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Physics challenges of the LHC

Physics challenges of the LHC. I. The LHC machine. II. The LHC experiments. III. B physics at the LHC. CP violation The LHCb experiment Hadron identification Expected results. XIV Swieca Summer School, Sao Paolo 29–31 January 2007. 1. CP violation.

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Physics challenges of the LHC

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  1. Physics challenges of the LHC I. The LHC machine II. The LHC experiments III. B physics at the LHC • CP violation • The LHCb experiment • Hadron identification • Expected results XIV Swieca Summer School, Sao Paolo 29–31 January 2007

  2. 1. CP violation • Recall the arguments for physics beyond the Standard Model (from before): • Dark matter • Astrophysical measurements of the rotations of galaxies indicate that normal “baryonic” matter makes up only ~ 4% of the total energy density of the Universe — what is the rest? • Gravity • Gravity is not part of the Standard ModelWhy is natural scale of gravity mP = √ħc/G ~ 1019 GeV (Planck mass) so much larger than the Electroweak scale ~ 102 GeV?Known as the “hierarchy problem” • Baryogenesis • Why is the world we observe made up almost entirely of matter,while it is expected that equal quantities of matter and antimatter were produced in the Big Bang? Hints may come from the high-pT experiments(eg Supersymmetryor extra dimensions) Connected with the field of B physics Physics challenges of the LHC (III)

  3. Baryogenesis • Big Bang (~ 14 billion years ago) → matter and antimatter equally producedFollowed by annihilation →nbaryon/ng ~ 10-10Why didn’t all the matter annihilate (luckily for us)? • No evidence found for an “antimatter world” elsewhere in the Universe • One of the requirements to produce an asymmetric final state (our world)from a symmetric matter/antimatter initial state (the Big Bang)is that CP symmetry must violated [Sakharov, 1967] • CP is violated in the Standard Model, through the weak mixing of quarksFor CP violation to occur there must be at least 3 generations of quarksSo problem of baryogenesis may be connected to why three generations exist, even though all normal matter is made up from the first (u, d, e, ne) • The way to probe CP violation is through the study of quark mixingIn particular, hadrons containing the b-quark show large CP asymmetriesHowever, the CP violation in the SM is not sufficient for baryogenesisOther sources of CP violation expected → good field to search for new physics Physics challenges of the LHC (III)

  4. Symmetries • Important concept in physics: if a system remains invariant under a continuous transformation, there is a corresponding conservation law eg invariance under spatial translation ↔ momentum conservation • Other transformations are discrete (eg reflection)Three important discrete transformations: P = parity spatial coordinates x, y, z –x, –y, –z T = time reversal time t –t C = charge conjugation particles ↔ antiparticles • Combined operation of all three discrete symmetries = CPTConservation of CPT is fundamental property of  all field theoriesGuarantees that particle has exactly the same mass as its antiparticle(tested to 1 part in 1018 by comparison of K0 and K0 masses) Physics challenges of the LHC (III)

  5. Parity violation • p0 = (uu-dd)L=0, S=0 C(p0) = +1 • B, E -B, -E C(g   • The Strong and Electromagnetic interactions conserve C, P and T • eg pion decay via the electromagnetic interaction: p0  gg but not ggg Initial state:C(p0) = +1 Final state:C(gg) = (-1)2 = +1 C(ggg) = (-1)3 = -1 • Weak interaction violates Parity(experiment of Wu et al in 1957) • Neutrinos are left-handedAntineutrinos are right-handed perhaps weak interaction conserves the combined operation, CP? eg G(p+m+nL) = G(p-m-nR) Physics challenges of the LHC (III)

  6. CP violation • Weak interaction appeared to conserve CP until the experiment of Christenson et al (1964): KL 3p (CP = -1) BR = 34%KLp+p- (CP = +1) BR = 2  10-3CP violation • BR (KLp-e+n) = 19.46% > BR (KLp+e-n) = 19.33% unambiguously differentiates matter from antimatter • In Standard Model, CP violation arises from quark mixingWeak eigenstates are “rotated” combination of flavour states V = unitary CKM matrix (Cabibbo-Kobayashi-Maskawa)Its elements give weak couplingsbetween quark flavours Physics challenges of the LHC (III)

  7. Unitarity of the CKM matrix gives relationships between rows and columns: SVijVik* = 0 (j k) • One of these relationships has terms of similar size:Vud Vub* + Vcd Vcb* + Vtd Vtb* = 0triangle relationship in the complex plane • (33) CKM matrix has 4 independent parameters:3 angles and one non-trivial phaseThe phase gives rise to CP violation — only present with  3 generations • CKM matrix observed to have a hierarchy of elementsParameterized [Wolfenstein] expanding in powers of the Cabibbo angle l = sin qC 0.22 Parameters (l, A, r, h) A 0.8, measured leaves r and h to be determinedh 0  CP violation Physics challenges of the LHC (III)

