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

SLIDE 1

Recent developments in the area of SoftQCD and Diffractive Physics at the ATLAS Experiment

Róbert Astaloš

(Comenius University Bratislava)

n behalf of the ATLAS Collaboration

56th International Winter Meeting on Nuclear Physics Bormio, Italy, January 22 – 26

January 26, 2018

1

SLIDE 2

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

SLIDE 3

Inelastic Proton-Proton Cross Section at √s = 13 TeV

3 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; MX > 13 GeV (fiducial region ξ = M2

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 ǫtrig:

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 (nMBTS ≥ 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

SLIDE 4

Monte Carlo models

4 RSS = number of events passing the singlesided selection

number of events passing the inclusive selection

used to adjust for each MC the fraction: fD = (σSD + σDD)/σinel

btained RSS = 10.4±0.4% (stat + syst)

nMBTS distributions in data compared to ones from MC utilizing the fitted fD value:

D

f 0.1 0.15 0.2 0.25 0.3 0.35 0.4

SS

R 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18

Data 2015 Pythia8 SS =0.085 ε Pythia8 DL, =0.060 ε Pythia8 DL, =0.10 ε Pythia8 DL, Pythia8 MBR EPOS LHC QGSJET-II

ATLAS

1

b µ =13 TeV, L=60.1 s

2 4 6 8 10 12 14 16 18 20 22 24

MBTS

n d

events

n d

events

n 1

2 −

10

1 −

10 1 Data Pythia8 SS = 0.06 ε Pythia8 DL, = 0.085 ε Pythia8 DL, = 0.10 ε Pythia8 DL, MBR EPOS LHC QGSJET-II ATLAS

1

b µ 13 TeV, 60.1 Inclusive selection

MBTS

n 2 4 6 8 10 12 14 16 18 20 22 24 MC/data 0.5 1 1.5 2 4 6 8 10 12

MBTS

n d

events

n d

events

n 1

1 −

10 Data Pythia8 SS = 0.06 ε Pythia8 DL, = 0.085 ε Pythia8 DL, = 0.10 ε Pythia8 DL, MBR EPOS LHC QGSJET-II ATLAS

1

b µ 13 TeV, 60.1 Single-sided selection

MBTS

n 2 4 6 8 10 12 MC/data 0.5 1 1.5

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

nly DL and MBR models considered for MC systematics

SLIDE 5

Fiducial inelastic cross section

fiducial cross section

σfid

inel(ξ > 10−6) = N−NBG ǫtrig×L × 1−fξ<10−6 ǫsel

CMC =

1−fξ<10−6 ǫsel

systematic uncertainties include: counter efficiency variations impact of the material uncertainty, uncertainty in the fitted value of fD, and variations in CMC found by comparing Pythia8 DL and MBR models measured fiducial cross section: σfid

inel = 68.1 ± 0.6(exp) ± 1.3(lum) mb

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 Pythia8 SS: 74.4 mb

5 – migration of events with ξ < 10−6 into the fiducial region – event selection efficiency Factor Value

Rel. uncertainty

Number of events passing the inclusive selection (N) 4159074 − Number of background events (NBG) 51187 ±50% Integrated luminosity [µb−1] (L) 60.1 ±1.9% Trigger efficiency (ǫtrig) 99.7% ±0.3% MC correction factor (CMC) 99.3% ±0.5%

SLIDE 6

Total inelastic cross section

6 total cross section

σinel = σfid

inel + σ7TeV(ξ < 5 × 10−6) × σMC(ξ<10−6) σ7TeV,MC(ξ<5×10−6)

σ7TeV(ξ < 5 × 10−6) is difference between σ7TeV

inel measured using ALFA detector

and σ7TeV(ξ > 5 × 10−6) measured using MBTS measured total cross section: σinel = 78.1 ± 0.6(exp) ± 1.3(lum) ± 2.6(extrap) mb

