TESTING THE STANDARD
MODEL IN THE FORWARD
REGION AT THE LHC
Ronan McNulty (UCD Dublin)
Irish Quantum Foundations,
Castletown House, 3,4th May 2013
Ronan McNulty, Irish Quantum Foundations
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Outline
• Theory: The Standard Model
• Experiment: LHCb detector
① Electroweak tests using W
,
Z
; probing the proton structure
② Electroweak tests using Z
;
sensitivity to Higgs
③ Test EM & QCD with exclusive
production of dimuons, J/ and χc.
Analysis
Paper
Luminosity
Wν
JHEP 06 (2012)
. 058
Z
CERN-LHCb-CONF-2013-007
1fb-1
Z
JHEP 1301 (2013) 111.
1fb-1
Higgs
arXiv:1304.2591
1fb-1
37pb-1
Exclusive J/ JPG 40 (2013) 045001
37pb-1
Exclusive χc
37pb-1
CERN-LHCb-CONF-2011-022
Ronan McNulty, Irish Quantum Foundations
This is what we want to test....
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fermion
mass
Higgs
mass
Z mass
W mass
Ronan McNulty, Irish Quantum Foundations
fermion
propagator
5
fermion
mass
W propagator
photon prop.
Z propagator
Higgs
mass
W mass
Higgs prop.
Z mass
gluon propagator
Ronan McNulty, Irish Quantum Foundations
fermion
propagator
fermion
mass
6
QED vertex
W,Z vertex
W propagator
photon prop.
W,Z,photon
interactions
Z propagator
Higgs
mass
W mass
Higgs
interactions
with fermions
and bosons
Higgs prop.
Z mass
QCD vertex
gluon propagator
Ronan McNulty, Irish Quantum Foundations
fermion
propagator
fermion
mass
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QED vertex
W,Z vertex
W propagator
photon prop.
W,Z,photon
interactions
Z propagator
Higgs
mass
W mass
Higgs
interactions
with fermions
and bosons
Higgs prop.
Z mass
QCD vertex
gluon propagator
Ronan McNulty, Irish Quantum Foundations
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The LHC
ALICE
ATLAS
CMS
LHCb
(10-15m)
Nominal Energy: 7TeV
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ATLAS and CMS surround the interaction
region
Fully instrumented in region: -2 <  < 2
Partial instrumentation:
-5 <  < 5
Ronan McNulty, Irish Quantum Foundations
The LHCb detector
0.4
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Complementarity of LHC detectors
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1. Electroweak tests using W
Z
;
probing the proton structure
u
u
d
u
u
d
,
Ronan McNulty, Irish Quantum Foundations
Parton Density Function (PDF):
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fq(x,Q2)
Probability that the proton contains this parton
with this momentum fraction
Q = Invariant mass of parton interaction
x = Qe±y/s
[y is rapidity, s c.o.m]
u
u
d
u
u
d
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Theory v Experiment at the LHC
Hadronic
Cross-section
PDFs
Partonic
Cross-section
• 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
Ronan McNulty, Irish Quantum Foundations
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8
7
6
5
log10(Q2)
[GeV2]
4
3
2
1
0
-1
-6
-5
-4
-3
log10(x)
-2
-1
0
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Ronan McNulty, Irish Quantum Foundations
9
8
7
6
5
log10(Q2)
[GeV2]
4
3
2
1
0
-1
-6
-5
-4
-3
log10(x)
-2
-1
0
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Ronan McNulty, Irish Quantum Foundations
9
8
7
6
5
log10(Q2)
[GeV2]
4
3
2
1
DGLAP
evolution
0
-1
-6
-5
-4
-3
log10(x)
-2
-1
0
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y:
8
6
4
2
0
2
4
6
7
6
5
log10(Q2)
[GeV2]
4
Production of
object of mass Q
at rapidity
3
2
1
DGLAP
evolution
0
-1
-6
-5
-4
-3
log10(x)
-2
-1
0
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y:
8
6
4
2
0
2
4
6
7
6
5
log10(Q2)
[GeV2]
4
ATLAS & CMS:
3
Collision between two
partons having similar
momentum fractions.
2
PDFs either already
measured by HERA or
Tevatron, or requiring
modest extrapolation
through DGLAP.
1
0
-1
-6
-5
-4
-3
log10(x)
-2
-1
0
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Ronan McNulty, Irish Quantum Foundations
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y:
8
6
4
2
0
2
4
6
7
6
5
log10(Q2)
[GeV2]
4
LHCb:
3
Collision between one well
understood parton and
one unknown or large
DGLAP evolved parton.
2
1
0
-1
-6
-5
-4
-3
log10(x)
-2
-1
0
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Ronan McNulty, Irish Quantum Foundations
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y:
8
6
4
2
0
2
4
6
7
6
5
log10(Q2)
[GeV2]
W,Z
4
ATLAS & CMS:
3
g*
Collision between two
partons having similar
momentum fractions.
2
PDFs either already
measured by HERA or
Tevatron, or requiring
modest extrapolation
through DGLAP.
1
0
-1
-6
-5
-4
-3
log10(x)
-2
-1
0
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Ronan McNulty, Irish Quantum Foundations
9
y:
8
6
4
2
0
2
4
6
7
6
5
log10(Q2)
[GeV2]
W,Z
LHCb:
4
Collision between one well
understood parton and
one unknown or large
DGLAP evolved parton.
