Transcript ppt - Rencontres de Blois

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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