testing the standard model in the forward region at the lhc
DESCRIPTION
Testing the standard Model in the forward region at the LHC. Ronan McNulty (UCD Dublin) Irish Quantum Foundations, Castletown House, 3,4 th May 2013. Outline. Theory : The Standard Model Experiment : LHCb detector Electroweak tests using W, Z; probing the proton structure - PowerPoint PPT PresentationTRANSCRIPT
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TESTING THE STANDARD MODEL IN THE FORWARD REGION AT THE LHCRonan McNulty (UCD Dublin)
Irish Quantum Foundations, Castletown House, 3,4th May 2013
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Ronan McNulty, Irish Quantum Foundations 2
Outline• Theory: The Standard Model• Experiment: LHCb detector① Electroweak tests using W,
Z; probing the proton structure② Electroweak tests using Z;
sensitivity to Higgs③ Test EM & QCD with exclusive
production of dimuons, J/ and χc.
.
Analysis Paper LuminosityWν JHEP 06 (2012) 058 37pb-1
Z CERN-LHCb-CONF-2013-007 1fb-1
Z JHEP 1301 (2013) 111. 1fb-1
Higgs arXiv:1304.2591 1fb-1
Exclusive J/ JPG 40 (2013) 045001 37pb-1
Exclusive χc CERN-LHCb-CONF-2011-022 37pb-1
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This is what we want to test....
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fermionmass
Higgsmass W mass
Z mass
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fermionpropagator
fermionmass
photon prop.
Higgsmass W mass
Z mass
gluon propagator
W propagator
Higgs prop.
Z propagator
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fermionpropagator QED vertex
fermionmass
W,Z vertex
photon prop.
Higgsmass W mass
Z mass
QCD vertex gluon propagator
Z propagator
W propagator
W,Z,photoninteractions
Higgsinteractionswith fermionsand bosons
Higgs prop.
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fermionpropagator QED vertex
fermionmass
W,Z vertex
photon prop.
Higgsmass W mass
Z mass
QCD vertex gluon propagator
Z propagator
W propagator
W,Z,photoninteractions
Higgsinteractionswith fermionsand bosons
Higgs prop.
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The LHC
(10-15m) Nominal Energy: 7TeV
CMS
LHCb
ATLASALICE
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ATLAS and CMS surround the interaction region
Fully instrumented in region: -2 < < 2Partial instrumentation: -5 < < 5
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The LHCb detector
0.4
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Complementarity of LHC detectors
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uud
uud
1. Electroweak tests using W, Z; probing the proton structure
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uud
uud
Parton Density Function (PDF):
Probability that the proton contains this parton with this momentum fraction
Q = Invariant mass of parton interactionx = Qe±y/s [y is rapidity, s c.o.m]
fq(x,Q2)
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Theory v Experiment at the LHC
Hadronic Cross-section
Partonic Cross-sectionPDFs
• Test the Standard Model at the highest energies. W/Z theory known to 1%• Constrain parton distribution functions.• Test QCD – of particular interest in regions with very soft gluons
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-6 -5 -4 -3 -2 -1 0
log10(x)
9
8
7
6
5log10(Q2)
[GeV2]4
3
2
1
0
-1
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-6 -5 -4 -3 -2 -1 0
log10(x)
9
8
7
6
5log10(Q2)
[GeV2]4
3
2
1
0
-1
LHC ki
nemati
c reg
ion
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-6 -5 -4 -3 -2 -1 0
log10(x)
9
8
7
6
5log10(Q2)
[GeV2]4
3
2
1
0
-1
LHC ki
nemati
c reg
ion
DGLAP evolution
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-6 -5 -4 -3 -2 -1 0
log10(x)
9
8
7
6
5log10(Q2)
[GeV2]4
3
2
1
0
-1
LHC ki
nemati
c reg
ion
DGLAP evolution
y: 6 4 2 0 2 4 6
Production of object of mass Qat rapidity
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-6 -5 -4 -3 -2 -1 0
log10(x)
9
8
7
6
5log10(Q2)
[GeV2]4
3
2
1
0
-1
LHC ki
nemati
c reg
ion
y: 6 4 2 0 2 4 6
ATLA
S &
CM
S
ATLAS & CMS:Collision between two partons having similar momentum fractions.
PDFs either already measured by HERA or Tevatron, or requiring modest extrapolation through DGLAP.
