Experimental Aspects of Deep-Inelastic Scattering. Kinematics, Techniques and Detectors

Transcription

1 1 Experimental Aspects of Deep-Inelastic Scattering Kinematics, Techniques and Detectors

2 2 Outline DIS Structure Function Measurements DIS Kinematics DIS Collider Detectors DIS process description Dirac Cross-Section Struck quark Hadrons Mott Cross-Section Rutherford Cross-Section Electron Proton DIS Cross-Section DIS Introduction Proton remnant Scattered Electron Summary and Outlook

3 DIS Introduction 3 General considerations on scattering experiments Rutherford Hofstadter Probing smaller distances requires larger momentum transfer q (small wavelength) SLAC FNAL CERN HERA Struck quark Hadrons Measurement of the final-state (e.g. scattered electron): Structure of target! Electron Proton Scatter point-like probe onto object (target) Proton remnant Scattered Electron

4 DIS Introduction 4 Fixed-target experiment: Endstation A at Stanford Linear Accelerator Center (SLAC)

5 5 DIS Introduction Collider experiment: Electron-Proton collisions at HERA (DESY, Hamburg, Germany) Equivalent to fixed target of Ee = GeV: Ee = 27.5 GeV Ep = 920 GeV Circumference: 6.3km

6 DIS Introduction 6 DIS major event classes Neutral-current event e p e X Charged-current event e p ν X

7 DIS Introduction 7 Diffractive events Ratio of diffractive to total cross-section (200<W<245GeV): 15% at Q 2 =4GeV 2 Dipole models: Successful description of inclusive and various diffractive measurements (e.g. Ratio of diffractive to inclusive cross-section, Diffractive Vector-Meson production)

8 DIS Process Description 8 Dirac Cross-Section (Electron-Mass M (Spin 1/2) Particle scattering) Recoil: E is kinematically determined by E and θ E Electron p 3 p 4 Scattering process: 2 2 E q p 1 p 2 Dirac-particle: Mass M / Spin 1/2 Dirac Cross-Section: Impact of electron-spin Impact of target-spin (mass M)

9 DIS Process Description 9 Mott Cross-Section (Electron-Heavy Mass M (Spin 1/2) Particle scattering) Mott condition: Electron scatters off a much heavier spin 1/2 particle of mass M >> m θ Calculate four-vector combinations:

10 DIS Process Description 10 Summary Re-write Dirac cross-section using the Mott cross-section with: Note: This is the result we would obtain if the proton would be a Dirac particle: M=m p Modifications to Rutherford cross-section: Relativistic effects (SPIN effects: Probe and Target (Negligible for very heavy target)!

11 DIS Process Description 11 Elastic ep scattering (1) Recall: Leptonic tensor p 3 p 4 Feynman graph: Note: The proton cannot be just a Dirac particle (g=2)! q p 1 p 2 Dirac-particle tensor Therefore: ep scattering Leptonic tensor p 3 p 4 We do not know exactly how this tensor looks like, but: q p 1 p 2 Hadronic tensor

12 DIS Process Description 12 Elastic ep scattering (2) High-energy limit: Dirac particle result: With: We find: For: Rosenbluth formula Form factors!

13 DIS Process Description 13 Elastic ep scattering (3) Summary Rosenbluth formula Rutherford cross section Recoil term! Mott cross section Note: For heavy target: Contribution from G 2 M term negligible: τ = q 2 /4M 2 0 For G 2 M = 1 and G2 E = 1 this reduces to the well-known Dirac particle case::

14 DIS Process Description 14 Elastic ep scattering (4) Scattering of electron (Spin 1/2) on pointcharge charge (Spin 0): Mott cross-section Homogeneous sphere with edge Exponential-like Point-like oscillating constant dipole constant Take into account finite charge distribution: Form factor 12C Ignore recoil! Hofstadter, 1953

15 DIS Process Description 15 Elastic ep scattering (5) Scattering of electron (Spin 1/2) on proton (Spin 1/2) dσ/dω [cm 2 /sr] Electron scattering on hydrogen target: 188MeV Nobel Prize 1961 Pointcharge, pointmoment Rosenbluth separation method: Mott Experiment Dirac Hofstadter θ

16 DIS Process Description 16 Inelastic ep scattering (1) Simplify this: Massless electron (m=0) of energy E strikes a stationary proton of mass M: Note: Leptonic tensor Hadronic tensor E is no longer kinematically determined by E and θ The total hadronic momentum can vary and is no longer constrained, i.e.:

17 DIS Process Description 17 Inelastic ep-scattering (2) Goal: Measure differential cross section in a particular energy range de Leptonic tensor: Ansatz for hadronic tensor: With We get: Therefore: Contraction with leptonic tensor yields: Note: W 1 and W 2 are functions of q2 and q p Negative four-momentum transfer squared! G E and G M are functions of q2 only! Bjorken scaling variable

18 DIS Process Description 18 Inelastic ep scattering (3) Scattering of electron (Spin 1/2) on proton (Spin 1/2) Scattering on point-like objects: Quarks! Here: Deep-inelastic scattering (DIS) Nobel Prize 1990 Friedman, Kendall and Taylor00

19 DIS Process Description 19 Summary Dirac cross-section Elastic ep cross-section Inelastic ep cross-section

20 DIS Kinematics 20 General considerations Four vectors: Neutral current exchange (NC): Charged current exchange (CC): Measurement of structure functions: NC: Scattered electron and/or hadronic final state CC: Hadronic final state (neutrino escapes detection) Determine kinematics!

