Far forward neutrinos at the Large Hadron Collider · the Large Hadron Collider Weidong Bai, Milind Diwan, Maria Vittoria Garzelli, Mary Hall Reno, Yu Seon Jeong March 25, 2020 ArXiv:

Far forward neutrinos at the Large Hadron

ColliderWeidong Bai, Milind Diwan, Maria Vittoria Garzelli, Mary Hall Reno,

Yu Seon JeongMarch 25, 2020

ArXiv: hep-ph/2002.03012 submitted to JHEP

Many thanks to Yu Seon Jeong and Maria Vittoria Garzelli for the slides

Outline• Brief introduction and reminder of LHC parameters

and geometry.

• Summary of current work on flux calculation and uncertainties

• Future plans for improvements and engagement

• Relation to CERN based projects This work started during the XIIIth Rencontres Du Vietnam (VietNus) July, 2017. MVD, MHR, MVG discussed if the LHCb data could be used to produce better predictions for both high energy atmospheric as well as far forward LHC neutrinos. It seemed feasible after some examination.

Peter Denton from HET has contributed to this effort independently.

The LHC descriptionparameter value

Circumference 27 km(r=4243 m)

depth 100 m

arcs 8 arcs each has 23 cells cell is 106.9 m

insertions8 insertions insertion is a straight section with transition regions at each end.

energy 14 TeV

bunches 3550 (with 7.5 m/25 ns spacing)effective bunches 2808

protons/bunch 1.15E+11

crossing 40 Mhz Luminosity 10^34 cm-2 sec-1

Min-bias event rate 50/crossing=1.6 GHz

inelastic rate ? ~20/crossing = 0.6 GHz inelastic cross sec 60 mbarn

bunch transverse size at IP 17 mu-m

bunch length at 7 TeV 7.5 cm

crossing angle 285 microrad

These parameters will change for HiLum-LHC with L = 10^35 cm-2 sec-1

LHC interaction region

The larger is the crossing angle, θc~285microrad , the smaller is the area of overlap and therefore smaller is the possibility of collision. It is worth noting that while σz is constant over the machine (~7.5 cm), σx varies and assumes its minimum in the Interaction Points.

We have ignored the crossing angle in our calculation, but will need to include it in detailed simulation.

𝜈𝜏

~few cm

17 μm

Ds X𝜏

For 𝛾~100, decay distances will be ~1-3 cm ⟹ size of the neutrino source for the LHC is ~10 cm. The LHC collision region is the most compact neutrino source ever made.

Production geometry

~4 GeV/14 TeV (momentum due to the crossing angle, ignored for the moment)

p

p

⌫, ⌫

gQ

g Q

hadronization

D/B mesons

decay

1

Prompt neutrinos

In pp collision at the LHC, various hadrons are produced.

A number of neutrinos are produced from subsequent decay of the secondary hadrons.

Neutrinos generated from the decay of charmed/bottom hadrons are called prompt neutrinos.

e . g.) π, K, D, B . . . → ν + X

Possible sites for detection

14 Ch.1 Motivation for GridPix

tunnel with a circumference of 27 km was built in order to host the Large Electron-Positron (LEP) collider. The LEP was able to deliver a centre of mass energy from91 GeV up to 209 GeV and was used for the detailed study of the electroweak theoryand the Standard Model in general through the production of the W± and the Zbosons. Until its decommission in 2000 the LEP collider has been used for detailedstudies of the electroweak theory and the precise determination of basic StandardModel quantities like the masses of the W and the Z bosons.

The same tunnel is used currently for the Large Hadron Collider (LHC), figure 1.2.The LHC is currently the world’s most powerful machine for high-energy physicsexperiments. The LHC was designed in order to collide proton beams with a centreof mass energy of 14 TeV and a luminosity of 1034 cm−2 s−1, [17]. In each one of thefour interaction points, one of the large experiments is installed, namely A LargeIon Collider Experiment (ALICE) [18], A Toroidal LHC Apparatus (ATLAS) [19],Compact Muon Solenoid (CMS) [20] and Large Hadron Collider Beauty (LHCb)experiment [21].

Figure 1.2: The current layout of the LHC showing the location of the ALICE,the ATLAS, the CMS and the LHCb experiments. The proton beams areaccelerated in several stages and reach 450 GeV at the Super Proton Syn-chrotron (SPS). Finally the beams are transferred in the LHC where they areaccelerated in opposite directions to their maximum energy.

