Beam-induced and cosmic-ray backgrounds observed in the ATLAS detector during the LHC 2012 proton-proton running period

This paper discusses various observations on beam-induced and cosmic-ray backgrounds in the ATLAS detector during the LHC 2012 proton-proton run. Building on published results based on 2011 data, the correlations between background and residual pressure of the beam vacuum are revisited. Ghost charge evolution over 2012 and its role for backgrounds are evaluated. New methods to monitor ghost charge with beam-gas rates are presented and observations of LHC abort gap population by ghost charge are discussed in detail. Fake jets from colliding bunches and from ghost charge are analysed with improved methods, showing that ghost charge in individual radio-frequency buckets of the LHC can be resolved. Some results of two short periods of dedicated cosmic-ray background data-taking are shown; in particular cosmic-ray muon induced fake jet rates are compared to Monte Carlo simulations and to the fake jet rates from beam background. A thorough analysis of a particular LHC fill, where abnormally high background was observed, is presented. Correlations between backgrounds and beam intensity losses in special fills with very high $\beta^*$ are studied.

30 March 2016

Contact: DAPR coordinators internal

JINST 11 (2016) P05013

e-print arXiv:1603.09202 - pdf from arXiv

Inspire record

Figures | Tables | Auxiliary Material

Figures

Figure 01

The general layout of the LHC [4]. The dispersion suppressors (DSL and DSR) are sections between the straight section and the regular arc. In this paper they are considered to be part of the arc, for simplicity. LSS denotes the Long Straight Section – roughly 500 m long parts of the ring without net bending. All insertions (experiments, cleaning, dump, RF) are located in the middle of these sections. Beams are injected through transfer lines TI2 and TI8. The ATLAS convention of labelling sides by `A' and `C' is indicated.

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Figure 02

Background and luminosity signal seen by LUCID in the high-β^* fill (discussed in section refsect:highbeta). Both data-sets average over three ATLAS luminosity blocks, i.e. about 180 s.

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Figure 03

Afterglow distribution, as seen by the BCM-TORx algorithm, at the end of the abort gap and in the region of the unpaired bunches. The signals associated with the unpaired bunches are seen in odd BCIDs from 1 to 23. The asymmetry of the rate distribution in them will be discussed in section refsect:ghostlumi. For the purpose of plotting the BCID numbers in the abort gap have been re-mapped to negative values by BCID = BCID_true - 3564.

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Figure 04a

Single sided BCM-TORx rates of unpaired and empty bunches during periods with separated full energy beams before (a) and after (b) noise subtraction and rescaling to match the primary beam-background rates of both beams, as explained in the text. For clarity, signals in odd and even BCIDs are shown with different symbols. The unpaired bunches are all in odd BCIDs.

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Figure 04b

Figure 05a

Response of the BCM y+ station in the BCIDs defined for beam-1 (a) and beam-2 (b) in the events triggered by L1_BCM_AC_CA_UNPAIRED_ISO. Data with 1368 colliding bunches until the BCM noise period are used.

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Figure 05b

Figure 06

Parameters A (a), b (b) and f (c), resulting from the fit of equation refeq:fit to data in periods 1--4, as detailed in table refbeam-periods.

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Figure 07

BCM background rate for both beams normalised by bunch intensity (a) and after an additional normalisation with the pressures at 22,m and 58,m (b) using Eq.,refeq:fit with the parameters as shown in figure,reffig:fitparams. The shaded area indicates the period when the BCM was noisy.

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Figure 08a

Left: rate of L1_BCM_Wide triggered events in unpaired BCIDs for the period prior to swapping the unpaired bunches. The solid symbols show the rate after vertex requirement and the open symbols with a vertex veto. Right: rate of L1_J10 triggered events in the same period, and after swapping the unpaired trains.

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Figure 08b

Figure 09

Charge in individual RF buckets (a) as measured by the LDM in LHC fill 3005. The buckets corresponding to the 50,ns beam structure are shown by the larger symbols. The signal of each bucket has been normalised by the sum of signals from all buckets of the beam. The product (b) of the bucket charges in beam-1 and beam-2.

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Figure 10

Ghost collision rate in unpaired bunches in the 2012 data, estimated from events triggered by L1_BCM_Wide and L1_J10 triggers. In both cases the presence of at least one reconstructed vertex is required. The early BCM points are not included, since the device was not properly timed in.

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Figure 11

The procedure of separating the background components in unpaired bunches. The open symbols in plot (a) show the total single-sided rate seen by the BCM while the solid symbols show the data after subtraction of the background and restricted to the BCID-range of the unpaired punches. Plot (b) shows the three background components separated, as explained in the text.

