Searches for Diffractive Dijet Production

Hardeep Bansil University of Birmingham

SM Soft QCD meeting24/10/2011

Contents• Theory & Motivation• Analysis• Plots• Next steps

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CMS Diffractive W/Z search• CMS have had difficulty in trying to

find diffractive W/Z signals– 300 of 40000 W/Z events have gap – Pythia 6 tunes (ND) plotted v data– No clear signal above ND

• More luck in studying asymmetry of W being in the same hemisphere as the gap– POMPYT (diffractive) incl. with ND– Determined that 50 ± 10% of events

with a forward gap > 1.9 are diffractive

– Harder to get result with Z

• Should have more opportunities studying dijets as source of diffractive hard scattering

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Diffractive dijets• Look for single diffractive events (pppX)

– Involve a rapidity gap due to colourless exchange with vacuum quantum numbers: “pomeron”

• Then look for dijet system within X– Hard diffraction

• Sensitive to the diffractive structure function (dPDF) of the proton

• Studied at HERA and Tevatron– At Tevatron, ratio of yields of SD to inclusive

dijets ≈ 1%– Likely to be smaller than this at LHC

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Motivation• Understand the structure of the diffractive exchange by

comparison with predictions from electron-proton data and be able to get a measure of FD

jj• Measure the ratio of the single diffractive to inclusive dijet

events • Gap Survival Probability – the chance of the gap between the

intact proton and diffractive system being lost due to scattering (affects measured structure function) – Tevatron have Gap Survival Factor of 10 smaller than H1 predictions– Khoze, Martin and Ryskin predict LHC to have GSF around 30

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Rescatter with p?

(ξ)

Comparison of Tevatron results to H1 predictions

Gap destruction by secondary scattering

Event display of candidate event

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+η MBTS counters filled

-η MBTS counters empty

Large area of calorimeter empty

Two jets, one in FCAL and one in HEC

Interesting variables• Calculate MX

2 ≈ Ep·(E±pz)X ξX = MX2 /s

• Calculate zIP ≈ (E±pz)jj/(E±pz)X • Additionally study jet (η, ET, Mjj) and gap properties

• Determine differential cross sections for as many of these variables as possible

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Mjj Mx

ξX

Gap Finding Algorithm• Gap finding based on Soft Diffraction analysis

– Divides calorimeter into 49 rings of 0.2 in η– Identifies calorimeter cells where energy significance (= cell

energy/noise) large enough that probability of noise cell studied in event is small

– Where no cells in ring found above ESig threshold ring is ‘empty’– Full details in blue box

• Determine the size of the biggest forward gap

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Example Single Diffractive TopologyDetector gap definition•Calorimeter: no cell above threshold E/σ > Sth - probability of noisy cell in ring smaller than 10-4 (electronic noise only, no pile-up environment)•Tracker: no good track above pT > 200 MeV, |η| < 2.5Truth gap definition•No stable particle above pT > 200 MeV

ΔηF:3.4|ηStart|:4.9

Analysis• Using Athena version AtlasProduction-16.6.4.2• Using MinBias stream data10 period A and B ESDs

– Run 153030 (period B) excluded (explained later) – Run 155118 (period B) excluded (large trigger prescales) – Total ∫L dt = 8.71 nb-1 - calculated using online iLumiCalc tool with

L1_MBTS_2 ref. trigger• Average <μ> for selected runs < 0.15 currently ignore pile-up

• Anti-Kt jets with R=0.6 or R=0.4:– Require >= 2 jets in event – ET Jet1,2 |η| < 4.5– ET Jet1 > 26 GeV, ET Jet2 > 20 GeV for asymmetric jet ET cuts (NLO), cut values

suggested based on work by Radek Zlebcik (Prague) – Jet ET Jet2 limit and η cuts based on jet energy scale systematic– Currently no requirements to ask about jet quality cuts

• Ask for a forward gap: |ηstart| = 4.9, ΔηF ≥ 2.0

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Asymmetric Jet Cuts• Parton level studies of single diffractive dijets• Using NLOJET++ and Frixione with cuts on right • Look at NLO negative/positive interference

contributions to cross section

• If pT of sub-leading jet is 20 GeV , then safe cut for leading jet is at 26 GeV (start of exponential drop in csx)

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NLO Cross section plots courtesy of Radek Zlebcik

Logarithmic y-axisLinear y-axis

Monte Carlo for Analysis• Currently using Pomwig LO generator as main comparison

– Modifies Herwig ep photoproduction so ee+γ becomes pp+IP – No rapidity gap destruction built in– Generates QCD 22 process within diffractive system in different pT ranges (8-17, 17-35, 35-70, 70+ GeV) for SD (system dissociating in ±z

direction)• Using MC samples generated by myself (4000 events of each

Pomwig sample) • Will need to get official Monte Carlo production done soon

• Have Pythia 6 and Pythia 8 Dijet samples to use as background (8-17, 17-35, 35-70, 70-140 GeV)– Pythia 8 J0 sample (8-17 GeV) not available without pile

up so not used here (needs official MC production too)

• Rapgap – Still trying to get this working in Rivet• Try Herwig++ samples soon

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Will asking for official MC prod now mean I have to migrate to Athena v17?

