Luminosity measurement at LHC

Transcription

1 Luminosity measurement at LHC Corsi di Dottorato congiunti BO-FE-PD Corso di Fisica delle Alte Energie Aprile 2012 Per Grafstrom CERN

2 Che cosa è la luminosita? Organizzazione Perche misurare la luminosita? Methods for absolute measurements Processes with know cross section Machine parameters Elastic scattering Relative measurements 2

3 Che cosa è la luminosita? 3

4 In Astronomia? Electromagnetic Energy radiated per unit time The luminosity of the sun W 1 Solar unit 4

5 In Accelerator Physics? Instantaneous Luminosity : Number of particles colliding per unit area and unit time Integrated Luminosity: The time integral over a well defined period 5

6 Che cosa è la luminosita? Organizzazione Perche misurare la luminosita? Methods for absolute measurements Processes with know cross section Machine parameters Elastic scattering Relative measurements 6

7 Luminosity measurements-why? Cross sections for Standard processes t-tbar production W/Z production. Theoretically known to better than 10% will improve in the future New physics manifesting in deviation of σ x BR relative the Standard Model predictions. Precision measurement becomes more important if new physics not directly seen (characteristic scale too high!) Important precision measurements Higgs production σ x BR tanβ measurement for MSSM Higgs. 7

8 Luminosity Measurement (cont.) Examples Higgs coupling tanβ measurement Relative precision on the measurement of σ H BR for various channels, as function of m H, at Ldt = 300 fb 1. The dominant uncertainty is from Luminosity: 10% (open symbols), 5% (solid symbols). (ATLAS-TDR-15, May 1999) Systematic error dominated by luminosity (ATLAS Physics TDR ) 8

9 Strategy: Absolute vs relative measurement 1. Measure the absolute luminosity with a precise method at optimal conditions 2. Calibrate luminosity monitor with this measurement, which can then be used at different conditions Luminosity Monitoring i.e. relative measurements: Using suitable observables in existing detectors Use dedicated luminosity monitors either provided by the experiments or by the machine Today we will mainly discuss methods for Absolute Luminosity Measuremet. but relative measurements are very important.. 9

10 Relative measurement we will come back to this./ 10

11 Absolute Luminosity Measurements Goal: Measure L with 3% accuracy (long term goal) How? Three major approaches LHC Machine parameters Rates of well-calculable processes: e.g. QED (like LEP), EW and QCD Elastic scattering Optical theorem: forward elastic rate + total inelastic rate: Luminosity from Coulomb Scattering Hybrids Use σ tot measured by others Combine machine luminosity with optical theorem We better pursue all options 11

12 Che cosa è la luminosita? Organizzazione Perche misurare la luminosita? Methods for absolute measurements Processes with known cross section Machine parameters Elastic scattering Relative measurements 12

13 Muon pairs Two photon production of muon pairs-qed γ γ p p µ µ Pure QED Theoretically well understood No strong interaction involving the muons Proton-proton re-scattering can be controlled Cross section known to better than 1 % 13

14 Muon pairs Two photon production of muon pairs P t > 3 GeV to reach the muon chambers P t >6 GeV to maintain trigger efficiency and reasonable rates µ - φ Centrally produced η < 2.5 P t (µµ) MeV Close to back to back in ϕ (background suppression) µ + 14

15 Muon pairs Strong interaction of a single proton Backgrounds Strong interaction between colliding proton Di-muons from Drell-Yan production Muons from hadron decay 15

16 Muon pairs Event selection-two kind of cuts Kinematic cuts P t of muons are equal within 2.5 σ of the measurement uncertainty Suppresses efficiently proton excitations and proton-proton re-scattering Good Vertex fit and no other charged track Suppress Drell-Yan background and hadron decays 16

