Measurement of antiproton production in p-He … · Measurement of antiproton production in p-He collisions at LHCb to constrain the secondary cosmic antiproton ﬂux Giacomo Graziani

Measurement of antiproton production inp-He collisions at LHCb to constrain the

secondary cosmic antiproton flux

He atrest

6.5 TeV proton

antiproton

Giacomo Graziani (INFN Firenze)on behalf of the LHCb Collaboration

STARS2017, La Habana, Cuba May 7, 2017

The LHCb ExperimentLHCb is the experiment devoted to heavy

flavours at the LHCFocused on CP violation and rare signaturesin b and c decaysExploiting LHC as the biggest b and c fac-tory on earth

Detector requirements:Forward geometry optimize acceptance

for bb pairsTracking : best possible proper time

and momentum resolutionParticle ID : excellent capabilities to select exclusive decaysTrigger : high flexibility and bandwidth (up to 15 kHz to disk)

è allowed to widen our physics program to include hadronspectroscopy, EW physics, kaon physics, heavy ion physics(pPb and PbPb collisions) . . .

G. Graziani slide 2 STARS2017

SMOG: the LHCb internal gas target

LHCb is the LHC experiment with“fixed-target like” geometryvery well suited for. . . fixed targetphysics!

JINST 3, (2008) S08005Int.J.Mod.Phys.A30 (2015) 1530022

The System for Measuring Overlap with Gas(SMOG) allows to inject small amount of no-ble gas (He, Ne, Ar, . . . ) inside the LHCbeam around (∼ ±20 m) the LHCb collisionregionExpected pressure ∼ 2× 10−7 mbar


SMOG applications

Originally conceived for the luminosity determi-nation with beam gas imagingJINST 9, (2014) P12005allows the most precise luminosity determination(1.2%) among the LHC experiments

Became the LHCb internal gas target for a rich and variedfixed target physics program:pA interactions @ 100 GeV scale: exploring cold nuclearmatter (CNM) effects in heavy flavour productionBridging the gap between SPS and RHIC/LHC energyscales!probing large-x nPDF (intrinsic charm): also relevant forneutrino astronomysoft QCD: relevant for modeling of cosmic ray showers inthe atmosphere and in cosmos

4

Ei2

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dotted grey: Conv. Atm. ie

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solid black: Conv. Atm. ie + Intrinsic Charm (H3A) + BERSS

IceCube astrophysical flux

IceCube atmospheric ie

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dotted grey: Conv. Atm. iµ solid grey: Conv. Atm. iµ + BERSS

solid black: Conv. Atm. iµ + Intrinsic Charm (H3A)

+ BERSS

IceCube astrophysical flux

IceCube iµ

FIG. 2. Left: Comparison of the total atmospheric νe + ν̄e data (IceCube-86 for 332 days) with calculations. The contributionto the νe + ν̄e flux from intrinsic charm for Case (A) for various cosmic ray spectra is shown by the dashed lines (H3A =magenta, H3P = green, H14A = brown, and H14B = magenta. H14A and H14B are on top of each other). The conventionalνe + ν̄e flux [123], conventional νe + ν̄e + BERSS (H3A), and conventional νe + ν̄e + BERSS + intrinsic charm contributionfor H3A are shown. Right: Same as the left panel, but for νµ + ν̄µ [6] (IceCube-79/ 86 for 2 years). This measurement alsoincludes the astrophysical neutrino flux. The astrophysical flux shown in these panels is from Refs. [4].

contribution follows the inelastic cross section [127].

We solve Eqs. 1 – 3 separately in the low and highenergy regime [57, 58, 64, 70, 72]. The final prompt neu-trino flux is a geometric interpolation of the low and highenergy solutions and includes the contribution of all thecharm hadrons, D0, D̄0, D±, D±s ,Λ

+c .

