Upload
quilla
View
18
Download
1
Embed Size (px)
DESCRIPTION
DIS and e+e- annihilation at strong coupling. Yoshitaka Hatta (U. Tsukuba). Based on:. arXiv:0710.2148 [hep-th] with E. Iancu and A. Mueller arXiv:0804.4733 [hep-th] with T. Matsuo arXiv:0807.0098 [hep-ph] with T. Matsuo. Contents. Introduction DIS in QCD DIS in N=4 SYM - PowerPoint PPT Presentation
Citation preview
DIS and e+e- annihilationat strong coupling
Yoshitaka Hatta
(U. Tsukuba)
arXiv:0710.2148 [hep-th] with E. Iancu and A. Mueller arXiv:0804.4733 [hep-th] with T. MatsuoarXiv:0807.0098 [hep-ph] with T. Matsuo
Based on:
Contents
Introduction DIS in QCD DIS in N=4 SYM e+e- annihilation and jets in QCD Jets in N=4 SYM ? Thermal (exponential) hadron spectrum
Motivations
Longstanding problems in the small- regime of QCD (Froissart’s theorem, soft vs. hard Pomerons, hadronization...) for which pQCD methods do not apply. How does the nature of scattering change as one goes from weak to strong coupling ? N=4 SYM
Possible applications to the phenomenology of `sQGP’ (finite-T ) at RHIC, or in the `unparticle’ sector at the LHC
BF xx
Regge limit of QCD
E
E
,...,,2 TpMtEs
One of the most challenging problems of QCD is the high energy limit.
Can we compute the total cross section, etc ?What are the properties of the final states ?
Experimental knowledge
Regge trajectory
2')0( mj
Quadratic relation between mass and spin of hadrons
Interpretable as a spinning stringwith tension
intercept slope
'1~ 0.5
Heuristic picture (life before QCD)
j
j
j
jj
j
jj tj
sg
tm
sg
')0(~
2
s
t
Suppose scattering is dominated by the t-channel exchange of particles on some Regge trajectory.
=
=
spin-j
Complex angular momentum
j
tjji
stj
s
j
edjtsT ')0(
')0(sin
1),(
j
1)0()0,(Im ss
sTtot
Analytically continue the partial waves to the complex J-plane and pick up the leading Regge pole
Regge limit in string theory (flat space)
The leading Regge trajectory (closed string) '
)2(22
j
m j
stot Amplitude dominated by the spin-2 graviton exchange
2'2sin
1),( 2
tj
s
j
edjgtsT
jji
s
2'22 1 ts sit
g
Deep inelastic scattering in QCD
P X
e
2Q
sQQmm
Q
qP
QB
pX
x 2222
22
2
x
High energy = small- )1(x
Two independent kinematic variables
02 Qqq Photon virtuality
Bjorken-
Physical meaning : momentum fraction of the constituents (`partons’)
DIS structure functions
Imaginary part of the forward Compton scattering amplitude
PJxJPexdi ememiqx |)0()(|Im 4
qP
QxFq
q
qPPq
q
qPPQxF
q
),(),(
22
222
12
p p
Probe of the parton distribution in the target hadron.
),(~),(),(),( 22222 QxxgQxqxQxxqQxF s
The operator product expansion
)(
2
24 ~~
2
j
jjQj
jj
ememixq QcOcJJexd
,)( 1qDqO jj
The moment sum rule)(
2
22
221
0~),(
j
j QQxFxdx
Invert
)(
2
212
2 )(2
~),(j
j Qxjc
i
djQxF
continued to arbitrary j
Twist-2, spin-j operators
)(22 jj
FDF j 2)(
The BFKL Pomeron Balitsky, Fadin, Kraev, Lipatov, `78
,1s 11ln xs
T = T+
5.0 1sx 2ln4
)(
2
21
0
1
2~
j
j Qx
jji
dj
),( 22 QxF
s2ln41
but )(j
‘Bootstrap’
`Phase diagram’ of QCD
2lnQ
s
xxQs
9.42 1)(
x
1ln Saturation
BF
KL
DGLAP
N=4 Super Yang-Mills
SU(Nc) local gauge symmetry Conformal symmetry SO(4,2)
The ‘t Hooft coupling doesn’t run. Global SU(4) R-symmetry
choose a U(1) subgroup and gauge it.
