Recent developments in the area of SoftQCD and Diffractive Physics - PowerPoint PPT Presentation

Recent developments in the area of SoftQCD and Diffractive Physics at the ATLAS Experiment Róbert Astaloš (Comenius University Bratislava) on behalf of the ATLAS Collaboration 56th International Winter Meeting on Nuclear Physics Bormio, Italy, January 22 – 26 January 26, 2018 1

Overview Measurement of the Inelastic Proton-Proton Cross Section at √ s = 13 TeV with the ATLAS Detector at the LHC Phys. Rev. Lett. 117 (2016) 182002, arXiv:1606.02625 Measurement of the exclusive γγ → µ + µ − process in pp collisions at √ s = 13 TeV with the ATLAS detector at the LHC Phys. Lett. B 777 (2018) 303, arXiv:1708.04053 Study of ordered hadron chains with the ATLAS detector Phys. Rev. D 96 (2017) 092008, arXiv:1709.07384 2

Inelastic Proton-Proton Cross Section at √ s = 13 TeV rise of total pp cross section with center-of-mass energy predicted by Heisenberg → probes the nonpertubative regime of QCD → confirmed by many experiments two sets of scintillation counters, elastic pp scattering out of their acceptance; M X > 13 GeV (fiducial region ξ = M 2 X / s > 10 − 6 ) → then extrapolated to total inelastic cross-section minimum-bias trigger scintillators (MBTS): installed on the frontface of each endcap calorimeter ( z = ± 3 . 6 m) cover region: 2 . 07 < | η | < 3 . 86; 149 < r < 445 & 445 . 5 < r < 895 mm two other forward detector used to measure trigger efficiency ǫ trig : forward Cherenkov detector LUCID ( z = ± 17 m): 5 . 6 < | η | < 5 . 9 tungsten-scintillator calorimeter det. LHCf ( z = ± 140 m): | η | > 8 . 4 inclusive selection: at least 2 MBTS counters with charge above 0.15 pC ( n MBTS ≥ 2) 4 159 074 events passing single-sided selection: hits in ≥ 2 counters on one side of the detector and no hits on the other to constrain diffractive component 442 192 events passing 3

Monte Carlo models SS Data 2015 ATLAS R 0.18 Pythia8 SS R SS = number of events passing the singlesided selection µ -1 s =13 TeV, L=60.1 b ε Pythia8 DL, =0.085 0.16 ε number of events passing the inclusive selection Pythia8 DL, =0.060 ε Pythia8 DL, =0.10 0.14 Pythia8 MBR used to adjust for each MC the fraction: EPOS LHC 0.12 QGSJET-II f D = ( σ SD + σ DD ) /σ inel 0.1 0.08 obtained R SS = 10.4 ± 0.4% (stat + syst) 0.06 0.04 0.1 0.15 0.2 0.25 0.3 0.35 0.4 f D n MBTS distributions in data compared to ones from MC utilizing the fitted f D value: 1 events MBTS events MBTS Data Pythia8 SS Data Pythia8 SS n n n n d d d d events ε ε events ε ε Pythia8 DL, = 0.06 Pythia8 DL, = 0.085 Pythia8 DL, = 0.06 Pythia8 DL, = 0.085 1 1 n ε n ε Pythia8 DL, = 0.10 MBR Pythia8 DL, = 0.10 MBR − 10 1 EPOS LHC QGSJET-II EPOS LHC QGSJET-II − 1 10 − 2 10 ATLAS ATLAS µ µ 13 TeV, 60.1 b -1 13 TeV, 60.1 b -1 Inclusive selection Single-sided selection MC/data 2 4 6 8 10 12 14 16 18 20 22 24 MC/data 2 4 6 8 10 12 1.5 1.5 1 1 0.5 0.5 2 4 6 8 10 12 14 16 18 20 22 24 2 4 6 8 10 12 n n MBTS MBTS best agreement with DL and MBR-based models; other do not describe data well Pythia8 DL with ε = 0 . 085 chosen as the nominal MC model only DL and MBR models considered for MC systematics 4

Fiducial inelastic cross section 1 − f ξ< 10 − 6 inel ( ξ > 10 − 6 ) = N − N BG σ fid ǫ trig ×L × fiducial cross section ǫ sel Factor Value Rel. uncertainty Number of events passing the inclusive selection ( N ) 4159074 − Number of background events ( N BG ) 51187 ± 50% Integrated luminosity [ µ b − 1 ] ( L ) 60.1 ± 1 . 9% Trigger efficiency ( ǫ trig ) 99 . 7% ± 0 . 3% MC correction factor ( C MC ) 99 . 3% ± 0 . 5% – migration of events with ξ < 10 − 6 into the fiducial region 1 − f ξ< 10 − 6 C MC = ǫ sel – event selection efficiency systematic uncertainties include: counter efficiency variations impact of the material uncertainty, uncertainty in the fitted value of f D , and variations in C MC found by comparing Pythia8 DL and MBR models σ fid inel = 68.1 ± 0.6(exp) ± 1.3(lum) mb measured fiducial cross section: MC predictions: Pythia8 DL: 71.0 mb ( ε =0.06); 69.1 mb ( ε =0.085); 68.1 mb ( ε =0.1) Pythia8 MBR: 70.1 mb EPOS LHC: 71.2 mb QGSJet-II: 72.7 mb 5 Pythia8 SS: 74.4 mb

