Do Particle Masses Change in the Nuclear Environment

1
Do Particle Masses Change in the Nuclear
Environment?

• Dr. Michael Wood
• University of South Carolina
• C. Djalali Univ. South Carolina
• R. Nasseripour George Washington Univ.
• D. P. Weygand Jefferson Lab
• and the CLAS Collaboration

2
Fundamental Masses
Most physicists today take a reductionist view
when it comes to describing matter. All matter
is made up of an indivisible quantity or particle.
In this picture, the indivisible particles are
the electrons, quarks, and the particles that
bind them. For this talk, I will focus on the
quarks and the strong nuclear interaction.
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Elementary Particles
The basic pieces of matter are leptons (eg.
electrons) and quarks. Current theory (Standard
Model) says that the forces holding matter
together are also particles, the bosons.
Note on units Albert Einstein told us that
energy and matter are related by the equation
Emc2 where E is energy,m is mass, and c is the
speed of light. So masses can be written in
energy units of GeV/c2. The electron mass is
9.1x10-31 kg or 0.511x10-3 GeV/c2. The proton
mass is 0.938 GeV/c2.
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Building Up Particle Families

• Particles made out of quarks are hadrons
• Hadrons come in two types
• Baryons 3 quarks
• Mesons quark-antiquark pairs
• Quarks carry a color charge
• 3 color charges red, green, blue
• Antiquarks carry anti-color charge
• Hadrons are color neutral.

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Mass of Composite Systems
Nucleon - mass not determined by sum of
constituent masses. Mass is given
by energy stored in motion of quarks
and by energy in color gluon fields
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The Vacuum
In order for ocean waves to propagate, they need
a medium, in this case water.
In the 19th century, the ether was proposed as
the electromagnetic medium. However, it was
never found. Our present understanding is that
the photon propagates by electron-positron
production and annihilation.
For hadrons to propagate, there exists a
quark-gluon nature to the vacuum analogous to the
electromagnetic nature. Unlike the EM vacuum,
the hadronic vacuum is large (300 MeV).
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Variable Quark Mass
Hadronic properties depend on the hadronic
vacuum can change with r
(density) and T (temperature). As
goes to zero, hadron masses go to zero.
How to measure modifications?
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Measured Mass
Consider two initial particles interacting and
forming an intermediate particle which decays
into two final particles.
If we think of A and B as waves and R as a
resonance (standing wave), the amplitude of the
resonant wave is given by the Breit-Wigner
formula
where Gwidth mparticle mass s(pApB)2 pmomentu
m
The probability of measuring the resonance is
called the cross section.
where Gwidth, Eenergy of the system ERresonance
energy
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First Evidence at ee- Measurements CERES/NA45 at
CERN
Large excess observed in PbAu below 0.7 GeV/c2.
Is it a r/w mass shift?
pAu understood in terms of pp superposition.
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Medium Modifications Seem to Explain RHI Results
However

• In AA collisions, the results are integrated
  over a whole range of r and T
• In AA collisions, the interesting phase of
  matter is produced in the
• very early stages of the reaction, generally far
  from equilibrium, making it hard
• to directly compare to the theoretical models
  which all assume equilibrium.
• 3) In AA collisions, many channels are involved

It is desirable to search for medium effects in
simpler reactions.
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Fixed Targets Experiments(T0 and rr0)

• The predicted medium modifications are so large
  that even at
• r normal nuclear density, they can be observed.
• Lets produce vector mesons in nuclei.
• Use beams that leave the nucleus in equilibrium
  (g,p,p???)
• g? A g V X
• ee-

Vector mesons r M768 MeV G 149 MeV
ct1.3 fm JP1- w M782 MeV G 8 MeV
ct23.4 fm f M1020 MeV G 4 MeV
ct44.4 fm
Note 1 fm 10-15 m 208Pb radius 7.4 fm
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KEK Results
pA g VX where Vr,w,f and decays in ee-.
Reported 20.5/- 0.5 MeV shift in a Cu
nucleus. However, their r meson is the excess in
the mass spectrum Background was not fixed by a
physical process.
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Photoproduction of Vector Mesons off of Nuclei
(g7 Experiment)
gA g VX g ee- X

• Photon beam (minimal disturbance to initial sate)
  
• Eg up to 3.8 GeV
• Photon beam illuminates the entire volume
• Targets LH2, LD2, C, Ti, Fe and Pb
• Leptonic decay
• Almost no final state interaction!
• Low branching ratio4.49x10-5
• Rare decay need to produce
• 100,000 rs to have 5 ee- decays.