3
2
g*
1
DGLAP
evolution
Potential to go to very
low x, where PDFs
essentially unknown
0
-1
-6
-5
-4
-3
log10(x)
-2
-1
0
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Pre-LHC precision on W,Z cross-sections
e.g. very roughly
W + ud u
=
»
W - du d
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W and Z production in the forward region
Experimentally: =N/L
Count number of events: Z, W
Master formula: s =
pN
eL
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The LHCb detector
0.4
25
First Z candidate
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Z cross-section measurement
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pN
s=
eL
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Efficiencies for W and Z analysis found
from tag-and-probe
First W candidate
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Purity of W selection
2<<2.5
+
2.5<<3
-
3.5<<4
4<<4.5
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pN
s=
eL
3<<3.5
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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
μe
μμ
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)
Ronan McNulty, Irish Quantum Foundations
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
Elastic
p
p
It’s QCD – but not as we normally see it. It’s colour-free
σelastic
≈ 40mb
σdiffractive ≈ 10mb
σinelastic ≈ 60mb
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Physics of the Vacuum
Pomeron (soft)
Elastic
p
0+
p
It’s QCD – but not as we normally see it. It’s colour-free
σelastic
≈ 40mb
σdiffractive ≈ 10mb
σinelastic ≈ 60mb
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Physics of the Vacuum
Pomeron (soft)
p
0+
p
It’s QCD – but not as we normally see it. It’s colour-free
σelastic
≈ 40mb
σdiffractive ≈ 10mb
σinelastic ≈ 60mb
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Physics of the Vacuum
Pomeron (hard and soft)
Diffractive
0+
p
No activity
“rapidity gap”
p
It’s QCD – but not as we normally see it. It’s colour-free
σelastic
≈ 40mb
σdiffractive ≈ 10mb
σinelastic ≈ 60mb
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Physics of the Vacuum
Pomeron (hard)
Central Exclusive
0+
p
“particle”
“rapidity gap”
“rapidity gap”
p
Elastic diffractive: clean environment to study vacuum, and
in particular, transition between soft and hard pomeron.
σelastic
≈ 40mb
σdiffractive ≈ 10mb
σinelastic ≈ 60mb
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Physics of the Vacuum
“particle”
Pomeron (hard)
Central Exclusive
0+
p
Photon
“rapidity gap”
“rapidity gap”
p
Elastic diffractive: clean environment to study vacuum, and
in particular, transition between soft and hard pomeron.
σelastic
≈ 40mb
σdiffractive ≈ 10mb
σinelastic ≈ 60mb
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Physics of the Vacuum
“particle”
Photon
Central Exclusive
QED !
“rapidity gap”
p
Photon
“rapidity gap”
p
Elastic diffractive: clean environment to study vacuum, and
in particular, transition between soft and hard pomeron.
σelastic
≈ 40mb
σdiffractive ≈ 10mb
σinelastic ≈ 60mb
Ronan McNulty, Irish Quantum Foundations
Sensitivity to gluon PDF
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xg µ x - l
Gluon PDF enters
squared
Leading order cross-section
Examples of dependence of Jpsi cross-section on PDF (left) and extraction
of gluon PDF (right) from Martin, Nockles, Ryskin, Teubner, arXiv:0709.4406v1
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The LHCb detector
(some sensitivity -3.5<η<-1.5)
0.4
Low multiplicity required. Restricts to single-interaction collisions
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VELO sub-detector measures particle positions to 5um
UCD helped build, commission and operate the VELO
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Graphical Representation
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Effect of rapidity gap requirement on
muon triggered events
JPG 40 (2013) 045001
JPG 40 (2013) 045001
All triggered events
With veto on backward tracks
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Central exclusive di-muon signals
SuperChic: L. Harland-Lang, V. Khoze, M. Ryskin, W. Stirling, EPJ.C65 (2010) 433-448
Starlight: S.R. Klein & J. Nystrand, PRL 92 (2004) 142003.
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Before and after requiring precisely two tracks
Tracks have
pT>400MeV
2<η<5
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Non-resonant background very small
JPG 40 (2013) 045001
Distributions are not background-subtracted.
37pb-1 of data: 1492 J/ψ and 40 ψ(2s)
JPG 40 (2013) 045001
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Cross-section measurement
Purity:
1. non-resonant bkg (1%)
2. Chi_c feeddown (9%)
3. Psi’ feedown (2%)
4. Inelastic Jpsi production (30%)
pN
s=
eL
Number of events
observed
Luminosity
Efficiency:
1. Trigger
2. Tracking & muon id.
3. Single interaction beam-crossing
n -m
m
P(n) = e
n!
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Feed-down backgrounds
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Inelastic background
Characterise pT spectrum of background using shapes with 3-8 tracks
and extrapolate to 2 track case.
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Inelastic background
Signal shape
Estimated from Superchic using exp(- b pT2) (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
JPG 40 (2013) 045001
Inelastic background shape
Purity of exclusive signal
below 900 MeV/c pT
= (70 ± 4 ± 6)%
Estimated from data.
Characterise shape for 3-8 tracks
and 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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LHCb compared to theory & experiment
*
All predictions (bar Schaefer&Szcaurek) have similar approach and give
similar results and are consistent with our data.
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e
LHCb compared to HERA
Twofold
ambiguity
LHCb c/s is HERA c/s weighted by photon spectrum + gap survival factor (r)
d
LHCb differential data fitted assuming power law dependence s (W ) = aW
a = 0.8+1.2
-0.5 nb
d = 0.92 ± 0.15
LHCb
Power law results
a = 3nb
d = 0.72
HERA
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LHCb compared to theory & experiment
JPG 40 (2013) 045001
measured
from fit
plotted
Ronan McNulty, Irish Quantum Foundations
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 muons
χc 0,1,2 decay to Jpsi+photon
Vacuum 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.
Ronan McNulty, Irish Quantum Foundations
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
LHCb preliminary results with 2010 data
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 determine
non-resonant background for each.