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CTEQ summer 01| brock | michigan state university | No. 1front nine
• we’re going to take a walk–“unspoiled”–on a tough course
• our walk will have only good lies, avoiding the rough
• we start in wide fairways, but dogleg to a difficult approach
• we’ll take drops, in order to stay in the fairway and avoid the rough
• we’ll use “gimmes” to quickly skip through material that Sopercovered (he’s already holed out)
• we’ll occasionally be schematic - it’s the broad lay of the land that Iwant to get across as this is highly technical stuff - doesn’t naturallyrecommend itself in full glory to lectures …
CTEQ summer 01| brock | michigan state university | No. 2front nine
W and Z Boson Production and Resummation
1. a bit of history
2. Drell-Yan formalism
3. multiple gluon emission - Sudakov exponential
4. resummation: “CSS” formalism
5. W/Z boson production
6. global fitting of all Drell-Yan data - new
7. conclusions
naïve DY - kinematics and cross section
kinematics, corrected for finite pT
Compton and annihilation cross sections
www.pa.msu.edu/ ~brock/cteq01.pdf
The game is the Royal and Ancient:
CTEQ summer 01| brock | michigan state university | No. 3front nine
It started innocuously enough…
Searching for the W
the idea was mature by 1960
• Heisenberg’s notion of an exchange force, 1932
• Fermi’s theory of b decay, 1934
• Yukawa’s notion of an exchanged boson, 1935
Feynman and GellMann, 1958
• an audacious paper - so enamored of the “Universal V-A”interaction that they introduced, that they concluded that along-standing experiment on He was wrong: A not themeasured T.
Lee and Yang
• didn’t predict parity violation in 1957, but indicated that ithadn’t been tested
• in 1960, fleshed out the properties of the W (W 0 & W ± ) andproposed production scenarios
CTEQ summer 01| brock | michigan state university | No. 4front nine
Lee and Yang considered W production
p
Fe
mn K, p decays a serious low pT bkgnd
but W should produce high pT m’s
pTMW/2
n m n
n
N N W
h N pW
Æ Æ
Æ Æ
( )
( )
l
lthey suggested
no evidence of W, but what Lederman and Pope found was a little unsettling:• unrelated to the W search, at a suggestion that there would be a gggg
continuum, they looked…
An experiment was mounted in 1970 at BNL with a 30 GeV/cproton beam.
the search was on…
CTEQ summer 01| brock | michigan state university | No. 5front nine
lotsa muons, 2 by 2
investigated by forming mass-pairs of muons
at very high pT’smomenta from range…
CTEQ summer 01| brock | michigan state university | No. 6front nine
so what was it?
it’s not the W
the only known source was photons…but at such high pT?
• much theoretical scurrying about
• one explanation stuck - that of Drell and Yan in 1971
they extrapolated from the (young, circa. 1970) parton model to suggestthat the mechanism was:
we can quickly reproduce their calculation
CTEQ summer 01| brock | michigan state university | No. 7front nine
immediate plans…
1. work out kinematics for the naïve model
2. calculate the naïve Drell-Yan cross section
– find some measurables
– check predictions
3. un-naïve the calculation a bit
– especially work on the kinematics for finite pT for theproduced photon
CTEQ summer 01| brock | michigan state university | No. 8front nine
the Drell-Yan ansatz:
parton-parton scattering inside of hadrons
i.e, buried inside
Let’s calculate it:first, kinematics.we’ll presume the simple CM:
rp pV V= 3 k
independence and incoherenceof the primary process:
is
CTEQ summer 01| brock | michigan state university | No. 9front nine
hadron/parton/V kinematics
