Transcript b,Q 2 - INFN - Torino Personal pages

Diffraction: a different window on
QCD and the proton structure
(an overview for non specialists)
M. Arneodo
University of Eastern Piedmont, Novara,
INFN Torino, Italy
University of Wisconsin, Madison
Dec 1st, 2003
1. Diffraction in terms of quarks, gluons and QCD
2. Diffraction as a tool to probe the proton
3. A look at the future: diffractive Higgs production ?
1
Diffraction in hadron scattering
Diffraction is a feature of hadron-hadron interactions (30% of stot):
a
a
vacuum
quantum
numbers
IP
b
Elastic
a
b
X
a
X
IP
IP
LRG
b
Y
b
b
Single Dissociation Double Dissociation
o) Beam particles emerge intact or dissociated into low-mass states.
Energy  beam energy (within a few %)
o) Final-state particles separated by large polar angle
(or pseudorapidity, ln tan(q/2)): Large Rapidity Gap (LRG)
o) Interaction mediated by t-channel exchange of object with vacuum
quantum numbers (no colour): the Pomeron (IP)
2
Pomeron ?!
Pomeron goes back to the ‘60s: Regge trajectory, ie a moving pole
in complex angular momentum plane. Would like to understand
diffraction in terms of quarks, gluons and QCD (need a hard process)
A worthwhile task:
•Diffraction is a significant part of stot
•Elastic cross section drives stot via optical theorem: dsel/dt|t=0 (stot)2
•Understanding diffraction in terms of QCD offers new insight into
QCD itself
In the last 5-10 years, we learned a lot about diffraction by scattering
pointlike probes (electrons) on Pomeron – the same technique used
for studying the structure of the proton
 now clear that diffraction has a well deserved place in QCD
NB in following will often refer to Pomeron as if it were real particle (it isn’t)
3
Part I
• The partonic structure of the Pomeron
as probed by a pointlike virtual photon
• Rather than Pomeron: diffractive PDFs
• Their applicability in ep and pp, pp
processes
4
Diffractive Deep Inelastic Scattering
e’
Q2
e
Q2 = virtuality of photon =
= (4-momentum exchanged at e vertex)2
g*
MX
W
LRG
IP
p’
p
t
= (4-momentum exchanged at p vertex)2
typically: |t|<1 GeV2
W = invariant mass of photon-proton system
MX= invariant mass of photon-Pomeron system
t
Diffractive peak
Uncorrected !
H1
e
p
27.5 GeV
xL=P’/Pbeam
920 GeV
s  320 GeV
5
Standard Deep Inelastic Scattering
For Q2<< MZ2:
In a frame in which the proton is very fast
(Breit frame):
Q2
x = Bjorken’s variable=
= fraction of proton’s momentum
carried by struck quark
 Q2/W2
W
W = photon-proton centre of mass energy
y = W2/s

d 2s
4 2 
y2
2

1

y

F
(
x
,
Q
)


2
2
4
2
dxdQ
xQ 
2[1  R( x, Q )] 
F2=Si[ei2 x fi(x,Q2)]
R=sL/sT
proton PDF
DIS probes the partonic
structure of the proton
6
Diffractive Deep Inelastic Scattering
e’
Q2
xIP = fraction of proton’s momentum
e
taken by Pomeron
g*
b
xIP
p
IP
t
d 4s
4 2

2
dbdQ dxIP dt
bQ 4
Naively, if IP were particle:
[Ingelman, Schlein]
p’
= x in Fermilab jargon
= Bjorken’s variable for the Pomeron
= fraction of Pomeron’s momentum
carried by struck quark
= x/xIP

 D ( 4)
y2
2
1

y

F
(
b
,
Q
, xIP , t )

D ( 4)  2
2(1  R
)