Our calculation improves over the previous esti-mates [56, 57, 78–80] in various important ways. Wenormalize our calculations to the ISR and the LEBC-MPS collaboration data [86, 87], which were not usedin the earliest works. We employ the latest cosmic rayflux measurement, and the experimentally measured nu-clear scaling of the cross section, and a theoreticallymotivated energy dependence of the cross section. Weuse a more updated calculation of the intrinsic charmcross section which takes into account the inherent non-perturbativeness of the process [124, 125] whereas someof these earlier works [78, 79] used a modified pQCD pre-scription to account for the high xF data.

Results: Our predictions for the flux of neutrinos (νµ+ν̄µ or νe + ν̄e) are shown in Fig. 1. The highest, interme-diate and the lowest flux are given by Case (A), Case (B),and Case (C) respectively. We also show the flux calcu-lated by BERSS [69], GMS [72], GRRST [70], HW1 [78],HW2 [79], and ERS w/G [6, 85]. Due to the uncertaintiesin parametrizing the g → cc̄ contribution, the resultingneutrino flux has an uncertainty of a factor of ∼ 5 [70].

Remarkably, we find that the atmospheric prompt neu-trino flux due to intrinsic charm is at the same level asthe pQCD contribution.

The neutrino fluxes due to intrinsic charm are largeenough to be detectable by IceCube. If IceCube does notdetect atmospheric prompt neutrinos at these flux lev-els, then it will imply strong constraints on the intrinsiccharm content of the proton.

In the intrinsic charm picture, the proton preferen-tially forms a charm hadron with a similar energy. Inthe g → cc̄ picture, due to its steeply falling dσ/dx dis-tribution, the charm hadron comes dominantly from aproton at much higher energy. A rapid energy depen-dence, disfavored by Refs. [124, 125], is used in Ref. [78],and this results in a much higher neutrino flux. Our re-sults are slightly lower than the calculation presented inRef. [79] due to the above mentioned refinements.

So far, IceCube has presented upper bounds on promptneutrinos. IceCube assumes that the prompt neutrinoflux is the ERS w/G spectrum and varies the normal-ization. IceCube takes into account the muon veto fordowngoing events via a likelihood analysis. The presentlimit on the prompt neutrino spectrum is 1.06 times theERS w/G flux [7]. These IceCube limits are close to theintrinsic charm prompt neutrino spectrum predictions,implying that IceCube can give information about in-trinsic charm content of the proton in the near future.

In Fig. 2 (left), we compare our calculation for Case (A)and the measurement of the atmospheric νe flux [123].

Laha and Brodsky, arXiv:1607.08240


Charm in p-Ar collisions @ 110 GeV LHCb-CONF-2017-001

D0 yield and J/ψ / D0 ratio vs pseudorapidity

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First result from the LHCb fixed target program,presented at the last Quark Matter conferenceObtained from the first small (few nb−1) p-Ar data sampleResult limited by statistics, but demonstrates the physics potentialDifferential shapes can already constrain high-x PDFs


https://cds.cern.ch/record/2255650

https://indico.cern.ch/event/433345/contributions/2358535/

Cosmic antiprotonsThe recent AMS-02 results provide unprecedentedaccuracy for measurement of p/p ratio in cosmicrays at high energies PRL 117, 091103 (2016)hint for a possible excess, and milder energy de-pendence than expectedprediction for p/p ratio from spallation of pri-mary cosmic rays on intestellar medium (H andHe) is presently limited by uncertainties on pproduction cross-sections, particularly for p-He

Giesen et al., JCAP 1509, 023 (2015)

�posed to the olderdata. The curve labelled ‘fiducial’ assumes

the reference values for the different contributions to the uncertainties: best fit proton and heliumfluxes, central values for the cross sections,propagation and central value for the Fisk potential.

We stress however that the whole uncertainty band can be spanned within the errors.

than primary, �p/p flux. Notice that the shaded yellow area does not coincide with the Min-Med-Max envelope (see in particular between 50 and 100 GeV): this is not surprising, as itjust reflects the fact that the choices of the parameters which minimize and maximize the p̄/psecondaries are slightly different from those of the primaries. However, the discrepancy is notvery large. We also notice for completeness that an additional source of uncertainty affects theenergy loss processes. Among these, the most relevant ones are the energy distribution in theoutcome of inelastic but non-annihilating interactions or elastic scatterings to the extent theydo not fully peak in the forward direction, as commonly assumed [55]. Although no detailedassessment of these uncertainties exists in the literature, they should affect only the sub-GeVenergy range, where however experimental errors are significantly larger, and which lies outsidethe main domain of interest of this article.