0CYMNg 2
(anomalous) dimension mass`t Hooft parameter curvature radius number of colors string coupling constant
The correspondence
N=4 SYM at strong coupling is dual to weak coupling type IIB string theory
on
Spectrums of the two theories match
Maldacena, `97
2'4 RCN1 sg
SYM string
55 SAdS
Our universe 5-th dim.
Dilaton localized at small
DIS at strong coupling
R-charge current excites metric fluctuations in the bulk, which then scatters off a dilaton (`glueball’)
r
r
Cut off the space at (mimic confinement)
2Q
Polchinski, Strassler, `02
We are here
Photon localized at large Qr
0rr
)( r
High energy string S-matrix
dilaton gauge bosonvertex op. vertex op.
Insert t-channel string statesdual to twist-2 operators
j
jj VV1
j
AdS version of the graviton Regge trajectory
Anomalous dimension in N=4 SYM
Gubser, Klebanov, Polyakov, `02
)(22
1
2)( 0jj
jj 2j Kotikov, Lipatov, Onishchenko, Velizhanin `05
Brower, Polchinski, Strassler, Tan, `06
)(22 jj
2~j
jj XXrV
`Pomeron vertex operators’ Twist—two operators
Diffusion in the 5th dimension, important when
Pomeron at strong couplingPhoton wavefunction
Dilaton wavefunction
Qr ~
~'r
220 j
~ln s
Pomeron = massive graviton in AdS with intercept
s
Q
N
s
c ln8
/lnexp
222
2
21
Graviton dominance — high energy behavior even worse than in QCD.
Multiple Pomeron exchange (higher genus) goes like
Keep Nc finite and study the onset of unitarity corrections. We shall be interested in the regime
Not suppressed when resummation
Beyond the single Pomeron exchange
212
1 sN
Tc
2~ cNs
but to comply with unitarity1T
ncn
s Nssg 22 ~)(
eNs c 2~
The real part strikes back
In DIS, typically 1~
'
Q
r
r
The saddle point in the j—integration
s
rrjsaddle 2
222
ln8
'ln22
0j
Resurrection of massless gravitons
j
2 4 60j
j
2 4 60j
small large 2Q 2Q
graviton propagator in AdS
Eikonal approximation
The real part much bigger than the imaginary part (unlike in QCD ! )Typical in theories with gravity
Large real part resummed by the eikonal approximation `t Hooft, `87 Amati, Ciafaloni, Veneziano `87Can be extended to AdS Cornalba, Costa, Penedones, Schiappa `06 Brower, Strassler, Tan `07
Valid when the impact parameter is large. This is the case in DIS at large !
Impact parameter
2Q
b
)()( biTebT
Diffractive dissociation
flat space 68)(
b
set
skdbT bik
AdS 2)(
r
srT
62
2
2
22 )(
b
s
b
lbT
bl ss
22
22 1
)(r
srT
rGl rr
s
Dominant source of inelasticity from cuts between pomerons.
Amati, et al, `87
Tidal force stretches a string,causing excitation.
Only mildly suppressed,comparable to the elastic phase
Giddings, `06
`Phase diagram’ of QCD
2lnQ
s
xxQs
9.42 1)(
x
1ln Saturation
BF
KL
DGLAP
Phase diagram at strong coupling
xY 1ln
Jets in QCD
ee
Average angular distributionreflecting fermionic degrees of freedom (quarks)
2cos1
Observation of jets in `75 marks one of the most striking confirmations of QCD and the parton picture
Fragmentation function
ee
P Count how many hadrons are there inside a quark.