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Figure 12

The BCM-TORx rate in unpaired bunches due to BIB, afterglow_BIB and ghost collisions for all 2012 fills with 1368 colliding bunches. Only the first 100 LB (∼ 100,minutes) of stable beams in each fill have been used in order to remove fill-length dependence.

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Figure 13

The total non-collision rate in the BCM_Wide trigger for 2012 LHC Fills with 1368 colliding bunches and the individual contributions to this rate. The open circles include a contribution of real ghost collisions due to the 1.1% vertex inefficiency. The jump in the pedestal-related components is due to the appearance of the BCM noise (shaded area), which then gradually decreased. Only the first 100 LB (∼ 100,minutes) of stable beams in each fill have been used in order to remove fill-length dependence due to the decreasing afterglow.

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Figure 14a

Jet times as a function of η in unpaired BCID before 3 August (a,b) and after 10 August (c,d). The (a,c) and (b,d) show unpaired BCIDs for beam-1 and beam-2, respectively. Only fills with 1368 colliding bunches are considered. Negative η corresponds to side C, i.e. outgoing beam-1.

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Figure 14b

Figure 14c

Figure 14d

Figure 15a

Jet times for fake jets at |η|>2 without a reconstructed vertex for the period before 3 August (a) and between 10 August and the swap of the unpaired trains (b). Only fills with 1368 colliding bunches are considered. The plots are normalised by the live-time, the rate is averaged over the six unpaired BCIDs per beam and the bin width is 0.5,ns.

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Figure 15b

Figure 16a

Rate of fake jets in the nominal RF buckets (-2 ns 2) are shown for the unpaired isolated bunches defined for beam-1 (a) and beam-2 (b). Each point corresponds to one LHC fill. Solid symbols represent jets with a timing consistent with the beam direction, while open symbols show jets with timing in the opposite direction. Events with jets at 4 ns
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Figure 16b

Figure 17a

Distributions of the leading jet φ (a) and E_T^miss (b) in the events from colliding bunches triggered by the E_T^miss trigger with an offline requirement of E_T^miss>160 GeV. Data is compared to the Standard Model expectation from the Z→νν+jets and W→ lν+jets electroweak processes. Other pp collision products, such as top and dibosons, that contribute to the total expectation by less than approximately 10% over the whole E_T^miss spectrum are not shown.

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Figure 17b

Figure 18a

The distribution of the ratio f_ch/f_max for leading jets in the events from colliding bunches triggered by the E_T^miss trigger with an offline requirement of E_T^miss>160 GeV (a). The value f_ch/f_max=0.1, used to separate NCB from pp collision processes, is indicated by the black line. The E_T^miss distribution for the NCB events determined by the f_ch/f_max<0.1 selection (b). Data is compared to the Standard Model expectation from the Z→νν+jets and W→ lν+jets electroweak processes. Other pp collision products, such as top and dibosons, that contribute to the total expectation by less than approximately 10% over the whole E_T^miss spectrum are not shown.

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Figure 18b

Figure 19

The rate of NCB in colliding bunches triggered by the E_T^miss trigger with an offline selection of E_T^miss>160 GeV. The sim1% contamination of pp collision products is not subtracted. Each point corresponds to one LHC fill. The technical stops are indicated in the plot.

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Figure 20

Evolution of the ratio of the NCB levels associated to beam-1 and beam-2 in the events triggered by the E_T^miss trigger with an offline selection of E_T^miss>160 GeV in colliding bunches. The beams are separated according to the leading jet η, where the beam-1 (beam-2) contribution is taken from η<-1.2 (η>1.2). Each point corresponds to one LHC fill. The technical stops are indicated in the plot.

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Figure 21

Distributions of the transverse momentum of the leading reconstructed jet in the selected events from the dedicated CRB data-taking. Data are compared to the CRB-muon Monte Carlo simulation.

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Figure 22

Distribution of the multiplicity of jets with p_T>10 GeV and |η|<4.5 in events containing at least one jet with p_T>40 GeV, using the dedicated CRB data, compared to the CRB-muon Monte Carlo simulation.

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Figure 23

Distributions of the leading jet transverse momentum in the fake jet samples obtained from the colliding bunches and the dedicated CRB data. The ratio indicates the fraction of CRB in the NCB sample. Only the fills with 1368 colliding bunches are taken in the former case.

