The EMC effect – history and future K. Rith, LNF Frascati 26.5.2008 Quark- and gluon-distributions are different for free nucleons and for bound nucleons inside nuclei
Feb 20, 2016
The EMC effect – history and futureK. Rith,
LNF Frascati
26.5.2008
Quark- and gluon-distributions are different for free nucleons and for bound nucleons inside nuclei
Specifically:
Open question: Do quarks and gluons play any role for the understanding of nuclear forces?
Can at least the short-range part be directly described by the exchange of quarks, gluons or multigluon states? (Analogue: Van der Waals force)Can the model of nuclear forces mediated by meson exchange currents be replaced by a fundamental theory based on the strong interaction between quarks and gluons?Is confinement influenced by the nuclear medium?Do nucleons swell due to the neighbourhood of other nucleons?Do they form multiquark clusters or even one big bag?
Deep-inelastic Lepton-Nucleon-Scattering
Q2 = -(k-k‘)2 = 2EE‘(1-cos) = E - E‘, y = /Ex = Q2/(2M) = fraction of nucleon‘s momentum P, carried by struck quark
From angular and momentum distribution of scattered leptons
P
hadrons
nucleonk= (E, k)
k‘= (E‘,k‘)
* xP1/ Q2
Internal structure of the nucleon Structure functions F1(x,Q2), F2(x,Q2), g(x,Q2)
d21/dxdQ2 = 42/Q4 F2(x,Q2)/x [1 –y –Q2/4E2 + (1 -2m2/Q2)(y2 + Q2/E2)/(2[1 + R(x,Q2)])]
F2(x,Q2) = x zq2 [ q(x,Q2) +
q(x,Q2) ] q = u,d,s,..
R(x,Q2) = [ F2(x,Q2) ( 1 + Q2/2 ) – 2xF1(x,Q2) ] / 2xF1(x,Q2)If RA1(x,Q2) = RA2(x,Q2) :(d21/dxdQ2)A1 / (d21/dxdQ2)A2 = F2
A1(x,Q2)/ F2A2(x,Q2)
End of the 1970‘s:Second generation of DIS experiments: CDHS, CHARM, CCFRR, CHIO, EMC, BCDMS majority used nuclear targets (Fe, CaCO3, C,.. ) Main aim: study scale breaking of structure functions predicted by QCD, determine QCD, gluon distribution g(x,Q2) via Altarelli-Parisi equations
Underlying assumption: Quark and gluon distributions obtained from nuclear targets are identical to those from free nucleons
Assumption: Nucleons do not change their internal properties (mass, radius, spin…) when being embedded in nuclei
Apart from Fermi-motion
qN(x) is convolution ofquark momentum distribution in free nucleon andnucleon momentum distribution in nucleus
Bodek, RitchieBerlad et al.
…………….. Frankfurt, Strikman
The EMC experiment at CERN
H2, D2
Fe calorimeter target
Fit to Fe-dataExpectation for D-data including Fermi-motion
EMC data for F2N(Fe) and
F2N(D)
The EMC effectJ.J. Aubert et al., Phys. Lett. 123B (1983) 275
A lot of excitement: up to now 814 citations
statistical errors
Published: March 31, 1983 25th anniversary
Consequence: Quark (and gluon) distributions are modified by the nuclear environmentBig surprise for high-energy physicists, but in principle expected by nuclear physicists and possible effects discussed in the 70th at several conferences about ‚Quarks in nuclei‘First review:
Proceedings of the 18th Rencontre de Moriond, March 13-19, 1983, pp. 207-222
!
Data from SLAC - 1
H
D
ee‘
N1 = NWalls + NH,D
N2 = NWalls
NH,D = N1-N2
H,D
1970-72
Archeology 1983
Fe,Al
empty
Data from SLAC-1, archeology
A. Bodek et al., PRL 50 (1983) 1431; PRL 51 (1983) 543
Data from SLAC-2, dedicated experimentR.G. Arnold et al., PRL 52 (1984) 727 ; J. Gomez et al., PRD 49 (1994)
4348
?
