V. Kundrát1 Bari-KFKI Budapest-Case Western Reserve Univ.-CERN-Genoa-Helsinki- Pisa/Siena-Prague-Tallinn (~ 80 physicists) Elastic pp scattering at energy of 7 TeV and total cross section – experiment TOTEM V. Kundrát, Institute of Physics, AS CR, v.v.i. (Based on reports of K. Eggert, M. Bozzo, S. Giani, G. Ruggiero) 1.Introduction – experimental set-up. 2.Measurement of elastic pp scattering at 7 TeV. 3.Measurement of pp total cross section at 7 TeV. 4.Outlook.

TOTEM V. Kundrát2 1. Introduction – experimental set-up.

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TOTEM detectors: V. Kundrát 4 pseudorapidity all detectors installed and work ….

Roman Pot detectors to measure very small p scattering angles (few μrad) scattered particles inside LHC tubes vertical and horizontal pots mounted as close as possible BPM fixed to RP … precise position of the beam TOTEM at RP: σ beam ~ 80 μm leading proton detection at distances (10 σ beam + d) ~ 1.5 mm from axis need “edgeless” detectors efficient up to physical edge to minimize “d” challenges of the Roman Pot technology for LHC: strenght, robustness, vacuum tightness, thin, flatness, radition length, RF pick up shielding workshop “Vakuum Praha” (vacuum parts) V. Kundrát 5

Horizontal Pot : physics, overlap for tracks alignment Integrated beam position monitor Interconnection vacuum bellow : bake out and RF Roman Pot detectors V. Kundrát 6

Compensation system Bypass to machine vacuum Atmospheric pressure Compensation system Roman Pot detectors V. Kundrát 7

Resolver with reduction gear Slide Ball Screw (2mm lead) Stepper Motor 400step/tour = 0.9 o resolution Sliding Guides full metal  switches LVDT position sensors Movement resolution 2 mm/400 steps = 5  m (  /16) Movements Roman Pot detectors V. Kundrát 8

TOTEM Roman Pot Station V. Kundrát 9

10 The window and the Detector Assembly Ferrite 500μm 150μm

V. Kundrát11 RP edgeless Silicon Detector 24 Detector Packages over >440m r/o channels 240 sensors (.3m2)

V. KundrátVila Lanna, Roman pot station at 147m Roman pots unit at 220m 12/11/201012Gennaro Ruggiero, PH/TOT

V. Kundrát13 The Roman Pots at 220 m Far stations at 220 m Near stations at 220 m

V. Kundrát14 T1 Telescope with Cathode Strip Chambers (CSCs) 7.5 m 10.5 m IP T1 CMS muon end-caps 5 planes with measurement of 3 coordinates per plane 3 deg rotation and overlap between adjacent planes Primary vertex reconstruction allows background rejection Trigger with anode wires 3m 3.1 < |η| < 4.7

V. Kundrát15 It is based on the GEM (Gas Electron Multiplier) detector technology 40 3-GEMs in total: 10 planes with semicircular GEMs around the beam- pipe on each side of the IP to cope with high particle fluxes. Beam Castor T2 Detector Castor TOTEM T2 integration with CMS Insertion design together with CMS T2: 5.3 < | η | < 6.5

V. Kundrát 16  The TOTEM detector set-up T1 T2

2. Measurement of elastic pp scattering at 7 TeV Roman Pot detectors (220 m) silicon sensors located symmetrically on either side of IP5; to maximize the acceptance of elastically scattered protons → RP can approach beam centre to transverse distance ~ 1 mm RP station composed of two units; each unit consists of 3 RP’s, two approaching beam vertically and one horizontally (allowing partial overlap between horizontal and vertical detectors); detectors in horizontal pots complete acceptance for diffractively scattered protons all RP’s are rigidly fixed within the unit together with BPM; delicate ask: to ensure precision and reproducibility of the alignment of all RP detector planes with respect to each other and to the beam centre each RP: stack of 10 silicon strip detectors design to reduce the insensitive area at the edge facing the beam only to a few tens of μm! 512 strips of each detector oriented at angle of + 45º (5 “u” planes) and - 45º (5 “v” planes) with respect to detector edge facing the beam; reduction of background at trigger level → requiring collinear hits in at least 3 of 5 planes for each projection. All detector planes were aligned and mounted with precision of 20 μm V. Kundrát 17