partons
the IVB
hadrons parton-hadron
connection:
CTEQ summer 01| brock | michigan state university | No. 10front nine
kinematics2
CTEQ summer 01| brock | michigan state university | No. 11front nine
invariants
hadron invariants
parton invariants
here: t x x= a b•which allows for a definition: t = M s2 /
from equation (3)
CTEQ summer 01| brock | michigan state university | No. 12front nine
rapidity
define:y
E p
E pVV V
V V
a
b
∫+
-
Ê
ËÁ
ˆ
¯˜
=Ê
ËÁ
ˆ
¯˜
12
12
ln
lnx
x
the familiar “rapidity”, additiveunder Lorentz boosts…
in this simple case:
= -( )x xa b P for thissimplecase
pM u
P
M t
PV3
2 2
4 4=
-Ê
ËÁ
ˆ
¯˜ -
-Ê
ËÁ
ˆ
¯˜so:
&
x ta bye, = ±using the above…
also, there are connections among invariants:
CTEQ summer 01| brock | michigan state university | No. 13front nine
kinematical boundaries
for one early Drell-Yan experiment - E288
xF = 0
fixed xb/xa or y
xF > 0
xF < 0
t
t
CTEQ summer 01| brock | michigan state university | No. 14front nine
world’s experimental regions
CTEQ summer 01| brock | michigan state university | No. 15front nine
the Drell-Yan calculation
simple Feynman diagram:
colinear beams
integrate away “d”
detect angular distribution of “c”
d
T d
flux normalizationfi
s
r
=◊( )
ÂÂ2
2
___
with cross section
In CM of a+b:
d P P p p p p dP dPc d a b c d c d242r p d
r r r r, ( )( ) = + - -( )and
flux normalization p p m m p sa b a b◊( ) = ◊( ) - =4 42where
CTEQ summer 01| brock | michigan state university | No. 16front nine
phase space reminder
my definitions…
d pc d p d p p d pc c c d d2 2 23r r r( ) ( , ) ( , )
r r r r= ∫ ÚW
ds
p
PT dc
fi
sp
= ÂÂ1
641
22
W___
so,
and then
d d p dp
p
sd
c c c
cc
2 2
216
r r
p
( ) ( )W
W
=
=
Úr
Kwhere
d p dp dE p
E p p p EE E
E E p p m
cc c c
a c a c cR c c
R
cR
a a c d
2 3 2
2
2
2 2
12 4
rp
d( )( )
( )r
r r
r r
=- ◊
Ê
ËÁ
ˆ
¯˜ -
= - -( ) +
W
CTEQ summer 01| brock | michigan state university | No. 17front nine
matrix element
the whole enchilada
with our frame choice:
indicative of 1/2-1/2 scattering: a prediction
CTEQ summer 01| brock | michigan state university | No. 18front nine
putting it together
the real inspiration:
a function which incoherently sums all quarks:
must average over colors - DY didn t know this
where is theprobability of finding an a-type parton in hadron Acarrying fraction of PA equalto xa
CTEQ summer 01| brock | michigan state university | No. 19front nine
find some measurables…
in order to get the differential cross section:
use
d d d dx dQa b Fx x tÆ Æ 2
which allows us to calculate the Jacobian to take
We have a prediction:
this quantity is
independent of
Q … only t
(or ) plus
CTEQ summer 01| brock | michigan state university | No. 20front nine
it worked!
Drell and Yan’s explanation of the Lederman/Poperesults proved substantially correct
early results demonstrated scaling…
…later results, even moreimpressively
…including the demonstrationof point -like 1/2-1/2 scattering
CTEQ summer 01| brock | michigan state university | No. 21front nine
why “naïve”?
well, because of two approximations:
1. scale invariant parton distribution functions
2. presumption of the g produced with pT=0
and
CTEQ summer 01| brock | michigan state university | No. 22front nine
loss of naiveté, 1
scale-violating pdf’s
The Factorization Theorem (Collins, Soper, Sterman) puts the physicallyappealing quark - parton model on a firm, formal basis:
physical cross section:
calculable, to a particular order in aS
separate short-distance (calculable) from long distance(not calculable) using a scale, mmmm f…hard part is only
implicitly dependent on mmmm f - in practice, mmmm f set = mmmm .