The proton beams are accelerated in opposite directions in two separate beampipes. The pipes are kept at high vacuum and for the steering of the beams strongsuperconducting electromagnets are used. During operation, the electromagnets arecooled down to a temperature of -271.3 ◦C which is even lower than the temperatureof the outer space (-270.5 ◦C).

Near CMS interaction point (IP)25 m from IP (quadruplet region)90 &120 m from IP (UJ53 & UJ57)240 m from IP (PR53 and PR57)

Near ATLAS IP480 m from IP (TI18 and TI12)

Preliminary site study: Beni et. al., 1903.06564

ATLAS IP

SPS

TI12

LHC

FASER

FIG. 1. Left panel: The arrow points to FASER’s location in service tunnel TI12, roughly 480m east of the ATLAS IP. Credit: CERN Geographical Information System. Right panel: Viewof FASER in tunnel TI12. The trench lowers the floor by 45 cm at the front of FASER to allowFASER to be centered on the beam collision axis. Credit: CERN Site Management and BuildingsDepartment.

installed in LS3 from 2024-26 in time to take data during the HL-LHC era from 2026-35. Despite their relatively small size, FASER and FASER 2 will complement the LHC’sexisting physics program, with remarkable sensitivity to dark photons, axion-like particles,and other proposed particles. In the following sections, we discuss FASER’s location, layout,and discovery potential. Additional details may be found in FASER’s Letter of Intent [9]and Technical Proposal [10]. In the Appendix, an Addendum to this document containsinformation about the interested community, anticipated construction and operating costs,and computing requirements.

II. LOCATION

The side tunnels TI12 and TI18 are nearly ideal locations for FASER [11]. These sidetunnels were formerly used to connect the SPS to the LEP (now LHC) tunnel, but they arecurrently unused. The LHC beam collision axis intersects TI12 and TI18 at a distance of 480m to the west and east of the ATLAS IP, respectively. Estimates based on detailed simula-tions using FLUKA [12, 13] by CERN’s Sources, Targets, and Interaction (STI) group [14],combined with in situ measurements using emulsion detectors, have now confirmed a lowrate of high-energy SM particles in these locations. Additionally, the FLUKA results com-bined with radiation monitor measurements have confirmed low radiation levels in thesetunnels. These locations, then, provide extremely low background environments for FASERto search for LLPs that are produced at or close to the IP, propagate in the forward directionclose to the beam collision axis, and decay visibly within FASER’s decay volume.

FASER is currently planned for installation in TI12. This location is shown in Fig. 1,and is roughly 480 m east of the ATLAS IP. The beam collision axis passes along the floor ofTI12, with its exact location depending on the beam crossing angle at ATLAS. TI12 slopesupward when leaving the LHC tunnel to connect to the shallower SPS tunnel. To placeFASER along the beam collision axis, the ground of TI12 must be lowered roughly 45 cmat the front of FASER, where particles from the ATLAS IP enter.

A schematic view of the far-forward region downstream of ATLAS is given in Fig. 2. Fromthe ATLAS IP, the LHC beam passes through a 270 m-long straight “insertion,” and thenenters an “arc” and bends. Far-forward charged particles are bent by the beam optics, and

3

Figure from 1901.04468

Preliminary study of radiation and backgrounds conducted to select TI12 location.

FASER/FASER-nu collaboration status: European/Japanese collaborations with limited US participation at the moment.

quadrupole absorbers

3.5T0.85 mRad

FASER-nu: 1908.02310: as currently envisioned: 25 cm x 25 cm x 1.25 meters. 1.2 tons of tungsten/emulsion

Detector concepts

Neutrino production All neutrino production is from meson decays. π ± decays (cτ=7.8 m) and K± decays (cτ=3.7 m) (and KL with cτ = 15m) contribute by decaying before they run into detector materials. We have ignored Muon decay. Charm and beauty decays are the most interesting contributions.D± → e / µ (semi)leptonic (33%) m=1870MeV, cτ=311 µm (decay to τ is very small)D0 → e / µ (semi)leptonic (13%) m=1865MeV, cτ=122 µm (no decay to τ due to mass) Ds

± → e / µ (semi)leptonic (6%) m = 1968MeV, cτ = 150µmDs

± → τντ (5.5%) pcm = 182 MeV. This would be the main source of ντ

B± → l±ν l X (11 %) m=5279 MeV, cτ=491 µm (most decays are to D which decay to neutrinos)B± → D X(> 95%)B0,B0 → l±ν l X (11 %) m=5279MeV, cτ=455µm B0,B0 → D X (>90%) Λc → lν l X (~ 10%) m=2286MeV, cτ=60µm (e/µ modes only) τ + → Xντ (100%) m=1776 MeV, cτ=87µm

All other higher mass mesons will cascade down to these to decay semileptonically. Our focus is on nu_tau.

