1Single Diffractive Cross SectionCalorimeter Gaps ApproachSoft QCD Meeting14th Oct 2010

Tim Martin, Paul NewmanUniversity of Birmingham

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Soft Diffraction

Diffractive Events

• Colour singlet exchange.• Can be Single or Double proton dissociation.• Diffractive mass can be anything from p+π0 up large systems with hundreds of GeV invariant mass.• Soft PT spectrum.• Large forward energy flow.• Less activity in the inner detector.

PythiaSingle Diff.TransverseMomentum

Transfer

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Diffractive Mass Distributions

Pythia8

• Measure dσ/dξ over the ξ range accessible at ATLAS• Constrain the relative weights of SD, DD and ND

At 7 TeV we should be

getting really big diffractive

systems!

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Truth Level EnergyFlow• Normalised to a single event

in ξ.• Differential in η.• Expect rapidity gap devoid of

all soft activity.Single Diffraction

• Linear relation between η of leading particle and log(ξ), slope = 1 for ln(ξ).

• Part hadronisation, mostly kinematics.

ηmax = η of Leading Truth Particle in X System

Are there any MC diffractive models which do not use the lund string hadronisation

model?

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Trigger Efficiency• Trigger events with the MBTS. Trigger is fully within

calorimeter acceptance so use loose trigger requirement. L1_MBTS_2.

`Here be dragons’

Unexploreable phase space with

Calo + MBTS analysis.

Correct for trigger

inefficiency in data using MC

here

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Experimental Re-definiton• In Double Diffraction, if the smaller system (MY) is too small

to get into the ATLAS calorimeters, then the event is experimentally indistinguishable (exc. ZDC) from a single diffractive event. This occurs around ξY of 10-6

Reclassify as Single

Diffractive.Name the new sample Single

Diffractive Like

New Definition:Double

Diffractive Like Systematic Check: Location of cut

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New Cross Sections• Single and Double Diffractive Like take into account the

migration of events from the Double Diffractive to Single Diffractive samples.

• Visible refers to the subset of the samples which can be triggered with the MBTS.

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Samples & Calo Cells• Current data sample of ~750k, Pythia8, Phojet, Pythia6; ND, SD,

DD • MC Empty • MinBias and RNDM stream for 1st 7 TeV run:

• Run 152166• ~600k L1_MBTS_2 & BCID = 1 Events• 7719 mb.

• Noise in TILE show non-Gaussian profile (double Gaussian).• TILE is not required for hermetic coverage of ATLAS and is

currently excluded from the analysis.mc09_7TeV.105003.pythia_sdiff.recon.ESD.e514_s764_s767_r1307/mc09_7TeV.105004.pythia_ddiff.recon.ESD.e514_s764_s767_r1307

mc09_7TeV.105001.pythia_minbias.recon.ESD.e517_s764_s767_r1307

data10_7TeV.00152166.physics_MinBias.recon.ESD.r1239/ data10_7TeV.00152166.physics_RNDM.recon.ESD.r1239/

mc09_7TeV.106096.PhojetNdiff.recon.ESD.e514_s764_s767_r1307mc09_7TeV.106097.PhojetSdiff.recon.ESD.e514_s764_s767_r1307mc09_7TeV.106098.PhojetDdiff.recon.ESD.e514_s764_s767_r1307

mc09_7TeV.108316.Pythia8_minbias_ND.recon.ESD.e533_s764_s767_r1307

mc09_7TeV.108317.Pythia8_minbias_SD.recon.ESD.e533_s764_s767_r1307

mc09_7TeV.108318.Pythia8_minbias_DD.recon.ESD.e533_s764_s767_r1307

Oldrich& Pavel

EM Barrel

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EM Barre

l

EMEndcapInner

EMEndcapOuter

FCALHEC

Tile

CaloCellSignificance

Data10 MBTS_2Data10 RD0_EMPTYMC Py8 InelasticMC Empty

Very Bad

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Cell Requirement• Using an Energy Veto• Currently require 1 cell above 5.5 E/σ.• Can perhaps improve our sensitivity by combining cells?

• No good for all cells in a slice.• E.G Barrel slice: 39,925 cells• RMS Noise of all cells combined: 8.6 GeV!• Swamps any minimum bias energy deposition.

• However, combination of cells with E/σ > 3 looks more promising.

• Order 50 cells per η slice, RMS noise of ~100 MeV – manageable.Under current investigation, may improve

systematics.