Finally, p̄’s have to penetrate into the heliosphere, where they are subject to the phenomenonof Solar modulation (abbreviated with ‘SMod’ when needed in the following figures“). We de-scribe this process in the usual force field approximation [52], parameterized by the Fisk po-tential φF , expressed in GV. As already mentioned in the introduction, the value taken by φFis uncertain, as it depends on several complex parameters of the Solar activity and thereforeultimately on the epoch of observation. In order to be conservative, we let φF vary in a wideinterval roughly centered around the value of the fixed Fisk potential for protons φpF (analo-gously to what done in [25], approach ‘B’). Namely, φF = [0.3, 1.0] GV ' φpF ± 50%φpF . Infig. 1, bottom right panel, we show the computation of the ratio with the uncertainties related

6

Lin et al., arXiv:1612.04001

TABLE I. Mean values and 1σ uncertainties of the propagation parameters derived through fitting

the data of B/C and 10Be/9Be ratios in three propagation models.

DR DR-2 DC

D0 (1028 cm2 s−1) 6.58 ± 1.27 3.59 ± 0.88 1.95 ± 0.50

δ 0.333 ± 0.011 0.423 ± 0.017 0.510 ± 0.034

R0 (GV) 4 4 4.71 ± 0.80

vA (km s−1) 37.8± 2.7 22.6± 3.1 /

dVc/dz (km s−1 kpc−1) / / 4.2 ± 3.2

zh (kpc) 4.7 ± 1.0 3.5 ± 0.8 2.5 ± 0.7

φB/C (MV) 326± 36 334± 37 182± 25

R(GV)1 10 210

/pp

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Tan&Ng + BP01 SYBILL 2.3

EPOS 1.99 EPOS LHC

QGSJETII-04 QGSJETII-04m

AMS-02

FIG. 5. The p̄/p ratio expected from different hadronic models in the DR propagation model,

in comparison with the AMS-02 measurement [4]. The propagation parameters used here give

the best-fit to the B/C ratio, adopted from Ref. [25]: D0 = 6.58 × 1028 cm2 s−1, δ = 0.33, vA =

37.8 km s−1, zh = 4.7 kpc, ν1 = 1.81, ν2 = 2.40. The solar modulation potential is set to 800 MV.

the two interaction models later.

However, even the other four hadronic models provide diverse predictions. Note that the

propagation parameters have been adjusted to fit the B/C data. Therefore the discrepancies

10

no previous measurement of p production in p-He,predictions from soft QCD models vary within afactor 2the LHC energy scale and LHCb +SMOG are verywell suited to perform this measurement


The p-He run LHCb-CONF-2017-002

Data collected in May 2016, with proton energy 6.5 TeV,√sNN = 110 GeV

Using fill for Van der Meer scan(parasitic data taking)Analysis from 9× 1020 protons on targetMost data from a single fill (5 hours)Minimum bias trigger, fully efficient on candidate eventslarge control samples (random triggers) to check trigger efficiencies, deadtime,pileupExploit excellent particle identification (PID) capabilities in LHCb to countantiprotons in (p, pT) bins within the kinematic range

12 < p < 110 GeV/c

pT > 0.4 GeV/c



Detector and AcceptanceJINST 3, (2008) S08005

Int.J.Mod.Phys.A30 (2015) 1530022

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Total acceptance × reconstructionefficiency for antiprotons

Tracking efficiency estimated fromsimulation, validated on (pp) data



RICH PerformanceEur. Phys. J. C 73 (2013) 2431

Particle separation in RICH1 K/p separation vs momentum


Antiproton identification strategyLHCb-CONF-2017-002

Build likelihood function for particle hypothesis using RICH responseUse difference of log likelihood (DLL) between p and K− and p and π−

