study of double parton scattering via w + 2- … · study of double parton scattering via w + 2-...

STUDY OF DOUBLE PARTON SCATTERING VIA W + 2-JET PROCESS USING CMS DETECTOR AT LHC

Ramandeep Kumar*, S. Bansal, M. Bansal,V. Bhatnagar, K. Mazumdar, J. B. Singh

Panjab University Chandigarh

Tata Institute of Fundamental Research Mumbai

(on behalf of the CMS collaboration)

IIT Guwahati

8-12 December, 2014

XXI HEP DAE Symposium IIT Guwahati, December 8-12, 2014Ramandeep Kumar 2

Multiple Parton Interactions (MPI)

● Multiple Parton Interactions (MPI): More than one parton-parton scatterings in a single proton-proton collision.

● At very high energy collisions, MPI matters more due to interactions at short distance scale.

● The presence of MPI in hadron collisions was experimentally established in Super Proton Synchrotron (SPS) experiments (UA1, UA2, UA5, etc. in p-pbar collisions).

● It contributes significantly to interesting single parton processes as background.● MPI studies provides information on matter overlap & multi-parton correlation.

MPI is significant at LHC


Double Parton Scattering (DPS)

In general, UE is a softer contribution, but... SOME MPI's can be hard.

► Double Parton Scattering (DPS)

Two hard parton-parton interactions in a single proton-proton collision leads to DPS.

DPS cross-section:

m = 1 when X = Ym = 2 when X ≠ Y

σXY =m

2.σ X .σY

σeff

, regarded as most natural link to the theories.σ eff

Measure of the matter overlap in hadron-hadron interactions.

● background for rare processes, e.g., Higgs, SUSY.● Provides infromation on transverse partonic

distribution of hadrons.

DPS studies are important:

● Theoretical prediction: ~ 20-30 mb. (input for event generators)σ eff

Independent of processes involved in first and second interaction, scale of the two processes and collision energy

Experimental Verification????


Effective cross-section

● Effective cross section is calculated by dividing the “non-differactive cross-section” with the “impact factor enhancement factor <f>”. σ eff=

σnon−differactive

f

Predictions from simulations:[PYTHIA8] ● A study to check the invariance of effective

cross section is done using MC events generated by using event generator PYTHIA8.

Effective cross-section (mb) = 23.7 + 3.3*log(√s[TeV])

● Effective cross section shows similar logarithmic increment variation for pp collisions or p-pbar collisions with increase in collision energy.

● Effective cross section is also found independant of scale of the two processes and the type of two processes leading to DPS in similar studies.


Effective cross-section: Experimental Measurements

Need of experimental verification to test dependance of effective cross-section on:● Processes involved in first and second interaction.

(if it is found independent, it can be measured using high rate processes and measured value can be used to estimate the contribution of rare processes.)

● Scale of the two processes & Collision energy

- Pre-LHC: Results available for collision energy from 63 GeV (AFS) to 1.96 TeV (Tevatron).

- Focus on (photon + 3-jet) process & (4-jet) process.

- LHC Measurements are there from ATLAS and CMS collaborations (7 TeV).

- Many ongoing measurements at 8 TeV with different processes (next talk)

Kinematic observables can be used to distinguish the DPS processes from the Single partonic scattering (SPS) processes.


DPS using W + 2-jet events

J1

W

SPS, Background

W

J1

J2J2

DPS, Signal

Signal: W from first hard parton-parton interaction, Exactly two jets from second one.

Decay of W in “muon” channel is studied.

Background: W +2-jet events from single interaction (SPS)

Proton-proton collision@ 7 TeV (LHC)CMS detector


Strategy towards DPS measurement

DPS

2. DPS measurements

1. corrected distributions

1. Correction of DPS-sensitive observables distributions to particle level.

2. Extraction of DPS fraction using corrected distributions and than calculation of effective cross-section.

2-step strategy

Δ pT

rel( j1 , j2)=(p⃗T ( j1)+ p⃗T ( j2)

∣pT ( j1)∣+∣pT ( j2)∣)

Δ S=arccos(p⃗T (μ ,MET )∘ p⃗T (dijet )

∣pT (μ ,MET )∣.∣pT (dijet )∣)

● ∆S, azimuthal angle between W(µν) and dijet vector

● transverse momentum imbalance b/w two jets

CMS PAS FSQ-12-028

σ eff=R

fDPS

.σ ' 2j

JHEP03 (2014) 032

Study of double parton scattering using W + 2-jet events in proton-

proton collisions at sqrt(s) = 7 TeV

MET = Missing Transverse Energy


Effective cross-section for W + 2-jet

σXY =m

2.σ X .σY

σeff

σ eff=σ 'W +0j

σ 'W +2j

DPS.σ ' 2j

σ eff=N 'W +0j

N 'W +2j

DPS.σ ' 2j

fDPS=

N 'W +2j

DPS

N 'W +2j

σ eff=N 'W +0j

fDPS

.N 'W +2j

.σ '2j

σ eff=R

fDPS

.σ ' 2j

R=N 'W+0j

N 'W+2j

In W+2-jetcross-sections are calculated

using same data sample, cross-sections terms can be re-written

in terms of number of events

● 'R' and Dijet cross-section is calculated from data.

