Two Lectures on Making Precision Measurements at Hadron Colliders
Making Precision Measurements at Hadron CollidersHenry Frisch
University of Chicago
Lake Louise Winter Institute, Feb. 17-23, 2006
Contents
1 Lecture I: The Electroweak Scale: Top, the W and Z, and the Higgs via MW and Mtop 3
2 Purpose 3
3 Some History and Cultural Background 43.1 Instrumental Sensitivity: Orders of Magnitude . . . . . . . . . . . . . . . . . . . . . . . . . 43.2 Hubris: The 50 GeV Top Quark and No Quarkonia . . . . . . . . . . . . . . . . . . . . . . 6
4 The Tevatron and the LHC 7
5 The Anatomy of Detectors at Hadron Collider: Basics 85.1 Basics: Kinematics and Coverage: pT vs P|| . . . . . . . . . . . . . . . . . . . . . . . . . . 85.2 Basics: Particle Detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
6 Calibration Techniques 126.1 Momentum and Energy Scales: E/p . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126.2 Higher-order momentum and energy corrections . . . . . . . . . . . . . . . . . . . . . . . . 14
7 W and Z0Production as Archetypes 14
8 ‘QCD’- Jet Production, Quark and Gluons, ISR, FSR 19
9 The MTop −MW Plane and the Higgs Mass 249.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249.2 What limits the precision on the W mass and the top mass measurements? . . . . . . . . . 24
10 Measuring the Top Quark Mass and Cross-section 2710.1 tt Production: Measuring the Top Cross-section Precisely . . . . . . . . . . . . . . . . . . . 2710.2 Total Cross-section for tt Production: Parsing the CDF and DØ Summary Plots . . . . . . 2910.3 Properties of the tt system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3210.4 Top Decays: ‘Lepton+Jets’, ‘Dileptons’, ‘All-Hadronic’ . . . . . . . . . . . . . . . . . . . . 3410.5 Precision Measurement of the Top Mass . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
11 Lecture II: Searching for Physics Beyond the SM, and Some Challenges for the Audi-ence 3511.1 Strategies: Signature-Based vs Model-Directed, Blind vs A Priori vs Myopic, etc. . . . . . . 3711.2 Lepton+Gamma+X: The `γ 6Et and ``γ Signatures . . . . . . . . . . . . . . . . . . . . . . . 39
HJF Lake Louise Winter Institute Feb. 17-23, 2006
Two Lectures on Making Precision Measurements at Hadron Colliders
11.3 Gamma+Gamma+X: The ``γ Signature . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4411.4 Inclusive High Pt W’s and Z’s: A Weak Boson Signature . . . . . . . . . . . . . . . . . . . 4611.5 The Tail of the W: Above the Pole- Wprimes . . . . . . . . . . . . . . . . . . . . . . . . . . 4911.6 An Indirect Search: Asymmetries above the Pole (CDF+D0) . . . . . . . . . . . . . . . . . 4911.7 A Classic SUSY Search: Trileptons at D0 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4911.8 A Classic SUSY Search: Met + Jets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
12 Direct Search for the Higgs 49
13 Expert Topics: Black and Double Black: Challenges for Students 5113.1 Fragmentation Near z = 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5113.2 Photon and Tau Fake Rates: Gluon and Quark Jets . . . . . . . . . . . . . . . . . . . . . . 5113.3 Monte Carlo Issues: QCD and QED, NLO and beyond . . . . . . . . . . . . . . . . . . . . 5113.4 B-jet Momentum Scale: Gamma-bjet Balancing . . . . . . . . . . . . . . . . . . . . . . . . 52
14 Beyond Expert: Out-of Boundary Area Topics: Challenges for Expert Groups 5314.1 Book-keeping: Rethinking Luminosity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5314.2 Rethinking Analysis Code and Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5314.3 Changing the Paradigm: W/Z ratios, Color Singlet/Color Triplet Ratios, and Other New
Precision Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5314.4 Particle ID: Distinguishing W → cs from W → ud, bb from b in Top Decays . . . . . . . . . 5314.5 Discrete Symmetry Tests: C, CP, and T above the W and Z poles . . . . . . . . . . . . . . 53
15 Credits 54
HJF Lake Louise Winter Institute Feb. 17-23, 2006
Two Lectures on Making Precision Measurements at Hadron Colliders
1 Lecture I: The Electroweak Scale: Top, the W and Z, and
the Higgs via MW and Mtop
2 Purpose
These two lectures are purely pedagogical. My intent is to enable non-experts
to get something out of the individual presentations on collider physics that
will follow- the Higgs, the W,Z, top, searches for SUSY, LED’s, etc. We are
presented with so many measurements and so much detail that we often forget
that we are talking about instruments and the measurements they have made.
The suprise is how precise the detectors themselves are; the challenge will be to
exploit that precision in the regime where statistics is no longer a problem, and
everything is dominated by the performance of the detector (‘systematics’).
This challenge also extends to the theoretical community- to look for
something new we will need to understand the non-new, i.e. the SM predictions,
at an unprecedented level of precision. Some amount of this can be done with
control samples- it is always best to use data rather than Monte Carlo, but
it’s not always possible. The detectors are already better than the theoretical
predictions- the theory community needs to catch up.
I work on CDF, and have used mostly CDF plots just because I know
them. No slight to DØ or the LHC experiments is meant. I have cut some
corners in places and been a little provocative in others, as teachers will. All
views presented here are my own.
I have intentionally used older public results from CDF and DØ instead
of the hot-off-the-press results generated for the 2006 ‘winter conferences’ so as
not to steal the thunder of the invited speakers who are here to present new
results from CDF and D0. The idea is to provide the understanding so that
you can ask them the hard questions, and to provoke discussion. This is going
to be really different from a raporteur’s talk...
