Transcript PREPARING for the Future QCD for LHC What We Need to Know/Learn: About Long Distances Go! S.D.

PREPARING for the Future
QCD for LHC
What We Need to Know/Learn:
About Long Distances
Go!
S.D. Ellis: West Coast LHC Theory Network
2/3/06
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Goals for QCD
• Be able to identify short distance structure of
event (presence of squarks, Higgs, etc.) from
measurements of long distance experimental
measurements (“objects”) – leptons, ’s, jets
(especially jets with heavy flavor – tagged jets)
and “”s (MET)
• Be able to reliably (< 10 % uncertainty) calculate
rates of signal and background processes
S.D. Ellis: West Coast LHC Theory Network
2/3/06
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Action Items
• Learn what is already known – who are the West
Coast experts? (Not me!)
• Study the essential issues that are not yet known –
who is working on what?
Today’s assistants –
• Short Distance QCD – Zvi
• Long Distance QCD - Me
S.D. Ellis: West Coast LHC Theory Network
2/3/06
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Outline
• Long Distance Before – Mapping Hadrons onto
Partons – Parton Distribution Functions (PDFs)
• Long Distance After – Mapping Hadrons (detectors)
onto Partons – Jets
Related Special Topics –
MC@NLO for Jets
Theory for energy correlations in Events
(Beyond perturbation theory)
S.D. Ellis: West Coast LHC Theory Network
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Resources
• HERA – LHC Workshop
http://www.desy.de/~heralhc/
• TeV4LHC Workshop
http://conferences.fnal.gov/tev4lhc/
• Computer Tools
http://www.cedar.ac.uk/
e.g., http://hepforge.cedar.ac.uk/ktjet/
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Long Distance QCD Primer
What’s the big deal?
QCD is gauge theory with “massless” particles  has
collinear (and soft) singularities (real & virtual)  Logs

QCD
Quark in proton
(parton model)
 Long Distance 
  s ln 
 Short Distance 
Quark in proton
after emitting gluon,
plus gluons in proton
Short Distance = 1/Hard Scattering Scale
And Long Distance   Bummer!!
S.D. Ellis: West Coast LHC Theory Network
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When in doubt, Sum those logs!!
Renormalized, evolving quark
distribution in proton:
q  x,  
where x = momentum fraction
(of quark in proton),
1/ is the Factorization Scale
1/Q is Short Distance (the
resolving scale)
1/ is long distance scale
DIS picture
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Renormalized PDF
Logs of (Q/) in short distance QCD (at fixed order in
perturbation theory)
[Must match Logs of  order by order]
Logs of (/) in PDF (to all orders in Perturbation theory)
QCD tells us how q(x,) changes with  (order by order in
perturbation theory - DGLAP), but, since we hid  (in q  x,  ),
we must measure q  x, 0 , i.e., at some scale.
But PDF is “universal” – fit to all data
S.D. Ellis: West Coast LHC Theory Network
2/3/06
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Parton Distribution Fcts
Internal (showering/singular) structure of partons in
hadrons, determined by
Perturbative evolution – LO, NLO, (NNLO)
Initial conditions  (Global) fits to data
e.g., CTEQ – See Soper & Tung
MRST
Alekhin – NNLO & DIS
 New tools for propagating errors in data to
uncertainties in cross sections calculated with
PDFs
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2/3/06
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Current Status
CTEQ

L   dx1dx2 g  x1 ,  g  x2 ,    x1x2  sˆ
Where s is the total hadronic
energy, and sˆ is the total
partonic, hard scattering energy
 Uncertainties > 10% except
for large x gluons (just where
we need them!)
Measures of parton
luminosity uncertainties
S.D. Ellis: West Coast LHC Theory Network
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s