[GeV] s

2

10

3

10

4

10 [mb]

inel

σ 30 40 50 60 70 80 90 100

ATLAS (MBTS) ATLAS (ALFA) TOTEM ALICE LHCb Auger pp (non-LHC) p p Pythia 8 EPOS LHC QGSJET-II

ATLAS

7000 8000 9000 10000 11000 12000 13000 65 70 75 80

LHC region

measured cross section agrees well with variety

f theoretical predictions

is consistent with the inelastic cross section increasing with √s

SLIDE 7

Exclusive γγ → µ+µ− production

7 γγ induced interactions provide unique opportunity to study high-energy electroweak processes

exclusive production single-proton dissociation double-proton dissociation (S-diss) (D-diss)

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 Q2 < 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: |d0|/σd0 < 3.0; |z0| 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 −

p p p p p p p p p ' X X

+ −

γ γ γ γ γ γ X ' '' µ µ

+ −

µ µ

+ −

µ µ

SLIDE 8

Exclusive selection

8

Tracks associated with dimuon vertex 10 Events 1 10

2

10

3

10

4

10

5

10

6

10

7

10

8

10 ATLAS

1

= 13 TeV, 3.2 fb s Baseline selection > 105 GeV

µ

+

µ

r m

< 70 GeV

µ

+

µ

12 GeV < m Data

model. uncertainty

ch

DY N

µ

+

µ → * γ Z/ Multijet

τ

+

τ → * γ Z/ t t

µ

+

µ → γ γ D-diss (post-fit)

µ

+

µ → γ γ S-diss (post-fit)

µ

+

µ → γ γ Exclusive

Tracks associated with dimuon vertex 2 3 4 5 6 7 8 910 20 30 40 Data / MC 0.8 1 1.2 [GeV]

µ

+

µ

m 20 40 60 80 100 120 140 160 180 Events / 5 GeV 1 10

2

10

3

10

4

10

5

10

6

10 ATLAS

1

= 13 TeV, 3.2 fb s + 1 mm vertex isolation Baseline selection

Data

model. uncertainty

ch

DY N

µ

+

µ → * γ Z/ Multijet

τ

+

τ → * γ Z/ t t

µ

+

µ → γ γ D-diss (post-fit)

µ

+

µ → γ γ S-diss (post-fit)

µ

+

µ → γ γ Exclusive

[GeV]

µ

+

µ

m 20 40 60 80 100 120 140 160 180 Data / MC 0.8 1 1.2 [GeV]

µ

+

µ T

p 5 10 15 20 25 30 Events / 0.75 GeV 1 10

2

10

3

10

4

10

5

10

6

10 ATLAS

1

= 13 TeV, 3.2 fb s < 70 GeV

µ

+

µ

+ 12 GeV < m + 1 mm vertex isolation Baseline selection

Data

model. uncertainty

ch

DY N

µ

+

µ → * γ Z/ Multijet

τ

+

τ → * γ Z/ t t

µ

+

µ → γ γ D-diss (post-fit)

µ

+

µ → γ γ S-diss (post-fit)

µ

+

µ → γ γ Exclusive

[GeV]

µ

+

µ T

p 5 10 15 20 25 30 Data / MC 0.8 1 1.2

Total Z/γ∗ Z/γ∗ Data Signal background S-diss D-diss → µ+µ− → τ +τ − Multijet t¯ 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 mµ+µ− < 70 GeV 12 395 4420 8800 6420 300 2000 30 50 pµ+µ−

T

< 1.5 GeV 7952 4370 4300 3550 60 670 7 10 Invariant mass range pµ

T requirement

|ηµ| requirement 12 GeV < mµ+µ− < 30 GeV > 6 GeV < 2.4 30 GeV < mµ+µ− < 70 GeV > 10 GeV < 2.4

typical signature of exclusive events: absence of other charged-particle tracks → a veto on additional charged-particle applied: no additional tracks with pT > 400 MeV and |η| < 2.5 near the dimuon vertex with |ztrk