HJF Lake Louise Winter Institute Feb. 17-23, 2006
Two Lectures on Making Precision Measurements at Hadron Colliders
Figure 1: A history of high-energy (no ISR) hadron colliders: integrated luminosity by year.
3 Some History and Cultural Background
3.1 Instrumental Sensitivity: Orders of Magnitude
A brief history of luminosity, starting with the SPPS and the race to discover
the W and the Z0, and then the race to discover the top, is shown in Figure 1.
At the Tevatron we feel we are in a race now to discover whatever is next-
for objects in the several hundred GeV mass range it’s all in accumulating
luminosity.
HJF Lake Louise Winter Institute Feb. 17-23, 2006
Two Lectures on Making Precision Measurements at Hadron Colliders
’88: Inverse Nanobarns ’06: Inverse Femtobarns
Figure 2: The integrated luminosity in the 1987 Tevatron run (Left), in Inverse Nanobarns, and in Run II(Right), in Inverse Femtobarns. Note that 1 fb−1= 103 pb−1= 106 nb−1. Note also the efficiency to tapehas improved substantially.
Figure 2 shows the luminosity ‘delivered’ and ‘to tape’ from the current Run
II, in inverse femtobarns (right), and from the 1987 run, in inverse nanobarns.
As a quick reminder, the W± → e±ν cross-section times BR is about 2.2 nb
for the left-hand plot, so 30 nb−1means that 66 W± → e±ν decays were cre-
ated in the recorded exposure. The cross-section for a 115 GeV Higgs in
W± → e±ν + H production is xxx fb, and so the right hand plot indicates
that xxx W± → e±ν +H events were created.
HJF Lake Louise Winter Institute Feb. 17-23, 2006
Two Lectures on Making Precision Measurements at Hadron Colliders
Figure 3: Left: The 1984 Top ‘discovery’; Right: The 1974 ‘no discovery’ announcement of the J/ψ andUpsilons.
3.2 Hubris: The 50 GeV Top Quark and No Quarkonia
Figure 3 is an historical reminder both that we should not be over-confident
about what we know, and that Nature has a rich menu of surprises. The left-
hand page is the discovery of something that did not exist- a top quark with
mass less than 50 GeV (it was largely W+jets, as shown by Steve Ellis). The
right-hand page is a prediction that there are no narrow states with masses
between 3 and 10 GeV decaying into lepton pairs (note both these guys did
well- Nature gave more chances!).
HJF Lake Louise Winter Institute Feb. 17-23, 2006
Two Lectures on Making Precision Measurements at Hadron Colliders
4 The Tevatron and the LHC
By now everybody should know about the Tevatron and LHC. I will spare
you pictures and boilerplate; The main differences that everybody, including
theorists, should know are:
Tevatron LHC
Parton Source Antiproton-Proton Proton-proton
Energy (TeV) 1.96 (not 2!) 14
Peak Luminosity (cm−2s−1) 2× 1032 1× 1034
Crossing Spacing (ns) 396 24.95
Peak Interactions/Crossing 5 19
Luminous Line σ (cm) 30 4.5 [3]
Luminosity Lifetime (hours) 3.8/23 [4] 15
< x > at MW 0.04 0.006
< x > at 2MT 0.18 0.025
An LHC upgrade to 1× 1035 is planned.
Figure 4: The CTEQ6.1M PDF’s at Q=100 (Joey Huston).
HJF Lake Louise Winter Institute Feb. 17-23, 2006
Two Lectures on Making Precision Measurements at Hadron Colliders
5 The Anatomy of Detectors at Hadron Collider: Basics
I start with a brief elementary introduction. Working at a hadron collider is
really different from at an e+e− machine!
5.1 Basics: Kinematics and Coverage: pT vs P||
The phase space for particle production at a hadron collider is traditionally
described in cylindrical coordinates with the z axis along the beam direction,
the radial direction called ‘transverse’, as in ‘Transverse Momentum’ (pT), and
the polar angle expressed as Pseudo-rapidity η, where η ≡ −ln(tanθ/2)).
Pseudo-rapidity is a substitute for the Lorentz-boost variable, y, where y ≡1/2ln(E + pz)/(E − pz) ≡ tanh−1(pz/E). Since in most cases one does not
know the mass of a particle produced in a hadron collision (most are light- pions,
kaons, baryons,..), we use pseudo-rapidity. (This is a common trap when doing
complex kinematics with W’s, Z’s, and top, where the mass truly matters).
Figure 5 shows an early sketch of the proposed coverage in η for CDF; note that
the big central detector seems very small, while the little luminosity monitors
seem big.
HJF Lake Louise Winter Institute Feb. 17-23, 2006
Two Lectures on Making Precision Measurements at Hadron Colliders
Figure 5: An early planning document (Hans Jensen) for the coverage in rapitidy for CDF
Two simple equations contain much of the physics for the production of heavy
states at a collider: the mass and longitudinal momentum of the heavy state
(e.g. a W, Z, tt pair, or WH) are determined by the fraction of the beam
momentum carried by the interacting partons. Note that for a heavy object
typically has a velocity β << 1, even though the longitudinal momentum is
typically not small (we’re not in the c.m! of the collision.). Note also that
the transverse momentum of the system is determined by the competition of
falling parton distribution functions (PDF’s- also known as structure functions)
as the total invariant mass of the system rises, and the increase in phase space
as the momentum of the system increases. The production thus peaks with a
total system energy above threshold by an amount characteristic of the slope
in x1 ∗ x2.
m2 = x1 ∗ x2s pz = (x1 − x2)pbeam (1)
HJF Lake Louise Winter Institute Feb. 17-23, 2006
Two Lectures on Making Precision Measurements at Hadron Colliders
5.2 Basics: Particle Detection
Here I deal with high-momentum particle detection. Low-momentum– typically
up to a few GeV– charged particles can be identified by processes that depend
on their velocity, β, as a simultaneous measurement of p = βγm and β allows
extracting the mass. However for momenta above a few GeV pions, kaons, and
protons cannot be separated. However electrons, muons, hadrons, and neutrinos
interact differently, as shown in Figure 5.2. The measurement of their energies
and/or momenta stem from their different modes of interaction.