F2D(4)  fIP (xIP,t) F2IP (b,Q2)
Flux of Pomerons
“Pomeron structure function” 7
Diffractive Structure Function vs b
Pomeron:
Proton:
!
bb
Weak b dependence – not a “normal” hadron !
x
8
Diffractive Structure Function vs b
Pomeron:
Proton
:
!
b
Weak b dependence – not a “normal” hadron !
x
9
Diffractive Structure Function vs Q2
Pomeron:
Q2
Positive scaling violations:
lots of gluons !
Proton
:
Q2
10
Diffractive PDFs
NLO DGLAP fit:
• Parametrise Flavour Singlet (quarks+antiquarks) and Gluons at Q2= 3 GeV2
• Evolve with NLO DGLAP and fit
z = fractional
momentum of
parton
Gluon dominated: integrated
fraction of exchanged momentum
carried by gluons (75  15)%
11
(Diffractive) hard scattering factorisation
Diffractive DIS, like inclusive DIS, is factorisable
Trentadue, Veneziano (1994); Berera, Soper (1996)…]:
[Collins (1998);
universal partonic cross section
diffractive parton distribution functions:
evolve according to DGLAP
fi/pD(z,Q2,xIP,t): probability to find, with probe of resolution Q2, in a
proton, parton i with momentum fraction z, under the condition that
proton remains intact, and emerges with small energy loss, xIP, and
momentum transfer t – diffractive PDFs are a feature of the proton
A new type of PDFs, with same dignity as standard PDFs. Applies
when vacuum quantum numbers are exchanged
Rather than IP exchange: probe diffractive PDFs of proton
12
Test factorisation in ep events
Use diffractive PDFs extracted from DGLAP fits of F2D
to predict, eg, diffractive dijet production cross section
jet
jet
4<Q2<80 GeV2
CDF cone algorithm
Ptjet 1(2) >5(4) GeV
xIP<0.05
gp centre-of-mass energy
• Normalisation and shape of data described ok
• Same conclusion for charm production
Hard scattering factorisation works in diffractive DIS
13
Test factorisation in pp events
?
jet
b
(x =xIP)
FDJJ
hard
scattering
(= F2D)
Factorisation of diffractive PDFs not expected to hold for pp,
pp scattering – indeed it does not:
jet
LRG
IP
Normalisation discrepancy (x10)
(depends on s [CDF, D0])
Hard scattering
factorisation violated in pp
(lots more evidence available)
14
Why is factorisation violated ?
Violation of factorisation understood in terms of (soft) rescattering
corrections of the spectator partons (Kaidalov, Khoze, Martin, Ryskin):
•Two-component eikonal model a`
la Good & Walker, Pumplin, Gribov
– pre-QCD !
•Main uncertainty is that on F2D
F2D
Predictions based
on rescattering
assuming HERA
diffractive PDFs
CDF data
b
15
Why is factorisation violated ? (cont’d)
• Understanding of breaking encouraging – lots of different
approaches investigated:
o) Bjorken (1993)
o) Gotsman, Levin, Maor (1993)
o) Goulianos (1995)
o) Buchmueller, Gehrmann, Hebecker (1997)
o) Cox, Forshaw, Loennblad (1999)
o) Enberg, Ingelman, Timneanu (2000)
o) Erhan, Schlein (2000)
o) Bialas, Peschanski (2002)
o) [list is incomplete]
IP flux renormalisation
Soft Colour Interactions
s-dependent IP trajectory
• If this works, can use at Tevatron the diffractive PDFs
from HERA, and vice versa – ie diffractive PDFs truly
universal
16
Summary I
• We have measured the partonic content of the exchange
responsible for elastic and diffractive interactions –
mainly gluons [ie we think we know what a Pomeron is]
• This has led to a new kind of PDFs which apply to the
class of QCD events where vacuum quantum numbers
are exchanged: diffractive PDFs
• Rather than consider diffraction as due to the exchange
of IP  exchange of partons belonging to the proton
• Hard scattering factorisation of diffractive PDFs works in
DIS. We are on the way to understanding the large
breaking of factorisation observed in ep vs pp
17
Open questions
• How safe is a QCD analysis of F2D ? Whole b range ok?
• Relevance of assuming F2D(4)  fIP (xIP,t) F2IP (b,Q2) ?
• Map the size of factorisation breaking as a function of
as many variables as possible – input from Tevatron
essential
• Understand rescattering corrections in terms of QCD
18
Part II
Diffraction as a tool to probe the proton:
• Consider ep diffractive scattering:
move to proton rest frame at HERA, find out that
sdiffr  [gluon density in proton]2
Example: exclusive vector meson production
Calculable in QCD !
• Correlations in the proton: GPDs
19
Diffractive DIS in the proton rest frame
Virtual photon fluctuates to qq, qqg states (colour dipoles)
g*
1
q

q
Q 2  M q2q
g*
 Eg ~ W 2 ~ 1 x
•Lifetime of dipoles very long because of large g boost (Eg  50TeV!)
 it is the dipole that interacts with the proton
•Transverse size proportional to 1/  (Q2+ Mqq2)
(for longitudinally polarised photons)
•cf Vector Meson Dominance (qq, qqg have JPC=1- - )
•This is why can do diffraction in ep collisions !
Transverse size of incoming hadron beam can be reduced
at will. Can be so small that strong interaction with proton
becomes perturbative (colour transparency) !
20
q
q
g
Diffractive DIS in the proton rest frame
g*
X
X
p
p
IP
p
p
+
X
p
2-gluon exchange:
LO realisation of vacuum
quantum numbers in QCD
Cross section proportional to
probability of finding 2 gluons
in the proton
!
s  [x g] 2
Gluon density in the proton
21
Example: Vector Meson production
(JPC=1--): r, f, J/y,U,...
V
V
IP
p
p
p
p
s  [ x g(x, Q  M )]
2
2
V
2
x  (Q 2  M 2V ) W 2
Growth of cross section
with decreasing x, hence with
increasing W, at large Q2+ MV2,
reflecting large gluon density
at low x
Ryskin (1993), Nikolaev et al (1994),
Brodsky et al (1994),...
22
VM: sensitivity to gluons in proton
s (gp
Vp),
Q2=0
s  [ x g(x, M 2V )]2
xM
2
V
W
V
2
p
p
 W0.2
xg(x)
MV

