Selected results related to forward
physics from the ATLAS experimentFrance-Japan SAKURA Workshop on small-x physics at LHC
29/10/2016
Yuji Yamazaki (Kobe)

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LHC operation 2016
• 𝑝𝑝 collisions
just finished• Heavy Ion run for
the rest of the year(𝑝 − Pb collisions
𝑠𝑁𝑁 = 5 and TeV)

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Run-2 2016 luminosity• 2015: 4.2 fb−1 delivered• 2016: almost 40 fb−1

Special runs
(low 𝛽*)– After all kind of troubles (injector, shield, ….)
the operation is very smooth from June onwards– Design luminosity (1 × 1034 cm2 s −1 ) achieved!
– Hopefully successfully collecting the Run-2 luminosity of 100 fb−1towards 20193

2016 challenge: trigger• Input rate: ~ 100 kHz(run1: 75 kHz)– with a bit of dead time

• Output rate: up to 2 kHz(run-1 design: 400 Hz)– Tight resource for computing etc.

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ATLAS Run1/2 standard model summary𝑡𝑡,ҧ
single top
W, Z
Diboson

W, Z + jets

Higgs
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Today’s selected topics• Diffractive dijet production• Double-parton interaction using 4-jets∗• The “𝜙ℓℓ” behaviour in W/Z• Inelastic cross section @ 13 TeV with MBTS (remnant tagger)• Elastic cross sections by the ALFA detector

6Phys. Lett. B 754 (2016) 214

Diffractive dijet cross sections• Main objective: to obtain the “survival probability”• Rapidity gap selection• Low 𝑝𝑇 jets to access small 𝑀𝑋– 𝑝𝑇 > 20 GeV and 𝜂 < 4.4• Cross sections in 𝜉ሚ (an estimator of 𝜉)and Δ𝜂 𝐹 (rapidity gap from the forward edge of the detector)

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Diffractive region: 𝚫𝜼𝑭 > 2.0• Significant excess over ND models (PYTHIA8, POWHEG+PYTHIA)• PYTHYA8 SD+DD explains the data without suppression factor– for various “Pomeron flux” models• POMWIG too high, giving 𝑆 2 = 0.16 ± 0.04 𝑠𝑡𝑎𝑡. ± 0.08(𝑒𝑥𝑝. 𝑠𝑦𝑠𝑡. )What is going on?

8cf. CMS result• You see a bit of excess over Pythia8 ND+SD+DD, in fact
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CERN-EP-2016-183 arXiv:1608.01857

Double-parton interactions through 4-jets• two types of double-parton scattering signal in 4-jet events– “cDPI”: complete-DPI, 2-jet ⨂ 2-jet– “sDPI”: semi-DPI, 3-jet ⨂ 1-jet• 1jet missing from detection for the “second” scattering– and generic 4-jet events from single parton-scatteringcDPI can be distinguished from the SPI, but sDPI not quite

10Effective cross section 𝝈𝐞𝐟𝐟
• 𝜎eff : transverse area of the hadron causing DPI–𝐴 𝐵
1 𝜎2𝑗 𝜎2𝑗DPS
𝜎4𝑗 = 2 𝜎 , which can be obtained using the relation:eff

DPS
𝜎4𝑗= 𝑓DPS ⋅ 𝜎4𝑗 and the dijet cross section 𝜎2𝑗+5.1• 𝜎eff = 14.9+1.2−1.0 𝑠𝑡𝑎𝑡. −3.8 (𝑠𝑦𝑠𝑡. ) mb

– obtained from the measurement
𝐷𝑃𝑆+0.033
𝑓4𝑗= 0.092+0.005−0.011 𝑠𝑡𝑎𝑡. −0.037 (𝑠𝑦𝑠𝑡. )• No trend in increase/decrease
as a function of 𝑠11

∗
Drell-Yan 𝒑ℓℓ
and“𝝓𝜼”𝑻Eur. Phys. J. C 76(5), 1-61 (2016)• 𝑝𝑇 of the Drell-Yan lepton pair is sensitive to radiation:for both soft (or forward) and hard regime•𝜋−Δ𝜙𝜙𝜂∗ = tan2

⋅ sin 𝜃𝜂∗≃ℓℓ𝑝𝑇2𝑀ℓℓ– only with angles of leptons, experimentally more robust

some structure against RESBOS, approximate NNLO+NNLL soft gluon
below and above the mass of the vector boson

12dependence on the vector-boson rapidity•The model knows the approximate
trend as a function of the bosonrapidity•the pattern of the deviations are similar
for all rapidity ranges

13comparison to other models• Quite different predictions: see– Powheg+Pythia and +Herwig– Powheg and Sherpa– … and RESBOS

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CERN-EP-2016-140 arXiv:1606.02625

Inelastic cross section @ 13 TeV• MBTS (Minimum-bias trigger scintillators) to tag inelastic,
SD (single-diffractive) and DD (double-) events– Covering 2.07 < 𝜂 < 3.86, corresp. to 𝜉 = 𝑀𝑋 Τ𝑠 > 5 × 10−6

• Other detectors including LHCf are used
to calibrate the trigger efficiency MBTS

Inclusive events

Single-sided events (mostly SD)

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Diffractive fraction
and MBTS hits

• 𝑅𝑆𝑆 = (single-sided)/inclusive– EPOS/QGSJET needs large
diffractive fraction for explaining
the observed 𝑅𝑆𝑆– Mainly because of large
multiplicity in the MBTS i.e. inforward rapidity
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13 TeV inelastic cross section 𝝈𝐢𝐧𝐞𝐥• Uncertainty due to the diffractive fraction 𝑓𝐷 is small• Extrapolation for 𝜉 < 5 × 10−6 : 9.9 ± 2.4 mb– “total” inelastic: 𝜎inel = 78.1 ± 0.6 exp ± 2.4 extrap. mb
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Phys. Lett. B (2016) 158

Total cross section from optical theorem• 𝑡 −distribution measured bydouble-arm Roman pots
– ALFA scintillating fibres byATLAS– TOTEM around the CMS IP• ALFA uses luminosity forabsolute cross section• TOTEM does not depend on
luminosity measurement𝑑𝑁𝑒𝑙ฬ
𝑑𝑡 𝑡=016𝜋
𝜎𝑡𝑜𝑡 =1 + 𝜌2 (𝑁𝑒𝑙 + 𝑁𝑖𝑛𝑒𝑙 )18

Results• Some tension between two results– slope results agree, though
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BACKUP20

Effect of Coulomb scattering•

Total cross section determined by extrapolating to 𝑡=0– need to take into account the Coulomb scattering and non-exponential slope•

interference around 𝑡 ~0.01•

cross section only important below this range (not measured by ATLAS)
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