diffractive dijet production with 2010 data hardeep bansil (1), oldřich kepka (2), vlastimil kůs...

$: Diffractive Dijet Production with 2010 data Hardeep Bansil (1), Oldřich Kepka (2), Vlastimil Kůs (2), Paul Newman (1), Marek Taševský (2) (1) University$
Diffractive Dijet Production with 2010 data

Hardeep Bansil(1), Oldřich Kepka(2), Vlastimil Kůs(2), Paul Newman(1), Marek

Taševský(2)

(1) University of Birmingham(2) Institute of Physics, Academy of Sciences of the Czech Republic

Standard Model Plenary Meeting24th April 2014

Current Status• Previously presented in SM Plenary meeting - July 2013

• CDS supporting note available since March 2014• https://cds.cern.ch/record/1670320

• (also see H. Bansil thesis)• http://cds.cern.ch/record/1696944

• Aiming for editorial board / paper

Diffractive Dijet Production with 2010 Data 224/04/2014

https://cds.cern.ch/record/1670320

http://cds.cern.ch/record/1696944

Introduction to diffraction• Total cross section in hadronic scattering experiments at 7 TeV

– Total = Elastic + Diffractive + Non-diffractive (ND)– 20% elastic, 80% inelastic (diffractive + ND)

• Diffractive channels together – 25-30% of the σinel

– Single-diffraction (SD: pp pX) – Main process of interest– Double-diffraction (DD: pp XY)

• Kinematic variables– invariant mass of the dissociated system MX (MY)

• at the LHC energy spans mp+mπ to approx. 1TeV

– fractional momentum loss ξX of the scattered proton

ξX = MX2 / s [ξY = MY

2 / s]

• Diffraction in the realm of soft QCD– Best described by phenomenological models (e.g. Regge theory)– Exchange of color singlet (Pomeron) large angular region in which no outgoing particles

(soft QCD radiation) are detected rapidity gap

Diffractive Dijet Production with 2010 Data 3

SD

DD

24/04/2014

Rapidity gaps in ATLAS detector

• Large Rapidity Gap (LRG) … Δη ~ -log10ξX … smaller ξX (MX) bigger gap– Region in η devoid of hadronic activity due to the exchange of colorless object (Pomeron)

• Detector-level LRG definition (ΔηF)– First defined by soft diffractive rapidity gaps analysis (Eur.Phys.J. C72(2012) 1926)– Biggest region in η (starting at the edge of the detector η=±4.9) absent of clusters and tracks

complying selection:– no tracks with pT>pT

cut (pTcut = 200 MeV)

– no TopoClusters; noise suppression …• (pT

cluster > 200 MeV)

• most significant cell in the cluster: Ecell/σnoise> Sthreshold

• No pile-up environment required– Pile-up could occupy the gap– Use early runs from 2010 (Period B)


ΔηF ~ 6 ↔ ξ ~ 10-4

LRG

24/04/2014

Hard diffraction• The aim

– Study single diffraction at hard scale, i.e. in high pT dijet events for first time in ATLAS

– Measure cross-section vs. gap size (ΔηF) and ξ (fractional momentum loss of intact proton)

– Independent measurement of gap survival probability in terms of both ΔηF & ξ

• Gap Survival Probability (S2)– Hard diffraction studied precisely at HERA (ep collisions). Diffractive PDFs (dPDFs) measured.

– Then Tevatron (pp collisions). Structure function ~10x smaller than HERA-based dPDFs predictions for Tevatron conditions. Explanation – rescattering of dissociated system with intact proton.

– S2 typical for hadron-hadron collisions

• What about S2 at LHC?– theoretical predictions (KMR) … S2 ~ 5-10 %


CDF data

Predictions based on HERA’s dPDFs

zIP, Momentum fraction of parton emitted by Pomeron

24/04/2014

Rescatter with p ?

ξ

PYTHIA8 + POMWIG• PYTHIA8 non-diffractive samples

• PYTHIA8 diffractive samples can use different Pomeron flux models – influences diffractive distributions

• Comparison to default Schuler-Sjostrand• H1 DPDFs better described by

Donnachie-Landshoff model• Can test against different models

• POMWIG samples• Modified HERWIG, Based on picture to right, using H1 DPDFs • Generated over kinematic range 10-6 < ξ < 0.1 and

10-6 < |t| < 10 GeV2


Event selection criteria• Starting with SM2010 inclusive measurement (next slide)• Good Runs List, Good primary vertex (ntracks>4), Triggers• Kinematic cuts (pT jet 1 > 30 GeV, pT jet 2 > 20 GeV, |ηjets|<4.4) , anti-kT R=0.4 or R=0.6