Data from SLAC-2, A-dependence
Data from neutrino experiments
EMC Spectrometer – phase 3
Problem with old H and D data at low x due to correlated inefficiencies of drift chambers W4/5, cured by additional proportional chambers P4/5
Data from EMC – phase 3
No enhancement at very low x,Some enhancement at 0.1 < x < 0.3
J. Ashman et al., PL B202 (1988) 603
Shadowing data from EMC – phase 3 M. Arneodo et al., PL B211 (1988)
493
Large-x behaviour Multiquarkclusters – Short Range
Correlations?
Origin: superfast nucleons and/or superfast quarks
SLAC
Large-x behaviour Multiquarkclusters – Short Range
Correlations?
To be studied in detail at JLAB12 – Hall C (E12-06-105)
CLAS, K.S. Egiyan et al., P.R.L. 96 (2006)082501
Overall picture of nuclear effects
Interpretation
Several hundred publications with different approachesNo unique model for the whole x-rangeComplications: ‚Any configuration of quarks, antiquarks and gluons coupled to overall color-singlet can be expanded in a basis of mesons, baryons and antibaryons‘ ‚Nobody knows how to boost the wavefunction of a bound system into the infinite momentum frame‘
Reviews: e.g.: M. Arneodo, Phys. Rep. 240 (1994) 301 D.F. Geesaman et al., Ann. Rev. Nucl. Part. Sci. 45 (1995) 337
Some approches
Convolution
F2A(x,Q2) = dy fc
A(y) F2c(x/y, Q2)
c = ‚cluster‘: N, , , 6q, ………fc
A(y): probability of finding ‚cluster‘ of momentum y in nucleus AF2
c(x/y, Q2): quark distribution in c
c x
A
Badly known, a lot of freedom
Change of confinement scale, swelling of nucleons, i.e., Q2 rescalingIdea: relevant quantity is (QR)Data should be identical for (QDRD)2 = (QARA)2
F2
Q2
small x
large x
Require increase of about 15%But: from quasielastic scattering (y-scaling): radius increase is at most ~3% (Sick et al.)
nucleus
Change of nucleon mass, x-rescaling
A
N pi = (M + Ei, pi )
xi‘ = Q2/2piq = Q2/[2(M+Ei) - 2 pi q]x‘ x / (1 + <Ei>/M) > x, <Ei> - 25 MeVContains both ‚binding correction‘ and ‚Fermi-motion‘
Ei = removal energy
Conventional nuclear physics with improved nucleon wavefuctions, removal energies and correlated many body approach (applicable for 0.3 < x < 0.9 ?)
Reasonable agreement for 0.3 < x < 0.7 room for additional contributions
C. Ciofi degli Atti and S. Liuti, PL B225 (1988) 215
Example:
2
Shadowing at high Q2Generalized vector-meson dominance
model in lab frame (property of photon)
mean free path: L = 1/( VN) 2.5 fm
fluctuation length:d = 2/(mv2 +
Q2) = 15 GeV d 10 fm
L
d
Absorption on surface
A/AN ~ A-1/3
d 1/Mx 1/(1 + mv
2/Q2)Effect dies out for x ~ 0.1
d >> L
Parton-parton fusion: ‚overcrowding‘ of low-x partons in infinite momentum frame (property of nucleus) d‘ d M/p
Lorentz contracted nucleon
DA‘
z
D ~ 1/Q2: transv. resolutionz ~ 1/xp: longt. size of gluonz > d‘, i.e., x < 1/Md 0.1
Low x gluons (and seaquarks) of different nucleons overlap and interact
Modified gluon and quark distributions
D
The NMC experiment at CERN
Main aims:Precision measurement of F2
p, F2D, F2
n/F2p, F2
p-F2n
Precision measurement of F2A1/F2
A2 (x,Q2) and (RA1-RA2)(x,Q2) for several nuclei; dependence on nuclear density and radius
Collected statistics: ~2 108 DIS events
Helium-4 4.00Lithium-6 6.05
Relevant publications from NMC:
P. Amaudruz et al., Z. Phys. C 51 (1991) 387 Z. Phys. C 53 (1992) 73 Phys. Lett. B 294 (1992) 120 Nucl. Phys. B 371 (1992) 553M. Arneodo et al., Phys. Lett. B 332 (1994) 3 Nucl. Phys. B 441 (1995) 3 Nucl. Phys. B 441 (1995) 12 Nucl. Phys. B. 481 (1996) 3-22 Nucl. Phys. B 481 (1996) 23-39
Complementary target setup: Minimize systematic errors due to incident flux I and acceptance A
I1
I2
A1
A2H
HD
D
I1
I2
I3
A1
A2 A3 A4 A5 A6(H)/(D) = (N11 N22)/(N12 N21)
/ = ………………..
