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«AB-Note-2005-030 BDI On the Potential Use of Zero Degree Calorimeters for LHC Luminosity Monitoring H. Schmickler – CERN 1211 Geneva 23 - CH S. ...»

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EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH

CERN ⎯ A&B DEPARTMENT

AB-Note-2005-030 BDI

On the Potential Use of Zero Degree Calorimeters for

LHC Luminosity Monitoring

H. Schmickler – CERN 1211 Geneva 23 - CH

S. White – Brookhaven National Lab., Upton, NY 11973 USA

Abstract

We discuss the ZDC role in commissioning proton running at LHC.

The ATLAS Zero Degree Calorimeters were designed to meet the needs of the Heavy Ion Program and follow closely experience at RHIC. In ATLAS, as at RHIC, they will be used primarily to measure absolute luminosity, determine reaction plane and centrality. They also provide a trigger sensitive to peripheral events- particularly those from diffractive photoproduction which is a promising area of research at LHC.

Experience at RHIC showed that the ZDC's provide a unique background-free measure of instantaneous luminosity during pp running also. When operated with a calorimeter threshold of

0.1 x pbeam the coincidence rate between calorimeters forward in both beam directions corresponds pp to ~0.4% x σ inelastic. This robust signal is commonly used for accelerator tuning and for vernier scans (where it reliably measures luminosity variations over at least 3 decades) at RHIC.

In this note we argue that the ATLAS ZDC (and similar devices - already in construction for ALICE and likely to be built for CMS also) will complement other available tools for commissioning the LHC with proton beams. In particular the ZDC discriminates effectively against beam-residual gas backgrounds as well as other sources which are not addressed in the ion chamber design and are notoriously hard to anticipate.

Geneva, Switzerland July, 2005 On the Potential Use of Zero Degree Calorimeters for LHC Luminosity Monitoring Hermann Schmickler CERN 1211 Geneva 23 Switzerland e–mail: Hermann.Schmickler@cern.ch Sebastian White Brookhaven National lab., Upton, NY 11973 USA e–mail:white1@bnl.gov Abstract We discuss the ZDC role in commissioning proton running at LHC.

The ATLAS Zero Degree Calorimeters were designed to meet the needs of the Heavy Ion Program and follow closely experience at RHIC. In ATLAS, as at RHIC, they will be used primarily to measure absolute luminosity, determine reaction plane and centrality. They also provide a trigger sensitive to peripheral events- particularly those from diffractive photoproduction which is a promising area of research at LHC.

Experience at RHIC showed that the ZDC’s provide a unique background-free measure of instantaneous luminosity during pp running also. When operated with a calorimeter threshold of 0.1 × pbeam the coincidence rate between calorimeters forward in both beam directions pp corresponds to ∼ 0.4% × σinelastic. This robust signal is commonly used for accelerator tuning and for vernier scans (where it reliably measures luminosity variations over at least 3 decades) at RHIC.

In this note we argue that the ATLAS ZDC (and similar devices - already in construction for ALICE and likely to be built for CMS also) will complement other available tools for commissioning the LHC with proton beams. In particular the ZDC discriminates effectively against beam-residual gas backgrounds as well as other sources which are not addressed in the ion chamber design and are notoriously hard to anticipate.

The outline of this note is as follows:

• comparison of RHIC and ATLAS ZDC geometries and their relative acceptance for forward physics.

• analysis of underlying physics contributing to the RHIC ZDC coincidence rate.

• evaluation of physics event generators used to predict rates at the LHC (MARS code uses DPMJETIII, we focus on Pythia).

• calculation of expected ZDC rates at the LHC.

• discussion of commissioning considerations.

A summary of the RHIC design and the Heavy Ion physics goals which it addresses can be found in references 1-4.

1 ATLAS vs. RHIC ZDC design Zero Degree Calorimeters are devices located in line with the colliding beam directions and placed downstream of the beam separator dipoles. Since the proton (or ion) beam is deflected in this dipole we have the opportunity to measure neutral energy with the calorimeters.

Fig. 1 shows the plan view of the beam layout at RHIC. The (small) available space between the beam tubes led to a design which maximizes uniformity of response with respect to impact location of incident particles. A tungsten Alloy (λI = 9.6cms) absorber and the ”quartz fiber” technique for shower sampling (which captures Cerenkov light produced by shower secondaries) were employed. This design resulted in a response uniformity of better than 90% for impact more than 1 cm from the calorimeter edges.

