Transcript Lepton Hadron Scattering, An Overview
Lepton Hadron Scattering, An Overview
Snowmass 2001 Working Group M5 July 3, 2001 Abhay Deshpande RIKEN BNL Research Center
RIKEN BNL Research Center
Experiments and The Standard Model
Standard Model (SM) of High Energy Physics:
1. Electroweak Physics: Electro-Magnetic-Weak Interactoins mediated by gauge bosons: g, W(+/-) and Z. Interactions between charged particles: quarks & leptons (3 families) 2. QCD Physics: Strong Interactions mediated by gauge boson: “g”, the gluon. Carry color charge! Distinct from EW carriers of forces in the fact that they can interact with other gluons “g-g” interaction 3. Gravitational Physics no direct connection found yet
Experimental Tests of the SM
1. Lepton-Lepton Scattering 2. Lepton-Hadron Scattering: Probing the structure of matter and strong interactions: Topic of this working group 3. Hadron-Hadron Scattering
All three methods are complementary
nature of above interactions.
.
Investigate the universality of physical laws leading towards a better understanding of structure of matter and Lepton Hadron Colliders, Snowmass01 July 3, 2001 2
Lepton Hadron Scattering Deep Inelastic Scattering
• A point like probe as in a lepton probes the inside of the hadron at high enough energies… exchange of g* or Z for neutral currents and W’s (not shown) for charged currents
Q
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2 (
k
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' ) 2
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2 2
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W s
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k
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q
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p
) 2
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E e E P
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x
) ( g * P CMS energy) ( eP CMS energy) 2 2
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x
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Lepton Hadron Scattering Deep Inelastic Scattering!
•
Fixed target experiments:
e-p, e-A scattering Unpolarized & Polarized DIS (SLAC/DESY) m -p, m -A scattering n -p, n -A scattering Unpolarized and polarized DIS ( Semi-un-polarized (FNAL) CERN /FNAL) •
Collider Experiments
e-p scattering Semi-un-polarized ONLY (DESY) Unpolarized Scattering Semi-un-polarized Scattering Polarized Scattering Lepton Hadron Colliders, Snowmass01 July 3, 2001 4
HERA at DESY
Proton Electron Proton Electron Lepton Hadron Colliders, Snowmass01 July 3, 2001 5
HERA Collider Detectors
ZEUS Detector Schematic H1 Detector Schematic Very close to 4 p acceptance Good calorimetry, tracking & vertex resolution Lepton Hadron Colliders, Snowmass01 July 3, 2001 6
HERA Kinematic Range with Collider Detectors
• Significant increase in kinematic range beyond the fixed target experimental era.
• Many new aspects of QCD were discovered and understood because of the extended range • Unresolved problems…..
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Experimental Excitement from HERA Future Experimental Measurements!
• • • •
Highlights from HERA I Proton structure & QCD
Unexpected rise of F2, Role of gluon Jet physics, Precise a S determination Electroweak physics
Low x phyisics
High parton density matter, Diffractive physics
Photon structure
Jet physics, Prompt photons
Physics beyond SM
Lepto-quarks, Contact interactions, excited leptons etc….
• • •
HERA II
Luminosity Upgrade Spin rotators for electron beams for ZEUS and H1 Polarization of electron beams
E E e p
H1 and ZEUS Detector upgrades 820 27 .
5 920 GeV GeV Apology: Will not cover all that HERA has done!
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Proton Structure function F
2
(x,Q2)
Rise in F 2 at low x Lepton Hadron Colliders, Snowmass01 • Scaling violation explicitly seen… Beyond the fixed target regime • H1 and ZEUS data in agreement. Further, pQCD predictions at NLO describe data impressively over many decades in x and Q2.
• July 3, 2001 Studies have resulted in the determination of gluon distribution, precise determination of a S 9
Gluon Determination from F 2 QCD Analysis
Recent results from DIS2001, Bologna Perturbative QCD analysis of F2 allows us to access the gluon Distribution in the proton.
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E. Elsen, H1, DIS2001
F
L
Longitudinal Structure Function
d
2
lN dxdQ
2
A
y
2 2 2
xF
1 (
x
,
Q
2 ) ( 1
y
)
F
2 (
x
,
Q
2 ) (
y
y
2 )
xF
3 (
x
,
Q
2 ) 2
F L
F
(
x
,
Q
2 ) 2
xF
(
x
,
Q
2 1 RUNNING ACCELERATORS AT LOWER ENERGY CONSEQUENCES TO THE LUMINOSITY?