6
NMC – Example of target arrangement
E665: M.R. Adams et al., Phys. Rev. Lett. 68 (1992) 3266; Z. Phys. C 67 (1995), 403
Detailed study of shadowing region
Dependence on nuclear mass A and density
Dependence on nuclear radius A1/3
a + b A-1/3
a + b A-1/3 + c A-2/3
Dependence on A1/3 or ?Ultimate experiment: Polarised 67Ho98 (J = 7/2)
4He(=0.089)/3He(=0.051)
4/3 =1.75JLAB-proposal E-03-103,…
But:
precise knowledge of F2
n/F2p at large x required
R4/R3 (4/3)1/3 =1.10 ??
Q2-dependence
Q2-dependence
F2A1/F2
A2 = a + b ln Q2
Sn/C
Gluon ‚overcrowding‘ in infinite momentum frame (property of nucleus)
d‘ d M/pLorentz contracted nucleon
DA‘
z
D ~ 1/Q2: transv. resolutionz ~ 1/xp: longt. size of gluonz > d‘, i.e., x < 1/Md 0.1
Low x gluons (and seaquarks) of different nucleons overlap and interact
Modified gluon and quark distributions
c
cc
J/e+, +
e-, - pt
g
*
(Hard scale: mass of c-quark)
Modification of gluon distributionQCD: If quark distributions are modified by the nuclear environment, then also the gluon distribution must change
Is enhancement at 0.1 < x < 0.3 due to ‚merged‘ gluons?Experimental tool: Inelastic J/-production
P. Amaudruz et al., Nucl. Phys. B 371 (1992) 553
Inelastic J/ production: GSN(x)/GC(x) = 1.13 0.08
Modification of gluon distribution
Modification of gluon distribution
f1(x) = F2Sn(x)/F2
C(x); r(x) = GSn(x)/GC(x) from Q2-dependence
T. Gousset, H.J. Pirner, PLB 375 (1996) 349
xtarget xbeam
proton
proton
}X
}X
-
+
Additional information from Drell-Yan
Additional information from Drell-Yan (E772)
Selection: x1 – x2 > 0.3 Ratio ~ qA1 / qA2
No indication of enhancement of sea-quarks , Valence-only effect?
Additional information from Drell-Yan
Very precise data expected from FNAL Main Injector DY
Additional information from neutrinos - MINERA
Linear combinations of () and ():Separate valence (xF3) and sea (q)
Additional information from neutrinos - MINERA
Also H,D
Overall picture of nuclear effects
Outlook - Polarized EMC effectI.C. Cloet, W. Bentz, A.W. Thomas, PLB 642 (2006) 210
Detailed study of shadowing region - 2
Dependence on nuclear radius A1/3 and scaling parameter n(x,Q2,A)
n number of gluons probed by hadronic fluctuations of photon B. Kopeliovich and B. Povh, PL B367 (1996) 329
a + b A-1/3
a + b A-1/3 + c A-2/3
Nuclear dependence of R = L/T
Dependence of R on A could indicate nuclear effects on g(x) or different higher twist contributions to RA1 and RA2
(d21/dxdQ2)A1 / (d21/dxdQ2)A2 = F2A1(x,Q2)/ F2
A2(x,Q2)requires RA1(x,Q2) = RA2(x,Q2)
Method: use different beam energies Ei
A1/A2(Ei) = (F2A1/F2
A2)[(1+RA2)(1+RA1)] [(1+ziRA1)(1+ziRA2)] (F2
A1/F2A2){1 – R (1-zi)/[(1+R)(1+ziR)]}
withR = RA1 - RA2, R = ½(RA1 + RA2)zi = [1 + ½ (yi
2 + Q2/Ei2)/(1 – yi – Q2/4Ei
2)] -1
Nuclear dependence of R = L/T
3 beam energies Ei: 120 GeV, 200 GeV, 280 GeV
R = RSn – RC = 0.040 0.021 (stat.) 0.026 (syst.)