17 Muon pairs What are the difficulties? The rate The kinematical constraints σ 1 pb A typical /cm 2 /sec year 6 fb -1 and 150 fills 40 events fill Luminosity MONITORING excluded What about LUMINOSITY calibration? 1 % statistical error more than a year of running Efficiencies Both trigger efficiency and detector efficiency must be known very precisely. Non trivial. Pile-up Running at /cm 2 /sec vertex cut and no other charged track cut will eliminate many good events CDF result First exclusive two-photon observed in e + e -.. but. 16 events for 530 pb -1 for a σ of 1.7 pb overall efficiency 1.6 % Summary Muon Pairs Cross sections well known and thus a potentially precise method. However it seems that statistics will always be a problem. 17

18 W and Z W and Z counting y ( W ± ) Z µµ 2 Hz at η ( l ± ) W µν 20 Hz at

19 W and Z W and Z counting Constantly increasing precision of QCD calculations makes counting of leptonic decays of W and Z bosons a possible way of measuring luminosity. In addition there is a very clean experimental signature through the leptonic decay channel. Use W in this discussion. σ (W) x BR(W lν) has more favourable rate. The rate is 10 x σ (Z) x BR(Z ll ). The Basic formula L = (N - BG)/ (ε x A W x σ th ) L is the integrated luminosity N is the number of W candidates BG is the number of back ground events ε is the efficiency for detecting W decay products A W is the acceptance σ th is the theoretical inclusive cross section 19

20 W and Z How to select events and eliminate background(n-bg) QCD background and heavy quarks Z e + e - where the second lepton is not identified Z τ + τ - where one τ decay in the electron channel Pseudorapidity η < 2.4 (no bias at edge) P t > 25 GeV (efficient electron ident) ttbar background Missing E t > 25 GeV W τ l ; τ decaying in the electron channel No jets with P t > 30 GeV (QCD background) 20

21 W and Z Uncertainties on σ th σ th is the convolution of the Parton Distribution Functions (PDF) and of the partonic cross section The uncertainty of the partonic cross section is available to NNLO in differential form with estimated scale uncertainty below 1 % (Anastasiou et al PRD 69, ) PDF s more controversial and complex 21

22 Basic processes at LHC q Jet q Jet Experimental Methods in Particle Physics 22

23 W and Z NNLO Calculations Bands indicate the uncertainty from varying the renormalization (µ R ) and factorization (µ F ) scales in the range: M Z /2 < (µ R = µ F ) < 2M Z Anastasiou et al., Phys.Rev. D69:094008, 2004 At LO: ~ % x-s error At NLO: ~ 6 % x-s error At NNLO: < 1 % x-s error Perturbative expansion is stabilizing and renormalization and factorization scales reduces to level of 1 % 23

24 W and Z x and Q 2 range of PDF s at LHC Sensistive to x values 10-1 > x > x10-4 Sea quarks and antiquark dominates g qqbar Gluon distribution at low x HERA result important 24

25 W and Z Sea(xS) and gluon (xg) PDF s PDF uncertainties reduced enormously with HERA. Most PDF sets quote uncertainties implying error in the W/Z cross section < 5 % However central values for different sets differs sometimes more! 25

26 W and Z Uncertainties in the acceptance A W The acceptance uncertainty depends on QCD theoretical error. Generator needed to study the acceptance The acceptance uncertainty depends on polarisation of W and on PDF s Uncertainty estimated to about 2 % Uncertainties on ε Uncertainty on trigger efficiency for isolated leptons Uncertainty on lepton identification cuts 26

27 W and Z Summary W and Z W and Z production has a high cross section and clean experimental signature making it a good candidate for luminosity measurements. The biggest uncertainties in the W/Z cross section comes from the PDF s. This contribution is sometimes quoted as big as 8 % taking into account different PDF s sets. Adding the experimental uncertainties we end up in the 10 % range. The precision might improve considerable if the LHC data themselves can help the understanding of the differences between different parameterizations The PDF s will hopefully get more constrained from early LHC data. Aiming at 3-5 % error in the error on the Luminosity from W/Z cross section after some time after the LHC start up 27

28 Che cosa è la luminosita? Organizzazione Perche misurare la luminosita? Methods for absolute measurements Processes with know cross section Machine parameters Elastic scattering Relative measurements 28

