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Overview of TOTEM Physics at LHC: Precision Measurements of Elastic Scattering and Diffraction

This presentation by Stefano Lami, on behalf of the TOTEM Collaboration, covers the key aspects of the TOTEM experiment at CERN's Large Hadron Collider (LHC). Highlighting the aims for high-precision measurements of total cross-section and elastic scattering, the talk elucidates the complexity of diffraction processes, including soft and hard scattering. The presentation focuses on the experimental setup, detector capabilities, and theoretical motivations that underpin the TOTEM's goals, particularly the pursuit of identifying competing models of hadronic interactions with a precision target of ~1%.

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Overview of TOTEM Physics at LHC: Precision Measurements of Elastic Scattering and Diffraction

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  1. The experiment at LHC • Stefano Lami • INFN Pisa • on behalf of the • TOTEM Collaboration

  2. jet b b jet TOTEM Physics Overview Total cross-section Ultimately ~1% precision Elastic Scattering Over a wide t range Forward physics Diffraction: soft and hard

  3. Physics Motivation (1) Dispersion relation fit Best fit TOTpp rises as ~ ln2 s • Predictions at 14 TeV: 90-130 mb • Available data not decisive • Aim of TOTEM: ~1% accuracy It will distinguish between several models COMPETE Collaboration fits all available hadronic data and predicts: LHC (14TeV): [PRL 89 201801 (2002)]

  4. Physics Motivation (2) … Islam Model: Elastic scattering (diffraction in general) • theoretical understanding not complete • number of approaches: Regge, geometrical, eikonal,QCD, . . . ⇒ rather incompatible predictions • intimately related to the structure of proton b* = 1540 m b* = 90 m b* = 2 m |t| Big uncertainties at large |t|: Models differ by ~ 3 orders of magnitude! 3-gluon exchange at large |t|: TOTEM will measure the 10-3< | t | ~(pq)2 <10 GeV2range with good statistics … and more

  5. p-p Interactions  = -lntg/2 x= Dp/p Non-diffractiveDiffractive Colour exchange Colourless exchange with vacuum quantum numbers dN / d  = exp (-) dN / d  = const  = -ln Incident hadrons acquire colour and break apart Incident hadrons retain their quantum numbers remaining colourless GOAL: understand the QCD nature of the diffractive exchange

  6. Elastic Scattering Single Diffraction M Double Diffraction Double Pomeron Exchange M Dominant Event Classes in p-p Collisions f ( = -lntg/2) ~65 mb~ 58 % h ~30mb ~10mb ~7 mb ~ 42 % Processes with large cross-sections ! ~1 mb << 1 mb Indicative cross-sections at 14 TeV

  7. TOTEM Experimental Apparatus • Physics requirements: forward proton detectors and large pseudorapidity coverage Roman Pots & tracking telescopes T1and T2 (inelastic evts + Vtxreco) • all detectors symmetrically on both sides of IP5 • all detectors L1 trigger capable T1: 3.1 < |h| < 4.7 T2: 5.3 < |h| < 6.5 Stefano Lami

  8. TOTEM Coverage Courtesy by Jan Kaspar Stefano Lami

  9. TOTEM Roman Pots Protons at few rad angles w. RPs at 10 + dfrom beam `Edgeless’ detectors to minimize d(sbeam~ 80 mm at RP220 w. b*=1540m) Each RP station has 2 units, 4m apart Each unit has 2 vertical insertions (‘pots’) + 1 horizontal Beam Thin window 150mm • Ongoing Detector Installation: • RP220 (147) fully (partially) equipped by June HALF OF THE RP STATION 10 silicon detector planes pitch 66mm Horizontal RP Vertical RP BPM Overlap for alignment

