Collider physics Introduction Some e + e - collider physics R(e + e - hadrons/ e + e - µ - µ + ) Z 0 and W at LEP Hadronic machines Total cross sections Hard and soft collisions Triggers An example: LHCb C4 Lecture 6 - Jim Libby 1
Why collide? Total centre-of-mass energy for proton beam on a fixed H target (p-p collision): Same beam energy but p and p colliding in lab frame The ratio: s s CM Fixed 2 2 s = ( E + m ) p = 2m + 2m FIXED 2E m 120 p p LAB p when m CM p s = << for 7 TeV protons p E p 2 4E p. (energy of Physics beyond the Standard Model lives at high mass therefore require highest energy possible Some other advantages (of many) Focussed high intensity e + e - collisions-clean with a tunable CoM energy 4π detectors Recycle bunches of particles fixed target is single shot 2 p LHC) p E p C4 Lecture 6 - Jim Libby 2
e + e - ep Types of collider LEP, CERN, Switzerland-(Electroweak physics) s = 90-200 GeV SLC, SLAC, US - s = 90 GeV (Linear collider) PEP-II, SLAC, US and KEK-B, KEK, Japan B factories s = m(ϒ(4s)) DAFNE, Frascati, Italy-K factory s = m(φ) CESR, Cornell, US-Charm and B factory VEPP, Novisibirsk, Russia and BES, Beijing, China light meson spectroscopy ILC,???,!!!!! - s = 500-1 TeV HERA, DESY-Deep Inelastic Scattering which probes proton structure proton - antiproton: pp SppS, CERN-Discovery of W and Z Tevatron, Fermilab-Current energy frontier machine s~1.8 TeV LHC, CERN- s = 14 TeV (starts 2008) Heavy Ion (Au-Au) Collisions RHIC, Brookhaven, US and LHC -Quark-Gluon Plasma Defunct Running Future Oxford collab. C4 Lecture 6 - Jim Libby 3
The total e + e - hadron cross section 10-2 10-3 ω φ J/ψ ψ(2s) 10-4 ρ ρ Z 10-5 10-6 σ mb 10-7 10-8 1 10 10 2 s 1/2 (GeV) e + e - qq hadrons no free quarks The quark level process can be compared to e + e - µ - µ + σ(e + e - µ - µ + ) = 4πα 2 /3s (88 nb)/s for s<<m z C4 Lecture 6 - Jim Libby 4
+ σ ( e e hadrons) R( s) = + + σ ( e e µ µ ) Calculated 10 3 R J/ψ ψ(2s) Z 10 2 ω φ 10 ρ 1 ρ 10-1 R = n 2 q Below charm threshold : n c c Q S GeV 1 10 10 2 where n = 3 is consistent with the data c is the number of colours and Q R = n Q c q= u, d, s 2 q = 2 3 n c q are the charges of the quarks produced. Above bottom threshold : R = n Q 2 c q q= u, d, s, c, b = 11 9 n c C4 Lecture 6 - Jim Libby 5
R in uds region C4 Lecture 6 - Jim Libby 6
R in charm region C4 Lecture 6 - Jim Libby 7
R in bottom threshold region C4 Lecture 6 - Jim Libby 8
LEP and the Z0 e - f Z 0 f=q, l - and ν e + Experiments at the LEP collider performed precise measurements of the total cross section near the Z 0 resonance One of several important results is that the resonance is compatible with only 3 neutrinos within the Standard Model Also, measurements are precise enough to probe loop effects Indirectly constrain new physics and mass of the Standard Model Higgs boson f Z 0 t b t b W C4 Lecture 6 - Jim Libby 9 Z 0 H Z 0 b b b
LEP and the W e - Z 0 /γ W + σ WW (pb) 30 20 LEP PRELIMINARY 11/07/2003 e + e - e + LEP also ran at energy above threshold for WW production Measured cross sections and W mass W mass important constraint on Higgs boson ν W + W - W - m W [GeV] 80.5 80.4 80.3 10 0 YFSWW/RacoonWW no ZWW vertex (Gentle) only ν e exchange (Gentle) 160 180 200 LEP1, SLD data LEP2 (prel.), pp data 68% CL m H [GeV] 114 300 1000 s (GeV) 150 175 200 C4 Lecture 6 - Jim Libby 10 m t [GeV] α
Multiplicity measurements Test and tune models of hadronisation These operate in a non-perturbative regime of strong interactions which is very hard/impossible to calculate At Z 0 multiplicity of different types of mesons is: Meson π ± <Multiplcity>/event π 0 16.99±0.27 K ± 9.42±0.32 K 0 2.242±0.063 D ± 2.049±0.026 D 0 0.175±0.016 0.454±0.030 B ± /B 0 0.165±0.026 N ch 40 35 30 25 20 15 10 5 e + e data p(p)-p data e ± p data γ γ 2, MARK I bubble chambers LENA JADE, TASSO CLEO HRS, TPC H1, ZEUS UA5 0 1 10 10 2 10 3 s (GeV) C4 Lecture 6 - Jim Libby 11 ISR ALEPH, DELPHI, L3, OPAL AMY MARK II
proton proton collision 10s of mb compared to nb in e+e- Resonance structure at low centre-of-mass energies over falling cross section Rising smoothly above s~10 GeV parton-parton collisions, in particular gluon gluon collisions contribute C4 Lecture 6 - Jim Libby 12
Proton antiproton collisions No purely elastic (pbar p->pbar p) regime in pbar p collisions because of annihilation C4 Lecture 6 - Jim Libby 13
Pion proton collisions resonances Rising at large p C4 Lecture 6 - Jim Libby 14