HOW CAN THE EFFETCS BE MINIMIZED?
extrapolations of pQCD in to a region of high y 2 )
r
• (
x
,
Q
2 )
F
2 (
x
,
Q
2 ) 1 ( 1
y
2
y
) 2
F L
(
x
,
Q
2 ) Ideally, to get FL free of assumptions, one needs data at different CM energies •
It has been suggested to run HERA at lower proton beam energy… 400 GeV
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Transition region: Low Q2 Photoproduction
pQCD
U. Fricke, ZEUS, DIS2001 Lepton Hadron Colliders, Snowmass01 July 3, 2001 12
Observations….
• • • • • • Is it really a hint of something new happening at low x?
If so we need to go past the HERA kinematics in low x and at the same time keep a “respectable” Q2 so that some of these perturbative calculations/fitting procedures are still valid
THERA/eVLHC Other ways
: There are models that predict that at low x the partonic densities might be very large, and as such the DGLAP formalism breaks down…. Predominantly gluonic matter at very high density.
Remember gluons are bosons, and at high enough densities they might form “Bose-Einstein Condensates” which form the high density matter.
Some have called this: Color Glass Condensate: A yet undiscovered state of matter…. When this (Glass) shatters…Quark Gluon Plasma is formed…
Bottom line is: to create the high gluon density objects we might as well work with HEAVY IONS for DIS.
This would be effectively high parton densities I.e. LOW x!
going to the Electon Ion Collider (BNL)
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Diffractive Events at HERA
Early observation at HERA Lepton Hadron Colliders, Snowmass01 July 3, 2001 The diffractive cross sections is unexpectedly large at HERA: ~typically 5-6% Explanations based on pomeron exchange exist.. But..
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Diffraction at HERA…
•
Larger than expected
Gluonic in origin • Consequences to interpretation at
low x, low-intermediate Q2
• Saturation models (Color Glass Condensate) do indeed account for the higher than expected diffractive events and predict that if e-A collisions are performed, due to even higher density of gluons in the nuclei, the expected diffractive cross sections might be as
large as 40%
• • A very clear and robust diffractive signal could be seen.
• Since these SAME high parton density models also address the low x structure function behavior in details… they provide a compelling alternative to the pomeron ideas…. How are they connected?
THERA/eVLHC and e-A physics at high enough energies such as EIC would allow us to directly look at this.
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a S
Determination from jet measurements…
ZEUS H1 Lepton Hadron Colliders, Snowmass01 July 3, 2001 16
a S
Determination from inclusive jet measurements…
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Universality!
• Determination of the strong coupling constant from different methods.
• Lead to astonishingly consistent values • We must be doing this right.. Beauty and complimentarity of measurement techniques! The smallest and the third smallest Uncertainty comes from DIS!
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Electroweak Physics at HERA
•At large Q2 the electromagnetic and weak currents assume equal strength.
•At high Q2 situation limited by statistics •Over 8 orders of magnitude these measurements have been compared with the predictions from SM •H1 and ZEUS data in excellent agreement •Further, data are in excellent agreement with the Standard Model.
These and such tests will improve further in HERA II. AND HERA will access Physics beyond the SM… if it exists… Lepton Hadron Colliders, Snowmass01 July 3, 2001 19
xF
3
Measurement at HERA
• First measurement of xF3 at HERA • Agrees well with the PDFs evolved from much lower Q2 • Statistics limited • HERA II will consolidate this measurement The polarized partner of xF3 is Structure function g 5 . Can be measured If we have polarized ep colliders…
Capable of having e+/e- beams!
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Rutherford would have been proud of this measurement…..
Lepton Hadron Colliders, Snowmass01 July 3, 2001 W. Marciano’s talk: Critical dimensions below which physics beyond the SM might appear are 10 e –17 cm 21
SM H1 Detector
Excess of high pT lepton events vs. SM
SM Lepton Hadron Colliders, Snowmass01 July 3, 2001 22
SM Processes Considered as Possible sources Of events Lepton Hadron Colliders, Snowmass01 July 3, 2001 23
High pT leptons
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Other searches of Beyond SM
• Contact Interaction • Various SUSY searches • R-parity violating lepto-quarks • Compositeness • Searches of extra-dimensions • ….. All of these would be possible with the large luminosity at HERA II and with future machines like THERA/eVLHC Lepton Hadron Colliders, Snowmass01 July 3, 2001 25
Photon Structure from High E
T
Photoprodction
• Sensitivity to a s • Color Dynamics • Sensitivity to Photon Structure function • Sensitivity to Proton structure function
Why high ET jets?
