Brian Foster
Oxford University
Heavy Quark Physics
@ HERA II
Cornell
05/13/05
What’s new for HERA II and what does it mean
for HQ physics – a reminder..
HQs in DIS, diffraction, photoproduction
Spectroscopy – is there a role for HERA II?
Production mechanisms
Single t production?
Summary
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Brian Foster - HQ physics @ HERA II
HERA & experiments
HERA @DESY
in Hamburg
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Brian Foster - HQ physics @ HERA II
HERA II Physics
Both ZEUS & H1 have made major upgrades in order to
utilise the increase in HERA luminosity to the full.
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Brian Foster - HQ physics @ HERA II
Vertex Region
The ZEUS MVD mostly 3 layers in barrel and 4
forward wheels; > 200K readout channels.
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Brian Foster - HQ physics @ HERA II
Vertex Region
H1 Si – 2 very thin CST layers; 5 disks covering 8 < q < 17o
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Brian Foster - HQ physics @ HERA II
Forward Physics
ZEUS major upgrade in forward direction replacement of
TRD’s with two stations of straw-tube chambers, each with
3 stereo layers.
H1 have made
improvements
to various parts
of their tracking
systems.
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Brian Foster - HQ physics @ HERA II
HERA II prospects
So optimistically HERA II promised factor 5 increase in
luminosity, with lepton polarisation, greatly improved
tracking, DAQ and triggering.
This implies generally order of magnitude improvement
in statistics and increased kinematic range and coverage.
The reality has been somewhat different. Major problems
with backgrounds delayed then achievement of the design
goals by almost 2 years - only now are we reaching,
sometimes, 1pb-1 per day.
What does all this mean for the physics reach of HERA II?
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Brian Foster - HQ physics @ HERA II
Low-x structure function data
e(k)
Kinematics
Q2 = xys
Q2
e'(k')
g *(q)
W2
xP
p(P)
 s = k+P =
energy in the ep c.m.s.
Q2 = -(k-k')2 = -q2 =
virtuality of the exchanged g
x = Q2/(2P • Q) =
fraction of proton momentum
carried by the struck quark
y = (P q)/(P k) =
fraction of beam lepton energy
transferred to the photon
W 2 = ys ~ Q2/ x
energy in the g*p c.m.s.
•
Brian Foster - HQ physics @ HERA II
•
8
Inside the proton
e
e
e
e
p remnant
q
Brian Foster - HQ physics @ HERA II
e
p
p remnantp
q
p
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Inside the proton
Low Q2 (large l)
Medium Q2 (medium l)
Heisenberg’s UP
allows gluons, and qq
pairs to be produced
for a very short time.
Large Q2 (short l)
At higher and higher
resolutions, the quarks
emit gluons, which also
emit gluons, which emit
quarks, which…….
At highest Q2, l ~ 1/Q ~ 10-18 m
Brian Foster - HQ physics @ HERA II
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Inside the proton
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Brian Foster - HQ physics @ HERA II
F2 data
ZEUS has very precise
F2 data over 6 orders of
magnitude in (x, Q2).
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Brian Foster - HQ physics @ HERA II
Inside the proton – the movie
QuickTime™ and a
BMP decompressor
are needed to see this picture.
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Brian Foster - HQ physics @ HERA II
Parton evolution
Factorization - hard processes can be regarded as convolution
of “sub-process” cross section with probability to find
participating partons in target & probe - subsequent
hadronisation ~ independent process
For DIS can (normally) consider
virtual photon as d-function
=> s ~ f  s where
s is sub-process cross section
f is parton dist. function,
satisfying f/ m ~ f  P
(m = renormalisation scale,
P is a splitting function)
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Brian Foster - HQ physics @ HERA II
Parton evolution
In general Ps are perturbative expansions to particular
orders, keeping terms most important for particular regions:
m

  n n

1
(
n
)

 s 


( n)
xP ( x, s ) =     Am
ln    x P ( x)