RP’s movement: via step motors (5 μm) during measurement the detectors in horizontal RP’s overlap with the ones in vertical RP’s → enable precise (10 μm) relative alignment of all three RP’s in the unit by correlating their positions via common particle tracks dedicated beam fill: to align all RP’s symmetrically owing to the beam centre by moving them against the sharp beam edge cut by the beam collimators each RP station: duplication of the RP units (i) local track angles in x- and y-planes ┴ to the beam are reconstructed with precision of 5 to 10 μrad; these angles are related via beam optics to the scattering angle of proton at the vertex (ii) proton trigger selection by track angle uses both units independently → high trigger efficiency (99 ± 1) % Data selection and analysis standard LHC 2010 optics: β*= 3.5 m, 4 proton bunches (7X10 10 p/bunch), total integrated luminosity 6.1 nb -1, 7σ distance from the beam V. Kundrát18

reconstructed track in both projections in the near and in the far vertical RP unit is required on each side of the IP. Two diagonals top left of IP – bottom right of IP and bottom left of IP – top right of IP (tagging possible elastic candidates) are used (yet constrained by alignment of RP’s). intercepts of selected tracks in scoring plane at 220 m ┴ beam direction: displacement along y –axis is proportional to vertical scattering angle; present standard LHC optics does not lead to sizeable horizontal displacement. protons with momentum loss ξ = ∆p/p are shifted in positive x-direction by amount x=ξ D (D is dispersion). Elastically scattered protons: x~0, diffractive protons: positive x values due to D. Requirement |x| < 0.4 mm … first criterion for selecting elastic candidate events V. Kundrát 19

V. Kundrát 20 using optical functions: vertical (θ * y ) and horizontal (θ * x ) … deduced from measurements at RP stations; θ* y … from track displacement in y (minimum angle corresponds to closest detector approach to beam), θ* x … from the track angle at RP stations; colinearity of elastically scattered protons → θ* x and θ* y should be the same on both sides of IP figs. demonstrate correlations between scatt. angles on both sides with a spread in agreement with the beam divergence; t –resolution of δt =0.1 GeV √ |t| has been deduced from t = - p 2 θ*. Colinearity at 3 σ … applied for reducing background

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V. Kundrát time-dependent instantaneous luminosity taken from CMS measurement (CMS Collab., CMS-PAS-EWK (2010), CMS-DP C (2011). Based on van der Meer scan (uncertainty 4 % for presented data). Recorded luminosity has been derived by integrating the luminosity, the trigger efficiency and the DAQ efficiency over all different runs. total acceptance: computed as a function of vertical direction y and the azimuth Φ alignment of RP’s has been optimized by reconstructing parallel tracks going trough the overlap between vertical and horizontal RP’s (final uncertainty is less than 10 μm statistical error in t is given by beam divergence; statistical error in dσ/dt by number of events systematic uncertainty in t … dominated by optics and alignment; systematic uncertainties in dσ/dt by uncertainty on the efficiency correction and resolution unfolding (depending on t measurement errors) 22

differential cross section after unfolding and inclusion of all systematic uncertainties: 0.36 < |t| < 2.5 GeV 2 G. Antchev et al.: Proton-proton elastic scattering at the LHC energy of √s = 7 TeV; EPL 95 (2011) V. Kundrát 23

model comparison J. Kašpar, V. Kundrát, M. Lokajíček, J. Procházka: Nucl. Phys. B 843 (2011) 84 (for pp at 14 TeV) V. Kundrát 24

3. Measurement of the pp total cross section at 7 TeV  special LHC optics quantities related to: (i) IP plane: A* (ii) to detector plane: A transverse vertex position: (x*,y*), scattering angle projections: (θ* x, θ * y ) displacement …(x,y) of the proton trajectory from the beam centre at the RP position s RP is given by x = L x θ* x + v x x*, y = L y θ* y + v y y* optical functions L x,y and v x,y at the RP position s RP are determined by the beta function L x,y = √ (β x,y β*) sin (∆ μ x,y ), v x,y = √ (β x,y / β*) cos (∆ μ x,y ) with phase advance ∆ μ x,y = ∫ IP s RP (1/β x,y (s)) ds relative to IP; (axis x ┴ screen) V. Kundrát 2525

2626 to maximize sensitivity of the position measurement to scattering angle while minimizing its dependence on vertex position special optics are designed to have: minimum beam divergence σ Θ * at the IP (imposing large values of β* via σ Θ * = √ ( ε n / β*) ), large values of L and v=0, and thus ∆μ = π/2 at least in one projection (“parallel-to-point-focusing”) β* = 90 m optics exhibits “parallel-to-point-focusing” only in the vertical plane (∆μ y ≈ π/2, L y ≈ 260 m, v y ≈ 0), whereas in horizontal plane ∆μ x ≈ π and hence L x ≈ 0 which helps separating elastic and diffractive events. Beam divergence σ Θ * ≈ 2.5 μrad. Vertical scattering angle Θ y * can be directly reconstructed from the track position y, whereas due to L x ≈ 0 horizontal component Θ x * is optimally reconstructed from track angle Θ x = dx/ds at RP:  data collection and event selection β* = 90 m optics, each beam had two bunches with populations of 1x10 10 protons and 2x10 10 protons, transverse emittances (1.8 – 2.6) μrad (depending on the bunch) → instantaneous luminosity 8x10 26 cm -2 s -1 ; RP’s at 220 m verifying the beam orbit did not differ from the one with nominal beam optics β*=1.5 m → RP positions defined relative to beam centre → events recorded → trigger requiring track segment in any of the vertical RPs in at least one of the two transverse

