LHCb status and physics

Transcription

1 Phenomenology club Theory Division, CERN June 13, 2002 LHCb status and physics Point 5 Olivier Schneider Olivier.Schneider@iphe.unil.ch CMS LHC Point 2 ALICE Point 1 ATLAS Point 8 LHCb a dedicated b physics experiment at CERN s Large Hadron Collider to perform precision measurements of CP violation and rare decays

2 LHCb collaboration O(500) participants from 45+3 institutes (Sept 2001) France: Annecy, Clermont-Ferrand, CPPM Marseille, LAL Orsay Germany: Tech.Univ. Dresden, Phys.Inst.Univ. Heidelberg, KIP Univ. Heidelberg, MPI Heidelberg Italy: Bologna, Cagliari, Ferrara, Firenze, Frascati, Genoa, Milan, Univ. Rome I (La Sapienza), Univ. Rome II (Tor Vergata) Netherlands: NIKHEF Poland: Cracow Inst. Nucl. Phys. & Tech. Univ. of Mining and Metallurgy, Warsaw Soltan Inst. Spain: Univ. Barcelona, Univ. Santiago de Compostela Switzerland: CERN, Univ. Lausanne, Univ. Zurich UK: Univ. Bristol, Univ. Cambridge, Univ. Edinburgh, Univ. Glasgow, IC London, Univ. Liverpool, Univ. Oxford, RAL Brazil: UFRJ, CPBF China: IHEP (Beijing), Tsinghua Univ. Russia: BINP, INR, ITEP, IHEP, PNPI Romania: IFIN-HH (Bucharest) Ukraine: Inst. Phys. Tech. (Kharkov), Inst. Nucl. Research (Kiev) Tech. ass: Espoo-Vantaa Inst. Tech. (Finland), Geneva Engineering School (Switzerland), CEFET-RJ (Brazil)

3 LHCb s physics goal Precision measurements of all parameters of the CKM matrix (in particular sides and angles of unitarity triangles) high statistics many different decay modes of B 0, B s,... Thorough test of CKM picture Is Standard Model the only source of CP violation? Is there new physics affecting b-hadron decays (Penguin loops) or B mixing (boxes,...)? V CKM V ud Vus V ub = Vcd Vcs V cb Vtd Vts V tb 1 λ2 / 2 λ Aλ3( ρ iη) = λ 1 λ2 / 2 Aλ2 + O( λ4) Aλ3( 1 ρ iη) Aλ2 1 η=η(1 λ 2 /2) η 0 ηλ 2 0 Im λ V cb (1 λ 2 /2)V ub Im V ub λ V cb γ α ρ=ρ(1 λ 2 /2) γ ρ V ud V ub + V cd V cb + V td V tb = 0 V td λ V cb β V tb V ub + V ts V us + V td V ud = 0 (1 λ 2 /2)V td λ V cb δγ V ts V cb 1 1 λ 2 /2+ρλ 2 argv ub = γ argv td = β Re argv ts = π+δγ Re

4 Pre-LHC players and programme Experimental results from Tevatron Run IIa: CDF + D0, each 2 fb -1 Asym. B factories: BABAR + Belle, each fb -1 Equivalent to several 10 8 B mesons Direct measurements of angles of unitarity triangle: σ(sin(2β)) 0.03 from B 0 J/ψ K S asymmetry no precise measurement of other angles Knowledge on sides of unitarity triangle σ( V cb ) few % error σ( V ub ) 5-10 % error σ( V td / V ts ) few-5 % error (assuming m s < 40 ps -1 ) 1 η sin 2 tan 1 ρ indirectly known to < 0.03

5 CKM triangle in 5 years (SM) η β from B J/Ψ K S ( 1 ρ) 2+ η 2 from m m d s 0 ρ2 η 2 from Γ( b u) Γ ( + b c) (ρ,η) 0 1 First stringent test of CKM ansatz! Sides dominated by theoretical uncertainties More statistics would improve sin(2β) or CKM picture might already appear inconsistent ρ

6 CKM triangle in 5 year (new physics) η β + new physics ( 1 ρ) 2+ η 2 + new physics 0 ρ + η 2 2 (ρ,η) γ 0 1 A precise measurement of γ, independent of possible new physics in the mixing, is needed to get the true CKM parameters (and provide evidence for the new physics) β Assume: additional diagrams for B mixing due to new physics; SM tree decays unaffected. ρ