•Soft effects minimized •Fragmentation/Hadronization effects small •Hard Scale reliable NLO Lepton Hadron Colliders, Snowmass01
A Great Test of QCD!
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Photon Structure… (Cont)
• NLO QCD predictions using available PDFs describe the measured cross sections.
• ZEUS sees deviations • Further interest and studies needed and expected…. HERAII • Ideal measurement for future colliders (polarized and un-polarized!) July 3, 2001 27
Summary Slide… for future unpolarized DIS
• Great anticipation based on present data towards HERA II • This however will be only a luminosity upgrade • Interesting physics may lie beyond the kinematic reach of HERA • Higher energy machines need to be designed… Plans for THERA, DIS with VLHC?
• High density partonic matter could be found in high energy e-A scattering… Diffractive signals from eA physics at high energies might tell us something about the very low x region Plan for EIC at RHIC?
Discovery of a new state of matter
… Color Glass Condensate…?
Detailed studies of diffraction, F L , in general gluons in heavy ions
Upcoming talks will tell us what these future machines could do….
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Polarized DIS
• Electron beams up to 50 GeV/c on fixed (solid and gaseous) targets were predominantly used at SLAC (EXXX serise) and DESY (HERMES) • Muon beams 100-200 GeV/c on fixed (solid) targets were used at CERN(EMC,SMC) Compared to un-polarized DIS, the kinematic range is extremely limited!
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How far does polarized DIS have to go!
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e
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k
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e s
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e p
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Polarized and Unpolarized DIS
e
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Cross Section
d u u d
unpol.
F
1 cos sin a a (
y
( 2 )
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Q g
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yg
1 ( ( 2
x
, ),
x
,
Q F
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Q
2 2 ( ) )
x
,
Q
2
y
g ) 2
g g
2 2 ( (
x
,
x
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Q Q
2 2 ) )
k
,
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: Lepton four momenta
Q
2 n (
k
k
) 2 : Four momentum
E
E
: Energy tra nsfer transfer
x
Q
2
y
n 2
M
n : Bjorken scaling variable
E
: Fractional energy tra nsfer g
Q
2 n Lepton Hadron Colliders, Snowmass01 Polarized Structure Functions:
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g
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) 1 2
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u
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s e i
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f i
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) (unpolariz ed) 1 2
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, ,
d s e i
2
f i
(
x
) (longitudi nal spin) Transversity Distributions:
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i
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d
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e i
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T f i
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Physics Goals of Past Experiments
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Polarized DIS … Towards a Collider.
• • These goals needed only inclusive DIS measurements Have lead to results which can only be improved by making new measurements at low x…. Higher sqrt(s) … higher energy beams and in collider mode:
Polarized Collider necessary
As experiments got more ambitious they started looking at the target fragmentation… and tried to identify the final states using sophisticated detectors: tracking, particle ID, vertexing etc. on hadronic final states.
Lea to the results on semi-inclusive DIS mostly relevant in intermediate to high x Next step could be to extend these to low x and a detailed study of target fragmentation region which would benefit a lot in Colllider mode DIS.
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pQCD analysis results using world data set
QCD Analysis of available Data Sets: (SMC Phys, Rev. D58:112002, 1998)
g
1 (
x
,
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2 ) 1 6
i
u
, ,
d s e i
2 1
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C q S
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x y
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C g
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j e
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Polarized Parton Distributions
• • • • Polarized Singlet quark distributions and the nonsinglet quark distributions determined rather accurately Polarized gluon distribution has largest uncertainty Principle uncertainties come from: a) assumed initial parton distribution functional form b) uncertainty in a S c) factorization and renormalization scale etc… All related to “ lack of data in the low x” region. Lepton Hadron Colliders, Snowmass01 July 3, 2001 35
Lack of low x data… consequences
Q2 = 10 GeV2 SMC Results
g
1 (
x
0 )
x
a as 0 a 0 .
5 Regge / QCD Lepton Hadron Colliders, Snowmass01 July 3, 2001 36
Neutron structure function… E154/SLAC
Lepton Hadron Colliders, Snowmass01 • Consequence: • Unertainties in low x for Bjorken sum rule… • Acceptable? No… measure low x!
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Ellis Jaffe Sum Rule
Violated!
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Ellis Jaffe Sum Rule….
Violated!
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Bjorken Sum Rule
Confirmed!
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Next Generation Experiments
How to access gluons…?
We know the quarks carry small amount of spin… and that gluons may have a significant role in this….
We saw what we need to determine gluon distributions… what other ways can we access gluon distributions? …..