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Figure 24a

Hits in the individual BCM stations in the events triggered by L1_BCM_CA_UNPAIRED_ISO for beam-1 BCIDs (a, c) and L1_BCM_AC_UNPAIRED_ISO for beam-2 BCIDs (b, d), i.e. in all cases opposite to the unpaired bunch direction. The plots (a) and (b) show data before 3 August while (c) and (d) show data from 10 August until the onset of the BCM noise. Only fills with 1368 colliding bunches are considered.

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Figure 24b

Figure 24c

Figure 24d

Figure 25

L1_BCM_AC (L1_BCM_CA) trigger rates per BCID from ghost-BIB before and after the swap of the unpaired trains. The BCM noise period has been excluded. From each fill the first five hours of stable beams have been considered. Fills shorter than that are not included. The mean values correspond to the average of several fills and the error bars indicate the RMS of the mean.

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Figure 26

Evolution of the opposite direction BCM background for both beams throughout 2012. Only ghost bunches in BCIDs with an unpaired bunch in the other beam are considered. The plot is restricted to fills with the 1368 colliding bunches pattern, which all have the same configuration of 6+6 unpaired bunches. Only the first five hours of stable beams are considered and shorter fills are ignored. Data in the period between 27 October and 26 November suffer from the BCM noise and should be ignored.

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Figure 27a

Rate of fake jets associated with ghost-BIB from the opposite beam after mid-August (a) and after the swap of the unpaired trains (b).

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Figure 27b

Rate of fake jets associated with ghost-BIB from the opposite beam after mid-August (a) and after the swap of the unpaired trains (b).

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Figure 28a

Jet times in the direction opposite to the unpaired bunch for fake jets at |η|>2 without a reconstructed vertex for the period before 3 August (a) and between 10 August and the swap of the unpaired bunches (b). Only fills with 1368 colliding bunches are considered. The plots are normalised by the live-time, the rate is averaged over the six unpaired BCIDs per beam and the bin width is 0.5,ns.

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Figure 28b

Figure 29

L1_BCM_AC (beam-1) and L1_BCM_CA (beam-2) background rates in the abort gap region. The points correspond to the average over all fills with the 1368 colliding bunch pattern, using the first five hours of stable beams and the error bars show the RMS of this average, thus reflecting any fill-to-fill variation. Fills shorter than five hours are not considered. Data shown are averages over periods before (a) and after (b) the LHC chromaticity changes. For plotting, the BCIDs in the abort gap have been re-mapped to negative values by BCID = BCID_true - 3564. The high rates just above BCID=0 correspond to the unpaired bunches.

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Figure 30

L1_BCM_CA (beam-2) background rates in the abort gap region for two time-ranges during stable beams. The open circles show the total data. The line represents an estimate of the random coincidence rate, as explained in the text. The solid circles show the data with the random rate subtracted. The fills used for plot (b) are a subset of those used for plot (a). Only fills between mid-August and the onset of the BCM noise have been considered. For plotting, the BCIDs in the abort gap have been re-mapped to negative values by BCID = BCID_true - 3564.

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Figure 31a

Time development (a) of the beam-2 hump sizes in the abort gap in a fill where abort gap cleaning was repeatedly active (shaded areas). The random coincidence background has been subtracted. The BCID-ranges corresponding to the three histograms are indicated in figure (b) where the background in the abort gap for both beams is shown. The data labelled [u] and [v] have been averaged over the periods indicated in (a), i.e. [u] shows an average over [u1]-[u3] of figure (a), and correspondingly for [v]. The hatched areas in figure (b) correspond to the three histograms, i.e. BCID-ranges, shown in figure (a). The curve indicates the estimated random coincidence background.

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Figure 31b

Figure 32

Beam-1 background as a function of time at the start of stable beams in LHC fill 3252, as seen by LUCID (a) and BCM (b). The lines show the averages over BCIDs, while the symbols show individual BCIDs. The bins correspond to ATLAS luminosity blocks, the widths of which varied from 10,s to the nominal 60,s. The dotted histograms are normalised per BCID, since there is no applicable bunch intensity.

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Figure 33

Background rate measured with the Pixel tagging method over 4 minutes during the period of highest rate. For beam-1 the average and the three unpaired isolated BCIDs are shown separately. For beam-2, with normal background, only the average is shown.

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Figure 34

BCM_AC (beam-1) and BCM_CA (beam-2) backgrounds in the high-background fill and a few fills before and after. Unlike the luminosity data in figure reffig:bcm₂13816, these triggers have negligible background contamination, but suffer from a time-resolution of only 300 s. Rates are shown before (a) and after (b) normalisation with the residual pressure. The high-background fill, 3252, is seen as the large excess in the middle of the plots.

