1. Vector Boson Production associated with jets @LHC (Atlas)
Monica Verducci CERN/CNAF
On behalf of Atlas Collaboration
IFAE 2006 Pavia

2. Vector Bosons with Jets @ ATLAS
Summary
LHC (ATLAS Detector)
Parton Density Function (PDFs) LHC
Vector Boson Production analysis as a possible constrain for PDFs: /W/Z+jets
Potential of the Z+jet analysis
Systematics of the measurement
First data: calibrations
Conclusions
Vector Bosons with ATLAS
Monica Verducci CERN

3. Vector Bosons with Jets @ ATLAS
LHC
Energy per proton
7 TeV
Bunch spacing
25 ns
Bunch size
15 m  12 cm
Protons per
Bunch
1011
Bunches per ring
2835
Lifetime
10 hours
Luminosity
1034 cm-2 s-1
Lenth of the ring
27 Km
Number of collisions per bunch 25
Vector Bosons with ATLAS
stot(pp) = 70 mb proton-proton
event rate R = s L = 109 eventi\sec
(ad alta luminosità)
Monica Verducci CERN

4. ATLAS@LHC Muon Spectrometer: Pt measurements and muon identification
Mounted on an air-core toroid with B field
Vector Bosons with ATLAS
Inner Tracker: Pt Measurements and charge of the particles with a solenoidal magnetic field of 2 T.
Calorimeters:
electromagnetic and hadronic
Monica Verducci CERN

5. Importance of PDFs at LHC
At a hadron collider, cross sections are a convolution of the partonic cross section with the PDFs.
PDFs are important for Standard Model physics, which will also be backgrounds to any new physics discovery: Higgs, Extra Dimensions…
Vector Bosons with ATLAS pA pB fa fb x1 x2 X
Monica Verducci CERN

6. Parton Kinematic Regime@LHC
The kinematic regime at the LHC is much broader than currently explored.
At the EW scale (ie W and Z masses) theoretical predictions for the LHC are dominated by low-x gluon uncertainty
Vector Bosons with ATLAS
At the TeV scale, uncertainties in cross section predictions for new physics are dominated by high-x gluon uncertainty
The x dependence of f(x,Q2) is determined by fits to data, the Q2 dependence is determined by the DGLAP equations.
Fits and evaluation of uncertainties performed by CTEQ, MRST, ZEUS etc.
Monica Verducci CERN

7. Constraining PDFs at LHC
Direct photon production
Studies ongoing to evaluate experimental uncertainties (photon identification, fake photon
rejection, backgrounds etc.)
(I.Dawson - Panic05,proc.)
W and Z rapidity distributions
Impact of PDF errors on W->en rapidity distributions investigated using HERWIG event generator with NLO corrections. Systematics < 5%
(A.Tricoli, hep-ex/ ,PHOTON05)
(A.Tricoli, Sarkar, Gwenlan CERN )
(A.C.Sarkar, hep-ph/ , Les Houches)
Z+b-jet
(Diglio,Tonazzo,Verducci-
ATL-COM-PHYS
AIP Conf 794:93-96, 2005,
hep-ph/ , CERN )
Compton
~90%
Annihilation
~10%
Vector Bosons with ATLAS
Monica Verducci CERN

8. Photons and W Boson Analysis
Photons production
Photon couples only to quarks, so potential good signal for studying underlying parton dynamics.
g Selection Efficiency~80%
CTQE6L-MRST01E ~ 16-18%
W Production
At y=0 the total PDF uncertainty is:
~ ±5.2% from ZEUS-S
~ ±3.6% from MRST01E
~ ±8.7% from CTEQ6.1M
ZEUS-S to MRST01E difference ~5%
ZEUS-S to CTEQ6.1 difference~3.5%
Vector Bosons with ATLAS
CTEQ61
MRST02
ZEUS02
e- rapidity
e+ rapidity
Generated
Monica Verducci CERN

9. The measurement: Z+jet (b)
Measurement of the b-quark PDF
Process sensitive to b content of
the proton
(J.Campbell et al. Phys.Rev.D69:074021,2004)
Tuning of the MonteCarlo tools for Standard Model
Background of new physics signatures
Calibration Tool (clean and high statistics signature)
(Santoni, Lefevre ATL-PHYS )
(Gupta et al. ATL-COM-PHYS )
Luminosity Monitor
Vector Bosons with ATLAS
Monica Verducci CERN

