Report of referee 1

The paper has a good overall structure and is in general conceptually clear and well written.

There are few typos or expressions which do not correspond to common jargon in the field:

Page 2 - At the end of Figure 1 caption: ”and (c) and (d) involve proton disassociation.” The usual terminology is ”proton dissociation” (see Ref [2])

Changed as suggested

Page 7 — At the end of the first paragraph: ”... in pp crossings preceeding and following...” The usual spelling is ”... preceding...” (single ”e”)

Changed as suggested

Page 13 — In the second line: ...”bunch bunch crossing”. The usual terminology is ...”bunch crossing”...

Changed as suggested

In addition, the referee observes a frequent usage of compound words, like ”high-rapidity”, ”beampipe”, ”single-diffractive”, ”double-diffractive” ”diffractive- production”, etc.; having recommended to use hyphens only when really needed, the referee accepts terms that have become common in the jargon of the field, suggesting to use the chosen spelling coherently throughout the text. At present both versions (hyphenated and unhyphenated) are used in the text, apparently without definite criteria.

We have checked every appearance of a hyphen and think them all to be well-placed.

So much concerning textual aspects of the paper; with respect to the content:

A - Central Exclusive Production (CEP) is a main goal of the HeRSCheL Forward Shower Counters (FSC), as stated in the Introduction; however the general reader doesn’t find references to literature on CEP and FSC there or further on in the paper; comprehensive reviews on CEP physics and exper- imental results, as well as on forward detectors at colliders, including FSC systems at the Tevatron and LHC, are given in a special issue of ”International Journal of Modern Physics A”: [1] M. Albrow, V. Khoze and C. Royon (Guest Editors) et al. : Central Exclusive Production in Hadron—Hadron Collisions (Special Issue); Int. J. Mod. Phys. A29, No. 28 (2014) [2] M. Albrow, P. Collins and A. Penzo: Forward shower counters for diffrac- tive physics at the LHC; Int. J. Mod. Phys. A 29, No. 28, 1446018 (2014) Ref. [1] could be cited at the end of the first sentence in the Introduction: ”... in particular Central Exclusive Production (CEP) analyses [1].” Ref. [2] could be cited in the second paragraph of the Introduction: ”... very forward particles interacting in the beampipe or other machine ele- ments [2].”

We are happy to add these references. Since it is impossible to cite the whole review issue you point to, we added [1] as "Prog.Part.Nucl.Phys. 65 (2010) 149-184" and [2] as proposed.

B — The role of FSC counters is to tag ”rapidity gaps”, regions devoid of particles, as signatures of CEP processes: the counters have mostly a ”veto” function. It is therefore important: - each counter having high efficiency for MIPs; - individual dark count being negligible. In section 4.1 the response of one HeRSCheL counter to cosmic muons was determined (Fig 9a) and seems slightly larger than 1pC at 1.1 kV. In the following sections the noise contribution and the empty detector re- sponse are analysed and given in figures in terms of ADC counts. But the ADC scale and dynamic range in pC are not given. Considering a MIP as unit of charge deposited in one counter, it would be interesting to know how many MIPs are equivalent to the noise level, for instance in Fig.11. The readjustment of the voltages in 2015 corresponds to which HV values and what average charge of the MIP signal?

We have added more details about the high-voltage settings used in 2015 and the average charge in units of "MIPs".

C - This information becomes important in the light of Fig. 10 and the reduction of the scintillator response (attenuation length) due to irradiation as a function of the integrated luminosity. This process is described qualitatively, but actual numbers are not given for the loss of signal, the voltage increase to compensate for it and the possible dark count increase due to the HV change

We have added more details about the HV increase in 2016. We did not observe an increase in dark count rate with HV.

In particular if these steps are repeated frequently, there may be an impact on the selection of rapidity gaps and consequently in the background rejection for CEP events. Any estimate? Simulations might probably provide an accurate description of most features discussed above.

Simulations do give us an estimate of the additional rapidity coverage that Herschel provides, but we do not rely on them given the difficulty in modelling the LHC elements between LHCb and points approximately 100m away. Ultimately, we do not consider the precise size of our rapidity gaps to be important; some background always remains where non-CEP activity escapes even Herschel. This background, the characteristics of which depend on the physics channel, necessitates careful treatment typically accomplished via an analysis of the candidate pt^2 spectrum. Herschel reduces the non-CEP background in our CEP analyses significantly, leading to reduced systematic uncertainties arising from analysis of the pt^2 spectrum, but requires no knowledge of the exact rapidity gap size. Since the pt^2 treatment is discussed in Section 5, we prefer not to introduce additional text.

D - Another important parameter for a good selection of rapidity gaps is the pile-up, which also depends on the luminosity; what is the situation of LHCb in this respect?

A typical number for the average number of visible interactions in LHCb is 1.1. Our CEP analyses have traditionally imposed a range of global-event requirements that limit the sample to that fraction of crossings where only 1 pp interaction occurred (around 33% of crossings in this case). The addition of a global event requirement using Herschel further restricts this sample (see the discussion at the bottom of page 16 and top of page 17 (and fig 16))

Report of referee 2

Due to my limited knowledge of the LHCb environment I ended the reading with a doubt in page 9 where in the third line counting from the bottom it is referred that proton beams were injected into the SPS. Is this correct or were the beams injected in the LHC ?

Changed from "injected into the SPS" to "extracted from the SPS".

I have also a suggestion, to increase the font size of the labels SIGNAL and integrated SIGNAL in the top central box of Fig. 8, since they are quite small and difficult to read in the current figure.

Changed, as suggested

Originality:

Scintillators have been used in the past as complementary systems for CEP studies, e.g., on CDF detector where the beam shower counters (BSC) were used as a veto system, as complementary detectors to the Roman Pots high granularity detectors. With this comment, independently of HERSCHEL being inspired or not in CDF or other previous similar detector, I think that the authors should put at least a reference to CDF BSCs. But HERSCHEL is quite original in the extension of the coverage up to almost +/-10 in eta, and in the usage of the scintillator system without Roman Pots in the LHC environment, profiting from a quite cheap detector to be effective in the trigger of CEP events in LHCb.

We agree, and have added a reference to "Int. J. Mod. Phys. A29, No. 28, (2014) 1446009" which provides a nice summary of CDF's use-case.

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