M. H. Wood, EMIN2009 1
The In-medium Widths of the w and f Mesons in Nuclei
17 Sep 2008
and the CLAS Collaboration
M. H. Wood (Canisius College, Buffalo, NY, USA)M. Paolone, R. Nasseripour, D. P. Weygand, C. Djalali
M. H. Wood, EMIN2009 2
Vector Meson-Nucleon Interaction
Elementary process – experimentally studied by beam incident on hydrogen
target. (T. H. Bauer, R. D. Spital, and D. R. Yennie, Rev. Mod. Phys. 50, 261 (1978))
In-medium interactions – Sum of elementary interactions? Medium effects (modified meson properties)?
Review of V-N properties in the medium (S. Leupold, V. Metag, and U. Mosel, arXiv:nucl-th/0907.2388 (2009))
Mass decrease – more nucleon resonances to couple. Width increase – modification of couplings and virtual loops.
17 Sep 2008
M. H. Wood, EMIN2009 317 Sep 2008
Mesons r: M=768 MeV G= 149 MeV JP=1- w: M=782 MeV G = 8 MeV f: M=1020 MeV G = 4 MeV
With the g7a experiment at Jefferson Lab (JLab), access to the in-medium meson widths.
Reaction : g+ A --> V X --> e+e- X ( no FSI) (Eg < 4 GeV) (Talk on r meson modifications – D. P. Weygand, 19 Sep)
The Widths of w and f Mesons in Nuclei
Nuclear transparency ratioTA=sA/AsN
Measurement : nuclear transparency ratio provides access to in-medium wN and fN total cross sections. Get width from s*
VN.
M. H. Wood, EMIN2009 417 Sep 2008
CBA
south linacnorth linac
injectorSuperconducting e- Accelerator Imax=200 mA Emax=6 GeV , dE/E=10-4. 1500 physicists, ~30 countries operational since 1997
Jefferson Lab (JLab)
M. H. Wood, EMIN2009 517 Sep 2008
Particle Detection In CLAS
Excellent /e discrimination: 5.4x10 -4 for one and 2.9x10-7 for two leptons.
M. H. Wood, EMIN2009 6
Superconducting Torus Magnet
6 Superconducting coils fordeflecting charged particles
Time-of-Flight Hodoscope48 Scintillators/sector
for measuring a particle’stravel time
Electromagnetic Calorimeter
Lead-Scintillator fordetecting electrons
Drift ChambersAr-CO2
6500 channels/sectorto measure the path of a
charged particle
Gas Cherenkov Counter
e/ separation
CEBAF Large Acceptance Spectrometer (CLAS)
e- : inbending trackse+: outbending tracks
EC e/ rejection factor : 10-2
CC e/ rejection factor : 10-1
EC/CC rejection factor : 10-3
Rejection factor for e+e- : 10-6
17 Sep 2008
M. H. Wood, EMIN2009 717 Sep 2008
Event Selection
e+
e-
p
Segmented targetFoils of Carbon, Iron, Titanium, and Lead LD2 (LH2) as control
Sample event
LD2C C C C
FeTi
Pb
M. H. Wood, EMIN2009 817 Sep 2008
e+e- Mass Spectra
Mass Spectra after subtraction of the r meson contribution.
Mass Spectrawith r, w, and fmesons
simulation
M. H. Wood, EMIN2009 917 Sep 2008
f Absorption
SPring8 g A gf A’ g K+K- A’ (Eg=1.5-2.4 GeV)T. Ishikawa et al. Phys. Lett. B 608, 215 (2005)
JLabSPring8
Giessen calculationsGiessen calculations w/ Spring8 absorption strengths
Normalized to carbon
Comparison to other work Comparison to Glauber calculations
In-medium: s*fN ~ 25 – 55 mb
Elementary: sfN < 10 mbNeeded for RHI calculations.
M. H. Wood, EMIN2009 1017 Sep 2008
w AbsorptionThe in-medium width is G=G0+Gcoll where Gcoll = grvs*
VN
Valencia calculations Kaskulov, Hernandez & Oset EPJ A 31, 245 (2007)
Giessen calculationsP. Mühlich and U. Mosel NPA 773, 156 (2006)
Normalized to carbon
JLab (upper/lower limits) CBELSA_TAPSM. Kotulla eta al., PRL 100, 192302 (2008)
M. H. Wood, EMIN2009 11
Momentum Dependence
17 Sep 2008
Possible explanation for disagreement is momentum dependence of the in-medium interactions.
Average w meson momentum:1.1 GeV (CBELSA-TAPS)1.7 GeV (JLab)
Instead of scaling each result, compare a common momentum bin.
CBELSA-TAPS has data in five momentum bins.
M. H. Wood, EMIN2009 1217 Sep 2008
In-medium r-w Interference
€
The e+e - amplitude is modeled as
F = fρ + ie iϕ fωwhere fρ ,ω are relativistic BW shapes.
Constructive
Destructive
Decay channelsJLab : w -> e+e-
CBELSA-TAPS : w -> 0 g
M. H. Wood, EMIN2009 1317 Sep 2008
r-w InterferenceInterference model after subtracting the r meson Data from the 2H target
M. H. Wood, EMIN2009 1417 Sep 2008
CLAS was an excellent tool for these studies: e+e- from rare leptonic decay of light vector mesons are
identified. Clear r, w and f signals in the invariant mass spectrum.
The w meson: From transparency ratios, width > 200 MeV! Disagreement with CBELSA-TAPS result
momentum dependence? in-medium destructive r-w interference?
The f meson: From transparency ratios, in medium total cross section ~ 25-55 mb Agreement with SPring8 result.Next at Jlab: High Statistics measurement of e+e- production on H2 (on tape) Conditionally approved g7b high statistics data (LD2, C, Fe, Nb, Sn)
Summary and Conclusions