Fit the 2-dimensional (DLL(p−K), DLL(p− π)) distributions using templates fromcalibration samples in each kinematic bin

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Raw yield for antiprotonsLHCb-CONF-2017-002

Units 103

20.7 25.7 36.8 18.7

37.0 35.9 40.0 31.4 27.9 25.0 22.4 18.7 28.6 21.2 11.4

20.9 21.2 21.3 22.2 19.9 17.5 15.6 12.9 11.9 13.8 12.2 8.4

13.2 16.1 17.3 15.6 15.6 15.9 15.3 13.3 10.1 10.6 10.9 9.0 6.7 2.4

11.6 15.8 17.7 17.2 17.5 17.2 17.6 16.0 14.1 12.4 12.0 12.0 11.3 6.6 3.1

6.7 8.4 10.5 11.9 11.1 12.3 12.2 12.4 11.7 10.1 9.1 8.1 7.6 7.0 5.5 2.0

5.4 6.8 8.3 10.2 11.1 12.6 12.7 11.8 12.9 13.1 12.0 10.8 9.5 9.6 8.3 6.7 3.9 1.8

1.7 2.1 2.9 3.4 4.1 5.3 5.5 6.6 6.4 7.1 7.3 7.4 6.8 6.4 5.8 5.4 4.8 4.6

0.2 0.3 0.3 0.4 0.6 0.8 1.0 1.4 1.7 1.8 2.0 2.1 2.4 2.4 2.4 2.3 2.1 2.0

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Background from hyperon decays

Current analysis limited to “prompt” component (direct production and pfrom strong resonance decays)Can be distinguished from p produced by weak decays of hyperons andsecondary interactions using the excellent LHCb vertexing capabilities

Non-prompt component issuppressed by requiringsmall impact parameter (IP)

]-1c [GeV T

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LHCb VELO

JINST 9 (2014) P09007

Residual detached component estimated to be (2.6± 0.6)% and subtractedSystematic uncertainty estimated from data/MC comparison of IP tails


Background from Residual Vacuum

Residual vacuum in LHC is not so small (∼ 10−9 mbar ) compared to SMOG pressureCan be a concern, especially for heavy contaminants (larger cross section than He), andbeam-induced local outgassingDirect measurement in data: about 15% of delivered protons on target acquired before Heinjection (but with identical vacuum pumping configuraton)

PV Track Multiplicity5 10 15 20 25 30 35 40

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LHCb-CONF-2017-002

Gas impurity found to be small:0.6± 0.2%PV multiplicity in residual vacuumevents is lower than in He events, buthas longer tails è confirm findingsfrom Rest Gas Analysis that resid-ual vacuum is mostly H2, with smallheavy contaminants



NormalizationUsing p-e− elastic scattering

Pro:LHCb sees the purely elastic regime: θ > 10mrad è ϑs < 29 mrad, Q2 < 0.01 GeV2

è cross-section very well known

distinct signature with single low-p andvery low pT electron track, and nothing elsebackground events mostly expected formvery soft collisions, where candidate comesfrom γ conversion or pion from CEP eventè background expected to be chargesymmetric, can use “single positrons” tomodel it in data

SPD hits5 10 15 20

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LHCb-CONF-2017-002Cons:

cross-section is small (order 100 µb, 3 orders of magnitude below hadronic cross section)electron has very low momentum and θ, it showers through beam pipe/detectorsè low acceptance and reconstruction efficiency



Event display of a candidate scattered electron


Electron spectraLHCb-CONF-2017-002

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Very good agreement withsimulation of single scat-tered electronsData confirm charge sym-metry of background

L = 0.443± 0.011± 0.027 nb−1

Systematic from variation of selection cuts, largest dependence is on azimuthal angleequivalent gas pressure is 2.4× 10−7 mbar, in agreement with the expected level in SMOG



Result for cross section: final uncertainties (relative)

LHCb-CONF-2017-002

Statistical:Yields in data/PID calibration 0.7− 10.8% (< 3% for most bins)Normalization 2.5%

