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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

TOTEMV. 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

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1. Introduction – experimental set-up.

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TOTEM detectors:

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)

2tan(ln

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: σbeam~ 80 μm

• leading proton detection at distances (10 σbeam + 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)

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Horizontal Pot : physics, overlap for tracks alignment

Integrated beam position monitor

Interconnection vacuum bellow : bake out and RF

Primary Vacuum

Detector

Thin window

Beam

Secondary Vacuum

Vacuum Chamber

Bellow

Beam

Roman Pot detectors

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Compensation system

Bypass to machine vacuum

Atmospheric pressure

Compensation systemRoman Pot detectors

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Resolver with reduction gear

Slide Ball Screw

(2mm lead)

Stepper Motor

400step/tour = 0.9o resolution

Sliding Guides

full metal

switchesLVDT position sensors

Movement resolution

2 mm/400 steps = 5 m (/16)

MovementsRoman Pot detectors

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TOTEM Roman Pot Station

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The window and the Detector Assembly

Ferrite

500μm

150μm

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RP edgeless Silicon Detector

24 Detector Packages over >440m122880 r/o channels240 sensors (.3m2)

19.04.2023V. Kundrát Vila Lanna, 15-12-2009 12Roman pot station at 147m

Roman pots unit at 220m

12/11/2010 12Gennaro Ruggiero, PH/TOT

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The Roman Pots at 220 m

Far stations at 220 m Near stations at 220 m

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T1 Telescope with Cathode Strip Chambers (CSCs)

7.5 m10.5 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

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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 CMSTOTEM T2 integration with CMS

Insertion designtogether with CMS

T2: 5.3 < | η | < 6.5

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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

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• 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 (7X1010 p/bunch) , total integrated luminosity 6.1 nb-1, 7σ distance from the beam

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• 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

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• 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 = - p2 θ*. Colinearity at 3 σ … applied for reducing background

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• time-dependent instantaneous luminosity taken from CMS measurement (CMS Collab., CMS-PAS-EWK-10-004 (2010), CMS-DP-2011-002 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)

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• differential cross section

• after unfolding and inclusion of all systematic uncertainties: 0.36 < |t| < 2.5 GeV2

G. Antchev et al.: Proton-proton elastic scattering at the LHC energy of √s = 7 TeV;

EPL 95 (2011) 41001

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• model comparison

J. Kašpar, V. Kundrát, M. Lokajíček, J. Procházka:

Nucl. Phys. B 843 (2011) 84

(for pp at 14 TeV)

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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 sRP is given by

x = Lx θ*x + vx x* , y = Ly θ*y + vy y*

optical functions Lx,y and vx,y at the RP position sRP are determined by the beta function

Lx,y = √ (βx,yβ*) sin (∆ μx,y ) , vx,y = √ (βx,y / β*) cos (∆ μx,y )

with phase advance ∆ μx,y = ∫IP

sRP (1/βx,y(s)) ds relative to IP; (axis x ┴ screen)

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• 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 σΘ* at the IP (imposing large values of β* via σΘ* = √ (εn / 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 (∆μy ≈ π/2, Ly ≈ 260 m, vy ≈ 0), whereas in horizontal plane ∆μx ≈ π and hence Lx ≈ 0 which helps separating elastic and diffractive events.

• Beam divergence σΘ* ≈ 2.5 μrad. Vertical scattering angle Θy* can be directly

reconstructed from the track position y , whereas due to Lx ≈ 0 horizontal component Θx* is optimally reconstructed from track angle Θx = dx/ds at RP:

data collection and event selection

• β* = 90 m optics, each beam had two bunches with populations of 1x1010 protons and 2x1010 protons, transverse emittances (1.8 – 2.6) μrad (depending on the bunch) → instantaneous luminosity 8x1026cm-2s-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 → 66950 events recorded → trigger requiring track segment in any of the vertical RPs in at least one of the two transverse

projections → 15973 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

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analysis

acceptance

theory – used formulas

• optical theorem: (*)

• elastic hadronic differential cross section:

(**)

• in forward direction (using (*) and (**))

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• differential cross section measured down to |t| = 2 x 10-2 GeV2

• 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) 21002

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• new data can be described by a single exponential fit (χ2/d.o.f.=0.8) over range (0.02,0.33) GeV2 with slope

B = (20.1 ± 0.2stat ± 0.3syst) GeV-2

• value of B increases wit energy √s (compared with ISR results)

• for t from (0.36, 0.47) GeV2 slope is larger

B = (23.6 ± 0.5stat ± 0.4syst) GeV-2

• dσ/dt at t=0

(503.7 ± 1.5stat ± 26.7syst) mb/GeV2

• integrating of elastic scattering cross section → (24.8 ± 0.2stat ± 1.2syst) mb out of which 16.5 mb was directly observed

• using COMPETE Collab. prediction for ρ = 0.14+0.01-.08 leads for value of total cross

section

σtot = (98.3 ± 0.2stat ± 2.8syst) mb

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3. 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| ~ 10-4 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 dσ/dt

• luminosity free determination of σtot (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

ddt

4 2 c 2G4 t t

2

totG2 t

te B t / 2

tot2 1 2

16 c 2e B ' t

= fine structure constant = relative Coulomb-nuclear phaseG(t) = nucleon em form factor = (1 + |t|/0.71)-2

= Re/Im f(pp)

• 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)

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possible source of discrepancy:

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• 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|;

FC+N(s,t) = FC(s,t) e i α Φ(s,t) + FN(s,t)

FC(s,t) … Coulomb (QED), FN(s,t) … hadronic amplitude

αΦ(s,t) … real relative phase;

α=1/137.036 … fine structure constant

pp at plab = 24 ÷ 2900 GeV/c

influence of both interactions (spins neglected) →

complete amplitude FC+N(s,t) (Bethe (1958))

pp 53 GeV

West-Yennie (generally complex function!!!)

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(V.K., M. Lokajíček, I. Vrkoč, Phys.Lett. B656 (2007) 182)

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more precise form of complete amplitude for determination of σtot ,, 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

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

• modulus and phase of amplitude FN(s,t) parameterized (at all t )

… peripheral

… central

eikonal model complete amplitude (optical theorem):

analysis of experimental data

(maximal flexibility…to include all possibilities)

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• results: pp at 53 GeV (V. K., M. Lokajíček, Z. Phys. C63 (1994) 619 –

values of σtot, B, ρ slightly different from WY analysis)

<btot2>1/2 = 1.03 fm; < bel

2>1/2 =0.68 fm; <bin2>1/2 = 1.09 fm … central (paradox!)

<bel2>1/2 = 1.80 fm; <bin

2>1/2 = 0.77 fm … peripheral

peripheral

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