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Particle physics for 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 Vi ê t Nam. The present state: major questions. Standard model of massless particles

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Particle physics for tomorrow : LHC

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  1. Particle physicsfor tomorrow :LHC Pierre Darriulat, Ha Noi, February 2007

  2. Content • The present state: major questions • Electron vs proton colliders • The Large Hadron Collider at CERN • Perspectives for Viêt Nam

  3. 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

  4. 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

  5. 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)

  6. 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

  7. 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.

  8. Current limit on Higgs mass (from LEP)

  9. 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.

  10. Major questions:three families, lepton-quark symmetryQuarks: (u,d) (c,s) (t,b)Leptons: (e, νe) (μ, νμ) (τ, ντ)

  11. 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 thatMHis 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

  12. 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+Wsusyf→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:Is the world supersymmetric?

  13. 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

  14. 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

  15. 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

  16. 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 cross-area, 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!

  17. 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.

  18. 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.

  19. 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 quark-antiquark 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

  20. 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: energy vs luminosity

  21. 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

  22. 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

  23. 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

  24. 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 RF-limited 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.

  25. The Large Hadron Collider at CERN OTHER RELEVANT PARAMETERS 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

  26. 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!

  27. Installation of the 1000th LHC dipole (out of 1232) September 5th 2005

  28. 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

  29. 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

  30. 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.

  31. 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.

  32. 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.

  33. 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.

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