0 | < 1 mm

definition of the fiducial region:

SLIDE 9

Fiducial cross section

9

acoplanarity

µ

+

µ 0.01 0.02 0.03 0.04 0.05 0.06

Events / 0.002

500 1000 1500 2000 2500 ATLAS

1

= 13 TeV, 3.2 fb s < 70 GeV

µ

+

µ

12 GeV < m

Data (post-fit)

µ

+

µ → γ γ Exclusive (post-fit)

µ

+

µ → γ γ S-diss

µ

+

µ → * γ + Z/

µ

+

µ → γ γ D-diss

acoplanarity

µ

+

µ 0.01 0.02 0.03 0.04 0.05 0.06 Data / MC 0.8 1 1.2

exclusive γγ → µ+µ− contribution extracted performing a binned maximum-likelihood fit to the measured dimuon acoplanarity distribution: 1 − |∆Φµ+µ−|/π the fiducial cross section in the dimuon invariant mass range of 12 GeV < mµ+µ− < 70 GeV is: σexcl.fid.

γγ→µ+µ− = 3.12 ± 0.07(stat) ± 0.14(syst) pb

fiducial cross section:

σexcl.fid.

γγ→µ+µ− = Nexcl. Lint×C

Nexcl. – total number of signal

events extracted using LL Lint – integral luminosity C – corr. factor for efficiencies and resolution effects differential fid. cross section as

funct. of dimuon invariant mass:
dσexcl.

γγ→µ+µ−

dmµ+µ−

=

Ni

excl.

Lint×Ci×(∆m)i

(∆m)i – width of the bin

SLIDE 10

Differential cross section and absorptive corrections

10

[GeV]

µ

+

µ

m 10 20 30 40 50 60 70

[pb/GeV]

µ

+

µ

/ dm σ d

0.05 0.1 0.15 0.2 0.25 0.3

1

= 13 TeV, 3.2 fb s ATLAS

Data

Stat. uncertainty
syst. uncertainty

⊕ Stat. EPA + finite-size correction SuperChic2 Theory uncertainty

[GeV]

µ

+

µ

m 10 20 30 40 50 60 70 Theo./ Data 0.9 1 1.1 1.2 s > /

µ

+

µ

<m

3

10

3

10 × 2

3

10 × 3

2

10

EPA

σ /

meas.

σ

0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 ATLAS

< 70 GeV

µ

+

µ

= 13 TeV, 12 < m s ATLAS > 11.5 GeV

µ

+

µ

= 7 TeV, m s CMS > 20 GeV

µ

+

µ

= 7 TeV, m s ATLAS > 45 GeV

µ

+

µ

= 8 TeV, m s ATLAS EPA + finite-size correction SuperChic2

Stat. uncertainty
syst. uncertainty

⊕ Stat.

Theo. uncertainty
3

10 × 5

expected that absorptive effects in two-photon interactions depend on the proton energy fractions passed to quasi-real photons → survival factor = ratio of measured cross section and to the bare EPA predictions → absorptive corrections tend to increase with energy fraction of protons passed to the initial-state photons

SLIDE 11

Ordered hadron chains

11 Correlation phenomena in hadron production: important source of information about early stages of hadron formation not yet understood from first principles 3D QCD string (helix-like shaped) fragmentation model & coherent emission

f adjacent hadrons:

cross-talk (causal constraint) between breakup vertices is imposed transverse shape of the string generates both the transverse momentum and the mass of the hadron quantization enables the build-up of the hadron mass spectrum assuming local homogenity of the fragmenting QCD field, Q between ground state pion can be predicted as function of their rank:

(Q(r) = 2pthr

T | sin(r∆Φ/2)|, pthr T

≃ 134 MeV, ∆Φ ≃ 2.82)

Phys. Rev. D 89, 015002 (2014)

Pair rank difference 1 2 3 4 5 Q expected [MeV] 266±8 91±3 236±7 171±5 178±5

SLIDE 12

Search of chains of correlated adjacent hadrons

12

Q [GeV]

