QCD at Hadron Colliders
Thomas J. LeCompte
Argonne National Laboratory
Why Is This Important?
At the LHC, we get events like this.
QCD is either your signal or your background; either way, it must be understood.
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Portrait of a Simple QCD Calculation
One part: the
calculation of the
“hard scatter”
PERTURBATIVE
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Portrait of a Simple QCD Calculation
One part: the
calculation of the
“hard scatter”
Another part:
connecting the
calculation (which
involves gluons)
to protons (which
contain gluons)
PERTURBATIVE
NON-PERTURBATIVE
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Portrait of a Simple QCD Calculation
One part: the
calculation of the
“hard scatter”
Another part:
connecting the
calculation (which
involves gluons)
to protons (which
contain gluons)
Last part: the fragmentation
of final-state gluons into jets
of particles
PERTURBATIVE
NON-PERTURBATIVE
NON-PERTURBATIVE
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Comparison with Experiment

Our experience has been that
progress is made when we already
know 2 of the 3 parts.
– Experiment then constrains the
third.

Hard
Scatter
Parton
densities
It is possible to gain information
when this is not true, but the
situation is much more confusing.
Fragmentation
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Outline

I will show a few examples from each category
– Initial state: direct photons, jets
– Hard scatter: W/Z + jets
– Fragmentation: the b-quark cross-section saga

I hope to set the stage for the new and exciting results that we will hear about
over the course of the week.

I apologize for not including results from RHIC in this talk.
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Parton Density Functions Today

One fit from CTEQ and one
from MRS is shown
– These are global fits from
all the data

Despite differences in
procedure, the conclusions
are remarkably similar
– Lends confidence to the
process

The gluon distribution is
enormous:
– The proton is mostly glue,
not mostly quarks
Want to know the uncertainties?
Use the Durham pdf plotter:
http://durpdg.dur.ac.uk/hepdata/pdf3.html
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HERA: Before and After

HERA revolutionized our knowledge of parton densities
– If you don’t believe me, look at papers pre-HERA and post-HERA
– It’s even more dramatic than it looks, as the shapes in the left plots are still informed by
the HERA data. Otherwise, the shapes would be more a function of imagination than of
physics.

There is still a need to explore for the LHC, where higher Q2 matters.
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Kinematic Reach of a 7 TeV LHC
LHC @ 7 TeV Reach with Jets & Photons
LHC @ 7 TeV Reach with W’s
In the 2010-11 Run, the
LHC has substantially
increased the kinematic
range available for study.
In particular, W production
allows probing low x, high
Q2 quarks and antiquarks.
The 7 TeV data “fills the
gap” between the Tevatron
and a 14 TeV LHC.
From CTEQ: these are the inputs to CTEQ5
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Direct Photons and the Gluon PDF


DIS and Drell-Yan are sensitive to the quark PDFs.
q
g
Gluon sensitivity is indirect
– The fraction of momentum not carried by the
quarks must be carried by the gluon.

It would be useful to have a direct measurement
of the gluon PDFs
– Even if it were less sensitive than the indirect
measurements, it would lend confidence to the
g
picture that is developing
q
– It also has the potential to probe higher Q2 than
Direct photon “Compton” process.
the indirect methods.
– This process depends on the (largely known) quark
distributions and the (less known) gluon
distribution
If this is such a good idea, how come this
process is unpopular with the global fits?
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Tevatron Data
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Tevatron Data


There is a discrepancy at low pT, seen by both experiments.
There are theoretical ideas on how to resolve this, but the cross-section
calculation and the PDF measurements have become intertwined.
– We are attempting to constrain 2 of the 3 parts simultaneously.
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Photons at the LHC
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Photons at the LHC

There is still something not understood going on below 50 GeV
–

But we know now that this is a function of ET, not of xT. We can separate PDF effects from the
calculational issues.
The additional kinematic reach of the LHC is apparent
–
–
–
–
For the same xT, the LHC goes out 3.5x farther in ET.
With only 1% of the data, the kinematic reach is the same as the Tevatron’s
This represents 1-10% of the data the LHC has already collected
The troublesome region below 50 GeV is a tiny piece of what will be studied
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Jets at the LHC

Cross-section measured over 10 orders of magnitude.
–
–

Reminder: this is a mix of qq, qg and gg states.
In 6 or 7 bins of rapidity
With jets with ET’s above 1 TeV.
How does this agree with QCD predictions?
–
Both experiments
already understand
their jet energy scales
to a few %.
You can’t tell from a log plot
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ATLAS data, so you can see it.
The agreement with NLO QCD is quite
good (over 8 orders of magnitude!).
Possibly something is going on in the far
forward region at high pT.
This is a tough region theoretically (low x,
high Q2) as well as experimentally (starved
for statistics to check the JES).
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CMS data, so you can see it.
Again, impressive agreement, with the possible exception of energetic forward jets.
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W/Z+Jets: Why You Care

This is the sort of event that
shows up in SUSY searches.
– This particular event has
no leptons, 3 jets and 420
GeV of missing ET.