Current PDF issues
• More precision for the Gluons
• Flavor, charge asymmetries, e.g., s vs s
• Heavy flavors (c,b)
experimental determination
include mass effects, defining thresholds
role of nonperturbative effects (i.e., besides
perturbative gluon splitting)
• Do we need NNLO fits? (global data probably not
that good yet)
S.D. Ellis: West Coast LHC Theory Network
2/3/06
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(The Dreaded) Underlying Event (UE)
Q: What happens to the other partons in the proton?
A: They interact (softly?) and make lots of other
hadronic stuff = THE UNDERLYING EVENT,
which contributes to jets
UE is approximately independent of the hard
scattering, but there must be color correlations!
The UE will likely be relevant at the LHC, as will
the simultaneous interactions of multiple protons
at high beam luminosity!
S.D. Ellis: West Coast LHC Theory Network
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Outgoing Long Distance

QCD
Naïve picture with short
distance partons, long
distance hadrons
(incoming F and
outgoing D), plus UE
 Long Distance 
  s ln 
 Short Distance 
Collinear singularities (real
and virtual) bite us again,
plus issue of hadronization
S.D. Ellis: West Coast LHC Theory Network
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Outgoing Long Distance
• Colored objects shower (into more colored objects) –
perturbative showering (large logs)
• Colored objects (eventually) organize into color singlet
hadrons – non-perturbative hadronization
“Hide” details (and large logs) with inclusive measure of final
state - sum over all emitted and hadronized objects in
direction of original parton so details (and logs) don’t
matter
 JETS
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2/3/06
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Associate “nearby” hadrons or partons into
JETS via ALGORITHMS, i.e., rules that can
be applied to data, MC and theory
• Cone Algorithms, e.g., Snowmass, based on fixed
geometry (well suited to hadron colliders with UEs)
• kT Algorithm, based on pairwise merging, nearest
in p-space, lowest pT first (familiar at e+e- colliders)
 Render PertThy IR & Collinear Safe
 But mapping of hadrons to partons can
never be 1 to 1, event-by-event!
colored states ≠ singlet states!
always some splash in/out!
S.D. Ellis: West Coast LHC Theory Network
2/3/06
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Think of the algorithm as a “microscope” for
seeing the (colorful) underlying structure -
S.D. Ellis: West Coast LHC Theory Network
2/3/06
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ATLAS 2-Jet Event
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Events in (D0) Detector
Associate nearby “stuff”, e.g., in circle in eta/phi.
This is a “cone” jet, presumed to arise from a hard
scattered parton with similar kinematic properties!
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How well does this work?
OK at ~10% level, but it ain’t as easy as that!
There are lots of details that matter when two jets
are nearby – (happens often in a collinear singular
theory):
Cone jets are defined by “stable” cones (geometric center =
pT weighted center) and they can overlap – must decide
how to split/merge
Experimenters only look around hot spots (seeds) and miss
2 cones that could fit in a single cone (RSEP)
Cone Jets
 kT jets
S.D. Ellis: West Coast LHC Theory Network
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NLO
NNLO
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Goals of IDEAL ALGORITHM
• Fully Specified: including defining in detail
any preclustering, merging, and splitting
issues
• Theoretically Well Behaved: the algorithm
should be infrared and collinear safe (and
insensitive) with no ad hoc clustering
parameters (e.g., RSEP)
• Detector Independence: there should be no
dependence on cell type, numbers, or size
• Order Independence: The algorithms should
behave equally at the parton, particle, and
detector levels.
• Uniformity: everyone uses the same
algorithms (at least some of the time)
S.D. Ellis: West Coast LHC Theory Network
2/3/06
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Fundamental Issue – Compare Experiments to
each other & to Theory
Warning:
We should all use the same algorithm!!
(as closely as humanly possible), i.e. both
ATLAS & CMS (and theorists).
This is NOT the case at the Tevatron, even in
Run II (nor was it in Run I)!!
S.D. Ellis: West Coast LHC Theory Network
2/3/06
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Current situation for Algorithms at the
Tevatron – see talk by Markus Wobisch at
Tev4LHC, 10/2005 at Fermilab
Merged
jets
Dark
towers
UN
Merged
jets
CDF Legacy Cone
Run II Cones
S.D. Ellis: West Coast LHC Theory Network
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Cone Algorithms in Run II
• DØ using the MidPoint Cone Algorithm
with fmerge = 0.50, a la the Run II
agreement
• CDF using the smaller “search cone” fix
(for Dark Towers) with fmerge = 0.75