21 DIS Kinematics 21 Kinematic variables (1) Mandelstam variables: s,t,u Centre-of-mass energy: s Q 2 Negative square of the momentum transfer q Determines wavelength of photon and therefore the size Δ which can be resolved Q 2 max = s x Bjorken scaling variable: Fraction of the proton momentum carried by the struck parton (Quark-Parton model)! W 2 Invariant mass squared of the hadronic final state system refers to the case where masses are ignored! Small x refers to large W 2

22 DIS Kinematics 22 Kinematic variables (2) ν Energy of the exchanged boson in the proton rest frame y Fraction of the incoming electron carried by the exchanged gauge boson also known as inelasticity in the proton rest t Momentum transfer at the hadronic vertex Note the above variables are connected by: For fixed x and y, ep collider allows to reach much large values in Q2 For fixed y and Q2, ep collider allows to reach much smaller values in x

23 DIS Kinematics 23 Collider kinematics (1)

24 DIS Kinematics 24 Collider kinematics (2) Electron method: scattered electron Jacquet-Blondel method: hadronic final state

25 DIS Kinematics 25 Collider kinematics (3) Lines of constant electron energy (E e ) Lines of constant electron angle (ϑ e ) Lines of constant hadron energy (F) Lines of constant hadron angle (γ)

26 DIS Kinematics 26 Collider kinematics (4) Low-x-low Q 2 : Electron and current jet (low energy) predominantly in rear direction High-x-low Q 2 : Electron in rear and current jet (High energy) in forward direction barrel forward rear High-x-high Q 2 : Electron predominantly in barrel/forward direction (High energy) and current jet in forward direction (High energy)

27 DIS Kinematics 27 Collider kinematics (5) Electron method: scattered electron Jacquet-Blondel method: hadronic final state

28 DIS Structure Function Measurements 28 Relativistic Invariant Cross-Section In terms of laboratory variables: Formulate this now in relativistic invariant quantities: Instead of W 1 and W 2, use: F 1 and F 2 : Longitudinal structure function: F L

29 DIS Structure Function Measurements 29 Essential idea bin in x and Q 2 1. Determination of kinematics Q 2 =4E ee e sin 2 ϑ e 2 x = Q2 sy (e.g. electron method): y =1 E e ϑ cos 2 e E e 2 2. Determination of crosssection and extraction of F 2 : ϑ e Number of selected Background events d 2 σ (N B) = dxdq2 L F 2 Luminosity Efficiency

30 DIS Structure Function Measurements 30 Results Three valence quarks F 2 x: Momentum fraction of struck quark Three bound valence quarks F 2 Three valence quarks and sea quarks + gluons F 2 QCD Valence quarks and QCD sea Discovery of asymptotic freedom in the theory of strong interaction (Quantum Chromo Dynamics): Nobel prize in physics 2004

31 DIS Collider Detectors 31 ep detector system: Here ZEUS Detector Central-Tracking detector: δp T p T = p T Inside superconducting solenoid of 1.43T Uranium calorimeter (barrel, rear and forward sections): electromagnetic part: δe E = 18% E hadronic part: δe E = 35% E Muon detection system in barrel, rear and forward direction

32 DIS Collider Detectors 32 ZEUS detector - Kinematic variable measurement Low Q 2 High Q 2 Electron variables Hadronic final-state variables

33 Summary 33 Connection of DIS cross-section and Dirac / Mott / Rutherford cross-sections Collider kinematics: Reconstruction of kinematics through electron or hadron method or combination of both Literature: Review on ep physics:, Eur. Phys. J. direct C1:2, Textbook on DIS: Robin Devenish and Amanda Cooper-Sarkar - Deep Inelastic Scattering

34 Backup - DIS Process Description 34 Basic aspects of scattering theory (1) Scattering process: Initial state: Final state: Scattering amplitude:

35 Backup - DIS Process Description 35 Basic aspects of scattering theory (2) Amplitude M: Dynamics Delta function enforces conservation of energy and momentum! Phase space: Kinematics Note: Four-momentum of the i th particle Statistical factor: 1/j! for each group of j identical particles in the final state

36 Backup - DIS Process Description 36 Basic aspects of scattering theory (3) Amplitude: Electron-Mass M (Spin 1/2) Particle scattering: Dirac scattering Momentum transfer: Approximation: Laboratory frame with the target particle of mass M at rest Electron with energy E scatters at an angle emerging with energy E Assumption: E,E >> mc 2 (m=0) Spin averaged amplitude:

37 Backup - DIS Process Description 37 Mott Cross-Section (1) Cross-section result: Impact of target spin for very heavy target drops out: Mott cross section: Scattering of spin 1/2 particle on heavy spin 0 heavy target Spin-averaged matrix element squared and cross-section for: M >> m Mott cross-section

38 Backup - DIS Process Description 38 Mott Cross-Section (2) Further simplification: m=0 Multiply by q 2 : Scaling behavior! Rutherford cross section!

39 Backup - DIS Process Description 39 Rutherford scattering (1) Non-relativistic limit: Incident electron is non-relativistic This can also be written as: Consequence of spin 1/2 nature of incoming probe particle! cos 2 θ term drops out in nonrelativistic limit: Or: Rutherford cross section!

40 Backup - DIS Process Description 40 Rutherford scattering (2) In natural units: Note: The Rutherford cross section is obtained from the Mott cross section assuming we are working in the non-relativistic limit: Spin effects of probe and target particle are negligible! The Mott cross section is obtained for the case of a target particle at rest (Heavy target): No recoil! Impact of spin 1/2 of probe particle taken into account - Spin effects of target particle negligible: Result is identical to scattering of spin 1/2 on spin 0 target! Difference between Rutherford and Mott cross section: cos 2 (θ/2) factor Factor is a consequence of angular momentum conservation: Helicity conservation for massless particles (β 1): Scattering by 180 requires spin flip (Impossible for spin 0 target)!