The RHIC design used PMMA fibers for the sampling layers because of their low cost and the low anticipated radiation doses. The RHIC design also oriented the absorber layers and readout fibers at 45o to the beam direction. The 45o orientation maximizes the light yield of the Calorimeter since the Cerenkov angle for forward going, β ∼ 1.0 shower secondaries in plastic fiber is about 45o.

In the ATLAS design the planes are oriented at 90o with respect to the beam direction.

This orientation was chosen to ensure mechanical compatibility with the LBNL Ion chamber lumi monitors that are to be installed between the 1st and 2nd ZDC modules(ie after 1.6 λI of absorber). The Atlas ZDC also uses glass core fiber instead of PMMA in order to deal with the higher expected radiation doses in the LHC environment.





For the purposes of this note the above differences between the RHIC and ATLAS ZDC design can be ignored as they only concern the actual light yield of the calorimeter for a given deposited energy. The transverse dimensions are the same in the RHIC and ATLAS case (9.4 cms wide by 15 cm high) and the distance from the nominal beam position to the center of the ZDC is 13 cms in the RHIC case and 8 cms in the LHC case..

Figure 1: RHIC interaction region layout.

–  –  –

2.1 A note on collision Geometry It is impossible to get a 2 arm ZDC coincidence from elastic scattered protons in either the RHIC or LHC case. This is due to Interaction region geometry whereby incoming beams start out at the outside of the ring and exit the IR on the inside or vice versa. In this case you can never balance pt since a proton scattered into the ZDC in one beam direction means that the other proton will be scattered away from its corresponding ZDC.

Generally, because of this feature of the collision scheme adopted for both RHIC and LHC, momentum conservation reduces the ZDC coincidence rate. This feature is reproduced in the simulations discussed below.

2.2 Simulations

The purpose of the simulation study is to use RHIC data to validate the available event generators and then use an event generator to extrapolate the physics from RHIC to LHC energies keeping in mind the simple detector description given above.

Our assumption, which we test using available RHIC(PHENIX) pp data, is that for the purposes of our calculation the event generator level adequately describes the coincidence trigger (5) ZDCN S = EZDCS EBeam × 10%”and”EZDCN Ebeam × 10% where EZDCS refers to energy deposition in the ZDC at the South side of IP1, for example.

A detailed simulation of secondary interactions in materials and transport by the accelerator optics of these secondaries is likely necessary for simulating performance of a tracking detectors. However for our case where we are considering energy flow with a large threshold (E 10% × pbeam ) we are not suprised to find that most features are reproduced at the generator level.

For the discussion below, we have used PYTHIA which turns out to agree well with the RHIC data.

2.3 Simulation model It is natural to divide the pp total cross section into 4 basic components which we characterize by distinguishing aspects of the event topology. We give the corresponding cross sections at RHIC energy below.

• elastic, 10 mbarn

• single diffractive, 14 mbarn

• double diffractive, 1 mbarn

• non-diffractive 25 mbarn These individual cross sections grow by roughly a factor of 2 or less at LHC energy (see discussion of R. Engel in Nucl. Phys. B 82 (2000)pp221-231, for example). We will find that σnon−dif f is the main contributor to the ZDCNS coincidence rate at LHC.

For this discussion we refer also to the PHENIX BBC hodoscope.

The BBC consists of 2 hodoscopes that each cover the rapidity interval 3 η 4 on each side of the interaction point.

We model the accelerator acceptance by a simple rule which approximates the beam tube diameters and DX magnet geometry and accept protons with p proton 90% × pbeam. There is essentially no cross section * acceptance for charged mesons in the forward region so we ignore them. The rest of the story is neutral energy in the form of photons and neutrons.

The PHENIX ZDC provides measurement of the total ZDC energy (with a resolution of 21% for 100 GeV neutrons), the distribution of energy within the ZDC transverse to the beam and the depth profile of energy deposit. During a small part of our data taking there was also a scintillator in front of the ZDC which was used to select leading protons.