• Focus on 2010 Period B data adding pile-up suppression cut L = 6.75 nb-1

• need for events with 1 interaction only … no PU vertices (having ntracks>1)• removes 5% of events in period B (σ-correction factors applied)

• Going below trigger efficiency plateau for jet triggers (next slide)• improving the use of available statistics for large gaps

• Going to lower jet-pT ranges• Increases statistics, allows smaller diffractive systems to be studied larger gaps• pT jet 1 > 20 GeV, pT jet 2 > 20 GeV , |ηjets|<4.4, anti-kT R=0.4 or R=0.6

• Forward gap definition (ΔηF)• after extensive studies “hybrid” gap-definition method:• “Measurements of the total transverse energy in pseudorapidity bins in proton-proton collisions at √s with ATLAS”• η-region devoid of activity (starting at either η=-4.8 or η=+4.8)• detector-level noise cuts: tracks with pT track > 200 MeV• TopoClusters with cell significance Ecell/σnoise > Sthr (~5.5)• Corrected (truth) cross section definition: pch (n) particle > 500 (200) MeV OR pT > 200 MeV


Trigger strategy• Start from Implementation of selection cuts and

trigger scheme from inclusive SM2010 dijet cross-section measurement

• Invariant mass spectrum successfully reproduced• presented during Inclusive Jet + Dijet Cross-Section meeting in April 2012

• However, event yields at large gaps unsatisfactory

• Optimise inclusive SM2010 measurement (OR of triggers associated with leading and sub-leading jet according to their pT, η)

– L1_J5 trigger for central jets (|η| < 2.8)

– Lower pT and forward jets triggered by L1_MBTS_1 (L1 forward jets not commissioned by then)

• Going below 99% trigger efficiency plateau (but staying above 70%) as MBTS_1 highly prescaled

• presented during Jet Trigger Signature Group Meeting in October 2012

(this allows to use jet triggers at lower pT) necessity to weight events by 1/εtrig


lower pT lower invariant mass larger gaps24/04/2014

L1_J5 (0.3<|η|<1.2)

R=0.4 jetsR=0.6 jetsFit to R=0.4Fit to R=0.6

ξproton reconstruction


• Truth level - fractional momentum loss of scattered proton– ξproton = (3.5TeV – pZ

proton) / 3.5TeV

• Cross section measured in terms of observable ξ± – approximation to real ξ– ξ± = Σ pT e±y / √s (TopoClusters / stable particles)– ξ+ … intact proton going in the +z

direction (system X in -z dir)– ξ- … intact proton going in the -z

direction (system X in +z dir)

• Approximation performs very wellfor ξ<0.01

Pythia 8 SD

ND, SD and DD ranges


• Data against PYTHIA8 ND, DD and SD – all scaled to L= 6.75 nb-1

• Non diffraction dominant for ΔηF < 2, ξ± > 0.01• Then diffractive contributions become more prominent

Comparison of groups• Comparison of uncorrected cross sections between Prague and

Birmingham groups– Completely independent code bases– Minor discrepancies exist work in progress


Unfolding


• Bayesian 2D unfolding technique– RooUnfold 1.1.1– input: pT of the leading jet vs. ΔηF or ξ±

• Detector-level PYTHIA8 ND:(SD+DD) ratio “fitted” to data to get the best possible shape description in both ΔηF and ξ± distributions– ΔηF distribution: ND×0.62, (SD+DD)×0.206– ξ± distribution: ND×0.577, (SD+DD)×0.283

Performance of 2D Unfolding


• Comprehensive tests of unfolding performance & stability done• closure tests

• sensitivity tests to mixture of ND, SD and DD (different distribution shapes)

• convergence of iterations (χ2)

• stability against choice of binning

• ... (more information in the back-up note)

• optimum 4 iterations

*Illustration based on 1D unfolding

2D unfolding actually uses severalinputs to account for migrations

Systematic uncertainties


• Jet Energy Scale (JES)• Jet Energy Resolution (JER)• Jet Angular Resolution (JAR)• Jet Reconstruction Efficiency (JRE)• Jet Cleaning Efficiency (JCE)• Unfolding• Trigger efficiency • Cluster energy scale (CES)• Cell significance threshold cut (CTC) uncertainty• Tracking - negligible• Vertex requirement• Luminosity

• Procedure:• 1) Run uncertainty-adjusted analysis on MC• 2) Produce new smearing matrix & reco-MC plots• 3) Unfold uncorrected data with new smearing matrix & reco-MC• 4) Compare new unfolding to standard procedure unfolding