Neutrino sources at the LHC

Figure from Buontempo et al, 1804.04413

This proposal studies the possibility of placing a detector around the beamline at 4< eta <5

Previous calculations• To understand the flux we must understand heavy meson production in the

far forward region.

• Two calculations exist -

• HyangKyu Park, JHEP 10 (2011) 092. Uses Pythia tuned for Tevatron data, and simply performed scaling to LHC energy 14 TeV. Calculations in the forward region have large uncertainties.

• De Rujula, Fernandez, Gomez-Cadenas, NP B405 (1993) 80-108. Performs a more fundamental analysis of production, but did not have the best collider data at that time to make accurate modeling of non-perturbative effects.

• Both above calculations have reasonable agreement. But there are quite likely very large unknown uncertainties.

• The FASER-nu collaboration has used new Monte Carlo codes and PYTHIA tunes.

• FASER-nu has also incorporated beamline geometry to perform estimates for pion and kaon produced neutrinos.

• Our goal is to understand the uncertainties and constraints on these fluxes.

Heavy quark production

• The HQ production cross section:

Perturbative QCD with collinear approximation

In collinear approximation, the partons are assumed to be collinear.

For very forward kinematic region, small transverse momentum corrections can be important for particle kinematics and trajectories.

⤇ kT smearing needs to be incorporated.

E d3σdpQ

3 = dx1 dx2 fi (x1,µF2 )∫

i, j=q,q ,g∑ f j (x2,µF

2 )Ed 3σ̂ ij (x1pp , x2pp , pQ ,mQ

2 ,µF2 ,µR

2

dpQ3

⎡

⎣⎢

⎤

⎦⎥

Transverse momentum smearingGaussian approximation of transverse momentum smearing

The heavy quark production cross section with K_T included:

Transverse momentum smearing:

Includes intrinsic kT due to Fermi motion of patrons inside initial-state protons. approximate effects of initial state showering

f (!kT ) =

1π kT

2 e− kT

2

kT2

⎛

⎝⎜⎜

⎞

⎠⎟⎟

1510.01707

FASER-nu calculation can be multiplied by 595 for comparison (without adjusting for the solid angle)

Our assumption is a 1 m diameter detector at 480 m with 35.6 tons.

This work

FASER-nu

Conclusion• LHC collision is a neutrino source; very compact, intense and very

high energy.

• This source provides a new scientific opportunity in the HL-LHC era

• Measure neutrino cross sections in the TeV range where they are not measured.

• Test for lepton universality with tau neutrinos

• Use the tau neutrino flux for new constraints on tau neutrino to sterile neutrino mixing.

• Constrain heavy quark content and production in the forward region. This is important for astrophysical neutrino experiments.

• The neutrino detector in the forward region is an interesting technical challenge.

Faser/Faser-nu collaborations• FASER was approved by CERN recently as a forward search

experiment for light weakly interacting particles. The detector is meant to see the decay of such particles to SM particles.

• Placement is 480 meters in a service tunnel TI12. The exact geometry is evolving, starting at 10 cm radius and Length of 1.5 m to 1 m radius and length of 5 m.

• FASER-nu will be a neutrino detector that adds to the program of FASER. Current dimensions: 25 cm x 25 cm X 1.35 m with mass of 1.2 tons.

• The FASER program will start in run 3 as approved.

References• Detecting and studying high energy collider

neutrinos with FASER… 1908.02310

• CMS note: 1903.06564

• FASER proposal: 1708.09389, 1812.09139, 1901.04468,

• FASER-nu: 1908.06564

• LHCb data: 1510.01707

simple kinematical considerations. Ds → τ +νmDs

= 1968 MeV and mτ = 1776 MeV therefore the center of mass momentum is relatively low ~182MeV

Eν =mD

2 −mτ2

2(ED − pD cos(θ ))

Eν ≈(mD

2 −mτ2 ) /mD

2

(1− γ 2θ 2 )ED = 0.185 × ED

(1− γ 2θ 2 )τ → X νmτ = 1776, typical center of mass momentum is ~880 MeV τ decays provide the higher energy tau neutrinos.

0 500 1000 1500 2000

0

100

200

300

Ds Energy GeV

NutauEnergyGeV

0

1 mrad

3 mrad

Ds → τ +ν