4 May 2004
Letter of Intent to CERN LHCC LHCC 2003-057/I-012rev.
Measurement of Photons and Neutral Pions in the
Very Forward Region of LHC
O. Adriani(1), A. Faus(2), M. Haguenauer(3), K. Kasahara(4), K. Masuda(5), Y. Matsubara(5), Y. Muraki(5), T. Sako(5), T. Tamura(6), S. Torii(6), W.C. Turner(7), J. Velasco(2)
( The LHCf collaboration (tentative) )
(1) INFN, Univ. di Firenze, Firenze, Italy
(2) IFIC,Centro Mixto CSIC-UVEG, Valencia, Spain
(3) Ecole-Polytechnique, Paris, France
(4) Shibaura Institute of Technology, Saitama, Japan
(5) STE laboratory, Nagoya University, Nagoya, Japan
(6) Kanagawa University, Yokohama, Japan
(7) LBNL, Berkeley, California, USA
Abstract
An energy calibration experiment is proposed for ultra high energy cosmic ray experiments in the energy range between 1017eV and 1020eV. Small calorimeterswill be located between the two beam pipes in the “Y vacuum chamber” 140m away from the interaction point of the Large Hadron Collider. Within an exposure time of a few hours at luminosity ≈1029 cm-2s-1, very important results will be obtained that will resolve long standing quests by the highest energy cosmic ray physics experiments.
- Research Purpose
Knowledge of the energy distribution of particles emitted in the very forward region is absolutely necessary for understanding cosmic ray phenomena. So far only one experiment has obtained data in the energy region exceeding 1014 eV; the CERN UA7 collaboration at 2x1014 eV. This experiment observed the energy distribution of photons and neutral pions in the rapidity range of y=5-7 [1].
A very interesting result has recently been reported by the AGASA cosmic ray experiment [2] that observed a considerable number of gigantic air showers in the energy region greater than 1020 eV(Fig. 1). It is quite difficult to confine cosmic ray protons with energies greater than 6x1019eV in our own Galaxy. Even if we assume the existence of a magnetic field of 3 x 10 -6 gauss in the halo of the Milky Way Galaxy, protons with energy over 6 x 1019eV escape from the halo which extends to a radius of 23kpc (Fig. 2). Furthermore the arrival directions of the highest energy cosmic rays do not correspond to any known celestial bodies. There is also difficulty attributing the origin of these highest energy cosmic rays to extra-galactic objects like Active Galactic Nuclei (AGN). Extragalactic protons of this extreme energy are not expected to arrive at the Earth due to photo-nuclear interactions with 2.7K photons by the 3-3 resonance interaction process (formation of Δ(1232) baryons). This is called the Greisen-Zatsepin-Kuzumin (GZK) cut-off. It is also difficult for extreme energy extragalactic particles other than protons to reach the Earth. Within the present scheme of physics, it is very hard to conceive of the source or the origin of such high-energy particles by a bottom-up scenario.
Therefore, the existence of the events above the GZK cut-off (super GZK events) must be explained by a top-down scenario invoking new physics such as the decay of cosmic strings, Z0 burst etc. [3] or by some yet unknown scenario. Within top down scenarios, a hypothesis is involved that Lorentz invariance might be violated [4]. On the basis of this situation it seems that detailed study of super GZK cosmic rays may lead to a break-through in the fields of fundamental particle physics and astrophysics. On the other hand the Fly’s-eye group of Utah has reported observation of a cosmic ray energy spectrum that is consistent with the GZK cut-off [5],[6] (Fig. 3).
At present, we cannot draw a definite conclusion on which result of the Fly’s eye and AGASA groups is correct. In view of this fact, new air shower projects - Auger [7] and TA [8] - are being started and the EUSO project is under consideration [9]. These groups use quite different experimental methods, each of which has advantages and drawbacks. Many of the experimental procedures for deriving the energy spectrum depend strongly on the model of nuclear interactions that is used in Monte Carlo simulations of the air showers. Therefore, in order to calibrate the nuclear interaction models in the Monte Carlo codes we think it is very important to establish the energy spectrum of particles emitted in the very forward region (which is effective for air shower development) at an energy much higher than the UA7 case. Since the laboratory equivalent energy of LHC is 1017eV, the calibration of Monte Carlo codes at such a high energy will give a firm base to explore the GZK problem. We will demonstrate this in more detail with Monte Carlo simulations later in this proposal.