  8. h • Rescaling the “Unitarity Triangle” by Vcd Vcb*: • Many of the measurements made of hadronscontaining the b-quark can be presented as constraints on this triangle eg measurements of their lifetime VcbThe fraction of charmless b decays  Vub • Neutral B mesons oscillate between their particle and antiparticle statesvia a second-order weak transition Frequency Dm of this oscillation Vtd • In addition, CP violation in B decays measures the relative phases of the matrix elements  measure the angles (a,b,g) r “Box diagram” for B0–B0 oscillation (= CP eigenstate)Decay “via mixing” with different phase Depends on phase of B0–B0 oscillation arg(Vtd)  angle b Physics challenges of the LHC (III)

  9. B Factories • The CP violation in B0 J/y KS decays has recently been beautifullymeasured by experiments BABAR and BELLE at the B factories These are machines (in the US and Japan) running on the (4S) resonance: e+e-(4S) B0B0 or B+B- • The CP asymmetry A(t) = G(B0 J/yKS) -G(B0 J/yKS) G(B0 J/yKS) +G(B0 J/yKS) A(t) = -sin2b sinDmtin the Standard Model • BABAR+BELLE measuresin2b = 0.674 ± 0.026 • This can be compared withthe indirect measurementfrom other constraints on theUnitarity Triangle Physics challenges of the LHC (III)

  10. Triumphant agreement! The Standard Model description of CP violation appears to be correct (at least to the level it has so far been tested) Physics challenges of the LHC (III)

  11. 2. The LHCb experiment • Cross section for bb production at 14 TeV: sbb ~ 500 mbEnormous production rate at LHCb: ~ 1012 bb pairs per year! much higher statistics than the current B factories • However, sbb < 1% of inelastic cross-section more background from non-b events  challenging triggerand high energy  more primary tracks, flavour tagging more difficult • Expect ~ 200,000 reconstructed B0 J/y KS events/yearcf current B-factory samples of ~ 4000 events precision on sin 2b ~ 0.02 in one year for LHCb (similar to current world average precision) • But in addition, all b-hadron species are produced: B0, B+, Bs, Bc , Lb …In particular can study the Bs (bs) system, inaccessible at the B factories • ATLAS and CMS are also planning to do B physics but will only have a lepton trigger, and poor hadron identification Physics challenges of the LHC (III)

  12. bb events b and b quarks are produced in pairs (mostly in the forward direction) Correlation between the b and bproduction angle (PYTHIA simulation) • Need to measure proper time of B decay: t=mB L / pchence decay length L (~ 1 cm in LHCb)and momentum p from decay products (which have ~ 1–100 GeV) • Also need to tag production state of B: whether it was B or BUse charge of lepton or kaon from decay of the other b hadron in the event Physics challenges of the LHC (III)

  13. Avoiding pileup • As discussed earlier, at the nominal LHC luminosity of 1034 cm-2s-1there are ~ 25 inelastic pp interactions per bunch crossing (every 25 ns) • The superimposed events can mimic the signature of B hadrons: tracks offset from the production vertex • LHCb chooses to run at few×1032 cm-2s-1 dominated by single interactions: • Makes it simpler to identify B decays from their vertex structureand will also reduce the radiation dose(which is an issue in the forward region) • Beams are defocussed locally for LHCb  can maintain optimal luminosity even when ATLAS/CMS run at 1034 cm-2s-1 Inelastic pp collisions/crossing LHCb Physics challenges of the LHC (III)

  14. LHCb in its cavern Offset interaction point (to make best use of existing cavern) Shielding wall(against radiation) Electronics + CPU farm Detectors can be moved away from beam-line for access Physics challenges of the LHC (III)

  15. LHCb detector ~ 300 mrad p p 10 mrad  Forward spectrometer (running in pp collider mode)Inner acceptance 10 mrad from conical beryllium beam pipe Physics challenges of the LHC (III)

  16. LHCb detector  Vertex locator around the interaction region Silicon strip detector with ~ 30 mm impact-parameter resolution Physics challenges of the LHC (III)

  17. LHCb detector  Tracking system and dipole magnet to measure angles and momenta Dp/p ~ 0.4 %, mass resolution ~ 14 MeV (for Bs DsK) Physics challenges of the LHC (III)

  18. LHCb detector  Two RICH detectors for charged hadron identification (discussed in more detail below) Physics challenges of the LHC (III)

  19. LHCb detector e h  Calorimeter system to identify electrons, hadrons and neutrals Important for the first level of the trigger Physics challenges of the LHC (III)

  20. LHCb detector m  Muon system to identify muons, also used in first level of trigger Physics challenges of the LHC (III)

  21. Vertex detector • Vertex detector has silicon microstrips with rf geometryapproaches to 8 mm from beam (inside complex secondary vacuum system) • Gives excellent proper time resolution of ~ 40 fs (important for Bs decays) Beam Vertex detector information is used in the trigger Physics challenges of the LHC (III)