Uncertainty inputs from SM 2010dijet analysis used here

Inherited from soft diffractiveanalysis

Selected systematic uncertainties


• Jet Energy Scale (JES)• Inherited from 2010 SM dijet analysis• Split into 7 components• JES can vary as much as 15% for

components as a function of η, pT • Added in quadrature• Uncertainty range: typically 25-30%

continuously rising up to large gaps

• ΔηF, Cell significance threshold cut (CTC)• Adjust noise suppression thresholds up and

down by 10%• Uncertainty typically 15-25%

• ξ±, Cluster Energy Scale (CES)• Cluster EM energy scale in MC adjusted by

correction factors • Uncertainty typically 10-20%

Differential cross sections• Differential cross sections calculated as for ΔηF

• Nweighted accounts for trigger efficiency per data event, prescales and unfolding• Compare against POMWIG and different PYTHIA8 flux models

• Requirement of hard scale affects kinematics no rapidity gap plateau• Cross sections slightly higher for R=0.6 but maintain the same shape• For PYTHIA8, ND ~1.3x larger in first bin, SD+DD/ND fairly even for ΔηF >2.5• POMWIG ~3x larger csx than data for ΔηF>3, slightly higher for R=0.4

16

)( F

weighted

F LN

dd

R=0.6R=0.4


Differential cross sections• Differential cross sections calculated as for ξ±

• Nweighted accounts for trigger efficiency per data event, prescales and unfolding• Compare against POMWIG and different PYTHIA8 flux models

• Forward gap requirement of 3 units removes majority of large ξ± events• POMWIG results significantly above data, PYTHIA8 roughly equal to data csx• Could be used to determine S2

LN

dd weighted

R=0.6, no gap req.


R=0.6, ΔηF > 3

Differential cross sections• Differential cross sections also calculated for jet variables• Without gap requirements, data compatible with PYTHIA8 ND• After Forward gap requirement of 3 units to enhance diffraction:

• Leading jet pt: diffractive states fall away faster than ND• Leading jet η: limited statistically but shows hints of double peak structure observed

in diffractive MCs, ND shows single central peak


Summary• Reproduced soft diffractive & inclusive dijet cross section

measurements• New selection cuts & trigger strategy developed• Analysis of 2010 period B data in ΔηF (gap size) and ξproton

(fractional momentum loss) distributions• Good agreement between Birmingham-Prague• Supporting note in advanced stage

• Looking to request editorial board as soon as possible• Some additional studies and MC generation (NLO calculations)

planned to improve measurement, extract gap survival probability (S2)


BACK UP SLIDES

24/04/2014 Diffractive Dijet Production with 2010 Data 20

Validation of new triggersPeriod B


Uncorrected distributions

RAW EVENTS SM2010 B New B

Gaps > 3 59 186

Gaps > 4 9 52

Agreement within 5%!

Significant grow of statistics at large gaps

24/04/2014

Diffractive dijet measurement by CMS• CMS measured diffractive contribution to dijet production at LHC

– based on L = 2.7 nb-1

– Measurement of ξ (approximates fractional momentum loss of scattered proton)– region of interest: ΔηF>1.9

• comparison to different MC models– ND (red): PYTHIA 6 & 8– SD (blue): PYTHIA 8, POMPYT, POMWIG– DD: PYTHIA 8– POWHEG for NLO comparisons

• Results– SD MCs predict more events than observed by

factor ≈5 in lowest ξ bin (S2)– data also consists of proton dissociative events

(scattered proton excited into low mass state escaping undetected into the forward region)

• Gap Survival Probability– S2 = 0.12 ± 0.05 (LO)– S2 = 0.08 ± 0.04 (NLO)


~

~

Phys. Rev. D 87 (2013) 012006

S2

Unfolding systematic• Method: creating “new” MC (by reweighting) describing reco-level data well• 1) Fit DataReco/MCReco by continuous function (data-MC scaled agreement doesn’t have to be perfect)• 2) Rerun MC analysis with weights from the fitting function• 3) Unfold scaled reco-MC by standard procedure• 4) Compare unfolded MC to truth MC (scaled) -> uncertainty

24/04/2014 Diffractive Dijet Production with 2010 Data 24

ΔηF measurement• Soft diffractive measurement • dσ/dΔηF ~ constant over several units of rapidity plateau


Soft diffraction

Diffractive plateau

diffractive dijet production with 2010 data hardeep bansil (1), oldřich kepka (2), vlastimil kůs...

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