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 ratio

TA=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

injector

Superconducting 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/p separation

CEBAF Large Acceptance Spectrometer (CLAS)

e- : inbending trackse+: outbending tracks

EC e/p rejection factor : 10-2

CC e/p 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)

JLab

SPring8

Giessen calculations

Giessen 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 coll = grv*

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 -> p0 g

M. H. Wood, EMIN2009 1317 Sep 2008

- r w Interference

Interference 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 , w and f signals in the invariant mass spectrum.

The meson: From transparency ratios, width > 200 MeV! Disagreement with CBELSA-TAPS result

momentum dependence? in-medium destructive -r w interference?

The 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