HJF Lake Louise Winter Institute Feb. 17-23, 2006
Two Lectures on Making Precision Measurements at Hadron Colliders
(a) Identifying a high ET-electron
(b) Identifying a high pT-muon
(c) Identifying a jet (d) Identifying a neutrino
HJF Lake Louise Winter Institute Feb. 17-23, 2006
Two Lectures on Making Precision Measurements at Hadron Colliders
6 Calibration Techniques
6.1 Momentum and Energy Scales: E/p
The Tevatron and the LHC are as different from LEP and other e+e− colliders
as night and day- it is a big disadvantage to have worked at LEP(!). One
key difference is that the overall mass (energy) scale is not set by the beam
energy- there is a continuum of c.m. energies in the parton-parton collisions.
Moreover the hard scattering is not at rest either longitudinally nor transverse
in the lab system- there is ‘intrinsic Kt’ as well as initial-state radiation (ISR).
Finally, the beam spot is a line and not a spot- the vertex point, used to
calculate transverse energies, has to be determined from the event, including
for neutrinos and photons for which no track is observed.
Dealing first with the issue of setting the scale for momentum, energy, and
mass measurements. All current detectors consist of a magnetic spectrometer
followed by calorimeters.
The magnetic spectrometer uses a precisely measured (NMR) magnetic field
and the precise geometry of the tracking chambers to measure the curvature
(1/PT )of the tracks of charged particles. This is an absolute measurement- if
perfect one has the momentum scale. One can then use particles with mea-
sured momentum as an in situ ‘test beam’ to calibrate the energy scale of the
calorimeters.
The momentum scale can be checked by measuring the masses of some calibra-
tion ‘lines’ thoughtfully provided by Mother Nature- the J/Psi and Υ systems,
and the Z0in its Z0 → µ+µ− decays (Z0 → e+e− doesn’t work for momentum
calibration!). Fig. 6 shows measured distributions from CDF. However the mo-
mentum scale can be incorrect due to mis-alignments in the tracking chamber.
The combination of a calorimeter and a magnetic spectrometer allows one to
remove the 1st-order errors in both [5] by measuring ‘E’ (calorimeter energy)
over ‘p’ (spectrometer moementum. With perfect resolution, no energy loss,
HJF Lake Louise Winter Institute Feb. 17-23, 2006
Two Lectures on Making Precision Measurements at Hadron Colliders
Figure 6: Left: The reconstructed JΨ invariant mass in dimuons (CDF). Right: The similar plot for theUpsilon system.
and no radiation these two should be equal: E/p = 1.0. Figure 7 shows the
measured spectrum in E/p for electrons.
The 1st-order error in momentum is due to a ‘false-curvature’- that is that
a straight line (zero-curvature= ∞ momentum) is reconstructed with a finite
momentum. The 1st-order error in calorimeter energy is an offset in the energy
scale, and does not depend on the sign (±) of the particle [6]. Expanding both
the curvature and calorimeter energies to first order:
1/p = 1/ptrue + 1/pfalse (µ+) 1/p = 1/ptrue − 1/pfalse (µ−) (2)
E = Etrue ∗ (1 + ε) (e+) E = Etrue ∗ (1− ε) (e−) (3)
The first-order false curvature pfalse then is derived by measuring E/p for pos-
itive and negative electrons with the same E
1/pfalse = ((E/p(e+)− E/p(e−))/2E (4)
The first-order calibration scale error ε then is removed by setting the calorime-
ter scale for electrons so that E/p agrees with expectations. In CDF, this is
done initially to make the calorimeter response uniform in φ− η.
1/pfalse = ((E/p(e+) + E/p(e−))/2 (5)
HJF Lake Louise Winter Institute Feb. 17-23, 2006
Two Lectures on Making Precision Measurements at Hadron Colliders
Figure 7:
6.2 Higher-order momentum and energy corrections
The momentum and energy calibrations at this point are good enough for ev-
erything at present exposures except the W mass measurement. There are three
higher-order effects that are taken care of at present:
1. ‘Twist’ between the 2 end-plates of the tracking chamber;
2. Systematic scale change in the z-measurements in the chamber;
3. Non-linearity of the calorimeter due to e(E/2) + γ(E/2) 6= e(E)
Figure 8 shows the use of the J/Ψ mass to correct for the first two of these
effects. What is plotted is the correction to the momentum scale versus the
cotan of the difference in polar (from the beam axis) angle of the two muons.
There is a linear correction to the curvature of δc = 6×10−7cot(θ) that corrects
for the twist between the endplates, and a change in the scale of the z-coordinate
by 2 parts in 104, zscale = 0.9998 ± 0.0001. This is precision tuning of a large
but exceptionally precise instrument!
7 W and Z0Production as Archetypes
Let us consider the production of the W and Z0vector bosons as archetypes of
hard processes. Figure 10 shows the dominant diagram and a ‘cartoon’ of the
HJF Lake Louise Winter Institute Feb. 17-23, 2006
Two Lectures on Making Precision Measurements at Hadron Colliders
Figure 8: Left: The correction to the momentum scale versus the cotan of the difference in polar angleof the two muons in J/psi decay before corrections: Right: The same after correcting the curvature byδc = 6× 10−7cot(θ) the scale of the z-coordinate by 2 parts in 104.