The dominant background
in these searches is
W/Z+jets
– Real missing energy from
real neutrinos.

History repeats itself. 16
years ago the discovery of
another particle depended
on understanding
the W+jets
background.
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Some Tevatron Results on Z+jets
With fb-1 sized datasets, comparisons
with theory possible up to the ~200 GeV
range for Z’s; a bit more for W’s.
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Things are a little different at the LHC
At the LHC, jet multiplicities are higher, and
with 3% of the data collected, the range of
pT’s accessible is about the same – maybe
already a little larger.
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I don’t want to forget W+jets
At the LHC, the top quark is a significant
background to W+jets. (At the Tevatron,
it’s the other way around.
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Z+b jets
CMS has a cute result where they
start with a Z+jets analysis and b-tag
a jet.
With the large number of W/Z+jets
events, expect to see more
examples of this kind of closer look
at the data.
Since this probes the bquark content of the
proton, this is perhaps a
better example of the 1st
part of a QCD
calculation than the 2nd.
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And an Interesting Result from CDF

This is the dijet mass in
W+2 jet events

Obviously, a bump is an
exciting thing.

However “I see bump” ≠
“there is a particle”
– It will be interesting to
hear what D0 says
when they are ready
– The LHC does not
report seeing this,
although it’s not clear
that it would be
expected to.
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Can This Be Explained by the JES?
From Tommaso Tabarelli de Fatis
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Lessons I Learned from this Bump

I do not believe CDF’s jet energy scale is off by 4%. This
would show up in too many other places to go
unnoticed.

I have no trouble believing ALPGEN’s predictions are
good to only ~4% today.
– This may just be a statement of my experimenter’s bias
– We are asking a very difficult thing of our generators –
predict QCD with enough accuracy that we can spot an
EWK-sized effect.
– I think this underscores the need to make as many
measurements as we can to compare with generators
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The b-quark cross-section saga
 At DPF92, CDF reported bottom quark
cross-sections a factor of at least two
greater than theory.
 This was at a center of mass energy of
1800 GeV.
 UA1 measurements at 630 GeV agreed
better with theory
– However, both theoretical and
experimental uncertainties were
substantially larger.
Community reaction: someone (i.e. CDF)
probably mismeasured something. Wait a while
and this will go away.
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But, it didn’t go away.

More recent CDF measurements
showed the same difficulty – the
theory underpredicts the data by
the same factor

This problem was not going away

Note that CDF (and also D0)
measures only the high pT tail of
the cross-section
– Most b’s were invisible.
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Commentary on measuring the top 10% of
something
Just how important
could the other 90%
be anyway?
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The Answer:
CDF measured J/y
from b decays
Allowing unfolding
to the b x-sec

s  29.4  0.443..19 b
NLO QCD predicts 20-40 b
Since the total cross-section agreed, but the high-pT portion did not, we had a
shape problem, not a size problem.
– The spectrum was stiffer than previously thought – causing us to mistake one for the
other.

Understanding fragmentation was the key – getting from the s(b-quark)
calculation to the s(b-hadron) measurement.
– Once that was understood, the other parts (PDFs and detailed calculation) followed.
– All three contributed at some level.
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Summary

There are three parts to a QCD calculation: the initial
state (PDF’s), the hard scatter, and the final state
(fragmentation). We make progress fastest when two
are well understood, and comparison with experiment
directly probes the third.

HERA revolutionized PDF’s.
– The Tevatron and the LHC are exploring regions in the (x,Q2) plane inaccessible to HERA.

Hard scattering calculations have been steadily improving – but so have the
demands we are placing on them.
– We are asking our W+2jets calculation good enough to see new EWK-sized physics.

Fragmentation usually doesn’t limit our understanding, but still has the capability
of surprising us.

At the LHC, whatever physics you intend to do, QCD is going to be part of you life.
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Thanks!
Thanks to the organizers for inviting me…
…and the ATLAS, CDF, CMS H1, D0 and Zeus collaborations, without whom I
would have had a very short talk.
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