  22. LHCb trigger Detector 40 MHz L0: high pT (m, e, g, h) [hardware, 4ms] 1 MHz HLT: high IP, high pT tracks [software] then full reconstruction of event 2 kHz Storage (event size ~ 50 kB) L0, HLT and L0×HLT efficiency Physics challenges of the LHC (III)

  23. 3. Hadron identification RICH detector Vertex locator • RICH detectors are the specialized detectors mentioned earlierthat allow charged hadrons (p, K, p)to be identified • Important for B physics, where there are many hadronic decay modes eg Bs0→ Ds- K+→ K+ K-p- K+ • Since ~ 7 more p than K are produced in pp events, making the mass combinations would give rise to large combinatorial background unless K and p tracks can be separated • RICH = Ring Imaging CHerenkov Physics challenges of the LHC (III)

  24. Cherenkov light • Radiation produced when a charged particle travels faster than the speed of light in the medium it is passing through (refractive index n v = c/n) P. Cherenkov received the Nobel Prize in 1958 for his study of this effect • Like the bow wave of a boat travelling over alake with speed greater than the wave velocity • Light produced in a cone with cosqC = 1/bnCan be detected as a ring image if the detectoris far from radiator, or if the light is focussed By measuring qC ( radius of ring)the velocity b of the particle is foundThen with knowledge of its momentumthe mass of the particle can be found Physics challenges of the LHC (III)

  25. Photon detectors 80 mm 1000 pixels • Need to detect single photons on the rings • Novel photon detector developed for the RICH detector system of LHCb • The Hybrid Photon Detector (HPD)combines a traditional vacuum phototubewith a pixellated silicon anode Test-beam image of Cherenkovrings from 50 GeV e + p beam Physics challenges of the LHC (III)

  26. The LHCb RICH system uses three different radiator materials:Cross section of one of the detectors Typical event: complex pattern recognition! Physics challenges of the LHC (III)

  27. Hadron ID performance • Performance of the p/K separation determined using simulated events • Example of how the RICH informationcan help to isolate B0 →p+ p- decays: Physics challenges of the LHC (III)

  28. 4. Expected results • Example of an early physics measurement that is expected from LHCb:Measurement of Bs–Bs oscillationsUse channel Bs Ds-p+ • Plot made for one year of data  80,000 selected eventsfor Dms = 20 ps-1 (SM preferred)Proper time distribution for eventsproduced as Bs (rather than Bs) • Need to take care of flavourtagging, proper-time resolution,background rejection andacceptance correction • Can measure frequency accurately cf recent result Dms = 17.8 ± 0.1 ps-1 [CDF] Next step: measure the phase of the oscillation, using Bs J/yf decays (Bs counterpart of B0 J/yKS), cleanly predicted in the SM: fs = -0.04 Physics challenges of the LHC (III)

  29. Unitarity Triangle • Constraints on the Unitarity Triangle that can be expected after ~ 5 years of LHCb data (10 fb-1), if all measurements agree with the Standard Model: • Accurate measurement (to a few degrees) of the CP angle g from Bs Ds±K±, B0 DK etc • Angle a from B0p+p-p0 • In addition, phase of Bs oscillation fs measured to ±0.01, i.e. precisely enough to see SM valueand therefore any new physics enhancements Physics challenges of the LHC (III)

  30. Penguin decays • These are another category of decays involving loop diagrams New particles might appear in those loops • Some indication from the B factory experiments that their results for penguindecays do not agree with expectations might be a hint of new physics? • LHCb should reach a precision of ±0.04on the asymmetry of Bs ff ExperimentTheory Physics challenges of the LHC (III)

  31. Rare decays • Profit from the enormous statisticsto search for very rare decays such as Bs m+m-Branching ratio ~ 310-9 in the Standard Model • BR can be strongly enhanced in SUSY[G. Kane et al, hep-ph/0310042] • LHCb can reach the SM predictionin a few years SUSY models LHCb 5 BR (x10-9) SM prediction 3 Integrated Luminosity (fb-1) Physics challenges of the LHC (III)

  32. Current status in the LHCb cavern: the experiment is almost complete! Physics challenges of the LHC (III)

  33. Summary (Part III) • The mechanism of baryogenesis is one of the open issues in physicsIt requires CP violation, which is present in the Standard Model • B physics has been a fertile field for checking the Standard Model picture of CP violation, both via constraining the Unitarity Triangle and now by direct measurements • The LHCb experiment is nearing completion, to take these studies to the next level of precision, and to extend them to other B hadron systemsIt includes RICH detectors that will allow charged hadrons to be identified over a large momentum range • The search for New Physics at LHCb is complementary to ATLAS/CMS,by the precision study of the influence of new particles in loop diagrams • We are eagerly awaiting the first LHC collisions at the end of this year,and the first physics run in 2008 I hope that some of you will join us! Physics challenges of the LHC (III)

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