10. Vector Bosons with Jets @ ATLAS
Why measure b-PDF?
LHC is ~5% of entire Z production -> Knowing σZ to about 1% requires a b-pdf precision of the order of 20%
Vector Bosons with ATLAS
Now we have only HERA
measurements, far from this precision
Monica Verducci CERN

11. Vector Bosons with Jets @ ATLAS
PDF Uncertainty
HERWING: MRST03CNNLO – CTEQ5M1 – Alehkin1000
Differences in total Z+b cross-section are of the order of 5-10%
The D0 collaboration has measured the ratio:
(Z+b)/ (Z+jet)
with Z→mm and Z → ee
Phys.Rev.Lett.94:161801,2005
Fitted values for selected sample in 184 pb-1
Vector Bosons with ATLAS
#Events
Pt b-jet (MeV)
NLO (J.Campbell et al.): /
Monica Verducci CERN

12. Z+jet: Impact to other measurements
Background to Higgs search
In models with enhanced (h+b)
and BR(h->mm)
(J.Campbell et al. Phys.Rev.D67:095002,2003)
Background to MS Higgs search
In models where pp -> ZH con H -> bb
Vector Bosons with ATLAS
Simple spread of existing PDFs gives up to 10% uncertainty on prediction of Higgs cross section.
Monica Verducci CERN

13. Vector Bosons with Jets @ ATLAS
Impact on New Physics
Susy Background:
Z(->nn) +jet
Effective Mass distribution for No-Leptons Mode after standard event selection M(g)≈M(q)≈1TeV
Black: ISAJET
Red: PYTHIA
Vector Bosons with ATLAS
Susy Atlas meetings
T.S.S.Asai U. of Tokyo
Event Topology
Monica Verducci CERN

14. Vector Bosons with Jets @ ATLAS
Z+jet(b) Analysis
Event selection: taking into account only Z→mm
Two isolated muons with
Pt > 20 GeV/c
opposite charge
invariant mass close to Mz
(70 <Mmm<110 GeV)
Two different b-tagging algorithms have been considered:
Soft muon
Inclusive b-tagging of jets
Vector Bosons with ATLAS
Analysis ATLAS Physics Workshop 2005
Monica Verducci CERN

15. Vector Bosons with Jets @ ATLAS
Backgrounds:
Cross Section (pb)
ZQ inclusive
13.40.9 0.8 0.8
Zj inclusive
89.7
13.8
49.2
6.83
Processes
LHC
TEVATRON
Vector Bosons with ATLAS
Signal:
(J.Campbell et al. Phys.Rev.D69:074021,2004)
Z Mass
RECO MC
Acceptance Efficiency = 59.6%
Trigger Efficiency > 95%
Cuts Efficiency ~ 40%
Monica Verducci CERN

16. Vector Bosons with Jets @ ATLAS
BTagging
All Jets
30 fb-1
b jet
other
# events
176642
204265
Vector Bosons with ATLAS
B Jets
BTagging Efficiency 59.5%
Purity 60.7%
Soft Muon Tagging
All Muons
30 fb-1
b jet
other
# events
22630
68088
B Muons
Soft MuonTagging Efficiency 7.2%
Purity 37.2%
Monica Verducci CERN

17. Vector Bosons with Jets @ ATLAS
Systematic Effects
Efficiency of b-tagging
To check b-tagging efficiency, we can use b-enriched samples. Experience at Tevatron & LEP indicates that we can expect:
Δεb/εb = 5%
Background from mistag
Check mistagging on a sample where no b-quark jets should be present
Vector Bosons with ATLAS
Monica Verducci CERN

18. Vector Bosons with Jets @ ATLAS
We use W+jet events, where there are not b jet
Jets will cover the whole Pt range
Statistics 30x Z+j (after selection of decays to muons)
The relative error on background from mistagging can be kept at the level of few-% in each bin of the Pt range
Diglio
2 Gev per bin
Vector Bosons with ATLAS
5 Gev per bin
5-2 Gev per bin
Full Simulation Rome Sample
Monica Verducci CERN

19. First Data Z+jet: Calibration
Calibration in situ of the jet energy scale -> jet energy absolute scale within 1%
This means calibrate the calorimeters using jets reconstructed in the exp.
Z+jet (b 5%)
high statistic -> 380pb
pjetT = pZT
balance criteria on
transverse plan
Vector Bosons with ATLAS
Truth
Reco
Gupta,et al.
Atlfast
Santoni,Lefevre
Monica Verducci CERN