Furthermore it is very important to establish the particle composition of cosmic rays in the energy range between 1017eV and 2x1019eV. According to the experimental results observed by the Utah Fly’s eye detector, the depth of the shower maximum varies from 600g/cm2 to 800g/cm2 in the energy range between 1017eV and 2x1019eV. When we compare these experimental results with a Monte Carlo calculation based on the QGS (Quark Gluon plasma Shower) jet model, the composition must be changed from iron dominant to proton dominant in the components with energies. However when we compare those results with the prediction by the DPM jet model, the composition of cosmic rays must not change in the energy range mentioned above (Fig. 4) [10]. Therefore the correct knowledge of the forward region is very important to produce a definite scientific result on the composition of cosmic rays.
Since the predominant energy flow in cosmic ray showers is in the very forward direction it is much more important to have knowledge of particles emitted near θ = 0 than to have knowledge of high transverse momentum jets near θ*= 90 degrees in the center of momentum frame for which ATLAS and CMS have been optimized. Filling this physics gap in the very forward direction is another very important purpose of the proposed experiment.
We propose to install a small scintillating fiber imaging calorimeter at a forward location 140m from the colliding beam intersection, for example, in the ATLAS intersection region. The final choice of intersection, however, will be determined after discussion with LHC machine people. We will be able to identify neutral pions by measuring individual photon energy (> 100 GeV), incident position and the two-photon invariant mass distribution that shows a clear peak at the neutral pion mass. Our proposal is described in the following sessions.
- Experimental Method
We propose to install a small electromagnetic shower detector in the forward direction 140m from the interaction point. At this location, there is a neutral absorber (TAN) containing a single large diameter beam pipe separated into two small beam pipes as shown in Fig. 5. Copper bar absorbers and a luminosity monitor are located in the space between the small beam pipes (Fig. 6). The dimensions of the space between beam pipes are: 96mm in wide, 190mm in height, and 1011mm in length (Fig. 7). The luminosity monitor will be located behind two or three of the 100mm long copper bars so the monitor is located near the shower maximum of high energy neutrons from the colliding beam intersection.
We propose to install a 54 radiation length (r.l.) tungsten shower calorimeter in place of three of thecopper bars in front of the luminosity monitor. The width of the space between the two small beam pipes is 96 mm as described above. The maximum size of the calorimeter that we want to install there is 90 mm (width) x 355 mm (height) x 290 mm (length). (Note; since the copper bars are each 100mm in length it is desirable to make the length of the calorimeter equal to an integer number of these Cu bars) The calorimeter will be located between the two beam pipes as indicated Fig. 8. The height of the calorimeter can be adjusted by a remote manipulator.
When the calorimeter is installed it will take the place of three copper bars at the front of the TAN instrumentationslot. The calorimeter would then be followed the luminosity monitor, which occupies the length of one copper bar. Six copper bars would then be installed behind the luminosity monitor to fill up the remainder of the TAN instrumentation slot. The number of nuclear interaction lengths of the calorimeter and the three copper bars which it replaces when inserted are nearly equal (to 2.0 nuclear interaction lengths). Consequently the performance of the luminosity monitor would not be strongly influenced by the interchange of calorimeter and copper bars.
The calorimeter is composed of 3 separate calorimeters with a tower structure, with each calorimeter having dimensions 2cm x 2cm x 28cm, 3cm x 3cm x 28cm and 4cm x 4cm x 28cm respectively (Fig. 9). The calorimeters will be mounted on a special aluminum support frame as shown in Fig. 10. The calorimeter is composed of tungsten plates, each plate having a thickness of 1 r.l.. Total weight of the calorimeter is expected to be 15kg. The total thickness is 54 r.l., including one r.l. projected thickness for the Cu beam tube. This length is sufficient to accurately measure the photon energy up to few TeV. A typical shower curve which will be expected to develop in the calorimeter is given in Fig. 11. Our Monte Carlo calculation shows that the energy of showers can be obtained with an accuracy of +/-2.8% for the photons with energy 1 TeV in case that the center of shower hits inside 1.5mm from the edge of the calorimeter. The results are shown in Fig. 12(See also Fig. 22).
