Soft QCD, Minimum bias and UE measurements at ATLAS

(1)

Notreviewed,forinternalcirculationonly

Soft QCD, Minimum bias and UE measurements at ATLAS

Oleg Zenin

On behalf of ATLAS Collaboration IHEP, Protvino, Russia

37th International Conference on High Energy Physics, 2-9 July 2014,

Valencia, Spain

(2)

Overview

Inclusive φ(1020) production in pp collisions at

√ s = 7 TeV

Underlying event measurements in jet and inclusive Z production.

MPI W + 2jet production at √

s = 7 TeV.

ALFA elastic d σ/dt results in the next talk by Hasko Stenzel

An emphasis on recent ATLAS soft-QCD results.

(3)

φ (1020) differential cross section arXiv:1402.6162 (accepted by EPJ C) pp → φ X , φ → K

⁺

K

⁻

, √

s = 7 TeV K

^±

are identified by

^dE_dx

in the pixel detector.

Fiducial volume:

500 < p

_T

(φ) < 1200 MeV, |y(φ)| < 0.8, p

T

(K

^±

) > 230 MeV, p(K

^±

) < 800 MeV Sensitive to s-quark and low-x gluon densities.

Also constrains fragmentation models.

dσ

dp_T

extrapolated to the full |y

_φ

| < 0.5 fiducial volume agrees with ALICE result:

More plots in backup

Large spread of predictions even between different tunes of Pythia 6 MC:

The fiducial cross section:

σ·Br(φ→K⁺K⁻) = 570±8_stat±66_syst±20_lumiµb.

(4)

Underlying event: definition

No strict definition.

Everything except the hard process:

soft ISR & FSR, MPI, beam remnants.

Can’t distinguish between various contributions. No reliable pQCD calculations.

Observables:

densities of particle multiplicitiy and E T

flow as functions of ∆φ angle and the scale of the hard scattering, p T . Leading object: jet or Z -boson.

Transverse and toward (in Z production only) regions are sensitive to the UE.

Further suppress jet activity by selecting the transverse side with lower ΣE T in each event.

−∆φ ∆φ

Leading object: p T

Toward

|∆φ| <

^π₃

Away

|∆φ| >

^2π₃

Transverse

π

3

< |∆φ| <

^2π₃

Transverse

π

3

< |∆φ| <

^2π₃

(5)

Underlying event: jet events arXiv:1406.0392 (accepted in EPJ C) Inclusive and exclusive jet topology:

pp → jet X and pp → 2jets, √

s = 7 TeV.

|η^jet|<2.8,p_T^jet= 20−800 GeV.

N

^ch

and Σp

_T^ch,ch+neu

densities vs. p

_T^lead.

and hp

_T^ch

i vs. p

_T^lead.

and N

^ch

With charged tracks:p^ch_T >0.5 GeV,|η^ch|<2.5 With calorimeter clusters:p^neu_T >0.2 GeV,|η^ch+neu|<4.8 Transverse regions are classified as min./max. in each event, according to Σp_T.

Transverse activity increases withp_T^lead, except for trans. min.

In exclusive dijet events this feature disappears.

FlathΣE_Tidensity in exclusive dijet events hints at MPI independent on the hard process.

MC models satisfactorily describe the data at|η|<2.5 but not in the full|η|<4.8 range.

⇐Exclusive dijets:

a sizable depletion of transversehΣE_Ti asp^lead_T is growing.

(6)

Underlying event: inclusive Z events (preliminary) pp → Z X , √

s = 7 TeV, Z → e

⁺

e

⁻

/µ

⁺

µ

⁻

NO exclusiveZ+ jet selection.

p^`_T>20 GeV,|η^`|<2.4,p_T^Z= 0−500 GeV.

N^chand Σp^ch_T densities vs.p_T^Zand hp_T^chivs.p^Z_TandN^ch

With charged tracks:p_T^ch>0.5 GeV,|η^ch|<2.5

Very similar to UE in inclusive jet events, up to kinematic selection bias as jets with p

^jet_T

> p

_T^Z

are allowed in inclusive Z selection, hence the long Σp

T

tail:

(7)

DPI in W + 2 jets New J. Phys. 15 (2013) 033038

√ s = 7 TeV, L = 36/pb.

W (`ν) + 2 jets with p

_T

> 20 GeV and |y| < 2.8.

Measure p

T

balance in the dijet system:

∆

jets

= |~ p

_T,1

+ ~ p

_T,2

|,

∆

ⁿ_jets

= ∆

_jets

|~ p

T,1

| + |~ p

T,2

|

Fraction of DPI contribution to W (`ν) + jj final state:

f

DPI

= 0.08 ± 0.01

stat

± 0.02

syst

(8)

DPI in W + 2 jets (continued) New J. Phys. 15 (2013) 033038

Effective area for hard DPI:

σ eff = 15 ± 3 stat +5 −3 syst mb

(9)

Summary

Non-perturbative QCD domain is probed in pp collisions at 0.9 and 7 TeV.

Access to very low x due to high √ s.

Hard jet, Z and W production is used to probe the underlying event and, particularly, multiple parton interactions.

I

UE is found to be independent from the hard process at high p

_T

scales.

Phenomenological MC models are tuned for 13 TeV LHC run using

data at lower energies.

(10)

Backup

(11)

φ(1020) differential cross section arXiv:1402.6162 (accepted by EPJ C)

Kaon identification:

To main slide

φ → K + K − signal extraction:

(12)

Underlying event: jet events arXiv:1406.0392 (accepted in EPJ C)

To main slide

(13)

To main slide

(14)

To main slide

(15)

Underlying event: jet events arXiv:1406.0392 (accepted in EPJ C)

To main slide

(16)

Underlying event: inclusive Z and jet events (preliminary)

To main slide

(17)

Underlying event: inclusive Z and jet events (preliminary)

To main slide

(18)

Underlying event: inclusive Z events, Σp T ch (preliminary)

To main slide

(19)

Underlying event: inclusive Z events, N ch (preliminary)

To main slide

(20)

Underlying event: inclusive Z events, Σp T ch vs MC

To main slide