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-6 -5 -4 -3 -2 -1 0
log10(x)
9
8
7
6
5log10(Q2)
[GeV2]4
3
2
1
0
-1
LHC ki
nemati
c reg
ion
y: 6 4 2 0 2 4 6
LHCb
LHCb
LHCb:Collision between one well understood parton and one unknown or large DGLAP evolved parton.
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-6 -5 -4 -3 -2 -1 0
log10(x)
9
8
7
6
5log10(Q2)
[GeV2]4
3
2
1
0
-1
LHC ki
nemati
c reg
ion
y: 6 4 2 0 2 4 6
ATLA
S &
CM
S
ATLAS & CMS:Collision between two partons having similar momentum fractions.
PDFs either already measured by HERA or Tevatron, or requiring modest extrapolation through DGLAP.
W,Z
g*
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-6 -5 -4 -3 -2 -1 0
log10(x)
9
8
7
6
5log10(Q2)
[GeV2]4
3
2
1
0
-1
LHC ki
nemati
c reg
ion
DGLAP evolution
y: 6 4 2 0 2 4 6
LHCb
LHCb
W,Z
g*
LHCb:Collision between one well understood parton and one unknown or large DGLAP evolved parton.
Potential to go to very low x, where PDFs essentially unknown
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Pre-LHC precision on W,Z cross-sections
e.g. very roughly
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W and Z production in the forward region
Experimentally: =N/LCount number of events: Z, W Master formula:
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The LHCb detector
0.4
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Firs
t Z c
andi
date
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Z cross-section measurement
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Efficiencies for W and Z analysis found from tag-and-probe
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Firs
t W c
andi
date
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Purity of W selection
+ -
2<<2.5 2.5<<3 3<<3.5
3.5<<4 4<<4.5
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W charge asymmetry
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Comparison to CMS
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Testing the theory
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Z differential cross-section as fn of rapidity and transverse momentum
compared to various PDF sets and different generators
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1. Summary Electroweak data from LHC is in good agreement with
Standard Model Predictions.
PDFs constrained and thus predict other processes (e.g. Higgs) with greater precision.
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2. Electroweak tests using Z; sensitivity to Higgs
e
Are these couplings the same?
Could something else produce ?
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Z->ττ signal and background
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Trigger and selection
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Estimated signal contributions
μμ
μμ μe
eμ μh eh
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Comparison of Z->μμ and Z->ττ results
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Reinterpretation in terms of Higgs
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Higgs boson in the forward region
Model independent limits SUSY limits (mhmax scenarios)
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2. Summary Lepton universality holds
SUSY parameter space severely constrained.
With more statistics we are sensitive to Higgs production in the forward region.
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3. Exclusive J/ψ and ψ(2S), χc and μμ
Results based on 37pb-1 of data taken in 2010
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Physics of the Vacuum
p
p
It’s QCD – but not as we normally see it. It’s colour-free
σelastic ≈ 40mbσdiffractive ≈ 10mbσinelastic ≈ 60mb
Elastic
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Physics of the Vacuum
p
p
It’s QCD – but not as we normally see it. It’s colour-free
σelastic ≈ 40mbσdiffractive ≈ 10mbσinelastic ≈ 60mb
0+
Pomeron (soft)Elastic
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Physics of the Vacuum
p
p
It’s QCD – but not as we normally see it. It’s colour-free
σelastic ≈ 40mbσdiffractive ≈ 10mbσinelastic ≈ 60mb
0+
Pomeron (soft)
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Physics of the Vacuum
p
p
It’s QCD – but not as we normally see it. It’s colour-free
σelastic ≈ 40mbσdiffractive ≈ 10mbσinelastic ≈ 60mb
0+
Pomeron (hard and soft)
No activity “rapidity gap”
Diffractive
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Physics of the Vacuum
p
p
Elastic diffractive: clean environment to study vacuum, andin particular, transition between soft and hard pomeron.
σelastic ≈ 40mbσdiffractive ≈ 10mbσinelastic ≈ 60mb
0+
Pomeron (hard)
“rapidity gap”
“rapidity gap”
“particle”
Central Exclusive
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Physics of the Vacuum
p
p
Elastic diffractive: clean environment to study vacuum, andin particular, transition between soft and hard pomeron.