To identify the position of a single photon orto resolve the positions of multiple photons, x and y detectors are prepared, each of which consists of a 1 mm x 1 mm square scintillating fiber (SciFi). Signals from SciFi are read out by using multi-anode (=64) photomultipliers (MAPMT), Hamamatsu H7546. The quantum efficiency of H7546 photomultipliers is 20%. The SciFi detectors are installed at depths of 8, 10, and 38 r.l.. The total number of fibers will be 512 which will require 8 MAPMTs.Signals from the MAPMTs are sent to a front-end circuit (FEC) (Fig. 13), including analog ASIC (VA32HDR14), FPGA and 16 bits ADC (Fig. 14). The Viking read-out system,VA32HDR14, has been developed to optimize the signals from the MAPMTs and the linearity is guaranteed up to 19 pC which corresponds to about 2000 MIPs. The FEC is necessary to sample and hold the input analog signal for 1.9 μs, while the analog signal is held, the pulse height will be measured by a 16bit ADC. A data taking rate up to - 3kHz is possible with the system described. . The space above the detector is open so the detector can be installed from above by a small remote manipulator similar to the ones used for Roman pots.
Thin plastic scintillator plates (0.3 cm in thickness) will also be installed at every 2-4 r.l. for triggering and for measuring the total deposited energy. The trigger signal will be derived by using these plate scintillators within a 100ns delay time after arrival of shower signal. The signal of the plastic scintillators will be read out by the small photomultipliers, Hamamatsu R1635 (Fig. 10 shows H3164-10 phototubes) which have a quantum efficiency of 25%. The photomultipliers have a dynamic range between 1 to 1000 particles (MIPs) and the pulse height will be measured by using normal CAMAC ADCs. (LeCroy 2249W) The photomultipliers will be set on a plate located 190mm from the bottom of the aluminum frame and high voltage will be supplied by using E1761 sockets for each photomultiplier.
In actual operation, after calibration of the energy deposited by a single MIP, the high voltage for the photomultipliers will be reduced from 900V to 500V. Then they can measure up to fifty thousand MIPs in the plastic scintillator. This corresponds to the number of electrons produced at the shower maximum of a 5 TeV photon. (see Fig. 11a, multiply the shower curve for 1TeV by five). The calibration of the gain of the photomultipliers is a very important task for this experiment. We will prepare a laser calibration system for this calibration. A short laser pulse with a typical duration of 10ns will be created and the light will be separately sent to each photomultiplier by the optical fiber, after passing through a shutter-filter system with the attenuation of the light intensityby 1 (0dB), 1/10 (20db) , 1/100 (40db) and 1/1000 (60db) times. Those four different intensities of the light will be read out under normal voltage (900V) where we can see the peak of the energy deposited by a single MIP and under lower voltage (500V) where we can measure the light produced by 50,000 MIPs.
- The Beam Condition, Exposure Time and Radiation Damage
To avoid the possibility of photons from multiple pp collisions entering the calorimeterduring a single bunch crossing it is desirable to operate the LHC at luminosity less than 1x1030/cm2sec. The expected collision rate should be one event per every ~10 microseconds. (one interaction of any kind every 10 microsec ) Since the ADCs for the multi-anode photomultipliers convert every charge entering within 10 microseconds, we prefer to put ≤10 approximately equally spaced bunches in the machine with ~2x1010 protons in each beam bunch. Under these conditions we expect the beam spread to be ≥ 60-80 microns (in r.m.s. radius). (Note : we shall develop in 2004-2005 years, a new ASIC which has an analogue gate of 100ns.)