Correlated Systematic:Normalization 6.0%GEC and PV cut 0.3%PV reco 0.8%Tracking 2.2%Residual Vacuum Background 0.1%Non-prompt background 0.3− 0.7%PID 1.2− 5.0%

Uncorrelated Systematic:Tracking 3.2%IP cut efficiency 1.0%PID 0− 26% (< 10% for most bins)MC statistics 0.8− 15% (< 4% for pT < 2 GeV/c)



Total relative uncertainty per bin, in per centLHCb-CONF-2017-002

24.7 13.3 27.6 19.1

9.5 9.6 17.5 8.2 8.8 7.9 19.9 22.2 17.0 12.4 9.8

8.5 9.4 9.5 15.0 9.7 9.4 10.2 10.4 15.7 10.2 10.9 10.8

10.7 8.1 9.3 7.9 8.5 8.6 8.0 13.6 23.3 10.7 12.2 9.5 9.8 13.9

14.7 9.6 9.8 8.2 8.5 7.9 8.0 8.1 15.1 21.0 10.4 9.4 11.1 9.7 10.6

17.3 13.7 10.8 11.7 11.4 9.8 9.9 9.2 11.4 11.1 12.1 9.6 9.6 10.4 9.7 13.4

12.4 11.0 10.1 8.7 8.5 10.3 9.8 15.8 9.8 9.2 9.2 9.2 9.5 12.8 11.6 10.7 12.6 17.2

17.5 11.6 12.9 10.7 9.1 12.5 10.3 9.4 10.1 9.4 9.4 9.4 9.5 9.5 10.2 14.0 20.0 25.7

20.6 17.0 16.9 14.5 13.0 12.0 15.8 11.1 10.8 10.8 10.8 11.3 11.2 10.5 12.2 12.3 12.5 25.1

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Result for cross section, compared with EPOS LHCLHCb-CONF-2017-002

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x (21.4 < p < 24.4 GeV/c)-410

x (24.4 < p < 27.7 GeV/c)-510

x (27.7 < p < 31.4 GeV/c)-610

x (31.4 < p < 35.5 GeV/c)-710

x (35.5 < p < 40.0 GeV/c)-810

x (40.0 < p < 45.0 GeV/c)-910

x (45.0 < p < 50.5 GeV/c)-1010

x (50.5 < p < 56.7 GeV/c)-1110

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LHCb Preliminary

Result for prompt production(excluding weak decays of hy-perons)

The total inelastic cross sectionis also measured to be

σLHCbinel = (140± 10) mb

The EPOS LHC prediction[T. Pierog at al, Phys. Rev. C92 (2015), 034906]

is 118 mb, ratio is 1.19± 0.08.



Result for cross section, ratio with modelsLHCb-CONF-2017-002

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Trasverse Momentum (GeV/c)

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Cross section is larger by factor∼ 1.5 wrt EPOS LHC (mostly from

larger p rate per collision).Better agreement with

EPOS 1.99 and HIJING 1.38

Many thanks to T. Pierogfor his advice with EPOS/CRMC!



Conclusions

LHCb started its fixed target programbecoming an unexpected contributor to cosmic ray physics!Many thanks to our colleaugues in cosmic rays community, O. Adriani,L. Bonechi, F. Donato and A. Tricomi for proposing this measurementThe p production measurement in p-He collisions is expected to narrow downsignificantly the uncertainty on the p/p prediction for cosmic rayslooking forward for updates of secondary p calculations


OutlookMore to come on p production:dataset with beam energy of 4 TeV also collectedwill also measure the detached (Λ decays) component

much more to harvest from the SMOG samples: charged particle yields,particle/antiparticle ratios, positrons, gamma, charm, deuterons. . .the fixed target program will be further developed in the coming years: manypossible unique measurements to better understand cold nuclear matter effectssoft QCD physics

the LHCb space missionjust started!


Documents

Measurement of antiproton production in p-He … · Measurement of antiproton production in p-He collisions at LHCb to constrain the secondary cosmic antiproton ﬂux Giacomo Graziani