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Figure 35a

Azimuthal distribution of clusters in events tagged as background during the high-background phase in fill 3252 (a) and during the normal background fill 3249 (b). Rates have been normalised by minimum-bias events in the same fill to compensate for inefficiencies and overlaps of individual modules. Taking into account the very different vertical scale on the two plots – the histograms for beam-2 are in fact almost identical in the two fills. The small dips reflect a residual effect of the module overlaps which have different effects for tracks from the IP and from upstream BIB events.

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Figure 35b

Figure 36

Background seen by BCM (a, b) and LUCID (c, d) in the high-β^* LHC fill 3216 in the paired (101, 1886) and unpaired (901, 992) bunches. Plots (a) and (c) show the hit rates from beam-1, while plots (b) and (d) show those from beam-2. Plots (e) and (f) give the corresponding bunch intensity loss rates for beam-1 and beam-2, respectively.

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Figure 37

Loss rates (BLMEI₆x3, BLMEI_C6x7) and collimator positions (TCP₆x3, TCP_C6x7 & TCP_D6x7) in IR3 (a, b) and in IR7 (c, d). Also shown in plots (c) and (d) are the positions of some roman pots (XRPV_A6x5) in IR5. In the instrument codes the ``x" stands for either ``R" or ``L", depending on the side of the experiment, as defined in section refsect:lhc. For all quantities the vertical unit is arbitrary. Only the relative loss rates are significant and the collimator positions only indicate the times of change.

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Figure 38

Correlation between beam loss rate and the background rates measured by LUCID in the two paired BCIDs 101 (a) and 1886 (d) and two unpaired BCIDs 901 for beam-1 (b) and 992 for beam-2 (c). The data-points falling on the horizontal at small loss rates represent the normal beam-gas and noise level. The points on the diagonal are associated to large losses due to cleaning in IR7. The dashed line is not a fit to the data, but just gives an indication of the slope. Especially in BCID 1886 some large background is seen also for small beam intensity loss. This is due to cleaning in IR3.

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Figure 39

Beam-gas rate per BCID in the events triggered by L1_AC_CA_UNPAIRED_ISO in the data with 50,ns (downward triangles) and 25,ns (upward triangles) bunch spacing. The BCID values corresponding to the unpaired bunches in the 50,ns (25,ns) data are indicated by the bottom (top) x-axis. For the 50,ns data only LHC fills with 1368 colliding bunches taken before the September technical stop are considered. Only the first well isolated unpaired bunches are shown for the bunch pattern used in the 25,ns fill.

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Tables

Table 01

Apertures of primary (TCP), secondary (TCS) and tertiary (TCT) collimators in units of nominal betatronic σ, corresponding to a normalised emittance of 3.5 upmu m [5]. Settings for normal high-luminosity optics (β^*=0.6 m) and the special high-β^* fill (β^*=1000 m), discussed in section refsect:highbeta, are given. ^&dagger;For the 1000 m optics, the given TCP settings varied and the values listed correspond to the tightest settings.

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Table 02

Periods with different numbers of colliding and unpaired bunches. Periods 3 and 4 are separated by TS3 during which changes to the BCM trigger logic were implemented. Since the pattern with 1380 colliding bunches had no unpaired bunches, background monitoring in that period was not possible.

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Table 03

Break-points, which had significant influence on rates or data quality for background monitoring based on BCM (top) and jets (bottom), during 2012 data taking with 1368 colliding bunches. Within a given period data from different fills should be comparable.

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Auxiliary material

Figure 01a

Response of the BCM x- station on side A in the events triggered by L1_BCM_Wide_UNPAIRED_NONISO before (a) and after (b) the September technical stop.

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Figure 01b

Response of the BCM x- station on side A in the events triggered by L1_BCM_Wide_UNPAIRED_NONISO before (a) and after (b) the September technical stop.

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Figure 02a

Ratio between BCM_Wide and J10 trigger rates before and after TS3 (a) and trend of BCM_AC_CA rates after scaling with the 22,m pressure (b).

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Figure 02b

Ratio between BCM_Wide and J10 trigger rates before and after TS3 (a) and trend of BCM_AC_CA rates after scaling with the 22,m pressure (b).

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Table 01

Summary of triggers used and their time windows (ns) with respect to the nominal collision time. The BCM_TORx is not a L1 trigger item, but the count rate provided for luminosity determination, which is used for some analyses of this paper.

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