## Measuring Vector Mesons with Dileptons 

Besides the direct measurement of open charm hadrons, the HFT will serve
as a powerful device to discriminate primordial electrons from
background electrons in the measurement of electromagnetic probes.

Photons and leptons emitted from the early stages of heavy ion
collisions suffer few interactions with their surroundings since they
interact via the electromagnetic force instead of the strong one.
Therefore, electromagnetic probes are ideal tools to study the
properties of matter created by relativistic heavy ion collisions.

On the other hand, processes which produce photons and leptons are rare
and are overwhelmed by photons and leptons from electromagnetic decays
of hadrons and subsequent γ conversions to leptons. In addition, the
final state electrons and positrons are the integrated yield of
radiation over the whole evolution of the colliding system. Despite all
the experimental difficulties, the low and intermediate mass dileptons
have been measured at the CERN-SPS and an excess of radiation above the
hadron cocktail has been observed in the invariant mass region at 0.2 \<
M~ee~ \< 0.6 GeV/c^2^ in semi-central Pb+Au collisions[^1]^,^[^2]. Also,
new and exciting results from the first three RHIC runs indicate that
jets have lost a large fraction of their energy in dense matter and the
hadronic phase is relatively short. We believe that these observations
at RHIC favor experimental measurements of photons and dileptons due to
thermal radiation and will result in a clearer signature than from lower
energy heavy ion collisions.

At STAR, electron identification is made possible by a combination of a
measurement of the energy lost by charged particles due to ionization of
the TPC gas (dE/dx) and a velocity measurement with the time-of-flight
system. The relativistic rise of the electron dE/dx separates the
electron dE/dx from those of hadrons except at the crossovers with pions
at momentum of \~ 0.2 GeV/c, kaons at \~ 0.6 GeV/c, protons at \~ 1.1
GeV/c and deuterons at \~ 1.5 GeV/c. A time-of-flight measurement, using
a requirement that \|1-β\| \< 0.03, eliminates slow hadrons and cleans
up the crossovers. This results in clean electron identification, as
shown in Figure 1. The plot on the top left shows the conventional dE/dx
measurement in the TPC gas as a function of particle momentum. The lower
plot shows the same dE/dx measurement but now requiring at TOF of
\|1-β\| \< 0.03. The upper electron band is clearly separated from the
lower pion band. Heavier particles (e.g. kaons and protons) are
completely removed by the TOF requirement. The right hand plot of Figure
1 shows a dE/dx projection in the transverse momentum range p~T~ =
1.0-1.5 GeV/c. Even at this rather large momentum, the dE/dx band of the
electrons is clearly separated from that of charged hadrons at lower
energy losses.

![](media/image1.jpeg){width="2.7944444444444443in"
height="2.7180555555555554in"}
![](media/image2.jpeg){width="2.9298611111111112in"
height="2.7055555555555557in"}

Figure 1: Left top: TPC dE/dx vs p in d+Au collisions. Left bottom: TPC
dE/dx vs p after TOFr PID selection of \| 1-β \| \< 0.03. Clean electron
identification is achieved. Right: dE/dx from TPC after TOFr PID
selection (left bottom panel) for 1.0 \< p \< 1.5 GeV/c.

These electrons are, to a large extent, from photons converting into
electron-positron pairs γ → e^+ ^e^-^ in the detector material. The HFT
detector will reduce the background electrons and positrons from these γ
conversions. By requiring hits in the HFT, electrons from photon
conversion outside the HFT, i.e. the SVT, SSD, and TPC inner field cage,
are rejected. Figure 2 shows the resulting yield from pure Monte Carlo
simulations (no tracking involved) of conversion electrons when
requiring hits in the TPC only (open circles), hits in the TPC and SVT
(open triangles) and hits in the TPC+SVT+HFT (closed circles). The
requirement of hits in the HFT reduces the detected yield of conversion
electrons by a factor of 50. Full simulation with appropriate pile-up
events shows that the rate of a random ghost track is 5%. This means
that we can achieve a maximum gamma rejection by a factor of 20. To
estimate the signal-to-background ratio in the vector meson
measurements, we take a reasonable conservative approach and assume that
HFT can reject gamma conversion by a factor of 10. Another source of
electron background stems from the semi-leptonic decays of heavy quarks,
charm and bottom. The dominant source at intermediate mass[^3]^,^[^4] is
from the semi-leptonic decays of open charm. Due to the large charm
yield at RHIC energies, the e+e- from charm-anti-charm decays are
comparable to that from gamma conversion and π and η Dalitz decays after
HFT rejection described above. Due to the lifetime of \~ 100 μm of
heavy-flavor hadrons, this charm semileptonic decay background can be
greatly reduced by measuring the displaced decay vertex with high
resolution as provided by the HFT detector. Detailed simulations show
that HFT is capable of rejecting 75% of e+e- pairs from the D0 decay
while preserving 50% of the primordial e+e- pairs. This increases the
signal-to-background ratio by a factor of 2.

![](media/image3.jpeg){width="5.000694444444444in"
height="3.410416666666667in"}

Figure 2: Electron p~T~ spectra from γ conversions reconstructed by
requiring TPC tracking, a TPC hit and 2 SVT hits, or a TPC hit and 2 SVT
hits and 2 HFT hits. The rejection factor with an additional SVT hit (2
hits) is 3 and with an additional HFT hit is 50.

The large reduction in electron background will enable us to observe
electromagnetic decays of short-lived vector meson, e.g. φ, ω →
*e*^+^*e*^−^, and intermediate mass dileptons with a few hundred
thousand central Au+Au events in STAR. The rejection of π^0^ and η
Dalitz decays by a factor of 3 (single track) can be achieved by
measuring both electrons in the pair which is possible due to the large
acceptance of the STAR TPC. Table 1 shows our estimate of central Au+Au
collisions needed in order to observe a signal with a statistical
significance of 3σ. Compared to measurements incorporating only the TPC
and TOF, the HFT will enhance the sensitivity of electro-magnetic vector
meson decay by a factor 20-40. We are in the process of simulating the
effects of the combinatorial background in more detail.

  ------------------------------------------- --------------- ------------
  Detectors                                   ω               φ

  TPC+TOF                                     8M              2M

  TPC+TOF+SVT+HFT                             200K            100K
  ------------------------------------------- --------------- ------------

Table 1: Number of central Au *+* Au events required to observe a 3-σ
signal for ω and φ in their leptonic decay channels under different
detector configurations.

[^1]: A. Marín et al., J. Phys. G: Nucl. Part. Phys. 30, S709 (2004).

[^2]: D. Adamova et al. (CERES Collaboration), Phys. Rev. Lett. 91
    (2003) 042301

[^3]: R. Rapp, *Phys. Rev.* C63, 054907 (2001).

[^4]: R. Rapp, *Pramana* 60, 675 (2003).