Figure 9: Measuring a higher-order correction to track curvature: the calorimeter to momentum ratio E/pversus cotθ for e+ and e−, before and after the curvature and z-scale corrections.
production process. Both the W and Z are observed in their leptonic decays
W± → l±ν and Z0 → ``. W and Z production thus provide a precise measure
of the up and down quark parton distribution functions (PDF’s). Since we
measure W’s and Z’s in their leptonic modes, the kinematics of the decay also
matter. Consider the W’s: they are polarized, as the u and d quarks are light
and couple through V-A so quarks have helicity -1 and antiquarks +1. The W
decays also by V-A, so the charged leptons come out opposite to the helicity
direction. However, the dominant effect, at least at the Tevatron, is that the
HJF Lake Louise Winter Institute Feb. 17-23, 2006
Two Lectures on Making Precision Measurements at Hadron Colliders
Figure 10: The dominant diagrams and a ‘cartoon’ of the production process for W and Z production.
W is moving in the rest frame, and since the (valence) u quark momentum is
generally higher than the (sea) d anti-quark; W+ go in the proton direction,
and W− in the p direction (the LHC, being proton-proton, doesn’t have this
useful asymmetry).
Figure 11 shows the distribution in the difference of e+ and e− versus η (pseudo-
rapidity) of the electron (e±) measured by CDF. The left-hand plot shows the
full range as well as the experimental uncertainty band; the right-hand plot
HJF Lake Louise Winter Institute Feb. 17-23, 2006
Two Lectures on Making Precision Measurements at Hadron Colliders
Figure 11: Left: The forward-backward charge asymmetry in W± → e±ν decays plotted versus pseudo-rapidity. The blue error band gives the experimental uncertainty; also shown is the prediction using theCTEQ5L parton distribution functions. Right: The same data, folded around zero in η (remember this ispp), compared to a prediction using the RESBOS MC generator and the CTEQ6.1M PDF’s.
shows a comparison with the predictions using the CTEQ6 PDF’s. One can
see that the PDF’s do not fit well, and so we are learning about the u and d
quark distributions from the W asymmetry.
The W and Z longitudinal momenta are determined by the structure functions;
the transverse momenta are determined by initial state radiation off of the in-
coming quarks (radiation off of the outgoing remnants is suppressed). Figure 12
HJF Lake Louise Winter Institute Feb. 17-23, 2006
Two Lectures on Making Precision Measurements at Hadron Colliders
Figure 12: Left: Right:
HJF Lake Louise Winter Institute Feb. 17-23, 2006
Two Lectures on Making Precision Measurements at Hadron Colliders
Figure 13: Left: Right:
8 ‘QCD’- Jet Production, Quark and Gluons, ISR, FSR
The dominant feature in the hadron collider landscape is the production of jets-
the hard scattering of partons. Figure 13 reproduces two pages from a seminal
paper in 1971, when the idea of partons was brand new, by Berman, Bjorken,
and Kogut, pointing out that the existence of partons would lead to point-like
scatterings and hence high pT phenomena, including ‘cores’ (jets).
HJF Lake Louise Winter Institute Feb. 17-23, 2006
Two Lectures on Making Precision Measurements at Hadron Colliders
Figure 14:
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Two Lectures on Making Precision Measurements at Hadron Colliders
Figure 15:
HJF Lake Louise Winter Institute Feb. 17-23, 2006
Two Lectures on Making Precision Measurements at Hadron Colliders
Figure 16:
HJF Lake Louise Winter Institute Feb. 17-23, 2006
Two Lectures on Making Precision Measurements at Hadron Colliders
Figure 17:
HJF Lake Louise Winter Institute Feb. 17-23, 2006
Two Lectures on Making Precision Measurements at Hadron Colliders
9 The MTop −MW Plane and the Higgs Mass
9.1 Motivation
The top quark is remarkable for its physics and useful as a tool for calibration.
It may also be a window into the world of heavy weakly-interacting particles
(such as a Higgs of one sort or another) in that it is produced strongly (i.e
with coupling Oαs) in pairs, but due to its strongly-conserved flavor quantum
number (top-ness), has to decay electro-weakly. Due to radiative corrections,
the masses of the W, Z, Higgs, and top quark are related in the SM; precise
measurements of the W and top quark masses determine the predicted Higgs
mass.
Figure 18: Left: The MW vs MT plane as of March 1998. Right: The MW vs MT plane as of the summerof 2005. Note the difference in the scales of the abscissas.
9.2 What limits the precision on the W mass and the top mass measure-ments?
Figure 20 gives the history of the uncertainty on the W mass as a function of
the square-root of luminosity. The statistical uncertainty is expected to scale
HJF Lake Louise Winter Institute Feb. 17-23, 2006
Two Lectures on Making Precision Measurements at Hadron Colliders
Figure 19: Left: The measured allowed region at 68% (1σ) in the MW −Mtop plane (the intersection ofinside the blue and solid-red contours), and the predicted dependence of the MW and Mtop on the SMHiggs mass. Right: The fit for the mass of the SM Higgs, showing the region excluded at 68% C.L.
as∫Ldt−1. The systematic uncertainties will be discussed below when we get
to the measurement of the W mass; however it is interesting to note that since
the systematics are studied with data, they also seem to scale with luminosity.
If the control of systematic uncertainties continues to scale with statistics as∫Ldt−1 the Tevatron can do as well as LHC projections [9], and with very
different systematics.
HJF Lake Louise Winter Institute Feb. 17-23, 2006
Two Lectures on Making Precision Measurements at Hadron Colliders
Figure 20: The total uncertainty on the W mass as measured at the Tevatron, versus integrated luminosity.If the control of systematic uncertainties continues to scale with statistics as
∫Ldt−1 the Tevatron can do
as well as LHC projections, and with different systematics.
