Transcript 20111107_SoftQcd - University of Birmingham

Searches for Diffractive Dijet
Production
Hardeep Bansil
University of Birmingham
SM Soft QCD meeting
07/11/2011
Contents
•
•
•
•
Theory & Motivation
Analysis
Plots
Next steps
2
CMS Diffractive W/Z search
• CMS have had difficulty in trying to
find diffractive W/Z signals
–
–
–
–
η~ = 5 - ∆ηF (similar to forward gap)
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
3
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
4
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
5
Event display of candidate event
-η MBTS
counters empty
+η MBTS
counters filled
Large area of
calorimeter empty
Two jets, one in FCAL
and one in HEC
6
Interesting variables
• Calculate MX2 ≈ Ep·(E±pz)X  ξX = MX2 /s
• Calculate zIP ≈ (E±pz)jj/(E±pz)X
• Additionally study jet (η, ET, Mjj) and gap properties
Mjj
Mx
ξX
• Determine differential cross sections for as many of
these variables as possible
7
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
8
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)
– 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
9
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
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 (start of exponential drop in csx)
NLO Cross section plots courtesy of Radek Zlebcik
Linear y-axis
Logarithmic y-axis
10
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)
• Will need to get official Monte Carlo production done soon
•
Will asking for
Have PYTHIA 6 and PYTHIA 8 Dijet samples to use official MC prod
as background (8-17, 17-35, 35-70, 70-140 GeV)
now mean I have
– PYTHIA 8 J0 sample (8-17 GeV) not available without pile to migrate to
up so not used here (needs official MC production too) Athena v17?
• RAPGAP – Still trying to get this working in Rivet
• Try HERWIG++ samples soon
• NLO comparison also coming soon
11
Reason to exclude run 153030
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
12
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
•
•
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
•
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
•
– Is that expected? Also need a check of the POMWIG normalisation
– Still observe forward gaps with sizes of 4, 5, 6 in PYTHIA 6/8 (unexpected)
– Is there any model of diffraction within these PYTHIA 6/8 Dijet events?
13
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
Before gap cuts, 20 GeV
•
•
MinBias Data
Pomwig SD
Pythia 6 Jets
Before gap cuts, 7 GeV
Not seeing flattening out of data or POMWIG SD  not really indicating
that 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
14
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
MinBias Data
Pomwig SD
Pythia 8 Jets
Pythia 6 Jets
MinBias Data
Pomwig SD
Pythia 8 Jets
Pythia 6 Jets
Before forward gap cuts
After forward gap cuts
•
Ratio of SD MC to MinBias Data suggests a Gap Survival Factor of 20
•
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, find that each MC sample, as well as data, have their own
distinct shape and asymmetry
–
Most likely due to smaller stats after gap cuts
15
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)
16
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)
17
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
7 GeV, MinBias Data – Pythia 6
Data – Py8
Pomwig SD
7 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 < 7.0 (GSF ≥ 20 here)
18
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
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. 10
19
Differential Cross Sections
• MC samples weighted to lumi of data runs - Differential cross
section as a function of forward gap size
• Still need to add in PYTHIA dijet samples as background
• No observed diffractive plateau – move to lower pT jets?
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)
20
Next steps
• Look at candidates with gaps of 4.0 < ΔηF < 6.0
• Move to lower pT jets for analysis to see stronger
case for diffractive dijet production?
– Will it be possible?
• Improve statistics of Data and MC samples
– Try later data (e.g. Period C, D) but now watch out for pileup 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
21
Diffractive Dijets
BACKUP SLIDES
22
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 -z
MX2 = 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 +z
MX2 = Ep·(E-pz)clus
zIP = (E-pz)jj/(E-pz)clus
xP = (E+pz)jj/(E+pz)clus
23