Transcript 20111208_BhamAtlasWeekly_Diff

Update on Diffractive Dijet
Production Search
Hardeep Bansil
University of Birmingham
Birmingham ATLAS Weekly Meeting
08/12/2011
Contents
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Theory & Motivation
Analysis
Plots
Comparison with Prague analysis
Next steps
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Diffractive dijets
• Look for single diffractive events (pppX)
– 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 FDjj
• 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
Rescatter
with p?
Comparison of
Tevatron results to
H1 predictions
(ξ)
Gap destruction
by secondary
scattering
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Interesting variables
• Calculate MX2 ≈ Ep·(E±pz)X  ξX = MX2 /s
• Calculate zIP ≈ (E±pz)jj/(E±pz)X
• Study jet (η, ET, Mjj) and forward gap properties
Mjj
Mx
ξX
• Determine differential cross sections for as many of
these variables as possible
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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
Detector 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)
Example Single Diffractive Topology
ΔηF:3.4
|ηStart|:4.9
•Tracker: no good track above pT >
200 MeV, |η| < 2.5
Truth gap definition
•No stable particle above pT > 200
MeV
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Analysis
• Using Athena version AtlasProduction-16.6.4.2
• Using MinBias stream data10 period A and B ESDs
– Run 153030 (period B) excluded (due to noise bursts)
– 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 passing loose jet quality cuts
– ET Jet1,2 |η| < 4.4
– 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 2010 jet energy scale systematic
• 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 interference terms
(order α3) to cross section
ETjet1, 2  20 GeV
 5   jet1, 2  3
x  0.03
t  1 GeV 2
• If pT of sub-leading jet is 20 GeV , then safe cut for leading jet
is at 26 GeV (well within exponential drop in csx)
NLO Cross section plots courtesy of Radek Zlebcik
Linear y-axis
Logarithmic y-axis
• Need someone to take over this work from Radek
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Monte Carlo for Analysis
• Currently using POMWIG LO generator as main comparison
– Modifies HERWIG ep photoproduction so ee+γ becomes pp+IP
– No rapidity gap destruction built in
– Generates QCD 22 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)
• 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) available only with pile up (not used)
• NLO comparison would be ideal
– Need someone to work on this
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Candidate event
-η MBTS
counters empty
+η MBTS
counters filled
∆ηF=5.2
Large area of
calorimeter empty
Two jets, one in FCAL (η=3.48)
and one in HEC (η=3.05)
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Uncorrected Gap Size Distribution
POMWIG SD,
PYTHIA 6 & 8 Jets
weighted relative
to luminosity of
data runs used and
then plotted
against MinBias
Data
MinBias Data
Pomwig SD
Pythia 8 Jets
Pythia 6 Jets
Drop in number of
events with ΔηF ≥ 6,
cuts into phase space
Biggest ND
contribution at
small ΔηF
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Ratio of MC to Data suggests a GSF of 15-25 in majority of bins (prev. ≈3)
Big factor between PYTHIA and data in the first bin
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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
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– Should be within a factor of 2 based on inclusive jets analysis
– Still observe forward gaps with sizes of 4, 5, 6 in PYTHIA 6/8
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Normalisation Issues
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To make A compatible with B, scale them to same luminosity as data by
applying weighting factor to variables (run dependent)
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Ldata – Luminosity of data
Ngen – Number of events generated
σgen – Csx of events generated
Nrec – Number of events reconstructed
to run over
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Ngen , Nrec,σgen all taken from AMI, checked multiple times but still get big
factor difference between data and
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Truth level cross sections plotted
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Weight 
Ldata
 N rec N gen 
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 N gen  
gen 
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Show smooth transitions between
different pT range samples
Agreement of Pythia samples at
high pT (missing P8 J0)
Need to understand and fix this
issue
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Uncorrected Gap Size Distribution
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POMWIG SD, PYTHIA 6 & 8 Jets weighted relative to luminosity of data
runs used and then plotted against MinBias Data
MinBias Data
Pomwig SD
Pythia 8 Jets
Pythia 6 Jets
Before gap cuts, 20 GeV
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MinBias Data
Pomwig SD
Pythia 6 Jets
Before gap cuts, 7 GeV
Unlike Soft Diffraction, by asking for jets (hard objects) in process, unlikely
to see flattening out of data (indicating a diffractive plateau is present)