HJF Lake Louise Winter Institute Feb. 17-23, 2006
Two Lectures on Making Precision Measurements at Hadron Colliders
Figure 21:
10 Measuring the Top Quark Mass and Cross-section
I will discuss two specific measurements as pedagogic examples of some specific
difficulties (challenges is the polite word) of doing precision measurements - the
measurements of the top cross-section and the top mass. The idea is make
it possible for you to ask really hard questions when you see these standard
busy-busy plots that speakers expect you to just let go by. First some basics.
10.1 tt Production: Measuring the Top Cross-section Precisely
The prime motivation for a precise measurement of the top cross-section is
that new physics could provide an additional source for the production (leading
to a larger cross-section than expected) or additional decay channels (leading
HJF Lake Louise Winter Institute Feb. 17-23, 2006
Two Lectures on Making Precision Measurements at Hadron Colliders
to a smaller measured cross-section into b) [8]. More prosaically, the cross-
section is a well-defined and in-principle easy-to-measure quantity that tests
many aspects of QCD and the underlying universe of hadron collider physics-
the PDF’s, LO, NLO and NNLO calculations, and provides a calibration point
for calorimeters and the energy scale (will be a key calibration for LHC). Lastly,
and less defensible scientifically, is the uneasy feeling that too low a cross-section
(e.g.) means that the top mass is really lighter than we measure, and the crucial
EWK fits and limits on the Higgs mass are thus probably not correct.
Figure 22 shows the dominant diagrams for top production. At the Tevatron
(left) the tt system, with a mass 400 GeV, samples the structure functions at
a typical x given by < x1x2 >= m2/√s =∼ (400/1960)2, giving < x > =
∼ 0.20,
well into the valence quark region. At the LHC, the corresponding value is
< x > =∼ 0.04, i.e. in a region dominated by gluons.
Figure 22: Left: The dominant diagram for tt production at the Tevatron; Right: The dominant diagramat the LHC. (from F. Maltoni [7]).
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Two Lectures on Making Precision Measurements at Hadron Colliders
10.2 Total Cross-section for tt Production: Parsing the CDF and DØ Sum-mary Plots
Figure 23: From CDF: The measured and predicted top cross-sections versus mass with approximately200 pb−1(Left) and now with approximately 350 pb−1(Right).
‘200’ pb-1 ‘350’ pb-1
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Two Lectures on Making Precision Measurements at Hadron Colliders
Figure 24: Left: The differential spectrum dN/dPT of t-quarks in tt production; Right:The differentialspectrum dN/dη, again for t-quarks.
‘200’ pb-1 ‘350’ pb-1
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Two Lectures on Making Precision Measurements at Hadron Colliders
Figure 25: Left: The distribution in ∆φ between the t and the t in tt production; Right: the analogousdistribution in ∆η.
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Two Lectures on Making Precision Measurements at Hadron Colliders
10.3 Properties of the tt system
The tt system is particularly interesting, as there may be new resonances decaying directly into tt or newpairs of particles each with a decay into top plus something. Either way there would be a feature in the ttmass spectrum and a change in shape in the tt pT spectrum. Figure26
Figure 26: Left: CDF’s ttbar mass spectrum from 320 pb−1. Right: The ttbar mass spectrum as measuredin 370 pb−1by DØ .
HJF Lake Louise Winter Institute Feb. 17-23, 2006
Two Lectures on Making Precision Measurements at Hadron Colliders
What is the probability for a lower-mass pair to be reconstructed at a higher
mass? Input a mean value for the pair, and look at the output. Abcissa runs
from 0 to 1200 GeV.
Figure 27: The output templates for an input ttbar pair mass; the abscissa runs between 0 and 1200 GeVin each plot (CDF).
450 GeV 500 GeV
550 GeV 600 GeV
650 GeV 700 GeV
750 GeV 800 GeV
850 GeV 900 GeV
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Two Lectures on Making Precision Measurements at Hadron Colliders
10.4 Top Decays: ‘Lepton+Jets’, ‘Dileptons’, ‘All-Hadronic’
The expectation is that the Top quark decays t → W+ + b (t → W−b); i.e.
Vtd = 1. The experimental limit on Vtd is Vtd = xx± xx [].
10.5 Precision Measurement of the Top Mass
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Two Lectures on Making Precision Measurements at Hadron Colliders
Figure 28:
11 Lecture II: Searching for Physics Beyond the SM, and Some
Challenges for the Audience
Our hope at the Tevatron is, of course, that we find something new before the
LHC. We had hints of new things in Run I:
1. the top dilepton sample looked odd (too many e-mu events, e-mu close in
phi, some odd kinematics;
2. The eeγγ 6Et event and the 2.8σ excess in ` + γ + 6Et
3. Perhaps more top to tau events than we deserved?
HJF Lake Louise Winter Institute Feb. 17-23, 2006
Two Lectures on Making Precision Measurements at Hadron Colliders
4. Top mass in dileptons was consistently lower than in lepton+jets
HJF Lake Louise Winter Institute Feb. 17-23, 2006
Two Lectures on Making Precision Measurements at Hadron Colliders
11.1 Strategies: Signature-Based vs Model-Directed, Blind vs A Priori vsMyopic, etc.
None of these was significant statistically- but made one want more data. We
now have 10-times the data! What to do: There are two major kinds of di-
rect searches, and in each three kinds of strategies have been followed (all this
categorization is arguable):
1. Signature-Based:
2. Model-Directed
Avoiding biases is important (see next slide)- two strategies are followed.
1. A Priori- use the same cuts as published in Run I, or in the 1st 1/3rd of
the data; then run on rest of data without changing anything (my favorite
for signature-based searches- look hard at your data!).
2. Blind- this is heavily used now-very useful and appropriate in some cases
(e.g. precision measurements: W mass, B lifetimes and masses, and classic
well-defined searches: B → µµ,...