W0.8

W1.7
W  1/  x
gp centre-of-mass energy
x
23
VM: sensitivity to gluons in proton
s (gp
Vp),
Q2=0
s  [ x g(x, M 2V )]2
xM
2
V
W
V
2
p
p
•At small MV (MV2  1 GeV2):
Incoming dipole behaves like a
normal-size hadron: the two
exchanged gluons are soft
– cf stot(gp)
MV
Flat s vs W reflects flat gluon
distribution for Q2  0
•At large MV :
W  1/  x
gp centre-of-mass energy
Fast growth of s with W reflects
growth of gluon distribution
with decreasing x
24
VM: sensitivity to gluons in proton
s (gp
Vp),
Q2=0
s  [ x g(x, M 2V )]2
xM
2
V
W
V
2
p
p
MV
W  1/  x
gp centre-of-mass energy
At large MV, data well reproduced
by pQCD
25
Summary II
• Hard diffraction sensitive to proton structure
and calculable in QCD
• (not discussed) Hard diffraction sensitive to
correlations in the proton:
g*
In general, x1  x2:
x1
p
s  [x g(x)] 2
x2
p
s  [H(x 1, x 2 )]2
Generalised parton distribution functions (GPD)
26
Open questions II
•Detailed understanding of higher orders:
vs
g*
r
sqq
pQCD
•Transition to (Q2+Mqq2)=0 and non-perturbative QCD:
Saturation
npQCD
r
connection to high-density QCD,
saturation of parton densities,
Colour Glass Condensate,
physics of RHIC
• ...
27
Part III
The future:
• Diffractive physics programs at
Fermilab, DESY, CERN
• Diffractive Higgs production – the
way to discover a light Higgs ?!
28
A look at the future
Aggressive diffractive programs at Fermilab, DESY and
CERN:
• CDF: new Beam Shower Counters, miniplug calorimeters
• D0: new Roman Pot spectrometer
• H1: new Very Forward Proton Spectrometer
• DESY: HERA-g proposal (glueballs & odderon with HERA-B detector)
HERA III after 2006 ?
• GTeV: Gluon Physics at the Tevatron
• CMS/TOTEM Forward Physics Project: study diffraction and
forward physics at full LHC luminosity.
Roman Pots and microstations
• ATLAS: studying the feasibility of forward detectors
29
CMS/TOTEM Forward Physics Project
beam
dipole
CMS
dipole
p’
roman pots
roman pots
p’
TOTEM
• Totem: Measure total & elastic cross sections, soft diffraction.
Inelastic detectors (CSC), Roman Pots at 150, 215 m, same I.P. as CMS
• CMS/Totem Forward Physics Project: hard diffraction, forward
physics at full LHC lumi. Roman Pots/mstations at 310/420 m (?), very
forward EM/HAD calorimeter, Si-tracker, Zero-degree calorimeter at 140 m
 EOI by summer ‘04
30
Diffractive Higgs at LHC
• For light Higgs ( 120 GeV), gg H, H  bb mode has highest branching
ratio, but signal swamped by gg bb
• Signal-to-background ratio improves dramatically for 2 rapidity gaps
and/or outgoing protons tagged: S/B~3; for 30 fb-1, observe 11 events
[Khoze, Martin, Ryskin]
p
p
LRG
IP
H
IP
p
LRG
p
b
b
Reconstruct MH from bb and/or from
scattered protons with missing mass
method
1-2 GeV resolution at LHC
(250 MeV resolution at Tevatron)
H tt, WW also OK
Major, but not insurmountable, experimental difficulties:
event pile-up at high lumi (23 interactions/bunch crossing) ‘spoils’
rapidity gaps; Roman Pot signals too late for L1 trigger
31
Diffractive Higgs at LHC
• Proton diffractive PDFs essential for
prediction
• Understanding of factorisation breaking
ep vs pp, pp essential,
including s dependence
• Wide range of theoretical predictions – consensus ?
Bialas and Landshoff, Cudell and Hernandez; Levin; Kharzeev, Levin;
Khoze, Martin and Ryskin; Cox, Forshaw and Heinemann,
Boonekamp et al, Enberg et al, Godizov et al, …
[some ruled out by Tevatron data]
• A very promising field – lots more theoretical and experimental
work necessary
32
Grand summary
• Diffraction is due to the exchange of partons from the
proton carrying the vacuum quantum numbers
 probe diffractive PDFs of the proton (mainly gluons)
• Hard scattering factorisation works in diffractive events
(but rescattering corrections to go from ep to pp, pp)
• Diffraction with a hard scale calculable in QCD
• Sensitivity to gluon density, correlations in proton (GPDs)
• Saturation: a window on the transition to npQCD
• Diffraction as a means to search for new physics
• Plenty more experimental and theoretical work necessary
33
Input from pp, pp, AA, ep, eA essential