29 Machine parameters Luminosity from Machine parameters Luminosity depends exclusively on beam parameters: Depends on f rev revolution frequency n b number of bunches N number of particles/bunch σ* beam size or rather overlap integral at IP Luminosity accuracy limited by The luminosity is reduced if there is a crossing angle ( 300 µrad ) 1 % for β* = 11 m and 20% for β* = 0.5 m extrapolation of σ x, σ y (or ε, β x *, β y *) from measurements of beam profiles elsewhere to IP; knowledge of optics, Precision in the measurement of the the bunch current beam-beam effects at IP, effect of crossing angle at IP, How precise can one measure the different parameters? 29

30 Machine parameters Use special calibration runs Calibration runs i.e calibrate the relative beam monitors of the experiments during dedicated calibration runs. Calibration runs with simplified LHC conditions to increase the precision Reduced intensity Fewer bunches No crossing angle Larger beam size. Simplified conditions that will optimize the condition for an accurate determination of both the beam sizes (overlap integral) and the bunch current. 30

31 Definition of σ vis the calibration constant 31

32 What is needed 32

33 Beam separation scans- VDM scans 33

34 Simon van der Meer Nobel Prize in 1984 for the contributions That led to the discoveries of the W and Z) (shared with Carlo Rubbia) Van der Meer s crucial contribution was the stochastic cooling for accumulating enough anti-protons in conditions to be accelerated later in the SPS together with protons to provide the 630 GeV collisions needed to discover the W and Z Particle Physics and Philosophy Maria in der Aue, March 2011, P. Jenni (CERN) Experimental Methods in Particle Physics 34

35 35

36 Precision NOW 2012 Bunch current about 0.5 % and total about 2 % 36

37 Che cosa è la luminosita? Organizzazione Perche misurare la luminosita? Methods for absolute measurements Processes with know cross section Machine parameters Elastic scattering Relative measurements 37

38 Optical theorem Elastic scattering and the Optical theorem The optical theorem relates the total cross section to the forward elastic rate σ tot = 4π Im f el (0) Thus we need Extrapolate the elastic cross section to t=o Measure the total rate Use best estimate of ρ ( ρ ~ % in L/L ) 38

39 Derivation of 39

40 Optical theorem What is required dn el /dt t =0 requires small t ~ 0.01 GeV 2 θ ~15 µrad ( nominal divergence is 32 µrad ) beam with smaller divergence large β* ~ 1000 m (divergence 1/ β* ) Zero crossing angle fewer bunches Special run at low luminosity N tot : need large coverage detectors to make accurate extrapolation over the full phase space (98% coverage requires η up 7-8 ) 40

41 Optical theorem Elastic Scattering 14 TeV exponential region Slide from M.Diele TOTEM squared 4-momentum transfer 41

42 Optical theorem TOTEM s Baseline Optics: β * = 1540 m Model-dependent systematic error of extrapolation of the elastic cross-section to t = 0: Uncertainty < 1 % (most cases < 0.2 %) t min(fit) = GeV 2 experimental systematics: % Slide from M.Diele TOTEM 42

43 Optical theorem Measurement of the Total Rate N el + N inel Trigger Losses σ [mb] T1/T2 double arm trigger loss [mb] T1/T2 single arm trigger loss [mb] Systematic error after extrapolation [mb] Minimum bias Single diffractive using β* = 1540, 90 m Double diffractive Double Pomeron Elastic Scattering (2) using β* = 1540, 90 β* = 1540 (90) m Extrapolation of diffractive cross-section to large 1/M 2 using dσ/dm 2 ~ 1/M 2. Total: 0.8 mb 0.8 β* = 1540 m 2 5 mb 2 5 β* = 90 m Loss at low diffractive masses M Acceptance single diffraction detected simulated extrapolated Slide from M.Deile TOTEM 43