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Gap Finding

Largestη Gap

E.G. SingleDiffractive

EventMX

MY

η

•Trigger on L1_MBTS_2.•Negligible pileup, no prescales, low beam backgrounds.•Divide the calorimeter into 10x rings spanning 1 unit of η.•Ring is full if 1 or more calo cells satisfy E/σCell > Threshold•Look for largest continuous gap of empty rings.•Plot Start of Gap [|η|] vs. Size of Gap [Δη].•The start of the gap is defined as the |η| of it’s farthest edge from η=0•Provides separation of diffractive and non-diffractive.

MBTS

Being Quantified

Systematic Check

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Overall Trigger Eff.o Trigger efficiency calculated

from MC on a bin-by-bin basis.

o Fractional breakdown of event types in each bin displayed below.

OverallTrigger

EfficiencyMC

Bin-by-binBreakdown ofEvent types

Key

Totally FullDetector Bin

Totally EmptyDetector Bin

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Tr. Eff Correctiono Trigger efficiency corrections are applied bin-by-bin to the

data histogram.o Bins with a MC trigger efficiency of < 50 % are not used in

the fit.o These are the `here be dragons bins’

Before Correction After Correction

Low Efficienc

y

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Formation of MC Templates

ND SDDD

o Map 2D distribution onto 1D plot for the purpose of fitting.o Pre-trigger MC distributions.o Normalise each MC shape to a single event.o Bins with overall trigger efficiency < 50% are plotted last – beyond the black line.o These bins are not

included in the fit.

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Check of Fit

oForm pseudo-data by applying bin-by-bin Poisson smear to MC prediction.oFit relative SD, DD and ND templates to pseudo-data.oFit constrained to sum to 1

One Trial

Residuals for one trial

1000 Trials

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Fit to Data – Pythia8

o Fit to data normalization in fitted bins.

oLog likelihood fit.

oMC shapes do not fully describe the data. Poor χ2

Pythia8:ND: 73.6%SD Like: 23.7%DD Like: 2.7%

Fit:ND: 74.2%SD Like: 18.1%DD Like: 7.74%

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Detector Acceptance• To convert to a cross-section, need to correct back to the

particle level.• Run gap finding algorithm over truth level data.

• Move from Calo Cells Truth Particles• Move from E/σ cut PT cut

• Build bin-by-bin correction histogram to apply to data. • c.f the trigger efficiency

correction.

Correction factors manageable in Single Diffractive Like (edge

gap) region.

Large correction factors in Double Diffractive Like region (central

gaps)

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Single-Diffractive• Comparison of algorithms at:

• Truth (PT > 200 MeV)• Reco (E/σ > 5.5)

• Hatched boxes denote located gaps.• Correct effects such as:

• Low PT particles not reaching callo.

• Multiple scattering.

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Cross Section• This slide 0 MeV PT cut gap definition• Data is...

• Corrected for Trigger Efficiency & for Detector Acceptance.

• Scaled to Lumi of run.• Plotted systematics, `best and worse case scenarios’

• E/σ > 5 noise AND +10% Cell energy scale AND 111% Lumi.

• E/σ > 6 noise AND -10% Cell energy scale AND 89% Lumi.

Last two bins have Trigger Efficiency < 50%

Suggestion from R. Barlow! Will check

adding in quad as well.

Systematic Band(systematics invert)

Default & fittedfractional visible

Xsec MC linesStatistical error

negligible

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Cross Section• This slide 100 MeV PT cut gap definition

• Interesting sensitivity to choice of PT cut defining cross section.

• Most effect at small gaps, explained as increasing pollution from non-diffractive events.

• Flick back and forth...

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Cross Section• This slide 200 MeV PT cut gap definition

• Flick back and forth...

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Cross Section• This slide 500 MeV PT cut gap definition

• Flick back and forth...

Work ongoing to match a sensible PT cut with the E/σ cut.

No particles in the gap (PT > 0 cut) looks sensible.Can also publish differential in PT

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Cross Section• Combination into a single result.

• Pythia8 taken as trigger and acceptance correction MC• Maximum deviation per data point for Pythia6 and

Phojet taken as systematic.• Added in quad., last two (low trigg. Eff.) bins not

plotted.

0 MeV PT Definition 100 MeV PT Definition

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Conclusion• Single diffractive like cross section dσ/dη = 1 mb η-1 • Measurement of the single diffractive like cross section at 7

TeV possible with very little minimum bias data.• Many cross checks, systematic checks,

systematic quantification is underway.• Still a bit more work to do with Calo Cell

selection & PT dependence on acceptancecorrection.

• We are working to compare with Tevatronresults at 2 TeV.

• We are writing, detailed internal note for the analysis being produced atthe moment.

• Details theory, all steps & cross checks.