projections → events characterized by the double-arm signature in the vertical RPs (top left of IP-bottom right of IP or bottom left of IP-top right of IP) collinearity of the two outgoing protons reconstructed with detector efficiency within 3 standard deviations in scattering angle correlation – correlation between reconstructed proton scattering angles on both sides of interaction points 7315 events 7370 events V. Kundrát 2727  analysis acceptance

 theory – used formulas optical theorem: (*) elastic hadronic differential cross section: (**) in forward direction (using (*) and (**)) V. Kundrát 2828 differential cross section measured down to |t| = 2 x GeV 2 extrapolation to t = 0

G. Antchev et al.: First measurement of the total proton-proton cross-section at the LHC energy of √s = 7 TeV; EPL 96 (2011) V. Kundrát 2929 new data can be described by a single exponential fit (χ 2 /d.o.f.=0.8) over range (0.02,0.33) GeV 2 with slope B = (20.1 ± 0.2 stat ± 0.3 syst ) GeV -2 value of B increases wit energy √s (compared with ISR results) for t from (0.36, 0.47) GeV 2 slope is larger B = (23.6 ± 0.5 stat ± 0.4 syst ) GeV -2 d σ/dt at t=0 (503.7 ± 1.5 stat ± 26.7 syst ) mb/GeV 2 integrating of elastic scattering cross section → (24.8 ± 0.2 stat ± 1.2 syst ) mb out of which 16.5 mb was directly observed using COMPETE Collab. prediction for ρ = leads for value of total cross section σ tot = (98.3 ± 0.2 stat ± 2.8 syst ) mb

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V. Kundrát Outlook (i)TOTEM experiment (elastic pp scattering) all detectors (RP’s at 147 and 220m, telescopes T1 and T2) are installed optics at higher values of β function enable to measure elastic events inside interference region, i.e., at |t| ~ GeV -2 (small distance of RP sensors from beam axis ~ 5 σ) RP’s at 147 m: enable to detect scattered protons at higher scattered angles → higher values of |t|; very important investigation of diffractive structure in d σ/dt luminosity free determination of σ tot (needs to measure total counting rate) however: determination of total cross section needs separation of Coulomb and hadronic elastic scattering → is always model dependent !!!

Coulomb scattering Nuclear scattering Coulomb- Nuclear interference  = fine structure constant  = relative Coulomb-nuclear phase G(t) = nucleon em form factor = (1 + |t|/0.71) -2  = Re/Im f (p  p) standard description of elastic pp scattering (only at small |t| values) at higher |t| values influence of Coulomb scattering neglected → only elastic hadronic amplitude taken into account (contradiction with model descriptions) V. Kundrát 32 possible source of discrepancy:

V. Kundrát 33 problems of model description of elastic pp scattering at the LHC experiments performed with ample statistics → precise data hadronic interactions at all t, Coulomb scattering at small |t|; F C+N (s,t) = F C (s,t) e i α Φ(s,t) + F N (s,t) F C (s,t) … Coulomb (QED), F N (s,t) … hadronic amplitude αΦ(s,t) … real relative phase; α=1/ … fine structure constant pp at p lab = 24 ÷ 2900 GeV/c  influence of both interactions (spins neglected) → complete amplitude F C+N (s,t) (Bethe (1958)) pp 53 GeV West-Yennie (generally complex function!!!) V. Kundrát (V.K., M. Lokajíček, I. Vrkoč, Phys.Lett. B656 (2007) 182)

V. Kundrát34 more precise form of complete amplitude for determination of σ tot,, B(t), ρ(t) (V. K., M. Lokajíček, Z. Phys. C63 (1994) 619) Use: either for performing analysis of data or for obtaining model predictions Predictions of 5 models

V. Kundrát35 J. Kašpar, V. Kundrát, M. Lokajíček, J. Procházka: Nucl. Phys. B 843 (2011) 84 – 106

modulus and phase of amplitude F N (s,t) parameterized (at all t ) … peripheral … central eikonal model complete amplitude (optical theorem): analysis of experimental data (maximal flexibility…to include all possibilities) V. Kundrát 36

results: pp at 53 GeV (V. K., M. Lokajíček, Z. Phys. C63 (1994) 619 – values of σ tot, B, ρ slightly different from WY analysis) 1/2 = 1.03 fm; 1/2 =0.68 fm; 1/2 = 1.09 fm … central (paradox!) 1/2 = 1.80 fm; 1/2 = 0.77 fm … peripheral peripheral V. Kundrát 37