  x 
n = 0  2  m = 0

(n )
where P are AP splitting functions
Leading ln Q2 terms come, in axial gauge, from evolution
along parton chain strongly ordered in transverse momenta,
terms - NLO sums
LO DGLAP sums up
terms which arise when two adjacent
kts become comparable, losing factor ln Q2.
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Brian Foster - HQ physics @ HERA II
QCD evolution
In small x region, leading terms in ln 1/x must be summed
independent of Q2. This is done by the BFKL equation.
LO (s ln 1/x)n terms arise from strong x ordering:
xn << x(n-1) << x(n-2) …
Generally, however, QCD coherence  angular ordering work in unintegrated f(x,kt2,m2) - 2 hard scales  more
complicated CCFM evolution equation. DGLAP/BFKL two
limits of angular ordering. DGLAP, q  kt/kl, q grows since kt
grows; in BFKL, q grows because kl  x falls.
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Brian Foster - HQ physics @ HERA II
Dipole Models
Much interest in dipole models & saturation.
prest
L.T.
Breit,  mom.
In principle offers unification of inclusive DIS, diffraction
1g
exchange
+
2 g octet
exchange
Inclusive F2
Brian Foster - HQ physics @ HERA II
2 g singlet
exchange
Diffraction
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HQ in DIS
Specialise to HQ structure functions.
There are many theoretical issues with how to include
heavy quarks, in particular the mass thresholds;
variations on the MVFN scheme a la Thorne&Roberts
are currently favoured and have been examined at NNLO.
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Brian Foster - HQ physics @ HERA II
HQ in DIS
We will need substantial part of the HERA II statistics to
resolve many of the interesting issues in charm in DIS, even
with substantial improvement in tagging efficiency.
Thorne, Roberts
1998
The mechanism to take charm mass into account can give
substantial mods. to theoretical expectation as function of Q2 at
higher x. Intrinsic charm can also modify these predictions
but unlikely we can get useful info. on this at HERA II.
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Brian Foster - HQ physics @ HERA II
HQ in DIS
The D* tagging method in D* → Ks will still be useful
although in general multiple scattering increases.
However this will be more than offset by ability to use
separated vertex or large impact parameter tag, as already
demonstrated at HERA I by H1.
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Brian Foster - HQ physics @ HERA II
HQ in DIS
HERA I has already told us a lot about the charm quark
density inside the proton; very nice new H1 results at DIS05.
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Brian Foster - HQ physics @ HERA II
HQ in DIS
H1 now using the power of their silicon tracker which was
present in a modified form for the last few years of HERA I
running - uses impact parameter tagging in 2-D to tag longlived quarks.
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Brian Foster - HQ physics @ HERA II
HQ in DIS
H1 analysed 57 pb-1 of HERA I data using impact parameter.
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Brian Foster - HQ physics @ HERA II
HQ in DIS
~0.2%
~20%
~1%
~30%
The fraction of c is
more or less
independent of Q2,
whereas that of b is
strongly rising.
Both agree well
with the the
predictions from
PDFs.
~4%
~20%
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Brian Foster - HQ physics @ HERA II
HQ in DIS
Some of the open questions are obvious.
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Brian Foster - HQ physics @ HERA II
HQ in DIS
Some were (much) more obscure.
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Brian Foster - HQ physics @ HERA II
HQ in DIS
Fortunately, this seems to have gone away at HERA II.
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Brian Foster - HQ physics @ HERA II
HQ in DIS
The promise of HERA II is great, since charm production
in DIS is sensitive probe to all sorts of dynamical models.
Saturation model
of Kowalski &
Teaney.
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Brian Foster - HQ physics @ HERA II
HQ in DIS - HERA II
First HERA II results starting to come through.
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Brian Foster - HQ physics @ HERA II
HQ in DIS - HERA II
First HERA II results starting to come through.
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Brian Foster - HQ physics @ HERA II
HQ in DIS
Very precise
measurements of F2C will
be possible for HERA II;
also gives accurate
gluon determination and
cross-check with the
more global QCD fits.
-1 -1
500
500
pbpb
Accurate b contribution
to F2 will become
possible – cross-check of
VFNS and clean test of
photon-gluon fusion
process.
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Brian Foster - HQ physics @ HERA II
HQ in DIS
HERA II should allow both collaborations to make a full
flavour decomposition of the inclusive F2 structure function.
For example, charm signal in
charged currents (expect ~ 50K