7 Measuring CKM angle γ (and possible new physics in B mixing) Γ(b u)/ Γ(b c) CP asym. in B s D s+ K CP asym. in B s D s K + CP asym. in B s J/ψ φ J/ψ η CP asym. in B d D + π CP asym. in B d D π + CP asym. in B d J/ψ K S m d γ 2δγ eff 2δγ eff γ + 2β eff 2β eff η 2 +ρ 2 γ atan(η/ρ) γ = 2 atan(η/(1 ρ)) + ϕ new η 2 +(1 ρ) 2 + r new SM values of η and ρ new physics parameters r new and ϕ new Cannot be measured at current experiments Need much higher statistics, including B s decays, and PID LHCb!

8 b production at LHC Huge cross section (but large theoretical uncertainty on prediction!): assume σ(pp bb) =500 µb at s=14 TeV strong angular correlation (bb pair either forward or backward) High luminosity LHC design: cm -2 s -1 LHCb choice to run at 2 x cm -2 s -1 to get mostly events without pileup (single pp interactions) All b hadron species produced B u (40%), B d (40%), B s (10%), B c, and b-baryons (10%) Large b-hadron momentum, hence boost Drawback: huge background non-bottom background: σ bb /σ inelastic 0.6% bottom background (want to study channels with visible BR as low as 10-7 ) bb pairs/ year 1 million every 10s 3 θb [rad] 2 1 Main experimental issues: trigger particle id. (e.g. K/π sep.) resolution (mass, proper time) θ b [rad]

9 LHCb detector (side view, end 2001) 1m B Forward acceptance: 1.9 < η < 4.9 Trigger: sensitive to B final states with and without leptons Particle identification: RICH K/π separation for 1 < p < 150 GeV/c Vertexing (Si): σ t =40 fs (for B s D s π) ~ 1/ m s

10 Particle identification LHCb K π separation for true π Extremely important for many exclusive channel Example: clean up of B 0 π + π signal Events / 20 MeV/c B d ππ B d πk B s πk B s KK Λ b pk Λ b pπ No RICH Events / 20 MeV/c B d ππ B d πk B s πk B s KK Λ b pk Λ b pπ LHCb With RICH ATLAS Invariant mass [ GeV/c 2 ] Invariant mass [ GeV/c 2 ]

11 LHCb trigger scheme L0 L1 L2 L3 40 MHz Medium p T hadron,µ,e,γ + pileup veto 1 MHz Detached vertex + IP of p T candidate* 40 khz Final state reconstruction 200 Hz (12.4 MHz of inel. interactions) calo+µ+pileup veto reduction ~40 (only ~10 from p T cut) vertex detector + L0 reduction ~25 * added since TP all detectors reduction ~ 8 x 25 Efficiencies on signal events passing offline selection (TP) L0 L1 L2 total B 0 J/ψ K S B 0 π + π B s D s K Muon chambers Calorimeters 3.6 k channels Primary vertex candidates 40 MHz 26 k channels High PT µ ± candidates Level-0 decision unit latency 4 µs RICH Tracker... L0 readout <1 MHz> 20k channels High ET hadron candidates e ± candidates γ candidates Level-0 yes Level-0 information 5 m 0 m Magnet... Readout Supervisor Level-1 yes L1 readout <40 khz> 205 k channels Detached vertices Level-1 decision unit latency 1.7 ms... Shield RICH Pileup detector... Vertex detector <1 MHz>

12 MC generation issues probability probability minimum bias events bb events Default PYTHIA (TP, 1998) 1.8 < η < minimum 60 bias 80 events 100 charged bb events multiplicity, N ch Tuned PYTHIA (since 1999) 1.8 < η < charged multiplicity, N ch (normalized distributions) Track multiplicity and p T distributions: will affect the performance of the trigger very important to have now realistic MC for signal and minimum bias events to assess trigger performance using PYTHIA 6 with multi-parton interactions model tuned on track multiplicities observed at lower energy (SPS, Tevatron) in non-single diffractive events at η ~ 0 minimum bias events bb events Tuned PYTHIA (since 1999) maximum P T (GeV/c) How reliable are these predictions? Should we use also different generators/tuning? (Which ones?)