2 Experiments still different!!
S.D. Ellis: West Coast LHC Theory Network
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The 2 algorithms are different in the
data (but not in the pert theory)!
 6%
difference at
large pT –
search cone
collects about
1% more pT
 At low pT all
the details
matter!! 
From Wobisch
S.D. Ellis: West Coast LHC Theory Network
2/3/06
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Comment on Heavy Flavor in Jets
• Do we understand how much heavy flavor (c,b) in
jets arises from Long Distance, i.e., gluon
showering? Either theoretically or
experimentally? Impacts estimates of tagging
• Current uncertainty seems to be of order 30%!
• Needs work!
S.D. Ellis: West Coast LHC Theory Network
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Comment for 4-vectors:
Jet Masses useful at LHC with large boosts
PT > 25 GeV
800
W/Z?
l jets PT 25,eta 2 5 GeV Bin
Original BB
light jet mass
distribution,
600
400
top
200
boost
0
100
150
200
l jet M ass GeV
S.D. Ellis: West Coast LHC Theory Network
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300
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Jet Summary
The largest initial challenge at the LHC will be
understanding how the jet algorithms work
Determining the “Jet Energy Scale”, i.e., the
correction factor from the observed energies of
stuff in jet to energy of underlying partons (e.g.,
using photons, Z’s recoiling against jets, etc.)
Ensuring the experiments use the “same” algorithm
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Other - MC@NLO
• JET studies clearly call for a conjoined NLO and MC tool:
(Frixione & Webber)
http://www.hep.phy.cam.ac.uk/theory/webber/MCatNLO/
• The MC includes dominant long distance QCD (showering
and hadronization) contributions “associated” with
participants in short distance QCD, and generates “full”
events that can be processed by a detector simulation (but
have used LO short distance QCD)
•
“Marry” this to precise NLO description of short distance
QCD to facilitate more accurate/realistic studies of final
states (including jets)
• MC@NLO v. 3.2 includes production of single & double
vector bosons, single (new) and pair top quarks, Higgs,
Higgs with W/Z and lepton pairs
S.D. Ellis: West Coast LHC Theory Network
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MC@NLO
• Issue is the merging of short distance with long
distance without double counting; first emission is
in short distance, not long distance (must subtract)
• Techniques for matching spelled out by F & W
(“similar” to matching of short distance and PDF)
• Many possibilities to double count for light jet
amplitudes, everything is colored (top quarks
never find out they are colored)
• Ellis & Kilgore, are working on this, but we ain’t
done! Any volunteers?
S.D. Ellis: West Coast LHC Theory Network
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Other - Energy Flow/Correlation Analyses
Sum lots of Logs
• Berger & Sterman, et al.
• Dokshitzer, Marchesini, Webber, et al.
• Dasgupta, Salam, et al.
Can these methods calculate “splash-in/out” down to nonperturbative level?
Can energy flow measures be used to isolate new physics?
Who, on the West Coast, is working on this?
S.D. Ellis: West Coast LHC Theory Network
2/3/06
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Suggests we do something very different –
not an integer jet count event-by-event?
Keep more information from each event – “extreme”
example –
JET ENERGY FLOW (JEF) –
Berger, et al. hep-ph/0202207, Snowmass 2001
F.V. Tkachov, hep-ph/9901444
Each event yields a distribution, E versus angle,
instead of a jet count!
Needs more study (or at least I need to understand it)!
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 “Summary”
Long Distance QCD is understood at the ~10% level, especially
where MC@NLO is available (need this for jets)
 PDF – Want better knowledge of glue & heavy flavors
 Run II data will help, especially precision W/Z versus pT
measurements
 Jets – Differences between experimental implementations in
Run II are probably as important as differences between
algorithms – Can we expect ATLAS & CMS to use the same
algorithm? Can we determine the long distance heavy
quark content?
 Can we find Something New & Different, e.g., Jet Energy
Flows? Can we use better the improved understanding of
energy flows to better define or replace jets?
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