## Simulations and Required Performance

3.2 Open Charm and Beauty Measurement

**Open Charm:**

Open charm and beauty spectra and yields are key for the physics goals
of spin gluon structure physics, structure function work in pA
reactions, and as probes of the early stages of heavy-ion collisions.
For heavy-ion collisions, the goal is to confirm that the excess of
electrons observedError: Reference source not found does indeed come
from charm decay, and to then extend the reach in pt to measure the
energy-loss of open charm at high pt as well as the production of
beauty.

![](media/image1.png){width="3.8188976377952755in"
height="2.430708661417323in"}The four layers of the central silicon
barrel provide an accurate measurement of the trajectory and impact
parameter of tracks near mid-rapidity. Single electrons at different
momenta were simulated through the strawman vertex detector. This
simulation was run at zero-field and the hits from the electrons tracked
back to calculate the transverse distance-of-closest approach (DCA) to
the known point-of-origin.

For all cases the DCA resolution is better than or comparable to the cτ 
of charm and beauty decays. The power of this resolution is seen by
comparing the distribution of DCA from charm, beauty and Dalitz decays
of π^0^ in figure 5. The spectra were generated from p+p events (PYTHIA)
passed through PISA.

In the bottom panels of figure 5 are the DCA distributions for electrons
above 1 GeV/c. A DCA cut of 200 μm removes the majority of Dalitz
contribution from the electron yield. Note this is the momentum of the
electron and given the large Q-value of the D-decay, these electrons
predominantly come from low-momentum D's. Since beauty decays have
longer lifetimes, the electrons from B decays dominate at large DCA
values. By fitting the full DCA distribution with the expected shapes
from the different cτ we should be able to simultaneously extract
integrated charm and beauty yields for electron momenta above 1 GeV/c.

This process is more difficult for electron momenta closer to 500 MeV/c,
but even at this momentum charm dominates the DCA distribution above 200
μm for Si thicknesses between 1 and 1.5% of a radiation length. In
particular the vertex detector will provide a dramatic improvement over
the previous measurement^15^, which was systematics limited (at 40%) due
to uncertainties in the background subtraction. Figure X shows the
"signal to noise" for the optimistic (0.01 X~0~) and pessimistic (0.02
X~0~) cases compared to the measurement without the vertex detector
available. This should allow a much cleaner extraction of the
background-subtracted electron spectra for p~T~\>0.4 GeV/c than was
previously available, including the charm-specific d^2^N~e~/dydp~T~ for
electrons up to about 2.5 GeV/c.

![](media/image2.jpeg){width="5.992361111111111in"
height="5.686111111111111in"}

![](media/image3.wmf){width="5.992361111111111in"
height="4.301388888888889in"}

Figure XX shows that there is a useful correlation between the p~T~ cut
applied to the electrons and the p~T~ of the parent D meson. The points
represent the most probable value of the parent p~T~ while the error bar
represents the FWHM spread.

![](media/image4.wmf){width="5.992361111111111in"
height="4.301388888888889in"}

For higher values of the electron p~T~, B-decays dominate the charm
yield (see below), and we will measure charm spectra via hadronic decay
channels. As an illustration of hadronic decay we plot the three body
invariant mass distribution for D=\>Kππ in Figure 6. This simulation is
for p+p from Pythia events embedded in a HIJING Au+Au background event.
Simultaneous cuts are placed on three displaced tracks (700μm), that the
tracks are with 300μm of each other and in addition the parent D must
point within 300μm of the collision vertex. These cuts produce a
reasonable signal/background. The challenge will be the low rates at
high-pt. As an estimate if we assume Au+Au 4\*blue-book luminosity, 50
full days/year, yield Au+Au= AA\*(yield p+p) then the D=\>Kπ above pt
\>4 GeV/c will produce into the PHENIX acceptance 30kevents/year in p+p
running and 10k events/year in Au+Au. This estimate is very preliminary
and requires more study regards TOF and aerogel acceptance. Triggering
on these events is discussed in section 3.3.

![](media/image5.jpeg){width="4.081889763779528in"
height="2.2992125984251968in"}

**Open Beauty:**

B meson production, while much rarer than D production, is somewhat
simpler to measure. The challenge is the relatively low rate. There seem
to be at least two possibilities;

-   Semileptonic decays: Since beauty mesons have a larger lifetime than
    charm mesons, it is possible to extract the beauty yield at low
    transverse momentum from the distribution of decay distances. At
    large transverse momentum beauty decays dominate the DCA
    distribution.

-   The decay channel B =\> J/ψ produces J/ψ that are displaced from the
    collision.

For momenta greater than 3 to 4 GeV/c electrons with displaced vertexes
are dominated by beauty decays. This is clearly seen in figure 8. By
placing a DCA cut on the order of 150μm we should be able to cleanly
separate electrons from beauty from all other sources. Note that at
these momenta it will not be possible to extract a charm source using
electron decay. As discussed in the previous section this will be done
via hadronic decays of charm.

![](media/image6.jpeg){width="5.993055555555555in"
height="6.127083333333333in"}

The signal to background ratio for beauty decays at high momentum is
shown in Figure Y. The effect of the DCA cut is even more favorable than
for charm.

![](media/image7.wmf){width="5.992361111111111in"
height="4.301388888888889in"}

**B =\> J/ψ=\>e^+^+e^−^  fell through the cracks? Pat?**