events) in principle measures
the s-quark density (+ leading
particles in NC etc.; but also
competing non-s diagrams from
gluon splitting in CC).
Constrain fit with s from
leading f?
At HERA II, both singlet &
non-singlet pdfs - u,d,s (CC DIS)
& c,b,g (NC DIS) - can be
determined with good accuracy.
Brian Foster - HQ physics @ HERA II
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HQ & Polarisation
Polarisation in DIS will be a unique tool @ HERA II
for exploring EW couplings.
In principle the spin orientation of the struck quark
can carry through a memory into the final-state
hadron.
In c and b production, one can be sure that the struck
quark is in one of the c or b hadrons and there is a
correlation between the detected heavy-quark hadron
and the struck quark.
Using weak decay of Lc, can analyse for spin orientation of
struck quark and hence disentangle spin-dependent ss.
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Brian Foster - HQ physics @ HERA II
HQ & Polarisation
But first signal reported for
inclusive Lc production
from a substantial
fraction of HERA I
data makes it difficult to
believe that statistically
significant results can be
obtained via this
technique.
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Brian Foster - HQ physics @ HERA II
HQ in Diffraction
The structure of diffraction and its mechanism and
explanation in the framework of pQCD is one of the most
fruitful areas of HERA I physics.
In the charm sector, very
strongly limited by statistics.
However, also very good
discrimination amongst
models – one of the areas
where we will most
benefit from the statistics
of HERA II.
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Brian Foster - HQ physics @ HERA II
HQ in Diffraction
Charm production cf inclusive F2 will also be additional
constraint on gluon pdf of diffractive exchange.
In exclusive processes,
the heavy c quark
gives clearly different
behaviour to light
quarks – opening the
relationship of QCD
models of diffraction.
One of the areas where
one clearly sees the effect
of mc as a hard scale.
Again, in all of these
studies, the statistics of
HERA II will be decisive.
Brian Foster - HQ physics @ HERA II
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HQ in Photoproduction
In general statistics not a problem with photoproduction
results, except where extra requirements on tagging – e.g.
double jet tags.
Can use impact parameter tagging to isolate HQ sample
in dijet events.
Visible range: pt jet1(2) >11(8) GeV
Measured quark fractions:
fuds~58%, fc~35% , fb~7%
Reach almost values expected
for massless u,d,s,c,b !
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Brian Foster - HQ physics @ HERA II
HQ in Photoproduction
Differential cross sections can be compared to NLO QCD
predictions.
Higher pT reached than for D*
measurements
Excess Data/NLO in
more forward direction
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Brian Foster - HQ physics @ HERA II
HQ in Photoproduction
Study of angular
distributions in dijet
events where one of
the jets contains a D*
can disentangle the
dynamics of the
c-production process.
In principle, this
method, with greatly
increased statistics
at HERA II, can lead
to determination of
c (and g) density in
g.
Brian Foster - HQ physics @ HERA II
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HQ in Photoproduction
Since charm tagging
works over a wide range
of momenta, has rather
high efficiency and high
purity, charm is a useful
method to look at
fragmentation.
The study of all these
charm particles has
allowed the determination
of fragmentation probs.
to u,d, ratios of P/V,
strangeness suppression
etc. to ~5 – 10%. This can
certainly be improved at HERA II.
Brian Foster - HQ physics @ HERA II
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HQ in Photoproduction
H1 data agrees well with
that from e+e- - not
surprisingly so do
the fragmentation
parameters deduced.
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Brian Foster - HQ physics @ HERA II
HQ in Photoproduction
ZEUS published very similar results some time ago. More
evidence that fragmentation factorises from the hard
production process.
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Brian Foster - HQ physics @ HERA II
HQ in Photoproduction
By looking at events
containing leading
neutrons and a charm
tag, we can get a
handle on the c (& g)
density in the .
This is a classic
“double-tag”
experiment which
needs the increased
statistics of HERA II
(and more!).
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Brian Foster - HQ physics @ HERA II
B production
B production both
in photoproduction
and DIS is really in
its infancy at
HERA I and will
be a major study
at HERA II.
Whether there is
really a problem
with the QCD
prediction of s(b)
will surely have
to wait for
HERA II.
Brian Foster - HQ physics @ HERA II
44
Charm spectroscopy in gp
The large HERA I data sample gave the opportunity to make
useful contributions to charm spectroscopy.