● DPS fraction(f DPS ) is extracted by fitting DPS sensitive observables taking signal and background templates from MC.

σ 'W +0j is particle level cross-section of events having W associated with no jet.

σ 'W +2j

DPS is particle level cross-section of DPS events producing W + 0-jet from 1st interaction and exactly two jets from 2nd interaction.

σ ' 2j is particle level cross-section of di-jet events.

● pT-balance between two jets ∆ rel p

T

● azimuthal angle between W and dijet system ∆S


W + 2-jet selection & validation (Data-MC)

● A data sample of pp collisions at √s = 7 TeV collected with CMS detector with integrated luminosity of 5 fb-1.

● 'W' Selection: single muon trigger is applied – muons: p

T > 35 GeV/c, |ƞ| < 2.1

– MET > 35 GeV/c, MT(W) > 50 GeV/c2.

● Jets selection: exactly two anti-kT jets within

cone size R = 0.5 with pT > 20 GeV/c, |ƞ| < 2.

Sample Relative contribution

(%)

Data -

W→μ+ν 91.5

W→τ+ν 1.48

DY 2.12

WW/WZ 1.02

top 3.74

multijet 0.46

∆ rel pT

∆S→ Nice agreement between data and MC predictions.

→ No DPS extraction at detector level, unfold distributions to stable particle level

JHEP03 (2014) 032


Systematics & Unfolding

● Background contribution is subtracted before unfolding.

● Method: Bayesian approach (cross checked with SVD method), consistent with 1-2%

● Systematic: – background; < 0.5% – jet energy scale; 1-3% – jet energy resolution; < 1% – missing energy resolution; 1-4% – pileup; 1-4%

● W + 2-jet cross-section also unfolded to particle level: – above sources + – luminosity; 2.2% – muon ID and trigger; 2.2%

Systematic uncertainties (%)

JHEP03 (2014) 032


Corrected data distribution (unfolded to particle level)

W + 2-jet cross section = 53.4 ± 0.1 (stat.) ± 7.6 (syst.) pb, consistent with MC

∆ rel pT

∆S

Pythia8 fails; Missing contribution

of higher order processes.

LO (MG + Pythia) and NLO (Powheg +

Pythia/Herwig6) MCs provide same level of agreement with measurement.

Both LO and NLO calculations fails in absence of MPI.

CMS PAS FSQ-12-028 Corrected data distributions approved JHEP03 (2014) 032


Strategy to extract DPS fraction

● Signal templates: Random mixture of W + 0-jet and dijet events from MCs. (Pythia8 DPS events can't be used due to presence of combinatorial events.)

● Background templates: – MadGraph + Pythia8; MPI parton tagged using status code (31-39). – NO jet-parton matching.

– NO overlap and/or missing phase space.– Remove events which can be identified as signal events at particle level i.e. two MPI partons should not be in

η acceptance (|η| < 2)

– NO pT (MPI parton) dependence for < 12-15 GeV

● Fractions with two observables are consistent within uncertainties.

● Simultaneous fit of observables; close with fDPS

evt (DPS fraction by default MPI model)

JHEP03 (2014) 032

XXI HEP DAE Symposium IIT Guwahati, December 8-12, 2014

∆S∆ rel pT

Ramandeep Kumar 13

DPS fraction in data & calculation of σeff

→ Background templates; remove events, which have leading MPI partons |η| < 2.

→ Simultaneous fit of ∆ rel pT and

∆S, effect of correlation is small (less than 5%).

fDPS

= 0.055

± 0.002 (stat.) ± 0.014 (syst.)


Summary● Extraction of fraction of W+ 2-jet events due to double parton scattering.

● The effective transverse area (σeff) of the hard partonic interaction is measured

in CMS experiment, using data collected at 7 TeV of center-of-mas energy.

FDPS

= 0.055 ± 0.002 (stat.) ± 0.014 (syst.)