Also tested with lower pT jets (7 GeV) but with similar results
– Cannot use PYTHIA 8 for comparison at 7 GeV due to missing 8-17 GeV sample
– Drop in events now after forward gap size of 7 as smaller MX is allowed
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First Bin Scaling of Gap Distribution
• POMWIG SD, PYTHIA 6 & 8 Jets weighted relative to
luminosity of data runs used and then plotted against
MinBias Data (all scaled by first bin)
MinBias Data
Pomwig SD
Pythia 8 Jets
Pythia 6 Jets
Before forward gap cuts,
20 GeV
MinBias Data
Pomwig SD
Pythia 6 Jets
Before forward gap cuts,
7 GeV
• Before gap cuts, observe that PYTHIA 6 is best describing
MinBias data (until larger forward gap sizes)
• Pythia 6 has poor model of diffraction so does that suggest
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First Bin Scaling & Background Subtraction
• First bin scaling used and then PYTHIA 6 & 8 Jets subtracted
from MinBias Data and plotted against POMWIG SD
• If (Data – PYTHIA) ≤ 0.0 then do not plot data point
• Best measure of determining minimum Gap Survival Factor
Data – Py6
Pomwig SD
20 GeV, MinBias Data – Pythia 6
Data – Py8
Pomwig SD
20 GeV, MinBias Data – Pythia 8
• Due to how PYTHIA samples describe MinBias data, get
unusual shape in first few bins (MC/Data too large at ΔηF=2.0)
• Best opportunity for studying diffractive dijets may lie with
selected candidates having 4.0 < ΔηF < 6.0 (GSF ≥ 20 here)
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Differential Cross Sections
• MC samples weighted to lumi of data runs - Differential cross
section as a function of forward gap size
• Some issues with acceptance and errors to fix soon
MinBias Data
Pomwig SD
Pythia 6
Pythia 8
Combined Acceptance
Weights applied to different
samples based on lumi Migrations can then cause the
acceptances to be larger or
smaller than expected
Needs more statistics
MC/Data ratio suggests GSF of approx. 15 (if not for acceptance issue)
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Noise Study in “Empty” Events
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Preliminary look at potential effects of noisy cells destroying gaps in
empty events by using RNDM stream
Define event as empty by having no reconstructed primary vertex (with
5+ associated tracks) + no MBTS counters fired
See that not all events have a full gap (∆ηF=9.8) – 47556/7380809 events
Significant Cells η
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Significant Cells φ
Looking at cells with high energy significance suggests slightly more
activity at +η and φ=-π
– Which events pass “Empty” cuts but still have activity?
– Is there a new noise burst to consider in these runs?
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Ratio of SD to ND Dijets
• Preliminary study to measure the ratio of the single
diffractive to inclusive dijet events based on events passing
jet cuts as well as gap cuts
• Only done on MinBias stream (RNDM stream has very low
statistics in comparison)
MinBias stream Data : Period A&B (91465899 events total)
AntiKt4 Jets (ND)
89004
AntiKt6 Jets (ND)
191682
AntiKt4 Jets + ∆ηF>2.0 (SD)
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AntiKt6 Jets + ∆ηF>2.0 (SD)
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AntiKt4 SD/ND ratio
0.00194
AntiKt6 SD/ND ratio
0.00165
• On current results, ratio around 0.002%, a lot smaller than at
Tevatron where ratio of yields of SD to inclusive dijets ≈ 1%
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Comparison with Prague
• Group in Prague also looking at diffractive dijets
• Earlier in analysis but already see difference in strategies
Birmingham
Prague
Data used & Stream
MinBias 2010 A&B
L1Calo/JetTauEtmiss - all 2010
Vertex
No requirement
Only 1 vtx (5+ tracks)
Trigger
MBTS_2
MBTS + Jet Pt dependent
Preferred SD MC
Pomwig SD
Herwig++ SD
Preferred ND MC
Pythia 8
Herwig++ ND
• Vertex requirement would force ourselves to smaller gap
sizes (less likely to see forward jets)
• Prague agree quite well with inclusive jets analysis for 2010
– How do I test this for only Periods A and B?
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Comparison with Prague
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Birmingham
Prague
Preferred SD MC
Pomwig SD
Herwig++ SD
Preferred ND MC
Pythia 8
Herwig++ ND
Herwig++ is C++ version of Pomwig but has known “feature”: factor of 3
increase in cross section
Issue with compatibility of hadronisation models between Pomwig
(cluster model) and Pythia (string model)
Herwig++ ND produces events with very large gaps
– Looks wrong
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Try comparing HERWIG++ & POMWIG SD samples soon
Will need to get official Monte Carlo production done soon
– Held up until settled on stats required / new filter to simulate larger gaps in
events with gaps / move from ESD to dAOD
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Next steps
SHORT TERM
• Fix normalisation issues
• Truth level studies to compare MC samples
• Get cross sections produced soon (with correct
errors & acceptances)
LONGER TERM
• Get official production of MC samples in new format
• Improve gap noise study
• Make NLO calculations?
• Check if OTX cuts are an issue
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