z production associated with jets @lhc (atlas)
DESCRIPTION
Z Production associated with jets @LHC (ATLAS). Monica Verducci CERN/INFN On behalf of Atlas Collaboration MCWS Frascati (Rome). Summary. Introduction @ LHC (ATLAS Detector) Parton Density Function (PDFs) measurements @ LHC Physics motivations for Z+jet measurement - PowerPoint PPT PresentationTRANSCRIPT
Z Production associated with jets @LHC (ATLAS)
Monica Verducci CERN/INFN On behalf of Atlas Collaboration
MCWS Frascati (Rome)
23th October 2006 Monica Verducci 2
Summary
Introduction @ LHC (ATLAS Detector) Parton Density Function (PDFs)
measurements @ 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
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
tot(pp) = 70 mb proton-proton event rate R L = 109 eventi\
sec (high luminosity)
Z(ll)+jet (~2Hz) γ+jet (~ 0.1 Hz) At low luminosity
Hierarchical trigger system ~MB/sec ~PB/year raw data
109 events/s =>1GHz1 event~ 1MB (~PB/s)
Bunch-crossing frequency: 40 MHz~ 20 collisions p-p per bunch crossing
23th October 2006 Monica Verducci 4
ATLAS@LHC Muon Spectrometer: Pt measurements and muon
identificationMounted 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 uncertaintyThe 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->e
rapidity distributions investigated using HERWIG event generator with NLO corrections. Systematics < 5%
(A.Tricoli, hep-ex/0511020,PHOTON05) (A.Tricoli, Sarkar, Gwenlan CERN-2005-01
(A.C.Sarkar, hep-ph/0512228, Les Houches)
eWud
eWdu
Compton~90%
Annihilation~10%
eWud
eWdu
Zdd
Zuu
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-2004-078 AIP Conf 794:93-96, 2005, hep-ph/0601164, CERN-2005-014)
(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-2002-026) (Gupta et al. ATL-COM-
PHYS-2005-067, Mehdiyev, Vichou, ATL-COM-PHYS-99-054)
Luminosity Monitor
23th October 2006 Monica Verducci 9
Why measure b-PDF?
bb->Z @ 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)
Pt b (MeV/c)
Nu
mbe
r of
eve
nts
23th October 2006 Monica Verducci 11
The D0 measurement of Zb/Zj
The D0 collaboration has recently measured:(Z+b)/ (Z+jet) with Z→ and Z → ee
→ Phys.Rev.Lett.94:161801,2005
Analysis flow:– select events with Z→ee or Z→ + 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
)(
004.0
005.0)(005.0024.0 syststat
jZ
bZ
NLO (J.Campbell et al.):
0.018 +/- 0.004
23th October 2006 Monica Verducci 12
LHC vs Tevatron
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)
J.Campbell et al. Phys.Rev.D69:074021,2004
Cross Section (pb) TEVATRON LHC
Processes ZQ inclusive
13.40.9 0.8 0.8
6.83 49.2
13.8 89.7
Zj inclusive
Zbgb
bZbgb
Zcgc
cZcgc
ZqgqZgqq , 7944122401010
30060900
50030060015870
406080701001390
3.12.1
8.15.1 1.03.20
30707050100601040
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-
(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(->jet Effective Mass distribution
for No-Leptons Mode after standard event selection M(g)≈M(q)≈1TeV
Black: ISAJET
Red: PYTHIA
Susy Atlas meetingsT.S.S.Asai U. of Tokyo
Event Topology
23th October 2006 Monica Verducci 15
Z+jet(b) Analysis
Event selection: taking into account only Z→ Two isolated muons with
• Pt > 20 GeV/c• opposite charge• invariant mass close to Mz
(70 110 GeV)
Two different b-tagging algorithms have been considered:• Soft muon• Inclusive b-tagging of jets
Analysis presented @ ATLAS Physics Workshop 2005ATL-COM-PHYS-2006-051 (Verducci, Diglio, Farilla, Tonazzo)
23th October 2006 Monica Verducci 16
How estimate the events…
Backgrounds: Signal:
othercutsaccPythiaotherother
bcutsacctablebb )(
tLN
tLZBRN
Acceptance Efficiency = 59.6%Trigger Efficiency > 95%Cuts Efficiency ~ 40%
mb106.2 6Pythiaother
23th October 2006 Monica Verducci 17
cuts # eventRome
# eventCSC
EffRome
EffCSC
2 mu 306129
67737 59.3% 48.6%
pt>20Eta cut
182628
67737 35.4% 48.6%
M range
157764
50721 30.5% 36.4%
# events (Rome) = 516550 (Layout for Rome Atlas Physics Workshop 2005)
# events (CSC) = 139400 (Computing System Commissioning 2006)
CSC
ROME
Z+1 jet reconstruction (I)
23th October 2006 Monica Verducci 18
Algo # eventRome
Eff Rome
# eventCSC
Eff CSC
Cone 0.7
51487
10% 16600
11.9%
Cone 0.4
51864
10% 16137
11.6%
Kt 20118
3.9% 8808 6.3%
Z+1 jet reconstruction (II)
Three different algorithms to select the jets with different radius.
Jet: pT > 15 GeV,|η|< 2.5
ATLANTIS DISPLAY (Rφ)ATLANTIS DISPLAY (Rφ)
CSC5145
23th October 2006 Monica Verducci 19
BTagging30 fb-1 b jet other
# events 176642 204265
BTagging Efficiency 59.5%
Purity 60.7%
30 fb-1 b jet other
# events 22630 68088
Soft MuonTagging Efficiency 7.2%
Purity 37.2%
Soft Muon Tagging
All Jets
B Jets
All Muons
B Muons
23th October 2006 Monica Verducci 20
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
23th October 2006 Monica Verducci 21
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
Full Simulation Rome Sample
Diglio
2 Gev per bin
5 Gev per bin
5-2 Gev per bin
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-1 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 pjet
T = pZT balance criteria on
transverse plan
-0.16 ± 0.01
(pT jet – pT zeta)/ pT zeta
23th October 2006 Monica Verducci 23
pT balance = (pT jet – pT boson)/ pT boson
bin2
pp
binp
0CalT
ZT
ZT
)~1(ppp
)pp(
)~1(ppp
)pp(
20Cal
TCalT0Cal
T
ZT
0CalT
2
1RawT
0CalTRaw
T
ZT
RawT
1
Jet
Z
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
Primary Vertex
d
Impact Parameter
Extrapolated track
Secondary Vertex,
B-hadron decays
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
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)
23th October 2006 Monica Verducci 28
Calibration in Situ (II)
Cone R=0.7 Et> 15 GeV Et(cell)=1.5 GeV E,: pt>5GeV
bin2
pp
binp
0CalT
ZT
ZT
)~1(pp
p
)pp(
)~1(ppp
)pp(
20Cal
TCalT0Cal
T
ZT
0CalT
2
1RawT
0CalTRaw
T
ZT
RawT
1
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:1. η depends only on
ISR 2. depends on both
resolution and ISR
22D
ZjetZT
jetTT
ZjetZT
jetTT
)2
cos()pp(K
)2
sin()pp(K
cpbpa)p( TTT