σeff (mb)= 20.7 ± 0.8 (stat.) ± 6.6 (syst.)

Thanks!

● Measured value is consistent(within uncertainties) with the previousresults by ATLAS, CDF, and D0.

● Large uncertainties: can not conclude dependence on collision energy.


Back-up

In high-energy proton-proton (pp) collisions at the Large Hadron Collider (LHC), semihard parton-parton scattering, producing particles with transverse momenta pT of a few GeV, dominates the inelastic cross section. In such processes longitudinal momentum fractions, given by x 2pT/ps, of values down to O(10−3) are probed.

At these values of x, the parton densities are large causing a sizable probability for two or more parton-parton scatterings within the same pp interaction [1]. Such multi-parton interactions (MPI) at semi-hard scales of a few GeVs have been observed in high-energy hadronic collisions [2].

Conversely, the evidence for hard double parton scattering (DPS) processes in the same pp collision at scales of a few tens of GeV is still relatively weak. In processes where a W and two jets are produced, the x values are larger, x 10−2, and the parton densities are lower. However, a sizable contribution to DPS can still be expected if the second scattering, yielding two jets, occurs at a high rate.

XXI HEP DAE Symposium IIT Guwahati, December 8-12, 2014 16Ramandeep Kumar

Tools to extract DPS fraction

DPS Fraction may be extarcted by:1. TFractionFitter2. rooFit

TFractionFitter is different from rooFit in the sense that it takes into account MC uncertainty as well while extracting DPS fraction.

Similar results are observed with these two different tools.

DPS fraction results are also seen by varying the number of bins for various templates.Negligible small effect is observed.

Bias Study:Bias Study is performed by generating Pseudo data by adding Signal and Background events in a particular fraction and than DPS fraction is extracted from this pseudo-data.

It is observed that fraction results are (5-6)% biased with both the tools.

XXI HEP DAE Symposium IIT Guwahati, December 8-12, 2014 17Ramandeep Kumar


Effective cross-section (simulations) : Scale invariance

'PhaseSpace:pTHatMin =x.0',.. PhaseSpace:pTHatMinSecond =x.0','PhaseSpace:pTHatMin =x.0',PhaseSpace:pTHatMinSecond =x.0',

Effective cross-section is independent of the scale of the interactions.

Scale of 1st interaction → Scale of 2nd interaction → Scale of Both interactions →

Effective cross-section is also calculated by varying the scale of 1st interaction and scale of 2nd interaction.


Effective cross-section (simulations) : Process invariance

'WeakSingleBoson:ffbar2W = on',

W+2-jet

'WeakSingleBoson:ffbar2gmZ = on',

Z+2-jet

'WeakDoubleBoson:ffbar2ZW = on

WZ+2-jet

'WeakDoubleBoson:ffbar2ZZ = on

ZZ+2-jet

'WeakDoubleBoson:ffbar2WW = on

WW+2-jet

Effective cross-section is independent of the process.

Center-of-mass energy = 7 TeV


Effective cross-section v/s collsion energy

Effective cross section shows similar logarithmic increment variation for pp collisions or p-pbar collisions with increase in collision energy

comEnergy = cms.double(x.0)...'Beams:idA = 2212','Beams:idB = -2212', 'WeakSingleBoson:ffbar2W = on',

comEnergy = cms.double(x.0)...'WeakSingleBoson:ffbar2W = on',

p-p collision

p-pbar collision

Effective cross-section (mb) = 23.7 + 3.3*log(√s[TeV])



In general, the more uneven the distribution the higher the <f_impact>. Also the pT0 parameter value has an impact, even if it is less important over a realistic range of values, although it implies that <f_impact> is energy-dependent. The origin of this effect is as follows. A lower pT0 implies more MPI activity at all impact parameters, so that the nondiffractive cross section sigma_ND increases, or equivalently the proton size. But if sigma_ND is fixed by data then the input radius of the matter overlap profile (not explicitly specified but implicitly adjusted at initialization) has to be shrunk so that the output value can stay constant. This means that the proton matter is more closely packed and therefore <f_impact> goes up.

Phase Space test

→ Def.2: No dependence on pT

max, fitted value of fraction is same as that of rejected

fraction while defining background.

→ Not true for approach used in ATLAS

Phase Space test

→ Same is true if we use matching for defining signal and background templates

Def.3Def.1: ATLAS approach

Def. 2

→ Def.2 and Def.3 seems to be covering phase-space proper.

→ Effective cross-section with Def.2 and Def.3 are in good agreement within 10%.

study of double parton scattering via w + 2- … · study of double parton scattering via w + 2-...

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