Reason to exclude run 153030

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AntiKt4 AntiKt6 • In an otherwise empty event, the problem modules create a large energy deposit in TileCal

• In AntiKt4 this can make 2 high ET jets passing the cuts, but with a larger cone size it will only create 1 jet

• Note the large amount of missing ET required to balance the event

• Affects most lumi blocks in run, DQ group informed as no DQ flags for this

Uncorrected Gap Size Distribution

• Ratio of ΣMonte Carlo to Data suggests a Gap Survival Factor of 20-30 in majority of bins (prev. ≈3)

• By ΔηF of 6, ξ = 10-4.5 MX = 39.4 GeV – cut out phase space for producing pair of 20 GeV jets so get drop in events after this point

• Dijet samples contribute less at higher gap sizes with new gap algorithm• Not seeing flattening out of data or Pomwig SD not really indicating that a diffractive

plateau is present• Need to move back to lower pT jets (7 GeV) in order improve chances of observing signal

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MinBias DataPomwig SDPythia 8 JetsPythia 6 Jets

Biggest ND contribution at small ΔηF

Drop in number of events with ΔηF ≥ 6, cuts into phase space

Pomwig SD, Pythia 6 & 8 Jets weighted relative to luminosity of data runs used and then plotted (stacked) against MinBias Data

Uncorrected Jet η Distribution• Pomwig SD, Pythia 6 & 8 Jets weighted relative to luminosity of data runs used and

then plotted (stacked) against MinBias Data

• Ratio of ΣMonte Carlo to Data suggests a Gap Survival Factor of 20-40

• Before gap cuts, get a small asymmetry in η (similar results for Jet 1 & 2)– Slightly more events with negative η for leading jet

• After gap cuts, get more events with positive η in data (apart from edge bins) and all MC samples have an unusual shape– May have something to do with jet ET cuts but would appreciate any suggestions as to why this occurs

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MinBias DataPomwig SDPythia 8 JetsPythia 6 Jets

MinBias DataPomwig SDPythia 8 JetsPythia 6 Jets

Before forward gap cuts After forward gap cuts

• SD weighted relative to luminosity of data runs used and then scaled to data integral (from plot) to make comparison of distribution shape

Shape Comparison

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ET Jet 1 η Jet 1

zIPGap size

Work more on this variable

Edge bins need to be studied

MinBias DataPomwig SDPythia 8 JetsPythia 6 Jets

MinBias DataPomwig SDPythia 8 JetsPythia 6 Jets

MinBias DataPomwig SDPythia 8 JetsPythia 6 Jets

MinBias DataPomwig SDPythia 8 JetsPythia 6 Jets

Differential Cross Sections• Pomwig SD weighted to lumi of data runs - Differential cross

section as a function of leading jet ET• Still need to add in Pythia dijet samples as background

16MC/Data ratio suggests GSF of approx. 20

MinBias DataPomwig SD

SD Combined AcceptanceStill need to determine whysome bins have high acceptances (and which SDsamples cause this to occur)

Differential Cross Sections• Pomwig SD weighted to lumi of data runs - Differential cross

section as a function of leading jet ET• Still need to add in Pythia dijet samples as background• No observed diffractive plateau – move to lower pT jets

17MC/Data ratio suggests GSF of approx. 30

SD Combined AcceptanceStill need to determine whysome bins have high acceptances (and which SDsamples cause this to occur)

MinBias DataPomwig SD

Next steps• Move to lower pT jets for analysis to see stronger

case for diffractive dijet production?– Will it be possible? Will need to evaluate JES systematic

for this• Try later data (e.g. Period C, D) but now watch out

for pile-up and MBTS prescales• Apply jet quality cuts• Test for calorimeter noise with RNDM stream

• Get official production of Pomwig SD samples• Get official production of Pythia 8 J0 sample• Get cross sections from Rapgap / Herwig++ and NLO

theory to compare with Pomwig

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BACKUP SLIDESDiffractive Dijets

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Mx, zIP, xP reconstruction• Based on E±pz method, which uses energy-momentum conservation and

fact that in SD, the intact proton loses almost none of its momentum

• Calculate Mx, xP and zIP using jets and calorimeter clusters on the correct side of the gap

• If X system goes to +z and intact proton to -zMX

2 = Ep·(E+pz)clus

zIP = (E+pz)jj/(E+pz)clus xP = (E-pz)jj/(E-pz)clus

• If X system goes to –z and intact proton to +zMX

2 = Ep·(E-pz)clus

zIP = (E-pz)jj/(E-pz)clus xP = (E+pz)jj/(E+pz)clus

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