σelastic ≈ 40mbσdiffractive ≈ 10mbσinelastic ≈ 60mb
0+
Pomeron (hard)
“rapidity gap”
“rapidity gap”
“particle”
Photon
Central Exclusive
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Physics of the Vacuum
p
p
Elastic diffractive: clean environment to study vacuum, andin particular, transition between soft and hard pomeron.
σelastic ≈ 40mbσdiffractive ≈ 10mbσinelastic ≈ 60mb
Photon
“rapidity gap”
“rapidity gap”
“particle”
Photon
Central Exclusive
QED !
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Sensitivity to gluon PDFLeading order cross-section
Gluon PDF enterssquared
Examples of dependence of Jpsi cross-section on PDF (left) and extractionof gluon PDF (right) from Martin, Nockles, Ryskin, Teubner, arXiv:0709.4406v1
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The LHCb detector
0.4
Low multiplicity required. Restricts to single-interaction collisions
(some sensitivity -3.5<η<-1.5)
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UCD helped build, commission and operate the VELO
VELO sub-detector measures particle positions to 5um
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Graphical Representation
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Effect of rapidity gap requirement on muon triggered events
All triggered events With veto on backward tracks
JPG 40 (2013) 045001JPG 40 (2013) 045001
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Central exclusive di-muon signals
SuperChic: L. Harland-Lang, V. Khoze, M. Ryskin, W. Stirling, EPJ.C65 (2010) 433-448Starlight: S.R. Klein & J. Nystrand, PRL 92 (2004) 142003.
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Before and after requiring precisely two tracks
Tracks havepT>400MeV2<η<5
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Non-resonant background very small
Distributions are not background-subtracted.37pb-1 of data: 1492 J/ψ and 40 ψ(2s)
JPG 40 (2013) 045001 JPG 40 (2013) 045001
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Cross-section measurementPurity:1. non-resonant bkg (1%)2. Chi_c feeddown (9%)3. Psi’ feedown (2%)4. Inelastic Jpsi production (30%) Number of events
observed
Luminosity
Efficiency:1. Trigger2. Tracking & muon id.3. Single interaction beam-crossing
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Feed-down backgrounds
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Inelastic background
Characterise pT spectrum of background using shapes with 3-8 tracksand extrapolate to 2 track case.
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Inelastic backgroundSignal shape Estimated from Superchic using exp(- b pT
2) (arXiv: 0909.4748)Take b from HERA data. Extrapolate to LHCb energies to get b= 6.1 +/- 0.3 GeV-2
Crosscheck: Fit to spectrum below with b free gives b = 5.8 +/- 1 GeV-2
Purity of exclusive signal below 900 MeV/c pT
= (70 ± 4 ± 6)%
Inelastic background shape
Estimated from data.Characterise shape for 3-8 tracksand extrapolate to 2 tracks.
This approach works for QED production of dimuons, tested using LPAIR simulation.Also checked with PYTHIA simulation of diffractive events.
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JPG 40 (2013) 045001
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LHCb compared to theory & experiment
*
All predictions (bar Schaefer&Szcaurek) have similar approach and givesimilar results and are consistent with our data.
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eLHCb compared to HERA
Twofold ambiguity
LHCb c/s is HERA c/s weighted by photon spectrum + gap survival factor (r)
LHCb differential data fitted assuming power law dependence
LHCb HERA
Power law results
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LHCb compared to theory & experiment
measured
from fit
plotted
JPG 40 (2013) 045001
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Deviations from power law
Saturation model (Motyka&Watt PRD 78 2008 014023) has deviation from pure power law.
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LHCb compared to theory & experiment
looks very like
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Other ways to fill the vacuum with muonsχc 0,1,2 decay to Jpsi+photonVacuum state should be 0
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3. Summary Nature of the pomeron investigated
Sensitive to gluon PDF
Prospect for new phenomena in QCD saturation odderon (3-bound gluons)
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Conclusions• Rich variety of physics available from LHC
• Standard Model has so far resisted all our attempts to break it!
• Precision physics and new energy regimes are key to improved understanding
• Irish physics very much involved at the energy frontier.
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Backup
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Determination of non-resonant background
pT for 3-track events
pT for 8-track events
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Exclusive pseudo-vector production
BR(χc0->J/ψγ)=1.2%BR(χc1->J/ψγ)=34.4%BR(χc2->J/ψγ)=19.5%
Dominance of Xc0 is confirmed.
Experimentally difficult to separate three resonances and determinenon-resonant background for each.
LHCb preliminary results with 2010 data