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G from Photon-Gluon-Fusion (PGF)
• Photon Gluon Fusion in DIS: Di-Jet events 2-High-pT hadron events HERMES Collaboration • At high Sqrt(s) the theoretical interpretation is without ambiguities or uncertainties!
• At low Sqrt(s) however things may not be as simple…..
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High pT Hadron:PGF
HERMES Results
G
/
G
0 .
41 0 .
18 (
stat
) 0 .
03 (
syst
) ???(
theory
) No estimate of theoretical Uncertainty Lepton Hadron Colliders, Snowmass01 July 3, 2001 43
G from high-pT hadron….
• PYTHIA used at very low scales… caution!
• VMD component’s reliability questionable, and no reliable uncertainty estimate possible.
•
High pT, possible in future lepton-hadron colliders will overcome this problems.
• H1 has already published such a result on G using high-pt hadrons.
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Scale dependencies in this business…
Lepton Hadron Colliders, Snowmass01 • M. Stratmann & W. Vogelsang • Proceedings of SPIN2000, Osaka, Japan.
• At low energies (such as HERMES/COMPASS) result strongly dependent on the value of the scale
At higher energies the scale dependence of the result significantly reduced… look at curves for eRHIC, HERA, RHIC, Tevatron…
Need a polarized high energy collider!
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Semi-Inclusive Physics: HERMES/SMC
SMC: Tags only charge of h HERMES: Has particle ID Flavor separated valence quark distributions in nucleon accessed Lepton Hadron Colliders, Snowmass01 July 3, 2001 46
Semi-Inclusive & Inclusive Asymmetries
Newly installed RICH would do excellent particle ID expect to see Asymmetries for K/ p instead of charged hadrons soon.
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Transversity
• Probability of finding a quark oriented transverse to the proton momentum • One of the last unmeasured spin structure function of the proton • Unmeasured: because it’s a chiral odd structure function and does not couple directly to observables in experiments • Suggested by Jaffe et al.: Why not measure a PRODUCT Transversity and something else the product of the two is a chiral even structure function Then if you can unfold that other function, to get at transversity!
• Approach will be tried by HERMES, COMPASS and RHIC Spin • The “other function” which needs to be measured is the Collin’s function. Hermes collaboration recently measured this for the first time for pions.
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Recent excitement in polarized DIS: Single spin azimuthal asymmetries…HERMES
Lepton Hadron Colliders, Snowmass01 July 3, 2001 • First attempt at measuring Collin’s function • Measured for positive and negative pions • Found that positive pion asymmetries are positive, while the negative pions are consistent with zero.
• This now in future can be used for getting to the transversity measurements.
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Crossing the x-Q2 Barrier in DIS: Low-x surprises!
• Elastic e-p scattering (SLAC, 1950s) Q2 ~ 1 GeV2
Finite Size of proton
• Inelastic e-p scattering (SLAC, 1960s) Q2 > 1 GeV2
Parton structure of the proton
• Inelastic m -p scattering off p/d at CERN (1980s) Q2 > 1 GeV2
Unpolarized EMC effect
• Inelastic e-p scattering at HERA/DESY (1990s) Q2 > 1 GeV2
Unexpected rise of F2, Study of pQCD and QCD through various physics processes, diffraction…
What NEXT?
A higher energy collider? Or Denser Target “A”?
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Physics with “Spin” full of Surprises!
•Stern & Gehrlach (1921): Space quantization associated with direction •Goudschmidt & Ulhenbeck (1926): Atomic fine structure and electron spin magnetic moment •Stern (1933): •Kusch (1947): Proton anomalous magnetic moment m
p
Electron anomalous magnetic moment / m
e
1 .
00119 m 0 •Prescott & Yale-SLAC Collaboration (1978): m
N
2 .
79 EW interference in polarized e-d DIS, parity non-conservation •EMC (1989): Proton spin crisis/puzzle Low x!
•HERMES (1999): ???? Transverse spin asymmetries ????
Future spin experiments may be best done In collider mode at high energies
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Challenges for Accelerator Physicists
How can we build upon all this?
• The accelerator physics will pave the way… -- High energy, high luminosity lepton hadron colliders, high polarizations of beam Access new kinematic regions for polarized and unpolarized DIS -- Variable beam energies… without much effect on luminosity… -- Ability in Linac as well as rings to have high -polarized e+/e beams… -- Highly dense interaction regions • We should know this months from all the specialists who have gathered here: -- How we can achieve this… and -- What journey we can embark upon within and outside of the Standard Model with such exotic tools… Lepton Hadron Colliders, Snowmass01 July 3, 2001 52