  10. T1 Telescope 3m • 1 station per side of IP5 • • each station: 5 planes • • each plane: 6 trapezoidal CSC detectors • • L1 trigger capability • • Cathode Strip Chamber detector • – 3 coordinates: 2 cathode strips, 1 anode wire • – Resolution ≈ 1 mm • • Primary Vtxreconstruction to discriminate • background (not beam-beam events) for the measurement of Ninel • Multiplicity measurement Installation foreseen for May/June: One arm eventually in September ¼ of T1 Stefano Lami

  11. T2 Telescope • 1 station on each side of IP5 • • each station: two halves (left/right) • • each half: 10 (5×2 back-to-back mounted) GEM detectors • • L1 trigger capability • • Triple Gas Electron Multiplier detectors • – double readout: strips (radial) and pads (coarse radial and azimuthal) • – resolution: radial 100 mm, azimuthal 0.8◦ • Vtxreco. + Multiplicity measurement Ongoing installation: fully done by May Stefano Lami

  12. TOTEM Physics and LHC Optics cross section luminosity • Feasible physics depends on running scenarios: • luminosity • beam optics (β*) • acceptance of proton detectors • precision in the measurement of scattering angle / beam divergence stot(~1%), low t elastic, stot(~5%), low t elastic, high t elastic, soft diffraction (semi)-hard diffraction hard diffraction EARLY mbb nb *(m)1540 902 0.5 L (cm2 s1) 1028 10301031 1033 TOTEM runsStandard runs Stefano Lami

  13. Details on Optics Proton transport equations: x = Lxx* + vxx* + D y = Lyy* + vyy * Optical functions: - L(effective length); - v(magnification); -D(machine dispersion) Describe the explicit path of particles through the magnetic elements as a function of the particle parameters at IP. Define t and  range (Acceptance)  = p/p;t = tx + ty; ti ~ -(pi*)2 (x*, y*): vertex position at IP (x*,y*): emission angle at IP Example Same sample of diffractive protons at different * - low *: p detected by momentum loss () - high *: p detected by trans. momentum (ty)

  14. Diffractive Proton Acceptance in (t, x): (contour lines at A = 10 %) RP220 Proton Acceptance RP 220m b*=1540 m Acceptance Detector distance to the beam: 10+0.5mm b*=90 m x resolved b = 0.5m - 2m b*=2 m -t = 0.01 GeV2 -t = 0.002 GeV2 * = 1540 m Beam 0.002 0.06 4.0 log(-t / GeV2) Low b*: see protons 0.02<x<0.2 or 2< |t| < 10 GeV2 Optics properties at RP220: 90m: see all protons 0.03 < |t| < 10 GeV2 Elastic Scattering Det. dist. 1.3 mm 6 mm

  15. Total Cross-Section and Elastic Scattering at low |t| Related by the Optical Theorem: Coulomb scattering Coulomb-Nuclear interference Nuclear scattering • a = fine structure constant • = relative Coulomb-nuclear phase G(t) = nucleon em form factor = (1 + |t|/0.71)-2 • r = Re/Im f(pp) TOTEM Approach: Measure the exp. slope B in the t-range 0.002 - 0.2 GeV2 , extrapolate d/dtto t=0,Measure total inelastic and elastic rates (all TOTEM detectors provide L1 triggers):

  16. simulated extrapolated Acceptance single diffraction Loss at low diffractive masses M detected Measurement of the Inelastic Rate • Inelastic double arm trigger: robust against background, inefficient at small M • Inelastic single arm trigger: suffers from beam-gas + halo background, best efficiency • Inelastic triggers and proton (SD, DPE): cleanest trigger, proton inefficiency to be extrapolated • Trigger on non-colliding bunches to determine beam-gas + halo rates. • Vertex reconstruction with T1, T2 to suppress background • Extrapolation of diffractive cross-section to large 1/M2 assuming ds/dM2 ~ 1/M2