1

10 1 (Q)

0.002

0.004 0.006

1

b µ = 7 TeV, 190 s Data, inclusive

< 0.59 GeV

3h

, m

3h

region of adjustment

ATLAS

based on minimization of mass of the shortest hadron chain containing a pair

f like-sign hadrons, a 3-hadron chain composed of 3 pairs

choosing the like-sign partner giving the minimal momentum difference adding an opposite charge hadron producing the overall minimum mass

∆Q = [N(Q)+− − N(Q)±±]/Nch

charge conservation constraint/ordering → [+ − +] or [− + −] correlation shape perfectly reproduced by selection of low mass hadron chains!

SLIDE 13

Low-mass three-hadron chains

13 upper limit on the mass of the triplet chain m3h obtained from the region of adjustment: ∆3h(Q) = fLS(Q; QLS, σLS)+fOS(Q; QOS, σOS) = nLS exp

−(Q−QLS)2

2σ2

LS

+nOS exp
−(Q−QOS)2

2σ2

OS

variations of multiplicity and correlation strength with change of acceptance region:

correlation strength: stable within restricted |η| region; reduced by a factor of 2 with pT threshold increased → in MB sample strings oriented mainly along the beam axis → correlated hadrons have small intrinsic pT → quantized fragm. model: ∼134 MeV

Parameter mcut

3h (input)

[MeV] 580 590 600 interpolation CCS/CS ±σ(stat) 0.88 ± 0.02 0.99 ± 0.02 1.09 ± 0.02 1.00 ± 0.02 (stat) ± 0.07 (syst) mcut

3h adjusted

591 ± 2 (stat) ± 7 (syst)

Measured Central Systematic uncertainty (by source) [MeV] parameter value [MeV] stat reconstruction unfolding acceptance combined mcut

3h

591 ± 2 ± 6 ± 4 −10 +7.5/−13 QLS 89.7 ± 2.1 −2.8 +2.1/−3.3 σLS 44.3 ± 0.8 −1.0 +0.8/−1.3 QOS 256.4 ± 5.5 −7.3 +5.5/−9.1 σOS 44.2 ± 1.9 −2.6 +1.9/−3.2

Acceptance pT >100 MeV pT >100 MeV pT >200 MeV variations |η| < 2.5 |η| < 1 |η| < 2.5 Nch/N main

ch

1 (by construction) 0.33 0.78 −

∆Q<0 d∆Q [%]

1.07±0.03(stat)+0.05

−0.17(syst)

1.24±0.07(stat)+0.06

−0.21(syst)

0.56±0.03(stat)+0.03

−0.10(syst)

SLIDE 14

Subtraction of selected three-hadron chains

14 ∆A(Q) = ∆(Q) − ∆3h(Q) ∆B(Q) = ∆(Q) − ∆3h(Q) − fOS(Q; QOS, σOS) data in agreement with the prediction of a thresholdlike behavior: after substraction of selected three-hadron chains from inclusive ∆(Q) no adjacent pairs up to a certain value B scenario: threshold value up to ∼0.25 GeV → coincides with threshold predicted by helical QCD string fragmentation model & fits position of the peak formed by closest

ppo-sign pairs

enhanced production of like-sign charge pairs traditionally atributed to Bose-Einstein effect R = N(Q)LS/N(Q)OS substraction of estimated contribution from

rdered hadron chains from both LS and OS

in both scenarios: the chain selection contains source of enhanced like-sign pair production → alternative explanation of the data

Q [GeV] 0.1 0.2 0.3 0.4 0.5 (Q)

0.002

0.004 0.006

1

b µ = 7 TeV, 190 s Data, (inclusive)

< 0.59 GeV

3h

, m

A

< 0.59 GeV

3h

, m

B

ATLAS

Q [GeV] 0.2 0.4 0.6 0.8 1 R(Q) 0.9 1 1.1 1.2

1

b µ = 7 TeV, 190 s Data, R (inclusive)