20. Vector Bosons with Jets @ ATLAS
Conclusions I
Precision Parton Distribution Functions are crucial for new physics discoveries at LHC and to tune MonteCarlo studies:
PDF uncertainties can compromise discovery potential (HERA-II: significant improvement to high-x PDF uncertainties)
At LHC the major source of errors will not be statistic but systematic uncertainties
To discriminate between conventional PDF sets we need to reach high experimental accuracy ( ~ few%) and to improve the detector performance and resolution
Standard Model processes like Direct Photon, Z and W productions are good processes:
to constrain PDF’s at LHC, especially the gluon
to calibrate the detector
Vector Bosons with ATLAS
Monica Verducci CERN

21. Vector Bosons with Jets @ ATLAS
Conclusions II
Z+b measurement in ATLAS will be possible with high statistics and good purity of the selected samples with two independent tagging methods
We will have data samples to control systematic errors related to b-tagging at the few-% level over the whole jet Pt distribution
b-tagging efficiency
Mistagging: from W+jet
Jet Calibration in situ: error within 1%
Vector Bosons with ATLAS
Monica Verducci CERN

22. Vector Bosons with Jets @ ATLAS
Many Thanks to
Patrizia Azzi & Fulvio Piccinini
Giacomo Polesello
Fabiola Gianotti
Alessandro Tricoli
Ada Farilla & Sara Diglio
Chiara Roda
Vector Bosons with ATLAS
Monica Verducci CERN

23. Vector Bosons with Jets @ ATLAS
Backup
Monica Verducci CERN

24. Event Selection Criteria for W+- ->l+- nl (TDR selection cuts)
Backgrounds sums:
QCD 2->2
Electrons: |η| < 2.4
Pt > 25 GeV
Missing Et > 25 GeV
To reject QCD bkg & high Pt W and Z due to I.S.R. :
No reconstructed jets in the event with Pt > 30 GeV
Recoil on transverse plane should satisfy |u|< 20 GeV
Z -> t+t-
Vector Bosons with ATLAS
Z -> e-e+
W -> tn -
Monica Verducci CERN

25. Vector Bosons with Jets @ ATLAS
W Analysis (I)
What is effect of including ATLAS W rapidity “pseudo-data” into global PDF fits.
Created 1M “data” sample, generated using CTEQ6.1 PDF and simulate ATLAS detector response using ATLFAST. Correct back to generator level using ZEUS-S PDF and use this “pseudo-data” in a global ZEUS-S PDF fit. Central value of ZEUS-S PDF prediction shifts and uncertainty is reduced:
Vector Bosons with ATLAS
~1day of data-taking at low Lumi
xg(x) ~ x –λ :
35% error reduction
BEFORE λ = ± 0.046
AFTER λ = ± 0.030
low-x gluon shape parameter λ:
Monica Verducci CERN

26. Vector Bosons with Jets @ ATLAS
W Analisys (II)
To improve on current PDF uncertainties:
Study of Rapidity distributions W+- -> e+-n
Vector Bosons with ATLAS
Cross section for pp→W+X with W→lν, l=e,μ is ~30 nb
(10 time larger than Tevatron)
300M evts/y at low Luminosity
Cuts acceptance ~25%
Assuming Lepton reconstruction efficiency ~ 90%
& identification efficiency ~ 80%
Total Selection
Efficiency ~20%
60 M W’s/y al low Luminosity (10 fb-1)
Monica Verducci CERN

27. BTagging Algorithm Inclusive jet b-tagging
Identification of a single jet in the event with b flavour
Vector Bosons with ATLAS
pT > 15 GeV
|η|< 2.5
Number of tracks > 0
Secondary vertex >3 (weight)
Primary Vertex
Secondary Vertex,
B-hadron decays d
Impact Parameter
Extrapolated track
Life time of a bottom hadron is about t ~ 1.5 ps long enought to permit to a hadron of 30 GeV of energy to do a distance of L ~ 3 mm before decaying
Monica Verducci CERN

28. Calibration in Situ (II)
Cone DR=0.7
Et> 15 GeV
Et(cell)=1.5 GeV
E,m,g: pt>5GeV
Vector Bosons with ATLAS
ISR Correction
Monica Verducci CERN

29. Calibration in Situ (III)
BiSector Method
Measurement of the resolution via estimation of the ISR contribution
Transverse plane:
η depends only on ISR
 depends on both resolution and ISR
Vector Bosons with ATLAS
Monica Verducci CERN