A brief anecdote about a blind analysis around 1900:
There was a controversy over 2 conflicting measurements of a line in the
solar spectrum. The famous spectroscopist at Princeton asked his machin-
ist to rule a grating at a non-standard (blind!) lines/inch, and to put the
value in a sealed envelope. The Prof. then measured the line in terms
of an unknown dispersion, wrote a Phys Rev with an accompanying letter
that said ‘under separate cover you will receive the grating spacing from
my machinist, Mr. Smith; take this number, multiply it by my number,
put it in the blank space in the paper, and publish it’. Now, that’s blind.
HJF Lake Louise Winter Institute Feb. 17-23, 2006
Two Lectures on Making Precision Measurements at Hadron Colliders
Much as in the search for the W and Z, there is a defining energy scale for new
physics beyond the SM. In the case of the W, Fermi’s ‘Standard Model’ (i.e. ‘ef-
fective field theory’) of a 4-fermion interaction predicted that νe+e− → νe+e
−
scattering violated S-wave unitarity at a c.m. energy =∼ 300 GeV (see Commins
and Bucksbaum, Chapter 1.6, e.g.). For the SM, it’s more complicated (see,
e.g. Gunion et al. in the Higgs Hunter’s Guide), but the conclusion is the
same- there must be something new at the TeV scale. We experimentalists
are consequently primed to find something new at the Tevatron and/or LHC.
New means comparing data to precise predictions of the SM. Figure 29 shows
what can happen when eagerness combines with insufficiently understood SM
predictions.
Figure 29: An example of why the careful calculation of SM predictions is so crucial: the announcementof the ‘discovery’ of SUSY at the 1986 Aspen Conference. The right explanation (S. Ellis) turned out tobe a cocktail of SM processes, in particular W+jets and Z+jets.
HJF Lake Louise Winter Institute Feb. 17-23, 2006
Two Lectures on Making Precision Measurements at Hadron Colliders
11.2 Lepton+Gamma+X: The `γ 6Et and ``γ Signatures
One of the anomalies of Run I was the famous eeγγ 6Et event. This spawned the
advent of ‘signature-based’ searches at the Tevatron. In particular there were
two follow-ups: γγ +X (Toback) and `γ +X (Berryhill). The `γ +X search
resulted in a 2.7σ excess over SM expectations.
Figure 30:
The analysis is being repeated with exactly the same kinematic cuts so this
time it is a priori- (i.e. not self-selected to be interesting).
HJF Lake Louise Winter Institute Feb. 17-23, 2006
Two Lectures on Making Precision Measurements at Hadron Colliders
Andrei Loginov Search for Lepton-Photon-X Events
Photon-Electron Flow-Chart
Lepton-Photon Sample1 Lepton and 1 Photon
ET > 25 GeV508 Events
??
Exactly 1 LeptonExactly 1 Photon∆φlγ > 1506ET < 25
397 Events
?
?
Inclusive Multi-Body Events(All Other Photon-Lepton)
111 Events
??
?
Z-Like lepton-photon81 Gev < Meγ < 101 Gev(Background Calibration)
209 Events
Exactly 1 LeptonExactly 1 Photon∆φlγ < 1506ET < 25 GeV
67 Events
Two-Body Events188 Events
Multi-Body lγET
Events
6ET > 25 GeV
25 Events
Multi-Photon and
Multi-Lepton Events
0 and 19 Events, resp.
Figure 2: Photon-Electron Sample: the subsets of inclusive γl events analyzed
Exotics Meeting -8- July 14, 2005
Andrei Loginov Search for Lepton-Photon-X Events
Photon-Muon Flow-Chart
Lepton-Photon Sample1 Lepton and 1 Photon
ET > 25 GeV71 Events
??
Exactly 1 LeptonExactly 1 Photon∆φlγ > 1506ET < 2528 Events
?
?
Inclusive Multi-Body Events(All Other Photon-Lepton)
43 Events
??
?
Z-Like lepton-photon81 Gev < Meγ < 101 Gev(Background Calibration)
10 Events
Exactly 1 LeptonExactly 1 Photon∆φlγ < 1506ET < 25 GeV
13 Events
Two-Body Events18 Events
Multi-Body lγET
Events
6ET > 25 GeV
18 Events
Multi-Photon and
Multi-Lepton Events
0 and 12 Events, resp.
Figure 1: Photon-Muon Sample: the subsets of inclusive γl events analyzed in this paper
Exotics Meeting -7- July 14, 2005
Figure 31: Left: The flow of the `+ γ +X signature based search in electrons. Right:The flow in muons.
HJF Lake Louise Winter Institute Feb. 17-23, 2006
Two Lectures on Making Precision Measurements at Hadron Colliders
Lepton+Photon+ 6ET Predicted Events
SM Source eγ 6ET µγ 6ET (e+ µ)γ 6ET
W±γ 11.9 ± 2.0 9.0 ± 1.4 20.9 ± 2.8Z0/γ + γ 1.2 ± 0.3 4.2 ± 0.7 5.4 ± 1.0W±γγ, Z0/γ + γγ 0.14 ± 0.02 0.18 ± 0.02 0.32 ± 0.04(W±γ or W±)→ τγ 0.7 ± 0.2 0.3 ± 0.1 1.0 ± 0.2Jet faking γ 2.8 ± 2.8 1.6 ± 1.6 4.4 ± 4.4Z0/γ → e+e−, e→ γ 2.5 ± 0.2 - 2.5 ± 0.2Jets faking `+ 6ET 0.6 ± 0.1 < 0.1 0.6 ± 0.1
Total SM
Prediction 19.8 ± 3.2 15.3 ± 2.2 35.1 ± 5.3
Observed in Data 25 18 43
Multi-Lepton+Photon Predicted Events
SM Source eeγ µµγ llγZ0/γ + γ 12.5 ± 2.3 7.3 ± 1.7 19.8 ± 4.0Z0/γ + γγ 0.24 ± 0.03 0.12 ± 0.02 0.36 ± 0.04Z0/γ+ Jet faking γ 0.3 ± 0.3 0.2 ± 0.2 0.5 ± 0.5Jets faking `+ 6ET 0.5 ± 0.1 < 0.1 0.5 ± 0.1
Total SM
Prediction 13.6 ± 2.3 7.6 ± 1.7 21.2 ± 4.0
Observed in Data 19 12 31
Table 1: A comparison of the numbers of events predicted by the Standard Model and the observations for the `γ 6ET and ``γ searches.The SM predictions for the two searches are dominated by Wγ and Zγ production, respectively [?, ?, ?]. Other contributions comefrom the tri-boson processes Wγγ and Zγγ, leptonic τ decays, and misidentified leptons, photons, or 6ET.