Hard and soft collisions Initial state radiation p Underlying event Outgoing parton-high p t pbar Underlying event Outgoing parton-high p t Final state radiation Hard scattering of partons within proton and (anti)proton yield two high transverse momentum (component perpendicular to beam direction) partons. Heavy intermediate states or high momentum transfer high Q 2 Drell Yan production of Z 0 /γ which then decays to a q anti-q pair gluon gluon fusion to produce bottom or top quarks (m t ~175 GeV/c) The remains of the breakup of the proton and antiproton form the underlying event (unlikely to have high p t ) Initial and final state radiation of gluons or photons contribute to the hardscattering process C4 Lecture 6 - Jim Libby 15
Hard and soft collisions Soft collisions: σ exp(-αp t ) Hard collisions: σ 1/p t 4 Dependence similar to Rutherford scattering Hard collisions a tiny piece of the total cross section Typical p t : Hard greater than ~3 GeV/c Soft ~0.3 GeV/c QCD models used in simulations of proton- (anti)proton collisions tuned on data Events selected in well understood regions of experiments with high efficiency for finding all tracks Central or low rapidity Pseudorapidity=η= -ln tan (θ/2) a Lorentz invariant quantity dη) (nb/gev) σ /(de T 2 d 10 4 10 3 10 2 10 1 10-1 10-2 10-3 10-4 10-5 10-6 10-7 R807 (pp at 45 GeV, η=0) R807 (pp at 63 GeV, η=0) UA2 (pp at 630 GeV, η <0.85) UA1 (pp at 630 GeV, η <0.7) CDF (pp at 540 GeV, 0.1< η <0.7) D (pp at 630 GeV, η <0.5) D (pp at 1.8 TeV, 0.1< η <0.7) CDF (pp at 1.8 TeV, 0.1< η <0.7) NLO-QCD, µ=, CTEQ4HJ 2 10 10 2 E T (GeV) C4 Lecture 6 - Jim Libby 16 E T
Triggering at hadron colliders At hadron machines: # of interesting events/# of bunch crossings<<1 interesting events are all hard, high transverse momentum Bunch crossings happen at a rate of 40 MHz at LHC Hardware limitations mean you cannot store all events to analyse at you leisure later Can only write at O(1 khz) to disk Limited storage space: each event 100 kbytes. (LHCb will have 40 10 13 events/year Limited CPU time for offline analysis Need to select/trigger interesting events in real time Rough analysis performed using a limited set of detectors components to make the decision whether to keep or discard an event C4 Lecture 6 - Jim Libby 17
Example: LHCb y 5m LHC experiment to collect large samples of events that contain bottom quarks Study CP violation the difference between matter and antimatter Vertex Locator TT Particle ID Magnet T1 T2T3 Electron, photon and hadron calorimetry RICH2 ECAL SPD/PS M1 HCAL M2 M3 M4 M5 Muon identification Precise tracking vertexing, impact parameters 5m Momentum measurement 10m 15m 20m C4 Lecture 6 - Jim Libby 18 z
Simulated LHCb events C4 Lecture 6 - Jim Libby 19
LHCb trigger architecture Several stages ( levels ) of the trigger with more complex and hence slower algorithms are run sequential If an event fails at any stage it is rejected Outline of the stages 1. Select single beam interactions at a rate of 10 MHz suppression by a factor of 4 On detector electronics perform first processing. There is a 4µs (160 clock periods) fixed wait ( latency ) for decision. All data collected in this period is stored in on-detector electronics ( pipelines ). Accept rate 1 MHz suppression by a further factor of 10. 2. All data transferred via fibre optic cables to a counting room 100 m away from the detector, which has far less radiation than around the detector. (Electronics simpler and cheaper.) This room is equipped with 1000s of CPUs, which perform the first analysis generic quantities then more de dedicated algorithms run for specific modes of interest. Variable time for decision. Events written to disk for offline analysis at ~2 khz suppression by a factor of 500. C4 Lecture 6 - Jim Libby 20
LHCb trigger algorithms Single beam crossings 2D primary vertex finding each primary vertex corresponds to a pp interaction Reject events with 2 or more because they are harder to analyse On-detector 1 st level: Recognise high transverse momentum µ ±, e ±, γ and hadrons in muon system and calorimeters. Signature of a hard scatter producing a b quark. C4 Lecture 6 - Jim Libby 21
LHCb trigger algorithms Off-detector 2 nd level: All subsystems data available for reconstructionbut first: B decays have a relatively long lifetime of 1.2 ps, so they fly some distance (cτ=360 µm) from primary vertex. Therefore, identify tracks with large impact parameter (originated from a decay some distance from the primary) Make crude transverse momentum measurement of these and associate with 1 st level µ ±, e ±, γ and hadrons. If OK detailed analysis with reconstruction of tracks for precise momentum and mass measurements and Pion and kaon ID to select decays of most interest 2 nd stage are C++ algorithms which are easy to evolve over time. Only 1 st level fixed when building the experiment C4 Lecture 6 - Jim Libby 22