54
P. Lecoq
therefore required, which will also improve the electron identification and allow
π 0 rejection with good efficiency in high multiplicity events. More generally, a
high stopping power is mandatory to longitudinally contain high energy showers
in a reasonable volume and cost (typically 20–25 X 0 are needed in high energy
calorimeters to contain at least 95% of the shower). Total lateral and longitudinal
containment of the showers is a prerequisite to minimize leakage fluctuations and to
achieve good energy resolution.
Fast scintillation is also an important parameter. In the search for rare events,
and at hadron colliders, one operates at high collision rates, which requires a
short time response of the detectors. Decay times of the order of the bunch
crossing time (typically 25 ns) or even less are necessary. Only optically allowed
(inter-configuration) transitions (like the transition 5d → 4f for Ce 3+ ), crossluminescence, which is intrinsically fast and temperature independent as observed
in Barium Fluoride (BaF 2 ), and strongly quenched intrinsic luminescence (as for
PWO) can give rise to a fast light signal.
The demand for a high light yield is less stringent at high energy (GeV range)
than at low energy (MeV range), because of the high number of scintillation photons
produced even by a poor scintillator, allowing a good signal detection above the
electronic noise. Such low light yield scintillators can therefore also be used for
calorimetric applications in magnetic spectrometers due the rapid development of
silicon photomultipliers (SiPM), with a gain comparable to photomultiplier tubes
(PMT) and with the additional advantages of being very compact and immune to
strong magnetic fields.
However, the high track density and event pile-up at high luminosity colliders
pose serious challenges for physics event reconstruction and analysis. At the Large
Hadron Collider (LHC) at CERN up to 40 pile-up events and more can be produced
at each bunch crossing at the design luminosity of 2.10 34 cm –2 s –1 , which will reach
200 pile-up events when the luminosity will be increased to 10 35 cm –2 s –1 at the
High Luminosity LHC [9]. For a collision region of about 10 cm (bunch length)
the collisions will be distributed over 300 ps (Fig. 3.4 left panel). Precise temporal
association of collision tracks or jets would help mitigate the pile-up. If this can
be done for charged particles at high transverse momentum with particle tracking
detectors this approach will be much more difficult in the forward-backward region
and even impossible for neutral particles. In this case only time-of-flight (TOF)
techniques can be applied as shown on the right panel of Fig. 3.4, where the two
crossing bunches are symbolized by blue and red bars while their overlapping area
is represented by a white bar. Events generated in the centre of the detector (z = 0)
will generate tracks arriving at the same time in the forward and backward regions.
On the other hand, events generated at any time off-centre of the bunch-overlapping
region will exhibit a TOF difference for the tracks generated in the forward and
backward regions, as shown on Fig. 3.4 (t 2 –t 4 , t 5 –t 7 , t 8 –t 10 ). A mitigation factor of
one order of magnitude necessitates a TOF precision of at least 30 ps [9].
P. Lecoq
therefore required, which will also improve the electron identification and allow
π 0 rejection with good efficiency in high multiplicity events. More generally, a
high stopping power is mandatory to longitudinally contain high energy showers
in a reasonable volume and cost (typically 20–25 X 0 are needed in high energy
calorimeters to contain at least 95% of the shower). Total lateral and longitudinal
containment of the showers is a prerequisite to minimize leakage fluctuations and to
achieve good energy resolution.
Fast scintillation is also an important parameter. In the search for rare events,
and at hadron colliders, one operates at high collision rates, which requires a
short time response of the detectors. Decay times of the order of the bunch
crossing time (typically 25 ns) or even less are necessary. Only optically allowed
(inter-configuration) transitions (like the transition 5d → 4f for Ce 3+ ), crossluminescence, which is intrinsically fast and temperature independent as observed
in Barium Fluoride (BaF 2 ), and strongly quenched intrinsic luminescence (as for
PWO) can give rise to a fast light signal.
The demand for a high light yield is less stringent at high energy (GeV range)
than at low energy (MeV range), because of the high number of scintillation photons
produced even by a poor scintillator, allowing a good signal detection above the
electronic noise. Such low light yield scintillators can therefore also be used for
calorimetric applications in magnetic spectrometers due the rapid development of
silicon photomultipliers (SiPM), with a gain comparable to photomultiplier tubes
(PMT) and with the additional advantages of being very compact and immune to
strong magnetic fields.
However, the high track density and event pile-up at high luminosity colliders
pose serious challenges for physics event reconstruction and analysis. At the Large
Hadron Collider (LHC) at CERN up to 40 pile-up events and more can be produced
at each bunch crossing at the design luminosity of 2.10 34 cm –2 s –1 , which will reach
200 pile-up events when the luminosity will be increased to 10 35 cm –2 s –1 at the
High Luminosity LHC [9]. For a collision region of about 10 cm (bunch length)
the collisions will be distributed over 300 ps (Fig. 3.4 left panel). Precise temporal
association of collision tracks or jets would help mitigate the pile-up. If this can
be done for charged particles at high transverse momentum with particle tracking
detectors this approach will be much more difficult in the forward-backward region
and even impossible for neutral particles. In this case only time-of-flight (TOF)
techniques can be applied as shown on the right panel of Fig. 3.4, where the two
crossing bunches are symbolized by blue and red bars while their overlapping area
is represented by a white bar. Events generated in the centre of the detector (z = 0)
will generate tracks arriving at the same time in the forward and backward regions.
On the other hand, events generated at any time off-centre of the bunch-overlapping
region will exhibit a TOF difference for the tracks generated in the forward and
backward regions, as shown on Fig. 3.4 (t 2 –t 4 , t 5 –t 7 , t 8 –t 10 ). A mitigation factor of
one order of magnitude necessitates a TOF precision of at least 30 ps [9].