13 LHCb Progress in experiment design... LHCb LHCb now under construction Technical Proposal Feb 1998 Dec 1999 Sept 2000 Sept 2000 May 2001 May 2001 Sept 2001 Dec 2001 O. Schneider, June 13, 2002 LHCb status and physics

14 Re-optimization Reducing secondary interactions ( detector occupancy): new beam pipe design: 25 mrad cone (Be?) + 10 mrad cone (Al-Be) Reducing material ( γ conversion, Bremsstrahlung, and hadron absorption!) hope to gain close to a factor 2 with new LHCb-light design LHCb heavy (TDRs) Radiation length (X 0 ) Interaction length (λ I ) Vertex detector 19% 4% RICH 1 14% 5% 9 tracking stations 27% 11% Total (before RICH2) 60% 20% Pattern recognition in the tracking not adressed until outer tracker TDR (e.g. assumed perfect in TP) effect on tracking design (long tracks, short tracks,...) Improving L1 trigger performance could be done if some rough momentum estimate was available for the L1 decision

15 Re-optimized detector (side view) To de described in a forthcoming TDR: B New beam pipe, less stations in VELO, less material in RICH1 Change in tracking philosophy: no stations in the magnet! Shielding wall removed: let B field extend in upstream region ( need local shielding of RICH1 photodetectors) New (Si) tracking station TT1 in L1 trigger (also useful for K S finding)

16 Re-optimization of L1 trigger retention on L0 accepted events TP (old generator) Using VELO+TT1+B field (20% momentum resolution): require one or two large impact parameter tracks with minimum p T Big performance improvement: affects all hadronic channels (B 0 π + π, B s D s K,...) efficiency for dimuon channels now > 90% (using L0 info at L1) Since efficiency is high (and steep curve ), might reduce L1 output rate need to optimize with L2 Trigger TDR foreseen early 2003 efficiency for B 0 π + π signal

17 New tracking Abandon continuous tracking à la HERAB Several pattern recognition approaches under study find tracks in VELO, extrapolate though magnet, and match with hits after the magnet (see figure) finds tracks after magnet (seeding stations) and match with VELO tracks find K S decaying outside VELO using TT1 hits Will reach typical efficiencies of 90% for long tracks (or more if from B decay), with reasonable ghost rate Efficiency Ghost rate

18 Short term plans (for the rest of 2002) Finalize optimized detector design Tune reconstruction and trigger algorithms Generate MC new samples Re-assess reconstruction and trigger performance Update events yields and signal/background ratios for benchmark channels Improvements will include: realistic design, including all material simulation of pileup and spillover new trigger algorithm & proper simulation of trigger full pattern recognition and reconstruction algorithms totally new software framework Forthcoming TDRs inner tracker optimization/performance trigger adding realism + real improvements!

19 Event yields Signal yields per year (10 7 s): triggered, reconstructed & selected untagged Flavour tagging (TP, 1998): only b l and b c s opposite side tags simple ranking 40% efficiency, 30% mistag Large uncertainties: cross section, BRs background estimates lack of MC stat., assumptions,... Future: Get more solid (realistic) estimates by end 2002 Can then start/repeat more refined physics performance studies Annual signal yield S/B Useful for Ref. Year B d J/ψ (µ + µ ) K S 73k 7 β YR 2000 B d J/ψ (e + e ) K S 15k 2 β YR 2000 B d D* (excl) π + 180k 6 γ+2β PhD 2001 B d D* (incl) π k 4 γ+2β PhD 2001 B s J/ψ (µ + µ ) φ 74k 30 δγ YR 2000 B s D s K 6k 12 γ 2δγ TP 1998 B d π + π 12k >1 α,γ YR 2000 B s K + K 12k γ YR 2000 B d ρ π 3k α YR 2000 B d D 0 K* 0 0.4k 1 γ TP 1998 B s D s π + 86k 20 m s TP 1998 B d K* 0 γ 26k 1 TP 1998 B d K* 0 µ + µ 8.6k >15 PhD 2002 b s µ + µ 24k 8 V ts /V td PhD 2002 b d µ + µ 0.5k 1.3 V ts /V td PhD 2002 B s µ + µ 16 S/ B=3.5 PhD 2002