  17. Combined Uncertainty in tot b* = 90 m 1540 m Extrapolation of elastic cross-section to t = 0: ± 4 % ± 0.2 % Total elastic rate (strongly correlated with extrapolation): ± 2 % ± 0.1 % Total inelastic rate: ± 1 % ± 0.8 % (error dominated by Single Diffractive trigger losses) Error contribution from (1+r2) using full COMPETE error band dr/r = 33 % ± 1.2 % Total uncertainty in stot including correlations in the error propagation: b* = 90 m : ± 5 %, b* = 1540 m : ± (1 ÷ 2) % . Slightly worse in L(~ total rate squared!) : ± 7 % (± 2 %). Precise Measurement with b* = 1540 m requires: improved knowledge of optical functions alignment precision < 50 mm

  18. b*=0.5 - 3 b*=1540 b*=90 Log(M) (GeV)‏ • Early Physics with TOTEM (Ep = 5 TeV & b* = 3m) RP220 Acceptance: - elastic scattering 1 < |t| < 12 GeV2 - diffractive protons 0.02 < x < 0.18 - resolution: σ(ξ) <∼ 6 · 10−3 very similar to Ep = 7 TeV !! DPE Protons seen for > 2% Stefano Lami

  19. 1 p 2 p p2 p1 p X  p p p  Rapidity Gap -ln MX2 =  s  Very first measurements with low b* optics • Using horizontal RPs • dSD/dM(SD events with high mass) • 0.02 <  < 0.18  2 < M < 6 TeV • (M)/M = 24 % • dDPE/dM(DPE high mass) • 250 < M < 2500 GeV • (M)/M = 2.13.5 % • Using vertical RPs:high t elastic scattering • dElastic/dt 1 < |t| < 12 GeV2 (t)  0.2 |t| SD DPE Rapidity Gap -ln1 Rapidity Gap -ln2  MX2 = 12s p1 p2 X 

  20. T1/T2: charged multiplicity • Measurement of charged multiplicity for different processes • Important also for cosmic ray physics (e.g. for MC generator validation) • Identification and measurement of rapidity gaps Acceptance: 3.1 <  < 4.7 (T1) & 5.3 <  < 6.5 (T2) () = 0.04  0.2, no mom. &  info dNch/dh [1/unit] Particelle cariche per evento minimum bias single diffractive

  21. Conclusions • TOTEMready for LHC restart • will run under all beam conditions • will need high β∗ optics → will require β∗ = 90m optics for early running • Early measurements • low*: • - study of SD and DPE at high masses • - elastic scattering at high |t| • - measurement of forward charged multiplicity • * = 90 m: • - first measurement of tot(and L ) with a precision of ~ 5% (~ 7%) • - elastic scattering in a wide |t| range • - inclusive studies of diffractive processes • - measurement of forward charged multiplicity • Later • Measurement of total pp cross-section (and L ) with a precision • of 1÷2 % (2 %) with * = 1540 m (dedicated short runs). • Measurement of elastic scattering in the range 10-3< |t| < 10 GeV2 • An extensive CMS/TOTEM Physics Programme

  22. The experiment at LHC • Backup Slides

  23. I1 I2 current terminating ring biasing ring Al - + SiO2 p+ p+ n-type bulk cut edge Al n+ 50µm Si CTS edgeless detectors Proton detection down to 10beam, + d (beam, = 80  600 m) To minimize d : detectors with highly reduced inactive edge (”edgeless”) Planar technology + CTS (Current Terminating Structure) Micro-strip Si detectors designed to reduce the inefficiency at the edge. Inefficient edge ~ 50 m 50 µm

  24. Optics and Beam Parameters • * = 90 m ideal for early running: • fits well into the LHC start-up running scenario; • uses standard injection (b* = 11m)  easier to commission than 1540 m optics • wide beam  ideal for training the RP operation (less sensitive to alignment) • * = 90 m optics proposal submitted to the LHCC and well received.