3h

, m < 0.59 GeV

A

R

3h

, m < 0.59 GeV

B

R ATLAS

SLIDE 15

Contribution of quadruplet chains

15 X = √ 3 T0−T2

2

i=0 Ti

Y =

3T1 2

i=0 Ti − 1

Ti - kinetic energy 0 and 2 form like-sign pair significant admixture

f opposite-sign pairs

with rank difference 3 (Q ∼ 0.236 GeV) → bias taken into account -9±5 MeV

X

1
0.5

0.5 1 Y

1
0.5

0.5 1 chains

π

+

π

π

and

+

π

π

+

π expected signal from 0.05 ± = 2.80 Φ Δ 4 MeV ± R = 68 κ X

1
0.5

0.5 1 Y

1
0.5

0.5 1 chains

π

)

+

π (

π

+

π and

π

+

π )

π

(

+

π expected signal from 0.05 ± = 2.80 Φ Δ 4 MeV ± R = 68 κ X

1
0.5

0.5 1 Y

1
0.5

0.5 1 chains

π

+

π π

π

and

+

π π

π

+

π expected signal from 0.05 ± = 2.80 Φ Δ 4 MeV ± R = 68 κ

X

1
0.5

0.5 1 Y

1
0.5

0.5 1

ch

/ N

3h

N 0.0005 0.001 ATLAS

1

b µ =7 TeV, 190 s Data, <0.59 GeV

3h

m prediction model

inclusive two-particle correlation pattern is reproduced by three-hadron chains below a mass limit of mcut

3h = 591 ± 2(stat)+7.5 −13 (syst) MeV

data show a threshold effect in the production of adjacent hadron pairs, it coincides with preferred momentum difference between opposite-sign pairs in the selected chains QOS = 256.4 ± 0.5(stat)±1.8(rec)+5.5

−9.1(chain selection) MeV

SLIDE 16

Summary

16

[GeV] s

2

10

3

10

4

10 [mb]

inel

σ 30 40 50 60 70 80 90 100

ATLAS (MBTS) ATLAS (ALFA) TOTEM ALICE LHCb Auger pp (non-LHC) p p Pythia 8 EPOS LHC QGSJET-II

ATLAS

7000 8000 9000 10000 11000 12000 13000 65 70 75 80

LHC region

[GeV]

µ

+

µ

m 10 20 30 40 50 60 70 [pb/GeV]

µ

+

µ

/ dm σ d 0.05 0.1 0.15 0.2 0.25 0.3

1

= 13 TeV, 3.2 fb s ATLAS

Data

Stat. uncertainty
syst. uncertainty

⊕ Stat. EPA + finite-size correction SuperChic2 Theory uncertainty

[GeV]

µ

+

µ

m 10 20 30 40 50 60 70 Theo./ Data 0.9 1 1.1 1.2 Q [GeV] 0.1 0.2 0.3 0.4 0.5 (Q)

0.002

0.004 0.006

1

b µ = 7 TeV, 190 s Data, (inclusive)

< 0.59 GeV

3h

, m

A

< 0.59 GeV

3h

, m

B

ATLAS

total inelastic pp cross section at √s = 13 TeV: σinel = 78.1 ± 0.6(exp) ± 1.3(lum) ± 2.6(extrap) mb

Phys. Rev. Lett. 117 (2016) 182002

the fiducial cross section for exclusive γγ → µ+µ− production in the dimuon invariant mass range of 12 GeV < mµ+µ− < 70 GeV is: σexcl.fid.

γγ→µ+µ− = 3.12 ± 0.07(stat) ± 0.14(syst) pb

Phys. Lett. B 777 (2018) 303

inclusive two-particle correlation pattern reproduced by three-hadron chains below a mass limit of: mcut

3h = 591 ± 2(stat)+7.5 −13 (syst) MeV

Phys. Rev. D 96 (2017) 092008