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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
XXI HEP DAE Symposium IIT Guwahati, December 8-12, 2014Ramandeep Kumar 3
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????
XXI HEP DAE Symposium IIT Guwahati, December 8-12, 2014Ramandeep Kumar 4
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.
XXI HEP DAE Symposium IIT Guwahati, December 8-12, 2014Ramandeep Kumar 5
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.
XXI HEP DAE Symposium IIT Guwahati, December 8-12, 2014Ramandeep Kumar 6
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
XXI HEP DAE Symposium IIT Guwahati, December 8-12, 2014Ramandeep Kumar 7
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
XXI HEP DAE Symposium IIT Guwahati, December 8-12, 2014Ramandeep Kumar 8
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
XXI HEP DAE Symposium IIT Guwahati, December 8-12, 2014Ramandeep Kumar 9
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
XXI HEP DAE Symposium IIT Guwahati, December 8-12, 2014Ramandeep Kumar 10
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
XXI HEP DAE Symposium IIT Guwahati, December 8-12, 2014Ramandeep Kumar 11
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
XXI HEP DAE Symposium IIT Guwahati, December 8-12, 2014Ramandeep Kumar 12
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.)
XXI HEP DAE Symposium IIT Guwahati, December 8-12, 2014Ramandeep Kumar 14
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.
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
XXI HEP DAE Symposium IIT Guwahati, December 8-12, 2014Ramandeep Kumar 18
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.
XXI HEP DAE Symposium IIT Guwahati, December 8-12, 2014Ramandeep Kumar 19
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
XXI HEP DAE Symposium IIT Guwahati, December 8-12, 2014Ramandeep Kumar 20
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])
XXI HEP DAE Symposium IIT Guwahati, December 8-12, 2014Ramandeep Kumar 21
Strategy towards DPS measurement
XXI HEP DAE Symposium IIT Guwahati, December 8-12, 2014Ramandeep Kumar 22
Strategy towards DPS measurement
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
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