1
Figure 32:
HJF Lake Louise Winter Institute Feb. 17-23, 2006
Two Lectures on Making Precision Measurements at Hadron Colliders
(GeV)TPhoton E20 40 60 80 100 120 140
Eve
nts
/10
GeV
0
5
10
15
20
25CDF Run II Preliminary
-1), 307 pbµ Data(e+TEγlγW
γZγe fake
γγ, Wγγ, QCD, ZγτW jet,
(a)
(GeV)TLepton E20 40 60 80 100 120 140 160 180 200
Eve
nts
/10
GeV
0
5
10
15
20
25(b)
(GeV)TE0 20 40 60 80 100 120 140
Eve
nts
/10
GeV
0
5
10
15
20
25(c)
) (GeV)γ, TE (l, TM0 50 100 150 200 250
Eve
nts
/15
GeV
02468
101214161820 (d)
(GeV)TPhoton E20 40 60 80 100 120 140
Eve
nts
/10
GeV
0
5
10
15
20
25 CDF Run II Preliminary-1), 307 pbµ Data(e+γll
γZ
γγZ jet, QCD, Z
(a)
(GeV)TLepton E20 40 60 80 100 120 140 160 180 200
Eve
nts
/10
GeV
0
5
10
15
20
25(b)
M (l, l) (GeV)0 50 100 150 200 250 300
Eve
nts
/10
GeV
0123456789 (c)
) (GeV)γM (l, l, 0 50 100 150 200 250 300 350 400
Eve
nts
/10
GeV
0
2
4
6
8
10 (d)
Figure 33:
(GeV)TE0 5 10 15 20 25 30 35
Eve
nts
/5 G
eV
0
2
4
6
8
10 CDF Run II Preliminary-1), 307 pbµData(
γZ
γγZ jet, Z
(a)
(GeV)TE0 5 10 15 20 25 30 35
Eve
nts
/5 G
eV
0
2
4
6
8
10 CDF Run II Preliminary-1Data(e), 307 pb
γZ
γγZ jet, QCD, Z
(b)
Figure 34:
No more ``γγ 6Et events with > 3 times the data and higher energy. Have
another factor of 3 in data ready.
HJF Lake Louise Winter Institute Feb. 17-23, 2006
Two Lectures on Making Precision Measurements at Hadron Colliders
However, this has proved another educational example of MC predictions being
the limiting factor in speed and sensitivity. We do not have a control sample-
depend on SM predictions, largely Wγ and Zγ. Have 2 MC generators- Mad-
Graph [?] and a program from UliBaur [?]. They agree beautifully.
Figure 35:
However after running them through Pythia they disagreed by 15% in yield,
including a different identification efficiency for muons (!). Problems were in the
interface (diagnosed by Loginov and Tsuno) for both- the Les Houches accord
format is not precisely defined. Lessons:
1. Always use 2 MC’s- you may find both samples are flawed.
2. CDF has lost huge amounts of time to the generator interfacing- needs
re-examination by the theoretical community.
HJF Lake Louise Winter Institute Feb. 17-23, 2006
Two Lectures on Making Precision Measurements at Hadron Colliders
11.3 Gamma+Gamma+X: The ``γ Signature
Figure 36:
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Two Lectures on Making Precision Measurements at Hadron Colliders
Figure 37:
HJF Lake Louise Winter Institute Feb. 17-23, 2006
Two Lectures on Making Precision Measurements at Hadron Colliders
11.4 Inclusive High Pt W’s and Z’s: A Weak Boson Signature
Idea: Many models of new physics- Extra Dimensions [?], Z-primes, Excited
Top, t′ → Wb, SUSY, Right-handed Quarks ([?]) naturally give a signature of
a high-Pt EWK boson- W, Z, or photon. Natural in strong production of pairs-
if decays, decays weakly. E.g. top
PtWEntries 1026
Mean 135
RMS 64.82
Underflow 0
Overflow 0
0 50 100 150 200 250 300 350 4000
20
40
60
80
100
120
PtWEntries 1026
Mean 135
RMS 64.82
Underflow 0
Overflow 0
Transverse Momentum of the W
Figure 38: The pT spectrum for Z’s from the decay of a 300 GeV right-handed singlet down quark QQ→uWdZ in the Bjorken-Pakvasa-Tuan model.
Figure 39: The inclusive search for high-¶T Z+X production (CDF). The cuts are frozen on the first 0.3fb−1: the rest will then be a priori.
HJF Lake Louise Winter Institute Feb. 17-23, 2006
Two Lectures on Making Precision Measurements at Hadron Colliders
However the inclusive Z+X is dominated by SM Z+jets- we cannot yet predict
this at the level needed. Figure 40
Figure 40: Inclusive high pT Z production and 3 monte-carlo predictions, showing that we cannot yet apriori test the data against the SM (work in progress).