),( 2QxDT
Q
E
Q
qPx 222
Feynman-x
First moment
nQxDdx T ),( 21
0 average multiplicity
2),( 21
0 QxxDdx T energy conservation
Second moment
022 Qq
Evolution equation
The fragmentation functions satisfy a DGLAP-type equation
),()(),(ln
222
QjDjQjDQ TTT
),(),( 211
0
2 QxDxdxQjD Tj
TTake a Mellin transform
Timelike anomalous dimension
)1(22 ),1( TQQDn T (assume )0
2122
,)(),(ln
Qz
xDzP
z
dzQxD
Q TTxT
Timelike anomalous dimension in QCD
Lowest order perturbation
Soft singularity
~x
11
)(
j
j sT
!!)1( T
ResummationAngle-ordering
)1(
8)1(
4
1)( 2 j
Njj s
T
Basseto, Ciafaloni, Marchesini, ‘80 Mueller, `81
Inclusive spectrum
largel-x small-x
),( 2QxxDdx
dxT
Roughly an inverse Gaussian in peaked at
Q
x
1
‘Hump-backed’ shape consistent with pQCD with coherence.
Mueller,Dokshitzer, Fadin, Khoze
x1ln
e+e- annihilation in strongly coupled N=4 SYM
Hofman, MaldacenaY.H., Iancu, Mueller,Y.H., Matsuo Y.H., Matsuo
arXiv:0803.1467 [hep-th]arXiv:0803.2481 [hep-th]arXiv:0804.4733 [hep-th]arXiv:0807.0098 [hep-ph]
022 Qq
5D Maxwell equation
Want to compute at strong coupling )( jT
But there is no corresponding operator in gauge theory…
DIS vs. e+e- : crossing symmetry
P
e 022 Qq
e
022 Qq
e
Bjorken variable Feynman variable
P
qP
QBx
2
2
2
2
Q
qPFx
Parton distribution function Fragmentation function
),( 2QxD BS ),( 2QxD FT
crossing
Reciprocity respecting evolution
),()(),(ln
2//
2/2
QjDjQjDQ TSTSTS
DGRAP equation
Dokshitzer, Marchesini, Salam, ‘06
The two anomalous dimensions derive from a single function
Basso, Korchemsky, ‘07Application to AdS/CFT
Assume this is valid at strong coupling and see where it leads to.
Confirmed up to three loops (!) in QCD Mitov, Moch, Vogt, `06
is consistent with the Gribov-Lipatov reciprocity)( jf
Average multiplicity at strong coupling
)(22
1
2)( 0jj
jjS
22
1)(
2
0
jjjjT
231)1(2 )()()( QQQn T
c.f. in perturbation theory, 22)(
QQn
crossing
c.f. heuristic argument QQn )( Y.H., Iancu, Mueller ‘08
Y.H., Matsuo ‘08
Jets at strong coupling?
The inclusive distribution is peaked at the kinematic lower limit
1Q QEx 2
Qx
FQQxDT22 ),( Rapidly decaying
for Qx
21)(
jjT in the supergravity limit
At strong coupling, branching is so fast and complete. There are no partons at large-x !
Q
Energy correlation functionHofman, Maldacena `08
Energy distribution is spherical for anyCorrelations disappear as
1)()3( OS
241 weak coupling
strong coupling
1
2
All the particles have the minimal four momentum~ and are spherically emitted. There are no jets at strong coupling !
Q
Thermal hadron spectrum
Identified particle yields are well described by a thermal model
T
MM
N
N **
exp
The model works in e+e- annihilation, hadron collisions, and heavy-ion collisions
Becattini,Chliapnikov,Braun-Munzinger et al.
The origin of the exponential law is not understood, though there are many speculations (QGP? Hawking radiation?) around…
A statistical modelBjorken and Brodsky, ‘70
Cross section to produce exactly n ‘pions’
Assume
Then Strikingly similar to theAdS/CFT predictions !
Total cross section given by the one-loop result !
Qn
n
nc
tot Q
Ne
2
24
32
Computing the matrix element
5D scalars5D photon
AdS metric
string amplitude
)()( )1(1 QzHNzA c
QnSince is large, do the saddle point approximation in the z-integral
)exp(iQz~
2.0c
The saddle point has an imaginary part.
This leads to
Probably absent once the decay width is Included in the propagators. Einhorn, ‘77
5.0c
for ‘mesons’
for `partons’
2
1
nV *)( 2
cNO )1(O
*zz
Summary
DIS and e+e- accessible at strong coupling The amplitude is dominantly real, final states mo
stly diffractive (unlike in QCD…) There are no jets, multiplicity rises linearly with
energy (unlike in QCD…) The multiplicity ratio exhibits ‘‘thermal’’ behavior
(like in QCD?)