  25. p Level-1 Trigger Schemes (L = 1.6 x 1028 cm-2 s-1) RP CMS RP T1/T2 p Elastic Trigger: Single Diffractive Trigger: Double Diffractive Trigger: Central Diffractive Trigger: Minimum Bias Trigger: p p p

  26. Extrapolation to the Optical Point (t = 0) at b*=90m (extrapol. - model) / model in d/dt |t=0Statistical extrapolation uncertainty ∫ L dt = 2 nb-1(5 hours @ 1029 cm-2 s-1) Common bias due to beam divergence : – 2 % (angular spread flattens dN/dt distribution) Spread between most of the models: ±1% Systematic error due to uncertainty of optical functions: ± 3% Different parameterizations for extrapolation tested (e.g. const. B, linear continuation of B(t)):negligible impact

  27. Diffractive scattering is a unique laboratory of confinement & QCD: A hard scale + protons which remain intact in the scattering process Forward protons observed, independent of their momentum losses b ‘wee’ parton cloud - grows logarithmically Elastic soft SD hard SD soft DPE hard DPE Lorentz contracted longitudinal view p jet jet Baryonic charge r~0.5fm ln s + b h Valence quarks in a bag with r ~ 0.2fm rapidity Soft Hard Diffractive Scattering large b small b p Rapidity gap survival & ”underlying” event structures are intimately connected with a geometrical view of the scattering - eikonal approach ! 29mb? 10mb? 1mb? • Soft processes: • Coherent • interactions • impact parameter • picture. Hard processes: Jet momenta correlated with the initial parton momenta. Cross sections are large Measure (M,,t)

  28. Possibilities of r measurement asymptotic behaviour: • 1 / ln s for s   pred. ~ 0.13 at LHC • Try to reach the Coulomb region and measure interference: • move the detectors closer to the beam than 10  + 0.5 mm • run at lower energy @ √s < 8 TeV

  29. Optical Functions (* = 90 m) L = (*)1/2 sin((s)) v = (/*)1/2 cos((s)) hit distribution (elastic) x = Lxx* + vxx* +D y = Lyy* + vyy*  Idea: Lylarge Lx=0 vy= 0 y(220) = /2 x(220) =  (parallel-to-point focussing on y) (m)‏ Optical functions: - L (effective length) - v (magnification) defined by  (betatron function) and  (phase advance); - D (machine dispersion)  describe the explicit path of particles through the magnetic elements as a function of the particle parameters at IP  = p/p (x*, y*): vertex position at IP (x*,y*): emission angle at IP t = tx + ty ti~ -(pi*)2

  30. Total multiplicity in T2 (5<<7) Forward Physics: VHE Cosmic Ray Connection p-p collisions @ LHC as predicted by generators tipically used to model hadronic showers generated by VHE CR Interpreting cosmic ray data depends on hadronic simulation programs. Forward region poorly known/constr. Models differ by factor 2 or more. Need forward particle/energy measurements e.g. dN/d, dE/d…

  31. Machine Induced Background T1/T2 Detectors: • beam-gas interactions: prel. ext. ~ 14 Hz per beam; ~ 19 KHz for MB events (MB = 80 mb, L = 2.4 ·1029 cm-2 s-1)  reduced by vertex reconstruction • muon halo (expected to be very small, not yet quantified) Roman Pot Detectors: • beam halo (protons out of design orbit): ext. (* = 1540m) ~ 12·10-4/bunch  reduced by requiring coincidence between RP arms • beam-gas interactions: ext. (* = 1540m) ~ 3·10-4/bunch after cuts  reduced with cuts on track angles and multiplicities • p-p collision (at IP) background: ext (* = 1540m) ~ (0.4  2)·10-4/bunch after cuts  reduced with cuts on track angles and hit multiplicities Tot. elast. evts ~ 3 KHz (L = 1029 cm-2 s-1); prel. expt. S/B ~ (0.6  0.7)·103

  32. Expected Radiation Dose in CMS/TOTEM In 1 year @ Linst = 1034 cm-2s-1 At RPs locations =>

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