Assuming the beam conditions described above have been achieved, the detector will be installed with the middle calorimeter positioned on the expected center of the photon flux from pp interactions (assuming zero crossing angle). We will then take data to establish where the center of the photon flux appears on the SciFi arrays. If everything goes well this will be finished in ten seconds. Of course it will take longer than this to understand the data and we also wish to repeat this procedure a few times, moving the calorimeter array vertically with the remote manipulator until we are satisfied that the calorimeters are operating properly.
Next we will fix the vertical position of the calorimeter and start the taking data. According to our Monte Carlo calculations, we need only 10 seconds to obtain the two photon invariant mass peak of the neutral pions with luminosity L≈ 1030 /cm2sec. However under the L≈ 1029 /cm2sec, we need 100 seconds as the data-taking time. Since we would like to measure the energy spectrum of the photons as a function of transverse position by changing the position of the calorimeter, probably 20 minutes net exposure time is necessary. These data taking times are undoubtedly shorter than the time needed for on-line verification of the quality of the data. So probably we need a few hours to be sure we have obtained good quality data.
Finally we would like to discuss the possibility ofradiation damage to this detector. Radiation damage characteristics for the silicon detector [11], the optical fiber [12], and the plastic scintillator in the calorimeter [13] have been reported. Typical radiation dose values given are 2x1014MIPs in the silicon detectorand 10kGy for the plastic scintillators. If the radiation doses exceed the above values, thedetectors do not show their proper character. The 10kGy corresponds to an energy deposition in the material of 108 erg /g. A minimum ionizing particle will loose 10-6 erg in scintillator with 3mm thickness, which corresponds to the thickness of present calorimeter. Since the density of plastic scintillator is ~ 1g/cm3, up to 1014MIPsin 3.3cm2 area of the detector, or 3.0x1013 MIPs/cm2 will not cause any remarkable damage. For the luminosity 1030cm-2 s-1, 105 MIPs will pass through the scintillators per second per cm2. This means that 1010 particles per day will pass through 1cm2 of the detector. The detector would not suffer any remarkableradiation damage for up to 3000 days operation which is orders of magnitude longer than we envision the calorimeter being installed. Furthermore during the early running of LHC, the luminosity will be even less, 5X1028 -1029 /cm2/sec [14]. It is clear that radiation damage is not an issue for our calorimeter.
4. Some Results from Monte Carlo Calculations
4.A The importance of measurement of production cross sections for interpretation of cosmic ray air shower experiments
. We introduce some results that are expected to be obtained from the proposed experiment and indicate how they will be useful for interpretation of cosmic ray shower experiments. These results have been obtained by Monte Carlo calculations. In Fig.15, we represent how important it is to measure the very forward region. The simulation has been done using the dpmjet model 3 which includes pythia and phojet. The Monte Carlo simulation has been done for showers with an inclination angle 60 degrees and for incident energy E0 = 1017eV. The bottom curve of Fig 15 shows the shower development contributed by pions and kaons emitted in a region of X < 0.1 and the middle curve represents the shower curve produced by photons emitted in a region of X < 0.05. The top curve is the shower curve without neglecting any components of the shower. From this graph you can understand the importance of the contribution of particles with large values of the Feynman variable X for total shower development.
For the next step, we artificially change the Monte Carlo generator in the region of X= 0.01-1.0. Of course the generator has been built to obey energy conservation. As shown in Fig. 16, the type A production cross-section deposits its energy in the deeper region of the atmosphere, while the type B cross-section leads to the early development of showers. If we do not know the production cross-section in the very forward region, we shall misunderstand an incident proton incident for curve A and an incident iron nucleus for curve B. Therefore the establishment of the very forward cross-section is very important. In Fig. 17a, we presentthe results of simulation with a more realistic cross-section model which could be obtained by actual experiments. In other words, Fig. 17a represents the differential cross-section possibly obtained by the proposed experiment. Fig. 17b is the same but for neutral pions. Fig. 17c shows that our experiment will be able to distinguish clearly production models presented by curve A and curve B in Fig. 16 for neutrons.