Rich spectrum - D1 ,D2* established;
D*' seen by DELPHI, not OPAL/CLEO.
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Brian Foster - HQ physics @ HERA II
Charm spectroscopy
However HERA II can reach states that other machines
cannot reach – e.g. states coupling strongly to gluons can
be copiously produced at HERA.
Not obvious why such
states should want to
decay to charm; but
then we didn’t expect
to see them in
light-quark decays
either.
HERA II can do charm
spectroscopy – we should
look in those places where
we are competitive.
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Brian Foster - HQ physics @ HERA II
Charm pentaquark?
Many experiments have seen the strange pentaquark
q(1530)K0s p ; many more haven’t. Some who did now
aren’t so sure.
H1 have signal for
a charmed
pentaquark.
Acceptance corr. event yields
Unambiguously ruled out by ZEUS. Needs HERA II.
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Brian Foster - HQ physics @ HERA II
Production mechanisms
As an electron-proton collider, HERA II has access to
unique production mechanisms that can provide strong
tests of QCD.
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Brian Foster - HQ physics @ HERA II
Production mechanisms
Interplay with
Tevatron can give
extra constraints on
relative importance of
CO & CS; large
uncertainties in MEs
mean not so obvious
that HERA II statistics
very helpful without
significantly more
theoretical work.
Inelastic J/y
photoproduction
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Brian Foster - HQ physics @ HERA II
Production mechanisms
One process at least
where we know
HERA II will be vital,
and which may well be
a help in unraveling the
J/y problems,
is Υ production – at
HERA I we barely (?)
saw elastic production,
let alone inelastic.
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Brian Foster - HQ physics @ HERA II
Single top production
HERA cannot produce tt pairs and single production
is highly suppressed in the SM – FCNC process.
Observation of events with high pt lepton, jet and
high missing pt sensitive to this process – and lots of other
things.
Brian Foster - HQ physics @ HERA II
51
Single top production
As is well known, there is an excess of such events in H1,
although not in ZEUS and not in either experiment
in the hadron channel.
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Brian Foster - HQ physics @ HERA II
Single top production
Summary of events seen to date - ZEUS numbers
consistent with background.
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Brian Foster - HQ physics @ HERA II
Single top production
This is perhaps
the clearest and
most important
area where just
the statistics gain
(and the better
b tagging) at
HERA II will
really pay
dividends
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Brian Foster - HQ physics @ HERA II
HERA II prospects
Heavy quark physics at HERA I has been exceptionally
rich. Not only have we measured production cross sections
over wide range of Q2, we have made contributions to
QCD understanding of heavy quarks, to spectroscopy
and to diffraction.
HERA II promises much. There will be factors of ~5 in
integrated luminosity; the tagging efficiency and kinematic
reach for heavy quark measurements will increase because
of the improvements to the detectors in general and the vertex
detectors in particular.
Many areas will benefit – particularly DIS and heavy quark
structure functions, and everything to do with B production.
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Brian Foster - HQ physics @ HERA II
HERA II prospects
The background problems are now solved and HERA is
starting to work well; however a series of bending magnets is
progressively failing and needs replacing in November.
There have been problems with some of the upgraded
parts of the detectors also.
The ZEUS Straw Tube Tracker is currently switched off
because insufficient cooling available and the thermal load
on the superconducting coil was causing instabilities. This
may be fixed during the shutdown coming in November.
The H1 silicon tracker also has problems. Backward and
forward detectors removed in summer and being upgraded
with radhard chips. The FST wafers are also being replaced
after damage in a beam accident. They will be reinstalled
in December.
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Brian Foster - HQ physics @ HERA II
Summary
There are only 2 years left until HERA will be switched off to
become a synchrotron radiation facility.
It’s going to be a race against time to achieve all the goals we
set in HERA II - but things are now looking good.
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Brian Foster - HQ physics @ HERA II
Dipole Models
Example of this type of model: Golec-Biernat & Wuesthoff predicts
l
2
 x  l Q 2  x  l Q 2 



x
Q


s g p ( x, Q 2 ) = s 0  0  02 ln   2  1  ln  0  02  1 
 x0  Q0

 x  Q
 
 x  Q
*
-Q2s0
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Brian Foster - HQ physics @ HERA II