44 Optical theorem The total cross section γ =2.2±σ ) (best fit) σ tot vs s and fit to (lns) γ γ =1.0 44

45 Optical theorem Summary optical theorem Measurements of the total rate in combination with the t-dependence of the elastic cross section is a well established and potentially powerful method for Luminosity calibration. Error contribution from extrapolation to t=0 < 1 % (theoretical and experimental) Error contribution from total rate ~ 0.8 % 1.6 % in luminosity Error from ρ ~ 0.5 % Luminosity determination of 2-3 % might be in reach Ultimate goal stated by TOTEM: Measurement of L and σ tot with Optical Theorem at the 1 % level. 45

46 Coulomb Elastic scattering at very small angles Measure elastic scattering at such small t-values that the cross section becomes sensitive to the Coulomb amplitude Effectively a normalization of the luminosity to the exactly calculable Coulomb amplitude No total rate measurement and thus no additional detectors near IP necessary UA4 used this method to determine the luminosity to 2-3 % 46

47 Coulomb Elastic scattering at very small angles 47

48 Coulomb What is required Need closest possible approach to the beam Need to measure extremely small angles using detectors in Roman pots far away from the IP Coulomb amplitude Strong amplitude for t= gev 2 This corresponds to 3.5 µrad -The Coulomb region at the collider at 120 µrad Two factors make it harder at the LHC Momentum larger ; t = (p θ) 2 factor 25 Cross section larger factor

49 Coulomb How to measure such small angles? Use optics with parallell to point from IP to detector and then measure the distance of the scattered particles from the beam axis and use Roman Pots far away from the IP to come as close as possible to the beam 49

50 Coulomb The Roman Pots Roman Pot Concept 50

51 ALFA Test beam Stations Alignment Vacuum Detectors Survey RP movement DCS TDAQ Commissioning Trigger Plans for 2011 The ALFA detector system ATLAS Elastic scattering 240 m 240 m Approach the beam to few mm Beam pipe Beam pipe Edge less detector NIELS BOHR INSTITUTE UNIVERSITY OF COPENHAGEN ATLAS week ALFA status and plans for 2011 Sune Jakobsen 2/17

52 ALFA Test beam Stations Alignment Vacuum Detectors Survey RP movement DCS TDAQ Commissioning Trigger Plans for 2011 Feedback from test beam of all 8 detectors Edge reconstruction efficiency Resolution without and with using non-hit layer σ = 46 µm σ = 30 µm Understanding the detector rotations improves resolution Including information of fiber layer with no hit improves resolution NIELS BOHR INSTITUTE UNIVERSITY OF COPENHAGEN ATLAS week ALFA status and plans for 2011 Sune Jakobsen 3/17

53 Coulomb Summary - Coulomb Getting the Luminosity through Coulomb normalization will be extremely challenging due to the small angles and the required closeness to the beam. Main challenge is not in the detectors but rather in the required beam properties Will the optics properties of the beam be know to the required precision? Will it be possible to decrease the emittance as much as we need? Will the beam halo allow approaches in the mm range? UA4 achieved a precision using this method at the level of 2-3 % but at the LHC it will be harder... 53

54 Che cosa è la luminosita? Organizzazione Perche misurare la luminosita? Methods for absolute measurements Processes with know cross section Machine parameters Elastic scattering Relative measurements 54

55 Overall conclusions on absolute measurements We have looked at the principle methods for luminosity determination at the LHC Each method has its weakness and its strength Accurate absolute luminosity determination is difficult We better exploit different options in parallell 55

56 Relative measurements In Principle one can use anything that is sensitive to the inelastic pp interactions as a source for relative measurements! However a big advantage to also have a dedicated Luminosity Monitor. STABILITY 56

57 Example from ATLAS - LUCID 57

58 58

59 What to measure? Need to measure µ vis for a given detector and algoritm. Examples from LUCID EVENT AND EVENT OR HIT The most suitable algoritm depends on factors like : µ-range, background. Small µ easy 59

60 EVENT counting 60

61 Due to pile up µ vis is not linear 61

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63 Back up 63

64 Alternative method for beam size measurement 64