measurable, universal not calculated, long-distanceeffects are absorbed into pdf’s
finite effects - the infamous “k-factor”
from now on, we’ll presume scale-violating pdf’s
CTEQ summer 01| brock | michigan state university | No. 23front nine
loss of naiveté, 2
finite pT … comes in 2 ways:
hard emission (think: “perturbative”)…(we’ll do it next)
soft emission (think: “complicated”!)…(we’ll do it later)
CTEQ summer 01| brock | michigan state university | No. 24front nine
take it a bit at a time…
to first order in aaaaS, the elementary processes are:
for example, emission is 1 gluon
This is a familiar fundamental process: annihilation
…one of a set of order-aaaaSaaaaEM hard processes which can be treated perturbatively
Compton graphs, “C”
Annihilation graphs, “A”
CTEQ summer 01| brock | michigan state university | No. 25front nine
more kinematics
it’s harder this time…
in principle, n states, for thei th particle…
presume cm and consider that the ith is V
CTEQ summer 01| brock | michigan state university | No. 26front nine
kinematics, 2
from solution for t,
express the
longitudinal p
as a fractional
difference of P
different from before:
CTEQ summer 01| brock | michigan state university | No. 27front nine
kinematics, 3
to get…
solving for M2:
so,
and therefore,
also,
CTEQ summer 01| brock | michigan state university | No. 28front nine
kinematics, 4
solving…
(recalling only in the zero pT case…)
CTEQ summer 01| brock | michigan state university | No. 29front nine
immediate plans…
1.set up the hadronic QCD “Compton” process
2.but, be lazy: calculate the eg Compton cross section
– everyone has done this, don’t need to really calculate…
– …except we’ll do it to a “heavy” photon
3.convert it to the QCD process by simple substitutions
4.use crossing to infer the Annihilation cross section
5.put it together…and then look at low pT for trouble
CTEQ summer 01| brock | michigan state university | No. 30front nine
from our earlier phase space outline:
Compton process:
set up to ignore d, and embed the process inside hadrons:
for the simple 2-2:
a+b V+d
a trick
integrate trivially:
note!
CTEQ summer 01| brock | michigan state university | No. 31front nine
compton, 2
mess with the delta function:
you can show that
so,
you remember
so…
is the root…
is the derivative…
so: allows us to do the xa integration
CTEQ summer 01| brock | michigan state university | No. 32front nine
compton, 3
so, putting it together:
for 2 body kinematics
where
integrating and changing variables…
the physics lives here…
CTEQ summer 01| brock | michigan state university | No. 33front nine
“regular” Compton scattering
textbook, but for keeping the outgoing photon mass
kinematics
rememberthe plan
CTEQ summer 01| brock | michigan state university | No. 34front nine
“regular” Compton scattering, 2
actually, an exercise in Halzen and Martin…
standard 2 body kinematics
Jacobian to go to invariants
so:
mass only
affects theinterference term
CTEQ summer 01| brock | michigan state university | No. 35front nine
from QED to QCD with hadrons…
now exploit our laziness...
morph from: to:
coupling change: decay to muons
CTEQ summer 01| brock | michigan state university | No. 36front nine
ÆÆÆÆ annihilation
use crossing symmetry to go Compton ÆÆÆÆ Annihilation:
“C” “A”
plus color changes…
CTEQ summer 01| brock | michigan state university | No. 37front nine
total order-aaaaS Drell-Yan cross section
putting it together…
where
this leads to trouble… the annihilation hard cross section goes like:
which, since leads to behavior for
CTEQ summer 01| brock | michigan state university | No. 38front nine
ubiquitous logarithms
perhaps not surprisingly, logs float to the surface…
factor out the 1/pT2 behavior and as xT Æ 0, the leading terms go like:
(There’ll be others later)
so,in that limit, the form of the order-aS Drell Yan cross section is:
where:
includes leading
log pdf’s
which live here…
here’s a scale (pT)
which cannot beabsorbed into the pdf’s:
the dreaded
“two scale problem”
BUT
kT dependenceas kTÆ 0
CTEQ summer 01| brock | michigan state university | No. 39front nine
sounds like a good theory
what about data? R209 at the ISR, 1982
badbehavior atlow pT
best fit for aGaussian-smeared model
hmm…
CTEQ summer 01| brock | michigan state university | No. 40front nine
immediate plans…
1. an interlude, of sorts…
– calculate the probability of the emission of a low-pt gluon
– then calculate the probability of an infinite number oflow-pt gluons
– call them “Sudakov” and identify the approximations asLeading Pole and Leading Double Log
CTEQ summer 01| brock | michigan state university | No. 41front nine
itty, bitty gluon radiation
a side calculations - dealing with soft gluon radiation(s)
define x1 and x2 to be themomentum fractionscarried by
q(p1) and q(p2)…then
x3 = 1 - x1 - x2 is the
fraction carried by g.