Total inelastic cross section σ MC ( ξ< 10 − 6 ) σ inel = σ fid inel + σ 7TeV ( ξ < 5 × 10 − 6 ) × total cross section σ 7TeV , MC ( ξ< 5 × 10 − 6 ) σ 7TeV ( ξ < 5 × 10 − 6 ) is difference between σ 7TeV inel measured using ALFA detector and σ 7TeV ( ξ > 5 × 10 − 6 ) measured using MBTS measured total cross section: σ inel = 78.1 ± 0.6(exp) ± 1.3(lum) ± 2.6(extrap) mb [mb] Pythia 8 ATLAS (MBTS) 100 ATLAS (ALFA) EPOS LHC inel measured cross section TOTEM QGSJET-II σ 90 ALICE agrees well with variety LHCb 80 of theoretical predictions Auger pp (non-LHC) 70 p p 60 is consistent with the inelastic cross section 50 LHC region increasing with √ s 80 75 40 70 30 65 ATLAS 7000 8000 9000 10000 11000 12000 13000 2 3 4 10 10 10 6 s [GeV]

Exclusive γγ → µ + µ − production γγ induced interactions provide unique opportunity to study high-energy electroweak processes exclusive production single-proton dissociation double-proton dissociation (S-diss) (D-diss) p p p p X ' X ' γ γ γ − − − µ µ µ + + + µ µ µ γ γ γ X '' p p p p p data set: pp collisions at √ s = 13 TeV, dimuon trigger, integrated luminosity 3.2 fb − 1 cross section calculations based on Equivalent Photon Approximation (EPA): colliding protons produce quasi-real photons with small virtuality of Q 2 < 0 . 1 GeV → convolving the photon fluxes with elementary cross section of γγ → µ + µ − muon candidates identified by matching complete tracks in MS to tracks in the ID muons required to be isolated, info from ID and calorimeters transverse and longitudinal impact parameters: | d 0 | /σ d 0 < 3 . 0; | z 0 | sin θ < 0 . 5 mm events required to have exactly 1 pair of opposite-sign charged muons background contributions: S-diss, D-diss, Z /γ ∗ → µ + µ − , Z /γ ∗ → τ + τ − 7

Exclusive selection 6 10 Events Events / 5 GeV Events / 0.75 GeV Data Data Data 8 ATLAS DY N model. uncertainty ATLAS ATLAS 10 ch 10 6 DY N model. uncertainty DY N model. uncertainty γ → µ + µ - ch Z/ * γ → ch µ + µ Z/ γ * → µ + µ - -1 -1 Z/ * - 10 5 -1 s = 13 TeV, 3.2 fb s = 13 TeV, 3.2 fb s = 13 TeV, 3.2 fb Multijet Multijet 7 Multijet 10 γ → τ + τ - + Z/ * 5 Z/ γ * → τ τ - 10 Z/ γ * → τ + τ - t t t t Baseline selection t t 4 10 6 γ γ → µ + µ - 10 γ γ → µ µ - D-diss D-diss + γ γ → µ + µ - γ γ → µ µ - D-diss γ γ → µ µ - S-diss + (post-fit) 4 S-diss + (post-fit) 10 γ γ → µ + µ - 5 γ γ → µ µ - S-diss (post-fit) γ γ → µ µ - 10 Exclusive + (post-fit) Exclusive + (post-fit) γ γ → µ µ - Exclusive + (post-fit) 3 10 Baseline selection 4 10 3 10 Baseline selection + 1 mm vertex isolation 10 2 3 + 1 mm vertex isolation + 12 GeV < m < 70 GeV 10 2 10 µ + µ - 10 2 10 10 10 12 GeV < m < 70 GeV µ + µ - 1 1 or m > 105 GeV 1 µ + µ - 0 20 40 60 80 100 120 140 160 180 0 5 10 15 20 25 30 Data / MC Data / MC 10 Data / MC 1.2 1.2 1.2 µ µ + - Tracks associated with dimuon vertex m [GeV] p [GeV] µ + µ - 1 1 1 T 0.8 0.8 0.8 0 20 40 60 80 100 120 140 160 180 0 5 10 15 20 25 30 2 3 4 5 6 7 8 910 20 30 40 µ µ - + m [GeV] p [GeV] Tracks associated with dimuon vertex µ µ + - T Total Z/γ ∗ Z/γ ∗ → µ + µ − → τ + τ − t ¯ Data Signal background S-diss D-diss Multijet t Baseline selection 2 933 384 5740 2 897 000 8640 8000 226 8000 10 900 590 000 12 200 1 mm vertex isolation 14 759 4560 11 100 6840 300 3900 30 50 0 m µ + µ − < 70 GeV 12 395 4420 8800 6420 300 2000 30 50 0 p µ + µ − < 1 . 5 GeV 7952 4370 4300 3550 60 670 7 10 0 T typical signature of exclusive events: absence of other charged-particle tracks → a veto on additional charged-particle applied: no additional tracks with p T > 400 MeV and | η | < 2 . 5 near the dimuon vertex with | z trk 0 | < 1 mm definition of the fiducial region: p µ | η µ | requirement Invariant mass range T requirement 12 GeV < m µ + µ − < 30 GeV > 6 GeV < 2 . 4 30 GeV < m µ + µ − < 70 GeV > 10 GeV < 2 . 4 8