HJF Lake Louise Winter Institute Feb. 17-23, 2006
Two Lectures on Making Precision Measurements at Hadron Colliders
To increase sensitivity, add objects to the signature- subsignatures of Z+Njets,
Z + γ, Z + `,... For example: a Z with 200 GeV Pt balanced by a photon with
200 GeV Pt from Run I (100 pb−1):
Figure 41:
From Run II- Z+N(photons) (300 pb−1now- soon 1000).
# of Photons0 1 2 3 4 5 6 7 8 9 10
en
trie
s
-210
-110
1
10
210
310
410
# of Photons0 1 2 3 4 5 6 7 8 9 10
en
trie
s
-210
-110
1
10
210
310
410 Inclusive Z’s
# of Photons0 1 2 3 4 5 6 7 8 9 10
en
trie
s
-210
-110
1
10
210
310
# of Photons0 1 2 3 4 5 6 7 8 9 10
en
trie
s
-210
-110
1
10
210
310
(Z) > 60 GeVTP
# of Photons0 1 2 3 4 5 6 7 8 9 10
en
trie
s
-210
-110
1
10
# of Photons0 1 2 3 4 5 6 7 8 9 10
en
trie
s
-210
-110
1
10(Z) > 120 GeVTP
-µ+µ →Z
DATA
Z + jets
WZ
ZZ
WW
ττ →Z
tt
Figure 42:
HJF Lake Louise Winter Institute Feb. 17-23, 2006
Two Lectures on Making Precision Measurements at Hadron Colliders
11.5 The Tail of the W: Above the Pole- Wprimes
Figure 43: The predicted tail of the W way above the pole (from D0).
11.6 An Indirect Search: Asymmetries above the Pole (CDF+D0)
11.7 A Classic SUSY Search: Trileptons at D0
11.8 A Classic SUSY Search: Met + Jets
12 Direct Search for the Higgs
We saw in Lecture I that the EWK precision data favor a light Higgs (too light,
even). I briefly summarize the current status of Higgs searches:
HJF Lake Louise Winter Institute Feb. 17-23, 2006
Two Lectures on Making Precision Measurements at Hadron Colliders
Figure 44: The cross-section limits from direct searches for the Higgs as of Sept 05 from CDF and D0
Figure 45: The ratio of cross-section limits from direct searches to SM predictions for the Higgs as of Sept05 from CDF and D0
HJF Lake Louise Winter Institute Feb. 17-23, 2006
Two Lectures on Making Precision Measurements at Hadron Colliders
13 Expert Topics: Black and Double Black: Challenges for Stu-
dents
13.1 Fragmentation Near z = 1
13.2 Photon and Tau Fake Rates: Gluon and Quark Jets
13.3 Monte Carlo Issues: QCD and QED, NLO and beyond
HJF Lake Louise Winter Institute Feb. 17-23, 2006
Two Lectures on Making Precision Measurements at Hadron Colliders
13.4 B-jet Momentum Scale: Gamma-bjet Balancing
The response of the calorimeter to the b-quark jets from top decay is critical for
the top mass; sharpening the resolution is also critical for discovering the Higgs.
One source of b’s of known momentum is Z0 → bb; even at the Tevatron this
is very difficult as the rate of 2-jet production prohibits an unprescaled trigger
threshold well below MZ/2. At the LHC this will be hopeless, I predict. How-
ever the ‘Compton’ process gluonb → γb will give a photon opposite a b-jet.
Figure 46 shows the flux of b-quarks versus x at Q = 100 GeV (CTEQ6.1M);
one can see that at x=0.01 (pT = 70 GeV at the LHC) the b-quark flux is
predicted to be only a factor of 3 lower than the gluon flux.
Figure 46: The PDF’s at Q = 100 GeV (CTEQ6.1M) showing that the b-quark flux is only half that ofthe u flux (Plot from Joey Huston).
HJF Lake Louise Winter Institute Feb. 17-23, 2006
Two Lectures on Making Precision Measurements at Hadron Colliders
14 Beyond Expert: Out-of Boundary Area Topics: Challenges
for Expert Groups
14.1 Book-keeping: Rethinking Luminosity
14.2 Rethinking Analysis Code and Structure
14.3 Changing the Paradigm: W/Z ratios, Color Singlet/Color Triplet Ra-tios, and Other New Precision Tests
14.4 Particle ID: Distinguishing W → cs from W → ud, bb from b in TopDecays
14.5 Discrete Symmetry Tests: C, CP, and T above the W and Z poles
HJF Lake Louise Winter Institute Feb. 17-23, 2006
Two Lectures on Making Precision Measurements at Hadron Colliders
15 Credits
HJF Lake Louise Winter Institute Feb. 17-23, 2006
Two Lectures on Making Precision Measurements at Hadron Colliders
References
[1] review
[2] orig bbk
[3] LHC Design Report CERN-2004-003 (June 2004), Section 2. I have taken the 7.75 cm quoted for theRMS bunch length, multiplied by the geometric luminosity reduction factor of 0.836, and divided by√
2. I hope this is correct.
[4] The initial luminosity has a lifetime of 3.8 hours, which crosses the longer lifetime after 2 hours, atwhich point the luminosity is half the peak.
[5] I first learned of this method from Aseet Mukherjee and Barry Wicklund, who used it in the CDFearly precise (at that time) measurement of the Z0mass.
[6] J. D. Jackson and R. McCarthy; ”Z3 Corrections to Energy Loss and Range”, Phys. Rev. B6,4131(1972).
[7] Fabio Maltoni, Top Physics: Theoretical Issues and Aims at theTevatron and LHC, HCP2005, July 8, Les Diablerets, Switz.;http://indico.cern.ch/getFile.py/access?contribId=51&sessionId=13&resId=0&materialId=slides&confId=0512
[8] Kane Mrenna top prod and dec
[9] Both are projections- time will tell.
[10]
[11]
[12]
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