism to generate all the masses in the Universe. • G(H->ff) ~ gf2MH ~ g2(Mf/MW)2MH , g = gW • G(H->WW) ~ g2MH3/MW2~ g2(MH/MW)2MH • G ~ MH3or G/MH ~ MH2==> G/MH ~ 1 @ 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 < 160 GeV use H --> ZZ --> 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 > 160 GeV. Coverage to || < 2.5 is required ( > 6 degrees)

26. CMS Trigger and DAQ System 1 GHz interactions 40 MHz crossing rate < 100 kHz L1 rate <10 kHz “L2” rate < 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 --> 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 << EW vev ~ 246 GeV << MGUT ~ 1016 GeV << 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 < 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 e+e- 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; 3/8[1 - 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 --> e+d , ud --> d. Thus p --> 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 < 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”