then:
log divergence when
x1,2 Æ 1/2
so,
define: (which is small)
(invariant qa-g mass, which is small)
CTEQ summer 01| brock | michigan state university | No. 42front nine
details, details…
wave some hands here…
change variables, for photon “lifetime”:
log divergence, jÆ0when j small (mqg small) and z near 1 (Eg small)
the middle term dominates…
called the “Leading PoleApproximation”, (LPA)
related to the Pqg splitting function
q qª = @tan/||
k
k
k
QT T
2and so,
small
CTEQ summer 01| brock | michigan state university | No. 43front nine
summing glue
add up a series of emissions
in the limit, the angle is small
define:
so…
as a function of kT, the probability of > q
so,
CTEQ summer 01| brock | michigan state university | No. 44front nine
calculate T a
change variables
from
likewise, then
so,
remember
again, with at least k2T<<Q2
leading double logapproximation, LDLA
CTEQ summer 01| brock | michigan state university | No. 45front nine
add ‘em up
treat radiations as independent
the probability of n, with < q
adding the probabilities for all possible n’s
which is an exponential:
SO
a significant result: as pT Æ 0, cross s Æ 0!
CTEQ summer 01| brock | michigan state university | No. 46front nine
but wait…
this is where we started
for the kT of the radiating quark has the same form as ourorder-aS cross section for the q…but including the effects of
copious radiation of soft glue
we can improve our Drell-Yan cross sectionthe same way:
what we got perturbatively at order-aS
adding infinite-order soft radiation
notice powers of aS
this “RESUMMATION” (the exponential) tames the bad behavior
CTEQ summer 01| brock | michigan state university | No. 47front nine
immediate plans…
1. move to W/Z production
– (ah, to) be naïve again in order to adjust to EWparameters
– get a sense of the rates
– use our results and calculate for finite pT
CTEQ summer 01| brock | michigan state university | No. 48front nine
W/Z production
Drell-Yan process is operative here too
the Feynman rules are slightly different
-ieQig m
vI Q
aI
ii i W
W W
ii
W W
=-
=
( ) sin
sin cos
( )
sin cos
32
3
2
2
2
q
q q
q qie v ai ig gm m-( )
ieW
g gqm ( )
sin1
12 25 2-
siblings
cousins
CTEQ summer 01| brock | michigan state university | No. 49front nine
from photons to W/Z:
just an EW lookup:
propagators:d
dQ Q
s2 4
1µ
12 2 2
s M MV V V-( ) + ( )G
couplings: W bosons Z bosons
plus, the connection between weak and electromagnetic couplings
CTEQ summer 01| brock | michigan state university | No. 50front nine
W cross section - more laziness
write down the, now standard, cross section:
make use of the “narrow width approximation”:
the hard part:
define W-specific parton density combination:
CTEQ summer 01| brock | michigan state university | No. 51front nine
W - cross section
combine with K-M matrix elements for the isospin-changing process
cabbibo cabibbo2 2+( ) angles, actually ~95%
~5%
so,
forget the Cabibbo-disallowed transition…
CTEQ summer 01| brock | michigan state university | No. 52front nine
done…
some details, delta function gymnastics:
defined as thedifferential PartonLuminosity
t = @ ª ¥ -Q
s
2 2
2380
20001 6 10.
s = 2 TeV
Tevatron luminosities
(royal)
and a
ncie
nt;
EH
LQ
st
t= ( ) Ê
ˈ¯
= ( )( ) ÊËÁ
ˆ¯˜
Ê
ËÁ
ˆ
¯˜
=
-
-
6
6 801
0 39
1
10
10100
9 9
22 3
9
nb
nb GeVGeV mb
mb
b
bnb
nb
. nb
ˆˆ
. ( )
ss
d
d
L
s(Z) is about 1/3 s(W)
so, we can integrate
ˆ . /
ˆs c
s
d
d@ fi @0 08 1002TeV nb
t
t
L
CTEQ summer 01| brock | michigan state university | No. 53front nine
Z cross section:
same idea: just follows the previous pattern…
where
so
CTEQ summer 01| brock | michigan state university | No. 54front nine
reasonable description, overall
the calculation is historical…the data and pdf fits aremuch advanced
s n◊ Æ ªBR nb( ) .W l 2 2EHLQ gives…
but, data and phenomenology deserve more attention thanpossible here…
hep-ph/0101051
CTEQ summer 01| brock | michigan state university | No. 55front nine
moving on…
let’s un-naïve the W calculation for finite pT and thenfor radiation:
so that:
everything goes through as before…
so, what’s wrong with this?
CTEQ summer 01| brock | michigan state university | No. 56front nine
aaacckk!!
The Sudakov factor includes the exponentiation of
leading double log approximation
I dropped the power of 2 in the exponentiated form
2
please put that in…the web will be correct…
CTEQ summer 01| brock | michigan state university | No. 57front nine
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