1. Z Production associated with jets @LHC (ATLAS)
Monica Verducci CERN/INFN
On behalf of Atlas Collaboration
MCWS Frascati (Rome)

2. Summary Introduction @ LHC (ATLAS Detector)
Parton Density Function (PDFs) LHC
Physics motivations for Z+jet measurement
Analysis on fully reconstructed MC samples
Possible checks on systematics from data
b-tagging efficiency
background
Jet energy scale
Conclusions and Outlook
23th October 2006
Monica Verducci

3. Large Hadron Collider stot(pp) = 70 mb proton-proton
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
stot(pp) = 70 mb proton-proton
event rate R = s L = 109 eventi\sec
(high luminosity)
Bunch-crossing frequency: 40 MHz
~ 20 collisions p-p per bunch crossing
Hierarchical trigger system
~MB/sec
~PB/year raw data
109 events/s =>1GHz
1 event~ 1MB (~PB/s)
Z(ll)+jet (~2Hz)
γ+jet (~ 0.1 Hz)
At low luminosity
23th October 2006
Monica Verducci

4. ATLAS@LHC Muon Spectrometer: Pt measurements and muon identification
Mounted on an air-core toroid with B field
Inner Tracker: Pt Measurements and charge of the particles with a solenoidal magnetic field of 2 T.
Calorimeters:
electromagnetic and hadronic
23th October 2006
Monica Verducci

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… pA pB fa fb x1 x2 X
23th October 2006
Monica Verducci

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
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.
23th October 2006
Monica Verducci

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)
Compton
~90%
Annihilation
~10%
23th October 2006
Monica Verducci

8. The measurement: Z+jet (b)
Measurement of the b-quark PDF
Process sensitive to b content of
the proton
(Diglio,Tonazzo,Verducci- ATL-COM-PHYS
AIP Conf 794:93-96, 2005, hep-ph/ , CERN )
(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 , Mehdiyev, Vichou, ATL-COM-PHYS )
Luminosity Monitor
23th October 2006
Monica Verducci

9. 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%
Now we have only HERA
measurements, far from this precision
23th October 2006
Monica Verducci

10. Z+b with different PDF sets
MRST5NLO, CTEQ5M1, Alehkin1000
(with LHAPDF in Herwig)
Differences in total Z+b cross-section are of the order of 5%
Some sensitivity from differential distributions: jet energy calibration crucial
Other PDF sets predict larger differences
(e.g., MRST5NNL0 >10%)
New studies are undergoing with different sets of PDFs function using NLO generators (Diglio, Farilla, Verducci)
Number of events
Pt b (MeV/c)
23th October 2006
Monica Verducci

11. The D0 measurement of Zb/Zj
The D0 collaboration has recently measured:
(Z+b)/ (Z+jet) with Z→mm and Z → ee
→ Phys.Rev.Lett.94:161801,2005
Analysis flow:
select events with Z→ee or
Z→mm + jet
apply b-tagging
extract content of b, c and light quarks
(assuming Nc/Nb from theory)
Fitted values for selected sample in 184 pb-1
NLO (J.Campbell et al.): /
23th October 2006
Monica Verducci

12. LHC vs Tevatron
J.Campbell et al.
Phys.Rev.D69:074021,2004
Cross Section (pb)
TEVATRON
LHC
Processes
ZQ inclusive
13.40.9 0.8 0.8
6.83
49.2
13.8
89.7
Zj inclusive
The measurement of Z+b should be more interesting at LHC than at Tevatron:
Signal cross-section larger (x80), and more luminosity
Relative background contribution smaller (x5)
23th October 2006
Monica Verducci

13. 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
Simple spread of existing PDFs gives up to 10% uncertainty on prediction of Higgs cross section.
23th October 2006
Monica Verducci

14. Impact on New Physics Susy Background: Z(->nn) +jet Black: ISAJET
Effective Mass distribution for No-Leptons Mode after standard event selection M(g)≈M(q)≈1TeV
Black: ISAJET
Red: PYTHIA
Susy Atlas meetings
T.S.S.Asai U. of Tokyo
Event Topology
23th October 2006
Monica Verducci

15. Z+jet(b) Analysis (70 <Mmm<110 GeV) Pt > 20 GeV/c
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
Analysis ATLAS Physics Workshop 2005
ATL-COM-PHYS (Verducci, Diglio, Farilla, Tonazzo)
23th October 2006
Monica Verducci

16. How estimate the events…
Backgrounds:
Signal:
Acceptance Efficiency = 59.6%
Trigger Efficiency > 95%
Cuts Efficiency ~ 40%
23th October 2006
Monica Verducci

17. Z+1 jet reconstruction (I)
# events (Rome) = (Layout for Rome Atlas Physics Workshop 2005)
# events (CSC) = (Computing System Commissioning 2006)
CSC
cuts
# event
Rome
CSC
Eff
2 mu
306129
67737
59.3%
48.6%
pt>20
Eta cut
182628
35.4%
M range
157764
50721
30.5%
36.4%
ROME
23th October 2006
Monica Verducci

18. Z+1 jet reconstruction (II)
CSC5145
Algo
# event
Rome
Eff Rome
CSC
Eff CSC
Cone 0.7
51487
10%
16600
11.9%
Cone 0.4
51864
16137
11.6% Kt
20118
3.9%
8808
6.3%
ATLANTIS DISPLAY (Rφ)
Three different algorithms to select the jets with different radius.
Jet: pT > 15 GeV,|η|< 2.5
23th October 2006
Monica Verducci

19. BTagging Soft Muon Tagging All Jets B Jets BTagging Efficiency 59.5%
30 fb-1
b jet
other
# events
176642
204265
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%
23th October 2006
Monica Verducci

20. Systematic Effects Efficiency of b-tagging Background from mistag
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
23th October 2006
Monica Verducci

21. Jets will cover the whole Pt range
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
5 Gev per bin
5-2 Gev per bin
Full Simulation Rome Sample
23th October 2006
Monica Verducci

22. Calibration in Situ
 and Z0 are well calibrated objects at EM scale balancing the recoiling hadronic system potentially large statistics available: L=1033cm-2s pT range from 20 GeV to ~60 GeV
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
-0.16 ± 0.01
(pT jet – pT zeta)/ pT zeta
23th October 2006
Monica Verducci

23. (pT jet – pT boson)/ pT boson
pT balance =
(pT jet – pT boson)/ pT boson
Jet Z Df
23th October 2006
Monica Verducci

24. 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
23th October 2006
Monica Verducci

25. 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%
23th October 2006
Monica Verducci

26. Backup
23th October 2006
Monica Verducci

27. Inclusive b-tagging Algorithm
Inclusive jet b-tagging
Identification of a single jet in the event with b flavour
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
23th October 2006
Monica Verducci

28. Calibration in Situ (II)
Cone DR=0.7
Et> 15 GeV
Et(cell)=1.5 GeV
E,m,g: pt>5GeV
ISR Correction
23th October 2006
Monica Verducci

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
23th October 2006
Monica Verducci