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CEBAF (Continuous Electron Beam Accelerator
Facility) at Jefferson Laboratory (JLab)
Superconducting Electron Accelerator (338
cavities), 100 duty cycle, Imax200 mA,
Emax6 GeV, dE/E10-4. 1500 physicists, 30
countries, operational since end of 1997
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Hall B

• Smallest experimental hall in size.
• Most complicated detector system.
• Six separate spectrometers for the most
• spatial coverage.
• Designed for detecting many particles in each
• event.
• Two beams electron or photon
• 120 physicists from about 10 countries

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CEBAF Large Acceptance Spectrometer (CLAS)
Electromagnetic Calorimeter Lead-Scintillator
for detecting electrons
Superconducting Torus Magnet 6 Superconducting
coils for deflecting charged particles
EC e/p rejection factor 10-2
e- inbending tracks e outbending tracks
Drift Chambers Ar-CO2 6500 channels/sector to
measure the path of a charged particle
Gas Cherenkov Counter e/p separation
CC e/p rejection factor 10-1
EC/CC rejection factor 10-3 Rejection factor
for ee- 10-6
Time-of-Flight Hodoscope 48 Scintillators/sector f
or measuring a particles travel time
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Event Display
Due to the magnetic field orientation, the
positively-charged particles bend away from the
beamline. The negatively-charged particles bend
inward.
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Multi-Segment Nuclear Target

• Contains materials with different average
  densities.
• LD2 and seven solid foils of C, Fe, Pb, and Ti.
• Each target material 1 g/cm2 and diameter 1.2 cm

• Proper spacing 2.5 cm to reduce multiple
  scattering
• Deuterium target as reference, small nucleus, no
  modification is expected.

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The Mass Spectrum

• Clearly observe w and f mesons.
• Some background remains in the low-mass region.
  It is determined by a mixed-event method.
• The r signal is extracted by removing the
  background and the w and f signals.

All targets combined
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Mass by Target
With the vertex information from the detector,
the mass spectrum was divided according to each
target. Evidence of target absorption the w
and f yields decreased as the number of target
nucleons increased.
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The r Mass Spectra
After removing the w, f, and background
contributions
The vacuum properties of the r meson are m770
MeV/c2 and G150 MeV. In Fe, upper limit with a
95 confidence level Dm 8/- 8
MeV/c2. Broadening of the width is consistent
with many-body effects.
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What Does It All Mean?
Our experiment successfully detected r mesons
being produced by a photon beam and decaying into
electron-positron pairs. Our results indicate a
small mass shift or possibly no shift at all. We
disagree with the KEK measurement which reported
a mass shift within their errors. We rule out
predictions of large mass shifts.

• We are working on a proposal to
• Study any momentum dependence to in-medium mass
  and width.
• Study the w and f mesons by means of absorption.
• Take data with Nb and Sn targets.

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Backup Slides
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Absorption of w Meson and its In-medium Width
The in-medium ? width is proportional to ?
absorption ?(?,p?) ? rvw?wN
Normalized to carbon
Normalized to carbon
M. Kaskulov, E.Hernandez and E. Oset EPJ A 31
(2007) 245
P. Mühlich and U. Mosel NPA 773 (2006) 156
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Comparison of w Meson Results
Normalized to carbon data.
Preliminary g7a result shows greater in-medium
broadening!!!
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Comparison to Expt. f Meson
Normalized to carbon data.
The Spring8 experiment was g A gf A g KK- A
in the photon energy range of 1.5-2.4 GeV.
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Contributions to Hadron Mass
The interesting feature of hadronic physics is
that the hadron mass is not a sum of the
constituent quark masses.
The hadron masses are formed dynamically
If the motion of the quark changes, the mass of
the hadrons will change.
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Simulations - BUU Transport Model

• Monte-Carlo simulations based on Giessen code
  using the BUU transport equations Mosel et al.
  Nucl. Phys. A671, 503 (2000)
• The code includes various decay channels and
  nuclear effects, and was coupled with the CLAS
  detector simulation software (GSIM)
• Generates 7 channels ee- decays of the ?????
  and ? Dalitz decays of the ?0, ?, ? and ?.
• Includes conventional medium effects such as
  Pauli blocking, shadowing for photon induced
  reactions, Fermi motion of nucleons, collisional
  broadening (targets other than proton).
• Mass shift added on demand.

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Fits to the Mass Spectra
Fits were performed with the simulated line
shapes to remove the w and f.
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The Extracted r Mass
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Extracting the Result

• Make ratio of mass spectra of heavy target to
  reference target.
• Fit the slope in region of r meson.
• Compare with relation of slope to the percentage
  change in mass.

Measured mass shift in Fe nucleus 8 /- 8 MeV.
This result is statistically consistent with no
mass shift. If there is a shift, it must be
small. The width is consistent with many-body
effects of the nucleus.
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Mass Ratios
ee- Invariant Mass (GeV/c2)
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Are the r Meson Properties Modified?
a 0.02/- 0.02
Preliminary
Masses are consistent with the PDG value
(m770.0/-0.8 MeV), with collisional broadening
in the widths (G150.7/-1.1MeV).