The main features of the deposited ZDC energy at RHIC are illustrated in Fig. 2 and 3 which show the total energy deposit and the neutral energy deposit in the ZDC for each of the 4 processes listed above (and derived from PYTHIA).

There are basically 2 classes of events:

• diffractive: in this case there is a leading proton sometimes detected in one ZDC opposite a relatively small energy in the other ZDC-presumably from decays into multiparticle final states which are partially detected.

• non-diffractive: in this case there is usually no leading particle and both arms have a continuous distribution of energy peaking at threshold.

Features of the diffractive events are seen clearly in PHENIX data shown in figures 4 and

5. In Fig.4 we show the coordinate of proton impacts in the ZDC shower maximum detector which peaks toward beam center as expected. In Fig. 5 we see the energy distribution of the proton tag candidates (in black) and the energy detected in the opposite ZDC.

Figure 2: Pythia simulation of Energy deposit in RHIC ZDC (red=Single diffractive, blue=double diffractive and green=non-diffractive). Note that relative normalization of 3 curves is arbitrary in both fig.2 and fig.3.

2.4 Comparison of rates, topologies

A comparison of rates predicted by Pythia is summarized in Fig. 6. The main conclusion is that our simulation with the Pythia event generator predicts an effective cross section for ZDCNS coincidence events in PHENIX of 0.5% of σinelastic whereas our measured rate is 0.36%. At RHIC 2/3 of the ZDCNS coincidence rate arises from the non-diffractive cross section. For our purposes this agreement at the level of 20% or so is perfectly adequate for estimating rates at the LHC.

As a further check we also show in Fig. 6 the expected fraction of ZDC coincidence events which also have a BBC coincidence as predicted by Pythia. With our calculated diffractive mix the BBC fraction should be about 33% whereas we measure 22%.

One can use this level of consistency to judge the precision of extrapolation to LHC rates in the ATLAS ZDC. We expect this calculation to be accurate to ± ∼ 50%.

The calculated ZDC coincidence rate for the LHC is found to be

–  –  –

Figure 4: Horizontal coordinate (in cms) of showers in PHENIX- all showers (left figure) and those showers identified as due to a proton (right figure). The blue curve shows the distribution for protons which deposit less than 50 GeV (presumably due to shower leakage). The red curve is for protons which deposit more than 50 GeV.

–  –  –

Figure 6: Comparison of rates, topologies in PHENIX pp data to Pythia predictions.

2.5 Luminosity Dynamic Range

From the above considerations one can draw a few simple conclusions:

• The ZDC rate is a linear measure of relative luminosity at least up to L = 10 33 cm−2 s−1.

–  –  –

ZDC coincidence rate remains linear with machine luminosity. On paper even at a luminosity of 1034 the ZDC rate would be 160% of the bunch frequency, which could be exploited for luminosity readings using mathematical corrections. On the other hand at these high rates the superposition of the signals from 20 events per crossing will generate an almost continous spectrum of deposited energy in the ZDC calorimeters, which render the coincidence technique and the discrimination on the forward signal from single leading hadrons unexploitable. The most reasonable way to use the ZDC as a luminosity monitor above 1033 would probably be to simply sum the total digitized energy per crossing, which will clearly extend measurements beyond 1034. Simulation studies based on recorded data at RHIC are possible, but have not been done yet. We therefore stay for the moment at a pessimistic limit of potential use up to luminosities of 1033.

• The background ZDC rate will be dominated by accidental coincidences between beamgas events in each beam. This is assured by the very large separation between ZDC’s.

No single beam gas interaction produces in-time energy in the 2 ZDC’s. The accidental coincidence rate depends on the square of the probability to have a ”beam gas” or ”beam scrape” event within a bunch crossing- ie (8) ZDC Rbackground = fcrossing × (fraction − of − beam − loss)2 hence the background rate will become negligible once beams are cleaned up. This accidental coinidence rate plagues every luminosity monitor. As long as the actual event rates are as low as a few % of the crossing frequency the accidental rate can be measured online by creating a coinidence of the signal with a one turn delayed signal of the opposite side monitor. Again this limits the use of ZDCs as luminosity monitor to a maximum L of 1033.

• We do not see a potential limitation in the luminosity range down to very low luminosities like 1028. Since the monitor is high bandwidth, it will detect any single interaction with the above efficiency.



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